Maximizing CD8+ T cell responses elicited by peptide vaccines containing CpG oligodeoxynucleotides

We assessed the ability of several factors to increase the size of tumor-antigen-specific CD8+ T cell responses elicited by vaccines incorporating peptides and CpG-containing oligodeoxynucleotides (CpG). Neither granulocyte-macrophage colony-stimulating factor (GM-CSF) nor an immunogenic MHC class II-presented “helper” peptide increased the size of epitope-specific CD8+ T cell responses elicited by peptide + CpG-containing vaccines. In contrast, low-dose subcutaneous interleukin (IL)-2 dramatically increased the size of splenic and peripheral blood epitope-specific CD8+ T cell responses generated by peptide + CpG-containing vaccines. Moreover, peptide + CpG-containing vaccines plus low-dose IL-2 mediated anti-tumor immunity. A prime-boost vaccination schedule elicited larger CD8+ T cell responses than a weekly vaccination schedule. Including larger doses of peptide in vaccines led to larger vaccine-elicited CD8+ T cell responses. Clinical trials of CpG-containing peptide vaccines are ongoing. These findings suggest strategies to increase the size of CD8+ T cell responses generated by CpG-containing peptide vaccines that could be tested in future clinical trials.