Flavivirus infection up-regulates the expression of class I and class II major histocompatibility antigens on and enhances T cell recognition of astrocytes in vitro

Gestational Viral Infections: Role of Host Immune System

Viral infections in pregnancy are major causes of maternal and fetal morbidity and mortality. Infections can develop in the neonate transplacentally, perinatally, or postnatally (from breast milk or other sources) and lead to different clinical manifestations, depending on the viral agent and the gestational age at exposure. Viewing the peculiar tolerogenic status which characterizes pregnancy, viruses could exploit this peculiar immunological status to spread or affect the maternal immune system, adopting several evasion strategies. In fact, both DNA and RNA virus might have a deep impact on both innate and acquired immune systems. For this reason, investigating the interaction with these pathogens and the host's immune system during pregnancy is crucial not only for the development of most effective therapies and diagnosis but mostly for prevention. In this review, we will analyze some of the most important DNA and RNA viruses related to gestational infections.

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.

Immunology of West Nile Virus Infection and the Role of Alpha-Synuclein as a Viral Restriction Factor

West Nile virus (WNV) is a single-stranded RNA flavivirus and is a major cause of viral encephalitis worldwide. Experimental models of WNV infection in mice are commonly used to define acute neuroinflammatory responses in the brain. Alpha-synuclein (Asyn) is a protein of primarily neuronal origin and is a major cause of Parkinson's disease (PD), a disorder characterized by loss of dopaminergic neurons. Both WNV and PD pathologies are largely mediated by inflammation of the central nervous system (neuroinflammation) and have overlapping inflammatory pathways. In this review, we highlight the roles of the immune system in both diseases while comparing and contrasting both protective and pathogenic roles of immune cells and their effector proteins. Additionally, we review the current literature showing that Asyn is an important mediator of the immune response with diverging roles in PD (pathogenic) and WNV disease (neuroprotective).

Zika Virus Escapes NK Cell Detection by Upregulating Major Histocompatibility Complex Class I Molecules

NK cells are innate lymphocytes that participate in many immune processes encompassing cancer, bacterial and fungal infection, autoimmunity, and even pregnancy and that specialize in antiviral defense. NK cells express inhibitory and activating receptors and kill their targets when activating signals overpower inhibitory signals. The NK cell inhibitory receptors include a uniquely diverse array of proteins named killer cell immunoglobulin-like receptors (KIRs), the CD94 family, and the leukocyte immunoglobulin-like receptor (LIR) family. The NK cell inhibitory receptors recognize mostly major histocompatibility complex (MHC) class I (MHC-I) proteins. Zika virus has recently emerged as a major threat due to its association with birth defects and its pandemic potential. How Zika virus interacts with the immune system, and especially with NK cells, is unclear. Here we show that Zika virus infection is barely sensed by NK cells, since little or no increase in the expression of activating NK cell ligands was observed following Zika infection. In contrast, we demonstrate that Zika virus infection leads to the upregulation of MHC class I proteins and consequently to the inhibition of NK cell killing. Mechanistically, we show that MHC class I proteins are upregulated via the RIGI-IRF3 pathway and that this upregulation is mediated via beta interferon (IFN-β). Potentially, countering MHC class I upregulation during Zika virus infection could be used as a prophylactic treatment against Zika virus.IMPORTANCE NK cells are innate lymphocytes that recognize and eliminate various pathogens and are known mostly for their role in controlling viral infections. NK cells express inhibitory and activating receptors, and they kill or spare their targets based on the integration of inhibitory and activating signals. Zika virus has recently emerged as a major threat to humans due to its pandemic potential and its association with birth defects. The role of NK cells in Zika virus infection is largely unknown. Here we demonstrate that Zika virus infection is almost undetected by NK cells, as evidenced by the fact that the expression of activating ligands for NK cells is not induced following Zika infection. We identified a mechanism whereby Zika virus sensing via the RIGI-IRF3 pathway resulted in IFN-β-mediated upregulation of MHC-I molecules and inhibition of NK cell activity. Countering MHC class I upregulation and boosting NK cell activity may be employed as prophylactic measures to combat Zika virus infection.

The role of astrocytes in multiple sclerosis pathogenesis

INTRODUCTION

Multiple sclerosis (MS) is a demyelinating autoimmune disease of the central nervous system (CNS), in which astrocytes play an important role as CNS immune cells. However, the activity of astrocytes as antigen-presenting cells (APC) continues to be subject to debate.

DEVELOPMENT

This review analyses the existing evidence on the participation of astrocytes in CNS inflammation in MS and on several mechanisms that modify astrocyte activity in the disease.

CONCLUSIONS

Astrocytes play a crucial role in the pathogenesis of MS because they express toll-like receptors (TLR) and major histocompatibility complex (MHC) classI andII. In addition, astrocytes participate in regulating the blood-brain barrier (BBB) and in modulating T cell activity through the production of cytokines. Future studies should focus on the role of astrocytes in order to find new therapeutic targets for the treatment of MS.

Transcriptional response to West Nile virus infection in the zebra finch (Taeniopygia guttata)

West Nile virus (WNV) is a widespread arbovirus that imposes a significant cost to both human and wildlife health. WNV exists in a bird-mosquito transmission cycle in which passerine birds act as the primary reservoir host. As a public health concern, the mammalian immune response to WNV has been studied in detail. Little, however, is known about the avian immune response to WNV. Avian taxa show variable susceptibility to WNV and what drives this variation is unknown. Thus, to study the immune response to WNV in birds, we experimentally infected captive zebra finches (Taeniopygia guttata). Zebra finches provide a useful model, as like many natural avian hosts they are moderately susceptible to WNV and thus provide sufficient viremia to infect mosquitoes. We performed RNAseq in spleen tissue during peak viremia to provide an overview of the transcriptional response. In general, we find strong parallels with the mammalian immune response to WNV, including upregulation of five genes in the Rig-I-like receptor signalling pathway, and offer insights into avian-specific responses. Together with complementary immunological assays, we provide a model of the avian immune response to WNV and set the stage for future comparative studies among variably susceptible populations and species.

Immunodominant West Nile Virus T Cell Epitopes Are Fewer in Number and Fashionably Late

Class I HLA molecules mark infected cells for immune targeting by presenting pathogen-encoded peptides on the cell surface. Characterization of viral peptides unique to infected cells is important for understanding CD8(+) T cell responses and for the development of T cell-based immunotherapies. Having previously reported a series of West Nile virus (WNV) epitopes that are naturally presented by HLA-A*02:01, in this study we generated TCR mimic (TCRm) mAbs to three of these peptide/HLA complexes-the immunodominant SVG9 (E protein), the subdominant SLF9 (NS4B protein), and the immunorecessive YTM9 (NS3 protein)-and used these TCRm mAbs to stain WNV-infected cell lines and primary APCs. TCRm staining of WNV-infected cells demonstrated that the immunorecessive YTM9 appeared several hours earlier and at 5- to 10-fold greater density than the more immunogenic SLF9 and SVG9 ligands, respectively. Moreover, staining following inhibition of the TAP demonstrated that all three viral ligands were presented in a TAP-dependent manner despite originating from different cellular compartments. To our knowledge, this study represents the first use of TCRm mAbs to define the kinetics and magnitude of HLA presentation for a series of epitopes encoded by one virus, and the results depict a pattern whereby individual epitopes differ considerably in abundance and availability. The observations that immunodominant ligands can be found at lower levels and at later time points after infection suggest that a reevaluation of the factors that combine to shape T cell reactivity may be warranted.

Antiviral macrophage responses in flavivirus encephalitis

Mosquito-borne flaviviruses are a major current and emerging threat, affecting millions of people worldwide. Global climate change, combined with increasing proximity of humans to animals and mosquito vectors by expansion into natural habitats, coupled with the increase in international travel, have resulted in significant spread and concomitant increase in the incidence of infection and severe disease. Although neuroinvasive disease has been well described for some viral infections such as Japanese Encephalitis virus (JEV) and West Nile virus (WNV), others such as dengue virus (DENV) have recently displayed an emerging pattern of neuroinvasive disease, distinct from the previously observed, systemically-induced encephalomyelopathy. In this setting, the immune response is a crucial component of host defence, in preventing viral dissemination and invasion of the central nervous system (CNS). However, subversion of the anti-viral activities of macrophages by flaviviruses can facilitate viral replication and spread, enhancing the intensity of immune responses, leading to severe immune-mediated disease which may be further exacerbated during the subsequent infection with some flaviviruses. Furthermore, in the CNS myeloid cells may be responsible for inducing specific inflammatory changes, which can lead to significant pathological damage during encephalitis. The interaction of virus and cells of the myeloid lineage is complex, and this interaction is likely responsible at least in part, for crucial differences between viral clearance and pathology. Recent studies on the role of myeloid cells in innate immunity and viral control, and the mechanisms of evasion and subversion used by flaviviruses are rapidly advancing our understanding of the immunopathological mechanisms involved in flavivirus encephalitis and will lead to the development of therapeutic strategies previously not considered.

Japanese Encephalitis Virus Generated Neurovirulence, Antigenicity, and Host Immune Responses

In response to a JE virus attack, infected body cells start secretion of different cytokines and activate innate immune response. Virus starts neuronal invasion by entering into nerve cells and inflecting the central nervous system. It avoids exposure of body’s natural immunity and generates neurotrophic effects. Virus causes acute susceptibility to CNS and establishes encephalitis syndrome that results in very high fatality in children. In survivors, JEV inhibits the growth and proliferation of NCPs and imposes permanent neuronal disorders like cognitive, motor, and behavioral impairments. However, body cells start TCR mediated interactions, to recognize viral antigens with class I MHC complex on specific target cells, and operate mass killing of virus infected cells by increased CTL activity. Thus, both cell mediated and antibody interactions plays a central role in protection against JEV. In the present review article virus generated neurovirulence, antigenicity, and host immune responses are described in detail. More emphasis is given on diagnosis, clinical care, and active immunization with well-designed potential antiflavivirus vaccines. Further, for achieving an elite success against JEV, global eradication strategies are to be needed for making vaccination program more responsible and effective in endemic areas.

Encephalitis due to emerging viruses: CNS innate immunity and potential therapeutic targets

The emerging viruses represent a group of pathogens that are intimately connected to a diverse range of animal vectors. The recent escalation of air travel climate change and urbanization has meant humans will have increased risk of contacting these pathogens resulting in serious CNS infections. Many RNA viruses enter the CNS by evading the BBB due to axonal transport from the periphery. The systemic adaptive and CNS innate immune systems express pattern recognition receptors PRR (TLRs, RiG-1 and MDA-5) that detect viral nucleic acids and initiate host antiviral response. However, several emerging viruses (West Nile Fever, Influenza A, Enterovirus 71 Ebola) are recognized and internalized by host cell receptors (TLR, MMR, DC-SIGN, CD162 and Scavenger receptor B) and escape immuno surveillance by the host systemic and innate immune systems. Many RNA viruses express viral proteins WNF (E protein), Influenza A (NS1), EV71 (protein 3C), Rabies (Glycoprotein), Ebola proteins (VP24 and VP 35) that inhibit the host cell anti-virus Interferon type I response promoting virus replication and encephalitis. The therapeutic use of RNA interference methodologies to silence gene expression of viral peptides and treat emerging virus infection of the CNS is discussed.

Anti-West Nile virus activity of in vitro expanded human primary natural killer cells

Background

Natural Killer (NK) cells are a crucial component of the host innate immune system with anti-viral and anti-cancer properties. However, the role of NK cells in West Nile virus (WNV) infection is controversial, with reported effects ranging from active suppression of virus to no effect at all. It was previously shown that K562-mb15-41BBL (K562D2) cells, which express IL-15 and 4-1BBL on the K562 cell surface, were able to expand and activate human primary NK cells of normal peripheral blood mononuclear cells (PBMC). The expanded NK cells were tested for their ability to inhibit WNV infection in vitro.

Results

Co-culture of PBMC with irradiated K562D2 cells expanded the NK cell number by 2-3 logs in 2-3 weeks, with more than 90% purity; upregulated NK cell surface activation receptors; downregulated inhibitory receptors; and boosted interferon gamma (IFN-γ) production by ~33 fold. The expanded NK (D2NK) cell has strong natural killing activity against both K562 and Vero cells, and killed the WNV infected Vero cells through antibody-dependent cellular cytotoxicity (ADCC). The D2NK cell culture supernatants inhibited both WNV replication and WNV induced cytopathic effect (CPE) in Vero cells when added before or after infection. Anti-IFN-γ neutralizing antibody blocked the NK supernatant-mediated anti-WNV effect, demonstrating a noncytolytic activity mediated through IFN-γ.

Conclusions

Co-culture of PBMC with K562D2 stimulatory cells is an efficient technique to prepare large quantities of pure and active NK cells, and these expanded NK cells inhibited WNV infection of Vero cells through both cytolytic and noncytolytic activities, which may imply a potential role of NK cells in combating WNV infection.

Pro-inflammatory functions of astrocytes correlate with viral clearance and strain-dependent protection from TMEV-induced demyelinating disease

Intracerebral infection of susceptible strains of mice, e.g. SJL/J, with Theiler's murine encephalomyelitis virus (TMEV) leads to a persistent CNS infection accompanied by development of a chronic-progressive inflammatory CNS autoimmune demyelinating disease which is clinically and pathologically similar to human multiple sclerosis. In contrast, resistant strains of mice, e.g. C57BL/6 (B6), effectively clear TMEV from the CNS and do not develop demyelinating disease. Although CD8(+) T cells are crucial for viral clearance in B6 mice, SJL mice also mount potent CD8(+) T cell responses against virus, thus the reason for the viral persistence in the CNS in these mice is unclear. Here, we examined innate anti-viral responses of CNS-resident astrocytes as a potential determinant of viral persistence and disease susceptibility. We demonstrate that B6 astrocytes produce significantly higher levels of cytokines, chemokines and adhesion molecules in response to TMEV infection, or stimulation with IFN-gamma and TNF-alpha or poly I:C than SJL mice. In addition, TMEV more effectively induces MHC I molecules on B6 astrocytes than SJL, corresponding with an increased ability to activate TMEV-specific CD8(+) T cells directly ex vivo. These results suggest that enhanced anti-viral responses of B6 astrocytes contribute to the ability of these mice to clear TMEV from the CNS and therefore to their resistance to the development of autoimmune demyelinating disease.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

West Nile virus-induced neuroinflammation: glial infection and capsid protein-mediated neurovirulence

West Nile virus (WNV) infection causes neurological disease at all levels of the neural axis, accompanied by neuroinflammation and neuronal loss, although the underlying mechanisms remain uncertain. Given the substantial activation of neuroinflammatory pathways observed in WNV infection, we hypothesized that WNV-mediated neuroinflammation and cell death occurred through WNV infection of both glia and neurons, which was driven in part by WNV capsid protein expression. Analysis of autopsied neural tissues from humans with WNV encephalomyelitis (WNVE) revealed WNV infection of both neurons and glia. Upregulation of proinflammatory genes, CXCL10, interleukin-1beta, and indolamine-2',3'-deoxygenase with concurrent suppression of the protective astrocyte-specific endoplasmic reticulum stress sensor gene, OASIS (for old astrocyte specifically induced substance), was evident in WNVE patients compared to non-WNVE controls. These findings were supported by increased ex vivo expression of these proinflammatory genes in glia infected by WNV-NY99. WNV infection caused endoplasmic reticulum stress gene induction and apoptosis in neurons but did not affect glial viability. WNV-infected astrocytic cells secreted cytotoxic factors, which caused neuronal apoptosis. The expression of the WNV-NY99 capsid protein in neurons and glia by a Sindbis virus-derived vector (SINrep5-WNVc) caused neuronal death and the release of neurotoxic factors by infected astrocytes, coupled with proinflammatory gene induction and suppression of OASIS. Striatal implantation of SINrep5-WNV(C) induced neuroinflammation in rats, together with the induction of CXCL10 and diminished OASIS expression, compared to controls. Moreover, magnetic resonance neuroimaging showed edema and tissue injury in the vicinity of the SINrep5-WNVc implantation site compared to controls, which was complemented by neurobehavioral abnormalities in the SINrep5-WNVc-implanted animals. These studies underscore the important interactions between the WNV capsid protein and neuroinflammation in the pathogenesis of WNV-induced neurological disorders.

CD8(+) T cell-mediated immune responses in West Nile virus (Sarafend strain) encephalitis are independent of gamma interferon

The flavivirus West Nile virus (WNV) can cause fatal encephalitis in humans and mice. It has recently been demonstrated, in an experimental model using WNV strain Sarafend and C57BL/6 mice, that both virus- and immune-mediated pathology is involved in WNV encephalitis, with CD8(+) T cells being the dominant subpopulation of lymphocyte infiltrates in the brain. Here, the role of activated WNV-immune CD8(+) T cells in mouse WNV encephalitis was investigated further. Passive transfer of WNV-immune CD8(+) T cells reduced mortality significantly and prolonged survival times of mice infected with WNV. Early infiltration of WNV-immune CD8(+) T cells into infected brains is shown, suggesting a beneficial contribution of these lymphocytes to recovery from encephalitis. This antiviral function was not markedly mediated by gamma interferon (IFN-gamma), as a deficiency in IFN-gamma did not affect mortality to two strains of WNV (Sarafend and Kunjin) or brain virus titres significantly. The cytolytic potential, as well as precursor frequency, of WNV-immune CD8(+) T cells were not altered by the absence of IFN-gamma. This was reflected in transfer experiments of WNV-immune CD8(+) T cells from IFN-gamma(-/-) mice into WNV-infected wild-type mice, which showed that IFN-gamma-deficient T cells were as effective as those from WNV-immune wild-type mice in ameliorating disease outcome. It is speculated here that one of the pleiotropic functions of IFN-gamma is mimicked by WNV-Sarafend-mediated upregulation of cell-surface expression of major histocompatibility complex antigens, which may explain the lack of phenotype of IFN-gamma(-/-) mice in response to WNV.

Immunopathology of flavivirus infections

With the recent emergence of the flavivirus, West Nile virus (WNV), in particular, the New York strain of Lineage I WNV in North America in 1999, there has been a significant increase in activity in neurotropic flavivirus research. These viruses cause encephalitis that can result in permanent neurological sequelae or death. Attempts to develop vaccines have made progress, but have been variably successful, despite considerable commercial underwriting. Thus, the discovery of ways and means to combat disease is no less urgent. As such, most recent work has been directed towards dissecting and understanding the pathogenesis of disease, as a way of informing possible approaches to abrogation or amelioration of illness. Whether inherent to flaviviruses or because humans are incidental, dead-end hosts, it is clear that these viruses interact with their human hosts in extremely complex ways. This occurs from the cellular level, at which infection must be established to produce disease, to its interaction with the adaptive immune response, which may result in its eradication, with or without immunopathological and consequent neurological sequelae. As human proximity to and contact with flavivirus insect vectors and amplifying hosts cannot practically be eliminated, our understanding of the pathogenesis of flavivirus-induced diseases, especially with regard to possible targets for treatment, is imperative.

Induction of classical and nonclassical MHC-I on mouse brain astrocytes by Japanese encephalitis virus

Infection with Flaviviruses upregulates the cell surface expression of MHC-I, MHC-II, ICAM-1 (CD54), VCAM-1 (CD106) and TAP proteins. Although all these studies have been confirmed using West Nile virus and other Flaviviruses, there are few reports that have examined the effects of Japanese encephalitis virus (JEV) infection directly on nonclassical and classical MHC expression in astrocytes. We show in this report that JEV infection of mouse brain astrocytes results in induction of the nonclassical MHC Class Ib genes, H-2T23, H-2Q4 and H-2T10 in addition to MHC-I, Type I (alpha/beta) IFNs, TAP-1, TAP-2, Tapasin, LMP-2, LMP-7 and LMP-10 but not IFNgamma, CD80, CD86 and MHC-II genes. The increased cell surface expression of these antigens as well as induction of the genes mentioned above as measured by RT-PCR suggests that JEV infection may lead to the induction of classical MHC Class Ia as well as nonclassical MHC Class Ib molecules.

Amyloid β up-regulates brain-derived neurotrophic factor production from astrocytes: Rescue from amyloid β-related neuritic degeneration

Astrocytes, the most abundant type of glia in the brain, are considered to play a key role in Alzheimer's disease (AD) pathologies. In a cell culture study, we have previously shown that astroglial responses against amyloid beta (Abeta) occur before obvious neuronal damage could be detected, suggesting the possibility that astrocytes might be an attractive therapeutic target for treating AD. In the present study, we investigated astroglial gene expression changes in response to Abeta to elucidate further the role of astrocytes in Abeta toxicity. By using real-time PCR and ELISA analyses, we found that Abeta rapidly induced astrocytes to produce brain-derived neurotrophic factor (BDNF). Abeta42 was more effective than Abeta40 in increasing astroglial BDNF production. Moreover, BDNF treatment rescued the neuronally differentiated human neuroblastoma cells from neuritic degeneration caused by Abeta toxicity. This is the first study to demonstrate that astrocytes are capable of increasing the production of a particular neurotrophic factor in response to Abeta. Our findings also identify BDNF as a potential therapeutic agent for preventing Abeta-related neuritic degeneration.

Neuronal CXCL10 directs CD8+ T-cell recruitment and control of West Nile virus encephalitis

The activation and entry of antigen-specific CD8(+) T cells into the central nervous system is an essential step towards clearance of West Nile virus (WNV) from infected neurons. The molecular signals responsible for the directed migration of virus-specific T cells and their cellular sources are presently unknown. Here we demonstrate that in response to WNV infection, neurons secrete the chemokine CXCL10, which recruits effector T cells via the chemokine receptor CXCR3. Neutralization or a genetic deficiency of CXCL10 leads to a decrease in CXCR3(+) CD8(+) T-cell trafficking, an increase in viral burden in the brain, and enhanced morbidity and mortality. These data support a new paradigm in chemokine neurobiology, as neurons are not generally considered to generate antiviral immune responses, and CXCL10 may represent a novel neuroprotective agent in response to WNV infection in the central nervous system.

Role of CD8+ T cells in control of West Nile virus infection

Infection with West Nile virus (WNV) causes fatal encephalitis more frequently in immunocompromised humans than in those with a healthy immune system. Although a complete understanding of this increased risk remains unclear, experiments with mice have begun to define how different components of the adaptive and innate immune response function to limit infection. Previously, we demonstrated that components of humoral immunity, particularly immunoglobulin M (IgM) and IgG, have critical roles in preventing dissemination of WNV infection to the central nervous system. In this study, we addressed the function of CD8(+) T cells in controlling WNV infection. Mice that lacked CD8(+) T cells or classical class Ia major histocompatibility complex (MHC) antigens had higher central nervous system viral burdens and increased mortality rates after infection with a low-passage-number WNV isolate. In contrast, an absence of CD8(+) T cells had no effect on the qualitative or quantitative antibody response and did not alter the kinetics or magnitude of viremia. In the subset of CD8(+)-T-cell-deficient mice that survived initial WNV challenge, infectious virus was recovered from central nervous system compartments for several weeks. Primary or memory CD8(+) T cells that were generated in vivo efficiently killed target cells that displayed WNV antigens in a class I MHC-restricted manner. Collectively, our experiments suggest that, while specific antibody is responsible for terminating viremia, CD8(+) T cells have an important function in clearing infection from tissues and preventing viral persistence.

Role of CD8+ T cells in control of West Nile virus infection

Infection with West Nile virus (WNV) causes fatal encephalitis more frequently in immunocompromised humans than in those with a healthy immune system. Although a complete understanding of this increased risk remains unclear, experiments with mice have begun to define how different components of the adaptive and innate immune response function to limit infection. Previously, we demonstrated that components of humoral immunity, particularly immunoglobulin M (IgM) and IgG, have critical roles in preventing dissemination of WNV infection to the central nervous system. In this study, we addressed the function of CD8(+) T cells in controlling WNV infection. Mice that lacked CD8(+) T cells or classical class Ia major histocompatibility complex (MHC) antigens had higher central nervous system viral burdens and increased mortality rates after infection with a low-passage-number WNV isolate. In contrast, an absence of CD8(+) T cells had no effect on the qualitative or quantitative antibody response and did not alter the kinetics or magnitude of viremia. In the subset of CD8(+)-T-cell-deficient mice that survived initial WNV challenge, infectious virus was recovered from central nervous system compartments for several weeks. Primary or memory CD8(+) T cells that were generated in vivo efficiently killed target cells that displayed WNV antigens in a class I MHC-restricted manner. Collectively, our experiments suggest that, while specific antibody is responsible for terminating viremia, CD8(+) T cells have an important function in clearing infection from tissues and preventing viral persistence.

Manipulation of cell surface macromolecules by flaviviruses

Cell surface macromolecules play a crucial role in the biology and pathobiology of flaviviruses, both as receptors for virus entry and as signaling molecules for cell–cell interactions in the processes of vascular permeability and inflammation. This review examines the cell tropism and pathogenesis of flaviviruses from the standpoint of cell surface molecules, which have been implicated as receptors in both virus–cell as well as cell–cell interactions. The emerging picture is one that encompasses extensive regulation and interplay among the invading virus, viral immune complexes, Fc receptors, major histocompatibility complex antigens, and adhesion molecules.

Astrocyte apoptosis: implications for neuroprotection

Astrocytes, the most abundant glial cell types in the brain, provide metabolic and trophic support to neurons and modulate synaptic activity. Accordingly, impairment in these astrocyte functions can critically influence neuronal survival. Recent studies show that astrocyte apoptosis may contribute to pathogenesis of many acute and chronic neurodegenerative disorders, such as cerebral ischemia, Alzheimer's disease and Parkinson's disease. We found that incubation of cultured rat astrocytes in a Ca(2+)-containing medium after exposure to a Ca(2+)-free medium causes an increase in intracellular Ca(2+) concentration followed by apoptosis, and that NF-kappa B, reactive oxygen species, and enzymes such as calpain, xanthine oxidase, calcineurin and caspase-3 are involved in reperfusion-induced apoptosis. Furthermore, we demonstrated that heat shock protein, mitogen-activated protein/extracellular signal-regulated kinase, phosphatidylinositol-3 kinase and cyclic GMP phosphodiesterase are target molecules for anti-apoptotic drugs. This review summarizes (1) astrocytic functions in neuroprotection, (2) current evidence of astrocyte apoptosis in both in vitro and in vivo studies including its molecular pathways such as Ca(2+) overload, oxidative stress, NF-kappa B activation, mitochondrial dysfunction, endoplasmic reticulum stress, and protease activation, and (3) several drugs preventing astrocyte apoptosis. As a whole, this article provides new insights into the potential role of astrocytes as targets for neuroprotection. In addition, the advance in the knowledge of molecular mechanisms of astrocyte apoptosis may lead to the development of novel therapeutic strategies for neurodegenerative disorders.

CD8+ T cells mediate recovery and immunopathology in West Nile virus encephalitis

C57BL/6J mice infected intravenously with the Sarafend strain of West Nile virus (WNV) develop a characteristic central nervous system (CNS) disease, including an acute inflammatory reaction. Dose response studies indicate two distinct kinetics of mortality. At high doses of infection (10(8) PFU), direct infection of the brain occurred within 24 h, resulting in 100% mortality with a 6-day mean survival time (MST), and there was minimal destruction of neural tissue. A low dose (10(3) PFU) of infection resulted in 27% mortality (MST, 11 days), and virus could be detected in the CNS 7 days postinfection (p.i.). Virus was present in the hypogastric lymph nodes and spleens at days 4 to 7 p.i. Histology of the brains revealed neuronal degeneration and inflammation within leptomeninges and brain parenchyma. Inflammatory cell infiltration was detectable in brains from day 4 p.i. onward in the high-dose group and from day 7 p.i. in the low-dose group, with the severity of infiltration increasing over time. The cellular infiltrates in brain consisted predominantly of CD8(+), but not CD4(+), T cells. CD8(+) T cells in the brain and the spleen expressed the activation markers CD69 early and expressed CD25 at later time points. CD8(+) T-cell-deficient mice infected with 10(3) PFU of WNV showed increased mortalities but prolonged MST and early infection of the CNS compared to wild-type mice. Using high doses of virus in CD8-deficient mice leads to increased survival. These results provide evidence that CD8(+) T cells are involved in both recovery and immunopathology in WNV infection.

Immune modulation by flaviviruses

Flaviviruses cause pleomorphic disease with significant morbidity and mortality worldwide. Interestingly, in contrast to most viruses, which subvert or avoid host immune systems, members of the neurotropic Japanese encephalitis serocomplex cause functional changes associated with increased efficacy of the immune response. These viruses induce increased cell surface expression of immune recognition molecules, including class I and II major histocompatibility complex (MHC) and various adhesion molecules. Increases are functional: infected cells are significantly more susceptible to both virus- and MHC-specific cytotoxic T cell lysis. Induced changes are modulated positively or negatively by Th1 and Th2 cytokines, as well as by cell cycle position and adherence status at infection. Infection also increases costimulatory molecule expression on Langerhans cells in the skin. Local interleukin-1 beta production causes accelerated migration of phenotypically altered Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of upregulation is unclear, but changes are associated with NF-kappa B activation and increased MHC and ICAM-1 gene transcription, independently of interferon (IFN) or other proinflammatory cytokines. Increased MHC and adhesion molecule expression may contribute to the pathogenesis of flavivirus encephalitis. Results from a murine model of flavivirus encephalitis developed in this laboratory suggest that fatal disease is immunopathological in nature, with IFN-gamma playing a crucial role. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity T cells, which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly upregulate cell surface molecules.

Innate and adaptive immune responses determine protection against disseminated infection by West Nile encephalitis virus

WNV continues to spread throughout the Western Hemisphere as virus activity in insects and animals has been reported in the United States, Canada, Mexico, and the Caribbean islands. West Nile virus (WNV) infects the central nervous system and causes severe disease primarily in humans who are immunocompromised or elderly. In this review, we discuss the mechanisms by which the immune system limits dissemination of WNV infection. Recent experimental studies in animals suggest important roles for both the innate and the adaptive immune responses in controlling WNV infection. Interferons, antibody, complement components and CD8+ T cells coordinate protection against severe infection and disease. These findings are analyzed in the context of recent approaches to vaccine development and immunotherapy against WNV.

Infection and injury of neurons by West Nile encephalitis virus

West Nile virus (WNV) infects neurons and leads to encephalitis, paralysis, and death in humans, animals, and birds. We investigated the mechanism by which neuronal injury occurs after WNV infection. Neurons in the anterior horn of the spinal cords of paralyzed mice exhibited a high degree of WNV infection, leukocyte infiltration, and degeneration. Because it was difficult to distinguish whether neuronal injury was caused by viral infection or by the immune system response, a novel tissue culture model for WNV infection was established in neurons derived from embryonic stem (ES) cells. Undifferentiated ES cells were relatively resistant to WNV infection. After differentiation, ES cells expressed neural antigens, acquired a neuronal phenotype, and became permissive for WNV infection. Within 48 h of exposure to an exceedingly low multiplicity of infection (5 x 10(-4)), 50% of ES cell-derived neurons became infected, producing nearly 10(7) PFU of infectious virus per ml, and began to die by an apoptotic mechanism. The establishment of a tractable virus infection model in ES cell-derived neurons facilitates the study of the molecular basis of neurotropism and the mechanisms of viral and immune-mediated neuronal injury after infection by WNV or other neurotropic pathogens.

Interaction of flaviviruses with cells of the vertebrate host and decoy of the immune response

Flaviviruses cause endemic and epidemic disease with significant morbidity and mortality throughout the world. In contrast to viruses that avoid the host immune response by down-regulating cell surface major histocompatibility complex expression, infection by members of the neurotropic Japanese encephalitis serogroup induce virus-directed functional increases in expression of class I and II major histocompatibility complex and various adhesion molecules, resulting in increased susceptibility to both virus- and major histocompatibility complex-specific cytotoxic T lymphocyte lysis. These changes are comodulated by T1 and T2 cytokines, as well as by cell cycle position and adherence status at infection. Infected skin dendritic (Langerhans) cells also show increased costimulatory molecule expression and local interleukin-1beta production causes accelerated migration of Langerhans cells to local draining lymph nodes, where initiation of antiviral immune responses occur. The exact mechanism(s) of up-regulation is unclear, but changes are associated with NF-kappaB activation and increased MHC and ICAM-1 gene transcription, independently of interferon or other pro-inflammatory cytokines. We hypothesize that these viruses may decoy the adaptive immune system into generating low-affinity, self-reactive T cells which clear virus poorly, as part of their survival strategy. This may enable viral growth and immune escape in cycling cells, which do not significantly up-regulate cell surface molecules. A possible side-effect of this might be immunopathology, caused by 'autoimmune' cross-reactive damage of uninfected high major histocompatibility complex and adhesion molecule-expressing cells, with consequent exacerbation of encephalitic disease. Results from a murine model of flavivirus encephalitis developed in this laboratory further suggest that interferon-gamma plays a crucial role in fatal immunopathology.

MHC class I up-regulation by flaviviruses: Immune interaction with unknown advantage to host or pathogen

In contrast to many other viruses that escape from cytotoxic T cell recognition by down-regulating major histocompatibility complex class I-restricted antigen presentation, flavivirus infection of mammalian cells up-regulates cell surface expression of major histocompatibility complex class I molecules. Two putative mechanisms for flavivirus-induced major histocompatibility complex class I up-regulation, one via activation of the transcription factor NF-kappaB, the second by augmentation of peptide import into the lumen of the endoplasmic reticulum, are reviewed, and the biological effect of the flavivirus-mediated phenomenon on target cell recognition by natural killer and cytotoxic T cells is addressed. Finally, we speculate on the physiological role of flavivirus-mediated modulation of major histocompatibility complex class I antigen presentation in the context of the biology of flavivirus transmission between the vertebrate host and arthropod vector and suggest that it may represent a strategy for immune evasion from the natural killer cell response or, alternatively, that up-regulation of major histocompatibility complex class I is a by-product of flavivirus replication without significance for virus growth.

Evasion of innate and adaptive immunity by flaviviruses

After a virus infects an animal, antiviral responses are generated that attempt to prevent dissemination. Interferons, antibody, complement, T and natural killer cells all contribute to the control and eradication of viral infections. Most flaviviruses, with the exception of some of the encephalitic viruses, cause acute disease and do not establish persistent infection. The outcome of flavivirus infection in an animal is determined by a balance between the speed of viral replication and spread, and the immune system response. Although many of the mechanistic details require further elucidation, flaviviruses have evolved specific tactics to evade the innate and adaptive immune response. A more thorough understanding of these principles could lead to improved models for viral pathogenesis and to strategies for the development of novel antiviral agents.

Regulation of immune recognition molecules by flavivirus, West Nile

We have shown the flaviviruses can up-regulate the cell surface expression of the immune recognition molecules, major histocompatability complex class-I and class-II (MHC-I, MHC-II), ICAM-1, VCAM, and E-selectin, in an interferon-independent and tumor necrosis factor-independent manner. This up-regulation is associated with an increased transcription of the relevant genes and is due to activation of the transcription factor, nuclear factor-kappa B. The level of up-regulation is determined in part by the cell cycle position of the cell when infected with the flavivirus, as quiescent cells show a greater increase in the level of expression of the immune recognition molecules, MHC-I and ICAM-1, than cells in other phases of the cell cycle. The resultant increased cell surface expression is functional with the increased expression resulting in increased recognition by flavivirus-specific and allo-specific cytotoxic T cells.

Pathogenesis of flavivirus encephalitis

Within the flavivirus family, viruses that cause natural infections of the central nervous system (CNS) principally include members of the Japanese encephalitis virus (JEV) serogroup and the tick-borne encephalitis virus (TBEV) serocomplex. The pathogenesis of diseases involves complex interactions of viruses, which differ in neurovirulence potential, and a number of host factors, which govern susceptibility to infection and the capacity to mount effective antiviral immune responses both in the periphery and within the CNS. This chapter summarizes progress in the field of flavivirus neuropathogenesis. Mosquito-borne and tickborne viruses are considered together. Flavivirus neuropathogenesis involves both neuroinvasiveness (capacity to enter the CNS) and neurovirulence (replication within the CNS), both of which can be manipulated experimentally. Neuronal injury as a result of bystander effects may be a factor during flavivirus neuropathogenesis given that microglial activation and elaboration of inflammatory mediators, including IL-1β and TNF-α, occur in the CNS during these infections and may accompany the production of nitric oxide and peroxynitrite, which can cause neurotoxicity.

Corking the bottleneck: the transporter associated with antigen processing as a target for immune subversion by viruses

In this chapter, mechanisms are reviewed that viruses use to inhibit the function of the peptide transporter associated with antigen processing (TAP), which translocates cytosolic peptides into the endoplasmic reticulum (ER) for binding to MHC class I molecules. Although some DNA viruses, such as adenovirus or EBV, downmodulate TAP expression on the transcriptional level, members of the alpha and beta subfamily of herpesviruses, such as herpes simplex virus (HSV) and human cytomegalovirus (HCMV), express proteins that bind to TAP and interfere with peptide translocation. The modes of action of the HSV-encoded cytosolic TAP inhibitor ICP47 and the HCMV-encoded ER-resident TAP inhibitor gpUS6 are discussed in detail. Viral interference with antigen presentation through TAP inhibition is not only relevant for the immunobiology of persistent viral infections but also contributes to the understanding of the translocation mechanism utilized by the ATP-binding cassette transporter TAP.

Modulation of transporter associated with antigen processing (TAP)-mediated peptide import into the endoplasmic reticulum by flavivirus infection

In contrast to many other viruses that escape the cellular immune response by downregulating major histocompatibility complex (MHC) class I molecules, flavivirus infection can upregulate their cell surface expression. Previously we have presented evidence that during flavivirus infection, peptide supply to the endoplasmic reticulum is increased (A. Müllbacher and M. Lobigs, Immunity 3:207-214, 1995). Here we show that during the early phase of infection with different flaviviruses, the transport activity of the peptide transporter associated with antigen processing (TAP) is augmented by up to 50%. TAP expression is unaltered during infection, and viral but not host macromolecular synthesis is required for enhanced peptide transport. This study is the first demonstration of transient enhancement of TAP-dependent peptide import into the lumen of the endoplasmic reticulum as a consequence of a viral infection. We suggest that the increased supply of peptides for assembly with MHC class I molecules in flavivirus-infected cells accounts for the upregulation of MHC class I cell surface expression with the biological consequence of viral evasion of natural killer cell recognition.

Mechanism of measles virus failure to activate NF-kappaB in neuronal cells

Lack of IFN-beta and MHC class I expression in measles virus (MV) infected neurons could impair the host antiviral defense mechanism and result in virus escape from recognition by cytotoxic T-cells. Induction of IFN-beta and MHC class I gene expression requires NF-kappaB activation which depends on degradation of IkappaBalpha, an inhibitory protein of NF-kappaB. In earlier studies we demonstrated that in contrast to glial cells, MV was unable to induce IkappaBalpha degradation in neuronal cells. It is unclear whether this failure is due to the presence of a neuron-specific IkappaBalpha isoform or a defect in the MV signaling cascade that leads to IkappaBalpha phosphorylation and degradation. In this study, an IkappaBalpha-wild type (WT) expression vector was transfected into neuronal and glial cells and subsequently exposed to MV. In contrast to glial cells, IkappaBalpha-WT was degraded in neuronal cells in response to TNFalpha but not MV. The findings eliminate the existence of an IkappaBalpha isoform in neuronal cells that is resistant to phosphorylation by MV. Blocking de novo protein synthesis with cyclohexamide had no effect on neuronal IkappaBalpha, indicating that lack of degradation rather than increased synthesis is responsible for IkappaBalpha accumulation in MV-stimulated neuronal cells. To determine if malfunction in the MV receptor CD46 is responsible for failure of IkappaBalpha phosphorylation and degradation, neuronal cells were transfected with a wild type CD46 (CD46-WT) expression vector. MV stimulation of CD46-WT transfected cells failed to induce IkappaBalpha degradation. Collectively these findings indicate that failure of MV to phosphorylate neuronal IkappaBalpha is not due to a presence of an IkappaBalpha isoform or malfunction of the MV receptor, and is more likely to be due to a defect in the signaling pathway that normally leads to IkappaBalpha phosphorylation and degradation.

Early E-selectin, VCAM-1, ICAM-1, and late major histocompatibility complex antigen induction on human endothelial cells by flavivirus and comodulation of adhesion molecule expression by immune cytokines

Expression of E-selectin (ELAM-1, CD62E) on human umbilical vein endothelial cells significantly increased 30 min postinfection with the flavivirus West Nile virus (WNV), was maximal by 2 h postinfection, and declined to baseline levels within 24 h. Expression of ICAM-1 (CD54) and VCAM-1 (CD106) was significantly increased by 2 h and maximal at 4 h after infection. P-selectin (CD62P) expression was unaffected by WNV. Upregulation occurred earlier than that caused by tumor necrosis factor alpha (TNF-alpha) or interleukin 1 (IL-1) and could not be inhibited by neutralizing TNF-alpha, IL-1alpha, or alpha/beta interferon (IFN-alpha/beta) antibodies, suggesting a direct, virus-mediated phenomenon. TNF-alpha significantly enhanced WNV-induced increases in E-selectin, P-selectin, ICAM-1, and VCAM-1 expression, while IFN-gamma enhanced WNV-induced ICAM-1 expression. In contrast, IL-4 abrogated WNV-induced E-selectin expression increases but acted in synergy with WNV to increase P-selectin and VCAM-1 expression. WNV increased the expression of class I and II major histocompatibility complex antigens (MHC-I and MHC-II, respectively) at 24 and 72 h, respectively. IFN-gamma, TNF-alpha, or IL-1 acted in synergy with WNV to produce greater increases in MHC-I expression than WNV or cytokines alone, while IFN-alpha/beta or IL-4 had no effect. MHC-II induction in cytokine-treated, WNV-infected cells was similar to that caused by cytokines alone. Neutralizing IFN-alpha/beta antibody inhibited WNV-induced MHC-I expression by 30% at 24 h and by 100% by 72 h. The differential kinetics of modulation suggest sequential adhesion of leukocyte subpopulations to infected endothelial cells, which may be important in initial viral spread in vivo.

Flavivirus-induced up-regulation of MHC class I antigens; implications for the induction of CD8+ T-cell-mediated autoimmunity

Infection of a wide variety of cells of human, mouse and other species' origin by flaviviruses such as WNV, YF, Den, MVE, KUN and JE, increases the cell-surface expression of MHC class I. This MHC class I up-regulation is not due to increased MHC class I synthesis per se, but the result of increased peptide availability in the ER for MHC class I assembly. This is most likely due to the interaction of the viral polyprotein with the ER membrane during viral replication. Flavivirus infection can overcome peptide deficiency in TAP-deficient or non-permissive cell lines such as RMA-S and Syrian hamster cells, BHK and NIL-2. The consequence of this increased MHC class I expression manifests itself in reduced susceptibility to NK cells and augmented lysis by Tc cells. In mice, long-term flavivirus-immune Tc cell memory formation is impaired, following the appearance of strong anti-self Tc cell reactivity observed in in vitro cultures from splenocytes of flavivirus-primed animals. We hypothesize that flavivirus-induced MHC class I up-regulation leads to transient T-cell autoimmunity, followed by down-regulation of both autoimmunity and virus-specific Tc cell memory. Furthermore, we speculate that flavivirus infections of humans in the tropics may be responsible for the observed lower incidence of overt autoimmunity in these geographic regions than in temperate climates where flaviviruses are not endemic.

Phenotypic changes in Langerhans' cells after infection with arboviruses: a role in the immune response to epidermally acquired viral infection?

The role of Langerhans cells (LC) in the initiation of an immune response to a viral infection remains unclear. In vivo epidermal infection with the arboviruses West Nile virus and Semliki Forest virus significantly increased the expression of major histocompatibility complex class II antigens, CD54, and CD80 on LC. Thus, during an epidermally acquired viral infection, local LC appear to mature to a phenotype approximating that of lymphoid dendritic cells. This change may be important in the activation of naïve T cells and the subsequent clearance of viral infection.

Modulation of immune-associated surface markers and cytokine production by murine retinal glial cells

Murine retinal glia are normally negative for major histocompatibility complex (MHC) Class II antigens and express low levels of MHC Class I and intercellular adhesion molecule-1 (ICAM-1) as detected by avidin-biotin-peroxidase immunohistochemistry. These surface molecules associated with immune function were either induced (Class II) or upregulated (Class I and ICAM-1) on cultured retinal glial cells in a dose- and time-dependent manner following exposure to recombinant interferon-gamma (rIFN-gamma). MHC Class I and II expression by passaged and primary cells was maximal (> 90% positive) after incubation with 100 U/m1 of rIFN-gamma for 48 h. ICAM-1 expression by primary and passaged cells tripled between 48 and 72 h after exposure to 25 or 50 U/m1 of rIFN-gamma. By 72 h after exposure to 100 U/m1 of rIFN-gamma, 62% of the retinal glia were positive for ICAM-1, whereas under normal culture conditions these molecules were detected on < 3% of the retinal glia. Bacterial lipopolysaccharide (LPS), a known stimulator of central nervous system (CNS) astrocytes, increased ICAM-1 expression only 3-fold to 9% of cells staining positively, but neither MHC Class I nor Class II expression was altered from baseline levels. Surface expression of ICAM-1, MHC Class I, and MHC Class II was unaffected by exposure to either rTNF-alpha (1000 U/m1) or rIL-6 (100 U/m1) for 24 h. Under normal culture conditions, intracellular interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) were detected immunohistochemically. Exposure to either rIFN-gamma or LPS induced more intense staining which correlated with increased secreted levels of both cytokines in culture supernatants. Levels of secreted TNF-alpha increased 6-fold after stimulation with LPS for 24 h, while secreted IL-6 increased over 9-fold. These results support the hypothesis that retinal glia may participate in intraretinal immune processes following stimulation during inflammatory and infections processes via either cell surface-or soluble mediator-dependent mechanisms or a combination of both.

Granulocyte macrophage colony stimulating factor stimulates in vitro proliferation of astrocytes derived from simian mature brains

In the brain, granulocyte-macrophage colony stimulating factor (GM-CSF) may be released by infiltrated cells of the immune system including T and B lymphocytes and mononuclear phagocytes, but also by nervous system resident cells such as microglia and astrocytes. Astrocyte-secreted GM-CSF may play an important role in enhancing the local inflammatory response to central nervous system (CNS) injury and in recruting microglia and activated macrophages. In this study, we demonstrated that GM-CSF, as TNF alpha and IL 6, stimulates in vitro proliferation of simian astrocytes in primary cultures. Results were confirmed by blocking experiments performed with a specific neutralizing mAb directed against GM-CSF. Furthermore, we demonstrated that GM-CSF mediates its effect on these cells through the alpha subunit of the GM-CSF receptor which is constitutively expressed at the membrane of the cultured simian astrocytes as assessed by immunofluorescence. GM-CSF effects on astrocytes could be involved in astrocytosis, a hallmark of various neurological injuries and in inflammatory processes in an autocrine manner.

Up-regulation of MHC class I by flavivirus-induced peptide translocation into the endoplasmic reticulum

Flavivirus infection of mammalian cells increases the cell surface expression of major histocompatibility complex (MHC) class I molecules, the recognition elements for cytotoxic T cells. Here, we show that the mechanism for flavivirus-induced up-regulation of class I MHC involves an increase in peptide supply to the endoplasmic reticulum. Flavivirus-mediated peptide supply for MHC class I assembly is independent of the peptide transporters for class I antigen presentation, since infection of class I MHC peptide transport-deficient cell lines with flaviviruses results in the cell surface expression of biologically functional class I MHC peptide complexes. The flavivirus-induced supply of antigenic peptides to the endoplasmic reticulum is not restricted to flavivirus-encoded peptides and independent of interferon. The data imply that peptide availability regulates surface expression of class I MHC restriction elements and suggests a mechanism for flavivirus-induced immunopathology.

Adherence status regulates the primary cellular activation responses to the flavivirus West Nile

Increases in cell-surface intercellular adhesion molecule-1 (ICAM-1; CD54) and major histocompatibility complex antigen class I (MHC-I) and class II (MHC-II) expression during flavivirus infection of murine macrophages was strongly dependent on adherence status. CD54 and MHC expression was significantly increased during infection with the flavivirus West Nile (WNV) on adherent, but not on non-adherent, macrophages. In contrast, increased CD54 and MHC-I expression was induced by interferon-gamma (IFN-gamma) in both cultures but was significantly greater on adherent cells than non-adherent cells. Adherent status was also important in human embryonic fibroblasts (HEF), adherent cells of non-immune origin. Similar to macrophages, WNV induced increased CD54 or MHC-I expression on adherent but not non-adherent HEF. Again, induction of these antigens by IFN-gamma occurred in both cultures but was significantly greater on adherent cells than non-adherent cells. Macrophages or HEF that aggregated when cultured at high density under non-adherent conditions, responded to WNV and IFN-gamma in a manner similar to adherent cells. Unresponsive non-adherent cells infected with WNV or treated with IFN-gamma under non-adherent conditions for 24 or 48 hr recovered if transferred to adherent culture conditions for 24 or 48 hr. Moreover, these cells expressed significantly higher cell-surface CD54 and MHC-I concentrations, compared to similarly treated HEF cultured under adherent conditions during the entire culture period. WNV infection also induced significant nitric oxide production in macrophages, and adherence status was similarly important in this response, with adherent cells producing higher amounts of nitrite/nitrate than non-adherent cells. These results suggest that adherent status may be critical for effective antiviral immune responses involving macrophages.

Coronavirus induction of class I major histocompatibility complex expression in murine astrocytes is virus strain specific

Neurotropic strains of mouse hepatitis viruses (MHV) such as MHV-A59 (A59) and MHV-4 (JHMV) cause acute and chronic encephalomyelitis and demyelination in susceptible strains of mice and rats. They are widely used as models of human demyelinating diseases such as multiple sclerosis (MS), in which immune mechanisms are thought to participate in the development of lesions in the central nervous system (CNS). The effects of MHV infection on target cell functions in the CNS are not well understood, but A59 has been shown to induce the expression of MHC class I molecules in glial cells after in vivo and in vitro infection. Changes in class I expression in infected cells may contribute to the immunopathogenesis of MHV infection in the CNS. In this communication, a large panel of MHV strains was tested for their ability to stimulate class I expression in primary astrocytes in vitro. The data show that the more hepatotropic strains, such as MHV-A59, MHV-1, MHV-2, MHV-3, MHV-D, MHV-K, and MHV-NuU, were potent inducers of class I expression in astrocytes during acute infection, measured by radioimmunoassay. The Kb molecule was preferentially expressed over Db. By contrast, JHMV and several viral strains derived from it did not stimulate the expression of class I molecules. Assays of virus infectivity indicated that the class I-inducing activity did not correlate with the ability of the individual viral strain to replicate in astrocytes. However, exposure of the viruses or the supernatants from infected astrocytes to ultraviolet light abolished the class I-inducing activity, indicating that infectious virus is required for class I expression. These data also suggest that class I expression was induced directly by virus infection, and not by the secretion of a soluble substance into the medium by infected astrocytes. Finally, analyses of A59/JHMV recombinant viral strains suggest that class I-inducing activity resides in one of the A59 structural genes.

CTL recognition of west Nile virus-infected fibroblasts is cell cycle dependent and is associated with virus-induced increases in class I MHC antigen expression

Confluent and non-confluent mouse embryo fibroblast (CMEF and NCMEF) monolayers were infected with West Nile virus (WNV) for 24 hr, and class I major histocompatibility complex antigen (MHC-I) concentrations measured by flow cytometry (FCM). Concentrations on CMEF increased significantly more than on NCMEF. This was not owing to differences in interferon-beta (IFN-beta)-mediated MHC induction, as the IFN-beta quantity secreted by each infected fibroblast was similar in each culture, and IFN-beta-mediated MHC-I induction on NCMEF was greater than on CMEF. Furthermore, despite neutralization of WNV-induced supernatant IFN-beta, CMEF increased MHC-I expression significantly more than NCMEF. Functionally, WNV-infected CMEF were lysed 10-fold better by WNV-specific and allospecific cytotoxic T cells, than infected NCMEF. FCM demonstrated 76% CMEF and 68% NCMEF distributed in G0/G1. This represented G0 in CMEF, and G1 in NCMEF, confirmed by ribonucleotide reductase M1 subunit labelling, where only 20% CMEF was labelled, compared to 84% NCMEF. The possible implications for antiviral immune responses are discussed.

A transgenic mouse model to assess the interaction of cytotoxic T lymphocytes with virally infected, class I MHC-expressing astrocytes

Astrocytes provide crucial support for neurons and their impairment by viruses or their interactions with anti-viral or autoimmune responses could contribute to neurological disease. We have developed a transgenic mouse model to assess lymphocyte-astrocyte interactions. The major histocompatibility complex (MHC) class I molecule, Db, was expressed in astrocytes under the transcriptional control of regulatory sequences from the glial fibrillary acidic protein (GFAP) gene. Baseline cerebral MHC class I mRNA levels from transgenic mice were elevated over those of non-transgenic controls, and a prominent increase in cerebral MHC class I expression occurred following focal, injury-induced astroglial activation within transgenic brains but not in non-transgenic controls. FACS analysis of explant astrocyte cultures from established transgenic lines demonstrated astroglial expression of the GFAP-Db fusion gene at the protein level. Functional antigen-presenting capacity was conferred by the Db transgene, as virus-infected primary astrocytes obtained from transgenic BALB/c mice (KdIdDdLd) expressing the Db molecule were lysed by Db-restricted anti-viral CTL.

Astrocytes: form, functions, and roles in disease

Astrocytes, once relegated to a mere supportive role in the central nervous system, are now recognized as a heterogeneous class of cells with many important and diverse functions. Major astrocyte functions can be grouped into three categories: guidance and support of neuronal migration during development, maintenance of the neural microenvironment, and modulation of immune reactions by serving as antigen-presenting cells. The concept of astrocytic heterogeneity is critical to understanding the functions and reactions of these cells in disease. Astrocytes from different regions of the brain have diverse biochemical characteristics and may respond in different ways to a variety of injuries. Astrocytic swelling and hypertrophy-hyperplasia are two common reactions to injury. This review covers the morphologic and pathophysiologic findings, time course, and determinants of these two responses. In addition to these common reactions, astrocytes may play a primary role in certain diseases, including epilepsy, neurological dysfunction in liver disease, neurodegenerative disorders such as Parkinson's and Huntington's diseases, and demyelination. Evidence supporting primary involvement of astrocytes in these diseases will be considered.