Toll/Toll-like receptors (TLRs) are key regulators of the innate immune system in both invertebrates and vertebrates. However, while mammalian TLRs directly recognize pathogen-associated molecular patterns, the insect Toll pathway is thought to be primarily activated by binding Spätzle cytokines that are processed from inactive precursors in response to microbial infection. Phylogenetic and structural data generated in this study supported earlier results showing that Toll9 members differ from other insect Tolls by clustering with the mammalian TLR4 group, which recognizes lipopolysaccharide (LPS) through interaction with myeloid differentiation-2 (MD-2)–like proteins. Functional experiments showed that BmToll9 from the silkmoth Bombyx mori also recognized LPS through interaction with two MD-2–like proteins, previously named BmEsr16 and BmPP, that we refer to in this study as BmMD-2A and BmMD-2B, respectively. A chimeric BmToll9–TLR4 receptor consisting of the BmToll9 ectodomain and mouse TLR4 transmembrane and Toll/interleukin-1 (TIR) domains also activated LPS-induced release of inflammatory factors in murine cells but only in the presence of BmMD-2A or BmMD-2B. Overall, our results indicate that BmToll9 is a pattern recognition receptor for LPS that shares conserved features with the mammalian TLR4–MD-2–LPS pathway.

Toll/Toll 样受体 (TLR) 是无脊椎动物和脊椎动物先天免疫系统的关键调节剂。然而，虽然哺乳动物 TLR 直接识别病原体相关的分子模式，但昆虫 Toll 通路被认为主要是通过结合 Spätzle 细胞因子来激活的，这些细胞因子是响应微生物感染而从无活性前体加工而来的。本研究中产生的系统发育和结构数据支持早期的结果，表明 Toll9 成员与其他昆虫 Tolls 不同，它们与哺乳动物 TLR4 组聚类，后者通过与髓样分化 2 (MD-2) 样蛋白的相互作用识别脂多糖 (LPS)。功能实验表明，来自家蚕 Bombyx mori的 BmToll9还通过与两种 MD-2 样蛋白（以前称为 BmEsr16 和 BmPP）相互作用来识别 LPS，我们在本研究中分别将其称为 BmMD-2A 和 BmMD-2B。由 BmToll9 胞外域和小鼠 TLR4 跨膜域和 Toll/白细胞介素-1 (TIR) 域组成的嵌合 BmToll9-TLR4 受体也激活 LPS 诱导的小鼠细胞炎症因子的释放，但仅在 BmMD-2A 或 BmMD-2B 存在的情况下. 总体而言，我们的结果表明 BmToll9 是 LPS 的模式识别受体，与哺乳动物 TLR4-MD-2-LPS 通路共享保守特征。