Influence of autoimmune ovarian disease pathogenesis on ZP3 contraceptive vaccine design

Zona pellucida (ZP) glycoproteins possess sperm receptor-binding activities. Antibodies against ZP can block sperm-egg interaction and thereby prevent fertilization. The feasibility of developing a safe contraceptive vaccine based on the ZP has been hampered by the finding that active immunization with autologous or heterologous ZP proteins results in infertility that is associated with ovarian dysfunction. A mouse model was used to investigate mechanisms of the ovarian pathology that is induced by active immunization with a 13mer peptide derived from mouse ZP3 (mZP3(330-342)). This peptide includes one native B-cell epitope and two nested T-cell epitopes. Ovarian pathology could be transferred into naive recipients by CD4+ T cells, but not by antibodies, from immunized mice, suggesting the importance of T cells in the mechanism of ovarian pathogenesis. Moreover, immune responses, as well as disease induction, were restricted to H-2a,k,u,s,axb haplotypes. On the basis of this mouse model, a strategy to generate a contraceptive anti-ZP antibody response without a pathogenic T-cell response, irrespective of H-2 haplotype, is described. The B-cell epitope was modified by amino acid substitution to eliminate the overlapping oophoritogenic T-cell epitope, and was linked to a promiscuous foreign T-cell epitope, bovine RNase94-104. The resultant chimaeric peptide (CP2) induced anti-ZP antibodies in 100% of the eight strains of inbred mice with different H-2 haplotypes without significant disease induction. An antifertility trial in B6AF1 female mice immunized with CP2 showed that the anti-ZP antibody was associated with a reduction in fertility. This infertility was reversed with a decline in anti-ZP antibody titre. Preliminary data show that this strategy of vaccine design may also be applied to primates.