Association of Spontaneous Thyroiditis with the Major Histocompatibility Complex of the Rat*

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Abstract
Overt insulin-dependent diabetes mellitus in the rat is associated with the u haplotype of the rat major histocompatibility complex (MHC), RT1. Thyroiditis of sufficient severity to result in elevation of TSH levels is seen in Buffalo rats (RT1b). In order to examine the association of autoimmune thyroid disease with MCH gene products, inbred Buffalo rats were crossbred with diabetic BB rats. The RT1 genotype, the histology of thyroid and pancreatic tissue, and indices of thyroid function were examined. The data indicate that animals having pancreatic lymphocytic infiltration and insulinopenic overt diabetes mellitus had at least 1 RT1u haplotype. All but one animal having severe histological thyroid lymphocytic infiltration had at least 1 RT1b haplotype. Rats with severe thyroiditis had higher mean TSH levels than rats with normal histology or rats with mild thyroiditis. It is concluded that gene products of the rat MHC affect the severity of spontaneous organ-specific autoimmune disease in terms of clinically apparent as well as tissue inflammatory disease.