Functional Characterization of the Primate Sperm Acrosomal Antigen (PSA‐63)

Abstract

ABSTRACT: Monoclonal antibody (HS‐63) raised in mice against human ejaculated sperm, polyclonal antibodies raised in rabbits against the cognate mouse testicular antigen (MSA‐63; or Fab) and polyclonal antibodies raised in the rabbit against recombinant fusion proteins (GST‐63) showed acrosomal localization in permeabilized rhesus monkey and human ejaculated sperm. Tail localization of the cognate prima+te sperm antigen (PSA‐63) was also seen with intact MSA‐63 antibodies and Fab fragments. The ability of these antibodies to inhibit sperm binding to the zona pellucida was measured with hemizona binding assays (HZAs). HS‐63 (1.2 mg/ml) inhibited rhesus monkey sperm binding (mean ± SEM) to homologous hemizonae (treatment, 15.5 ± 3.3; control, 58.9 ± 9.4; P < 0.025), whereas comparable concentrations of protein from nonimmunized mouse preparations were inactive (ascites fluid, 67.6 ± 43.5; no ascites fluid, 72.0 ± 44.6). Intact MSA‐63 antibodies inhibited (up to 99%) monkey sperm‐zona binding in a concentration‐dependent manner. Moreover, inhibition in this case by intact MSA‐63 antibody was limited to capacitated sperm. Similarly, intact MSA‐63 antibodies inhibited (up to 85%) human sperm binding to homologous zonae in an antibody concentration‐dependent manner. Fab fragments derived from MSA‐63, when present in insemination mixtures (0.5 mg/ml), inhibited (P < 0.01) primate sperm binding to homologous hemizonae (monkey, 9.6 ± 3; human sperm 9.4 ± 2) compared with matched hemizona controls (monkey, 117 ± 29; human, 20.4 ± 3). Furthermore, rhesus monkey sperm‐zona binding was reduced by 84% in the presence of rabbit anti‐GST‐63 antibodies. Purified rabbit immunoglobulins, used as controls, did not influence sperm‐hemizonae binding and sperm agglutination did not contribute to the antibody effect. These results suggest that an accessible pool of PSA‐63 is important for an initial step in mammalian fertilization and this antigen is a legitimate candidate for an antifertility vaccine.