Gene fusion of molecular adjuvant C3d to hCGβ enhances the anti-hCGβ antibody response in DNA immunization

Abstract Objective: To express the hCGβ-C3d3 fusion protein in a CHO cell continual expression system to investigate further the adjuvant effects of C3d on contraceptive vaccination. Method: We constructed a plasmid pcDNA3-hCGβ-C3d3 which contains three copies of murine C3d cDNA and the hCGβ gene by cloning the chimerical hCGβ-C3d3 cDNA into the eukaryotic vector pcDNA3 downstream of the CMV promoter. The plasmid was transfected into a COS-7 cell transient expression system and a CHO cell continual expression system. RIA was used to detect hCGβ in the culture supernatant. Western blot and Raji cell immunohistochemical assays were performed to evaluate the expressed protein. Then, 6–8-week-old female BALB/c mice were inoculated intramuscularly with pcDNA3-hCGβ and pcDNA3-hCGβ-C3d3, and ELISA was used to assess anti-hCGβ IgG antibody in serum. Results: In 72 h after COS-7 cells were transfected with the plasmid pcDNA3-hCGβ-C3d3, 1.0×10 5 cells could secrete 152 ng of the recombinant protein (calculated by hCGβ contained). The transfected CHO cells, which were then screened by G418, could continuously secrete the fusion protein at 660 ng/10 6 cells/48 h. The hCGβ-C3d3 protein was purified by anti-hCGβ immunoaffinity chromatography. Raji cell immunohistochemical assay demonstrated that both the hCGβ and C3d3 were successfully fused. After DNA immunization intramuscularly, the anti-hCGβ IgG antibody titer in the pcDNA3-hCGβ-C3d3 immunized group was 243-fold higher than that of the pcDNA3-hCGβ immunized group. Conclusion: We have expressed the hCGβ-C3d3 protein successfully, both in a transient expression system (COS-7 cells) and in a stable expression system (CHO cells). The C3d3 molecular adjuvant can enhance significantly the immunogenecity of hCGβ antigen in DNA immunization.