Cytotoxic T lymphocytes responding to low dose TRP2 antigen are induced against B16 melanoma by liposome-encapsulated TRP2 peptide and CpG DNA adjuvant.

The induction of a potent and specific T cell response is a major challenge in the development of efficacious cancer vaccine strategies. We applied a novel liposomal formulation (AVE3) for efficient delivery of antigenic peptides into APCs of the skin. These liposomes resulted in a long-lasting deposition of encapsulated compounds at the injection site and the draining lymph nodes. Using a peptide from the melanocyte differentiation antigen tyrosinase-related protein (TRP2) 2 we could show that vaccination with liposome-encapsulated peptide in combination with oligodeoxynucleotides containing unmethylated CpG motifs (CpG ODNs) as adjuvant leads to the induction of tumor cell-specific cytotoxic T cells. The most potent immune response was observed when both, TRP2 peptide and CpG ODNs, were encapsulated into AVE3. Importantly, in contrast to vaccination with free TRP2 liposomal TRP2 peptide generated T cells which respond to 1000-fold lower antigen concentration. Using the poorly immunogenic B16 melanoma model we could demonstrate that vaccination with liposomal TRP2 peptide plus CpG ODNs but not vaccination with free peptide or adjuvant alone resulted in tumor protection in subcutaneous and metastatic tumor models. In summary, vaccination with liposome-encapsulated peptide antigen and CpG ODN allows for the in vivo loading and activation of DC, thereby generating reactive CTL populations even against poorly immunogenic self-peptide presenting tumors resulting in a potent anti-tumor immune response.