Characterization and functional analysis of a novel C1q domain-containing protein from grass carp (Ctenopharyngodon idella) in response to bacterial challenge

C1q domain-containing (C1qDC) proteins are significant pattern recognition receptors in the innate immune system that play important roles in regulating immune responses in vertebrates and invertebrates. In this study, a gene encoding the fish C1qDC (designated CiC1ql2) from the grass carp Ctenopharyngodon idella was characterized for the first time. Like other reported C1ql proteins, CiC1ql2 contains a conserved C1q domain and a signal peptide at its 5′ end. Expression profile analysis revealed that CiC1ql2 was mostly expressed in the intestine but was expressed at low levels in the brain and gill. In vivo injection experiments directly revealed that CiC1ql2 exhibited strong responsiveness to Aeromonas hydrophila, Aeromonas veronii and pathogen-associated molecular pattern (PAMP) challenge, as indicated by the presence of lipopolysaccharide (LPS) and peptidoglycan (PGN), and that the expression of these genes was significantly upregulated in the intestine of C. idella. Additionally, the recombinant CiC1ql2 protein obviously promoted the agglutination of the gram-negative bacteria A. hydrophila and A. veronii and the gram-positive bacterium Bacillus subtilis, as shown by in vitro binding experiments. Moreover, CiC1ql2 protein injection experiment revealed that CiC1ql2 significantly enhanced the expression levels of classic complement genes (C3, C5, and C7) and inflammatory cytokines (TNF-α, IL-1β, and IL-8), while knockdown of CiC1ql2 decreased the expression of these immune-related genes. Taken together, our results suggested that CiC1ql2 could act as a pathogenic pattern recognition receptor involved in complement activation and that this might be related to the intestinal defense of C. idella against bacterial infection.