Electron microscopic immunolocalization of a conserved sperm acrosomal antigen recognized by HS-63 monoclonal...

HS-63 monoclonal antibody was shown to react with a sperm-specific acrosomal antigen from spermatozoa of a variety of mammalian species. Indirect immunofluorescence showed that HS-63 was associated with acrosome-intact sperm after sperm had been fixed with methanol. Electron microscopy (EM) was employed to determine the ultrastructural localization of this sperm antigen. When the immunogold labelled goat anti-mouse IgG was used as a probe, we demonstrated that HS-63 monoclonal antibody did not bind to the freshly prepared human spermatozoa. However, gold particles were observed in the intra-acrosomal region, when the spermatozoa had been pre-treated with 0.5% Triton X-100 prior to incubation with HS-63. We further observed that the immunogold did not stain the inner acrosome membrane when the spermatozoa became acrosome-reacted. A good correlation was obtained between the percentage of spermatozoa which did not react with HS-63 as determined by indirect immunofluorescence assay and that of acrosome-reacted spermatozoa as quantitated by electron microscopy. The results of this EM study were consistent with those obtained by indirect immunofluorescence assay and both indicated that HS-63 reacts only with the capacitated and acrosome-intact spermatozoa. Therefore, HS-63 monoclonal antibody is a useful probe for rapid evaluation of acrosomal status in human spermatozoa.