A poxvirus-encoded pyrin domain protein interacts with ASC-1 to inhibit host inflammatory and apoptotic responses to infection

An orthogonal proteomic-genomic screen identifies AIM2 as a cytoplasmic DNA sensor for the inflammasome

Cytoplasmic DNA triggers activation of the innate immune system. Although 'downstream' signaling components have been characterized, the DNA-sensing components remain elusive. Here we present a systematic proteomics screen for proteins that associate with DNA, 'crossed' to a screen for transcripts induced by interferon-beta, which identified AIM2 as a candidate cytoplasmic DNA sensor. AIM2 showed specificity for double-stranded DNA. It also recruited the inflammasome adaptor ASC and localized to ASC 'speckles'. A decrease in AIM2 expression produced by RNA-mediated interference impaired DNA-induced maturation of interleukin 1beta in THP-1 human monocytic cells, which indicated that endogenous AIM2 is required for DNA recognition. Reconstitution of unresponsive HEK293 cells with AIM2, ASC, caspase-1 and interleukin 1beta showed that AIM2 was sufficient for inflammasome activation. Our data suggest that AIM2 is a cytoplasmic DNA sensor for the inflammasome.

Inflammasome recognition of influenza virus is essential for adaptive immune responses

Influenza virus infection is recognized by the innate immune system through Toll like receptor (TLR) 7 and retinoic acid inducible gene I. These two recognition pathways lead to the activation of type I interferons and resistance to infection. In addition, TLR signals are required for the CD4 T cell and IgG2a, but not cytotoxic T lymphocyte, responses to influenza virus infection. In contrast, the role of NOD-like receptors (NLRs) in viral recognition and induction of adaptive immunity to influenza virus is unknown. We demonstrate that respiratory infection with influenza virus results in the activation of NLR inflammasomes in the lung. Although NLRP3 was required for inflammasome activation in certain cell types, CD4 and CD8 T cell responses, as well as mucosal IgA secretion and systemic IgG responses, required ASC and caspase-1 but not NLRP3. Consequently, ASC, caspase-1, and IL-1R, but not NLRP3, were required for protective immunity against flu challenge. Furthermore, we show that caspase-1 inflammasome activation in the hematopoietic, but not stromal, compartment was required to induce protective antiviral immunity. These results demonstrate that in addition to the TLR pathways, ASC inflammasomes play a central role in adaptive immunity to influenza virus.

Innate sensors of influenza virus: clues to developing better intranasal vaccines

Mucosal immunity acquired by natural infection with influenza viruses at the respiratory tract is more effective and cross-protective against subsequent variant virus infection than systemic immunity induced by parenteral immunization with inactivated vaccines. To develop an effective influenza vaccine, it is beneficial to mimic the process of natural infection that bridges innate and adaptive immune systems. The innate immune system that recognizes influenza virus infection consists of several classes of pattern-recognition receptors, including the Toll-like receptors, the retinoic acid-inducible gene-I-like receptors and the NOD-like receptors. Here, we review our current understanding of the mechanism of innate recognition of influenza and how the signals emanating from the innate sensors control adaptive immunity. Further, we discuss the potential roles of these receptors in developing intranasal influenza vaccines.

The caspase-1 inflammasome: a pilot of innate immune responses

The inflammasome is a large multiprotein complex whose assembly leads to the activation of caspase-1, which promotes the maturation of proinflammatory cytokines interleukin-1beta (IL-1beta) and IL-18. Proteins encoded by the nucleotide-binding domain and leucine-rich repeat (NLR) containing gene family form the central components of inflammasomes and act as intracellular sensors to detect cytosolic microbial components and "danger" signals (such as ATP and toxins). The inflammasome not only plays a pivotal role in innate immune responses toward pathogens but also mediates the activity of aluminum adjuvants. Thus, the inflammasome and associated signaling pathways are attractive targets for new therapeutics and vaccines.

Caspases as therapeutic targets

The development of small molecules to modulate caspase activity offers a novel therapeutic strategy in the treatment of apoptosis-related and inflammatory diseases. Caspases are key mediators of apoptosis and inflammation; deregulation of their activation or expression can lead to the development of conditions such as neurodegenerative and autoinflammatory disorders. This review details the different caspase-associated disorders while focusing on caspase-1 inhibition as a potential therapeutic strategy. Problems facing the development of effective and safe caspase therapeutics will also be addressed.

Poxvirus host range genes

As a family of viruses, poxviruses collectively exhibit a broad host range and most of the individual members are capable of replicating in a wide array of cell types from various host species, at least in vitro. At the cellular level, poxvirus tropism is dependent not upon specific cell surface receptors, but rather upon: (1) the ability of the cell to provide intracellular complementing factors needed for productive virus replication, and (2) the ability of the specific virus to successfully manipulate intracellular signaling networks that regulate cellular antiviral processes downstream of virus entry. The large genomic coding capacity of poxviruses enables the virus to express a unique collection of viral proteins that function as host range factors, which specifically target and manipulate host signaling pathways to establish optimal cellular conditions for viral replication. Functionally, the known host range factors from poxviruses have been associated with manipulation of a diverse array of cellular targets, which includes cellular kinases and phosphatases, apoptosis, and various antiviral pathways. To date, only a small number of poxvirus host range genes have been identified and studied, and only a handful of these have been functionally characterized. For this reason, poxvirus host range factors represent a potential gold mine for the discovery of novel pathogen-host protein interactions. This review summarizes our current understanding of the mechanisms by which the known poxvirus host range genes, and their encoded factors, expand tropism through the manipulation of host cell intracellular signaling pathways.

Overexpression of IL-1alpha in skin differentially modulates the immune response to scarification with vaccinia virus

Transepidermal inoculation of vaccinia virus (VV), or scarification, has been used effectively for the induction of specific and long-lasting immunity to smallpox and is superior to other routes of immunization. Scarification of individuals with atopic skin disease or immune deficiency, however, can lead to persistent viral replication and result in significant morbidity and mortality. These effects of scarification presumably reflect the unique immunological properties of skin and the immune cells resident in, or recruited to, the site of inoculation. To explore these phenomena, we utilized transgenic mice engineered to overexpress IL-1alpha, a critical mediator of cutaneous inflammation, in the epidermis. Following scarification with VV, both transgenic and wild-type mice develop local pox. At high doses of VV, IL-1alpha transgenic mice recruited immune cells to the inoculation site more rapidly and demonstrated enhanced T-cell and humoral immune responses. At limiting doses, however, IL-1alpha transgenic mice could effectively control virus replication without formation of pox lesions or activation of a memory response. This study suggests that IL-1 might be useful as an adjuvant to enhance antiviral immunity and promote safer vaccination strategies; however, understanding the balance of IL-1 effects on innate and adaptive immune functions will be critical to achieve optimal results.

Mycobacterium tuberculosis prevents inflammasome activation

Mycobacterium tuberculosis (Mtb) parasitizes host macrophages and subverts host innate and adaptive immunity. Several cytokines elicited by Mtb are mediators of mycobacterial clearance or are involved in tuberculosis pathology. Surprisingly, interleukin-1beta (IL-1beta), a major proinflammatory cytokine, has not been implicated in host-Mtb interactions. IL-1beta is activated by processing upon assembly of the inflammasome, a specialized inflammatory caspase-activating protein complex. Here, we show that Mtb prevents inflammasome activation and IL-1beta processing. An Mtb gene, zmp1, which encodes a putative Zn(2+) metalloprotease, is required for this process. Infection of macrophages with zmp1-deleted Mtb triggered activation of the inflammasome, resulting in increased IL-1beta secretion, enhanced maturation of Mtb containing phagosomes, improved mycobacterial clearance by macrophages, and lower bacterial burden in the lungs of aerosol-infected mice. Thus, we uncovered a previously masked role for IL-1beta in the control of Mtb and a mycobacterial system that prevents inflammasome and, therefore, IL-1beta activation.

Acute neurodegeneration and the inflammasome: central processor for danger signals and the inflammatory response?

Activation of the inflammatory response is a crucial event in the adverse outcome of cerebral ischemia, which is promoted by proinflammatory cytokines such as interleukin (IL)-1beta. Although caspase-1 is necessary for IL-1beta processing, the 'upstream' signaling pathways were, until recently, essentially unknown. Fortunately, the inflammasome, a multiprotein complex responsible for activating caspase-1 and caspase-5, has recently been characterized. The activation of the inflammasome can result in one of several consequences such as cytokine secretion, cell death, or the development of a stress-resistant state. The significance of the inflammasome for the initiation of the inflammatory response during systemic diseases has already been shown and members of the inflammasome complex were recently found to be induced in acute brain injury. However, the specific pathophysiologic role of the inflammasome in neurodegenerative disorders still remains to be clarified. The underlying theories (e.g., danger signal theory) along with the signaling pathways that link the inflammasome to acute neurodegeneration will be discussed here. Furthermore, the stimuli that potentially activate the inflammasome in cerebral ischemia will be specified, as well as their relation to well-known pathways activating the innate immune response (e.g., Toll-like receptor signaling) and the consequences that result from their activation (beneficial versus deleterious).

Mapping of POP1-binding site on pyrin domain of ASC

Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) is an essential adaptor protein in the formation of a multiprotein complex that activates procaspase-1. ASC is also known as a modulator of NF-kappaB activation pathways. ASC has a bipartite domain structure, consisting of an N-terminal pyrin domain (PYD) and a C-terminal caspase-recruitment domain. The PYD of ASC (ASC_PYD) is known to interact with various PYD-containing intracellular danger signal sensors and PYD-only proteins. Using purified proteins, we characterized the in vitro interaction of ASC_PYD with PYD-only protein 1 (POP1). POP1 specifically interacts with ASC_PYD with a dissociation constant of 4.08 +/- 0.52 microm but does not interact with Cryopyrin. NMR and mutagenesis experiments show that a negative electrostatic potential surface patch (EPSP) on ASC_PYD, consisting of the first (H1) and fourth (H4) helices, is essential in the interaction with POP1. A positive EPSP on POP1, consisting of the second (H2) and third (H3) helices, is a counterpart of this interaction. The interaction between ASC_PYD and POP1 is similar to the interaction between caspase recruitment domains of Apaf-1 and procaspase-9. In addition, we present evidence that conformational changes at the long loop of ASC_PYD between the H2 and H3 helices can affect its interaction with POP1. Based on our observations, we propose that the positive EPSP of ASC_PYD, including the H2 and H3 helices, may be the binding site for Cryopyrin, and the interaction with Cryopyrin may induce the dissociation of POP1 from ASC_PYD.

Murine cytomegalovirus infection and apoptosis in organotypic retinal cultures

PURPOSE

An organotypic retinal culture model was used to determine the pattern of murine cytomegalovirus (MCMV) infection and whether apoptosis is induced in MCMV-infected cultured retinas.

METHODS

Retinas harvested from C57BL/6 mice were individually cultured at 37 degrees C on 3-microm filter inserts placed in 24-well plates. Some retinas were infected with MCMV (5 x 10(5) PFU/well). At days 4, 7, and 11 after infection (pi), the culture medium and cultured retinas were collected for examination.

RESULTS

Replicating virus was recovered and viral early antigen (EA)- and late antigen (LA)-positive cells were observed in the MCMV-infected retinal cultures. Most MCMV-infected cells were glia and horizontal cells. Infection resulted in atrophy of the photoreceptor cells and cytomegaly. Apoptosis of uninfected bystander cells, including photoreceptor cells and horizontal cells, was observed. TNF-alpha was produced by activated microglia during MCMV infection of the retina. Mouse apoptosis microarray studies, caspase activity studies, and RT-PCR studies showed that the genes involved in both the death receptor-mediated apoptotic pathway and the mitochondrial pathway were upregulated.

CONCLUSIONS

Many aspects of MCMV infection of retinal cultures parallel those observed during MCMV retinitis in mice. Thus, this in vitro system may be used to explore the role of apoptosis of uninfected retinal cells and the contribution of cytokines and other modulators to the pathogenesis of CMV retinitis.

The inflammasome recognizes cytosolic microbial and host DNA and triggers an innate immune response

The innate immune system recognizes nucleic acids during infection and tissue damage. Whereas viral RNA is detected by endosomal toll-like receptors (TLR3, TLR7, TLR8) and cytoplasmic RIG-I and MDA5, endosomal TLR9 and cytoplasmic DAI bind DNA, resulting in the activation of nuclear factor-kappaB and interferon regulatory factor transcription factors. However, viruses also trigger pro-inflammatory responses, which remain poorly defined. Here we show that internalized adenoviral DNA induces maturation of pro-interleukin-1beta in macrophages, which is dependent on NALP3 and ASC, components of the innate cytosolic molecular complex termed the inflammasome. Correspondingly, NALP3- and ASC-deficient mice display reduced innate inflammatory responses to adenovirus particles. Inflammasome activation also occurs as a result of transfected cytosolic bacterial, viral and mammalian (host) DNA, but in this case sensing is dependent on ASC but not NALP3. The DNA-sensing pro-inflammatory pathway functions independently of TLRs and interferon regulatory factors. Thus, in addition to viral and bacterial components or danger signals in general, inflammasomes sense potentially dangerous cytoplasmic DNA, strengthening their central role in innate immunity.

Pathogen subversion of cell-intrinsic innate immunity

The mammalian immune system has evolved under continuous selective pressure from a wide range of microorganisms that colonize and replicate in animal hosts. A complex set of signaling networks initiate both innate and adaptive immunity in response to the diverse pathogens that mammalian hosts encounter. In response, viral and microbial pathogens have developed or acquired sophisticated mechanisms to avoid, counteract and subvert sensors, signaling networks and a range of effector functions that constitute the host immune response. This balance of host response and pathogen countermeasures contributes to chronic infection in highly adapted pathogens that have coevolved with their host. In this review we outline some of the themes that are beginning to emerge in the mechanisms by which pathogens subvert the early innate immune response.

Molecular cloning and expression analysis of the ASC gene from mandarin fish and its regulation of NF-kappaB activation

Apoptosis-associated speck-like protein containing a CARD (ASC) is an adaptor protein that has a bipartite domain structure, an N-terminal PYRIN domain and a C-terminal caspase-recruitment domain (CARD). In this study, we cloned the mandarin fish ASC cDNA (mfASC), which consisted of 899bp with a 115bp 5'-UTR and a 181bp 3'-UTR. The open reading frame encoded 201 amino acids. The mfASC shows 37% identity to an ASC orthologue from zebrafish. The mfASC has two protein-protein interaction domains, an N-terminal PYRIN domain and a C-terminal CARD domain. The mfASC gene structure was determined and had a length of 3954bp with four exons separated by three introns. Northern blot analysis showed that mfASC mRNA is constitutively expressed in the head kidney, gill, hind kidney, spleen and intestine. In vitro studies, mfASC fused with green fluorescent protein appeared as a speck in the transfected 293T cells. When transiently overexpressed in 293T cells, mfASC inhibited NF-kappaB activity with or without tumor necrosis factor (TNFalpha) or lipopolysacharide (LPS) stimulation.

COPs and POPs: modulators of inflammasome activity

Inflammasomes represent molecular platforms for the activation of inflammatory caspases and are essential for processing and secretion of the inflammatory cytokines IL-1beta and IL-18. Multiple key proteins of inflammasomes contain caspase recruitment domains (CARDs) or pyrin domains (PYDs). Dissecting CARD- and PYD-mediated interactions substantially improved our understanding of the mechanisms by which these protein platforms are activated and emphasized their essential role during the inflammatory cytokine response. However, their precise regulation is still poorly understood. A family of small proteins that are composed of either a CARD or a PYD only emerged as important inflammasome regulators. These CARD-only proteins (COPs) and PYD-only proteins (POPs) function as endogenous dominant negative proteins that modulate the activity of inflammasomes in response to pathogen infection and tissue destruction. In this review we will summarize the most recent advances in the regulation of inflammasomes and highlight their importance for immunity and inflammatory disease.

A Shope Fibroma virus PYRIN-only protein modulates the host immune response

PYRIN domain (PYD) proteins have recently emerged as important signaling molecules involved in the development of innate immunity to intracellular pathogens through activation of inflammatory mediator pathways. ASC is the central adaptor protein, which links pathogen recognition by PYD-containing pathogen recognition receptors to the activation of downstream effectors, including activation of Caspase-1 and NF-kappaB. The cellular PYD-only protein 1 (cPOP1) can block the recruitment of ASC to activated PAN receptors and thereby functions as an endogenous inhibitor of the PYD-mediated signal transduction pathway. Here we describe the identification and characterization of a Shope Fibroma homolog to cPOP1. Like cPOP1, a Shope Fibroma virus-encoded POP (vPOP), co-localizes and directly associates with ASC and inhibits PYD-mediated signal transduction. Poxviruses are known to encode immune evasive proteins to promote host cell infection and suppression of the host immune response. Poxvirus-encoded vPOPs represent a novel class of immune evasive proteins and impair the host response by blocking Cryopyrin and ASC inflammasome-mediated activation of pro-Caspase-1 and subsequent processing of pro-interleukin (IL)-1beta, and expression of vPOPs causes activation of NF-kappaB.

A phylogenetic and functional overview of inflammatory caspases and caspase-1-related CARD-only proteins

Caspase 1 is a cysteinyl aspartate-specific proteinase involved in the maturation of inflammatory cytokines such as pro-IL-1β (interleukin-1β) and pro-IL-18. Caspase 1 clusters phylogenetically together with human caspases 4, 5 and 12 and murine caspases 11 and 12, and forms the group of the so-called inflammatory caspases. Caspase 1 consists of an N-terminal CARD (caspase recruitment domain) and a proteolytic domain containing the catalytic residues. The CARD-containing prodomain is involved in the formation of the protease-activating inflammasome complex. We have also found that the prodomain is necessary and sufficient for the activation of NF-κB (nuclear factor κB). The human genome also contains three caspase-1-related CARD-only decoy proteins [COP (CARD-only protein), INCA (inhibitory CARD) and ICEBERG], which are located near the caspase 1 locus. In this mini-review, we focus on the evolutionary aspects of the inflammatory caspase locus in the human, chimpanzee, Rhesus monkey, mouse and rat. Furthermore, we discuss the functional characteristics of the caspase-1-related CARD-only proteins in relation to caspase-1-mediated IL-1β maturation and NF-κB activation.

Cryopyrin-associated periodic syndromes and autoinflammation

Autoinflammatory syndromes are a distinct class of inherited diseases of cytokine dysregulation with important cutaneous features. Several disorders, including familial cold autoinflammatory syndrome (FCAS), Muckle-Wells syndrome and neonatal onset multisystem inflammatory disorder (NOMID), are associated with mutations in a common gene, CIAS-1. These disorders are now believed to represent related conditions along a spectrum of disease severity, in which FCAS is the mildest and NOMID is the most severe phenotype. Patients typically present with lifelong atypical urticaria with systemic symptoms, with potential for developing end-organ damage due to chronic inflammation. Advances in the understanding of the genetic basis of these syndromes have also revealed cytokine signalling molecules that are critical to normal regulation of inflammatory pathways. The dramatic response of these syndromes to anakinra, an interleukin (IL)-1 antagonist, highlights the important role of IL-1 cytokine signalling in the pathogenesis of this rare but fascinating class of diseases.

Regulatory molecules involved in inflammasome formation with special reference to a key mediator protein, ASC

The recent identification of cytosolic pattern recognition receptors (PRRs) with leucine-rich repeats, which recognize pathogen-associated molecular patterns (PAMPs), has been garnering considerable attention. Activated PRRs form molecular complexes called inflammasomes, consisting of related proteins that include procaspase 1[interleukin (IL) 1beta converting enzyme (ICE)]. Inflammasomes have been shown to facilitate molecular proximity, stimulate activation of procaspase 1, which consequently produces inflammatory cytokines IL-1beta and IL-18 and ultimately lead to the initiation of innate immunity. An adaptor protein, apoptosis-associated speck-like protein containing a CARD (ASC), which recruits PRRs carrying the pyrin homologous domain (PYD) and procaspase 1 through PYD and CARD, respectively, is responsible for the formation of some inflammasomes and also activation of procaspase 1. In this review, our main attention will be directed to PYD region analysis of ASC to understand the interaction between PYD-carrying PRRs and ASC. Taking into consideration the other aspects of the ASC gene in the proapoptotic ability and down-regulation by methylation, the biological function of ASC will be discussed in relation to the epigenetic aspects of infection, inflammation, and cancer.

Strategies that modulate inflammasomes: insights from host-pathogen interactions

The innate immune system is a dynamic and complex network for recognizing and responding to cellular insult or tissue damage after infection or injury. The primary effector mechanism of innate immunity is the generation of acute and chronic inflammatory responses through regulation of the processing and activation of proinflammatory caspases, particularly caspase 1, and cytokines, most notably IL-1beta and IL-18. Inflammasomes, cytosolic multi-protein complexes that function as molecular scaffolds for caspase activation, have recently emerged as the pivotal mechanism by which host innate immune and inflammatory responses are regulated. In this review, we investigate the mechanisms by which inflammasomes are modulated, both by endogenous host systems and by microbial pathogens.

NALP inflammasomes: a central role in innate immunity

Inflammasomes are cytoplasmic multiprotein complexes that mediate the maturation of the proinflammatory cytokines interleukin-1beta (IL-1beta), IL-18, and possibly IL-33 by controlling the activation of the inflammatory caspases-1 and -5. Assembly of inflammasomes depends on NOD-like receptor (NLR) family members such as NALPs, NAIP, and IPAF. Various microbial and endogenous stimuli activate different types of inflammasomes. This article focuses on the Pyrin domain containing NLRs, known as NALP proteins. Recent findings provide exciting insights into how these proteins might be activated and also provide evidence of the critical role of the NALP inflammasomes in innate immunity and inflammatory diseases.

The death domain superfamily in intracellular signaling of apoptosis and inflammation

The death domain (DD) superfamily comprising the death domain (DD) subfamily, the death effector domain (DED) subfamily, the caspase recruitment domain (CARD) subfamily, and the pyrin domain (PYD) subfamily is one of the largest domain superfamilies. By mediating homotypic interactions within each domain subfamily, these proteins play important roles in the assembly and activation of apoptotic and inflammatory complexes. In this chapter, we review the molecular complexes assembled by these proteins, the structural and biochemical features of these domains, and the molecular interactions mediated by them. By analyzing the potential molecular basis for the function of these domains, we hope to provide a comprehensive understanding of the function, structure, interaction, and evolution of this important family of domains.

Pyrin-only protein 2 modulates NF-kappaB and disrupts ASC:CLR interactions

NF-kappaB is pivotal for transactivation of cell-cycle regulatory, cytokine, and adhesion molecule genes and is dysregulated in many cancers, neurodegenerative disorders, and inflammatory diseases. Proteins with pyrin and/or caspase recruitment domains have roles in apoptosis, innate immunity, and inflammation. Many pyrin domain (PYD) proteins modulate NF-kappaB activity as well as participate in assembling both the perinuclear "apoptotic speck" and the pro-IL1beta/IL-18-converting inflammasome complex. "Pyrin-only" proteins (POP) are attractive as negative regulators of PYD-mediated functions and one such protein, POP1, has been reported. We report the identification and initial characterization of a second POP. POP2 is a 294 nt single exon gene located on human chromosome 3 encoding a 97-aa protein with sequence and predicted structural similarity to other PYDs. Highly similar to PYDs in CATERPILLER (CLR, NLR, NALP) family proteins, POP2 is less like the prototypic pyrin and ASC PYDs. POP2 is expressed principally in peripheral blood leukocytes and displays both cytoplasmic and nuclear expression patterns in transfected cells. TNF-alpha-stimulated and p65 (RelA)-induced NF-kappaB-dependent gene transcription is inhibited by POP2 in vitro by a mechanism involving changes in NF-kappaB nuclear import or distribution. While colocalizing with ASC in perinuclear specks, POP2 also inhibits the formation of specks by the CLR protein CIAS1/NALP3. Together, these observations demonstrate that POP2 is a negative regulator of NF-kappaB activity that may influence the assembly of PYD-dependent complexes.

Myxoma virus in the European rabbit: interactions between the virus and its susceptible host

Myxoma virus (MV) is a poxvirus that evolved in Sylvilagus lagomorphs, and is the causative agent of myxomatosis in European rabbits (Oryctolagus cuniculus). This virus is not a natural pathogen of O. cuniculus, yet is able to subvert the host rabbit immune system defenses and cause a highly lethal systemic infection. The interaction of MV proteins and the rabbit immune system has been an ideal model to help elucidate host/poxvirus interactions, and has led to a greater understanding of how other poxvirus pathogens are able to cause disease in their respective hosts. This review will examine how MV causes myxomatosis, by examining a selection of the identified immunomodulatory proteins that this virus expresses to subvert the immune and inflammatory pathways of infected rabbit hosts.

Cellular pyrin domain-only protein 2 is a candidate regulator of inflammasome activation

Pyrin domain (PYD) proteins have recently emerged as important signaling molecules involved in the development of innate immunity against intracellular pathogens through activation of inflammatory mediator pathways. ASC is the central adaptor protein, which links pathogen recognition by PYD-containing pathogen recognition receptors, known as PYD-Nod-like receptors (NLR), PAN, PYPAF, NALP, Nod, and Caterpiller proteins, to the activation of downstream effectors, including activation of caspase-1 and NF-kappaB. Activation of these effectors occurs when specific protein complexes, known as inflammasomes, are formed. PYD signal transduction leads to inflammasome assembly and activation of specific effector proteins. It is modulated by a cellular PYD-only protein (cPOP1), which binds to ASC and interferes with the recruitment of ASC to activated PYD-NLRs. Here we describe the identification and characterization of a second cellular POP (cPOP2), which shows highest homology to the PYD of PAN1. cPOP2 binds to ASC and PAN1, thereby blocking formation of cryopyrin and PAN1-containing inflammasomes, activation of caspase-1, and subsequent processing and secretion of bioactive interleukin-1beta. Existence of a second cPOP provides additional insights into inflammasome formation and suggests that POPs might be a common regulatory mechanism to "fine-tune" the activity of specific PYD-NLR family protein-containing inflammasomes.

Inflammasome adaptors and sensors: intracellular regulators of infection and inflammation

The NOD-like receptors have important roles in innate immunity as intracellular sensors of microbial components and cell injury. It has been proposed that these cytosolic proteins regulate the cysteine protease caspase-1 within a multiprotein complex known as the 'inflammasome'. Activation of caspase-1 leads to the cleavage and activation of pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) and IL-18, as well as host-cell death. The analysis of mice that are deficient in various inflammasome components has revealed that the inflammasome is a dynamic entity that is assembled from different adaptors in a stimulus-dependent manner. Here we review recent work on the activation of the inflammasome in response to various bacterial pathogens and tissue damage.

Caspase-1-dependent processing of pro-interleukin-1beta is cytosolic and precedes cell death

The pro-inflammatory cytokine interleukin-1beta is a key mediator of inflammation and is implicated in the pathogenesis of diverse disease states. Despite its biological importance, the mechanisms of its processing to an active form and its trafficking to the extracellular compartment remain poorly understood. Interleukin-1beta secretion is proposed to occur via several distinct mechanisms including microvesicle shedding and the regulated secretion of lysosomes. In this study, we report for the first time that caspase-1-dependent processing of pro-interleukin-1beta can occur in the cytosol following activation of P2X7-receptor. We also provide evidence that the pathway of secretion in this model is independent of the lysosomal trafficking regulator, a protein involved in lysosome secretion. Although release of interleukin-1beta occurred before the appearance of significant levels of lactate dehydrogenase in the supernatant, the cells ultimately died. It is clear that structural changes preceding cell death, occurring after caspase-1 activation, promote the cellular release of interleukin-1beta. We investigated the involvement of lipid rafts in this process and discovered that depleting the plasma membrane of cholesterol did not adversely affect interleukin-1beta secretion in response to ATP. We propose that, in macrophages, ATP-induced interleukin-1beta processing occurs in the cytosol by a mechanism that resembles pyroptosis.

Inflammation et virus: déclenchement,contournements et détournements de la réponse inflammatoire au cours des infections virales

The inflammatory process aims at opposing an early responseto the viral infections. Inflammation is supposed to delay or limit viral multiplication and dissemination until a specific immune response can be raised. This review introduces the basis of virus-induced inflammation and presents various strategies that are used by viruses to circumvent or exploit inflammation for their own benefit.

The phylogenetic analysis of hepatitis C virus isolates obtained from two Iranian carriers revealed evidence for a new subtype of HCV genotype 3

Classification of hepatitis C virus is based on phylogenetic analysis of the strains reported world wide. Different strains are classified within 6 major genotypes and several minor groups (subtypes). In addition to epidemiologic value of determining genotype/subtype of this virus, the result may change the therapeutic strategy used for a patient. During a survey on hepatitis C in Iran, we found two cases assigned as 1b genotype by PCR-RFLP on 5' UTR, but three based on core region sequencing. Fragments from 5' UTR, Core and NS5b regions were PCR-amplified and sequenced followed by phylogenetic analysis. Although the 5' UTR of this new strain is very similar to genotypes 1 and 6, analysis of core region classifies it in a separate branch of genotype 3, close to subtypes h and k. Further analysis of NS5b region put this new strain in a separate branch near other subtypes of genotype 3 and 4. These data are suggestive of a new subtype within genotype 3.

The inflammasome: first line of the immune response to cell stress

The NALP3-inflammasome is a protein complex that stimulates caspase-1 activation to promote the processing and secretion of proinflammatory cytokines. Recent work indicates that the NALP3-inflammasome can be activated by endogenous "danger signals" as well as compounds associated with pathogens (Kanneganti et al., 2006; Mariathasan et al., 2006, Martinon et al., 2006; Sutterwala et al., 2006). Here, we discuss new insights into the regulation of caspase-1 activity in the inflammatory response.

Critical role for Cryopyrin/Nalp3 in activation of caspase-1 in response to viral infection and double-stranded RNA

Viral infection induces the production of interleukin (IL)-1beta and IL-18 in macrophages through the activation of caspase-1, but the mechanism by which host cells sense viruses to induce caspase-1 activation is unknown. In this report, we have identified a signaling pathway leading to caspase-1 activation that is induced by double-stranded RNA (dsRNA) and viral infection that is mediated by Cryopyrin/Nalp3. Stimulation of macrophages with dsRNA, viral RNA, or its analog poly(I:C) induced the secretion of IL-1beta and IL-18 in a cryopyrin-dependent manner. Consistently, caspase-1 activation triggered by poly(I:C), dsRNA, and viral RNA was abrogated in macrophages lacking cryopyrin or the adaptor ASC (apoptosis-associated speck-like protein containing a caspase-activating and recruitment domain) but proceeded normally in macrophages deficient in Toll-like receptor 3 or 7. We have also shown that infection with Sendai and influenza viruses activates the cryopyrin inflammasome. Finally, cryopyrin was required for IL-1beta production in response to poly(I:C) in vivo. These results identify a mechanism mediated by cryopyrin and ASC that links dsRNA and viral infection to caspase-1 activation resulting in IL-1beta and IL-18 production.

Inflammatory caspases and inflammasomes: master switches of inflammation

Fifteen years have passed since the cloning and characterization of the interleukin-1beta-converting enzyme (ICE/caspase-1), the first identified member of a family of proteases currently known as caspases. Caspase-1 is the prototypical member of a subclass of caspases involved in cytokine maturation termed inflammatory caspases that also include caspase-4 caspase -5, caspase -11 and caspase -12. Efforts to elucidate the molecular mechanisms involved in the activation of these proteases have uncovered an important role for the NLR family members, NALPs, NAIP and IPAF. These proteins promote the assembly of multiprotein complexes termed inflammasomes, which are required for activation of inflammatory caspases. This article will review some evolutionary aspects, biochemical evidences and genetic studies, underlining the role of inflammasomes and inflammatory caspases in innate immunity against pathogens, autoinflammatory syndromes and in the biology of reproduction.

TLRs, NLRs and RLRs: a trinity of pathogen sensors that co-operate in innate immunity

Significant advances in our understanding of innate immunity have been made following the identification of three families of pathogen sensors: Toll-like receptors (TLRs), NOD-like receptors (NLRs) and RIG-I-like receptors (RLRs). Members of the TLR family recognize bacteria, viruses, fungi and protozoa; NLRs with known functions detect bacteria, and RLRs are anti-viral. It is likely that interplay between these families ensures the efficient co-ordination of innate immune responses, through either synergistic or co-operative signalling. Important interactions occur between TLRs and certain NLRs for inducing the pro-inflammatory cytokine interleukin (IL)-1beta. TLRs induce pro-IL-1beta production and prime NLR-containing multi-protein complexes, termed "inflammasomes", to respond to bacterial products and products of damaged cells. This results in caspase-1 activation and the subsequent processing of pro-IL-1beta to its active form. In this article, we hypothesize that during the first phase of the host response to infection, an important interplay occurs between these families, providing a substantial combinatorial repertoire in innate immunity.

Poxviruses aren't stuPYD

Pathogens utilize many strategies to dampen the host inflammatory response. In this issue of Immunity, a report by Johnston and colleagues reveals a poxvirus strategy that inhibits the inflammasome, arresting secretion of interleukin-1-related cytokines, thus silencing key alarms that mobilize host defenses.