Characterization of the toll-like receptor 13 homologs in American bullfrogs, Lithobates catesbeianus, and their involvement in antimicrobial immunity

Toll-like receptors (TLRs) are an important family of pattern-recognition receptors that recognize a series of pathogen-associated molecular patterns to trigger the host immune response. In the present study, we identified three TLR13 genes (i.e., LcLTR13a, LcTLR13b, and LcTLR13c) in American bullfrogs, Lithobates catesbeianus, and explored their constitutive and pathogen-induced expression profile in different tissues. The open reading frames of LcLTR13a, LcTLR13b and LcTLR13c are 2811, 2844, and 2814 bp long, encoding 955, 947, and 933 amino acids, respectively. Phylogenetic analysis showed that three LcTLR13 genes grouping in different subclades indicate the gene duplication of bullfrog TLR13 as in teleost. The three LcTLR13 genes were expressed in all the tissues examined, with the liver and spleen expressing the higher levels. Moreover, the expression levels of LcLTR13a, LcTLR13b, and LcTLR13c were significantly induced in the liver and spleen after bullfrogs were challenged with bacterial (i.e., Citrobacter freundii, Aeromonas hydrophila, and Edwardsiella tarda) pathogens. These results indicate that LcTLR13 genes play an essential role in the bullfrog antibacterial innate immune response, providing new insights into immune mechanisms of bullfrogs.