Maturational changes of the CD52-like epididymal glycoprotein on cynomolgus monkey sperm and their apparent reversal in capacitation conditions

A major epididymal secretory protein in men has a colinear cDNA sequence wi th lymphocyte CD52, a sialylated glycoprotein. Immunostaining and flow cyto metric detection of cynomolgus monkey sperm CD52 during epididymal maturati on showed increases from 20 to 85% stained sperm from the caput to the corp us with staining intensities doubled. Freshly prepared cauda sperm showed o nly 10% staining while they markedly increased in percentage and intensity of staining upon incubation at 37 degrees C under capacitating conditions, but not at 4 degrees C. Western blotting of proteins from fresh cauda sperm revealed no less antigen than corpus sperm. Staining of ejaculated sperm e xhibited similar increases during incubation. Further washing with a high s alt medium before staining to remove any electrostatically-bound molecules masking the antigen showed no effect. Incubation-induced increases in antig en binding were accelerated by the addition of neuraminidase (0.25 and 0.5 U/ml), but not affected by the sialyl residue-rich fetuin (5 mg/ml) competi ng for any endogenous neuraminidase. There were no concomitant decreases in the staining of sialic acid residues during capacitation-incubation. These findings suggest a cryptic antigen epitope site as a consequence of sperm maturation and subsequent re-exposure under capacitation conditions, but no t due to the removal of sialic acid residues by endogenous neuraminidase. I nvolvement of endogenous proteases was also ruled out, as incubation in the presence of protease inhibitors did not hinder the increases but resulted in a dose-dependent enhancement in staining, suggesting some protease-sensi tive unmasking process. In conclusion, the monkey epididymal secreted CD52 on sperm underwent changes in antigenic characteristics during sperm matura tion which were reversed under capacitation conditions. Mel. Reprod. Dev. 5 7:280-289, 2000. (C) 2000 Wiley-Liss, Inc.