The Role of NLRP3 Inflammasome in Pneumococcal Infections

Endogenous innate sensor NLRP3 is a key component in peritoneal macrophage dynamics required for cestode establishment

The NLRP3 receptor can assemble inflammasome platforms to trigger inflammatory responses; however, accumulating evidence suggests that it can also display anti-inflammatory properties. Here, we explored the role of nucleotide-binding oligomerization domain pyrin-containing protein 3 (NLRP3) in Taenia crassiceps experimental infection, which requires immune polarization into a Th2-type profile and peritoneal influx of suppressive macrophages for successful colonization. NLRP3 deficient mice (NLRP3-/-) were highly resistant against T. crassiceps, relative to wild-type (WT) mice. Resistance in NLRP3-/- mice was associated with a diminished IL-4 output, high levels of IL-15, growth factor for both innate and adaptive lymphocytes, and a dramatic decrease in peritoneum-infiltrating suppressive macrophages. Also, a transcriptional analysis on bone marrow-derived macrophages exposed to Taenia-secreted antigens and IL-4 revealed that NLRP3-/- macrophages express reduced transcripts of relm-α and PD-1 ligands, markers of alternative activation and suppressive ability, respectively. Finally, we found that the resistance displayed by NLRP3-/- mice is transferred through intestinal microbiota exchange, since WT mice co-housed with NLRP3-/- mice were significantly more resistant than WT animals preserving their native microbiota. Altogether, these data demonstrate that NLRP3 is a component of innate immunity required for T. crassiceps to establish, most likely contributing to macrophage recruitment, and controlling lymphocyte-stimulating cytokines such as IL-15.

Structural properties of immune complexes formed by viral antigens and specific antibodies shape the inflammatory response of macrophages

Data on the course of viral infections revealed severe inflammation as a consequence of antiviral immune response. Despite extensive research, there are insufficient data on the role of innate immune cells in promoting inflammation mediated by immune complexes (IC) of viral antigens and their specific antibodies. Recently, we demonstrated that antigens of human polyomaviruses (PyVs) induce an inflammatory response in macrophages. Here, we investigated macrophage activation by IC. We used primary murine macrophages as a cell model, virus-like particles (VLPs) of PyV capsid protein as antigens, and a collection of murine monoclonal antibodies (mAbs) of IgG1, IgG2a, IgG2b subclasses. The inflammatory response was investigated by analysing inflammatory chemokines and activation of NLRP3 inflammasome. We observed a diverse pattern of chemokine secretion in macrophages treated with different IC compared to VLPs alone. To link IC properties with cell activation status, we characterised the IC by advanced optical and acoustic techniques. Ellipsometry provided precise real-time kinetics of mAb-antigen interactions, while quartz crystal microbalance measurements showed changes in conformation and viscoelastic properties during IC formation. These results revealed differences in mAb-antigen interaction and mAb binding parameters of the investigated IC. We found that IC-mediated cell activation depends more on IC characteristics, including mAb affinity, than on mAb affinity for the activating Fc receptor. IC formed by the highest affinity mAb showed a significant enhancement of inflammasome activation. This may explain the hyperinflammation related to viral infection and vaccination. Our findings demonstrate that IC promote the viral antigen-induced inflammatory response depending on antibody properties.

ALK-JNK signaling promotes NLRP3 inflammasome activation and pyroptosis via NEK7 during Streptococcus pneumoniae infection

Streptococcus pneumoniae (S. pneumoniae), a clinically important pathogen worldwide, causes serious invasive diseases, such as pneumonia, otitis media, and meningitis. The NLR family pyrin domain-containing 3 (NLRP3) inflammasome, an important component of the innate immune system, plays a key role in defense against pathogen infection; however the specific activation mechanism induced by S. pneumoniae infection is not fully understood. Here, primary mouse macrophages were selected as the in vitro cell model, and the effect of kinases on S. pneumoniae infection-induced NLRP3 inflammasome activation was investigated in vivo and in vitro using the western blot/RT-PCR/Co-IP/immunofluorescence staining/ELISA with or without kinase inhibitor or siRNA pretreatment. In this study, we found that the formation of the NEK7-NLRP3 complex significantly increased during S. pneumoniae infection and that anaplastic lymphoma kinase (ALK) and Jun N-terminal kinase (JNK) were phosphorylated rapidly. ALK and JNK inhibitors significantly reduced the ability of bacterial killing, the gene expression of NLRP3 inflammasome, the formation of apoptosis-associated speck-like protein containing caspase-recruitment domain (ASC) specks and the NEK7-NLRP3 complex, which in turn decreased the activation level of NLRP3 inflammasome-associated molecules and the maturation of interleukin-1β (IL-1β). In addition, ALK regulated the phosphorylation of JNK. Interestingly, the ALK/JNK/NEK7-NLRP3 signaling pathway is also involved in regulating pyroptosis and IL-1β secretion triggered by S. pneumoniae infection. In conclusion, our data suggest, for the first time, that the ALK/JNK/NEK7-NLRP3 signaling pathway may play an important role in NLRP3 inflammasome activation and pyroptosis and consequently regulate the host immune response upon S. pneumoniae infection.

NLRP3 Inflammasome: A key contributor to the inflammation formation

Inflammation is an important part of the development of various organ diseases. The inflammasome, as an innate immune receptor, plays an important role in the formation of inflammation. Among various inflammasomes, the NLRP3 inflammasome is the most well studied. The NLRP3 inflammasome is composed of skeletal protein NLRP3, apoptosis-associated speck-like protein (ASC) and pro-caspase-1. There are three types of activation pathways: (1) "classical" activation pathway; (2) "non-canonical" activation pathway; (3) "alternative" activation pathway. The activation of NLRP3 inflammasome is involved in many inflammatory diseases. A variety of factors (such as genetic factors, environmental factors, chemical factors, viral infection, etc.) have been proved to activate NLRP3 inflammasome and promote the inflammatory response of the lung, heart, liver, kidney and other organs in the body. Especially, the mechanism of NLRP3 inflammation and its related molecules in its associated diseases remains not to be summarized, namely they may promote or delay inflammatory diseases in different cells and tissues. This article reviews the structure and function of the NLRP3 inflammasome and its role in various inflammations, including inflammations caused by chemically toxic substances.

The quorum sensing com system regulates pneumococcal colonisation and invasive disease in a pseudo-stratified airway tissue model

BACKGROUND

The effects of the com quorum sensing system during colonisation and invasion of Streptococcus pneumoniae (Spn) are poorly understood.

METHODS

We developed an ex vivo model of differentiated human airway epithelial (HAE) cells with beating ciliae, mucus production and tight junctions to study Spn colonisation and translocation. HAE cells were inoculated with Spn wild-type TIGR4 (wtSpn) or its isogenic ΔcomC quorum sensing-deficient mutant.

RESULTS

Colonisation density of ΔcomC mutant was lower after 6 h but higher at 19 h and 30 h compared to wtSpn. Translocation correlated inversely with colonisation density. Transepithelial electric resistance (TEER) decreased after pneumococcal inoculation and correlated with increased translocation. Confocal imaging illustrated prominent microcolony formation with wtSpn but disintegration of microcolony structures with ΔcomC mutant. ΔcomC mutant showed greater cytotoxicity than wtSpn, suggesting that cytotoxicity was likely not the mechanism leading to translocation. There was greater density- and time-dependent increase of inflammatory cytokines including NLRP3 inflammasome-related IL-18 after infection with ΔcomC compared with wtSpn. ComC inactivation was associated with increased pneumolysin expression.

CONCLUSIONS

ComC system allows a higher organisational level of population structure resulting in microcolony formation, increased early colonisation and subsequent translocation. We propose that ComC inactivation unleashes a very different and possibly more virulent phenotype that merits further investigation.

Inflammasome activation by Gram-positive bacteria: Mechanisms of activation and regulation

The inflammasomes are intracellular multimeric protein complexes consisting of an innate immune sensor, the adapter protein ASC and the inflammatory caspases-1 and/or -11 and are important for the host defense against pathogens. Activaton of the receptor leads to formation of the inflammasomes and subsequent processing and activation of caspase-1 that cleaves the proinflammatory cytokines IL-1β and IL-18. Active caspase-1, and in some instances caspase-11, cleaves gasdermin D that translocates to the cell membrane where it forms pores resulting in the cell death program called pyroptosis. Inflammasomes can detect a range of microbial ligands through direct interaction or indirectly through diverse cellular processes including changes in ion fluxes, production of reactive oxygen species and disruption of various host cell functions. In this review, we will focus on the NLRP3, NLRP6, NLRC4 and AIM2 inflammasomes and how they are activated and regulated during infections with Gram-positive bacteria, including Staphylococcus spp., Streptococcus spp. and Listeria monocytogenes.

Comparison of systemic inflammatory profiles in COVID-19 and community-acquired pneumonia patients: a prospective cohort study

BACKGROUND

Inflammatory responses contribute to tissue damage in COVID-19 and community-acquired pneumonia (CAP). Although predictive values of different inflammatory biomarkers have been reported in both, similarities and differences of inflammatory profiles between these conditions remain uncertain. Therefore, we aimed to determine the similarities and differences of the inflammatory profiles between COVID-19 and CAP, and their association with clinical outcomes.

METHODS

We report a prospective observational cohort study; conducted in a reference hospital in Latin America. Patients with confirmed COVID-19 pneumonia and CAP were included. Multiplex (Luminex) cytokine assays were used to measure the plasma concentration of 14 cytokines at hospital admission. After comparing similarities and differences in the inflammatory profile between COVID-19 and CAP patients, an unsupervised classification method (i.e., hierarchical clustering) was used to identify subpopulations within COVID-19 and CAP patients.

RESULTS

A total of 160 patients were included, 62.5% were diagnosed with COVID-19 (100/160), and 37.5% with CAP (60/160). Using the hierarchical clustering, COVID-19 and CAP patients were divided based on its inflammatory profile: pauci, moderate, and hyper-inflammatory immune response. COVID-19 hyper-inflammatory subpopulation had the highest mortality. COVID-19 hyper-inflammatory subpopulation, compared to pauci-inflammatory, had higher levels of IL-10 (median [IQR] 61.4 [42.0-109.4] vs 13.0 [5.0-24.9], P: < 0.001), IL-6 (48.1 [22.3-82.6] vs 9.1 [0.1-30.4], P: < 0.001), among others. Hyper-inflammatory vs pauci-inflammatory CAP patients were characterized by elevation of IFN2 (48.8 [29.7-110.5] vs 3.0 [1.7-10.3], P: < 0.001), TNFα (36.3 [24.8-53.4] vs 13.1 [11.3-16.9], P: < 0.001), among others. Hyper-inflammatory subpopulations in COVID-19 and CAP compared to the corresponding pauci-inflammatory subpopulations had higher MCP-1.

CONCLUSIONS

Our data highlights three distinct subpopulations in COVID-19 and CAP, with differences in inflammatory marker profiles and risks of adverse clinical outcomes.

TRIAL REGISTRATION

This is a prospective study, therefore no health care intervention were applied to participants and trial registration is not applicable.

Chicken cathelicidin-2 promotes NLRP3 inflammasome activation in macrophages

Chicken cathelicidin-2 (CATH-2) as a host defense peptide has been identified to have potent antimicrobial and immunomodulatory activities. Here, we reported the mechanism by which CATH-2 modulates NLRP3 inflammasome activation. Our results show that CATH-2 and ATP as a positive control induced secretion of IL-1β and IL-1α in LPS-primed macrophages but did not affect secretion of IL-6, IL-12 and TNF-α. Furthermore, CATH-2 induced caspase-1 activation and oligomerization of apoptosis-associated speck-like protein containing a carboxy- terminal caspase recruitment domain (ASC), which is essential for NLRP3 inflammasome activation. However, CATH-2 failed to induce IL-1β secretion in Nlrp3^-/-, Asc^-/- and Casp1^-/- macrophages. Notably, IL-1β and NLRP3 mRNA expression were not affected by CATH-2. In addition, CATH-2-induced NLRP3 inflammasome activation was mediated by K⁺ efflux but independent of the P2X7 receptor that is required for ATP-mediated K⁺ efflux. Gene interference of NEK7 kinase which has been identified to directly interact with NLRP3, significantly reduced IL-1β secretion and caspase-1 activation induced by CATH-2. Furthermore, confocal microscopy shows that CATH-2 significantly induced lysosomal leakage with the diffusion of dextran fluorescent signal. Cathepsin B inhibitors completely abrogated IL-1β secretion and caspase-1 activation as well as attenuating the formation of ASC specks induced by CATH-2. These results all indicate that CATH-2-induced activation of NLRP3 inflammasome is mediated by K⁺ efflux, and involves the NEK7 protein and cathepsin B. In conclusion, our study shows that CATH-2 acts as a second signal to activate NLRP3 inflammasome. Our study provides new insight into CATH-2 modulating immune response.

VSIG4 Attenuates NLRP3 and Ameliorates Neuroinflammation via JAK2-STAT3-A20 Pathway after Intracerebral Hemorrhage in Mice

Intracerebral hemorrhage (ICH) is a fatal cerebrovascular disease. Neuroinflammation plays an important pathological role in brain injury after ICH. NLRP3 contributes to the pathogenesis of ICH, but the underlying mechanisms regulating of NLRP3 remain elusive. V-set and immunoglobulin domain containing 4 (VSIG4), specifically expressed in resting tissue-resident macrophages, can deliver anti-inflammatory signals into various inflammatory diseases. However, the interaction between VSIG4 and NLRP3, as well as the underlying mechanisms after ICH have not been reported. C57BL/6 mice were subjected to the autologous blood injection ICH model. VSIG4 and NLRP3 levels of macrophages were detected following ICH. Ad-VSIG4 or controls were administered via intracerebroventricular (i.c.v) injection before ICH induction. STAT3 inhibitor (S31-201), JAK2 inhibitor (TG101348), or Ad-A20 RNAi was administered to investigate the role of JAK2-STAT3-A20 pathway in VSIG4-mediated neuroinflammation after ICH. Pro-inflammatory cytokine production, BBB disruption, brain water content, and neurological test were examined in ICH mice. VSIG4 levels were significantly decreased, and NLRP3 levels were significantly increased in the perihematomal brain tissues after ICH. Ad-VSIG4 attenuated NLRP3 levels and inhibited inflammation, as well as improved neurological function and reduced BBB disruption and brain water content. Furthermore, Ad-VSIG4 increased the protein levels of phosphorylated JAK2 and STAT3, and A20 levels at 24 h after ICH. STAT3 inhibitor, JAK2 inhibitor, and A20 RNAi abolished the beneficial effects of Ad-VSIG4 after ICH. In summary, these data suggested that VSIG4 attenuated NLRP3 and ameliorated neuroinflammation via JAK2-STAT3-A20 pathway after intracerebral hemorrhage in mice. VSIG4 might be an ideal therapeutic target for ICH patients.

Connexin 43: A Target for the Treatment of Inflammation in Secondary Complications of the Kidney and Eye in Diabetes

Of increasing prevalence, diabetes is characterised by elevated blood glucose and chronic inflammation that precedes the onset of multiple secondary complications, including those of the kidney and the eye. As the leading cause of end stage renal disease and blindness in the working population, more than ever is there a demand to develop clinical interventions which can both delay and prevent disease progression. Connexins are membrane bound proteins that can form pores (hemichannels) in the cell membrane. Gated by cellular stress and injury, they open under pathophysiological conditions and in doing so release 'danger signals' including adenosine triphosphate into the extracellular environment. Linked to sterile inflammation via activation of the nod-like receptor protein 3 inflammasome, targeting aberrant hemichannel activity and the release of these danger signals has met with favourable outcomes in multiple models of disease, including secondary complications of diabetes. In this review, we provide a comprehensive update on those studies which document a role for aberrant connexin hemichannel activity in the pathogenesis of both diabetic eye and kidney disease, ahead of evaluating the efficacy of blocking connexin-43 specific hemichannels in these target tissues on tissue health and function.

Clinical characteristics and risk factors associated with mortality in patients with severe community-acquired pneumonia and type 2 diabetes mellitus

BACKGROUND

The present study was performed to investigate the impacts of type 2 diabetes mellitus (T2DM) on severe community-acquired pneumonia (SCAP) and to develop a novel prediction model for mortality in SCAP patients with T2DM.

METHODS

This was a retrospective observational study conducted in consecutive adult patients with SCAP admitted to the intensive care unit (ICU) of West China Hospital, Sichuan University, China, between September 2011 and September 2019. The primary outcome was hospital mortality. A propensity score matching (PSM) analysis model with a 1:2 ratio was used for the comparisons of clinical characteristics and outcomes between T2DM and nondiabetic patients. The independent risk factors were identified via univariate and then multivariable logistic regression analysis and were then used to establish a nomogram.

RESULTS

In total, 1262 SCAP patients with T2DM and 2524 matched patients without T2DM were included after PSM. Patients with T2DM had longer ICU length of stay (LOS) (13 vs. 12 days, P = 0.016) and higher 14-day mortality (15% vs. 10.8%, P < 0.001), 30-day mortality (25.7% vs. 22.7%, P = 0.046), ICU mortality (30.8% vs. 26.5%, P = 0.005), and hospital mortality (35.2% vs. 31.0%, P = 0.009) than those without T2DM. In SCAP patients with T2DM, the independent risk factors for hospital mortality were increased numbers of comorbidities and diabetes-related complications; elevated C-reactive protein (CRP), neutrophil to lymphocyte ratio (NLR), brain natriuretic peptide (BNP) and blood lactate; as well as decreased blood pressure on admission. The nomogram had a C index of 0.907 (95% CI: 0.888, 0.927) in the training set and 0.873 (95% CI: 0.836, 0.911) in the testing set, which was superior to the pneumonia severity index (PSI, AUC: 0.809, 95% CI: 0.785, 0.833). The calibration curve and decision curve analysis (DCA) also demonstrated its accuracy and applicability.

CONCLUSIONS

SCAP patients with T2DM had worse clinical outcomes than nondiabetic patients. The nomogram has good predictive performance for hospital mortality and might be generally applied after more external validations.

IL-38-mediated NLRP3/caspase-1 inhibition is a disease-modifying treatment for TMJ inflammation

Recently, interleukin-38 (IL-38) was identified as an important anti-inflammatory and immunosuppressive factor, but its functional role in temporomandibular joint (TMJ) inflammation remains unknown. This study aimed to elucidate how IL-38 affects chondrocytes and the underlying mechanism that contributes to anti-inflammatory processes in the TMJ. Western blotting, quantitative real-time PCR, enzyme-linked immunosorbent assay, and immunofluorescence analysis were used to verify that IL-38 has anti-inflammatory effects on chondrocytes, and the related key pathways were analyzed by western blotting. SiRNA-IL-38, siRNA-NLRP3, and MCC950 were used to investigate the mechanism underlying the anti-inflammatory effects of IL-38. Inflammation models were induced by injection of complete Freund's adjuvant in TMJ with mouse recombinant IL-38 in in vivo studies. Histological and immunohistochemical analyses were used to investigate histological changes in the cartilage. The results showed that IL-38 inhibited the expression of inflammatory cytokines and MMPs. IL-38 limited inflammation by inhibiting the expression of MAPKs/NF-κB and the NLRP3/caspase-1 pathway. In vivo, IL-38 reduced chondrocyte inflammation and limited cartilage degeneration. This study shows for the first time that IL-38 plays a protective role in TMJ cartilage. IL-38 exerts anti-inflammatory effects through the NLRP3/caspase-1 pathway and may be a promising agent for treating TMJ inflammation.

Innate immune responses at the asymptomatic stage of influenza A viral infections of Streptococcus pneumoniae colonized and non-colonized mice

Seasonal Influenza A virus (IAV) infections can promote dissemination of upper respiratory tract commensals such as Streptococcus pneumoniae to the lower respiratory tract resulting in severe life-threatening pneumonia. Here, we aimed to compare innate immune responses in the lungs of healthy colonized and non-colonized mice after IAV challenge at the initial asymptomatic stage of infection. Responses during a severe bacterial pneumonia were profiled for comparison. Cytokine and innate immune cell imprints of the lungs were analyzed. Irrespective of the colonization status, mild H1N1 IAV infection was characterized by a bi-phasic disease progression resulting in full recovery of the animals. Already at the asymptomatic stage of viral infection, the pro-inflammatory cytokine response was as high as in pneumococcal pneumonia. Flow cytometry analyses revealed an early influx of inflammatory monocytes into the lungs. Neutrophil influx was mostly limited to bacterial infections. The majority of cells, except monocytes, displayed an activated phenotype characterized by elevated CCR2 and MHCII expression. In conclusion, we show that IAV challenge of colonized healthy mice does not automatically result in severe co-infection. However, a general local inflammatory response was noted at the asymptomatic stage of infection irrespective of the infection type.

Hydrogen Peroxide Is Crucial for NLRP3 Inflammasome-Mediated IL-1β Production and Cell Death in Pneumococcal Infections of Bronchial Epithelial Cells

Epithelial cells play a crucial role in detection of the pathogens as well as in initiation of the host immune response. Streptococcus pneumoniae (pneumococcus) is a typical colonizer of the human nasopharynx, which can disseminate to the lower respiratory tract and subsequently cause severe invasive diseases such as pneumonia, sepsis, and meningitis. Hydrogen peroxide (H2O2) is produced by pneumococci as a product of the pyruvate oxidase SpxB. However, its role as a virulence determinant in pneumococcal infections of the lower respiratory tract is not well understood. In this study, we investigated the role of pneumococcal-derived H2O2 in initiating epithelial cell death by analyzing the interplay between 2 key cell death pathways, namely, apoptosis and pyroptosis. We demonstrate that H2O2 primes as well as activates the NLRP3 inflammasome and thereby mediates IL-1β production and release. Furthermore, we show that pneumococcal H2O2 causes cell death via the activation of both apoptotic as well as pyroptotic pathways which are mediated by the activation of caspase-3/7 and caspase-1, respectively. However, H2O2-mediated IL-1β release itself occurs mainly via apoptosis.

Mangiferin Mitigates Lipopolysaccharide-Induced Lung Injury by Inhibiting NLRP3 Inflammasome Activation

Scope

Mangiferin (MF) is a natural phytopolyphenol, which displays potential pharmacological properties involving antibacterial, anti-inflammation, antioxidant and anti-tumor. However, little is known about the roles of MF in lung injury. The aim of this study is to demonstrate the modulatory effects and molecular mechanisms by which MF operates in sepsis-induced lung injury.

Methods and Results

To examine the protective properties of MF, an in vivo model of lipopolysaccharide (LPS)-induced lung injury in mice and an in vitro model of LPS-treated J774A.1 cells were established, respectively. The results revealed that MF treatment significantly relieved LPS-induced pathological injury and inflammatory response in murine lung tissues. Meanwhile, MF treatment also inhibited nucleotide-binding oligomerization domain (NOD)-like receptor family, pyrin domain-containing protein 3 (NLRP3) inflammasome activation and pyroptosis induced by LPS. In macrophage-specific NLRP3 deficiency mice treated with LPS, MF showed little protective effects. NLRP3 overexpression by adenovirus could also offset the beneficial effects of MF in LPS-treated J774A.1 cells. Furthermore, we found that MF could suppress the expression of NLPR3 and pyroptosis of macrophages by inhibiting the nuclear translocation of the nuclear factor-κB (NF-κB) subunits P50 and P65.

Conclusion

MF protects against lung injury and inflammatory response by inhibiting NLRP3 inflammasome activation in a NF-κB-dependent manner in macrophages, which provides a promising therapeutic candidate for the treatment of lung injury.

The NLRP3-Inflammasome-Caspase-1 Pathway Is Upregulated in Idiopathic Pulmonary Fibrosis and Acute Exacerbations and Is Inducible by Apoptotic A549 Cells

Idiopathic pulmonary fibrosis (IPF) is a relentlessly progressive disease harboring significant morbidity and mortality despite recent advances in therapy. Regardless of disease severity acute exacerbations (IPF-AEs) may occur leading to considerable loss of function and are the leading cause of death in IPF. Histologic features of IPF-AE are very similar to acute respiratory distress syndrome (ARDS), but the underlying mechanisms are incompletely understood. We investigated the role of the NLRP3 inflammasome in IPF and IPF-AE. Bronchoalveolar lavage (BAL) cells were sampled from patients with IPF (n = 32), IPF-AE (n = 10), ARDS (n = 7) and healthy volunteers (HV, n = 37) and the NLRP3-inflammasome was stimulated in-vitro. We found the NLRP3 inflammasome to be hyper-inducible in IPF compared to HV with increased IL-1ß and pro-IL-1ß levels on ELISA upon stimulation as well as increased caspase-1 activity measured by caspase-1p20 immunoblotting. In IPF-AE, IL-1ß was massively elevated to an extent similar to ARDS. To evaluate potential mechanisms, we co-cultured BAL cells with radiated A549 cells (a model to simulate apoptotic alveolar epithelial cells), which led to increased NLRP3 mRNA expression and increased caspase-1 dependent IL-1ß production. In the presence of a reactive oxygen species (ROS) inhibitor (diphenyleneiodonium) and a cathepsin B inhibitor (E64D), NLRP3 expression was suppressed indicating that induction of NLRP3 activation following efferocytosis of apoptotic A549 cells is mediated via ROS and cathepsin-B. In summary, we present evidence of involvement of the NLRP3 inflammasome-caspase pathway in the pathogenesis of IPF-AE, similarly to ARDS, which may be mediated by efferocytosis of apoptotic alveolar epithelial cells in IPF.

The Critical Role of NLRP6 Inflammasome in Streptococcus pneumoniae Infection In Vitro and In Vivo

Streptococcus pneumoniae (S. pneumoniae) causes severe pulmonary diseases, leading to high morbidity and mortality. It has been reported that inflammasomes such as NLR family pyrin domain containing 3 (NLRP3) and absent in melanoma 2 (AIM2) play an important role in the host defense against S. pneumoniae infection. However, the role of NLRP6 in vivo and in vitro against S. pneumoniae remains unclear. Therefore, we investigated the role of NLRP6 in regulating the S. pneumoniae-induced inflammatory signaling pathway in vitro and the role of NLRP6 in the host defense against S. pneumoniae in vivo by using NLRP6^−/− mice. The results showed that the NLRP6 inflammasome regulated the maturation and secretion of IL-1β, but it did not affect the induction of IL-1β transcription in S. pneumoniae-infected macrophages. Furthermore, the activation of caspase-1, caspase-11, and gasdermin D (GSDMD) as well as the oligomerization of apoptosis-associated speck-like protein (ASC) were also mediated by NLRP6 in S. pneumoniae-infected macrophages. However, the activation of NLRP6 reduced the expression of NF-κB and ERK signaling pathways in S. pneumoniae-infected macrophages. In vivo study showed that NLRP6^−/− mice had a higher survival rate, lower number of bacteria, and milder inflammatory response in the lung compared with wild-type (WT) mice during S. pneumoniae infection, indicating that NLRP6 plays a negative role in the host defense against S. pneumoniae. Furthermore, increased bacterial clearance in NLRP6 deficient mice was modulated by the recruitment of macrophages and neutrophils. Our study provides a new insight on S. pneumoniae-induced activation of NLRP6 and suggests that blocking NLRP6 could be considered as a potential therapeutic strategy to treat S. pneumoniae infection.