Type I interferon inhibits interleukin-1 production and inflammasome activation

Innate Immune Signaling Activated by MDR Bacteria in the Airway

Health care-associated bacterial pneumonias due to multiple-drug resistant (MDR) pathogens are an important public health problem and are major causes of morbidity and mortality worldwide. In addition to antimicrobial resistance, these organisms have adapted to the milieu of the human airway and have acquired resistance to the innate immune clearance mechanisms that normally prevent pneumonia. Given the limited efficacy of antibiotics, bacterial clearance from the airway requires an effective immune response. Understanding how specific airway pathogens initiate and regulate innate immune signaling, and whether this response is excessive, leading to host-induced pathology may guide future immunomodulatory therapy. We will focus on three of the most important causes of health care-associated pneumonia, Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae, and review the mechanisms through which an inappropriate or damaging innate immune response is stimulated, as well as describe how airway pathogens cause persistent infection by evading immune activation.

The kinase activity of PKR represses inflammasome activity

The protein kinase R (PKR) functions in the antiviral response by controlling protein translation and inflammatory cell signaling pathways. We generated a transgenic, knock-in mouse in which the endogenous PKR is expressed with a point mutation that ablates its kinase activity. This novel animal allows us to probe the kinase-dependent and -independent functions of PKR. We used this animal together with a previously generated transgenic mouse that is ablated for PKR expression to determine the role of PKR in regulating the activity of the cryopyrin inflammasome. Our data demonstrate that, in contradiction to earlier reports, PKR represses cryopyrin inflammasome activity. We demonstrate that this control is mediated through the established function of PKR to inhibit protein translation of constituents of the inflammasome to prevent initial priming during innate immune signaling. These findings identify an important role for PKR to dampen inflammation during the innate immune response and caution against the previously proposed therapeutic strategy to inhibit PKR to treat inflammation.

Negative regulation of the inflammasome: keeping inflammation under control

In addition to its roles in controlling infection and tissue repair, inflammation plays a critical role in diverse and distinct chronic diseases, such as cancer, metabolic syndrome, and neurodegenerative disorders, underscoring the harmful effect of an uncontrolled inflammatory response. Regardless of the nature of the stimulus, initiation of the inflammatory response is mediated by assembly of a multimolecular protein complex called the inflammasome, which is responsible for the production of inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18. The different stimuli and mechanisms that control inflammasome activation are fairly well understood, but the mechanisms underlying the control of undesired inflammasome activation and its inactivation remain largely unknown. Here, we review recent advances in our understanding of the molecular mechanisms that negatively regulate inflammasome activation to prevent unwanted activation in the resting state, as well as those involved in terminating the inflammatory response after a specific insult to maintain homeostasis.

Altered signaling in systemic juvenile idiopathic arthritis monocytes

Systemic juvenile idiopathic arthritis (sJIA) is characterized by systemic inflammation and arthritis. Monocytes are implicated in sJIA pathogenesis, but their role in disease is unclear. The response of sJIA monocytes to IFN may be dysregulated. We examined intracellular signaling in response to IFN type I (IFNα) and type II (IFNγ) in monocytes during sJIA activity and quiescence, in 2 patient groups. Independent of disease activity, monocytes from Group 1 (collected between 2002 and 2009) showed defective STAT1 phosphorylation downstream of IFNs, and expressed higher transcript levels of SOCS1, an inhibitor of IFN signaling. In the Group 2 (collected between 2011 and 2014), monocytes of patients with recent disease onset were IFNγ hyporesponsive, but in treated, quiescent subjects, monocytes were hyperresponsive to IFNγ. Recent changes in medication in sJIA may alter the IFN hyporesponsiveness. Impaired IFN/pSTAT1 signaling is consistent with skewing of sJIA monocytes away from an M1 phenotype and may contribute to disease pathology.

IFN-γ Primes Keratinocytes for HSV-1-Induced Inflammasome Activation

Inflammasomes are immune complexes that induce an inflammatory response upon sensing of different stress signals. This effect is mainly mediated by activation and secretion of the proinflammatory cytokines proIL-1β and -18. Here we report that infection of human primary keratinocytes with the double-stranded DNA viruses modified vaccinia virus Ankara (MVA) or herpes simplex virus type 1 (HSV-1)-induced secretion of mature IL-1β and -18. This secretion was dependent on several inflammasome complexes; however, the absent in melanoma 2 (AIM2) inflammasome, which is activated by binding of double-stranded DNA, played the most important role. Whereas prestimulation of keratinocytes with IFN-γ moderately increased MVA-induced IL-1β and IL-18 secretion, it was essential for substantial secretion of these cytokines in response to herpes simplex virus type 1 infection. IFN-γ partially restored HSV-1 suppressed proIL-1β expression and was also required for inflammasome activation. Most importantly, IFN-γ strongly suppressed virus replication in keratinocytes in vitro and ex vivo, which was independent of inflammasome activation. Our results suggest that, similar to Herpesviridae infection in mice, HSV-1 replication in human skin is controlled by a positive feedback loop of keratinocyte-derived IL-1/IL-18 and IFN-γ expressed by immune cells.

Galectins regulate the inflammatory response in airway epithelial cells exposed to microbial neuraminidase by modulating the expression of SOCS1 and RIG1

Influenza patients frequently display increased susceptibility to Streptococcus pneumoniae co-infection and sepsis, the prevalent cause of mortality during influenza pandemics. However, the detailed mechanisms by which an influenza infection predisposes patients to suffer pneumococcal pneumonia are not fully understood. A murine model for influenza infection closely reflects the observations in human patients, since if the animals that have recovered from influenza A virus (IAV) sublethal infection are challenged with S. pneumoniae, they undergo a usually fatal uncontrolled cytokine response. We have previously demonstrated both in vitro and in vivo that the expression and secretion of galectin-1 (Gal1) and galectin-3 (Gal3) are modulated during IAV infection, and that the viral neuraminidase unmasks galactosyl moieties in the airway epithelia. In this study we demonstrate in vitro that the binding of secreted Gal1 and Gal3 to the epithelial cell surface modulates the expression of SOCS1 and RIG1, and activation of ERK, AKT or JAK/STAT1 signaling pathways, leading to a disregulated expression and release of pro-inflammatory cytokines. Our results suggest that the activity of the viral and pneumococcal neuraminidases on the surface of the airway epithelial cells function as a "danger signal" that leads to rapid upregulation of SOCS1 expression to prevent an uncontrolled inflammatory response. The binding of extracellular Gal1 or Gal3 to the galactosyl moieties unmasked on the surface of airway epithelial cells can either "fine-tune" or severely disregulate this process, respectively, the latter potentially leading to hypercytokinemia.

Ion efflux and influenza infection trigger NLRP3 inflammasome signaling in human dendritic cells

The nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome, a multiprotein complex, is an essential intracellular mediator of antiviral immunity. In murine dendritic cells, this complex responds to a wide array of signals, including ion efflux and influenza A virus infection, to activate caspase-1-mediated proteolysis of IL-1β and IL-18 into biologically active cytokines. However, the presence and function of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome in human dendritic cells, in response to various triggers, including viral infection, has not been defined clearly. Here, we delineate the contribution of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome to the secretion of IL-1β, IL-18, and IL-1α by human dendritic cells (monocyte-derived and primary conventional dendritic cells). Activation of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome in human dendritic cells by various synthetic activators resulted in the secretion of bioactive IL-1β, IL-18, and IL-1α and induction of pyroptotic cell death. Cellular IL-1β release depended on potassium efflux and the activity of proteins nucleotide-binding oligomerization domain-like receptor protein 3 and caspase-1. Likewise, influenza A virus infection of dendritic cells resulted in priming and activation of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome and secretion of IL-1β and IL-18 in an M2- and nucleotide-binding oligomerization domain-like receptor protein 3-dependent manner. The magnitude of priming by influenza A virus varied among different strains and inversely corresponded to type I IFN production. To our knowledge, this is the first report describing the existence and function of the nucleotide-binding oligomerization domain-like receptor protein 3 inflammasome in human dendritic cells and the ability of influenza A virus to prime and activate this pathway in human dendritic cells, with important implications for antiviral immunity and pathogenesis.

Update on psoriasis immunopathogenesis and targeted immunotherapy

Over recent years, significant progress has been made in characterisation of the underlying pathogenic mechanisms in psoriasis, a common cutaneous disease that is associated with major systemic co-morbidity and reduced life expectancy. Basic science discoveries have informed the design of novel therapeutic approaches, many of which are now under evaluation in late-stage clinical trials. Here we describe the complex interplay between immune cell types and cytokine networks that acts within self-perpetuating feedback loops to drive cutaneous inflammation in psoriasis. Genetic studies have been pivotal in the construction of the disease model and more recently have uncovered a distinct aetiology for rare, pustular variants of psoriasis. The translation of mechanistic insights into potential advancements in clinical care will also be described, including several treatments that target the interleukin-23 (IL-23)/T17 immune axis.

NLRP3 inflammasome and its inhibitors: a review

Inflammasomes are newly recognized, vital players in innate immunity. The best characterized is the NLRP3 inflammasome, so-called because the NLRP3 protein in the complex belongs to the family of nucleotide-binding and oligomerization domain-like receptors (NLRs) and is also known as “pyrin domain-containing protein 3”. The NLRP3 inflammasome is associated with onset and progression of various diseases, including metabolic disorders, multiple sclerosis, inflammatory bowel disease, cryopyrin-associated periodic fever syndrome, as well as other auto-immune and auto-inflammatory diseases. Several NLRP3 inflammasome inhibitors have been described, some of which show promise in the clinic. The present review will describe the structure and mechanisms of activation of the NLRP3 inflammasome, its association with various auto-immune and auto-inflammatory diseases, and the state of research into NLRP3 inflammasome inhibitors.

Gadolinium-based compounds induce NLRP3-dependent IL-1β production and peritoneal inflammation

OBJECTIVE

Nephrogenic systemic fibrosis (NSF) is a progressive fibrosing disorder that may develop in patients with chronic kidney disease after administration of gadolinium (Gd)-based contrast agents (GBCAs). In the setting of impaired renal clearance of GBCAs, Gd deposits in various tissues and fibrosis subsequently develops. However, the precise mechanism by which fibrosis occurs in NSF is incompletely understood. Because other profibrotic agents, such as silica or asbestos, activate the nucleotide-binding oligomerisation domain (NOD)-like receptor protein 3 (NLRP3) inflammasome and initiate interleukin (IL)-1β release with the subsequent development of fibrosis, we evaluated the effects of GBCAs on inflammasome activation.

METHODS

Bone marrow derived macrophages from C57BL/6, Nlrp3(-/-) and Asc(-/-) mice were incubated with three Gd-containing compounds and IL-1β activation and secretion was detected by ELISA and western blot analysis. Inflammasome activation and regulation was investigated in IL-4- and interferon (IFN)γ-polarised macrophages by ELISA, quantitative real time (qRT)-PCR and NanoString nCounter analysis. Furthermore, C57BL/6 and Nlrp3(-/-)mice were intraperitoneally injected with GBCA and recruitment of inflammatory cells to the peritoneum was analysed by fluorescence-activated cell sorting (FACS).

RESULTS

Free Gd and GBCAs activate the NLRP3 inflammasome and induce IL-1β secretion in vitro. Gd-diethylenetriaminepentaacetic acid also induces the recruitment of neutrophils and inflammatory monocytes to the peritoneum in vivo. Gd activated IL-4-polarised macrophages more effectively than IFNγ-polarised macrophages, which preferentially expressed genes known to downregulate inflammasome activity.

CONCLUSIONS

These data suggest that Gd released from GBCAs triggers a NLRP3 inflammasome-dependent inflammatory response that leads to fibrosis in an appropriate clinical setting. The preferential activation of IL-4-differentiated macrophages is consistent with the predominantly fibrotic presentation of NSF.

Innate immune perturbations, accumulating DAMPs and inflammasome dysregulation: A ticking time bomb in ageing

Ageing has pronounced effects on the immune system, including on innate immune cells. Whilst most studies suggest that total numbers of different innate immune cell populations do not change dramatically during ageing, many of their functions such as phagocytosis, antigen presentation and inflammatory molecule secretion decline. In contrast, many endogenous damage-associated molecular patterns (DAMPs) accumulate during ageing. These include reactive oxygen species (ROS) released from damaged mitochondria, extracellular nucleotides like ATP, high mobility group box (HMGB) 1 protein, oxidized low density lipoprotein, amyloid-beta (Aβ), islet amyloid polypeptide and particulates like monosodium urate (MSU) crystals and cholesterol crystals. Some of these DAMPs trigger the activation of inflammasomes, cytosolic danger sensing signalling platforms that drive both the maturation of specific pro-inflammatory mediators such as IL-1β, as well as the initiation of pro-inflammatory pyroptotic cell death. Herein, we review the evidence that dysregulated inflammasome activation, via altered innate immune cell functions and elevated levels of DAMPs, contributes to the establishment of chronic, low-grade inflammation (characterized by elevated levels of IL-6 and C-reactive protein) and the development of age-related pathological processes.

The emerging role of nuclear viral DNA sensors

Detecting pathogenic DNA by intracellular receptors termed "sensors" is critical toward galvanizing host immune responses and eliminating microbial infections. Emerging evidence has challenged the dogma that sensing of viral DNA occurs exclusively in sub-cellular compartments normally devoid of cellular DNA. The interferon-inducible protein IFI16 was shown to bind nuclear viral DNA and initiate immune signaling, culminating in antiviral cytokine secretion. Here, we review the newly characterized nucleus-originating immune signaling pathways, their links to other crucial host defenses, and unique mechanisms by which viruses suppress their functions. We frame these findings in the context of human pathologies associated with nuclear replicating DNA viruses.

Type I IFNs Act upon Hematopoietic Progenitors To Protect and Maintain Hematopoiesis during Pneumocystis Lung Infection in Mice

Although acquired bone marrow failure (BMF) is considered a T cell-mediated autoimmune disease, few studies have considered contributing roles of innate immune deviations following otherwise innocuous infections as a cause underlying the immune defects that lead to BMF. Type I IFN signaling plays an important role in protecting hematopoiesis during systemic stress responses to the opportunistic fungal pathogen Pneumocystis. During Pneumocystis lung infection, mice deficient in both lymphocytes and type I IFN receptor (IFrag(-/-)) develop rapidly progressing BMF associated with accelerated hematopoietic cell apoptosis. However, the communication pathway eliciting the induction of BMF in response to this strictly pulmonary infection has been unclear. We developed a conditional-null allele of Ifnar1 and used tissue-specific induction of the IFrag(-/-) state and found that, following Pneumocystis lung infection, type I IFNs act not only in the lung to prevent systemic immune deviations, but also within the progenitor compartment of the bone marrow to protect hematopoiesis. In addition, transfer of sterile-filtered serum from Pneumocystis-infected mice as well as i.p. injection of Pneumocystis into uninfected IFrag(-/-) mice induced BMF. Although specific cytokine deviations contribute to induction of BMF, immune-suppressive treatment of infected IFrag(-/-) mice ameliorated its progression but did not prevent loss of hematopoietic progenitor functions. This suggested that additional, noncytokine factors also target and impair progenitor functions; and interestingly, fungal β-glucans were also detected in serum. In conclusion, our data demonstrate that type 1 IFN signaling protects hematopoiesis within the bone marrow compartment from the damaging effects of proinflammatory cytokines elicited by Pneumocystis in the lung and possibly at extrapulmonary sites via circulating fungal components.

Inflammasome/IL-1β Responses to Streptococcal Pathogens

Inflammation mediated by the inflammasome and the cytokine IL-1β are some of the earliest and most important alarms to infection. These pathways are responsive to the virulence factors that pathogens use to subvert immune processes, and thus are typically activated only by microbes with potential to cause severe disease. Among the most serious human infections are those caused by the pathogenic streptococci, in part because these species numerous strategies for immune evasion. Since the virulence factor armament of each pathogen is unique, the role of IL-1β and the pathways leading to its activation varies for each infection. This review summarizes the role of IL-1β during infections caused by streptococcal pathogens, with emphasis on emergent mechanisms and concepts countering paradigms determined for other organisms.

Interferon-α curbs production of interleukin-22 by human peripheral blood mononuclear cells exposed to live Borrelia burgdorferi

Cytokine networks initiated by means of innate immunity are regarded as a major determinant of host defence in response to acute infection by bacteria including Borrelia burgdorferi. Herein, we demonstrate that interferon (IFN)-α, either endogenously produced after exposure of cells to toll-like receptor-9-activating CpG oligonucleotides or provided as recombinant cytokine, weakens activation of the anti-bacterial interleukin (IL)-1/IL-22 axis in human peripheral blood mononuclear cells exposed to viable B. burgdorferi. As IFN-α has been related to pathological dissemination of the spirochaete, data suggest an immunoregulatory role of type I IFN in this context that is able to significantly modify cytokine profiles thereby possibly determining early course of B. burgdorferi infection.

Chronic TLR Stimulation Controls NLRP3 Inflammasome Activation through IL-10 Mediated Regulation of NLRP3 Expression and Caspase-8 Activation

While the molecular mechanisms promoting activation of the Nod-like Receptor (NLR) family member NLRP3 inflammasome are beginning to be defined, little is known about the mechanisms that regulate the NLRP3 inflammasome. Acute (up to 4 hours) LPS stimulation, followed by ATP is frequently used to activate the NLRP3 inflammasome in macrophages. Interestingly, we observed that the ability of LPS to license NLRP3 is transient, as prolonged (12 to 24 hours) LPS exposure was a relatively ineffective priming stimulus. This suggests that relative to acute LPS, chronic LPS exposure triggers regulatory mechanisms to dampen NLRP3 activation. Transfer of culture supernatants from macrophages stimulated with LPS for 24 hours dramatically reduced ATP- and nigericin-induced NLRP3 inflammasome activation in naïve macrophages. We further identified IL-10 as the secreted inflammasome-tolerizing factor that acts in an autocrine manner to control activation of the NLRP3 inflammasome. Finally, we demonstrated that IL-10 dampens NLRP3 expression to control NLRP3 inflammasome activation and subsequent caspase-8 activation. In conclusion, we have uncovered a mechanism by which chronic, but not acute, LPS exposure induces IL-10 to dampen NLRP3 inflammasome activation to avoid overt inflammation.

Looking for Pyroptosis-Modulating miRNAs as a Therapeutic Target for Improving Myocardium Survival

Pyroptosis is the most recently identified type of regulated cell death with inflammatory response and has characteristics distinct from those of apoptosis or necrosis. Recently, independent studies have reported that small noncoding RNAs termed microRNAs (miRNAs) are involved in the regulation of pyroptosis. Nevertheless, only a handful of empirical data regarding miRNA-dependent regulation of pyroptosis is currently available. This review is aimed to provide a current update on the role of miRNAs in pyroptosis and to offer suggestions for future studies probing miRNAs as a linker connecting pyroptosis to various cardiovascular diseases (CVDs) and their potential as a therapeutic target for preventing excessive cell death of myocardium during CVDs.

HIV-tuberculosis-associated immune reconstitution inflammatory syndrome is characterized by Toll-like receptor and inflammasome signalling

Patients with HIV-associated tuberculosis (TB) initiating antiretroviral therapy (ART) may develop immune reconstitution inflammatory syndrome (TB-IRIS). No biomarkers for TB-IRIS have been identified and the underlying mechanisms are unclear. Here we perform transcriptomic profiling of the blood samples of patients with HIV-associated TB. We identify differentially abundant transcripts as early as week 0.5 post ART initiation that predict downstream activation of proinflammatory cytokines in patients who progress to TB-IRIS. At the characteristic time of TB-IRIS onset (week 2), the signature is characterized by over-representation of innate immune mediators including TLR signalling and TREM-1 activation of the inflammasome. In keeping with the transcriptional data, concentrations of plasma cytokines and caspase-1/5 are elevated in TB-IRIS. Inhibition of MyD88 adaptor and group 1 caspases reduces secretion of cytokines including IL-1 in TB-IRIS patients. These data provide insight on the pathogenesis of TB-IRIS and may assist the development of specific therapies.

Some patients with HIV-associated tuberculosis develop an immune reconstitution inflammatory syndrome (TB-IRIS) in response to antiretroviral therapy. Here the authors identify genes differentially expressed in patients likely to progress to TB-IRIS and find activation of Toll-like receptor and inflammasome pathways.

The pathogenic role of the inflammasome in neurodegenerative diseases

The inflammasome is a large macromolecular complex that contains multiple copies of a receptor or sensor of pathogen-derived or damage-derived molecular patterns, pro-caspase-1, and an adaptor called ASC (apoptotic speck containing protein with a CARD), which results in caspase-1 maturation. Caspase-1 then mediates the release of pro-inflammatory cytokines such as IL-1β and IL-18. These cytokines play critical roles in mediating immune responses during inflammation and innate immunity. Broader studies of the inflammasome over the years have implicated their roles in the pathogenesis of a variety of inflammatory diseases. Recently, studies have shown that the inflammasome modulates neuroinflammatory cells and the initial stages of neuroinflammation. A secondary cascade of events associated with neuroinflammation (such as oxidative stress) has been shown to activate the inflammasome, making the inflammasome a promising therapeutic target in the modulation of neurodegenerative diseases. This review will focus on the pathogenic role that inflammasomes play in neurologic diseases such as Alzheimer's disease, traumatic brain injury, and multiple sclerosis. We here review the role of the inflammasome in the pathogenesis of traumatic brain injury (TBI). TBI is initiated by physical force exerted to head, resulting in neuronal injury and death. Primary insult is followed by a secondary cascade of events following neuroinflammation such as mitochondrial dysfunction, production of reactive oxygen species, potassium effluxes, and release of circulating DNA. These events can potentially trigger the activation of NLRP3, NLRP1, and AIM2 during TBI but have yet to be confirmed (dashed lines). NLRP3, NLRP1, and AIM2 associate with the adaptor protein ASC, which initiates the cleavage of pro-caspase-1 to the mature form of caspase-1 which cleaves pro-IL-1β and pro-IL-18 into their mature forms of IL-1β and IL-18.

Multiple Cathepsins Promote Pro-IL-1β Synthesis and NLRP3-Mediated IL-1β Activation

Sterile particles induce robust inflammatory responses that underlie the pathogenesis of diseases like silicosis, gout, and atherosclerosis. A key cytokine mediating this response is IL-1β. The generation of bioactive IL-1β by sterile particles is mediated by the NOD-like receptor containing a pyrin domain 3 (NLRP3) inflammasome, although exactly how this occurs is incompletely resolved. Prior studies have found that the cathepsin B inhibitor, Ca074Me, suppresses this response, supporting a model whereby ingested particles disrupt lysosomes and release cathepsin B into the cytosol, somehow activating NLRP3. However, reports that cathepsin B-deficient macrophages have no defect in particle-induced IL-1β generation have questioned cathepsin B's involvement. In this study, we examine the hypothesis that multiple redundant cathepsins (not just cathepsin B) mediate this process by evaluating IL-1β generation in murine macrophages, singly or multiply deficient in cathepsins B, L, C, S and X. Using an activity-based probe, we measure specific cathepsin activity in living cells, documenting compensatory changes in cathepsin-deficient cells, and Ca074Me's dose-dependent cathepsin inhibition profile is analyzed in parallel with its suppression of particle-induced IL-1β secretion. Also, we evaluate endogenous cathepsin inhibitors cystatins C and B. Surprisingly, we find that multiple redundant cathepsins, inhibited by Ca074Me and cystatins, promote pro-IL-1β synthesis, and to our knowledge, we provide the first evidence that cathepsin X plays a nonredundant role in nonparticulate NLRP3 activation. Finally, we find cathepsin inhibitors selectively block particle-induced NLRP3 activation, independently of suppressing pro-IL-1β synthesis. Altogether, we demonstrate that both small molecule and endogenous cathepsin inhibitors suppress particle-induced IL-1β secretion, implicating roles for multiple cathepsins in both pro-IL-1β synthesis and NLRP3 activation.

Pathogenic NLRP3 Inflammasome Activity during Candida Infection Is Negatively Regulated by IL-22 via Activation of NLRC4 and IL-1Ra

Candida albicans is a well-tolerated resident of human mucosal tissues. This implies that host defense mechanisms cooperate to limit inflammation while controlling fungal burden. The cytokine IL-22 and inflammasomes are essential components of the mucosal responses to C. albicans. How these components cooperate to mediate the balance of inflammation and host defense is not explored. We find that NLRP3 inflammasome activation promotes neutrophil recruitment and inflammation during infection and that this activity is counteracted by IL-22. Mechanistically, IL-22 activated NLRC4 for sustained production of the IL-1 receptor antagonist IL-1Ra, which restrained NLRP3 activity. Symptomatic infection in mice and humans occurred under conditions of IL-1Ra deficiency and was rescued in mice by replacement therapy with the recombinant IL-1Ra anakinra. Thus, pathogenic inflammasome activity during Candida infection is negatively regulated by the IL-22/NLRC4/IL-1Ra axis. Our findings offer insights into the pathogenesis of C. albicans and suggest therapeutic avenues for candidiasis.

Type I IFN Induction via Poly-ICLC Protects Mice against Cryptococcosis

Cryptococcus neoformans is the most common cause of fungal meningoencephalitis in AIDS patients. Depletion of CD4 cells, such as occurs during advanced AIDS, is known to be a critical risk factor for developing cryptococcosis. However, the role of HIV-induced innate inflammation in susceptibility to cryptococcosis has not been evaluated. Thus, we sought to determine the role of Type I IFN induction in host defense against cryptococci by treatment of C. neoformans (H99) infected mice with poly-ICLC (pICLC), a dsRNA virus mimic. Unexpectedly, pICLC treatment greatly extended survival of infected mice and reduced fungal burdens in the brain. Protection from cryptococcosis by pICLC-induced Type I IFN was mediated by MDA5 rather than TLR3. PICLC treatment induced a large, rapid and sustained influx of neutrophils and Ly6Chigh monocytes into the lung while suppressing the development of eosinophilia. The pICLC-mediated protection against H99 was CD4 T cell dependent and analysis of CD4 T cell polyfunctionality showed a reduction in IL-5 producing CD4 T cells, marginal increases in Th1 cells and dramatic increases in RORγt+ Th17 cells in pICLC treated mice. Moreover, the protective effect of pICLC against H99 was diminished in IFNγ KO mice and by IL-17A neutralization with blocking mAbs. Furthermore, pICLC treatment also significantly extended survival of C. gattii infected mice with reduced fungal loads in the lungs. These data demonstrate that induction of type I IFN dramatically improves host resistance against the etiologic agents of cryptococcosis by beneficial alterations in both innate and adaptive immune responses.

The ASK family kinases differentially mediate induction of type I interferon and apoptosis during the antiviral response

Viral infection activates host defense mechanisms, including the production of type I interferon (IFN) and the apoptosis of infected cells. We investigated whether these two antiviral responses were differentially regulated in infected cells. We showed that the mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) apoptosis signal-regulating kinase 1 (ASK1) was activated in cells by the synthetic double-stranded RNA analog polyinosinic:polycytidylic acid [poly(I:C)] and by RNA viruses, and that ASK1 played an essential role in both the induction of the gene encoding IFN-β (IFNB) and apoptotic cell death. In contrast, we found that the MAPKKK ASK2, a modulator of ASK1 signaling, was essential for ASK1-dependent apoptosis, but not for inducing IFNB expression. Furthermore, genetic deletion of either ASK1 or ASK2 in mice promoted the replication of influenza A virus in the lung. These results indicated that ASK1 and ASK2 are components of the antiviral defense mechanism and suggested that ASK2 acts as a key modulator that promotes apoptosis rather than the type I IFN response. Because ASK2 is selectively present in epithelium-rich tissues, such as the lung, ASK2-dependent apoptosis may contribute to an antiviral defense in tissues with a rapid repair rate in which cells could be readily replaced.

Treating rheumatological diseases and co-morbidities with interleukin-1 blocking therapies

The inflammatory cytokines IL-1α and IL-1β orchestrate local and systemic inflammatory responses underlying a broad spectrum of diseases. Three agents for reducing IL-1 activities are currently available. Anakinra is a recombinant form of the naturally occurring IL-1 receptor antagonist. Anakinra binds to the IL-1 receptor and prevents the activity of IL-1α and IL-1β. The soluble decoy receptor rilonacept and the neutralizing mAb canakinumab block IL-1β. A mAb directed against the IL-1 receptor and a neutralizing anti-human IL-1α are in clinical trials. The availability of therapies specifically targeting IL-1 unveiled the pathological role of IL-1-mediated inflammation in a broadening list of diseases. Conditions effectively treated with agents blocking IL-1 range from classic rheumatic diseases, such as RA and gout, to autoinflammatory syndromes, such as systemic JIA and FMF. However, IL-1 antagonism is also effective against highly prevalent inflammatory diseases, namely cardiovascular diseases and type 2 diabetes, conditions that are frequently encountered as co-morbidities in patients with rheumatic diseases. Thereby, IL-1 inhibition has the potential to lift the burden of disease for patients with rheumatic conditions, but also to provide clinical benefits beyond the efficacy on osteoarticular manifestations.

Salmonella Typhimurium Co-Opts the Host Type I IFN System To Restrict Macrophage Innate Immune Transcriptional Responses Selectively

Innate immune inflammatory responses are subject to complex layers of negative regulation at intestinal mucosal surfaces. Although the type I IFN system is critical for amplifying antiviral immunity, it has been shown to play a homeostatic role in some models of autoimmune inflammation. Type I IFN is triggered in the gut by select bacterial pathogens, but whether and how the type I IFN might regulate innate immunity in the intestinal environment have not been investigated in the context of Salmonella enterica serovar Typhimurium (ST). ST infection of human or murine macrophages reveals that IFN-β selectively restricts the transcriptional responses mediated by both the TLRs and the NOD-like receptors. Specifically, IFN-β potently represses ST-dependent innate induction of IL-1 family cytokines and neutrophil chemokines. This IFN-β-mediated transcriptional repression was independent of the effects of IFN-β on ST-induced macrophage cell death, but significantly dependent on IL-10 regulation. We further evaluated ST pathogenesis in vivo following oral inoculation of mice lacking IFN-β. We show that IFN-β(-/-) mice exhibit greater resistance to oral ST infection and a slower spread of ST to distal sterile sites. This work provides mechanistic insight into the relationship between ST and type I IFN, and demonstrates an additional mechanism by which IFN-β may promote spread of enteric pathogens.

Cardiac fibroblasts as sentinel cells in cardiac tissue: Receptors, signaling pathways and cellular functions

Cardiac fibroblasts (CF) not only modulate extracellular matrix (ECM) proteins homeostasis, but also respond to chemical and mechanical signals. CF express a variety of receptors through which they modulate the proliferation/cell death, autophagy, adhesion, migration, turnover of ECM, expression of cytokines, chemokines, growth factors and differentiation into cardiac myofibroblasts (CMF). Differentiation of CF to CMF involves changes in the expression levels of various receptors, as well as, changes in cell phenotype and their associated functions. CF and CMF express the β2-adrenergic receptor, and its stimulation activates PKA and EPAC proteins, which differentially modulate the CF and CMF functions mentioned above. CF and CMF also express different levels of Angiotensin II receptors, in particular, AT1R activation increases collagen synthesis and cell proliferation, but its overexpression activates apoptosis. CF and CMF express different levels of B1 and B2 kinin receptors, whose stimulation by their respective agonists activates common signaling transduction pathways that decrease the synthesis and secretion of collagen through nitric oxide and prostacyclin I2 secretion. Besides these classical functions, CF can also participate in the inflammatory response of cardiac repair, through the expression of receptors commonly associated to immune cells such as Toll like receptor 4, NLRP3 and interferon receptor. The activation by their respective agonists modulates the cellular functions already described and the release of cytokines and chemokines. Thus, CF and CMF act as sentinel cells responding to a plethora of stimulus, modifying their own behavior, and that of neighboring cells.

Inflammasomes: mechanism of action, role in disease, and therapeutics

The inflammasomes are innate immune system receptors and sensors that regulate the activation of caspase-1 and induce inflammation in response to infectious microbes and molecules derived from host proteins. They have been implicated in a host of inflammatory disorders. Recent developments have greatly enhanced our understanding of the molecular mechanisms by which different inflammasomes are activated. Additionally, increasing evidence in mouse models, supported by human data, strongly implicates an involvement of the inflammasome in the initiation or progression of diseases with a high impact on public health, such as metabolic disorders and neurodegenerative diseases. Finally, recent developments pointing toward promising therapeutics that target inflammasome activity in inflammatory diseases have been reported. This review will focus on these three areas of inflammasome research.

Deficient NLRP3 and AIM2 Inflammasome Function in Autoimmune NZB Mice

Inflammasomes are protein complexes that promote caspase activation, resulting in processing of IL-1β and cell death, in response to infection and cellular stresses. Inflammasomes have been anticipated to contribute to autoimmunity. The New Zealand Black (NZB) mouse develops anti-erythrocyte Abs and is a model of autoimmune hemolytic anemia. These mice also develop anti-nuclear Abs typical of lupus. In this article, we show that NZB macrophages have deficient inflammasome responses to a DNA virus and fungal infection. Absent in melanoma 2 (AIM2) inflammasome responses are compromised in NZB by high expression of the AIM 2 antagonist protein p202, and consequently NZB cells had low IL-1β output in response to both transfected DNA and mouse CMV infection. Surprisingly, we also found that a second inflammasome system, mediated by the NLR family, pyrin domain containing 3 (NLRP3) initiating protein, was completely lacking in NZB cells. This was due to a point mutation in an intron of the Nlrp3 gene in NZB mice, which generates a novel splice acceptor site. This leads to incorporation of a pseudoexon with a premature stop codon. The lack of full-length NLRP3 protein results in NZB being effectively null for Nlrp3, with no production of bioactive IL-1β in response to NLRP3 stimuli, including infection with Candida albicans. Thus, this autoimmune strain harbors two inflammasome deficiencies, mediated through quite distinct mechanisms. We hypothesize that the inflammasome deficiencies in NZB alter the interaction of the host with both microflora and pathogens, promoting prolonged production of cytokines that contribute to development of autoantibodies.

Interfering with immunity: detrimental role of type I IFNs during infection

Type I IFNs are known to inhibit viral replication and mediate protection against viral infection. However, recent studies revealed that these cytokines play a broader and more fundamental role in host responses to infections beyond their well-established antiviral function. Type I IFN induction, often associated with microbial evasion mechanisms unique to virulent microorganisms, is now shown to increase host susceptibility to a diverse range of pathogens, including some viruses. This article presents an overview of the role of type I IFNs in infections with bacterial, fungal, parasitic, and viral pathogens and discusses the key mechanisms mediating the regulatory function of type I IFNs in pathogen clearance and tissue inflammation.

Tr1 Cells, but Not Foxp3+ Regulatory T Cells, Suppress NLRP3 Inflammasome Activation via an IL-10-Dependent Mechanism

The two best-characterized types of CD4(+) regulatory T cells (Tregs) are Foxp3(+) Tregs and Foxp3(-) type 1 regulatory (Tr1) cells. The ability of Foxp3(+) Tregs and Tr1 cells to suppress adaptive immune responses is well known, but how these cells regulate innate immunity is less defined. We discovered that CD44(hi)Foxp3(-) T cells from unmanipulated mice are enriched in Tr1 cell precursors, enabling differentiation of cells that express IL-10, as well as Tr1-associated cell surface markers, CD49b and LAG-3, and transcription factors, cMaf, Blimp-1, and AhR. We compared the ability of Tr1 cells versus Foxp3(+) Tregs to suppress IL-1β production from macrophages following LPS and ATP stimulation. Surprisingly, Tr1 cells, but not Foxp3(+) Tregs, inhibited the transcription of pro-IL-1β mRNA, inflammasome-mediated activation of caspase-1, and secretion of mature IL-1β. Consistent with the role for IL-10 in Tr1 cell-mediated suppression, inhibition of inflammasome activation and IL-1β secretion was abrogated in IL-10R-deficient macrophages. Moreover, IL-1β production from macrophages derived from Nlrp3(A350V) knockin mice, which carry a mutation found in cryopyrin-associated periodic syndrome patients, was suppressed by Tr1 cells but not Foxp3(+) Tregs. Using an adoptive transfer model, we found a direct correlation between Tr1 cell engraftment and protection from weight loss in mice expressing a gain-of-function NLRP3. Collectively, these data provide the first evidence for a differential role of Tr1 cells and Foxp3(+) Tregs in regulating innate immune responses. Through their capacity to produce high amounts of IL-10, Tr1 cells may have unique therapeutic effects in disease-associated inflammasome activation.

Inflammasomes and human autoimmunity: A comprehensive review

Inflammasomes are multi-protein complexes composed of a NOD-like receptor (NLR)/an AIM-like receptor (ALR), the adapter molecule apoptosis-associated speck-like protein that contains a CARD (ASC), and caspase-1. Active caspase-1 cleaves pro-IL-1β and pro-IL-18 to IL-1β and IL-18, resulting in inflammation. Genetic mutations in inflammasomes were first recognized to result in autoinflammatory diseases, which are characterized by the absence of both autoantibodies and autoreactive-T/B cells. However, there is increasing attention being placed on genetic polymorphisms that are involved in the components of inflammasomes, and these have implications for innate immunity and the natural history of autoimmune diseases. For example, while the NOD-like receptor family, pyrin domain containing 1 (NLRP1) haplotypes contributes to susceptibility to developing vitiligo; there are other single nucleotide polymorphisms (SNPs) that alters the susceptibility and severity of rheumatoid arthritis (RA) and juvenile idiopathic arthritis. Indeed, there are multiple factors that contribute to lowering the threshold of immunity and inflammasomes play a key role in this threshold. For example, IL-1β and IL-18 further perpetuate Th17 responses and endothelial cell damage, which potentiate a number of autoimmune diseases, including synovitis in RA, cardiovascular disease, and systemic lupus erythematosus (SLE). There is also increasing data on the role of innate immunity in experimental autoimmune encephalomyelitis (EAE), in lupus nephritis, and in a variety of autoimmune pathologies in which activation of the innate immune system is the driver for the adaptive system. Indeed, it is likely that the chronic pathology of autoimmunity is mediated in part by otherwise innocent bystander cells, augmented by inflammasomes.

Emerging complexity and new roles for the RIG-I-like receptors in innate antiviral immunity

Innate immunity is critical for the control of virus infection and operates to restrict viral susceptibility and direct antiviral immunity for protection from acute or chronic viral-associated diseases including cancer. RIG-I like receptors (RLRs) are cytosolic RNA helicases that function as pathogen recognition receptors to detect RNA pathogen associated molecular patterns (PAMPs) of virus infection. The RLRs include RIG-I, MDA5, and LGP2. They function to recognize and bind to PAMP motifs within viral RNA in a process that directs the RLR to trigger downstream signaling cascades that induce innate immunity that controls viral replication and spread. Products of RLR signaling also serve to modulate the adaptive immune response to infection. Recent studies have additionally connected RLRs to signaling cascades that impart inflammatory and apoptotic responses to virus infection. Viral evasion of RLR signaling supports viral outgrowth and pathogenesis, including the onset of viral-associated cancer.

Perception of self: distinguishing autoimmunity from autoinflammation

Rheumatic diseases can be divided in two groups, autoinflammatory and autoimmune disorders. The clinical presentation of both types of diseases overlap, but the pathological pathways underlying rheumatic autoinflammation and autoimmunity are distinct and are the subject of ongoing research. There are a number of ways in which these groups of diseases differ in terms of disease mechanisms and therapeutic responses. First, autoinflammatory diseases are driven by endogenous danger signals, metabolic mediators and cytokines, whereas autoimmunity involves the activation of T and B cells, the latter requiring V-(D)-J recombination of receptor-chain gene segments for maturation. Second, the efficacy of biologic agents directed against proinflammatory cytokines (for example IL-1β and TNF) also highlights differences between autoinflammatory and autoimmune processes. Finally, whereas autoinflammatory diseases are mostly driven by inflammasome-induced IL-1β and IL-18 production, autoimmune diseases are associated with type I interferon (IFN) signatures in blood. In this Review, we provide an overview of the monocyte intracellular pathways that drive autoinflammation and autoimmunity. We convey recent findings on how the type I IFN pathway can modulate IL-1β signalling (and vice versa), and discuss why IL-1β-mediated autoinflammatory diseases do not perpetuate into autoimmunity. The origins of intracellular autoantigens in autoimmune disorders are also discussed. Finally, we suggest how new mechanistic knowledge of autoinflammatory and autoimmune diseases might help improve treatment strategies to benefit patient care.

Macrophages monitor tissue osmolarity and induce inflammatory response through NLRP3 and NLRC4 inflammasome activation

Interstitial osmolality is a key homeostatic variable that varies depending on the tissue microenvironment. Mammalian cells have effective mechanisms to cope with osmotic stress by engaging various adaptation responses. Hyperosmolality due to high dietary salt intake has been linked to pathological inflammatory conditions. Little is known about the mechanisms of sensing the hyperosmotic stress by the innate immune system. Here we report that caspase-1 is activated in macrophages under hypertonic conditions. Mice with high dietary salt intake display enhanced induction of Th17 response upon immunization, and this effect is abolished in caspase-1-deficient mice. Our findings identify an unknown function of the inflammasome as a sensor of hyperosmotic stress, which is crucial for the induction of inflammatory Th17 response.

Identification of the inflammasome Nlrp1b as the candidate gene conferring diabetes risk at the Idd4.1 locus in the nonobese diabetic mouse

Type 1 diabetes in the NOD mouse model has been linked to >30 insulin-dependent diabetes (Idd) susceptibility loci. Idd4 on chromosome 11 consists of two subloci, Idd4.1 and Idd4.2. Using congenic analysis of alleles in NOD and NOD-resistant (NOR) mice, we previously defined Idd4.1 as an interval containing >50 genes that controlled expression of genes in the type 1 IFN pathway. In this study, we report refined mapping of Idd4.1 to a 1.1-Mb chromosomal region and provide genomic sequence analysis and mechanistic evidence supporting its role in innate immune regulation of islet-directed autoimmunity. Genetic variation at Idd4.1 was mediated by radiation-sensitive hematopoietic cells, and type 1 diabetes protection conferred by the NOR allele was abrogated in mice treated with exogenous type 1 IFN-β. Next generation sequence analysis of the full Idd4.1 genomic interval in NOD and NOR strains supported Nlrp1b as a strong candidate gene for Idd4.1. Nlrp1b belongs to the Nod-like receptor (NLR) gene family and contributes to inflammasome assembly, caspase-1 recruitment, and release of IL-1β. The Nlrp1b of NOR was expressed as an alternative spliced isoform that skips exon 9, resulting in a premature stop codon predicted to encode a truncated protein. Functional analysis of the truncated NOR Nlrp1b protein demonstrated that it was unable to recruit caspase-1 and process IL-1β. Our data suggest that Idd4.1-dependent protection from islet autoimmunity is mediated by differences in type 1 IFN- and IL-1β-dependent immune responses resulting from genetic variation in Nlrp1b.

Prostaglandin E2 Inhibits NLRP3 Inflammasome Activation through EP4 Receptor and Intracellular Cyclic AMP in Human Macrophages

PGE2 is a potent lipid mediator involved in maintaining homeostasis but also promotion of acute inflammation or immune suppression in chronic inflammation and cancer. Nucleotide-binding domain, leucine-rich repeat-containing protein (NLR)P3 inflammasome plays an important role in host defense. Uncontrolled activation of the NLRP3 inflammasome, owing to mutations in the NLRP3 gene, causes cryopyrin-associated periodic syndromes. In this study, we showed that NLRP3 inflammasome activation is inhibited by PGE2 in human primary monocyte-derived macrophages. This effect was mediated through PGE2 receptor subtype 4 (EP4) and an increase in intracellular cAMP, independently of protein kinase A or exchange protein directly activated by cAMP. A specific agonist of EP4 mimicked, whereas its antagonist or EP4 knockdown reversed, PGE2-mediated NLRP3 inhibition. PGE2 caused an increase in intracellular cAMP. Blockade of adenylate cyclase by its inhibitor reversed PGE2-mediated NLRP3 inhibition. Increase of intracellular cAMP by an activator of adenylate cyclase or an analog of cAMP, or a blockade of cAMP degradation by phosphodiesterase inhibitor decreased NLRP3 activation. Protein kinase A or exchange protein directly activated by cAMP agonists did not mimic, and their antagonists did not reverse, PGE2-mediated NLRP3 inhibition. Additionally, constitutive IL-1β secretion from LPS-primed PBMCs of cryopyrin-associated periodic fever syndromes patients was substantially reduced by high doses of PGE2. Moreover, blocking cytosolic phospholipase A2α by its inhibitor or small interfering RNA or inhibiting cyclooxygenase 2, resulting in inhibition of endogenous PGE2 production, caused an increase in NLRP3 inflammasome activation. Our results suggest that PGE2 might play a role in maintaining homeostasis during the resolution phase of inflammation and might serve as an autocrine and paracrine regulator.

Stress-activated Dendritic Cells (DC) Induce Dual Interleukin (IL)-15- and IL1β-mediated Pathways, Which May Elicit CD4+ Memory T Cells and Interferon (IFN)-stimulated Genes

The prevailing evidence suggests that immunological memory does not require antigenic re-stimulation but is maintained by low level tonic stimulation. We examined the hypothesis that stress agents contribute to tonic cellular activation and maintain immunological memory. Stimulation of monocyte-derived dendritic cells (DC) with stress agents elicits reactive oxygen species and HSP70. NFκB is activated, which up-regulates membrane-associated (ma) IL-15, caspase-1 and IL-1β. Co-culture of stress-treated DC with mononuclear cells activates IL-15 and IL-1β receptors on CD4(+) T cells, eliciting CD40L, proliferation, and up-regulation of CD45RO(+) memory T cells. The transcription factors Tbet(high) and RORγt are up-regulated, whereas FoxP3 is down-regulated, resulting in enhanced Th1 and Th17 expression and the corresponding cytokines. The interaction between maIL-15 expressed by DC and IL-15R on CD4(+) T cells results in one pathway and the corresponding cells expressing IL-1β and IL1βR as a second pathway. Importantly, inhibition studies with IL-15 antibodies and IL-1βR inhibitor suggest that both pathways may be required for optimum CD4(+) CD45RO(+) memory T cell expression. Type 1 IFN expression in splenic CD11c DC of stress-treated mice demonstrated a significant increase of IFN-α in CD11c CD317(+) and CD8α(+) DC. Analysis of RNA in human CD4(+) memory T cells showed up-regulation of type 1 IFN-stimulated genes and inhibition with histone methyltransferase inhibitor. We suggest the paradigm that stress-induced tonic stimulation might be responsible for the robust persistence of the immune response in vaccination and that epigenetic changes are involved in maintaining CD4(+) T cell memory.

The epithelial danger signal IL-1α is a potent activator of fibroblasts and reactivator of intestinal inflammation

Intestinal epithelial cell (IEC) death is typical of inflammatory bowel disease (IBD). We investigated: i) whether IEC-released necrotic cell products (proinflammatory mediators) amplify mucosal inflammation, ii) the capacity of necrotic cell lysates from HT29 cells or human IECs to induce human intestinal fibroblasts' (HIF) production of IL-6 and IL-8, and iii) whether IL-1α, released by injured colonocytes, exacerbated experimental IBD. Necrotic cell lysates potently induced HIF IL-6 and IL-8 production independent of Toll-like receptors 2 and 4, receptor for advanced glycation end-products, high-mobility group box 1, uric acid, IL-33, or inflammasome activation. IL-1α was the key IEC-derived necrotic cell product involved in HIF cytokine production. IL-1α-positive cells were identified in the epithelium in human IBD and dextran sulfate sodium (DSS)-induced colitis. IL-1α was detected in the stool of colitic mice before IL-1β. IL-1α enemas reactivated inflammation after DSS colitis recovery, induced IL-1 receptor expression in subepithelial fibroblasts, and activated de novo inflammation even in mice without overt colitis, after the administration of low-dose DSS. IL-1α amplifies gut inflammation by inducing cytokine production by mesenchymal cells. IL-1α-mediated IEC-fibroblast interaction may be involved in amplifying and perpetuating inflammation, even without obvious intestinal damage. IL-1α may be a target for treating early IBD or preventing the reactivation of IBD.

Type I interferons in infectious disease

Type I interferons (IFNs) have diverse effects on innate and adaptive immune cells during infection with viruses, bacteria, parasites and fungi, directly and/or indirectly through the induction of other mediators. Type I IFNs are important for host defence against viruses. However, recently, they have been shown to cause immunopathology in some acute viral infections, such as influenza virus infection. Conversely, they can lead to immunosuppression during chronic viral infections, such as lymphocytic choriomeningitis virus infection. During bacterial infections, low levels of type I IFNs may be required at an early stage, to initiate cell-mediated immune responses. High concentrations of type I IFNs may block B cell responses or lead to the production of immunosuppressive molecules, and such concentrations also reduce the responsiveness of macrophages to activation by IFNγ, as has been shown for infections with Listeria monocytogenes and Mycobacterium tuberculosis. Recent studies in experimental models of tuberculosis have demonstrated that prostaglandin E2 and interleukin-1 inhibit type I IFN expression and its downstream effects, demonstrating that a cross-regulatory network of cytokines operates during infectious diseases to provide protection with minimum damage to the host.

Murine Gammaherpesvirus 68 Pathogenesis Is Independent of Caspase-1 and Caspase-11 in Mice and Impairs Interleukin-1β Production upon Extrinsic Stimulation in Culture

Gammaherpesviruses establish lifelong infections that are associated with the development of cancer. These viruses subvert many aspects of the innate and adaptive immune response of the host. The inflammasome, a macromolecular protein complex that controls inflammatory responses to intracellular danger signals generated by pathogens, is both activated and subverted during human gammaherpesvirus infection in culture. The impact of the inflammasome response on gammaherpesvirus replication and latency in vivo is not known. Caspase-1 is the inflammasome effector protease that cleaves the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18. We infected caspase-1-deficient mice with murine gammaherpesvirus 68 (MHV68) and observed no impact on acute replication in the lung or latency and reactivation from latency in the spleen. This led us to examine the effect of viral infection on inflammasome responses in bone marrow-derived macrophages. We determined that infection of macrophages with MHV68 led to a robust interferon response but failed to activate caspase-1 or induce the secretion of IL-1β. In addition, MHV68 infection led to a reduction in IL-1β production after extrinsic lipopolysaccharide stimulation or upon coinfection with Salmonella enterica serovar Typhimurium. Interestingly, this impairment occurred at the proIL-1β transcript level and was independent of the RTA, the viral lytic replication and transcription activator. Taken together, MHV68 impairs the inflammasome response by inhibiting IL-1β production during the initial stages of infection.

IMPORTANCE

Gammaherpesviruses persist for the lifetime of the host. To accomplish this, they must evade recognition and clearance by the immune system. The inflammasome consists of proteins that detect foreign molecules in the cell and respond by secreting proinflammatory signaling proteins that recruit immune cells to clear the infection. Unexpectedly, we found that murine gammaherpesvirus pathogenesis was not enhanced in mice lacking caspase-1, a critical inflammasome component. This led us to investigate whether the virus actively impairs the inflammasome response. We found that the inflammasome was not activated upon macrophage cell infection with murine gammaherpesvirus 68. Infection also prevented the host cell inflammasome response to other pathogen-associated molecular patterns, indicated by reduced production of the proinflammatory cytokine IL-1β upon bacterial coinfection. Taken together, murine gammaherpesvirus impairment of the inflammatory cytokine IL-1β in macrophages identifies one mechanism by which the virus may inhibit caspase-1-dependent immune responses in the infected animal.

Dopamine controls systemic inflammation through inhibition of NLRP3 inflammasome

Inflammasomes are involved in diverse inflammatory diseases, so the activation of inflammasomes needs to be tightly controlled to prevent excessive inflammation. However, the endogenous regulatory mechanisms of inflammasome activation are still unclear. Here, we report that the neurotransmitter dopamine (DA) inhibits NLRP3 inflammasome activation via dopamine D1 receptor (DRD1). DRD1 signaling negatively regulates NLRP3 inflammasome via a second messenger cyclic adenosine monophosphate (cAMP), which binds to NLRP3 and promotes its ubiquitination and degradation via the E3 ubiquitin ligase MARCH7. Importantly, in vivo data show that DA and DRD1 signaling prevent NLRP3 inflammasome-dependent inflammation, including neurotoxin-induced neuroinflammation, LPS-induced systemic inflammation, and monosodium urate crystal (MSU)-induced peritoneal inflammation. Taken together, our results reveal an endogenous mechanism of inflammasome regulation and suggest DRD1 as a potential target for the treatment of NLRP3 inflammasome-driven diseases.

Protein kinase R and the inflammasome

Protein kinase R (PKR) was first identified as a mediator of the double-stranded RNA (dsRNA)-mediated inhibition of protein synthesis in extracts from interferon-treated cells. In a physiological context, viral replication results in production of dsRNA, activation of PKR by autophosphorylation, and phosphorylation of the protein synthesis initiation factor eIF2α. Subsequent biochemical, structural, and genetic analyses have identified the dsRNA and kinase domain structure of PKR, and shown that its deletion from the germline of mice results in impaired resistance to infection by many different viruses. These studies have also opened up roles for PKR in different signaling pathways, the most recent being regulation of the inflammasome. Here we review evidence for this newly ascribed function for PKR and discuss roles in inflammasome regulation and associated diseases.

NLRC5 exclusively transactivates MHC class I and related genes through a distinctive SXY module

MHC class II (MHCII) genes are transactivated by the NOD-like receptor (NLR) family member CIITA, which is recruited to SXY enhancers of MHCII promoters via a DNA-binding "enhanceosome" complex. NLRC5, another NLR protein, was recently found to control transcription of MHC class I (MHCI) genes. However, detailed understanding of NLRC5's target gene specificity and mechanism of action remained lacking. We performed ChIP-sequencing experiments to gain comprehensive information on NLRC5-regulated genes. In addition to classical MHCI genes, we exclusively identified novel targets encoding non-classical MHCI molecules having important functions in immunity and tolerance. ChIP-sequencing performed with Rfx5(-/-) cells, which lack the pivotal enhanceosome factor RFX5, demonstrated its strict requirement for NLRC5 recruitment. Accordingly, Rfx5-knockout mice phenocopy Nlrc5 deficiency with respect to defective MHCI expression. Analysis of B cell lines lacking RFX5, RFXAP, or RFXANK further corroborated the importance of the enhanceosome for MHCI expression. Although recruited by common DNA-binding factors, CIITA and NLRC5 exhibit non-redundant functions, shown here using double-deficient Nlrc5(-/-)CIIta(-/-) mice. These paradoxical findings were resolved by using a "de novo" motif-discovery approach showing that the SXY consensus sequence occupied by NLRC5 in vivo diverges significantly from that occupied by CIITA. These sequence differences were sufficient to determine preferential occupation and transactivation by NLRC5 or CIITA, respectively, and the S box was found to be the essential feature conferring NLRC5 specificity. These results broaden our knowledge on the transcriptional activities of NLRC5 and CIITA, revealing their dependence on shared enhanceosome factors but their recruitment to distinct enhancer motifs in vivo. Furthermore, we demonstrated selectivity of NLRC5 for genes encoding MHCI or related proteins, rendering it an attractive target for therapeutic intervention. NLRC5 and CIITA thus emerge as paradigms for a novel class of transcriptional regulators dedicated for transactivating extremely few, phylogenetically related genes.

Increased inflammasome related gene expression profile in PBMC may facilitate T helper 17 cell induction in multiple sclerosis

The NLRP3 inflammasome is a macromolecular complex importantly involved in IL-1β processing. A role for this has been described in multiple sclerosis (MS). One mechanism by which IL-1β might be involved in MS is by inducing pathogenic Th17 cells, i.e. GM-CSF+ Th17 cells. In the present study, we show that expression of the inflammasome related genes, NLRP3, caspase-1, IL-1β and the IL-1β/IL-1Ra ratio, was increased in PBMC from MS patients compared to healthy controls (HC). However, in an in vitro inflammasome activity assay with PBMC, IL-1β protein secretion and the IL-1β/IL-1Ra protein ratio were similar in MS patients and HC. Th cells cultured in the presence of supernatant derived from LPS/ATP inflammasome activated PBMC showed increased Th17 and GM-CSF+ Th17 cell frequencies in HC and MS patients and decreased anti-inflammatory IL-10+Th cell frequency in HC compared to Th cells cultured in the presence of control supernatant. Moreover, addition of the immune modulator calcitriol to the former condition resulted in reduced frequencies of Th17 and GM-CSF+Th17 cells, and also of IL-10+ Th cells. Evidently, our data indicate that inflammasome activity can skew the Th cell population toward a more pro-inflammatory composition, an effect that might be inhibited by vitamin D, and that might be importantly involved in inflammation within the central nervous system.

Deficiency in lymphotoxin β receptor protects from atherosclerosis in apoE-deficient mice

RATIONALE

Lymphotoxin β receptor (LTbR) regulates immune cell trafficking and communication in inflammatory diseases. However, the role of LTbR in atherosclerosis is still unclear.

OBJECTIVE

The aim of this study was to elucidate the role of LTbR in atherosclerosis.

METHODS AND RESULTS

After 15 weeks of feeding a Western-type diet, mice double-deficient in apolipoprotein E and LTbR (apoE(-/-)/LTbR(-/-)) exhibited lower aortic plaque burden than did apoE(-/-) littermates. Macrophage content at the aortic root and in the aorta was reduced, as determined by immunohistochemistry and flow cytometry. In line with a decrease in plaque inflammation, chemokine (C-C motif) ligand 5 (Ccl5) and other chemokines were transcriptionally downregulated in aortic tissue from apoE(-/-)/LTbR(-/-) mice. Moreover, bone marrow chimeras demonstrated that LTbR deficiency in hematopoietic cells mediated the atheroprotection. Furthermore, during atheroprogression, apoE(-/-) mice exhibited increased concentrations of cytokines, for example, Ccl5, whereas apoE(-/-)/LTbR(-/-) mice did not. Despite this decreased plaque macrophage content, flow cytometric analysis showed that the numbers of circulating lymphocyte antigen 6C (Ly6C)(low) monocytes were markedly elevated in apoE(-/-)/LTbR(-/-) mice. The influx of these cells into atherosclerotic lesions was significantly reduced, whereas apoptosis and macrophage proliferation in atherosclerotic lesions were unaffected. Gene array analysis pointed to chemokine (C-C motif) receptor 5 as the most regulated pathway in isolated CD115(+) cells in apoE(-/-)/LTbR(-/-) mice. Furthermore, stimulating monocytes from apoE(-/-) mice with agonistic anti-LTbR antibody or the natural ligand lymphotoxin-α1β2, increased Ccl5 mRNA expression.

CONCLUSIONS

These findings suggest that LTbR plays a role in macrophage-driven inflammation in atherosclerotic lesions, probably by augmenting the Ccl5-mediated recruitment of monocytes.

Endogenous interferon-β-inducible gene expression and interferon-β-treatment are associated with reduced T cell responses to myelin basic protein in multiple sclerosis

Autoreactive CD4⁺ T-cells are considered to play a major role in the pathogenesis of multiple sclerosis. In experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis, exogenous and endogenous type I interferons restrict disease severity. Recombinant interferon-β is used for treatment of multiple sclerosis, and some untreated multiple sclerosis patients have increased expression levels of type I interferon-inducible genes in immune cells. The role of endogenous type I interferons in multiple sclerosis is controversial: some studies found an association of high expression levels of interferon-β-inducible genes with an increased expression of interleukin-10 and a milder disease course in untreated multiple sclerosis patients, whereas other studies reported an association with a poor response to treatment with interferon-β. In the present study, we found that untreated multiple sclerosis patients with an increased expression of interferon-β-inducible genes in peripheral blood mononuclear cells and interferon-β-treated multiple sclerosis patients had decreased CD4⁺ T-cell reactivity to the autoantigen myelin basic protein ex vivo. Interferon-β-treated multiple sclerosis patients had increased IL10 and IL27 gene expression levels in monocytes in vivo. In vitro, neutralization of interleukin-10 and monocyte depletion increased CD4⁺ T-cell reactivity to myelin basic protein while interleukin-10, in the presence or absence of monocytes, inhibited CD4⁺ T-cell reactivity to myelin basic protein. Our findings suggest that spontaneous expression of interferon-β-inducible genes in peripheral blood mononuclear cells from untreated multiple sclerosis patients and treatment with interferon-β are associated with reduced myelin basic protein-induced T-cell responses. Reduced myelin basic protein-induced CD4⁺ T-cell autoreactivity in interferon-β-treated multiple sclerosis patients may be mediated by monocyte-derived interleukin-10.

Viral sensing at the blood-brain barrier: new roles for innate immunity at the CNS vasculature

Neurotropic viral infections are a major source of disease worldwide and represent a growing burden to public health. While the central nervous system (CNS) is normally protected from viral infection by the blood-brain barrier (BBB), many viruses are able to cross the BBB and establish CNS infection through processes that largely remain poorly understood. A growing body of recent research has begun to shed light on the viral and host factors that modulate BBB function, contributing to both protective and pathological disease processes. Central to these studies have been the actions of host cytokines and chemokines, which have increasingly been shown to be key regulators of BBB physiology. This review summarizes recent advances in understanding how BBB function governs both viral pathogenesis and host immune responses during neurotropic viral infections.