Nucleotide-binding oligomerization domain 2 and Toll-like receptor 2 function independently in a murine model of arthritis triggered by intraarticular peptidoglycan.

The NOD-like receptor family (NLR) plays a critical role in innate immunity. The members of the NLR family share many functional and structural characteristics and are thought to cooperate with Toll-like receptors (TLRs) in host defense. While much of the focus has been on the role of TLRs and their involvement in autoinflammatory diseases such as arthritis, the NLR family is emerging as an important participant in inflammation—perhaps even more so than TLRs in light of the association of the many NLR family members and inflammatory diseases (1, 2). One NLR family member in particular, NOD2 (also known as NLRC2 or CARD15), plays an important role in the health and function of diathrodial joints as evidenced by the fact that a single amino acid change in NOD2 causes Blau syndrome (3), which is characterized by inflammatory arthritis, uveitis and dermatitis (4, 5). In addition, most patients previously diagnosed with early onset sarcoidosis have been shown to have a mutation in the nucleotide oligomerization domain (NOD) of NOD2 (6). As such, understanding the function of NOD2 in the joints could clarify the pathogenesis of Blau syndrome and quite possibly other more common forms of arthritis. NOD2 plays an important role in bacterial infections. It is understood that NOD2 functions as an intracellular sensor of muramyl dipeptide (MDP) (7-9), which is a breakdown product of peptidoglycan (PGN)—an ubiquitous component of bacterial cell walls. Once activated NOD2 plays a role in the induction of signal transduction pathways involving the kinase, RIP2, the transcription factor, NF-κB, and CARD9 and MAP kinases (10-13). Cross-talk between NOD2 and several different TLRs, including TLR2, has been observed, further linking TLR and NLR functioning and perhaps regulation. In some settings NOD2 amplifies the function of TLRs since suboptimal concentrations of specific TLR ligands and MDP delivered simultaneously can produce synergistic cytokine responses (14-17). Conversely, in models of colitis, activation of NOD2 by MDP has ability to suppress inflammation triggered by TLR activation (18). Interestingly, different polymorphisms in NOD2 increase the risk for developing Crohn's disease, a chronic inflammatory disorder of the intestinal tract (6). Mouse models of colitis support a negative regulatory role of NOD2 in intestinal inflammation and mice deficient in NOD2 have lost this negative regulatory function, making them prone to murine colitis when TLR2 is activated in the gut (18-20). Despite our understanding of the cellular function of NOD2, whether NOD2 exerts a similar regulatory capacity in the joint is far from understood. We have previously demonstrated that NOD2 deficiency did not alter a T-cell dependent model of chronic and sterile arthritis induced by immunization with the cartilage component proteoglycan (21). However, our finding that MDP activation of NOD2 exacerbated proteoglycan-induced disease prompted us to explore the role of NOD2 in an acute model of inflammatory arthritis triggered by innate immunity. Evidence demonstrating expression of NOD2 within joint tissue (22, 23) along with the presence of bacterial cell wall components such as PGN and MDP within the joints of patients with rheumatoid arthritis (RA) (23, 24) would support the notion that bacterial components could directly activate NOD2 within the joints themselves to trigger local inflammation. In order to gain insight into this question, we have studied arthritis induced following intra-articular injection of two TLR2 ligands, peptidoglycan (PGN) and synthetic lipopeptide, N, palmityol(S)-[2,3-bis(palmitoyloxy)-(2Rs)-propyl]Cys-Ser-Lys4 (Pam3CSK4), along with muramyl dipeptide (MDP), a breakdown product of PGN that is sensed by NOD2. Using mice deficient in TLR2 and NOD2, we have explored the functioning of these proteins in acute inflammatory arthritis.