The antimicrobial peptide MetchnikowinII enhances Ptfa antigen immune responses against avian Pasteurella multocida in chickens.

Immunostimulants and vaccines are the main means for controlling infectious diseases and searching highly effective and low toxic immunestimulants has always been the focus of researchers. The MetchnikowinII (MetII) had been expressed by us and exhibited both antibacterial and antifungal activities, in this study, we evaluated its potential for an adjuvant effect. In chickens, antigen-specific immunoglobulin Gs (IgGs) were increased after MetII adjuvanted vaccination using the Ptfa protein. Compared to group Ptfa + iFA, which was only adjuvanted with incomplete Freund's adjuvant (iFA), the antibody titers of the group Ptfa + iFA + Met20 µg·mL-1 (PFM20) and Ptfa + iFA + Propolis (PFP) significantly increased (P<0.05). Likewise, Interleukin-2 (IL-2) and Interferon-γ (IFN-γ) cytokines in group Ptfa + iFA + Met20 µg·mL-1 (PFM20) and Ptfa + iFA + Propolis (PFP) were significantly higher than those of the other three experimental groups (P<0.05). The stimulation index (SI) value in chickens of group PFM20 was significantly higher than that of the other four experimental groups (P<0.05). Chickens that received MetII adjuvanted vaccinations benefitted from higher protection rate (88%) when challenged with Pasteurella multocida (P. multocida), which was significantly higher than those of group PF and PFP (P<0.05). These results suggested that the antimicrobial peptide MetII may play an adjuvant role in the immune response in chickens but need a proper usage, because the higher usage of 40 µg·mL-1 and 60 µg·mL-1 resulted poor effect. Whether MetII could be a potential adjuvant or a biomolecule as part of a complex adjuvant for vaccines needs more experimental evidence, the study still provides an examples for understanding vaccine adjuvants.