C5a drives the inflammatory response with bacterial dose effect by binding to C5aR1 in zebrafish infected with Aeromonas hydrophila

The complement system is essential to host defense, but its excessive activation caused by severe pathogen invasion is a driving force in adverse inflammatory. The binding of complement component 5a (C5a) and complement component 5a receptor 1 (C5aR1) is the key to trigger complement-mediated inflammatory response in mammals. However, the role of C5a-C5aR1 axis in fish immune response remains obscure. In this study, the role of C5a-C5aR1 axis of zebrafish (Danio rerio) after serious infection with Aeromonas hydrophila was investigated. C5a and C5aR1 of zebrafish were cloned, with CDS sequences of 228 and 1041 bp, respectively, and they were widely expressed in various tissues with the highest expression in the liver and spleen, respectively. The survival of zebrafish was closely correlated to the dose of A. hydrophila. The cytokine storm occurred at high concentrations of A. hydrophila infection. At 24 h post infection (hpi), the expression of C5a and C5aR1 in the spleen increased 26.8-fold and 9.9-fold in treatment group 1 (TG1, 3.0 × 107 CFU/mL) (P < 0.01), and 4.7-fold and 3.4-fold in treatment group 2 (TG2, 1.0 × 107 CFU/mL) (P < 0.05), respectively. Correspondingly, proinflammatory cytokines interleukin-1β (IL-1β), interleukin-8 (IL-8), and interleukin-17 (IL-17) were positively correlated to C5a and C5aR1 at mRNA and protein expression levels. The expression of IL-1β was significantly increased in the spleen at 6 hpi, with a 599.2-fold and 203.2-fold upregulation in TG1 and TG2 (P < 0.001), respectively. Moreover, after inhibition of C5a-C5aR1 binding treated with C5aR1 antagonist (W-54011), zebrafish showed lower expression of C5a, C5aR1, and cytokines, less intestinal damage, and significantly enhancement of survival (P < 0.05) after A. hydrophila challenge. This study revealed that the inflammatory effect of C5a was achieved by binding to C5aR1 in zebrafish, providing novel insights into using C5a-C5aR1 axis as an effective target to reduce bacterial inflammation and disease in fish.