Xenogeneic thyroid-stimulating hormone-like activity of the human natural anti-Gal antibody. Interaction of anti-Gal with porcine thyrocytes and with recombinant human thyroid-stimulating hormone receptors expressed on mouse cells.

Anti-Gal is a natural polyclonal antibody that constitutes 1% of circulating IgG in all humans and that interacts specifically with the mammalian carbohydrate epitope Gal alpha 1-3Gal beta 1-4GlcNAc-R (termed the alpha-galactosyl epitope). This epitope is abundant on thyrocytes, as well as, on other cells of nonprimate mammals, prosimians and New World monkeys, but its expression is diminished in Old World monkey, ape, and human tissues. We hypothesized that anti-Gal may bind in vitro to alpha-galactosyl epitopes on xenogeneic TSH receptors (TSHR) and mimic the effect of TSH on xenogeneic thyrocytes. Assays performed with porcine thyrocytes have indicated that anti-Gal can mimic in vitro TSH effects in stimulation for cAMP synthesis, 125I uptake, and cell proliferation. Furthermore, depletion of anti-Gal from serum of patients with Graves' disease resulted in elimination of a large proportion of the thyroid stimulating immunoglobulin activity and half of the thyroglobulin binding inhibiting Ig activity, when the sera were assayed with porcine thyrocytes. The effect of anti-Gal binding to alpha-galactosyl epitopes on TSHR was further demonstrated by the antibody-mediated stimulation for cAMP synthesis in mouse 3T3 cells (cells expressing alpha-galactosyl epitopes), which were transfected with recombinant human TSHR. CHO cells (cells lacking alpha-galactosyl epitopes), transfected with recombinant human TSHR were not stimulated by anti-Gal. It is, therefore, suggested that in studies on antibodies in Graves' disease sera, the effect of anti-Gal may be excluded by using target cells that are devoid of alpha-galactosyl epitopes. Alternatively, anti-Gal could be removed from the tested sera, before the assay with xenogeneic thyrocytes.