Immunogenicity and contraceptive potential of recombinant human sperm associated antigen (SPAG9)

Human sperm-associated antigen 9 (hSPAG9) is a potential target for sperm-based contraceptive vaccine in lieu of its location on the sperm acrosomal compartment and its implication in sperm-egg interaction. SPAG9 is an acrosomal molecule which is not only restricted to a specific region (domain) of the acrosome but also undergoes relocation to the equatorial region in a stage-specific manner during acrosome reaction, demonstrating its potential role in sperm-egg binding. Human SPAG9 nucleotide sequence revealed 94% identity with macaque SPAG9 and 96.8% with baboon SPAG9 over the entire sequence. The amino acid sequence comparison of human SPAG9 with macaque and baboon revealed an overall homology of 84.9% and 90.6%, respectively. The presence of a high level of homology at the amino acid and nucleotide levels indicates that SPAG9 is conserved in macaque, baboon and human sharing common function and common origin in the biological past. Immunogenicity studies were carried in rats, which demonstrated that recombinant hSPAG9 protein adsorbed on alum is highly immunogenic. Antibodies thus generated after immunization reacted with recombinant human SPAG9 (rhSPAG9) and native SPAG9 protein from human sperm in Western blot analysis. In an in vitro assay, anti-rhSPAG9 antibodies inhibited sperm adherence to or penetration in zona-free hamster egg penetration test. Further, anti-SPAG9 antibodies inhibited the binding of human sperm to intact human oocyte as well as to matched hemi-zonae, indicating that the recombinant protein is a suitable contraceptive vaccinogen. Together these results demonstrate that the rhSPAG9 adsorbed on alum is immunogenic in nature, which is a permissible adjuvant for immunogenicity and fertility trials in non-human primates.