Antigen-specific immunoregulation and autoimmune thyroiditis

Molecular mimicry and clonal deletion: A fresh look

In this article, I trace the historic background of clonal deletion and molecular mimicry, two major pillars underlying our present understanding of autoimmunity and autoimmune disease. Clonal deletion originated as a critical element of the clonal selection theory of antibody formation in order to explain tolerance of self. If we did have complete clonal deletion, there would be major voids, the infamous "black holes", in our immune repertoire. For comprehensive, protective adaptive immunity, full deletion is necessarily a rare event. Molecular mimicry, the sharing of epitopes among self and non-self antigens, is extraordinary common and provides the evidence that complete deletion of self-reactive clones is rare. If molecular mimicry were not common, protective adaptive immunity could not be all-encompassing. By taking a fresh look at these two processes together we can envision their evolutionary basis and understand the need for regulatory devices to prevent molecular mimicry from progressing to autoimmune disease.

The genetics of autoimmune thyroiditis: the first decade

Most of our current understanding of the genetic predisposition to autoimmune disease can be traced to experiments performed in the decade from 1971 to 1981. Chella David was a key contributor to this research. Many of these early steps came from studies of experimental autoimmune thyroiditis. This model has been especially valuable because essentially the same disease can occur spontaneously in selected strains of animals or can be induced by deliberate immunization. From a genetic point of view, the disease has been investigated in three different species: mice, rats and chickens. The same antigen, thyroglobulin, initiates the disease in all three species. Among the main discoveries were the relationship of autoimmune disease to the major histocompatibility complex (MHC), the interplay of different subregions within the MHC in promoting or retarding development of disease, the differing roles of MHC class II and MHC I class genes in induction and effector phases, respectively, and the cumulative effect of non-MHC genes, each of which represents a small addition to overall susceptibility. Other experiments revealed that genetic differences in thyroglobulin allotypes influence susceptibility to thyroiditis. Thyroid glands differed in different strains in vulnerability to passive transfer of antibody. The first evidence of modulatory genes on the sex-related X chromosome emerged. All of these genetic findings were concurrently translated to the human disease, Hashimoto's thyroiditis, where thyroglobulin is also the initiating antigen.

Alternatively activated macrophages in infection and autoimmunity

Macrophages are innate immune cells that play an important role in activation of the immune response and wound healing. Pathogens that require T helper-type 2 (Th2) responses for effective clearance, such as parasitic worms, are strong inducers of alternatively activated or M2 macrophages. However, infections such as bacteria and viruses that require Th1-type responses may induce M2 as a strategy to evade the immune system. M2 are particularly efficient at scavenging self tissues following injury through receptors like the mannose receptor and scavenger receptor-A. Thus, M2 may increase autoimmune disease by presenting self tissue to T cells. M2 may also exacerbate immune complex (IC)-mediated pathology and fibrosis, a hallmark of autoimmune disease in women, due to the release of profibrotic factors such as interleukin-1beta, transforming growth factor-beta, fibronectin and matrix metalloproteinases. We have found that M2 comprise anywhere from 30% to 70% of the infiltrate during acute viral or experimental autoimmune myocarditis, and shifts in M2 populations correlate with increased IC deposition, fibrosis and chronic autoimmune pathology. Thus, women may be at an increased risk of M2-mediated autoimmunity due to estrogen's ability to increase Th2 responses.

Thymus function, ageing and autoimmunity

We suggest that the thymus is the critical time-keeper in the ageing process with respect to immune responses. Because the thymus involutes asymmetrically, a clonal imbalance occurs with ageing since the proportion of autoantigen-specific helper/inducer T cells increases relative to the number of autoantigen-specific regulatory T cells. As a result, circulating autoantibody levels rise with age. On the other hand, nonspecific immunoregulatory mechanisms increase with age. As the thymic cortex atrophies, the response to foreign antigens declines, whereas the response to self-antigen rises, generating the ageing paradox.

Immunoregulation in autoimmune thyroid disease

We have postulated over many years that autoimmune thyroid diseases (AITD) are disorders of immunoregulation due to antigen specific defect(s) in suppressor (regulatory) T (Ts) lymphocyte function. Despite earlier skepticism, there is recent increased evidence to support this view. Several investigators working with animal models have demonstrated T lymphocyte subsets that are regulatory, i.e., will prevent AITD; conversely, depletion of these cells precipitates the lesion in the experimental models. These cells have been shown to be inadequately activated by specific antigen. In human AITD, recent studies have demonstrated that CD8+ (suppressor/cytotoxic) and CD8+CD11b+ ("pure suppressor") cells are activated by irrelevant antigen normally, but are significantly less well activated in response to thyroglobulin or thyroperoxidase. In further similar studies, CD8+ cells from patients with Graves' disease (GD) are induced normally in response to glutamic acid decarboxylase-65 (GAD-65), the putative beta cell antigen important in insulin-dependent diabetes mellitus (IDDM), but significantly less to synthetic TSH receptor (TSHR). Conversely, CD8+ cells from patients with IDDM are activated normally in response to TSHR, but significantly less to GAD-65. While these reductions in activation are partial only, and other additive factors playing on the immune system may be necessary to precipitate AITD, this disorder in the activation of Ts cells may be fundamental to the development of these disorders. This in turn may be due to molecular disturbances in MHC-related genes that dictate the mechanisms of presentation of specific antigen.

Suppressor T lymphocyte dysfunction is important in the pathogenesis of autoimmune thyroid disease: a perspective

Over the past decade, there was considerable scepticism regarding the existence, nature, and function of suppressor T (Ts) lymphocytes, particularly antigen-specific Ts lymphocytes. The receptors of putative antigen-specific murine Ts hybrids revealed that most either lacked the T cell receptor beta (TCR beta) chain or had failed to properly rearrange them and thus could not make a functional receptor. Moreover, studies of the DNA of sequences of the MHC I-J region (considered an important determinant for suppression) fail to show evidence for a unique open reading frame capable of encoding the I-J restriction element. In addition, no molecular basis for suppression had been characterized. These criticisms have now been satisfactorily resolved and the pendulum is swinging in favor of participation by antigen-specific Ts lymphocytes in immune tolerance. Ts lymphocytes have now been cloned and cell lines have been found to be idiotypic-specific. Indeed, specific Ts lymphocytes in the periphery have been shown to prevent the appearance of autoreactivity. It is, therefore, legitimate to consider their control as part of the mechanisms of maintaining the integrity of the immune system. Moreover, all human primary Ts clones and the majority of murine Ts clones have now been shown to rearrange TCR alpha beta. Ts lymphocytes are induced by antigen via antigen-presenting cells (APCs), and may do so in the context of MHC class II antigens, a concept not previously accepted. Moreover, there is now considerable evidence that such cells many well be involved in various autoimmune disease states.(ABSTRACT TRUNCATED AT 250 WORDS)