Viewpoints on inflammasomes
The NLRP3 inflammasome, a target for therapy in diverse disease states
A role for NLRP3 inflammasome in recurrent and chronic inflammation was initially described in a group of rare autoinflammatory conditions, termed cryopyrin-associated periodic syndrome. Subsequently, inflammasomes have been implicated in the pathology of many common diseases, including cancer, gout and diabetes. Despite diverse pathologies, the central role of the inflammasome in innate defences and tumour elimination suggests common therapeutic approaches to reduce inflammation where appropriate.
A paradigm shift in our understanding of a broad spectrum of immunological diseases has resulted from the discovery of genetic susceptibility loci for a number of hereditary periodic fever (HPF) syndromes and the realisation that these conditions are linked by dysregulation of the innate immune system. The concept of autoinflammatory disorders as a new disease classification was introduced with the discovery of mutations in TNFRSF1A as the genetic basis for TRAPS (TNFR-associated periodic syndrome) 1. An integrated classification of immunological diseases, based on the concepts of autoinflammation and autoimmunity being at opposite ends of the immunological disease continuum, has subsequently emerged 2.
The genes responsible for five autoinflammatory HPF syndromes have been identified, and include MEFV (encoding pyrin) responsible for familial mediterranean fever; TNFRSF1A for TRAPS; mevalonate kinase for HIDS (hyperimmunoglobulin D and periodic fever syndrome); the PSTPIP1 gene for PAPA syndrome (pyogenic arthritis, pyoderma gangrenosum, acne); and NLRP3/CIASI responsible for three related conditions (FCAS, familial cold-induced autoinflammatory syndrome; Muckle–Wells syndrome; and NOMID, neonatal onset multisystem inflammatory disorder), collectively termed the cryopyrin-associated periodic syndrome (CAPS) 3. The NLRP3/CIASI mutations cause spontaneous activation of the NLRP3 inflammasome complex (also known as the NALP3 or cryopyrin inflammasome or the caspase-1 inflammasome), leading to excessive IL-1β secretion 4 and associated sterile inflammation. This is responsible for the clinical manifestations of CAPS, as well as playing a major role in a number of other autoinflammatory diseases, including familial mediterranean fever 5, 6.
Treatment of autoinflammatory disease
The effectiveness of IL-1 inhibition in a variety of disorders has resulted in marked patient benefit. This approach was used for CAPS initially 7, but is currently the treatment of choice for most HPF. Not surprisingly, use of recombinant IL-1R antagonist (IL-1Ra), known as anakinra, is particularly effective in treating deficiency of IL-1Ra (DIRA) syndrome. Recessive mutations in the IL1RN gene (encoding IL-1Ra) were shown to result in an inability to secrete IL-1Ra and hyper-responsiveness to IL-1β 8, 9. These studies suggest that treating DIRA patients promptly with anakinra may prevent the development of painful and debilitating bone abnormalities observed in this disease 8.
Until recently, anakinra has been the mainstay of treatment of CAPS 10. Two alternative IL-1 antagonists are currently available. Rilonacept, which acts as a soluble decoy receptor for both IL-1β and IL-1α, can produce rapid symptomatic improvement 11, and a fully humanised mAb against IL-1β, canakinumab, has also been approved for the use in FCAS and Muckle–Wells syndrome. A phase III clinical study has demonstrated the efficacy of canakinumab in CAPS patients 12.
A pilot study has shown that IL-1β inhibition by anakinra is also effective in acute gout 13 and resistant pseudogout 14. Following on from this success, a proof-of-concept study of rilonacept was conducted in patients with chronic gout; the first controlled and blinded study of an IL-1 blocking agent in this condition 15. Rilonacept has the advantage of a long plasma half-life, and the ability to bind to IL-1β with high affinity 16, but it also binds to both IL-1α and IL-1Ra, with lower affinity. This ensures that rilonacept has the potential to inhibit IL-1 in vivo with better efficiency than other IL-1-targeted therapies. IL-1 blocking agents are currently in widespread use to treat the HPF syndrome (Table 1).
Table 1. Inflammasome activation in autoinflammatory and other disorders
|CAPS||NLRP3/CIASI (NLRP3/cryopyrin)||Yes 7, 10–12|
|DIRA syndrome||IL1RN (IL-1Ra)||Yes 8|
|Gout||Monosodium urate crystals||Yes 13|
|Pseudogout||Calcium pyrophosphate dihydrate crystals||Yes 14|
|Allergic contact dermatitis||Sensitising allergens||Not tested|
|T2D||Polygenic/metabolic stress and inflammation||30, 31|
|Liver damage||Chemically induced||39 (animal model)|
|Cancer (myeloma)||IL-1 production by tumour cells induces IL-6 that acts as a growth factor for myeloma cell proliferation||36|
A subset of systemic onset juvenile idiopathic arthritis (SOJIA) has also been classified as an autoinflammatory disease in recent years. Gene expression studies of SOJIA patients identified a unique IL-1β signature 17, which changed significantly in patients undergoing IL-1β blockade. However, subsequent studies have failed to replicate the IL-1β signature 18, and excessive IL-1β secretion was not found in SOJIA patients at any stage of therapy in one report 19.
The three IL-1 antagonists currently available act over different time periods; short-acting anakinra has a half-life of 4–6 h, rilonacept a half-life of 6–7 days, and long-acting canakinumab has a half-life of 28–30 days. The decision to use a short, intermediate or long acting agent in these various disorders will be a key question in the future, as will the point at which IL-1 blockade will be initiated, to prevent the development of skeletal and CNS defects, in conditions such as CAPS and DIRA.
Allergic contact hypersensitivity
The skin is constantly subjected to environmental insults (microbial, chemical and physical) that may trigger immune responses 20. It has been proposed that the presence of NLRP3 in the skin (keratinocytes and tissue resident dendritic cells) provides a first line of defence by enabling the rapid sensing of invading pathogens, thereby triggering an innate immune response via NLRP3 inflammasome activation 21, 22.
Sensitising allergens that penetrate the skin surface induce a delayed type hypersensitivity reaction, called contact hypersensitivity (CHS) 23, 24. Evidence has been presented for the involvement of NOD-like receptors (NLR) as well as IL-1β, IL-18 and caspase-1 in the mouse CHS model 25, 26. Recent work has also suggested that IL-18 plays an important role by distinguishing the presence of contact allergens from irritants 27 (Table 1). The outcome of skin immune responses with respect to tolerance or immunity is dependent on skin NLRP3 inflammasome activation, and secreted IL-1β and IL-18 may regulate the quality of an allergen-specific T-cell response in CHS 25. Furthermore, mice deficient in IL-1β have impaired CHS to trinitrochlorobenzone 28. These discoveries suggest that modulation of the NLRP3 inflammasome may offer a therapeutic strategy to modulate T-cell responses in patients suffering from allergic CHS. Excitingly, manipulation of the NLRP3 inflammasome may also offer a perspective to induce tolerance towards a given contact allergen.
Type 2 diabetes
Type 2 diabetes (T2D) occurs when beta cells in the pancreas fail to produce sufficient insulin to overcome insulin resistance. Several lines of evidence support the role of IL-1β in the pathogenesis of T2D; expression of the IL-1Ra is reduced in the pancreatic islets of these patients, with IL-1β being produced in response to high glucose concentrations, leading to decreased cell proliferation and apoptosis 29. Larsen et al. have reported that anakinra treatment results in decreased glycated haemoglobin (HbA1c) levels and increased insulin production in T2D patients 30. An IL-1β antibody, Xoma 052, was shown to restore glycemic control in T2D patients in a double-blind, placebo-controlled, dose-escalation study 31.
In this regard, it is also relevant that glyburide, a sulphonylurea drug used to treat T2D, inhibits the NLRP3 inflammasome 32. T2D is a burgeoning global health problem and this advance in understanding the pathogenesis will offer novel therapeutic avenues in the future.
Inflammation appears to provide a local environment in which many tumours flourish and IL-1β has a key role in this process 33. Inflammasome-mediated pathogen recognition 34 provides a potential, but as yet unproven, link between infection-induced inflammation and cancer. However, a role for the inflammasome has been revealed in environmental carcinogenesis; silica and asbestos both activate the NLRP3 inflammasome, increasing IL-1β production via caspase-1 activation and causing lung inflammation 35. In humans, chronic exposure to asbestos is a key risk factor for development of mesothelioma, suggesting that inflammasome-mediated inflammation might underlie the pathogenesis of this tumour.
The link between inflammation and cancer has prompted the evaluation of anti-inflammatory agents in tumour therapy 33. In myeloma, a plasma cell neoplasia localised to the bone marrow, there is a evidence that myeloma-derived IL-1β induces IL-6 production by bone marrow stromal cells, and this acts as a growth factor for proliferation of the myeloma cells. Blocking IL-1β with anakinra diminishes IL-6 production and, in a clinical trial, this treatment significantly reduced disease progression 36. Myeloma is often treated with thalidomide, and this agent has recently been shown to inhibit caspase-1 activity (and IL-1β secretion) in keratinocytes 37. This suggests that thalidomide, might act in myeloma via caspase-1 inhibition and the breaking of the IL-1β-IL-6 loop, targeted by Lust et al. 36. Furthermore, these studies suggest that targeting the action of IL-1β (e.g. by using anakinra or longer acting IL-1β inhibitors) might be a useful alternative to thalidomide therapy.
The link between inflammation and cancer should not always be viewed as detrimental, as there is a likely balance between inflammation that triggers productive anti-tumour immune responses and inflammation that promotes tumour progression 33. This has been demonstrated most strikingly by Ghiringhelli et al. 38. Extracellular ATP activates the inflammasome via purinergic receptors 26; ATP derived from dying tumour cells stimulates dendritic cell production of IL-1β, via the NLRP3 inflammasome, and IL-1β is required for optimal IFN-γ production by CD8 T cells and tumour elimination in vivo38. Although the study was performed using animal models, the authors also demonstrated that breast cancer patients harbouring a P2X7 receptor variant, with reduced affinity for ATP, were more likely to develop metastases. These results suggest that this pathway of ATP activation of the NLRP3 inflammasome, via purinergic receptors, is likely to emerge as a major player in the regulation of anti-tumour immunity.
IL-1β is important in mediating chemically induced liver damage and progression from an acute injury to liver fibrosis. This was demonstrated in IL-1R-deficient mice, which, following thioacetamide treatment, were partially protected against liver damage and had reduced fibrogenesis 39. The synthesis of IL-1β typically depends on the activation of two danger sensing pathways; the TLR pathway to stimulate production of IL-1 propeptide, and the NLRP3 inflammasome complex to process propeptide into a mature cytokine. The role of the NLRP3 inflammasome in this process, and in the context of the liver disease, was studied by two groups. Watanabe et al. found that hepatic stellate cells, which are responsible for inducing liver fibrosis when activated, express the NLRP3 inflammasome. They also found that mice lacking either NLRP3 or the ASC component of the inflammasome were protected against tetrachloride- or thioactamide-induced liver damage 40. Imaeda and co-workers found that IL-1β synthesis in the liver is dependent on TLR9- and NLRP3-mediated pathways 41. They used acetaminophen-induced liver injury and various mouse gene knockouts to demonstrate that DNA released by the damaged hepatocytes activates the TLR9 pathway to produce pro-IL-1 and IL-18, and that NLRP3 inflammasome components (NLRP3, ASC or caspase-1) are required to produce the mature cytokines. Knockout mice lacking either TLR9 or one of the NLRP3 inflammasome components show reduced synthesis of IL-1β and IL-18, and subsequent reduced mortality and liver injury after acetaminophen treatment. The authors also found that liver injury could be significantly reduced if the animals were treated with aspirin before or concordantly with acetaminophen. The beneficial effect of aspirin in this case was found to be mainly due to its ability to downregulate pro-IL-1β and pro-IL-18 transcription. These studies confirm that, apart from the direct cytotoxic effects of, for example, acetaminophen, IL-1β- and IL-18-mediated innate immune responses play a significant role in causing liver damage. These cytokines are, therefore, logical targets to be considered when deciding how to best treat acute and chronic liver damage in the future. The main reservation regarding the potential success of this approach is the reported finding that, under certain circumstances, NLRP3/ASC/caspase-1 complex activation may directly lead to cell death rather than IL-1β production 42; this mechanism may also have contributed to liver damage in the experimental animals.
Rheumatoid arthritis (RA) was the first major disease in which IL-1 blockade was tested. Anakinra was well tolerated in patients with active RA, and moderately effective when used as monotherapy, or in combination with methotrexate 43, 44. However, a systematic review, published in 2009, concluded that the utility of anakinra for the treatment of RA is likely to be limited; only modest improvements have been reported, compared with other biological medications, such as anti-TNF therapy 45. It seems plausible therefore that unlike TNF and IL-6, IL-1β is not pivotal in the hierarchy of cytokines orchestrating the marked immunological perturbations in autoimmune conditions such as RA.
Anti-IL-1β therapy has had a major impact on the treatment of a number of autoinflammatory diseases, particularly the HPF, although it would appear be less effective in treatment of autoimmune disease. However, increasing knowledge of the function of the NLRP3 inflammasome in other complex disorders is suggesting that a niche will also be found for this approach in a subset of these disorders.
G. Cook is supported by Yorkshire Cancer Research, S. Savic by the NIHR-Leeds Musculoskeletal Biomedical Research Unit (LMBRU), M. Wittmann by The Royal Society, and M. F. McDermott by FP7-HEALTH-2007-2.4.4-1 grant; both G. Cook and M. F. McDermott are supported by the Charitable Foundation of the Leeds Teaching Hospitals and the Arthritis Research Campaign (arc).
Conflict of interest: The authors declare no financial or commercial conflict of interest.