The alpha-galactosyl epitope on human normal and autoimmune thyroid cells

As much as 1% of circulating IgG in man (the natural anti-Gal antibody) is directed against the alpha-galactosyl epitope, with the structure Gal alpha 1----3Gal beta 1----4GlcNAc-R. The alpha-galactosyl epitope is abundantly expressed on cells of nonprimate mammals, prosimians, and New World monkeys. Its expression is diminished in Old World monkeys, apes, and humans. It has been previously suggested that interaction between anti-Gal and aberrantly expressed alpha-galactosyl epitopes on thyroid cells may contribute to the initiation of autoimmune thyroid disorders. To study this possibility, alpha-galactosyl epitope expression on thyroid cell membranes of normal individuals and patients with Graves' disease was assessed by a sensitive radioimmunoassay. alpha-Gal-actosyl epitopes were found both on normal and diseased thyroid cells. Whereas the concentration of these epitopes on Graves' disease thyroid membranes was somewhat higher than that observed in normal glands, the difference was not significant. The activity of the enzyme, alpha 1-3-galactosyltransferase, which synthesizes the alpha-galactosyl epitope, was higher in microsomal fractions obtained from some patients as compared with healthy controls, but not significantly different. In view of the abundance of anti-Gal antibody in the circulation, it is argued that, under physiologic conditions, the interaction of this antibody with alpha-galactosyl epitopes does not elicit pathologic effects. However, aberrant expression of the alpha-galactosyl epitope may result in effective anti-Gal binding to thyroid cells (e.g., rearrangement of this structure on the cell membrane or its increased expression).(ABSTRACT TRUNCATED AT 250 WORDS)