Characteristics and functions of interleukin 21 in channel catfish (Ictalurus punctatus).

Interleukin-21 (IL-21) is mainly produced by CD4+ T cells and NKT cells, belonging to the gamma chain (γc) family. It promotes the proliferation, activation, and differentiation of immune cells and activates the JAK-STAT, MAPK signaling, and PI3K pathways to regulate immune responses, crucially impacting anti-infectious inflammatory responses. In this study, IL-21 was successfully identified in channel catfish (Ictalurus punctatus, Ip), with a coding sequence (CDS) length of 438 bp encoding a 145-amino acid (aa) protein. The signal peptide comprised 19 aa, while the mature peptide consists of 126 aa, featuring four α-helix structures and two pairs of disulfide bonds from four conserved cysteine residues. qPCR data revealed highest IpIL-21 expression in channel catfish gills and spleen. Bacterial and viral infections upregulated IpIL-21 expression across multiple tissues. Poly(I:C) specifically enhanced IpIL-21 expression in channel catfish kidney cells (CCK) at 48 h post-infection (hpi). In vitro, recombinant IpIL-21 (rIpIL-21) protein induced upregulation of IL-10, IL-21, IL-1β, and STAT3 expression in CCK, while inhibiting IL-22 and IL-26 expression. IL-6 and IL-20 expressions were inhibited at low doses and induced at high doses, demonstrating a dose-dependent pattern. These findings underscored IpIL-21's significance as a critical immune factor in channel catfish, pivotal for the antibacterial inflammatory response defense system.