Outer membrane protein A: An immunogenic protein induces highly protective efficacy against Vibrio ichthyoenteri

Vibrio ichthyoenteri was an important causative agent of bacterial enteritis in flounder (Paralichthys olivaceus). Outer membrane protein A (OmpA) of Gram-negative pathogen was a major cell surface antigen. In the present study, OmpA of V. ichthyoenteri was recombinantly expressed in Escherichia coli, and the immunogenicity of OmpA was identified by western blotting using flounder anti-rOmpA and anti-V. ichthyoenteri antibodies. The vaccine potential of rOmpA was tested in a flounder model, and a high relative percentage of survival rate was obtained with 73.1% after challenge with V. ichthyoenteri. Meanwhile, the immune response of flounder induced by rOmpA was also investigated, and the results showed that the sIg + lymphocytes in blood, spleen, and pronephros significantly proliferated, and the peak levels occurred at week 4 after immunization. Moreover, rOmpA could induce higher levels of specific serum antibodies than the control group after immunization, and the peak level occurred at week 5 after immunization. Meanwhile, qRT-PCR analysis showed that the expressions of CD4-1, CD8α, IL-1β, IFN-γ, MHCIα and MHCIIα genes were significantly up-regulated after immunization with rOmpA. Taking together, these results demonstrated that rOmpA could evoke highly protective effects against V. ichthyoenteri challenge and induce strong immune response of flounder, which indicated that OmpA was a promising vaccine candidate.