Endobiotic glycolipid structure of gut symbionts modulates host inflammatory response

Introduction/Rationale: Commensal microbes in the intestine continuously shape the immunological landscape by producing diverse microbial products with immunomodulatory potential. Among these, outer membrane glycolipids serve as key mediators of host–microbe interaction. Lipidomic analysis revealed that symbiotic microbes possess diverse lipid A species differing in acyl chain number and phosphorylation degree, whereas pathobionts show limited diversity, predominantly producing hexa-acylated, diphosphorylated species. This compositional complexity across species has hindered detailed investigation of structure–activity relationships.

Methods: To dissect the immunological roles of symbiotic lipid A structures, we employed chemically synthesized analogs resembling glycolipids from commensals.

Results: These analogs, varying in acylation and phosphorylation, induced distinct transcriptional responses in dendritic cells. Notably, under-acylated (tri- and tetra-acylated) and monophosphorylated lipid A analogs strongly activated type I interferons and IFN-stimulated genes, whereas penta-acylated species lacked this activity. This structure-driven response correlated with preferential engagement of endosomal TLR4, leading to sustained IFN-β production and intracellular lipid droplet formation. The lipid A–induced IFN-β response was essential for promoting colonic RORγt⁺ regulatory T cells while suppressing Th17 differentiation and gut inflammation. Consistently, lipooligosaccharides from a B. fragilis strain engineered to overproduce tetra-acylated lipid A showed stronger IFN-β–inducing capacity than those from the wild type.

Conclusion: Collectively, our work delineates the mechanistic basis linking commensal glycolipid structure to host immunomodulatory responses. Structural nuances in symbiont-derived lipid A variants regulate the maintenance of colonic Tregs, sustaining a balanced and healthy endobiotic state.