Induction of experimental autoimmune thyroiditis by heat-denatured porcine thyroglobulin: a Tc1-mediated disease

Experimental rat models for Hashimoto's thyroiditis

PURPOSE

Hashimoto's thyroiditis (HT) is an autoimmune thyroid disease characterized by T lymphocyte-mediated destruction of thyroid follicles. To study the pathogenesis of HT and the efficacy of new substances for its treatment, an easily obtained and adequate to the human disease experimental model is needed. The aim of our study was to find out whether it is possible to induce experimental autoimmune thyroiditis (EAT) similar to Hashimoto's thyroiditis by injecting with thyroglobulin (Tg) without using agents that enhance its thyroiditogenicity and without taking into account the genetic sensitivity of animals.

METHODS

Wistar rats were immunized with freshly isolated rat Tg or porcine Tg. In 8 weeks, histological studies of the thyroid and parathyroid glands were performed. Thyroid function and total serum calcium level were also evaluated.

RESULTS

Immunization with both rat and porcine freshly isolated Tg caused T lymphocytic infiltration of the thyroid gland, thyroid follicle atrophy and degradation in Wistar rats. EAT caused by porcine Tg was characterized by greater severity than EAT induced with rat Tg. In 55% of rats with porcine Tg-induced EAT, oxyphilic metaplasia was detected in the parathyroid glands. In addition, low total serum calcium was observed in these rats.

CONCLUSION

Two rat models of autoimmune thyroiditis were obtained. EAT caused in Wistar rats by immunization with rat Tg is similar to Hashimoto's thyroiditis. EAT induced with porcine Tg was accompanied by oxyphil cell metaplasia in the parathyroids and hypocalcemia.

Immunology of alopecia areata

Alopecia areata is a condition that affects hair follicles and leads to hair loss ranging from small well-defined patches to complete loss of all body hair. Despite its high incidence, the pathobiology is not fully understood, and no single concept could be universally accepted.

Alopecia areata is mostly considered to be an autoimmune disease, in which the collapse of hair follicle immune privilege plays a key role. Higher incidence rate in the female population and increased overall risk of other autoimmune disorders militate in favor of autoimmune hypothesis. Antibodies against multiple components of hair follicles almost exclusively attack in anagen phase, where melanogenesis takes place. It suggests involvement of melanogenesis-associated autoantigens as a target epitope.

Some investigators believed that alopecia areata is not a truly autoimmune disease but is only ‘consistent with’ autoimmune mechanisms. High frequency of a positive family history up to 42% may reflects the contribution of heredity factors. In addition, no specific target autoantigen has been identified so far, and autoantibodies to hair follicle-associated antigens are detectable in normal individuals.

Recent advances in the pathogenesis of autoimmune hair loss disease alopecia areata

Alopecia areata is considered to be a cell-mediated autoimmune disease, in which autoreactive cytotoxic T cells recognize melanocyte-associated proteins such as tyrosinase. This review discusses recent advances in the understanding of the pathogenesis of alopecia areata, focusing on immunobiology and hormonal aspects of hair follicles (HFs). The HF is a unique “miniorgan” with its own immune and hormonal microenvironment. The immunosuppressive milieu of the anagen hair bulb modulated by immunosuppressive factors is known as “hair follicle immune privilege.” The collapse of the hair follicle immune privilege leads to autoimmune reactions against hair follicle autoantigens. Alopecia areata is sometimes triggered by viral infections such as influenza that causes excess production of interferons (IFN). IFN-γ is one of the key factors that lead to the collapse of immune privilege. This paper reviews the interactions between the endocrine and immune systems and hair follicles in the pathogenesis of alopecia areata.

Cross-priming in health and disease

Cross-priming is an important mechanism to activate cytotoxic T lymphocytes (CTLs) for immune defence against viruses and tumours. Although it was discovered more than 25 years ago, we have only recently gained insight into the underlying cellular and molecular mechanisms, and we are just beginning to understand its physiological importance in health and disease. Here we summarize current concepts on the cross-talk between the immune cells involved in CTL cross-priming and on its role in antimicrobial and antitumour defence, as well as in immune-mediated diseases.

Cloning and characterization of canine thyroglobulin complementary DNA

Canine thyroglobulin (cTg) is one of the thyroid autoantigens associated with hypothyroidism caused by autoimmune thyroiditis in dog. To identify canine-specific areas in cTg, we cloned, by reverse transcriptase PCR, and sequenced the complete cDNA of cTg. It revealed an open reading frame of 8289 nucleotides, which encode a polypeptide of 2762 amino acids that is 78.9 and 78.1% identical to bovine and human thyroglobulin, respectively. This complete cTg sequence may be useful to promote the understanding of the primary structure of cTg and, it will be informative data in the further search about antigenic epitopes associated with autoimmune thyroiditis and pathogenesis of cTg-associated thyroid diseases in dog.

Cytotoxic T cells

The immune system is a complex arrangement of cells and molecules that preserve the integrity of the organism by elimination of all elements judged dangerous. Within the immune system, a humoral and a cellular as well as an innate and an adaptive arm can be differentiated. The key players of adaptive cellular immune responses are T lymphocytes in general and, for the effector function, cytotoxic T lymphocytes (CTLs) in particular. T lymphocytes arise in the bone marrow and migrate to the thymus for maturation. During this process, T cells somatically rearrange gene segments, eventually leading to the expression of a unique antigen-binding molecule, the T-cell receptor (TCR). This receptor allows them to monitor all cells of the body, ready to destroy any cell posing a threat to the organism. Cytotoxicity is exerted directly through the Fas or perforin pathway and/or indirectly by the release of cytokines. Obviously, the activity of such a potent cell is tightly regulated. Indeed, a predominance of stimulatory over inhibitory signals is required for effective immune responses to pathogens, and a predominance of inhibitory over stimulatory signals is required for maintenance of self-tolerance. Still, several situations occur in which an inappropriate CTL response leads to either autoimmune disease or persistence of pathogens.

Newer insights into the pathogenesis of experimental autoimmune thyroiditis

Experimental autoimmune thyroiditis (EAT), produced in the mouse by immunization with murine thyroglobulin plus complete Freund's adjuvant, represents a valuable model for studying the pathogenesis of human chronic (Hashimoto's) thyroiditis. A major issue requiring clarification is the difference between benign autoimmunity, characterized solely by production of autoantibodies to thyroglobulin, and pathogenic autoimmunity where injury occurs to the thyroid cells. In this article, we describe the role of two key cytokines, IL12 and IFNgamma, in modifying the pathogenic immune response. EAT, defined by cellular infiltration of the thyroid and the development of thyroglobulin-specific autoantibodies, is a dynamic process. Consequently, a cytokine may exert a different effect at different times during the disease process. For purposes of discussion, we propose that there are three stages in the development of EAT: priming; initiation; and progression. Administration of anti-IL12 during the priming stage and initiation dramatically decreases disease and lowers autoantibody levels. In contrast, injection of recombinant IL12 after disease was established significantly decreases the severity of disease and reduces autoantibody levels. Unlike IL-12, IFNgamma was not essential for the priming of EAT. However, the severity of disease in the anti-IFNgamma-treated initiation- and progression-treated animals was higher than in controls, implying a regulatory role for IFNgamma. These findings emphasize that EAT involves a complex array of pathogenic mechanisms. The balance of cytokines produced during the early phase of the autoimmune reaction probably determines the progression from a harmless autoimmune response to autoimmune disease.