Douglas F. Lake, PhD & Adrienne D. Briggs, MD
A 30-year-old woman has one living child, age 6. Her child and her husband are Rh positive and she is Rho(D) and Du negative. She is now in her ninth month of pregnancy and is in the labor room having frequent contractions. Her Rh antibody test taken earlier in the pregnancy was negative. What immunotherapy is appropriate for this patient? When and how should it be administered?
Agents that suppress the immune system play an important role in preventing the rejection of organ or tissue grafts and in the treatment of certain diseases that arise from dysregulation of the immune response. While precise details of the mechanisms of action of a number of these agents are still obscure, knowledge of the elements of the immune system is useful in understanding their effects. Agents that augment the immune response or selectively alter the balance of various components of the immune system are also becoming important in the management of certain diseases such as cancer, AIDS, and autoimmune or inflammatory diseases. A growing number of other conditions (infections, cardiovascular diseases, organ transplantation) may also be candidates for immune manipulation.
ELEMENTS OF THE IMMUNE SYSTEM
NORMAL IMMUNE RESPONSES
The immune system has evolved to protect the host from invading pathogens and to eliminate disease. When functioning at its best, the immune system is exquisitely responsive to invading pathogens while retaining the capacity to recognize self tissues and antigens to which it is tolerant. Protection from infection and disease is provided by the collaborative efforts of the innate and adaptive immune systems.
The Innate Immune System
The innate immune system is the first line of defense against invading pathogens (eg, bacteria, viruses, fungi, parasites) and consists of mechanical, biochemical, and cellular components. Mechanical components include skin/epidermis and mucus; biochemical components include antimicrobial peptides and proteins (eg, defensins), complement, enzymes (eg, lysozyme, acid hydrolases), interferons, acidic pH, and free radicals (eg, hydrogen peroxide, superoxide anions); cellular components include neutrophils, monocytes, macrophages, natural killer (NK), and natural killer-T (NKT) cells. Unlike adaptive immunity, the innate immune response exists prior to infection, is not enhanced by repeated infection, and is generally not antigen-specific. An intact skin or mucosa is the first barrier to infection. When this barrier is breached, an immediate innate immune response, referred to as “inflammation” is provoked that ultimately leads to destruction of the pathogen. The process of pathogen destruction can be accomplished, for example, by biochemical components such as lysozyme (which breaks down bacterial peptidoglycan cell walls) and complement activation. Complement components (Figure 55–1) enhance macrophage and neutrophil phagocytosis by acting as opsonins (C3b) and chemoattractants (C3a, C5a), which recruit immune cells from the bloodstream to the site of infection. The activation of complement eventually leads to pathogen lysis via the generation of a membrane attack complex that creates holes in the pathogen membrane, killing it.
FIGURE 55–1 Role of complement in innate immunity. Complement is made up of nine proteins (C1–C9), which are split into fragments during activation. A: Complement components (C3a, C5a) attract phagocytes (1) to inflammatory sites (2), where they ingest and degrade pathogens (3). B: Complement components C5b, C6, C7, C8, and C9 associate to form a membrane attack complex (MAC) that lyses bacteria, causing their destruction. C:Complement component C3b is an opsonin that coats bacteria (1) and facilitates their ingestion (2) and digestion (3) by phagocytes.
During the inflammatory response triggered by infection, neutrophils and monocytes enter the tissue sites from the peripheral circulation. This cellular influx is mediated by the action of chemoattractant cytokines (chemokines) (eg, interleukin-8 [IL-8; CXCL8], macrophage chemotactic protein-1 [MCP-1; CCL2], and macrophage inflammatory protein-1α [MIP-1α CCL3]) released from activated endothelial cells and immune cells (mostly tissue macrophages) at the inflammatory site. Egress of the immune cells from blood vessels into the inflammatory site is mediated by adhesive interactions between cell surface receptors (eg, L-selectin, integrins) on the immune cells and ligands (eg, sialyl-Lewis x, intercellular adhesion molecule-1 [ICAM-1]) on the activated endothelial cell surface. The tissue macrophages as well as dendritic cells express pattern recognition receptors (PRRs) that include Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs), scavenger receptors, mannose receptors, and lipopolysaccharide (LPS)-binding protein, which recognize key evolutionarily conserved pathogen components referred to as pathogen-associated molecular patterns (PAMPs). Examples of PAMPs include microbe-derived unmethylated CpG DNA, flagellin, double-stranded RNA, peptidoglycan, and LPS. The PRRs recognize PAMPs in various components of pathogens and stimulate the release of proinflammatory cytokines, chemokines, and interferons. If the innate immune response is successfully executed, the invading pathogen is ingested, degraded, and eliminated, and disease is either prevented or is of short duration.
In addition to monocytes and neutrophils, natural killer (NK), natural killer-T (NKT), and gamma-delta T (γδT) cells recruited to the inflammatory site contribute to the innate response by secreting interferon-gamma (IFN-γ) and interleukin-17 (IL-17),* which activate resident tissue macrophages and dendritic cells and recruit neutrophils respectively to successfully eliminate invading pathogens. NK cells are so called because they are able to recognize and destroy virus-infected normal cells as well as tumor cells without prior stimulation. This activity is regulated by “killer cell immunoglobulin-like receptors” (KIRs) on the NK cell surface that are specific for major histocompatibility complex (MHC) class I molecules. When NK cells bind self MHC class I proteins (expressed on all nucleated cells), these receptors deliver inhibitory signals, preventing them from killing normal host cells. Tumor cells or virus-infected cells that have down-regulated MHC class I expression do not engage these KIRs, resulting in activation of NK cells and subsequent destruction of the target cell. NK cells kill target cells by releasing cytotoxic granules such as perforins and granzymes that induce programmed cell death.
NKT cells express T-cell receptors as well as receptors commonly found on NK cells. NKT cells recognize microbial lipid antigens presented by a unique class of MHC-like molecules known as CD1 and have been implicated in host defense against microbial agents, autoimmune diseases, and tumors.
The Adaptive Immune System
The adaptive immune system is mobilized by cues from the innate response when the innate processes are incapable of coping with an infection. The adaptive immune system has a number of characteristics that contribute to its success in eliminating pathogens. These include the ability to (1) respond to a variety of antigens, each in a specific manner; (2) discriminate between foreign (“non-self”) antigens (pathogens) and self antigens of the host; and (3) respond to a previously encountered antigen in a learned way by initiating a vigorous memory response. This adaptive response culminates in the production of antibodies, which are the effectors of humoral immunity; and the activation of T lymphocytes, which are the effectors of cell-mediated immunity.
The induction of specific adaptive immunity requires the participation of professional antigen-presenting cells (APCs), which include dendritic cells (DCs), macrophages, and B lymphocytes. These cells play pivotal roles in the induction of an adaptive immune response because of their capacity to phagocytize particulate antigens (eg, pathogens) or endocytose protein antigens, and enzymatically digest them to generate peptides, which are then loaded onto class I or class II MHC proteins and “presented” to the cell surface T-cell receptor (TCR) (Figure 55–2). CD8 T cells recognize class I-MHC peptide complexes while CD4 T cells recognize class II-MHC peptide complexes. At least two signals are necessary for the activation of T cells. The first signal is delivered following engagement of the TCR with peptide-bound MHC molecules. In the absence of a second signal, the T cells become unresponsive (anergic) or undergo apoptosis. The second signal involves binding of costimulatory molecules (CD40, CD80 [also known as B7-1], and CD86 [also known as B7-2]) on the APC to their respective ligands (CD40L for CD40, CD28 for CD80 or CD86). Activation of T cells is regulated via a negative feedback loop involving another molecule known as T-lymphocyte-associated antigen 4 (CTLA-4). Following engagement of CD28 with CD80 or CD86, CTLA-4 in the cytoplasm is mobilized to the cell surface where, because of its higher affinity of binding to CD80 and CD86, it outcompetes or displaces CD28 resulting in suppression of T-cell activation and proliferation. This property of CTLA-4 has been exploited as a strategy for sustaining a desirable immune response such as that directed against cancer. A recombinant humanized antibody (ipilimumab) that binds CTLA-4 prevents its association with CD80/CD86. In so doing, the activated state of T cells is sustained. Recently completed vaccine trials in metastatic melanoma patients receiving anti-CTLA-4 antibody reported objective and durable clinical responses in a few patients. Unfortunately, these beneficial responses were associated with the development of autoimmune toxicity in some patients, raising concern about this approach.
FIGURE 55–2 T-cell activation by an antigen-presenting cell requires engagement of the T-cell receptor by the MHC-peptide complex (signal 1) and binding of the costimulatory molecules (CD80, CD86) on the dendritic cell to CD28 on the T cell (signal 2). The activation signals are strengthened by CD40/CD40L and ICAM-1/LFA-1 interactions. In a normal immune response, T-cell activation is regulated by T-cell-derived CTLA-4, which binds to CD80 or CD86 with higher affinity than CD28 and sends inhibitory signals to the nucleus of the T cell.
T lymphocytes develop and learn to recognize self and non-self antigens in the thymus; those T cells that bind with high affinity to self antigens in the thymus undergo apoptosis (negative selection), while those that are capable of recognizing foreign antigens in the presence of self MHC molecules are retained and expanded (positive selection) for export to the periphery (lymph nodes, spleen, mucosa-associated lymphoid tissue, peripheral blood), where they become activated after encountering MHC-presented peptides (Figures 55–2 and 55–3).
FIGURE 55–3 Scheme of cellular interactions during the generation of cell-mediated and humoral immune responses (see text). The cell-mediated arm of the immune response involves the ingestion and digestion of antigen by antigen-presenting cells such as macrophages. Activated Th cells secrete IL-2, which causes proliferation and activation of cytotoxic T lymphocytes, and Th1 and Th2 cell subsets. Th1 cells also produce IFN-™ and TNF-™, which can directly activate macrophages and NK cells. The humoral response is triggered when B lymphocytes bind antigen via their surface immunoglobulin. They are then induced by Th2-derived IL-4 and IL-5 to proliferate and differentiate into memory cells and antibody-secreting plasma cells. Regulatory cytokines such as IFN-γ and IL-10 down-regulate Th2 and Th1 responses, respectively.
Studies using murine T-cell clones have demonstrated the presence of two subsets of T helper lymphocytes (Th1 and Th2) based on the cytokines they secrete after activation. The TH1 subset characteristically produces IFN-γ, IL-2, and IL-12 and induces cell-mediated immunity by activation of macrophages, cytotoxic T cells (CTLs), and NK cells. The TH2 subset produces IL-4, IL-5, IL-6, and IL-10 (and sometimes IL-13), which induce B-cell proliferation and differentiation into antibody-secreting plasma cells. IL-10 produced by TH2 cells inhibits cytokine production by TH1 cells via the down-regulation of MHC expression by APCs. Conversely, IFN-γ produced by TH1 cells inhibits the proliferation of TH2 cells (Figure 55–3). Although these subsets have been well described in vitro, the nature of the antigenic challenge that elicits a TH1 or TH2 phenotype is less clear. Extracellular bacteria typically cause the elaboration of TH2 cytokines, culminating in the production of neutralizing or opsonic antibodies. In contrast, intracellular organisms (eg, mycobacteria) elicit the production of TH1 cytokines, which lead to the activation of effector cells such as macrophages. A less well-defined T-cell subset (TH3) has been described that produces transforming growth factor-β (TGF-β), whose numerous functions include down-regulation of proliferation and differentiation of T lymphocytes.
Recently, a subset of CD4 T cells that secrete IL-17 (TH17) has been implicated in neutrophil recruitment to sites of inflammation. The regulatory T (Treg) cells comprise a population of CD4 T cells that is essential for preventing autoimmunity and allergy as well as maintaining homeostasis and tolerance to self antigens. This cell population exists as natural Treg (nTreg), derived directly from the thymus, and induced (adaptive) Treg (iTreg), generated from naïve CD4 T cells in the periphery. Both populations have also been shown to inhibit antitumor immune responses and are implicated in fostering tumor growth and progression. Recent attempts to distinguish both populations have resulted in the discovery of a transcription factor, Helios, in nTreg but not in iTreg cells.
CD8 T lymphocytes recognize endogenously processed peptides presented by virus-infected cells or tumor cells. These peptides are usually nine-amino-acid fragments derived from virus or protein tumor antigens in the cytoplasm and are loaded onto MHC class I molecules (Figure 55–2) in the endoplasmic reticulum. In contrast, class II MHC molecules present peptides (usually 11–22 amino acids) derived from extracellular (exogenous) pathogens to CD4 T helper cells. In some instances, exogenous antigens, upon ingestion by APCs, can be presented on class I MHC molecules to CD8 T cells. This phenomenon, referred to as “cross-presentation,” involves retro-translocation of antigens from the endosome to the cytosol for peptide generation in the proteosome and is thought to be useful in generating effective immune responses against infected host cells that are incapable of priming T lymphocytes. Upon activation, CD8 T cells induce target cell death via lytic granule enzymes (“granzymes”), perforin, and the Fas-Fas ligand (Fas-FasL) apoptosis pathways.
B lymphocytes undergo selection in the bone marrow, during which self-reactive B lymphocytes are clonally deleted while B-cell clones specific for foreign antigens are retained and expanded. The repertoire of antigen specificities by T cells is genetically determined and arises from T-cell receptor gene rearrangement while the specificities of B cells arise from immunoglobulin gene rearrangement; for both types of cells, these determinations occur prior to encounters with antigen. Upon an encounter with antigen, a mature B cell binds the antigen, internalizes and processes it, and presents its peptide—bound to class II MHC—to CD4 helper cells, which in turn secrete IL-4 and IL-5. These interleukins stimulate B-cell proliferation and differentiation into memory B cells and antibody-secreting plasma cells. The primary antibody response consists mostly of IgM-class immunoglobulins. Subsequent antigenic stimulation results in a vigorous “booster” response accompanied by class (isotype) switching to produce IgG, IgA, and IgE antibodies with diverse effector functions (Figure 55–3). These antibodies also undergo affinity maturation, which allows them to bind more efficiently to the antigen. With the passage of time, this results in accelerated elimination of microorganisms in subsequent infections. Antibodies mediate their functions by acting as opsonins to enhance phagocytosis and cellular cytotoxicity and by activating complement to elicit an inflammatory response and induce bacterial lysis (Figure 55–4).
FIGURE 55–4 Antibody has multiple functions. The prototypical antibody consists of two heavy (H) and two light (L) chains, each subdivided into constant (CL, CH) and variable (VL, VH) domains. The structure is held together by intra- and interchain disulfide bridges. A: The complementarity-determining region (CDR) of the antigen-binding portion of the antibody engages the antigenic determinant (epitope) in a lock and key fashion. B: Antigen-antibody complexes activate complement to produce split complement components that cause bacterial lysis. C: The Fc portion of antibodies binds to Fc receptors on phagocytes (eg, macrophages, neutrophils) and facilitates uptake of bacteria (opsonization).
ABNORMAL IMMUNE RESPONSES
Whereas the normally functioning immune response can successfully neutralize toxins, inactivate viruses, destroy transformed cells, and eliminate pathogens, inappropriate responses can lead to extensive tissue damage (hypersensitivity) or reactivity against self antigens (autoimmunity); conversely, impaired reactivity to appropriate targets (immunodeficiency) may occur and abrogate essential defense mechanisms.
Hypersensitivity can be classified as antibody-mediated or cell-mediated. Three types of hypersensitivity are antibody-mediated (types I–III), while the fourth is cell-mediated (type IV). Hypersensitivity occurs in two phases: the sensitization phase and the effector phase. Sensitization occurs upon initial encounter with an antigen; the effector phase involves immunologic memory and results in tissue pathology upon a subsequent encounter with that antigen.
1. Type I—Immediate, or type I, hypersensitivity is IgE-mediated, with symptoms usually occurring within minutes following the patient’s reencounter with antigen. Type I hypersensitivity results from cross-linking of membrane-bound IgE on blood basophils or tissue mast cells by antigen. This cross-linking causes cells to degranulate, releasing substances such as histamine, leukotrienes, and eosinophil chemotactic factor, which induce anaphylaxis, asthma, hay fever, or urticaria (hives) in affected individuals (Figure 55–5). A severe type I hypersensitivity reaction such as systemic anaphylaxis (eg, from insect envenomation, ingestion of certain foods, or drug hypersensitivity) requires immediate medical intervention.
FIGURE 55–5 Mechanism of type I hypersensitivity. Initial exposure to allergen (sensitization phase) leads to production of IgE by plasma cells differentiated from allergen-specific B cells (not shown). The secreted IgE binds IgE-specific receptors (Fc?R) on blood basophils and tissue mast cells. Re-exposure to allergen leads to cross-linking of membrane-bound IgE (effector phase). This cross-linking causes degranulation of cytoplasmic granules and release of mediators that induce vasodilation, smooth muscle contraction, and increased vascular permeability. These effects lead to the clinical symptoms characteristic of type I hypersensitivity.
2. Type II—Type II hypersensitivity results from the formation of antigen-antibody complexes between foreign antigen and IgM or IgG immunoglobulins. One example of this type of hypersensitivity is a blood transfusion reaction that can occur if blood is not cross-matched properly. Preformed antibodies bind to red blood cell membrane antigens that activate the complement cascade, generating a membrane attack complex that lyses the transfused red blood cells. In hemolytic disease of the newborn, anti-Rh IgG antibodies produced by an Rh-negative mother cross the placenta, bind to red blood cells of an Rh-positive fetus, and damage them. The disease is prevented in subsequent pregnancies by the administration of anti-Rh antibodies to the mother 24–48 hours after delivery (see Immunosuppressive Antibodies, below). Type II hypersensitivity can also be drug-induced and may occur during the administration of penicillin (for example) to allergic patients. In these patients, penicillin binds to red blood cells or other host tissue to form a neoantigen that evokes production of antibodies capable of inducing complement-mediated red cell lysis. In some circumstances, subsequent administration of the drug can lead to systemic anaphylaxis (type I hypersensitivity).
3. Type III—Type III hypersensitivity is due to the presence of elevated levels of antigen-antibody complexes in the circulation that ultimately deposit on basement membranes in tissues and vessels. Immune complex deposition activates complement to produce components with anaphylatoxic and chemotactic activities (C5a, C3a, C4a) that increase vascular permeability and recruit neutrophils to the site of complex deposition. Complex deposition and the action of lytic enzymes released by neutrophils can cause skin rashes, glomerulonephritis, and arthritis in these individuals. If patients have type III hypersensitivity against a particular antigen, clinical symptoms usually occur 3–4 days after exposure to the antigen.
4. Type IV: Delayed-type hypersensitivity—Unlike type I, II, and III hypersensitivities, delayed-type hypersensitivity (DTH) is cell-mediated, and responses occur 2–3 days after exposure to the sensitizing antigen. DTH is caused by antigen-specific DTH TH1 cells and induces a local inflammatory response that causes tissue damage characterized by the influx of antigen-nonspecific inflammatory cells, especially macrophages. These cells are recruited under the influence of TH1-produced cytokines (Figure 55–6), which chemoattract circulating monocytes and neutrophils, induce myelopoiesis, and activate macrophages. The activated macrophages are primarily responsible for the tissue damage associated with DTH. Although widely considered to be deleterious, DTH responses are very effective in eliminating infections caused by intracellular pathogens such as Mycobacterium tuberculosis and Leishmania species. Clinical manifestations of DTH include tuberculin and contact hypersensitivities. Tuberculosis exposure is determined using a DTH skin test. Positive responses show erythema and induration caused by accumulation of macrophages and DTH T (TDTH) cells at the site of the tuberculin injection. Poison ivy is the most common cause of contact hypersensitivity, in which pentadecacatechol, the lipophilic chemical in poison ivy, modifies cellular tissue and results in a DTH T-cell response.
FIGURE 55–6 Mechanism of type IV hypersensitivity (DTH). In the sensitization phase, the processed allergen (eg, from poison ivy) is presented to CD4 Th1 cells by antigen-presenting cells in association with class II MHC. T cells are induced to express IL-2 receptors and are stimulated to proliferate and differentiate into memory TDTH cells. Secondary contact with antigen triggers the effector phase, in which memory TDTH cells release cytokines that attract and activate nonspecific inflammatory macrophages and neutrophils. These cells display increased phagocytic and microbicidal activities and release large quantities of lytic enzymes that cause extensive tissue damage.
Autoimmune disease arises when the body mounts an immune response against itself due to failure to distinguish self tissues and cells from foreign (nonself) antigens or loss of tolerance to self. This phenomenon derives from the activation of self-reactive T and B lymphocytes that generate cell-mediated or humoral immune responses directed against self antigens. The pathologic consequences of this reactivity constitute several types of autoimmune diseases. Autoimmune diseases are highly complex due to MHC genetics, environmental conditions, infectious entities, and dysfunctional immune regulation. Examples of such diseases include rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, and insulin-dependent diabetes mellitus (type 1 diabetes). In rheumatoid arthritis, IgM antibodies (rheumatoid factors) are produced that react with the Fc portion of IgG and may form immune complexes that activate the complement cascade, causing chronic inflammation of the joints and kidneys. In systemic lupus erythematosus, antibodies are made against DNA, histones, red blood cells, platelets, and other cellular components. In multiple sclerosis and type 1 diabetes, cell-mediated autoimmune attack destroys myelin surrounding nerve cells and insulin-producing islet beta cells of the pancreas, respectively. In type 1 diabetes, activated CD4 TDTH cells that infiltrate the islets of Langerhans and recognize self islet beta cell peptides are thought to produce cytokines that stimulate macrophages to produce lytic enzymes, which destroy islet beta cells. Autoantibodies directed against the islet beta cell antigens are produced, but do not contribute significantly to disease.
A number of mechanisms have been proposed to explain autoimmunity:
1.Exposure of antigens previously sequestered from the immune system (eg, lens protein, myelin basic protein) to self-reactive T lymphocytes.
2.Molecular mimicry by invading pathogens, in which immune responses are directed at antigenic determinants on pathogens that share identical or similar epitopes with normal host tissue. This phenomenon occurs in rheumatic fever following Streptococcus pyogenes infection, in which heart damage is thought to arise from an immune response directed against streptococcal antigens shared with heart muscle. The suggested viral etiology of autoimmune diseases has been ascribed to immune responses (both cell-mediated and humoral) directed against virus epitopes that mimic self antigens.
3.Inappropriate expression of class II MHC molecules on the membranes of cells that normally do not express class II MHC (eg, islet beta cells). Increased expression of MHC II may increase presentation of self peptides to T helper cells, which in turn induce CTL, TDTH, and B-lymphocyte cells that react against self antigens.
Immunodeficiency diseases result from inadequate function in the immune system; the consequences include increased susceptibility to infections and prolonged duration and severity of disease. Immunodeficiency diseases are either congenital or arise from extrinsic factors such as bacterial or viral infections or drug treatment. Affected individuals frequently succumb to infections caused by opportunistic organisms of low pathogenicity for the immunocompetent host. Examples of congenitally acquired immunodeficiency diseases include X-linked agammaglobulinemia, DiGeorge’s syndrome, and severe combined immunodeficiency disease (SCID) due to adenosine deaminase (ADA) deficiency.
X-linked agammaglobulinemia is a disease affecting males that is characterized by a failure of immature B lymphocytes to mature into antibody-producing plasma cells. These individuals are susceptible to recurrent bacterial infections, although the cell-mediated responses directed against viruses and fungi are preserved. DiGeorge’s syndrome is due to failure of the thymus to develop, resulting in diminished T-cell responses (TDTH, CTL), while the humoral response remains functional, but does not benefit from T-cell help.
The ADA enzyme normally prevents the accumulation of toxic deoxy-ATP in cells. Deoxy-ATP is particularly toxic to lymphocytes, and leads to death of T and B cells. Absence of the enzyme therefore results in SCID. Infusion of the purified enzyme (pegademase, from bovine sources) and transfer of ADA gene-modified lymphocytes have both been used successfully to treat this disease.
AIDS represents the classic example of immunodeficiency disease caused by extrinsic viral infection, in this instance the human immunodeficiency virus (HIV). This virus exhibits a strong tropism for CD4 T helper cells; these become depleted, giving rise to increased frequency of opportunistic infections and malignancies in infected individuals. AIDS is also characterized by an imbalance in TH1 and TH2 cells, and the ratios of cells and their functions are skewed toward TH2. This results in loss of cytotoxic T-lymphocyte activity, loss of delayed hypersensitivity, and hypergammaglobulinemia.
Immunosuppressive agents have proved very useful in minimizing the occurrence or impact of deleterious effects of exaggerated or inappropriate immune responses. Unfortunately, these agents also have the potential to cause disease and to increase the risk of infection and malignancies.
Glucocorticoids (corticosteroids) were the first hormonal agents recognized as having lympholytic properties. Administration of any glucocorticoid reduces the size and lymphoid content of the lymph nodes and spleen, although it has no toxic effect on proliferating myeloid or erythroid stem cells in the bone marrow.
Glucocorticoids are thought to interfere with the cell cycle of activated lymphoid cells. The mechanism of their action is described in Chapter 39. Glucocorticoids are quite cytotoxic to certain subsets of T cells, but their immunologic effects are probably due to their ability to modify cellular functions rather than to direct cytotoxicity. Although cellular immunity is more affected than humoral immunity, the primary antibody response can be diminished, and with continued use, previously established antibody responses are also decreased. Additionally, continuous administration of corticosteroid increases the fractional catabolic rate of IgG, the major class of antibody immunoglobulins, thus lowering the effective concentration of specific antibodies. Contact hypersensitivity mediated by DTH T cells, for example, is usually abrogated by glucocorticoid therapy.
Glucocorticoids are used in a wide variety of conditions (Table 55–1). It is thought that the immunosuppressive and anti-inflammatory properties of corticosteroids account for their beneficial effects in diseases like idiopathic thrombocytopenic purpura and rheumatoid arthritis. Glucocorticoids modulate allergic reactions and are useful in the treatment of diseases like asthma or as premedication for other agents (eg, blood products, chemotherapy) that might cause undesirable immune responses. Glucocorticoids are first-line immunosuppressive therapy for both solid organ and hematopoietic stem cell transplant recipients, with variable results. The toxicities of long-term glucocorticoid therapy can be severe and are discussed in Chapter 39.
TABLE 55–1 Clinical uses of immunosuppressive agents.
Cyclosporine (cyclosporin A, CSA) is an immunosuppressive agent with efficacy in human organ transplantation, in the treatment of graft-versus-host (GVH) disease after hematopoietic stem cell transplantation, and in the treatment of selected autoimmune disorders. Cyclosporine is a peptide antibiotic that appears to act at an early stage in the antigen receptor-induced differentiation of T cells and blocks their activation. Cyclosporine binds to cyclophilin, a member of a class of intracellular proteins called immunophilins. Cyclosporine and cyclophilin form a complex that inhibits the cytoplasmic phosphatase, calcineurin, which is necessary for the activation of a T-cell-specific transcription factor. This transcription factor, NF-AT, is involved in the synthesis of interleukins (eg, IL-2) by activated T cells. In vitro studies have indicated that cyclosporine inhibits the gene transcription of IL-2, IL-3, IFN-γ, and other factors produced by antigen-stimulated T cells, but it does not block the effect of such factors on primed T cells nor does it block interaction with antigen.
Cyclosporine may be given intravenously or orally, though it is slowly and incompletely absorbed (20–50%). The absorbed drug is primarily metabolized by the P450 3A enzyme system in the liver with resultant multiple drug interactions. This propensity for drug interactions contributes to significant interpatient variability in bioavailability, such that cyclosporine requires individual patient dosage adjustments based on steady-state blood levels and the desired therapeutic ranges for the drug. Cyclosporine ophthalmic solution is now available for severe dry eye syndrome, as well as ocular GVH disease. Inhaled cyclosporine is being investigated for use in lung transplantation.
Toxicities are numerous and include nephrotoxicity, hypertension, hyperglycemia, liver dysfunction, hyperkalemia, altered mental status, seizures, and hirsutism. Cyclosporine causes very little bone marrow toxicity. While an increased incidence of lymphoma and other cancers (Kaposi’s sarcoma, skin cancer) have been observed in transplant recipients receiving cyclosporine, other immunosuppressive agents may also predispose recipients to cancer. Some evidence suggests that tumors may arise after cyclosporine treatment because the drug induces TGF-β, which promotes tumor invasion and metastasis.
Cyclosporine may be used alone or in combination with other immunosuppressants, particularly glucocorticoids. It has been used successfully as the sole immunosuppressant for cadaveric transplantation of the kidney, pancreas, and liver, and it has proved extremely useful in cardiac transplantation as well. In combination with methotrexate, cyclosporine is a standard prophylactic regimen to prevent GVH disease after allogeneic stem cell transplantation. Cyclosporine has also proved useful in a variety of autoimmune disorders, including uveitis, rheumatoid arthritis, psoriasis, and asthma. Its combination with newer agents is showing considerable efficacy in clinical and experimental settings where effective and less toxic immunosuppression is needed. Newer formulations of cyclosporine have been developed that are improving patient compliance (smaller, better tasting pills), and increasing bioavailability.
Tacrolimus (FK 506) is an immunosuppressant macrolide antibiotic produced by Streptomyces tsukubaensis. It is not chemically related to cyclosporine, but their mechanisms of action are similar. Both drugs bind to cytoplasmic peptidylprolyl isomerases that are abundant in all tissues. While cyclosporine binds to cyclophilin, tacrolimus binds to the immunophilin FK-binding protein (FKBP). Both complexes inhibit calcineurin, which is necessary for the activation of the T-cell-specific transcription factor NF-AT.
On a weight basis, tacrolimus is 10–100 times more potent than cyclosporine in inhibiting immune responses. Tacrolimus is utilized for the same indications as cyclosporine, particularly in organ and stem cell transplantation. Multicenter studies in the USA and in Europe indicate that both graft and patient survival are similar for the two drugs. Tacrolimus has proven to be effective therapy for preventing rejection in solid-organ transplant patients even after failure of standard rejection therapy, including anti-T-cell antibodies. It is now considered a standard prophylactic agent (usually in combination with methotrexate or mycophenolate mofetil) for GVH disease.
Tacrolimus can be administered orally or intravenously. The half-life of the intravenous form is approximately 9–12 hours. Like cyclosporine, tacrolimus is metabolized primarily by P450 enzymes in the liver, and there is potential for drug interactions. The dosage is determined by trough blood level at steady state. Its toxic effects are similar to those of cyclosporine and include nephrotoxicity, neurotoxicity, hyperglycemia, hypertension, hyperkalemia, and gastrointestinal complaints.
Because of the effectiveness of systemic tacrolimus in some dermatologic diseases, a topical preparation is now available. Tacrolimus ointment is currently used in the therapy of atopic dermatitis and psoriasis.
PROLIFERATION SIGNAL INHIBITORS
A new class of immunosuppressive agents called proliferation-signal inhibitors (PSIs) includes sirolimus (rapamycin) and its derivative everolimus. The mechanism of action of PSIs differs from that of the calcineurin inhibitors. PSIs bind the circulating immunophilin FK506-binding protein 12, resulting in an active complex that blocks the molecular target of rapamycin (mTOR). The mTOR is a key component of a complex intracellular signaling pathway involved in cellular processes such as cell growth and proliferation, angiogenesis, and metabolism. Thus, blockade of mTOR ultimately can lead to inhibition of interleukin-driven T-cell proliferation. Both everolimus and sirolimus also may inhibit B-cell proliferation and immunoglobulin production.
Sirolimus is available only as an oral drug. Its half-life is about 60 hours, while that of everolimus is about 43 hours. Both drugs are rapidly absorbed and elimination is similar to that of cyclosporine and tacrolimus, being substrates for both cytochrome P450 3A and P-glycoprotein. Hence, significant drug interactions can occur. For example, use with cyclosporine can increase the plasma levels of both sirolimus and everolimus such that drug levels need to be monitored. Target dose-ranges of these drugs vary depending on clinical use.
Sirolimus has been used effectively alone and in combination with other immunosuppressants (corticosteroids, cyclosporine, tacrolimus, and mycophenolate mofetil) to prevent rejection of solid organ allografts. It is used as prophylaxis and as therapy for steroid-refractory acute and chronic GVH disease in hematopoietic stem cell transplant recipients. Topical sirolimus is also used in some dermatologic disorders and, in combination with cyclosporine, in the management of uveoretinitis. Recently, sirolimus-eluting coronary stents have been shown to reduce restenosis and additional adverse cardiac events in patients with severe coronary artery disease, due to the drug’s antiproliferative effects. Everolimus is a newer drug that has shown clinical efficacy similar to sirolimus in solid organ transplant recipients; it is under investigation as an additional therapeutic agent for the treatment of chronic cardiac allograft vasculopathy.
Toxicities of the PSIs can include profound myelosuppression (especially thrombocytopenia), hepatotoxicity, diarrhea, hypertriglyceridemia, pneumonitis, and headache. Because nephrotoxicity is of major concern when administering calcineurin inhibitors, and since renal toxicity is less common with PSIs, there is interest in increased early use of the latter agents. However, increased use in stem cell transplantation regimens as GVH disease prophylaxis, particularly when combined with tacrolimus, has revealed an increased incidence of hemolytic-uremic syndrome.
Mycophenolate mofetil (MMF) is a semisynthetic derivative of mycophenolic acid, isolated from the mold Penicillium glaucus. In vitro, it inhibits T- and B-lymphocyte responses, including mitogen and mixed lymphocyte responses, probably by inhibition of de novo synthesis of purines. Mycophenolate mofetil is hydrolyzed to mycophenolic acid, the active immunosuppressive moiety; it is synthesized and administered as MMF to enhance bioavailability.
Mycophenolate mofetil is available in both oral and intravenous forms. The oral form is rapidly metabolized to mycophenolic acid. Although the cytochrome P450 3A system is not involved, some drug interactions still occur. Plasma drug levels are frequently monitored.
Mycophenolate mofetil is used in solid organ transplant patients for refractory rejection and, in combination with prednisone, as an alternative to cyclosporine or tacrolimus in patients who do not tolerate those drugs. Its antiproliferative properties make it the first-line drug for preventing or reducing chronic allograft vasculopathy in cardiac transplant recipients. Mycophenolate mofetil is used as prophylaxis for and treatment of both acute and chronic GVH disease in hematopoietic stem cell transplant patients. Newer immunosuppressant applications for MMF include lupus nephritis, rheumatoid arthritis, inflammatory bowel disease, and some dermatologic disorders.
Toxicities include gastrointestinal disturbances (nausea and vomiting, diarrhea, abdominal pain) headache, hypertension, and reversible myelosuppression (primarily neutropenia).
Thalidomide is an oral sedative drug that was withdrawn from the market in the 1960s because of disastrous teratogenic effects when used during pregnancy. Nevertheless, it has significant immunomodulatory actions and is currently in active use or in clinical trials for over 40 different illnesses. Thalidomide inhibits angiogenesis and has anti-inflammatory and immunomodulatory effects. It inhibits tumor necrosis factor-alpha (TNF-α), reduces phagocytosis by neutrophils, increases production of IL-10, alters adhesion molecule expression, and enhances cell-mediated immunity via interactions with T cells. The complex actions of thalidomide continue to be studied as its clinical use evolves.
Thalidomide is currently used in the treatment of multiple myeloma at initial diagnosis and for relapsed-refractory disease. Patients generally show signs of response within 2–3 months of starting the drug, with response rates of 20–70%. When combined with dexamethasone, the response rates in myeloma are 90% or more in some studies. Many patients have durable responses—up to 12–18 months in refractory disease and even longer in some patients treated at diagnosis. The success of thalidomide in myeloma has led to numerous clinical trials in other diseases such as myelodysplastic syndrome, acute myelogenous leukemia, and GVH disease, as well as in solid tumors like colon cancer, renal cell carcinoma, melanoma, and prostate cancer, with variable results to date. Thalidomide has been used for many years in the treatment of some manifestations of leprosy and has been reintroduced in the USA for erythema nodosum leprosum; it is also useful in management of the skin manifestations of lupus erythematosus.
The adverse effect profile of thalidomide is extensive. The most important toxicity is teratogenesis. Because of this effect, thalidomide prescription and use is closely regulated by the manufacturer. Other adverse effects of thalidomide include peripheral neuropathy, constipation, rash, fatigue, hypothyroidism, and increased risk of deep vein thrombosis. Thrombosis is sufficiently frequent, particularly in the hematologic malignancy population, that most patients are placed on some type of anticoagulant when thalidomide treatment is initiated.
Owing to thalidomide’s serious toxicity profile, considerable effort has been expended in the development of analogs. Immunomodulatory derivatives of thalidomide are termed IMiDs. Some IMiDs are much more potent than thalidomide in regulating cytokines and affecting T-cell proliferation. Lenalidomide is an oral IMiD that in animal and in vitro studies has been shown to be similar to thalidomide in action, but with less toxicity, especially teratogenicity. Lenalidomide was approved by the FDA when trials showed its effectiveness in the treatment of the myelodysplastic syndrome with the chromosome 5q31 deletion. Clinical trials using lenalidomide to treat multiple myeloma showed similar efficacy, leading to approval for both primary and relapsed/refractory myeloma. Pomalidomide (originally called CC-4047) is the newest oral IMiD to be FDA approved. Like the other IMiDs, it has myriad mechanisms of actions including antiangiogenic activity, inhibition of TNF-α, and stimulation of apoptosis and cytotoxic T-cell activity. Most clinical trials of pomalidomide have targeted patients with relapsed/refractory multiple myeloma, for which the FDA approved the drug in 2013. Both lenalidomide and pomalidomide have side effect profiles similar to thalidomide.
Azathioprine is a prodrug of mercaptopurine and, like mercaptopurine, functions as an antimetabolite (see Chapter 54). Although its action is presumably mediated by conversion to mercaptopurine and further metabolites, it has been more widely used than mercaptopurine for immunosuppression in humans. These agents represent prototypes of the antimetabolite group of cytotoxic immunosuppressive drugs, and many other agents that kill proliferative cells appear to work at a similar level in the immune response.
Azathioprine is well absorbed from the gastrointestinal tract and is metabolized primarily to mercaptopurine. Xanthine oxidase converts much of the active material to 6-thiouric acid prior to excretion in the urine. After administration of azathioprine, small amounts of unchanged drug and mercaptopurine are also excreted by the kidney, and as much as a twofold increase in toxicity may occur in anephric or anuric patients. Since much of the drug’s inactivation depends on xanthine oxidase, patients who are also receiving allopurinol (see Chapters 36 and 54) for control of hyperuricemia should have the dose of azathioprine reduced to one-fourth to one-third the usual amount to prevent excessive toxicity.
Azathioprine and mercaptopurine appear to produce immunosuppression by interfering with purine nucleic acid metabolism at steps that are required for the wave of lymphoid cell proliferation that follows antigenic stimulation. The purine analogs are thus cytotoxic agents that destroy stimulated lymphoid cells. Although continued messenger RNA synthesis is necessary for sustained antibody synthesis by plasma cells, these analogs appear to have less effect on this process than on nucleic acid synthesis in proliferating cells. Cellular immunity as well as primary and secondary serum antibody responses can be blocked by these agents.
Azathioprine and mercaptopurine appear to be of definite benefit in maintaining renal allografts and may be of value in transplantation of other tissues. These antimetabolites have also been used with some success in the management of acute glomerulonephritis, in the renal component of systemic lupus erythematosus, and in some cases of rheumatoid arthritis, Crohn’s disease, and multiple sclerosis. The drugs have been of occasional use in prednisone-resistant antibody-mediated idiopathic thrombocytopenic purpura and autoimmune hemolytic anemias.
The chief toxic effect of azathioprine and mercaptopurine is bone marrow suppression, usually manifested as leukopenia, although anemia and thrombocytopenia may occur. Skin rashes, fever, nausea and vomiting, and sometimes diarrhea occur, with the gastrointestinal symptoms seen mainly at higher dosages. Hepatic dysfunction, manifested by very high serum alkaline phosphatase levels and mild jaundice, occurs occasionally, particularly in patients with preexisting hepatic dysfunction.
The alkylating agent cyclophosphamide is one of the most efficacious immunosuppressive drugs available. Cyclophosphamide destroys proliferating lymphoid cells (see Chapter 54) but also appears to alkylate some resting cells. It has been observed that very large doses (eg, > 120 mg/kg intravenously over several days) may induce an apparent specific tolerance to a new antigen if the drug is administered simultaneously with, or shortly after, the antigen. In smaller doses, it has been effective against autoimmune disorders (including systemic lupus erythematosus) and in patients with acquired factor XIII antibodies and bleeding syndromes, autoimmune hemolytic anemia, antibody-induced pure red cell aplasia, and Wegener’s granulomatosis.
Treatment with large doses of cyclophosphamide carries considerable risk of pancytopenia and therefore is generally combined with stem cell rescue (transplant) procedures. Although cyclophosphamide appears to induce tolerance for marrow or immune cell grafting, its use does not prevent the subsequent GVH syndrome, which may be serious or lethal if the donor is a poor histocompatibility match (despite the severe immunosuppression induced by high doses of cyclophosphamide). The drug may also cause hemorrhagic cystitis, which can be prevented or treated with mesna. Other adverse effects of cyclophosphamide include nausea, vomiting, cardiac toxicity, and electrolyte disturbances.
Pyrimidine Synthesis Inhibitors
Leflunomide is a prodrug of an inhibitor of pyrimidine synthesis. Teriflunomide is the principal active metabolite of leflunomide. They both reversibly inhibit the mitochondrial enzyme dihydroorotate dehydrogenase, which is involved in pyrimidine synthesis and ultimately results in decreased lymphocyte activation. They have anti-inflammatory activity in addition to immunomodulatory properties.
Leflunomide is orally active, and the active metabolite has a long half-life of several weeks. Thus, the drug should be started with a loading dose, but it can be taken once daily after reaching steady state. It is approved only for rheumatoid arthritis at present, though studies are underway combining leflunomide with mycophenolate mofetil for a variety of autoimmune and inflammatory skin disorders, as well as preservation of allografts in solid organ transplantation. Leflunomide also appears (from murine data) to have antiviral activity. Toxicities include elevation of liver enzymes with some risk of liver damage and renal impairment. Patients with severe liver disease should not receive leflunomide. This drug is teratogenic and contraindicated in pregnancy. A low frequency of cardiovascular effects (angina, tachycardia) has been reported.
Teriflunomide is FDA-approved for the treatment of relapsing-remitting multiple sclerosis. Although immunomodulatory, its exact mechanism of action in the treatment of multiple sclerosis is unclear. It is hypothesized to decrease the number of activated lymphocytes in the central nervous system. It is a once-daily oral drug that, unlike leflunomide, does not require a loading dose. Teriflunomide’s side effect profile is similar to that of leflunomide and it is contraindicated in pregnancy and severe liver disease. The incidence of neutropenia in patients taking the drug is 15%, and 10% of patients have a decrease in platelet counts.
Hydroxychloroquine is an antimalarial agent with immunosuppressant properties. It is thought to suppress intracellular antigen processing and loading of peptides onto MHC class II molecules by increasing the pH of lysosomal and endosomal compartments, thereby decreasing T-cell activation.
Because of these immunosuppressant activities, hydroxychloroquine is used to treat some autoimmune disorders (see Chapter 36), eg, rheumatoid arthritis and systemic lupus erythematosus. It has also been used to both treat and prevent GVH disease after allogeneic stem cell transplantation.
Other Cytotoxic Agents
Other cytotoxic agents, including methotrexate, vincristine, and cytarabine (see Chapter 54), also have immunosuppressive properties. Methotrexate has been used extensively in rheumatoid arthritis (see Chapter 36) and in the treatment of GVH disease. Although the other agents can be used for immunosuppression, their use has not been as widespread as the purine antagonists, and their indications for immunosuppression are less certain. The use of methotrexate (which can be given orally) appears reasonable in patients with idiosyncratic reactions to purine antagonists. The antibiotic dactinomycin has also been used with some success at the time of impending renal transplant rejection. Vincristine appears to be quite useful in idiopathic thrombocytopenic purpura refractory to prednisone. The related vinca alkaloid vinblastine has been shown to prevent mast cell degranulation in vitro by binding to microtubule units within the cell and to prevent release of histamine and other vasoactive compounds.
Pentostatin is an adenosine deaminase inhibitor that has been used mainly as an antineoplastic agent for lymphoid malignancies; it produces a profound lymphopenia. It is now frequently used for steroid-resistant GVH disease after allogeneic stem cell transplantation, as well as in preparative regimens prior to those transplants to provide severe immunosuppression to prevent allograft rejection.
Three other FDA-approved immunomodulators are used exclusively in the treatment of relapsing remitting multiple sclerosis.
Dimethyl fumarate (DMF) is the methyl ester of fumaric acid. Its exact mechanism of action is unknown, though it appears to activate the nuclear factor (erythroid-derived)-like-2 (NFR-2) transcriptional pathway. Activation of the NFR-2 pathway results in reduction of the oxidative stress that contributes to demyelination; it also appears to help protect the nerve cells from inflammation. DMF is given orally. Lymphopenia may be significant, so blood counts must be monitored regularly and the drug may be withheld if active infection is present. Flushing is common with treatment initiation and usually improves with time. Other less common adverse effects include nausea, diarrhea, abdominal pain, increased hepatic enzymes, and eosinophilia.
Glatiramer acetate (GA) is a mixture of synthetic polypeptides and four amino acids (L-glutamic acid, L-alanine, L-lysine and L-tyrosine) in a fixed molar ratio. Its mechanism of immunomodulation in multiple sclerosis is unknown. Studies suggest that GA downregulates the immune response to myelin antigens by induction and activation of suppressor T-cells that migrate to the central nervous system. It is given as a subcutaneous injection (not intravenously) in variable dosages and schedules. Toxicities include skin hypersensitivity, and rarely lipoatrophy and skin necrosis at the injection site. Other adverse effects include flushing, chest pain, dyspnea, throat constriction, and palpitations, all of which are usually mild and self-limited.
Fingolimod hydrochloride (FH) is an orally active sphingosine 1-phosphate (S1P) receptor modulator that is derived from the fungal metabolite myriocin. The S1P receptor (subtype 1) controls the release of lymphocytes from lymph nodes and the thymus. FH is metabolized to fingolimod phosphate, which subsequently binds the S1P receptor and ultimately decreases circulating lymphocyte numbers in the periphery and central nervous system. S1P receptors are also expressed on neurons, such that FH may also be affecting neurodegeneration, gliosis, and endogenous repair mechanisms as well as resulting in lymphopenia to modify disease activity in multiple sclerosis. FH can cause serious cardiac toxicity including bradycardia, prolongation of the QT interval, and other arrhythmias. Because of these potential complications, the drug requires cardiac monitoring for 6 hours after the first dose is given. FH is contraindicated in patients with preexisting conditions such as type II or III heart block, prolonged QTc, recent myocardial infarction, or heart failure. Less common adverse effects include macular edema, elevated hepatic enzymes, headache, diarrhea, and cough. The drug is metabolized primarily by the cytochrome P450 system; thus caution is needed when it is used in combination with other drugs metabolized in the same manner.
The development of hybridoma technology by Milstein and Köhler in 1975 revolutionized the antibody field and radically increased the purity and specificity of antibodies used in the clinic and for diagnostic tests in the laboratory. Hybridomas are B cells fused to immortal plasmacytoma cells that secrete monoclonal antibodies specific for a target antigen. Large-scale hybridoma culture facilities are employed by the pharmaceutical industry to produce diagnostic and clinical grade monoclonal antibodies.
More recently, molecular biology has been used to develop monoclonal antibodies. Combinatorial libraries of cDNAs encoding immunoglobulin heavy and light chains expressed on bacteriophage surfaces are screened against purified antigens. The result is an antibody fragment with specificity and high affinity for the antigen of interest. This technique has been used to develop antibodies specific for viruses (eg, HIV), bacterial proteins, tumor antigens, and even cytokines. Several antibodies developed in this manner are FDA-approved for use in humans.
Other genetic engineering techniques involve production of chimeric and humanized versions of murine monoclonal antibodies in order to reduce their antigenicity and increase the half-life of the antibody in the patient. Murine antibodies administered as such to human patients elicit production of human antimouse antibodies (HAMAs), which clear the original murine proteins very rapidly. Humanization involves replacing most of the murine antibody with equivalent human regions while keeping only the variable, antigen-specific regions intact. Chimeric mouse-human antibodies have similar properties with less complete replacement of the murine components. The current naming convention for these engineered substances uses the suffix “-umab” or “-zumab” for humanized antibodies, and “-imab” or “-ximab” for chimeric products. These procedures have been successful in reducing or preventing HAMA production for many of the antibodies discussed below.
Antilymphocyte & Antithymocyte Antibodies, & Chimeric Molecules
Antisera directed against lymphocytes have been prepared sporadically for over 100 years. With the advent of human organ transplantation as a realistic therapeutic option, heterologous antilymphocyte globulin (ALG) took on new importance. ALG and antithymocyte globulin (ATG) are now in clinical use in many medical centers, especially in transplantation programs. The antiserum is usually obtained by immunization of horses, sheep, or rabbits with human lymphoid cells.
ALG acts primarily on the small, long-lived peripheral lymphocytes that circulate between the blood and lymph. With continued administration, “thymus-dependent” (T) lymphocytes from lymphoid follicles are also depleted, as they normally participate in the recirculating pool. As a result of the destruction or inactivation of T cells, an impairment of delayed hypersensitivity and cellular immunity occurs while humoral antibody formation remains relatively intact. ALG and ATG are useful for suppressing certain major compartments (ie, T cells) of the immune system and play a definite role in the management of solid organ and bone marrow transplantation.
Monoclonal antibodies directed against specific cell surface proteins such as CD2, CD3, CD25, CD40, and various integrins much more selectively influence T-cell subset function. The high specificity of these antibodies improves selectivity and reduces toxicity of the therapy, and alters the disease course in several different autoimmune disorders.
In the management of transplants, ALG and monoclonal antibodies can be used in the induction of immunosuppression, in the treatment of initial rejection, and in the treatment of steroid-resistant rejection. There has been some success in the use of ALG and ATG plus cyclosporine to prepare recipients for bone marrow transplantation. In this procedure, the recipient is treated with ALG or ATG in large doses for 7–10 days prior to transplantation of bone marrow cells from the donor. ALG appears to destroy the T cells in the donor marrow graft, and the probability of severe GVH disease is reduced.
The adverse effects of ALG are mostly those associated with injection of a foreign protein. Local pain and erythema often occur at the injection site (type III hypersensitivity). Since the humoral antibody response remains active in the recipient, skin-reactive and precipitating antibodies may be formed against the foreign ALG. Similar reactions occur with monoclonal antibodies of murine origin caused by the release of cytokines by T cells and monocytes.
Anaphylactic and serum sickness reactions to ALG and murine monoclonal antibodies have been observed and usually require cessation of therapy. Complexes of host antibodies with horse ALG may precipitate and localize in the glomeruli of the kidneys causing kidney damage.
Immune Globulin Intravenous (IGIV)
A different approach to immunomodulation is the intravenous use of polyclonal human immunoglobulin. This immunoglobulin preparation (usually IgG) is prepared from pools of thousands of healthy donors, and no single, specific antigen is the target of the “therapeutic antibody.” Rather, one expects that the pool of different antibodies will have a normalizing effect upon the patient’s immune networks.
IGIV in high doses (2 g/kg) has proved effective in a variety of different applications ranging from immunoglobulin deficiencies to autoimmune disorders to HIV disease to bone marrow transplantation. In patients with Kawasaki’s disease, it has been shown to be safe and effective, reducing systemic inflammation and preventing coronary artery aneurysms. It has also brought about good clinical responses in systemic lupus erythematosus and refractory idiopathic thrombocytopenic purpura. Possible mechanisms of action of IGIV include a reduction of T helper cells, increase of regulatory T cells, decreased spontaneous immunoglobulin production, Fc receptor blockade, increased antibody catabolism, and idiotypic-anti-idiotypic interactions with “pathologic antibodies.” Although its precise mechanism of action is still unknown, IGIV brings undeniable clinical benefit to many patients with a variety of immune syndromes.
Rho(D) Immune Globulin
One of the earliest major advances in immunopharmacology was the development of a technique for preventing Rh hemolytic disease of the newborn. The technique is based on the observation that a primaryantibody response to a foreign antigen can be blocked if specific antibody to that antigen is administered passively at the time of exposure to antigen. Rho(D) immune globulin is a concentrated (15%) solution of human IgG containing high titer antibodies against the Rho(D) antigen of the red cell.
Sensitization of Rh-negative mothers to the D antigen occurs usually at the time of birth of an Rho(D)-positive or Du-positive infant, when fetal red cells leak into the mother’s bloodstream. Sensitization might also occur occasionally with miscarriages or ectopic pregnancies. In subsequent pregnancies, maternal antibody against Rh-positive cells is transferred to the fetus during the third trimester, leading to the development of erythroblastosis fetalis (hemolytic disease of the newborn).
If an injection of Rho(D) antibody is administered to the Rh-negative mother within 24–72 hours after the birth of an Rh-positive infant, the mother’s own antibody response to the foreign Rho(D)-positive cells is suppressed because the infant’s red cells are cleared from circulation before the mother can generate a B-cell response against Rho(D). Therefore she has no memory B cells that can activate upon subsequent pregnancies with an Rho(D)-positive fetus.
When the mother has been treated in this fashion, Rh hemolytic disease of the newborn has not been observed in subsequent pregnancies. For this prophylactic treatment to be successful, the mother must be Rho(D)-negative and Du-negative and must not already be immunized to the Rho(D) factor. Treatment is also often advised for Rh-negative mothers antepartum at 26–28 weeks’ gestation who have had miscarriages, ectopic pregnancies, or abortions, when the blood type of the fetus is unknown. Note: Rho(D) immune globulin is administered to the mother and must not be given to the infant.
The usual dose of Rho(D) immune globulin is 2 mL intramuscularly, containing approximately 300 mcg anti-Rho(D) IgG. Adverse reactions are infrequent and consist of local discomfort at the injection site or, rarely, a slight temperature elevation.
Hyperimmune immunoglobulins are IGIV preparations made from pools of selected human or animal donors with high titers of antibodies against particular agents of interest such as viruses or toxins (see also Appendix). Various hyperimmune IGIVs are available for treatment of respiratory syncytial virus, cytomegalovirus, varicella zoster, human herpesvirus 3, hepatitis B virus, rabies, tetanus, and digoxin overdose. Intravenous administration of the hyperimmune globulins is a passive transfer of high titer antibodies that either reduces risk or reduces the severity of infection. Rabies hyperimmune globulin is injected around the wound and given intravenously. Tetanus hyperimmune globulin is administered intravenously when indicated for prophylaxis. Rattlesnake and coral snake hyperimmune globulins (antivenoms) are of equine or ovine origin and are effective for North and South American rattlesnakes and some coral snakes (but not Arizona coral snake). Equine and ovine antivenoms are available for rattlesnake envenomations, but only equine antivenom is available for coral snake bite. An Arizona bark scorpionantivenom is also available as equine (Fab)™2. This preparation prevents neurologic manifestations of scorpion envenomation and is generally used in young children and infants.
MONOCLONAL ANTIBODIES (MABS)
Advances in the ability to manipulate the genes for immunoglobulins have resulted in development of a wide array of humanized and chimeric monoclonal antibodies directed against therapeutic targets. As described above, the only murine elements of humanized monoclonal antibodies are the complementarity-determining regions in the variable domains of immunoglobulin heavy and light chains. Complementarity-determining regions are primarily responsible for the antigen-binding capacity of antibodies. Chimeric antibodies typically contain antigen-binding murine variable regions and human constant regions. The following are brief descriptions of the engineered antibodies that have been approved for clinical use; they are presented alphabetically.
Alemtuzumab is a humanized IgG1 with a kappa chain that binds to CD52 found on normal and malignant B and T lymphocytes, NK cells, monocytes, macrophages, and a small population of granulocytes. Alemtuzumab was previously approved for the treatment of B-cell chronic lymphocytic leukemia (CLL) in patients who have been treated with alkylating agents and have failed fludarabine therapy. Alemtuzumab appears to deplete leukemic (and normal) cells by direct antibody-dependent lysis. More recently, alemtuzumab was approved by the EU for the treatment of patients diagnosed with relapsing remitting multiple sclerosis. In the latter, alemtuzumab depletes autoimmune inflammatory T and B cells while the drug is in the circulation. Repopulating lymphocytes appear to temporarily rebalance the immune system. Patients receiving this antibody become lymphopenic and may also become neutropenic, anemic, and thrombocytopenic. As a result patients should be closely monitored for opportunistic infections and hematologic toxicity.
Bevacizumab is a humanized IgG1 monoclonal antibody that binds to vascular endothelial growth factor (VEGF) and inhibits VEGF from binding to its receptor, especially on endothelial cells. It is an antiangiogenic drug that has been shown to inhibit growth of blood vessels (angiogenesis) in tumors. It is approved for first- and second-line treatment of patients with metastatic colorectal cancer alone or in combination with appropriate chemotherapy. It is also approved for treatment of non-small cell lung cancer, glioblastoma multiforme that has progressed after prior treatment, and metastatic kidney cancer when used with interferon-alfa. Since bevacizumab is antiangiogenic, it should not be administered until patients heal from surgery. Patients taking the drug should be watched for hemorrhage, gastrointestinal perforations, and wound healing problems. Bevacizumab has also been used off label by intravitreal injection to slow progression of neovascular macular degeneration (see ranibizumab under Other MABs, below).
Catumaxomab is a recombinant bi-specific trifunctional rat-mouse IgG hybrid monoclonal antibody that targets the epithelial cell adhesion molecule (EpCAM) on tumor cells and the CD3 protein on T cells. This bi-specific monoclonal antibody is approved in the EU and in the USA as an orphan drug for treating abdominal ascites in ovarian and gastric cancers. The rationale behind the bi-specific characteristics of catumaxomab is that it brings CD3-expressing anti-tumor T cells into close proximity of tumor cells expressing EpCAM. The Fc portion of the antibody also recruits phagocytic cells that mediate antibody-dependent cellular cytotoxicity and complement resulting in complement-dependent cytotoxicity of tumor cells.
Cetuximab is a human-mouse chimeric monoclonal antibody that targets epidermal growth factor receptor (EGFR). Binding of cetuximab to EGFR inhibits tumor cell growth by a variety of mechanisms, including decreases in kinase activity, matrix metalloproteinase activity, and growth factor production, and increased apoptosis. It is approved for use in patients with EGFR-positive head and neck squamous cell carcinoma in combination with radiotherapy or appropriate chemotherapy. It is also approved for treatment of kRas-negative, EGFR-positive metastatic colorectal cancer in combination with radiotherapy or appropriate chemotherapy, or as a single agent in patients who cannot tolerate certain chemotherapies. Cetuximab may be administered in combination with irinotecan or alone in patients who cannot tolerate irinotecan. HAMAs are generated by about 4% of patients being treated with cetuximab.
Ofatumumab is a human IgG1 monoclonal antibody directed against an epitope on CD20 on lymphocytes. Rituximab, the first approved CD20 monoclonal antibody (see below), binds a different CD20 epitope. Ofatumumab is approved for patients with CLL who are refractory to fludarabine and alemtuzumab. Ofatumumab binds to all B cells including B-CLL. It is thought to lyse B-CLL cells in the presence of complement and to mediate antibody-dependent cellular cytotoxicity. There is a slight risk of hepatitis B virus reactivation in patients taking ofatumumab.
Panitumumab is a fully human IgG2 kappa light chain monoclonal antibody. It is approved for the treatment of EGFR-expressing metastatic colorectal carcinoma with disease progression on or following fluoropyrimidine-, oxaliplatin-, and irinotecan-containing chemotherapy regimens. Panitumumab binds to EGFR (similar to cetuximab), inhibiting epidermal growth factor from binding to its receptor, and prevents ligand-induced receptor autophosphorylation and activation of receptor-associated kinases. It inhibits cell growth, induces apoptosis, decreases vascular growth factor production, and suppresses internalization of the EGFR. Although dermatologic and infusion-related toxicities are common following infusion of panitumumab, the distinct advantage over cetuximab is that it is fully human (ie, does not elicit HAMAs) and thus has an extended half-life in circulation. This is the first FDA-approved monoclonal antibody produced from transgenic mice expressing the human immunoglobulin gene loci.
Pertuzumab is a recombinant humanized IgG1 monoclonal antibody. It is approved for the treatment of metastatic or locally advanced HER-2/neu-positive breast cancer in combination with trastuzumab (see below) and docetaxel as neoadjuvant therapy. This antibody suppresses tumor growth by preventing heterodimerization of the human epidermal growth factor receptor HER-2/neu with other HER family members, thus inhibiting ligand-mediated intracellular signaling through MAP kinase and PI3 kinase pathways. Pertuzumab also mediates antibody-dependent cell-mediated cytotoxicity on HER-2/neu-positive tumor cells.
Rituximab is a chimeric murine-human monoclonal IgG1 (human Fc) that binds to the CD20 molecule on normal and malignant B lymphocytes and is approved for the therapy of patients with CD20-positive large-B-cell diffuse non-Hodgkin’s lymphoma, and relapsed or refractory low-grade or follicular B-cell non-Hodgkin’s lymphoma as a single agent or in combination with appropriate chemotherapy. It is approved for treatment of CLL in combination with chemotherapy. It is also approved for the treatment of rheumatoid arthritis in combination with methotrexate in patients for whom anti-TNF-α therapy has failed. The most recent indication for rituximab is for the treatment of Wegener’s granulomatosis and microscopic polyangiitis. The mechanism of action includes complement-mediated lysis, antibody-dependent cellular cytotoxicity, and induction of apoptosis in malignant lymphoma cells and in B cells involved in the pathogenesis of rheumatoid arthritis and granulomatosis and polyangiitis. In lymphoma this drug appears to be synergistic with chemotherapy (eg, fludarabine, CHOP, see Chapter 54). Anemia or neutropenia is an important adverse effect, which can be countered with granulocyte colony-stimulating factor (G-CSF). Other adverse effects include hypotension, rash, gastrointestinal disturbance, fever, and fatigue.
Trastuzumab is a recombinant DNA-derived, humanized monoclonal antibody that binds to the extracellular domain of HER-2/neu. This antibody blocks the natural ligand from binding and down-regulates the receptor. Trastuzumab is approved for the treatment of HER-2/neu-positive tumors in patients with breast cancer and patients with metastatic gastric or gastroesophageal junction adenocarcinoma. As a single agent it induces remission in 15–20% of breast cancer patients; in combination with chemotherapy, it increases response rates and duration as well as 1-year survival. Trastuzumab is under investigation for other tumors that express HER-2/neu (see Chapter 54). Patients should be monitored for potential cardiomyopathy while taking this drug.
MABs Used to Deliver Isotopes & Toxins to Tumors
Ado-trastuzumab emtansine is an antibody-drug conjugate in which the anti-HER-2/neu antibody, trastuzumab (see above), is chemically linked to the cytotoxic agent, mertansine, a microtubule disruptor. Ado-trastuzumab emtansine is approved for patients with HER-2/neu-positive breast cancer who have previously received trastuzumab and a taxane separately or in combination, and whose disease recurred or progressed during prior treatment. Toxicities are identical to trastuzumab alone, but also include hepatotoxicity due to emtansine.
Arcitumomab is a murine Fab fragment from an anti-carcinoembryonic antigen (CEA) antibody labeled with technetium 99m (99mTc) that is used for imaging patients with metastatic colorectal carcinoma (immunoscintigraphy) to determine extent of disease. CEA is often upregulated in patients with gastrointestinal carcinomas. The use of the Fab fragment decreases the immunogenicity of the agent so that it can be given more than once; intact murine monoclonal antibodies would elicit stronger HAMA.
Brentuximab vedotin is a new antibody-drug conjugate that binds CD30, a cell surface marker in the TNF receptor superfamily that is expressed on anaplastic large T-cell lymphomas and on Reed-Sternberg cells in Hodgkin lymphoma; it may also be expressed on activated leukocytes. Brentuximab vedotin consists of a chimeric (mouse-human) IgG1 linked to monomethylauristatin E (MMAE), a microtubule-disrupting agent that induces cell cycle arrest and apoptosis. When this ADC binds CD30 on the cell surface, the complex is internalized followed by proteolytic cleavage of MMAE from the IgG. Brentuximab is approved for treatment of patients with Hodgkin’s lymphoma after failure of autologous stem cell transplantation or after failure of at least two previous chemotherapy regimens. It is also approved for patients with systemic anaplastic large cell lymphoma after failure of at least one previous multiagent chemotherapy regimen. Patients taking brentuximab vedotin should be monitored primarily for neutropenia and peripheral sensory neuropathy.
Capromab pendetide is a murine monoclonal antibody specific for prostate specific membrane antigen. It is coupled to isotopic indium (111In) and is used in immunoscintigraphy for patients with biopsy-confirmed prostate cancer and post-prostatectomy in patients with rising prostate specific antibody level to determine extent of disease.
Ibritumomab tiuxetan is an anti-CD20 murine monoclonal antibody labeled with isotopic yttrium (90Y) or 111In. The radiation of the isotope coupled to the antibody provides the major antitumor activity of this drug. Ibritumomab is approved for use in patients with relapsed or refractory low-grade, follicular, or B-cell non-Hodgkin’s lymphoma, including patients with rituximab-refractory follicular disease. It is used in conjunction with rituximab in a two-step therapeutic regimen.
MABs Used as Immunosuppressants & Anti-Inflammatory Agents
Adalimumab, certolizumab pegol, etanercept, golimumab, and infliximab are antibodies that bind TNF-α, a proinflammatory cytokine that is important in rheumatoid arthritis and similar inflammatory diseases. Abatacept is a recombinant fusion protein composed of the extracellular domain of cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) fused to hinge, CH2, and CH3 domains of the Fc domains of human IgG1. Tocilizumab is recombinant humanized IgG1 that binds to soluble and membrane-associated IL-6 receptors. It inhibits IL-6-mediated signaling on lymphocytes, suppressing inflammatory processes. These drugs are approved for use in rheumatoid and other forms of arthritis and are discussed in Chapter 36.
Alefacept is an engineered protein consisting of the CD2-binding portion of leukocyte-function-associated antigen-3 (LFA-3) fused to a human IgG1 Fc region (hinge, CH2, and CH3). It is approved for the treatment of plaque psoriasis. It inhibits activation of T cells by binding to cell surface CD2, inhibiting the normal CD2/LFA-3 interaction (Figure 55–7). Treatment of patients with alefacept also results in a dose-dependent reduction of the total number of circulating T cells, especially CD4 and CD8 memory effector subsets that predominate in psoriatic plaques. Peripheral T-cell counts of patients receiving alefacept must be monitored and the drug discontinued if CD4 lymphocyte levels fall below 250 cells/μL.
FIGURE 55–7 Actions of some monoclonal antibodies (shown in red). CTLA-4-lgFc fusion protein (CTLA-4-lg, abatacept) binds to CD80/86 on DC and inhibits T-cell costimulation. Alefacept inhibits activation of T cells by blocking the interaction of LFA-3 and CD2. Basiliximab and daclizumab block IL-2 from binding to the IL-2 receptor (CD25) on T cells, preventing activation; CD25 is also important for the survival of T regulatory cells. T-cell activation can be maintained or restored if CTLA-4 interaction with CD80/86 is blocked using an anti-CTLA-4 antibody (ipilimumab); ipilimumab inhibits CTLA-4 signaling and prolongs activation.
Basiliximab is a chimeric mouse-human IgG1 that binds to CD25, the IL-2 receptor alpha chain on activated lymphocytes. Daclizumab is a humanized IgG1 that also binds to the α subunit of the IL-2 receptor. Both agents function as IL-2 antagonists, blocking IL-2 from binding to activated lymphocytes, and are therefore immunosuppressive. They are indicated for prophylaxis of acute organ rejection in renal transplant patients and either drug may be used as part of an immunosuppressive regimen that also includes glucocorticoids and cyclosporine.
Canakinumab is a human IgG kappa chain monoclonal antibody that prevents IL-1β from binding to its receptor. It is approved for cryopyrin-associated periodic syndromes (CAPS) in adults and children 4 years old and older. CAPS includes familial cold autoinflammatory syndrome, Muckle-Wells syndrome, and systemic juvenile idiopathic arthritis in children 2 years old or older. These diseases are caused by mutations in a gene (NLRP-3) that encodes cryopyrin, an important component of the inflammasome. NLRP-3 mutations cause excessive release of IL-1β, causing autoimmune inflammation resulting in fever, urticarial-like rash, arthralgia, myalgia, fatigue, and conjunctivitis.
Natalizumab is a humanized IgG4 monoclonal antibody that binds to the α4-subunit of α4β1 and α4β7 integrins expressed on the surfaces of all leukocytes except neutrophils, and inhibits the α4-mediated adhesion of leukocytes to their cognate receptor. It is indicated for patients with multiple sclerosis and Crohn’s disease who have not tolerated or had inadequate responses to conventional treatments. Natalizumab should not be used with any of the anti-TNF-α drugs listed above.
Omalizumab is an anti-IgE recombinant humanized monoclonal antibody that is approved for the treatment of allergic asthma in adult and adolescent patients whose symptoms are refractory to inhaled corticosteroids (see Chapter 20). The drug is also approved for chronic urticaria. The antibody blocks the binding of IgE to the high-affinity Fcε receptor on basophils and mast cells, which suppresses IgE-mediated release of type I allergy mediators such as histamine and leukotrienes. Total serum IgE levels may remain elevated in patients for up to 1 year after administration of omalizumab.
Ustekinumab is a human IgG1 monoclonal antibody that binds to the p40 subunit of IL-12 and IL-23 cytokines. It blocks IL-12 and IL-23 from binding to their receptors, therefore inhibiting receptor-mediated signaling in lymphocytes. Ustekinumab is indicated for adult patients with moderate to severe plaque psoriasis either alone or with methotrexate. The advantage of ustekinumab over anti-TNF-α drugs for psoriasis is faster and longer term improvement in symptoms along with very infrequent dosing.
Vedolizumab is a humanized monoclonal antibody that targets the α4β7 integrin in the gastrointestinal tract. It does not appear to induce systemic immunosuppression of other α4β7 integrin-binding antibodies such as natalizumab because it does not bind to the majority of α4β7 integrin on lymphocytes. It has been recommended for approval for the treatment of Crohn’s disease and ulcerative colitis.
Abciximab is a Fab fragment of a murine-human monoclonal antibody that binds to the integrin GPIIb/IIIa receptor on activated platelets and inhibits fibrinogen, von Willebrand factor, and other adhesion molecules from binding to activated platelets, thus preventing their aggregation. It is indicated as an adjunct to percutaneous coronary intervention in combination with aspirin and heparin for the prevention of cardiac ischemic complications. See Chapter 34for additional details.
Denosumab is a human IgG2 monoclonal antibody specific for human RANKL (receptor activator of nuclear factor kappa-B ligand; see Chapter 42). By binding RANKL it inhibits the maturation of osteoclasts, the cells responsible for bone resorption. Denosumab is indicated for treatment of postmenopausal women with osteoporosis at high risk for fracture. Before starting denosumab, patients must be evaluated to be sure they are not hypocalcemic. During treatment, patients should receive supplements of calcium and vitamin D.
Eculizumab is a humanized IgG monoclonal antibody that binds the C5 complement component, inhibiting its cleavage into C5a and C5b thereby inhibiting the terminal pore-forming lytic activity of complement. Eculizumab is approved for patients with paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). It dramatically reduces the need for red blood cell transfusions. It prevents PNH symptoms of anemia, fatigue, thrombosis, and hemoglobinemia by inhibiting intravascular hemolysis. Similarly in aHUS eculizumab prevents complement-mediated thrombotic microangiopathy. Clinicians must be aware of increased risk of meningococcal infection in patients receiving this anti-C5 monoclonal antibody.
Palivizumab is a humanized IgG1 monoclonal antibody that binds to the fusion protein of respiratory syncytial virus (RSV), preventing serious lower respiratory tract disease. It is used in neonates at risk for this viral infection and reduces the frequency of infection and hospitalization by about 50% (see Chapter 49).
Ranibizumab is a recombinant human IgG1 Fab that binds to VEGF-A. It prevents new blood vessel formation by blocking VEGF from binding to its receptor. Ranibizumab is approved for intravitreal injection in patients with neovascular age-related macular degeneration, diabetic macular edema, and sudden blurring or vision loss secondary to macular edema following retinal vein occlusion. Pegaptanib is a pegylated oligonucleotide that binds extracellular VEGF and is also given by intravitreous injection to slow macular degeneration.
Raxibacumab is a human IgG1 lambda chain monoclonal antibody that binds to the PA protein of Bacillus anthracis, preventing cellular entry of the anthrax toxins (lethal and edema factors). Raxibacumab is approved for the treatment or prophylaxis of adults and children with inhalational anthrax in combination with appropriate antibacterial drugs. Interestingly, raxibacumab was not tested in humans because exposing a control cohort to inhalational anthrax is unethical and there are too few naturally infected persons to conduct a proper clinical trial.
CLINICAL USES OF IMMUNOSUPPRESSIVE DRUGS
Immunosuppressive agents are commonly used in two clinical circumstances: transplantation and autoimmune disorders. The agents used differ somewhat for the specific disorders treated (see specific agents and Table 55–1), as do administration schedules. Because autoimmune disorders are very complex, optimal treatment schedules have yet to be established in many of them.
SOLID ORGAN & BONE MARROW TRANSPLANTATION
In organ transplantation, tissue typing—based on donor and recipient histocompatibility matching with the human leukocyte antigen (HLA) haplotype system—is required. Close histocompatibility matching reduces the likelihood of graft rejection and may also reduce the requirements for intensive immunosuppressive therapy. Prior to transplant, patients may receive an immunosuppressive regimen, including antithymocyte globulin, daclizumab, or basiliximab. Four types of rejection can occur in a solid organ transplant recipient: hyperacute, accelerated, acute, and chronic. Hyperacute rejection is due to preformed antibodies against the donor organ, such as anti-blood group antibodies. Hyperacute rejection occurs within hours of the transplant and cannot be stopped with immunosuppressive drugs. It results in rapid necrosis and failure of the transplanted organ. Accelerated rejection is mediated by both antibodies and T cells, and it also cannot be stopped by immunosuppressive drugs. Acute rejection of an organ occurs within days to months and involves mainly cellular immunity. Reversal of acute rejection is usually possible with general immunosuppressive drugs such as azathioprine, mycophenolate mofetil, cyclosporine, tacrolimus, glucocorticoids, cyclophosphamide, methotrexate, and sirolimus. Recently, biologic agents such as anti-CD3 monoclonal antibodies have been used to stem acute rejection. Chronic rejection usually occurs months or even years after transplantation. It is characterized by thickening and fibrosis of the vasculature of the transplanted organ, involving both cellular and humoral immunity. Chronic rejection is treated with the same drugs as those used for acute rejection.
Allogeneic hematopoietic stem cell transplantation is a well-established treatment for many malignant and nonmalignant diseases. An HLA-matched donor, usually a family member, is located, patients are conditioned with high-dose chemotherapy or radiation therapy, and then donor stem cells are infused. The conditioning regimen is used not only to kill cancer cells in the case of malignant disease, but also to totally suppress the immune system so that the patient does not reject the donor stem cells. As patients’ blood counts recover (after reduction by the conditioning regimen) they develop a new immune system that is created from the donor stem cells. Rejection of donor stem cells is uncommon, and can only be treated by infusion of more stem cells from the donor.
GVH disease, however, is very common, occurring in the majority of patients who receive an allogeneic transplant. GVH disease occurs because donor T cells fail to recognize the patient’s skin, liver, and gut (usually) as self and attack those tissues. Although patients are given immunosuppressive therapy (cyclosporine, methotrexate, and others) early in the transplant course to help prevent this development, it usually occurs despite these medications. Acute GVH disease occurs within the first 100 days, and is usually manifested as a skin rash, severe diarrhea, or hepatotoxicity. Additional medications are added, invariably starting with high-dose corticosteroids, and adding drugs such as mycophenolate mofetil, sirolimus, tacrolimus, daclizumab, and others, with variable success rates. Patients generally progress to chronic GVH disease (after 100 days) and require therapy for variable periods thereafter. Unlike solid organ transplant patients, however, most stem cell transplant patients are able to eventually discontinue immunosuppressive drugs as GVH disease resolves (usually 1–2 years after their transplant).
The effectiveness of immunosuppressive drugs in autoimmune disorders varies widely. Nonetheless, with immunosuppressive therapy, remissions can be obtained in many instances of autoimmune hemolytic anemia, idiopathic thrombocytopenic purpura, type 1 diabetes, Hashimoto’s thyroiditis, and temporal arteritis. Improvement is also often seen in patients with systemic lupus erythematosus, acute glomerulonephritis, acquired factor VIII inhibitors (antibodies), rheumatoid arthritis, inflammatory myopathy, scleroderma, and certain other autoimmune states.
Immunosuppressive therapy is utilized in chronic severe asthma, where cyclosporine is often effective and sirolimus is another alternative. Omalizumab (anti-IgE antibody) has been approved for the treatment of severe asthma (see previous section). Tacrolimus is currently under clinical investigation for the management of autoimmune chronic active hepatitis and of multiple sclerosis, where IFN-β has a definitive role.
The development of agents that modulate the immune response rather than suppress it has become an important area of pharmacology. The rationale underlying this approach is that such drugs may increasethe immune responsiveness of patients who have either selective or generalized immunodeficiency. The major potential uses are in immunodeficiency disorders, chronic infectious diseases, and cancer. The AIDS epidemic has greatly increased interest in developing more effective immunomodulating drugs.
The cytokines are a large and heterogeneous group of proteins with diverse functions. Some are immunoregulatory proteins synthesized within lymphoreticular cells and play numerous interacting roles in the function of the immune system and in the control of hematopoiesis. The cytokines that have been clearly identified are summarized in Table 55–2. In most instances, cytokines mediate their effects through receptors on relevant target cells and appear to act in a manner similar to the mechanism of action of hormones. In other instances, cytokines may have antiproliferative, antimicrobial, and antitumor effects.
TABLE 55–2 The cytokines.
The first group of cytokines discovered, the interferons (IFNs), were followed by the colony-stimulating factors (CSFs, discussed in Chapter 33). The latter regulate the proliferation and differentiation of bone marrow progenitor cells. Most of the more recently discovered cytokines have been classified as interleukins (ILs) and numbered in the order of their discovery. Pharmaceutical cytokines are produced using gene cloning techniques.
Most cytokines (including TNF-α, IFN-γ, IL-2, G-CSF, and granulocyte-macrophage colony-stimulating factor [GM-CSF]) have very short serum half-lives (minutes). The usual subcutaneous route of administration provides slower release into the circulation and a longer duration of action. Each cytokine has its own unique toxicity, but some toxicities are shared. For example, IFN-α, IFN-β, IFN-γ, IL-2, and TNF-α all induce fever, flu-like symptoms, anorexia, fatigue, and malaise.
Interferons are proteins that are currently grouped into three families: IFN-α, IFN-β, and IFN-γ. The IFN-α and IFN-β families comprise type I IFNs, ie, acid-stable proteins that act on the same receptor on target cells. IFN-γ, a type II IFN, is acid-labile and acts on a separate receptor on target cells. Type I IFNs are usually induced by virus infections, with leukocytes producing IFN-α. Fibroblasts and epithelial cells produce IFN-β. IFN-γ is usually the product of activated T lymphocytes.
IFNs interact with cell receptors to produce a wide variety of effects that depend on the cell and IFN types. IFNs, particularly IFN-γ, display immune-enhancing properties, which include increased antigen presentation and macrophage, NK cell, and cytotoxic T-lymphocyte activation. IFNs also inhibit cell proliferation. In this respect, IFN-α and IFN-β are more potent than IFN-γ. Another striking IFN action is increased expression of MHC molecules on cell surfaces. While all three types of IFN induce MHC class I molecules, only IFN-γ induces class II expression. In glial cells, IFN-β antagonizes this effect and may, in fact, decrease antigen presentation within the nervous system.
IFN-α is approved for the treatment of several neoplasms, including hairy cell leukemia, chronic myelogenous leukemia, malignant melanoma, and Kaposi’s sarcoma, and for use in hepatitis B and C infections. It has also shown activity as an anti-cancer agent in renal cell carcinoma, carcinoid syndrome, and T-cell leukemia. IFN-β is approved for use in relapsing-type multiple sclerosis. IFN-γ is approved for the treatment of chronic granulomatous disease and IL-2, for metastatic renal cell carcinoma and malignant melanoma. Clinical investigations of other cytokines, including IL-1, -3, -4, -6, -10, -11, and -12, are ongoing. Toxicities of IFNs, which include fever, chills, malaise, myalgias, myelosuppression, headache, and depression, can severely restrict their clinical use.
TNF-α has been extensively tested in the therapy of various malignancies, but results have been disappointing due to dose-limiting toxicities. One exception is the use of intra-arterial high-dose TNF-α for malignant melanoma and soft tissue sarcoma of the extremities. In these settings, response rates greater than 80% have been noted.
Cytokines have been under clinical investigation as adjuvants to vaccines, and IFNs and IL-2 have shown some positive effects in the response of human subjects to hepatitis B vaccine. Denileukin diftitox is IL-2 fused to diphtheria toxin, used for the treatment of patients with CD25+ cutaneous T-cell lymphomas. IL-12 and GM-CSF have also shown adjuvant effects with vaccines. GM-CSF is of particular interest because it promotes recruitment of professional antigen-presenting cells such as the dendritic cells required for priming naive antigen-specific T-lymphocyte responses. There are some claims that GM-CSF can itself stimulate an antitumor immune response, resulting in tumor regression in melanoma and prostate cancer.
It is important to emphasize that cytokine interactions with target cells often result in the release of a cascade of different endogenous cytokines, which exert their effects sequentially or simultaneously. For example, IFN-γ exposure increases the number of cell-surface receptors on target cells for TNF-α. Therapy with IL-2 induces the production of TNF-α, while therapy with IL-12 induces the production of IFN-γ.
An important application of immunomodulation therapy involves the use of cytokine inhibitors for inflammatory diseases (see Chapter 36) and septic shock, conditions in which cytokines such as IL-1 and TNF-α (see above) are involved in the pathogenesis. Drugs now in use or under investigation include anticytokine antibodies and soluble cytokine receptors. Anakinra is a recombinant form of the naturally occurring IL-1 receptor antagonist that prevents IL-1 from binding to its receptor, stemming the cascade of cytokines that would otherwise be released. Anakinra is approved for use in adult rheumatoid arthritis patients who have failed treatment with one or more disease-modifying antirheumatic drugs but is no longer much used for this indication. As stated above, canakinumab is a recombinant human anti-IL-1β monoclonal antibody. It binds to human IL-1β and prevents it from binding to IL-1 receptors. Rilonacept is a dimeric fusion protein consisting of the ligand-binding domains of the extracellular portions of the human interleukin-1 receptor component (IL-1RI) and IL-1 receptor accessory protein (IL-1RAcP) fused to the Fc portion of human IgG1. These molecules are indicated for treatment of cryopyrin-associated periodic syndromes.
Patients must be carefully monitored for serious infections or malignancies if they are also taking an anti-TNF-α drug, have chronic infections, or are otherwise immunosuppressed.
IMMUNOLOGIC REACTIONS TO DRUGS & DRUG ALLERGY
The basic immune mechanism and the ways in which it can be suppressed or stimulated by drugs have been discussed in previous sections of this chapter. Drugs also activate the immune system in undesirable ways that are manifested as adverse drug reactions. These reactions are generally grouped in a broad classification as “drug allergy.” Indeed, many drug reactions such as those to penicillin, iodides, phenytoin, and sulfonamides are allergic in nature. These drug reactions are manifested as skin eruptions, edema, anaphylactoid reactions, glomerulonephritis, fever, and eosinophilia.
Drug reactions mediated by immune responses can have several different mechanisms. Thus, any of the four major types of hypersensitivity discussed earlier in this chapter (pages 950–951) can be associated with allergic drug reactions:
•Type I: IgE-mediated acute allergic reactions to stings, pollens, and drugs, including anaphylaxis, urticaria, and angioedema. IgE is fixed to tissue mast cells and blood basophils, and after interaction with antigen the cells release potent mediators.
•Type II: Drugs often modify host proteins, thereby eliciting antibody responses to the modified protein. These allergic responses involve IgG or IgM in which the antibody becomes fixed to a host cell, which is then subject to complement-dependent lysis or to antibody-dependent cellular cytotoxicity.
•Type III: Drugs may cause serum sickness, which involves immune complexes containing IgG complexed with a foreign antigen and is a multisystem complement-dependent vasculitis that may also result in urticaria.
•Type IV: Cell-mediated allergy is the mechanism involved in allergic contact dermatitis from topically applied drugs or induration of the skin at the site of an antigen injected intradermally.
In some drug reactions, several of these hypersensitivity responses may occur simultaneously. Some adverse reactions to drugs may be mistakenly classified as allergic or immune when they are actually genetic deficiency states or are idiosyncratic and not mediated by immune mechanisms (eg, hemolysis due to primaquine in glucose-6-phosphate dehydrogenase deficiency, or aplastic anemia caused by chloramphenicol).
IMMEDIATE (TYPE I) DRUG ALLERGY
Type I (immediate) sensitivity allergy to certain drugs occurs when the drug, not capable of inducing an immune response by itself, covalently links to a host carrier protein (hapten). When this happens, the immune system detects the drug-hapten conjugate as “modified self” and responds by generating IgE antibodies specific for the drug-hapten. It is not known why some people mount an IgE response to a drug, while others mount IgG responses. Under the influence of IL-4, -5, and -13 secreted by TH2 cells, B cells specific for the drug secrete IgE antibody. The mechanism for IgE-mediated immediate hypersensitivity is diagrammed in Figure 55–5.
Fixation of the IgE antibody to high-affinity Fc receptors (FcεRs) on blood basophils or their tissue equivalent (mast cells) sets the stage for an acute allergic reaction. The most important sites for mast cell distribution are skin, nasal epithelium, lung, and gastrointestinal tract. When the offending drug is reintroduced into the body, it binds and cross-links basophil and mast cell-surface IgE to signal release of the mediators (eg, histamine, leukotrienes; see Chapters 16 and 18) from granules. Mediator release is associated with calcium influx and a fall in intracellular cAMP within the mast cell. Many of the drugs that block mediator release appear to act through the cAMP mechanism (eg, catecholamines, glucocorticoids, theophylline), others block histamine release, and still others block histamine receptors. Other vasoactive substances such as kinins may also be generated during histamine release. These mediators initiate immediate vascular smooth muscle relaxation, increased vascular permeability, hypotension, edema, and bronchoconstriction.
Drug Treatment of Immediate Allergy
One can test an individual for possible sensitivity to a drug by a simple scratch test, ie, by applying an extremely dilute solution of the drug to the skin and making a scratch with the tip of a needle. If allergy is present, an immediate (within 10–15 minutes) wheal (edema) and flare (increased blood flow) will occur. However, skin tests may be negative in spite of IgE hypersensitivity to a hapten or to a metabolic product of the drug, especially if the patient is taking steroids or antihistamines.
Drugs that modify allergic responses act at several links in this chain of events. Prednisone, often used in severe allergic reactions, is immunosuppressive; it blocks proliferation of the IgE-producing clones and inhibits IL-4 production by T helper cells in the IgE response, since glucocorticoids are generally toxic to lymphocytes. In the efferent limb of the allergic response, isoproterenol, epinephrine, and theophylline reduce the release of mediators from mast cells and basophils and produce bronchodilation. Epinephrine opposes histamine; it relaxes bronchiolar smooth muscle and contracts vascular muscle, relieving both bronchospasm and hypotension. As noted in Chapter 8, epinephrine is the drug of choice in anaphylactic reactions. The antihistamines competitively inhibit histamine, which would otherwise produce bronchoconstriction and increased capillary permeability in end organs. Glucocorticoids may also act to reduce tissue injury and edema in the inflamed tissue, as well as facilitating the actions of catecholamines in cells that may have become refractory to epinephrine or isoproterenol. Several agents directed toward the inhibition of leukotrienes may be useful in acute allergic and inflammatory disorders (see Chapter 20).
Desensitization to Drugs
When reasonable alternatives are not available, certain drugs (eg, penicillin, insulin) must be used for life-threatening illnesses even in the presence of known allergic sensitivity. In such cases, desensitization (also called hyposensitization) can sometimes be accomplished by starting with very small doses of the drug and gradually increasing the dose over a period of hours or days to the full therapeutic range (see Chapter 43). This practice is hazardous and must be performed under direct medical supervision with epinephrine available for immediate injection, as anaphylaxis may occur before desensitization has been achieved. It is thought that slow and progressive administration of the drug gradually binds all available IgE on mast cells, triggering a gradual release of granules. Once all of the IgE on the mast cell surfaces has been bound and the cells have been degranulated, therapeutic doses of the offending drug may be given with minimal further immune reaction. Therefore, a patient is only desensitized during administration of the drug.
AUTOIMMUNE (TYPE II) REACTIONS TO DRUGS
Certain autoimmune syndromes can be induced by drugs. Examples include systemic lupus erythematosus following hydralazine or procainamide therapy, “lupoid hepatitis” due to cathartic sensitivity, autoimmune hemolytic anemia resulting from methyldopa administration, thrombocytopenic purpura due to quinidine, and agranulocytosis due to a variety of drugs. As indicated in other chapters of this book, a number of drugs are associated with type I and type II reactions. In these drug-induced autoimmune states, IgG antibodies bind to drug-modified tissue and are destroyed by the complement system or by phagocytic cells with Fc receptors. Fortunately, autoimmune reactions to drugs usually subside within several months after the offending drug is withdrawn. Immunosuppressive therapy is warranted only when the autoimmune response is unusually severe.
SERUM SICKNESS & VASCULITIC (TYPE III) REACTIONS
Immunologic reactions to drugs resulting in serum sickness are more common than immediate anaphylactic responses, but type II and type III hypersensitivities often overlap. The clinical features of serum sickness include urticarial and erythematous skin eruptions, arthralgia or arthritis, lymphadenopathy, glomerulonephritis, peripheral edema, and fever. The reactions generally last 6–12 days and usually subside once the offending drug is eliminated. Antibodies of the IgM or IgG class are usually involved. The mechanism of tissue injury is immune complex formation and deposition on basement membranes (eg, lung, kidney), followed by complement activation and infiltration of leukocytes, causing tissue destruction. Glucocorticoids are useful in attenuating severe serum sickness reactions to drugs. In severe cases plasmapheresis can be used to remove the offending drug and immune complexes from circulation.
Immune vasculitis can also be induced by drugs. The sulfonamides, penicillin, thiouracil, anticonvulsants, and iodides have all been implicated in the initiation of hypersensitivity angiitis. Erythema multiforme is a relatively mild vasculitic skin disorder that may be secondary to drug hypersensitivity. Stevens-Johnson syndrome is probably a more severe form of this hypersensitivity reaction and consists of erythema multiforme, arthritis, nephritis, central nervous system abnormalities, and myocarditis. It has frequently been associated with sulfonamide therapy. Administration of nonhuman monoclonal or polyclonal antibodies such as rattlesnake antivenom may cause serum sickness.
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CASE STUDY ANSWER
Within 24–72 hours postpartum, the woman should be given a 2 mL intramuscular injection of 300 mcg anti-Rho(D) immune globulin. This will clear any fetal Rh-positive red cells from her circulation so she does not generate anti-Rho(D) B cells that might jeopardize any future pregnancy.