ULTRASTRUCTURAL-LOCALIZATION OF IGG AND TPO IN AUTOIMMUNE THYROCYTES REFERRING TO THE TRANSCYTOSIS OF IGG AND THE ANTIGEN PRESENTATION OF TPO

While autoantibodies against thyroid peroxidase (TPO) are known to pro duce cytotoxicity in vitro, their in vivo effects are still obscure. I n addition, the mechanism of TPO autoantibody creation needs to be dis closed because the localization of TPO on thyrocytes is considered to be restricted to the apical membrane, which is not in contact with imm unocompetent cells. In order to study these crucial processes in the p athogenesis of thyroid autoimmunity, the ultrastructural localization of TPO and IgG was determined and quantified in thyrocytes of normal t hyroid gland and thyroid tissue of patients suffering from Graves' dis ease. This was done by using ultrathin frozen sections and the immunog old method. IgGs were detected in the follicular lumen, close to the a pical membrane, in transport vesicles, the endoplasmic reticulum, and the Golgi apparatus of thyrocytes from patients with Graves' disease. The labeling of TPO in the basolateral membrane was distinctly lower t han that of the apical membrane, but was significant in comparison to the plasma membrane labeling of fibroblasts present in the same sectio ns. These data indicate that thyroid autoantibodies may perform their cytotoxic function in intracellular compartments besides the plasma me mbrane. TPO molecules on the basolateral membrane of HLA class II anti gen-positive thyrocytes may initiate antigen presentation of TPO as we ll as the formation and uptake of TPO autoantibodies.