REVIEWS Emerging inflammasome effector mechanisms Mohamed Lamkanfi Abstract | Caspase 1 activation by inflammasome complexes in response to pathogen- associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) induces the maturation and secretion of the pro-inflammatory cytokines interleukin-1β (IL-1β) and IL-18. Recent reports have begun to identify additional inflammasome effector mechanisms that proceed independently of IL-1β and IL-18. These include the induction of pyroptotic cell death, the restriction of bacterial replication, the activation of lipid metabolic pathways for cell repair and the secretion of DAMPs and leaderless cytokines. These non-canonical functions of caspase 1 illustrate the diverse mechanisms by which inflammasomes might contribute to innate immunity, repair responses and host defence. (BOX 1) Cryopyrinopathies Inflammasomes are emerging as key regulators viral and bacterial pathogens that target caspase 1 activa‑ 24 A spectrum of hereditary of the innate immune response, and the activity of these tion in inflammasomes . Indeed, IL‑1β and IL‑18 were autoinflammatory diseases multiprotein complexes has been linked to inflamma‑ recognized early on for their ability to cause a wide variety that are caused by mutations tory bowel diseases1–5, vitiligo6, gouty arthritis7, type 1 of biological effects associated with infection, inflamma‑ in the gene encoding NLR and type 2 diabetes8,9, and less common autoinflamma‑ tion and autoimmunity25. IL‑1β regulates systemic and family, pyrin domain- containing 3 (NLRP3) that tory disorders that are collectively referred to as cryopy- local responses to infection, injury and immunological trigger continuous activation rinopathies10,11. Inflammasome complexes are thought to challenge by generating fever, activating lymphocytes and of the NLRP3 inflammasome. be assembled around members of the NOD-like receptor promoting leukocyte transmigration into sites of injury Based on the severity and (NLR) or HIN‑200 protein families12 (FIG. 1). These or infection25. Although IL‑18 lacks the pyrogenic activity spectrum of the symptoms — pathogen- which can include urticarial receptors are thought to detect microbial of IL‑1β, it induces interferon‑γ (IFNγ) production by skin rashes, prolonged associated molecular patterns (PAMPs) and endogenous activated T cells and natural killer cells in the presence episodes of fever, sensorineural damage-associated molecular patterns (DAMPs) in intra‑ of IL‑12, thereby contributing to T helper 1 (TH1) cell hearing loss, headaches, cellular compartments, similar to the role of mammalian polarization25,26. In the absence of IL‑12, IL‑18 can pro‑ cognitive deficits and renal Toll‑like receptors (TLRs) at the cell surface and within mote T 2 cell responses through the production of T 2 amyloidosis — these diseases H H 13 are classified as familial cold endosomes . Although it is incompletely understood cell cytokines such as IL‑4, IL‑5 and IL‑10 (REFS 26–28). autoinflammatory syndrome, how NLRs detect microbial ligands and DAMPs14,15, it More recently, IL‑18 has also been implicated in driving Muckle–Wells syndrome or is evident that inflammasome assembly results in the TH17 cell responses because it synergizes with IL‑23 chronic infantile neurological activation of caspase 1 (BOX 2). This evolutionarily con‑ to induce IL‑17 production by already committed cutaneous articular syndrome. 29,30 served cysteine protease is mainly known for its role TH17 cells . Thus, IL‑1β and IL‑18 are important in the maturation of the pro‑inflammatory cytokines inflammasome effector molecules, as illustrated by the interleukin‑1β (IL‑1β) and IL‑18 (REFS 16–19). marked response to therapy with IL‑1 inhibitors found IL‑1β and IL‑18 are related cytokines that are pro‑ in patients with cryopyrinopathies, who have increased duced as cytosolic precursors and usually require inflammasome activation31,32. caspase 1‑mediated cleavage for full activation and However, not all inflammasome functions can be 16–19 Department of Biochemistry, secretion . However, additional proteases, including abrogated by neutralization of IL‑1β and IL‑18. For exam‑ Ghent University, and VIB caspase 8, myeloblastin (also known as proteinase 3) ple, caspase 1‑deficient mice are resistant to lipopoly‑ Department of Medical and granzyme A, have been shown to convert pro‑IL‑1β saccharide (LPS)‑induced shock, whereas mice lacking Protein Research, Albert into a biologically active cytokine in several established both IL‑1β and IL‑18 are susceptible33. Moreover, a recent Baertsoenkaai 3, B-9000 mouse models of human disease20–23. This indicates that study showed that IL‑1β and IL‑18 are not required for Ghent, Belgium. e-mail: Mohamed.Lamkanfi@ caspase 1 is not always required for the maturation of caspase 1‑mediated clearance of several bacterial patho‑ VIB-UGent.be IL‑1β, and such redundancy in controlling IL‑1β matura‑ gens (namely, modified Salmonella enterica subsp. enterica doi:10.1038/nri2936 tion might safeguard the host immune response against serovar Typhimurium strains that constitutively express NATURE REVIEWS | IMMUNOLOGY VOLUME 11 | MARCH 2011 | 213 © 2011 Macmillan Publishers Limited. All rights reserved REVIEWS Box 1 | Inflammasomes inflammatory, cell survival and repair responses through activation of cell surface receptors, such as FGF recep‑ Inflammasomes are intracellular multiprotein complexes that mediate the tor 1, the IL‑1 and IL‑18 receptors and the receptor for proximity-induced autoactivation of caspase 1. Inflammasome-mediated caspase 1 advanced glycation end‑products (RAGE). activation has been shown to occur in macrophages, dendritic cells, epithelial cells and Although the molecular mechanism by which IL‑1β, possibly other cell types during bacterial, viral, fungal and parasitic infections. Inflammasomes are activated in response to stimulation with damage-associated IL‑18 and other proteins that lack signal peptides are molecular patterns (DAMPs), such as uric acid and ATP, and upon exposure to crystalline secreted remains obscure, several models have been substances, such as monosodium urate, silica and asbestos particles12,24. The molecular proposed to explain the release of such ‘leaderless pro‑ composition of inflammasome complexes is stimulus dependent, with certain members teins’ in microvesicles that are shed from the plasma of the NOD-like receptor (NLR) and HIN-200 receptor families functioning as the membrane, or in secretory lysosomes or exosomes38. activating platform in these complexes. Genetic studies in mice indicate the existence of Interestingly, adherent monocytes from caspase 1‑ at least four types of inflammasome (FIG. 1). Three of these contain NLR proteins, namely deficient mice and peritoneal macrophages from mice NLR family, pyrin domain-containing 1B (NLRP1B), NLR family, CARD-containing 4 lacking two inflammasome components — namely, (NLRC4) and NLRP3. The fourth type of inflammasome contains the HIN-200 protein NLR family, pyrin domain‑containing 3 (NLRP3) and absent in melanoma 2 (AIM2)12. The bipartite adaptor protein ASC (apoptosis-associated apoptosis‑associated speck‑like protein containing a speck-like protein containing a CARD; also known as PYCARD) probably has a key role in inflammasome assembly and caspase 1 activation by bridging the interaction between CARD (ASC; also known as PYCARD) — not only failed 16,18,19,40 NLRs or HIN-200 proteins and caspase 1, although the precise role of ASC in the activation to secrete IL‑1β and IL‑18 after LPS stimulation , of the NLRP1B and NLRC4 inflammasomes is debated. NLRP1B and NLRC4 contain a but were also partially defective in the secretion of the caspase recruitment domain (CARD) at their carboxyl and amino termini, respectively leaderless cytokine IL‑1α17,40. Unlike IL‑1β and IL‑18, (unlike AIM2 and NLRP3, which have a pyrin domain) and can therefore interact directly IL‑1α does not undergo caspase 1‑mediated cleavage26. with caspase 1 when overexpressed, without requiring ASC. However, evidence of a role This might indicate that caspase 1 modulates IL‑1α for ASC in the activation of the endogenous NLRC4 inflammasome is provided by the secretion indirectly by regulating the secretory path‑ observation that robust caspase 1 activation and the production of interleukin-1β (IL-1β) way of this cytokine, and may point to a broader role for and IL-18 are markedly decreased in ASC-deficient macrophages infected with viral or caspase 1 in regulating unconventional protein secretion. bacterial pathogens, or exposed to a variety of DAMPs and crystalline substances50,63,88. Indeed, pharmacological inhibition, RNA interference (RNAi)‑mediated downregulation and targeted deletion of caspase 1 were all recently shown to block the secre‑ flagellin, Legionella pneumophila and Burkholderia thai- tion of IL‑1β, IL‑1α and FGF2 by macrophages, UVA‑ landensis)34. Similarly, caspase 1‑deficient mice are more irradiated fibroblasts and UVB‑irradiated keratinocytes, susceptible to infection with Francisella tularensis than respectively41. In addition, caspase 1 activation by either mice lacking both IL‑1β and IL‑18 (REF. 35), and this the NLRP3 inflammasome or the NLR family, CARD‑ indicates that additional caspase 1‑dependent mecha‑ containing 4 (NLRC4) inflammasome was required for nisms might contribute to the control of infection. In secretion of the nuclear DAMP HMGB1 from activated this regard, several recent publications have begun to and infected macrophages33. Because