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MAJOR HISTOCOMPATIBILITY ANTIGENS AND SUSCEPTIBILITY TO DISEASE R. M. Zinkernagel Department of Immunopathology, Scripps Clinic and Research Foundation, La Jolla, California 92037
CONTENTS INTRODUCTION . . ... . . .. . . ..... .. .... . .. . ... .. .. .. .. . . EMPIRICAL CLINICAL FINDINGS ................... .................................................... MHC, ITS ROLE IN T-CELL-MEDIATED IMMUNITY, AND AN EXPLANATION FOR MHC POLYMORPHISM .................................... MHC-RESTRICTED T-CELL-MEDIATED IMMUNITY AGAINST INTRACELLULAR PARASITES: A MECHANISTIC FUNCTION FOR MHC PRODUCTS .... .. ... ..... ..... ...... ........ .. .. ...... . ... ........ ... . . MHC-LINKED IR GENES .......................................................... ....................... ....... WHY MHC POLYMORPHISM? MHC POLYMORPHISM IS LINKED TO THE MHC RESTRICTION OF T-CELLS . .............................................. A SPECULATION: MHC-ASSOCIATED DISEASES ARE OF IMMUNOPATHOLOGICAL ORIGIN AND CAUSED BY AUTOAGGRESSIVE MHC-RBSTRICTED T-CELLS .................. ......... CONCLUSION................................................................................................................ .... .
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INTRODUCTION
The incidence of certain diseases is somehow linked with major transplanta tion antigens coded by the major histocompatibility gene complex (MHC) (reviewed in 15,40,47, 55, 63, 73). Why the two phenomena are associated and how T-cell immunity, which also seems closely related, fits into the picture are still unclear and subject to many speculations (1, 2, 7, 12, 15, 201
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20,29,40,64, 67-69). The explanatory hypotheses span several possibilities: Infectious agents mimic MHC products, so disease is a consequence of tolerance (61,61a); MHC products act as virus-receptors and so determine susceptibility to infection (25,61,61a); and MHC-linked immune response (Ir) genes-including immune surveillance against altered self-reflect both the polymorphism of MHC and the restriction specificity and MHC regulated responsiveness of T-cells (2, 3, 20, 32a, 40, 61a, 68, 69). These models have all been described in detail and have been reviewed extensively. This review emphasizes work with acute infectious viral diseases in which immunoprotective as well as immunodestructive mechanisms function. The experimental diseases described here are mediated principally by T-cells, subject to control by MHC-linked (Ir) genes. The discussion and explana tions are obviously biased and are based on the following assumptions. (a) The association between MHC and susceptibility to disease is relatively direct; thus we largely disregard the possibility that such susceptibility must be a multifactorial phenomenon in which the MHC is only one decisive factor among many [e.g. macrophages,immunoglobulin (Ig) allotypes,con current infection, etc] (b) The association between MHC and disease re flects an MHC dependence of the host's immunologic responsiveness. To simplify the present argument, I assume that the association in question is caused by defects affecting only one class of immune responses, in this case, responses mediated by cytotoxic thymus-derived (T) lymphocytes. I do not deal principally with non lytic cellular responses here,but the argument can be readily extended to all T-cell responses and combinations thereof. (c) Viral or other intracellular infections are often the instigators of diseases for which susceptibility is linked to the MHC. The empirical clinical findings, some experimental analytical models,and arguments on the role of polymorphic MHC products in cellular immunity are reviewed briefly. Thereby the stage is set for the proposal: Many diseases preferentially associated with certain MHC haplotypes are of autoaggres sivel character, and increased or decreased susceptibility may directly re tlect MHC restriction of T-cells. EMPIRICAL CLINICAL FINDINGS
As summarized in recent reviews (15, 47, 55, 63), susceptibility to many diseases characterizes individuals of particular MHC haplotypes (HLA II use the term autoaggressive here to describe the fact that T-cells attack host cells when the latter express foreign antigenic determinants; the term autoimmune is avoided because of
its implied meaning of immune reactivity against normal cells or self-structures. I think that
autoimmunity in the strict sense may exist only very rarely and that most of the autoimmune pathology is in fact of autoaggressive character
as
defined here.
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MHC DISEASE ASSOCIATION
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type in man). The association and incidence of specific diseases with certain HLA antigens have been compiled in a report from the HLA and Disease Registry 0/ Copenhagen and in a summary thereof (55, 63). The most significant (relative risk, >5) degrees of susceptibility or resistance linked to certain HLA antigens have been found for the following diseases: an kylosing spondylitis; Reiter's disease; Yersinia arthritis; Salmonella arthri tis; psoriatic arthritis; acute anterior uveitis; psoriasis vulgaris; and dermatitis herpetiformis. Other less pointed linkages (relative risk between 2 and 5) have been described for systemic lupus erythematosus, thyrotoxicosis, juvenile insulin dependent diabetes, ulcerative colitis, psoriasis vulgaris, myasthenia gravis,
and multiple sclerosis. Even though this list is incomplete,each disease presents salient features regarded as characteristically autoimmune or autoaggressive. MHC, ITS ROLE IN T-CELL-MEDIATED IMMUNITY, AND AN EXPLANATION FOR MHC POLYMORPHISM
The murine MHC (H-2) is a gene region located on chromosome 17 (the human HLA counterpart is on chromosome 6) and is of a size approximat ing the genome of Escherichia coli. More than 10 loci are known in H-2 coded in some 10 subregions, H-2K, I-A, I-B, I-J, I-E, I-C, S, G, and D, but other subregions (e.g. TI, Q,etc) that code for similar products are included in the MHC (reviewed in 23, 30, 32, 59, 62). The murine K,D, regions (corresponding to HLA-A,B in man) code for the serologically defined classical major transplantation antigens that are expressed on all cells and serve as targets for lytic T-cells. The murine I region (correspond ing to the HLA-D,DWR regions in man) codes for serologically defined determinants expressed only by lymphohemopoietic cells (la-antigens), for antigens involved in proliferative and other nonlytic T-cell responses,and for genes that regulate several other nonlytic T-cell responses. The I region also codes for Ir genes that regulate immune responsiveness of nonlytic T-cells. MHC products were originally detected in their role as the cell surface antigens most responsible for rejection of foreign cell and tissue grafts, therefore their name, major histocompatibility or transplantation antigens. However, during the last 10 to 15 years, it has become increasingly clear that the prime biological function of MHC products is not to frustrate transplantation surgery, but rather to function in all immune responses mediated by T-lymphocytes (reviewed in 18,20,29,35,44,45,48,52,53, 58, 60, 65, 68).
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All research on T-cell function reaffirms that T-cells are specific not only for a foreign antigen but also for a self-MHC product (self-H). This phe nomenon is perhaps best illustrated by the following example: During acute virus infections in mice,virus-specificT-cells are generated that can destroy (lyse) virus-infected target cells in vitro (reviewed in 4, 50). Interestingly, virus-specific cytotoxicT-cells from an inbred H-2k mouse lyse only infected H-2k target cells,not target cells infected with the same virus but originating from different inbred strains of mice (72). Thus,T-cells express two specifi cities, one for self-H and one for a foreign antigenic determinant, X. It is not clear whether this dual specificity lies in two separate receptor sites, one for self-H and one for the foreign antigenic determinant X, or is combined into a single receptor site specific for a neoantigenic determinant of the complex between self-H and X (18, 57,58, 72). Whether or not any of these receptors are products of genes that code variable regions of Ig heavy chains is equally unclear. However, several facts have emerged. (a) Different classes of effector T-cells are specific for different self-H antigens; lytic T-cells are specific for H-2K,D products in mice (or HLA-A,B in man), whereas nonlytic (proliferative, cooperative, etc) T-cells are specific for self-H-2I (HLA-D,DWR). Thus, the effector function of T-cells is deter mined by their restriction specificity. (b) In general, the responsiveness of T-cells against particular antigens is regulated by genes coded in the MHC (3,36,40-43,53,68). These lr genes and the genes coding for the restricting self·H seem to map to the same MHC region and therefore may be identical (3,32a,68,71). (c) The capacities to recognize self-H and the Ir-phenotype are not determined by the genotype of the stem cells from which T·cells derive,but rather by the MHC of the thymus (9,28,68). Thus,recognition of self·H is selected ontogenetically in the thymus and is independent of antigen recognition. These findings have been explained with the two theories ofT-ceIl recog nition mentioned. Unfortunately,since the molecular nature ofT·cell recep tors is still unknown, these interpretations have not been too revealing. However,these findings and speculations have led to a rationale as to how to explain MHC polymorphism, i.e. the fact that in a population multiple allelic forms of MHC gene products exist. One hypothesis proposes that MHC products influence antigen presentation. Self-H binds to foreign anti gens (or fragments thereot) to form or expose the immunogenic deter minant(s); low responder MHC alleles fail to complex immunogeneically and therefore do not induce a proper response. In this model polymorphism and gene duplication optimally guarantee the formation of immunogenic antigen presentation in association with self-H. This idea was applied first to an altered-self model of T-cell recognition (20, 68) and, subsequently formulated differently, as a theory of determinant selection (2, 53). AI-
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MHC DISEASE ASSOCIATION
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though the possibility that MHC products determine the quality of antigen presentation and thus the degree of antigenicity is not formally excluded (and may explain some phenomena), currently this proposal is not accepted as a general explanation. The fact that recognition of self-H is acquired independently of foreign antigen X in the thymus is more readily compati ble with a model in which T-cells have two receptor sites. Within a two-receptor site model of T-cell recognition, it is proposed that the Ir defect is not expressed at the level of antigen presentation, but rather reflects a defect in the T-cell receptor repertoire for foreign antigen X. Since this defect is determined by the MHC this means that thymic selection of a receptor for a particular self-H limits this T-cells' receptor repertoire for X [several possible models have been proposed by Langman (34), Cohn & Epstein (13), and von Boehmer et al (9) and have been widely discussed or reviewed (68)]. For the present argument it is not really crucial which of the T-cell receptor models is correct. However, to link MHC polymorphism with T-cell restriction and the receptor repertoire, I prefer to define MHC-linked Ir defects as holes in the receptor repertoire and explain them as a direct consequence of T-cells being MHC restricted. Therefore, MHC polymor phism is essentially linked to the size of the T-cell receptor repertoire of the species and distributes the holes in the repertoire randomly in the popula tion. In addition, gene duplication (e.g. K and D in H-2 or A and B in HLA) guarantees that in each individual at least two overlapping T-cell receptor repertoires are generated, thus minimizing the danger that a hole becomes apparent in the repertoire. MHC polymorphism, together with gene dupli cation, maximizes overall responsiveness in the species population and in the individual, minimizing the chance that the population (or any individ ual) is a nonresponder to, for example, a highly pathogenic virus. The extent of polymorphism (and duplication) of MHC products and the size of the T-cell receptor repertoire in a given population therefore must be linked and, probably, have co-evolved. The two following sets of experimental observations illustrate these arguments and are crucial for our explanation of MHC-associated disease. One set of observations concerns the role of T-cell-mediated immunity and the MHC products in handling intracellular parasites, and the second elucidates Ir gene functions in this context. MHC-RESTRICTED T-CELL-MEDIATED IMMUNITY AGAINST INTRACELLULAR PARASITES: A MECHANISTIC FUNCTION FOR MHC PRODUCTS
Infectious diseases are dealt with here briefly and in terms of natural selec tion. Infections by extracellular bacteria or viruses and intracellular bac-
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teria are among the most acute and life-endangering episodes of infancy and childhood, a fact easily forgotten in the Western world shaped by hygiene, preventive vaccinations,and antibiotics (to, 16, 22). Other infections, par ticularly parasitic diseases such as malaria, trypanosomiasis, leishmaniasis, filariasis, etc, are more chronic and, even when fatal, usually allow the patient to survive long enough to reproduce. From this point of view, the finding that T-cell-mediated immunity is often crucial for overcoming viral infections or infections with intracellular bacteria (4-6, 33, 37, 38, 49, 50) whereas antibodies and complement are essential for overcoming infections with extracellular bacteria such as pneumo-, staphylo-, gono-, or strep tococci is very revealing. That is, the most polymorphic systems known in higher vertebrates are MHC products and Ig allotypes (34), and this diver sity is apparently intimately linked to the fact that the species and the individual can respond to and rid themselves of the widest possible range of infectious agents. This discussion deals with cellular immune responsive ness in relation to the MHC and proposes that MHC restriction and MHC polymorphism have evolved because T-cells control against acute intracel lular parasites. However, the fact that antibody-mediated responsiveness (particularly to polysaccarides as encountered on bacterial capsules) is linked to the Ig allotype indicates that polymorphism of Ig allotypes may have evolved under a similar selective pressure exerted by extracellular bacteria (reviewed in 24). Over the last few years the following picture has emerged for T-cells involved in recovery from infections by intracellular parasites. Cytotoxic T-cells specific for viral antigens are also specific for self-H; thus, T-cells generally express this dual specificity when assayed in vitro in 51Cr release tests or in vivo for adoptive transfer of protection (reviewed in 18,68). The self-H involved is coded by H-2K or D (HLA-A or B in man). Clearly, recovery from viral infections is not solely promoted by cytotoxic T-cells, but rather in concert with recruited inflammatory cells such as macro phages, and with antibodies (4, 6, 50). However, cytotoxic T-cells may function critically early in viral infection by slowing the replication and spread of virus. T-cells destroy infected target cells during the eclipse phase of virus infection and eliminate the virus-producing cell before viral progeny are assembled. The important fact to keep in mind here is that viruses are eliminated by destruction of host cells, i.e. via immunological autoaggres sion (19, 68, 69). Interestingly, T-cells involved in recovery from intracellular bacteria (e.g. Listeria monocytogenes) are also MHC-restricted, but to H-2I (70). These T-cells are not obviously cytotoxic, but act specifically to activate macro phages to increased bactericidal activity (37,38). A similar mechanism may allow some T-cells to react against some viruses via activated macrophages.
MHC DISEASE ASSOCIATION
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Conceptually, these findings fit the following explanation for the function of MHC products
(67):
Viruses that infect phagocytic as well as non
phagocytic cells may be eliminated during the eclipse phase of the infectious
cy cle by cytotoxic T-cells. To fulfill this function, polymorphic K,D deter minants, which are expressed universally on all cells, seem to have evolved (presumably from cell surface molecules involved in cell interactions or mediating differentiation signals) as receptors for lytic signals on a parallel
T-cells. In contrast, polymorphic I region determi expressed on selected cells of mainly lymphohemopoietic origin (including macrophages). These determinants have evolved to function as receptors of nonlytic differentiation signals on macrophages to cause en zyme activation and on B-cells to cause antibody production or the switch from IgM to IgG production (67).
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course with cytotoxic
nants are
MHC-LINKED IR GENES
The fact that MHC genes influence immune responsiveness has been known for at least 10 years (3, 36, 40-43, 53, 68). These Ir genes have been studied most extensively with respect to their role in the antibody response and have the following characteristics:
(a)
Their regulatory role is antigen specific,
(b) respon siveness phenotype is dominant (but nonresponder interaction cannot be rescued); and (c) the genes probably exert their effect at the level of T helper cells (either at the l evel of repertoir e or at the level of antigen presentation and induction). Some similar effects have been mapped to H-21 for T-cell dose dependent, and MHC haplotype (H-21 region) dependent;
responses measured by proliferation or delayed-type hypersensitivity (re viewed in
45, 53).
Recently, similar Ir regulation of expression andlor generation of cyto
toxic effector T-cells has been detected for cytotoxic T-cell responses against trinitrophenyl (reviewed in 58), the male H-Y antigen (8, 60), as well as for viral antigens (17, 71). The latter types of Ir genes act antigen specifically (to a certain degree), MHC haplotype dependently, and map to K or D [i.e. to the same MHC regions that code for the restricting self-H (K,D) for cytotoxic T-cells]. In these reactions a nonresponsiveness linked to a nonre sponder
K or D gene cannot be rescued and has a dominant character. Thus, MHC-linked Ir genes that regulate responsiveness of T-cells and the genes coding for the res tric ting self-H determinant are apparently iden tical or very closely linked. This conclusion is strengthened by the recent finding that T-cell specificity for self-H is selected in the thymus and by the thymic MHC. Similarly, the Ir-phenotype of maturing T-cells is deter mined by the MHC of the thymus rather than that of the precursor T-cells
(9, 28, 68). Therefore, selection of the restriction specificity
for
thymic
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self-H also automatically determines the If' phenotype. Although not proven, the idea that the If' gene product and the restricting self-H are identical is very attractive indeed, because this concept would reduce some If' gene phenomena to a direct consequence of the MHC restriction of T-cells (68). WHY MHC POLYMORPHISM? MHC POLYMORPHISM IS LINKED TO THE MHC RESTRICTION OF T-CELLS
The extreme polymorphism of MHC products has been explained in many different ways. Most hypotheses assume that the interest of viruses in K,D products of H-2 may stem from the fact that the latter have evolved from cell and organ growth-regulating differentiation antigens (18, 31, 61, 61a, 68). The theories differ mainly in that some imply immunologic reasons for polymorphism and other do not. Arguments that polymorphism is not related to immunity are that polymorphism may serve as a marker system of individuality that prevents mutual fusion or parasitism between members of the same or other species (12, 64), or that when coupled with the immune system, polymorphism may prevent the spread of infectious tumors (11). Alternatively, polymorphism may have developed as a pure accident of nature and is maintained only because relatively closely linked loci are polymorphic (e.g. Tit in the mouse) (see 1, 7, 30). A different hypothesis is that since many viruses replicate best in mUltiplying cells, interaction of a virus with cell-surface antigens (e.g. 25, 51, 61) involved in cell differentia tion and proliferation may influence susceptibility to infections. However, this argument (see 51, 61, 62) does not really explain why the MHC prod ucts are polymorphic. None of these mechanisms can be disproven, and some in fact may operate partially and simultaneously. Nevertheless, as developed in a previous section, the most compelling idea is that MHC polymorphism and MHC-restricted cell-mediated immunity to intracellular parasites are intimately linked; therefore, because T-cell effector function is determined by self-MHC products, size of the species' T-cell receptor reper toire for foreign antigens is directly dependent on the polymorphism of the MHC products (20, 27, 34, 68). The inbred mice infected with any of the viruses tested so far generate strong cytotoxic T-cell responses. If this immune activity is separated into an H-2K-restricted response and an H-2D-restricted response, and one tests T-cell activity against self-K plus virus and against self-D plus virus, great differences are detected. For example, Kk is associated with very high response to poxvirus but rather weak response to lymphocytic choriomenin gitis virus. In contrast, Dk is associated with high response to lymphocytic choriomeningitis virus, but no measurable response to poxvirus. Similar
MHC DISEASE ASSOCIAnON
209
examples may be found for I-restrictedT-cell function as exemplified in the H-Y model
(9),
but not for I-restricted virus-specific T-cells (71).
Although the present example deals only with expression of virus-specific cytotoxic T-cells, it is understood that I-restricted T-cell responsiveness parallels this. If the species mouse possessed only one single self-H marker as a receptor for lytic signals delivered by specific T-cells, for example
Ok, the consequences would be disastrous for the species' survival. The first Annu. Rev. Microbiol. 1979.33:201-213. Downloaded from www.annualreviews.org by Laurentian University on 10/31/13. For personal use only.
poxvirus pandemic would eliminate all mice. Polymorphism of self-H alone would reduce this chance substantially, but not eliminate it entirely, since deaths would accumulate as one after another of many possible highly mutable viruses attacks. Only duplication of self-H together with polymor phism deals effectively with the problem, because it becomes difficult for any virus to mutate in such a way as to mimic two or four self-H markers at the same time or otherwise escape MHC-restricted immune surveillance. This concept implies that MHC products presently functioning in T-cell mediated immunity fulfilled other but related functions earlier during phy logeny. In fact, duplication of these original MHC loci may have allowed some of them to be sequestered functionally to co-evolve withT-cell immu nity and become highly polymorphic, whereas others have remained with original functions as markers for cell interactions, cell differentiation, organ formation, etc. Some of the many MHC-linked T/,
Qa, T, L,
etc, loci may
well represent points in case (32a). In essence, the fact that MHC-coded self-H defines the effector function ofT-cells, and because T-cells are MHC restricted, with the MHC influenc ing the T-cell receptor repertoire, implies that development of MHC poly morphism is intimately linked to T-cell-mediated immunity, both having formed under selective pressure by intracellular parasites.
A SPECULATION: MHC-ASSOCIATED DISEASES ARE OF IMMUNOPATHOLOGICAL ORIGIN AND CAUSED BY AUTOAGGRESSIVE MHC-RESTRICTED T-CELLS As the introduction states, the association between MHC and disease relates not to acute infectious diseases but to chronic diseases with an aura of autoaggressiveness. From this starting point the concept was developed (a) T-cell mediated immunity is intimately tied to MHC products, because effector T-cells function via MHC-coded cell surface receptors. (b) T-cell effector functions-that is, lytic functions, which lead to host cell destruc tion or inflammation (but not lysis) via recruitment and activation of macro phages-result in recovery from intracellular parasites.
(c) Ir gene products
and restricting self-H are probably identical. Ir gene phenomena therefore are a direct consequence of MHC restriction. Whether T-cells recognize
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self-H and foreign antigen X as a complex neoantigen via a single receptor site or as two distinct entities via two separate recognition sites is irrelevant here. Recovery from infectious diseases caused by intracellular bacteria and viruses, therefore, can be viewed as resulting from the balance between the viruses' ability to destroy cells (cytopathic effect) and the T-cells' ability to kill cells and destroy tissue (cytotoxicity and/or recruitment of inflamma
4,6,19,46,50,65). Lymphocytic choriomeningitis (14, 19, 26, 54) and autoaggressive hepatitis in humans (39)
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tory cells) (reviewed in in the mouse
may serve as examples of the latter case. This balance is obviously in fluenced on one hand by the viruses' cytopathogenicity, tropism, rapidity of spread, generation time, and susceptibility to other factors such as anti body-mediated modulation, and on the other hand by the host's degree of immunocompetence and MHC linked
Ir
gene-dependent immune respon
siveness. In response to acute infection by highly cytopathic viruses that most commonly afllict young individuals who may die before reproducing, the host's only alternative to death is to eliminate the virus. Therefore, life threatening infectious intracellular agents to which a population is exposed normally will eliminate low responders. Survivors will consist only of phenotypic high responders, and no associations between MHC and suscep tibility to disease are noticeable. The finding that all mice are high respon ders to pox, lymphocytic choriomeningitis virus, or parainfluenza virus, three of the most prevalent infectious agents of the species, fits this concept very well indeed. Therefore I propose that MHC-associated diseases are generally found only in relation to noncytopathic or poorly cytopathic agents that cause chronic infections and do not usually interfere with reproduction. Only these types of infections leave a certain leeway for the balance of immuno protection versus damage caused either by the infectious agents or more importantly by the ensuing immune response. From this point of view we would consider MHC-associated diseases to be autoaggressive diseases and vice versa. The association between susceptibility to disease and the MHC may develop as follows, depending again on the viruses' characteristics, its sus ceptibility to immune modulation, and the host's immunocompetence at the time of contact. (a) MHC-linked low responsiveness to a poorly cytopatho genic virus may magnify the spread of virus and subsequent extensive and chronic cell-mediated destruction of host tissue. Here, low-responder MHC alleles are associated with increased susceptibility when compared with high responder MHC alleles.
(b)
MHC-linked low responsiveness to a
poorly cytopathogenic virus that has already spread widely (under cover of prenatal immunoincompetence, for example, or because of temporary im-
MHC DISEASE ASSOCIATION
211
mune modulation by passive maternal immunity or concurrent infection with cross-reactive agents) may be associated with decreased susceptibility to disease. The latter example would fit the fact that many MHC disease associations are dominant. An important characteristic of the association between MHC type and susceptibility to disease is that it involves often entire haplotype configura tions rather than single MHC alleles. Perhaps this is best explained by the
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following proposal: Since MHC-restricted T-cell activities are intercon nected (e.g. I-restricted interactions between T helper cells and other lym phocytes or between T-cells and macrophages may ultimately generate K,D-restricted cytotoxic T-cells), certain combinations of K plus I or D plus I or various I region alleles may influence the overall result of immune responsiveness
(9,29, 68).
Also, intergenic complementation in MHC may
influence T-celJ responsiveness
(21, 56, 66).
Whether the few examples
known indicate that the MHC codes for more than only restricting self-H (e.g. parts of the T-cell receptors?) is unknown. Either mechanism may cause linkage with haplotype rather than single loci. The significance is that T-cell responses may be subject to far more complicated influences than was originally thought.
CONCLUSION Diseases whose susceptibility is associated with certain alleles of MHC reflect (a)T-cell effector function determined by the restricting MHC prod ucts,
(b)
an intimate link between MHC polymorphism and size of the
T-cell receptor repertoire, and (c) immunopathologic effects of T-cells. Autoaggressive T-cell-dependent diseases, therefore, are likely to be asso ciated with the MHC, and MHC association indicates T-cell-mediated autoaggressive disease. What are the consequences of such a proposal? As mentioned above, immunopathology may be favored by various mechanisms and only some individuals may profit from immunosuppressive therapy, although at this time no guidelines are at hand. The present speculation also implies a caveat concerning the use of attenuated live vaccines. These viruses may shift the balance of immune protection versus immune destruction in favor of the latter because of the attenuated character (e.g. a poorer cytopathogenicity) of the virus. Fortu nately, the general high responder status to most of the wild-type viruses is probably also valid for the attenuated virus, and genetic selection of low responders protected by vaccination may require some time and many generations. In contrast, if the hypothetical infectious agent causing an autoaggressive disease becomes known, development of appropriate vac cines could induce efficient elimination of this agent before widespread
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immunologic autoaggression occurs. On the other hand, tolerization of individuals to these particular agents may be an attractive therapeutic alternative possibility. Ultimately, to understand autoaggressive disease and its linkage to MHC and to devise treatment, we must have at hand a comprehensive biochemical analysis ofT-cell receptors and a refined knowl edge of viral physiology and of the interaction between virus and vertebrate
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host. ACKNOWLEDGMENTS
I thank Andrea Rothman and Phyllis Minick for their invaluable editorial assistance and Annette Parson for her devoted help in preparing this manu script. Part of this work was supported by USPHSG This is Publication no.
1703
13779 and AI-00273.
of the Immunology Departments of Scripps
Clinic and Research Foundation and was completed on January
11, 1979.
Literature Cited 1. Amos, D. B., Bodmer, W. F., Ceppel Iini, R., Condliffe, P. G., Dausset, J., Fahey, J. L., Goodman, H. C., Klein, G., Klein, J., Lilly, F., Mann, D. L., McDevitt, R., Nathenson, S., Palm, J., Reisfeld, R. A., Rogentine, G. N., Sand erson, A. R., ShrefDer, D. C., Simonsen, M., Van Rood, J. J. 1972. Fed. hoc 3 1:1087-104 2. Benacerraf, B. 1978. J. 1mmunoL 120:1809-12 3. Benacerraf, B., Germain, R. 1978. 1m munoL Rev. 38:70-119 4. Blanden, R. V. 1974. Transplant. Rev. 19:56-88 S. Blanden, R. V., Langman, R. E. 1972. Scand. J. 1mmunoL 1:379-91 6. Bloom, B. R., Rager-Zisman, B. 1975. In Viral Immunology and Immunopa thology, ed. A. L. Notkins pp. 11 3 1 36. New York: Academic 7. Bodmer, W. F. 1972. Nature 237:139 8. Boehmer, R. von, 1977. The Cytotoxic Immune Response Against Male Cells: Control by Two Genes in the Murine Major Histocompatibility Complex, pp. 1-18. Basel: Inst. Immunol. 9. Boehmer, H. von, Haas, W., Jerne, N. K. 1978. hoc. Natl. Acad. Sci. USA 75:2439-42 10. Burnet, F. M. 1962. In NaturalHistoryof Infectious Disease, 3rd ed. Cam bridge: Cambridge Univ. Press 11. Burnet, F. M. 1971. In Immunological Surveillance. Sidney, Aust: Pergamon 12. Burnet, F. M. 1973. Nature 245:359-65 13. Cohn, M., Epstein, R. 1978. Cell Im munol 39:125-53 .
,
-
14. Cole, G. A., Nathanson, N. 1975. Prog. Med. ViroL 18:94 15. Dausset, J., Svejgaard, A. 1977. In HLA and Disease, ed. J. Dausset, A. Svejg aard, pp. 1-316. Copenhagen: Munksg aard 16. Defoe, D. 1722. In A Journal of the Plague Year Written by a Citizen Who Continued All the While in London, ed. E. Rbys. New York: Dutton. (1908) 17. Doherty, P. C., Biddison, W. E., Ben nink, J. R., Knowles, B. B. 1978. J. Exp. Med. 148:534-43 18. Doherty, P. C., Blanden, R. V., Zinker nagel, R. M. 1976. Transplant. Rev. 29:89-124 19. Doherty, P. C., Zinkernagel, R. M. 1974. Transplant. Rev. 19:89-20 20. Doherty, P. C., Zinkernagel, R. M. 1975. Lancet 1:1406-9 21. Dorf, M. E., Benacerraf, B. 1975. hoc Natl. Acad. Sci. USA 72:3671-75 22. Fenner, F. 1968. In The Biology of Ani mal Viruses, Vol. III. New York: Aca demic 23. Festenstein, H., Demant, P. 1978. Curro Top. Immunol. 9:1-150. London: Ar nold 24. Heidelberger, M. 1956. In Lectures in Immunochemistry. New York: Aca demic 25. Helenius, A., Morein, B., Fries, E., Si· mons, K., Robinson, P., Schirrmacher, V., Terhorst, c., Strominger, J. L. 1978. hoc. NatL Acad. Sci USA 75:3846-50 26. Hotchin, J. 1963. Cold Spring Harbor Symp. Quant. Bioi. 27:479-99 .
Annu. Rev. Microbiol. 1979.33:201-213. Downloaded from www.annualreviews.org by Laurentian University on 10/31/13. For personal use only.
MHC DISEASE ASSOCIATION 27. Ierne, N. K. 1971. Eur. J. Immunol. 1:1-11 28. Kappler, J. W., Marrack, P. 1978. J. Exp. Med. 148:1510-22 29. Katz, D. H., Benacerraf, B. 1975. Transplant. Rev. 22:175-95 30. Klein, J. 1975. In Biology of the Mouse Histocompatibility-2 Complex. New York: Springer 31. Klein, J. 1976. Curro Top. Immunobiol. 5:297-336 32. Klein, J. 1978. Springer Semin. Im munopathoL 1:31-39 32a. Klein, J. 1979. Science 203:516--21 33. Lane, F. C., Unanue, E. R. 1972. J. Exp. Med. 135:1104-12 34. Langman, R. E. 1978. Rev. Phys. Bio chern. PharmacoL 81:1 35. Lawrence, H. S. 1959. PhysioL Rev. 39:811-59 36. Levine, B. B., Ojeda, A. P., Benacerraf, B. 1963. J. Exp. Med. 118:953-57 37. Mackaness, G. B. 1964. J. Exp. Med. 120:105-20 38. Mackaness, G. B. 1969. J. Exp. Med. 129:973-92 39. Mackay, I. R. 1974. N Y Acad. Sci. 52:453 40. McDevitt, H. 0., Bodmer, W. F. 1974. Lancet 1:1269-75 41. McDevitt, H. 0., Chinitz, A. 1969. Science 163:1207-8 42. McDevitt, H. 0., Deak, B. D., Shreffler, D. C., Klein, I., Stimpfling, I. H., Snell, G. D. 1972. J. Exp. Med. 135:1259-78 43. McDevitt, H. 0., Sela, M. 1965. J. Exp. ' Med. 122:517-31 44. McMichael, A. J., Ting, A., Zweerink, H. I., Askonas, B. A. 1977. Nature 270:524-26 45. Miller, J. F. A. P., Vadas, M. A. 1977. Scand. J. ImmunoL 6:771-78 46. Mims, C. A. 1964. Bacteriol Rev. 28:30 47. Morris, P. 1974. Contemp. Top. Im munobiol. 3:141 48. Munro, A. I., Bright, S. 1976. Nature 264:145-52 49. North, R. I. 1973. Cell Immunol 7:166--76 50. Notkins, A. L. 1975. In Viral Im munology and Immunopathology, ed. A. L. Notkins. New York: Academic 51. Ohno, S. 1977. Transplant. Rev. 33: 59-69 52. Paul, W. E., Benacerraf, B. 1977. Science 195:1293-300
213
53. Rosenthal, A. S. 1978. ImmunoL Rev. 40:136--52 54. Rowe, W. P. 1954. Naval Med. Res. Inst. Res. Rep. 12:167-220 55. Ryder, L. P., Svejgaard, A. 1976. Re part from the HLA and Disease Registry of Copenhagen. Copenhagen: Ryder & Svejgaard. 34 pp. 56. Schwartz, R. H., David, C. S., Dorf, M. E., Benacerraf, B., Paul, W. E. 1978. Proc. Natl. Acad. Sci. USA 75:2387-91 57. Shearer, G. M. 1974. Eur. J. ImmunoL 4:257 58. Shearer, G. M., Schmitt-Verhulst, A. M. 1977. Adv. ImmunoL 25:55-91 59. Shreffler, D. C., David, C. S. 1975. Adv. ImmunoL 20:125 60. Simpson, E., Gordon, R. D. 1977. Im munoL Rev. 35:59-75 61. Snell, G. D. 1968. Folia BioL 14:335 61a. Snell, G. D. 1978. The Harvey Lec tures. In press 62. Snell, G. D., Dausset, J., Nathenson, S. 1976. In Histocompatibility. New York: Academic 63. Svejgaard, A., Platz, P., Ryder, L. P., Nielsen, L. S., Thomsen, M. 1975. Transplant. Rev. 22:1-43 64. Theodor, J. L. 1970. Nature 227:690702 65. Thomas, D. W., Clement, L., Shevach, E. M. 1978. ImmunoL Rev. 40:181-204 66. Warner, C. M., Mcivor, I. L., Maurer, P. H., Merryman, C. F. 1977. J. Exp. Med. 145:766 67. Zinkernagel, R. M. 1977. Transplant. Proc. 9:1835-38 68. Zinkernagel, R. M. 1978. ImmunoL Rev. 42:224-70 69. Zinkernagel, R. M. 1979. Transplant. Proc. 11:624-27 70. Zinkernagel, R. M., Althage, A., Adler, B., Blanden, R. V., Davidson, W. F., Kees, U., Dunlop, M. B. C., Shreffier, D. C. 1977. J. Exp. Med. 145:1353-67 71. Zinkernagel, R. M., Althage, A., Cooper, A. A. S., Kreeb, G., Klein, P. A., Sefton, B., Flaherty, L., Stimpfling, I., Shreffler, D., Klein, I. 1978. J. Exp. Med. 148:592-606 72. Zinkernagel, R. M., Doherty, P. C. 1974. Nature 248:701-2 73. Zinkernagel, R. M., Doherty, P. C. 1977. In HLA and Disease, ed. I. Daus set, A. Svejgaard, pp. 256--68. Copen hagen: Munksgaard
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MAJOR HISTOCOMPATIBILITY ANTIGENS AND SUSCEPTIBILITY TO DISEASE R. M. Zinkernagel Department of Immunopathology, Scripps Clinic and Research Foundation, La Jolla, California 92037
CONTENTS INTRODUCTION . . ... . . .. . . ..... .. .... . .. . ... .. .. .. .. . . EMPIRICAL CLINICAL FINDINGS ................... .................................................... MHC, ITS ROLE IN T-CELL-MEDIATED IMMUNITY, AND AN EXPLANATION FOR MHC POLYMORPHISM .................................... MHC-RESTRICTED T-CELL-MEDIATED IMMUNITY AGAINST INTRACELLULAR PARASITES: A MECHANISTIC FUNCTION FOR MHC PRODUCTS .... .. ... ..... ..... ...... ........ .. .. ...... . ... ........ ... . . MHC-LINKED IR GENES .......................................................... ....................... ....... WHY MHC POLYMORPHISM? MHC POLYMORPHISM IS LINKED TO THE MHC RESTRICTION OF T-CELLS . .............................................. A SPECULATION: MHC-ASSOCIATED DISEASES ARE OF IMMUNOPATHOLOGICAL ORIGIN AND CAUSED BY AUTOAGGRESSIVE MHC-RBSTRICTED T-CELLS .................. ......... CONCLUSION................................................................................................................ .... .
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INTRODUCTION
The incidence of certain diseases is somehow linked with major transplanta tion antigens coded by the major histocompatibility gene complex (MHC) (reviewed in 15,40,47, 55, 63, 73). Why the two phenomena are associated and how T-cell immunity, which also seems closely related, fits into the picture are still unclear and subject to many speculations (1, 2, 7, 12, 15, 201
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20,29,40,64, 67-69). The explanatory hypotheses span several possibilities: Infectious agents mimic MHC products, so disease is a consequence of tolerance (61,61a); MHC products act as virus-receptors and so determine susceptibility to infection (25,61,61a); and MHC-linked immune response (Ir) genes-including immune surveillance against altered self-reflect both the polymorphism of MHC and the restriction specificity and MHC regulated responsiveness of T-cells (2, 3, 20, 32a, 40, 61a, 68, 69). These models have all been described in detail and have been reviewed extensively. This review emphasizes work with acute infectious viral diseases in which immunoprotective as well as immunodestructive mechanisms function. The experimental diseases described here are mediated principally by T-cells, subject to control by MHC-linked (Ir) genes. The discussion and explana tions are obviously biased and are based on the following assumptions. (a) The association between MHC and susceptibility to disease is relatively direct; thus we largely disregard the possibility that such susceptibility must be a multifactorial phenomenon in which the MHC is only one decisive factor among many [e.g. macrophages,immunoglobulin (Ig) allotypes,con current infection, etc] (b) The association between MHC and disease re flects an MHC dependence of the host's immunologic responsiveness. To simplify the present argument, I assume that the association in question is caused by defects affecting only one class of immune responses, in this case, responses mediated by cytotoxic thymus-derived (T) lymphocytes. I do not deal principally with non lytic cellular responses here,but the argument can be readily extended to all T-cell responses and combinations thereof. (c) Viral or other intracellular infections are often the instigators of diseases for which susceptibility is linked to the MHC. The empirical clinical findings, some experimental analytical models,and arguments on the role of polymorphic MHC products in cellular immunity are reviewed briefly. Thereby the stage is set for the proposal: Many diseases preferentially associated with certain MHC haplotypes are of autoaggres sivel character, and increased or decreased susceptibility may directly re tlect MHC restriction of T-cells. EMPIRICAL CLINICAL FINDINGS
As summarized in recent reviews (15, 47, 55, 63), susceptibility to many diseases characterizes individuals of particular MHC haplotypes (HLA II use the term autoaggressive here to describe the fact that T-cells attack host cells when the latter express foreign antigenic determinants; the term autoimmune is avoided because of
its implied meaning of immune reactivity against normal cells or self-structures. I think that
autoimmunity in the strict sense may exist only very rarely and that most of the autoimmune pathology is in fact of autoaggressive character
as
defined here.
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type in man). The association and incidence of specific diseases with certain HLA antigens have been compiled in a report from the HLA and Disease Registry 0/ Copenhagen and in a summary thereof (55, 63). The most significant (relative risk, >5) degrees of susceptibility or resistance linked to certain HLA antigens have been found for the following diseases: an kylosing spondylitis; Reiter's disease; Yersinia arthritis; Salmonella arthri tis; psoriatic arthritis; acute anterior uveitis; psoriasis vulgaris; and dermatitis herpetiformis. Other less pointed linkages (relative risk between 2 and 5) have been described for systemic lupus erythematosus, thyrotoxicosis, juvenile insulin dependent diabetes, ulcerative colitis, psoriasis vulgaris, myasthenia gravis,
and multiple sclerosis. Even though this list is incomplete,each disease presents salient features regarded as characteristically autoimmune or autoaggressive. MHC, ITS ROLE IN T-CELL-MEDIATED IMMUNITY, AND AN EXPLANATION FOR MHC POLYMORPHISM
The murine MHC (H-2) is a gene region located on chromosome 17 (the human HLA counterpart is on chromosome 6) and is of a size approximat ing the genome of Escherichia coli. More than 10 loci are known in H-2 coded in some 10 subregions, H-2K, I-A, I-B, I-J, I-E, I-C, S, G, and D, but other subregions (e.g. TI, Q,etc) that code for similar products are included in the MHC (reviewed in 23, 30, 32, 59, 62). The murine K,D, regions (corresponding to HLA-A,B in man) code for the serologically defined classical major transplantation antigens that are expressed on all cells and serve as targets for lytic T-cells. The murine I region (correspond ing to the HLA-D,DWR regions in man) codes for serologically defined determinants expressed only by lymphohemopoietic cells (la-antigens), for antigens involved in proliferative and other nonlytic T-cell responses,and for genes that regulate several other nonlytic T-cell responses. The I region also codes for Ir genes that regulate immune responsiveness of nonlytic T-cells. MHC products were originally detected in their role as the cell surface antigens most responsible for rejection of foreign cell and tissue grafts, therefore their name, major histocompatibility or transplantation antigens. However, during the last 10 to 15 years, it has become increasingly clear that the prime biological function of MHC products is not to frustrate transplantation surgery, but rather to function in all immune responses mediated by T-lymphocytes (reviewed in 18,20,29,35,44,45,48,52,53, 58, 60, 65, 68).
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All research on T-cell function reaffirms that T-cells are specific not only for a foreign antigen but also for a self-MHC product (self-H). This phe nomenon is perhaps best illustrated by the following example: During acute virus infections in mice,virus-specificT-cells are generated that can destroy (lyse) virus-infected target cells in vitro (reviewed in 4, 50). Interestingly, virus-specific cytotoxicT-cells from an inbred H-2k mouse lyse only infected H-2k target cells,not target cells infected with the same virus but originating from different inbred strains of mice (72). Thus,T-cells express two specifi cities, one for self-H and one for a foreign antigenic determinant, X. It is not clear whether this dual specificity lies in two separate receptor sites, one for self-H and one for the foreign antigenic determinant X, or is combined into a single receptor site specific for a neoantigenic determinant of the complex between self-H and X (18, 57,58, 72). Whether or not any of these receptors are products of genes that code variable regions of Ig heavy chains is equally unclear. However, several facts have emerged. (a) Different classes of effector T-cells are specific for different self-H antigens; lytic T-cells are specific for H-2K,D products in mice (or HLA-A,B in man), whereas nonlytic (proliferative, cooperative, etc) T-cells are specific for self-H-2I (HLA-D,DWR). Thus, the effector function of T-cells is deter mined by their restriction specificity. (b) In general, the responsiveness of T-cells against particular antigens is regulated by genes coded in the MHC (3,36,40-43,53,68). These lr genes and the genes coding for the restricting self·H seem to map to the same MHC region and therefore may be identical (3,32a,68,71). (c) The capacities to recognize self-H and the Ir-phenotype are not determined by the genotype of the stem cells from which T·cells derive,but rather by the MHC of the thymus (9,28,68). Thus,recognition of self·H is selected ontogenetically in the thymus and is independent of antigen recognition. These findings have been explained with the two theories ofT-ceIl recog nition mentioned. Unfortunately,since the molecular nature ofT·cell recep tors is still unknown, these interpretations have not been too revealing. However,these findings and speculations have led to a rationale as to how to explain MHC polymorphism, i.e. the fact that in a population multiple allelic forms of MHC gene products exist. One hypothesis proposes that MHC products influence antigen presentation. Self-H binds to foreign anti gens (or fragments thereot) to form or expose the immunogenic deter minant(s); low responder MHC alleles fail to complex immunogeneically and therefore do not induce a proper response. In this model polymorphism and gene duplication optimally guarantee the formation of immunogenic antigen presentation in association with self-H. This idea was applied first to an altered-self model of T-cell recognition (20, 68) and, subsequently formulated differently, as a theory of determinant selection (2, 53). AI-
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though the possibility that MHC products determine the quality of antigen presentation and thus the degree of antigenicity is not formally excluded (and may explain some phenomena), currently this proposal is not accepted as a general explanation. The fact that recognition of self-H is acquired independently of foreign antigen X in the thymus is more readily compati ble with a model in which T-cells have two receptor sites. Within a two-receptor site model of T-cell recognition, it is proposed that the Ir defect is not expressed at the level of antigen presentation, but rather reflects a defect in the T-cell receptor repertoire for foreign antigen X. Since this defect is determined by the MHC this means that thymic selection of a receptor for a particular self-H limits this T-cells' receptor repertoire for X [several possible models have been proposed by Langman (34), Cohn & Epstein (13), and von Boehmer et al (9) and have been widely discussed or reviewed (68)]. For the present argument it is not really crucial which of the T-cell receptor models is correct. However, to link MHC polymorphism with T-cell restriction and the receptor repertoire, I prefer to define MHC-linked Ir defects as holes in the receptor repertoire and explain them as a direct consequence of T-cells being MHC restricted. Therefore, MHC polymor phism is essentially linked to the size of the T-cell receptor repertoire of the species and distributes the holes in the repertoire randomly in the popula tion. In addition, gene duplication (e.g. K and D in H-2 or A and B in HLA) guarantees that in each individual at least two overlapping T-cell receptor repertoires are generated, thus minimizing the danger that a hole becomes apparent in the repertoire. MHC polymorphism, together with gene dupli cation, maximizes overall responsiveness in the species population and in the individual, minimizing the chance that the population (or any individ ual) is a nonresponder to, for example, a highly pathogenic virus. The extent of polymorphism (and duplication) of MHC products and the size of the T-cell receptor repertoire in a given population therefore must be linked and, probably, have co-evolved. The two following sets of experimental observations illustrate these arguments and are crucial for our explanation of MHC-associated disease. One set of observations concerns the role of T-cell-mediated immunity and the MHC products in handling intracellular parasites, and the second elucidates Ir gene functions in this context. MHC-RESTRICTED T-CELL-MEDIATED IMMUNITY AGAINST INTRACELLULAR PARASITES: A MECHANISTIC FUNCTION FOR MHC PRODUCTS
Infectious diseases are dealt with here briefly and in terms of natural selec tion. Infections by extracellular bacteria or viruses and intracellular bac-
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teria are among the most acute and life-endangering episodes of infancy and childhood, a fact easily forgotten in the Western world shaped by hygiene, preventive vaccinations,and antibiotics (to, 16, 22). Other infections, par ticularly parasitic diseases such as malaria, trypanosomiasis, leishmaniasis, filariasis, etc, are more chronic and, even when fatal, usually allow the patient to survive long enough to reproduce. From this point of view, the finding that T-cell-mediated immunity is often crucial for overcoming viral infections or infections with intracellular bacteria (4-6, 33, 37, 38, 49, 50) whereas antibodies and complement are essential for overcoming infections with extracellular bacteria such as pneumo-, staphylo-, gono-, or strep tococci is very revealing. That is, the most polymorphic systems known in higher vertebrates are MHC products and Ig allotypes (34), and this diver sity is apparently intimately linked to the fact that the species and the individual can respond to and rid themselves of the widest possible range of infectious agents. This discussion deals with cellular immune responsive ness in relation to the MHC and proposes that MHC restriction and MHC polymorphism have evolved because T-cells control against acute intracel lular parasites. However, the fact that antibody-mediated responsiveness (particularly to polysaccarides as encountered on bacterial capsules) is linked to the Ig allotype indicates that polymorphism of Ig allotypes may have evolved under a similar selective pressure exerted by extracellular bacteria (reviewed in 24). Over the last few years the following picture has emerged for T-cells involved in recovery from infections by intracellular parasites. Cytotoxic T-cells specific for viral antigens are also specific for self-H; thus, T-cells generally express this dual specificity when assayed in vitro in 51Cr release tests or in vivo for adoptive transfer of protection (reviewed in 18,68). The self-H involved is coded by H-2K or D (HLA-A or B in man). Clearly, recovery from viral infections is not solely promoted by cytotoxic T-cells, but rather in concert with recruited inflammatory cells such as macro phages, and with antibodies (4, 6, 50). However, cytotoxic T-cells may function critically early in viral infection by slowing the replication and spread of virus. T-cells destroy infected target cells during the eclipse phase of virus infection and eliminate the virus-producing cell before viral progeny are assembled. The important fact to keep in mind here is that viruses are eliminated by destruction of host cells, i.e. via immunological autoaggres sion (19, 68, 69). Interestingly, T-cells involved in recovery from intracellular bacteria (e.g. Listeria monocytogenes) are also MHC-restricted, but to H-2I (70). These T-cells are not obviously cytotoxic, but act specifically to activate macro phages to increased bactericidal activity (37,38). A similar mechanism may allow some T-cells to react against some viruses via activated macrophages.
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Conceptually, these findings fit the following explanation for the function of MHC products
(67):
Viruses that infect phagocytic as well as non
phagocytic cells may be eliminated during the eclipse phase of the infectious
cy cle by cytotoxic T-cells. To fulfill this function, polymorphic K,D deter minants, which are expressed universally on all cells, seem to have evolved (presumably from cell surface molecules involved in cell interactions or mediating differentiation signals) as receptors for lytic signals on a parallel
T-cells. In contrast, polymorphic I region determi expressed on selected cells of mainly lymphohemopoietic origin (including macrophages). These determinants have evolved to function as receptors of nonlytic differentiation signals on macrophages to cause en zyme activation and on B-cells to cause antibody production or the switch from IgM to IgG production (67).
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course with cytotoxic
nants are
MHC-LINKED IR GENES
The fact that MHC genes influence immune responsiveness has been known for at least 10 years (3, 36, 40-43, 53, 68). These Ir genes have been studied most extensively with respect to their role in the antibody response and have the following characteristics:
(a)
Their regulatory role is antigen specific,
(b) respon siveness phenotype is dominant (but nonresponder interaction cannot be rescued); and (c) the genes probably exert their effect at the level of T helper cells (either at the l evel of repertoir e or at the level of antigen presentation and induction). Some similar effects have been mapped to H-21 for T-cell dose dependent, and MHC haplotype (H-21 region) dependent;
responses measured by proliferation or delayed-type hypersensitivity (re viewed in
45, 53).
Recently, similar Ir regulation of expression andlor generation of cyto
toxic effector T-cells has been detected for cytotoxic T-cell responses against trinitrophenyl (reviewed in 58), the male H-Y antigen (8, 60), as well as for viral antigens (17, 71). The latter types of Ir genes act antigen specifically (to a certain degree), MHC haplotype dependently, and map to K or D [i.e. to the same MHC regions that code for the restricting self-H (K,D) for cytotoxic T-cells]. In these reactions a nonresponsiveness linked to a nonre sponder
K or D gene cannot be rescued and has a dominant character. Thus, MHC-linked Ir genes that regulate responsiveness of T-cells and the genes coding for the res tric ting self-H determinant are apparently iden tical or very closely linked. This conclusion is strengthened by the recent finding that T-cell specificity for self-H is selected in the thymus and by the thymic MHC. Similarly, the Ir-phenotype of maturing T-cells is deter mined by the MHC of the thymus rather than that of the precursor T-cells
(9, 28, 68). Therefore, selection of the restriction specificity
for
thymic
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self-H also automatically determines the If' phenotype. Although not proven, the idea that the If' gene product and the restricting self-H are identical is very attractive indeed, because this concept would reduce some If' gene phenomena to a direct consequence of the MHC restriction of T-cells (68). WHY MHC POLYMORPHISM? MHC POLYMORPHISM IS LINKED TO THE MHC RESTRICTION OF T-CELLS
The extreme polymorphism of MHC products has been explained in many different ways. Most hypotheses assume that the interest of viruses in K,D products of H-2 may stem from the fact that the latter have evolved from cell and organ growth-regulating differentiation antigens (18, 31, 61, 61a, 68). The theories differ mainly in that some imply immunologic reasons for polymorphism and other do not. Arguments that polymorphism is not related to immunity are that polymorphism may serve as a marker system of individuality that prevents mutual fusion or parasitism between members of the same or other species (12, 64), or that when coupled with the immune system, polymorphism may prevent the spread of infectious tumors (11). Alternatively, polymorphism may have developed as a pure accident of nature and is maintained only because relatively closely linked loci are polymorphic (e.g. Tit in the mouse) (see 1, 7, 30). A different hypothesis is that since many viruses replicate best in mUltiplying cells, interaction of a virus with cell-surface antigens (e.g. 25, 51, 61) involved in cell differentia tion and proliferation may influence susceptibility to infections. However, this argument (see 51, 61, 62) does not really explain why the MHC prod ucts are polymorphic. None of these mechanisms can be disproven, and some in fact may operate partially and simultaneously. Nevertheless, as developed in a previous section, the most compelling idea is that MHC polymorphism and MHC-restricted cell-mediated immunity to intracellular parasites are intimately linked; therefore, because T-cell effector function is determined by self-MHC products, size of the species' T-cell receptor reper toire for foreign antigens is directly dependent on the polymorphism of the MHC products (20, 27, 34, 68). The inbred mice infected with any of the viruses tested so far generate strong cytotoxic T-cell responses. If this immune activity is separated into an H-2K-restricted response and an H-2D-restricted response, and one tests T-cell activity against self-K plus virus and against self-D plus virus, great differences are detected. For example, Kk is associated with very high response to poxvirus but rather weak response to lymphocytic choriomenin gitis virus. In contrast, Dk is associated with high response to lymphocytic choriomeningitis virus, but no measurable response to poxvirus. Similar
MHC DISEASE ASSOCIAnON
209
examples may be found for I-restrictedT-cell function as exemplified in the H-Y model
(9),
but not for I-restricted virus-specific T-cells (71).
Although the present example deals only with expression of virus-specific cytotoxic T-cells, it is understood that I-restricted T-cell responsiveness parallels this. If the species mouse possessed only one single self-H marker as a receptor for lytic signals delivered by specific T-cells, for example
Ok, the consequences would be disastrous for the species' survival. The first Annu. Rev. Microbiol. 1979.33:201-213. Downloaded from www.annualreviews.org by Laurentian University on 10/31/13. For personal use only.
poxvirus pandemic would eliminate all mice. Polymorphism of self-H alone would reduce this chance substantially, but not eliminate it entirely, since deaths would accumulate as one after another of many possible highly mutable viruses attacks. Only duplication of self-H together with polymor phism deals effectively with the problem, because it becomes difficult for any virus to mutate in such a way as to mimic two or four self-H markers at the same time or otherwise escape MHC-restricted immune surveillance. This concept implies that MHC products presently functioning in T-cell mediated immunity fulfilled other but related functions earlier during phy logeny. In fact, duplication of these original MHC loci may have allowed some of them to be sequestered functionally to co-evolve withT-cell immu nity and become highly polymorphic, whereas others have remained with original functions as markers for cell interactions, cell differentiation, organ formation, etc. Some of the many MHC-linked T/,
Qa, T, L,
etc, loci may
well represent points in case (32a). In essence, the fact that MHC-coded self-H defines the effector function ofT-cells, and because T-cells are MHC restricted, with the MHC influenc ing the T-cell receptor repertoire, implies that development of MHC poly morphism is intimately linked to T-cell-mediated immunity, both having formed under selective pressure by intracellular parasites.
A SPECULATION: MHC-ASSOCIATED DISEASES ARE OF IMMUNOPATHOLOGICAL ORIGIN AND CAUSED BY AUTOAGGRESSIVE MHC-RESTRICTED T-CELLS As the introduction states, the association between MHC and disease relates not to acute infectious diseases but to chronic diseases with an aura of autoaggressiveness. From this starting point the concept was developed (a) T-cell mediated immunity is intimately tied to MHC products, because effector T-cells function via MHC-coded cell surface receptors. (b) T-cell effector functions-that is, lytic functions, which lead to host cell destruc tion or inflammation (but not lysis) via recruitment and activation of macro phages-result in recovery from intracellular parasites.
(c) Ir gene products
and restricting self-H are probably identical. Ir gene phenomena therefore are a direct consequence of MHC restriction. Whether T-cells recognize
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self-H and foreign antigen X as a complex neoantigen via a single receptor site or as two distinct entities via two separate recognition sites is irrelevant here. Recovery from infectious diseases caused by intracellular bacteria and viruses, therefore, can be viewed as resulting from the balance between the viruses' ability to destroy cells (cytopathic effect) and the T-cells' ability to kill cells and destroy tissue (cytotoxicity and/or recruitment of inflamma
4,6,19,46,50,65). Lymphocytic choriomeningitis (14, 19, 26, 54) and autoaggressive hepatitis in humans (39)
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tory cells) (reviewed in in the mouse
may serve as examples of the latter case. This balance is obviously in fluenced on one hand by the viruses' cytopathogenicity, tropism, rapidity of spread, generation time, and susceptibility to other factors such as anti body-mediated modulation, and on the other hand by the host's degree of immunocompetence and MHC linked
Ir
gene-dependent immune respon
siveness. In response to acute infection by highly cytopathic viruses that most commonly afllict young individuals who may die before reproducing, the host's only alternative to death is to eliminate the virus. Therefore, life threatening infectious intracellular agents to which a population is exposed normally will eliminate low responders. Survivors will consist only of phenotypic high responders, and no associations between MHC and suscep tibility to disease are noticeable. The finding that all mice are high respon ders to pox, lymphocytic choriomeningitis virus, or parainfluenza virus, three of the most prevalent infectious agents of the species, fits this concept very well indeed. Therefore I propose that MHC-associated diseases are generally found only in relation to noncytopathic or poorly cytopathic agents that cause chronic infections and do not usually interfere with reproduction. Only these types of infections leave a certain leeway for the balance of immuno protection versus damage caused either by the infectious agents or more importantly by the ensuing immune response. From this point of view we would consider MHC-associated diseases to be autoaggressive diseases and vice versa. The association between susceptibility to disease and the MHC may develop as follows, depending again on the viruses' characteristics, its sus ceptibility to immune modulation, and the host's immunocompetence at the time of contact. (a) MHC-linked low responsiveness to a poorly cytopatho genic virus may magnify the spread of virus and subsequent extensive and chronic cell-mediated destruction of host tissue. Here, low-responder MHC alleles are associated with increased susceptibility when compared with high responder MHC alleles.
(b)
MHC-linked low responsiveness to a
poorly cytopathogenic virus that has already spread widely (under cover of prenatal immunoincompetence, for example, or because of temporary im-
MHC DISEASE ASSOCIATION
211
mune modulation by passive maternal immunity or concurrent infection with cross-reactive agents) may be associated with decreased susceptibility to disease. The latter example would fit the fact that many MHC disease associations are dominant. An important characteristic of the association between MHC type and susceptibility to disease is that it involves often entire haplotype configura tions rather than single MHC alleles. Perhaps this is best explained by the
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following proposal: Since MHC-restricted T-cell activities are intercon nected (e.g. I-restricted interactions between T helper cells and other lym phocytes or between T-cells and macrophages may ultimately generate K,D-restricted cytotoxic T-cells), certain combinations of K plus I or D plus I or various I region alleles may influence the overall result of immune responsiveness
(9,29, 68).
Also, intergenic complementation in MHC may
influence T-celJ responsiveness
(21, 56, 66).
Whether the few examples
known indicate that the MHC codes for more than only restricting self-H (e.g. parts of the T-cell receptors?) is unknown. Either mechanism may cause linkage with haplotype rather than single loci. The significance is that T-cell responses may be subject to far more complicated influences than was originally thought.
CONCLUSION Diseases whose susceptibility is associated with certain alleles of MHC reflect (a)T-cell effector function determined by the restricting MHC prod ucts,
(b)
an intimate link between MHC polymorphism and size of the
T-cell receptor repertoire, and (c) immunopathologic effects of T-cells. Autoaggressive T-cell-dependent diseases, therefore, are likely to be asso ciated with the MHC, and MHC association indicates T-cell-mediated autoaggressive disease. What are the consequences of such a proposal? As mentioned above, immunopathology may be favored by various mechanisms and only some individuals may profit from immunosuppressive therapy, although at this time no guidelines are at hand. The present speculation also implies a caveat concerning the use of attenuated live vaccines. These viruses may shift the balance of immune protection versus immune destruction in favor of the latter because of the attenuated character (e.g. a poorer cytopathogenicity) of the virus. Fortu nately, the general high responder status to most of the wild-type viruses is probably also valid for the attenuated virus, and genetic selection of low responders protected by vaccination may require some time and many generations. In contrast, if the hypothetical infectious agent causing an autoaggressive disease becomes known, development of appropriate vac cines could induce efficient elimination of this agent before widespread
212
ZINKERNAGEL
immunologic autoaggression occurs. On the other hand, tolerization of individuals to these particular agents may be an attractive therapeutic alternative possibility. Ultimately, to understand autoaggressive disease and its linkage to MHC and to devise treatment, we must have at hand a comprehensive biochemical analysis ofT-cell receptors and a refined knowl edge of viral physiology and of the interaction between virus and vertebrate
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host. ACKNOWLEDGMENTS
I thank Andrea Rothman and Phyllis Minick for their invaluable editorial assistance and Annette Parson for her devoted help in preparing this manu script. Part of this work was supported by USPHSG This is Publication no.
1703
13779 and AI-00273.
of the Immunology Departments of Scripps
Clinic and Research Foundation and was completed on January
11, 1979.
Literature Cited 1. Amos, D. B., Bodmer, W. F., Ceppel Iini, R., Condliffe, P. G., Dausset, J., Fahey, J. L., Goodman, H. C., Klein, G., Klein, J., Lilly, F., Mann, D. L., McDevitt, R., Nathenson, S., Palm, J., Reisfeld, R. A., Rogentine, G. N., Sand erson, A. R., ShrefDer, D. C., Simonsen, M., Van Rood, J. J. 1972. Fed. hoc 3 1:1087-104 2. Benacerraf, B. 1978. J. 1mmunoL 120:1809-12 3. Benacerraf, B., Germain, R. 1978. 1m munoL Rev. 38:70-119 4. Blanden, R. V. 1974. Transplant. Rev. 19:56-88 S. Blanden, R. V., Langman, R. E. 1972. Scand. J. 1mmunoL 1:379-91 6. Bloom, B. R., Rager-Zisman, B. 1975. In Viral Immunology and Immunopa thology, ed. A. L. Notkins pp. 11 3 1 36. New York: Academic 7. Bodmer, W. F. 1972. Nature 237:139 8. Boehmer, R. von, 1977. The Cytotoxic Immune Response Against Male Cells: Control by Two Genes in the Murine Major Histocompatibility Complex, pp. 1-18. Basel: Inst. Immunol. 9. Boehmer, H. von, Haas, W., Jerne, N. K. 1978. hoc. Natl. Acad. Sci. USA 75:2439-42 10. Burnet, F. M. 1962. In NaturalHistoryof Infectious Disease, 3rd ed. Cam bridge: Cambridge Univ. Press 11. Burnet, F. M. 1971. In Immunological Surveillance. Sidney, Aust: Pergamon 12. Burnet, F. M. 1973. Nature 245:359-65 13. Cohn, M., Epstein, R. 1978. Cell Im munol 39:125-53 .
,
-
14. Cole, G. A., Nathanson, N. 1975. Prog. Med. ViroL 18:94 15. Dausset, J., Svejgaard, A. 1977. In HLA and Disease, ed. J. Dausset, A. Svejg aard, pp. 1-316. Copenhagen: Munksg aard 16. Defoe, D. 1722. In A Journal of the Plague Year Written by a Citizen Who Continued All the While in London, ed. E. Rbys. New York: Dutton. (1908) 17. Doherty, P. C., Biddison, W. E., Ben nink, J. R., Knowles, B. B. 1978. J. Exp. Med. 148:534-43 18. Doherty, P. C., Blanden, R. V., Zinker nagel, R. M. 1976. Transplant. Rev. 29:89-124 19. Doherty, P. C., Zinkernagel, R. M. 1974. Transplant. Rev. 19:89-20 20. Doherty, P. C., Zinkernagel, R. M. 1975. Lancet 1:1406-9 21. Dorf, M. E., Benacerraf, B. 1975. hoc Natl. Acad. Sci. USA 72:3671-75 22. Fenner, F. 1968. In The Biology of Ani mal Viruses, Vol. III. New York: Aca demic 23. Festenstein, H., Demant, P. 1978. Curro Top. Immunol. 9:1-150. London: Ar nold 24. Heidelberger, M. 1956. In Lectures in Immunochemistry. New York: Aca demic 25. Helenius, A., Morein, B., Fries, E., Si· mons, K., Robinson, P., Schirrmacher, V., Terhorst, c., Strominger, J. L. 1978. hoc. NatL Acad. Sci USA 75:3846-50 26. Hotchin, J. 1963. Cold Spring Harbor Symp. Quant. Bioi. 27:479-99 .
Annu. Rev. Microbiol. 1979.33:201-213. Downloaded from www.annualreviews.org by Laurentian University on 10/31/13. For personal use only.
MHC DISEASE ASSOCIATION 27. Ierne, N. K. 1971. Eur. J. Immunol. 1:1-11 28. Kappler, J. W., Marrack, P. 1978. J. Exp. Med. 148:1510-22 29. Katz, D. H., Benacerraf, B. 1975. Transplant. Rev. 22:175-95 30. Klein, J. 1975. In Biology of the Mouse Histocompatibility-2 Complex. New York: Springer 31. Klein, J. 1976. Curro Top. Immunobiol. 5:297-336 32. Klein, J. 1978. Springer Semin. Im munopathoL 1:31-39 32a. Klein, J. 1979. Science 203:516--21 33. Lane, F. C., Unanue, E. R. 1972. J. Exp. Med. 135:1104-12 34. Langman, R. E. 1978. Rev. Phys. Bio chern. PharmacoL 81:1 35. Lawrence, H. S. 1959. PhysioL Rev. 39:811-59 36. Levine, B. B., Ojeda, A. P., Benacerraf, B. 1963. J. Exp. Med. 118:953-57 37. Mackaness, G. B. 1964. J. Exp. Med. 120:105-20 38. Mackaness, G. B. 1969. J. Exp. Med. 129:973-92 39. Mackay, I. R. 1974. N Y Acad. Sci. 52:453 40. McDevitt, H. 0., Bodmer, W. F. 1974. Lancet 1:1269-75 41. McDevitt, H. 0., Chinitz, A. 1969. Science 163:1207-8 42. McDevitt, H. 0., Deak, B. D., Shreffler, D. C., Klein, I., Stimpfling, I. H., Snell, G. D. 1972. J. Exp. Med. 135:1259-78 43. McDevitt, H. 0., Sela, M. 1965. J. Exp. ' Med. 122:517-31 44. McMichael, A. J., Ting, A., Zweerink, H. I., Askonas, B. A. 1977. Nature 270:524-26 45. Miller, J. F. A. P., Vadas, M. A. 1977. Scand. J. ImmunoL 6:771-78 46. Mims, C. A. 1964. Bacteriol Rev. 28:30 47. Morris, P. 1974. Contemp. Top. Im munobiol. 3:141 48. Munro, A. I., Bright, S. 1976. Nature 264:145-52 49. North, R. I. 1973. Cell Immunol 7:166--76 50. Notkins, A. L. 1975. In Viral Im munology and Immunopathology, ed. A. L. Notkins. New York: Academic 51. Ohno, S. 1977. Transplant. Rev. 33: 59-69 52. Paul, W. E., Benacerraf, B. 1977. Science 195:1293-300
213
53. Rosenthal, A. S. 1978. ImmunoL Rev. 40:136--52 54. Rowe, W. P. 1954. Naval Med. Res. Inst. Res. Rep. 12:167-220 55. Ryder, L. P., Svejgaard, A. 1976. Re part from the HLA and Disease Registry of Copenhagen. Copenhagen: Ryder & Svejgaard. 34 pp. 56. Schwartz, R. H., David, C. S., Dorf, M. E., Benacerraf, B., Paul, W. E. 1978. Proc. Natl. Acad. Sci. USA 75:2387-91 57. Shearer, G. M. 1974. Eur. J. ImmunoL 4:257 58. Shearer, G. M., Schmitt-Verhulst, A. M. 1977. Adv. ImmunoL 25:55-91 59. Shreffler, D. C., David, C. S. 1975. Adv. ImmunoL 20:125 60. Simpson, E., Gordon, R. D. 1977. Im munoL Rev. 35:59-75 61. Snell, G. D. 1968. Folia BioL 14:335 61a. Snell, G. D. 1978. The Harvey Lec tures. In press 62. Snell, G. D., Dausset, J., Nathenson, S. 1976. In Histocompatibility. New York: Academic 63. Svejgaard, A., Platz, P., Ryder, L. P., Nielsen, L. S., Thomsen, M. 1975. Transplant. Rev. 22:1-43 64. Theodor, J. L. 1970. Nature 227:690702 65. Thomas, D. W., Clement, L., Shevach, E. M. 1978. ImmunoL Rev. 40:181-204 66. Warner, C. M., Mcivor, I. L., Maurer, P. H., Merryman, C. F. 1977. J. Exp. Med. 145:766 67. Zinkernagel, R. M. 1977. Transplant. Proc. 9:1835-38 68. Zinkernagel, R. M. 1978. ImmunoL Rev. 42:224-70 69. Zinkernagel, R. M. 1979. Transplant. Proc. 11:624-27 70. Zinkernagel, R. M., Althage, A., Adler, B., Blanden, R. V., Davidson, W. F., Kees, U., Dunlop, M. B. C., Shreffier, D. C. 1977. J. Exp. Med. 145:1353-67 71. Zinkernagel, R. M., Althage, A., Cooper, A. A. S., Kreeb, G., Klein, P. A., Sefton, B., Flaherty, L., Stimpfling, I., Shreffler, D., Klein, I. 1978. J. Exp. Med. 148:592-606 72. Zinkernagel, R. M., Doherty, P. C. 1974. Nature 248:701-2 73. Zinkernagel, R. M., Doherty, P. C. 1977. In HLA and Disease, ed. I. Daus set, A. Svejgaard, pp. 256--68. Copen hagen: Munksgaard
