CpG DNA is a Potent Enhancer of Humoral and Cell-Mediated Immune Responses against Orientia tsutsugamushi in C3H/HeN Mice

Background: Synthetic oligodeoxynucleotide (ODNs) encoding CpG motifs mimic the activity of bacterial CpG motifs that trigger innate immune responses, and their use as an adjuvant has been demonstrated to significantly improve humoral immune response. Methods: CpG ODNs were evaluated for their immunostimulatory activities on mice vaccinated with recombinant outer membrane protein P56 (rP56Δ) of Orientia tsutsugamushi. Results: CpG ODNs significantly enhanced immune responses of mice against rP56Δ post three doses of immunization, with IgG titer ranging from 80,000 to more than 640,000. Furthermore, they also improved the immunization efficiency of rP56Δ by shortening the immunization schedule. Only two doses of immunization of rP56Δ plus CpG ODNs were needed to induce equally high antibody level as the three-dose immunization scheme of rP56Δ plus Freund's adjuvant. Isotypic analysis of antibodies revealed that rP56Δ plus CpG ODNs enhanced mice humoral immune responses as rP56Δ plus Freund's adjuvant did. IgG1, IgG2a, IgG2b and k light chain were induced significantly. Interestingly, rP56Δ stimulated spleen cells of vaccinated BALB/c mice to produce more IL-12 and gamma interferon than IL-4. In contrast, spleen cells stimulated with ConA, IL-4 was dominantly induced. Furthermore, CpG ODNs could cooperatively elicit memory immune responses in rP56Δ-immunized mice, as demonstrated by quick IgM induction boost shot. Conclusion: Results from this study revealed that rP56Δ could stimulate C3H/HeN mice to produce immune responses; and CpG ODN adjuvant enhanced its immunization effect and shortened the immunization time.