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Autoimmune Disease: Reflections and Projections
C H A P T E R
1 Autoimmune Disease: Reflections and Projections Noel R. Rose Department of Pathology, Brigham and Women’s Hospital/Harvard Medical School, Boston, MA, United States
O U T L I N E Foreword
3
Genetics and Exposures
6
Personal Introduction
3
Epidemiology and Prediction
7
Autoimmunity and Autoimmune Disease
4
Acknowledgment
8
Clonal Balance and Regulation
5
FOREWORD The overriding message of this book is that autoimmunity is a common phenomenon in normal individuals, but that it may be a signal of disease, and play a primary or secondary role in the disease process. Autoimmune diseases are diverse because they occur in differing anatomical sites, but they share many key pathogenetic features. In human medicine, they represent a category of disease and benefit from considering them as a family, sharing a genetic pool and experiencing many similar exposures. Most of this book, therefore, is devoted to descriptions of the human diseases that are most firmly established to be the consequence of an autoimmune response. Other chapters of the book detail the common features, their genetic, molecular, and pathologic significance, and their application to diagnoses and treatments.
PERSONAL INTRODUCTION I began my career in immunology when I was a student at the University of Pennsylvania School of Medicine in the years 1948 51. I was fortunate to receive a fellowship at the University’s Center for the Study of Venereal Diseases. Syphilis and other sexually transmitted diseases had undergone the expected increase during World War II and remained a major public health problem in the immediate postwar years. Most of my time was devoted to my basic research on Treponema pallidum My sole clinical responsibility at the center was to assist in a study of the efficacy of the newly available drug, penicillin, in patients with primary syphilis. We were provided with samples of chancre fluid on admission to the clinic. Following penicillin therapy, samples were taken daily until all the spirochetes were inactive. My job was to enumerate the proportion of viable T. pallidum under the darkfield microscope. With a little practice, it was quite easy to recognize the living T. pallidum by its well-named “queenly motion,” a dignified profile worthy of royalty. Other dermal spirochetes showed jerky movements suggestive of plebeians. The Autoimmune Diseases, 6th. DOI: https://doi.org/10.1016/B978-0-12-812102-3.00001-4
3
Copyright © 2020 Elsevier Inc. All rights reserved.
4
1. AUTOIMMUNE DISEASE: REFLECTIONS AND PROJECTIONS
At this time, the field of syphilis serology was undergoing a major resolution, occasioned by the introduction of the treponemal immobilization test (TPI) by Robert Nelson and Manfred Mayer at the Johns Hopkins University. As the name suggested, the assay was based on the loss of motion of viable T. pallidum produced by antibodies (and complement) from patients. This test represented the first diagnostic procedure for demonstrating antibodies specific for the pathogen rather than the standard “Wassermann test” used in various formats in the diagnosis of syphilis. Intrigued by the fact that the TPI depended upon a loss of motility, I wondered if the antibody reacted with flagella of the organism. It was not known whether T. pallidum had flagella or moved by its spiral motion. My first publication was an electron microscope study of spirochetes showing that they do have flagella-like structures. I never had an opportunity to go on with the hunch that syphilitic patients produce antibodies to flagellar antigen.
AUTOIMMUNITY AND AUTOIMMUNE DISEASE The Wassermann test mentioned above was devised by Wassermann et al. at the Robert Koch Institute in Berlin in the early 1900s (Chapter 2, Autoimmunity: A History of the Early Struggle for Recognition). In its many modifications, it quickly became the standard serologic test to assist in diagnosis and in following the treatment of patients with syphilis (Chapter 69). The antigen to which the Wassermann antibody is directed was later characterized by Pangborn at the New York State Health Department as a phospholipid and named cardiolipin because of its relative abundance in heart tissues. Cardiolipin, a component of cell wall, is found in differing amounts in virtually any organ of the body and in any vertebrate. Despite a great deal of speculation over the years, cardiolipin has never been shown to play a pathogenic role in syphilis even though its specific antibody is still of great value in recognizing and following the infection. It represents the first autoantibody widely used as a diagnostic reagent in the clinical laboratory. One might speculate that had T. pallidum not already been identified as the cause of syphilis, cardiolipin, or a replica might have been considered the causative agent of an “autoimmune” disease, syphilis. The gradual acceptance of autoimmunity as a frequent consequence of a normal immune response and of disease as an occasional consequence of autoimmunity is described in Chapter 2. Briefly, studies of human immunology began near the end of the 19th century with experiments on infection by Roux and Yersin at the Pasteur Institute showing that several major human diseases such as tetanus and diphtheria are attributable to production by the respective pathogen of a specific toxin. Behring and Kitasato produced a disease-specific antibody to the toxin could treat or even prevent the disease. Even in other human diseases in which there is no special toxin, an immune response could provide protection. Soon afterwards, Jules Bordet showed that an immune response was generated by parenteral injection of even harmless substances, such as red blood cells from another species. Thus immunology became the physiological science not only of infection but of recognition of substances foreign to the host. A natural follow-on question raised at the time is whether the immune response can be generated by injection of molecules of the host itself. Early experiments clearly showed that antibodies could be induced quite regularly to isolated sites such as the eye or the testis. Production of these antibodies were sometimes associated with disease (Chapter 2). Paul Ehrlich found that he could produce antibodies to red blood cells of most other goats but never to the immunized donor goat itself. He suggested that there were natural barriers to such autoimmune responses, because they could result in harm. These experiments were widely interpreted by most immunologists to suggest that immune responses to the host itself were not possible. Reports that autoantibodies could be the agents of disease were greeted with the greatest skepticism. A change in thinking followed research on inflammation as a consequence of immunization. The concept of inflammation as the body’s response infection or other cell injury goes back to the earliest days of medicine. Metchnikoff pointed out that, although normally a method of protection, inflammation that exceeds its normal boundaries can itself cause disease. In contrast to immunity, inflammation is nonspecific; that is, it does not target to the specific inducing molecule. The major cells initiating the inflammatory process such as polymorphonuclear and mononuclear cells broadly respond to nonself or altered-self substances. They are not capable of distinguishing fine differences among molecules as can the specialized lymphocytes of the immune system (Chapters 4, 10 and 11).
I. GENERAL CONSIDERATIONS
CLONAL BALANCE AND REGULATION
5
By the 1950s, immunology crossed the line and recognized that uncontrolled inflammation induced by an autoimmune response can cause disease in the host. This dramatic change in thinking can be related to the introduction of a number of new technical advances in immunology that opened previously impossible opportunities. They included novel methods for separating mixtures of antibodies or antigens in gels, localizing antibodies in tissues by labeling them and providing more sensitive procedures for identifying and quantitating circulating antibodies. These methods showed that antibodies capable of reacting with antigens of the host are commonly found normal animals and in healthy individuals. When present in heightened amounts, beyond a populationbased “normal” range autoantibodies were often associated with, and predictive of, disease (Chapters 69 and 70). On occasion autoantibodies can be acknowledged directly as a causative agent of pathology based on accidental or deliberate transfer of the autoantibodies between human subjects. Most of the advancement in the field, however, has depended on investigating genetically defined animals in which cellular manipulation and transfer can prove that autoantibodies or selected lymphocytes, even in the absence of antibody, can cause of similar disease. Animal experiments were particularly convincing in instances where the human disease involves inflammation in a specific organ site such as the brain or the thyroid gland [Chapters 40 and 51]. Other diseases more often systemic can be modeled in animals by selection or by genetic manipulation [Chapters 30 and 31]. By the early 1980s, the list of diseases reputed to be caused or significantly aggravated by autoimmunity, based on firm experimental evidence, increased several folds. Concurrently, the immune response itself was dissected with greater precision. The clones of B cells as the source of specific autoantibodies (Chapters 8 and 9) provide the basis for clonal-based therapies (Chapters 71 and 72). Clonal definition of T cells based on antigen specificity is a bit more difficult because additional limitations are placed on T-cell recognition. It requires help from cells bearing the corresponding major transplantation markers which serve as antigen-presenting cells, to be discussed later. Despite their clear association with autoimmune-induced inflammatory diseases, autoantigen-specific T cells are more often more conveniently identified and measured through cell surface markers or by the cytokine products generated by living, activated cells rather than by direct antigen binding (Chapters 5 and 6). The realization that self-reactive B cells and T cells are plentiful in normal individuals raised the question that Ehrlich had considered: Will they do harm? Disease due to autoimmunity is relatively uncommon compared to the frequency of autoimmune lymphocytes because evolution has provided the host with devises to tolerate them. Much of the knowledge of tolerance came from studies on transplantation which proceeded in tandem with research on autoimmune disease in the 1950s and 60s.
CLONAL BALANCE AND REGULATION The experimental basis of specific immune tolerance was established by Medawar et al. Their first experiments were inspired by the original observation of Ray Owen. He reported that cattle exposed in utero to nonidentical blood cells can tolerate these cells without the expected immune response. Medawar supported these observations by transferring spleen cells into genetically different new-born mice. He found that the recipient mouse tolerated skin grafts from donor mice. MacFarlane Burnet realized that these experiments provided an alternative to the then-prevalent instructive theories of antibody production. Those earlier theories suggested that the broad, almost infinite, recognition capacity of the cells of the immune system was achieved by instructions given by the antigen itself. Burnet proposed that the role of antigen may be to select its counterpart “immunocyte” (lymphocyte) from a large repertoire of T cells and B cells. The selected ones then multiplied and produced a clone of identical antigen-specific lymphocytes. Burnet further envisioned that an antigen-driven negative selective process takes place during the embryonic development of immunologically active lymphocytes. In that way the immune repertoire encompasses the universe of nonself-antigens but eliminates reaction to self-antigens. The antigen-selection model became the prevalent explanation for self-tolerance but left open the question of how autoimmunity could ever occur. Burnet’s first suggestion was that autoimmunity may represent a chance mutation, resulting in production of a “forbidden clone.” Experimental evidence supporting this view, however, has been meager because autoimmune responses are rarely mono- or pauciclonal. More commonly negative selection is incomplete, so that a small population, mainly of low affinity T cells or B cells, escapes complete elimination. Small numbers of self-reactive lymphocytes are not uncommon in the blood. Based on their low frequency and low affinity, they may never inflict pathologic effects. They do, however, represent a potential risk for pathogenic autoimmunity.
I. GENERAL CONSIDERATIONS
6
1. AUTOIMMUNE DISEASE: REFLECTIONS AND PROJECTIONS
If the self-antigen is presented in a particularly potent manner even low affinity T cells and B cells can be stimulated into action. These conditions prevail when an antigen is given with a powerful adjuvant to potentiate the response. In inducing experimental models of autoimmune disease, the appropriate antigen is commonly administered with complete Freund adjuvant containing both a mycobacterial peptide and selected mineral oil. This method is used in producing models of organ-specific autoimmune disease in experimental animals. Autoimmune diseases have often been associated with a prior infection that provides an adjuvant effect (Chapters 21 and 70). In other instances the stimulus from a viral infection or environmental agent may serve as an adjuvant (Chapters 20 and 64). Good health of the immune system depends upon maintaining homeostasis through a proper balance between the signals that may stimulate clones of self-reactive lymphocytes and those that tend to suppress them. I suggested several years ago that the term “clonal balance” is more appropriate than clonal selection for describing a healthy immune system. It is likely that multiple mechanisms are in place in most individuals most of the time that maintain favorable clonal balance. Some of these regulatory agents may be found within the immune system itself such as suppressor B cells, T cells, and monocytes. Additional signals come from other regulatory systems such as the endocrine system. The sex bias, usually favoring females, and age-related susceptibilities represent the close interaction between endocrine and immune systems (Chapter 24). Recent studies of neurologic and psychological disorders and autoimmune disease exemplify comparable interactions, particularly through the generation of mediators that affect both immunologic and neurologic responses. The availability of multiple, overlapping regulatory mechanisms is the most effective mechanism for assuring a well-balanced immune system. Strong evidence supporting the importance of clonal balance in human subjects has come from recent investigations on immunologic approaches on cancer therapy. The oncologist may administer drugs designed to inhibit critical steps (checkpoints) that regulate self-directed immune responses, including antigens on cancer cells. Checkpoint inhibitors are drugs that facilitate the development of a cancer-specific immune response. In some instances, however, they can also trigger an immune response to normal self-antigens and cause autoimmune disease. Two diseases that were considered rare are relatively frequent outcomes of efforts to alter the normal immune homeostasis. Well-studied examples are hypophysitis (Chapter 43) and myocarditis (Chapter 64). Why does the body nurture and maintain self-reactive immune cells? One possibility is based on the well-described phenomenon of molecular mimicry; that is, antigenic determinants (epitopes) found in other organisms, even in plants, may closely resemble epitopes present in the body of the host. In some instances, the mimicry may be involved in inducing a pathogenic autoimmune response (Chapter 21). The complete elimination of these cross-reacting lymphocytes could significantly deplete the extensive immunologic repertoire that is the basis of clonal selection theory.
GENETICS AND EXPOSURES The next theme for research on autoimmune disease reached prominence in the 1970s based on understanding that immune system function follows the principles of Darwinian selection. The growing acceptance of autoimmune diseases as a family of relats diseases reflected itself in a growing number of clinical reports of clustering of different autoimmune disorders. A patient with one autoimmune disease tends to have a greater likelihood of a second or even third autoimmune disease suggesting shared heredity and/or environment. Some of the reported cooccurrences were quite striking; for example, an individual with autoimmune thyroid disease a heightened risk of autoimmune diabetes due to shared genes or common exposures (Chapters 40 and 70). A concurrent pathway of research supporting a genetic component of autoimmune disease came from longstanding investigations of genetically defined rodents, especially rats and mice. These investigations, dating back to the 1930s by Peter Gorer in London and George Snell at the Jackson Laboratory, recognized that transplantation of tumors between experimental animals depended mainly upon the genetic constitution of the animals rather than on tumor-specific antigens. The studies led to development of inbred strains of rodents in which histocompatibility genes were defined. By the1960s, it was recognized that there is a cluster of genes that are preeminent in controlling acceptance of tumor and tissue transplants. The major histocompatibility complex (MHC) dominated acceptance of allografts in all of the animals investigated, including humans (Chapters 23 and 26). Hugh McDevitt and Michael Sela, taking advantage of these genetically defined animals, showed that the MHC regulated the immune response to simple synthetic or small natural peptides.
I. GENERAL CONSIDERATIONS
EPIDEMIOLOGY AND PREDICTION
7
Investigations on autoimmune disease began in the late 1960s when Vladutiu and I were able to induce experimental autoimmune thyroiditis in inbred mice, taking advantage of the many defined strains provided by the Jackson Laboratory. We were able to discern that the autoimmune response to a large protein molecule, thyroglobulin, was determined genetically by the mouse MHC. Later we were able to report that a similar MHCrelated predominance was present in spontaneously developing thyroiditis in partially inbred chickens. Parallel studies of other autoimmune diseases including both experimentally induced models and in spontaneous examples rapidly followed and genetic studies in humans became the focus of extensive investigation. In addition to the MHC as the dominant inherited determinant of resistance to autoimmune disease in experimental animals and humans, many other genes make small but significant contributions. Frequently these genes turn out to be the controlling factors already known to be important in regulating the normal immune response as well as controls of cancer immunity. These genes, acting collectively, contribute to the broad susceptibility to autoimmune diseases. Yet a few examples have been described in which a single gene locus dictates susceptibility to a group of autoimmune diseases. This turned out to be particularly striking in the polyendocrine syndromes where autoimmune regulatory genes predominate by preventing expression in the thymus of certain organ-specific antigens the endocrines during negative selection (Chapter 39). Despite the importance of genetic regulation, susceptibility to autoimmune disease is not attributable mainly to heredity. Among even highly inbred rodents and identical human twins, there are still differences in the occurrences of autoimmune disease. Monozygotic twins may show a statistically significant cooccurrence but rarely develop the same autoimmune disease at the same time. These differences may be attributable to the well-known postgermline changes. There is already solid evidence that epigenetics plays an important role in determining autoimmune susceptibility (Chapter 25). Genetically identical animals differing only in the sex-related genes can be dramatically different in susceptibility (Chapter 24). Although in most instances, the bias favors females, few autoimmune diseases are more prevalent and sometimes more severe in males. In addition to their own genome, humans carry a second population with its own genetic constitution, the microbiome (Chapter 19). The interaction between a human host and its bacterial inhabitants calls attention to the importance of nutrition in immune response. The microbial population, once established during infancy, tends to be quite stable in health but can change remarkably with illness. Working out the cause-and-effect relationship remains an intriguing problem for research. One of the great voids in research on autoimmune diseases is the lack of firm knowledge of when and how noninfectious agents in the environment may instigate a pathogenic autoimmune response (Chapter 2,). Studies of experimental animals have made it clear that exposure to nonviable agents such as mercury have vastly different effects on autoimmune responses. Studies on humans have been difficult because it is rare that a single environmental agent can be identified and studied in isolation. Generally, large populations must be investigated over long periods of time to minimize the effects of background confounders. Other than governmental agencies, few institutions are able to provide the infrastructure for such large-scale, long-term studies. Yet it is likely that such exposures represent the reason that many of the autoimmune diseases appear to be increasing in prevalence, particularly in the more industrialized societies. One example of reductionist experiments carried out on human subjects is the administration of vaccines (Chapter 22). Because of their importance in maintaining a high standard of public health, vaccine safety remains a matter of great concern. Despite the hundreds of millions of vaccinations administered around the world, rarely is there is a statistically valid increase in a well-recognized disease. A small cohort of individuals receiving seasonal influenza vaccine in 1967 developed Guillain Barre´ syndrome. The basic biologic for this unique event has not been elucidated. Ongoing studies of narcolepsy may provide a second example of influenza vaccine related autoimmune disease. Here the potential antigen must be better defined. Vaccination is immunology’s greatest success. Vigilance must be maintained for public health purposes but also for opportunities to identify critical early steps in the induction of pathogenic autoimmune responses in humans.
EPIDEMIOLOGY AND PREDICTION As indicated previously, remarkable progress has been made in detailed genetic studies of susceptibility to autoimmune disease in experimental animals. In contrast, epidemiologic investigations in humans have too often been indecisive. Large populations must be studied over time to track all but the most prominent genes.
I. GENERAL CONSIDERATIONS
8
1. AUTOIMMUNE DISEASE: REFLECTIONS AND PROJECTIONS
Future volumes of “The Autoimmune Diseases” will certainly need to provide a greater understanding of the epidemiology of autoimmune disease. Only in that way can we determine who is most likely to develop one of these diseases, what the outcome may be, and what methods could be instituted to treat or prevent them. Such epidemiologic study is emerging through technologic advances in handling large amounts of information. They permit us for the first time to study enough individuals over long periods of time to look for correlations and associations with environmental and dietary exposures. At the same time, the availability of clinical data collected over the life-span allows us to focus with more precision on individuals and permit a rational definition on a personal basis of normality and departure from normality. The clinical laboratory will not have to depend upon population-based normal ranges but may also look more precisely at departures from personal norms, representing an individual’s established homeostasis. At this time, the best predictor of autoimmune disease remains the antibody. It is well established that in many autoimmune disease elevated levels of autoantibodies appear well before clinical evidence. Combined with family history or genetic analysis, they open the possibility to earlier intervention. The most advanced studies have been carried out with autoimmune diabetes (Chapter 70). A combination of genetic data obtained either from family history or actual genome studies have been coupled with appearance or rise of multiple relevant autoantibodies. These early warning signs convey with a high degree of probability that a child will develop diabetes. The next goal will be the design of benign interventions; that is, procedures that could be instituted in a clinically well child that will prevent the occurrence of a disease without itself producing injury. New, more targeted treatments appear almost daily and promising opportunities for the specific inhibitors of key steps of pathogenesis are on the horizon (Chapters 70 and 72). The future management of the autoimmune diseases will certainly be intervention before irreversible damage has occurred. Ian Mackay and I hope that this book will help to achieve that goal.
Acknowledgment I thank Arthur Silverstein and Jorge Kalil for their corrections, suggestions, and wise advice.
I. GENERAL CONSIDERATIONS
1 Autoimmune Disease: Reflections and Projections Noel R. Rose Department of Pathology, Brigham and Women’s Hospital/Harvard Medical School, Boston, MA, United States
O U T L I N E Foreword
3
Genetics and Exposures
6
Personal Introduction
3
Epidemiology and Prediction
7
Autoimmunity and Autoimmune Disease
4
Acknowledgment
8
Clonal Balance and Regulation
5
FOREWORD The overriding message of this book is that autoimmunity is a common phenomenon in normal individuals, but that it may be a signal of disease, and play a primary or secondary role in the disease process. Autoimmune diseases are diverse because they occur in differing anatomical sites, but they share many key pathogenetic features. In human medicine, they represent a category of disease and benefit from considering them as a family, sharing a genetic pool and experiencing many similar exposures. Most of this book, therefore, is devoted to descriptions of the human diseases that are most firmly established to be the consequence of an autoimmune response. Other chapters of the book detail the common features, their genetic, molecular, and pathologic significance, and their application to diagnoses and treatments.
PERSONAL INTRODUCTION I began my career in immunology when I was a student at the University of Pennsylvania School of Medicine in the years 1948 51. I was fortunate to receive a fellowship at the University’s Center for the Study of Venereal Diseases. Syphilis and other sexually transmitted diseases had undergone the expected increase during World War II and remained a major public health problem in the immediate postwar years. Most of my time was devoted to my basic research on Treponema pallidum My sole clinical responsibility at the center was to assist in a study of the efficacy of the newly available drug, penicillin, in patients with primary syphilis. We were provided with samples of chancre fluid on admission to the clinic. Following penicillin therapy, samples were taken daily until all the spirochetes were inactive. My job was to enumerate the proportion of viable T. pallidum under the darkfield microscope. With a little practice, it was quite easy to recognize the living T. pallidum by its well-named “queenly motion,” a dignified profile worthy of royalty. Other dermal spirochetes showed jerky movements suggestive of plebeians. The Autoimmune Diseases, 6th. DOI: https://doi.org/10.1016/B978-0-12-812102-3.00001-4
3
Copyright © 2020 Elsevier Inc. All rights reserved.
4
1. AUTOIMMUNE DISEASE: REFLECTIONS AND PROJECTIONS
At this time, the field of syphilis serology was undergoing a major resolution, occasioned by the introduction of the treponemal immobilization test (TPI) by Robert Nelson and Manfred Mayer at the Johns Hopkins University. As the name suggested, the assay was based on the loss of motion of viable T. pallidum produced by antibodies (and complement) from patients. This test represented the first diagnostic procedure for demonstrating antibodies specific for the pathogen rather than the standard “Wassermann test” used in various formats in the diagnosis of syphilis. Intrigued by the fact that the TPI depended upon a loss of motility, I wondered if the antibody reacted with flagella of the organism. It was not known whether T. pallidum had flagella or moved by its spiral motion. My first publication was an electron microscope study of spirochetes showing that they do have flagella-like structures. I never had an opportunity to go on with the hunch that syphilitic patients produce antibodies to flagellar antigen.
AUTOIMMUNITY AND AUTOIMMUNE DISEASE The Wassermann test mentioned above was devised by Wassermann et al. at the Robert Koch Institute in Berlin in the early 1900s (Chapter 2, Autoimmunity: A History of the Early Struggle for Recognition). In its many modifications, it quickly became the standard serologic test to assist in diagnosis and in following the treatment of patients with syphilis (Chapter 69). The antigen to which the Wassermann antibody is directed was later characterized by Pangborn at the New York State Health Department as a phospholipid and named cardiolipin because of its relative abundance in heart tissues. Cardiolipin, a component of cell wall, is found in differing amounts in virtually any organ of the body and in any vertebrate. Despite a great deal of speculation over the years, cardiolipin has never been shown to play a pathogenic role in syphilis even though its specific antibody is still of great value in recognizing and following the infection. It represents the first autoantibody widely used as a diagnostic reagent in the clinical laboratory. One might speculate that had T. pallidum not already been identified as the cause of syphilis, cardiolipin, or a replica might have been considered the causative agent of an “autoimmune” disease, syphilis. The gradual acceptance of autoimmunity as a frequent consequence of a normal immune response and of disease as an occasional consequence of autoimmunity is described in Chapter 2. Briefly, studies of human immunology began near the end of the 19th century with experiments on infection by Roux and Yersin at the Pasteur Institute showing that several major human diseases such as tetanus and diphtheria are attributable to production by the respective pathogen of a specific toxin. Behring and Kitasato produced a disease-specific antibody to the toxin could treat or even prevent the disease. Even in other human diseases in which there is no special toxin, an immune response could provide protection. Soon afterwards, Jules Bordet showed that an immune response was generated by parenteral injection of even harmless substances, such as red blood cells from another species. Thus immunology became the physiological science not only of infection but of recognition of substances foreign to the host. A natural follow-on question raised at the time is whether the immune response can be generated by injection of molecules of the host itself. Early experiments clearly showed that antibodies could be induced quite regularly to isolated sites such as the eye or the testis. Production of these antibodies were sometimes associated with disease (Chapter 2). Paul Ehrlich found that he could produce antibodies to red blood cells of most other goats but never to the immunized donor goat itself. He suggested that there were natural barriers to such autoimmune responses, because they could result in harm. These experiments were widely interpreted by most immunologists to suggest that immune responses to the host itself were not possible. Reports that autoantibodies could be the agents of disease were greeted with the greatest skepticism. A change in thinking followed research on inflammation as a consequence of immunization. The concept of inflammation as the body’s response infection or other cell injury goes back to the earliest days of medicine. Metchnikoff pointed out that, although normally a method of protection, inflammation that exceeds its normal boundaries can itself cause disease. In contrast to immunity, inflammation is nonspecific; that is, it does not target to the specific inducing molecule. The major cells initiating the inflammatory process such as polymorphonuclear and mononuclear cells broadly respond to nonself or altered-self substances. They are not capable of distinguishing fine differences among molecules as can the specialized lymphocytes of the immune system (Chapters 4, 10 and 11).
I. GENERAL CONSIDERATIONS
CLONAL BALANCE AND REGULATION
5
By the 1950s, immunology crossed the line and recognized that uncontrolled inflammation induced by an autoimmune response can cause disease in the host. This dramatic change in thinking can be related to the introduction of a number of new technical advances in immunology that opened previously impossible opportunities. They included novel methods for separating mixtures of antibodies or antigens in gels, localizing antibodies in tissues by labeling them and providing more sensitive procedures for identifying and quantitating circulating antibodies. These methods showed that antibodies capable of reacting with antigens of the host are commonly found normal animals and in healthy individuals. When present in heightened amounts, beyond a populationbased “normal” range autoantibodies were often associated with, and predictive of, disease (Chapters 69 and 70). On occasion autoantibodies can be acknowledged directly as a causative agent of pathology based on accidental or deliberate transfer of the autoantibodies between human subjects. Most of the advancement in the field, however, has depended on investigating genetically defined animals in which cellular manipulation and transfer can prove that autoantibodies or selected lymphocytes, even in the absence of antibody, can cause of similar disease. Animal experiments were particularly convincing in instances where the human disease involves inflammation in a specific organ site such as the brain or the thyroid gland [Chapters 40 and 51]. Other diseases more often systemic can be modeled in animals by selection or by genetic manipulation [Chapters 30 and 31]. By the early 1980s, the list of diseases reputed to be caused or significantly aggravated by autoimmunity, based on firm experimental evidence, increased several folds. Concurrently, the immune response itself was dissected with greater precision. The clones of B cells as the source of specific autoantibodies (Chapters 8 and 9) provide the basis for clonal-based therapies (Chapters 71 and 72). Clonal definition of T cells based on antigen specificity is a bit more difficult because additional limitations are placed on T-cell recognition. It requires help from cells bearing the corresponding major transplantation markers which serve as antigen-presenting cells, to be discussed later. Despite their clear association with autoimmune-induced inflammatory diseases, autoantigen-specific T cells are more often more conveniently identified and measured through cell surface markers or by the cytokine products generated by living, activated cells rather than by direct antigen binding (Chapters 5 and 6). The realization that self-reactive B cells and T cells are plentiful in normal individuals raised the question that Ehrlich had considered: Will they do harm? Disease due to autoimmunity is relatively uncommon compared to the frequency of autoimmune lymphocytes because evolution has provided the host with devises to tolerate them. Much of the knowledge of tolerance came from studies on transplantation which proceeded in tandem with research on autoimmune disease in the 1950s and 60s.
CLONAL BALANCE AND REGULATION The experimental basis of specific immune tolerance was established by Medawar et al. Their first experiments were inspired by the original observation of Ray Owen. He reported that cattle exposed in utero to nonidentical blood cells can tolerate these cells without the expected immune response. Medawar supported these observations by transferring spleen cells into genetically different new-born mice. He found that the recipient mouse tolerated skin grafts from donor mice. MacFarlane Burnet realized that these experiments provided an alternative to the then-prevalent instructive theories of antibody production. Those earlier theories suggested that the broad, almost infinite, recognition capacity of the cells of the immune system was achieved by instructions given by the antigen itself. Burnet proposed that the role of antigen may be to select its counterpart “immunocyte” (lymphocyte) from a large repertoire of T cells and B cells. The selected ones then multiplied and produced a clone of identical antigen-specific lymphocytes. Burnet further envisioned that an antigen-driven negative selective process takes place during the embryonic development of immunologically active lymphocytes. In that way the immune repertoire encompasses the universe of nonself-antigens but eliminates reaction to self-antigens. The antigen-selection model became the prevalent explanation for self-tolerance but left open the question of how autoimmunity could ever occur. Burnet’s first suggestion was that autoimmunity may represent a chance mutation, resulting in production of a “forbidden clone.” Experimental evidence supporting this view, however, has been meager because autoimmune responses are rarely mono- or pauciclonal. More commonly negative selection is incomplete, so that a small population, mainly of low affinity T cells or B cells, escapes complete elimination. Small numbers of self-reactive lymphocytes are not uncommon in the blood. Based on their low frequency and low affinity, they may never inflict pathologic effects. They do, however, represent a potential risk for pathogenic autoimmunity.
I. GENERAL CONSIDERATIONS
6
1. AUTOIMMUNE DISEASE: REFLECTIONS AND PROJECTIONS
If the self-antigen is presented in a particularly potent manner even low affinity T cells and B cells can be stimulated into action. These conditions prevail when an antigen is given with a powerful adjuvant to potentiate the response. In inducing experimental models of autoimmune disease, the appropriate antigen is commonly administered with complete Freund adjuvant containing both a mycobacterial peptide and selected mineral oil. This method is used in producing models of organ-specific autoimmune disease in experimental animals. Autoimmune diseases have often been associated with a prior infection that provides an adjuvant effect (Chapters 21 and 70). In other instances the stimulus from a viral infection or environmental agent may serve as an adjuvant (Chapters 20 and 64). Good health of the immune system depends upon maintaining homeostasis through a proper balance between the signals that may stimulate clones of self-reactive lymphocytes and those that tend to suppress them. I suggested several years ago that the term “clonal balance” is more appropriate than clonal selection for describing a healthy immune system. It is likely that multiple mechanisms are in place in most individuals most of the time that maintain favorable clonal balance. Some of these regulatory agents may be found within the immune system itself such as suppressor B cells, T cells, and monocytes. Additional signals come from other regulatory systems such as the endocrine system. The sex bias, usually favoring females, and age-related susceptibilities represent the close interaction between endocrine and immune systems (Chapter 24). Recent studies of neurologic and psychological disorders and autoimmune disease exemplify comparable interactions, particularly through the generation of mediators that affect both immunologic and neurologic responses. The availability of multiple, overlapping regulatory mechanisms is the most effective mechanism for assuring a well-balanced immune system. Strong evidence supporting the importance of clonal balance in human subjects has come from recent investigations on immunologic approaches on cancer therapy. The oncologist may administer drugs designed to inhibit critical steps (checkpoints) that regulate self-directed immune responses, including antigens on cancer cells. Checkpoint inhibitors are drugs that facilitate the development of a cancer-specific immune response. In some instances, however, they can also trigger an immune response to normal self-antigens and cause autoimmune disease. Two diseases that were considered rare are relatively frequent outcomes of efforts to alter the normal immune homeostasis. Well-studied examples are hypophysitis (Chapter 43) and myocarditis (Chapter 64). Why does the body nurture and maintain self-reactive immune cells? One possibility is based on the well-described phenomenon of molecular mimicry; that is, antigenic determinants (epitopes) found in other organisms, even in plants, may closely resemble epitopes present in the body of the host. In some instances, the mimicry may be involved in inducing a pathogenic autoimmune response (Chapter 21). The complete elimination of these cross-reacting lymphocytes could significantly deplete the extensive immunologic repertoire that is the basis of clonal selection theory.
GENETICS AND EXPOSURES The next theme for research on autoimmune disease reached prominence in the 1970s based on understanding that immune system function follows the principles of Darwinian selection. The growing acceptance of autoimmune diseases as a family of relats diseases reflected itself in a growing number of clinical reports of clustering of different autoimmune disorders. A patient with one autoimmune disease tends to have a greater likelihood of a second or even third autoimmune disease suggesting shared heredity and/or environment. Some of the reported cooccurrences were quite striking; for example, an individual with autoimmune thyroid disease a heightened risk of autoimmune diabetes due to shared genes or common exposures (Chapters 40 and 70). A concurrent pathway of research supporting a genetic component of autoimmune disease came from longstanding investigations of genetically defined rodents, especially rats and mice. These investigations, dating back to the 1930s by Peter Gorer in London and George Snell at the Jackson Laboratory, recognized that transplantation of tumors between experimental animals depended mainly upon the genetic constitution of the animals rather than on tumor-specific antigens. The studies led to development of inbred strains of rodents in which histocompatibility genes were defined. By the1960s, it was recognized that there is a cluster of genes that are preeminent in controlling acceptance of tumor and tissue transplants. The major histocompatibility complex (MHC) dominated acceptance of allografts in all of the animals investigated, including humans (Chapters 23 and 26). Hugh McDevitt and Michael Sela, taking advantage of these genetically defined animals, showed that the MHC regulated the immune response to simple synthetic or small natural peptides.
I. GENERAL CONSIDERATIONS
EPIDEMIOLOGY AND PREDICTION
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Investigations on autoimmune disease began in the late 1960s when Vladutiu and I were able to induce experimental autoimmune thyroiditis in inbred mice, taking advantage of the many defined strains provided by the Jackson Laboratory. We were able to discern that the autoimmune response to a large protein molecule, thyroglobulin, was determined genetically by the mouse MHC. Later we were able to report that a similar MHCrelated predominance was present in spontaneously developing thyroiditis in partially inbred chickens. Parallel studies of other autoimmune diseases including both experimentally induced models and in spontaneous examples rapidly followed and genetic studies in humans became the focus of extensive investigation. In addition to the MHC as the dominant inherited determinant of resistance to autoimmune disease in experimental animals and humans, many other genes make small but significant contributions. Frequently these genes turn out to be the controlling factors already known to be important in regulating the normal immune response as well as controls of cancer immunity. These genes, acting collectively, contribute to the broad susceptibility to autoimmune diseases. Yet a few examples have been described in which a single gene locus dictates susceptibility to a group of autoimmune diseases. This turned out to be particularly striking in the polyendocrine syndromes where autoimmune regulatory genes predominate by preventing expression in the thymus of certain organ-specific antigens the endocrines during negative selection (Chapter 39). Despite the importance of genetic regulation, susceptibility to autoimmune disease is not attributable mainly to heredity. Among even highly inbred rodents and identical human twins, there are still differences in the occurrences of autoimmune disease. Monozygotic twins may show a statistically significant cooccurrence but rarely develop the same autoimmune disease at the same time. These differences may be attributable to the well-known postgermline changes. There is already solid evidence that epigenetics plays an important role in determining autoimmune susceptibility (Chapter 25). Genetically identical animals differing only in the sex-related genes can be dramatically different in susceptibility (Chapter 24). Although in most instances, the bias favors females, few autoimmune diseases are more prevalent and sometimes more severe in males. In addition to their own genome, humans carry a second population with its own genetic constitution, the microbiome (Chapter 19). The interaction between a human host and its bacterial inhabitants calls attention to the importance of nutrition in immune response. The microbial population, once established during infancy, tends to be quite stable in health but can change remarkably with illness. Working out the cause-and-effect relationship remains an intriguing problem for research. One of the great voids in research on autoimmune diseases is the lack of firm knowledge of when and how noninfectious agents in the environment may instigate a pathogenic autoimmune response (Chapter 2,). Studies of experimental animals have made it clear that exposure to nonviable agents such as mercury have vastly different effects on autoimmune responses. Studies on humans have been difficult because it is rare that a single environmental agent can be identified and studied in isolation. Generally, large populations must be investigated over long periods of time to minimize the effects of background confounders. Other than governmental agencies, few institutions are able to provide the infrastructure for such large-scale, long-term studies. Yet it is likely that such exposures represent the reason that many of the autoimmune diseases appear to be increasing in prevalence, particularly in the more industrialized societies. One example of reductionist experiments carried out on human subjects is the administration of vaccines (Chapter 22). Because of their importance in maintaining a high standard of public health, vaccine safety remains a matter of great concern. Despite the hundreds of millions of vaccinations administered around the world, rarely is there is a statistically valid increase in a well-recognized disease. A small cohort of individuals receiving seasonal influenza vaccine in 1967 developed Guillain Barre´ syndrome. The basic biologic for this unique event has not been elucidated. Ongoing studies of narcolepsy may provide a second example of influenza vaccine related autoimmune disease. Here the potential antigen must be better defined. Vaccination is immunology’s greatest success. Vigilance must be maintained for public health purposes but also for opportunities to identify critical early steps in the induction of pathogenic autoimmune responses in humans.
EPIDEMIOLOGY AND PREDICTION As indicated previously, remarkable progress has been made in detailed genetic studies of susceptibility to autoimmune disease in experimental animals. In contrast, epidemiologic investigations in humans have too often been indecisive. Large populations must be studied over time to track all but the most prominent genes.
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1. AUTOIMMUNE DISEASE: REFLECTIONS AND PROJECTIONS
Future volumes of “The Autoimmune Diseases” will certainly need to provide a greater understanding of the epidemiology of autoimmune disease. Only in that way can we determine who is most likely to develop one of these diseases, what the outcome may be, and what methods could be instituted to treat or prevent them. Such epidemiologic study is emerging through technologic advances in handling large amounts of information. They permit us for the first time to study enough individuals over long periods of time to look for correlations and associations with environmental and dietary exposures. At the same time, the availability of clinical data collected over the life-span allows us to focus with more precision on individuals and permit a rational definition on a personal basis of normality and departure from normality. The clinical laboratory will not have to depend upon population-based normal ranges but may also look more precisely at departures from personal norms, representing an individual’s established homeostasis. At this time, the best predictor of autoimmune disease remains the antibody. It is well established that in many autoimmune disease elevated levels of autoantibodies appear well before clinical evidence. Combined with family history or genetic analysis, they open the possibility to earlier intervention. The most advanced studies have been carried out with autoimmune diabetes (Chapter 70). A combination of genetic data obtained either from family history or actual genome studies have been coupled with appearance or rise of multiple relevant autoantibodies. These early warning signs convey with a high degree of probability that a child will develop diabetes. The next goal will be the design of benign interventions; that is, procedures that could be instituted in a clinically well child that will prevent the occurrence of a disease without itself producing injury. New, more targeted treatments appear almost daily and promising opportunities for the specific inhibitors of key steps of pathogenesis are on the horizon (Chapters 70 and 72). The future management of the autoimmune diseases will certainly be intervention before irreversible damage has occurred. Ian Mackay and I hope that this book will help to achieve that goal.
Acknowledgment I thank Arthur Silverstein and Jorge Kalil for their corrections, suggestions, and wise advice.
I. GENERAL CONSIDERATIONS