Immune effects of a bivalent expressed outer membrane protein to American eels (Anguilla rostrota)

• Bivalent OMP enhance the immunity of lymphocytes.
• Bivalent OMP improve the titers of specific antibody.
• Bivalent OMP generally increased lysozyme activity in the suspension of liver and kidney.
• Enhanced immunity causes less mortality when challenged by Aeromonas hydrophila or Edwardsilla tarda.
• Bivalent OMP can be applied as a new vaccine in eels farming.

The specific and non-specific immune parameters and protection of American eels (Anguilla rostrata) were evaluated after immunized eels with a bivalent expressed out membrane protein (OMP) of porin Ⅱ of Aeromonas hydrophila and ompS2 of Edwardsiella tarda. One hundred eighty eels were distributed into 3 equal groups and intraperitoneal (i.p) injection with phosphate-buffered saline (PBS group), formalin-killed-whole-cell (FKC) of A. hydrophila and E. tarda (FKC group) or the bivalent OMP (OMP group). The lymphocytes and red blood cells collected on 14, 21 and 42 days post-vaccination were used to evaluate the stimulation index (SI) and the sera collected on 14, 21, 28 and 42 days were used to assize the titers of specific antibody as well as lysozyme activity. Lysozyme activities in skin mucus, suspension of liver and kidney were also recorded on 14, 21 and 28 days. On 28 d post-vaccination, eels from all three groups were challenged by i.p injection of live A. hydrophila or E. tarda. The results show that, compared with the PBS group, proliferation of lymphocytes in OMP group was significantly (P < 0.05) enhanced on 21 days, and the serum titers of anti-A. hydrophila and anti- E. tarda antibody in eels of FKC and OMP group were significant increased (P < 0.05 or P < 0.01) on 14, 21 and 28 days. Activity of the lysozyme in serum, skin mucus, liver and kidney were significant changed (P < 0.05 or P < 0.01) between the three groups. Relative Percent Survival (RPS) after challenged with A. hydrophila on 28 days post immunization in two vaccinated groups vs. PBS group were 50%, and the RPS challenge E. tarda in FKC and OMP vs. PBS group were 50% and 37.5% respectively. These results suggest that American eels immunized with the bivalent OMP would positively affect specific as well as non-specific immune parameters and protect against infection by the two pathogens in freshwater farming.