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.

中文翻译：

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美国牛蛙 Lithobates catesbeianus 中 Toll 样受体 13 同源物的特征及其与抗菌免疫的关系

Toll 样受体 (TLR) 是一个重要的模式识别受体家族，可识别一系列与病原体相关的分子模式以触发宿主免疫反应。在本研究中，我们在美国牛蛙Lithobates catesbeianus中鉴定了三个 TLR13 基因（即 LcLTR13a、LcTLR13b 和 LcTLR13c），并探讨了它们在不同组织中的组成型和病原体诱导的表达谱。 LcLTR13a、LcTLR13b和LcTLR13c的开放阅读框长2811、2844和2814bp，分别编码955、947和933个氨基酸。系统发育分析表明，三个 LcTLR13 基因分在不同的亚支中，表明牛蛙 TLR13 与硬骨鱼中一样存在基因重复。三个 LcTLR13 基因在所有检查的组织中都有表达，其中肝脏和脾脏的表达水平较高。此外，在牛蛙受到细菌（即弗氏柠檬酸杆菌、嗜水气单胞菌和迟缓爱德华氏菌）病原体攻击后，肝脏和脾脏中LcLTR13a、LcTLR13b和LcTLR13c的表达水平显着诱导。这些结果表明LcTLR13基因在牛蛙抗菌先天免疫反应中发挥着重要作用，为牛蛙免疫机制提供了新的见解。