Pathogen recognition and anti-infective immune responses mediated by TLR22 in largemouth bass Micropterus salmoides

To investigate the pathogen-associated molecular recognition characteristics and immune response of Toll-like receptor 22 (TLR22) in Micropterus salmoides, sequence characterization was performed, and its potential immune functions were further evaluated through tissue expression profiling, pathogen-associated stimulation response analysis, recombinant protein binding assays, and in vivo challenge experiments. The results showed that the open reading frame of TLR22 in M. salmoides was 2880 bp in length and encoded 959 amino acids. The deduced protein exhibited the typical structural features of teleost TLR22 and showed high sequence homology with TLR22 from other teleost species. TLR22 was expressed in all examined tissues, with relatively high expression levels in the head kidney and spleen. Stimulation with poly(I:C), lipopolysaccharide (LPS), and Aeromonas hydrophila significantly induced temporal changes in TLR22 expression in the head kidney, liver, and spleen. Recombinant TLR22 protein was able to bind multiple bacterial species and pathogen-associated molecular patterns, and displayed differential binding activities toward different ligands. In vivo challenge experiments showed that pretreatment with recombinant TLR22 protein enhanced the expression responses of immune-related genes, including TLR22, traf6, IL-6, IL-1β, and hepcidin, in relevant tissues following A. hydrophila infection. The TLR22-Ah group also exhibited a numerically higher final survival rate than the PBS-Ah and TrxA-Ah groups, although the pairwise differences were not statistically significant. Collectively, these results suggest that TLR22 in M. salmoides may participate in pathogen stimulus-induced innate immune responses and play important roles in pathogen recognition and immune regulation. This study provides a basis for further elucidating the immunological function of TLR22 in M. salmoides.