Three CpG oligodeoxynucleotide classes differentially enhance antigen-specific humoral and cellular immune responses...

Synthetic oligodeoxynucleotides containing unmethylated CpG-dinucleotides (CpG-ODNs) are immunostimulatory in a broad spectrum of species. Extensive studies provide evidence that CpG-ODNs are effective as immunotherapeutics and vaccine adjuvants in various clinical settings. Three major classes of immunostimulatory CpG-ODNs are well characterized according to their in vitro activities and chemical compositions. However, it remains largely unclear whether and how these differences translate in vivo and in particular when used as vaccine adjuvants. In the present study, a panel of CpG-ODNs, including four representative sequences respectively from each class, was used to characterize their adjuvant activities in mice. The results demonstrated that three CpG-ODN classes can differentially affect antigen-specific humoral and cellular immune responses. Specifically, the B- and C-class CpG-ODNs induce a potent Th1-biased immunity with comparable antibody levels as well as CD4⁺ and CD8⁺ T cell responses. In contrast, although the A-class CpG-ODNs can weakly enhance antibody titers and CD8⁺ T cell response regarding cytotoxic activity, they are not able to change the IgG1/IgG2a ratio or elicit antigen-specific, IFN-γ-secreting CD4⁺ and CD8⁺ T cells. Consistent with this, three CpG-ODN classes provide differential antigen-specific protection against Listeria monocytogenes, an intracellular bacterial infection. In conclusion, our study provides not only better knowledge about the adjuvant activities of three CpG-ODN classes but also implications for the rational design of CpG-ODN adjuvants.