Exploring hemopexin's antibacterial activity in bony fish: An immunological study of PoHpx-a

Hemopexin (Hpx) is a conserved acute-phase protein that participates in the immune regulation and anti-bacterial response of higher vertebrates. However, the immunological function of Hpx in bony fish remains largely unclear. In this study, we found that Paralichthys olivaceus Hpx-a (PoHpx-a) contains a signal peptide and four hemopexin-like domains, with double β-propeller structure. PoHpx-a was found to be broadly occurred at 9 tissues, with the most abundant expression in liver, and its transcription was significantly influence in immune-related tissues upon bacterial challenge. Recombinant PoHpx-a (rPoHpx-a) exhibited binding activity to different types of bacteria. Furthermore, rPoHpx-a notably regulated serum complement activity. rPoHpx-a could bind to peripheral blood leukocytes (PBLs), inhibit cells LDH release, promote cells proliferation, and enhance cells phagocytic activity. In vivo, rPoHpx-a prominently reduced bacterial loads in the liver, spleen, and kidney, downregulated the expression of related pro-inflammatory cytokines, alleviated pathological damage of tissues, and improved the survival rate of hosts infected by Edwardsiella piscicida. These results indicate that PoHpx-a acts as a key immunoregulatory factor in antibacterial defense by regulating complement activity, enhancing leukocyte function, suppressing excessive inflammation, and inhibiting bacterial spread. This study provides new insights into the immune function of Hpx in teleost fish.