Immunogenicity of zona pellucida vaccines

Development of zona pellucida (ZP) based contraceptive vaccines raises a number of complications that challenge current immunological capabilities. Our research examines two aspects of these immunological problems. First, recent studies demonstrate that one bacterially expressed rabbit ZP recombinant vaccine (rec55) induces autoantibodies in primates that prevent sperm-ZP binding and induction of the acrosome reaction in the homologous ZP in vitro. Immunization with rec55 does not induce ovarian pathology, and the duration of antibody titres indicates that this vaccine would have reversible effects on fertility. However, the immunogenicity of the rec55 protein produced in the pEX bacterial expression vector is low. We have therefore expressed the cDNA encoding rec55 using the pGEX vector for the following reasons: (i) the pGEX expressed recombinant proteins are soluble in aqueous solution; (ii) affinity purification of recombinant proteins on glutathione Sepharose columns is more effective for obtaining larger quantities of purified protein; and (iii) the availability of protease cleavage sites between the ZP and glutathione S transferase fusion proteins should eliminate the possibility of carrier-mediated suppression of immune responses. These improvements in protein production and purification yield immunogen which is more malleable to immunological studies. Second, while it is clear that ZP recombinant vaccines can eliminate the problem of ovarian pathology, it is important to understand how such pathology results from immunization with native ZP proteins and certain recombinant ZP proteins. To this end, we have initiated immunization studies in baboons comparing rabbit ZP and pig ZP immunogens in Titremax adjuvant. Unilateral ovariectomies at different time points after immunization (1.5, 2, 4 and 6 months) will allow studies on the time course of pathology within the ovary. Comparison of these results with ongoing rabbit ZP immunizations will help elucidate the differences in immunogenicity of ZP proteins isolated from the two species.