Harnessing PoSAP, a serum amyloid P component in Japanese flounder (Paralichthys olivaceus) for superior antibacterial immunity in teleost fish

Abstract

Serum amyloid P component (SAP), a member of the short pentraxin family, plays a multifaceted role in immune responses. In this study, we investigated the antibacterial immunity of SAP in Paralichthys olivaceus (PoSAP). PoSAP comprises 215 amino acids and shares 55.6–76.1% overall sequence identity with other fish species. In vitro, recombinant PoSAP (rPoSAP) showed differential binding ability to eight bacterial species and agglutinated S. aureus and P. fluorescens. rPoSAP demonstrated bacteriostatic activity against seven bacterial species and bacterial activity against E. tarda. In serum, the addition of rPoSAP or blocking endogenous PoSAP significantly regulated hemolytic and bactericidal activity, potentially resulting from the interaction of PoSAP with complement C1qA and C1qC chains, thereby inducing activation of the classical complement pathway. Regarding leukocyte vitality, rPoSAP bound to peripheral blood leukocytes and activated their chemotaxis, proliferation, inflammatory gene expression, phagocytosis, and reactive oxygen species production, leading to leukocyte-mediated antibacterial effects. Besides, neutrophils stimulated with rPoSAP extruded neutrophil extracellular traps. In vivo, PoSAP expression was significantly regulated by bacterial infection, and rPoSAP enhanced antibacterial effects in flounder tissues. In summary, these observations provide novel insights into the superior antibacterial immunity conferred by PoSAP in teleost fish.