Immunological identification of sperm antigens that participate in fertilization

Using monoclonal antibodies (mAbs) as probes for specific components of the spermatozoon, we have initiated an investigation aimed at identifying those sperm components that participate in the events of gamete interaction. We intend to exploit these mAbs not only for identifying functional sperm components, but also for defining the constituents of discrete domains that comprise the structure of this highly differentiated cell. Among the large group of anti-sperm mAbs that we have generated, we have focused to date upon two categories. The first category consists of mAbs that localize to the acrosomal crescent, a restricted region of plasma membrane overlying the acrosome. Within this category, the mAbs share many similarities with regard to subclass, species and tissue cross-reactivity, and antigen solubility, in addition to cellular distribution. Nevertheless, despite these similarities, some mAbs in this category (e.g., M42) inhibit fertilization, whereas others (e.g., M41) are non-inhibitory. The block to fertilization observed in the presence of M42 is dependent upon the zona pellucida surrounding the egg. The specific event prevented by M42 appears to be the induction of the acrosome reaction at its physiological site, the surface of the zona pellucida. The sperm components recognized specifically by M42 are a cluster of high molecular weight moieties, ranging from approximately 220,000 to 240,000. The mAbs described in the second category display common localization at the equatorial segment of the sperm head. The pair of mAbs discussed from this category, M2 and M29, again bear considerable similarity to each other, yet differ significantly in their ability to inhibit fertilization. M2 does not inhibit, whereas M29 causes a marked inhibition of fertilization. With M29, however, the block to fertilization is independent of the zona pellucida. The M29 mAb interferes with sperm interaction with the egg plasma membrane subsequent to sperm attachment; since M29 does not prevent sperm binding to the egg plasma membrane, the specific event affacted, in all likelihood, is gamete membrane fusion. M29 recognizes a single sperm component, with subunit molecular weight of approx. 40,000. A variety of experiments are underway currently, both to characterize the antigens recognized by these mAbs further as well as to identify additional sperm components that participate in the fertilization process.