Riemerella anatipestifer OMP85, a BamA family outer membrane protein, enhances virulence through recruiting host complement regulator vitronectin to mediate complement evasion

Riemerella anatipestifer infection causes septicemia and polyserositis in poultry, leading to serious economic losses to the global poultry industry. The ability to break through the host complement defense barrier is an important characteristic of R. anatipestifer. However, the underlying mechanisms are still not well understood at present. This study identifies outer membrane protein 85, a conserved member in the OMP family, as a key mediator that recruits of host complement regulatory factor vitronectinto inhibit formation of the membrane attack complex allowing R. anatipestifer to evade host serum complement killing. Western blotting and flow cytometry confirmed that OMP85 mediated the binding of duck vitronectin (dVn) to R. anatipestifer. ELISA and pull-down assays identified amino acids 553 to 579 in OMP85 as critical for recognition of dVn. Moreover, the interaction of OMP85 with dVn inhibited MAC formation in both R. anatipestifer and E.coli strains producing OMP85. Serum survival of also correlated with production of OMP85. OMP85 exhibited strong immunogenicity, inducing the production of bactericidal antibodies and enhancing peripheral blood leukocyte killing, as shown by serum bactericidal activity and opsonophagocytosis assays, respectively. In addition, adhesion and invasion defects in R. anatipestifer OMP85 deletion mutant caused a 32-fold reduction in virulence and attenuated tissue and pathological changes compared with the wild-type strain. In conclusion, this study demonstrates the crucial role of OMP85 in complement evasion and virulence of R. anatipestifer. In view of its immunogenicity, OMP85 has significant potential as a candidate antigen for cross-protective vaccines against R. anatipestifer.