NLRP3 inflammasome: key mediator of neuroinflammation in murine Japanese encephalitis

Inflammasomes in the CNS

Microglia and macrophages in the CNS contain multimolecular complexes termed inflammasomes. Inflammasomes function as intracellular sensors for infectious agents as well as for host-derived danger signals that are associated with neurological diseases, including meningitis, stroke and Alzheimer's disease. Assembly of an inflammasome activates caspase 1 and, subsequently, the proteolysis and release of the cytokines interleukin-1β and interleukin-18, as well as pyroptotic cell death. Since the discovery of inflammasomes in 2002, there has been burgeoning recognition of their complexities and functions. Here, we review the current understanding of the functions of different inflammasomes in the CNS and their roles in neurological diseases.

MicroRNA-29b modulates Japanese encephalitis virus-induced microglia activation by targeting tumor necrosis factor alpha-induced protein 3

Japanese encephalitis virus (JEV), a single-stranded RNA (ssRNA) virus, is the leading cause of encephalitis in Asia. Microglial activation is one of the key events in JEV-induced neuroinflammation. Although the various microRNAs (miRNAs) has been shown to regulate microglia activation during pathological conditions including neuroviral infections, till date, the involvement of miRNAs in JEV infection has not been evaluated. Hence, we sought to evaluate the possible role of miRNAs in mediating JEV-induced microglia activation. Initial screening revealed significant up-regulation of miR-29b in JEV-infected mouse microglial cell line (BV-2) and primary microglial cells. Furthermore, using bioinformatics tools, we identified tumor necrosis factor alpha-induced protein 3, a negative regulator of nuclear factor-kappa B signaling as a potential target of miR-29b. Interestingly, in vitro knockdown of miR-29b resulted in significant over-expression of tumor necrosis factor alpha-induced protein 3, and subsequent decrease in nuclear translocation of pNF-κB. JEV infection in BV-2 cell line elevated inducible nitric oxide synthase, cyclooxygenase-2, and pro-inflammatory cytokine expression levels, which diminished after miR-29b knockdown. Collectively, our study demonstrates involvement of miR-29b in regulating JEV- induced microglial activation.

A Nod to disease vectors: mitigation of pathogen sensing by arthropod saliva

Arthropod saliva possesses anti-hemostatic, anesthetic, and anti-inflammatory properties that facilitate feeding and, inadvertently, dissemination of pathogens. Vector-borne diseases caused by these pathogens affect millions of people each year. Many studies address the impact of arthropod salivary proteins on various immunological components. However, whether and how arthropod saliva counters Nod-like (NLR) sensing remains elusive. NLRs are innate immune pattern recognition molecules involved in detecting microbial molecules and danger signals. Nod1/2 signaling results in activation of the nuclear factor-κB and the mitogen-activated protein kinase pathways. Caspase-1 NLRs regulate the inflammasome~– a protein scaffold that governs the maturation of interleukin (IL)-1β and IL-18. Recently, several vector-borne pathogens have been shown to induce NLR activation in immune cells. Here, we provide a brief overview of NLR signaling and discuss clinically relevant vector-borne pathogens recognized by NLR pathways. We also elaborate on possible anti-inflammatory effects of arthropod saliva on NLR signaling and microbial pathogenesis for the purpose of exchanging research perspectives.

Platelets mediate increased endothelium permeability in dengue through NLRP3-inflammasome activation

Dengue is the most frequent hemorrhagic viral disease and re-emergent infection in the world. Although thrombocytopenia is characteristically observed in mild and severe forms of dengue, the role of platelet activation in dengue pathogenesis has not been fully elucidated. We hypothesize that platelets have major roles in inflammatory amplification and increased vascular permeability during severe forms of dengue. Here we investigate interleukin (IL)-1β synthesis, processing, and secretion in platelets during dengue virus (DV) infection and potential contribution of these events to endothelial permeability during infection. We observed increased expression of IL-1β in platelets and platelet-derived microparticles from patients with dengue or after platelet exposure to DV in vitro. We demonstrated that DV infection leads to assembly of nucleotide-binding domain leucine rich repeat containing protein (NLRP3) inflammasomes, activation of caspase-1, and caspase-1-dependent IL-1β secretion. Our findings also indicate that platelet-derived IL-1β is chiefly released in microparticles through mechanisms dependent on mitochondrial reactive oxygen species-triggered NLRP3 inflammasomes. Inflammasome activation and platelet shedding of IL-1β-rich microparticles correlated with signs of increased vascular permeability. Moreover, microparticles from DV-stimulated platelets induced enhanced permeability in vitro in an IL-1-dependent manner. Our findings provide new evidence that platelets contribute to increased vascular permeability in DV infection by inflammasome-dependent release of IL-1β.

Expression of the NLRP3 inflammasome in cerebral cortex after traumatic brain injury in a rat model

Inflammatory response plays an important role in the pathogenesis of secondary damage after traumatic brain injury (TBI). The inflammasome is a multiprotein complex involved in innate immunity and a number of studies have suggested that the inflammasome plays a critical role in a host inflammatory signaling. Nucleotide-binding domain, leucine-rich repeat, pyrin domain containing 3 (NLRP3) is a key component of the NLRP3-inflammasome, which also includes apoptotic speck-containing protein (ASC) with a cysteine protease (caspase)-activating recruitment domain and pro-caspase1. Activation of the NLRP3-inflammasome causes the processing and release of the interleukin 1 beta (IL-1β) and interleukin 18 (IL-18). Based on this, we hypothesized that the NLRP3-inflammasome could participate in the inflammatory response following TBI. However, the expression of NLRP3-inflammasome in cerebral cortex after TBI is not well known. Rats were randomly divided into control, sham and TBI groups (including 6 h, 1 day, 3 day and 7 day sub-group). TBI model was induced, and animals were sacrificed at each time point respectively. The expression of NLRP3-inflammasome was measured by quantitative real-time polymerase chain reaction, western blot and immunohistochemistry respectively. Immunofluorescent double labeling was performed to identify the cell types of NLRP3-inflammasome's expression. Moreover, enzyme linked immunosorbent assay was used to detect the alterations of IL-1β and IL-18 at each time point post-injury. The results showed that, TBI could induce assembly of NLRP3-inflammasome complex, increased expression of ASC, activation of caspase1, and processing of IL-1β and IL-18. These results suggested that NLRP3-inflammasome might play an important role in the inflammation induced by TBI and could be a target for TBI therapy.

The inflammasome: pathways linking psychological stress, depression, and systemic illnesses

Stress is a common occurrence in everyday life and repeated or traumatic stress can be a precipitating factor for illnesses of the central nervous system, as well as peripheral organ systems. For example, severe or long-term psychological stress can not only induce depression, a leading illness worldwide, but can also cause psychosomatic diseases such as asthma and rheumatoid arthritis. Related key questions include how psychological stress influences both brain and peripheral systems, and what detection mechanisms underlie these effects? A clue is provided by the discovery of the pathways underlying the responses to host "danger" substances that cause systemic diseases, but can also contribute to depression. The inflammasome is a protein complex that can detect diverse danger signals and produce the accompanying immune-inflammatory reactions. Interestingly, the inflammasome can detect not only pathogen-associated molecules, but also cell damage-associated molecules such as ATP. Here, we propose a new inflammasome hypothesis of depression and related comorbid systemic illnesses. According to this hypothesis, the inflammasome is a central mediator by which psychological and physical stressors can contribute to the development of depression, and as well as a bridge to systemic diseases. This hypothesis includes an explanation for how psychological stress can influence systemic diseases, and conversely how systemic diseases can lead to psychiatric illnesses. The evidence suggests that the inflammasome may be a new target for the development of treatments for depression, as well as psychosomatic and somato-psycho diseases.

Innate immunity and neuroinflammation

Inflammation of central nervous system (CNS) is usually associated with trauma and infection. Neuroinflammation occurs in close relation to trauma, infection, and neurodegenerative diseases. Low-level neuroinflammation is considered to have beneficial effects whereas chronic neuroinflammation can be harmful. Innate immune system consisting of pattern-recognition receptors, macrophages, and complement system plays a key role in CNS homeostasis following injury and infection. Here, we discuss how innate immune components can also contribute to neuroinflammation and neurodegeneration.

IL-1β production through the NLRP3 inflammasome by hepatic macrophages links hepatitis C virus infection with liver inflammation and disease

Chronic hepatitis C virus (HCV) infection is a leading cause of liver disease. Liver inflammation underlies infection-induced fibrosis, cirrhosis and liver cancer but the processes that promote hepatic inflammation by HCV are not defined. We provide a systems biology analysis with multiple lines of evidence to indicate that interleukin-1β (IL-1β) production by intrahepatic macrophages confers liver inflammation through HCV-induced inflammasome signaling. Chronic hepatitis C patients exhibited elevated levels of serum IL-1β compared to healthy controls. Immunohistochemical analysis of healthy control and chronic hepatitis C liver sections revealed that Kupffer cells, resident hepatic macrophages, are the primary cellular source of hepatic IL-1β during HCV infection. Accordingly, we found that both blood monocyte-derived primary human macrophages, and Kupffer cells recovered from normal donor liver, produce IL-1β after HCV exposure. Using the THP-1 macrophage cell-culture model, we found that HCV drives a rapid but transient caspase-1 activation to stimulate IL-1β secretion. HCV can enter macrophages through non-CD81 mediated phagocytic uptake that is independent of productive infection. Viral RNA triggers MyD88-mediated TLR7 signaling to induce IL-1β mRNA expression. HCV uptake concomitantly induces a potassium efflux that activates the NLRP3 inflammasome for IL-1β processing and secretion. RNA sequencing analysis comparing THP1 cells and chronic hepatitis C patient liver demonstrates that viral engagement of the NLRP3 inflammasome stimulates IL-1β production to drive proinflammatory cytokine, chemokine, and immune-regulatory gene expression networks linked with HCV disease severity. These studies identify intrahepatic IL-1β production as a central feature of liver inflammation during HCV infection. Thus, strategies to suppress NLRP3 or IL-1β activity could offer therapeutic actions to reduce hepatic inflammation and mitigate disease.

Hepatitis C virus (HCV) causes chronic infection of the liver and is a leading cause of liver inflammation, cirrhosis and liver cancer in nearly 200 million people worldwide. Importantly, hepatic inflammation during chronic HCV infection is considered to be the primary catalyst for progressive liver disease and development of liver cancer. However, the underlying molecular mechanism(s) of HCV-mediated hepatic inflammation are not well understood. The goal of this study was to determine the mechanisms of HCV-induced inflammation. We found that serum IL-1β levels are elevated in chronic hepatitis C patients. Furthermore, we found that hepatic macrophages or Kupffer cells are the major IL-1β-producing cell population within HCV infected livers. Our studies, using the THP1 cell culture model of HCV exposure, reveal that exposure of macrophages to HCV induces IL-1β through a process of infection-independent phagocytic virus uptake that triggers signaling through MyD88/TLR7 and NLRP3 inflammasome pathways to drive IL-1β expression and maturation/secretion, respectively. RNA sequencing (RNA-seq) analysis of patient liver biopsies shows that viral triggering of these signaling pathways drives an inflammatory response linked with liver disease in patients with chronic hepatitis C. Our results identify HCV-induced IL-1β production by hepatic macrophages as a critical and central process that promotes liver inflammation and disease.

Rabies virus is recognized by the NLRP3 inflammasome and activates interleukin-1β release in murine dendritic cells

Inflammasome activation is important for the development of an effective host defense against many pathogens, including RNA viruses. However, the mechanism by which the inflammasome recognizes RNA viruses and its role in rabies virus (RABV) pathogenicity and immunogenicity remain poorly defined. To determine the function of the inflammasome in response to RABV infection, we infected murine bone marrow-derived dendritic cells (BMDCs) with RABV. Our results indicate that the infection of BMDCs with RABV induces both the production of pro-interleukin-1β (pro-IL-1β) and its processing, resulting in the secretion of active IL-1β through activation of the NLRP3-, ASC-, and caspase-1-dependent inflammasome. As previously shown for the induction of type I interferon by RABV, the induction of pro-IL-1β also depends upon IPS-1. We demonstrate that both the production of pro-IL-1β and activation of the inflammasome require viral replication. We also demonstrate that increased viral replication in BMDCs derived from IFNAR-deficient mice resulted in significantly more IL-1β release. Additionally, IL-1 receptor-deficient mice show an increase in RABV pathogenicity. Taken together, these results indicate an important role of the inflammasome in innate immune recognition of RABV.

IL-1β signaling promotes CNS-intrinsic immune control of West Nile virus infection

West Nile virus (WNV) is an emerging flavivirus capable of infecting the central nervous system (CNS) and mediating neuronal cell death and tissue destruction. The processes that promote inflammation and encephalitis within the CNS are important for control of WNV disease but, how inflammatory signaling pathways operate to control CNS infection is not defined. Here, we identify IL-1β signaling and the NLRP3 inflammasome as key host restriction factors involved in viral control and CNS disease associated with WNV infection. Individuals presenting with acute WNV infection displayed elevated levels of IL-1β in their plasma over the course of infection, suggesting a role for IL-1β in WNV immunity. Indeed, we found that in a mouse model of infection, WNV induced the acute production of IL-1β in vivo, and that animals lacking the IL-1 receptor or components involved in inflammasome signaling complex exhibited increased susceptibility to WNV pathogenesis. This outcome associated with increased accumulation of virus within the CNS but not peripheral tissues and was further associated with altered kinetics and magnitude of inflammation, reduced quality of the effector CD8⁺ T cell response and reduced anti-viral activity within the CNS. Importantly, we found that WNV infection triggers production of IL-1β from cortical neurons. Furthermore, we found that IL-1β signaling synergizes with type I IFN to suppress WNV replication in neurons, thus implicating antiviral activity of IL-1β within neurons and control of virus replication within the CNS. Our studies thus define the NLRP3 inflammasome pathway and IL-1β signaling as key features controlling WNV infection and immunity in the CNS, and reveal a novel role for IL-1β in antiviral action that restricts virus replication in neurons.

Since its introduction into North America in 1999, West Nile virus (WNV) has emerged as a leading cause of viral encephalitic disease in the United States. While low level inflammation is important for clearance of WNV, high levels of inflammation are associated with increased disease. The goal of this study was to identify host signaling pathways that control the balance of inflammation and protective immunity to WNV. Using a mouse model of infection, we identified a central nervous system (CNS)-intrinsic requirement for the NLRP3 inflammasome and IL-1β signaling in limiting WNV associated disease within the CNS. First, IL-1β signaling was essential for regulating the magnitude and kinetics of inflammation within CNS. Secondly, the absence of IL-1β signaling disrupted the quality of the effector T lymphocyte response against the virus. Finally, these dysregulated immune responses were linked to a direct ability for IL-1β signaling to synergize with type I IFN signaling and limit virus replication within cortical neurons, key target cells of WNV infection within the CNS. Together this study identifies the NLRP3 inflammasome and IL-1β signaling as key restriction factors that act to regulate viral load and the quality of inflammatory responses within the CNS to impart protective immunity against WNV infection.

The involvement of microglial cells in Japanese encephalitis infections

Despite the availability of effective vaccines, Japanese encephalitis virus (JEV) infections remain a leading cause of encephalitis in many Asian countries. The virus is transmitted to humans by Culex mosquitoes, and, while the majority of human infections are asymptomatic, up to 30% of JE cases admitted to hospital die and 50% of the survivors suffer from neurological sequelae. Microglia are brain-resident macrophages that play key roles in both the innate and adaptive immune responses in the CNS and are thus of importance in determining the pathology of encephalitis as a result of JEV infection.