Purification and immunogenicity study of human papillomavirus type 16 L1 protein in Saccharomyces cerevisiae

Human papillomavirus 16 virus-like particle (HPV16 VLP) vaccines expressed in Saccharomyces cerevisiae are under Phase III trial and are expected to be on the market in the near future. We have established a convenient and economical system for the prophylactic study of vaccines derived from HPV16 VLPs, and neutralization tests to standardize HPV serological methodology as a measure of validation. To purify HPV16 VLPs, yeast cells expressing HPV16 L1 protein were cultured and purified on a small scale by ultracentrifugation and size-exclusion and cation-exchange chromatography using open columns. The highly purified HPV16 L1 protein was identified by SDS-PAGE and Western blotting, and electron microscopic analysis confirmed that they self-assembled into VLPs. To test the efficacy of the purified VLPs as a vaccine and their ability to induce humoral immunity, we performed ELISA assays and observed a significant increase in the titer of anti-HPV16 VLPs antibodies in the sera of immunized mice. High anti-HPV16 neutralizing titers were found in the sera of vaccinated mice, as measured by a SEAP-based pseudovirus neutralization assay. These results would be useful in the evaluation of the immunogenicity of HPV vaccine candidates, and provide an international reference standard for HPV serological methods.