Molecular biology approaches to evaluate species variation in immunogenicity and antigenicity of zona pellucida proteins

Immunocontraception using the glycoproteins of the mammalian zona pellucida (ZP) has held great promise because antibodies specific to ZP antigens would inhibit fertility and not be abortive. It has been shown, however, that some ZP proteins will elicit adverse effects since immunization may affect ovarian follicular development. These effects vary among different mammalian species as well as on the source of the ZP immunogen. Therefore, the use of molecular biology has been essential in identifying specific ZP protein(s) that inhibit fertility without altering ovarian follicular development and in defining the relationships of ZP proteins among different species. Use of recombinant ZP proteins has allowed us to begin to dissect antigenic domains of ZP proteins and to evaluate their potential roles in the fertilization process. Recent studies using recombinant rabbit ZP proteins to immunize cynomolgus monkeys (Maraca fascicularis) have shown that the 55 kDa ZP protein will elicit antibodies that inhibit sperm binding while not altering ovarian function, in contrast to immunization with a recombinant truncated protein of the 75 kDa ZP protein which causes ovarian dysgenesis. The rabbit 55 kDa protein is the homologue of the pig ZP3 alpha sperm receptor and the human ZPB protein but is distinct from the mouse ZP3 sperm receptor. Expression of this protein using the baculovirus expression system has further shown that the 55 kDa protein binds to capacitated rabbit spermatozoa over the acrosomal region and induces the acrosome reaction. Antibodies against this recombinant form of the ZP also inhibit rabbit spermatozoa from binding to rabbit egg in vitro. These studies demonstrate the need to determine the structure and function of ZP proteins of different mammalian species to evaluate their potential for contraceptive vaccines.