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cASCading specks Lori Broderick & Hal M Hoffman
npg
© 2014 Nature America, Inc. All rights reserved.
Inflammasome-driven inflammation extends into the extracellular space and to neighboring cells through the passive release of specks consisting of the adaptor ASC; this perpetuates the innate immune response and adds a dimension beyond interleukin 1 to autoinflammation.
T
he first line of host defense is the innate immune system, which has developed numerous extracellular mechanisms to combat pathogens, including complement, antimicrobial peptides and neutrophil traps. Cells of the innate immune system also have receptors that detect conserved molecular motifs associated with microbes (pathogenassociated molecular patterns) and endogenous danger signals (damage-associated molecular patterns). Several families of such pattern-recognition receptors are present both on the cell surface and in the cytoplasm, including the Toll-like receptors (TLRs), C-type lectin receptors, RIG-I-like receptors and Nod leucine-rich repeat–containing receptors (NLRs)1. Together these receptors recognize infectious particles, chemicals and crystals and damaged tissues, forming a complex system to protect the host from persistent onslaught by a multitude of agents. Two studies in this issue of Nature Immunology by Latz and colleagues2 and by Pelegrin and colleagues3 provide exciting new evidence for the unification of extracellular and intracellular inflammatory cascades that amplify the host response to danger signals. While expression of the TLR family is split, with expression of TLR1, TLR2, TLR4,
Lori Broderick is in the Department of Pediatrics, University of California, La Jolla, San Diego, California, USA, and Rady Children’s Hospital San Diego, San Diego, California, USA. Hal M. Hoffman is in the Department of Pediatrics, University of California, La Jolla, San Diego, California, USA, the Department of Medicine, University of California, La Jolla, San Diego, California, USA, and Rady Children’s Hospital San Diego, San Diego, California, USA. e-mail: [email protected]
698
TLR5 and TLR6 on the cell surface and expression of TLR3, TLR7, TLR8 and TLR9 in endocytic compartments, the NLRs have traditionally been thought of as strictly cytoplasmic pattern-recognition receptors that can nucleate multimeric proteins known as ‘inflammasomes’. The intracellular formation of inflammasomes that include a sensor NLR, such as NLRP3, induces polymerization of the adaptor ASC into large cytoplasmic fibrils, cleaves caspase-1 and ultimately cascades to the maturation and release of the cyto kines interleukin 1β (IL-1β) and IL-18. These extracellular mediators promote downstream inflammation through interactions with their receptors and by inducing the expression of other proinflammatory cytokines (Fig. 1). The new publications by Latz and Pelegrin and their colleagues challenge that model of compartmentalization and demonstrate that inflammasome component particles, or ‘specks’, are released during cell death 2,3. These specks have the ability to recruit and activate pro-caspase-1 and pro-IL-1β in the extracellular compartment and to act as endogenous danger signals. This multistep multicellular pathway perpetuates inflammation, similar to a hydropower cascade, providing numerous opportunities for regulation and therapeutic targeting. Specks are visible protein aggregates of inflammasome components and have been reported in several in vitro cell systems4–6. Rather than a simple experimental observation, studies by the laboratories of Latz and Pelegrin demonstrate that the ASC specks themselves are an important mediator of inflammation driven by the innate immune system2,3. ASC specks normally form in primary macrophages and monocytic cell lines in response to activation of inflammasomes. Although they are not actively secreted, ASC
specks are released into the extracellular space secondary to caspase-1-dependent cell death (pyroptosis). Despite their passive release, these specks accumulate and may subsequently be taken up by neighboring macrophages via phagocytosis and lead to the release of IL-1β, which suggests that specks serve as an inflammation-initiating danger signal. Furthermore, these papers demonstrate that preformed, extracellular specks can induce the maturation of IL-1β in the absence of known inflammasome triggers such as K+ flux, reactive oxygen species and cathepsin B, essentially bypassing the sensor-activating portion of the inflammasome cascade. Since new formation of inflammasomes is not required in the cell taking up the specks, the active phagocytosis of these migrating specks indicates that cells that do not express inflammasome components can perform classic inflammasome-mediated functions. Through the extensive use of cell-free systems and fluorescence imaging, both groups identify extracellular inflammasome components that are further able to prolong inflammatory responses. Independently of the primary activated cell, free ASC specks can recruit NLRP3 and pro-caspase-1. This cell-independent inflammasome functionally acts to cleave precursor (‘pro-’) forms, activating caspase-1 and IL-1β outside of the cell. These data provide evidence of a local microenvironmental response to certain inflammasome triggers that may help to answer some persisting questions about known triggers of inflammation. For example, large crystals (such as monosodium urate or asbestos) that are too large to be ingested by macrophages nonetheless activate intracellular pathways that trigger the release of IL-1β, which leads to inflammation. The finding that extracellular specks can both recruit
volume 15 number 8 august 2014 nature immunology
n e w s and v i e w s Stimuli Cathepsin B Crystals
Ions
Ca2+ K+
Sensor activation
ROS
cAMP Pro-IL-1β Pro-IL-18
IL-1β
IL-1β
IL-18
IL-18
ASC speck formation
NLRP3
Cold ASC
Procaspase 1
Cytokine release and/or cell death
Active caspase-1
Release of ASC specks Pyroptosis
npg
Debbie Maizels/Nature Publishing Group
© 2014 Nature America, Inc. All rights reserved.
IL-1β
Extracellular activation of caspase-1 and IL-1β Pro-IL-1β Phagocytosis of speck Pro-IL-1β Pro-IL-18
IL-1β
Active caspase-1
IL-18 Perpetuation of inflammation via neighboring cell
Figure 1 The release of ASC specks beyond macrophage borders can trigger an inflammatory cascade. Upon sensing any of a multitude of danger signals, the sensor NLRP3 polymerizes with ASC and pro-caspase-1 to form the NLRP3 inflammasome, which leads to the cleavage and release of IL-1β and IL-18. In cells undergoing pyroptotic cell death, inflammasomes are released as ASC specks. This release of specks propels the inflammasome beyond the activated cell into the extracellular space, where it may activate free pro-IL-1β or be ingested by neighboring cells via phagocytosis, leading to further activation of caspase-1, release of cytokines and inflammation. ROS, reactive oxygen species.
and activate pro-caspase-1 and pro-IL-1β might suggest a mechanism for an amplified response to exposure to urate crystals whereby neighboring cells assist in the immune response. In vivo evaluation of this complex process in mouse models shows that the injection of fluorescence-labeled ASC specks into the intraperitoneal space leads to the recruitment of neutrophils and monocytes in an IL-1-dependent manner. Interestingly, ASC-deficient mice lack such inflammatory effects, which suggests that speck-driven inflammation is contingent on the recruitment of additional ASC molecules. However, specks remain visible in tissues for up to 96 hours following injection, which indicates a resistance to proteases and suggests that the effects of initial activation of the inflammasome might have long-lasting effects on the inflammatory microenvironment via a ‘depot’ effect.
These two groundbreaking reports2,3 appear on the heels of two manuscripts defining structural features and molecular activity of inflammasome components7,8. One of these shows that relatively few molecular sensors such as NLRP3 may lead to the polymerization of numerous molecules of ASC and caspase-1 in a self-propagating fashion in a host cell7. The independent findings from the laboratories of Latz and Pelegrin about the influence of free ASC specks after the demise of the primary cell extends the innate immune response beyond the borders of individual cells2,3. Together these groups provide further evidence of prion-like activity by inflammasome fibrils that leads to the amplification of inflammatory signaling cascades. This new evidence might have important clinical implications. Traditionally, dysregulation of the innate immune system has been associated with immunodeficiency, sepsis and the induction of autoimmunity. The novel finding
nature immunology volume 15 number 8 august 2014
that ASC specks can act as danger signals expands their inflammation-driving abilities beyond the cell membrane, which leads to additional questions about interactions between the innate immune system and adaptive immune system. Latz and colleagues identify antibodies to ASC in patients with autoimmune diseases as well as in a mouse model of lupus2. Opsonization of specks by autoantibodies would further propagate the inflammatory response, which leaves open the question of what undefined regulatory mechanisms keep inflammasome pathways in check. The study of rare hereditary inflammatory syndromes involving mainly dysregulation of the innate immune system has defined a new classification of immunological disorders known as ‘autoinflammatory disorders’. These are often characterized by a failure to control self-perpetuating IL-1β-mediated inflammation. One of the best examples of these is are the cryopyrin-associated periodic syndromes (CAPS), which are due to gain-of-function mutations in the gene that encodes NLRP3. The success of IL-1-targeted therapy supports the concept of autoinflammatory pathways driven by IL-1β. However, the finding that extracellular inflammasomes perpetuate inflammation by activating other cells provides a new potential mechanism of autoinflammation. Somatic mosaicism has been identified in patients with the typical CAPS phenotype who lack easily identifiable mutations in NLRP3, which demonstrates that mutations in only a small portion of a patient’s cells are necessary for complete disease presentation. The identification, by Pelegrin and colleagues, of ASC specks in the serum of patients with mutations in NLRP3 and specifically in patients with CAPS who have somatic mosaicism3, combined with the new evidence of the ability of ASC specks to activate bystander cells, provides intriguing mechanistic support for how patients with mutations in as few as 4% of their leukocytes can have full-blown CAPS9. As more inflammasome-driven diseases are identified, clinicians are faced with diagnostic and therapeutic dilemmas. For example, in the study from Latz and colleagues, bronchoalveolar lavage fluid from human patients with chronic obstructive pulmonary disease and pneumonia contains extracellular specks, while similar fluid from patients with pulmonary hypertension or healthy donors does not2; this suggests that ASC specks may have a role in the chronic inflammatory response in these more common diseases. These discoveries provide support for the development of new anti-inflammasome therapies targeted upstream of IL-1β formation10. 699
n e w s and v i e w s COMPETING FINANCIAL INTERESTS The authors declare competing financial interests: details are available in the online version of the paper. 1. Netea, M.G. & van der Meer, J.W. N. Engl. J. Med. 364, 60–70 (2011). 2. Franklin, B. et al. Nat. Immunol. 15, 727–737 (2014).
3. Baroja-Mazo, A. et al. Nat. Immunol. 15, 738–748 (2014). 4. Masumoto, J. et al. J. Biol. Chem. 274, 33835–33838 (1999). 5. Richards, N. et al. J. Biol. Chem. 276, 39320–39329 (2001). 6. Stutz, A., Horvath, G.L., Monks, B.G. & Latz, E. Methods Mol. Biol. 1040, 91–101 (2013).
7. Lu, A. et al. Cell 156, 1193–1206 (2014). 8. Cai, X. et al. Cell 156, 1207–1222 (2014). 9. Tanaka, N. et al. Arthritis Rheum. 63, 3625–3632 (2011). 10. Brydges, S.D. et al. J. Clin. Invest. 123, 4695–4705 (2013).
Peroxisomal MAVS activates IRF1-mediated IFN-λ production Siyuan Ding & Michael D Robek
I
nterferons represent an evolutionarily conserved group of cytokines with pivotal roles in innate immunity against microbial pathogens. On the basis of similarity in amino acid sequence and ligand-receptor interactions, the interferons are classified as type I (IFN-α, IFN-β, IFN-ε, IFN-κ and IFN-ω), type II (IFN-γ) and type III (IFN-λ). Since the discovery of IFN-λ in 2003, research on this interferon has focused mainly on the cellular responses to it, including signal transduction via the kinase Jak and transcription factor STAT induction of the expression of interferon-stimulated genes and broad inhibition of viral replication, all of which are hallmarks shared by type I interferons. Although a tissue-specific antiviral role for IFN-λ at epithelial surfaces, such as those in the lungs, liver and intestine, has been appreciated1,2, how the types I and III interferons diverge and complement each other, and especially how IFN-λ may be induced differently, have remained poorly defined. In this issue of Nature Immunology, Odendall and colleagues identify a unique signaling pathway that originates in peroxisomes and specifically triggers expression of IFN-λ but not of IFN-β in response to various viral and bacterial pathogens3. Continuing published work on the characterization of peroxisomal MAVS (MAVSpex)4, the authors demonstrate that the antiviral activity downstream of signaling via MAVS-pex can be acquired by other cells through transfer of the culture medium, which indicates the secretion of soluble factors that confer protection against viruses. This finding is consistent with the idea that Siyuan Ding and Michael D. Robek are in the Department of Pathology, Yale University School of Medicine, New Haven, Connecticut, USA. e-mail: [email protected]
700
the cell-to-cell transmission of antiviral molecules in a paracrine manner has an essential role in innate immunity. Many factors can potentially account for this transferable cellextrinsic antiviral activity, including interferoninducible bioactive lipids (25-hydroxycholesterol), second messengers (such as cGAMP) induced by pathogen-associated molecular patterns, or exosomes carrying cargo such as mRNA and protein encoded by interferon-stimulated genes or autophagy-inducing microRNA clusters5–8. Through the use of a combination of pharmacological inhibitors and knockdown via small interfering RNA, the authors observe that the antiviral property of MAVS-pex is mediated through STAT1 and the tyrosine kinases Jak1 and Jak2 and is not species specific, reminiscent of IFN-λ activity. Through studies of a wide variety of cell types and intracellular pathogens, Odendall et al.3 confirm and extend the published observation that IFN-λ is highly inducible by activators of the cytosolic receptor RIG-I and exerts potent early antiviral activity. Notably, the authors further mechanistically delineate the signaling pathways and transcription factors involved in the induction of type I and III interferons, represented by IFN-β and IFN-λ1, respectively. While RIG-I, MAVS, the transcription factors IRF3 and NF-κB and the mitogen-activated protein kinase p38 are shared by both interferon types, inhibition of the kinase Erk by the small-molecule inhibitor PD98059 or knockdown of IRF1 through the use of small interfering RNA specifically cripples the expression of either IFN-β or IFN-λ1, respectively, suggestive of a distinct route for the activation of IFN-λ (Fig. 1). In line with a report showing that IRF1 mediates the induction of IFN-λ after infection of lung epithelial cells with influenza virus or rhinovirus9, together these two pieces of evidence indicate that the involvement of IRF1 in the activation
of type III interferons is a universal mechanism independent of cell type or stimulus. The authors next investigate whether the subcellular localization of MAVS to peroxisomes is associated with IRF1-mediated induction of IFN-λ. By complementing cells in which expression of wild-type MAVS is
5′-ppp-RNA (SeV, DenV, reovirus)
RIG-I
MAVS-mito
p38
MAVS-pex
NF-κB
IRF3 IRF1
Erk
IFN-β
IFN-λ1
Figure 1 IRF1-dependent induction of IFN-λ expression by MAVS-pex. Upon infection with an RNA virus, as typified by Sendai virus (SeV), dengue virus (DenV) and reovirus, cytosolic RIG-I is activated by short 5′-triphosphate RNA (5′-ppp-RNA) and interacts with MAVS, thereby initiating a series of signaling cascades that lead to the production of type I and type III interferons. Mitochondrial MAVS (MAVS-mito) mediates the expression of both IFN-β and IFN-λ1 through common signaling molecules such as p38, NF-κB and IRF3. In contrast, MAVSpex induces IFN-λ via a mechanism involving IRF1, while activation of Erk by mitochondrial MAVS contributes only to IFN-β expression. A maximal antiviral response requires the coordination of MAVS activation from both cellular organelles.
volume 15 number 8 august 2014 nature immunology
Debbie Maizels/Nature Publishing Group
npg
© 2014 Nature America, Inc. All rights reserved.
Infection with an RNA virus induces the interferons IFN- and IFN- via the adaptor MAVS located in mitochondria, while peroxisomal MAVS selectively activates an IFN- response.
cASCading specks Lori Broderick & Hal M Hoffman
npg
© 2014 Nature America, Inc. All rights reserved.
Inflammasome-driven inflammation extends into the extracellular space and to neighboring cells through the passive release of specks consisting of the adaptor ASC; this perpetuates the innate immune response and adds a dimension beyond interleukin 1 to autoinflammation.
T
he first line of host defense is the innate immune system, which has developed numerous extracellular mechanisms to combat pathogens, including complement, antimicrobial peptides and neutrophil traps. Cells of the innate immune system also have receptors that detect conserved molecular motifs associated with microbes (pathogenassociated molecular patterns) and endogenous danger signals (damage-associated molecular patterns). Several families of such pattern-recognition receptors are present both on the cell surface and in the cytoplasm, including the Toll-like receptors (TLRs), C-type lectin receptors, RIG-I-like receptors and Nod leucine-rich repeat–containing receptors (NLRs)1. Together these receptors recognize infectious particles, chemicals and crystals and damaged tissues, forming a complex system to protect the host from persistent onslaught by a multitude of agents. Two studies in this issue of Nature Immunology by Latz and colleagues2 and by Pelegrin and colleagues3 provide exciting new evidence for the unification of extracellular and intracellular inflammatory cascades that amplify the host response to danger signals. While expression of the TLR family is split, with expression of TLR1, TLR2, TLR4,
Lori Broderick is in the Department of Pediatrics, University of California, La Jolla, San Diego, California, USA, and Rady Children’s Hospital San Diego, San Diego, California, USA. Hal M. Hoffman is in the Department of Pediatrics, University of California, La Jolla, San Diego, California, USA, the Department of Medicine, University of California, La Jolla, San Diego, California, USA, and Rady Children’s Hospital San Diego, San Diego, California, USA. e-mail: [email protected]
698
TLR5 and TLR6 on the cell surface and expression of TLR3, TLR7, TLR8 and TLR9 in endocytic compartments, the NLRs have traditionally been thought of as strictly cytoplasmic pattern-recognition receptors that can nucleate multimeric proteins known as ‘inflammasomes’. The intracellular formation of inflammasomes that include a sensor NLR, such as NLRP3, induces polymerization of the adaptor ASC into large cytoplasmic fibrils, cleaves caspase-1 and ultimately cascades to the maturation and release of the cyto kines interleukin 1β (IL-1β) and IL-18. These extracellular mediators promote downstream inflammation through interactions with their receptors and by inducing the expression of other proinflammatory cytokines (Fig. 1). The new publications by Latz and Pelegrin and their colleagues challenge that model of compartmentalization and demonstrate that inflammasome component particles, or ‘specks’, are released during cell death 2,3. These specks have the ability to recruit and activate pro-caspase-1 and pro-IL-1β in the extracellular compartment and to act as endogenous danger signals. This multistep multicellular pathway perpetuates inflammation, similar to a hydropower cascade, providing numerous opportunities for regulation and therapeutic targeting. Specks are visible protein aggregates of inflammasome components and have been reported in several in vitro cell systems4–6. Rather than a simple experimental observation, studies by the laboratories of Latz and Pelegrin demonstrate that the ASC specks themselves are an important mediator of inflammation driven by the innate immune system2,3. ASC specks normally form in primary macrophages and monocytic cell lines in response to activation of inflammasomes. Although they are not actively secreted, ASC
specks are released into the extracellular space secondary to caspase-1-dependent cell death (pyroptosis). Despite their passive release, these specks accumulate and may subsequently be taken up by neighboring macrophages via phagocytosis and lead to the release of IL-1β, which suggests that specks serve as an inflammation-initiating danger signal. Furthermore, these papers demonstrate that preformed, extracellular specks can induce the maturation of IL-1β in the absence of known inflammasome triggers such as K+ flux, reactive oxygen species and cathepsin B, essentially bypassing the sensor-activating portion of the inflammasome cascade. Since new formation of inflammasomes is not required in the cell taking up the specks, the active phagocytosis of these migrating specks indicates that cells that do not express inflammasome components can perform classic inflammasome-mediated functions. Through the extensive use of cell-free systems and fluorescence imaging, both groups identify extracellular inflammasome components that are further able to prolong inflammatory responses. Independently of the primary activated cell, free ASC specks can recruit NLRP3 and pro-caspase-1. This cell-independent inflammasome functionally acts to cleave precursor (‘pro-’) forms, activating caspase-1 and IL-1β outside of the cell. These data provide evidence of a local microenvironmental response to certain inflammasome triggers that may help to answer some persisting questions about known triggers of inflammation. For example, large crystals (such as monosodium urate or asbestos) that are too large to be ingested by macrophages nonetheless activate intracellular pathways that trigger the release of IL-1β, which leads to inflammation. The finding that extracellular specks can both recruit
volume 15 number 8 august 2014 nature immunology
n e w s and v i e w s Stimuli Cathepsin B Crystals
Ions
Ca2+ K+
Sensor activation
ROS
cAMP Pro-IL-1β Pro-IL-18
IL-1β
IL-1β
IL-18
IL-18
ASC speck formation
NLRP3
Cold ASC
Procaspase 1
Cytokine release and/or cell death
Active caspase-1
Release of ASC specks Pyroptosis
npg
Debbie Maizels/Nature Publishing Group
© 2014 Nature America, Inc. All rights reserved.
IL-1β
Extracellular activation of caspase-1 and IL-1β Pro-IL-1β Phagocytosis of speck Pro-IL-1β Pro-IL-18
IL-1β
Active caspase-1
IL-18 Perpetuation of inflammation via neighboring cell
Figure 1 The release of ASC specks beyond macrophage borders can trigger an inflammatory cascade. Upon sensing any of a multitude of danger signals, the sensor NLRP3 polymerizes with ASC and pro-caspase-1 to form the NLRP3 inflammasome, which leads to the cleavage and release of IL-1β and IL-18. In cells undergoing pyroptotic cell death, inflammasomes are released as ASC specks. This release of specks propels the inflammasome beyond the activated cell into the extracellular space, where it may activate free pro-IL-1β or be ingested by neighboring cells via phagocytosis, leading to further activation of caspase-1, release of cytokines and inflammation. ROS, reactive oxygen species.
and activate pro-caspase-1 and pro-IL-1β might suggest a mechanism for an amplified response to exposure to urate crystals whereby neighboring cells assist in the immune response. In vivo evaluation of this complex process in mouse models shows that the injection of fluorescence-labeled ASC specks into the intraperitoneal space leads to the recruitment of neutrophils and monocytes in an IL-1-dependent manner. Interestingly, ASC-deficient mice lack such inflammatory effects, which suggests that speck-driven inflammation is contingent on the recruitment of additional ASC molecules. However, specks remain visible in tissues for up to 96 hours following injection, which indicates a resistance to proteases and suggests that the effects of initial activation of the inflammasome might have long-lasting effects on the inflammatory microenvironment via a ‘depot’ effect.
These two groundbreaking reports2,3 appear on the heels of two manuscripts defining structural features and molecular activity of inflammasome components7,8. One of these shows that relatively few molecular sensors such as NLRP3 may lead to the polymerization of numerous molecules of ASC and caspase-1 in a self-propagating fashion in a host cell7. The independent findings from the laboratories of Latz and Pelegrin about the influence of free ASC specks after the demise of the primary cell extends the innate immune response beyond the borders of individual cells2,3. Together these groups provide further evidence of prion-like activity by inflammasome fibrils that leads to the amplification of inflammatory signaling cascades. This new evidence might have important clinical implications. Traditionally, dysregulation of the innate immune system has been associated with immunodeficiency, sepsis and the induction of autoimmunity. The novel finding
nature immunology volume 15 number 8 august 2014
that ASC specks can act as danger signals expands their inflammation-driving abilities beyond the cell membrane, which leads to additional questions about interactions between the innate immune system and adaptive immune system. Latz and colleagues identify antibodies to ASC in patients with autoimmune diseases as well as in a mouse model of lupus2. Opsonization of specks by autoantibodies would further propagate the inflammatory response, which leaves open the question of what undefined regulatory mechanisms keep inflammasome pathways in check. The study of rare hereditary inflammatory syndromes involving mainly dysregulation of the innate immune system has defined a new classification of immunological disorders known as ‘autoinflammatory disorders’. These are often characterized by a failure to control self-perpetuating IL-1β-mediated inflammation. One of the best examples of these is are the cryopyrin-associated periodic syndromes (CAPS), which are due to gain-of-function mutations in the gene that encodes NLRP3. The success of IL-1-targeted therapy supports the concept of autoinflammatory pathways driven by IL-1β. However, the finding that extracellular inflammasomes perpetuate inflammation by activating other cells provides a new potential mechanism of autoinflammation. Somatic mosaicism has been identified in patients with the typical CAPS phenotype who lack easily identifiable mutations in NLRP3, which demonstrates that mutations in only a small portion of a patient’s cells are necessary for complete disease presentation. The identification, by Pelegrin and colleagues, of ASC specks in the serum of patients with mutations in NLRP3 and specifically in patients with CAPS who have somatic mosaicism3, combined with the new evidence of the ability of ASC specks to activate bystander cells, provides intriguing mechanistic support for how patients with mutations in as few as 4% of their leukocytes can have full-blown CAPS9. As more inflammasome-driven diseases are identified, clinicians are faced with diagnostic and therapeutic dilemmas. For example, in the study from Latz and colleagues, bronchoalveolar lavage fluid from human patients with chronic obstructive pulmonary disease and pneumonia contains extracellular specks, while similar fluid from patients with pulmonary hypertension or healthy donors does not2; this suggests that ASC specks may have a role in the chronic inflammatory response in these more common diseases. These discoveries provide support for the development of new anti-inflammasome therapies targeted upstream of IL-1β formation10. 699
n e w s and v i e w s COMPETING FINANCIAL INTERESTS The authors declare competing financial interests: details are available in the online version of the paper. 1. Netea, M.G. & van der Meer, J.W. N. Engl. J. Med. 364, 60–70 (2011). 2. Franklin, B. et al. Nat. Immunol. 15, 727–737 (2014).
3. Baroja-Mazo, A. et al. Nat. Immunol. 15, 738–748 (2014). 4. Masumoto, J. et al. J. Biol. Chem. 274, 33835–33838 (1999). 5. Richards, N. et al. J. Biol. Chem. 276, 39320–39329 (2001). 6. Stutz, A., Horvath, G.L., Monks, B.G. & Latz, E. Methods Mol. Biol. 1040, 91–101 (2013).
7. Lu, A. et al. Cell 156, 1193–1206 (2014). 8. Cai, X. et al. Cell 156, 1207–1222 (2014). 9. Tanaka, N. et al. Arthritis Rheum. 63, 3625–3632 (2011). 10. Brydges, S.D. et al. J. Clin. Invest. 123, 4695–4705 (2013).
Peroxisomal MAVS activates IRF1-mediated IFN-λ production Siyuan Ding & Michael D Robek
I
nterferons represent an evolutionarily conserved group of cytokines with pivotal roles in innate immunity against microbial pathogens. On the basis of similarity in amino acid sequence and ligand-receptor interactions, the interferons are classified as type I (IFN-α, IFN-β, IFN-ε, IFN-κ and IFN-ω), type II (IFN-γ) and type III (IFN-λ). Since the discovery of IFN-λ in 2003, research on this interferon has focused mainly on the cellular responses to it, including signal transduction via the kinase Jak and transcription factor STAT induction of the expression of interferon-stimulated genes and broad inhibition of viral replication, all of which are hallmarks shared by type I interferons. Although a tissue-specific antiviral role for IFN-λ at epithelial surfaces, such as those in the lungs, liver and intestine, has been appreciated1,2, how the types I and III interferons diverge and complement each other, and especially how IFN-λ may be induced differently, have remained poorly defined. In this issue of Nature Immunology, Odendall and colleagues identify a unique signaling pathway that originates in peroxisomes and specifically triggers expression of IFN-λ but not of IFN-β in response to various viral and bacterial pathogens3. Continuing published work on the characterization of peroxisomal MAVS (MAVSpex)4, the authors demonstrate that the antiviral activity downstream of signaling via MAVS-pex can be acquired by other cells through transfer of the culture medium, which indicates the secretion of soluble factors that confer protection against viruses. This finding is consistent with the idea that Siyuan Ding and Michael D. Robek are in the Department of Pathology, Yale University School of Medicine, New Haven, Connecticut, USA. e-mail: [email protected]
700
the cell-to-cell transmission of antiviral molecules in a paracrine manner has an essential role in innate immunity. Many factors can potentially account for this transferable cellextrinsic antiviral activity, including interferoninducible bioactive lipids (25-hydroxycholesterol), second messengers (such as cGAMP) induced by pathogen-associated molecular patterns, or exosomes carrying cargo such as mRNA and protein encoded by interferon-stimulated genes or autophagy-inducing microRNA clusters5–8. Through the use of a combination of pharmacological inhibitors and knockdown via small interfering RNA, the authors observe that the antiviral property of MAVS-pex is mediated through STAT1 and the tyrosine kinases Jak1 and Jak2 and is not species specific, reminiscent of IFN-λ activity. Through studies of a wide variety of cell types and intracellular pathogens, Odendall et al.3 confirm and extend the published observation that IFN-λ is highly inducible by activators of the cytosolic receptor RIG-I and exerts potent early antiviral activity. Notably, the authors further mechanistically delineate the signaling pathways and transcription factors involved in the induction of type I and III interferons, represented by IFN-β and IFN-λ1, respectively. While RIG-I, MAVS, the transcription factors IRF3 and NF-κB and the mitogen-activated protein kinase p38 are shared by both interferon types, inhibition of the kinase Erk by the small-molecule inhibitor PD98059 or knockdown of IRF1 through the use of small interfering RNA specifically cripples the expression of either IFN-β or IFN-λ1, respectively, suggestive of a distinct route for the activation of IFN-λ (Fig. 1). In line with a report showing that IRF1 mediates the induction of IFN-λ after infection of lung epithelial cells with influenza virus or rhinovirus9, together these two pieces of evidence indicate that the involvement of IRF1 in the activation
of type III interferons is a universal mechanism independent of cell type or stimulus. The authors next investigate whether the subcellular localization of MAVS to peroxisomes is associated with IRF1-mediated induction of IFN-λ. By complementing cells in which expression of wild-type MAVS is
5′-ppp-RNA (SeV, DenV, reovirus)
RIG-I
MAVS-mito
p38
MAVS-pex
NF-κB
IRF3 IRF1
Erk
IFN-β
IFN-λ1
Figure 1 IRF1-dependent induction of IFN-λ expression by MAVS-pex. Upon infection with an RNA virus, as typified by Sendai virus (SeV), dengue virus (DenV) and reovirus, cytosolic RIG-I is activated by short 5′-triphosphate RNA (5′-ppp-RNA) and interacts with MAVS, thereby initiating a series of signaling cascades that lead to the production of type I and type III interferons. Mitochondrial MAVS (MAVS-mito) mediates the expression of both IFN-β and IFN-λ1 through common signaling molecules such as p38, NF-κB and IRF3. In contrast, MAVSpex induces IFN-λ via a mechanism involving IRF1, while activation of Erk by mitochondrial MAVS contributes only to IFN-β expression. A maximal antiviral response requires the coordination of MAVS activation from both cellular organelles.
volume 15 number 8 august 2014 nature immunology
Debbie Maizels/Nature Publishing Group
npg
© 2014 Nature America, Inc. All rights reserved.
Infection with an RNA virus induces the interferons IFN- and IFN- via the adaptor MAVS located in mitochondria, while peroxisomal MAVS selectively activates an IFN- response.
