A TLR9 agonist enhances the anti-tumor immunity of peptide and lipopeptide vaccines via different mechanisms

The toll-like receptor 9 (TLR9) agonists CpG oligodeoxynucleotides (CpG ODNs) have been recognized as promising adjuvants for vaccines against infectious diseases and cancer. However, the role of TLR9 signaling in the regulation of antigen uptake and presentation is not well understood. Therefore, to investigate the effects of TLR9 signaling, this study used synthetic peptides (IDG) and lipopeptides (lipoIDG), which are internalized by dendritic cells (DCs) via endocytosis-dependent and endocytosis-independent pathways, respectively. Our data demonstrated that the internalization of lipoIDG and IDG by bone marrow-derived dendritic cells (BMDCs) was not enhanced in the presence of CpG ODNs; however, CpG ODNs prolonged the co-localization of IDG with CpG ODNs in early endosomes. Surprisingly, CpG ODNs enhanced CD8(+) T cell responses, and the anti-tumor effects of IDG immunization were stronger than those of lipoIDG immunization. LipoIDG admixed with CpG ODNs induced low levels of CD8(+) T cells and partially inhibit tumor growth. Our findings suggest that CpG ODNs increase the retention of antigens in early endosomes, which is important for eliciting anti-tumor immunity. These results will facilitate the application of CpG adjuvants in the design of different vaccines.