Roles of phagocytosis activating protein (PAP) in Aeromonas hydrophila infected Cyprinus carpio

• PAP gene was highly expressed in the Cyprinus carpio that were injected by inactivated Aeromonas hydrophila.
• PAP-phMGFP DNA immunostimulant increased the phagocytic index and survival rate of A. hydrophila Infected fish.
• A chitosan-PAP-phMGFP nanoparticle was produced as a feed for fish.
• A chitosan-PAP-phMGFP nanoparticle can be an immunostimulant in fish for A. hydrophila infection.

Cyprinus carpio (koi) is one of the most popular ornamental fish. A major problem for C. carpio farming is bacterial infections especially by Aeromonas hydrophila. Previously studies had shown that the Phagocytosis Activating Protein (PAP) gene was involved in the innate immune response of animals. Therefore, we attempted to identify a role for the PAP gene in the immunology of C. carpio. The expression of the PAP was found in C. carpio whole blood and increased when the fish were stimulated by inactivated A. hydrophila. In addition, PAP-phMGFP DNA was injected as an immunostimulant. The survival rate and the phagocytic index were significantly increased in the A. hydrophila infected fish that received the PAP-phMGFP DNA immunostimulant. A chitosan-PAP-phMGFP nanoparticle was then developed and feeded into fish which infected with A. hydrophila. These fish had a significantly lower mortality rate than the control. Therefore, this research confirmed a key role for PAP in protection fish from bacterial infection and the chitosan-PAP-phMGFP nanoparticle could be a good prototype for fish immunostimulant in the future.