Taming the inflammasome

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The NLRP3 inflammasome is involved in the molecular etiology of multiple autoinflammatory diseases. Two studies identify inhibitors of NLRP3 activation and might pave the way for new treatment options for a variety of diseases.

A decade ago, the NLRP3 inflammasome was identified as a multi-protein complex of the innate immune system, consisting of the NOD-like receptor (NLR) NLRP3, the adaptor protein ASC and caspase-1. The NLRP3 inflammasome functions primarily in myeloid cells. It is activated by a variety of stimuli, including bacterial pore-forming toxins and molecules related to cellular damage, such as HMGB1. Upon oligomerization, caspase-1 becomes active and cleaves the pro-inflammatory cytokines IL-1β and IL-18 into their active secreted forms (Fig. 1). The discovery that this complex can also be activated by a variety of inducers of sterile inflammation, such as ATP, solid crystals, and environmental irritants, has provided the missing link between these substances and a range of IL-1β–driven autoinflammatory disorders, among them gout, atherosclerosis, obesity, and neurodegenerative diseases1. Furthermore, gain-of-function mutations in NLRP3 cause cryopyrin-associated periodic syndromes (CAPSs), including familial cold autoinflammatory syndrome (FCAS) and Muckle–Wells syndrome (MWS), rare human conditions that are characterized by chronic fever, rash, joint pain, and neurological complications2. Deletion of the NLRP3 inflammasome was shown to ameliorate atherosclerosis, multiple sclerosis, Alzheimer disease, type 2 diabetes, and gout in mouse models of these diseases3.

Figure 1: Inhibitors of the NLRP3 inflammasome.
figure1

In response to activation of innate immune receptors by stimuli such as microbial ligands, transcription of pro-inflammatory genes including those encoding NLRP3 and pro-IL1β is induced. Transcription of proinflammatory genes primes components of the NLRP3 inflammasome complex. Upon stimulation with a variety of endogenous and exogenous signals, a common characteristic of which is the induction of K+ efflux from the activated cell, the NLRP3 inflammasome assembles as a complex with ASC and pro-caspase-1. As a consequence, caspase-1 cleaves pro-IL-1β into its active form for secretion. Youm et al.6 suggest that β-hydroxybutyrate inhibits K+ efflux and prevents NLRP3 activation. Coll et al.7 describe the small molecule MCC950 as a specific inhibitor of NLRP3 inflammasome assembly. PYD, pyrin domain; CARD, caspase activation and recruitment domain.

The central involvement of the inflammasome in human disease has incited efforts to identify potent and specific ways to interfere with NLRP3 activation in the setting of auto-inflammatory diseases. Although both endogenous3,4 and synthetic5 inflammasome inhibitors have been described, the only clinically available therapeutics for NLRP3-driven autoinflammatory diseases are antibodies targeting IL-1β signaling, namely anakinra, rilonacept, and canakinumab, which are not specific to NLRP3 activity and cause general immunosuppression2. In this issue of Nature Medicine, Youm et al.6 and Coll et al.7 describe two new molecules that function as potent and specific inhibitors of NLRP3, thus constituting promising candidates for clinical testing.

During starvation, systemic metabolic alterations induce the production and utilization of ketone bodies, small acidic metabolites that serve as sources of ATP in periods of glucose shortage8. Youm et al.6 report that one such ketone body, β-hydroxybutyrate, but not the structurally related acetoacetate or butyrate, specifically inhibits NLRP3 inflammasome activation and downstream cytokine production by numerous known NLRP3 activators in mouse bone marrow–derived macrophages and human monocytes in vitro. When administered at physiological concentrations to mice in complex with nanolipogens that improve its bioavailability, β-hydroxybutyrate blocked NLRP3 inflammasome activation in response to monosodium ureate (MSU) crystals, the causative agent of gout. Similarly, in mouse models with human gain-of-function mutations leading to the manifestations of MWS and FCAS, β-hydroxybutyrate potently reduced IL-1β secretion.

This finding extends previous reports of the close interconnection between the metabolic and innate immune systems9. Secreted metabolites are sensed by the immune system to mediate a concerted whole-organismal energy and defense program. One such example is the inhibitory effect of short-chain fatty acids on macrophage inflammatory responses10. The identification of β-hydroxybutyrate as an NLRP3 inhibitor provides a rationale for the investigation of the effectiveness of a ketogenic, anti-inflammatory diet for the treatment of NLRP3-dependent autoinflammatory diseases. Indeed, Youm et al.6 observed an amelioration of inflammatory symptoms in the mouse model of FCAS when mice were fed a ketogenic diet. This mouse-based study is an exciting development in the emerging field of dietary approaches as modulators of immunity11 and should prompt further investigation in human studies. However, to avoid the long-term effects that a ketogenic diet may impose on other aspects of metabolic homeostasis in humans, further modification of the ketogenic diet may be needed to enable elevation of ketones while avoiding systemic side effects.

Coll et al.7 report the identification of a synthetic NLRP3 inhibitor of the inflammasome, MCC950, which was first described in a screen of IL-1β–processing inhibitors more than a decade ago12. Similarly to β-hydroxybutyrate, MCC950 blocks NLRP3 in mouse macrophages in response to canonical in vitro activators and prevents ASC speck formation, a characteristic microscopic feature of inflammasome assembly and ASC oligomerization. Interestingly, and in contrast to β-hydroxybutyrate, Coll et al.7 find that MCC950 also inhibits the recently discovered caspase-11–dependent noncanonical pathway of IL-1β release and pyroptosis induction. In mice, MCC950 potently reduces serum IL-1β levels in response to lipopolysaccharide (LPS) and improves disease manifestations in a mouse model of multiple sclerosis, including the suppression of disease-driving pathogenic T cell responses in the brain. In addition, Coll et al.7 use MCC950 to ameliorate the symptoms of MWS in a mouse model and to block NLRP3 activation in blood cells in vitro from a small cohort of people with MWS.

Despite their structural dissimilarity, both β-hydroxybutyrate and MCC950 seem to be specific inhibitors of NLRP3, not affecting Toll-like receptor (TLR) signaling or other inflammasome-forming NLRs. The effect of β-hydroxybutyrate seems to be independent of any common immunomodulatory mechanism of starvation, such as autophagy or the production of reactive oxygen species (ROS). At a molecular level, the numerous endogenous and microbial stimuli that activate the NLRP3 inflammasome converge at the induction of potassium (K+) efflux from activated cells, suggesting that this molecular event might initiate inflammasome complex formation13. Youm et al.6 find that β-hydroxybutyrate blocks K+ efflux from macrophages and prevents ASC complex formation (Fig. 1). It remains to be studied whether β-hydroxybutyrate may directly regulate K+ channels, or whether it regulates K+ homeostasis through other intracellular processes. Coll et al.7 find that MCC950 also blocks ASC assembly, but in this case the inhibitory mechanism seems to be independent of intracellular ion levels or NLRP3 oligomerization, pointing toward the possibility that this small molecule acts downstream of known NLRP3 activators and interferes with the process of mature inflammasome complex formation (Fig. 1). Further understanding of the mechanisms of action of these newly identified NLRP3 inhibitors will shed new light on the various intracellular regulatory steps controlling inflammasome assembly and signaling.

With remaining questions about the underlying molecular mechanisms notwithstanding, the discovery of these two NLRP3 inflammasome inhibitors prompts exploration of the clinical potential of both molecules. A therapeutic approach involving specific inhibition of NLRP3 inflammasome signaling may be superior to the currently used antibody-based therapies, by avoiding universal suppression of IL-1β activation by other inflammasome-forming NLRs, which is crucially important in multiple processes such as the response to infection. As a preliminary assessment of the clinical potential of MCC950, Coll et al.7. demonstrate encouraging parameters of pharmacokinetics and bioavailability of this compound in mice. The exciting discoveries presented by the studies of Youm et al.6 and Coll et al.7 represent a starting point for an era in which using NLRP3-inhibiting therapeutics may serve as a potential future means of prevention and treatment of inflammasome-dependent multifactorial diseases.

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Correspondence to Eran Elinav.

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Levy, M., Thaiss, C. & Elinav, E. Taming the inflammasome. Nat Med 21, 213–215 (2015). https://doi.org/10.1038/nm.3808

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