Characterization and functional analysis of CsTLR13M/13S, two novel teleost toll-like receptor 13 in antimicrobial immunity

In mammals, toll-like receptors 13 (TLR13), as an intracellular endosomal TLR, recognizes various pathogenic microorganism, playing vital roles in the first line of host immunity through myeloid differentiation factor 88 (MyD88) signaling pathway. However, the anti-microbial effect of teleost TLR13 is largely obscure. Here, we reported the expression patterns, signaling pathway and immune property of tongue sole (Cynoglossus semilaevis) TLR13 (CsTLR13). The membrane form of CsTLR13 (CsTLR13M) contains three domain, i.e. extracellular domain (ECD), cytoplasmic TIR domain and transmembrane (TM) domain; the soluble form of CsTLR13 (CsTLR13S) only contains single ECD. CsTLR13M/CsTLR13S share 77.5 % identity with each other, with the same 19 LRRs in the ECD. CsTLR13M/CsTLR13S were constitutively existed in different tissues with similar expression patterns, and regulated by bacterial and viral infection. Recombinant CsTLR13S (rCsTLR13S) could bind to bacteria, with the highest affinity to P. fluorescens, and inhibited the growth of bacteria. In vivo, knockdown of CsTLR13M/CsTLR13S markedly enhanced bacterial infection. Consistently, CsTLR13M/CsTLR13S-deficient peripheral blood leukocytes (PBLs) revealed reduced expression of pro-inflammatory genes, MyD88-signaling related genes, and ROS production, which in line with rCsTLR13S activated the expression of pro-inflammatory and MyD88-signaling related genes. Overall, our findings give new insights into the signaling pathway and function of CsTLR13 in teleost immunity.