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Enteropathogenic and enterohemorrhagic bacterial infections in humans are a severe cause of morbidity and mortality. Although NOD-like receptors (NLRs) NOD2 and NLRP3 have important roles in the generation of protective immune responses to enteric pathogens, whether there is crosstalk among NLRs to regulate immune signaling is not known. Here, we show that mice and macrophages deficient in NOD2, or the downstream adaptor RIP2, have enhanced NLRP3- and caspases-11-dependent non-canonical inflammasome activation in a mouse model of enteropathogenic Citrobacter rodentium infection. Mechanistically, NOD2 and RIP2 regulate reactive oxygen species (ROS) production. Increased ROS in Rip2-deficient macrophages subsequently enhances c-Jun N-terminal kinase (JNK) signaling resulting in increased caspase-11 expression and activation, and more non-canonical NLRP3-dependant inflammasome activation. Intriguingly, this leads to protection of the colon epithelium for up to 10 days in Rip2-deficient mice infected with C. rodentium. Our findings designate NOD2 and RIP2 as key regulators of cellular ROS homeostasis and demonstrate for the first time that ROS regulates caspase-11 expression and non-canonical NLRP3 inflammasome activation through the JNK pathway.

Viral-bacterial coinfections, such as with influenza A virus and Streptococcus pneumoniae (S.p.), are known to cause severe pneumonia. It is well known that the host response has an important role in disease. Interleukin-1β (IL-1β) is an important immune signaling cytokine responsible for inflammation and has been previously shown to contribute to disease severity in numerous infections. Other studies in mice indicate that IL-1β levels are dramatically elevated during IAV-S.p. coinfection. However, the regulation of IL-1β during coinfection is unknown. Here, we report the NLRP3 inflammasome is the major inflammasome regulating IL-1β activation during coinfection. Furthermore, elevated IL-1β mRNA expression is due to enhanced TLR2-MYD88 signaling, which increases the amount of pro-IL-1β substrate for the inflammasome to process. Finally, NLRP3 and high IL-1β levels were associated with increased bacterial load in the brain. Our results show the NLRP3 inflammasome is not protective during IAV-S.p. coinfection.

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in young children and elderly, worldwide and poses significant risks to immunocompromised individuals. To elucidate host-virus interactions at the transcriptional level, we analyzed differential gene expression in HEp-2 cells infected with RSV using cDNA microarray analysis complemented by quantitative PCR (qPCR). HEp-2 cells were infected with RSV at a multiplicity of infection of 1, and total RNA was isolated 24 hours post-infection for gene expression profiling. Radiolabeled cDNA probes from RSV-infected and mock-infected cells were hybridized to Atlas ® Human Cancer cDNA arrays, and differential gene expression was quantified by densitometry. We identified 12 host genes that were significantly upregulated in RSV-infected cells from the cDNA microarray (≥2-fold increase, P <0.01), confirmed by qPCR, encompassing functional categories including cell cycle regulation, cytoskeletal organization, apoptosis modulation, immune evasion, and inflammation. Notably, the cyclin-dependent kinase inhibitor CDKN1A was induced ~14-fold, suggesting RSV triggers a host cell cycle arrest. The intermediate filament protein, vimentin was up ~6-fold, consistent with cytoskeletal rearrangements observed during viral syncytium formation. Anti-apoptotic MCL1 increased ~11-fold, while pro-apoptotic caspase-4 showed a more modest 1.6-fold rise, indicating a complex regulation of cell death pathways. We also observed marked upregulation of a fibronectin receptor subunit (~24-fold) and complement regulatory protein CD59 (~2-fold), highlighting potential mechanisms of enhanced cell-cell fusion and viral immune evasion. The proinflammatory cytokine interleukin-6 was elevated ~7-fold, underscoring the inflammatory response to RSV. These findings provide a global snapshot of the host transcriptomic response to RSV infection and yield insights into how RSV modulates host cellular machinery to favor viral replication and spread. Understanding these host-virus interactions may unveil novel targets for antiviral therapy and inform strategies to mitigate RSV disease pathogenesis.

Autophagocytic disposal of mitochondria (mitophagy) is important for regulating inflammation. Kanneganti and colleagues show that RIP2 kinase–initiated mitophagy is critical for dampening virally triggered immunopathology. NOD2 receptor and the cytosolic protein kinase RIPK2 regulate NF-κB and MAP kinase signaling during bacterial infections, but the role of this immune axis during viral infections has not been addressed. We demonstrate that Nod2 −/− and Ripk2 −/− mice are hypersusceptible to infection with influenza A virus. Ripk2 −/− cells exhibited defective autophagy of mitochondria (mitophagy), leading to enhanced mitochondrial production of superoxide and accumulation of damaged mitochondria, which resulted in greater activation of the NLRP3 inflammasome and production of IL-18. RIPK2 regulated mitophagy in a kinase-dependent manner by phosphorylating the mitophagy inducer ULK1. Accordingly, Ulk1 −/− cells exhibited enhanced mitochondrial production of superoxide and activation of caspase-1. These results demonstrate a role for NOD2-RIPK2 signaling in protection against virally triggered immunopathology by negatively regulating activation of the NLRP3 inflammasome and production of IL-18 via ULK1-dependent mitophagy.

[...]Zhao and Zhaoevaluated the vast literature describing the roles of NLRP3 during viral infection. Others have specifically expanded our understanding of NLR signaling pathways with their functions during different infectious and inflammatory diseases. [...]the results, hypotheses, and postulations made by the authors of the articles in this collection provide exciting avenues of future research, namely, the development of viable therapeutics to target NLRs and their downstream signaling pathways, the need to more carefully examine NLR receptor specificity for specific pathogens, and the need for more detailed characterization of the molecular mechanisms involved in NLR signaling and activation. Conflict of Interest The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. 1.ZhuSDingSWangPWeiZPanWPalmNW.Nlrp9b inflammasome restricts rotavirus infection in intestinal epithelial cells.Nature.

Human Respiratory Syncytial Virus (HRSV) is a leading cause of bronchopneumonia in infants and the elderly. To date, knowledge of viral and host protein interactions within HRSV is limited and are critical areas of research. Here, we show that HRSV Matrix (M) protein interacts with the cellular adaptor protein complex 3 specifically via its medium subunit (AP-3Mu3A). This novel protein-protein interaction was first detected via yeast-two hybrid screen and was further confirmed in a mammalian system by immunofluorescence colocalization and co-immunoprecipitation. This novel interaction is further substantiated by the presence of a known tyrosine-based adaptor protein MU subunit sorting signal sequence, YXXФ: where Ф is a bulky hydrophobic residue, which is conserved across the related RSV M proteins. Analysis of point-mutated HRSV M derivatives indicated that AP-3Mu3A- mediated trafficking is contingent on the presence of the tyrosine residue within the YXXL sorting sequence at amino acids 197-200 of the M protein. AP-3Mu3A is up regulated at 24 hours post-infection in infected cells versus mock-infected HEp2 cells. Together, our data suggests that the AP-3 complex plays a critical role in the trafficking of HRSV proteins specifically matrix in epithelial cells. The results of this study add new insights and targets that may lead to the development of potential antivirals and attenuating mutations suitable for candidate vaccines in the future.

Influenza A virus (IAV) causes seasonal epidemics annually and pandemics every few decades. Most antiviral treatments used for IAV are only effective if administered during the first 48 h of infection and antiviral resistance is possible. Therapies that can be initiated later during IAV infection and that are less likely to elicit resistance will significantly improve treatment options. Pyruvate, a key metabolite, and an end product of glycolysis, has been studied for many uses, including its anti-inflammatory capabilities. Sodium pyruvate was recently shown by us to decrease inflammasome activation during IAV infection. Here, we investigated sodium pyruvate’s effects on IAV in vivo. We found that nebulizing mice with sodium pyruvate decreased morbidity and weight loss during infection. Additionally, treated mice consumed more chow during infection, indicating improved symptoms. There were notable improvements in pro-inflammatory cytokine production (IL-1β) and lower virus titers on day 7 post-infection in mice treated with sodium pyruvate compared to control animals. As pyruvate acts on the host immune response and metabolic pathways and not directly on the virus, our data demonstrate that sodium pyruvate is a promising treatment option that is safe, effective, and unlikely to elicit antiviral resistance.

Clinical studies have indicated that subvirion inactivated vaccines against avian influenza viruses, particularly H5N1, are poorly immunogenic in humans. As a consequence, the use of adjuvants has been championed for the efficient vaccination of a naïve population against avian influenza. Aluminum salts (alum) and the oil-in-water emulsion MF59 are safe and effective adjuvants that are being used with influenza vaccines, but the mechanism underlying their stimulation of the immune system remains poorly understood. It was shown recently that activation of a cytosolic innate immune-sensing complex known as \"NLR-Pyrin domain containing 3\" (NLRP3) inflammasome, also known as \"cryopyrin,\" \"cold-induced autoinflammatory syndrome 1\" (CIAS1), or nacht domain-, leucine-rich repeat-, and PYD-containing protein 3 (Nalp3), is essential for the adjuvant effect of alum. Here we show that the inflammasome component apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), an adapter protein within the NLRP3 inflammasome, is a crucial element in the adjuvant effect of MF59 when combined with H5N1 subunit vaccines. In the absence of ASC, H5-specific IgG antibody responses are significantly reduced, whereas the responses are intact in NLRP3⁻/⁻ and caspase-1⁻/⁻ mice. This defect is caused mainly by the failure of antigen-specific B cells to switch from IgM to IgG production. We conclude that ASC plays an inflammasome-independent role in the induction of antigen-specific humoral immunity after vaccination with MF59-adjuvanted influenza vaccines. These findings have important implications for the rational design of next-generation adjuvants.

Viral-bacterial coinfections, such as with influenza A virus and Streptococcus pneumoniae (S.p.), are known to cause severe pneumonia. It is well known that the host response has an important role in disease. Interleukin-1[beta] (IL-1[beta]) is an important immune signaling cytokine responsible for inflammation and has been previously shown to contribute to disease severity in numerous infections. Other studies in mice indicate that IL-1[beta] levels are dramatically elevated during IAV-S.p. coinfection. However, the regulation of IL-1[beta] during coinfection is unknown. Here, we report the NLRP3 inflammasome is the major inflammasome regulating IL-1[beta] activation during coinfection. Furthermore, elevated IL-1[beta] mRNA expression is due to enhanced TLR2-MYD88 signaling, which increases the amount of pro-IL-1[beta] substrate for the inflammasome to process. Finally, NLRP3 and high IL-1[beta] levels were associated with increased bacterial load in the brain. Our results show the NLRP3 inflammasome is not protective during IAV-S.p. coinfection.

Ketone bodies are elevated in response to fasting, a low-carbohydrate ketogenic diet or high-intensity exercise. Vishwa Deep Dixit and colleagues report that one metabolite, β-hydroxybutyrate, inhibits the NLRP3 inflammasome. In vivo , β-hydroxybutyrate is anti-inflammatory and suppresses NLRP3-mediated inflammatory disease. The ketone bodies β-hydroxybutyrate (BHB) and acetoacetate (AcAc) support mammalian survival during states of energy deficit by serving as alternative sources of ATP 1 . BHB levels are elevated by starvation, caloric restriction, high-intensity exercise, or the low-carbohydrate ketogenic diet 2 . Prolonged fasting reduces inflammation; however, the impact that ketones and other alternative metabolic fuels produced during energy deficits have on the innate immune response is unknown 2 , 3 , 4 , 5 , 6 . We report that BHB, but neither AcAc nor the structurally related short-chain fatty acids butyrate and acetate, suppresses activation of the NLRP3 inflammasome in response to urate crystals, ATP and lipotoxic fatty acids. BHB did not inhibit caspase-1 activation in response to pathogens that activate the NLR family, CARD domain containing 4 (NLRC4) or absent in melanoma 2 (AIM2) inflammasome and did not affect non-canonical caspase-11, inflammasome activation. Mechanistically, BHB inhibits the NLRP3 inflammasome by preventing K + efflux and reducing ASC oligomerization and speck formation. The inhibitory effects of BHB on NLRP3 are not dependent on chirality or starvation-regulated mechanisms like AMP-activated protein kinase (AMPK), reactive oxygen species (ROS), autophagy or glycolytic inhibition. BHB blocks the NLRP3 inflammasome without undergoing oxidation in the TCA cycle, and independently of uncoupling protein-2 (UCP2), sirtuin-2 (SIRT2), the G protein–coupled receptor GPR109A or hydrocaboxylic acid receptor 2 (HCAR2). BHB reduces NLRP3 inflammasome–mediated interleukin (IL)-1β and IL-18 production in human monocytes. In vivo , BHB or a ketogenic diet attenuates caspase-1 activation and IL-1β secretion in mouse models of NLRP3-mediated diseases such as Muckle–Wells syndrome, familial cold autoinflammatory syndrome and urate crystal–induced peritonitis. Our findings suggest that the anti-inflammatory effects of caloric restriction or ketogenic diets may be linked to BHB-mediated inhibition of the NLRP3 inflammasome.