Ornithine lipids from Akkermansia muciniphila are dynamically modulated in colitis and shape macrophage inflammatory responses

The gut microbiota is a key modulator of host immunity, in part through the production of structurally diverse and largely still uncharacterized bacterial lipids and metabolites with potential immunoregulatory properties. Using a gnotobiotic Oligo-Mouse-Microbiota (OMM¹²) mouse model infected with the Citrobacter rodentium pathogen, we investigated metabolomic changes associated with colitis. Untargeted metabolomics revealed an accumulation of host-derived lipids in the inflamed colon, while several bacterial lipid classes, including sphingolipids, glycerophospholipids, and fatty acyls were depleted. Among the bacterial lipids, ornithine-containing lipids (OLs) produced by Akkermansia muciniphila were significantly reduced during inflammation. Isolation, structural characterization, and chemical synthesis revealed OL 16:0/15:0 as a membrane-associated lipid from A. muciniphila. This lipid contains an L-ornithine head group, with its α-amino group forming an amide bond with 3(R)-hydroxypalmitic acid, while the 3(R)-hydroxyl position is esterified with pentadecanoic acid. Functional studies showed that macrophages internalize and partially metabolize OL 16:0/15:0 into N^α-(3-hydroxypalmitoyl)-L-ornithine and 3(R)-hydroxypalmitic acid. In LPS-stimulated macrophages, a 1:1 mixture of OL diastereomers (3R,S + 3S,S) reduced Il6 and Il1b gene expression and decreased IL-6 secretion, without triggering IL-1β release. Interestingly, this diastereomeric mixture exhibited an opposite effect to the natural (3R,S)-epimer, which selectively promoted IL-1β secretion in LPS-primed macrophages. These results uncover a possible stereoselective modulation of IL-1β production by bacterial OLs. Overall, OL 16:0/15:0 is dynamically regulated during inflammation and may play a role in the immunomodulation of host-microbiota interactions.