The structural characteristics and expression characteristics of C1S in response to GCRV infection in grass carp

The complement system, a critical component of innate immunity in fish, plays a pivotal role in the defense against Grass Carp Reovirus (GCRV) infection in grass carp. This study explores the structural characteristics of C1S, a crucial molecule in the classical pathway of the complement system, and its involvement in the response to GCRV infection. We found that the grass carp C1S gene comprises six domains similar to those in mammals: two CUB (Complement C1r/C1s, Uegf, Bmp1) domains, two CCP (Complement control protein) domains, one EGFCA (Calcium-binding epidermal growth factor) domain, and one Tryp_SPc (Trypsin-like serine protease) domain, albeit without chromosomal collinearity to humans. Comparative analysis revealed that the identity and similarity of this gene with those in other species range from 30.6 to 89.4 % and 30.7-89.7 %, respectively. Phylogenetic analysis positioned C1S in close relation with R. klamathensis and D. rerio. Tissue expression profiles in both healthy and GCRV-infected grass carp indicated primary expression of C1S in the liver, with expression peaks at day 7 post-infection in the liver and spleen, and at day 5 in the kidney. Functional assays demonstrated that C1S activates the complement system via cleavage of complement component 3 (C3) into C3b, further inhibiting GCRV replication and upregulating antiviral genes IFN1, IRF3, and IRF7. These findings elucidate the mechanism by which the complement system mediates resistance to GCRV infection in grass carp, offering a substantial theoretical foundation for further research.