Heterogeneity of human sperm surface antigens identified by indirect immunoprecipitation of antisperm antibody bound to biotinylated sperm.

Target Ag recognized by clinically important C-fixing anti-sperm antibodies (ASA) were identified by indirect immunoprecipitation after incubation of biotinylated, motile sperm with ASA-positive sera from autoimmune, isoimmune, and vasectomized patients. This method assessed native protein thus permitting analysis of conformation-dependent membrane Ag that would be missed by previously reported Ag identification techniques. Sera from 35 ASA-positive and 10 ASA-negative patients were exposed to capacitated, surface-biotinylated motile sperm. After detergent-extraction of the membrane fraction, the immune complexes were precipitated with protein G-agarose. The Ag were eluted under reducing conditions, electrophoresed on SDS-10% PAGE, electroblotted onto nitrocellulose membranes, and visualized by the biotin/avidin-peroxidase detection method. By this method, only high titered ASA-positive sera demonstrated variable reactivity restricted to a total of eight to nine protein bands. Individual sera revealed a differential reactivity to a constant set of protein bands (63/61, 58/56, 29, and 21/19 kDa) and a preferential reactivity to either 45-, 43-, or 41-kDa protein Ag that were patient specific. None of these bands were precipitable by ASA-negative sera. ASA directed against the 63/61-kDa and 58/56-kDa polypeptides were found in 85% of immunoprecipitable sera; the 29-kDa was detectable with 70% of sera. Immunoreactivity to the 45-kDa to 41-kDa bands was detectable with 30% of the immunoprecipitable sera. Although the pattern of Ag recognized by ASA from autoimmune, isoimmune, and vasectomized patients was similar, our results indicate that in addition to the recognition of common human sperm surface determinants, distinct sperm surface epitopes are also recognized as foreign by the patient's immune system. Sera that immunoprecipitated the predominant monomeric 29-kDa or the dimeric 58/56-kDa Ag when preincubated with capacitated sperm markedly inhibited sperm binding and penetration to the human zona when compared with ASA-negative serum controls. It is suggested that C-fixing ASA directed to these surface determinants may account for some of the clinical manifestations of ASA-mediated infertility.