Immunology (Lippincott Illustrated Reviews Series) 2nd Edition

Chapter 17: Transplantation

I. OVERVIEW

The ability to replace or restore damaged tissues, or even entire body parts, has long been a dream of the healing professions. The broad application of transplantation in human medicine has been available only for the past five or six decades. Among the obstacles that had to be overcome were infection control, the genetic matching of donors with hosts, an understanding of the immunologic processes involved, and the development of agents that could inhibit the immune system. The development of antiseptic techniques coupled with antibiotics reduced the risk of infection, whereas tissue typing and immunosuppressive drugs increased the probability of transplant success.

II. GENETIC BASIS OF TRANSPLANTATION

The genetic basis for transplantation was recognized in the early twentieth century by pioneers such as Loeb, Tyzzer, and Little. The genetic match (similarity/disparity) between the donor and the host is perhaps the most important factor determining the likelihood of a successful transplant. The recipient’s immune system looks for certain genetically encoded molecules (histocompatibility antigens) on the surfaces of the donor cells. Thus, the response against transplanted cells and tissues has parallels to the body’s response to foreign infectious organisms.

A. Histocompatibility genes and antigens

Histocompatibility genes encode histocompatibility antigens. It is estimated that there are several scores of such loci, probably more than a hundred. Among these are the MHC class I and II molecules encoded within the major histocompatibility complex (MHC). With the possible exception of a few loci whose expression is not understood, the products of histocompatibility genes are codominantly expressed. Codominance means that they are expressed whether present as a single copy (heterozygous or hemizygous) or two copies (homozygous). Thus, an individual heterozygous at a particular histocompatibility locus (e.g., H1a/H1b) would simultaneously express both H1a and H1b molecules on the same surface cell surface (Fig. 17.1). The same would be true for other histocompatibility loci (e.g., H2a/H2bH3a/H3). The terminology here is that applied to humans. The H2 in humans should not be confused with the H2 of mice, which is the murine MHC. The MHC of humans is termed HLA (see Chapter 6).

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Figure 17.1

Histocompatibility antigens.

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Figure 17.2

Display of histocompatibility antigens. Peptide fragments that result from proteasome degradation of cytoplasmic molecules and their subsequent loading onto MHC I molecules in the endoplasmic reticulum are present on the surface of all nucleated cells.

The structures and functions are known for only a very few of these molecules, namely, the MHC class I and II molecules. Little is known about the other non-MHC histocompatibility antigens except that they include molecules encoded by a large number of genes scattered among all of the chromosomes (including X and Y). In principle, any peptide fragment brought to the cell surface and presented by either MHC class I or II molecules could serve as a histocompatibility antigen (Fig. 17.2). Such fragments could be derived from cytosolic proteins or from cell debris ingested and degraded by phagocytic cells. The important distinction is that the molecules are encoded within the transplanted donor cells and not derived from infectious agents.

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Figure 17.3

Classification of grafts by donor–recipient genetic relationship.

B. Types of grafts

Transplants may be categorized by location or by the genetic relationship between the recipient and the donor. With respect to location, tissues or organs that are placed in their normal anatomic location are called orthotopic grafts. However, many transplanted tissues or organs can function quite well in other sites as well. Grafts that are placed into a site other than their normal one are called heterotopic grafts. Heterotopic grafts are especially useful in cases in which orthotopic placement may be technically difficult.

Classification of grafts by the donor–recipient genetic relationship (Fig. 17.3) is more complex. Autografts are those transferred from one part of an individual to another location on that same individual. Syngeneic grafts are those transferred between different individuals who are genetically identical or nearly so (e.g., identical twins or members of an inbred strain). Allogeneic grafts (or allografts) are transferred between two genetically disparate individuals of the same species (e.g., brother and sister, parent and child, or totally unrelated individuals). Finally, xenogeneic grafts (or xenografts) are those exchanged between members of different species (e.g., the placement of primate hearts into human recipients).

C. The laws of transplantation

The laws of transplantation were originally established in experimental studies, particularly in mice, but are applicable to human transplantation as well. Genetic diversity in humans virtually ensures that no two individuals are genetically identical (identical twins are an exception). The histocompatibility antigens of concern in transplantation vary from one case to another, depending on what specific genetic differences are present in each donor–recipient combination (Fig. 17.4). Experimental animals and plants can be deliberately bred to reduce their genetic heterogeneity so that genetic variability becomes a controlled variable rather than an uncontrolled one. This process, called inbreeding, is accomplished by mating of closely related individuals. When laboratory mice are subjected to brother–sister matings for 20 or more consecutive generations, inbred strains are produced. The animals within a given inbred strain are hypothetically homozygous for more than 99% of their genetic loci and, for practical purposes, are all genetically identical.

Transplants between members of the same inbred strains and between members of different inbred strains were used to deduce the laws of transplantation, which can be summarized as a host can recognize as foreign and mount a response against any histocompatibility antigen not encoded within its own cells (Fig. 17.5). Grafts exchanged between individuals of the same species who are completely different (homozygous for different alleles) at a histocompatibility locus can potentially be rejected. Such differences do not necessarily cause rejection on every occasion, for various reasons, but the potential is always present. Each member in the exchange will recognize the allelic form of the histocompatibility antigen expressed by the other as foreign. Heterozygous recipients, on the other hand, will see nothing foreign on grafts received from homozygous parental donors. Heterozygous grafts placed onto either type of homozygous parental type recipients will be rejected, as they express histocompatibility antigens that are foreign to one or the other parental recipient.

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Figure 17.4

Histocompatibility differences vary among donor–recipient combinations. Which donor antigens stimulate the recipient immune response depends on the specific combination of donor and recipient histocompatibility genes involved.

The utility of inbred strains can be extended by further subjecting them to programs of selection and breeding that use normal genetic recombination for the transfer of small chromosomal segments from one inbred strain to another. These new sets of inbred animals are called congenic strains, and they permit comparisons among organisms that differ from one another by only a small section of a chromosome or, conversely, that have only a small chromosomal segment in common (Fig. 17.6). Comparisons among congenic strains allow the mapping and analysis of individual histocompatibility genes within the transferred segment.

The most thoroughly characterized histocompatibility genes are those encoding the MHC class I and II molecules. As was discussed in Chapter 6, the MHC class I and II molecules are normally quite polymorphic within populations. As a result, MHC class I and II molecules (or, more precisely, fragments of different MHC I and II molecules being presented on intact and normally functioning MHC class I and II molecules) that differ between a host and a donor are readily recognized as foreign and trigger host immune responses directed against the donor cells (Figs. 17.7A and 17.7B). Foreign MHC molecules (especially the class I molecules) present a strong barrier to transplant survival, and it has been estimated that 5% to 10% of an individual’s CD8+ T cells can recognize and bind fragments of foreign MHC class I.

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Figure 17.5

Laws of transplantation.

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Figure 17.6

Congenic strains. Congenic strains, attained by systemic breeding and selection, differ only by small chromosomal regions.

Several different characteristics have been noted that distinguish the effects on transplantation of differences between host and donor at MHC I and II loci from those of differences at non-MHC (or minor) histocompatibility loci (Table 17.1). Although exceptions can often be found, valid generalizations can be made.

III. TISSUE REJECTION

The recipient immune system recognizes peptide fragments presented by MHC class I or II molecules, whether those fragments are derived from infectious organisms or from the degradation of self-molecules encoded by host genes (see Figs. 10.6 and 10.7). In the case of transplanted tissues, the genes of the engrafted cells may encode nonself molecules that also can be detected by the recipient immune system and function as histocompatibility antigens. T cells can detect and be activated against histocompatibility antigens through two different pathways of recognition: direct or indirect (Fig. 17.8). Direct recognition involves antigen presentation by donor antigen-presenting cells (APCs) to recipient T cells, whereas indirect recognition involves antigen presentation by recipient APCs to recipient T cells.

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Figure 17.7

Fragments of MHC I and II molecules can be presented as histocompatibility antigens by intact MHC I and MHC II molecules. A. Erroneously translated, misfolded or, otherwise damaged cytoplasmically synthesized MHC class I and II molecules may be ubiquinated and degraded by proteasomes. The resulting fragments can be transported into the endoplasmic reticulum by TAP and loaded onto nascent MHC I molecules for eventual presentation to CD8+ T cells. B. Ingested MHC class I and II molecules may be degraded and loaded onto MHC II molecules for presentation.

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Figure 17.8

Direct recognition and indirect recognition.

Direct recognition can occur only when some of the MHC class I or II molecules on the donor cells are identical to those on recipient cells. Like other cytosolic proteins, MHC class I and II molecules can be degraded by proteasomes and the resulting fragments presented on the cell surface by intact MHC class I molecules. If the donor and recipient have MHC class I molecules in common, APCs of donor origin may be able to present those peptide fragments directly to the TCRs of recipient CD8+ T cells. Because the MHC class I molecules on the donor cells are the same as those present in the host thymus during thymic education, the recipient TCRs are able to recognize and bind the pMHC I molecules on the donor cells. Direct recognition may also occur if donor APCs ingest cellular debris of donor origin and process/present it via MHC class II molecules to recipient CD4+ T cells. Indirect recognition occurs when recipient APCs process and present peptide fragments derived from the ingestion, processing, and presentation of cellular debris from donor cells—debris that contains the donor histocompatibility antigens—and present it to recipient T cells.

Thus, the recognition of foreign histocompatibility antigens and the activation of T cells against them involve processes very similar to those involved in the initiation of responses against antigens derived from infectious organisms. Indeed, the recipient immune system may view the transplanted cells as just another batch of infected cells—infected by nonself-genes.

A. Types of rejection

Rejection responses fall into three general categories—chronic, acute, and hyperacute—depending on timing and intensity. Each type involves particular sets of immune responses and is determined in part by the genetic mismatch between donor and recipient.

B. Immune responses involved in rejection

Chronic rejections are the slowest and the least vigorous type of rejection. The transplanted tissues or organs establish a vascular connection and proceed to function for weeks, months, and even years before signs of deterioration due to immune attack become evident. Even after the first signs of rejection appear, the graft destruction proceeds slowly and gradually as the graft tissue is replaced by intracellular matrix and scar tissue. Chronic rejections are typical of situations in which the donor and recipient differ by only non-MHC histocompatibility gene differences, although there are exceptions.

Acute rejections occur much sooner after graft emplacement than do chronic rejections. The grafts establish vascular connections and function normally for a relatively short period (e.g., 2 to 4 weeks) before the first signs of rejection appear. Unlike chronic rejections, acute rejections proceed rapidly once underway. The grafts become edematous and inflamed, with an influx of blood and mononuclear cell infiltrates, and complete destruction and sloughing of the grafted tissues may take only a very few days following the first signs of deterioration. Acute rejections are commonly seen when the donor and recipient differ at MHC histocompatibility genes, especially those involving the MHC class I loci.

CLINICAL APPLICATION

Renal transplantation

Doug, a 42-year-old male, presented initially 3 years ago with weakness. He developed type 1 diabetes at age 18 and hypertension at age 32. He had been taking insulin and an antihypertensive medication. Blood tests revealed low hemoglobin and decreased renal function. He was diagnosed with anemia associated with chronic kidney disease. He was referred to a nephrologist, who managed the patient’s hypertension with an angiotensin-converting enzyme inhibitor, erythropoietin for anemia, dietary protein restriction, and vitamin D supplementation. Now, 3 years later, he has developed end-stage renal disease with worsening renal function requiring replacement therapy. The patient is advised to undergo dialysis or renal transplantation. He is advised that renal transplantation provides a good quality of life and is less expensive overall than chronic hemodialysis.

Fortunately, his brother has volunteered to be a kidney donor and is found to be an appropriate genetic match. The patient received his brother’s healthy kidney together with immunosuppressive therapy. The kidney is functioning satisfactorily as he recovers from surgery.

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Figure 17.9

Second set rejections. Initial allografts between different inbred strains usually undergo acute rejection. If the rejected graft combination is repeated, the newly placed graft is rejected in an accelerated (“second set”) manner.

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Figure 17.10

Naturally occurring antibodies. Naturally occurring antibodies against A and B antigens were so named because they were already present at the time of transfusion, prior to any known exposure or immunization.

Hyperacute rejections are the most rapid type of rejection. They are initiated and completed within a few days of graft placement, usually before the grafted tissue or organs can establish connections with the recipient vasculature. The immune attack is typically directed at the vasculature of the graft and is mediated (in various situations) by complement, natural killer (NK) cells, and/or preexisting antibodies. Hyperacute rejections have also been called “white grafts” because in the case of skin, the failure to establish a vascular connection gives the engrafted skin a blanched appearance. The term can be misleading; it does not describe the comparable condition of other rejected tissues. A hyperacutely rejected kidney, for example, may be bluish in color owing to the large amount of deteriorating blood trapped within it.

Like responses to infectious organs, immune responses against transplanted tissues or organs can display memory. Attempts to repeat grafts that have previously been rejected usually result in an accelerated graft rejection, a phenomenon termed second set rejection (Fig. 17.9). Grafts that are rejected chronically on the initial occasion may be rejected acutely when repeated. During the initial rejection, activated T and B lymphocytes can generate populations of memory cells that provide the basis for accelerated and heightened secondary responses. Second set responses are therefore simply secondary immune responses directed against histocompatibility antigens.

Although not every type of immune response is necessarily generated for every allograft or xenograft, almost every relevant type of immune response has been observed among various rejection episodes: antibodies, T-cell responses, complement, and even NK cells.

Antibodies against graft antigens occur from two primary sources. Natural antibodies are preexisting antibodies that are present in the absence of known exposure or immunization. They provide, for example, the basis for transfusion reactions against ABO antigens on red blood cells, a topic that is discussed later in this chapter. Natural antibodies are produced, probably by B-1 B cells, following stimulation by antigenic molecules on the natural flora found in the body (Fig. 17.10). They are of the IgM isotype and are directed against carbohydrate antigens. These antibodies are stimulated by microbial carbohydrate molecules but may cross-react with carbohydrate molecules on eukaryotic cells (e.g., human). Thus, for example, they can act immediately to damage erythrocytes in transfusions that are mismatched for carbohydrate ABO antigens. Similarly, in the case of xenografts, they can bind immediately to some carbohydrate molecules associated with the graft vasculature and initiate fatal damage to the graft.

The second source of antibodies involved in graft rejection occurs by the activation of B cells and generation of plasma cells synthesizing antibodies against histocompatibility antigens on graft tissue. Typically, sufficient amounts of antibodies to affect graft survival are generated only after prolonged or repeated exposures. Antibodies usually have little or no role in chronic or acute rejections unless they have been elevated by previous rejections of grafts bearing the same histocompatibility antigens. Acute rejection of first-time grafts is mediated by T-cell responses. Although it is relatively easy to generate significant levels of antibodies against MHC class I and II molecules by repeated exposures to allogeneic grafts (or injected cells), it has been difficult to demonstrate the consistent generation of antibodies against minor (non-MHC) histocompatibility antigens. Binding of antibodies to graft cells can initiate destructive actions such as complement activation, opsonization, and antibody-dependent cell-mediated cytotoxicity. The effects of these actions can vary depending on the nature of the targeted tissue.

Development of delayed (-type) hypersensitivity (DTH) and cytotoxic T-lymphocyte (CTL) responses directed against histocompatibility antigens has been demonstrated in both acute and chronic rejections. The inflammatory nature of the DTH response, with the recruitment and activation of macrophages, suggests that it plays a significant role. Although CTLs specifically directed against histocompatibility antigens are clearly generated, how much they contribute to a given rejection can be difficult to discern because their killing is directed against a single target cell at a time. Both DTH and CTL responses can be generated against MHC (class I and II) and non-MHC histocompatibility antigens.

Complement activation and the ensuing inflammation can inflict considerable injury and even death on grafted cells. As was mentioned, this inflammation can be targeted through the attachment of graft-specific IgG and IgM molecules. However, complement has also been found to have an important impact on xenografts that does not involve the classical pathway of activation. Host cells are protected from the potential threat of deposition of complement fragments (e.g., C3b and C4b) on host cell membranes by the presence of various cell receptors and membrane-associated enzymes that continuously break them down and remove them. These protective mechanisms, however, are species-specific. Thus, when a graft from a miniature swine is placed on a human recipient, the enzymes and receptors that effectively protect the pig cells from pig complement are not effective against human complement, and the graft cells can be rapidly attacked by fragments of human complement initiating opsonization and formation of the membrane attack complex. The rapid action of these preexisting complement components leads to hyperacute rejection of xenografts.

NK cells recognize molecules produced by damaged or stressed cells and prepare to kill those cells. They refrain from doing so, however, if they recognize sufficient levels of appropriate MHC class I molecules on the targeted cells. In the case of xenografts, host NK may recognize stress molecules on graft cells but will not find appropriate host MHC class I molecules on the graft cells to inhibit them. As a result, host NK cells can cause considerable injury to the graft and constitute another significant barrier to successful xenotransplantation.

C. Therapeutic intervention

The initial effort to minimize the risk of rejection is to genetically match the donor and recipient as closely as possible. However, some degree of mismatch is present in most transplants. The next step that can be taken is to inhibit the ability of the recipient immune system to attack and damage the engrafted tissues. This inhibition is approached in two general ways:

• Specific immune tolerance involves a selective inhibiting of the responsiveness to a given antigen or set of antigens.

• Immune suppression (or immunosuppression) involves inhibiting general immune responsiveness without regard to the specificity.

Although specific immune tolerance to foreign grafts can be induced in experimental systems, it usually requires some advance information about the precise genetic differences involved and sufficient lead time to prepare the recipient. These requirements have limited its use in humans so far. In addition, some of the techniques are ethically inappropriate in humans. Therefore, immunotherapy for transplant patients still relies on immunosuppression.

Immunosuppressive techniques such as whole-body irradiation or the use of toxic drugs effectively eliminate immune responses that could damage transplanted organs and tissues (Table 17.2). The treated recipients, however, are then open to opportunistic infections that can be fatal if not successfully monitored and controlled. Over the past few decades, additional drugs (e.g., cyclosporine, tacrolimus, and rapamycin) have been developed that have more restricted effects on the immune system. Their effects are targeted more closely on cells that react to graft antigens while leaving the remainder of the immune system relatively uninhibited in its ability to deal with infectious agents. They are not without risk, however. Patients must often receive the drugs for an extended period. If a significant infection occurs during this period, the immune cells responding to the infectious agent could be inhibited in the same way as those responding to graft alloantigens. In addition, extended use of these drugs is sometimes associated with damage to organs such as the liver. These and other therapeutic drugs are discussed in greater detail in Chapter 18.

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A second approach to inducing a less than global inhibition of the immune response has been the use of antibodies directed at molecules on the surface of the cells involved in immune responses, particularly lymphocytes and APCs. Antibodies against MHC class I or class II molecules can inhibit with T-cell activation. Antibodies against CD4 or CD8 molecules, when administered during active rejection, have been shown to inhibit or destroy T cells and halt the rejection at least temporarily; however, antibodies against broad categories of T lymphocytes (e.g., anti-CD3 antibodies) have problems similar to those seen with immunosuppressive drugs, and their long-term use can reduce the body’s ability to respond to infectious agents.

IV. TISSUE-SPECIFIC CONSIDERATIONS

Special problems may arise when particular tissues are transplanted. We will discuss two of these situations: those involving blood transfusions and the transfer of bone marrow.

A. Transfusion

Transfusion is essentially the transplantation of blood. Erythrocytes and white cells in the transfused blood bear hundreds of molecules that can vary among individuals and act as histocompatibility antigens on these cells. Erythrocytes alone are estimated to express over 400 such types of antigens. Fortunately, mismatches for most of these antigens seldom have clinical consequences, and those tend to be of minimal severity when they do occur. There are, however, two antigen systems that are of major clinical concern: the ABO and Rh systems.

1. ABO: The ABO antigen system is a set of carbohydrate structures on erythrocyte surfaces and on some endothelial and epithelial cells. They are synthesized by glycosyl transferases encoded by two loci: the H locus and the ABO locus (Fig. 17.11) (Table 17.3). The H locus has two alleles: dominant H and recessive h. The recessive h allele encodes a nonfunctional product, but the H allele encodes a fucosyl transferase that attaches fucose to a precursor molecule normally present on erythrocyte surfaces to produce H substance. H substance is the precursor for the glycosyltransferases encoded by the alleles of the ABO locus that modify the H substance to produce A and B antigens (Fig. 17.11).

A and B antigens are recognized and bound by natural antibodies (also called naturally occurring antibodies) present in the serum without any stimulation from prior transfusions or intentional immunizations. These natural antibodies, of the IgM isotype, are probably generated against carbohydrates on normal body flora, and their role in transfusion is probably because of cross-reaction with certain carbohydrates on erythrocytes that share structural similarities with those on the microbial flora. Individuals who have neither A nor B on their own erythrocytes generate IgM antibodies against both A and B. Individuals of blood type A, tolerant to their own A antigens, will produce only anti-B antibodies. Similarly, type B individuals are tolerant to their own B antigens and therefore generate only anti-A antibodies.

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Figure 17.11

Synthesis of ABO blood group antigens.

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Mismatched transfusions (e.g., type A erythrocytes given to a type B recipient) can have serious consequences. The naturally occurring IgM antibodies react almost immediately with the transfused erythrocytes to initiate agglutination and complement-mediated lysis. It is the agglutination that produces the clumping seen in demonstrations of ABO typing commonly performed in laboratories (see Chapter 20). ABO mismatching can result in massive destruction of transfused red blood cells (transfusion reaction) and, if severe enough, can produce a type of transfusion reaction known as an acute hemolytic reaction within 24 hours of transfusion. This reaction is caused by widespread hemolysis within the vasculature from the binding of IgM to erythrocytes and the ensuing complement activation. Clinical signs include fever, chills, shortness of breath, and urticaria. If it is extensive enough, a potentially fatal condition known as disseminated intravascular coagulation can develop.

Such situations emphasize the necessity of correct typing and matching of donors and recipients. Type A individuals can safely be given blood of phenotypes A and O, whereas type B recipients can safely receive blood of phenotypes B or O (Table 17.3). Type O recipients should receive erythrocytes only from other type O donors. AB individuals are “universal recipients” and can safely receive transfusions from donors of phenotypes A, B, O, or AB (Table 17.4).

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2. Rh: The Rh (“Rhesus”) antigens on erythrocyte surfaces are proteins. When an Rh-negative (Rh) individual is exposed to Rh-positive (Rh+) erythrocytes, he or she can generate antibodies, some of which are of the IgG isotype. Rh antigens can be typed prior to transfusion, and Rh-related transfusion reactions can be avoided by avoiding the transfusion of Rh recipients with Rh+ blood. Rh incompatibility during pregnancy presents a special concern for an Rh mother who carries an Rh+ fetus. Fetal blood immunizes the mother’s immune system to make IgG antibodies that may cross the placenta and destroy fetal erythrocytes in utero.

Rh antigens are encoded by a series of closely linked loci (D and CE) with dominant alleles (e.g., D) and recessive alleles (e.g., d), the most important of which is D. DD or Dd individuals have the Rh+phenotype, whereas those with dd are Rh (Table 17.5). When the father is Rh+, an Rh mother may carry an Rh+ fetus (Fig. 17.12). The maternal immune system is exposed to fetal blood as early as the first trimester of pregnancy and begins to generate anti-Rh IgG antibodies. The first Rh+ fetus is rarely at risk because of the time needed for injurious levels of anti-Rh antibodies to develop. However, subsequent Rh+ fetuses are at risk because maternal anti-Rh antibodies can increase rapidly and enter the fetus. Binding to fetal erythrocytes can lead to anemia and damage to other fetal organs. This is called hemolytic disease of the newborn (HDN) or sometimes erythroblastosis fetalis. The Rh antigen is a protein and elicits an IgG response. Every conception between an Rh+ male and an Rh female has the potential to produce an Rh-incompatible fetus. Aborted (spontaneous or induced) conceptions can also lead to the development of an IgG antibody response to Rh0 (D).

CLINICAL APPLICATION

Blood transfusion reaction

Aileen, a 55-year-old female, has had breast cancer for several years requiring chemotherapy. She is hospitalized for chemotherapy-induced anemia requiring a blood transfusion.

Within minutes after beginning the blood transfusion, she develops fever, nausea, back pain, and hypotension. The blood transfusion is immediately stopped. She is given intravenous fluid and acetaminophen. The patient’s blood type is retested, and the original typing is found to be erroneous, confirming that the reaction was caused by a transfusion reaction. Fortunately, her symptoms resolve without any complications, such as acute kidney failure.

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Figure 17.12

Hemolytic disease of the newborn. An Rh mother carrying an Rh+ fetus can be exposed to fetal erythrocytes during pregnancy and delivery. The maternal immune system can generate anti-Rh IgG antibodies that cross the placenta and bind to Rh+ fetal erythrocytes. Upon binding, these antibodies can induce destruction of fetal erythrocytes that may lead to anemia and other consequences.

In HDN, binding of anti-Rh antibodies to erythrocytes activates fetal complement, causing lysis of erythrocytes. The resulting anemia may become so severe that the fetus sustains severe damage or dies in utero. To compensate for the anemia, the fetal bone marrow releases immature erythrocytes (or erythroblasts). The abnormal presence of these erythroblasts in the fetal circulation is the hallmark of the disease (hence the term erythroblastosis fetalis).

Preventive therapy, especially the use of Rh0 (D) immune globulin to minimize the risk of the mother becoming sensitized against Rh, is now routinely available for this situation. This involves the injection of a high-titer anti-Rh antibody preparation such as RhoGAM or MICRhoGAM. These preparations contain pooled anti-Rh antibodies, prepared from human serum obtained from mothers who have made antibodies to Rh antigens. Rh0 (D) immune globulin should be administered after the 12th gestational week for ongoing pregnancy was well as for spontaneous or induced abortion. RhoGAM and MICRhoGAM remove fetal cells from the maternal circulation quickly enough to avoid sensitizing the mother’s own immune system against Rh. Use of Rh0 (D) immune globulin may also be appropriate after a blood transfusion of an Rh female.

CLINICAL APPLICATION

Hemolytic disease of the newborn

Kim, a 30-year-old female is pregnant for the third time. Her first pregnancy resulted in a miscarriage, and she did not follow-up with any additional testing. During her second pregnancy, the baby was jaundiced at birth and exhibited anemia and hepatosplenomegaly consistent with hemolytic disease of the newborn. Kim was found to be Rh, and the baby’s father was Rh+. Elevated levels of anti-Rh antibodies were found in Kim’s blood.

During her third pregnancy, she has been very concerned. To prevent complications, she receives injections of RhoGAM, a high-titer anti-Rh antibody prepared from human serum from mothers who have made antibodies against Rh antigens. RhoGAM is given at 28 weeks of pregnancy and again within 72 hours of delivery if the baby is Rh+. Kim delivers a healthy baby boy.

Although apparently healthy, the child should be followed to check for sequelae that may not be apparent at birth. Where appropriate prenatal care is observed, HDN has become rare. However, it is still a danger where appropriate prenatal care is avoided or unavailable.

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Figure 17.13

Bone marrow transplantation. Immunocompetent T cells in the donor bone marrow may recognize host antigens as foreign and initiate a graft-versus-host (GVH) response. The risk of GVH can be greatly reduced by removing mature T cells from the bone marrow inoculate prior to its introduction.

B. Bone marrow

The bone marrow carries stem cells for the entire hematopoietic system and (at least hypothetically) could be used to treat individuals in whom some or all of these tissues are intrinsically defective or may have been damaged. Examples include those with immune deficiency diseases, some anemias, and the effects of cancer therapies. Indeed, transplantation of bone marrow can provide benefits to some of these patients, but it also carries unique risks. Bone marrow transplantation involves the placement of an immunocompetent tissue into a recipient who is usually immunodeficient for natural or therapeutic reasons. Even recipients with intact immune systems, however, undergo procedures that deliberately damage their immune systems to enable the transplanted bone marrow to establish itself in its new environment.

Under these circumstances, the immunocompetent cells in the transplanted bone marrow may recognize histocompatibility antigens on recipient cells as foreign and attack the host tissues (Fig. 17.13). This is a graft-versus-host (GVH) response, and the resulting damage is graft-versus-host disease (GVHD), which can be potentially fatal. GVHD can develop from two sources within the transplanted bone marrow: the stem cells and the mature T cells present in the implanted bone marrow. The most immediate and serious threat comes from the mature T cells because they are capable of generating rapid and severe GVH responses. These responses can be minimized by pretreatment of the bone marrow inoculate to remove the T cells prior to implantation. It is hoped that lymphocytes generated from the implanted stem cells will become tolerant to host histocompatibility antigens as they undergo positive and negative selection in the recipient thymus. Tolerance is sometimes imperfect, but when GVH responses do result from the activity of donor stem cell-derived lymphocytes, they are usually transient and less severe than the GVH responses initiated by mature T cells within the bone marrow inoculum.

Although genetic matching of donor and recipient can minimize the risk of GVHD, it is also important for another reason. T cells generated by implanted stem cells must undergo thymic education in the recipient thymus. Some degree of matching between the MHC class I and II genes of the host and donor is required for the positive and negative selection events of thymic education to proceed effectively. Bone marrow recipients are also vulnerable to opportunistic infection while the new marrow becomes established and must be carefully monitored for infection and treated appropriately. Once established and functioning, the transplanted hematopoietic stem cells can often provide a normal or near-normal condition to recipients for the remainder of their lives.

C. Immune-privileged sites

Some anatomic sites are “permissive” in tolerating genetic mismatches between donor and recipient that would lead to prompt rejection in most parts of the body. Allogeneic and xenogeneic grafts that would be rapidly rejected at most sites in the body can often survive when placed into these areas. These sites are termed immune-privileged sites, and each has features that limit the immune response to cells and molecules within them. The immune-privileged sites include the eye, the testicular tubules, the brain, and perhaps the placenta.

The eye has several features that make it a privileged site. The aqueous humor of the anterior chamber allows cells and molecules to exist without close contact with the vasculature, thus hiding them from the immune system to some extent. In addition, the immunologic processes that inhibit immune responses, such as the apoptotic death of lymphocytes attacking tissues in the eye, operate very rapidly, perhaps protecting the eye from inflammatory damage. This mechanism may also account, at least in part, for the ease with which corneas can be transplanted between individuals with genetic differences that would be difficult to overcome with other tissues.

The lumen of the testes also provides an immunologically privileged site. The testicular tubules are developed and closed prior to the development of the immune system. Because the Sertoli cells and other tubular elements prevent any subsequent passage of immune cells into the testicular tubules, the molecules and cells that are unique to that environment (e.g., spermatogonia and developing sperm) are never recognized as self by the immune system. As a result, if the testicular tubules are breached by infection, injury, or surgical intervention, the immune system can react against the seemingly foreign antigens that become exposed. It is estimated that a portion of male infertility cases may stem from immune responses against exposed testicular tubule elements.

The brain is sometimes cited as an immune-privileged site, because the blood–brain barrier can limit the exchange of cells and large molecules between the vasculature and the nervous system. The extent of this isolation is still somewhat unclear, as is the extent to which the cells of the immune system recirculate through nervous tissue. It appears that mechanisms that rapidly suppress potentially dangerous immune-mediated injury, similar to those seen in the eye, may also exist in the brain.

The placenta presents an interesting conundrum. The developing fetus typically expresses numerous histocompatibility antigens that are foreign to the mother. Why, therefore, does the maternal immune system not attach and destroy the fetus? Although the basis for the sheltering of the fetus from the maternal immune system (aside from maternal IgG crossing the placenta to provide passive protection to the fetus) has yet to be clarified, several structural and biochemical features of the fetal/uterine environment have been suggested as contributing factors.

V. TISSUE SOURCES

Tissues available for transplantation can come from various different sources. Traditionally, they have been harvested from voluntary living donors or from cadavers. In the case of cadaver donors, permission must usually be obtained through the documented permission of the donor given prior to death or through the agreement of family or guardians. Depending on the nature of the tissue, donated cells can sometimes be expanded or modified in vitro prior to implantation. Increasing research into the use of stem cells, either from adult or embryonic sources, provides yet another potential source but has had limited use in humans to this point. Finally, the search for available organs has been extended to other species, and the use of primate and swine donors has provided some benefits, although it has been limited by other problems inherent in xenogeneic exchanges.

A. Human tissues and organs

The total number of transplants that have been performed now exceeds a half million worldwide. The increased efficacy of transplantation has been made possible by continual improvements in techniques, in the ability to genetically match donors and recipients, and in the ongoing development of immunosuppressive and antibiotic agents that can be used to manipulate the recipient immune system to permit graft survival without an accompanying overwhelming sepsis.

1. Organ procurement and distribution: A growing imbalance exists between the number of organs available for transplantation and the number of patients awaiting them, and the effective distribution of available organs has become increasingly complicated. In the United States, organ distribution is managed through the United Network for Organ Sharing (UNOS), the organization that administers the federally funded Organ Procurement and Transplantation Network. UNOS uses various factors, including the degree of genetic matching, potential benefit to the recipient, and geographical priorities to prioritize the assignment of donated organs as they become available. UNOS maintains a regularly updated web site available to the public that details the types of organs, number of transplants performed, success rates, waiting lists, and other criteria.

2. Stem cell and fetal sources: The ability to transfer healthy stem cells that are self-renewing and capable of generating new cells and/or tissues offers benefit to various injuries (e.g., burn wounds, spinal cord injuries) and diseases (e.g., arthritis, diabetes, cardiovascular disease, and neurologic diseases such as Alzheimer disease and Parkinson disease). In some cases, these represent new forms of therapy; in other cases, they extend the effectiveness of previous therapies. For example, transplantation of pancreatic islet cells has been used to treat diabetes, but the transplanted cells have finite life spans. The transplantation of stem cells that are capable of generating these cells provides a potentially permanent replacement therapy.

Adult stem cells have already been used in a limited number of human cases, but their ability to generate various new tissues is more limited. In addition, much has yet to be learned about how best to obtain and prepare them for use. Their primary application thus far has been the use of hematopoietic stem cells in bone marrow transplantation. Embryonic stem cells have a broader capacity for regeneration, as has been demonstrated in experimental animal models, but their use in humans has been restricted by practical and ethical considerations.

3. Ethical considerations: Transplantation involves decisions that may create ethical difficulties for some individuals. In some cases, cultural or religious customs forbid individuals from participating as either donors or recipients of transplantation and even blood transfusion. Even when there are no such general limitations, individuals are often personally reluctant to offer themselves as potential donors. As a result, the need for donated organs greatly exceeds the supply, creating the need for a system such as UNOS that regulates their distribution to prevent availability from becoming dependent on a potential recipient’s wealth or social/political influence.

The potential use of embryonic stem cells faces social and religious opposition from some segments of the scientific, religious, and general communities. Currently, this opposition has imposed severe limitations on obtaining and using human embryonic stem cells for either research or therapeutic use.

B. Nonhuman (xeno-) tissues and organs

The shortage of available human organs has spurred research into the use of nonhuman alternatives. Numerous attempts have been made to use animal donors. Primates are an obvious donor choice because of their close genetic relationship to humans. Pigs have many physiologic similarities to humans, and some breeds have organs that are an appropriate size for use in human recipients. Pig skin has also been used on occasion for temporary coverage of damaged areas in human burn victims.

Xenotransplantation has not been very successful or widely used, however. Xenografts face significant immunologic obstacles. Concern also exists about the potential for introducing zoonotic infections (infections passed from one species to another) through xenotransplantation. Finally, some individuals oppose the use of xenografts on ethical grounds.

Naturally existing antibodies in human serum, such as those against ABO antigens on human erythrocytes, can react with xenogeneic tissues to produce hyperacute rejections. NK cells can detect stress molecules on xenograft cells and bind to them via their killer activation receptors. However, the absence of human MHC class I molecules on the xenografts prevents the NK cells from ceasing the killing response through binding of their killer inhibition receptors. Xenogeneic cells lack enzymes that protect them against the attachment of human complement components that lead to cell lysis. These various mechanisms often destroy xenografts before the T cell-mediated responses typically associated with allograft rejection are even generated. Attempts to resolve these problems have used genetic engineering of the animal donors to introduce various human genes. Although there have been promising experimental advances, they have not yet significantly increased the clinical application of xenotransplantation.

Chapter Summary

• The genetic match (similarity/disparity) between the donor and the host is a very important factor in determining the likelihood of a successful transplant.

• Histocompatibility genes encode histocompatibility antigens. Among these are the MHC class I and II molecules encoded within the major histocompatibility complex (MHC).

• Grafts that are placed in their normal anatomic location are called orthotopic grafts. Grafts that are placed into a site other than their normal one are called heterotopic grafts.

• Autografts are those transferred from one part of an individual to another location on that same individual. Syngeneic grafts are those transferred between different individuals who are genetically identical or members of the same inbred strain of experimental animals. Allogeneic grafts (or allografts) are transferred between two genetically disparate individuals of the same species. Xenogeneic grafts (or xenografts) are those exchanged between members of different species.

• The laws of transplantation can be summarized as follows: A host can recognize as foreign and mount a response against any histocompatibility antigen not encoded within its own cells.

• The recipient immune system recognizes peptide fragments presented by MHC class I or II molecules. In the case of transplanted tissues, the genes of the engrafted cells may encode molecules that also can be detected by the recipient immune system and function as histocompatibility antigens. The recognition of foreign histocompatibility antigens and the activation of T cells against them involve processes that are very similar to those involved in the initiation of responses against antigens derived from infectious organisms.

• Chronic rejections are the slowest and the least vigorous type of rejection. Chronic rejections are typical of situations in which the donor and recipient differ by only non-MHC histocompatibility gene differences. Acute rejections occur much sooner after graft emplacement than do chronic rejections (e.g., 2 to 4 weeks). Hyperacute rejections are the most rapid type of rejection. They are initiated and completed within a very few days of graft placement, usually before the grafted tissue or organs can establish connections with the recipient vasculature. Second set rejection are grafts that are rejected more rapidly when repeated on a recipient who rejected the same type of graft on a previous occasion.

• Development of delayed (-type) hypersensitivity (DTH) and cytotoxic T-lymphocyte (CTL) responses directed against histocompatibility antigens have been demonstrated in both acute and chronic rejections.

• Steps can be taken to inhibit the ability of the recipient immune system to attack and damage the engrafted tissues. Specific immune tolerance involves a selective inhibition of the responsiveness to a given antigen or set of antigens. Immune suppression (or immunosuppression) is a broad and general inhibition of immune responsiveness without regard to specificity. A second approach to inducing a less than global inhibition of the immune response has been the use of antibodies directed at molecules on the surface of the cells involved in immune responses, particularly lymphocytes and antigen-presenting cells.

• ABO mismatching can result in massive destruction of transfused red blood cells (transfusion reaction) and, if severe enough, can produce a type of transfusion reaction known as an acute hemolytic reaction within 24 hours of transfusion.

• When an Rh-negative (Rh) individual is exposed to Rh-positive (Rh+) erythrocytes, he or she can generate antibodies, some of which are of the IgG isotype. In the case of an Rh mother carrying an Rh+ fetus, the maternal anti-Rh IgG antibodies can cross the placenta and bind to fetal erythrocytes. This can lead to hemolytic disease of the newborn.

• Graft-versus-host disease (GVHD) can develop from two sources within transplanted bone marrow: the stem cells and the mature T cells present in the implanted bone marrow. The latter present the most serious risk of developing GVHD, but the risk can be minimized by removing them from the bone marrow inoculate prior to its infusion.

• Tissues available for transplantation can come from various different sources. Traditionally, they have been harvested from voluntary living donors or from cadavers.

Study Questions

17.1. A 23-year-old female has HLA genotype A3/A8, B1/B8, C4/C1. For each locus, the maternal allele is listed first and the paternal allele second. Several potential donors are available for an organ graft. Which of the following donors would be the closest match?

A. Donor A: A8/A27, B24/B8, C4/C9

B. Donor B: A3/A3, B27/B8, C1/C1

C. Donor C: A8/A6, B44/B8, C4/C1

D. Donor D: A6/A27, B1/B8, C4/C2

E. Donor E: A3/A8, B1/B27, C9/C4

The answer is B. The closest match will have the fewest mismatched HLA genes not present in the recipient. For donor B, only HLA B27 is not already present in the recipient. Donor A has three mismatches, Donor C has two mismatches, Donor D has three mismatches, and Donor E has two mismatches.

17.2. After receiving a kidney transplant from the most appropriate available donor, a 38-year-old female is administered immunosuppressive drugs, including cyclosporine, in order to

A. decrease T-cell production of IL-2.

B. destroy stem cells in her bone marrow.

C. induce involution of her thymus.

D. inhibit macrophage release of IFN-γ.

E. reduce plasma cell secretion of IgG antibodies.

The answer is A. Cyclosporine decreases T-cell production of IL-2, resulting in decreased T-cell proliferation. Cyclosporine treatment does not destroy bone marrow stem cells, nor does it induce thymic involution. Neither macrophage release of IFN-γ nor plasma cell secretion of IgG antibodies is affected by cyclosporine.

17.3. A 6-year-old male receives a bone marrow transplant from his father during treatment for acute myelogenous leukemia. Of primary concern will be the potential development of

A. acute rejection.

B. an allergic reaction.

C. autoimmune responses.

D. graft-versus-host disease.

E. immediate hypersensitivity.

The answer is D. Graft-versus-host (GVH) disease is a risk because bone marrow contains immunocompetent tissue. The GVH response is directed against host antigens that are not present in the donor bone marrow. Recipients of bone marrow transplants are usually immunocompromised or immunosuppressed, resulting in little risk for development of host-versus-graft responses such as acute rejection. Allergic reactions, also described as type I or immediate hypersensitivity reactions, do not occur in response to bone marrow transplantation. An autoimmune response is one directed by the immune system against self-antigens.

17.4. With no therapeutic intervention, the most likely outcome for a transplanted skin graft obtained from an unrelated donor who is HLA identical to the recipient is

A. acute rejection.

B. chronic rejection.

C. graft-versus-host disease.

D. hyperacute rejection.

E. long-term success.

The answer is B. Chronic rejection is most likely to occur, over months to years, in such a situation. Unrelated HLA identical individuals will have numerous mismatches of minor histocompatibility genes. Because the major histocompatibility genes match, hyperacute and acute rejections are unlikely to occur. Skin does not contain immunocompetent tissue and cannot mount a graft versus host response. Even with identical major histocompatibility genes, long-term success of a transplanted skin graft will require immunosuppressive therapy.

17.5. What are the possible ABO blood types of children to the union of a man who has blood type AB and a woman who has blood type O?

A. A only

B. A and B only

C. A, B, and AB only

D. A, B, AB, and O

E. O only

The answer is B. Blood types A and B are both possible in children of parents with type AB and type O. The genders of the parents and of the children are inconsequential because inheritance of ABO blood group is autosomal. A and B are codominant and are both dominant to O. In this example, children will inherit either the A or B allele from the father and the O allele from their mother and will have either blood type A or blood type B. Inheritance of both A and B or of O only is not possible, eliminating types AB and O as possible blood types among the children of this couple.