The hidden language of gut-derived lipopolysaccharides: fine chemistry, huge immunological consequences.

Lipopolysaccharides (LPSs) from Gram-negative bacteria are traditionally viewed as potent "endotoxins" recognized by the immune system and capable of triggering robust inflammation. However, increasing evidence from gut commensals is dismantling this one-dimensional view. The gastrointestinal tract is indeed the major reservoir of LPSs, owing to the dense Gram-negative community inhabiting the small and large intestine, with total weight in healthy individuals estimated to exceed one gram. This necessarily means that the mere presence of LPSs cannot be directly linked to inflammation. Moreover, chronic exposure to low-potency or atypical LPSs can recalibrate innate immunity, fostering tolerance or, conversely, failing to provide adequate tonic stimulation and thereby predisposing the system to aberrant activation. Understanding this delicate balance and the structural and cellular mechanisms that sustain it, is essential to interpret the immunological impact of the gut LPSs in health and disease. In this Perspective, we highlight recent advances revealing the remarkable chemical diversity of commensal-derived LPSs and illustrate how subtle variations in LPS lipid A acylation and phosphorylation, core oligosaccharide architecture, O-antigen composition, and overall supramolecular organization profoundly rewire receptor usage and downstream immune outcomes. These insights underscore the enormous, still largely untapped potential of gut LPS chemistry to reveal unifying structural hallmarks that distinguish inflammatory, tolerogenic, and immunologically "tuned" features. Although fragments of this logic are beginning to emerge, a comprehensive framework remains urgently needed. Decoding the chemical language adopted by LPSs in the gut will be essential to reclassify LPSs not merely as dangerous molecules, but as a potential source of immunomodulators and as a blueprint for next-generation tools enabling precision control of host-microbe interactions.