The Pseudomonas aeruginosa Type III Secretion System Exoenzyme Effector ExoU Induces Mitochondrial Damage in a Murine Bone Marrow-Derived Macrophage Infection Model.

Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen that causes nosocomial pneumonia, urinary tract infections, and bacteremia. A hallmark of P. aeruginosa pathogenesis is disruption of host cell function by the type III secretion system (T3SS) and its cognate exoenzyme effectors. The T3SS effector ExoU is phospholipase A2 (PLA2) that targets the host cell plasmalemmal membrane to induce cytolysis and is an important virulence factor that mediates immune avoidance. In addition, ExoU has been shown to subvert the host inflammatory response in a noncytolytic manner. In primary bone marrow-derived macrophages (BMDMs), P. aeruginosa infection is sensed by the nucleotide-binding domain containing leucine-rich repeats-like receptor 4 (NLRC4) inflammasome, which triggers caspase-1 activation and inflammation. ExoU transiently inhibits NLRC4 inflammasome-mediated activation of caspase-1 and its downstream target, interleukin 1ß (IL-1ß), to suppress activation of inflammation. In the present study, we sought to identify additional noncytolytic virulence functions for ExoU and discovered an unexpected association between ExoU, host mitochondria, and NLRC4. We show that infection of BMDMs with P. aeruginosa strains expressing ExoU elicited mitochondrial oxidative stress. In addition, mitochondria and mitochondrion-associated membrane fractions enriched from infected cells exhibited evidence of autophagy activation, indicative of damage. The observation that ExoU elicited mitochondrial stress and damage suggested that ExoU may also associate with mitochondria during infection. Indeed, ExoU phospholipase A2 enzymatic activity was present in enriched mitochondria and mitochondrion-associated membrane fractions isolated from P. aeruginosa-infected BMDMs. Intriguingly, enriched mitochondria and mitochondrion-associated membrane fractions isolated from infected Nlrc4 homozygous knockout BMDMs displayed significantly lower levels of ExoU enzyme activity, suggesting that NLRC4 plays a role in the ExoU-mitochondrion association. These observations prompted us to assay enriched mitochondria and mitochondrion-associated membrane fractions for NLRC4, caspase-1, and IL-1ß. NLRC4 and pro-caspase-1 were detected in enriched mitochondria and mitochondrion-associated membrane fractions isolated from noninfected BMDMs, and active caspase-1 and active IL-1ß were detected in response to P. aeruginosa infection. Interestingly, ExoU inhibited mitochondrion-associated caspase-1 and IL-1ß activation. The implications of ExoU-mediated effects on mitochondria and the NLRC4 inflammasome during P. aeruginosa infection are discussed.

Mitochondrial cardiolipin is required for Nlrp3 inflammasome activation.

Nlrp3 inflammasome activation occurs in response to numerous agonists but the specific mechanism by which this takes place remains unclear. All previously evaluated activators of the Nlrp3 inflammasome induce the generation of mitochondrial reactive oxygen species (ROS), suggesting a model in which ROS is a required upstream mediator of Nlrp3 inflammasome activation. Here we have identified the oxazolidinone antibiotic linezolid as a Nlrp3 agonist that activates the Nlrp3 inflammasome independently of ROS. The pathways for ROS-dependent and ROS-independent Nlrp3 activation converged upon mitochondrial dysfunction and specifically the mitochondrial lipid cardiolipin. Cardiolipin bound to Nlrp3 directly and interference with cardiolipin synthesis specifically inhibited Nlrp3 inflammasome activation. Together these data suggest that mitochondria play a critical role in the activation of the Nlrp3 inflammasome through the direct binding of Nlrp3 to cardiolipin.

NOD-like receptor C4 Inflammasome Regulates the Growth of Colon Cancer Liver Metastasis in NAFLD.

Nonalcoholic fatty liver disease (NAFLD) enhances the growth and recurrence of colorectal cancer (CRC) liver metastasis. With the rising prevalence of NAFLD, a better understanding of the molecular mechanism underlying NAFLD-associated liver metastasis is crucial. Tumor-associated macrophages (TAMs) constitute a large portion of the tumor microenvironment that promotes tumor growth. NOD-like receptor C4 (NLRC4), a component of an inflammasome complex, plays a role in macrophage activation and interleukin (IL)-1ß processing. We aimed to investigate whether NLRC4-mediated TAM polarization contributes to metastatic liver tumor growth in NAFLD. Wild-type and NLRC4-/- mice were fed low-fat or high-fat diet for 6 weeks followed by splenic injection of mouse CRC MC38 cells. The tumors were analyzed 2 weeks after CRC cell injection. High-fat diet-induced NAFLD significantly increased the number and size of CRC liver metastasis. TAMs and CD206-expressing M2 macrophages accumulated markedly in tumors in the presence of NAFLD. NAFLD up-regulated the expression of IL-1ß, NLRC4, and M2 markers in tumors. In NAFLD, but not normal livers, deletion of NLRC4 decreased liver tumor growth accompanied by decreased M2 TAMs and IL-1ß expression in tumors. Wild-type mice showed increased vascularity and vascular endothelial growth factor (VEGF) expression in tumors with NAFLD, but these were reduced in NLRC4-/- mice. When IL-1 signaling was blocked by recombinant IL-1 receptor antagonist, liver tumor formation and M2-type macrophages were reduced, suggesting that IL-1 signaling contributes to M2 polarization and tumor growth in NAFLD. Finally, we found that TAMs, but not liver macrophages, produced more IL-1ß and VEGF following palmitate challenge. Conclusion: In NAFLD, NLRC4 contributes to M2 polarization, IL-1ß, and VEGF production in TAMs, which promote metastatic liver tumor growth.

Cutting Edge: Mitochondrial Assembly of the NLRP3 Inflammasome Complex Is Initiated at Priming.

The NLRP3 inflammasome is activated in response to microbial and danger signals, resulting in caspase-1-dependent secretion of the proinflammatory cytokines IL-1ß and IL-18. Canonical NLRP3 inflammasome activation is a two-step process requiring both priming and activation signals. During inflammasome activation, NLRP3 associates with mitochondria; however, the role for this interaction is unclear. In this article, we show that mouse NLRP3 and caspase-1 independently interact with the mitochondrial lipid cardiolipin, which is externalized to the outer mitochondrial membrane at priming in response to reactive oxygen species. An NLRP3 activation signal is then required for the calcium-dependent association of the adaptor molecule ASC with NLRP3 on the mitochondrial surface, resulting in inflammasome complex assembly and activation. These findings demonstrate a novel lipid interaction for caspase-1 and identify a role for mitochondria as supramolecular organizing centers in the assembly and activation of the NLRP3 inflammasome.

Nlrp12 Mediates Adverse Neutrophil Recruitment during Influenza Virus Infection.

Exaggerated inflammatory responses during influenza A virus (IAV) infection are typically associated with severe disease. Neutrophils are among the immune cells that can drive this excessive and detrimental inflammation. In moderation, however, neutrophils are necessary for optimal viral control. In this study, we explore the role of the nucleotide-binding domain leucine-rich repeat containing receptor family member Nlrp12 in modulating neutrophilic responses during lethal IAV infection. Nlrp12-/- mice are protected from lethality during IAV infection and show decreased vascular permeability, fewer pulmonary neutrophils, and a reduction in levels of neutrophil chemoattractant CXCL1 in their lungs compared with wild-type mice. Nlrp12-/- neutrophils and dendritic cells within the IAV-infected lungs produce less CXCL1 than their wild-type counterparts. Decreased CXCL1 production by Nlrp12-/- dendritic cells was not due to a difference in CXCL1 protein stability, but instead to a decrease in Cxcl1 mRNA stability. Together, these data demonstrate a previously unappreciated role for Nlrp12 in exacerbating the pathogenesis of IAV infection through the regulation of CXCL1-mediated neutrophilic responses.

TIMP-1 Promotes the Immune Response in Influenza-Induced Acute Lung Injury.

INTRODUCTION: Influenza infects millions of people each year causing respiratory distress and death in severe cases. On average, 200,000 people annually are hospitalized in the United States for influenza related complications. Tissue inhibitor of metalloproteinase-1 (TIMP-1), a secreted protein that inhibits MMPs, has been found to be involved in lung inflammation. Here, we evaluated the role of TIMP-1 in the host response to influenza-induced lung injury. METHODS: Wild-type (WT) and Timp1-deficient (Timp1-/-) mice that were 8-12 weeks old were administered A/PR/8/34 (PR8), a murine adapted H1N1 influenza virus, and euthanized 6 days after influenza installation. Bronchoalveolar lavage fluid and lungs were harvested from each mouse for ELISA, protein assay, PCR, and histological analysis. Cytospins were executed on bronchoalveolar lavage fluid to identify immune cells based on morphology and cell count. RESULTS: WT mice experienced significantly more weight loss compared to Timp1-/- mice after influenza infection. WT mice demonstrated more immune cell infiltrate and airway inflammation. Interestingly, PR8 levels were identical between the WT and Timp1-/- mice 6 days post-influenza infection. CONCLUSION: The data suggest that Timp1 promotes the immune response in the lungs after influenza infection facilitating an injurious phenotype as a result of influenza infection.

Yellow Fever Virus, but Not Zika Virus or Dengue Virus, Inhibits T-Cell Receptor-Mediated T-Cell Function by an RNA-Based Mechanism.

The Flavivirus genus within the Flaviviridae family is comprised of many important human pathogens including yellow fever virus (YFV), dengue virus (DENV), and Zika virus (ZKV), all of which are global public health concerns. Although the related flaviviruses hepatitis C virus and human pegivirus (formerly named GBV-C) interfere with T-cell receptor (TCR) signaling by novel RNA and protein-based mechanisms, the effect of other flaviviruses on TCR signaling is unknown. Here, we studied the effect of YFV, DENV, and ZKV on TCR signaling. Both YFV and ZKV replicated in human T cells in vitro; however, only YFV inhibited TCR signaling. This effect was mediated at least in part by the YFV envelope (env) protein coding RNA. Deletion mutagenesis studies demonstrated that expression of a short, YFV env RNA motif (vsRNA) was required and sufficient to inhibit TCR signaling. Expression of this vsRNA and YFV infection of T cells reduced the expression of a Src-kinase regulatory phosphatase (PTPRE), while ZKV infection did not. YFV infection in mice resulted in impaired TCR signaling and PTPRE expression, with associated reduction in murine response to experimental ovalbumin vaccination. Together, these data suggest that viruses within the flavivirus genus inhibit TCR signaling in a species-dependent manner.

Inflammasome-independent IL-1ß mediates autoinflammatory disease in Pstpip2-deficient mice.

Chronic recurrent multifocal osteomyelitis (CRMO) is a human autoinflammatory disorder that primarily affects bone. Missense mutation (L98P) of proline-serine-threonine phosphatase-interacting protein 2 (Pstpip2) in mice leads to a disease that is phenotypically similar to CRMO called chronic multifocal osteomyelitis (cmo). Here we show that deficiency of IL-1RI in cmo mice resulted in a significant reduction in the time to onset of disease as well as the degree of bone pathology. Additionally, the proinflammatory cytokine IL-1ß, but not IL-1α, played a critical role in the pathology observed in cmo mice. In contrast, disease in cmo mice was found to be independent of the nucleotide-binding domain, leucine-rich repeat-containing family, pyrin domain-containing 3 (NLRP3) inflammasome as well as caspase-1. Neutrophils, but not bone marrow-derived macrophages, from cmo mice secreted increased IL-1ß in response to ATP, silica, and Pseudomonas aeruginosa compared with neutrophils from WT mice. This aberrant neutrophil response was sensitive to inhibition by serine protease inhibitors. These results demonstrate an inflammasome-independent role for IL-1ß in disease progression of cmo and implicate neutrophils and neutrophil serine proteases in disease pathogenesis. These data provide a rationale for directly targeting IL-1RI or IL-1ß as a therapeutic strategy in CRMO.

Immune complexes inhibit IL-1 secretion and inflammasome activation.

IgG immune complexes have been shown to modify immune responses driven by APCs in either a pro- or anti-inflammatory direction depending upon the context of stimulation. However, the ability of immune complexes to modulate the inflammasome-dependent innate immune response is unknown. In this study, we show that IgG immune complexes suppress IL-1α and IL-1ß secretion through inhibition of inflammasome activation. The mechanism by which this inhibition occurs is via immune complex ligation of activating FcγRs, resulting in prevention of both activation and assembly of the inflammasome complex in response to nucleotide-binding domain leucine-rich repeat (NLR) P3, NLRC4, or AIM2 agonists. In vivo, administration of Ag in the form of an immune complex during priming of the immune response inhibited resultant adaptive immune responses in an NLRP3-dependent model of allergic airway disease. Our data reveal an unexpected mechanism regulating CD4(+) T cell differentiation, by which immune complexes suppress inflammasome activation and the generation of IL-1α and IL-1ß from APCs, which are critical for the Ag-driven differentiation of CD4(+) T cells.

NFIL3/E4BP4 controls type 2 T helper cell cytokine expression.

Type 2 T helper (T(H)2) cells are critical for the development of allergic immune responses; however, the molecular mechanism controlling their effector function is still largely unclear. Here, we report that the transcription factor NFIL3/E4BP4 regulates cytokine production and effector function by T(H)2 cells. NFIL3 is highly expressed in T(H)2 cells but much less in T(H)1 cells. Production of interleukin (IL)-13 and IL-5 is significantly increased in Nfil3(-/-) T(H)2 cells and is decreased by expression of NFIL3 in wild-type T(H)2 cells. NFIL3 directly binds to and negatively regulates the Il13 gene. In contrast, IL-4 production is decreased in Nfil3(-/-) T(H)2 cells. Increased IL-13 and IL-5 together with decreased IL-4 production by antigen-stimulated splenocytes from the immunized Nfil3(-/-) mice was also observed. The ability of NFIL3 to alter T(H)2 cytokine production is a T-cell intrinsic effect. Taken together, these data indicate that NFIL3 is a key regulator of T(H)2 responses.

Sensing damage by the NLRP3 inflammasome.

The NLRP3 inflammasome is activated in response to a variety of signals that are indicative of damage to the host including tissue damage, metabolic stress, and infection. Upon activation, the NLRP3 inflammasome serves as a platform for activation of the cysteine protease caspase-1, which leads to the processing and secretion of the proinflammatory cytokines interleukin-1ß (IL-1ß) and IL-18. Dysregulated NLRP3 inflammasome activation is associated with both heritable and acquired inflammatory diseases. Here, we review new insights into the mechanism of NLRP3 inflammasome activation and its role in disease pathogenesis.

Cutting edge: Nlrp10 is essential for protective antifungal adaptive immunity against Candida albicans.

Nucleotide-binding domain leucine-rich repeat containing receptors (NLRs) are cytosolic receptors that initiate immune responses to sterile and infectious insults to the host. Studies demonstrated that Nlrp3 is critical for the control of Candida albicans infections and in the generation of antifungal Th17 responses. In this article, we show that the NLR family member Nlrp10 also plays a unique role in the control of disseminated C. albicans infection in vivo. Nlrp10-deficient mice had increased susceptibility to disseminated candidiasis, as indicated by decreased survival and increased fungal burdens. In contrast to Nlrp3, Nlrp10 deficiency did not affect innate proinflammatory cytokine production from macrophages and dendritic cells challenged with C. albicans. However, Nlrp10-deficient mice displayed a profound defect in Candida-specific Th1 and Th17 responses. These results demonstrate a novel role for Nlrp10 in the generation of adaptive immune responses to fungal infection.

Francisella tularensis live vaccine strain folate metabolism and pseudouridine synthase gene mutants modulate macrophage caspase-1 activation.

Francisella tularensis is a Gram-negative bacterium and the causative agent of the disease tularemia. Escape of F. tularensis from the phagosome into the cytosol of the macrophage triggers the activation of the AIM2 inflammasome through a mechanism that is not well understood. Activation of the AIM2 inflammasome results in autocatalytic cleavage of caspase-1, resulting in the processing and secretion of interleukin-1ß (IL-1ß) and IL-18, which play a crucial role in innate immune responses to F. tularensis. We have identified the 5-formyltetrahydrofolate cycloligase gene (FTL_0724) as being important for F. tularensis live vaccine strain (LVS) virulence. Infection of mice in vivo with a F. tularensis LVS FTL_0724 mutant resulted in diminished mortality compared to infection of mice with wild-type LVS. The FTL_0724 mutant also induced increased inflammasome-dependent IL-1ß and IL-18 secretion and cytotoxicity in macrophages in vitro. In contrast, infection of macrophages with a F. tularensis LVS rluD pseudouridine synthase (FTL_0699) mutant resulted in diminished IL-1ß and IL-18 secretion from macrophages in vitro compared to infection of macrophages with wild-type LVS. In addition, the FTL_0699 mutant was not attenuated in vivo. These findings further illustrate that F. tularensis LVS possesses numerous genes that influence its ability to activate the inflammasome, which is a key host strategy to control infection with this pathogen in vivo.

Tim-1 regulates Th2 responses in an airway hypersensitivity model.

T-cell immunoglobulin mucin-1 (Tim-1) is a transmembrane protein postulated to be a key regulator of Th2-type immune responses. This hypothesis is based in part upon genetic studies associating Tim-1 polymorphisms in mice with a bias toward airway hyperrespon-siveness (AHR) and the development of Th2-type CD4(+) T cells. Tim-1 expressed by Th2 CD4(+) T cells has been proposed to function as a co-stimulatory molecule. Tim-1 is also expressed by B cells, macrophages, and dendritic cells, but its role in responses by these cell types has not been firmly established. Here, we generated Tim-1-deficient mice to determine the role of Tim-1 in a murine model of allergic airway disease that depends on the development and function of Th2 effector cells and results in the generation of AHR. We found antigen-driven recruitment of inflammatory cells into airways is increased in Tim-1-deficient mice relative to WT mice. In addition, we observed increased antigen-specific cytokine production by splenocytes from antigen-sensitized Tim-1-deficient mice relative to those from controls. These data support the conclusion that Tim-1 functions in pathways that suppress recruitment of inflammatory cells into the airways and the generation or activity of CD4(+) T cells.

A SOCS-1 promoter variant is associated with total serum IgE levels.

SOCS-1 is a critical regulator of multiple signaling pathways, including those activated by cytokines that regulate Ig H chain class switching to IgE. Analysis of mice with mutations in the SOCS-1 gene demonstrated that IgE levels increase with loss of SOCS-1 alleles. This suggested that overall SOCS-1 acts as an inhibitor of IgE expression in vivo. A genetic association study was performed in 474 children enrolled in the Tucson Children's Respiratory Study to determine if genetic variation in the SOCS-1 locus correlates with altered levels of IgE. Carriers of the C-allele for a novel, 3' genomic single nucleotide polymorphism (SNP) in the SOCS-1 gene (SOCS1+1125G > C; rs33932899) were found to have significantly lower levels of serum IgE compared with those of homozygotes for the G-allele. Analysis demonstrated that the SOCS1+1125G > C SNP was in complete linkage disequilibrium with an SNP at position SOCS1-820G > T (rs33977706) of the SOCS-1 promoter. Carriers of the T-allele at the SOCS1-820G > T were also found to be associated with the decreased IgE. The promoter SNP increased transcriptional activity of the SOCS-1 promoter in reporter assays and human B cells. Consistent with this observation, the presence of this polymorphism within the promoter abolished binding of yin yang-1, which is identified as a negative regulator of SOCS-1 transcriptional activity. These data suggest that genetic variation in the SOCS-1 promoter may affect IgE production.

The NLRP3 inflammasome: a sensor of immune danger signals.

The innate immune system senses danger signals via evolutionary conserved receptors. The nucleotide-binding domain leucine-rich repeat containing receptor (NLR) family is a group of intracellular receptors that drive a wide variety of inflammatory responses. A number of the NLR family members can form inflammasomes, which are multiprotein complexes that can activate caspase-1 and ultimately lead to the processing and secretion of interleukin (IL)-1beta, IL-18 and IL-33. One of the best-studied members of the NLR family is NLRP3 for which a number of divergent activators have recently been described. These and other studies examining the NLRP3 inflammasome will be discussed in this review.

Inflammasome-Independent Roles of NLR and ALR Family Members.

Pattern recognition receptors, including members of the NLR and ALR families, are essential for recognition of both pathogen- and host-derived danger signals. Several members of these families, including NLRP1, NLRP3, NLRC4, and AIM2, are capable of forming multiprotein complexes, called inflammasomes, that result in the activation of pro-inflammatory caspase-1. However, in addition to the formation of inflammasomes, a number of these family members exert inflammasome-independent functions. Here, we will discuss inflammasome-independent functions of NLRC4, NLRP12, and AIM2 and examine their roles in regulating innate and adaptive immune processes.

Incidence of interruptive penicillin allergy alerts in patients with previously documented beta-lactam exposure: Potential for leveraging the electronic health record to identify erroneous allergies.

BACKGROUND: Approximately 10% of patients report allergies to penicillin, yet >90% of these allergies are not clinically significant. Patients reporting penicillin allergies are often treated with second-line, non-ß-lactam antibiotics that are typically broader spectrum and more toxic. Orders for ß-lactam antibiotics for these patients trigger interruptive alerts, even when there is electronic health record (EHR) data indicating prior ß-lactam exposure. OBJECTIVE: To describe the rate that interruptive penicillin allergy alerts display for patients who have previously had a ß-lactam exposure. DESIGN: Retrospective EHR review from January 2013 through June 2018. SETTING: A nonprofit health system including 1 large tertiary-care medical center, a smaller associated hospital, 2 emergency departments, and ˜250 outpatient clinics. PARTICIPANTS: All patients with EHR-documented of penicillin allergies. METHODS: We examined interruptive penicillin allergy alerts and identified the number and percentage of alerts that display for patients with a prior administration of a penicillin class or other ß-lactam antibiotic. RESULTS: Of 115,081 allergy alerts that displayed during the study period, 8% were displayed for patients who had an inpatient administration of a penicillin antibiotic after the allergy was noted, and 49% were displayed for patients with a prior inpatient administration of any ß-lactam. CONCLUSIONS: Many interruptive penicillin allergy alerts display for patients who would likely tolerate a penicillin, and half of all alerts display for patients who would likely tolerate another ß-lactam.

Sterile inflammatory responses mediated by the NLRP3 inflammasome.

Through pattern recognition receptors the innate immune system detects disruption of the normal function of the organism and initiates responses directed at correcting these derangements. Cellular damage from microbial or non-microbial insults causes the activation of nucleotide-binding domain leucine-rich repeat containing receptors in multiprotein complexes called inflammasomes. Here we discuss the role of the NLRP3 inflammasome in the recognition of cellular damage and the initiation of sterile inflammatory responses.