IFN1 Enhances Thrombocyte Phagocytosis through IFN Receptor Complex-JAK/STAT-Complement C3.3-CR1 Pathway and Facilitates Antibacterial Immune Regulation in Teleost

Type I IFNs with strong positive charges exhibit robust bactericidal activity and a protective effect against bacterial infections. However, the antibacterial mechanism in vivo remains unknown. In this study, Ab blockade of IFN1, a member of type I IFNs in grass carp (Ctenopharyngodon idella), resulted in high mortality, tissue bacterial loads, and low expression of immune factors after bacterial challenge, which indicates that the antibacterial activity of IFN1 has physiological significance. Meanwhile, we injected grass carp with the recombinant and purified intact IFN1 protein after bacterial injection, and the result demonstrated a remarkable therapeutic effect. Furthermore, we found that IFN1 expression was remarkably induced in blood cells after bacterial challenge, and prophagocytosis via IFN1 mostly increased in thrombocytes. Then, we isolated peripheral blood thrombocytes by polyclonal Ab of CD41 and stimulated thrombocytes with recombinant IFN1, and the results indicated that immune factors and complement components (especially C3.3) were induced. Unexpectedly, complements demonstrated not only bacteriolysis but also bacterial aggregation. Furthermore, Ab blockades of the three subunits (CRFB1/CRFB2/CRFB5) of the IFN1 receptor or inhibition of STAT1 almost abolished the prophagocytosis via IFN1 and reduced C3.3 and immune factor expression in thrombocytes. Meanwhile, Ab blockade of the complement receptor CR1 greatly attenuated the prophagocytosis of IFN1. In contrast, mouse IFN-β did not show the promotion of antibacterial activity. These results clarify the prophagocytosis and immune regulation pathways of IFN1 in antibacterial immunity in teleosts. This study reveals the antibacterial mechanisms of type I IFNs in vivo and inspires functional studies of IFN in bacterial infections.