Prospects of zona pellucida glycoproteins as immunogens for contraceptive vaccine

The zona pellucida (ZP) surrounding a mammalian oocyte mediates the initial recognition and binding of spermatozoon to oocyte in a relatively species-specific manner and plays an important role in the subsequent activation events during the fertilization process. The ZP comprises three biochemically and immunologically distinct glycoproteins termed ZP1, ZP2 and ZP3. The critical role of ZP glycoproteins in reproduction together with their tissue-specific nature have led to their being considered as potential candidate antigens for immunocontraception. Immunization of females with ZP glycoproteins leads to a block of fertility in several animal models. However, it is invariably associated with either a transient or an irreversible alteration in the cyclicity, hormonal profile and follicular development in the ovary. To overcome these problems, attempts are being made to delineate relevant 'B' cell epitopes on ZP proteins so as to design immunocontraceptive vaccines based on synthetic peptides devoid of oophoritogenic 'T' cell epitopes. Monoclonal antibodies capable of inhibiting the gamete interaction are being employed to delineate such regions. Additionally, DNA-recombinant technology has made it feasible to obtain, in reasonably large quantities, the ZP glycoproteins from human and non-human primates. Availability of sequence information of these zona proteins and the availability of recombinant antigens (devoid of other ovarian-associated proteins) will further help in understanding more precisely their functions during fertilization and make it feasible to undertake immunization studies to determine their prospects as immunogens for fertility regulation.