Characterization of immune response elicited by the recombinant outer membrane protein OmpF of Aeromonas hydrophila, a potential vaccine candidate in murine model

Porins, the outer membrane proteins of gram negative bacteria, perform vital roles in bacterial survival and virulence, such as nutrient transportation across the membrane as well as adhesion to host cells during infection. The outer membrane proteins, OmpF and OmpC, are part of a two-component regulatory system, essential for the maintenance of solute concentrations in the cytoplasmic milieu of bacteria, and are thus considered vital for bacterial survival. Exposed on the surface of gram-negative bacteria, these channel proteins are highly immunogenic and can thus be exploited as vaccine candidates. In the present study, we have cloned, characterized, and expressed outer membrane protein OmpF of Aeromonas hydrophila, a major fish pathogen and also known to cause severe infections in humans. The cloned ompF gene of A. hydrophila consisting of an open reading frame corresponding to mature OmpF was expressed and purified from the heterologous host, E. coli. High level of expression resulted in recovery of 120 mg/L of the purified rOmpF at shake flask level. Polyclonal antisera raised against the recombinant OmpF showed a very high endpoint titer (> 1:80,000) and were able to specifically agglutinate live A. hydrophila. Further, anti-OmpF antisera cross-reacted with the cell lysates of various Aeromonas isolates, suggesting that anti-rOmpF antibodies can be used to identify different A. hydrophila isolates in infected conditions. Antibody isotyping, cytokine ELISA, and ELISPOT assay indicated predominantly Th1 type of immune response. The recombinant OmpF reported in the present study thus has the potential to be used as a vaccine candidate against A. hydrophila.