Immunoelectron microscopic study of thyroid microsomal antigen

Using human IgG F(ab') 2 prepared from pooled sera containing a high titer of thyroid microsomal antibody (TMAb), localization of thyroid microsomal antigen (TMAg) was studied electron microscopically in dissociated follicular cells obtained from 25 thyroids of patients with Basedow's disease. Twenty out of the 25 cases were positive for serum-TMAb. In 18 of these cases, the reaction product for spontaneous membrane-bound IgG was demonstrated only on the apical plasma membrane of a small number of follicular cells, but was not observed anywhere in 5 cases without serum-TMAb. After masking the spontaneous membrane-bound IgG with anti-human IgG-goat IgG Fab, deposits of the reaction product disappeared or were significantly decreased. After the masking, all cases incubated in the above-mentioned TMAb-positive IgG F(ab')2 showed an intense reaction product located linearly on the apical plasma membrane of a large number of follicular cells, but not on the lateral and basal plasma membrane and in the cytoplasm. On the other hand, after the masking, the control group incubated in human IgG F(ab')2 from a normal subject revealed the same result as that in the group with masked spontaneous membrane-bound IgG. These findings indicate that TMAg is located only on the apical plasma membrane of follicular cells.