Gut microbiota-derived GlcNAc-MurNAc is a TLR4 agonist that protects the host gut
Gut microbiota-derived peptidoglycan fragments (PGNs) are key signaling molecules that regulate multiple aspects of the host's health. Yet the exact structures of natural PGNs in hosts have not been fully elucidated. Herein, we developed an LC-HRMS/MS analytical platform for global quantification and profiling of natural PGN subtypes in host gut and sera, unexpectedly revealing the abundance of PGN-derived saccharide moieties that do not resemble canonical ligands of mammalian NOD1/2 receptors. Focusing on the disaccharide GlcNAc-MurNAc (GM), which does not activate NOD1/2 yet still exhibits immunostimulatory effects in host immune cells, we established GM as a mild TLR4 agonist, illustrating an alternate PGN sensing mechanism other than NOD signaling. Importantly, the administration of GM mitigates colonic inflammation in the DSS-induced colitis model in mice via a TLR4-dependent manner, highlighting the in vivo significance of gut microbiota-derived PGN saccharides in maintaining host intestinal homeostasis. Overall design: Bone marrow derived macrophages (BMDMs)  from wildtype or TLR4-knockout C57BL/6J mice were differentiated by L929 cell culture supernatants-added complete media. BMDMs on Day 7 since the start of differentiation were treated with water, GlcNAc-MurNAc (GM) or MurNAc-D-Ala-L-Gln (MDP) for 24h in IMDM (serum-free) media.