Complement factor D of Paralichthys olivaceus in serum enhances C3 deposition to activate complement against bacterial infection.

The complement system has an important role in the response to bacterial infection. Complement factor D (CFD) is a serine protease involved in the alternative pathway activation by promoting complement factor B cleavage. In teleosts, the biological functions of CFD are poorly understood. In this study, we examined the immunological properties of a CFD homologue from Paralichthys olivaceus (PoCFD). PoCFD comprises 277 amino acids and contains a trypsin-like serine protease (Tryp) domain. PoCFD was constitutively expressed in ten different tissues and was upregulated by bacterial challenge. Recombinant PoCFD (rPoCFD) exhibited high apparent binding capacities to a broad-spectrum of bacteria in a concentration-dependent manner. rPoCFD treatment increased the hemolytic and bactericidal activity of serum and enhanced the serum killing to Vibrio harveyi by increasing C3 deposition on the bacterial surface. When P. olivaceus was infected with V. harveyi, rPoCFD treatment significantly reduced tissue bacterial loads, pathological features, and mortality of fish. Consistently, when PoCFD expression in P. olivaceus was knocked down, the bacterial replication and mortality of fish were significantly enhanced. This study provided new insights indicating that CFD in teleosts activates complement by enhancing C3 deposition, and is involved in the host immune defense against bacterial infection.