Immune Responses and Antitumor Effect through Delivering to Antigen Presenting Cells by Optimized Conjugates Consisting of CpG-DNA and Antigenic Peptide.

Immunotherapy using antigen-specific cytotoxic T lymphocytes (CTLs) has become one of the most attractive strategies for cancer treatment. For the induction of antigen-specific CTLs in vivo, the co-delivery of CpG-DNAs and antigens to the same antigen-presenting cells (APCs) is a promising strategy. In this study, we prepared conjugates consisting of 40mer of CpG-DNA (CpG40) and antigenic peptide (OVA257-264), which have the following distinctive features: (1) multiple CpG motifs in a molecule; (2) cleavage in the cytosol because of the disulfide bonding via cysteine residue between peptide and CpG-DNA; (3) conjugation designed to induce antigen presentation on MHC class I molecules. Immunization with the conjugate CpG40-C-OVA257-264 at the mouse tail base induced strong CTL activity at a very low peptide dose of 20 ng/head. It was found that the conjugates were internalized into C-type mannose receptor 1 (MRC1)-expressing cells in inguinal lymph nodes, indicating that the CpG portion in the conjugate acts as not only an adjuvant for the activation of TLR9 but also a carrier to APCs expressing MRC1. In a tumor-bearing mice model, mice immunized with CpG40-C-OVA257-264 conjugates exhibited long delays in tumor growth compared with those treated with PBS, OVA257-264 alone, or a mixture of CpG40 and OVA257-264. Therefore, CpG-C-peptide conjugates could be a new and effective platform for peptide vaccine for the treatment of cancers and infectious diseases.