Intrathyroidal mast cells express major histocompatibility complex class-II antigens.

We studied the expression of major histocompatibility complex (MHC) class-II antigen in mast cells of rat thyroid glands. In the normal rat thyroid, mast cells express MHC class-II antigen. In BioBreeding Wistar (BB/W) rats, the strain of animals prone to develop spontaneous autoimmune thyroiditis, the number of MHC class-II-positive mast cells was significantly higher than in normal rats (p less than 0.01). Furthermore, intrathyroidal mast cells in BB/W rats showed an increased MHC class-II expression before the appearance of circulating thyroid antibodies or the infiltration of tissue with mononuclear cells. These data suggest that mast cells in the rat thyroid gland may have a function as antigen-presenting cells.