NOD2 and rheumatoid arthritis: Bacterial products such as lipopolysaccharide (LPS) or peptidoglycan are known to activate FLSs by interacting with PRRs present on these cells [109,110], and ligands for PRRs such as the NOD-2 ligand MDP or bacterial peptidoglycans have been identified in the joints of patients with RA [111,112].