Immune protection and safety of Aeromonas veronii fliD deletion mutant as a live attenuated vaccine in Cyprinus carpio

Aeromonas veronii is one of the important pathogens causing motile Aeromonas septicemia (MAS) in aquaculture, leading to high fish mortality and severe economic losses. Live attenuated vaccines are considered to be an effective method to control bacterial infections and have been widely used in veterinary vaccines. In order to develop a safe and effective vaccine against MAS, the safety and protective efficacy of the fliD gene deletion strain of A. veronii as a live attenuated vaccine for Cyprinus carpio were evaluated in this study. After intraperitoneal injection vaccination, the A. veronii & UDelta;fliD was able to colonize temporarily in the head kidney and spleen, and was finally cleared by C. carpio without causing clinical symptoms and affecting the growth performance of C. carpio. The significantly increased IgM levels in the serum indicated that both intraperitoneal injection and immersion vaccination induced a significant humoral immune response. The pro-inflammatory factors IL-1 & beta;, TNF-& alpha; and IL-6 were significantly increased at the early stage after intraperitoneal injection vaccination, indicating that A. veronii & UDelta;fliD induced an inflammatory response in C. carpio, which facilitated bacterial clearance. In addition, a significant proliferation of head kidney lymphocytes was also observed after stimulation with A. veronii soluble antigen. The survival rates of C. carpio vaccinated by intraperitoneal injection and immersion were 83.33% and 56.67% after intraperitoneal challenge with wild-type A. veronii, compared with 13.33% and 16.67% in the control group. The histopathological examination showed that A. veronii & UDelta;fliD could protect the liver and spleen against tissue lesion after challenge. These results indicate that attenuated strain A. veronii & UDelta;fliD can induce an effective humoral and cellular immune response and provide protection against A. veronii for C. carpio without adverse effects, which can be developed as an effective live attenuated vaccine candidate to control the A. veronii infection.