Human sperm surface glycoprotein involved in sperm-zona pellucida interaction

To identify peptide-specific antibodies which define sperm surface antigens, hybridomas were derived from the splenocytes of mice immunized with swollen human spermatozoa which had been subjected to limited proteolytic cleavage under reducing conditions prior to immunization. A total of 13.7% of the hybrid clones secreted antibodies which reacted with deglycosylated human seminal plasma glycoproteins when screened by an ELISA. A monoclonal antibody, designated mAb 4A8 sp., specifying a peptide epitope of human epididymal and a sperm surface glycoprotein was selected which inhibited human sperm-egg binding in a dose-dependent manner, and totally blocked sperm penetration in vitro. This inhibition did not result from an effect of the antibody on the motility of spermatozoa, nor was it due to premature induction of the acrosome reaction. Exclusion of oligosaccharide chains by chemical hydrolysis with trifluoromethane sulphonic acid (TFMS), enzymatic degradation and binding of lectins, did not abrogate the reactivity of mAb 4A8 to the cognate epitope whereas antibody binding was precluded upon digestion with proteolytic enzymes. In Western immunoblots of human seminal plasma glycoproteins, the antigen presented as a set of immunoreactive polypeptides, a major glycoprotein of M(r) 78 kDa and less prominent bands of M(r) 56 and 44 kDa. Immunocytochemical staining of a number of human reproductive and somatic tissues revealed strong immunostaining of the luminal epithelium of the epididymis as well as of spermatozoa in the lumen. Immunolocalization to the plasma membrane of ejaculated human spermatozoa was demonstrated by immunofluorescence, although on undigested spermatozoa the antigen epitope was less accessible. Upon capacitation the antigen persisted on the sperm surface and was present on the head of capacitated acrosome-intact spermatozoa. The pronounced peripheral immunostaining of the sperm head was accentuated after DTT/trypsin treatment, implicating the dynamic accessibility of the epitope on the plasma membrane of capacitated spermatozoa. It is suggested that the protein in question appears on the sperm membrane as a consequence of its modification in the epididymis (insertion and processing), and may be involved in the processes leading to sperm attachment and interaction with the human zona pellucida.