Encephalomyocarditis virus viroporin 2B activates NLRP3 inflammasome

HCV genomic RNA activates the NLRP3 inflammasome in human myeloid cells

BACKGROUND

Elevated plasma levels of IL-1β and IL-18 from patients with hepatitis C virus (HCV) infection indicate a possible activation of inflammasome by HCV.

METHODOLOGY/PRINCIPAL FINDINGS

To demonstrate whether HCV infection activates the inflammasome, we investigated inflammasome activation from HCV infected hepatic Huh7 cells, or monocytic cells and THP-1 derived macrophages challenged with HCV virions, but no any inflammasome activation was detected in these cells. However, when we transfected HCV genomic RNA into monocytes or macrophages, IL-1β was secreted in a dose-dependent manner. We also detected ASC oligomerization and caspase-1 cleavage in HCV RNA transfected macrophages. Using shRNA-mediated gene silencing or specific inhibitors, we found that HCV RNA-induced IL-1β secretion was dependent on the presence of inflammasome components such as NLRP3, ASC and caspase-1. Furthermore, we also found that RIG-I was dispensable for HCV RNA-induced NLRP3 inflammasome activation, while reactive oxygen species (ROS) production was required.

CONCLUSIONS

Our results indicate that HCV RNA activates the NLRP3 inflammasome in a ROS-dependent manner, and RIG-I is not required for this process.

Inflammasomes in antiviral immunity: clues for influenza vaccine development

Inflammasomes are cytosolic multiprotein complexes that sense microbial motifs or cellular stress and stimulate caspase-1-dependent cytokine secretion and cell death. Recently, it has become increasingly evident that both DNA and RNA viruses activate inflammasomes, which control innate and adaptive immune responses against viral infections. In addition, recent studies suggest that certain microbiota induce inflammasomes-dependent adaptive immunity against influenza virus infections. Here, we review recent advances in research into the role of inflammasomes in antiviral immunity.

An Update on PYRIN Domain-Containing Pattern Recognition Receptors: From Immunity to Pathology

Cytosolic pattern recognition receptors (PRRs) sense a wide range of endogenous danger-associated molecular patterns as well as exogenous pathogen-associated molecular patterns. In particular, Nod-like receptors containing a pyrin domain (PYD), called NLRPs, and AIM2-like receptors (ALRs) have been shown to play a critical role in host defense by facilitating clearance of pathogens and maintaining a healthy gut microflora. NLRPs and ALRs both encode a PYD, which is crucial for relaying signals that result in an efficient innate immune response through activation of several key innate immune signaling pathways. However, mutations in these PRRs have been linked to the development of auto-inflammatory and autoimmune diseases. In addition, they have been implicated in metabolic diseases. In this review, we summarize the function of PYD-containing NLRPs and ALRs and address their contribution to innate immunity, host defense, and immune-linked diseases.

Rift Valley fever virus infection induces activation of the NLRP3 inflammasome

Inflammasome activation is gaining recognition as an important mechanism for protection during viral infection. Here, we investigate whether Rift Valley fever virus, a negative-strand RNA virus, can induce inflammasome responses and IL-1β processing in immune cells. We have determined that RVFV induces NLRP3 inflammasome activation in murine dendritic cells, and that this process is dependent upon ASC and caspase-1. Furthermore, absence of the cellular RNA helicase adaptor protein MAVS/IPS-1 significantly reduces extracellular IL-1β during infection. Finally, direct imaging using confocal microscopy shows that the MAVS protein co-localizes with NLRP3 in the cytoplasm of RVFV infected cells.

Mechanisms of NOD-like receptor-associated inflammasome activation

A major function of a subfamily of NLR (nucleotide-binding domain, leucine-rich repeat containing, or NOD-like receptor) proteins is in inflammasome activation, which has been implicated in a multitude of disease models and human diseases. This work will highlight key progress in understanding the mechanisms that activate the best-studied NLRs (NLRP3, NLRC4, NAIP, and NLRP1) and in uncovering inflammasome NLRs.

Viroporin activity of the JC polyomavirus is regulated by interactions with the adaptor protein complex 3

Viroporins, which are encoded by a wide range of animal viruses, oligomerize in host cell membranes and form hydrophilic pores that can disrupt a number of physiological properties of the cell. Little is known about the relationship between host cell proteins and viroporin activity. The human JC polyomavirus (JCV) is the causative agent of progressive multifocal leukoencephalopathy. The JCV-encoded agnoprotein, which is essential for viral replication, has been shown to act as a viroporin. Here we demonstrate that the JCV agnoprotein specifically interacts with adaptor protein complex 3 through its δ subunit. This interaction interrupts adaptor protein complex 3-mediated vesicular trafficking with suppression of the targeting of the protein to the lysosomal degradation pathway and instead permits the transport of agnoprotein to the cell surface with resulting membrane permeabilization. The findings demonstrate a previously undescribed paradigm in virus-host interactions allowing the host to regulate viroporin activity and suggest that the viroporins of other viruses may also be highly regulated by specific interactions with host cell proteins.

Hepatitis C virus induces interleukin-1β (IL-1β)/IL-18 in circulatory and resident liver macrophages

Hepatitis C virus (HCV)-mediated chronic liver disease is a global health problem, and inflammation is believed to be an important player in disease pathogenesis. HCV infection often leads to severe fibrosis/cirrhosis and hepatocellular carcinoma, although the mechanisms for advancement of disease are not fully understood. The proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 have critical roles in establishment of inflammation. In this study, we examined induction of IL-1β/IL-18 secretion following HCV infection. Our results demonstrated that monocyte-derived human macrophages (THP-1) incubated with cell culture-grown HCV enhance the secretion of IL-1β/IL-18 into culture supernatants. A similar cytokine release was also observed for peripheral blood mononuclear cell (PBMC)-derived primary human macrophages and Kupffer cells (liver-resident macrophages) upon incubation with HCV. THP-1 cells incubated with HCV led to caspase-1 activation and release of proinflammatory cytokines. Subsequent studies demonstrated that HCV induces pro-IL-1β and pro-IL-18 synthesis via the NF-κB signaling pathway in macrophages. Furthermore, introduction of HCV viroporin p7 RNA into THP-1 cells was sufficient to cause IL-1β secretion. Together, our results suggested that human macrophages exposed to HCV induce IL-1β and IL-18 secretion, which may play a role in hepatic inflammation.

Mitochondrial protein mitofusin 2 is required for NLRP3 inflammasome activation after RNA virus infection

Nod-like receptor family, pyrin domain-containing 3 (NLRP3), is involved in the early stages of the inflammatory response by sensing cellular damage or distress due to viral or bacterial infection. Activation of NLRP3 triggers its assembly into a multimolecular protein complex, termed "NLRP3 inflammasome." This event leads to the activation of the downstream molecule caspase-1 that cleaves the precursor forms of proinflammatory cytokines, such as interleukin 1 beta (IL-1β) and IL-18, and initiates the immune response. Recent studies indicate that the reactive oxygen species produced by mitochondrial respiration is critical for the activation of the NLRP3 inflammasome by monosodium urate, alum, and ATP. However, the precise mechanism by which RNA viruses activate the NLRP3 inflammasome is not well understood. Here, we show that loss of mitochondrial membrane potential [ΔΨ(m)] dramatically reduced IL-1β secretion after infection with influenza, measles, or encephalomyocarditis virus (EMCV). Reduced IL-1β secretion was also observed following overexpression of the mitochondrial inner membrane protein, uncoupling protein-2, which induces mitochondrial proton leakage and dissipates ΔΨ(m). ΔΨ(m) was required for association between the NLRP3 and mitofusin 2, a mediator of mitochondrial fusion, after infection with influenza virus or EMCV. Importantly, the knockdown of mitofusin 2 significantly reduced the secretion of IL-1β after infection with influenza virus or EMCV. Our results provide insight into the roles of mitochondria in NLRP3 inflammasome activation.

Unsolved Mysteries in NLR Biology

NOD-like receptors (NLRs) are a class of cytoplasmic pattern-recognition receptors. Although most NLRs play some role in immunity, their functions range from regulating antigen presentation (NLRC5, CIITA) to pathogen/damage sensing (NLRP1, NLRP3, NLRC1/2, NLRC4) to suppression or modulation of inflammation (NLRC3, NLRP6, NLRP12, NLRX1). However, NLRP2, NLRP5, and NLRP7 are also involved in non-immune pathways such as embryonic development. In this review, we highlight some of the least well-understood aspects of NLRs, including the mechanisms by which they sense pathogens or damage. NLRP3 recognizes a diverse range of stimuli and numerous publications have presented potential unifying models for NLRP3 activation, but no single mechanism proposed thus far appears to account for all possible NLRP3 activators. Additionally, NLRC3, NLRP6, and NLRP12 inhibit NF-κB activation, but whether direct ligand sensing is a requirement for this function is not known. Herein, we review the various mechanisms of sensing and activation proposed for NLRP3 and other inflammasome activators. We also discuss the role of NLRC3, NLRP6, NLRP12, and NLRX1 as inhibitors and how they are activated and function in their roles to limit inflammation. Finally, we present an overview of the emerging roles that NLRP2, NLRP5, and NLRP7 play during embryonic development and postulate on the potential pathways involved.

Enter at your own risk: how enteroviruses navigate the dangerous world of pattern recognition receptor signaling

Enteroviruses are the most common human viral pathogens worldwide. This genus of small, non-enveloped, single stranded RNA viruses includes coxsackievirus, rhinovirus, echovirus, and poliovirus species. Infection with these viruses can induce mild symptoms that resemble the common cold, but can also be associated with more severe syndromes such as poliomyelitis, neurological diseases including aseptic meningitis and encephalitis, myocarditis, and the onset of type I diabetes. In humans, polarized epithelial cells lining the respiratory and/or digestive tracts represent the initial sites of infection by enteroviruses. Control of infection in the host is initiated through the engagement of a variety of pattern recognition receptors (PRRs). PRRs act as the sentinels of the innate immune system and serve to alert the host to the presence of a viral invader. This review assembles the available data annotating the role of PRRs in the response to enteroviral infection as well as the myriad ways by which enteroviruses both interrupt and manipulate PRR signaling to enhance their own replication, thereby inducing human 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.

Herpes simplex virus 1 infection induces activation and subsequent inhibition of the IFI16 and NLRP3 inflammasomes

Inflammasomes are multiprotein complexes that recognize pathogens and pathogen- or danger-associated molecular patterns. They induce the maturation and secretion of powerful proinflammatory interleukin-1B (IL-1β), IL-18, and IL-33 cytokines, which in turn activate expression of other immune genes and lymphocyte recruitment to the site of primary infection, thereby controlling invading pathogens. Inflammasomes are comprised of cytoplasmic sensor molecules, such as NLRP3 and AIM2 or nuclear sensor IFI16, the adaptor protein ASC (apoptosis-associated speck-like protein containing CARD), and the effector protein procaspase-1. Herpes simplex virus 1 (HSV-1), a ubiquitous virus that infects humans and establishes life-long latency, has evolved numerous mechanisms to evade host detection and immune responses. Here, we show that early during in vitro infection of human foreskin fibroblasts (2 to 4 h), HSV-1 induced the activation of the IFI16 and NLRP3 inflammasomes and maturation of IL-1β. Independent of viral gene expression, IFI16 recognized the HSV-1 genome in infected cell nuclei, relocalized, and colocalized with ASC in the cytoplasm. However, HSV-1 specifically targeted IFI16 for rapid proteasomic degradation at later times postinfection, which was dependent on the expression of ICP0, an immediate early protein of HSV-1. In contrast, NLRP3, AIM2, and ASC levels were not decreased. Also, caspase-1 was "trapped" in actin clusters at later time points that likely blocked the NLRP3/IFI16 inflammasome activity. In addition, the secretion of mature IL-1β was inhibited. These results suggest that though the host cell responds to HSV-1 infection by IFI16 and NLRP3 inflammasomes early during infection, HSV-1 has evolved mechanisms to shut down these responses to evade the proinflammatory consequences.

Caspases and immunity in a deadly grip

Caspases are crucial for the execution of apoptotic cell death. However, caspase-1, the first identified mammalian caspase, was not discovered in the context of apoptosis, but rather as an enzyme that processes the proinflammatory cytokine interleukin (IL)-1β. More recently, additional nonapoptotic roles of apoptotic caspases have been uncovered. For example, caspase-8 can counteract necroptosis, an inflammatory mode of cell death induced by receptor-interacting protein (RIP)3. Here, we explore how caspases and their associated proteins and substrates mediate multiple cellular immune processes that extend beyond cell death. We focus on caspases-1, -8 and -11 because of the growing excitement in considering these caspases through the lens of regulators of immunity rather than primarily as arbiters of cell fate.