Efficient T-cell surveillance of the CNS requires expression of the CXC chemokine receptor 3

CXCR3 signaling reduces the severity of experimental autoimmune encephalomyelitis by controlling the parenchymal distribution of effector and regulatory T cells in the central nervous system

The chemokine receptor CXCR3 promotes the trafficking of activated T and NK cells in response to three ligands, CXCL9, CXCL10, and CXCL11. Although these chemokines are produced in the CNS in multiple sclerosis and experimental autoimmune encephalomyelitis (EAE), their role in the pathogenesis of CNS autoimmunity is unresolved. We examined the function of CXCR3 signaling in EAE using mice that were deficient for CXCR3 (CXCR3(-/-)). The time to onset and peak disease severity were similar for CXCR3(-/-) and wild-type (WT) animals; however, CXCR3(-/-) mice had more severe chronic disease with increased demyelination and axonal damage. The inflammatory lesions in WT mice consisted of well-demarcated perivascular mononuclear cell infiltrates, mainly in the spinal cord and cerebellum. In CXCR3(-/-) mice, these lesions were more widespread throughout the CNS and were diffused and poorly organized, with T cells and highly activated microglia/macrophages scattered throughout the white matter. Although the number of CD4(+) and CD8(+) T cells infiltrating the CNS were similar in CXCR3(-/-) and WT mice, Foxp3(+) regulatory T cells were significantly reduced in number and dispersed in CXCR3(-/-) mice. The expression of various chemokine and cytokine genes in the CNS was similar in CXCR3(-/-) and WT mice. The genes for the CXCR3 ligands were expressed predominantly in and/or immediately surrounding the mononuclear cell infiltrates. We conclude that in EAE, CXCR3 signaling constrains T cells to the perivascular space in the CNS and augments regulatory T cell recruitment and effector T cell interaction, thus limiting autoimmune-mediated tissue damage.

Analysis of the pharmacokinetic/pharmacodynamic relationship of a small molecule CXCR3 antagonist, NBI-74330, using a murine CXCR3 internalization assay

BACKGROUND AND PURPOSE

Pharmacokinetic/pharmacodynamic (PK/PD) models are necessary to relate the degree of drug exposure in vivo to target blockade and pharmacological efficacy. This manuscript describes a murine agonist-induced CXCR3 receptor internalization assay and demonstrates its utility for PK/PD analyses.

EXPERIMENTAL APPROACH

Activated murine DO11.10 cells were incubated with agonist in the presence or absence of a CXCR3 antagonist and changes in surface CXCR3 expression were detected by flow cytometry. For PK/PD analysis, mice were dosed with a small molecule CXCR3 antagonist, NBI-74330, (100 mg kg(-1)) orally or subcutaneously and plasma samples taken at specified timepoints for the CXCR3 internalization assay.

KEY RESULTS

Surface CXCR3 expression was specifically decreased in response to CXCL9, CXCL10 and CXCL11. CXCL11 was the most potent CXCR3 agonist in buffer (pA50=9.23+/-0.26) and the pA50 for CXCL11 was unaltered in murine plasma (pA50=9.17+/-0.15). The affinity of a small molecule CXCR3 antagonist, NBI-74330, was obtained in the absence or presence of plasma (buffer pA2 value: 7.84+/-0.14; plasma pKB) value 6.36+/-0.01). Administration of NBI-74330 to mice resulted in the formation of an N-oxide metabolite, also an antagonist of CXCR3. Both antagonists were detectable up to 7 h post oral dose and 24 h post subcutaneous dose. Measurement of CXCR3 internalization demonstrated significant antagonism of this response ex vivo, 24 h following subcutaneous administration of NBI-74330.

CONCLUSIONS AND IMPLICATIONS

The CXCR3 receptor internalization assay provides a robust method for determining agonist potency orders, antagonist affinity estimates and PK/PD analyses, which discriminate between dosing regimens for the CXCR3 antagonist NBI-74330.

Expression of CXC chemokine ligand 10 from the mouse hepatitis virus genome results in protection from viral-induced neurological and liver disease

Using the recombinant murine coronavirus mouse hepatitis virus (MHV) expressing the T cell-chemoattractant CXCL10 (MHV-CXCL10), we demonstrate a potent antiviral role for CXCL10 in host defense. Instillation of MHV-CXCL10 into the CNS of CXCL10-deficient (CXCL10(-/-)) mice resulted in viral infection and replication in both brain and liver. Expression of virally encoded CXCL10 within the brain protected mice from death and correlated with increased infiltration of T lymphocytes, enhanced IFN-gamma secretion, and accelerated viral clearance when compared with mice infected with an isogenic control virus, MHV. Similarly, viral clearance from the livers of MHV-CXCL10-infected mice was accelerated in comparison to MHV-infected mice, yet was independent of enhanced infiltration of T lymphocytes and NK cells. Moreover, CXCL10(-/-) mice infected with MHV-CXCL10 were protected from severe hepatitis as evidenced by reduced pathology and serum alanine aminotransferase levels compared with MHV-infected mice. CXCL10-mediated protection within the liver was not dependent on CXC-chemokine receptor 2 (CXCR2) signaling as anti-CXCR2 treatment of MHV-CXCL10-infected mice did not modulate viral clearance or liver pathology. In contrast, treatment of MHV-CXCL10-infected CXCL10(-/-) mice with anti-CXCL10 Ab resulted in increased clinical disease correlating with enhanced viral recovery from the brain and liver as well as increased serum alanine aminotransferase levels. These studies highlight that CXCL10 expression promotes protection from coronavirus-induced neurological and liver disease.

Presence of HSV-1 immediate early genes and clonally expanded T-cells with a memory effector phenotype in human trigeminal ganglia

The latent persistence of herpes simplex virus type 1 (HSV-1) in human trigeminal ganglia (TG) is accompanied by a chronic CD8 T-cell infiltrate. The focus of the current work was to look for HSV-1 transcription activity as a potential trigger of the immune response and to characterize the immune cell infiltrates by this feature. We combined in situ hybridization, laser cutting microscopy, and single cell RT-PCR to demonstrate the expression of the HSV-1 immediate early (IE) genes ICP0 and ICP4 in human trigeminal neurons. Using CDR3 spectratyping, we showed that the infiltrating T-cells are clonally expanded, indicating an antigen-driven immune response. Moreover, the persisting CD8+ T-cells had features of the memory effector phenotype. The voltage-gated potassium channel Kv1.3, a marker of chronic activated memory effector cells, and the chemokines CCL5 and CXCL10 were expressed by a subpopulation of infiltrating cells. The corresponding chemokine receptors CCR5 and CXCR3 were co-expressed on virtually all CD8 T-cells. In addition, T-cells expressed granzymes and perforin. In contrast to animal models of HSV-1 latency, hardly any FoxP3-positive regulatory T-cells were detected in human TG. Thus, HSV-1 IE genes are expressed in human TG and the infiltrating T-cells bear several characteristics that suggest viral antigenic stimulation.

CCR5 and CXCR3 are dispensable for liver infiltration, but CCR5 protects against virus-induced T-cell-mediated hepatic steatosis

CCR5 and CXCR3 are important molecules in regulating the migration of activated lymphocytes. Thus, the majority of tissue-infiltrating T cells found in the context of autoimmune conditions and viral infections express CCR5 and CXCR3, and the principal chemokine ligands are expressed within inflamed tissues. Accordingly, intervention studies have pointed to nonredundant roles of these receptors in models of allograft rejection, viral infection, and autoimmunity. In spite of this, considerable controversy exists, with many studies failing to support a role for CCR5 or CXCR3 in disease pathogenesis. One possible explanation is that different chemokine receptors may take over in the absence of any individual receptor, thus rendering individual receptors redundant. We have attempted to address this issue by analyzing CCR5(-/-), CXCR3(-/-), and CCR5/CXCR3(-/-) mice with regard to virus-induced liver inflammation, generation and recruitment of effector cells, virus control, and immunopathology. Our results indicate that CCR5 and CXCR3 are largely dispensable for tissue infiltration and virus control. In contrast, the T-cell response is accelerated in CCR5(-/-) and CCR5/CXCR3(-/-) mice and the absence of CCR5 is associated with the induction of CD8(+) T-cell-mediated immunopathology consisting of marked hepatic microvesicular steatosis.

NK cells stimulate recruitment of CXCR3+ T cells to the brain during Plasmodium berghei-mediated cerebral malaria

NK cells are cytotoxic lymphocytes that also secrete regulatory cytokines and can therefore influence adaptive immune responses. NK cell function is largely controlled by genes present in a genomic region named the NK complex. It has been shown that the NK complex is a genetic determinant of murine cerebral malaria pathogenesis mediated by Plasmodium berghei ANKA. In this study, we show that NK cells are required for cerebral malaria disease induction and the control of parasitemia. NK cells were found infiltrating brains of cerebral malaria-affected mice. NK cell depletion resulted in inhibition of T cell recruitment to the brain of P. berghei-infected animals. NK cell-depleted mice displayed down-regulation of CXCR3 expression and a significant reduction of T cells migrating in response to IFN-gamma-inducible protein 10, indicating that this chemokine pathway plays an essential role in leukocyte trafficking leading to cerebral disease and fatalities.

T cell antiviral effector function is not dependent on CXCL10 following murine coronavirus infection

The chemokine CXCL10 is expressed within the CNS in response to intracerebral infection with mouse hepatitis virus (MHV). Blocking CXCL10 signaling results in increased mortality accompanied by reduced T cell infiltration and increased viral titers within the brain suggesting that CXCL10 functions in host defense by attracting T cells into the CNS. The present study was undertaken to extend our understanding of the functional role of CXCL10 in response to MHV infection given that CXCL10 signaling has been implicated in coordinating both effector T cell generation and trafficking. We show that MHV infection of CXCL10(+/+) or CXCL10(-/-) mice results in comparable levels of T cell activation and similar numbers of virus-specific CD4+ and CD8+ T cells. Subsequent analysis revealed no differences in T cell proliferation, IFN-gamma secretion by virus-specific T cells, or CD8+ T cell cytolytic activity. Analysis of chemokine receptor expression on CD4+ and CD8+ T cells obtained from MHV-immunized CXCL10(+/+) and CXCL10(-/-) mice revealed comparable levels of CXCR3 and CCR5, which are capable of responding to ligands CXCL10 and CCL5, respectively. Adoptive transfer of splenocytes acquired from MHV-immunized CXCL10(-/-) mice into MHV-infected RAG1(-/-) mice resulted in T cell infiltration into the CNS, reduced viral burden, and demyelination comparable to RAG1(-/-) recipients of immune CXCL10(+/+) splenocytes. Collectively, these data imply that CXCL10 functions primarily as a T cell chemoattractant and does not significantly influence T cell effector response following MHV infection.

Toll-like receptor 2 signaling in response to brain injury: an innate bridge to neuroinflammation

Reactive gliosis is a prominent feature of neurodegenerative and neuroinflammatory disease in the CNS, yet the stimuli that drive this response are not known. There is growing appreciation that signaling through Toll-like receptors (TLRs), which is key to generating innate responses to infection, may have pathogen-independent roles. We show that TLR2 was selectively upregulated by microglia in the denervated zones of the hippocampus in response to stereotactic transection of axons in the entorhinal cortex. In mice lacking TLR2, there were transient, selective reductions in lesion-induced expression of cytokines and chemokines. Recruitment of T cells, but not macrophages, was delayed in TLR2-deficient mice, as well as in mice lacking TNFR1 (tumor necrosis factor receptor 1). TLR2 deficiency also affected microglial proliferative expansion, whereas all of these events were unaffected in TLR4-mutant mice. Consistent with the fact that responses in knock-out mice had all returned to wild-type levels by 8 d, there was no evidence for effects on neuronal plasticity at 20 d. These results identify a role for TLR2 signaling in the early glial response to brain injury, acting as an innate bridge to neuroinflammation.

Both CXCR3 and CXCL10/IFN-inducible protein 10 are required for resistance to primary infection by dengue virus

We examined the extent to which CXCR3 mediates resistance to dengue infection. Following intracerebral infection with dengue virus, CXCR3-deficient (CXCR3(-/-)) mice showed significantly higher mortality rates than wild-type (WT) mice; moreover, surviving CXCR3(-/-) mice, but not WT mice, often developed severe hind-limb paralysis. The brains of CXCR3(-/-) mice showed higher viral loads than those of WT mice, and quantitative analysis using real-time PCR, flow cytometry, and immunohistochemistry revealed fewer T cells, CD8(+) T cells in particular, in the brains of CXCR3(-/-) mice. This suggests that recruitment of effector T cells to sites of dengue infection was diminished in CXCR3(-/-) mice, which impaired elimination of the virus from the brain and thus increased the likelihood of paralysis and/or death. These results indicate that CXCR3 plays a protective rather than an immunopathological role in dengue virus infection. In studies to identify critical CXCR3 ligands, CXCL10/IFN-inducible protein 10-deficient (CXCL10/IP-10(-/-)) mice infected with dengue virus showed a higher mortality rate than that of the CXCR3(-/-) mice. Although CXCL10/IP-10, CXCL9/monokine induced by IFN-gamma, and CXCL11/IFN-inducible T cell alpha chemoattractant share a single receptor and all three of these chemokines are induced by dengue virus infection, the latter two could not compensate for the absence of CXCL10/IP-10 in this in vivo model. Our results suggest that both CXCR3 and CXCL10/IP-10 contribute to resistance against primary dengue virus infection and that chemokines that are indistinguishable in in vitro assays differ in their activities in vivo.

CXCL10 is the key ligand for CXCR3 on CD8+ effector T cells involved in immune surveillance of the lymphocytic choriomeningitis virus-infected central nervous system

IFN-gamma-inducible protein 10/CXCL10 is a chemokine associated with type 1 T cell responses, regulating the migration of activated T cells through binding to the CXCR3 receptor. Expression of both CXCL10 and CXCR3 are observed during immunopathological diseases of the CNS, and this receptor/ligand pair is thought to play a central role in regulating T cell-mediated inflammation in this organ site. In this report, we investigated the role of CXCL10 in regulating CD8(+) T cell-mediated inflammation in the virus-infected brain. This was done through analysis of CXCL10-deficient mice infected intracerebrally with lymphocytic choriomeningitis virus, which in normal immunocompetent mice induces a fatal CD8(+) T cell-mediated meningoencephalitis. We found that a normal antiviral CD8(+) T cell response was generated in CXCL10-deficient mice, and that lack of CXCL10 had no influence on the accumulation of mononuclear cells in the cerebrospinal fluid. However, analysis of the susceptibility of CXCL10-deficient mice to lymphocytic choriomeningitis virus-induced meningitis revealed that these mice just like CXCR3-deficient mice were partially resistant to this disease, whereas wild-type mice invariably died. Furthermore, despite marked up-regulation of the two remaining CXCR3 ligands: CXCL9 and 11, we found a reduced accumulation of CD8(+) T cells in the brain parenchyma around the time point when wild-type mice succumb as a result of CD8(+) T cell-mediated inflammation. Thus, taken together these results indicate a central role for CXCL10 in regulating the accumulation of effector T cells at sites of CNS inflammation, with no apparent compensatory effect of other CXCR3 ligands.

Differential roles for CXCR3 in CD4+ and CD8+ T cell trafficking following viral infection of the CNS

Lymphocyte infiltration into the central nervous system (CNS) following viral infection represents an important component of host defense and is required for control of viral replication. However, the mechanisms governing inflammation in response to viral infection of the CNS are not well understood. Following intracranial (i.c.) infection of susceptible mice with mouse hepatitis virus (MHV), mice develop an acute encephalomyelitis followed by a chronic demyelinating disease. The CXC chemokine ligand 10 (CXCL10) is expressed following MHV infection and signals T cells to migrate into the CNS. The functional contribution of the CXCL10 receptor CXCR3 in host defense and disease in response to MHV infection was evaluated. The majority of CD4+ and CD8+ T cells infiltrating the CNS following MHV infection express CXCR3. Administration of anti-CXCR3 antibody reduced CD4+ T cell infiltration (p<or=0.05), while CD8+ T cell trafficking was not affected. Anti-CXCR3 treatment during chronic disease correlated with improved motor skills and reduced demyelination. The selective effect of anti-CXCR3 treatment on CD4+ T cells was not the result of either reduced proliferation or modulation in chemokine receptor gene expression. Therefore, CXCR3 signaling has a non-redundant role in T cell subset trafficking in response to viral infection.

Perforin-deficient CD8+ T cells mediate fatal lymphocytic choriomeningitis despite impaired cytokine production

Intracerebral (i.c.) infection with lymphocytic choriomeningitis virus (LCMV) is one of the most studied models for virus-induced immunopathology, and based on results from perforin-deficient mice, it is currently assumed that fatal disease directly reflects perforin-mediated cell lysis. However, recent studies have revealed additional functional defects within the effector T cells of LCMV-infected perforin-deficient mice, raising the possibility that perforin may not be directly involved in mediating lethal disease. For this reason, we decided to reevaluate the role of perforin in determining the outcome of i.c. infection with LCMV. We confirmed that the expansion of virus-specific CD8(+) T cells is unimpaired in perforin-deficient mice. However, despite the fact that the virus-specific CD8(+) effector T cells in perforin-deficient mice are broadly impaired in their effector function, these mice invariably succumb to i.c. infection with LCMV strain Armstrong, although a few days later than matched wild-type mice. Upon further investigation, we found that this delay correlates with the delayed recruitment of inflammatory cells to the central nervous system (CNS). However, CD8(+) effector T cells were not kept from the CNS by sequestering in infected extraneural organ sites such as liver or lungs. Thus, the observed dysfunctionality regarding the production of proinflammatory mediators probably results in the delayed recruitment of effector cells to the CNS, and this appears to be the main explanation for the delayed onset of fatal disease in perforin-deficient mice. However, once accumulated in the CNS, virus-specific CD8(+) T cells can induce fatal CNS pathology despite the absence of perforin-mediated lysis and reduced capacity to produce several key cytokines.

Chemokine receptor CXCR3: an unexpected enigma

CXCR3, the receptor for CXCL9/MIG, CXCL10/IP-10, and CXCL11/I-TAC, is preferentially expressed on activated Th1 T cells and has been predicted to play an important role in their trafficking. However, this simplistic view of the function of CXCR3 and its ligands has not been borne out by studies of disease models, including experimental autoimmune encephalomyelitis (EAE), using varied methods of receptor blockade, as well as knockout or transgenic mice. This review focuses on the current understanding of the enigmatic role of CXCR3 and its ligands in CNS inflammatory/autoimmune disorders. The conflicting results among varied models of CNS inflammation suggest complex and multiple roles for CXCR3 and its ligands in the pathogenesis of CNS inflammatory/autoimmune diseases. Thus, further study is needed to determine how CXCL10 neutralizing agents or CXCR3 receptor antagonists might be applied to treating human disease.

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 CXCR3 in the immune response to murine gammaherpesvirus 68

The chemokine IP-10 (CXCL10) and its cellular receptor CXCR3 are upregulated in the lung during murine gammaherpesvirus 68 (MHV-68) infection. In order to determine the role of the CXCR3 chemokine receptor in the immune response to MHV-68, CXCR3-/- mice were infected with the virus. CXCR3-/- mice showed delayed clearance of replicating MHV-68 from the lungs. This correlated with delayed T-cell recruitment to the lungs and reduced cytolytic activity prior to viral clearance. Splenomegaly and the numbers of latently infected cells per spleen were transiently increased. However, CXCR3-/- mice showed normal virus-specific antibody titers and effective long-term control of MHV-68 infection.

Opposing Effects of CXCR3 and CCR5 Deficiency on CD8+ T Cell-Mediated Inflammation in the Central Nervous System of Virus-Infected Mice

T cells play a key role in the control of viral infection in the CNS but may also contribute to immune-mediated cell damage. To study the redundancy of the chemokine receptors CXCR3 and CCR5 in regulating virus-induced CD8+ T cell-mediated inflammation in the brain, CXCR3/CCR5 double-deficient mice were generated and infected intracerebrally with noncytolytic lymphocytic choriomeningitis virus. Because these chemokine receptors are mostly expressed by overlapping subsets of activated CD8+ T cells, it was expected that absence of both receptors would synergistically impair effector T cell invasion and therefore protect mice against the otherwise fatal CD8+ T cell-mediated immune attack. Contrary to expectations, the accumulation of mononuclear cells in cerebrospinal fluid was only slightly delayed compared with mice with normal expression of both receptors. Even more surprising, CXCR3/CCR5 double-deficient mice were more susceptible to intracerebral infection than CXCR3-deficient mice. Analysis of effector T cell generation revealed an accelerated antiviral CD8+ T cell response in CXCR3/CCR5 double-deficient mice. Furthermore, while the accumulation of CD8+ T cells in the neural parenchyma was significantly delayed in both CXCR3- and CXCR3/CCR5-deficient mice, more CD8+ T cells were found in the parenchyma of double-deficient mice when these were analyzed around the time when the difference in clinical outcome becomes manifest. Taken together, these results indicate that while CXCR3 plays an important role in controlling CNS inflammation, other receptors but not CCR5 also contribute significantly. Additionally, our results suggest that CCR5 primarily functions as a negative regulator of the antiviral CD8+ T cell response.

Virus-specific and bystander CD8 T cells recruited during virus-induced encephalomyelitis

Neurotropic coronavirus-induced encephalitis was used to evaluate recruitment, functional activation, and retention of peripheral bystander memory CD8+ T cells. Mice were first infected with recombinant vaccinia virus expressing a non-cross-reactive human immunodeficiency virus (HIV) epitope, designated p18. Following establishment of an endogenous p18-specific memory CD8+ T-cell population, mice were challenged with coronavirus to directly compare recruitment, longevity, and activation characteristics of both primary coronavirus-specific and bystander memory populations trafficking into the central nervous system (CNS). HIV-specific memory CD8+ T cells were recruited early into the CNS as components of the innate immune response, preceding CD8+ T cells specific for the dominant coronavirus epitope, designated pN. Although pN-specific T-cell numbers gradually exceeded bystander p18-specific CD8+ T-cell numbers, both populations peaked concurrently within the CNS. Nevertheless, coronavirus-specific CD8+ T cells were preferentially retained. By contrast, bystander CD8+ T-cell numbers declined to background numbers following control of CNS virus replication. Furthermore, in contrast to highly activated pN-specific CD8+ T cells, bystander p18-specific CD8+ T cells recruited to the site of inflammation maintained a nonactivated memory phenotype and did not express ex vivo cytolytic activity. Therefore, analysis of host CD8+ T-cell responses to unrelated infections demonstrates that bystander memory CD8+ T cells can comprise a significant proportion of CNS inflammatory cells during virus-induced encephalitis. However, transient CNS retention and the absence of activation suggest that memory bystander CD8+ T cells may not overtly contribute to pathology in the absence of antigen recognition.

Analysis of the Contributions of l -Selectin and CXCR3 in Mediating Leukocyte Homing to Pregnant Mouse Uterus

PROBLEM

Dynamic changes occur in endometrial immune cell populations during pregnancy but regulatory events promoting cell recruitment to the uterus are not established. Ovarian steroid hormones promote l-selectin and alpha4-integrin-mediated interactions between human peripheral natural killer (NK) cells and uterine endothelium while CXCR3, CXCR4 and their ligands are implicated in homing of human uNK cells to decidua.

METHOD OF STUDY

Mice genetically-ablated for l-selectin or CXCR3 were studied. Morphometric analyses of implantation sites and assays of cell function (in vitro adhesion; in vivo homing following adoptive cell transfer) were undertaken.

RESULTS

Leukocytes home to anti-mesometrial decidua with l-selectin making an early (<6 hr in vivo) contribution. Unexpectedly, CXCR3 deletion had no effect on homing but reduced the ability of uNK cells to modify placental spiral arterioles.

CONCLUSIONS

Redundant mechanisms underlie homing of leukocytes to the uterus and their importance can be evaluated by an in vivo approach described herein.

Differential responses of human brain cells to West Nile virus infection

In recent years, West Nile virus (WNV) has emerged as a major cause of encephalitis in the United States. However, the neuropathogenesis of this flavivirus is poorly understood. In the present study, the authors used primary human brain cell cultures to investigate two neuropathogenic features: viral replication and induction of cytokines. Although neurons and astrocytes were found to support productive WNV infection, viral growth was poorly permissive in microglial cells. Compared to neuronal cultures that sustained viral growth for at least 2 weeks, replication peaked in astrocytes by 72 h post infection. In response to viral infection, astrocytes produced chemokines (CXCL10 and CCL5), but none of the cytokines (tumor necrosis factor [TNF]-alpha, interleukin [IL]-1beta, IL-6, interferon alpha or gamma) tested could be detected. Although microglial cells failed to support viral replication, WNV induced production of the proinflammatory cytokines IL-6 and TNF-alpha. Microglial cells also released robust amounts of the chemokines CXCL10 and CCL2, as well as lower levels of CCL5, in response to WNV infection. WNV-induced chemokine and cytokine production by microglia was coupled with activation of mitogen-activated protein kinase (MAPK) intracellular signaling pathways. Inhibition of p38 MAPK decreased chemokine production in response to WNV. Taken together, these findings suggest that microglial cell responses may influence the neuropathogenesis of WNV infection.