Measles virus V protein inhibits NLRP3 inflammasome-mediated interleukin-1β secretion

Sphingosine-1-phosphate signaling mediates shedding of measles virus-infected respiratory epithelial cells

Measles virus (MeV) is an extremely infectious respiratory virus and a major cause of childhood morbidity and mortality worldwide. MeV infection of the respiratory epithelium induces shedding of multinucleate epithelial cells from the apical surface of the epithelium without compromising epithelial barrier integrity. To study the mechanisms driving the apical extrusion of MeV-infected respiratory epithelial cells, we used primary differentiated tracheal epithelial cell cultures (rhTECs) and respiratory samples from rhesus macaques infected with wild-type MeV (WT MeV) or live-attenuated MeV (LAMV). We show that sphingosine-1-phosphate (S1P) signaling, rather than cell death or inflammasome activation, plays a key role in WT MeV and LAMV-induced cell shedding. Inhibiting S1P signaling resulted in delayed shedding of clusters of infected cells and higher viral titers within the epithelium, suggesting that cell extrusion impacts viral dynamics within the respiratory tract. We also found that shedding of individual infected cells began early after apical infection, prior to the formation of infected cell clusters within the epithelium. These findings offer new insights into MeV biology and pathogenesis within the respiratory tract.

IMPORTANCE

Despite the availability of a safe and effective vaccine, measles virus (MeV) still has a significant global impact, and in 2022 alone led to over 136,000 deaths. MeV is one of the most contagious known viruses and spreads via the respiratory route. When respiratory epithelial cells are infected, they are shed into the lumen of the respiratory tract, but this process is poorly understood. Here, we use primary differentiated respiratory epithelial cells from rhesus macaques to show that sphingosine-1-phosphate (S1P) signaling, and not cell death or inflammasome activation, plays a role in cell shedding during both wild-type and live-attenuated MeV infection. Through this mechanism, MeV-infected cells are extruded without disrupting the integrity of the respiratory epithelium. Inhibiting S1P signaling resulted in delayed shedding of infected cells and higher viral titers in the epithelium. These findings indicate that host cellular responses play an important role in MeV infectivity.

Ubiquitin Ligases in Control: Regulating NLRP3 Inflammasome Activation

Ubiquitin ligases play pivotal roles in the regulation of NLR family pyrin domain containing 3 (NLRP3) inflammasome activation, a critical process in innate immunity and inflammatory responses. This review explores the intricate mechanisms by which various E3 ubiquitin ligases exert both positive and negative influences on NLRP3 inflammasome activity through diverse post-translational modifications. Negative regulation of NLRP3 inflammasome assembly is mediated by several E3 ligases, including F-box and leucine-rich repeat protein 2 (FBXL2), tripartite motif-containing protein 31 (TRIM31), and Casitas B-lineage lymphoma b (Cbl-b), which induce K48-linked ubiquitination of NLRP3, targeting it for proteasomal degradation. Membrane-associated RING-CH 7 (MARCH7) similarly promotes K48-linked ubiquitination leading to autophagic degradation, while RING finger protein (RNF125) induces K63-linked ubiquitination to modulate NLRP3 function. Ariadne homolog 2 (ARIH2) targets the nucleotide-binding domain (NBD) domain of NLRP3, inhibiting its activation, and tripartite motif-containing protein (TRIM65) employs dual K48 and K63-linked ubiquitination to suppress inflammasome assembly. Conversely, Pellino2 exemplifies a positive regulator, promoting NLRP3 inflammasome activation through K63-linked ubiquitination. Additionally, ubiquitin ligases influence other components critical for inflammasome function. TNF receptor-associated factor 3 (TRAF3) mediates K63 polyubiquitination of apoptosis-associated speck-like protein containing a CARD (ASC), facilitating its degradation, while E3 ligases regulate caspase-1 activation and DEAH-box helicase 33 (DHX33)-NLRP3 complex formation through specific ubiquitination events. Beyond direct inflammasome regulation, ubiquitin ligases impact broader innate immune signaling pathways, modulating pattern-recognition receptor responses and dendritic cell maturation. Furthermore, they intricately control NOD1/NOD2 signaling through K63-linked polyubiquitination of receptor-interacting protein 2 (RIP2), crucial for nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) activation. Furthermore, we explore how various pathogens, including bacteria, viruses, and parasites, have evolved sophisticated strategies to hijack the host ubiquitination machinery, manipulating NLRP3 inflammasome activation to evade immune responses. This comprehensive analysis provides insights into the molecular mechanisms underlying inflammasome regulation and their implications for inflammatory diseases, offering potential avenues for therapeutic interventions targeting the NLRP3 inflammasome. In conclusion, ubiquitin ligases emerge as key regulators of NLRP3 inflammasome activation, exhibiting a complex array of functions that finely tune immune responses. Understanding these regulatory mechanisms not only sheds light on fundamental aspects of inflammation but also offers potential therapeutic avenues for inflammatory disorders and infectious diseases.

Inflammasome Activation by RNA Respiratory Viruses: Mechanisms, Viral Manipulation, and Therapeutic Insights

Respiratory viruses, including SARS-CoV-2, influenza, parainfluenza, rhinovirus, and respiratory syncytial virus (RSV), are pathogens responsible for lower respiratory tract infections, particularly in vulnerable populations such as children and the elderly. Upon infection, these viruses are recognized by pattern recognition receptors, leading to the activation of inflammasomes, which are essential for mediating inflammatory responses. This review discusses the mechanisms by which these RNA respiratory viruses activate inflammasomes, emphasizing the roles of various signaling pathways and components involved in this process. Additionally, we highlight the specific interactions between viral proteins and inflammasome sensors, elucidating how these viruses manipulate the host immune response to facilitate infection. Understanding the dynamics of inflammasome activation in response to respiratory viruses provides critical insights for developing immunomodulatory therapeutic strategies aimed at mitigating inflammation and improving outcomes in respiratory tract infections.

Activation and evasion of inflammasomes during viral and microbial infection

The inflammasome is a cytoplasmic multiprotein complex that induces the maturation of the proinflammatory cytokines interleukin-1β (IL-1β) and interleukin-18 (IL-18) or pyroptosis by activating caspases, which play critical roles in regulating inflammation, cell death, and various cellular processes. Multiple studies have shown that the inflammasome is a key regulator of the host defence response against pathogen infections. During the process of pathogenic microbe invasion into host cells, the host's innate immune system recognizes these microbes by activating inflammasomes, triggering inflammatory responses to clear the microbes and initiate immune responses. Moreover, microbial pathogens have evolved various mechanisms to inhibit or evade the activation of inflammasomes. Therefore, we review the interactions between viruses and microbes with inflammasomes during the invasion process, highlight the molecular mechanisms of inflammasome activation induced by microbial pathogen infection, and highlight the corresponding strategies that pathogens employ to evade inflammasome activity. Finally, we also discuss potential therapeutic strategies for the treatment of pathogenic microbial infections via the targeting of inflammasomes and their products.

Anti-inflammatory effects of cyclodextrin nanoparticles enable macrophage repolarization and reduce inflammation

Inflammation plays a critical role in the pathophysiology of many diseases, and dysregulation of the involved signaling cascades often culminates in uncontrollable disease progression and, ultimately, chronic manifestation. Addressing these disorders requires balancing inflammation control while preserving essential immune functions. Cyclodextrins (CDs), particularly β-CD, have gained attention as biocompatible biomaterials with intrinsic anti-inflammatory properties, and chemical modification of their backbone offers a promising strategy to enhance their physicochemical properties, adaptability, and therapeutic potential. This study evaluated and characterized the immunomodulatory effects of amphiphilic CD derivatives, which self-assemble into nanoparticles, compared to soluble parent β-CD. In a human macrophage model, CD nanoparticles demonstrated superior anti-inflammatory activity, with derivative-specific effects tied to their physicochemical properties, surpassing the soluble β-CD control. Alongside the downregulation of key pro-inflammatory markers, significant reductions in inflammasome activation and changes in lipid profiles were observed. The findings of this study underscore the potential of cyclodextrin-based nanoparticles as versatile biomaterials for treating the complex pathophysiology of various acute and chronic inflammation-associated disorders.

Immunological landscape of human lymphoid explants during measles virus infection

In humans, lymph nodes are the primary site of measles virus (MeV) replication. To understand the immunological events that occur at this site, we infected human lymphoid tissue explants using a pathogenic strain of MeV that expresses GFP. We found that MeV infected 5%-15% of cells across donors. Using single-cell RNA-Seq and flow cytometry, we found that while most of the 29 cell populations identified in the lymphoid culture were susceptible to MeV, there was a broad preferential infection of B cells and reduced infection of T cells. Further subsetting of T cells revealed that this reduction may be driven by the decreased infection of naive T cells. Transcriptional changes in infected B cells were dominated by an interferon-stimulated gene (ISG) signature. To determine which of these ISGs were most substantial, we evaluated the proteome of MeV-infected Raji cells by mass spectrometry. We found that IFIT1, IFIT2, IFIT3, ISG15, CXCL10, MX2, and XAF1 proteins were the most highly induced and positively correlated with their expression in the transcriptome. These data provide insight into the immunological events that occur in lymph nodes during infection and may lead to the development of therapeutic interventions.

Autophagy and Inflammation: Regulatory Roles in Viral Infections

Autophagy is a highly conserved intracellular degradation pathway in eukaryotic organisms, playing an adaptive role in various pathophysiological processes throughout evolution. Inflammation is the immune system's response to external stimuli and tissue damage. However, persistent inflammatory reactions can lead to a range of inflammatory diseases and cancers. The interaction between autophagy and inflammation is particularly evident during viral infections. As a crucial regulator of inflammation, autophagy can either promote or inhibit the occurrence of inflammatory responses. In turn, inflammation can establish negative feedback loops by modulating autophagy to suppress excessive inflammatory reactions. This interaction is pivotal in the pathogenesis of viral diseases. Therefore, elucidating the regulatory roles of autophagy and inflammation in viral infections will significantly enhance our understanding of the mechanisms underlying related diseases. Furthermore, it will provide new insights and theoretical foundations for disease prevention, treatment, and drug development.

Inhibition of caspase-1-dependent apoptosis suppresses peste des petits ruminants virus replication

BACKGROUND

Peste des petits ruminants (PPR), caused by the PPR virus (PPRV), is an acute and fatal contagious disease that mainly infects goats, sheep, and other artiodactyls. Peripheral blood mononuclear cells (PBMCs) are considered the primary innate immune cells.

OBJECTIVES

PBMCs derived from goats were infected with PPRV and analyzed to detect the relationship between PPRV replication and apoptosis or the inflammatory response.

METHODS

Quantitative real-time polymerase chain reaction was used to identify PPRV replication and cytokines expression. Flow cytometry was conducted to detect apoptosis and the differentiation of CD4+ and CD8+ T cells after PPRV infection.

RESULTS

PPRV stimulated the differentiation of CD4+ and CD8+ T cells. In addition, PPRV induced apoptosis in goat PBMCs. Furthermore, apoptosis and the inflammatory response induced by PPRV could be suppressed by Z-VAD-FMK and Z-YVAD-FMK, respectively. Moreover, the virus titer of PPRV was attenuated by inhibiting caspase-1-dependent apoptosis and inflammation.

CONCLUSIONS

This study showed that apoptosis and the inflammatory response play an essential role in PPR viral replication in vitro, providing a new mechanism related to the cell host response.

The NLRP3 inflammasome in viral infection (Review)

The interplay between pathogen and host determines the immune response during viral infection. The Nod‑like receptor (NLR) protein 3 inflammasome is a multiprotein complex that induces the activation of inflammatory caspases and the release of IL‑1β, which play an important role in the innate immune responses. In the present review, the mechanisms of the NLR family pyrin domain containing 3 inflammasome activation and its dysregulation in viral infection were addressed.

Inflammasome activation by viral infection: mechanisms of activation and regulation

Viral diseases are the most common problems threatening human health, livestock, and poultry industries worldwide. Viral infection is a complex and competitive dynamic biological process between a virus and a host/target cell. During viral infection, inflammasomes play important roles in the host and confer defense mechanisms against the virus. Inflammasomes are polymeric protein complexes and are considered important components of the innate immune system. These immune factors recognize the signals of cell damage or pathogenic microbial infection after activation by the canonical pathway or non-canonical pathway and transmit signals to the immune system to initiate the inflammatory responses. However, some viruses inhibit the activation of the inflammasomes in order to replicate and proliferate in the host. In recent years, the role of inflammasome activation and/or inhibition during viral infection has been increasingly recognized. Therefore, in this review, we describe the biological properties of the inflammasome associated with viral infection, discuss the potential mechanisms that activate and/or inhibit NLRP1, NLRP3, and AIM2 inflammasomes by different viruses, and summarize the reciprocal regulatory effects of viral infection on the NLRP3 inflammasome in order to explore the relationship between viral infection and inflammasomes. This review will pave the way for future studies on the activation mechanisms of inflammasomes and provide novel insights for the development of antiviral therapies.

Race between virus and inflammasomes: inhibition or escape, intervention and therapy

The inflammasome is a multiprotein complex that further regulates cell pyroptosis and inflammation by activating caspase-1. The assembly and activation of inflammasome are associated with a variety of diseases. Accumulative studies have shown that inflammasome is a key modulator of the host's defense response to viral infection. Indeed, it has been established that activation of inflammasome occurs during viral infection. At the same time, the host has evolved a variety of corresponding mechanisms to inhibit unnecessary inflammasome activation. Therefore, here, we review and summarize the latest research progress on the interaction between inflammosomes and viruses, highlight the assembly and activation of inflammosome in related cells after viral infection, as well as the corresponding molecular regulatory mechanisms, and elucidate the effects of this activation on virus immune escape and host innate and adaptive immune defenses. Finally, we also discuss the potential therapeutic strategies to prevent and/or ameliorate viral infection-related diseases via targeting inflammasomes and its products.

Dying in self-defence: a comparative overview of immunogenic cell death signalling in animals and plants

Host organisms utilise a range of genetically encoded cell death programmes in response to pathogen challenge. Host cell death can restrict pathogen proliferation by depleting their replicative niche and at the same time dying cells can alert neighbouring cells to prepare environmental conditions favouring future pathogen attacks. As expected, many pathogenic microbes have strategies to subvert host cell death to promote their virulence. The structural and lifestyle differences between animals and plants have been anticipated to shape very different host defence mechanisms. However, an emerging body of evidence indicates that several components of the host-pathogen interaction machinery are shared between the two major branches of eukaryotic life. Many proteins involved in cell death execution or cell death-associated immunity in plants and animals exert direct effects on endomembrane and loss of membrane integrity has been proposed to explain the potential immunogenicity of dying cells. In this review we aim to provide a comparative view on how cell death processes are linked to anti-microbial defence mechanisms in plants and animals and how pathogens interfere with these cell death programmes. In comparison to the several well-defined cell death programmes in animals, immunogenic cell death in plant defence is broadly defined as the hypersensitive response. Our comparative overview may help discerning whether specific types of immunogenic cell death exist in plants, and correspondingly, it may provide new hints for previously undiscovered cell death mechanism in animals.

Infection of Pro- and Anti-Inflammatory Macrophages by Wild Type and Vaccine Strains of Measles Virus: NLRP3 Inflammasome Activation Independent of Virus Production

In humans and non-human primates, wild type (WT) measles virus (MeV) replicates extensively in lymphoid tissue and induces an innate response characteristic of NF-κB and inflammasome activation without type I interferon. In contrast, the live attenuated MeV vaccine (LAMV) replicates poorly in lymphoid tissue with little detectable in vivo cytokine production. To characterize the innate responses of macrophages to WT MeV and LAMV infection, we analyzed primary human monocyte-derived macrophages and phorbol myristic acid-matured monocytic THP-1 cells (M0) polarized to inflammatory (M1) and anti-inflammatory (M2) phenotypes 24 h after MeV infection. LAMV infected macrophages more efficiently than WT MeV but produced less virus than WT MeV-infected macrophages. Both strains induced production of NF-κB-responsive cytokines IL-6 and TNFα and inflammasome products IL-1β and IL-18 without evidence of pyroptosis. Analysis of THP-1 cells deficient in inflammasome sensors NOD-like receptor pyrin (NLRP)3, IFN-γ-inducible protein 16 (IFI16) or absent in melanoma (AIM)2; adaptor apoptosis-associated speck-like protein containing a CARD (ASC) or effector caspase 1 showed that IL-18 production was dependent on NLRP3, ASC, and caspase 1. However, M1 cells produced IL-1β in the absence of ASC or caspase 1 indicating alternate pathways for MeV-induced pro-IL-1β processing. Therefore, the innate response to in vitro infection of macrophages with both LAMV and WT MeV includes production of IL-6 and TNFα and activation of the NLRP3 inflammasome to release IL-1β and IL-18. LAMV attenuation impairs production of infectious virus but does not reduce ability to infect macrophages or innate responses to infection.

Pyroptosis in Antiviral Immunity

Pyroptosis is a form of lytic, programmed cell death that functions as an innate immune effector mechanism to facilitate host defense against pathogenic microorganisms, including viruses. This type of proinflammatory cell death is orchestrated by proteolytic activation of human or mouse caspase-1, mouse caspase-11 and human caspase-4 and caspase-5 in response to infectious and inflammatory stimuli. Induction of pyroptosis requires either a canonical inflammasome responsible for caspase-1 activation or a noncanonical complex composed of caspase-11 in mice or caspase-4 or caspase-5 in humans. Recent studies have identified the pore-forming protein gasdermin D, a substrate of these inflammatory caspases, as an executioner of pyroptosis. The membrane pores formed by gasdermin D facilitate release of proinflammatory cytokines IL-1β and IL-18 and consequent biologic effects of these cytokines together with other released components. Pyroptosis, like other forms of programmed cell death, helps eliminate infected cells and thereby restricts the replicative niche, undermining survival and proliferation of intracellular pathogens. This includes viruses as well as bacteria, where ample evidence supports a critical role for inflammasome effector functions and cell death in host defense. Viruses have evolved their own mechanisms to modulate inflammasome signaling and pyroptosis. Here, we review the current literature regarding the role of pyroptosis in antiviral immune responses.

Crosstalk between mitophagy and innate immunity in viral infection

Mitochondria are important organelles involved in cell metabolism and programmed cell death in eukaryotic cells and are closely related to the innate immunity of host cells against viruses. Mitophagy is a process in which phagosomes selectively phagocytize damaged or dysfunctional mitochondria to form autophagosomes and is degraded by lysosomes, which control mitochondrial mass and maintain mitochondrial dynamics and cellular homeostasis. Innate immunity is an important part of the immune system and plays a vital role in eliminating viruses. Viral infection causes many physiological and pathological alterations in host cells, including mitophagy and innate immune pathways. Accumulating evidence suggests that some virus promote self-replication through regulating mitophagy-mediated innate immunity. Clarifying the regulatory relationships among mitochondria, mitophagy, innate immunity, and viral infection will shed new insight for pathogenic mechanisms and antiviral strategies. This review systemically summarizes the activation pathways of mitophagy and the relationship between mitochondria and innate immune signaling pathways, and then discusses the mechanisms of viruses on mitophagy and innate immunity and how viruses promote self-replication by regulating mitophagy-mediated innate immunity.

Promiscuous Inflammasomes: The False Dichotomy of RNA/DNA Virus-Induced Inflammasome Activation and Pyroptosis

It is well-known that viruses activate various inflammasomes, which can initiate the programmed cell death pathway known as pyroptosis, subsequently leading to cell lysis and release of inflammatory cytokines IL-1β and IL-18. This pathway can be triggered by various sensors, including, but not limited to, NLRP3, AIM2, IFI16, RIG-I, and NLRC4. Many viruses are known either to activate or inhibit inflammasomes as a part of the innate immune response or as a mechanism of pathogenesis. Early research in the field of virus-induced pyroptosis suggested a dichotomy, with RNA viruses activating the NLRP3 inflammasome and DNA viruses activating the AIM2 inflammasome. More recent research has shown that this dichotomy may not be as distinct as once thought. It seems many viruses activate multiple inflammasome sensors. Here, we detail which viruses fit the dichotomy as well as many that appear to defy this clearly false dichotomy. It seems likely that most, if not all, viruses activate multiple inflammasome sensors, and future research should focus on expanding our understanding of inflammasome activation in a variety of tissue types as well as virus activation of multiple inflammasomes, challenging biases that stemmed from early literature in this field. Here, we review primarily research performed on human viruses but also include details regarding animal viruses whenever possible.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

Peste Des Petits Ruminants Virus N Protein Is a Critical Proinflammation Factor That Promotes MyD88 and NLRP3 Complex Assembly

Inflammatory responses play a central role in host defense against invading pathogens. Peste des petits ruminants virus (PPRV) causes highly contagious acute or subacute disease of small ruminants. However, the precise mechanism by which PPRV regulates inflammatory responses remains unknown. Here, we revealed a novel mechanism by which PPRV induces inflammation. Our study showed that PPRV induced the secretion of interleukin 1β (IL-1β) by activating the NF-κB signaling pathway and the NLRP3 inflammasome. Moreover, PPRV replication and protein synthesis were essential for NLRP3 inflammasome activation. Importantly, PPRV N protein promoted NF-κB signaling pathway and NLRP3 inflammasome via direct binding of MyD88 and NLPR3, respectively, and induced caspase-1 cleavage and IL-1β maturation. Biochemically, N protein interacted with MyD88 to potentiate the assembly of MyD88 complex and interacted with NLPR3 to facilitate NLRP3 inflammasome complex assembly by forming an N-NLRP3-ASC ring-like structure, leading to IL-1β secretion. These findings demonstrate a new function of PPRV N protein as an important proinflammation factor and identify a novel underlying mechanism modulating inflammasome assembly and function induced by PPRV. IMPORTANCE An important part of the innate immune response is the activation of NF-κB signaling pathway and NLPR3 inflammasome, which is induced upon exposure to pathogens. Peste des petits ruminants virus (PPRV) is a highly contagious virus causing fever, stomatitis, and pneumoenteritis in goats by inducing many proinflammatory cytokines. Although the NF-κB signaling pathway and NLRP3 inflammasome play an important role in regulating host immunity and viral infection, the precise mechanism by which PPRV regulates inflammatory responses remains unknown. This study demonstrates that PPRV induces inflammatory responses. Mechanistically, PPRV N protein facilitates the MyD88 complex assembly by directly binding to MyD88 and promotes the NLRP3 inflammasome complex assembly by directly binding to NLRP3 to form ring-like structures of N-NLRP3-ASC. These findings provide insights into the prevention and treatment of PPRV infection.

Innate Sensors Trigger Regulated Cell Death to Combat Intracellular Infection

Intracellular pathogens pose a significant threat to animals. In defense, innate immune sensors attempt to detect these pathogens using pattern recognition receptors that either directly detect microbial molecules or indirectly detect their pathogenic activity. These sensors trigger different forms of regulated cell death, including pyroptosis, apoptosis, and necroptosis, which eliminate the infected host cell niche while simultaneously promoting beneficial immune responses. These defenses force intracellular pathogens to evolve strategies to minimize or completely evade the sensors. In this review, we discuss recent advances in our understanding of the cytosolic pattern recognition receptors that drive cell death, including NLRP1, NLRP3, NLRP6, NLRP9, NLRC4, AIM2, IFI16, and ZBP1. Expected final online publication date for the Annual Review of Immunology, Volume 40 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Focus on the Mechanisms and Functions of Pyroptosis, Inflammasomes, and Inflammatory Caspases in Infectious Diseases

Eukaryotic cells can initiate several distinct self-destruction mechanisms to display essential roles for the homeostasis maintenance, development, and survival of an organism. Pyroptosis, a key response mode in innate immunity, also referred to as caspase-1-dependent proinflammatory programmed necrotic cell death activated by human caspase-1/4/5, or mouse caspase-1/11, plays indispensable roles in response to cytoplasmic insults and immune defense against infectious diseases. These inflammatory caspases are employed by the host to eliminate pathogen infections such as bacteria, viruses, protozoans, and fungi. Gasdermin D requires to be cleaved and activated by these inflammatory caspases to trigger the pyroptosis process. Physiological rupture of cells results in the release of proinflammatory cytokines, the alarmins IL-1β and IL-18, symbolizing the inflammatory potential of pyroptosis. Moreover, long noncoding RNAs play direct or indirect roles in the upstream of the pyroptosis trigger pathway. Here, we review in detail recently acquired insights into the central roles of inflammatory caspases, inflammasomes, and pyroptosis, as well as the crosstalk between pyroptosis and long noncoding RNAs in mediating infection immunity and pathogen clearance.

Deciphering Respiratory-Virus-Associated Interferon Signaling in COPD Airway Epithelium

COPD is a chronic lung disorder characterized by a progressive and irreversible airflow obstruction, and persistent pulmonary inflammation. It has become a global epidemic affecting 10% of the population, and is the third leading cause of death worldwide. Respiratory viruses are a primary cause of COPD exacerbations, often leading to secondary bacterial infections in the lower respiratory tract. COPD patients are more susceptible to viral infections and associated severe disease, leading to accelerated lung function deterioration, hospitalization, and an increased risk of mortality. The airway epithelium plays an essential role in maintaining immune homeostasis, and orchestrates the innate and adaptive responses of the lung against inhaled and pathogen insults. A healthy airway epithelium acts as the first line of host defense by maintaining barrier integrity and the mucociliary escalator, secreting an array of inflammatory mediators, and initiating an antiviral state through the interferon (IFN) response. The airway epithelium is a major site of viral infection, and the interaction between respiratory viruses and airway epithelial cells activates host defense mechanisms, resulting in rapid virus clearance. As such, the production of IFNs and the activation of IFN signaling cascades directly contributes to host defense against viral infections and subsequent innate and adaptive immunity. However, the COPD airway epithelium exhibits an altered antiviral response, leading to enhanced susceptibility to severe disease and impaired IFN signaling. Despite decades of research, there is no effective antiviral therapy for COPD patients. Herein, we review current insights into understanding the mechanisms of viral evasion and host IFN antiviral defense signaling impairment in COPD airway epithelium. Understanding how antiviral mechanisms operate in COPD exacerbations will facilitate the discovery of potential therapeutic interventions to reduce COPD hospitalization and disease severity.

The evolution of regulated cell death pathways in animals and their evasion by pathogens

The coevolution of host-pathogen interactions underlies many human physiological traits associated with protection from or susceptibility to infections. Among the mechanisms that animals utilize to control infections are the regulated cell death pathways of pyroptosis, apoptosis, and necroptosis. Over the course of evolution these pathways have become intricate and complex, coevolving with microbes that infect animal hosts. Microbes, in turn, have evolved strategies to interfere with the pathways of regulated cell death to avoid eradication by the host. Here, we present an overview of the mechanisms of regulated cell death in Animalia and the strategies devised by pathogens to interfere with these processes. We review the molecular pathways of regulated cell death, their roles in infection, and how they are perturbed by viruses and bacteria, providing insights into the coevolution of host-pathogen interactions and cell death pathways.

Immune Response to Viruses

Viruses are essentially, obligate intracellular parasites. They require a host to replicate their genetic material, spread to other cells, and eventually to other hosts. For humans, most viral infections are not considered lethal, regardless if at the cellular level, the virus can obliterate individual cells. Constant genomic mutations, (which can alter the antigenic content of viruses such as influenza or coronaviruses), zoonosis or immunosuppression/immunocompromisation, is when viruses achieve higher host mortality. Frequent examples of the severe consequenses of viral infection can be seen in children and the elderly. In most instances, the immune system will take a multifaceted approach in defending the host against viruses. Depending on the virus, the individual, and the point of entry, the immune system will initiate a robust response which involves multiple components. In this chapter, we expand on the total immune system, breaking it down to the two principal types: Innate and Adaptive Immunity, their different roles in viral recognition and clearance. Finally, how different viruses activate and evade different arms of the immune system.

Immune response to SARS-CoV-2 Variants: A focus on severity, susceptibility, and preexisting immunity

The heterogeneous phenotypes among patients with coronavirus disease 2019 (COVID-19) has drawn worldwide attention, especially those with severe symptoms without comorbid conditions. Immune responses to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative virus of COVID-19, occur mainly by the innate immune response via the interferon (IFN)-mediated pathways, and the adaptive immunity via the T lymphocyte and the antibody mediated pathways. The ability of the original Wuhan SARS-CoV-2 strain, and possibly more so with new emerging variants, to antagonize IFN-mediated antiviral responses can be behind the higher early viral load, higher transmissibility, and milder symptoms compared to SARS-CoV and are part of the continued clinical evolution of COVID-19. Since it first emerged, several variants of SARS-CoV-2 have been circulating worldwide. Variants that have the potential to elude natural or vaccine-mediated immunity are variants of concern. This review focuses on the main host factors that may explain the immune responses to SARS-CoV-2 and its variants in the context of susceptibility, severity, and preexisting immunity.

Viral manipulation of host cell necroptosis and pyroptosis

Cell death forms an essential component of the antiviral immune response. Viral infection elicits different forms of host cell death, including the lytic and inflammatory cell death modes necroptosis or pyroptosis. The induction of both types of cell death not only eliminates virus-infected cells but also contributes to the development of innate and adaptive immunity through the release of inflammatory mediators. The importance of necroptosis and pyroptosis in host defence is evident from the numerous viral evasion mechanisms that suppress these cell death pathways. Here, we review the emerging principles by which viruses antagonise host cell necroptosis and pyroptosis to promote their spread and block host immunity.

Autophagy Modulation by Viral Infections Influences Tumor Development

Autophagy is a self-degradative process important for balancing cellular homeostasis at critical times in development and/or in response to nutrient stress. This is particularly relevant in tumor model in which autophagy has been demonstrated to have an important impact on tumor behavior. In one hand, autophagy limits tumor transformation of precancerous cells in early stage, and in the other hand, it favors the survival, proliferation, metastasis, and resistance to antitumor therapies in more advanced tumors. This catabolic machinery can be induced by an important variety of extra- and intracellular stimuli. For instance, viral infection has often been associated to autophagic modulation, and the role of autophagy in virus replication differs according to the virus studied. In the context of tumor development, virus-modulated autophagy can have an important impact on tumor cells' fate. Extensive analyses have shed light on the molecular and/or functional complex mechanisms by which virus-modulated autophagy influences precancerous or tumor cell development. This review includes an overview of discoveries describing the repercussions of an autophagy perturbation during viral infections on tumor behavior.

Innate immunity, inflammation activation and heat-shock protein in COVID-19 pathogenesis

SARS-CoV-2-induced COVID-19 is a serious pandemic of the 21st century, which has caused a devastating loss of lives and a global economic catastrophe. A successful vaccine against SARS-CoV-2 has suffered a delay due to lack of substantial knowledge about its mechanisms of action. Understanding the innate immune system against SARS-CoV-2 and the role of heat shock proteins' (HSP) inhibiting and resolution of inflammatory pathways may provide information to the low SARS-CoV-2 mortality rates in Africa. In addition, bats being a host to different viruses, including SARS-CoV-2 possess a well specialized IFN-innate antiviral inflammatory response, showing no signs of disease or pro-inflammatory cytokine storm. We discuss the molecular pathways in COVID-19 with a focus on innate immunity, inflammation, HSP responses, and suggest appropriate candidates for therapeutic targets and The contribution of the innate immune system to the efficacy of mRNA or vector based Corona immunizations.

Dual RNA-Seq Enables Full-Genome Assembly of Measles Virus and Characterization of Host-Pathogen Interactions

Measles virus (MeV) has a negative-sense 15 kb long RNA genome, which is generally conserved. Recent advances in high-throughput sequencing (HTS) and Dual RNA-seq allow the analysis of viral RNA genomes and the discovery of viral infection biomarkers, via the simultaneous characterization of the host transcriptome. However, these host-pathogen interactions remain largely unexplored in MeV infections. We performed untargeted Dual RNA-seq in 6 pharyngeal and 6 peripheral blood mononuclear cell (PBMCs) specimens from patients with MeV infection, as confirmed via routine real-time PCR testing. Following optimised DNase treatment of total nucleic acids, we used the pharyngeal samples to build poly-A-enriched NGS libraries. We reconstructed the viral genomes using the pharyngeal datasets and we further conducted differential expression, gene-ontology and pathways enrichment analysis to compare both the pharyngeal and the peripheral blood transcriptomes of the MeV-infected patients vs. control groups of healthy individuals. We obtained 6 MeV genotype-B3 full-genome sequences. We minutely analyzed the transcriptome of the MeV-infected pharyngeal epithelium, detecting all known viral infection biomarkers, but also revealing a functional cluster of local antiviral and inflammatory immune responses, which differ substantially from those observed in the PBMCs transcriptome. The application of Dual RNA-seq technologies in MeV-infected patients can potentially provide valuable information on the virus genome structure and the cellular innate immune responses and drive the discovery of new targets for antiviral therapy.

Activation and Inhibition of the NLRP3 Inflammasome by RNA Viruses

Inflammation refers to the response of the immune system to viral, bacterial, and fungal infections, or other foreign particles in the body, which can involve the production of a wide array of soluble inflammatory mediators. It is important for the development of many RNA virus-infected diseases. The primary factors through which the infection becomes inflammation involve inflammasome. Inflammasomes are proteins complex that the activation is responsive to specific pathogens, host cell damage, and other environmental stimuli. Inflammasomes bring about the maturation of various pro-inflammatory cytokines such as IL-18 and IL-1β in order to mediate the innate immune defense mechanisms. Many RNA viruses and their components, such as encephalomyocarditis virus (EMCV) 2B viroporin, the viral RNA of hepatitis C virus, the influenza virus M2 viroporin, the respiratory syncytial virus (RSV) small hydrophobic (SH) viroporin, and the human rhinovirus (HRV) 2B viroporin can activate the Nod-like receptor (NLR) family pyrin domain-containing 3 (NLRP3) inflammasome to influence the inflammatory response. On the other hand, several viruses use virus-encoded proteins to suppress inflammation activation, such as the influenza virus NS1 protein and the measles virus (MV) V protein. In this review, we summarize how RNA virus infection leads to the activation or inhibition of the NLRP3 inflammasome.

SARS-CoV-2 Nonstructural Proteins 1 and 13 Suppress Caspase-1 and the NLRP3 Inflammasome Activation

Viral infection-induced activation of inflammasome complexes has both positive and negative effects on the host. Proper activation of inflammasome complexes induces down-stream effector mechanisms that inhibit viral replication and promote viral clearance, whereas dysregulated activation has detrimental effects on the host. Coronaviruses, including SARS-CoV and MERS-CoV, encode viroporins that activate the NLRP3 inflammasome, and the severity of coronavirus disease is associated with the inflammasome activation. Although the NLRP3 inflammasome activation is implicated in the pathogenesis of coronaviruses, these viruses must evade inflammasome-mediated antiviral immune responses to establish primary replication. Screening of a complementary DNA (cDNA) library encoding 28 SARS-CoV-2 open reading frames (ORFs) showed that two nonstructural proteins (NSPs), NSP1 and NSP13, inhibited caspase-1-mediated IL-1β activation. NSP1 amino acid residues involved in host translation shutoff and NSP13 domains responsible for helicase activity were associated with caspase-1 inhibition. In THP-1 cells, both NSP1 and NSP13 significantly reduced NLRP3-inflammasome-induced caspase-1 activity and IL-1β secretion. These findings indicate that SARS-CoV-2 NSP1 and NSP13 are potent antagonists of the NLRP3 inflammasome.

Transcriptional Immune Signatures of Alveolar Macrophages and the Impact of the NLRP3 Inflammasome on Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) Replication

Porcine Reproductive and Respiratory Syndrome (PRRS) is a contagious viral (PRRSV) disease in pigs characterized by poor reproductive health, increased mortality, and reductions in growth rates. PRRSV is known to implement immuno-antagonistic mechanisms to evade detection and mute host responses to infection. To better understand the cellular immunosignature of PRRSV we have undertaken transcriptome and immunomodulatory studies in PRRSV-infected porcine alveolar macrophages (PAMs). We first used genome-wide transcriptome profiling (RNA-seq) to elucidate PRRSV-induced changes in the PAM transcriptome in response to infection. We found a number of cellular networks were altered by PRRSV infection, including many associated with innate immunity, such as, the NLRP3 inflammasome. To further explore the role(s) of innate immune networks in PRRSV-infected PAMs, we used an NLRP3-specific inhibitor, MCC950, to identify the potential functionality of the inflammasome during PRRSV replication. We found that PRRSV does quickly induce expression of inflammasome-associated genes in PAMs. Treatment of PAMs with MCC950 suggests NLRP3 inflammasome activation negatively impacts viral replication. Treatment of PAMs with cell culture supernatants from macrophages subjected to NLRP3 inflammasome activation (via polyinosinic-polycytidylic acid (poly I:C) transfection), prior to PRRSV infection resulted in significantly reduced viral RNA levels compared to PAMs treated with cell culture supernatants from macrophages subjected to NLRP3 inflammasome inhibition (MCC950 treatment/poly I:C transfection). This further supports a role for NLRP3 inflammasome activation in the innate macrophagic anti-PRRSV immune response and suggests that PRRSV is sensitive to the effects of NLRP3 inflammasome activity. Taken together, these transcriptome and immunoregulatory data highlight the complex changes PRRSV infection induces in the molecular immune networks of its cellular host.

NLRP3 Sensing of Diverse Inflammatory Stimuli Requires Distinct Structural Features

The NLRP3 inflammasome is central to host defense and implicated in various inflammatory diseases and conditions. While the favored paradigm of NLRP3 inflammasome activation stipulates a unifying signal intermediate that de-represses NLRP3, this view has not been tested. Further, structures within NLRP3 required for inflammasome activation are poorly defined. Here we demonstrate that while the NLRP3 LRRs are not auto-repressive and are not required for inflammasome activation by all agonists, distinct sequences within the NLRP3 LRRs positively and negatively modulate inflammasome activation by specific ligands. In addition, elements within the HD1/HD2 “hinge” of NLRP3 and the nucleotide-binding domain have contrasting functions depending upon the specific agonists. Further, while NLRP3 1–432 is minimally sufficient for inflammasome activation by all agonists tested, the pyrin, and linker domains (1–134) function cooperatively and are sufficient for inflammasome activation by certain agonists. Conserved cysteines 8 and 108 appear important for inflammasome activation by sterile, but not infectious insults. Our results define common and agonist-specific regions of NLRP3 that likely mediate ligand-specific responses, discount the hypothesis that NLRP3 inflammasome activation has a unified mechanism, and implicate NLRP3 as an integrator of agonist-specific, inflammasome activating signals.

Lysosome-Associated Membrane Proteins Support the Furin-Mediated Processing of the Mumps Virus Fusion Protein

Mumps virus (MuV), an enveloped RNA virus of the Paramyxoviridae family and the causative agent of mumps, affects the salivary glands and other glandular tissues as well as the central nervous system. The virus enters the cell by inducing the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by MuV envelope proteins: the hemagglutinin-neuraminidase and fusion (F) protein. Cleavage of the MuV F protein (MuV-F) into two subunits by the cellular protease furin is a prerequisite for fusion and virus infectivity. Here, we show that 293T (a derivative of HEK293) cells do not produce syncytia upon expression of MuV envelope proteins or MuV infection. This failure is caused by the inefficient MuV-F cleavage despite the presence of functional furin in 293T cells. An expression cloning strategy revealed that overexpression of lysosome-associated membrane proteins (LAMPs) confers on 293T cells the ability to produce syncytia upon expression of MuV envelope proteins. The LAMP family comprises the ubiquitously expressed LAMP1 and LAMP2, the interferon-stimulated gene product LAMP3, and the cell type-specific proteins. The expression level of the LAMP3 gene, but not of LAMP1 and LAMP2 genes, differed markedly between 293T and HEK293 cells. Overexpression of LAMP1, LAMP2, or LAMP3 allowed 293T cells to process MuV-F efficiently. Furthermore, these LAMPs were found to interact with both MuV-F and furin. Our results indicate that LAMPs support the furin-mediated cleavage of MuV-F and that, among them, LAMP3 may be critical for the process, at least in certain cells.IMPORTANCE The cellular protease furin mediates proteolytic cleavage of many host and pathogen proteins and plays an important role in viral envelope glycoprotein maturation. MuV, an enveloped RNA virus of the Paramyxoviridae family and an important human pathogen, enters the cell through the fusion of its envelope with the plasma membrane of the target cell. Membrane fusion is mediated by the viral attachment protein and the F protein. Cleavage of MuV-F into two subunits by furin is a prerequisite for fusion and virus infectivity. Here, we show that LAMPs support the furin-mediated cleavage of MuV-F. Expression levels of LAMPs affect the processing of MuV-F and MuV-mediated membrane fusion. Among LAMPs, the interferon-stimulated gene product LAMP3 is most critical in certain cells. Our study provides potential targets for anti-MuV therapeutics.

STING promotes NLRP3 localization in ER and facilitates NLRP3 deubiquitination to activate the inflammasome upon HSV-1 infection

One of the fundamental reactions of the innate immune responses to pathogen infection is the release of pro-inflammatory cytokines, including IL-1β, processed by the NLRP3 inflammasome. The stimulator of interferon genes (STING) has the essential roles in innate immune response against pathogen infections. Here we reveal a distinct mechanism by which STING regulates the NLRP3 inflammasome activation, IL-1β secretion, and inflammatory responses in human cell lines, mice primary cells, and mice. Interestingly, upon HSV-1 infection and cytosolic DNA stimulation, STING binds to NLRP3 and promotes the inflammasome activation through two approaches. First, STING recruits NLRP3 and facilitates NLRP3 localization in the endoplasmic reticulum, thereby facilitating the inflammasome formation. Second, STING interacts with NLRP3 and attenuates K48- and K63-linked polyubiquitination of NLRP3, thereby promoting the inflammasome activation. Collectively, we demonstrate that the cGAS-STING-NLRP3 signaling is essential for host defense against HSV-1 infection.

The innate immune system is a primary host defense strategy to suppress the pathogen infections. One of the fundamental reactions of the innate immunity is the release of pro-inflammatory cytokines, including interleukine-1β (IL-1β), regulated by inflammasomes. The best-characterized inflammasomes is the NLRP3 inflammasome. STING has the essential roles in innate immune response against pathogen infections and is required for pathogen-induced inflammasome activation and IL-1β secretion. This study explores how STING regulates the NLRP3 inflammasome and reveals a distinct mechanism underlying such regulation upon herpes simplex virus type 1 (HSV-1) infection and cytosolic DNA stimulation. The authors propose that the cGAS-STING-NLRP3 axis is essential for host defense against HSV-1 infection.

NLRP3 Inflammasome-A Key Player in Antiviral Responses

The NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome is an oligomeric complex comprised of the NOD-like receptor NLRP3, the adaptor ASC, and caspase-1. This complex is crucial to the host's defense against microbes as it promotes IL-1β and IL-18 secretion and induces pyroptosis. NLRP3 recognizes variety of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) generated during viral replication that triggers the NLRP3 inflammasome-dependent antiviral immune responses and facilitates viral eradication. Meanwhile, several viruses have evolved elaborate strategies to evade the immune system by targeting the NLRP3 inflammasome. In this review, we will focus on the crosstalk between the NLRP3 inflammasome and viruses, provide an overview of viral infection-induced NLRP3 inflammasome activation, and the immune escape strategies of viruses through their modulation of the NLRP3 inflammasome activity.

Advances in the molecular mechanisms of NLRP3 inflammasome activators and inactivators

NLRP3 inflammasome is an intracellular protein complex that initiates cellular injury via assembly of NLRP3, ASC and caspase-1 in response to microbial infection and sterile stressors. The importance of NLRP3 inflammasome in immunity and human diseases has been well documented. Up to now, targeted inhibition of the assembly of NLRP3 inflammasome complex and of its activation was thought to be therapeutic strategy for associated diseases. Recent studies show that a host of molecules such as NIMA-related kinase 7 (Nek7) and DEAD-box helicase 3 X-linked (DDX3X) and a large number of biological mediators including cytokines, microRNAs, nitric oxide, carbon monoxide, nuclear factor erythroid-2 related factor 2 (Nrf2) and cellular autophagy participate in the activation and inactivation of NLRP3 inflammasome. This review summarizes current understanding of the molecular basis of NLRP3 inflammasome activation and inactivation. This knowledge may lead to development of new therapies directed at NLRP3 inflammasome related diseases.

Inflammasome in HIV infection: Lights and shadows

The importance of inflammasome, and related cytokines IL-1ß and IL-18, in host defense against pathogens is well documented, however, at the same time, dysregulation of inflammasome has been associated to multifactorial diseases characterized by chronic inflammation (i.e.: metabolic disorders, cardiovascular diseases, neurodegenerative diseases, autoimmunity, cancer). Inflammasome activation has been described in response to HIV-1 and possibly contributes to the resistance against virus establishment, however, on the other hand, when viral infection becomes chronic, independently from antiretroviral therapy, the increase constitutive activation of inflammasome has been eventually associated to a worse prognosis, raising the question about the role played by inflammasome and/or some specific receptors in this context. Due to the chance to imply targeted therapies that inhibit inflammasome activation and/or cytokines release, it will be important to define the impact of the complex in the pathogenesis of HIV. The purpose of this review is to depict the double-faced inflammasome role in HIV-1 infection, trying to unveil whether besides its role in first line defense against the virus, it exerts a harmful effect during the chronic phase of infection.

Dengue Virus M Protein Promotes NLRP3 Inflammasome Activation To Induce Vascular Leakage in Mice

Dengue virus (DENV) infection causes serious clinical symptoms, including dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Vascular permeability change is the main feature of the diseases, and the abnormal expression of proinflammatory cytokines is the important cause of vascular permeability change. However, the mechanism underlying vascular permeability induced by DENV has not been fully elucidated. Here, we reveal a distinct mechanism by which DENV infection promotes NLRP3 inflammasome activation and interleukin-1 beta (IL-1β) release to induce endothelial permeability and vascular leakage in mice. DENV M protein interacts with NLRP3 to facilitate NLRP3 inflammasome assembly and activation, which induce proinflammatory cytokine IL-1β activation and release. Notably, M can induce vascular leakage in mouse tissues by activating the NLRP3 inflammasome and IL-1β. More importantly, inflammatory cell infiltration and tissue injuries are induced by M in wild-type (WT) mouse tissues, but they are not affected by M in NLRP3 knockout (NLRP3^-/-) mouse tissues. Evans blue intensities in WT mouse tissues are significantly higher than in NLRP3^-/- mouse tissues, demonstrating an essential role of NLRP3 in M-induced vascular leakages in mice. Therefore, we propose that upon DENV infection, M interacts with NLRP3 to facilitate inflammasome activation and IL-1β secretion, which lead to the induction of endothelial permeability and vascular leakage in mouse tissues. The important role of the DENV-M-NLRP3-IL-1β axis in the induction of vascular leakage provides new insights into the mechanisms underlying DENV pathogenesis and DENV-associated DHF and DSS development.IMPORTANCE Dengue virus (DENV) is a mosquito-borne pathogen, and infections by this virus are prevalent in over 100 tropical and subtropical countries or regions, with approximately 2.5 billion people at risk. DENV infection induces a spectrum of clinical symptoms, ranging from classical dengue fever (DF) to severe dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). Therefore, it is important to understand the mechanisms underlying DENV pathogenesis. In this study, we reveal that the DENV membrane protein (M) interacts with the host NLRP3 protein to promote NLRP3 inflammasome activation, which leads to the activation and release of a proinflammatory cytokine, interleukin-1 beta (IL-1β). More importantly, we demonstrate that M protein can induce vascular permeability and vascular leakage and that NLRP3 is required for M-induced vascular leakage in mouse tissues. Collectively, this study reveals a distinct mechanism underlying DENV pathogeneses and provides new insights into the development of therapeutic agents for DENV-associated diseases.

Biophysical characterization and single-chain Fv construction of a neutralizing antibody to measles virus

The measles virus (MV) is a major cause of childhood morbidity and mortality worldwide. We previously established a mouse monoclonal antibody, 2F4, which shows high neutralizing titers against eight different genotypes of MV. However, the molecular basis for the neutralizing activity of the 2F4 antibody remains incompletely understood. Here, we have evaluated the binding characteristics of a Fab fragment of the 2F4 antibody. Using the MV infectious assay, we demonstrated that 2F4 Fab inhibits viral entry via either of two cellular receptors, SLAM and Nectin4. Surface plasmon resonance (SPR) analysis of recombinant proteins indicated that 2F4 Fab interacts with MV hemagglutinin (MV-H) with a K_D value at the nm level. Furthermore, we designed a single-chain Fv fragment of 2F4 antibody as another potential biopharmaceutical to target measles. The stable 2F4 scFv was successfully prepared by the refolding method and shown to interact with MV-H at the μm level. Like 2F4 Fab, scFv inhibited receptor binding and viral entry. This indicates that 2F4 mAb uses the receptor-binding site and/or a neighboring region as an epitope with high affinity. These results provide insight into the neutralizing activity and potential therapeutic use of antibody fragments for MV infection.

Virome and Inflammasomes, a Finely Tuned Balance with Important Consequences for the Host Health

BACKGROUND

The virome is a network of viruses normally inhabiting humans. It forms a conspicuous portion of the so-called microbiome, once generically referred to as resident flora. Indeed, viruses infecting humans without leading to clinical disease are increasingly recognized as part of the microbiome and have an impact on the development of our immune system. In addition, they activate inflammasomes, multiprotein complexes that assemble in cells and that are responsible for the downstream effects of sensing pathogens.

OBJECTIVE

This review aims at summarizing the evidence on the role of the virome in modulating inflammation and emphasizes evidence for Anelloviruses as useful molecular markers to monitor inflammatory processes and immune system competence.

METHOD

We carried out a review of the literature published in the last 5 years and summarized older literature to take into account ground-breaking discoveries concerning inflammasome assembly and virome.

RESULTS

A massive amount of data recently emerging demonstrate that the microbiome closely reflects what we eat, and many other unexpected variables. Composition, location, and amount of the microbiome have an impact on innate and adaptive immune defences. Viruses making up the virome contribute to shaping the immune system. Anelloviruses, the best known of such viruses, are present in most human beings, persistently without causing apparent disease. Depending on their interplay with such viruses, inflammasomes instruct host defences to tolerate or forfeit a specific microorganism.

CONCLUSION

The virome plays an important role in shaping human immune defences and contributes to inflammatory processes by quenching or increasing them.

NLRP3 inflammasome as a potential treatment in ischemic stroke concomitant with diabetes

The NLRP3 (nucleotide-binding oligomerization domain-like receptor [NLR] family pyrin domain-containing 3) inflammasome is a member of the NLR family of innate immune cell sensors. These are crucial regulators of cytokine secretions, which promote ischemic cell death and insulin resistance. This review summarizes recent progress regarding the NLRP3 inflammasome as a potential treatment for ischemic stroke in patients with diabetes, two complicated diseases that often occur together. Stroke worsens glucose metabolism abnormalities, and the outcomes after stroke are more serious for diabetic patients compared with those without diabetes. Inflammation contributes to organ injury after ischemic stroke and diabetes. Recent research has focused on inhibiting the activation of inflammasomes and thus reducing the maturation of proinflammatory cytokines such as interleukin (IL)-1β and IL-18. Studies suggest that inhibition of NLRP3 prevents or alleviates both ischemic stroke and diabetes. Targeting against the assembly and activity of the NLRP3 inflammasome is a potential and novel therapy for inflammasome-associated diseases, including ischemic stroke concomitant with diabetes.

Human metapneumovirus activates NOD-like receptor protein 3 inflammasome via its small hydrophobic protein which plays a detrimental role during infection in mice

NOD-like receptor protein 3 (NLRP3) inflammasome activation triggers caspase-1 activation-induced maturation of interleukin (IL)-1β and IL-18 and therefore is important for the development of the host defense against various RNA viral diseases. However, the implication of this protein complex in human metapneumovirus (HMPV) disease has not been fully studied. Herein, we report that NLRP3 inflammasome plays a detrimental role during HMPV infection because NLRP3 inflammasome inhibition protected mice from mortality and reduced weight loss and inflammation without impacting viral replication. We also demonstrate that NLRP3 inflammasome exerts its deleterious effect via IL-1β production since we observed reduced mortality, weight loss and inflammation in IL-1β-deficient (IL-1β^-/-) mice, as compared to wild-type animals during HMPV infection. Moreover, the effect on these evaluated parameters was not different in IL-1β^-/- and wild-type mice treated with an NLRP3 inflammasome inhibitor. The production of IL-1β was also abrogated in bone marrow derived macrophages deficient for NLRP3. Finally, we show that small hydrophobic protein-deleted recombinant HMPV (HMPV ΔSH) failed to activate caspase-1, which is responsible for IL-1β cleavage and maturation. Furthermore, HMPV ΔSH-infected mice had less weight loss, showed no mortality and reduced inflammation, as compared to wild-type HMPV-infected mice. Thus, NLRP3 inflammasome activation seems to be triggered by HMPV SH protein in HMPV disease. In summary, once activated by the HMPV SH protein, NLRP3 inflammasome promotes the maturation of IL-1β, which exacerbates HMPV-induced inflammation. Therefore, the blockade of IL-1β production by using NLRP3 inflammasome inhibitors might be a novel potential strategy for the therapy and prevention of HMPV infection.

Human metapneumovirus (HMPV), a negative-stranded, enveloped RNA virus, is recognized as one of the leading causes of acute respiratory disease in children since its discovery in 2001. Nevertheless, there is currently no licensed vaccine for the prevention of HMPV infection and treatment modalities are limited to the use of ribavirin, a weak antiviral agent or immunoglobulins. NOD-like receptor protein 3 (NLRP3) inflammasome has been shown to be involved in the pathogenesis of several RNA viral diseases but its role during HMPV infection has not been fully studied. Here, we report for the first time that NLRP3 inflammasome is activated by the small hydrophobic protein of HMPV, leading to the release of IL-1β, which has the potential to exacerbate inflammation. However, NLRP3 inflammasome has no direct influence on viral replication. Thus, IL-1β-mediated inflammatory process plays an important role during HMPV infection and, therefore, anti-IL-1β strategies such as the use of NLRP3 inhibitors may be a novel potential approach for the prevention and therapy of HMPV disease.

Principles of inflammasome priming and inhibition: Implications for psychiatric disorders

The production of inflammatory proteins by the innate immune system is a tightly orchestrated procedure that allows the body to efficiently respond to exogenous and endogenous threats. Recently, accumulating evidence has indicated that disturbances in the inflammatory response system not only provoke autoimmune disorders, but also can have deleterious effects on neuronal function and mental health. As inflammation in the brain is primarily mediated by microglia, there has been an expanding focus on the mechanisms through which these cells initiate and propagate neuroinflammation. Microglia can enter persistently active states upon their initial recognition of an environmental stressor and are thereafter prone to elicit amplified and persistent inflammatory responses following subsequent exposures to stressors. A recent focus on why primed microglia cells are susceptible to environmental insults has been the NLRP3 inflammasome. Its function within the innate immune system is regulated in such a manner that supports a role for the complex in gating neuroinflammatory responses. The activation of NLRP3 inflammasome in microglia results in the cleavage of zymogen inflammatory interleukins into functional forms that elicit a number of consequential effects in the local neuronal environment. There is evidence to support the principle that within primed neuroimmune systems a lowered threshold for NLRP3 activation can cause persistent neuroinflammation or the amplified production of inflammatory cytokines, such as IL-1β and IL-18. Over the course of an individual's lifetime, persistent neuroinflammation can subsequently lead to the pathophysiological signatures that define psychological disorders. Therefore, targeting the NLRP3 inflammasome complex may represent an innovative and consequential approach to limit neuroinflammatory states in psychiatric disorders, such as major depressive disorder.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.

Sendai Virus V Protein Inhibits the Secretion of Interleukin-1β by Preventing NLRP3 Inflammasome Assembly

Inflammasomes play a key role in host innate immune responses to viral infection by caspase-1 (Casp-1) activation to facilitate interleukin-1β (IL-1β) secretion, which contributes to the host antiviral defense. The NLRP3 inflammasome consists of the cytoplasmic sensor molecule NLRP3, adaptor protein ASC, and effector protein pro-caspase-1 (pro-Casp-1). NLRP3 and ASC promote pro-Casp-1 cleavage, leading to IL-1β maturation and secretion. However, as a countermeasure, viral pathogens have evolved virulence factors to antagonize inflammasome pathways. Here we report that V gene knockout Sendai virus [SeV V(-)] induced markedly greater amounts of IL-1β than wild-type SeV in infected THP1 macrophages. Deficiency of NLRP3 in cells inhibited SeV V(-)-induced IL-1β secretion, indicating an essential role for NLRP3 in SeV V(-)-induced IL-1β activation. Moreover, SeV V protein inhibited the assembly of NLRP3 inflammasomes, including NLRP3-dependent ASC oligomerization, NLRP3-ASC association, NLRP3 self-oligomerization, and intermolecular interactions between NLRP3 molecules. Furthermore, a high correlation between the NLRP3-binding capacity of V protein and the ability to block inflammasome complex assembly was observed. Therefore, SeV V protein likely inhibits NLRP3 self-oligomerization by interacting with NLRP3 and inhibiting subsequent recruitment of ASC to block NLRP3-dependent ASC oligomerization, in turn blocking full activation of the NLRP3 inflammasome and thus blocking IL-1β secretion. Notably, the inhibitory action of SeV V protein on NLRP3 inflammasome activation is shared by other paramyxovirus V proteins, such as Nipah virus and human parainfluenza virus type 2. We thus reveal a mechanism by which paramyxovirus inhibits inflammatory responses by inhibiting NLRP3 inflammasome complex assembly and IL-1β activation.IMPORTANCE The present study demonstrates that the V protein of SeV, Nipah virus, and human parainfluenza virus type 2 interacts with NLRP3 to inhibit NLRP3 inflammasome activation, potentially suggesting a novel strategy by which viruses evade the host innate immune response. As all members of the Paramyxovirinae subfamily carry similar V genes, this new finding may also lead to identification of novel therapeutic targets for paramyxovirus infection and related diseases.

Viral strategies for triggering and manipulating mitophagy

Viral infection causes many physiological alterations in the host cell, and many of these alterations can affect the host mitochondrial network, including mitophagy induction. A substantial amount of literature has been generated that advances our understanding of the relationship between mitophagy and several viruses. Some viruses trigger mitophagy directly, and indirectly and control the mitophagic process via different strategies. This enables viruses to promote persistent infection and attenuate the innate immune responses. In this review, we discuss the events of virus-regulated mitophagy and the functional relevance of mitophagy in the pathogenesis of viral infection and disease. Abbreviation: ATG: autophagy related; BCL2L13: BCL2 like 13; BNIP3L/NIX: BCL2 interacting protein 3 like; CL: cardiolipin; CSFV: classical swine fever virus; CVB: coxsackievirus B; DENV: dengue virus; DNM1L: dynamin 1 like; FIS1: fission, mitochondrial 1; FUNDC1: FUN14 domain containing 1; HPIV3: human parainfluenza virus 3; HSV-1: herpes simplex virus type 1; IMM: inner mitochondrial membrane; IAV: influenza A virus; IFN: interferon; IKBKE/IKKε: inhibitor of nuclear factor kappa B kinase subunit epsilon; LUBAC: linear ubiquitin assembly complex; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MeV: measles virus; MAVS: mitochondrial antiviral signaling protein; MFF: mitochondria fission factor; NLRP3: NLR family pyrin domain containing 3; NDV: Newcastle disease virus; NR4A1: nuclear receptor subfamily 4 group A member 1; OMM: outer mitochondrial membrane; OPA1: OPA1, mitochondrial dynamin like GTPase; PRKN: parkin RBR E3 ubiquitin protein ligase; PINK1: PTEN induced putative kinase 1; PHB2: prohibitin 2; PRRSV: porcine reproductive and respiratory syndrome virus; PRRs: pattern-recognition receptors; RLRs: RIG-I-like receptors; ROS: reactive oxygen species; RIPK2: receptor interacting serine/threonine kinase 2; SESN2: sestrin 2; SNAP29: synaptosome associated protein 29; STX17: syntaxin 17; TGEV: transmissible gastroenteritis virus; TUFM: Tu translation elongation factor, mitochondrial; TRAF2: TNF receptor associated factor 2; TRIM6: tripartite motif containing 6; Ub: ubiquitin; ULK1: unc-51 like autophagy activating kinase 1; VZV: varicella-zoster virus.

Zika virus elicits inflammation to evade antiviral response by cleaving cGAS via NS1-caspase-1 axis

Viral infection triggers host innate immune responses, which primarily include the activation of type I interferon (IFN) signaling and inflammasomes. Here, we report that Zika virus (ZIKV) infection triggers NLRP3 inflammasome activation, which is further enhanced by viral non-structural protein NS1 to benefit its replication. NS1 recruits the host deubiquitinase USP8 to cleave K11-linked poly-ubiquitin chains from caspase-1 at Lys134, thus inhibiting the proteasomal degradation of caspase-1. The enhanced stabilization of caspase-1 by NS1 promotes the cleavage of cGAS, which recognizes mitochondrial DNA release and initiates type I IFN signaling during ZIKV infection. NLRP3 deficiency increases type I IFN production and strengthens host resistance to ZIKVin vitro and in vivo Taken together, our work unravels a novel antagonistic mechanism employed by ZIKV to suppress host immune response by manipulating the interplay between inflammasome and type I IFN signaling, which might guide the rational design of therapeutics in the future.

Immune Ecosystem of Virus-Infected Host Tissues

Virus infected host cells serve as a central immune ecological niche during viral infection and replication and stimulate the host immune response via molecular signaling. The viral infection and multiplication process involves complex intracellular molecular interactions between viral components and the host factors. Various types of host cells are also involved to modulate immune factors in delicate and dynamic equilibrium to maintain a balanced immune ecosystem in an infected host tissue. Antiviral host arsenals are equipped to combat or eliminate viral invasion. However, viruses have evolved with strategies to counter against antiviral immunity or hijack cellular machinery to survive inside host tissue for their multiplication. However, host immune systems have also evolved to neutralize the infection; which, in turn, either clears the virus from the infected host or causes immune-mediated host tissue injury. A complex relationship between viral pathogenesis and host antiviral defense could define the immune ecosystem of virus-infected host tissues. Understanding of the molecular mechanism underlying this ecosystem would uncover strategies to modulate host immune function for antiviral therapeutics. This review presents past and present updates of immune-ecological components of virus infected host tissue and explains how viruses subvert the host immune surveillances.