CELLS OF THE IMMUNE SYSTEM
B and T lymphocytes
Immunity mediated by B lymphocytes is referred to as humoral immunity because transfer of immunity from one individual to another can be achieved via transfer of blood plasma or serum. T lymphocyte receptors can only recognize antigens if they are expressed on the surface of a cell in association with a protein of the major histocompatability complex (MHC) (Table 9.1).
Cells of the immune system are present in various tissues of the body, especially those in contact with the external environment. Immune system cells prevent establishment of infections and clear up cancerous or damaged cells in the host. These cells, organized in specialized tissues such as the thymus, spleen and lymph nodes, circulate in the blood (Figures 9.1-9.3).
Anatomical sites for immune responses
Activation of immune responses can occur at several levels in the body, including the site of entry of microorganisms (skin, lungs, etc.). Antigens that enter the bloodstream are concentrated in the spleen where T and B cell responses can be initiated. Similarly, another circulatory system, the lymphatic system, drains extracellular fluid from the tissues. Antigen collected in the lymphatic vessels is captured in the lymph nodes, which also contain both T and B cells. Activated effector and memory T cells leave the spleen and lymph nodes, circulate throughout the body, and move specifically to sites of antigen entry. B cells can also be present in peripheral organs, particularly mucosal tissues such as the intestine and reproductive tract. However, many activated cells remain in the spleen and lymph nodes, perhaps because the actions of B cells are dependent upon antibodies, which circulate in the blood (Hansen, 2000).
Table 9.1 Some cellular components of the immune system
|
Cell type |
Key function |
|
Phagocytes Macrophage |
Phagocytosis, antigen presentation |
|
Dendritic cell |
Endocytosis and phagocytosis, antigen presentation |
|
Neutrophil |
Phagocytosis |
|
Lymphocytes B lymphocyte |
Secretion of antibody, antigen presentation |
|
T helper lymphocyte type 1 (Th1) |
Secretion of cytokines that stimulate macrophages and cytotoxic T cells |
|
T helper lymphocyte type 2 (Th2) |
Secretion of cytokines that stimulate B cells and antibody responses |
|
Cytotoxic T lymphocyte |
Lysis of target cells |
|
Gamma-delta (yδ) T lymphocyte* |
Lysis of target cells, secretion of cytokines and growth factors |
*T-cell receptor is of the yδ type; other T lymphocytes have aβ type
COMPONENTS OF THE IMMUNE SYSTEM
The reproductive tract contains macrophages, T lymphocytes, B lymphocytes, neutrophils and other cells that participate in maintaining the reproductive tract in a sterile state. Moreover, the reproductive tract is drained by the lymphatic system. Lymph vessels draining the uterus join with similar vessels from the ovary so that concentrations of progesterone in utero-ovarian lymph on the side ipsilateral to an ovary with a functional corpus luteum can be 10-1000-fold higher than concentrations in jugular blood.
Figure 9.1 Sites of antigen origin and absorption, and lymphocyte and antibody entry into the male reproductive tract. (I) Testis: production of testis- and sperm-specific antigen; absorption of antigens after trauma; seminiferous tubules can be invaded by immune lymphocytes, whereas antibody enters the rete testes and efferent ducts; autoantibodies can cause agglutination and immobilization of spermatozoa. (2) Epididymis: secretion of coating antigens; removal of non-ejaculated sperm by phagocytosis by macrophages; inflammation can occlude duct resulting in sperm extravasation and absorption of antigens; entry site for immune cells and antibodies. (3) Vasectomy: occludes sperm passage resulting in sperm extravasation and granulomas and antigen absorption in epididymis and at site of vas sectioning if not ligated. (4) Prostate: secretion of prostatic antigens; inflammation results in occlusion of ejaculatory duct and absorption of prostatic and sperm antigens; entry of antibodies into ejaculate. (5) Seminal vesicle: major secretion of sperm-coating antigens; inflammation results in occlusion of ejaculatory duct and absorption of seminal and sperm antigens; entry and secretion of antibodies into ejaculate. Autoantibodies against seminal and prostatic secretions cause sperm agglutination. (6) Active immunization: use of sperm or testis antigens can cause autoantibodies and immune lymphocytes, depressed sperm quality and allergic orchitis in man and animals. Use of prostatic and seminal antigens may result in autoagglutinins and possible lesions in respective accessory glands
Innate immune system
The most immediate responses to invasion of foreign organisms are those mediated by phagocytic cells of the innate immune system. Macrophages and neutrophils engulf foreign particles and then kill the organism. This process, called phagocytosis, can be stimulated by other components of the immune system. For example, phagocytes have receptors for antibodies (produced by B lymphocytes) and complement (produced by the liver), and more easily recognize microorganisms coated with these molecules. Also, various regulatory molecules called cytokines produced by T lymphocytes can activate phagocytes and make them more effective. One such cytokine is tumor necrosis factor (TNF).
Natural killer (NK) cells are a type of lymphocyte; however, unlike other cells of this lineage, they do not have receptors that recognize specific antigens. Rather, NK cells kill cells that do not display MHC class I antigens on their cell surfaces or have MHC class I but without the self-peptide usually present in association with the MHC protein (Figure 9.4).
The most common types of cell with either of these conditions are those infected with virus. Like phagocytes, activity of NK cells can be increased by certain cytokines from other lymphocytes. One such molecule is interferon-y. Also, NK cells contain immunoglobulin receptors and have increased killing activity against cells that have antibody on their cell surfaces. In addition to cellular components, the innate immune system also includes proteins, such as complement which promotes phagocytosis and lysis of microbes, and lactoferrin which can inhibit bacterial growth.
Figure 9.3 Schematic representation of primary and secondary immune response in infertility. (1) Sperm elements are absorbed by macrophages and taken to the reticuloendothelial system (RES) which produces the large molecular IgM antibody or primary response (2), which then protects the body by attacking the immunogen. (3) On repeated exposure to this antigen the macrophages stimulate this large lymphocyte to produce many circulatory antibodies of IgG type which also attack the antigen (4)
Figure 9.4 Model illustrating how lymphocyte inhibitory molecules from the endometrium and trophoblast inhibit activation and effector functions of maternal lymphocyte.There are at least two cells that could potentially cause cytolysis of trophoblast cells. These are aβ cytotoxic T cells that possess T cell receptors (TCR) specific for trophoblast transplantation antigens (Trans. Ag.), and natural killer (NK) cells which recognize trophoblast without major histocompatibility antigens. These cells are inhibited by molecules secreted from the trophoblast and endometrium. Prostaglandin E2 (PGE2), secreted by trophoblast, is among the lymphocyte-inhibitory molecules at the fetal-maternal interface. UTMP, uterine milk protein;TGF, transforming growth factor
When antigens are placed in the reproductive tract, initial removal is accomplished by phagocytic cells of the innate immune system. Macrophages and dendritic-like cells are located in the reproductive tract. Microbial infection, deposition of semen or other inflammatory stimuli cause the release of chemotactic molecules that stimulate the movement of neutrophils out of the blood and into the lumen of the uterus. Opsonins (from the Greek, opsonein, to prepare food) are molecules that bind to particulate antigens and increase the affinity of phagocytes for that antigen. For example, serum contains complement C3b which can cause opsonization by binding to microorganisms; neutrophils and macrophages have receptors for C3b on their cell surfaces.
Microbial infection is also associated with inflammation accompanied by vasodilatation, increased vascular permeability, movement of serum proteins into the uterine lumen and production of a uterine discharge. These effects may be mediated by mast cells, basophils and eosinophils, which release vasoactive molecules upon activation.
Responses mediated by the innate immune system occur rapidly; however, they do not provide any long-term adjustments for improving the effectiveness of immune responses on subsequent exposure to an antigen.
Cell-mediated immunity (Table 9.2)
Tlymphocytes
The actions of T lymphocytes are collectively termed cell-mediated immunity. T cell-dependent immunity from an immunized to a non-immunized animal requires the transfer to T cells themselves. There are two classes of T cells:
(1) Cytotoxic T cells, which are CD8 T lymphocytes, kill cells expressing antigen recognized by the T lymphocyte’s receptor. Lysis is caused by secretion of toxic proteins: ‘perforin’ and ‘granzymes’. CD8 lymphocytes are activated by antigen bound to MHC class I antigens, i.e. intracellular antigens such as virus. Thus, cytotoxic T cells kill cells that contain intracellular antigens that are not found by antibody or phagocytosed by macrophages or neutrophils.
(2) T helper (Th) cells regulate the activity of other lymphocytes by providing ‘help’ in the form of secretion of various cytokines. This activity is exerted primarily by CD4 T cells (that recognize extracellular antigens). In rodents, there are at least two types of T helper cells.
Recognition of antigen by T lymphocytes
One property of T lymphocytes is that the T cell receptor can only recognize antigen if it is expressed on the surface of a cell in association with a protein of the MHC. Thus, most T cells cannot recognize antigen themselves, but require so-called antigen-presenting cells that can process foreign proteins in such a way as to place the processed antigen within an MHC protein. Cells that can function as antigen-presenting cells include macrophages, dendritic cells (phagocytic and endocytic cells located in many tissues) and B lymphocytes (Hansen, 2000).
Role of T helper cells
Th1 cells’ function is to promote cell-mediated immunity. Th1 cells produce cytokines, such as interferon-y, that preferentially stimulate bacterial killing by macrophages; lymphotoxin, which stimulates neutrophils; and interleukin-2, which promotes growth of cytotoxic T cells. The Th2 cells promote primarily antibody-dependent immunity. The characteristic cytokine produced by Th2 cells is interleukin- 4, which stimulates B lymphocytes to produce antibody (Hansen, 2000; Pinkert, 2000).
Acquired immune system
Acquired immune responses often take longer to exert their functions but responses are specific for individual foreign molecules (antigen) and result in a process called immunological memory, so that subsequent exposure to the same antigen will lead to a stronger response to that antigen.
The two main types of cells of the acquired immune system are B and T lymphocytes. B cells are derived from the bone marrow. T lymphocytes also are formed from precursors in the bone marrow but undergo a period of differentiation in the thymus before seeding various lymphoid tissues. Both T and B cells express receptors on their cell surfaces that are specific for a particular antigen. For B cells, the antigen receptor is an immunoglobulin molecule, while for T cells, the receptor is a protein called the T cell receptor. Two types of T cells can be identified. Some T cells have a receptor made of an a- and β-subunit.
Table 9.2 Cells and soluble mediators of cell-mediated immune responses (adapted from Anderson and Hill, 1988)
|
Cell type |
Characteristics |
Principal relevant soluble factors |
|
T lymphocytes |
Found primarily in circulation and lymphoid tissues; will home to sites of foreign antigenic stimulus in response to bacterial and leukocytic chemotactic factors; may also be responsive to hormonal influences; recognize foreign antigens when presented with MHC class I or II membrane antigens; long-lived memory cells rapidly respond to specific foreign antigens to mount rapid effective secondary immune response; subsets with a variety of effector and immunoregulatory functions |
Lymphokines (interleukin (IL)-2,IL-3,IL-4,IL-5, IL-6, у-interferon (y-IFN), colony-stimulating factor (CSF); prostaglandin E2 (PGE2), diamine oxidase, perforans |
|
Monocyte |
High phagocytic and pinocytic capacity; long life in tissue; capacity to differentiate locality; ability to interact specifically with lymphocytes to promote antigen-specific response; usually become prominent in inflammatory lesions after 8-12 h;have Fc receptors for IgG antibodies, use IgG to identify foreign antigens (antibody-dependent cellular cytotoxicity (ADCC)), opsinization mechanism) |
Monokines (IL-1, tumor necrosis factor (TNF)); complement components (C1, C2, C3, C4, C5, factor B, properdin); PGE2, PGF2, leukotrienes B and C; peroxidase; superoxide, hydrogen peroxide, hydroxyl radicals; monohydroxy-eicosatetraenoic acids; prostacyclin; elastase collagenase, plasminogen activator, lysozyme, procoagulant activity (PCA) factor; fibronectin |
|
Neutrophils |
Rapid migration; available in large numbers; short life-span in tissues; high phagocytic capacity; rapid burst of oxidative metabolism after stimulation; potent repertoire of oxidative metabolites and digestive enzymes; major elements in acute inflammation; have Fc receptors for IgG subclass antibodies (IgG1 and IgG3 in humans) and participate in ADCC responses |
Peroxidase, polymorphonuclear leukocytes elastase, collagenase, lysozyme, myeloperoxidase, lactoferrin, alkaline phosphatase, β-glucuronidase, acid glycerophosphatase, cathepsins, a-fucosidase, 5'-nucleotidase, β-galactosidase, arylsulfatase, N-acetyl- β-glucosaminidase, mucopolysaccharides, trypsin-like proteases, histamine-release inhibitors, histaminase, PGE2, superoxide, hydrogen peroxide, hydroxyl radicals |
|
Eosinophils |
Frequently found in tissues infected with parasites or with high levels of IgE; cytotoxic activity directed toward parasites; surface receptors for IgG, IgE and complement components (providing recognition sites for cell activation and release of components) |
Peroxidase, superoxide, hydrogen peroxide, PGE2, leukotrienes B and C, diamine oxidase, kinase, lysophospholipase, arylsulfatase B, phospholipase D |
|
Basophils |
High-affinity receptors for IgE; degranulation triggered by IgE receptors cross-linking and also by some lymphokines; circulating cells, localize in tissue only under special circumstances; cutaneous basophil hypersensitivity reactions in response to soluble antigens, parasites, tumor cells or allogeneic cells; activated by soluble products of lymphocytes, polymorphonuclear leukocytes, macrophages |
Histamine, serotonin, dopamine, plasminogen activator, heparin, diverse proteolytic enzymes, acidic proteoglycans, chemotactic factors for neutrophils, limited amount of classical lysosomal enzymes listed for neutrophils, peroxidase, superoxide, hydrogen peroxide |
|
Mast cells |
High-affinity receptors for IgE; sessile cells, located primarily in blood vessels, lymphatics and connective tissue; especially abundant in skin and mucosal tissues |
Similar to those produced by basophils |
|
Platelets |
Circulating non-replicating cells derived from megakaryocytes; may undergo activation during immune reactions; activated platelets become sticky and aggregrate, trapping leukocytes in areas of inflammation and cell-mediated immunity reactions; secrete clotting and growth factors, vasoactive amines and lipids, neutral and acid hydrolases, which contribute to inflammatory responses |
Platelet-derived growth factor, serotonin, catecholamines, fibrinogen, cathepsin, alkaline phosphatase, β-glucuronidase |
(Continued)
|
Cell type |
Characteristics |
Principal relevant soluble factors |
|
Plasma cells |
Cells of B lymphoid lineage that reside primarily in lymphoid tissues and at mucosal surfaces; presence in reproductive tissues may be hormonally controlled; are directed by specific antigen to any lymphokine to produce a variety of immunoglobulins, many of which bind to immunoglobulin in Fc receptors on macrophages, NK cells, and neutrophils and allow them to participate in ADCC cell-mediated immunity responses |
Immunoglobulins (IgG, IgA, IgE, IgM) |
These cells, called aβT cells, predominate in lymphoid organs, such as the spleen and lymph node, and are the major cells in blood. Other lymphocytes have a T-cell receptor made of a y- and δ-subunit. These yδT cells are numerous in epithelia, such as the skin and the lining of the intestinal and reproductive tracts. One key feature of lymphocytes is their capacity for immunological memory. When a lymphocyte recognizes an antigen specific for its receptor, it undergoes a process called activation whereby the cell proliferates. Some of these daughter cells differentiate into effector cells that perform various activities, such as antibody production or lysis of target cells. Several diseases are associated with autoimmune responses (Table 9.3).
B lymphocytes
Immunity mediated by B lymphocytes is referred to as humoral immunity because transfer of immunity from one animal to the next can be achieved via transfer of blood plasma or serum. B cells inhibit microbial growth by secreting soluble proteins called antibodies that circulate in the blood and are present in extracellular fluids. Antibodies exert a variety of actions that inhibit microbial growth. Some activated B cells secrete antibody but persist in the animal for months or years. Thus, subsequent exposure to the same antigen can result in heightened immune response.
Structurally, antibodies are members of a family of proteins called immunoglobulins. Each immunoglobulin is made up of four separate protein subunits (two heavy chains and two light chains) organized to give the molecule two sites for binding antigen, and one site called the Fc portion that is involved in binding to specific cells and to complement (Figure 9.5). There are several types of immunoglobulin molecules secreted by B cells including IgG, IgM, IgA, IgD and IgE. Each activated B cell secretes only one immunoglobulin
Table 9.3 Classification of some human autoimmune diseases (adapted from Rao, 2002)
|
Disease |
Self-antigen |
|
Organ-specific autoimmune disease Hashimoto’s thyroiditis |
Thyroid proteins and cells |
|
Autoimmune hemolytic anemia |
RBC membrane proteins |
|
Goodpasture’s syndrome |
Renal and lung basement membrane |
|
Graves’ disease |
Thyroid-stimulating hormone receptor |
|
Addison’s disease |
Adrenal cells |
|
Idiopathic thrombocytopenic purpura |
Platelet membrane proteins |
|
Insulin-dependent diabetes mellitus |
Pancreatic beta cells |
|
Pernicious anemia |
Gastric parietal cells |
|
Myasthenia gravis |
Acetylcholine receptors |
|
Myocardial infarction |
Heart |
|
Post-streptococcal glomerulonephritis |
Kidney |
|
Spontaneous infertility |
Sperm |
|
Systemic autoimmune disease Multiple sclerosis |
Brain or white matter |
|
Rheumatoid arthritis |
Connective tissue, IgG |
|
Ankylosing spondylitis |
Vertebrae |
|
Scleroderma |
Nuclei, heart, lung, kidney, gastrointestinal tract |
|
Systemic lupus erythematosus |
DNA, nuclear proteins, RBC and platelet membrane |
|
Sjogren’s syndrome |
Salivary gland, liver, kidney, thyroid |
|
RBC, red blood cell |
|
Figure 9.5 Schematic representations of the structure of IgG, IgA and IgM. Each immunoglobulin is formed by two heavy chain molecules and two light chain molecules held together by disulfide bonds (indicated by dashed lines).The loops in each chain are formed by intramolecular disulfide bonds.There are two antibody sites for each immunoglobulin monomer. The Fc portion of the protein is responsible for binding to complement and to immunoglobulin receptors on neutrophils, macrophages and other cells. IgG is secreted as a monomer, whereas IgA is a dimer and IgM exists as a pentamer. Multimeric forms of immunoglobulin are held together by a protein called J-chain. In addition, IgA secreted across epithelial surfaces (not shown here) contains another protein called secretory component. Adapted from Abbas et al., 1997
type specific for one antigen. The most common immunoglobulin in blood is IgG. Another immunoglobulin molecule of particular interest for the reproductive tract is IgA, which is a major antibody in several mucosal sites, such as the gastrointestinal and reproductive tracts. Immunoglobulin A exists as two separate antibody molecules joined by a short protein called J-chain. In addition, IgA secreted across epithelia contains an additional protein, called secretory component, that is added as IgA crosses epithelial cells on its way to the mucosal surface.
Antibody binding to a microorganism can result in several actions that lead to the clearance of the microbe. For many microbes, successful colonization of a host requires binding of the microbe to cell membranes of the host. This can be made more difficult when the bacteria are coated with antibody. In addition, the Fc portion of IgG and IgA can be bound by specific receptors on macrophages and neutrophils, so that the rate of phagocytosis can be enhanced when bacteria are coated with antibody. Proteins of the complement system can also bind to the Fc portion of IgG and lead to complement-mediated lysis of the microbial cell. Cells infected with virus or bacteria can be destroyed in a process called antibody-dependent cell cytotoxicity, which occurs when NK cells or eosinophils release toxic granules upon binding of their surface Fc receptors to antibody attached to infected cells. Receptors for the Fc portion of immunoglobulins also exist on T and B cells, and this provides for additional regulation of lymphocyte function by antibody-bound particles.
Sperm immunology
Coitus is associated with microbial contamination because the penis, and sometimes the seminal plasma, contain microorganisms. Despite these microbial invasions, microorganisms can be rapidly removed from the reproductive tract and the tract usually remains free from infection. A sterile environment in the reproductive tract is maintained by the presence of an effective antimicrobial defense system that includes physical barriers, phagocytes that engulf and kill microorganisms, B lymphocytes that produce antibodies against invading microbes, and T lymphocytes that can kill virus- and bacteria-infected host cells (Hansen, 2000). Immune function in the reproductive tract is regulated by hormones to maximize antimicrobial function. The immune system consists of different cell types which prevent establishment of infection by microorganisms, and clear cancerous or damaged cells in the host. These cells are organized into specialized tissues such as the thymus, spleen and lymph nodes, circulate in the blood and are present in various tissues of the body, especially those in contact with the external environment.
Responses to sperm
Sperm are antigenic when injected subcutaneously into females. Therefore, mechanisms must be found to remove sperm from the reproductive tract following coitus in a way that will not lead to the development of humoral or cellular immunity to sperm. The main fate of sperm in the reproductive tract is phagocytosis by neutrophils; deposition of sperm induces an influx of phagocytic cells (neutrophils, macrophages and dendritic-like cells) into the reproductive tract. The lymphocytes in the lymph nodes draining the uterus can become activated by seminal plasma, perhaps because immunosuppresants are inactivated as sperm antigens reach the lymph nodes.
Figure 9.6 Distribution of DNA, IgM, IgG within the sperm
Despite the presence of mechanisms to prevent immune responses to sperm, females can develop antisperm immunity, and antibodies directed towards sperm can occasionally be recovered from the reproductive tract.
Sperm antibodies and autoimmune responses
Spermatozoa are capable of inducing an antibody response (Figures 9.6 and 9.7). Despite unsuccessful attempts to use immunity to sperm as a method of male contraception, sperm antibodies can cause reproductive failure. Immunopathological mechanisms of sperm antibodies cause autoimmune responses:
(1) Arming macrophages/enhancing of sperm from male genital tract;
(2) Mediating cytotoxic (immobilizing) effects on sperm in the presence of high titer of complement cascade components;
(3) Sperm unable to penetrate cervical mucus because of agglutination or related pathological mechanisms;
(4) Interfering with sperm capacitation, decapacita- tion and sperm hyperactivation;
(5) Influencing sperm selection within the female genital tract.
Figure 9.7 (a) Agglutination by antibody: when two sperm surface antigenic sites on separate cells are cross-linked by antibody, immunological agglutination occurs. (b) Antiglobulin agglutination occurs when sperm are secondarily cross-linked through antisperm by bivalent antiglobulin
Several immunopathological markers can detect certain autoimmune responses. IgA sperm-specific antibodies are the main indicators of poor semen penetration (Figure 9.8). Antigenic components of semen originate in the testis, epididymis, vas deferens and accessory glands. They can be broadly classified as those in the seminal plasma and those that are sperm bound. Sperm carry a mixture of antigens, including sperm-specific antigens, histocompatibility antigens (i.e. those responsible for the rejection of tissue grafts), blood group antigens and other somatic tissue antigens. Sperm antigens may be antigenic within the male (autoantigens) or the female (isoantigens) reproductive system. Of the sperm antigens, those on the surface of the plasma membrane are probably responsible for the reproductive failure. To be effective, antibodies against sperm must enter the seminal fluid or the cervical mucus following deposition in the female tract.
Figure 9.8 Capillary mucus penetration test (Kremer as modified by Fjallbrant). Cervical mucus is drawn up into capillary tubes (0.5 mm internal diameter, 40 mm long) and one end sealed with plasticine or modeling clay. The capillary tube is mounted on a special calibrated microslide with the open end placed into a chamber containing semen.The slide is placed into a humid chamber and incubated at 37°C for 1 h. The slide is examined under low-power microscopy to determine the furthest penetration of sperm. Donor semen and husband semen are tested against donor’s mucus and wife’s mucus. Penetration ratings: poor < 5 mm; fair 6-19 mm; and good > 20 mm. Sperm from men with circulating and seminal plasma antisperm antibodies show poor penetration
An autoimmune response is normally prevented by the relative isolation of the seminiferous tubules from the rest of the body by the blood-testis barrier.
If the barrier is breached, antisperm antibodies are produced that might attack sperm. Autoantibodies against sperm are found in the serum of infertile men. Antisperm antibodies can prevent fertilization by immobilizing sperm, impairing sperm penetration of cervical mucus, inactivating acrosomal enzymes presumed essential for fertilization, inhibiting the attachment of sperm to the zona pellucida, or interfering with embryonic viability. This problem has been overcome by the development of monoclonal antibodies against sperm surface components.