Native zona pellucida reactivity and in-vitro effect on human sperm-egg binding with antisera against bonnet monkey ZP1 and ZP3 synthetic peptides

In mammals, zona pellucida glycoprotein-3 (ZP3) is the putative ligand for primary sperm binding and induces the acrosome reaction. Recent evidence suggests that zona pellucida glycoprotein-1 (ZP1) also play an important role, in some species, during fertilization. In order to identify synthetic peptide immunogens capable of inducing antibodies reactive with native zona and inhibiting sperm-oocyte interaction, peptide encompassing the amino acid (aa) residues 334-343 of bonnet monkey ZP3 (bmZP3) was synthesized co-linearly with a 'promiscuous' T-cell epitope of circumsporozoite protein (CSP, 378-398 aa) of Plasmodium falciparum. In addition, four peptides corresponding to bonnet monkey ZP1 (bmZP1((58-79 aa)), bmZP1((136-153 aa)), bmZP1((212-228 aa)) and bmZP1((251-273 aa))) were synthesized. The synthetic peptides corresponding to bmZP1 were conjugated with diphtheria toxoid. Immunization of female BALB/cJ mice with the above conjugates and CSP-bmZP3((334-343 aa)) peptide led to the generation of an adequate antibody response against the respective zona peptide. Antibodies against bmZP1((251-273 aa)) and CSP-bmZP3((334-343 aa)) recognized bonnet monkey and human zona pellucida in an indirect immunofluorescence assay. Further, these antibodies when tested independently or in combination also significantly inhibited the binding of human spermatozoa to zona pellucida in a hemizona assay. These studies will further help in the design of synthetic peptide immunogens comprising of multiple B cell epitope from different zona proteins for better immunocontraceptive efficacy.