DNA immunization with plasmids expressing hCGβ-chimeras

Abstract

Human chorionic gonadotropin has been used as an anti-fertility vaccine and as a target for cancer immunotherapy. We have explored the use of DNA immunization with the aim of improving the immunogenicity of this hormone. Stimulating the muscle with electric pulses following intramuscular injection of plasmids expressing hCGβ resulted in higher levels of human chorionic gonadotropin (hCG)-specific antibodies, which could be further enhanced following a protein boost with hCG mixed with adjuvant. DNA vaccines encoding a membrane attached or a secreted form of hCGβ produced similar—albeit relatively modest—antibody responses. Providing hCGβ with additional T cell help by vaccinating with a plasmid encoding a hCGβ-hFc fusion protein did not further increase the antibody levels in the immunized animals. However, immunization of mice with a construct encoding hCGβ fused to C3d 3 produced significantly lower antibody levels relative to mice immunized with the hCGβ-alone expression plasmid, even though the hCGβ-C3d 3 chimera was expected to facilitate cross-linking of the antigen-specific B-cell receptor and CR2 thereby lowering the threshold of activation. Thus the limiting factor determining the antibody levels following hCGβ immunization, at least for DNA immunization, is related to the amount of protein available rather than the form of protein produced or lack of T cell epitopes.