Introduction to Medical Immunology - part 4 pps

Page 250 1. In the few hours after hatching, there is a rapid decrease in the surface antigens, as reflected by a progressive decrease in the binding of purified and fluorescein-labeled heterologous antischistosomular antibodies to the parasite membrane. This is observed even in parasites cultured in serumfree medium. 2. Schistosomula recovered from infected animals have host proteins bound to the outer layers, masking the remaining parasite antigens. E. Antigenic Variation has been characterized in bacteria (Borrelia recurrentis), parasites (trypanosomes, the agents of African sleeping sickness; Giardia lamblia), and viruses (human immunodeficiency virus, HIV). 1. African trypanosomes have a surface coat constituted mainly of a single glycoprotein (variant-specific surface glycoprotein or VSG), for which there are about 10 3 genes in the chromosome. At any given time, only one of those genes is expressed, the others remain silent. For every 10 6 or 10 7 trypanosome divisions, a mutation occurs that replaces the active VSG gene on the expression site by a previously silent VSG gene. The previously expressed gene is destroyed, and a new VSG protein is coded, which is antigenically different. The emergence of a new antigenic coat allows the parasite to multiply unchecked. As antibodies emerge to the newly expressed VSG protein, parasitemia will decline, only to increase as soon as a new mutation occurs and a different VSG protein is synthesized. 2. Giardia lamblia has a similar mechanism of variation but the rate of surface antigen replacement is even faster (once every 10 3 divisions). 3. Borrelia recurrentis, the agent of relapsing fever carries genes for at least 26 different variable major proteins (VMP) which are sequentially activated by duplicative transposition to an expression site. The successive waves of bacteremia and fever correspond to the emergence of new mutants which, for a while, can proliferate unchecked until antibodies are formed. 4. HIV exhibits a high degree of antigenic variation, which seems to be the result of errors introduced by the reverse transcriptase when synthesizing viral DNA from the RNA template. The mutation rate is relatively high (one in every 10 3 progeny particles), and the immune response selects the mutant strains that present new configurations in the outer envelope proteins, allowing the mutant to proliferate unchecked by preexisting neutralizing antibodies. F. Cell-to-Cell Spread allows infectious agents to propagate without being exposed to specific antibodies or phagocytic cells. 1. Listeria monocytogenes, after becoming intracellular, can travel along the cytoskeleton and promote the fusion of the membrane of an infected cell with the membrane of a neighboring noninfected cell, which is subsequently invaded. 2. Herpes viruses, retroviruses, and paramyxoviruses cause the fusion of infected cells with noninfected cells allowing viral particles to pass from cell to cell. G. Integration of Microbial Genomes is a tactic exclusive of viruses, particularly DNA viruses and retroviruses. The integration of viral genomes in the host genome allows the virus to cause persistent or latent infections, with minimal Page 251 replication of the integrated virus and minimal expression of viral proteins in the membranes of infected cells. H. Immunosuppressive Effects of Infection. Although immunosuppressive effects have been described in association with bacteria and parasitic infections, the best documented examples of infection-associated immunosuppression are those described in viral infections. 1. Measles. Patients in the acute phase of measles are more susceptible to bacterial infections, such as pneumonia. Both delayed hypersensitivity responses and then in vitro lymphocyte proliferation in response to mitogens and antigens are significantly depressed during the acute phase of measles and the immediate convalescence period, usually returning to normal after 4 weeks. Recent investigations suggest that infection of monocytes/macrophages with the measles virus is associated with a down-regulation of interleukin-12 synthesis, which can explain the depression of cell-mediated immunity associated with measles. 2. Cytomegalovirus. Mothers and infants infected with cytomegalovirus show depressed responses to CMV virus, but normal responses to T-cell mitogens, suggesting that, in some cases, the immunosuppression may be antigen- specific, while in measles it is obviously nonspecific. 3. Influenza virus has been found to depress CMI in mice, apparently due to an increase in the suppressor activity of T lymphocytes. 4. Epstein-Barr virus releases a specific protein that has extensive sequence homology with interleukin-10. The biological properties of this viral protein are also analogous to those of interleukin-10; both are able to inhibit lymphokine synthesis by T-cell clones. 5. Human immunodeficiency virus. HIV infection is associated with depletion of CD4+ cells, which are the primary target of the virus. The depletion of CD4 cells results mostly from viral replication itself and by the priming of infected cells for apoptosis, which is triggered by stimuli that normally would cause T -cell proliferation. In addition, FcR -dependent phagocytosis is depressed in HIV-infected monocyte-derived macrophages, further contributing to the immunological compromise of the infected patients. X. Abnormal Consequences of the Immune Response A. The Activation of T lymphocytes by Bacterial Superantigens 1. A variety of bacterial exotoxins, such as staphylococcal enterotoxins -A and -B (SE-A and SE-B), staphylococcal toxic shock syndrome toxin-1 (TSST-1), exfoliating toxin, and streptococcal exotoxin A, as well as other unrelated bacterial proteins (such as streptococcal M proteins) have been characterized as “superantigens.” 2. Superantigens are defined by their ability to stimulate T cells without being processed. The stimulation of T cells is polyclonal; thus the designation of “superantigen” is a misnomer, but it has gained popularity and is widely used in the literature. 3. The best studied superantigens are the staphylococcal enterotoxins, which Page 252 are potent polyclonal activators of murine and human T lymphocytes, inducing T-cell proliferation and cytokine release. TSST-1 also appears to activate monocytes and is a potent B-cell mitogen, inducing B-cell proliferation and differentiation. 4. Mechanism of action. Superantigens bind directly and simultaneously to the nonpolymorphic area of class II MHC on professional accessory cells (macrophages and related cells) and to the V β chain of the α / β TcR (Fig. 13.4). a. For example, staphylococcal enterotoxins bind exclusively to specific subfamilies of V β chains that are expressed only by certain individuals. When expressed, these V β chain regions can be found on 2–20% of a positive individual's T cells, and the cross - linking of the TcR2 and of the APC by the enterotoxin activates all T cells (both CD4 and CD8+) expressing the specific V β region recognized by the enterotoxin. b. The massive T-cell activation induced by superantigens results in the release of large amounts of IL-2, interferon- γ , lymphotoxin (TNF- β ), and TNF- α . c. Patients infected by bacteria able to release large amounts of superantigens (e.g., S. aureus- releasing enterotoxins or TSST-1 and Group A Streptococcus- releasing) may develop septic shock as a consequence of the systemic effects of these cytokines, which include fever, endothelial damage, profound hypotension, disseminated intravascular coagulation, multiorgan failure, and death. d. After the initial burst of cytokine release, the stimulated T cells either undergo apoptosis or become anergic. This effect could severely disturb Figure 13.4 Diagrammatic comparison of the mechanisms of T-cell stimulation by conventional antigens and staphylococcal enterotoxins. While conventional antigens are processed into oligopeptides, which bind to MHC-II molecules, and then bind specifically to a TcR binding site (left panel), bacterial superantigens interact with nonpolymorphic areas of the V β chain of the TcR and of the MHC-II molecule on an APC (right panel). Notice that superantigen binding overrides the need for TcR recognition of the MHC-II- associated oligopeptide, and thus T cells of many different specificities can be activated. It is also important to note that both APC and T cells can be stimulated as a consequence of the extensive cross-linking of membrane proteins. (Modified from Johnson, H.M., Russell, J.K., and Pontzer, C.H. Sci. Am. 266 (4):92, 1992.) Page 253 the ability of the immune system to adequately respond to superantigen-releasing baceria. B. Infection as a Consequence of the Uptake of Antigen-Antibody Complexes. The immune response, in some cases, facilitates the access of infectious agents to cells in which they will be able to proliferate. 1. Macrophages are often infected by intracellular organisms that are ingested as a consequence of opsonization. 2. Babesia rodhaini, a bovine intraerythrocytic parasite, penetrates the host's cells after it has bound complement, particularly C3. Absorption of C3b -containing circulating antigen-antibody complexes to the CR1 expressed by red cells allows the parasite to gain access to the red cell, which becomes its permanent location. C. Post-Infectious Tissue Damage. Several examples of deleterious consequences of the immune response have to do with the predominant role of the immune response in the pathogenesis of the disease. 1. Immune-complex-induced inflammation. Antigen-antibody complexes, if formed in large amounts, can cause disease by being trapped in different capillary networks and leading to inflammation. The clinical expression of immune complex-related inflammation depends on the localization of the trapped complexes: vasculitis and purpura, when the skin is predominantly affected, glomerulonephritis if trapping takes place on the glomerular capillaries, arthritis when the joints are affected (see Chap. 21). Viruses are often involved in the formation of circulating antigen-antibody complexes. 2. Immune destruction of infected cells and tissues. An immune response directed against an infectious agent may be the main cause of damage to the infected tissue. a. In subacute sclerosing panencephalitis, a degenerative disease of the nervous system associated with persistent infection with the measles virus, the response against viral epitopes expressed in infected neurons is believed to be the primary mechanism of disease. b. In some forms of chronic active hepatitis (see Chap. 23), the immune response directed against viral epitopes expressed by infected hepatocytes seems to cause more tissue damage than the infection itself. c. Cross-reactions with tissue antigens have been proposed as the basis for the association of streptococcal infections with rheumatic carditis and glomerulonephritis. Antibodies to type 1 streptococcal M protein cross- react with epitopes of myocardium and kidney mesangial cells and cause inflammatory changes in the heart and glomeruli, respectively. X. Epilogue The outcome of an infectious process depends on a very complex set of interactions with the immune system. A successful pathogen has developed mechanisms that avoid fast elimination by an immunocompetent host. These mechanisms allow the infectious agent to replicate, cause disease, and spread to other individuals before the immune response is induced. The immune response, on the other hand, is a powerful weapon that, once set in Page 254 motion, may destroy friendly targets. Thus, the therapeutic strategies in infectious diseases have to consider all these questions, such as the particular survival strategy of the infectious agents, the effects of the infection on the immune system, and the possibility that the immune response may be more of a problem than the infection itself. Self-Evaluation Questions Choose the ONE best answer. 13.1 Which one of the following variables is most important as a determinant of the possible use of active immunization to protect an individual from an infectious disease after a known exposure? A. Existence of memory cells stimulated in a previous immunization B. Immunogenicity of the antigen used for vaccination C. Length of the incubation time of the disease D. Preexisting levels of protective antibody E. Resistance of the infectious agent to nonimmune phagocytosis 13.2 The elimination of an intracellular organism from an infected human macrophage is most likely to depend on the: A. Formation of syntitia by fusion of infected and noninfected cells B. Intracellular diffusion of complement-fixing antibodies C. Phagocytosis of the infected cells by noninfected macrophages D. Release of enzymes and superoxide radicals directly into the cytoplasm E. Sensitization of T cells against microbial-derived peptides presented in association with MHC-II molecules on the membrane of the infected cell 13.3 Which of the following mechanisms is the basis for the massive activation of the immune system caused by staphylococcal enterotoxins? A. Ability to interact nonspecifically with MHC-II molecules and α / β TcR B. Activation of APC, followed by release of massive amounts of lymphocyte-activating cytokines C. Binding to APC, followed by activation of all T cells expressing TcR with specific V β regions D. Mitogenic activation of the TH2 helper subpopulation E. Promiscuous binding to the peptide-binding grooves of MHC-II molecules 13.4 The finding of a generalized depression of cellular immunity during the acute phase of measles should be considered as: A. A poor prognosis indicator B. Evidence of a primary immune deficiency syndrome C. Reflecting a transient state of immunosuppression D. The result of a concurrent bacterial infection E. The result of a laboratory error 13.5 Which of the following approaches would be useful to treat an intracellular infection such as the one caused by Mycobacterium tuberculosis? A. Administration of a mixture of interleukins 4, 5, and 6 to activate a B-cell response B. Administration of in vitro-activated NK cells C. Administration of interferon- γ to activate macrophages Page 255 D. Administration of interleukin-2 to induce a predominant response of TH1 cells E. Transfusion of T lymphocytes from patients who survived M. tuberculosis infections 13.6 An antibody to tetanus to toxoid will prevent the clinical manifestations of tetanus by: A. Binding to the antigenic portion of the toxin molecule and inhibiting the interaction between the toxin and its receptor B. Causing the destruction of Clostridium tetani before it releases significant amounts of toxin C. Facilitating the onset of a recall response to the toxin D. Inhibiting the binding of the toxin to its receptor by blocking the toxin's active site E. Promoting ADCC reactions against C. tetani 13.7 Herpes simplex virus escapes immune defenses by: A. Being a very weak immunogen B. Causing immunosuppression C. Infecting immunocompetent cells D. Producing an excess of soluble antigen that “blocks” the corresponding antibody E. Spreading from cell to cell with minimal exposure to the extracellular environment 13.8 The ABO isohemagglutinins are synthesized as a result of: A. Blood transfusions with incompatible blood B. Cross-immunization with polysaccharide-rich enterobacteriaceae C. Genetic predisposition D. Mixing of placental and fetal blood at the time of birth E. Repeated pregnancies 13.9 In a newborn baby with blood typed as A, Rh positive, the lack of anti-B isoagglutinins is: A. A reflection of lack of exposure to intestinal flora during intrauterine life B. A very exceptional finding, identifying the baby as a nonresponder to blood group B substance C. Evidence of maternal immunoincompetence D. Evidence suggestive of fetal immunoincompetence E. Unlikely; the test should be repeated 13.10 The parasite-killing properties of eosinophils are linked to the production and secretion of: A. Histamine B. Leukotrienes C. Major basic protein D. Perforin E. Prostaglandins Answers 13.1 (C) If the incubation period exceeds that of the time necessary for eliciting an immune response (primary or secondary), then active immunization can be used to prevent the development of the disease after a known exposure. Page 256 13.2 (E) Sensitized T cells will become activated and release interferon- γ , which can enhance the killing properties of macrophages and related cells. 13.3 (C) After binding to MHC-II molecules, staphylococcal enterotoxins are able to interact and activate T cells expressing TcR with specific V β regions. 13.4 (C) Measles is characteristically associated with a transient depression of cellular immunity during the acute phase; a patient with such immune depression should not be considered as having a primary immune deficiency or as necessarily having a poor prognosis. 13.5 (C) Administration of interferon- γ , the natural macrophage activator, would be the most promising of the listed approaches. For activation of TH1 cells, IL-12 would be the indicated cytokine. Activated NK cells have not been proven to be important in the elimination of cells infected by intracellular bacteria. Transfusion of T cells from recovering patients has the disadvantage over interferon- γ administration of potential reactions due to donor-recipient histoincompatibility. 13.6 (A) An antitetanus toxoid antibody is neither cytotoxic to Clostridium tetani, since it reacts with the toxin, which is an exotoxin, nor able to bind to the active site of the toxin, since the toxoid used for immunization has lost (as a consequence of detoxification) the active site. Memory responses depend on an increased number of memory cells, and not on the presence of circulating antibody. 13.7 (E) Herpes simplex viruses are immunogenic, do not infect immunocompetent cells, do not cause immunosuppression, and do not induce the release of large amounts of soluble antigens from infected cells. However, they can cause fusion of infected and noninfected cells, and this allows them to spread from cell to cell without being exposed to humoral defense mechanisms. 13.8 (B) Mixing of maternal and fetal blood at birth may cause maternal sensitization to Rh blood groups, but does not appear to cause sensitization of the newborn, probably due to a combination of factors such as the immaturity of the immune system and the fact that the maternal red cells directly enter the fetal circulation, which is not a very immunogenic route of presentation for any given antigen. 13.9 (A) Since the newborn's intestine has not been colonized by enterobacteriaceae, the antigenic stimulation for production of isoagglutinins has not taken place before birth and negative titers of isohemagglutinins are normal. Maternal isoagglutinins, being in a large majority of cases of the IgM class, do not cross the placenta. 13.10 (C) Major basic protein is the main parasiticidal compound released by eosinophils recognizing IgE-coated parasites. Bibliography Borst, P., and Graves, D.R. Programmed gene rearrangements altering gene expression. Science, 235: 658, 1987. Cook, D.N., Beck, M., Coffman, T.M., et al., Requirement of MIP-la for an inflammatory response to viral infection. Science, 269: 1583, 1995. Eze, M.O. Avoidance and inactivation of reactive oxygen species: Novel microbial immune evasion strategies. Med. Hypotheses, 34: 252, 1991. Page 257 Karp, C.L., Wysocka, M., Wahl, L.M., Ahearn, J.M., Cuomo, P.J., Sherry, B., Tirnchieri, G., and Griffin, D. Mechanism of suppression of cell-mediated immunity by measles virus. Science, 273: 228, 1996. Kim, J., Urban, R.G., Strominger, J.L., and Wiley, D.C. Toxic shock syndrome toxin-1 complexed with a class II major histocompatibility molecule HLA-DR1. Science, 266: 1870, 1994. Kraus, W., Dale, J.B., and Beachey, E.H. Identification of an epitope of type 1 streptococcal M protein that is shared with a 43-kDa protein of human myocardium and renal glomeruli. J. Immunol., 145: 4089, 1990. Louzir, H., Ternynck, T., Gorgi, Y., Tahar, S., Ayed, K., and Avrameas, S. Autoantibodies and circulating immune complexes in sera from patients with hepatitis B virus-related chronic liver disease. Clin. Immunol. Immunopathol., 62: 160, 1992. Mauel, J. Macrophage-parasite interactions in Leishmania infections. J. Leukoc. Biol., 47: 187, 1990. Miethke, T., Wahl, C., Heeg, K., and Wagner, H. Superantigens: The paradox of T-cell activation vs. inactivation. Int. Arch. Allergy Immunol., 106: 3, 1995. Research Brief. Disarming the immune system: HIV uses multiple strategies. J. NIH Res., 8: 33, 1996. [...]... Page 2 84 Table 15.1 Normal Values for Human Immunoglobulinsa IgG IgA 0 IgM Newborn 636–1606 6–25 1–2 mo 250–900 1–53 20–87 4 6 mo 196–558 4 73 27–100 10–12 mo 2 94 1069 16– 84 41–150 1–2 yr 345 –1210 14 106 43 –173 3 4 yr 44 0–1135 21–159 47 –200 5–18 yr 630–1280 33–200 48 –207 8–10 yr 608–1572 45 –236 52– 242 Greater than 10 yrs 639–1 349 70–312 57–352 aIn mg/dL, as determined by immunonephelometry in the Department... detection of anti-Ig factors (rheumatoid factors) in the rheumatoid arthritis (RA) test (Fig 14. 7) Figure 14. 7 Detection of the rheumatoid factor by the latex agglutination technique A suspension of IgG-coated latex particles is mixed with a 1:20 dilution of three sera Obvious clumping is seen in A, corresponding to a strongly positive serum; no clumping is seen in B, corresponding to a negative serum;... present, will bind to the kinetoplast and will be revealed with fluorescein-labeled anti-IgG antibody g With the introduction of fluorometers, a wide new array of quantitative immunofluorescence assays has been developed The principles are similar to those just mentioned: antigen is bound to a solid phase, exposed to a serum sample containing specific antibody, unbound immu- Page 2 74 noglobulins are... diagnostic procedures such as a test to detect antithyroid antibodies, the Rose Waaler test for anti-Ig factors present in the serum of patients with Figure 14. 9 Diagrammatic representation of a passive hemagglutination test to detect anti-IgA antibodies Purified IgA was coated to chromium chloride-treated RBC; the IgA-coated red cells will be agglutinated by anti-IgA antibodies (Original diagram from... insulin-antibody complexes from free insulin by precipitation with polyethylene glycol 14. 9 To develop a test to screen circulating antitreponemal antibodies by immuno-fluorescence, you need all of the following EXCEPT: A A fluorescence microscope B A positive serum control, known to contain antitreponemal antibodies C A suspension of formalin-killed Treponema pallidum D Fluorescein-labeled anti-human... illustrated in Figure 14. 3, a patient's serum was being tested for the presence of an IgG monoclonal protein Such protein, by definition, has to be homogeneous in mobility, has to react with anti-IgG antibodies, and either with anti-kappa or with anti-lambda antibodies The diagram shows stained precipitates corresponding to the lanes that were overlaid with anti-IgG and with anti-κ light chains This... quality control, and needs to be carried out by specialized personnel in certified laboratories Page 2 64 Figure 14. 4 Diagrammatic representation of a Western blot study to confirm the existence of anti HIV antibodies In the first step, a mixture of HIV antigens is separated by size (large antigens remain close to the origin where the sample is applied, smaller antigens move deep into the acrylamide gel... Book Medical Publishers, Chicago, 1983 Kaplan, L.A., and Pesce, A.J., eds Non-Isotopic Alternatives to Radioimmunoassay Marcel Dekker, New York, 1981 Phillips, T.M Analytical Techniques in Immunochemistry Marcel Dekker, New York, 1992 Turgeon, M.L Immunology and Serology in Laboratory Medicine, 2nd ed Mosby, St Louis, 1996 Wreghitt, T.J., and Morgan-Capner, P ELISA in the Clinical Microbiology Laboratory... popularized by its use in the diagnosis of HIV infection 1 Principle (Fig 14. 4) a The first step in the preparation of an immunoblot (also known as a Western blot) is to separate the different viral antigens (gp160 to p16) according to their molecular size [the numbers to the right of “gp” or “p” refer to the protein mass in kilodaltons (kD)] This is achieved by performing electrophoresis in the presence... different, non-competing epitopes in human chorionic gonadotrophin One antibody Page 278 Figure 14. 14 Diagrammatic representation of an enzymoimmunoassay test for the diagnosis of HIV infection HIV antigens are adsorbed to the solid phase, then incubated with patient's sera and calibrated positive sera; After washing unbound proteins, a second enzyme-labeled, antihuman immunoglobulin antibody is added to the . bacterial exotoxins, such as staphylococcal enterotoxins -A and -B (SE-A and SE-B), staphylococcal toxic shock syndrome toxin-1 (TSST-1), exfoliating toxin, and streptococcal exotoxin A, as. IL-2, interferon- γ , lymphotoxin (TNF- β ), and TNF- α . c. Patients infected by bacteria able to release large amounts of superantigens (e.g., S. aureus- releasing enterotoxins or TSST-1. reactions due to donor-recipient histoincompatibility. 13.6 (A) An antitetanus toxoid antibody is neither cytotoxic to Clostridium tetani, since it reacts with the toxin, which is an exotoxin, nor