CXCR3 mediates region-specific antiviral T cell trafficking within the central nervous system during West Nile virus encephalitis

Balancing functions of regulatory T cells in mosquito-borne viral infections

Mosquito-borne viral infections are on the rise worldwide and can lead to severe symptoms such as haemorrhage, encephalitis, arthritis or microcephaly. A protective immune response following mosquito-borne viral infections requires the generation of a controlled and balanced immune response leading to viral clearance without immunopathology. Here, regulatory T cells play a central role in restoring immune homeostasis. In current review, we aim to provide an overview and summary of the phenotypes of FOXP3+ Tregs in various mosquito-borne arboviral disease, their association with disease severity and their functional characteristics. Furthermore, we discuss the role of cytokines and Tregs in the immunopathogenesis of mosquito-borne infections. Lastly, we discuss possible novel lines of research which could provide additional insight into the role of Tregs in mosquito-borne viral infections in order to develop novel therapeutic approaches or vaccination strategies.

Orchestration of antiviral responses within the infected central nervous system

Many newly emerging and re-emerging viruses have neuroinvasive potential, underscoring viral encephalitis as a global research priority. Upon entry of the virus into the CNS, severe neurological life-threatening conditions may manifest that are associated with high morbidity and mortality. The currently available therapeutic arsenal against viral encephalitis is rather limited, emphasizing the need to better understand the conditions of local antiviral immunity within the infected CNS. In this review, we discuss new insights into the pathophysiology of viral encephalitis, with a focus on myeloid cells and CD8+ T cells, which critically contribute to protection against viral CNS infection. By illuminating the prerequisites of myeloid and T cell activation, discussing new discoveries regarding their transcriptional signatures, and dissecting the mechanisms of their recruitment to sites of viral replication within the CNS, we aim to further delineate the complexity of antiviral responses within the infected CNS. Moreover, we summarize the current knowledge in the field of virus infection and neurodegeneration and discuss the potential links of some neurotropic viruses with certain pathological hallmarks observed in neurodegeneration.

T Cells Trafficking into the Brain in Aging and Alzheimer's Disease

The meninges, choroid plexus (CP) and blood-brain barrier (BBB) are recognized as important gateways for peripheral immune cell trafficking into the central nervous system (CNS). Accumulation of peripheral immune cells in brain parenchyma can be observed during aging and Alzheimer's disease (AD). However, the mechanisms by which peripheral immune cells enter the CNS through these three pathways and how they interact with resident cells within the CNS to cause brain injury are not fully understood. In this paper, we review recent research on T cells recruitment in the brain during aging and AD. This review focuses on the possible pathways through which T cells infiltrate the brain, the evidence that T cells are recruited to the brain, and how infiltrating T cells interact with the resident cells in the CNS during aging and AD. Unraveling these issues will contribute to a better understanding of the mechanisms of aging and AD from the perspective of immunity, and hopefully develop new therapeutic strategies for brain aging and AD.

Neonatal Chlamydia muridarum respiratory infection causes neuroinflammation within the brainstem during the early postnatal period

Respiratory infections are one of the most common causes of illness and morbidity in neonates worldwide. In the acute phase infections are known to cause wide-spread peripheral inflammation. However, the inflammatory consequences to the critical neural control centres for respiration have not been explored. Utilising a well characterised model of neonatal respiratory infection, we investigated acute responses within the medulla oblongata which contains key respiratory regions. Neonatal mice were intranasally inoculated within 24 h of birth, with either Chlamydia muridarum or sham-infected, and tissue collected on postnatal day 15, the peak of peripheral inflammation. A key finding of this study is that, while the periphery appeared to show no sex-specific effects of a neonatal respiratory infection, sex had a significant impact on the inflammatory response of the medulla oblongata. There was a distinct sex-specific response in the medulla coincident with peak of peripheral inflammation, with females demonstrating an upregulation of anti-inflammatory cytokines and males showing very few changes. Microglia also demonstrated sex-specificity with the morphology of females and males differing based upon the nuclei. Astrocytes showed limited changes during the acute response to neonatal infection. These data highlight the strong sex-specific impact of a respiratory infection can have on the medulla in the acute inflammatory phase.

Role of inflammatory cytokine burst in neuro-invasion of Japanese Encephalitis virus infection: an immunotherapeutic approaches

Japanese Encephalitis remains a significant global health concern, contributing to millions of deaths annually worldwide. Microglial cells, as key innate immune cells within the central nervous system (CNS), exhibit intricate cellular structures and possess molecular phenotypic plasticity, playing pivotal roles in immune responses during CNS viral infections. Particularly under viral inflammatory conditions, microglial cells orchestrate innate and adaptive immune responses to mitigate viral invasion and dampen inflammatory reactions. This review article comprehensively summarizes the pathophysiology of viral invasion into the CNS and the cellular interactions involved, elucidating the roles of various immune mediators, including pro-inflammatory cytokines, in neuroinflammation. Leveraging this knowledge, strategies for modulating inflammatory responses and attenuating hyperactivation of glial cells to mitigate viral replication within the brain are discussed. Furthermore, current chemotherapeutic and antiviral drugs are examined, elucidating their mechanisms of action against viral replication. This review aims to provide insights into therapeutic interventions for Japanese Encephalitis and related viral infections, ultimately contributing to improved outcomes for affected individuals.

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.

Temporal tracking of microglial and monocyte single-cell transcriptomics in lethal flavivirus infection

As the resident parenchymal myeloid population in the central nervous system (CNS), microglia are strategically positioned to respond to neurotropic virus invasion and have been implicated in promoting both disease resolution and progression in the acute and post-infectious phase of virus encephalitis. In a mouse model of West Nile virus encephalitis (WNE), infection of the CNS results in recruitment of large numbers of peripheral immune cells into the brain, the majority being nitric oxide (NO)-producing Ly6C^hi inflammatory monocyte-derived cells (MCs). In this model, these cells enhance immunopathology and mortality. However, the contribution of microglia to this response is currently undefined. Here we used a combination of experimental tools, including single-cell RNA sequencing (scRNA-seq), microglia and MC depletion reagents, high-dimensional spectral cytometry and computational algorithms to dissect the differential contribution of microglia and MCs to the anti-viral immune response in severe neuroinflammation seen in WNE. Intriguingly, analysis of scRNA-seq data revealed 6 unique microglia and 3 unique MC clusters that were predominantly timepoint-specific, demonstrating substantial transcriptional adaptation with disease progression over the course of WNE. While microglia and MC adopted unique gene expression profiles, gene ontology enrichment analysis, coupled with microglia and MC depletion studies, demonstrated a role for both of these cells in the trafficking of peripheral immune cells into the CNS, T cell responses and viral clearance. Over the course of infection, microglia transitioned from a homeostatic to an anti-viral and then into an immune cell-recruiting phenotype. Conversely, MC adopted antigen-presenting, immune cell-recruiting and NO-producing phenotypes, which all had anti-viral function. Overall, this study defines for the first time the single-cell transcriptomic responses of microglia and MCs over the course of WNE, demonstrating both protective and pathological roles of these cells that could potentially be targeted for differential therapeutic intervention to dampen immune-mediated pathology, while maintaining viral clearance functions.

Interleukins, Chemokines, and Tumor Necrosis Factor Superfamily Ligands in the Pathogenesis of West Nile Virus Infection

West Nile virus (WNV) is a mosquito-borne pathogen that can lead to encephalitis and death in susceptible hosts. Cytokines play a critical role in inflammation and immunity in response to WNV infection. Murine models provide evidence that some cytokines offer protection against acute WNV infection and assist with viral clearance, while others play a multifaceted role WNV neuropathogenesis and immune-mediated tissue damage. This article aims to provide an up-to-date review of cytokine expression patterns in human and experimental animal models of WNV infections. Here, we outline the interleukins, chemokines, and tumor necrosis factor superfamily ligands associated with WNV infection and pathogenesis and describe the complex roles they play in mediating both protection and pathology of the central nervous system during or after virus clearance. By understanding of the role of these cytokines during WNV neuroinvasive infection, we can develop treatment options aimed at modulating these immune molecules in order to reduce neuroinflammation and improve patient outcomes.

Involvement of chemokine receptor CXCR3 in the defense mechanism against Neospora caninum infection in C57BL/6 mice

C-X-C motif chemokine receptor 3 (CXCR3) is an important receptor controlling the migration of leukocytes, although there is no report regarding its role in Neospora caninum infection. Herein, we investigated the relevance of CXCR3 in the resistance mechanism to N. caninum infection in mice. Wild-type (WT) C57BL/6 mice and CXCR3-knockout (CXCR3KO) mice were used in all experiments. WT mice displayed a high survival rate (100%), while 80% of CXCR3KO mice succumbed to N. caninum infection within 50 days. Compared with WT mice, CXCR3KO mice exhibited significantly lower body weights and higher clinical scores at the subacute stage of infection. Flow cytometric analysis revealed CXCR3KO mice as having significantly increased proportions and numbers of CD11c-positive cells compared with WT mice at 5 days post infection (dpi). However, levels of interleukin-6 and interferon-γ in serum and ascites were similar in all groups at 5 dpi. Furthermore, no differences in parasite load were detected in brain, spleen, lungs or liver tissue of CXCR3KO and WT mice at 5 and 21 dpi. mRNA analysis of brain tissue collected from infected mice at 30 dpi revealed no changes in expression levels of inflammatory response genes. Nevertheless, the brain tissue of infected CXCR3KO mice displayed significant necrosis and microglial activation compared with that of WT mice at 21 dpi. Interestingly, the brain tissue of CXCR3KO mice displayed significantly lower numbers of FoxP3⁺ cells compared with the brain tissue of WT mice at 30 dpi. Accordingly, our study suggests that the lack of active regulatory T cells in brain tissue of infected CXCR3KO mice is the main cause of these mice having severe necrosis and lower survival compared with WT mice. Thus, CXCR3⁺ regulatory T cells may play a crucial role in control of neosporosis.

The dynamics of γδ T cell responses in nonhuman primates during SARS-CoV-2 infection

Although most SARS-CoV-2 infections are mild, some patients develop systemic inflammation and progress to acute respiratory distress syndrome (ARDS). However, the cellular mechanisms underlying this spectrum of disease remain unclear. γδT cells are T lymphocyte subsets that have key roles in systemic and mucosal immune responses during infection and inflammation. Here we show that peripheral γδT cells are rapidly activated following aerosol or intra-tracheal/intra-nasal (IT/IN) SARS-CoV-2 infection in nonhuman primates. Our results demonstrate a rapid expansion of Vδ1 γδT cells at day1 that correlate significantly with lung viral loads during the first week of infection. Furthermore, increase in levels of CCR6 and Granzyme B expression in Vδ1 T cells during viral clearance imply a role in innate-like epithelial barrier-protective and cytotoxic functions. Importantly, the early activation and mobilization of circulating HLA-DR+CXCR3+ γδT cells along with significant correlations of Vδ1 T cells with IL-1Ra and SCF levels in bronchoalveolar lavage suggest a novel role for Vδ1 T cells in regulating lung inflammation during aerosol SARS-CoV-2 infection. A deeper understanding of the immunoregulatory functions of MHC-unrestricted Vδ1 T cells in lungs during early SARS-CoV-2 infection is particularly important in the wake of emerging new variants with increased transmissibility and immune evasion potential.

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.

Influenza A Virus (H1N1) Infection Induces Microglial Activation and Temporal Dysbalance in Glutamatergic Synaptic Transmission

Influenza A virus (IAV) causes respiratory tract disease and is responsible for seasonal and reoccurring epidemics affecting all age groups. Next to typical disease symptoms, such as fever and fatigue, IAV infection has been associated with behavioral alterations presumably contributing to the development of major depression. Previous experiments using IAV/H1N1 infection models have shown impaired hippocampal neuronal morphology and cognitive abilities, but the underlying pathways have not been fully described. In this study, we demonstrate that infection with a low-dose non-neurotrophic H1N1 strain of IAV causes ample peripheral immune response followed by a temporary blood-brain barrier disturbance. Although histological examination did not reveal obvious pathological processes in the brains of IAV-infected mice, detailed multidimensional flow cytometric characterization of immune cells uncovered subtle alterations in the activation status of microglial cells. More specifically, we detected an altered expression pattern of major histocompatibility complex classes I and II, CD80, and F4/80 accompanied by elevated mRNA levels of CD36, CD68, C1QA, and C3, suggesting evolved synaptic pruning. To closer evaluate how these profound changes affect synaptic balance, we established a highly sensitive multiplex flow cytometry-based approach called flow synaptometry. The introduction of this novel technique enabled us to simultaneously quantify the abundance of pre- and postsynapses from distinct brain regions. Our data reveal a significant reduction of VGLUT1 in excitatory presynaptic terminals in the cortex and hippocampus, identifying a subtle dysbalance in glutamatergic synapse transmission upon H1N1 infection in mice. In conclusion, our results highlight the consequences of systemic IAV-triggered inflammation on the central nervous system and the induction and progression of neuronal alterations. IMPORTANCE Influenza A virus (IAV) causes mainly respiratory tract disease with fever and fatigue but is also associated with behavioral alterations in humans. Here, we demonstrate that infection with a low-dose non-neurotrophic H1N1 strain of IAV causes peripheral immune response followed by a temporary blood-brain barrier disturbance. Characterization of immune cells uncovered subtle alterations in the activation status of microglia cells that might reshape neuronal synapses. We established a highly sensitive multiplex flow cytometry-based approach called flow synaptometry to more closely study the synapses. Thus, we detected a specific dysbalance in glutamatergic synapse transmission upon H1N1 infection in mice. In conclusion, our results highlight the consequences of systemic IAV-triggered inflammation on the central nervous system and the induction and progression of neuronal alterations.

Involvement of various chemokine/chemokine receptor axes in trafficking and oriented locomotion of mesenchymal stem cells in multiple sclerosis patients

Multiple sclerosis (MS) is a specific type of chronic immune-mediated disease in which the immune responses are almost run against the central nervous system (CNS). Despite intensive research, a known treatment for MS disease yet to be introduced. Thus, the development of novel and safe medications needs to be considered for the disease management. Application of mesenchymal stem cells (MSCs) as an emerging approach was recruited forthe treatment of MS. MSCs have several sources and they can be derived from the umbilical cord, adipose tissue, and bone marrow. Chemokines are low molecular weight proteins that their functional activities are achieved by binding to the cell surface G protein-coupled receptors (GPCRs). Chemokine and chemokine receptors are of the most important and effective molecules in MSC trafficking within the different tissues in hemostatic and non-hemostatic circumstances. Chemokine/chemokine receptor axes play a pivotal role in the recruitment and oriented trafficking of immune cells both towards and within the CNS and it appears that chemokine/chemokine receptor signaling may be the most important leading mechanisms in the pathogenesis of MS. In this article, we hypothesized that the chemokine/chemokine receptor axes network have crucial and efficacious impacts on behavior of the MSCs, nonetheless, the exact responsibility of these axes on the targeted tropism of MSCs to the CNS of MS patients yet remained to be fully elucidated. Therefore, we reviewed the ability of MSCs to migrate and home into the CNS of MS patients via expression of various chemokine receptors in response to chemokines expressed by cells of CNS tissue, to provide a great source of knowledge.

Intrinsic Innate Immune Responses Control Viral Growth and Protect against Neuronal Death in an Ex Vivo Model of West Nile Virus-Induced Central Nervous System Disease

Recruitment of immune cells from the periphery is critical for controlling West Nile virus (WNV) growth in the central nervous system (CNS) and preventing subsequent WNV-induced CNS disease. Neuroinflammatory responses, including the release of proinflammatory cytokines and chemokines by CNS cells, influence the entry and function of peripheral immune cells that infiltrate the CNS. However, these same cytokines and chemokines contribute to tissue damage in other models of CNS injury. Rosiglitazone is a peroxisome proliferator-activated receptor gamma (PPARγ) agonist that inhibits neuroinflammation. We used rosiglitazone in WNV-infected ex vivo brain slice cultures (BSC) to investigate the role of neuroinflammation within the CNS in the absence of peripheral immune cells. Rosiglitazone treatment inhibited WNV-induced expression of proinflammatory chemokines and cytokines, interferon beta (IFN-β), and IFN-stimulated genes (ISG) and also decreased WNV-induced activation of microglia. These decreased neuroinflammatory responses were associated with activation of astrocytes, robust viral growth, increased activation of caspase 3, and increased neuronal loss. Rosiglitazone had a similar effect on in vivo WNV infection, causing increased viral growth, tissue damage, and disease severity in infected mice, even though the number of infiltrating peripheral immune cells was higher in rosiglitazone-treated, WNV-infected mice than in untreated, infected controls. These results indicate that local neuroinflammatory responses are capable of controlling viral growth within the CNS and limiting neuronal loss and may function to keep the virus in check prior to the infiltration of peripheral immune cells, limiting both virus- and immune-mediated neuronal damage. IMPORTANCE West Nile virus is the most common cause of epidemic encephalitis in the United States and can result in debilitating CNS disease. There are no effective vaccines or treatments for WNV-induced CNS disease in humans. The peripheral immune response is critical for protection against WNV CNS infections. We now demonstrate that intrinsic immune responses also control viral growth and limit neuronal loss. These findings have important implications for developing new therapies for WNV-induced CNS disease.

Blockade of Autocrine CCL5 Responses Inhibits Zika Virus Persistence and Spread in Human Brain Microvascular Endothelial Cells

Zika virus (ZIKV) is a neurovirulent flavivirus that uniquely causes fetal microcephaly, is sexually transmitted, and persists in patients for up to 6 months. ZIKV persistently infects human brain microvascular endothelial cells (hBMECs) that form the blood-brain barrier (BBB) and enables viral spread to neuronal compartments. We found that CCL5, a chemokine with prosurvival effects on immune cells, was highly secreted by ZIKV-infected hBMECs. Although roles for CCL5 in endothelial cell (EC) survival remain unknown, the presence of the CCL5 receptors CCR3 and CCR5 on ECs suggested that CCL5 could promote ZIKV persistence in hBMECs. We found that exogenous CCL5 induced extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in hBMECs and that ERK1/2 cell survival signaling was similarly activated by ZIKV infection. Neutralizing antibodies to CCL5, CCR3, or CCR5 inhibited persistent ZIKV infection of hBMECs. While knockout (KO) of CCL5 failed to prevent ZIKV infection of hBMECs, at 3 days postinfection (dpi), we observed a >90% reduction in ZIKV-infected CCL5-KO hBMECs and a multilog reduction in ZIKV titers. In contrast, the addition of CCL5 to CCL5-KO hBMECs dose-dependently rescued ZIKV persistence in hBMECs. Inhibiting CCL5 responses using CCR3 (UCB35625) and CCR5 (maraviroc) receptor antagonists reduced the number of ZIKV-infected hBMECs and ZIKV titers (50% inhibitory concentrations [IC₅₀s] of 2.5 to 12 μM), without cytotoxicity (50% cytotoxic concentration [CC₅₀] of >80 μM). These findings demonstrate that ZIKV-induced CCL5 directs autocrine CCR3/CCR5 activation of ERK1/2 survival responses that are required for ZIKV to persistently infect hBMECs. Our results establish roles for CCL5 in ZIKV persistence and suggest the potential for CCL5 receptor antagonists to therapeutically inhibit ZIKV spread and neurovirulence.

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.

Common Dysregulation of Innate Immunity Pathways in Human Primary Astrocytes Infected With Chikungunya, Mayaro, Oropouche, and Zika Viruses

Arboviruses pose a major threat throughout the world and represent a great burden in tropical countries of South America. Although generally associated with moderate febrile illness, in more severe cases they can lead to neurological outcomes, such as encephalitis, Guillain-Barré syndrome, and Congenital Syndromes. In this context astrocytes play a central role in production of inflammatory cytokines, regulation of extracellular matrix, and control of glutamate driven neurotoxicity in the central nervous system. Here, we presented a comprehensive genome-wide transcriptome analysis of human primary astrocytes infected with Chikungunya, Mayaro, Oropouche, or Zika viruses. Analyses of differentially expressed genes (DEGs), pathway enrichment, and interactomes have shown that Alphaviruses up-regulated genes related to elastic fiber formation and N-glycosylation of glycoproteins, with down-regulation of cell cycle and DNA stability and chromosome maintenance genes. In contrast, Oropouche virus up-regulated cell cycle and DNA maintenance and condensation pathways while down-regulated extracellular matrix, collagen metabolism, glutamate and ion transporters pathways. Zika virus infection only up-regulated eukaryotic translation machinery while down-regulated interferon pathways. Reactome and integration analysis revealed a common signature in down-regulation of innate immune response, antiviral response, and inflammatory cytokines associated to interferon pathway for all arboviruses tested. Validation of interferon stimulated genes by reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) corroborated our transcriptome findings. Altogether, our results showed a co-evolution in the mechanisms involved in the escape of arboviruses to antiviral immune response mediated by the interferon (IFN) pathway.

Chemokines and chemokine receptors during COVID-19 infection

Chemokines are crucial inflammatory mediators needed during an immune response to clear pathogens. However, their excessive release is the main cause of hyperinflammation. In the recent COVID-19 outbreak, chemokines may be the direct cause of acute respiratory disease syndrome, a major complication leading to death in about 40% of severe cases. Several clinical investigations revealed that chemokines are directly involved in the different stages of SARS-CoV-2 infection. Here, we review the role of chemokines and their receptors in COVID-19 pathogenesis to better understand the disease immunopathology which may aid in developing possible therapeutic targets for the infection.

Ifit2 deficiency restricts microglial activation and leukocyte migration following murine coronavirus (m-CoV) CNS infection

The interferon-induced tetratricopeptide repeat protein (Ifit2) protects mice from lethal neurotropic viruses. Neurotropic coronavirus MHV-RSA59 infection of Ifit2^-/- mice caused pronounced morbidity and mortality accompanied by rampant virus replication and spread throughout the brain. In spite of the higher virus load, induction of many cytokines and chemokines in the brains of infected Ifit2^-/- mice were similar to that in wild-type mice. In contrast, infected Ifit2^-/- mice revealed significantly impaired microglial activation as well as reduced recruitment of NK1.1 T cells and CD4 T cells to the brain, possibly contributing to the lack of viral clearance. These two deficiencies were associated with a lower level of microglial expression of CX3CR1, the receptor of the CX3CL1 (Fractalkine) chemokine, which plays a critical role in both microglial activation and leukocyte recruitment. The above results uncovered a new potential role of an interferon-induced protein in immune protection.

Interferons (IFNs) are known to protect from virus dissemination and pathogenesis. Several IFN stimulated genes (ISG) regulate neuropathogenesis but the mechanisms underlying the antiviral effects are not clearly understood. IFN induced tetratricopeptide repeats (Ifit) are a class of ISGs. Among the Ifits, Ifit2 is known to play a beneficial role in restricting neurotropic viral replication. To provide better cellular insights into the protective mechanisms of Ifit2 functions, using a neurotropic coronavirus infection in Ifit2 depleted mice we report that in the absence of Ifit2, viral replication is dramatically increased and mice develop severe clinical signs and symptoms of neurological deficit. Despite the enormous viral load, Ifit2 deficient mice are impaired in microglial activation and recruitment of peripheral leukocytes into the CNS. This impaired leuocyte infiltration in Ifit2 deficient mice was also associated with reduced expression of a novel chemokine receptor CX3CR1,which is important for viral induced microglial activation and maintaining tissue homeostasis.

Effector CD8 T Cell-Dependent Zika Virus Control in the CNS: A Matter of Time and Numbers

Zika virus (ZIKV), a mosquito-borne flavivirus, came into the spotlight in 2016 when it was found to be associated with an increased rate of microcephalic newborns in Brazil. The virus has further been recognized to cause neurologic complications in children and adults in the form of myelitis, encephalitis, acute disseminated encephalomyelitis (ADEM) and Guillain Barre Syndrome in a fraction of infected individuals. With the ultimate goal of identifying correlates of protection to guide the design of an effective vaccine, the study of the immune response to ZIKV infection has become the focus of research worldwide. Both innate and adaptive immune responses seem to be essential for controlling the infection. Induction of sufficient levels of neutralizing antibodies has been strongly correlated with protection against reinfection in various models, while the role of CD8 T cells as antiviral effectors in the CNS has been controversial. In an attempt to improve our understanding regarding the role of ZIKV-induced CD8 T cells in protective immunity inside the CNS, we have expanded on previous studies in intracranially infected mice. In a recent study, we have demonstrated that, peripheral ZIKV infection in adult C57BL/6 mice induces a robust CD8 T cell response that peaks within a week. In the present study, we used B cell deficient as well as wild-type mice to show that there is a race between CXCR3-dependent recruitment of the effector CD8 T cells and local ZIKV replication, and that CD8 T cells are capable of local viral control if they arrive in the brain early after viral invasion, in appropriate numbers and differentiation state. Our data highlight the benefits of considering this subset when designing vaccines against Zika virus.

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.

Sex differences in cytokine production following West Nile virus infection: Implications for symptom manifestation

West Nile virus (WNV) infection outcomes vary among individuals, with most infections resulting in asymptomatic or mild flu-like symptoms. We previously reported an association between early cytokine production and symptom outcome following WNV infection in US blood donors. In this meta-analysis, we found that WNV-infected females reported more symptoms than WNV-infected males, despite similar initial viremia and type I interferon responses. As the infection progressed, males exhibited a protracted cytokine response-marked by sustained CCL2 (MCP-1), CCL11 (eotaxin-1), CXCL10 (IP-10) and IL-15-that was absent in females. Our results suggest that sex differences may be a factor in sustaining WNV immunity.

Immune Correlates of Protection From West Nile Virus Neuroinvasion and Disease

BACKGROUND

A challenge to the design of improved therapeutic agents and prevention strategies for neuroinvasive infection and associated disease is the lack of known natural immune correlates of protection. A relevant model to study such correlates is offered by the Collaborative Cross (CC), a panel of recombinant inbred mouse strains that exhibit a range of disease manifestations upon infection.

METHODS

We performed an extensive screen of CC-F1 lines infected with West Nile virus (WNV), including comprehensive immunophenotyping, to identify groups of lines that exhibited viral neuroinvasion or neuroinvasion with disease and lines that remained free of WNV neuroinvasion and disease.

RESULTS

Our data reveal that protection from neuroinvasion and disease is multifactorial and that several immune outcomes can contribute. Immune correlates identified include decreased suppressive activity of regulatory T cells at steady state, which correlates with peripheral restriction of the virus. Further, a rapid contraction of WNV-specific CD8+ T cells in the brain correlated with protection from disease.

CONCLUSIONS

These immune correlates of protection illustrate additional networks and pathways of the WNV immune response that cannot be observed in the C57BL/6 mouse model. Additionally, correlates of protection exhibited before infection, at baseline, provide insight into phenotypic differences in the human population that may predict clinical outcomes upon infection.

Cellular microRNA-155 Regulates Virus-Induced Inflammatory Response and Protects against Lethal West Nile Virus Infection

West Nile virus (WNV) is a flavivirus that has disseminated globally as a significant cause of viral encephalitis in humans. MircoRNA-155 (miR-155) regulates various aspects of innate and adaptive immune responses. We previously reported that WNV infection induces upregulation of miR-155 in mice brains. In the current study, we demonstrate the critical role of miR-155 in restricting the pathogenesis of WNV infection in mice. Compared to wild-type (WT) mice, miR-155 knockout mice exhibited significantly higher morbidity and mortality after infection with either a lethal strain, WNV NY99, or a non-lethal strain, WNV Eg101. Increased mortality in miR-155^−/− mice was associated with significantly high WNV burden in the serum and brains. Protein levels of interferon (IFN)-α in the serum and brains were higher in miR-155^−/− mice. However, miR-155^−/− mice exhibited significantly lower protein levels of anti-viral interleukin (IL)-1β, IL-12, IL-6, IL-15, and GM-CSF despite the high viral load. Primary mouse cells lacking miR-155 were more susceptible to infection with WNV compared to cells derived from WT mice. Besides, overexpression of miR-155 in human neuronal cells modulated anti-viral cytokine response and resulted in significantly lower WNV replication. These data collectively indicate that miR-155 restricts WNV production in mouse and human cells and protects against lethal WNV infection in mice.

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.

Genotypic and phenotypic characterization of West Nile virus NS5 methyltransferase mutants

Although West Nile virus (WNV) causes annual cases of neurological disease and deaths in humans, a vaccine has not been licensed for human use. Several WNV genes have been targeted for mutagenesis in attempts to generate live attenuated vaccine candidates, including the non-structural protein NS5. Specifically, mutation of WNV NS5-K61A or NS5-E218A in the catalytic tetrad of the methyltransferase decreases enzyme activity of the NS5 protein and correspondingly attenuates the virus in mice. In this report, NS5-K61A, NS5-E218A, and a double mutant encoding both mutations (NS5-K61A/E218A) were compared both in vitro and in vivo. Each single mutant was strongly attenuated in highly susceptible outbred mice, whereas the double mutant unexpectedly was not attenuated. Sequencing analysis demonstrated that the double mutant was capable of reversion at both residues NS5-61 and NS5-218, whereas the genotype of the single mutants did not show evidence of reversion. Overall, either NS5-K61A or NS5-E218A methyltransferase mutations could be potential mutations to include in a candidate live WNV vaccine; however, multiple mutations in the catalytic tetrad of the methyltransferase are not tolerated.

Antibody-suppressor CD8+ T Cells Require CXCR5

BACKGROUND

We previously reported the novel activity of alloprimed CD8 T cells that suppress posttransplant alloantibody production. The purpose of the study is to investigate the expression and role of CXCR5 on antibody-suppressor CD8 T-cell function.

METHODS

C57BL/6 mice were transplanted with FVB/N hepatocytes. Alloprimed CD8 T cells were retrieved on day 7 from hepatocyte transplant recipients. Unsorted or flow-sorted (CXCR5CXCR3 and CXCR3CXCR5) alloprimed CD8 T-cell subsets were analyzed for in vitro cytotoxicity and capacity to inhibit in vivo alloantibody production following adoptive transfer into C57BL/6 or high alloantibody-producing CD8 knock out (KO) hepatocyte transplant recipients. Alloantibody titer was assessed in CD8 KO mice reconstituted with naive CD8 T cells retrieved from C57BL/6, CXCR5 KO, or CXCR3 KO mice. Antibody suppression by ovalbumin (OVA)-primed monoclonal OVA-specific t-cell receptor transgenic CD8+ T cells (OT-I) CXCR5 or CXCR3 CD8 T-cell subsets was also investigated.

RESULTS

Alloprimed CXCR5CXCR3CD8 T cells mediated in vitro cytotoxicity of alloprimed "self" B cells, while CXCR3CXCR5CD8 T cells did not. Only flow-sorted alloprimed CXCR5CXCR3CD8 T cells (not flow-sorted alloprimed CXCR3CXCR5CD8 T cells) suppressed alloantibody production and enhanced graft survival when transferred into transplant recipients. Unlike CD8 T cells from wild-type or CXCR3 KO mice, CD8 T cells from CXCR5 KO mice do not develop alloantibody-suppressor function. Similarly, only flow-sorted CXCR5CXCR3 (and not CXCR3CXCR5) OVA-primed OT-I CD8 T cells mediated in vivo suppression of anti-OVA antibody production.

CONCLUSIONS

These data support the conclusion that expression of CXCR5 by antigen-primed CD8 T cells is critical for the function of antibody-suppressor CD8 T cells.

Neuroinflammation During RNA Viral Infections

Neurotropic RNA viruses continue to emerge and are increasingly linked to diseases of the central nervous system (CNS) despite viral clearance. Indeed, the overall mortality of viral encephalitis in immunocompetent individuals is low, suggesting efficient mechanisms of virologic control within the CNS. Both immune and neural cells participate in this process, which requires extensive innate immune signaling between resident and infiltrating cells, including microglia and monocytes, that regulate the effector functions of antiviral T and B cells as they gain access to CNS compartments. While these interactions promote viral clearance via mainly neuroprotective mechanisms, they may also promote neuropathology and, in some cases, induce persistent alterations in CNS physiology and function that manifest as neurologic and psychiatric diseases. This review discusses mechanisms of RNA virus clearance and neurotoxicity during viral encephalitis with a focus on the cytokines essential for immune and neural cell inflammatory responses and interactions. Understanding neuroimmune communications in the setting of viral infections is essential for the development of treatments that augment neuroprotective processes while limiting ongoing immunopathological processes that cause ongoing CNS disease.

Network meta-analysis correlates with analysis of merged independent transcriptome expression data

Background

Using meta-analysis, high-dimensional transcriptome expression data from public repositories can be merged to make group comparisons that have not been considered in the original studies. Merging of high-dimensional expression data can, however, implicate batch effects that are sometimes difficult to be removed. Removing batch effects becomes even more difficult when expression data was taken using different technologies in the individual studies (e.g. merging of microarray and RNA-seq data). Network meta-analysis has so far not been considered to make indirect comparisons in transcriptome expression data, when data merging appears to yield biased results.

Results

We demonstrate in a simulation study that the results from analyzing merged data sets and the results from network meta-analysis are highly correlated in simple study networks. In the case that an edge in the network is supported by multiple independent studies, network meta-analysis produces fold changes that are closer to the simulated ones than those obtained from analyzing merged data sets. Finally, we also demonstrate the practicability of network meta-analysis on a real-world data example from neuroinfection research.

Conclusions

Network meta-analysis is a useful means to make new inferences when combining multiple independent studies of molecular, high-throughput expression data. This method is especially advantageous when batch effects between studies are hard to get removed.

Electronic supplementary material

The online version of this article (10.1186/s12859-019-2705-9) contains supplementary material, which is available to authorized users.

High neopterin and IP-10 levels in cerebrospinal fluid are associated with neurotoxic tryptophan metabolites in acute central nervous system infections

Background

The host response to intruders in the central nervous system (CNS) may be beneficial but could also be harmful and responsible for neurologic symptoms and sequelae in CNS infections. This immune response induces the activation of the kynurenine pathway (KP) with the production of neuroactive metabolites. Herein, we explored cytokine and KP responses in cerebrospinal fluid (CSF) and serum in patients with encephalitis, aseptic, and bacterial meningitis.

Methods

Cytokines were measured in CSF and serum by multiplex assay in adult patients with encephalitis of infectious, autoimmune or unknown etiology (n = 10), aseptic meningitis (ASM, n = 25), acute bacterial meningitis (ABM, n = 6), and disease control patients with similar symptoms but without pleocytosis in CSF (n = 42). Liquid chromatography-tandem mass spectrometry (LC-MS/ MS) was used to measure KP metabolites in CSF and serum.

Results

A characteristic pattern of increasing cytokine levels and KP metabolites was found in CSF from encephalitis to ASM, with the highest levels in ABM. In ASM and ABM, most inflammatory mediators, including IL-6, IL-8, and IFN-inducible protein-10 (IP-10), showed markedly elevated levels in CSF compared with serum, indicating production within the CNS. In contrast to most mediators, the highest level of IP-10 was found in the ASM group, suggesting a potential role for IP-10 in aseptic/viral meningitis. Neopterin and IP-10 were associated with marked changes in KP metabolites in CSF with increasing kynurenine/tryptophan ratio reflecting indoleamine 2,3-dioxygenase activity. Neopterin, a marker of IFN-γ activity, was associated with an unfavorable balance between neuroprotective and neurotoxic tryptophan metabolites.

Conclusion

We show that parenchymal and meningeal inflammations in CNS share a characteristic cytokine profile with a general immune response in the CSF with limited influence from the systemic circulation. IFN-γ activity, assessed by neopterin and IP-10 levels, may play a role in the activation of the KP pathway in these patients, potentially mediating neurotoxic effects.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1366-3) contains supplementary material, which is available to authorized users.

Viral Encephalitis and Neurologic Diseases: Focus on Astrocytes

Neurotropic RNA virus infections cause a major neurological disease burden. Due to the morbidity and mortality rates of viral encephalitides worldwide, there is a need to develop clinical treatments. Features of the central nervous system (CNS), including interconnected cell types and limited regeneration, provide unique challenges. Viral encephalitis and antiviral immunity can disrupt the CNS environment, leaving patients with poor neurological outcomes despite virologic control. The cellular mechanism(s) underlying neurological recovery are not fully understood, but involve neuroimmune interactions that, until recently, primarily focused on microglia. With increasing evidence that astrocytes also have significant roles in inflammatory responses to viruses, here we summarize recent astrocyte contributions to acute virologic control and neurological impairments during recovery from viral infection.

Host genetic control of mosquito-borne Flavivirus infections

Flaviviruses are arthropod-borne viruses, several of which represent emerging or re-emerging pathogens responsible for widespread infections with consequences ranging from asymptomatic seroconversion to severe clinical diseases and congenital developmental deficits. This variability is due to multiple factors including host genetic determinants, the role of which has been investigated in mouse models and human genetic studies. In this review, we provide an overview of the host genes and variants which modify susceptibility or resistance to major mosquito-borne flaviviruses infections in mice and humans.

DNMT1 regulates expression of MHC class I in post-mitotic neurons

Major Histocompability Complex I (MHC-I) molecules present cellularly derived peptides to the adaptive immune system. Generally MHC-I is not expressed on healthy post-mitotic neurons in the central nervous system, but it is known to increase upon immune activation such as viral infections and also during neurodegenerative processes. MHC-I expression is known to be regulated by the DNA methyltransferase DNMT1 in non-neuronal cells. Interestingly DNMT1 expression is high in neurons despite these being non-dividing. This suggests a role for DNMT1 in neurons beyond the classical re-methylation of DNA after cell division. We thus investigated whether DNMT1 regulates MHC-I in post-mitotic neurons. For this we used primary cultures of mouse cerebellar granule neurons (CGNs). Our results showed that knockdown of DNMT1 in CGNs caused upregulation of some, but not all subtypes of MHC-I genes. This effect was synergistically enhanced by subsequent IFNγ treatment. Overall MHC-I protein level was not affected by knockdown of DNMT1 in CGNs. Instead our results show that the relative MHC-I expression levels among the different MHC subtypes is regulated by DNMT1 activity. In conclusion, we show that while the mouse H2-D1/L alleles are suppressed in neurons by DNMT1 activity under normal circumstances, the H2-K1 allele is not. This finding is particularly important in two instances. One: in the context of CNS autoimmunity with epitope presentation by specific MHC-I subtypes where this allele specific regulation might become important; and two: in amyotropic lateral sclerosis (ALS) where H2-K but not H2-D protects motor neurons from ALS astrocyte-induced toxicity in a mouse model of ALS.

T Cell Receptor-Major Histocompatibility Complex Interaction Strength Defines Trafficking and CD103+ Memory Status of CD8 T Cells in the Brain

T cell receptor-major histocompatibility complex (TCR-MHC) affinities span a wide range in a polyclonal T cell response, yet it is undefined how affinity shapes long-term properties of CD8 T cells during chronic infection with persistent antigen. Here, we investigate how the affinity of the TCR-MHC interaction shapes the phenotype of memory CD8 T cells in the chronically Toxoplasma gondii-infected brain. We employed CD8 T cells from three lines of transnuclear (TN) mice that harbor in their endogenous loci different T cell receptors specific for the same Toxoplasma antigenic epitope ROP7. The three TN CD8 T cell clones span a wide range of affinities to MHCI-ROP7. These three CD8 T cell clones have a distinct and fixed hierarchy in terms of effector function in response to the antigen measured as proliferation capacity, trafficking, T cell maintenance, and memory formation. In particular, the T cell clone of lowest affinity does not home to the brain. The two higher affinity T cell clones show differences in establishing resident-like memory populations (CD103+) in the brain with the higher affinity clone persisting longer in the host during chronic infection. Transcriptional profiling of naïve and activated ROP7-specific CD8 T cells revealed that Klf2 encoding a transcription factor that is known to be a negative marker for T cell trafficking is upregulated in the activated lowest affinity ROP7 clone. Our data thus suggest that TCR-MHC affinity dictates memory CD8 T cell fate at the site of infection.

Niches for the Long-Term Maintenance of Tissue-Resident Memory T Cells

Tissue-resident memory T cells (T_RM cells) are a population of immune cells that reside in the lymphoid and non-lymphoid organs without recirculation through the blood. These important cells occupy and utilize unique anatomical and physiological niches that are distinct from those for other memory T cell populations, such as central memory T cells in the secondary lymphoid organs and effector memory T cells that circulate through the tissues. CD8⁺ T_RM cells typically localize in the epithelial layers of barrier tissues where they are optimally positioned to act as sentinels to trigger antigen-specific protection against reinfection. CD4⁺ T_RM cells typically localize below the epithelial layers, such as below the basement membrane, and cluster in lymphoid structures designed to optimize interactions with antigen-presenting cells upon reinfection. A key feature of T_RM populations is their ability to be maintained in barrier tissues for prolonged periods of time. For example, skin CD8⁺ T_RM cells displace epidermal niches originally occupied by γδ T cells, thereby enabling their stable persistence for years. It is also clear that the long-term maintenance of T_RM cells in different microenvironments is dependent on multiple tissue-specific survival cues, although the specific details are poorly understood. However, not all T_RM persist over the long term. Recently, we identified a new spatial niche for the maintenance of CD8⁺ T_RM cells in the lung, which is created at the site of tissue regeneration after injury [termed repair-associated memory depots (RAMD)]. The short-lived nature of RAMD potentially explains the short lifespans of CD8⁺ T_RM cells in this particular tissue. Clearly, a better understanding of the niche-dependent maintenance of T_RM cells will be important for the development of vaccines designed to promote barrier immunity. In this review, we discuss recent advances in our understanding of the properties and nature of tissue-specific niches that maintain T_RM cells in different tissues.

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.

Chemokines: Critical Regulators of Memory T Cell Development, Maintenance, and Function

Memory T cells are central to orchestrating antigen-specific recall responses in vivo. Compared to naïve T cells, memory T cells respond more quickly to cognate peptide:MHC with a shorter lag time for entering the cell cycle and exerting effector functions. However, it is now well established that this enhanced responsiveness is not the only mechanism whereby memory T cells are better equipped than naïve T cells to rapidly and robustly induce inflammation. In contrast to naïve T cells, memory T cells are composed of distinct subsets with unique trafficking patterns and localizations. Tissue-resident memory T cells persist in previously inflamed tissue and function as first responders to cognate antigen reexposure. In addition, a heterogeneous group of circulating memory T cells augment inflammation by either rapidly migrating to inflamed tissue or responding to cognate antigen within secondary lymphoid organs and producing additional effector T cells. Defining the mechanisms regulating T cell positioning and trafficking and how this influences the development, maintenance, and function of memory T cell subsets is essential to improving vaccine design as well as treatment of immune-mediated diseases. In this chapter, we will review our current knowledge of how chemokines, critical regulators of cell positioning and migration, govern memory T cell biology in vivo. In addition, we discuss areas of uncertainty and future directions for further delineating how T cell localization influences memory T cell biology.

Cytokines Interleukin 4 (IL-4) and Interleukin 10 (IL-10) Gene Polymorphisms as Potential Host Susceptibility Factors in Virus-Induced Encephalitis

Background

This study aimed to analyze and explore the relationship between the cytokines IL-4 and IL-10 in relation to gene polymorphism and their respective effects on the susceptibility to virus-induced encephalitis.

Material/Methods

From January 2012 to June 2013, 112 patients with virus-induced encephalitis (the case group and 109 healthy individuals (the control group) were recruited for the purposes of this study. The functional variations that IL-4 and IL-10 genes exhibit were detected through the use of a function analysis and selection tool for single-nucleotide polymorphisms (FASTSNP). The genotypes of IL-4 were rs2227283 and IL-4 rs2227288, and the genotypes of IL-10 were rs1800871 and IL-10 rs1800872. These genotypes were respectively assessed using direct sequencing.

Results

IL-4 rs2227283 and IL-10 rs1800871 have no correlation in with risk of virus-induced encephalitis (both P>0.05)

GA and AA genotypes were related to IL-4 rs2227288 and GT, while TT and GT + TT genotypes were related to IL-10 rs1800872. These were highlighted as being risk factors in virus-induced encephalitis (all P<0.05). However, the duration of fever, white blood cell (WBC) count, C-reactive protein (CRP), neutrophils, and lymphocytes and monocytes of virus-induced encephalitis patients with IL-4 rs2227288 and IL-10 rs1800872 all displayed significant differences (all P<0.05). Frequencies of GAGT and CAGT haplotypes were evaluated and deemed to be of statistical significance and subsequently were highlighted as being risk factors in virus-induced encephalitis (all P<0.05).

Conclusions

IL-4 rs2227288 and IL-10 rs1800872 may contribute to an increased risk for virus-induced encephalitis. Through use of direct sequencing, we showed that genotypes of IL-4 rs2227288 and IL-10 rs1800872 may have particular host susceptibility to virus-induced encephalitis.

Sequential transmigration of polymorphonuclear cells and naive CD3+ T lymphocytes across the blood-cerebrospinal-fluid barrier in vitro following infection with Echovirus 30

Viral meningitis by non-polio enteroviruses (NPEV) is a major public health burden causing fatal outcomes especially in the younger population. Strong evidence exists that the blood-cerebrospinal-fluid (CSF) barrier (BCSFB) serves as an entry point for enterovirus and leucocytes into the central nervous system (CNS). Moreover, analysis of clinical CSF specimens of patients with a NPEV infection revealed a predominance of polymorphonuclear granulocytes (PMN) in the early phase and mononuclear cells in the later course of meningitis. By applying a functional in vitro model of the BCSFB consisting of human choroid plexus papilloma (HIBCPP) cells, we aimed to analyse the mechanisms of sequential migration of PMN and naive CD3⁺ T lymphocytes following infection with Echovirus 30 (EV30). EV30 infection led to increased transmigration of PMN and naive CD3⁺ T lymphocytes. Transmigration of PMN was significantly enhanced in the presence of naive CD3⁺ T lymphocytes, but not vice versa. The barrier function was not differentially altered under the respective conditions. Infection with EV30 led to an upregulation of CXCL3 and CXCL11 on the RNA-level. Additional analysis of cytokine secretion revealed relatively high concentrations of IL-8, CCL20, CXCL3, CXCL10 and M-CSF. Overall, there was a predominantly polar direction of cytokine secretion to the basolateral side. IL-7 was the only cytokine which was strongly secreted to the apical side and that was enhanced following EV30 infection in our model. In conclusion, this study highlights the role of the choroid plexus and cytokines in regulating leucocyte entry into the CNS in the context of EV30 infection.

T cell responses in the central nervous system

T cells are required for immune surveillance of the central nervous system (CNS); however, they can also induce severe immunopathology in the context of both viral infections and autoimmunity. The mechanisms that are involved in the priming and recruitment of T cells to the CNS are only partially understood, but there has been renewed interest in this topic since the 'rediscovery' of lymphatic drainage from the CNS. Moreover, tissue-resident memory T cells have been detected in the CNS and are increasingly recognized as an autonomous line of host defence. In this Review, we highlight the main mechanisms that are involved in the priming and CNS recruitment of CD4⁺ T cells, CD8⁺ T cells and regulatory T cells. We also consider the plasticity of T cell responses in the CNS, with a focus on viral infection and autoimmunity.

Enhanced viral clearance and reduced leukocyte infiltration in experimental herpes encephalitis after intranasal infection of CXCR3-deficient mice

Herpes simplex virus type 1 (HSV-1) encephalitis (HSE) is the most common fatal sporadic encephalitis in developed countries. There is evidence from HSE animal models that not only direct virus-mediated damage caused but also the host's immune response contributes to the high mortality of the disease. Chemokines modulate and orchestrate this immune response. Previous experimental studies in HSE models identified the chemokine receptor CXCR3 and its ligands as molecules with a high impact on the course of HSE in mouse models. In this study, the role of the chemokine receptor CXCR3 was evaluated after intranasal infection with the encephalitogenic HSV-1 strain 17 syn⁺ using CXCR3-deficient mice (CXCR3^-/-) and wild-type controls. We demonstrated a neurotropic viral spread into the CNS of after intranasal infection. Although viral load and histological distribution of infected neurons were independent from CXCR3 signaling early after infection, CXCR3-deficient mice cleared HSV-1 more efficiently 14 days after infection. Furthermore, CXCR3 deficiency led to a decreased weight loss in mice after HSV-1 infection. T cell infiltration and microglial activation was prominently reduced by inhibition of CXCR3 signaling. Quantitative PCR of proinflammatory cytokines and chemokines confirmed the reduced neuroinflammatory response in CXCR3-deficient mice during HSE. Our results demonstrate that the recruitment of peripheral immune cells into the CNS, induction of neuroinflammation, and consecutive weight loss during herpes encephalitis is modulated by CXCR3 signaling. Interruption of the CXCR3 pathway ameliorates the detrimental host immune response and in turn, leads paradoxically to an enhanced viral clearance after intranasal infection. Our data gives further insight into the role of CXCR3 during HSE after intranasal infection.

TGF-β Controls the Formation of Kidney-Resident T Cells via Promoting Effector T Cell Extravasation

Tissue-resident memory T (T_RM) cells, a population of noncirculating memory T cells, are one of the essential components of immunological memory in both mouse and human. Although CD69⁺CD103⁺ T_RM cells represent a major T_RM cell population in barrier tissues including the mucosal surface and the skin, CD69⁺CD103^- T_RM cells dominate most nonbarrier tissues, such as the kidney. TGF-β is required for the differentiation of CD69⁺CD103⁺ T_RM cells in barrier tissues. However, the developmental control of CD69⁺CD103^- T_RM cells in nonbarrier tissues remains largely unknown and the involvement of TGF-β signaling is less clear. In this study we demonstrated that TGF-β promoted the formation of kidney-resident T cells via enhancing the tissue entry of effector T cells. Mechanistically, TGF-β enhanced E- and P-selectin and inflammatory chemokine-mediated extravasation of effector T cells. Thus TGF-β controls the first developmental checkpoint of T_RM cell differentiation in nonbarrier tissues.

Age-related alterations in immune responses to West Nile virus infection

West Nile virus (WNV) is the most important causative agent of viral encephalitis worldwide and an important public health concern in the United States due to its high prevalence, severe disease, and the absence of effective treatments. Infection with WNV is mainly asymptomatic, but some individuals develop severe, possibly fatal, neurological disease. Individual host factors play a role in susceptibility to WNV infection, including genetic polymorphisms in key anti-viral immune genes, but age is the most well-defined risk factor for susceptibility to severe disease. Ageing is associated with distinct changes in immune cells and a decline in immune function leading to increased susceptibility to infection and reduced responses to vaccination. WNV is detected by pathogen recognition receptors including Toll-like receptors (TLRs), which show reduced expression and function in ageing. Neutrophils, monocyte/macrophages and dendritic cells, which first recognize and respond to infection, show age-related impairment of many functions relevant to anti-viral responses. Natural killer cells control many viral infections and show age-related changes in phenotype and functional responses. A role for the regulatory receptors Mertk and Axl in blood-brain barrier permeability and in facilitating viral uptake through phospholipid binding may be relevant for susceptibility to WNV, and age-related up-regulation of Axl has been noted previously in human dendritic cells. Understanding the specific immune parameters and mechanisms that influence susceptibility to symptomatic WNV may lead to a better understanding of increased susceptibility in elderly individuals and identify potential avenues for therapeutic approaches: an especially relevant goal, as the world's populating is ageing.

T Cell Interstitial Migration: Motility Cues from the Inflamed Tissue for Micro- and Macro-Positioning

Effector T cells exit the inflamed vasculature into an environment shaped by tissue-specific structural configurations and inflammation-imposed extrinsic modifications. Once within interstitial spaces of non-lymphoid tissues, T cells migrate in an apparent random, non-directional, fashion. Efficient T cell scanning of the tissue environment is essential for successful location of infected target cells or encounter with antigen-presenting cells that activate the T cell's antimicrobial effector functions. The mechanisms of interstitial T cell motility and the environmental cues that may promote or hinder efficient tissue scanning are poorly understood. The extracellular matrix (ECM) appears to play an important scaffolding role in guidance of T cell migration and likely provides a platform for the display of chemotactic factors that may help to direct the positioning of T cells. Here, we discuss how intravital imaging has provided insight into the motility patterns and cellular machinery that facilitates T cell interstitial migration and the critical environmental factors that may optimize the efficiency of effector T cell scanning of the inflamed tissue. Specifically, we highlight the local micro-positioning cues T cells encounter as they migrate within inflamed tissues, from surrounding ECM and signaling molecules, as well as a requirement for appropriate long-range macro-positioning within distinct tissue compartments or at discrete foci of infection or tissue damage. The central nervous system (CNS) responds to injury and infection by extensively remodeling the ECM and with the de novo generation of a fibroblastic reticular network that likely influences T cell motility. We examine how inflammation-induced changes to the CNS landscape may regulate T cell tissue exploration and modulate function.

Arbovirus Infections

PURPOSE OF REVIEW

Arbovirus (arthropod-borne virus) infections are increasingly important causes of neurologic disease in the United States through both endemic transmission and travel-associated infections. This article reviews the major arbovirus infections that can cause neurologic disease likely to be encountered in the United States.

RECENT FINDINGS

West Nile virus continues to be an important cause of epidemic encephalitis, while emerging arbovirus infections such as dengue and chikungunya have rapidly expanded their geographic distribution. As emerging arboviruses expand in new geographic regions, neurologic abnormalities are reported in new patient populations.

SUMMARY

Emerging arbovirus infections are increasingly important causes of neurologic disease throughout the world and in the United States. While no US Food and Drug Administration (FDA)-approved therapy is yet available for these infections, prompt recognition and diagnosis from the consulting neurologist will ensure appropriate supportive care for the patient.

CCR5 ameliorates Japanese encephalitis via dictating the equilibrium of regulatory CD4(+)Foxp3(+) T and IL-17(+)CD4(+) Th17 cells

Background

CCR5 is a CC chemokine receptor involved in the migration of effector leukocytes including macrophages, NK, and T cells into inflamed tissues. Also, the role of CCR5 in CD4⁺Foxp3⁺ regulatory T cell (Treg) homing has recently begun to grab attention. Japanese encephalitis (JE) is defined as severe neuroinflammation of the central nervous system (CNS) following infection with mosquito-borne flavivirus JE virus. However, the potential contribution of CCR5 to JE progression via mediating CD4⁺Foxp3⁺ Treg homing has not been investigated.

Methods

Infected wild-type (Ccr5^+/+) and CCR5-deficient (Ccr5^−/−) mice were examined daily for mortality and clinical signs, and neuroinflammation in the CNS was evaluated by infiltration of inflammatory leukocytes and cytokine expression. In addition, viral burden, NK- and JEV-specific T cell responses were analyzed. Adoptive transfer of CCR5⁺CD4⁺Foxp3⁺ Tregs was used to evaluate the role of Tregs in JE progression.

Results

CCR5 ablation exacerbated JE without altering viral burden in the extraneural and CNS tissues, as manifested by increased CNS infiltration of Ly-6C^hi monocytes and Ly-6G^hi granulocytes. Compared to Ccr5^+/+ mice, Ccr5^−/− mice unexpectedly showed increased responses of IFN-γ⁺NK and CD8⁺ T cells in the spleen, but not CD4⁺ T cells. More interestingly, CCR5-ablation resulted in a skewed response to IL-17⁺CD4⁺ Th17 cells and correspondingly reduced CD4⁺Foxp3⁺ Tregs in the spleen and brain, which was closely associated with exacerbated JE. Our results also revealed that adoptive transfer of sorted CCR5⁺CD4⁺Foxp3⁺ Tregs into Ccr5^−/− mice could ameliorate JE progression without apparently altering the viral burden and CNS infiltration of IL-17⁺CD4⁺ Th17 cells, myeloid-derived Ly-6C^hi monocytes and Ly-6G^hi granulocytes. Instead, adoptive transfer of CCR5⁺CD4⁺Foxp3⁺ Tregs into Ccr5^−/− mice resulted in increased expression of anti-inflammatory cytokines (IL-10 and TGF-β) in the spleen and brain, and transferred CCR5⁺ Tregs were found to produce IL-10.

Conclusions

CCR5 regulates JE progression via governing timely and appropriate CNS infiltration of CD4⁺Foxp3⁺ Tregs, thereby facilitating host survival. Therefore, this critical and extended role of CCR5 in JE raises possible safety concerns regarding the use of CCR5 antagonists in human immunodeficiency virus (HIV)-infected individuals who inhabit regions in which both HIV and flaviviruses, such as JEV and West Nile virus, are endemic.

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.