REVIEWS Unleashing the therapeutic potential of NOD-like receptors Kaoru Geddes*, João G. Magalhães* and Stephen E. Girardin‡ Abstract | Nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are a family of intracellular sensors that have key roles in innate immunity and inflammation. Whereas some NLRs — including NOD1, NOD2, NAIP (NLR family, apoptosis inhibitory protein) and NLRC4 — detect conserved bacterial molecular signatures within the host cytosol, other members of this family sense ‘danger signals’, that is, xenocompounds or molecules that when recognized alert the immune system of hazardous environments, perhaps independently of a microbial trigger. In the past few years, remarkable progress has been made towards deciphering the role and the biology of NLRs, which has shown that these innate immune sensors have pivotal roles in providing immunity to infection, adjuvanticity and inflammation. Furthermore, several inflammatory disorders have been associated with mutations in human NLR genes. Here, we discuss the effect that research on NLRs will have on vaccination, treatment of chronic inflammatory disorders and acute bacterial infections. Nuclear factor-κβ Innate immunity to microbial pathogens relies on the current research suggests that they are essential for the A transcription factor activated specific host-receptor detection of pathogen- and danger- induction and regulation of the caspase 1 inflammasome by NLR or TLR signalling derived molecular signatures (collectively referred to as through their N-terminal pyrin domain7. Another that mediates expression of pathogen-associated molecular patterns (PAMPs) and important aspect of NLR biology is that a number of the cytokines and chemokines. danger-associated molecular patterns (DAMPs), respec- genes that encode these proteins are mutated in human Inflammasome tively). The field of innate immunity research is currently chronic inflammatory disorders, including Crohn’s disease A multi-protein complex that experiencing a remarkable expansion due to the identifi- (for NOD2), Muckle–Wells syndrome (for NLRP3), processes pro-caspase 1 into cation of several families of these PAMP and DAMP atopic disorders (for NOD1) and vitiligo (for NLRP1)5. mature caspase 1. sensors, which are collectively termed pattern recognition This suggests that NLRs have a key role in linking host molecules1,2. The best-studied family of pattern recognition innate immunity to microbes and the regulation of molecules is the Toll-like receptor (TLR) family, which inflammatory pathways8. in mammals encodes 10–12 membrane-spanning mol- The PAMPs and DAMPs that are detected by some ecules with diverse specificities for PAMPs or DAMPs3. NLRs have recently been identified (TABLE 1). NOD1 and More recently, another family of pattern recognition NOD2 sense peptidoglycan, a heterogeneous polymer molecules — named the nucleotide-binding and oligo- found in the cell walls of bacteria. These two NLRs merization domain (NOD)-like receptor (NLR) family detect distinct entities within the peptidoglycan poly- *Department of — has received considerable attention. mer: whereas NOD2 detects muramyl dipeptide (MDP), Immunology and In humans, the NLR family is composed of 22 intra- which is a motif common to Gram-positive and Gram- ‡Department of Laboratory cell ular pattern recognition molecules (FIG. 1) that share negative bacterial peptidoglycan9,10, NOD1 specifically Medicine and Pathobiology, Medical Sciences Building, a central NACHT domain (domain present in NAIP, detects diaminopimelic acid (DAP)-type peptidoglycan, University of Toronto, CIITA, HET-E and TP1) and a carboxy-terminal leucine- which is found almost exclusively in Gram-negative bac- 1 King’s College Circle, rich repeat (LRR) region4–6. One subfamily of NLRs is teria11,12. NLRP3 (also known as NALP3 or cryopyrin) has Toronto, Ontario CANADA composed of NOD1 and NOD2, which have an amino- been shown to detect a range of PAMPs (such as MDP, M5S 1A8, Canada terminal caspase recruitment domain (CARD) required poly(I–C), double-stranded RNA from viruses, bacterial Corresponence to S.E.G. nuclear factor-κB (FIG. 2) e-mail: to trigger (NF-κB) signalling . The RNA and pore-forming toxins), as well as a number of [email protected] NLR family, pyrin domain-containing (NLRP) proteins DAMPs (such as extracellular ATP, uric acid, asbestos, doi:10.1038/nrd2783 constitute another homogenous NLR subfamily, and silica, aluminium hydroxide and amyloid-β peptide), NATURE REVIEWS | DRUG DISCOVERY VOLUME 8 | JUNE 2009 | 465 © 2009 Macmillan Publishers Limited. All rights reserved REVIEWS which suggests the existence of a common stimulus, (sensed by NLRP1) (TABLE 1)20. Following detection of triggered downstream of all these PAMPs and/or DAMPs their specific PAMP or DAMP, NLRs trigger a number (see REFS 13,14 for recent reviews on this topic). It has of signalling pathways that, overall, contribute to the been proposed that this common trigger could be stimu- host response to microbes and xenocompounds. Several lated by reactive oxygen species or lysosomal damage15–17. review articles have focused on NLRs and their biological The other recently identified molecular patterns that function5,13,21. Indeed, this family of intracellular sensors trigger NLRs include flagellin (sensed by NLR family, is emerging as a crucial hub for the regulation of host CARD domain-containing 4 (NLRC4; also known as responses to bacterial pathogens, inflammation and IPAF) and NLR family, apoptosis inhibitory protein adaptive immunity. (NAIP))18,19 and the lethal toxin of Bacillus anthracis In this Review, only the most extensively studied NLRs are discussed; however, research on other NLRs asso- a ciated with disease and inflammation, such as NLRP1 22 NLRA (the mutation of which leads to vitiligo) and NLRP12 subfamily CIITA (the mutation of which results in hereditary periodic fever syndromes)23, is still emerging. In addition, mutations in b NOD2 have been associated with a growing number of 24–26 NLRB malignant diseases, including early-onset breast cancer , subfamily NAIP non-Hodgkin’s lymphoma27, melanoma28 and lung can- cer26, and with negative outcomes following transplant surgery29,30; however, the molecular mechanisms under- c lying these associations are still unclear. Therefore, NLRP research in the field of NLRs has a broad clinical relevance. Crohn’s disease subfamily NLRP1 A type of inflammatory bowel This article reviews literature in the field of NLR research disease characterized by NLRP2–NLRP9 that is pertinent to the development of improved vaccina- granulomatous inflammation NLRP11–NLRP14 tion strategies, the treatment of inflammatory disorders in any region of the — for example, inflammatory bowel disease including gastro intestinal tract NLRP10 (although most frequently Crohn’s disease, asthma, gout and rheumatoid arthritis — occurring in the terminal ileum). and possibly acute and chronic bacterial infections. The d recent exciting discoveries in NLR research mean that NLRC/X Atopic disorders subfamily NLRC3 this emerging area of investigation may have matured to Allergic hypersensitivity NLRC5 a point at which transferring our fundamental knowledge that can manifest as NLRX1 eczema, rhinitis, asthma or to the development of clinical interventions is realistic and conjunctivitis, in which the NLRC4 appropriate. Here, we highlight the therapeutic potential affected organ does not come NOD1 of targeting NLRs, their ligands or the pathways that into direct contact with the they trigger. allergen. NOD2 Muramyl dipeptide The role of NLRs in adjuvanticity Caspase recruitment Baculoviral inhibitory (MDP). A component domain repeat Historically, vaccination has proved to be one of the most of peptidoglycan effective medical interventions, allowing for the control Pyrin domain Acid transactivation (N-acetylmuramyl-L-alanyl- or eradication of major diseases. However, traditional D-isoglutamine) that is Undefined domain domain specifically detected by NOD2. vaccination strategies such as the use of live attenuated Synthetic MDP is frequently Leucine-rich repeat NACHT domain pathogens, whole inactivated organisms or inactivated used in experiments as a toxins present certain limitations. For example, some means to directly stimulate Figure 1 | The NOD-like receptor (NLR) family. pathogens are difficult or even impossible to grow in NOD2. Schematic representation of the 22 members of the Nature Reviews | Drug Discovery culture, live attenuated vaccines can cause disease in nucleotide-binding and oligomerization domain Vitiligo (NOD)-like receptor (NLR) family as found in the human immune-compromised individuals by reverting to a A skin disorder that genome. Common to all NLRs is a central NACHT domain more virulent strain, and whole inactivated vaccines often involves dysfunction of (domain present in NAIP, CIITA, HET-E and TP1) flanked on contain components that have unwanted side effects. melanocytes, resulting the carboxy-terminus by a leucine-rich repeat domain. Therefore, the optimization of vaccination strategies in loss of pigmentation. NLRs can be divided into four subfamilies, mainly on the remains an area of intense clinical research. Hereditary periodic fever basis of their amino-terminal domain: (a) the sole member One aspect of vaccine development concerns the syndromes of the NLRA family, CIITA, is unique in that it acts as a immuno genic molecules and/or particles themselves: Rare heritable disorders transcription factor; (b)