Effects Of Different Adjuvants On CTL Immune Response Induced By OVA

CD8~+ CTL response plays a critical role in immunotherapies against tumors and chronic infections. DNA vaccines and live vector vaccines were conventionally used to induce cellular immunity through the classical endogenous MHCⅠclass antigen-presenting pathway. Exogenous antigens, like proteins and peptides, can induce CD8~+ CTL response by means of cross-presentation and cross-priming. However, effective vaccine adjuvants are needed to enhance antigen presentation and co-stimulate signals as well as provide the Th1 type cytokine milleu. In the present study, CpG-ODN alone, or in combination with Al(OH)3 or Montanide ISA 720, were evaluated for their capacity to promoting CD8~+ CTL response through cross-presentation and cross-priming of protein antigens. This study would provide implications for the rational design of novel therapeutic vaccines based on protein and peptide antigens.Firstly, C57BL/6 mice were immunized intramuscularly with chicken ovalbumin (OVA) as a model antigen, and CpG-ODN, Al(OH)3, Montanide ISA 720, CpG-ODN + Al (OH)3 or CpG-ODN + Montanide ISA 720 as adjuvants, respectively. The cellular immune responses were evaluated by intracellular cytokine staining assay and in vivo CTL killing assay. On this basis, in combination with Al(OH )3 as composite adjuvants, three CpG-ODN classes (A-, B- and C-class) with different chemical compositions and biological activities were further compared for their immuno-potentiating effects on cellular immune responses against OVA antigen.The results showed that after two immunizations, compared with the control group without adjuvant, Al(OH)3 alone was not able to effectively induce cellular immunity. In contrast, CpG-ODN or Montanide ISA 720 alone enhanced antigen-specific CD8~+ T cell response (as indicated by IFN-γsecretion and CTL activity) to some extent. However, they were not able to enhance antigen-specific Th1 type CD4~+ T cell response. Both composite adjuvants showed more stronger adjuvant effects, in which CpG-ODN + Montanide ISA 720 could only enhance IFN-γ-secreting CD4~+ and CD8~+ T cell responses, while CpG-ODN + Al (OH) 3 could not only enhance IFN-γsecretion of CD4~+ and CD8~+ T, but also promote the CD8 + CTL responses.Of three different classes of CpG-ODN, the B- and C-class sequences exhibited similar adjuvant effects on cellular immune responses, not only promoting IFN-γsecretion by antigen-specific CD4~+ and CD8~+ T cells, but also inducing antigen-specific CD8~+ CTL response. In contrast, although the A-class CpG-ODN could weakly enhance antigen-specific CD8~+ T cell response regarding cytotoxic activity, they were not able to elicit antigen-specific, IFN-γ-secreting CD4~+ and CD8~+ T cells. Consistent with this, B- and C-class CpG-ODN also provided better antigen-specific immune protection against Listeria monocytogenes challenge.In conclusion, either B- or C-class CpG-ODN, in combination with Al(OH)3 as composite adjuvants, showed the strongest ability to promote CD8~+ CTL response in mice via cross-presentation and cross-priming of protein antigens.