Immunization of a novel outer membrane protein from Aeromonas hydrophila simultaneously resisting A. hydrophila and Edwardsiella anguillarum infection in European eels (Angullia angullia)

In cultivated European eels, Aeromonas hydrophila, Edwardsiella anguillarum and Vibrio vulnificus are three important bacterial pathogens. In this study, an expressed recombinant Outer membrane proteinII (rOmpII) from A. hydrophila was intraperitoneally injected into European eels (Anguilla angullia). All examined eels were equally divided into three groups. One group was injected with PBS only (PBS group), one group was injected with 1:1 mixture of PBS and Freund's incomplete adjuvant (PBS + F, adjuvant group), and the third group was injected with 1:1 mixture of 1 mg mL(-1) rOmpII and Freund's incomplete adjuvant (rOmpII + F, OmpII group). The immunogenicity of OmpII was studied by detecting the expression of 4 immune-related genes, stimulation index (SI) of the whole blood cell, serum antibody titer, lysozyme and Superoxide Dismutase (SOD) activity, and relative percent of survival (RPS) rate. The results showed that gene expression of MHC-II, LysC, SOD and IgM in the OmpII group significantly increased in liver, spleen, kidney and intestine. At 28 days post the immunization (dpi), the SI of whole blood cells in the OmpII group increased significantly; at 14, 21, 28 and 42 dpi, the serum antibody titers against A. hydrophila and E. anguillarum in the OmpII group were significantly higher than that of the PBS and the adjuvant group; the SOD in the OmpII group was found increased significantly in liver, kidney, mucus and serum. On the 28 dpi, eels were challenged by A. hydrophila, E. anguillarum and V. vulnificus for cross protection study. The results showed that the RPS of the OmpII group were 83.33%, 55.56% and 33.33% respectively. These results showed that the expressed OmpII from A. hydrophila significantly improve the immune function of Europena eels and their resistance to the infection of A. hydrophila and E. anguillarum simultaneously.