The PorV protein as a cross-protective antigen against Riemerella anatipestifer infection

Screening and identifying cross-protective antigens are key to developing new vaccines, particularly for bacterial pathogens with multiple serotypes. In this study, the adhesins PorV and PosF were evaluated as subunit vaccine candidates against Riemerella anatipestifer (R. anatipestifer, RA) infection. Ducks immunized with PorV, PosF, PorV+PosF or inactivated RA bacterin produced high levels of specific antibodies, and antibodies induced by inactivated bacterin also responded specifically to the PorV and PosF proteins. Serum from PorV protein-immunized ducks significantly increased the phagocytic activity of peripheral blood mononuclear cells, which was comparable to that of bacterin-immunized serum. In the serum bactericidal assay, the survival rates of RA in the presence of serum from PorV, PosF, PorV+PosF and bacterin-immunized ducks were 22%, 16%, 8% and 0.2%, respectively, compared with 73% in serum from non-immunized healthy ducks. These results indicate that the PorV and PosF proteins elicited functional antibodies that worked in conjunction with the complement to kill RA. After challenge with 5 × LD₅₀ of RA-YM by footpad injection, immunization with PorV, PosF, or PorV+PosF resulted in protection rates of 40%, 20%, and 20%, respectively, compared with 100% survival in bacterin-vaccinated ducklings. Although the protection rate conferred by PorV protein was only 40%, it significantly reduced the levels of inflammatory cytokines induced by RA-YM challenge and alleviated pathological damage. Moreover, immunization with the PorV protein protected ducks against challenge with field isolates of serotype 5 RA. These results demonstrate that PorV exhibits cross-protective potential against different serotypes of RA and represents a promising candidate for subunit vaccine development.