Role of mannose-binding lectin in regulating monocytes/macrophages functions during Aeromonas hydrophila infection in grass carp, Ctenopharyngodon idella.

Mannose-binding lectin (MBL) is a vital component in host's innate immune system and the initiator of the lectin pathway of complement cascade. However, its opsonic role has rarely been reported. In this study, we revealed the biological function of Ctenopharyngodon idella MBL (CiMBL) in regulating monocytes/macrophages (MO/MФ) in the grass carp (C. idella). Flow cytometry results indicated that recombinant CiMBL (rCiMBL) significantly enhanced the phagocytotic activity of MO/MФ. Recombinant CiMBL also enhanced bactericidal activity and respiratory burst capacity in Aeromonas hydrophila-infected MO/MФ, regulated A. hydrophila-induced polarization of MO/MФ including down- and up-regulated pro- and anti-inflammatory cytokines, respectively, suppressed the inducible nitric oxide synthase activity, and enhanced the arginase activity. In addition, rCiMBL suppressed the bacteria burden in tissues and blood in vivo and enhanced the survival rate of juvenile A. hydrophila-infected grass carp. We provide evidence that CiMBL was synthesized by MO/MФ, regulating the biological function of MO/MФ against A. hydrophila infection.