Enhanced expression of complement regulatory proteins on thyroid epithelial cells of Graves' disease

Cytotoxic anti-thyroid microsomal autoantibodies are highly prevalent in sera of patients with Graves' disease, but in Graves' disease thyroid tissues rarely show destructive changes. We postulated that this might be due to membrane-associated complement regulatory proteins which protect target cells from injury by complement activation. We, therefore, investigated the expression of membrane attack complex inhibitory factor (MACIF) and decay accelerating factor (DAF) in the thyroid tissues from patients with Graves' disease, Hashimoto's thyroiditis, thyroid adenocarcinoma and normal human thyroid tissues. We found a high level of expression of MACIF and DAF in Graves' thyroid tissues. Using the membrane immunofluorescence and cell-ELISA techniques, we also investigated the factors which enhanced the MACIF and DAF expression in cultured thyroid cells. Thyroid stimulating hormone, phorbol 12, 13-dibutyrate and thyroid stimulating autoantibody enhanced the MACIF and DAF expression. These findings suggest that the membrane complement regulatory proteins increase in response to the thyroid stimulating factors such as thyroid stimulating autoantibody in Graves' disease and that this increase then protects the cells from damage due to complement activation by thyroid autoantibodies.