Identification and characterization of a 27 kDa acrosome protein of human sperm defined by a monoclonal antibody with fertilization-blocking effect.

A monoclonal antibody, MAb 1G1, possessing a strong fertilization-blocking activity was prepared by immunizing a BALB/c mouse with capacitated human sperm in order to study the molecular nature of sperm antigens relevant to fertilization. MAb 1G1 inhibited human sperm penetration into zona-free hamster eggs. It reacted to the apical portion of acrosome-reacted human sperm, but did not react to live sperm before the acrosome reaction as demonstrated by immunofluorescence staining. In paraffin-embedded testis sections, the round spermatids, spermatocytes and spermatozoa were stained with MAb 1G1, but the spermatogonia were not stained. Neither Sertoli cells, Leydig cells nor other somatic tissues were stained. The sperm of Japanese monkey, bull, boar, hamster and mouse were not stained. Therefore, the staining of sperm was species specific. The antigen corresponding to MAb 1G1 showed a band at 27 kDa by immunoblotting. The reactivity of the antigenic component was not destroyed by periodic acid treatment. From the results obtained, it was postulated that the antigenic molecule might be a polypeptide. These results indicated that this MAb might be a useful tool for studying the mechanism of human sperm-egg fertilization and the development of a contraceptive vaccine.