Deficient IFN signaling by myeloid cells leads to MAVS-dependent virus-induced sepsis

Equine lentivirus Gag protein degrades mitochondrial antiviral signaling protein via the E3 ubiquitin ligase Smurf1

Equine infectious anemia virus (EIAV) and HIV-1 are both members of the Lentivirus genus and are similar in virological characters. EIAV is of great concern in the equine industry. Lentiviruses establish a complex interaction with the host cell to counteract the antiviral responses. There are various pattern recognition receptors in the host, for instance, the cytosolic RNA helicases interact with viral RNA to activate the mitochondrial antiviral signaling protein (MAVS) and subsequent interferon (IFN) response. However, viruses also exploit multiple strategies to resist host immunity by targeting MAVS, but the mechanism by which lentiviruses are able to target MAVS has remained unclear. In this study, we found that EIAV infection induced MAVS degradation, and that EIAV Gag protein recruited the E3 ubiquitin ligase Smurf1 to polyubiquitinate and degrade MAVS. The CARD domain of MAVS and the WW domain of Smurf1 are responsible for the interaction with Gag. EIAV Gag is a precursor polyprotein of the membrane-interacting matrix p15, the capsid p26, and the RNA-binding nucleocapsid proteins p11 and p9. Therefore, we analyzed which protein domain of Gag could interact with MAVS and Smurf1. We found that p15 and p26, but not p11 or p9, target MAVS for degradation. Moreover, we identified the key amino acid residues that support the interactions between p15 or p26 and MAVS or Smurf1. The present study describes a novel role of the EIAV structural protein Gag in targeting MAVS to counteract innate immunity, and reveals the mechanism by which the equine lentivirus can antagonize against MAVS.IMPORTANCEHost anti-RNA virus innate immunity relies mainly on the recognition by retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5), and subsequently initiates downstream signaling through interaction with mitochondrial antiviral signaling protein (MAVS). However, viruses have developed various strategies to counteract MAVS-mediated signaling, although the method of antagonism of MAVS by lentiviruses is still unknown. In this article, we demonstrate that the precursor (Pr55gag) polyprotein of EIAV and its protein domains p15 and p26 target MAVS for ubiquitin-mediated degradation through E3 ubiquitin ligase Smurf1. MAVS degradation leads to the inhibition of the downstream IFN-β pathway. This is the first time that lentiviral structural protein has been found to have antagonistic effects on MAVS pathway. Overall, our study reveals a novel mechanism by which equine lentiviruses can evade host innate immunity, and provides insight into potential therapeutic strategies for the control of lentivirus infection.

Viral sepsis: diagnosis, clinical features, pathogenesis, and clinical considerations

Sepsis, characterized as life-threatening organ dysfunction resulting from dysregulated host responses to infection, remains a significant challenge in clinical practice. Despite advancements in understanding host-bacterial interactions, molecular responses, and therapeutic approaches, the mortality rate associated with sepsis has consistently ranged between 10 and 16%. This elevated mortality highlights critical gaps in our comprehension of sepsis etiology. Traditionally linked to bacterial and fungal pathogens, recent outbreaks of acute viral infections, including Middle East respiratory syndrome coronavirus (MERS-CoV), influenza virus, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), among other regional epidemics, have underscored the role of viral pathogenesis in sepsis, particularly when critically ill patients exhibit classic symptoms indicative of sepsis. However, many cases of viral-induced sepsis are frequently underdiagnosed because standard evaluations typically exclude viral panels. Moreover, these viruses not only activate conventional pattern recognition receptors (PRRs) and retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) but also initiate primary antiviral pathways such as cyclic guanosine monophosphate adenosine monophosphate (GMP-AMP) synthase (cGAS)-stimulator of interferon genes (STING) signaling and interferon response mechanisms. Such activations lead to cellular stress, metabolic disturbances, and extensive cell damage that exacerbate tissue injury while leading to a spectrum of clinical manifestations. This complexity poses substantial challenges for the clinical management of affected cases. In this review, we elucidate the definition and diagnosis criteria for viral sepsis while synthesizing current knowledge regarding its etiology, epidemiology, and pathophysiology, molecular mechanisms involved therein as well as their impact on immune-mediated organ damage. Additionally, we discuss clinical considerations related to both existing therapies and advanced treatment interventions, aiming to enhance the comprehensive understanding surrounding viral sepsis.

Type I interferon signaling in dendritic cells limits direct antigen presentation and CD8+ T cell responses against an arthritogenic alphavirus

Ross River virus (RRV) and other alphaviruses cause chronic musculoskeletal syndromes that are associated with viral persistence, which suggests deficits in immune clearance mechanisms, including CD8+ T-cell responses. Here, we used a recombinant RRV-gp33 that expresses the immunodominant CD8+ T-cell epitope of lymphocytic choriomeningitis virus (LCMV) to directly compare responses with a virus, LCMV, that strongly induces antiviral CD8+ T cells. After footpad injection, we detected fewer gp33-specific CD8+ T cells in the draining lymph node (DLN) after RRV-gp33 than LCMV infection, despite similar viral RNA levels in the foot. However, less RRV RNA was detected in the DLN compared to LCMV, with RRV localizing principally to the subcapsular region and LCMV to the paracortical T-cell zones. Single-cell RNA-sequencing analysis of adoptively transferred gp33-specific transgenic CD8+ T cells showed rapid differentiation into effector cells after LCMV but not RRV infection. This defect in RRV-specific CD8+ T effector cell maturation was corrected by local blockade of type I interferon (IFN) signaling, which also resulted in increased RRV infection in the DLN. Studies in Wdfy4-/-, CD11c-Cre B2mfl/fl, or Xcr1-Cre Ifnar1fl/fl mice that respectively lack cross-presenting capacity, MHC-I antigen presentation by dendritic cells (DCs), or type I IFN signaling in the DC1 subset show that RRV-specific CD8+ T-cell responses can be improved by enhanced direct antigen presentation by DCs. Overall, our experiments suggest that antiviral type I IFN signaling in DCs limits direct alphavirus infection and antigen presentation, which likely delays CD8+ T-cell responses.IMPORTANCEChronic arthritis and musculoskeletal disease are common outcomes of infections caused by arthritogenic alphaviruses, including Ross River virus (RRV), due to incomplete virus clearance. Unlike other viral infections that are efficiently cleared by cytotoxic CD8+ T cells, RRV infection is surprisingly unaffected by CD8+ T cells as mice lacking or having these cells show similar viral persistence in joint and lymphoid tissues. To elucidate the basis for this deficient response, we measured the RRV-specific CD8+ T-cell population size and activation state relative to another virus known to elicit a strong T-cell response. Our findings reveal that RRV induces fewer CD8+ T cells due to limited infection of immune cells in the draining lymph node. By increasing RRV susceptibility in antigen-presenting cells, we elicited a robust CD8+ T-cell response. These results highlight antigen availability and virus tropism as possible targets for intervention against RRV immune evasion and persistence.

Single-cell analysis reveals an antiviral network that controls Zika virus infection in human dendritic cells

Zika virus (ZIKV) is a mosquito-borne flavivirus that caused an epidemic in the Americas in 2016 and is linked to severe neonatal birth defects, including microcephaly and spontaneous abortion. To better understand the host response to ZIKV infection, we adapted the 10× Genomics Chromium single-cell RNA sequencing (scRNA-seq) assay to simultaneously capture viral RNA and host mRNA. Using this assay, we profiled the antiviral landscape in a population of human monocyte-derived dendritic cells infected with ZIKV at the single-cell level. The bystander cells, which lacked detectable viral RNA, expressed an antiviral state that was enriched for genes coinciding predominantly with a type I interferon (IFN) response. Within the infected cells, viral RNA negatively correlated with type I IFN-dependent and -independent genes (the antiviral module). We modeled the ZIKV-specific antiviral state at the protein level, leveraging experimentally derived protein interaction data. We identified a highly interconnected network between the antiviral module and other host proteins. In this work, we propose a new paradigm for evaluating the antiviral response to a specific virus, combining an unbiased list of genes that highly correlate with viral RNA on a per-cell basis with experimental protein interaction data.

IMPORTANCE

Zika virus (ZIKV) remains a public health threat given its potential for re-emergence and the detrimental fetal outcomes associated with infection during pregnancy. Understanding the dynamics between ZIKV and its host is critical to understanding ZIKV pathogenesis. Through ZIKV-inclusive single-cell RNA sequencing (scRNA-seq), we demonstrate on the single-cell level the dynamic interplay between ZIKV and the host: the transcriptional program that restricts viral infection and ZIKV-mediated inhibition of that response. Our ZIKV-inclusive scRNA-seq assay will serve as a useful tool for gaining greater insight into the host response to ZIKV and can be applied more broadly to the flavivirus field.

Single cell analysis reveals an antiviral network that controls Zika virus infection in human dendritic cells

Zika virus (ZIKV) is a mosquito-borne flavivirus that caused an epidemic in the Americas in 2016 and is linked to severe neonatal birth defects, including microcephaly and spontaneous abortion. To better understand the host response to ZIKV infection, we adapted the 10x Genomics Chromium single cell RNA sequencing (scRNA-seq) assay to simultaneously capture viral RNA and host mRNA. Using this assay, we profiled the antiviral landscape in a population of human moDCs infected with ZIKV at the single cell level. The bystander cells, which lacked detectable viral RNA, expressed an antiviral state that was enriched for genes coinciding predominantly with a type I interferon (IFN) response. Within the infected cells, viral RNA negatively correlated with type I IFN dependent and independent genes (antiviral module). We modeled the ZIKV specific antiviral state at the protein level leveraging experimentally derived protein-interaction data. We identified a highly interconnected network between the antiviral module and other host proteins. In this work, we propose a new paradigm for evaluating the antiviral response to a specific virus, combining an unbiased list of genes that highly correlate with viral RNA on a per cell basis with experimental protein interaction data. Our ZIKV-inclusive scRNA-seq assay will serve as a useful tool to gaining greater insight into the host response to ZIKV and can be applied more broadly to the flavivirus field.

Mitochondrial antiviral signaling protein: a potential therapeutic target in renal disease

Mitochondrial antiviral signaling protein (MAVS) is a key innate immune adaptor on the outer mitochondrial membrane that acts as a switch in the immune signal transduction response to viral infections. Some studies have reported that MAVS mediates NF-κB and type I interferon signaling during viral infection and is also required for optimal NLRP3 inflammasome activity. Recent studies have reported that MAVS is involved in various cancers, systemic lupus erythematosus, kidney diseases, and cardiovascular diseases. Herein, we summarize the structure, activation, pathophysiological roles, and MAVS-based therapies for renal diseases. This review provides novel insights into MAVS's role and therapeutic potential in the pathogenesis of renal diseases.

Blockade of interferon signaling decreases gut barrier integrity and promotes severe West Nile virus disease

The determinants of severe disease caused by West Nile virus (WNV) and why only ~1% of individuals progress to encephalitis remain poorly understood. Here, we use human and mouse enteroids, and a mouse model of pathogenesis, to explore the capacity of WNV to directly infect gastrointestinal (GI) tract cells and contribute to disease severity. At baseline, WNV poorly infects human and mouse enteroid cultures and enterocytes in mice. However, when STAT1 or type I interferon (IFN) responses are absent, GI tract cells become infected, and this is associated with augmented GI tract and blood-brain barrier (BBB) permeability, accumulation of gut-derived molecules in the brain, and more severe WNV disease. The increased gut permeability requires TNF-α signaling, and is absent in WNV-infected IFN-deficient germ-free mice. To link these findings to human disease, we measured auto-antibodies against type I IFNs in serum from WNV-infected human cohorts. A greater frequency of auto- and neutralizing antibodies against IFN-α2 or IFN-ω is present in patients with severe WNV infection, whereas virtually no asymptomatic WNV-infected subjects have such antibodies (odds ratio 24 [95% confidence interval: 3.0 - 192.5; P = 0.003]). Overall, our experiments establish that blockade of type I IFN signaling extends WNV tropism to enterocytes, which correlates with increased gut and BBB permeability, and more severe disease.

Interleukin-36γ is causative for liver damage upon infection with Rift Valley fever virus in type I interferon receptor-deficient mice

Type I interferons (IFN) are pro-inflammatory cytokines which can also exert anti-inflammatory effects via the regulation of interleukin (IL)-1 family members. Several studies showed that interferon receptor (IFNAR)-deficient mice develop severe liver damage upon treatment with artificial agonists such as acetaminophen or polyinosinic:polycytidylic acid. In order to investigate if these mechanisms also play a role in an acute viral infection, experiments with the Bunyaviridae family member Rift Valley fever virus (RVFV) were performed. Upon RVFV clone (cl)13 infection, IFNAR-deficient mice develop a severe liver injury as indicated by high activity of serum alanine aminotransferase (ALT) and histological analyses. Infected IFNAR-/- mice expressed high amounts of IL-36γ within the liver, which was not observed in infected wildtype (WT) animals. In line with this, treatment of WT mice with recombinant IL-36γ induced ALT activity. Furthermore, administration of an IL-36 receptor antagonist prior to infection prevented the formation of liver injury in IFNAR-/- mice, indicating that IL-36γ is causative for the observed liver damage. Mice deficient for adaptor molecules of certain pattern recognition receptors indicated that IL-36γ induction was dependent on mitochondrial antiviral-signaling protein and the retinoic acid-inducible gene-I-like receptor. Consequently, cell type-specific IFNAR knockouts revealed that type I IFN signaling in myeloid cells is critical in order to prevent IL-36γ expression and liver injury upon viral infection. Our data demonstrate an anti-inflammatory role of type I IFN in a model for virus-induced hepatitis by preventing the expression of the novel IL-1 family member IL-36γ.

Clodronate is not protective in lethal viral encephalitis despite substantially reducing inflammatory monocyte infiltration in the CNS

Bone marrow (BM)-derived monocytes induce inflammation and tissue damage in a range of pathologies. In particular, in a mouse model of West Nile virus (WNV) encephalitis (WNE), nitric oxide-producing, Ly6Chi inflammatory monocytes from the BM are recruited to the central nervous system (CNS) and contribute to lethal immune pathology. Reducing the migration of these cells into the CNS using monoclonal antibody blockade, immune-modifying particles or CSF-1R inhibitors reduces neuroinflammation, improving survival and/or clinical outcomes. Macrophages can also be targeted more broadly by administration of clodronate-encapsulated liposomes, which induce apoptosis in phagocytes. In this study, clodronate reduced the inflammatory infiltrate by 70% in WNE, however, surprisingly, this had no effect on disease outcome. More detailed analysis demonstrated a compensatory increase in neutrophils and enhanced activation status of microglia in the brain. In addition, we observed increased numbers of Ly6Chi BM monocytes with an increased proliferative capacity and expression of SCA-1 and CD16/32, potentially indicating output of immature cells from the BM. Once in the brain, these cells were more phagocytic and had a reduced expression of antigen-presenting molecules. Lastly, we show that clodronate also reduces non-myeloid cells in the spleen and BM, as well as ablating red blood cells and their proliferation. These factors likely impeded the therapeutic potential of clodronate in WNE. Thus, while clodronate provides an excellent system to deplete macrophages in the body, it has larger and broader effects on the phagocytic and non-phagocytic system, which must be considered in the interpretation of data.

Endogenous Retroviruses (ERVs): Does RLR (RIG-I-Like Receptors)-MAVS Pathway Directly Control Senescence and Aging as a Consequence of ERV De-Repression?

Bi-directional transcription of Human Endogenous Retroviruses (hERVs) is a common feature of autoimmunity, neurodegeneration and cancer. Higher rates of cancer incidence, neurodegeneration and autoimmunity but a lower prevalence of autoimmune diseases characterize elderly people. Although the re-expression of hERVs is commonly observed in different cellular models of senescence as a result of the loss of their epigenetic transcriptional silencing, the hERVs modulation during aging is more complex, with a peak of activation in the sixties and a decline in the nineties. What is clearly accepted, instead, is the impact of the re-activation of dormant hERV on the maintenance of stemness and tissue self-renewing properties. An innate cellular immunity system, based on the RLR-MAVS circuit, controls the degradation of dsRNAs arising from the transcription of hERV elements, similarly to what happens for the accumulation of cytoplasmic DNA leading to the activation of cGAS/STING pathway. While agonists and inhibitors of the cGAS-STING pathway are considered promising immunomodulatory molecules, the effect of the RLR-MAVS pathway on innate immunity is still largely based on correlations and not on causality. Here we review the most recent evidence regarding the activation of MDA5-RIG1-MAVS pathway as a result of hERV de-repression during aging, immunosenescence, cancer and autoimmunity. We will also deal with the epigenetic mechanisms controlling hERV repression and with the strategies that can be adopted to modulate hERV expression in a therapeutic perspective. Finally, we will discuss if the RLR-MAVS signalling pathway actively modulates physiological and pathological conditions or if it is passively activated by them.

STING1 in sepsis: Mechanisms, functions, and implications

Sepsis is a life-threatening clinical syndrome and one of the most challenging health problems in the world. Pathologically, sepsis and septic shock are caused by a dysregulated host immune response to infection, which can eventually lead to multiple organ failure and even death. As an adaptor transporter between the endoplasmic reticulum and Golgi apparatus, stimulator of interferon response cGAMP interactor 1 (STING1, also known as STING or TMEM173) has been found to play a vital role at the intersection of innate immunity, inflammation, autophagy, and cell death in response to invading microbial pathogens or endogenous host damage. There is ample evidence that impaired STING1, through its immune and non-immune functions, is involved in the pathological process of sepsis. In this review, we discuss the regulation and function of the STING1 pathway in sepsis and highlight it as a suitable drug target for the treatment of lethal infection.

Shiftless inhibits flavivirus replication in vitro and is neuroprotective in a mouse model of Zika virus pathogenesis

Flaviviruses such as Zika virus and West Nile virus have the potential to cause severe neuropathology if they invade the central nervous system. The type I interferon response is well characterized as contributing to control of flavivirus-induced neuropathogenesis. However, the interferon-stimulated gene (ISG) effectors that confer these neuroprotective effects are less well studied. Here, we used an ISG expression screen to identify Shiftless (SHFL, C19orf66) as a potent inhibitor of diverse positive-stranded RNA viruses, including multiple members of the Flaviviridae (Zika, West Nile, dengue, yellow fever, and hepatitis C viruses). In cultured cells, SHFL functions as a viral RNA-binding protein that inhibits viral replication at a step after primary translation of the incoming genome. The murine ortholog, Shfl, is expressed constitutively in multiple tissues, including the central nervous system. In a mouse model of Zika virus infection, Shfl ^-/- knockout mice exhibit reduced survival, exacerbated neuropathological outcomes, and increased viral replication in the brain and spinal cord. These studies demonstrate that Shfl is an important antiviral effector that contributes to host protection from Zika virus infection and virus-induced neuropathological disease.

The MAVS Immune Recognition Pathway in Viral Infection and Sepsis

Significance: It is estimated that close to 50 million cases of sepsis result in over 11 million annual fatalities worldwide. The pathognomonic feature of sepsis is a dysregulated inflammatory response arising from viral, bacterial, or fungal infections. Immune recognition of pathogen-associated molecular patterns is a hallmark of the host immune defense to combat microbes and to prevent the progression to sepsis. Mitochondrial antiviral signaling protein (MAVS) is a ubiquitous adaptor protein located at the outer mitochondrial membrane, which is activated by the cytosolic pattern recognition receptors, retinoic acid-inducible gene I (RIG-I) and melanoma differentiation associated gene 5 (MDA5), following binding of viral RNA agonists. Recent Advances: Substantial progress has been made in deciphering the activation of the MAVS pathway with its interacting proteins, downstream signaling events (interferon [IFN] regulatory factors, nuclear factor kappa B), and context-dependent type I/III IFN response. Critical Issues: In the evolutionary race between pathogens and the host, viruses have developed immune evasion strategies for cleavage, degradation, or blockade of proteins in the MAVS pathway. For example, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) M protein and ORF9b protein antagonize MAVS signaling and a protective type I IFN response. Future Directions: The role of MAVS as a sensor for nonviral pathogens, host cell injury, and metabolic perturbations awaits better characterization in the future. New technical advances in multidimensional single-cell analysis and single-molecule methods will accelerate the rate of new discoveries. The ultimate goal is to manipulate MAVS activities in the form of immune-modulatory therapies to combat infections and sepsis. Antioxid. Redox Signal. 35, 1376-1392.

Emerging roles of the complement system in host-pathogen interactions

The complement system has historically been entertained as a fluid-phase, hepatically derived system which protects the intravascular space from encapsulated bacteria. However, there has been an increasing appreciation for its role in protection against non-encapsulated pathogens. Specifically, we have an improved understanding of how pathogens are recognized by specific complement proteins, as well as how they trigger and evade them. Additionally, we have an improved understanding of locally derived complement proteins, many of which promote host defense. Moreover, intracellular complement proteins have been identified that facilitate local protection and barrier function despite pathogen invasion. Our review aims to summarize these advances in the field as well as provide an insight into the pathophysiological changes occurring when the system is dysregulated in infection.

West Nile Virus-Associated Hospitalizations, California, 2004-2017

BACKGROUND

West Nile virus (WNV) is the most commonly reported mosquito-borne disease in the USA. California reports more WNV disease than any other state.

METHODS

We identified WNV-associated hospitalizations from 2004 through 2017 in California and estimated hospitalization incidence using Patient Discharge Data. We described demographic, geographic, and clinical characteristics of WNV hospitalizations; identified risk factors for in-hospital death; and tabulated hospitalization charges.

RESULTS

From 2004 through 2017, 3109 Californians were hospitalized with WNV (median, 214 patients/year; range, 72-449). The majority were male (1983; 63.8%) and aged ≥60 years (1766; 56.8%). The highest median annual hospitalization rate (0.88 hospitalizations/100 000 persons) was in the Central Valley, followed by southern California (0.59 hospitalizations/100 000 persons). Most patients (2469; 79.4%) had ≥1 underlying condition, including hypertension, cardiovascular disease, diabetes, chronic kidney disease, or immunosuppression due to medications or disease. Median hospitalization length of stay was 12 days (interquartile range, 6-23 days). During hospitalization, 1317 (42%) patients had acute respiratory failure and/or sepsis/septic shock, 772 (24.8%) experienced acute kidney failure, and 470 (15.1%) had paralysis; 272 (8.8%) patients died. Nearly 47% (1444) of patients were discharged for additional care. During these 14 years, $838 680 664 (mean $59.9 million/year) was charged for WNV hospitalizations, 73.9% through government payers at a median charge of $142 321/patient.

CONCLUSIONS

WNV-associated hospitalizations were substantial and costly in California. Hospitalization incidence was higher in males, elderly persons, and patients with underlying conditions. WNV persists as a costly and severe public health threat in California.

Type I IFN signaling limits hemorrhage-like disease after infection with Japanese encephalitis virus through modulating a prerequisite infection of CD11b+Ly-6C+ monocytes

Background

The crucial role of type I interferon (IFN-I, IFN-α/β) is well known to control central nervous system (CNS) neuroinflammation caused by neurotrophic flaviviruses such as Japanese encephalitis virus (JEV) and West Nile virus. However, an in-depth analysis of IFN-I signal-dependent cellular factors that govern CNS-restricted tropism in JEV infection in vivo remains to be elucidated.

Methods

Viral dissemination, tissue tropism, and cytokine production were examined in IFN-I signal-competent and -incompetent mice after JEV inoculation in tissues distal from the CNS such as the footpad. Bone marrow (BM) chimeric models were used for defining hematopoietic and tissue-resident cells in viral dissemination and tissue tropism.

Results

The paradoxical and interesting finding was that IFN-I signaling was essentially required for CNS neuroinflammation following JEV inoculation in distal footpad tissue. IFN-I signal-competent mice died after a prolonged neurological illness, but IFN-I signal-incompetent mice all succumbed without neurological signs. Rather, IFN-I signal-incompetent mice developed hemorrhage-like disease as evidenced by thrombocytopenia, functional injury of the liver and kidney, increased vascular leakage, and excessive cytokine production. This hemorrhage-like disease was closely associated with quick viral dissemination and impaired IFN-I innate responses before invasion of JEV into the CNS. Using bone marrow (BM) chimeric models, we found that intrinsic IFN-I signaling in tissue-resident cells in peripheral organs played a major role in inducing the hemorrhage-like disease because IFN-I signal-incompetent recipients of BM cells from IFN-I signal-competent mice showed enhanced viral dissemination, uncontrolled cytokine production, and increased vascular leakage. IFN-I signal-deficient hepatocytes and enterocytes were permissive to JEV replication with impaired induction of antiviral IFN-stimulated genes, and neuron cells derived from both IFN-I signal-competent and -incompetent mice were vulnerable to JEV replication. Finally, circulating CD11b⁺Ly-6C⁺ monocytes infiltrated into the distal tissues inoculated by JEV participated in quick viral dissemination to peripheral organs of IFN-I signal-incompetent mice at an early stage.

Conclusion

An IFN-I signal-dependent model is proposed to demonstrate how CD11b⁺Ly-6C⁺ monocytes are involved in restricting the tissue tropism of JEV to the CNS.

The Role of Type I Interferon Signaling in Regulating Cytokine Production and Cell Survival in Bone Marrow-Derived Macrophages

During viral infections, cells produce type I interferons (IFNs), which are detected by the IFNα/β receptor (IFNAR). To survive in hosts, viruses have strategies to downregulate IFN-mediated signaling. We hypothesized that macrophages, which are among the first myeloid cells to respond to viral infections, would produce a different cytokine profile if responding to ligation of pattern recognition receptors (PRRs) while IFNAR-mediated signaling was compromised. Specifically, IFNAR-mediated regulation of interleukin (IL)-1α, IL-6, IL-12, and tumor necrosis factor-α was studied in cultured murine bone marrow-derived macrophages. Since viruses like vesicular stomatitis virus can ligate PRRs such as Toll-like receptor (TLR)4 and 7, macrophages were stimulated with the TLR4 and TLR7 agonists lipopolysaccharide (LPS) or imiquimod, respectively, with or without antibody-mediated IFNAR-blockade. Cytokines and viability were assessed for 3 days poststimulation. Blocking IFNARs acutely exacerbated cytokine production by macrophages and aided their survival when they were treated with LPS. In contrast, cytokine concentrations were unaffected or slightly reduced by IFNAR blockade, but macrophages died at a greater rate when imiquimod was the stimulus. This demonstrated a differential role of IFNAR signaling in regulating PRR-induced cytokines. This suggests potential mechanisms whereby macrophages responding to viruses that inhibit type I IFN responses might contribute to excessive inflammation in some cases and inappropriately low-magnitude responses in others. This also provides a well-defined cell-based model for further dissecting the role of type I IFN signaling in macrophages responding to viral and other infections.

Pathogenicity and virulence of West Nile virus revisited eight decades after its first isolation

West Nile virus (WNV) is a flavivirus which transmission cycle is maintained between mosquitoes and birds, although it occasionally causes sporadic outbreaks in horses and humans that can result in serious diseases and even death. Since its first isolation in Africa in 1937, WNV had been considered a neglected pathogen until its recent spread throughout Europe and the colonization of America, regions where it continues to cause outbreaks with severe neurological consequences in humans and horses. Although our knowledge about the characteristics and consequences of the virus has increased enormously lately, many questions remain to be resolved. Here, we thoroughly update our knowledge of different aspects of the WNV life cycle: virology and molecular classification, host cell interactions, transmission dynamics, host range, epidemiology and surveillance, immune response, clinical presentations, pathogenesis, diagnosis, prophylaxis (antivirals and vaccines), and prevention, and we highlight those aspects that are still unknown and that undoubtedly require further investigation.

Context Is Key: Delineating the Unique Functions of IFNα and IFNβ in Disease

Type I interferons (IFNs) are critical effector cytokines of the immune system and were originally known for their important role in protecting against viral infections; however, they have more recently been shown to play protective or detrimental roles in many disease states. Type I IFNs consist of IFNα, IFNβ, IFNϵ, IFNκ, IFNω, and a few others, and they all signal through a shared receptor to exert a wide range of biological activities, including antiviral, antiproliferative, proapoptotic, and immunomodulatory effects. Though the individual type I IFN subtypes possess overlapping functions, there is growing appreciation that they also have unique properties. In this review, we summarize some of the mechanisms underlying differential expression of and signaling by type I IFNs, and we discuss examples of differential functions of IFNα and IFNβ in models of infectious disease, cancer, and autoimmunity.

mRNA induced expression of human angiotensin-converting enzyme 2 in mice for the study of the adaptive immune response to severe acute respiratory syndrome coronavirus 2

The novel human coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a pandemic. Critical to the rapid evaluation of vaccines and antivirals against SARS-CoV-2 is the development of tractable animal models to understand the adaptive immune response to the virus. To this end, the use of common laboratory strains of mice is hindered by significant divergence of the angiotensin-converting enzyme 2 (ACE2), which is the receptor required for entry of SARS-CoV-2. In the current study, we designed and utilized an mRNA-based transfection system to induce expression of the hACE2 receptor in order to confer entry of SARS-CoV-2 in otherwise non-permissive cells. By employing this expression system in an in vivo setting, we were able to interrogate the adaptive immune response to SARS-CoV-2 in type 1 interferon receptor deficient mice. In doing so, we showed that the T cell response to SARS-CoV-2 is enhanced when hACE2 is expressed during infection. Moreover, we demonstrated that these responses are preserved in memory and are boosted upon secondary infection. Importantly, using this system, we functionally identified the CD4+ and CD8+ structural peptide epitopes targeted during SARS-CoV-2 infection in H2b restricted mice and confirmed their existence in an established model of SARS-CoV-2 pathogenesis. We demonstrated that, identical to what has been seen in humans, the antigen-specific CD8+ T cells in mice primarily target peptides of the spike and membrane proteins, while the antigen-specific CD4+ T cells target peptides of the nucleocapsid, membrane, and spike proteins. As the focus of the immune response in mice is highly similar to that of the humans, the identification of functional murine SARS-CoV-2-specific T cell epitopes provided in this study will be critical for evaluation of vaccine efficacy in murine models of SARS-CoV-2 infection.

Human iPSC-Derived Neuronal Cells From CTBP1-Mutated Patients Reveal Altered Expression of Neurodevelopmental Gene Networks

A recurrent de novo mutation in the transcriptional corepressor CTBP1 is associated with neurodevelopmental disabilities in children (Beck et al., 2016, 2019; Sommerville et al., 2017). All reported patients harbor a single recurrent de novo heterozygous missense mutation (p.R342W) within the cofactor recruitment domain of CtBP1. To investigate the transcriptional activity of the pathogenic CTBP1 mutant allele in physiologically relevant human cell models, we generated induced pluripotent stem cells (iPSC) from the dermal fibroblasts derived from patients and normal donors. The transcriptional profiles of the iPSC-derived “early” neurons were determined by RNA-sequencing. Comparison of the RNA-seq data of the neurons from patients and normal donors revealed down regulation of gene networks involved in neurodevelopment, synaptic adhesion and anti-viral (interferon) response. Consistent with the altered gene expression patterns, the patient-derived neurons exhibited morphological and electrophysiological abnormalities, and susceptibility to viral infection. Taken together, our studies using iPSC-derived neuron models provide novel insights into the pathological activities of the CTBP1 p.R342W allele.

In the Crosshairs: RNA Viruses OR Complement?

Complement, a part of the innate arm of the immune system, is integral to the frontline defense of the host against innumerable pathogens, which includes RNA viruses. Among the major groups of viruses, RNA viruses contribute significantly to the global mortality and morbidity index associated with viral infection. Despite multiple routes of entry adopted by these viruses, facing complement is inevitable. The initial interaction with complement and the nature of this interaction play an important role in determining host resistance versus susceptibility to the viral infection. Many RNA viruses are potent activators of complement, often resulting in virus neutralization. Yet, another facet of virus-induced activation is the exacerbation in pathogenesis contributing to the overall morbidity. The severity in disease and death associated with RNA virus infections shows a tip in the scale favoring viruses. Growing evidence suggest that like their DNA counterparts, RNA viruses have co-evolved to master ingenious strategies to remarkably restrict complement. Modulation of host genes involved in antiviral responses contributed prominently to the adoption of unique strategies to keep complement at bay, which included either down regulation of activation components (C3, C4) or up regulation of complement regulatory proteins. All this hints at a possible “hijacking” of the cross-talk mechanism of the host immune system. Enveloped RNA viruses have a selective advantage of not only modulating the host responses but also recruiting membrane-associated regulators of complement activation (RCAs). This review aims to highlight the significant progress in the understanding of RNA virus–complement interactions.

Zika Virus Replication in Myeloid Cells during Acute Infection Is Vital to Viral Dissemination and Pathogenesis in a Mouse Model

Zika virus (ZIKV) can establish infection in immune privileged sites such as the testes, eye, and placenta. Whether ZIKV infection of white blood cells is required for dissemination of the virus to immune privileged sites has not been definitively shown. To assess whether initial ZIKV replication in myeloid cell populations is critical for dissemination during acute infection, recombinant ZIKVs were generated that could not replicate in these specific cells. ZIKV was cell restricted by insertion of a complementary sequence to a myeloid-specific microRNA in the 3' untranslated region. Following inoculation of a highly sensitive immunodeficient mouse model, crucial immune parameters, such as quantification of leukocyte cell subsets, cytokine and chemokine secretion, and viremia, were assessed. Decreased neutrophil numbers in the spleen were observed during acute infection with myeloid-restricted ZIKV that precluded the generation of viremia and viral dissemination to peripheral organs. Mice inoculated with a nontarget microRNA control ZIKV demonstrated increased expression of key cytokines and chemokines critical for neutrophil and monocyte recruitment and increased neutrophil influx in the spleen. In addition, ZIKV-infected Ly6Chi monocytes were identified in vivo in the spleen. Mice inoculated with myeloid-restricted ZIKV had a decrease in Ly6Chi ZIKV RNA-positive monocytes and a lack of inflammatory cytokine production compared to mice inoculated with control ZIKV.IMPORTANCE Myeloid cells, including monocytes, play a crucial role in immune responses to pathogens. Monocytes have also been implicated as "Trojan horses" during viral infections, carrying infectious virus particles to immune privileged sites and/or to sites protected by physical blood-tissue barriers, such as the blood-testis barrier and the blood-brain barrier. In this study, we found that myeloid cells are crucial to Zika virus (ZIKV) pathogenesis. By engineering ZIKV clones to encode myeloid-specific microRNA target sequences, viral replication was inhibited in myeloid cells by harnessing the RNA interference pathway. Severely immunodeficient mice inoculated with myeloid-restricted ZIKV did not demonstrate clinical signs of disease and survived infection. Furthermore, viral dissemination to peripheral organs was not observed in these mice. Lastly, we identified Ly6Cmid/hi murine monocytes as the major myeloid cell population that disseminates ZIKV.

mRNA induced expression of human angiotensin-converting enzyme 2 in mice for the study of the adaptive immune response to severe acute respiratory syndrome coronavirus 2

The novel human coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a pandemic resulting in nearly 20 million infections across the globe, as of August 2020. Critical to the rapid evaluation of vaccines and antivirals is the development of tractable animal models of infection. The use of common laboratory strains of mice to this end is hindered by significant divergence of the angiotensin-converting enzyme 2 (ACE2), which is the receptor required for entry of SARS-CoV-2. In the current study, we designed and utilized an mRNA-based transfection system to induce expression of the hACE2 receptor in order to confer entry of SARS-CoV-2 in otherwise non-permissive cells. By employing this expression system in an in vivo setting, we were able to interrogate the adaptive immune response to SARS-CoV-2 in type 1 interferon receptor deficient mice. In doing so, we showed that the T cell response to SARS-CoV-2 is enhanced when hACE2 is expressed during infection. Moreover, we demonstrated that these responses are preserved in memory and are boosted upon secondary infection. Interestingly, we did not observe an enhancement of SARS-CoV-2 specific antibody responses with hACE2 induction. Importantly, using this system, we functionally identified the CD4+ and CD8+ peptide epitopes targeted during SARS-CoV-2 infection in H2b restricted mice. Antigen-specific CD8+ T cells in mice of this MHC haplotype primarily target peptides of the spike and membrane proteins, while the antigen-specific CD4+ T cells target peptides of the nucleocapsid, membrane, and spike proteins. The functional identification of these T cell epitopes will be critical for evaluation of vaccine efficacy in murine models of SARS-CoV-2. The use of this tractable expression system has the potential to be used in other instances of emerging infections in which the rapid development of an animal model is hindered by a lack of host susceptibility factors.

The complement system in COVID-19: friend and foe?

Coronavirus disease 2019 (COVID-19), the disease caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has resulted in a global pandemic and a disruptive health crisis. COVID-19-related morbidity and mortality have been attributed to an exaggerated immune response. The role of complement activation and its contribution to illness severity is being increasingly recognized. Here, we summarize current knowledge about the interaction of coronaviruses with the complement system. We posit that (a) coronaviruses activate multiple complement pathways; (b) severe COVID-19 clinical features often resemble complementopathies; (c) the combined effects of complement activation, dysregulated neutrophilia, endothelial injury, and hypercoagulability appear to be intertwined to drive the severe features of COVID-19; (d) a subset of patients with COVID-19 may have a genetic predisposition associated with complement dysregulation; and (e) these observations create a basis for clinical trials of complement inhibitors in life-threatening illness.

A SARS-CoV-2 Infection Model in Mice Demonstrates Protection by Neutralizing Antibodies

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a pandemic with millions of human infections. One limitation to the evaluation of potential therapies and vaccines to inhibit SARS-CoV-2 infection and ameliorate disease is the lack of susceptible small animals in large numbers. Commercially available laboratory strains of mice are not readily infected by SARS-CoV-2 because of species-specific differences in their angiotensin-converting enzyme 2 (ACE2) receptors. Here, we transduced replication-defective adenoviruses encoding human ACE2 via intranasal administration into BALB/c mice and established receptor expression in lung tissues. hACE2-transduced mice were productively infected with SARS-CoV-2, and this resulted in high viral titers in the lung, lung pathology, and weight loss. Passive transfer of a neutralizing monoclonal antibody reduced viral burden in the lung and mitigated inflammation and weight loss. The development of an accessible mouse model of SARS-CoV-2 infection and pathogenesis will expedite the testing and deployment of therapeutics and vaccines.

Interferon-β Plays a Detrimental Role in Experimental Traumatic Brain Injury by Enhancing Neuroinflammation That Drives Chronic Neurodegeneration

DNA damage and type I interferons (IFNs) contribute to inflammatory responses after traumatic brain injury (TBI). TBI-induced activation of microglia and peripherally-derived inflammatory macrophages may lead to tissue damage and neurological deficits. Here, we investigated the role of IFN-β in secondary injury after TBI using a controlled cortical impact model in adult male IFN-β-deficient (IFN-β^-/-) mice and assessed post-traumatic neuroinflammatory responses, neuropathology, and long-term functional recovery. TBI increased expression of DNA sensors cyclic GMP-AMP synthase and stimulator of interferon genes in wild-type (WT) mice. IFN-β and other IFN-related and neuroinflammatory genes were also upregulated early and persistently after TBI. TBI increased expression of proinflammatory mediators in the cortex and hippocampus of WT mice, whereas levels were mitigated in IFN-β^-/- mice. Moreover, long-term microglia activation, motor, and cognitive function impairments were decreased in IFN-β^-/- TBI mice compared with their injured WT counterparts; improved neurological recovery was associated with reduced lesion volume and hippocampal neurodegeneration in IFN-β^-/- mice. Continuous central administration of a neutralizing antibody to the IFN-α/β receptor (IFNAR) for 3 d, beginning 30 min post-injury, reversed early cognitive impairments in TBI mice and led to transient improvements in motor function. However, anti-IFNAR treatment did not improve long-term functional recovery or decrease TBI neuropathology at 28 d post-injury. In summary, TBI induces a robust neuroinflammatory response that is associated with increased expression of IFN-β and other IFN-related genes. Inhibition of IFN-β reduces post-traumatic neuroinflammation and neurodegeneration, resulting in improved neurological recovery. Thus, IFN-β may be a potential therapeutic target for TBI.SIGNIFICANCE STATEMENT TBI frequently causes long-term neurological and psychiatric changes in head injury patients. TBI-induced secondary injury processes including persistent neuroinflammation evolve over time and can contribute to chronic neurological impairments. The present study demonstrates that TBI is followed by robust activation of type I IFN pathways, which have been implicated in microglial-associated neuroinflammation and chronic neurodegeneration. We examined the effects of genetic or pharmacological inhibition of IFN-β, a key component of type I IFN mechanisms to address its role in TBI pathophysiology. Inhibition of IFN-β signaling resulted in reduced neuroinflammation, attenuated neurobehavioral deficits, and limited tissue loss long after TBI. These preclinical findings suggest that IFN-β may be a potential therapeutic target for TBI.

Frequent Homozygous Deletions of Type I Interferon Genes in Pleural Mesothelioma Confer Sensitivity to Oncolytic Measles Virus

INTRODUCTION

Oncolytic immunotherapy is based on the use of nonpathogenic replicative oncolytic viruses that infect and kill tumor cells exclusively. Recently, we found that the spontaneous oncolytic activity of the Schwarz strain of measles virus (MV) against human malignant pleural mesothelioma (MPM) depends on defects in the antiviral type I interferon (IFN-I) response in tumor cells.

METHODS

In this study, we studied three independent human MPM bio-collections to identify the defects in the IFN-I responses in tumor cells.

RESULTS

We show that the most frequent defect is the homozygous deletions (HDs) of all the 14 IFN-I genes (IFN-α and IFN-β) that we found in more than half of MV-sensitive MPM cell lines. These HDs occur together with the HDs of the tumor suppressor gene CDKN2A also located in the 9p21.3 chromosome region. Therefore, the IFN-I^-/- MPM cell lines develop a partial and weak IFN-I response when they are exposed to the virus compared with that of normal cells and MV-resistant MPM cell lines. This response consists of the expression of a restricted number of IFN-stimulated genes that do not depend on the presence of IFN-I. In addition, the IFN-I^-/- MPM cell lines infected by MV also develop a pro-inflammatory response associated with stress of the endoplasmic reticulum.

CONCLUSION

Our study emphasizes the link between HDs of IFN-I encoding genes and the CDKN2A gene in MPM and sensitivity to MV oncolytic immunotherapy.

Distinct Roles of Interferon Alpha and Beta in Controlling Chikungunya Virus Replication and Modulating Neutrophil-Mediated Inflammation

Type I interferons (IFNs) are key mediators of the innate immune response. Although members of this family of cytokines signal through a single shared receptor, biochemical and functional variation exists in response to different IFN subtypes. While previous work has demonstrated that type I IFNs are essential to control infection by chikungunya virus (CHIKV), a globally emerging alphavirus, the contributions of individual IFN subtypes remain undefined. To address this question, we evaluated CHIKV pathogenesis in mice lacking IFN-β (IFN-β knockout [IFN-β-KO] mice or mice treated with an IFN-β-blocking antibody) or IFN-α (IFN regulatory factor 7 knockout [IRF7-KO] mice or mice treated with a pan-IFN-α-blocking antibody). Mice lacking either IFN-α or IFN-β developed severe clinical disease following infection with CHIKV, with a marked increase in foot swelling compared to wild-type mice. Virological analysis revealed that mice lacking IFN-α sustained elevated infection in the infected ankle and in distant tissues. In contrast, IFN-β-KO mice displayed minimal differences in viral burdens within the ankle or at distal sites and instead had an altered cellular immune response. Mice lacking IFN-β had increased neutrophil infiltration into musculoskeletal tissues, and depletion of neutrophils in IFN-β-KO but not IRF7-KO mice mitigated musculoskeletal disease caused by CHIKV. Our findings suggest disparate roles for the IFN subtypes during CHIKV infection, with IFN-α limiting early viral replication and dissemination and IFN-β modulating neutrophil-mediated inflammation.IMPORTANCE Type I interferons (IFNs) possess a range of biological activity and protect against a number of viruses, including alphaviruses. Despite signaling through a shared receptor, there are established biochemical and functional differences among the IFN subtypes. The significance of our research is in demonstrating that IFN-α and IFN-β both have protective roles during acute chikungunya virus (CHIKV) infection but do so by distinct mechanisms. IFN-α limits CHIKV replication and dissemination, whereas IFN-β protects from CHIKV pathogenesis by limiting inflammation mediated by neutrophils. Our findings support the premise that the IFN subtypes have distinct biological activities in the antiviral response.

West Nile Virus Infection Blocks Inflammatory Response and T Cell Costimulatory Capacity of Human Monocyte-Derived Dendritic Cells

West Nile virus (WNV) is a neurotropic flavivirus and the leading cause of mosquito-borne encephalitis in the United States. Recent studies in humans have found that dysfunctional T cell responses strongly correlate with development of severe WNV neuroinvasive disease. However, the contributions of human dendritic cells (DCs) in priming WNV-specific T cell immunity remains poorly understood. Here, we demonstrate that human monocyte derived DCs (moDCs) support productive viral replication following infection with a pathogenic strain of WNV. Antiviral effector gene transcription was strongly induced during the log phase of viral growth, while secretion of type I interferons (IFN) occurred with delayed kinetics. Activation of RIG-I like receptor (RLR) or type I IFN signaling prior to log phase viral growth significantly diminished viral replication, suggesting that early activation of antiviral programs can block WNV infection. In contrast to the induction of antiviral responses, WNV infection did not promote transcription or secretion of proinflammatory (interleukin-6 [IL-6], granulocyte-macrophage colony-stimulating factor [GM-CSF], CCL3, CCL5, and CXCL9) or T cell modulatory (IL-4, IL-12, and IL-15) cytokines. There was also minimal induction of molecules associated with antigen presentation and T cell priming, including the costimulatory molecules CD80, CD86, and CD40. Functionally, WNV-infected moDCs dampened allogenic CD4 and CD8 T cell activation and proliferation. Combining these observations, we propose a model whereby WNV subverts human DC activation to compromise priming of WNV-specific T cell immunity.IMPORTANCE West Nile virus (WNV) is an encephalitic flavivirus that remains endemic in the United States. Previous studies have found dysfunctional T cell responses correlate to severe disease outcomes during human WNV infection. Here, we sought to better understand the ability of WNV to program human dendritic cells (DCs) to prime WNV-specific T cell responses. While productive infection of monocyte-derived DCs activated antiviral and type I interferon responses, molecules associated with inflammation and programming of T cells were minimally induced. Functionally, WNV-infected DCs dampened T cell activation and proliferation during an allogeneic response. Combined, our data support a model whereby WNV infection of human DCs compromises WNV-specific T cell immunity.

STAT5: a Target of Antagonism by Neurotropic Flaviviruses

Flaviviruses are a diverse group of arthropod-borne viruses responsible for numerous significant public health threats; therefore, understanding the interactions between these viruses and the human immune response remains vital. West Nile virus (WNV) and Zika virus (ZIKV) infect human dendritic cells (DCs) and can block antiviral immune responses in DCs. Previously, we used mRNA sequencing and weighted gene coexpression network analysis (WGCNA) to define molecular signatures of antiviral DC responses following activation of innate immune signaling (RIG-I, MDA5, or type I interferon [IFN] signaling) or infection with WNV. Using this approach, we found that several genes involved in T cell cosignaling and antigen processing were not enriched in DCs during WNV infection. Using cis-regulatory sequence analysis, STAT5 was identified as a regulator of DC activation and immune responses downstream of innate immune signaling that was not activated during either WNV or ZIKV infection. Mechanistically, WNV and ZIKV actively blocked STAT5 phosphorylation downstream of RIG-I, IFN-β, and interleukin-4 (IL-4), but not granulocyte-macrophage colony-stimulating factor (GM-CSF), signaling. Unexpectedly, dengue virus serotypes 1 to 4 (DENV1 to DENV4) and the yellow fever 17D vaccine strain (YFV-17D) did not antagonize STAT5 phosphorylation. In contrast to WNV, ZIKV inhibited JAK1 and TYK2 phosphorylation following type I IFN treatment, suggesting divergent mechanisms used by these viruses to inhibit STAT5 activation. Combined, these findings identify STAT5 as a target of antagonism by specific pathogenic flaviviruses to subvert the immune response in infected DCs.IMPORTANCE Flaviviruses are a diverse group of insect-borne viruses responsible for numerous significant public health threats. Previously, we used a computational biology approach to define molecular signatures of antiviral DC responses following activation of innate immune signaling or infection with West Nile virus (WNV). In this work, we identify STAT5 as a regulator of DC activation and antiviral immune responses downstream of innate immune signaling that was not activated during either WNV or Zika virus (ZIKV) infection. WNV and ZIKV actively blocked STAT5 phosphorylation downstream of RIG-I, IFN-β, and IL-4, but not GM-CSF, signaling. However, other related flaviviruses, dengue virus serotypes 1 to 4 and the yellow fever 17D vaccine strain, did not antagonize STAT5 phosphorylation. Mechanistically, WNV and ZIKV showed differential inhibition of Jak kinases upstream of STAT5, suggesting divergent countermeasures to inhibit STAT5 activation. Combined, these findings identify STAT5 as a target of antagonism by specific pathogenic flaviviruses to subvert antiviral immune responses in human DCs.

The Pathogenesis of Sepsis and Potential Therapeutic Targets

Sepsis is defined as “a life-threatening organ dysfunction caused by a host’s dysfunctional response to infection”. Although the treatment of sepsis has developed rapidly in the past few years, sepsis incidence and mortality in clinical treatment is still climbing. Moreover, because of the diverse manifestations of sepsis, clinicians continue to face severe challenges in the diagnosis, treatment, and management of patients with sepsis. Here, we review the recent development in our understanding regarding the cellular pathogenesis and the target of clinical diagnosis of sepsis, with the goal of enhancing the current understanding of sepsis. The present state of research on targeted therapeutic drugs is also elaborated upon to provide information for the treatment of sepsis.

Protective and Pathogenic Effects of Interferon Signaling During Pregnancy

Immune regulation at the maternal-fetal interface is complex due to conflicting immunological objectives: protection of the fetus from maternal pathogens and prevention of immune-mediated rejection of the semiallogeneic fetus and placenta. Interferon (IFN) signaling plays an important role in restricting congenital infections as well as in the physiology of healthy pregnancies. In this review, we discuss the antiviral and pathogenic effects of type I IFN (IFN-α, IFN-β), type II IFN (IFN-γ), and type III IFN (IFN-λ) during pregnancy, with an emphasis on mouse and non-human primate models of congenital Zika virus infection. In the context of these animal model systems, we examine the role of IFN signaling during healthy pregnancy. Finally, we review mechanisms by which dysregulated type I IFN responses contribute to poor pregnancy outcomes in humans with autoimmune disease, including interferonopathies and systemic lupus erythematosus.

STING is required for host defense against neuropathological West Nile virus infection

West Nile Virus (WNV), an emerging and re-emerging RNA virus, is the leading source of arboviral encephalitic morbidity and mortality in the United States. WNV infections are acutely controlled by innate immunity in peripheral tissues outside of the central nervous system (CNS) but WNV can evade the actions of interferon (IFN) to facilitate CNS invasion, causing encephalitis, encephalomyelitis, and death. Recent studies indicate that STimulator of INterferon Gene (STING), canonically known for initiating a type I IFN production and innate immune response to cytosolic DNA, is required for host defense against neurotropic RNA viruses. We evaluated the role of STING in host defense to control WNV infection and pathology in a murine model of infection. When challenged with WNV, STING knock out (-/-) mice displayed increased morbidity and mortality compared to wild type (WT) mice. Virologic analysis and assessment of STING activation revealed that STING signaling was not required for control of WNV in the spleen nor was WNV sufficient to mediate canonical STING activation in vitro. However, STING-/- mice exhibited a clear trend of increased viral load and virus dissemination in the CNS. We found that STING-/- mice exhibited increased and prolonged neurological signs compared to WT mice. Pathological examination revealed increased lesions, mononuclear cellular infiltration and neuronal death in the CNS of STING-/- mice, with sustained pathology after viral clearance. We found that STING was required in bone marrow derived macrophages for early control of WNV replication and innate immune activation. In vivo, STING-/- mice developed an aberrant T cell response in both the spleen and brain during WNV infection that linked with increased and sustained CNS pathology compared to WT mice. Our findings demonstrate that STING plays a critical role in immune programming for the control of neurotropic WNV infection and CNS disease.

Dendritic cell-associated MAVS is required to control West Nile virus replication and ensuing humoral immune responses

Mitochondrial antiviral signaling protein (MAVS) is a critical innate immune signaling protein that directs the actions of the RIG-I-like receptor (RLR) signaling pathway of RNA virus recognition and initiation of anti-viral immunity against West Nile virus (WNV). In the absence of MAVS, mice die more rapidly after infection with the pathogenic WNV-Texas (TX) strain, but also produce elevated WNV-specific IgG concomitant with increased viral burden. Here we investigated whether there was a B cell intrinsic role for MAVS during the development of protective humoral immunity following WNV infection. MAVS-/- mice survived infection from the non-pathogenic WNV-Madagascar (MAD) strain, with limited signs of disease. Compared to wildtype (WT) controls, WNV-MAD-infected MAVS-/- mice had elevated serum neutralizing antibodies, splenic germinal center B cells, plasma cells and effector T cells. We found that when rechallenged with the normally lethal WNV-TX, MAVS-/- mice previously infected with WNV-MAD were protected from disease. Thus, protective humoral and cellular immune responses can be generated in absence of MAVS. Mice with a conditional deletion of MAVS only in CD11c+ dendritic cells phenocopied MAVS whole body knockout mice in their humoral responses to WNV-MAD, displaying elevated virus titers and neutralizing antibodies. Conversely, a B cell-specific deletion of MAVS had no effect on immune responses to WNV-MAD compared to WT controls. Thus, MAVS in dendritic cells is required to control WNV replication and thereby regulate downstream humoral immune responses.

Therapeutic efficacy of favipiravir against Bourbon virus in mice

Bourbon virus (BRBV) is an emerging tick-borne RNA virus in the orthomyxoviridae family that was discovered in 2014. Although fatal human cases of BRBV have been described, little is known about its pathogenesis, and no antiviral therapies or vaccines exist. We obtained serum from a fatal case in 2017 and successfully recovered the second human infectious isolate of BRBV. Next-generation sequencing of the St. Louis isolate of BRBV (BRBV-STL) showed >99% nucleotide identity to the original reference isolate. Using BRBV-STL, we developed a small animal model to study BRBV-STL tropism in vivo and evaluated the prophylactic and therapeutic efficacy of the experimental antiviral drug favipiravir against BRBV-induced disease. Infection of Ifnar1^-/- mice lacking the type I interferon receptor, but not congenic wild-type animals, resulted in uniformly fatal disease 6 to 10 days after infection. RNA in situ hybridization and viral yield assays demonstrated a broad tropism of BRBV-STL with highest levels detected in liver and spleen. In vitro replication and polymerase activity of BRBV-STL were inhibited by favipiravir. Moreover, administration of favipiravir as a prophylaxis or as post-exposure therapy three days after infection prevented BRBV-STL-induced mortality in immunocompromised Ifnar1^-/- mice. These results suggest that favipiravir may be a candidate treatment for humans who become infected with BRBV.

Bourbon virus (BRBV) is a novel tick-borne RNA virus that can cause fatal disease in humans. No approved antiviral treatment is available. We have cultured the second human isolate of BRBV and with it developed a small animal disease model. In this mouse model, BRBV causes severe disease as measured by weight loss after infection and uniform death 6 to 10 days after infection. Virus replication occurred predominantly in the spleen and the liver of the infected animals, with additional organs infected at later time points after infection. This disease model was used to test the efficacy of favipiravir, a viral RNA polymerase inhibitor that was developed for the related Influenza A virus. Prophylactic and therapeutic treatment with favipiravir resulted in complete protection from a lethal BRBV infection. These data suggest that favipiravir and perhaps other RNA polymerase inhibitors could be used to treat BRBV infections in humans.

West Nile Virus-Inclusive Single-Cell RNA Sequencing Reveals Heterogeneity in the Type I Interferon Response within Single Cells

West Nile virus (WNV) is a neurotropic mosquito-borne flavivirus of global importance. Neuroinvasive WNV infection results in encephalitis and can lead to prolonged neurological impairment or death. Type I interferon (IFN-I) is crucial for promoting antiviral defenses through the induction of antiviral effectors, which function to restrict viral replication and spread. However, our understanding of the antiviral response to WNV infection is mostly derived from analysis of bulk cell populations. It is becoming increasingly apparent that substantial heterogeneity in cellular processes exists among individual cells, even within a seemingly homogenous cell population. Here, we present WNV-inclusive single-cell RNA sequencing (scRNA-seq), an approach to examine the transcriptional variation and viral RNA burden across single cells. We observed that only a few cells within the bulk population displayed robust transcription of IFN-β mRNA, and this did not appear to depend on viral RNA abundance within the same cell. Furthermore, we observed considerable transcriptional heterogeneity in the IFN-I response, with genes displaying high unimodal and bimodal expression patterns. Broadly, IFN-stimulated genes negatively correlated with viral RNA abundance, corresponding with a precipitous decline in expression in cells with high viral RNA levels. Altogether, we demonstrated the feasibility and utility of WNV-inclusive scRNA-seq as a high-throughput technique for single-cell transcriptomics and WNV RNA detection. This approach can be implemented in other models to provide insights into the cellular features of protective immunity and identify novel therapeutic targets.IMPORTANCE West Nile virus (WNV) is a clinically relevant pathogen responsible for recurrent epidemics of neuroinvasive disease. Type I interferon is essential for promoting an antiviral response against WNV infection; however, it is unclear how heterogeneity in the antiviral response at the single-cell level impacts viral control. Specifically, conventional approaches lack the ability to distinguish differences across cells with varying viral abundance. The significance of our research is to demonstrate a new technique for studying WNV infection at the single-cell level. We discovered extensive variation in antiviral gene expression and viral abundance across cells. This protocol can be applied to primary cells or in vivo models to better understand the underlying cellular heterogeneity following WNV infection for the development of targeted therapeutic strategies.

Characterization of Pathogenic Sepsis Etiologies and Patient Profiles: A Novel Approach to Triage and Treatment

Pathogenic sepsis is not a monolithic condition. Three major types of sepsis exist within this category: bacterial, viral, and fungal, each with its own mechanism of action. While similar in symptoms, the etiologies and immune mechanisms of these types differ enough that a discrete patient base can be recognized for each one. Non-specific treatment, such as broad-spectrum antibiotics, without determination of sepsis origins may worsen sepsis symptoms and leads to increased morbidity and mortality in patients. However, recognition of current and historical patterns in likely patients for each sepsis type may aid in differentiation between pathogens prior to definitive blood testing. Clinicians may ultimately be able to diagnose and treat bacterial, viral, and fungal sepsis using analysis of previous patient patterns and circumstances in addition to standard care. This method is likely to decrease incidence of multidrug-resistant organisms, organ failure due to ineffective treatment, and turnaround time to the correct treatment for each sepsis patient. Ultimately, we aim to provide classification information on these patient populations and to suggest epidemiology-based screening methods that can be integrated into critical care medicine, specifically triage and treatment of sepsis.

Rotavirus VP3 targets MAVS for degradation to inhibit type III interferon expression in intestinal epithelial cells

Rotaviruses (RVs), a leading cause of severe diarrhea in young children and many mammalian species, have evolved multiple strategies to counteract the host innate immunity, specifically interferon (IFN) signaling through RV non-structural protein 1 (NSP1). However, whether RV structural components also subvert antiviral response remains under-studied. Here, we found that MAVS, critical for the host RNA sensing pathway upstream of IFN induction, is degraded by the RV RNA methyl- and guanylyl-transferase (VP3) in a host-range-restricted manner. Mechanistically, VP3 localizes to the mitochondria and mediates the phosphorylation of a previously unidentified SPLTSS motif within the MAVS proline-rich region, leading to its proteasomal degradation and blockade of IFN-λ production in RV-infected intestinal epithelial cells. Importantly, VP3 inhibition of MAVS activity contributes to enhanced RV replication and to viral pathogenesis in vivo. Collectively, our findings establish RV VP3 as a viral antagonist of MAVS function in mammals and uncover a novel pathogen-mediated inhibitory mechanism of MAVS signaling.

The Role of Nucleic Acid Sensing in Controlling Microbial and Autoimmune Disorders

Innate immunity, the first line of defense against invading pathogens, is an ancient form of host defense found in all animals, from sponges to humans. During infection, innate immune receptors recognize conserved molecular patterns, such as microbial surface molecules, metabolites produces during infection, or nucleic acids of the microbe's genome. When initiated, the innate immune response activates a host defense program that leads to the synthesis proteins capable of pathogen killing. In mammals, the induction of cytokines during the innate immune response leads to the recruitment of professional immune cells to the site of infection, leading to an adaptive immune response. While a fully functional innate immune response is crucial for a proper host response and curbing microbial infection, if the innate immune response is dysfunctional and is activated in the absence of infection, autoinflammation and autoimmune disorders can develop. Therefore, it follows that the innate immune response must be tightly controlled to avoid an autoimmune response from host-derived molecules, yet still unencumbered to respond to infection. In this review, we will focus on the innate immune response activated from cytosolic nucleic acids, derived from the microbe or host itself. We will depict how viruses and bacteria activate these nucleic acid sensing pathways and their mechanisms to inhibit the pathways. We will also describe the autoinflammatory and autoimmune disorders that develop when these pathways are hyperactive. Finally, we will discuss gaps in knowledge with regard to innate immune response failure and identify where further research is needed.

Splenic macrophages are required for protective innate immunity against West Nile virus

Although the spleen is a major site for West Nile virus (WNV) replication and spread, relatively little is known about which innate cells in the spleen replicate WNV, control viral dissemination, and/or prime innate and adaptive immune responses. Here we tested if splenic macrophages (MΦs) were necessary for control of WNV infection. We selectively depleted splenic MΦs, but not draining lymph node MΦs, by injecting mice intravenously with clodronate liposomes several days prior to infecting them with WNV. Mice missing splenic MΦs succumbed to WNV infection after an increased and accelerated spread of virus to the spleen and the brain. WNV-specific Ab and CTL responses were normal in splenic MΦ-depleted mice; however, numbers of NK cells and CD4 and CD8 T cells were significantly increased in the brains of infected mice. Splenic MΦ deficiency led to increased WNV in other splenic innate immune cells including CD11b- DCs, newly formed MΦs and monocytes. Unlike other splenic myeloid subsets, splenic MΦs express high levels of mRNAs encoding the complement protein C1q, the apoptotic cell clearance protein Mertk, the IL-18 cytokine and the FcγR1 receptor. Splenic MΦ-deficient mice may be highly susceptible to WNV infection in part to a deficiency in C1q, Mertk, IL-18 or Caspase 12 expression.

Distinct Effects of Type I and III Interferons on Enteric Viruses

Interferons (IFNs) are key host cytokines in the innate immune response to viral infection, and recent work has identified unique roles for IFN subtypes in regulating different aspects of infection. Currently emerging is a common theme that type III IFNs are critical in localized control of infection at mucosal barrier sites, while type I IFNs are important for broad systemic control of infections. The intestine is a particular site of interest for exploring these effects, as in addition to being the port of entry for a multitude of pathogens, it is a complex tissue with a variety of cell types as well as the presence of the intestinal microbiota. Here we focus on the roles of type I and III IFNs in control of enteric viruses, discussing what is known about signaling downstream from these cytokines, including induction of specific IFN-stimulated genes. We review viral strategies to evade IFN responses, effects of IFNs on the intestine, interactions between IFNs and the microbiota, and briefly discuss the role of IFNs in controlling viral infections at other barrier sites. Enhanced understanding of the coordinate roles of IFNs in control of viral infections may facilitate development of antiviral therapeutic strategies; here we highlight potential avenues for future exploration.

Innate Immune Basis for Rift Valley Fever Susceptibility in Mouse Models

Rift Valley fever virus (RVFV) leads to varied clinical manifestations in animals and in humans that range from moderate fever to fatal illness, suggesting that host immune responses are important determinants of the disease severity. We investigated the immune basis for the extreme susceptibility of MBT/Pas mice that die with mild to acute hepatitis by day 3 post-infection compared to more resistant BALB/cByJ mice that survive up to a week longer. Lower levels of neutrophils observed in the bone marrow and blood of infected MBT/Pas mice are unlikely to be causative of increased RVFV susceptibility as constitutive neutropenia in specific mutant mice did not change survival outcome. However, whereas MBT/Pas mice mounted an earlier inflammatory response accompanied by higher amounts of interferon (IFN)-α in the serum compared to BALB/cByJ mice, they failed to prevent high viral antigen load. Several immunological alterations were uncovered in infected MBT/Pas mice compared to BALB/cByJ mice, including low levels of leukocytes that expressed type I IFN receptor subunit 1 (IFNAR1) in the blood, spleen and liver, delayed leukocyte activation and decreased percentage of IFN-γ-producing leukocytes in the blood. These observations are consistent with the complex mode of inheritance of RVFV susceptibility in genetic studies.

Regional astrocyte IFN signaling restricts pathogenesis during neurotropic viral infection

Type I IFNs promote cellular responses to viruses, and IFN receptor (IFNAR) signaling regulates the responses of endothelial cells of the blood-brain barrier (BBB) during neurotropic viral infection. However, the role of astrocytes in innate immune responses of the BBB during viral infection of the CNS remains to be fully elucidated. Here, we have demonstrated that type I IFNAR signaling in astrocytes regulates BBB permeability and protects the cerebellum from infection and immunopathology. Mice with astrocyte-specific loss of IFNAR signaling showed decreased survival after West Nile virus infection. Accelerated mortality was not due to expanded viral tropism or increased replication. Rather, viral entry increased specifically in the hindbrain of IFNAR-deficient mice, suggesting that IFNAR signaling critically regulates BBB permeability in this brain region. Pattern recognition receptors and IFN-stimulated genes had higher basal and IFN-induced expression in human and mouse cerebellar astrocytes than did cerebral cortical astrocytes, suggesting that IFNAR signaling has brain region-specific roles in CNS immune responses. Taken together, our data identify cerebellar astrocytes as key responders to viral infection and highlight the existence of distinct innate immune programs in astrocytes from evolutionarily disparate regions of the CNS.

The Complement System in Flavivirus Infections

The incidence of flavivirus infections has increased dramatically in recent decades in tropical and sub-tropical climates worldwide, affecting hundreds of millions of people each year. The Flaviviridae family includes dengue, West Nile, Zika, Japanese encephalitis, and yellow fever viruses that are typically transmitted by mosquitoes or ticks, and cause a wide range of symptoms, such as fever, shock, meningitis, paralysis, birth defects, and death. The flavivirus genome is composed of a single positive-sense RNA molecule encoding a single viral polyprotein. This polyprotein is further processed by viral and host proteases into three structural proteins (C, prM/M, E) and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5) that are involved in viral replication and pathogenicity. The complement system has been described to play an important role in flavivirus infection either by protecting the host and/or by influencing disease pathogenesis. In this mini-review, we will explore the role of complement system inhibition and/or activation against infection by the Flavivirus genus, with an emphasis on dengue and West Nile viruses.

Zika Virus Antagonizes Type I Interferon Responses during Infection of Human Dendritic Cells

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus that is causally linked to severe neonatal birth defects, including microcephaly, and is associated with Guillain-Barre syndrome in adults. Dendritic cells (DCs) are an important cell type during infection by multiple mosquito-borne flaviviruses, including dengue virus, West Nile virus, Japanese encephalitis virus, and yellow fever virus. Despite this, the interplay between ZIKV and DCs remains poorly defined. Here, we found human DCs supported productive infection by a contemporary Puerto Rican isolate with considerable variability in viral replication, but not viral binding, between DCs from different donors. Historic isolates from Africa and Asia also infected DCs with distinct viral replication kinetics between strains. African lineage viruses displayed more rapid replication kinetics and infection magnitude as compared to Asian lineage viruses, and uniquely induced cell death. Infection of DCs with both contemporary and historic ZIKV isolates led to minimal up-regulation of T cell co-stimulatory and MHC molecules, along with limited secretion of inflammatory cytokines. Inhibition of type I interferon (IFN) protein translation was observed during ZIKV infection, despite strong induction at the RNA transcript level and up-regulation of other host antiviral proteins. Treatment of human DCs with RIG-I agonist potently restricted ZIKV replication, while type I IFN had only modest effects. Mechanistically, we found all strains of ZIKV antagonized type I IFN-mediated phosphorylation of STAT1 and STAT2. Combined, our findings show that ZIKV subverts DC immunogenicity during infection, in part through evasion of type I IFN responses, but that the RLR signaling pathway is still capable of inducing an antiviral state, and therefore may serve as an antiviral therapeutic target.

Encephalitic Arboviruses: Emergence, Clinical Presentation, and Neuropathogenesis

Arboviruses are arthropod-borne viruses that exhibit worldwide distribution, contributing to systemic and neurologic infections in a variety of geographical locations. Arboviruses are transmitted to vertebral hosts during blood feedings by mosquitoes, ticks, biting flies, mites, and nits. While the majority of arboviral infections do not lead to neuroinvasive forms of disease, they are among the most severe infectious risks to the health of the human central nervous system. The neurologic diseases caused by arboviruses include meningitis, encephalitis, myelitis, encephalomyelitis, neuritis, and myositis in which virus- and immune-mediated injury may lead to severe, persisting neurologic deficits or death. Here we will review the major families of emerging arboviruses that cause neurologic infections, their neuropathogenesis and host neuroimmunologic responses, and current strategies for treatment and prevention of neurologic infections they cause.

A Mouse Model of Zika Virus Pathogenesis

The ongoing Zika virus (ZIKV) epidemic and unexpected clinical outcomes, including Guillain-Barré syndrome and birth defects, has brought an urgent need for animal models. We evaluated infection and pathogenesis with contemporary and historical ZIKV strains in immunocompetent mice and mice lacking components of the antiviral response. Four- to six-week-old Irf3(-/-)Irf5(-/-)Irf7(-/-) triple knockout mice, which produce little interferon α/β, and mice lacking the interferon receptor (Ifnar1(-/-)) developed neurological disease and succumbed to ZIKV infection, whereas single Irf3(-/-), Irf5(-/-), and Mavs(-/-) knockout mice exhibited no overt illness. Ifnar1(-/-) mice sustained high viral loads in the brain and spinal cord, consistent with evidence that ZIKV causes neurodevelopmental defects in human fetuses. The testes of Ifnar1(-/-) mice had the highest viral loads, which is relevant to sexual transmission of ZIKV. This model of ZIKV pathogenesis will be valuable for evaluating vaccines and therapeutics as well as understanding disease pathogenesis.

Type I interferons in viral control and immune regulation

Type 1 interferons (IFN-I) exert pleiotropic biological effects during viral infections, all which contribute to balancing virus control and immune pathology. Despite extensive antiviral functions that subdue virus replication, recent studies demonstrate pathogenic and pro-viral roles for IFN-I signaling during acute and persistent virus infection. IFN-I signaling can promote morbidity and mortality through induction of aberrant inflammatory responses during acute viral infection. In contrast, IFN-I signaling during persistent viral infection supports immune suppression, lymphoid tissue disorganization and CD4 T cell dysfunction. Systematic characterization of the cellular populations and intricacies of IFN-I signaling that promote pathology or immune suppression during acute and persistent viral infections, respectively, should inform the development of treatments and modalities to control viral associated pathologies.