Immune response to human papillomavirus 16 L1E7 chimeric virus-like particles: induction of cytotoxic T cells and specific tumor protection.

Expression of human papillomavirus type 16 (HPV 16) fusion proteins LI deltaCE7(1-55) and LI deltaCE7(1-60) (carboxy-terminal deletion of LI replaced by 55 or 60 amino-terminal amino acids of E7) leads to formation of chimeric papillomavirus-like particles (CVLPs). After "infection" of cells by CVLPs, the chimeric proteins can be detected in the cytosol and the endoplasmic reticulum (ER), suggesting that they are intracellularly processed via the MHC class I pathway and, therefore, able to activate cytotoxic T lymphocytes (CTLs). To investigate the cytotoxic immune response against HPV 16 LI deltaCE7(1-60) and LI deltaCE7(1-55) CVLPs, we immunized C57Bl/6 mice with various CVLP doses without adjuvant. Two weeks after immunization, spleen cells were prepared and stimulated in vitro using HPV 16 E7-expressing transfectants of the tumor cell line RMA. In 51Cr-release cytotoxicity assays, spleen cells of mice vaccinated with LI deltaCE7(1-60) CVLPs specifically lysed the RMA-E7 transfectants as well as RMA cells loaded with the peptide E7(49-57), which represents an H2-Db-restricted CTL epitope. This demonstrates that CVLPs induce an E7-specific CTL response in mice in the absence of an adjuvant. Furthermore, immunization with CVLPs prevented outgrowth of E7-expressing tumor cells even if inoculation of cells was performed 2 weeks before vaccination. We conclude from our data that CVLPs show promise for therapy of HPV-associated lesions.