Generation of antitumor immunity by cytotoxic T lymphocyte epitope peptide vaccination, CpG-oligodeoxynucleotide adjuvant, and CTLA-4 blockade.

Although peptide immunization often leads to the induction of strong T-cell responses, it is seldom effective against established tumors. One possibility is that these T-cell responses are not strong enough or do not last sufficiently long to have an effect in tumor eradication. Here, we examined the role of synthetic oligodeoxynucleotide (ODN) adjuvants containing unmethylated cytosine-guanine motifs (CpG-ODN) and CTLA-4 blockade in enhancing the antitumor effectiveness of peptide vaccines intended to elicit CTL responses. The results show that combination immunotherapy consisting of vaccination with a synthetic peptide corresponding to an immunodominant CTL epitope derived from tyrosinase-related protein-2 administered with CpG-ODN adjuvant and followed by systemic injection of anti-CTLA-4 antibodies increased the survival of mice against the poorly immunogenic B16 melanoma. Interestingly, whereas this combination therapy was effective when administered to tumor-bearing mice (therapeutic protocol), it had no significant effect when applied in the prophylactic mode (i.e., before the tumor challenge). Moreover, the antitumor effect of the combination immunotherapy required the participation of CD4+ and CD8+ T lymphocytes and was accompanied by the induction of antitumor CD4+ T-cell responses. The overall results suggest that peptide vaccination of tumor-bearing mice, applied in combination with a strong adjuvant and CTLA-4 blockade, is capable of eliciting durable antitumor T cell responses that provide survival benefit. These findings bear clinical significance for the design of peptide-based therapeutic vaccines for human cancer patients.