Molecular characterization and expression analysis of IL-21 and its four receptor genes in yellow catfish (Pelteobagrus fulvidraco) in response to Edwardsiella ictaluri infection.

Interleukin 21 (IL-21) is a member of the IL-2 family and a pleiotropic cytokine that exerts its effects by binding to its receptor complex (IL-21Rα and γ chain). Due to the critical role and multifunctionality of IL-21, its functions have been extensively studied in mammals; however, there has been limited research on IL-21 in teleosts. In this study, the cDNA sequences of Pf_IL-21, Pf_IL-21Rα.1, Pf_IL-21Rα.2, Pf_γc1 and Pf_γc2 genes were cloned from yellow catfish (Pelteobagrus fulvidraco). Four α-helices and two η-helices were found in Pf_IL-21, one α-helices and three η-helices were found in Pf_IL-21Rα.1/2, and four α-helices and three η-helices were found in Pf_γc1/2. Quantitative real-time PCR analysis revealed high levels of mRNA expressions of Pf_IL-21 and its four receptors in mucosal and immune-related tissues of healthy yellow catfish. The mRNA expressions of these five genes in peripheral blood leukocytes (PBLs) were found to be regulated after stimulation with lipopolysaccharides (LPS), phytohaemagglutinin (PHA) and polyinosinic-polycytidylic acid (Poly I:C). Following Edwardsiella ictaluri infection, the mRNA expressions of these genes were significantly up-regulated in peripheral blood, hindgut, swim bladder, and other mucosal or immune-related tissues. In addition, the impact of recombinant (r) Pf_IL-21 protein on the intestinal microbiota and histological structure following E. ictaluri infection was examined. The results showed that E. ictaluri infection resulted in a decline in intestinal microbiota richness and diversity, while Fusobacteriota and Cetobacterium abnormally increased, and hindgut exhibited significant pathological alterations. However, the injection of rPf_IL-21 led to a restoration of intestinal microbiota richness and diversity, accompanied by an increase in certain beneficial bacteria and a better recovery of the hindgut structure. These findings suggest that Pf_IL-21 and its four receptor genes may play vital roles in immune defense against pathogens or stimulants in fish, and Pf_IL-21 may contribute to gut microbiota stability and participate in intestinal immune regulation in fish.