Host Recognition of Bacterial Muramyl Dipeptide Mediated through NOD2

TLDR

NOD2, linked to Crohn’s disease susceptibility, mediates immune responses to bacterial lipopolysaccharide and peptidoglycan, yet the specific ligand it recognizes has been unclear. Biochemical and functional studies identified the peptidoglycan‑derived muramyl dipeptide MurNAc‑L‑Ala‑D‑isoGln as the minimal structure required for NOD2 activation. The study showed that NOD2 stereospecifically recognizes this dipeptide, that disease‑associated NOD2 mutations abolish this recognition, and that this interaction underlies host defense against Crohn’s disease and informs adjuvant and vaccine design.

Abstract

NOD2, a protein associated with susceptibility to Crohn's disease, confers responsiveness to bacterial preparations of lipopolysaccharide and peptidoglycan, but the precise moiety recognized remains elusive. Biochemical and functional analyses identified muramyl dipeptide (MurNAc-L-Ala-D-isoGln) derived from peptidoglycan as the essential structure in bacteria recognized by NOD2. Replacement of L-Ala for D-Ala or D-isoGln for L-isoGln eliminated the ability of muramyl dipeptide to stimulate NOD2, indicating stereoselective recognition. Muramyl dipeptide was recognized by NOD2 but not by TLR2 or co-expression of TLR2 with TLR1 or TLR6. NOD2 mutants associated with susceptibility to Crohn's disease were deficient in their recognition of muramyl dipeptide. Notably, peripheral blood mononuclear cells from individuals homozygous for the major disease-associated L1007fsinsC NOD2 mutation responded to lipopolysaccharide but not to synthetic muramyl dipeptide. Thus, NOD2 mediates the host response to bacterial muropeptides derived from peptidoglycan, an activity that is important for protection against Crohn's disease. Because muramyl dipeptide is the essential structure of peptidoglycan required for adjuvant activity, these results also have implications for understanding adjuvant function and effective vaccine development.