CD8+ T cells require perforin to clear West Nile virus from infected neurons

NK cell profiling in West Nile virus encephalitis reveals potential metabolic basis for functional inhibition

Natural killer (NK) cells are cytotoxic lymphocytes important for viral defense. West Nile virus (WNV) infection of the central nervous system (CNS) causes marked recruitment of bone marrow (BM)-derived monocytes, T cells and NK cells, resulting in severe neuroinflammation and brain damage. Despite substantial numbers of NK cells in the CNS, their function and phenotype remain largely unexplored. Here, we demonstrate that NK cells mature from the BM to the brain, upregulate inhibitory receptors and show reduced cytokine production and degranulation, likely due to the increased expression of the inhibitory NK cell molecule, MHC-I. Intriguingly, this correlated with a reduction in metabolism associated with cytotoxicity in brain-infiltrating NK cells. Importantly, the degranulation and killing capability were restored in NK cells isolated from WNV-infected tissue, suggesting that WNV-induced NK cell inhibition occurs in the CNS. Overall, this work identifies a potential link between MHC-I inhibition of NK cells and metabolic reduction of their cytotoxicity during infection.

Antigen non-specific CD8+ T cells accelerate cognitive decline in aged mice following respiratory coronavirus infection

Primarily a respiratory infection, numerous patients infected with SARS-CoV-2 present with neurologic symptoms, some continuing long after viral clearance as a persistent symptomatic phase termed "long COVID". Advanced age increases the risk of severe disease, as well as incidence of long COVID. We hypothesized that perturbations in the aged immune response predispose elderly individuals to severe coronavirus infection and post-infectious sequelae. Using a murine model of respiratory coronavirus, mouse hepatitis virus strain A59 (MHV-A59), we found that aging increased clinical illness and lethality to MHV infection, with aged animals harboring increased virus in the brain during acute infection. This was coupled with an unexpected increase in activated CD8+ T cells within the brains of aged animals but reduced antigen specificity of those CD8+ T cells. Aged animals demonstrated spatial learning impairment following MHV infection, which correlated with increased neuronal cell death and reduced neuronal regeneration in aged hippocampus. Using primary cell culture, we demonstrated that activated CD8+ T cells induce neuronal death, independent of antigen-specificity. Specifically, higher levels of CD8+ T cell-derived IFN-γ correlated with neuronal death. These results support the evidence that CD8+ T cells in the brain directly contribute to cognitive dysfunction following coronavirus infection in aged individuals.

Regulation and distribution of European sea bass perforins point to their role in the adaptive cytotoxic response against NNV

Cell-mediated cytotoxicity is a complex immune mechanism that involves the release of several killing molecules, being perforin (PRF) one of the most important effector players. Perforin is synthesized by T lymphocytes and natural killer cells in mammals and responsible for the formation of pores on the target cell membrane during the killing process. Although perforin has been extensively studied in higher vertebrates, this knowledge is very limited in fish. Therefore, in this study we have identified four prf genes in European sea bass (Dicentrarchus labrax) and evaluated their mRNA levels. All sea bass prf genes showed the typical and conserved domains of its human orthologue and were closely clustered by the phylogenetic analysis. In addition, all genes showed constitutive and ubiquitous tissular expression, being prf1.9 gene the most highly expressed in immune tissues. Subsequently, in vitro stimulation of head-kidney (HK) cells with phytohemagglutinin, a T-cell activator, showed an increase of all prf gene levels, except for prf1.3 gene. European sea bass HK cells increased the transcription of prf1.2 and prf1.9 during the innate cell-mediated cytotoxic activity against xenogeneic target cells. In addition, sea bass infected with nodavirus (NNV) showed a similar expression pattern of all prf in HK and brain at 15 days post-infection, except for prf1.3 gene and in the gonad. Finally, the use of a polyclonal antibody against PRF1.9 showed an increase of positive cells in HK, brain and gonad from NNV-infected fish. Taken together, the data seem to indicate that all prf genes, except prf1.3, appear to be involved in the European sea bass immunity, and probably in the cell-mediated cytotoxic response, with PRF1.9 playing the most important role against nodavirus. The involvement of the PRFs and the CMC activity in the vertical transmission success of the virus is also discussed.

A Pocket Guide to CCR5-Neurotropic Flavivirus Edition

CCR5 is among the most studied chemokine receptors due to its profound significance in human health and disease. The notion that CCR5 is a functionally redundant receptor was challenged through the demonstration of its unique protective role in the context of West Nile virus in both mice and humans. In the nearly two decades since this initial discovery, numerous studies have investigated the role of CCR5 in the context of other medically important neurotropic flaviviruses, most of which appear to support a broad neuroprotective role for this receptor, although how CCR5 exerts its protective effect has been remarkably varied. In this review, we summarize the mechanisms by which CCR5 controls neurotropic flaviviruses, as well as results from human studies evaluating a genetic link to CCR5, and propose unexplored areas of research that are needed to unveil even more exciting roles for this important receptor.

Molecular and Cellular Mechanisms Underlying Neurologic Manifestations of Mosquito-Borne Flavivirus Infections

Flaviviruses are a family of enveloped viruses with a positive-sense RNA genome, transmitted by arthropod vectors. These viruses are known for their broad cellular tropism leading to infection of multiple body systems, which can include the central nervous system. Neurologic effects of flavivirus infection can arise during both acute and post-acute infectious periods; however, the molecular and cellular mechanisms underlying post-acute sequelae are not fully understood. Here, we review recent studies that have examined molecular and cellular mechanisms that may contribute to neurologic sequelae following infection with the West Nile virus, Japanese encephalitis virus, Zika virus, dengue virus, and St. Louis encephalitis virus. Neuronal death, either from direct infection or due to the resultant inflammatory response, is a common mechanism by which flavivirus infection can lead to neurologic impairment. Other types of cellular damage, such as oxidative stress and DNA damage, appear to be more specific to certain viruses. This article aims to highlight mechanisms of cellular damage that are common across several flavivirus members and mechanisms that are more unique to specific members. Our goal is to inspire further research to improve understanding of this area in the hope of identifying treatment options for flavivirus-associated neurologic changes.

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.

NK cell-derived extracellular granzyme B drives epithelial ulceration during HSV-2 genital infection

Genital herpes is characterized by recurrent episodes of epithelial blistering. The mechanisms causing this pathology are ill defined. Using a mouse model of vaginal herpes simplex virus 2 (HSV-2) infection, we show that interleukin-18 (IL-18) acts upon natural killer (NK) cells to promote accumulation of the serine protease granzyme B in the vagina, coinciding with vaginal epithelial ulceration. Genetic loss of granzyme B or therapeutic inhibition by a specific protease inhibitor reduces disease and restores epithelial integrity without altering viral control. Distinct effects of granzyme B and perforin deficiency on pathology indicates that granzyme B acts independent of its classic cytotoxic role. IL-18 and granzyme B are markedly elevated in human herpetic ulcers compared with non-herpetic ulcers, suggesting engagement of these pathways in HSV-infected patients. Our study reveals a role for granzyme B in destructing mucosal epithelium during HSV-2 infection, identifying a therapeutic target to augment treatment of genital herpes.

Potential of Interleukin (IL)-12 Group as Antivirals: Severe Viral Disease Prevention and Management

The interleukin (IL)-12 family consists of pro- and anti-inflammatory cytokines that are able to signal the activation of host antiviral immunity while preventing over-reactive immune reactions due to active virus replication and viral clearance. Amongst others, IL-12 and IL-23 are produced and released by innate immune cells such as monocytes and macrophages to signal the proliferation of T cells and release of effector cytokines, which subsequently activate host defence against virus infections. Interestingly, the dualities of IL-27 and -35 are evidently shown in the course of virus infections; they regulate the synthesis of cytokines and antiviral molecules, proliferation of T cells, and viral antigen presentation in order to maximize virus clearance by the host immune system. In terms of anti-inflammatory reactions, IL-27 signals the formation of regulatory T cells (Treg) which in turn secrete IL-35 to control the scale of inflammatory response that takes place during virus infections. Given the multitasking of the IL-12 family in regards to the elimination of virus infections, its potential in antiviral therapy is unequivocally important. Thus, this work aims to delve deeper into the antiviral actions of the IL-12 family and their applications in antiviral therapies.

Single-cell RNA transcriptome analysis of CNS immune cells reveals CXCL16/CXCR6 as maintenance factors for tissue-resident T cells that drive synapse elimination

Background

Emerging RNA viruses that target the central nervous system (CNS) lead to cognitive sequelae in survivors. Studies in humans and mice infected with West Nile virus (WNV), a re-emerging RNA virus associated with learning and memory deficits, revealed microglial-mediated synapse elimination within the hippocampus. Moreover, CNS-resident memory T (T_RM) cells activate microglia, limiting synapse recovery and inducing spatial learning defects in WNV-recovered mice. The signals involved in T cell-microglia interactions are unknown.

Methods

Here, we examined immune cells within the murine WNV-recovered forebrain using single-cell RNA sequencing to identify putative ligand-receptor pairs involved in intercellular communication between T cells and microglia. Clustering and differential gene analyses were followed by protein validation and genetic and antibody-based approaches utilizing an established murine model of WNV recovery in which microglia and complement promote ongoing hippocampal synaptic loss.

Results

Profiling of host transcriptome immune cells at 25 days post-infection in mice revealed a shift in forebrain homeostatic microglia to activated subpopulations with transcriptional signatures that have previously been observed in studies of neurodegenerative diseases. Importantly, CXCL16/CXCR6, a chemokine signaling pathway involved in T_RM cell biology, was identified as critically regulating CXCR6 expressing CD8⁺ T_RM cell numbers within the WNV-recovered forebrain. We demonstrate that CXCL16 is highly expressed by all myeloid cells, and its unique receptor, CXCR6, is highly expressed on all CD8⁺ T cells. Using genetic and pharmacological approaches, we demonstrate that CXCL16/CXCR6 not only is required for the maintenance of WNV-specific CD8 T_RM cells in the post-infectious CNS, but also contributes to their expression of T_RM cell markers. Moreover, CXCR6⁺CD8⁺ T cells are required for glial activation and ongoing synapse elimination.

Conclusions

We provide a comprehensive assessment of the role of CXCL16/CXCR6 as an interaction link between microglia and CD8⁺ T cells that maintains forebrain T_RM cells, microglial and astrocyte activation, and ongoing synapse elimination in virally recovered animals. We also show that therapeutic targeting of CXCL16 in mice during recovery may reduce CNS CD8⁺ T_RM cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13073-022-01111-0.

The role of antiviral CD8+ T cells in cognitive impairment

The impact of the immune system on the etiopathogenesis of neurodegenerative diseases, including Alzheimer's disease, is a rapidly growing area of investigation. Evidence from human patients and animal models implicates neurotropic viral infections, and specifically the antiviral immune response of brain-infiltrating CD8⁺ T cells, as potential drivers of disease pathology. While infiltration and retention of CD8⁺ T cells within the brain following viral infection is associated with improved survival, CD8⁺ T cells also contribute to neuronal death and gliosis which underlie cognitive impairment in several disease models. Here we review the role of antiviral CD8⁺ T cells as potential mediators of cognitive impairment and highlight the mechanisms by which brain-resident CD8⁺ T cells may contribute to neurodegenerative disease pathology.

Lifelong cytomegalovirus and early-LIFE irradiation synergistically potentiate age-related defects in response to vaccination and infection

While whole-body irradiation (WBI) can induce some hallmarks of immune aging, (re)activation of persistent microbial infection also occurs following WBI and may contribute to immune effects of WBI over the lifespan. To test this hypothesis in a model relevant to human immune aging, we examined separate and joint effects of lifelong latent murine cytomegalovirus (MCMV) and of early-life WBI over the course of the lifespan. In late life, we then measured the response to a West Nile virus (WNV) live attenuated vaccine, and lethal WNV challenge subsequent to vaccination. We recently published that a single dose of non-lethal WBI in youth, on its own, was not sufficient to accelerate aging of the murine immune system, despite widespread DNA damage and repopulation stress in hematopoietic cells. However, 4Gy sub-lethal WBI caused manifest reactivation of MCMV. Following vaccination and challenge with WNV in the old age, MCMV-infected animals experiencing 4Gy, but not lower, dose of sub-lethal WBI in youth had reduced survival. By contrast, old irradiated mice lacking MCMV and MCMV-infected, but not irradiated, mice were both protected to the same high level as the old non-irradiated, uninfected controls. Analysis of the quality and quantity of anti-WNV immunity showed that higher mortality in MCMV-positive WBI mice correlated with increased levels of MCMV-specific immune activation during WNV challenge. Moreover, we demonstrate that infection, including that by WNV, led to MCMV reactivation. Our data suggest that MCMV reactivation may be an important determinant of increased late-life mortality following early-life irradiation and late-life acute infection.

MAVS mediates a protective immune response in the brain to Rift Valley fever virus

Rift Valley fever virus (RVFV) is a highly pathogenic mosquito-borne virus capable of causing hepatitis, encephalitis, blindness, hemorrhagic syndrome, and death in humans and livestock. Upon aerosol infection with RVFV, the brain is a major site of viral replication and tissue damage, yet pathogenesis in this organ has been understudied. Here, we investigated the immune response in the brain of RVFV infected mice. In response to infection, microglia initiated robust transcriptional upregulation of antiviral immune genes, as well as increased levels of activation markers and cytokine secretion that is dependent on mitochondrial antiviral-signaling protein (MAVS) and independent of toll-like receptors 3 and 7. In vivo, Mavs^-/- mice displayed enhanced susceptibility to RVFV as determined by increased brain viral burden and higher mortality. Single-cell RNA sequence analysis identified defects in type I interferon and interferon responsive gene expression within microglia in Mavs^-/- mice, as well as dysregulated lymphocyte infiltration. The results of this study provide a crucial step towards understanding the precise molecular mechanisms by which RVFV infection is controlled in the brain and will help inform the development of vaccines and antiviral therapies that are effective in preventing encephalitis.

Rift Valley fever virus causes severe disease in humans and livestock and in some cases can be fatal. There is concern about the use of Rift Valley fever virus as a bioweapon since it can be transmitted through the air, and there are no vaccines or antiviral treatments. Airborne transmission of the virus causes severe inflammation of the brain, yet little is known about the immune response against the virus in this organ. Here, we investigated the immune response in the brain to Rift Valley fever virus following intranasal infection. We determined that microglia, the resident immune cells of the brain, initiate a robust response to Rift Valley fever virus infection and identified a key immune pathway that is critical for the ability of microglia to respond to infection. When this immune pathway is rendered non-functional, mice have a dysregulated response to infection in the brain. This study provides insight into how the immune response can control Rift Valley fever virus infection of the brain.

West Nile Virus Vaccination Protects against Usutu Virus Disease in Mice

West Nile virus (WNV) and Usutu virus (USUV) are mosquito-borne flaviviruses that can cause neuroinvasive disease in humans. WNV and USUV circulate in both Africa and Europe and are closely related. Due to antigenic similarity, WNV-specific antibodies and USUV-specific antibodies have the potential to bind heterologous viruses; however, it is unclear whether this interaction may offer protection against infection. To investigate how prior WNV exposure would influence USUV infection, we used an attenuated WNV vaccine that contains the surface proteins of WNV in the backbone of a dengue virus 2 vaccine strain and protects against WNV disease. We hypothesized that vaccination with this attenuated WNV vaccine would protect against USUV infection. Neutralizing responses against WNV and USUV were measured in vitro using sera following vaccination. Sera from vaccinated CD-1 and Ifnar1-/- mice cross-neutralized with WNV and USUV. All mice were then subsequently challenged with an African or European USUV strain. In CD-1 mice, there was no difference in USUV titers between vaccinated and mock-vaccinated mice. However, in the Ifnar1-/- model, vaccinated mice had significantly higher survival rates and significantly lower USUV viremia compared to mock-vaccinated mice. Our results indicate that exposure to an attenuated form of WNV protects against severe USUV disease in mice and elicits a neutralizing response to both WNV and USUV. Future studies will investigate the immune mechanisms responsible for the protection against USUV infection induced by WNV vaccination, providing critical insight that will be essential for USUV and WNV vaccine development.

Flavivirus Persistence in Wildlife Populations

A substantial number of humans are at risk for infection by vector-borne flaviviruses, resulting in considerable morbidity and mortality worldwide. These viruses also infect wildlife at a considerable rate, persistently cycling between ticks/mosquitoes and small mammals and reptiles and non-human primates and humans. Substantially increasing evidence of viral persistence in wildlife continues to be reported. In addition to in humans, viral persistence has been shown to establish in mammalian, reptile, arachnid, and mosquito systems, as well as insect cell lines. Although a considerable amount of research has centered on the potential roles of defective virus particles, autophagy and/or apoptosis-induced evasion of the immune response, and the precise mechanism of these features in flavivirus persistence have yet to be elucidated. In this review, we present findings that aid in understanding how vector-borne flavivirus persistence is established in wildlife. Research studies to be discussed include determining the critical roles universal flavivirus non-structural proteins played in flaviviral persistence, the advancement of animal models of viral persistence, and studying host factors that allow vector-borne flavivirus replication without destructive effects on infected cells. These findings underscore the viral–host relationships in wildlife animals and could be used to elucidate the underlying mechanisms responsible for the establishment of viral persistence in these animals.

Microglia and Perivascular Macrophages Act as Antigen Presenting Cells to Promote CD8 T Cell Infiltration of the Brain

CD8 T cell infiltration of the central nervous system (CNS) is necessary for host protection but contributes to neuropathology. Antigen presenting cells (APCs) situated at CNS borders are thought to mediate T cell entry into the parenchyma during neuroinflammation. The identity of the CNS-resident APC that presents antigen via major histocompatibility complex (MHC) class I to CD8 T cells is unknown. Herein, we characterize MHC class I expression in the naïve and virally infected brain and identify microglia and macrophages (CNS-myeloid cells) as APCs that upregulate H-2Kb and H-2Db upon infection. Conditional ablation of H-2Kb and H-2Db from CNS-myeloid cells allowed us to determine that antigen presentation via H-2Db, but not H-2Kb, was required for CNS immune infiltration during Theiler's murine encephalomyelitis virus (TMEV) infection and drives brain atrophy as a consequence of infection. These results demonstrate that CNS-myeloid cells are key APCs mediating CD8 T cell brain infiltration.

Zika virus infection during pregnancy protects against secondary infection in the absence of CD8+ cells

While T cell immunity is an important component of the immune response to Zika virus (ZIKV) infection generally, the efficacy of these responses during pregnancy remains unknown. Here, we tested the capacity of CD8 lymphocytes to protect from secondary challenge in four macaques, two of which were depleted of CD8+ cells prior to rechallenge with a heterologous ZIKV isolate. The initial challenge during pregnancy produced transcriptional signatures suggesting complex patterns of immune modulation as well as neutralizing antibodies that persisted until rechallenge, which all animals efficiently controlled, demonstrating that the primary infection conferred adequate protection. The secondary challenge promoted activation of innate and adaptive immune cells, possibly suggesting a brief period of infection prior to clearance. These data confirm that ZIKV infection during pregnancy induces sufficient immunity to protect from a secondary challenge and suggest that this protection is not dependent on CD8 T cells.

CARD9 Deficiency Increases Hippocampal Injury Following Acute Neurotropic Picornavirus Infection but Does Not Affect Pathogen Elimination

Neurotropic viruses target the brain and contribute to neurologic diseases. Caspase recruitment domain containing family member 9 (CARD9) controls protective immunity in a variety of infectious disorders. To investigate the effect of CARD9 in neurotropic virus infection, CARD9^−/− and corresponding C57BL/6 wild-type control mice were infected with Theiler’s murine encephalomyelitis virus (TMEV). Brain tissue was analyzed by histology, immunohistochemistry and molecular analyses, and spleens by flow cytometry. To determine the impact of CARD9 deficiency on T cell responses in vitro, antigen presentation assays were utilized. Genetic ablation of CARD9 enhanced early pro-inflammatory cytokine responses and accelerated infiltration of T and B cells in the brain, together with a transient increase in TMEV-infected cells in the hippocampus. CARD9^−/− mice showed an increased loss of neuronal nuclear protein⁺ mature neurons and doublecortin⁺ neuronal precursor cells and an increase in β-amyloid precursor protein⁺ damaged axons in the hippocampus. No effect of CARD9 deficiency was found on the initiation of CD8⁺ T cell responses by flow cytometry and co-culture experiments using virus-exposed dendritic cells or microglia-enriched glial cell mixtures, respectively. The present study indicates that CARD9 is dispensable for the initiation of early antiviral responses and TMEV elimination but may contribute to the modulation of neuroinflammation, thereby reducing hippocampal injury following neurotropic virus infection.

MyD88 signaling by neurons induces chemokines that recruit protective leukocytes to the virus-infected CNS

Viral encephalitis initiates a series of immunological events in the brain that can lead to brain damage and death. Astrocytes express IFN-β in response to neurotropic infection, whereas activated microglia produce proinflammatory cytokines and accumulate at sites of infection. Here, we observed that neurotropic vesicular stomatitis virus (VSV) infection causes recruitment of leukocytes into the central nervous system (CNS), which requires MyD88, an adaptor of Toll-like receptor and interleukin-1 receptor signaling. Infiltrating leukocytes, and in particular CD8⁺ T cells, protected against lethal VSV infection of the CNS. Reconstitution of MyD88, specifically in neurons, restored chemokine production in the olfactory bulb as well as leukocyte recruitment into the infected CNS and enhanced survival. Comparative analysis of the translatome of neurons and astrocytes verified neurons as the critical source of chemokines, which regulated leukocyte infiltration of the infected brain and affected survival.

Immunoinformatics and molecular dynamics approaches: Next generation vaccine design against West Nile virus

West Nile Virus (WNV) is a life threatening flavivirus that causes significant morbidity and mortality worldwide. No preventive therapeutics including vaccines against WNV are available for human use. In this study, immunoinformatics approach was performed to design a multi epitope-based subunit vaccine against this deadly pathogen. Human (HLA) and Mice (H-2) allele specific potential T-cell and B-cell epitopes were shortlisted through a stringent procedure. Molecular docking showed selected epitopes that have stronger binding affinity with human TLR-4. Molecular dynamics simulation confirmed the stable nature of the docked complex. Furthermore, in silico cloning analysis ensures efficient expression of desired gene in the microbial system. Interestingly, previous studies showed that two of our selected epitopes have strong immune response against WNV. Therefore, selected epitopes could be strong vaccine candidates to prevent WNV infections in human. However, further in vitro and in vivo investigations could be strengthening the validation of the vaccine candidate against WNV.

Complex Roles of Neutrophils during Arboviral Infections

Arboviruses are known to cause large-scale epidemics in many parts of the world. These arthropod-borne viruses are a large group consisting of viruses from a wide range of families. The ability of their vector to enhance viral pathogenesis and transmission makes the development of treatments against these viruses challenging. Neutrophils are generally the first leukocytes to be recruited to a site of infection, playing a major role in regulating inflammation and, as a result, viral replication and dissemination. However, the underlying mechanisms through which neutrophils control the progression of inflammation and disease remain to be fully understood. In this review, we highlight the major findings from recent years regarding the role of neutrophils during arboviral infections. We discuss the complex nature of neutrophils in mediating not only protection, but also augmenting disease pathology. Better understanding of neutrophil pathways involved in effective protection against arboviral infections can help identify potential targets for therapeutics.

Correlation of Regulatory T Cell Numbers with Disease Tolerance upon Virus Infection

The goal of a successful immune response is to clear the pathogen while sparing host tissues from damage associated with pathogen replication and active immunity. Regulatory T cells (Treg) have been implicated in maintaining this balance as they contribute both to the organization of immune responses as well as restriction of inflammation and immune activation to limit immunopathology. To determine if Treg abundance prior to pathogen encounter can be used to predict the success of an antiviral immune response, we used genetically diverse mice from the collaborative cross infected with West Nile virus (WNV). We identified collaborative cross lines with extreme Treg abundance at steady state, either high or low, and used mice with these extreme phenotypes to demonstrate that baseline Treg quantity predicted the magnitude of the CD8 T cell response to WNV infection, although higher numbers of baseline Tregs were associated with reduced CD8 T cell functionality in terms of TNF and granzyme B expression. Finally, we found that abundance of CD44⁺ Tregs in the spleen at steady state was correlated with an increased early viral load within the spleen without an association with clinical disease. Thus, we propose that Tregs participate in disease tolerance in the context of WNV infection by tuning an appropriately focused and balanced immune response to control the virus while at the same time minimizing immunopathology and clinical disease. We hypothesize that Tregs limit the antiviral CD8 T cell function to curb immunopathology at the expense of early viral control as an overall host survival strategy.

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.

CD8 T cell-Derived Perforin and TNF-α Are Crucial Mediators of Neuronal Destruction in Experimental Autoimmune Enteric Ganglionitis

In neuron-specific ovalbumin-transgenic CKTAC mice, antigen-specific OT-I CD8 T cells home to the enteric nervous system, where they attack and destroy neurons of the myenteric and submucosal plexus. Clinically, experimental autoimmune enteric ganglionitis (EAEG) manifests with gastrointestinal dysmotility and rapidly progresses to lethal ileus. Although interferon-γ has been identified as capable of damaging neurons in EAEG, the role of perforin, Fas/FasL, and tumor necrosis factor-α (TNF-α) in this disease is still a matter of debate. Thus, CKTAC mice were adoptively transferred with either perforin^-/- or wild-type OT-I CD8 T cells. In addition, CKTAC mice that had received wild-type OT-I CD8 T cells were treated by either anti-TNF-α or anti-FasL. Furthermore, wild-type OT-I CD8 T cells were adoptively transferred into CKTAC mice with neuron-specific deletion of Fas. Although neither inactivation of enteric neuronal Fas nor anti-FasL treatment improved the disease, the absence of perforin from OT-I CD8 T cells and anti-TNF-α treatment significantly ameliorated EAEG and prevented lethal ileus by rescue of enteric neurons. Thus, these experiments identify perforin and TNF-α as important in the pathogenesis of EAEG.

Immunity to TBEV Related Flaviviruses with Reduced Pathogenicity Protects Mice from Disease but Not from TBEV Entry into the CNS

Tick-borne encephalitis virus (TBEV) is a leading cause of vector-borne viral encephalitis with expanding endemic regions across Europe. In this study we tested in mice the efficacy of preinfection with a closely related low-virulent flavivirus, Langat virus (LGTV strain TP21), or a naturally avirulent TBEV strain (TBEV-280) in providing protection against lethal infection with the highly virulent TBEV strain (referred to as TBEV-Hypr). We show that prior infection with TP21 or TBEV-280 is efficient in protecting mice from lethal TBEV-Hypr challenge. Histopathological analysis of brains from nonimmunized mice revealed neuronal TBEV infection and necrosis. Neuroinflammation, gliosis, and neuronal necrosis was however also observed in some of the TP21 and TBEV-280 preinfected mice although at reduced frequency as compared to the nonimmunized TBEV-Hypr infected mice. qPCR detected the presence of viral RNA in the CNS of both TP21 and TBEV-280 immunized mice after TBEV-Hypr challenge, but significantly reduced compared to mock-immunized mice. Our results indicate that although TBEV-Hypr infection is effectively controlled in the periphery upon immunization with low-virulent LGTV or naturally avirulent TBEV 280, it may still enter the CNS of these animals. These findings contribute to our understanding of causes for vaccine failure in individuals vaccinated with TBE vaccines.

New advances in immune components mediating viral control in the CNS

Protective immune responses in the central nervous system (CNS) must act efficiently but need to be tightly controlled to avoid excessive damage to this vital organ. Under homeostatic conditions, the immune surveillance of the CNS is mediated by innate immune cells together with subsets of memory lymphocytes accumulating over lifetime. Accordingly, a wide range of immune responses can be triggered upon pathogen infection that can be associated with devastating clinical outcomes, and which most frequently are due to neurotropic viruses. Here, we discuss recent advances about our understanding of anti-viral immune responses with special emphasis on mechanisms operating in the various anatomical compartments of the CNS.

T cell engagement of cross-presenting microglia protects the brain from a nasal virus infection

The neuroepithelium is a nasal barrier surface populated by olfactory sensory neurons that detect odorants in the airway and convey this information directly to the brain via axon fibers. This barrier surface is especially vulnerable to infection, yet respiratory infections rarely cause fatal encephalitis, suggesting a highly evolved immunological defense. Here, using a mouse model, we sought to understand the mechanism by which innate and adaptive immune cells thwart neuroinvasion by vesicular stomatitis virus (VSV), a potentially lethal virus that uses olfactory sensory neurons to enter the brain after nasal infection. Fate-mapping studies demonstrated that infected central nervous system (CNS) neurons were cleared noncytolytically, yet specific deletion of major histocompatibility complex class I (MHC I) from these neurons unexpectedly had no effect on viral control. Intravital imaging studies of calcium signaling in virus-specific CD8+ T cells revealed instead that brain-resident microglia were the relevant source of viral peptide-MHC I complexes. Microglia were not infected by the virus but were found to cross-present antigen after acquisition from adjacent neurons. Microglia depletion interfered with T cell calcium signaling and antiviral control in the brain after nasal infection. Collectively, these data demonstrate that microglia provide a front-line defense against a neuroinvasive nasal infection by cross-presenting antigen to antiviral T cells that noncytolytically cleanse neurons. Disruptions in this innate defense likely render the brain susceptible to neurotropic viruses like VSV that attempt to enter the CNS via the nose.

The lymphocyte populations and their migration into the central nervous system in tick-borne encephalitis

In tick-borne encephalitis (TBE) the cerebrospinal fluid (CSF) cytosis is dominated by T CD3+CD4+ and T CD3+CD8+ lymphocytes, but their pathogenetic roles and mechanisms of migration into central nervous system (CNS) are unclear. Currently, we have studied CSF lymphocyte subsets and chemotactic axes in TBE patients stratified according to the clinical presentation. Blood and CSF were obtained from 51 patients with TBE (presenting as meningitis in 30, meningoencephalitis in 18 and meningoencephalomyelitis in 3), 20 with non-TBE meningitis and 11 healthy controls. We have studied: (1) abundances of the main lymphocyte subsets and (2) CXCR3 and CCR5 expression on CD3+CD4+ and CD3+CD8+ lymphocytes cytometrically with fluorochrome-stained monoclonal antibodies; (3) concentrations of chemotactic cytokines: CCL5 (CCR5 ligand), CXCL10 (CXCR3 ligand), IL-16, CCL2, CCL20 and CXCL5 with ELISA. Cytokine concentrations were additionally studied in 8 pediatric TBE patients. Data were analyzed with non-parametric tests, p < 0.05 considered significant. The higher CSF lymphocyte counts were associated with symptoms of CNS involvement, especially with altered consciousness (B, Th and Tc cells) and focal neurologic deficits (B cells). The minor fraction of double-positive T CD4+CD8+ cells was unique in associating negatively with encephalitis and altered consciousness. CSF CD3+CD4+ and CD3+CD8+ lymphocyte population was enriched in CCR5-positive cells and CCL5 concentration in CSF was increased and associated with a milder presentation. Although CXCL10 was vividly up-regulated intrathecally and correlated with CSF T lymphocyte counts, the CXCR3 expression in CSF T lymphocytes was low. Serum and CSF concentrations of CCL2, CXCL5 and IL-16 were increased in adult TBE patients, CCL2 created a chemotactic gradient towards CSF and both CCL2 and IL-16 concentrations correlated positively with CSF lymphocyte counts. The particular lymphoid cell populations in CSF associate differently with the clinical presentation of TBE, suggesting their distinct roles in pathogenesis. CCR5/CCL5 axis probably contributes to T lymphocyte migration into CNS. CXCL10 mediates the intrathecal immune response, but is probably not directly responsible for T cell migration. Additional chemotactic factors must be involved, probably including CCL2 and IL-16.

The continued threat of emerging flaviviruses

Flaviviruses are vector-borne RNA viruses that can emerge unexpectedly in human populations and cause a spectrum of potentially severe diseases including hepatitis, vascular shock syndrome, encephalitis, acute flaccid paralysis, congenital abnormalities and fetal death. This epidemiological pattern has occurred numerous times during the last 70 years, including epidemics of dengue virus and West Nile virus, and the most recent explosive epidemic of Zika virus in the Americas. Flaviviruses are now globally distributed and infect up to 400 million people annually. Of significant concern, outbreaks of other less well-characterized flaviviruses have been reported in humans and animals in different regions of the world. The potential for these viruses to sustain epidemic transmission among humans is poorly understood. In this Review, we discuss the basic biology of flaviviruses, their infectious cycles, the diseases they cause and underlying host immune responses to infection. We describe flaviviruses that represent an established ongoing threat to global health and those that have recently emerged in new populations to cause significant disease. We also provide examples of lesser-known flaviviruses that circulate in restricted areas of the world but have the potential to emerge more broadly in human populations. Finally, we discuss how an understanding of the epidemiology, biology, structure and immunity of flaviviruses can inform the rapid development of countermeasures to treat or prevent human infections as they emerge.

Interferon-Gamma Modulation of the Local T Cell Response to Alphavirus Encephalomyelitis

Infection of mice with Sindbis virus (SINV) provides a model for examining the role of the immune response to alphavirus infection of the central nervous system (CNS). Interferon-gamma (IFN-γ) is an important component of this response, and we show that SINV-infected differentiated neurons respond to IFN-γ in vitro by induction of antiviral genes and suppression of virus replication. To determine the in vivo effects of IFN-γ on SINV clearance and T cell responses, C57BL/6 mice lacking IFN-γ or IFN-γ receptor-1 were compared to wild-type (WT) mice after intracranial SINV infection. In WT mice, IFN-γ was first produced in the CNS by natural killer cells and then by CD4+ and CD8+ T cells. Mice with impaired IFN-γ signaling initiated clearance of viral RNA earlier than WT mice associated with CNS entry of more granzyme B-producing CD8+ T cells. However, these mice established fewer CD8+ tissue-resident memory T (TRM) cells and were more likely to experience reactivation of viral RNA synthesis late after infection. Therefore, IFN-γ suppresses the local development of granzyme B-expressing CD8+ T cells and slows viral RNA clearance but promotes CD8+ TRM cell establishment.

Comparative Pathology of West Nile Virus in Humans and Non-Human Animals

West Nile virus (WNV) continues to be a major cause of human arboviral neuroinvasive disease. Susceptible non-human vertebrates are particularly diverse, ranging from commonly affected birds and horses to less commonly affected species such as alligators. This review summarizes the pathology caused by West Nile virus during natural infections of humans and non-human animals. While the most well-known findings in human infection involve the central nervous system, WNV can also cause significant lesions in the heart, kidneys and eyes. Time has also revealed chronic neurologic sequelae related to prior human WNV infection. Similarly, neurologic disease is a prominent manifestation of WNV infection in most non-human non-host animals. However, in some avian species, which serve as the vertebrate host for WNV maintenance in nature, severe systemic disease can occur, with neurologic, cardiac, intestinal and renal injury leading to death. The pathology seen in experimental animal models of West Nile virus infection and knowledge gains on viral pathogenesis derived from these animal models are also briefly discussed. A gap in the current literature exists regarding the relationship between the neurotropic nature of WNV in vertebrates, virus propagation and transmission in nature. This and other knowledge gaps, and future directions for research into WNV pathology, are addressed.

Oral Antibiotic Treatment of Mice Exacerbates the Disease Severity of Multiple Flavivirus Infections

Although the outcome of flavivirus infection can vary from asymptomatic to lethal, environmental factors modulating disease severity are poorly defined. Here, we observed increased susceptibility of mice to severe West Nile (WNV), Dengue, and Zika virus infections after treatment with oral antibiotics (Abx) that depleted the gut microbiota. Abx treatment impaired the development of optimal T cell responses, with decreased levels of WNV-specific CD8⁺ T cells associated with increased infection and immunopathology. Abx treatments that resulted in enhanced WNV susceptibility generated changes in the overall structure of the gut bacterial community and in the abundance of specific bacterial taxa. As little as 3 days of treatment with ampicillin was sufficient to alter host immunity and WNV outcome. Our results identify oral Abx therapy as a potential environmental determinant of systemic viral disease, and they raise the possibility that perturbation of the gut microbiota may have deleterious consequences for subsequent flavivirus infections.

Current Understanding of West Nile Virus Clinical Manifestations, Immune Responses, Neuroinvasion, and Immunotherapeutic Implications

West Nile virus (WNV) is the most common mosquito-borne virus in North America. WNV-associated neuroinvasive disease affects all ages, although elderly and immunocompromised individuals are particularly at risk. WNV neuroinvasive disease has killed over 2300 Americans since WNV entered into the United States in the New York City outbreak of 1999. Despite 20 years of intensive laboratory and clinical research, there are still no approved vaccines or antivirals available for human use. However, rapid progress has been made in both understanding the pathogenesis of WNV and treatment in clinical practices. This review summarizes our current understanding of WNV infection in terms of human clinical manifestations, host immune responses, neuroinvasion, and therapeutic interventions.

Identification of Protective CD8 T Cell Responses in a Mouse Model of Zika Virus Infection

Many flaviviruses including dengue (DENV), and Zika (ZIKV) have attracted significant attention in the past few years. As many flaviviruses are spread by arthropods, most of the world's population is at risk of encountering a flavivirus, and infection with these viruses has created a significant disease burden worldwide. Vaccination against flaviviruses is thought to be one of the most promising avenues for reducing the disease burden associated with these viruses. The optimism surrounding a vaccine approach is supported by the highly successful vaccines for yellow fever and Japanese encephalitis. Central to the development of new successful vaccines is the understanding of the correlates of protection that will be necessary to engineer into new vaccines. To aid in this endeavor we have directed our efforts to identify correlates of protection that will reduce the disease burden associated with ZIKV and DENV. Within this study we have identified a novel murine ZIKV specific CD8⁺ T cell epitope, and shown that the ZIKV epitope specific CD8⁺ T cell response has a distinct immunodominance hierarchy present during acute infection and is detectible as part of the memory T cell responses. Our studies confirm that ZIKV-specific CD8⁺ T cells are an important correlate of protection for ZIKV and demonstrate that both naïve and ZIKV immune CD8⁺ T cells are sufficient for protection against a lethal ZIKV infection. Overall this study adds to the body of literature demonstrating a role for CD8⁺ T cells in controlling flavivirus infection.

T Lymphocytes as Measurable Targets of Protection and Vaccination Against Viral Disorders

Continuous epidemiological surveillance of existing and emerging viruses and their associated disorders is gaining importance in light of their abilities to cause unpredictable outbreaks as a result of increased travel and vaccination choices by steadily growing and aging populations. Close surveillance of outbreaks and herd immunity are also at the forefront, even in industrialized countries, where previously eradicated viruses are now at risk of re-emergence due to instances of strain recombination, contractions in viral vector geographies, and from their potential use as agents of bioterrorism. There is a great need for the rational design of current and future vaccines targeting viruses, with a strong focus on vaccine targeting of adaptive immune effector memory T cells as the gold standard of immunity conferring long-lived protection against a wide variety of pathogens and malignancies. Here, we review viruses that have historically caused large outbreaks and severe lethal disorders, including respiratory, gastric, skin, hepatic, neurologic, and hemorrhagic fevers. To observe trends in vaccinology against these viral disorders, we describe viral genetic, replication, transmission, and tropism, host-immune evasion strategies, and the epidemiology and health risks of their associated syndromes. We focus on immunity generated against both natural infection and vaccination, where a steady shift in conferred vaccination immunogenicity is observed from quantifying activated and proliferating, long-lived effector memory T cell subsets, as the prominent biomarkers of long-term immunity against viruses and their associated disorders causing high morbidity and mortality rates.

The immune response against flaviviruses

Arthropod-borne flaviviruses are important human pathogens that cause a diverse range of clinical conditions, including severe hemorrhagic syndromes, neurological complications and congenital malformations. Consequently, there is an urgent need to develop safe and effective vaccines, a process requiring better understanding of the immunological mechanisms involved during infection. Decades of research suggest a paradoxical role of the immune response against flaviviruses: although the immune response is crucial for the control, clearance and prevention of infection, poor clinical outcomes are commonly associated with virus-specific immunity and immunopathogenesis. This relationship is further complicated by the high homology among viruses and the implication of cross-reactive immune responses in protection and pathogenesis. This Review examines the dual role of the adaptive immune response against flaviviruses, particularly emphasizing the most recent findings regarding cross-reactive T cell and antibody responses, and the effects that these concepts have on vaccine-development endeavors.

Molecular characterization of three novel perforins in common carp (Cyprinus carpio L.) and their expression patterns during larvae ontogeny and in response to immune challenges

Background

In the host immune system, perforin is a cytotoxic effector molecule that eliminate virus-infected and malignant cells. Moreover, some recent studies also imply the involvement of perforin in antibacterial immunity. Common carp (Cyprinus carpio L.), one of the most economically important fish species in China, has a high susceptibility to viruses and bacteria. Thus far, in common carp, no data are available regarding the identification and immunologic function of the perforin.

Results

In the present study, the cDNA and genomic DNA sequences of three perforin isoform genes were cloned and characterized in common carp, named CcPRF1, CcPRF2 and CcPRF3. Amino acid sequences of the three CcPRFs were quite different, with identities ranged from 37.3 to 39.5%. Phylogenetic analysis showed that three CcPRFs, each in a separate sub-branch, possessed closer evolutionary relationship with other teleost perforins, especially with cyprinid fishes, than higher vertebrates. Expression analysis revealed that each CcPRF gene was differentially expressed in all of the nine tested tissues. During larvae ontogeny, each CcPRF displayed a distinct expression pattern, while with a common expression peak at 22 days post hatching (dph). Moreover, in vivo or in vitro, after stimulation with polyI:C, LPS and Aeromonas hydrophila, each CcPRF was induced significantly, with differential expression dynamics.

Conclusions

Our findings suggest that perforin might play significant roles in larval immune system and in the immune defense of common carp against viral and bacterial pathogens. Meantime, the differential expression dynamics seem to imply possible different cellular locations or functional differences across various CcPRF isoforms.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1613-y) contains supplementary material, which is available to authorized users.

Cell-Mediated Immune Responses and Immunopathogenesis of Human Tick-Borne Encephalitis Virus-Infection

Tick-borne encephalitis virus (TBEV) is a flavivirus that belongs to the Flaviviridae family. TBEV is transmitted to humans primarily from infected ticks. The virus causes tick-borne encephalitis (TBE), an acute viral disease that affects the central nervous system (CNS). Infection can lead to acute neurological symptoms of significant severity due to meningitis or meningo(myelo)encephalitis. TBE can cause long-term suffering and has been recognized as an increasing public health problem. TBEV-affected areas currently include large parts of central and northern Europe as well as northern Asia. Infection with TBEV triggers a humoral as well as a cell-mediated immune response. In contrast to the well-characterized humoral antibody-mediated response, the cell-mediated immune responses elicited to natural TBEV-infection have been poorly characterized until recently. Here, we review recent progress in our understanding of the cell-mediated immune response to human TBEV-infection. A particular emphasis is devoted to studies of the response mediated by natural killer (NK) cells and CD8 T cells. The studies described include results revealing the temporal dynamics of the T cell- as well as NK cell-responses in relation to disease state and functional characterization of these cells. Additionally, we discuss specific immunopathological aspects of TBEV-infection in the CNS.

Pharmacologic Depletion of Microglia Increases Viral Load in the Brain and Enhances Mortality in Murine Models of Flavivirus-Induced Encephalitis

Flaviviruses account for most arthropod-borne cases of human encephalitis in the world. However, the exact mechanisms of injury to the central nervous system (CNS) during flavivirus infections remain poorly understood. Microglia are the resident immune cells of the CNS and are important for multiple functions, including control of viral pathogenesis. Utilizing a pharmacologic method of microglia depletion (PLX5622 [Plexxikon Inc.], an inhibitor of colony-stimulating factor 1 receptor), we sought to determine the role of microglia in flaviviral pathogenesis. Depletion of microglia resulted in increased mortality and viral titer in the brain following infection with either West Nile virus (WNV) or Japanese encephalitis virus (JEV). Interestingly, microglial depletion did not prevent virus-induced increases in the expression of relevant cytokines and chemokines at the mRNA level. In fact, the expression of several proinflammatory genes was increased in virus-infected, microglia-depleted mice compared to virus-infected, untreated controls. In contrast, and as expected, expression of the macrophage marker triggering receptor expressed on myeloid cells 2 (TREM2) was decreased in virus-infected, PLX5622-treated mice compared to virus-infected controls.IMPORTANCE As CNS invasion by flaviviruses is a rare but life-threatening event, it is critical to understand how brain-resident immune cells elicit protection or injury during disease progression. Microglia have been shown to be important in viral clearance but may also contribute to CNS injury as part of the neuroinflammatory process. By utilizing a microglial depletion model, we can begin to parse out the exact roles of microglia during flaviviral pathogenesis with hopes of understanding specific mechanisms as potential targets for therapeutics.

Dynamics of Tissue-Specific CD8+ T Cell Responses during West Nile Virus Infection

The mouse model of West Nile virus (WNV), which is a leading cause of mosquito-borne encephalitis worldwide, has provided fundamental insights into the host and viral factors that regulate viral pathogenesis and infection outcome. In particular, CD8⁺ T cells are critical for controlling WNV replication and promoting protection against infection. Here, we present the characterization of a T cell receptor (TCR)-transgenic mouse with specificity for the immunodominant epitope in the WNV NS4B protein (here referred to as transgenic WNV-I mice). Using an adoptive-transfer model, we found that WNV-I CD8⁺ T cells behave similarly to endogenous CD8⁺ T cell responses, with an expansion phase in the periphery beginning around day 7 postinfection (p.i.) followed by a contraction phase through day 15 p.i. Through the use of in vivo intravascular immune cell staining, we determined the kinetics, expansion, and differentiation into effector and memory subsets of WNV-I CD8⁺ T cells within the spleen and brain. We found that red-pulp WNV-I CD8⁺ T cells were more effector-like than white-pulp WNV-I CD8⁺ T cells, which displayed increased differentiation into memory precursor cells. Within the central nervous system (CNS), we found that WNV-I CD8⁺ T cells were polyfunctional (gamma interferon [IFN-γ] and tumor necrosis factor alpha [TNF-α]), displayed tissue-resident characteristics (CD69⁺ and CD103⁺), persisted in the brain through day 15 p.i., and reduced the viral burden within the brain. The use of these TCR-transgenic WNV-I mice provides a new resource to dissect the immunological mechanisms of CD8⁺ T cell-mediated protection during WNV infection.IMPORTANCE West Nile Virus (WNV) is the leading cause of mosquito-borne encephalitis worldwide. There are currently no approved therapeutics or vaccines for use in humans to treat or prevent WNV infection. CD8⁺ T cells are critical for controlling WNV replication and protecting against infection. Here, we present a comprehensive characterization of a novel TCR-transgenic mouse with specificity for the immunodominant epitope in the WNV NS4B protein. In this study, we determine the kinetics, proliferation, differentiation into effector and memory subsets, homing, and clearance of WNV in the CNS. Our findings provide a new resource to dissect the immunological mechanisms of CD8⁺ T cell-mediated protection during WNV infection.

Minocycline Has Anti-inflammatory Effects and Reduces Cytotoxicity in an Ex Vivo Spinal Cord Slice Culture Model of West Nile Virus Infection

West Nile virus (WNV) is a neurotropic flavivirus that can cause significant neurological disease. Mouse models of WNV infection demonstrate that a proinflammatory environment is induced within the central nervous system (CNS) after WNV infection, leading to entry of activated peripheral immune cells. We utilized ex vivo spinal cord slice cultures (SCSC) to demonstrate that anti-inflammatory mechanisms may also play a role in WNV-induced pathology and/or recovery. Microglia are a type of macrophage that function as resident CNS immune cells. Similar to mouse models, infection of SCSC with WNV induces the upregulation of proinflammatory genes and proteins that are associated with microglial activation, including the microglial activation marker Iba1 and CC motif chemokines CCL2, CCL3, and CCL5. This suggests that microglia assume a proinflammatory phenotype in response to WNV infection similar to the proinflammatory (M1) activation that can be displayed by other macrophages. We now show that the WNV-induced expression of these and other proinflammatory genes was significantly decreased in the presence of minocycline, which has antineuroinflammatory properties, including the ability to inhibit proinflammatory microglial responses. Minocycline also caused a significant increase in the expression of anti-inflammatory genes associated with alternative anti-inflammatory (M2) macrophage activation, including interleukin 4 (IL-4), IL-13, and FIZZ1. Minocycline-dependent alterations to M1/M2 gene expression were associated with a significant increase in survival of neurons, microglia, and astrocytes in WNV-infected slices and markedly decreased levels of inducible nitric oxide synthase (iNOS). These results demonstrate that an anti-inflammatory environment induced by minocycline reduces viral cytotoxicity during WNV infection in ex vivo CNS tissue.IMPORTANCE West Nile virus (WNV) causes substantial morbidity and mortality, with no specific therapeutic treatments available. Antiviral inflammatory responses are a crucial component of WNV pathology, and understanding how they are regulated is important for tailoring effective treatments. Proinflammatory responses during WNV infection have been extensively studied, but anti-inflammatory responses (and their potential protective and reparative capabilities) following WNV infection have not been investigated. Minocycline induced the expression of genes associated with the anti-inflammatory (M2) activation of CNS macrophages (microglia) in WNV-infected SCSC while inhibiting the expression of genes associated with proinflammatory (M1) macrophage activation and was protective for multiple CNS cell types, indicating its potential use as a therapeutic reagent. This ex vivo culture system can uniquely address the ability of CNS parenchymal cells (neurons, astrocytes, and microglia) to respond to minocycline and to modulate the inflammatory environment and cytotoxicity in response to WNV infection without peripheral immune cell involvement.

Virulence determinants of West Nile virus: how can these be used for vaccine design?

West Nile virus (WNV), a neurotropic mosquito-borne flavivirus, has become endemic in the USA and parts of Europe since 1999. There is no licensed WNV vaccine for humans. Considering the robust immunity from immunization with live, attenuated vaccines, a live WNV vaccine is an ideal platform for disease control. Animal and mosquito studies have identified a number of candidate attenuating mutations, including the structural proteins premembrane/membrane and envelope, and the nonstructural proteins NS1, NS2A, NS3, NS4A, NS4B and NS5, and the 3' UTR. Many of the mutations that have been examined attenuate WNV using different mechanisms, thus providing a greater understanding of WNV virulence while also identifying specific mutations as candidates to include in a WNV live vaccine.

Chemokine Receptor Ccr7 Restricts Fatal West Nile Virus Encephalitis

West Nile virus (WNV) is a mosquito-transmitted flavivirus that can cause debilitating encephalitis. To delineate the mechanisms behind this pathology, we studied Ccr7-deficient mice, which afforded us the capacity to study infection in mice with disrupted peripheral cellular trafficking events. The loss of Ccr7 resulted in an immediate pan-leukocytosis that remained elevated throughout the infection. This leukocytosis resulted in a significant enhancement of leukocyte accumulation within the central nervous system (CNS). Despite an excess of virus-specific T cells in the CNS, Ccr7-deficient mice had significantly higher CNS viral loads and mortality rates than wild-type animals. Mechanistically, the elevated trafficking of infected myeloid cells into the brain in Ccr7-deficient mice resulted in increased levels of WNV in the CNS, thereby effectively contributing to neuroinflammation and lowering viral clearance. Combined, our experiments suggest that during WNV infection, Ccr7 is a gatekeeper for nonspecific viral transference to the brain.IMPORTANCE In this study, we show that Ccr7 is required for the sufficient migration of dendritic cells and T cells into the draining lymph node immediately following infection and for the restriction of leukocyte migration into the brain. Further, the severe loss of dendritic cells in the draining lymph node had no impact on viral replication in this organ, suggesting that WNV may migrate from the skin into the lymph node through another mechanism. Most importantly, we found that the loss of Ccr7 results in a significant leukocytosis, leading to hypercellularity within the CNS, where monocytes/macrophages contribute to CNS viremia, neuroinflammation, and increased mortality. Together, our data point to Ccr7 as a critical host defense restriction factor limiting neuroinflammation during acute viral infection.

The natural killer cell response to West Nile virus in young and old individuals with or without a prior history of infection

West Nile virus (WNV) typically leads to asymptomatic infection but can cause severe neuroinvasive disease or death, particularly in the elderly. Innate NK cells play a critical role in antiviral defenses, yet their role in human WNV infection is poorly defined. Here we demonstrate that NK cells mount a robust, polyfunctional response to WNV characterized by cytolytic activity, cytokine and chemokine secretion. This is associated with downregulation of activating NK cell receptors and upregulation of NK cell activating ligands for NKG2D. The NK cell response did not differ between young and old WNV-naïve subjects, but a history of symptomatic infection is associated with more IFN-γ producing NK cell subsets and a significant decline in a specific NK cell subset. This NK repertoire skewing could either contribute to or follow heightened immune pathogenesis from WNV infection, and suggests that NK cells could play an important role in WNV infection in humans.

Interleukin-17A Promotes CD8+ T Cell Cytotoxicity To Facilitate West Nile Virus Clearance

CD8⁺ T cells are crucial components of immunity and play a vital role in recovery from West Nile virus (WNV) infection. Here, we identify a previously unrecognized function of interleukin-17A (IL-17A) in inducing cytotoxic-mediator gene expression and promoting CD8⁺ T cell cytotoxicity against WNV infection in mice. We find that IL-17A-deficient (Il17a^-/-) mice are more susceptible to WNV infection and develop a higher viral burden than wild-type (WT) mice. Interestingly, the CD8⁺ T cells isolated from Il17a^-/- mice are less cytotoxic and express lower levels of cytotoxic-mediator genes, which can be restored by supplying recombinant IL-17A in vitro and in vivo Importantly, treatment of WNV-infected mice with recombinant IL-17A, as late as day 6 postinfection, significantly reduces the viral burden and increases survival, suggesting a therapeutic potential for IL-17A. In conclusion, we report a novel function of IL-17A in promoting CD8⁺ T cell cytotoxicity, which may have broad implications in other microbial infections and cancers.

IMPORTANCE

Interleukin-17A (IL-17A) and CD8⁺ T cells regulate diverse immune functions in microbial infections, malignancies, and autoimmune diseases. IL-17A is a proinflammatory cytokine produced by diverse cell types, while CD8⁺ T cells (known as cytotoxic T cells) are major cells that provide immunity against intracellular pathogens. Previous studies have demonstrated a crucial role of CD8⁺ T cells in recovery from West Nile virus (WNV) infection. However, the role of IL-17A during WNV infection remains unclear. Here, we demonstrate that IL-17A protects mice from lethal WNV infection by promoting CD8⁺ T cell-mediated clearance of WNV. In addition, treatment of WNV-infected mice with recombinant IL-17A reduces the viral burden and increases survival of mice, suggesting a potential therapeutic. This novel IL-17A-CD8⁺ T cell axis may also have broad implications for immunity to other microbial infections and cancers, where CD8⁺ T cell functions are crucial.

Acute systemic DNA damage in youth does not impair immune defense with aging

Aging‐related decline in immunity is believed to be the main driver behind decreased vaccine efficacy and reduced resistance to infections in older adults. Unrepaired DNA damage is known to precipitate cellular senescence, which was hypothesized to be the underlying cause of certain age‐related phenotypes. Consistent with this, some hallmarks of immune aging were more prevalent in individuals exposed to whole‐body irradiation (WBI), which leaves no anatomical repository of undamaged hematopoietic cells. To decisively test whether and to what extent WBI in youth will leave a mark on the immune system as it ages, we exposed young male C57BL/6 mice to sublethal WBI (0.5–4 Gy), mimicking human survivor exposure during nuclear catastrophe. We followed lymphocyte homeostasis thorough the lifespan, response to vaccination, and ability to resist lethal viral challenge in the old age. None of the irradiated groups showed significant differences compared with mock‐irradiated (0 Gy) animals for the parameters measured. Even the mice that received the highest dose of sublethal WBI in youth (4 Gy) exhibited equilibrated lymphocyte homeostasis, robust T‐ and B‐cell responses to live attenuated West Nile virus (WNV) vaccine and full survival following vaccination upon lethal WNV challenge. Therefore, a single dose of nonlethal WBI in youth, resulting in widespread DNA damage and repopulation stress in hematopoietic cells, leaves no significant trace of increased immune aging in a lethal vaccine challenge model.

CCL2, but not its receptor, is essential to restrict immune privileged central nervous system-invasion of Japanese encephalitis virus via regulating accumulation of CD11b(+) Ly-6C(hi) monocytes

Japanese encephalitis virus (JEV) is a re‐emerging zoonotic flavivirus that poses an increasing threat to global health and welfare due to rapid changes in climate and demography. Although the CCR2–CCL2 axis plays an important role in trafficking CD11b⁺ Ly‐6C^hi monocytes to regulate immunopathological diseases, little is known about their role in monocyte trafficking during viral encephalitis caused by JEV infection. Here, we explored the role of CCR2 and its ligand CCL2 in JE caused by JEV infection using CCR2‐ and CCL2‐ablated murine models. Somewhat surprisingly, the ablation of CCR2 and CCL2 resulted in starkly contrasting susceptibility to JE. CCR2 ablation induced enhanced resistance to JE, whereas CCL2 ablation highly increased susceptibility to JE. This contrasting regulation of JE progression by CCR2 and CCL2 was coupled to central nervous system (CNS) infiltration of Ly‐6C^hi monocytes and Ly‐6G^hi granulocytes. There was also enhanced expression of CC and CXC chemokines in the CNS of CCL2‐ablated mice, which appeared to induce CNS infiltration of these cell populations. However, our data revealed that contrasting regulation of JE in CCR2‐ and CCL2‐ablated mice was unlikely to be mediated by innate natural killer and adaptive T‐cell responses. Furthermore, CCL2 produced by haematopoietic stem cell‐derived leucocytes played a dominant role in CNS accumulation of Ly‐6C^hi monocytes in infected bone marrow chimeric models, thereby exacerbating JE progression. Collectively, our data indicate that CCL2 plays an essential role in conferring protection against JE caused by JEV infection. In addition, blockage of CCR2, but not CCL2, will aid in the development of strategies for prophylactics and therapeutics of JE.

Protective effects of a Modified Vaccinia Ankara-based vaccine candidate against Crimean-Congo Haemorrhagic Fever virus require both cellular and humoral responses

Crimean-Congo Haemorrhagic Fever (CCHF) is a severe tick-borne disease, endemic in many countries in Africa, the Middle East, Eastern Europe and Asia. There is no approved vaccine currently available against CCHF. The most promising candidate, which has previously been shown to confer protection in the small animal model, is a modified Vaccinia Ankara virus vector expressing the CCHF viral glycoprotein (MVA-GP). It has been shown that MVA-GP induces both humoral and cellular immunogenicity. In the present study, sera and T-lymphocytes were passively and adoptively transferred into recipient mice prior to challenge with CCHF virus. Results demonstrated that mediators from both arms of the immune system were required to demonstrate protective effects against lethal challenge.

Dual Function of Ccr5 during Langat Virus Encephalitis: Reduction in Neutrophil-Mediated Central Nervous System Inflammation and Increase in T Cell-Mediated Viral Clearance

Tick-borne encephalitis virus (TBEV) is a vector-transmitted flavivirus that causes potentially fatal neurologic infection. There are thousands of cases reported annually, and despite the availability of an effective vaccine, the incidence of TBEV is increasing worldwide. Importantly, up to 30% of affected individuals develop long-term neurologic sequelae. We investigated the role of chemokine receptor Ccr5 in a mouse model of TBEV infection using the naturally attenuated tick-borne flavivirus Langat virus (LGTV). Ccr5-deficient mice presented with an increase in viral replication within the CNS and decreased survival during LGTV encephalitis compared with wild-type controls. This enhanced susceptibility was due to the temporal lag in lymphocyte migration into the CNS. Adoptive transfer of wild-type T cells, but not Ccr5-deficient T cells, significantly improved survival outcome in LGTV-infected Ccr5-deficient mice. Concomitantly, a significant increase in neutrophil migration into the CNS in LGTV-infected Ccr5(-/-) mice was documented at the late stage of infection. Ab-mediated depletion of neutrophils in Ccr5(-/-) mice resulted in a significant improvement in mortality, a decrease in viral load, and a decrease in overall tissue damage in the CNS compared with isotype control-treated mice. Ccr5 is crucial in directing T cells toward the LGTV-infected brain, as well as in suppressing neutrophil-mediated inflammation within the CNS.

West Nile Virus Infection in the Central Nervous System

West Nile virus (WNV), a neurotropic single-stranded flavivirus has been the leading cause of arboviral encephalitis worldwide.  Up to 50% of WNV convalescent patients in the United States were reported to have long-term neurological sequelae.  Neither antiviral drugs nor vaccines are available for humans.  Animal models have been used to investigate WNV pathogenesis and host immune response in humans.  In this review, we will discuss recent findings from studies in animal models of WNV infection, and provide new insights on WNV pathogenesis and WNV-induced host immunity in the central nervous system.