Association of spontaneous thyroiditis with the major histocompatibility complex of the rat

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 MHC gene products, we have crossed inbred Buffalo rats with diabetic BB rats and examined the RT1 genotype, the histology of thyroid and pancreatic tissue, and two indices of thyroid function. The data indicate that animals having pancreatic lymphocytic infiltration and insulinopenic overt diabetes mellitus had at least one RT1u haplotype. All but one animal having severe histological thyroid lymphocytic infiltration had at least one RT1b haplotype. Rats with severe thyroiditis had higher mean TSH levels than rats with normal histology or rats with mild thyroiditis. We conclude 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.