Thyroid disease and Yersinia

Current perspective on the pathogenesis of Graves' disease and ophthalmopathy

Graves' disease (GD) is a very common autoimmune disorder of the thyroid in which stimulatory antibodies bind to the thyrotropin receptor and activate glandular function, resulting in hyperthyroidism. In addition, some patients with GD develop localized manifestations including ophthalmopathy (GO) and dermopathy. Since the cloning of the receptor cDNA, significant progress has been made in understanding the structure-function relationship of the receptor, which has been discussed in a number of earlier reviews. In this paper, we have focused our discussion on studies related to the molecular mechanisms of the disease pathogenesis and the development of animal models for GD. It has become apparent that multiple factors contribute to the etiology of GD, including host genetic as well as environmental factors. Studies in experimental animals indicate that GD is a slowly progressing disease that involves activation and recruitment of thyrotropin receptor-specific T and B cells. This activation eventually results in the production of stimulatory antibodies that can cause hyperthyroidism. Similarly, significant new insights have been gained in our understanding of GO that occurs in a subset of patients with GD. As in GD, both environmental and genetic factors play important roles in the development of GO. Although a number of putative ocular autoantigens have been identified, their role in the pathogenesis of GO awaits confirmation. Extensive analyses of orbital tissues obtained from patients with GO have provided a clearer understanding of the roles of T and B cells, cytokines and chemokines, and various ocular tissues including ocular muscles and fibroblasts. Equally impressive is the progress made in understanding why connective tissues of the orbit and the skin in GO are singled out for activation and undergo extensive remodeling. Results to date indicate that fibroblasts can act as sentinel cells and initiate lymphocyte recruitment and tissue remodeling. Moreover, these fibroblasts can be readily activated by Ig in the sera of patients with GD, suggesting a central role for them in the pathogenesis. Collectively, recent studies have led to a better understanding of the pathogenesis of GD and GO and have opened up potential new avenues for developing novel treatments for GD and GO.

Infections and autoimmune endocrine disease

The literature examined in this review points to the possible involvement of infectious agents in the pathogenesis of autoimmune endocrine diseases, primarily autoimmune thyroid disease and diabetes mellitus. Various mechanisms have been proposed to explain induction of autoimmunity by infection but it seems that three possibilities may be important in individuals susceptible to developing autoimmune disease: molecular mimicry (perhaps to retroviruses); polyclonal T cell activation (by an endogenous superantigen or an infecting organism); and MHC class II antigen induction. It seems reasonable that all three mechanisms operate together or separately in different individuals. Data continue to accumulate in favour of infectious agents being important initiators of autoimmune disease.

A perspective on human autoimmune thyroid disease: is there an abnormality of the target cell which predisposes to the disorder?

It has been suggested recently that autoimmunity could be regarded as a physiological response of the normal immune system to autoantigens caught up in an inflammatory response to viral or bacterial antigen expressed in the target tissue. Other theories to explain autoimmunity include molecular mimicry whereby a viral or microbial hapten similar to an autoantigen initiates the production of autoantibodies that cross react with an autoantigen, with a subsequent immune response reacting with autologous cell structures which are homologous with the particular microorganism. There has also been a suggestion that there may be a genetic abnormality of the target cell which is necessary for the initiation of autoimmune thyroid disease. The present review examines these proposals and provides evidence against an antigen-driven origin for autoimmune thyroid disease (AITD). Currently, there is no valid evidence for viral involvement, and likewise the evidence for molecular mimicry as an initiating factor does not hold up to scrutiny. While a genetic abnormality of the thyrocyte may be important in certain animal models of AITD, in the human there is no evidence for such an abnormality. Evidence that AITD is derived from a disturbance of immunoregulatory mechanisms has been documented elsewhere and would appear to be the most appropriate explanation for these disorders. The immunoregulatory disturbance itself may be related to an abnormality of the mechanism of specific antigen (i.e. normal autoantigen) presentation to appropriately induce T lymphocytes and that theory will require further illumination.

The immunoregulatory disturbance in autoimmune thyroid disease

There is now considerable evidence for a genetically induced antigen-specific defect in suppressor T lymphocytes as the basis for AITD, derived from several laboratories and via different types of experimental techniques. In addition, there is now evidence for additive effects on reducing generalized suppressor T lymphocyte numbers and/or function by environmental factors as well as hyperthyroidism itself, and these effects would be superimposed upon the organ-specific defect. Such effects on generalized suppressor T lymphocyte numbers may act as precipitating and self-perpetuating factors, in Graves' disease at least. Presentation of the antigen by the thyroid cell via HLA-DR expression on its membrane does occur as a result of interferon gamma production by T lymphocytes. This in turn appears to be secondary to the initial specific immune assault and is not a primary inductive step. However, it may be important as an amplifying intermediate factor but cannot perpetuate the process in the absence of the underlying immune disorder. There is, however, no evidence for an underlying antigenic abnormality or stimulus in human autoimmune thyroid disease, and the initiating event would appear to be due to perturbation of the generalized immune system superimposed on the organ-specific abnormality. Variations in the serological and clinical expression of AITD would appear to depend on the severity of the original organ-specific disturbance in suppressor T lymphocyte function, plus the added factor of environmental influences playing upon generalized suppressor T lymphocyte function and numbers. Remissions in Graves' disease brought about by antithyroid drugs may well be via their effect on modulating the thyroid cell activity; this will then reduce thyrocyte-immunocyte signalling, allowing remission to occur in those patients with a partial organ-specific defect in suppressor T lymphocytes.

Humoral immune stimulation and antiepithelial antibodies in Yersinia infection

Results consistent with a general humoral immune stimulation were found when 127 sera from 89 patients with Yersinia enterocolitica infection were studied. Significantly increased gammaglobulin concentrations and elevated isohemagglutinin titers were seen in these sera as compared to sera from normal blood donors and patients with streptococcal infection. Antinuclear and anti-smooth muscle antibodies were demonstrated in both yersinia and streptococcal infection. The prevalence of non-organ specific antiepithelial antibodies reacting with gastrointestinal and thyroid epithelial cells was significantly increased in yersinia infection.

Cell-mediated immunity of Yersinia enterocolitica serotype 3 in patients with thyroid diseases

The cellular immunity to Yersinia enterocolitica serotype 3 and crude human thyroid extract in 64 patients with thyroid diseases and 25 controls was studied by the leucocyte migration test. In the patient group as a whole and in patients with Graves' disease and nontoxic diffuse goitre a significantly reduced leucocyte migration towards Yersinia was found when compared with the controls. In controls the migration index was not related to the presence or titre of circulating yersinia antibodies, whereas the migration index of patients with yersinia antibodies was lower than the migration index of patients without yersinia antibodies as well as that of the controls. The leucocyte migration inhibition in two patients with recent yersinosis was normal during the recovery phase. In the presence of thyroid extract leucocyte migration inhibition differed only significantly in Graves' disease. However, a significantly positive correlation between inhibition of migration by thyroid extract and by Yersinia was found, while no correlation could be demonstrated in the controls. The cell-mediated immunity towards Yersinia in thyroid diseases thus demonstrated adds further evidence to the association between Yersinia and thyroid disease.