Inflammasomes in Myeloid Cells: Warriors Within.

The inflammasome is a large multimeric protein complex comprising an effector protein that demonstrates specificity for a variety of activators or ligands; an adaptor molecule; and procaspase-1, which is converted to caspase-1 upon inflammasome activation. Inflammasomes are expressed primarily by myeloid cells and are located within the cell. The macromolecular inflammasome structure can be visualized by cryo-electron microscopy. This complex has been found to play a role in a variety of disease models in mice, and several have been genetically linked to human diseases. In most cases, the effector protein is a member of the NLR (nucleotide-binding domain leucine-rich repeat-containing) or NOD (nucleotide oligomerization domain)-like receptor protein family. However, other effectors have also been described, with the most notable being AIM-2 (absent in melanoma 2), which recognizes DNA to elicit inflammasome function. This review will focus on the role of the inflammasome in myeloid cells and its role in health and disease.

Expression profile of innate immune receptors, NLRs and AIM2, in human colorectal cancer: correlation with cancer stages and inflammasome components.

NLRs (nucleotide-binding domain leucine-rich repeat proteins or NOD-like receptors) are regulators of inflammation and immunity. A subgroup of NLRs and the innate immune receptor, AIM2 (absent-in-melanoma 2), can induce the assembly of a large caspase-1 activating complex called the inflammasome. Other NLRs regulate key signaling pathways such as NF-kB and MAPK. Since inflammation is a central component of colorectal cancer (CRC), this work was undertaken to analyze NLR and AIM2 expression in human CRC by combining bioinformatics analysis and experimental verification using clinical tissue samples. Additional experiments analyzed the association of (i) gene expression and cancer staging, and (ii) gene expression among inflammasome components.Ten public CRC datasets from the Oncomine® Platform were analyzed. Genes analyzed include NLRP1, NLRP3, NLRP6, NLRP12, NLRC3, NLRC4, NLRC5, NOD1, NOD2 and AIM2. Additionally, forty case-matched cancer samples and adjacent healthy control tissues isolated from a cohort of Chinese CRC patients were profiled.Three patterns of gene expression in CRC are shown. The expression of NLRC3, a checkpoint of inflammation, and the inflammasome components NLRP1, NLRP3, NLRC4 and AIM2 were reduced in CRC. NOD1 and NOD2 expression was increased in CRC, while NLRC5, NLRP6 and NLRP12 showed little difference compared to controls. Reduced expression of NLRC3 in CRC was verified in all available databases analyzed and confirmed with our patient cohort. Furthermore, the extent of NLRC3 and AIM2 gene reduction was correlated with cancer progression. This report reveals the potential value of NLR and AIM2 genes as biomarkers of CRC and cancer progression.

The NLR adaptor ASC/PYCARD regulates DUSP10, mitogen-activated protein kinase (MAPK), and chemokine induction independent of the inflammasome.

ASC/PYCARD is a common adaptor for a diverse set of inflammasomes that activate caspase-1, most prominently the NLR-based inflammasome. Mounting evidence indicates that ASC and these NLRs also elicit non-overlapping functions, but the molecular basis for this difference is unclear. To address this, we performed microarray and network analysis of ASC shRNA knockdown cells. In pathogen-infected cells, an ASC-dependent interactome is centered on the mitogen-activated protein kinase (MAPK) ERK and on multiple chemokines. ASC did not affect the expression of MAPK but affected its phosphorylation by pathogens and Toll-like receptor agonists via suppression of the dual-specificity phosphatase, DUSP10/MKP5. Chemokine induction, DUSP function, and MAPK phosphorylation were independent of caspase-1 and IL-1ß. MAPK activation by pathogen was abrogated in Asc(-/-) but not Nlrp3(-/-), Nlrc4(-/-), or Casp1(-/-) macrophages. These results demonstrate a function for ASC that is distinct from the inflammasome in modulating MAPK activity and chemokine expression and further identify DUSP10 as a novel ASC target.

Deletion of ripA alleviates suppression of the inflammasome and MAPK by Francisella tularensis.

Francisella tularensis is a facultative intracellular pathogen and potential biothreat agent. Evasion of the immune response contributes to the extraordinary virulence of this organism although the mechanism is unclear. Whereas wild-type strains induced low levels of cytokines, an F. tularensis ripA deletion mutant (LVSΔripA) provoked significant release of IL-1ß, IL-18, and TNF-α by resting macrophages. IL-1ß and IL-18 secretion was dependent on inflammasome components pyrin-caspase recruitment domain/apoptotic speck-containing protein with a caspase recruitment domain and caspase-1, and the TLR/IL-1R signaling molecule MyD88 was required for inflammatory cytokine synthesis. Complementation of LVSΔripA with a plasmid encoding ripA restored immune evasion. Similar findings were observed in a human monocytic line. The presence of ripA nearly eliminated activation of MAPKs including ERK1/2, JNK, and p38, and pharmacologic inhibitors of these three MAPKs reduced cytokine induction by LVSΔripA. Animals infected with LVSΔripA mounted a stronger IL-1ß and TNF-α response than that of mice infected with wild-type live vaccine strain. This analysis revealed novel immune evasive mechanisms of F. tularensis.

Critical role of apoptotic speck protein containing a caspase recruitment domain (ASC) and NLRP3 in causing necrosis and ASC speck formation induced by Porphyromonas gingivalis in human cells.

Periodontal disease is a chronic inflammatory disorder that leads to the destruction of tooth-supporting tissue and affects 10-20 million people in the U.S. alone. The oral pathogen Porphyromonas gingivalis causes inflammatory host response leading to periodontal and other secondary inflammatory diseases. To identify molecular components that control host response to P. gingivalis in humans, roles for the NLR (NBD-LRR) protein, NLRP3 (cryopyrin, NALP3), and its adaptor apoptotic speck protein containing a C-terminal caspase recruitment domain (ASC) were studied. P. gingivalis strain A7436 induces cell death in THP1 monocytic cells and in human primary peripheral blood macrophages. This process is ASC and NLRP3 dependent and can be replicated by P. gingivalis LPS and Escherichia coli. P. gingivalis-induced cell death is caspase and IL-1 independent and exhibits morphological features consistent with necrosis including loss of membrane integrity and release of cellular content. Intriguingly, P. gingivalis-induced cell death is accompanied by the formation of ASC aggregation specks, a process not previously described during microbial infection. ASC specks are observed in P. gingivalis-infected primary human mononuclear cells and are dependent on NLRP3. This work shows that P. gingivalis causes ASC- and NLRP3-dependent necrosis, accompanied by ASC speck formation.

Blimp-1/PRDM1 mediates transcriptional suppression of the NLR gene NLRP12/Monarch-1.

NLR (nucleotide-binding domain, leucine-rich repeat) proteins are intracellular regulators of host defense and immunity. One NLR gene, NLRP12 (NLR family, pyrin domain containing 12)/Monarch-1, has emerged as an important inhibitor of inflammatory gene expression in human myeloid cells. This is supported by genetic analysis linking the loss of a functional NLRP12 protein to hereditary periodic fever. NLRP12 transcription is diminished by specific TLR stimulation and myeloid cell maturation, consistent with its role as a negative regulator of inflammation. The NLRP12 promoter contains a novel Blimp-1 (B lymphocyte-induced maturation protein-1)/PRDM1 (PR domain-containing 1, with ZNF domain) binding site, and Blimp-1 reduces NLRP12 promoter activity, expression, and histone 3 acetylation. Blimp-1 associates with the endogenous NLRP12 promoter in a TLR-inducible manner and mediates the down-regulation of NLRP12 expression by TLR agonists. As expected, the expression of NLRP12 and Blimp-1 is inversely correlated. Analysis of Blimp-1(-/-) murine myeloid cells provides physiologic evidence that Blimp-1 reduces NLRP12 gene expression during cell differentiation. This demonstrates a novel role for Blimp-1 in the regulation of an NLR gene.

Enhancement of DNA vaccine potency through coadministration of CIITA DNA with DNA vaccines via gene gun.

Administration of DNA vaccines via gene gun has emerged as an important form of Ag-specific immunotherapy. The MHC CIITA is a master regulator of MHC class II expression and also induces expression of class I molecules. We reasoned that the gene gun administration of CIITA DNA with DNA vaccines employing different strategies to improve MHC I and II processing could enhance DNA vaccine potency. We observed that DC-1 cells transfected with CIITA DNA lead to higher expression of MHC I and II molecules, leading to enhanced Ag presentation through the MHC I/II pathways. Furthermore, our data suggested that coadministration of DNA-encoding calreticulin (CRT) linked to human papillomavirus (HPV) 16 E6 Ag (CRT/E6) with CIITA DNA leads to enhanced E6-specific CD8(+) T cell immune responses in vaccinated mice. In addition, coadministration of the combination of CRT/E6 DNA with CIITA DNA and DNA encoding the invariant chain (Ii) linked to the pan HLA-DR-reactive epitope (Ii-PADRE) further enhanced E6-specific CD8(+) T cell immune responses in vaccinated mice. Treatment with the combination vaccine was also shown to enhance the antitumor effects and to prolong survival in TC-1 tumor-bearing mice. Vaccination with the combination vaccine also led to enhanced E6-specific CD8(+) memory T cells and to long-term protection against TC-1 tumors and prolonged survival in vaccinated mice. Thus, our findings suggest that the combination of CIITA DNA with CRT/E6 and Ii-PADRE DNA vaccines represents a potentially effective means to combat tumors in the clinical setting.

NLR, the nucleotide-binding domain leucine-rich repeat containing gene family.

The NLR (nucleotide-binding domain leucine-rich repeat containing) family is found in plants and animals, and serves as crucial regulators of inflammatory and innate immune response, though its functions are likely to extend greatly beyond innate immunity, and even beyond the immune system. This review discusses recent findings regarding the function of NLR proteins in the control of IL-1, NF-kappaB, and host response to pathogens including distinct forms of cell death. The review also covers recent advances regarding the biochemical nature of NLRs, its regulation by intracellular nucleotides and extracellular ATP, by the chaperone protein HSP90, and the ubiquitin ligase-associated protein SGT1. Its role in inflammation is linked to the formation of biochemical complexes such as the inflammasome, and its roles in cell death might be linked to the proposed formation of pyroptosome and necrosome.

Osteoblasts express NLRP3, a nucleotide-binding domain and leucine-rich repeat region containing receptor implicated in bacterially induced cell death.

UNLABELLED: Bacterially induced osteoblast apoptosis may be a major contributor to bone loss during osteomyelitis. We provide evidence for the functional expression in osteoblasts of NLRP3, a member of the NLR family of cytosolic receptors that has been implicated in the initiation of programmed cell death. INTRODUCTION: Osteoblasts undergo apoptosis after exposure to intracellular bacterial pathogens commonly associated with osteomyelitis. Death of this bone-forming cell type, in conjunction with increased numbers and activity of osteoclasts, may underlie the destruction of bone tissue at sites of bacterial infection. To date, the mechanisms responsible for bacterially induced apoptotic osteoblast cell death have not been resolved. MATERIALS AND METHODS: We used flow cytometric techniques to determine whether intracellular invasion is needed for maximal apoptotic cell death in primary osteoblasts after challenge with Salmonella enterica. In addition, we used real-time PCR and immunoblot analyses to assess osteoblast expression of members of the nucleotide-binding domain leucine-rich repeat region-containing family of intracellular receptors (NLRs) that have been predicted to be involved in the induction of programmed cell death. Furthermore, we have used co-immunoprecipitation and siRNA techniques to confirm the functionality of such sensors in this cell type. RESULTS: In this study, we showed that invasion of osteoblasts by Salmonella is necessary for maximal induction of apoptosis. We showed that murine and human osteoblasts express NLRP3 (previously known as CIAS1, cryopyrin, PYPAF1, or NALP3) but not NLRC4 (IPAF) and showed that the level of expression of this cytosolic receptor is modulated after bacterial challenge. We showed that osteoblasts express ASC, an adaptor molecule for NLRP3, and that these molecules associate after Salmonella infection. In addition, we showed that a reduction in the expression of NLRP3 attenuates Salmonella-induced reductions in the activity of an anti-apoptotic transcription factor in osteoblasts. Furthermore, we showed that NLRP3 expression is needed for caspase-1 activation and maximal induction of apoptosis in osteoblasts after infection with Salmonella. CONCLUSIONS: The functional expression of NLRP3 in osteoblasts provides a potential mechanism underlying apoptotic cell death of this cell type after challenge with intracellular bacterial pathogens and may be a significant contributory factor to bone loss at sites of infection.

Rebuilding host-pathogen interaction from the ground up: in vitro reconstitution of the inflammasome.

The inflammasome is a macromolecular complex responsible for the proteolytic processing and activation of the secreted cytokine IL-1beta. It is assembled and activated in response to upstream intracellular sensors of microbial components and cell injury. Now, Faustin et al. describe an in vitro cell-free reconstitution of the inflammasome, opening up a new avenue to better understand this innate immune pathway of microbe sensing.

Cryopyrin/NALP3 binds ATP/dATP, is an ATPase, and requires ATP binding to mediate inflammatory signaling.

The CATERPILLER (CLR/NLR) gene family encodes a family of putative nucleotide-binding proteins important for host defense. Although nucleotide binding is thought to be central to this family, this aspect is largely unstudied. The CATERPILLER protein cryopyrin/NALP3 regulates IL-1beta processing by assembling the multimeric inflammasome complex. Mutations within the exon encoding the nucleotide-binding domain are associated with hereditary periodic fevers characterized by constitutive IL-1beta production. We demonstrate that purified cryopyrin binds ATP, dATP, and ATP-agarose, but not CTP, GTP, or UTP, and exhibits ATPase activity. Mutation of the nucleotide-binding domain reduces ATP binding, caspase-1 activation, IL-1beta production, cell death, macromolecular complex formation, self-association, and association with the inflammasome component ASC. Disruption of nucleotide binding abolishes the constitutive activation of disease-associated mutants, identifying nucleotide binding by cryopyrin as a potential target for antiinflammatory pharmacologic intervention.

Monarch-1/PYPAF7 and other CATERPILLER (CLR, NOD, NLR) proteins with negative regulatory functions.

CATERPILLER is a mammalian gene family with signature NBD and LRR domains. Several members of this family are positive regulators of inflammatory responses. Others, however, exert negative effects on proinflammatory responses. These data are particularly convincing when shRNA/siRNA are used. This review focuses on the Monarch-1/PYPAF7 gene with brief discussions of CLR16.2/NOD3, PYPAF2/PAN1/NALP2, and PYPAF3.

Cutting edge: ASC mediates the induction of multiple cytokines by Porphyromonas gingivalis via caspase-1-dependent and -independent pathways.

Porphyromonas gingivalis (Pg) is a major etiologic agent for chronic periodontitis. Tissue destruction by Pg results partly from induction of host inflammatory responses through TLR2 signaling. This work examines the role of apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC), an adaptor molecule important for TLR-mediated caspase-1 activation. Results demonstrate that ASC levels are stable upon infection of human THP1 monocytic cells with Pg but decrease after cytokine induction. Using short hairpin RNA, we demonstrate an essential role for ASC in induction of IL-1beta by TLR2, 4, and 5 agonists, live Escherichia coli, and Pg. Induction of IL-6, IL-8, IL-10, and TNF also requires ASC, but this induction is not inhibited by IL-1 receptor antagonist or caspase-1 inhibitor. Similar results in U937 indicate broad applicability of these findings. Pg-infected ASC knockdown THP1 cells exhibit reduced transcript levels and NF-kappaB activation. These results suggest a role for ASC in cytokine induction by Pg involving both caspase-1-dependent and -independent mechanisms.

Genetic control of MHC class II expression.

The presentation of peptides to T cells by MHC class II molecules is of critical importance in specific recognition by the immune system. Expression of class II molecules is exquisitely controlled at the transcriptional level. A large set of proteins interact with the promoters of class II genes. The most important of these is CIITA, a master controller that orchestrates expression but does not bind directly to the promoter. The transcriptosome complex formed at class II promoters is a model for induction of gene expression.