The induction of cytotoxic T-lymphocyte precursor cells by recombinant vaccinia virus expressing human papillomavirus type 16 L1.

Expression of the major coat protein L1 of human papillomavirus type 16 by recombinant vaccinia viruses using a vaccinia late promoter and their use in generating antibodies have already been reported (Zhou et al., 1990). We have now constructed recombinant vaccinia viruses (VVs) which express the L1 protein from an early promoter with the intention of inducing cell-mediated immunity. This necessitated the removal of sequence motifs (TTTTTNT) from the L1 gene which would otherwise have caused transcription termination when expressed from a vaccinia virus early promoter. The nucleotide sequence was mutated to retain the correct amino acid sequence of the L1 protein. Full-length mRNA and L1 protein were generated in cells infected with the recombinant virus containing the mutant sequence, whereas the wild-type sequence generated only truncated mRNA and no detectable protein. Mice were immunized with VV expressing L1 from the mutant sequence and from the wild-type sequence in constructs with either early or late vaccinia virus promoters. Only the early promoter constructs were effective in priming cytotoxic T lymphocytes (CTL). Moreover the mutant sequence was significantly more effective than the wild-type sequence. The same L1 sequences, expressed from a vaccinia virus late promoter or coexpressed with MHC Class I molecules also expressed from a late promoter, produced high levels of L1 protein in both cases but nevertheless failed to elicit CTL activity. This is the first report of an HPV-specific CTL response and we have reaffirmed the importance of choosing the correct promoter and sequence expressed when using recombinant vaccinia viruses to induce cytotoxic T lymphocytes. These data are relevant for the design of vaccines to generate cell-mediated immunity against human papillomavirus infection.