Development of a highly immunogenic recombinant candidate vaccine against human chorionic gonadotropin.

Human chorionic gonadotropin (hCG) is synthesized soon after fertilization and is essential for embryonic implantation. A vaccine targeting hCG would be an ideal choice for immuno-contraception; an anti-hCG vaccine developed by Talwar et al., has previously undergone Phase II efficacy trials, providing proof of principle. These trials established the threshold levels of bio-neutralizing anti-hCG antibody titers required to prevent pregnancy; however, these titers (>50 ng/ml) were achieved in only 80% of immunized women. In this communication, we report a novel recombinant anti-hCG vaccine which demonstrates improved immunogenicity. hCGβ was genetically fused at C-terminal to the B-subunit of E. coli heat-labile enterotoxin. The recombinant fusion protein (hCGβ-LTB) was expressed in Pichia pastoris and, upon adsorption on Alhydrogel along with Mycobacterium indicus pranii (MIP) as an immuno-modulator, evoked a very high anti-hCG immune response in 100% of immunized BALB/c mice. This recombinant vaccine is expected to reduce cost as well as facilitate production of a molecularly consistent conjugate on a large scale.