Abstract:[Background] Riemerella anatipestifer (RA), a major pathogen that causes losses in the poultry industry, can cause serositis and septicemia in ducks and other birds. The available vaccines generally have weak reaction and low immunogenicity and induce low antibody levels. [Objective] To explore the immune response and protective effect of CpG adjuvant and provide evidence for the prevention of duck serositis with CpG adjuvant. [Methods] The proliferation test of lymphocytes in duck peripheral blood and real-time fluorescence quantitative PCR were employed to determine the optimal sequence. The recombinant plasmid pUC18-CpG was constructed and compared with CpG ODN for immunogenicity. Ducklings were immunized via subcutaneous injection twice with inactivated RA-GH5 as the antigen and pUC18-CpG as the adjuvant, and then the serum antibody titer and cytokine level were determined. The ducklings were then challenged by RA-GH5 strain through intramuscular injection, and the histopathological changes and immune protection rate were examined. [Results] The optimal sequence CpG-3 was screened out. The immunostimulatory activity of pUC18-CpG containing CpG-3 was not significantly different from that of CpG-3. Compared with RA vaccine alone, the addition of pUC18-CpG significantly increased the serum antibody titer and the levels of interleukin-2 (IL-2) and IL-4. The immune protection rates of 80 μg, 20 μg, and 40 μg pUC18-CpG to the ducklings were 70%, 60%, and 60%, respectively. [Conclusion] The combination of pUC18-CpG and RA-GH5 inactivated vaccine can induce stronger humoral and cellular immune responses.
