Evaluation of an outer membrane protein as a vaccine against Edwardsiella anguillarum in Japanese eels (Anguilla japonica)

Edwardsiella anguillarum is a novel bacterial pathogen causing hemorrhagic septicemia in Japanese eels (Anguilla japonica) and results in significant economic loss to eels farming. Currently, there is no applied vaccine against E. anguillarum. In this study, the outer membrane protein A (OmpA) of E. anguillarum was expressed and purified, then Japanese eels (Anguilla japonica) were intraperitoneally injected (i.p) with bovine serum albumin (BSA group), formalin killed-whole-cell of E. anguillarum (FKC group) or the expressed OmpA of E. anguillarum (OMP group) respectively. On 14, 21, 28 and 42 d post immunization (dpi) respectively, the whole blood cells were collected to evaluate the stimulation index and bactericidal activity; the serum were obtained to assess the titers of specific antibody, lysozyme activity, complement activity and bactericidal activity; lysozyme activities in the suspension of kidney, skin mucus and liver in eels were ascertained. The results showed that the stimulation index of the whole blood cells in OMP group was significantly enhanced compared with the BSA group on 28 dpi; the titers of specific antibody in OMP group were significantly increased compared with FKC and BSA group on 28 d and 42 dpi respectively; lysozyme activity in serum of the OMP group was significantly higher than that of BSA group; complement activity of the OMP group was significantly higher than that of the BSA group; the OMP group showed strong bactericidal capacity on 21 and 28 dpi. Simultaneously, the relative percent survival (RPS) of Omp vs BSA group was 77.7% after eels were challenged by E. anguillarum on 28 dpi. These results showed that the expressed rOmpA was a good antigen of E. anguillarum and offered an alternative strategy for making applied aquaculture vaccines.