CsSAP, a teleost serum amyloid P component, interacts with bacteria, promotes phagocytosis, and enhances host resistance against bacterial and viral infection

•CsSAP shares 46%–58% overall sequence identities with fish serum amyloid P component.•CsSAP expression in fish tissues was upregulated by bacterial and viral infection.•rCsSAP bound to nine different bacteria and promoted phagocytosis.•Fish administered with rCsSAP exhibited enhanced resistance to pathogen infection.•Fish treated with CsSAP antibody showed weakened ability to clear invading bacteria.

Serum amyloid P component (SAP) is a member of the pentraxins family that plays important roles in innate immunity in vertebrates. In fish, the biological function of SAP is essentially unknown. In this study, we examined the expression and function of a SAP homologue (CsSAP) from tongue sole Cynoglossus semilaevis. CsSAP shares 46%–58% overall sequence identities with known fish SAP and was upregulated in expression by bacterial and viral infection. Recombinant CsSAP (rCsSAP) exhibited differential binding capacities to a wide range of Gram-positive and Gram-negative bacteria and promoted uptake of the bound bacteria by host phagocytes. When introduced in vivo, rCsSAP enhanced host resistance not only to bacterial infection but also to viral infection. Consistently, antibody blockage of CsSAP significantly weakened the ability of tongue sole to clear invading bacteria. These results provide the first evidence that fish SAP contributes significantly to both antibacterial and antiviral immunities.