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 circu lating IgG in all humans and that interacts specifically with the mamm alian carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (termed the a lpha-galactosyl epitope). This epitope is abundant on thyrocytes, as w ell as, on other cells of nonprimate mammals, prosimians and New World monkeys, but its expression is diminished in Old World monkey, ape, a nd human tissues. We hypothesized that anti-Gal may bind in vitro to a lpha-galactosyl epitopes on xenogeneic TSH receptors (TSHR) and mimic the effect of TSH on xenogeneic thyrocytes. Assays performed with porc ine thyrocytes have indicated that anti-Gal can mimic in vitro TSH eff ects in stimulation for cAMP synthesis, I-125 uptake, and cell prolife ration. 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 thyroglob ulin binding inhibiting Ig activity, when the sera were assayed with p orcine thyrocytes. The effect of anti-Gal binding to alpha-galactosyl epitopes on TSHR was further demonstrated by the antibody-mediated sti mulation for cAMP synthesis in mouse 3T3 cells (cells expressing alpha -galactosyl epitopes), which were transfected with recombinant human T SHR. CHO cells (cells lacking alpha-galactosyl epitopes) transfected w ith recombinant human TSHR were not stimulated by anti-Gal. It is, the refore, suggested that in studies on antibodies in Graves' disease ser a, the effect of anti-Gal may be excluded by using target cells that a re devoid of alpha galactosyl epitopes. Alternatively, anti-Gal could be removed from the tested sera, before the assay with xenogeneic thyr ocytes.