Examination of the immunocontraceptive potential of recombinant rabbit fertilin subunits in rabbit

Recombinant fertilin subunits produced in a bacterial expression systems were used to test fertilin as an immunocontraceptive antigen in the European rabbit (Oryctolagus cuniculus). Wild female rabbits (n = 40) were immunized with either recombinant rabbit fertilin alpha or beta subunits by the s.c. or intra-Peyer's patch route. High titers of serum anti-fertilin polyclonal IgG antibodies were achieved in all rabbits after repeated boosts, with fertilin-specific IgG but not IgA antibodies detected in vaginal lavages of all animals. The serum IgG antibodies recognized polypeptides in detergent extracts of rabbit sperm with relative molecular masses of 48, 53, and 85 kDa on reducing SDS-PAGE gels and were shown to bind to the head region of methanol-fixed and live caudal rabbit sperm. Preincubation of rabbit sperm with these anti-fertilin IgG antibodies at concentrations of 400 micrograms/ml blocked sperm binding to zona-intact oocytes and inhibited fertilization in vitro by 60-80%. However, despite the levels of circulating and vaginal IgG antibodies achieved, only 4 immunized does failed to become pregnant out of 33 that ovulated. The remaining animals either showed no effect on fertility (n = 29) relative to control animals or failed to ovulate (n = 7). All control animals ovulated and were either fully fertile (n = 15) or were mated to infertile males (n = 4). In addition, proven-fertile male domestic rabbits (n = 3) were immunized s.c. and boosted three times with fertilin beta. Only one animal subsequently showed impaired fertility. These results show that in the rabbit, high levels of circulating sperm-reactive anti-fertilin antibodies and the presence of vaginal IgG does not ensure infertility.