Complement-activated fragment Ba functions as an antibacterial protein and mediates immune responses in lower vertebrates

The complement system plays an important role in antibacterial infection and immune regulation. Ba, an important complement component, is produced and released by the cleavage of complement factor B during complement activation. However, the immune functions of Ba are unclear. In this study, we reported that recombinant Ba exerted direct bactericidal and immune regulatory effects. Recombinant Paralichthys olivaceus Ba (rPoBa) bound bacteria via interaction with the bacterial wall component lipopolysaccharide, resulting in bacterial membrane permeabilization and bacterial death. Furthermore, rPoBa exhibited bactericidal activity against Gram-negative bacteria in a manner that depended on concentration, time, temperature, pH, and metal ions. Structure prediction analysis showed that PoBa contained three distinct complement control protein (CCP) domains. CCP1 was mainly responsible for binding to lipopolysaccharide, and both CCP1 and CCP3 might be required for bacterial membranous permeabilization. The bactericidal effects of Ba were observed only in lower vertebrates, with no such effects observed in mammals. In addition, rPoBa could protect P. olivaceus against Vibrio harveyi infection both in vitro and in vivo by significantly improving the immune activity of peripheral blood leukocytes and reducing tissue bacterial loads. Consistently, when PoCFB expression in P. olivaceus was knocked down, the PoBa production and complement activity were decreased, and bacterial replication was significantly enhanced. In conclusion, this study revealed that the complement-activated recombinant Ba fragment improved the immune defense against bacterial infection and provided a potential strategy to control disease in lower vertebrates.