Lipoglycans Contribute to Innate Immune Detection of Mycobacteria

Innate immune recognition is based on the detection, by pattern recognition receptors (PRRs), of molecular structures that are unique to microorganisms. Lipoglycans are macromolecules specific to the cell envelope of mycobacteria and related genera. They have been described to be ligands, as purified molecules, of several PRRs, including the C-type lectins Mannose Receptor and DC-SIGN, as well as TLR2. However, whether they are really sensed by these receptors in the context of a bacterium infection remains unclear. To address this question, we used the model organism Mycobacterium smegmatis to generate mutants altered for the production of lipoglycans. Since their biosynthesis cannot be fully abrogated, we manipulated the biosynthesis pathway of GDP-Mannose to obtain some strains with either augmented (∼1.7 fold) or reduced (∼2 fold) production of lipoglycans. Interestingly, infection experiments demonstrated a direct correlation between the amount of lipoglycans in the bacterial cell envelope on one hand and the magnitude of innate immune signaling in TLR2 reporter cells, monocyte/macrophage THP-1 cell line and human dendritic cells, as revealed by NF-κB activation and IL-8 production, on the other hand. These data establish that lipoglycans are bona fide Microbe-Associated Molecular Patterns contributing to innate immune detection of mycobacteria, via TLR2 among other PRRs.固有免疫识别依赖于模式识别受体（pattern recognition receptors, PRRs）对微生物特异性分子结构的识别。脂聚糖（lipoglycans）是分枝杆菌（mycobacteria）及相关属细菌细胞壁包膜所特有的大分子物质。已有研究证实，纯化状态下的脂聚糖可作为多种模式识别受体的配体，包括C型凝集素（C-type lectins）家族的甘露糖受体（Mannose Receptor）、DC-SIGN以及Toll样受体2（TLR2）。然而，在细菌感染的生理语境中，这些受体是否能够真正识别脂聚糖，目前仍未有定论。为解答这一科学问题，本研究以耻垢分枝杆菌（Mycobacterium smegmatis）作为模式生物，构建脂聚糖合成异常的突变菌株。由于脂聚糖的生物合成通路无法被完全阻断，我们通过调控GDP-甘露糖（GDP-Mannose）的生物合成途径，成功获得了脂聚糖产量分别升高（约1.7倍）与降低（约2倍）的工程菌株。有趣的是，后续感染实验结果表明：细菌细胞壁包膜中脂聚糖的含量，与TLR2报告细胞、单核细胞/巨噬细胞THP-1细胞系以及人树突状细胞的固有免疫信号激活强度之间存在直接正相关关系，该关联通过NF-κB激活水平与IL-8分泌量得以验证。本研究数据证实，脂聚糖是确凿的微生物相关分子模式（microbe-associated molecular patterns, MAMPs），可通过包括TLR2在内的多种模式识别受体参与分枝杆菌的固有免疫识别。