Human cytomegalovirus mediates APOBEC3B relocalization early during infection through a ribonucleotide reductase-independent mechanism

The APOBEC3 family of DNA cytosine deaminases comprises an important arm of the innate antiviral defense system. The gamma-herpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus and the alpha-herpesviruses herpes simplex virus (HSV)-1 and HSV-2 have evolved an efficient mechanism to avoid APOBEC3 restriction by directly binding to APOBEC3B and facilitating its exclusion from the nuclear compartment. The only viral protein required for APOBEC3B relocalization is the large subunit of the ribonucleotide reductase (RNR). Here, we ask whether this APOBEC3B relocalization mechanism is conserved with the beta-herpesvirus human cytomegalovirus (HCMV). Although HCMV infection causes APOBEC3B relocalization from the nucleus to the cytoplasm in multiple cell types, the viral RNR (UL45) is not required. APOBEC3B relocalization occurs rapidly following infection suggesting the involvement of an immediate early or early (IE/E) viral protein. In support of this possibility, genetic (IE1 mutant) and pharmacologic (cycloheximide) strategies that prevent the expression of IE/E viral proteins also block APOBEC3B relocalization. In comparison, the treatment of infected cells with phosphonoacetic acid, which interferes with viral late protein expression, still permits A3B relocalization. These results combine to indicate that the beta-herpesvirus HCMV uses an RNR-independent, yet phenotypically similar, molecular mechanism to antagonize APOBEC3B. IMPORTANCE Human cytomegalovirus (HCMV) infections can range from asymptomatic to severe, particularly in neonates and immunocompromised patients. HCMV has evolved strategies to overcome host-encoded antiviral defenses to achieve lytic viral DNA replication and dissemination and, under some conditions, latency and long-term persistence. Here, we show that HCMV infection causes the antiviral factor, APOBEC3B, to relocalize from the nuclear compartment to the cytoplasm. This overall strategy resembles that used by related herpesviruses. However, the HCMV relocalization mechanism utilizes a different viral factor(s) and available evidence suggests the involvement of at least one protein expressed at the early stages of infection. This knowledge is important because a greater understanding of this mechanism could lead to novel antiviral strategies that enable APOBEC3B to naturally restrict HCMV infection.

Dysregulated brain regulatory T cells fail to control reactive gliosis following repeated antigen stimulation

This study was undertaken to investigate the role of CD4+FoxP3+ regulatory T cells (Tregs) in regulating neuroinflammation during viral Ag-challenge and re-challenge. CD8⁺ lymphocytes persisting within tissues are designated tissue-resident memory T cells (T_RM), within brain: bT_RM. Reactivation of bT_RM with T cell epitope peptides generates rapid antiviral recall, but repeated stimulation leads to cumulative dysregulation of microglial activation, proliferation, and prolonged neurotoxic mediator production. Here, we show Tregs were recruited into murine brains following prime-CNS boost, but displayed altered phenotypes following repeated Ag-challenge. In response to repeated Ag, brain Tregs (bTregs) displayed inefficient immunosuppressive capacity, along with reduced expression of suppression of tumorigenicity 2 (ST2) and amphiregulin (Areg). Ex vivo Areg treatment revealed reduced production of neurotoxic mediators such as iNOS, IL-6, and IL-1β, and decreased microglial activation and proliferation. Taken together, these data indicate bTregs display an unstable phenotype and fail to control reactive gliosis in response to repeated Ag-challenge.

Human cytomegalovirus mediates APOBEC3B relocalization early during infection through a ribonucleotide reductase-independent mechanism

The APOBEC3 family of DNA cytosine deaminases comprises an important arm of the innate antiviral defense system. The gamma-herpesviruses EBV and KSHV and the alpha-herpesviruses HSV-1 and HSV-2 have evolved an efficient mechanism to avoid APOBEC3 restriction by directly binding to APOBEC3B and facilitating its exclusion from the nuclear compartment. The only viral protein required for APOBEC3B relocalization is the large subunit of the ribonucleotide reductase (RNR). Here, we ask whether this APOBEC3B relocalization mechanism is conserved with the beta-herpesvirus human cytomegalovirus (HCMV). Although HCMV infection causes APOBEC3B relocalization from the nucleus to the cytoplasm in multiple cell types, the viral RNR (UL45) is not required. APOBEC3B relocalization occurs rapidly following infection suggesting involvement of an immediate early or early (IE-E) viral protein. In support of this mechanism, cycloheximide treatment of HCMV-infected cells prevents the expression of viral proteins and simultaneously blocks APOBEC3B relocalization. In comparison, the treatment of infected cells with phosphonoacetic acid, which is a viral DNA synthesis inhibitor affecting late protein expression, still permits A3B relocalization. These results combine to show that the beta-herpesvirus HCMV uses a fundamentally different, RNR-independent molecular mechanism to antagonize APOBEC3B.

Importance

Human cytomegalovirus (HCMV) infections can range from asymptomatic to severe, particularly in neonates and immunocompromised patients. HCMV has evolved strategies to overcome host-encoded antiviral defenses in order to achieve lytic viral DNA replication and dissemination and, under some conditions, latency and long-term persistence. Here, we show that HCMV infection causes the antiviral factor, APOBEC3B, to relocalize from the nuclear compartment to the cytoplasm. This overall strategy resembles that used by related herpesviruses. However, the HCMV relocalization mechanism utilizes a different viral factor(s) and available evidence suggests the involvement of at least one protein expressed at the early stages of infection. This knowledge is important because a greater understanding of this mechanism could lead to novel antiviral strategies that enable APOBEC3B to naturally restrict HCMV infection.

Differential Cytokine-Induced Responses of Polarized Microglia

The role of select pro- and anti-inflammatory mediators in driving microglial cell polarization into classically (M1), or alternatively, (M2) activated states, as well as the subsequent differential responses of these induced phenotypes, was examined. Expression of PD-L1, MHC-II, MHC-I, arginase 1 (Arg-1), and inducible nitric oxide synthase (iNOS) was assessed using multi-color flow cytometry. We observed that both pro- and anti-inflammatory mediators induced PD-L1 expression on non-polarized microglia. Moreover, IFN-γ stimulated significant MHC class I and II expression on these cells. Interestingly, we observed that only IL-4 treatment induced Arg-1 expression, indicating M2 polarization. These M2 cells were refractory to subsequent depolarization and maintained their alternatively activated state. Furthermore, PD-L1 expression was significantly induced on these M2-polarized microglia after treatment with pro-inflammatory mediators, but not anti-inflammatory cytokines. In addition, we observed that only LPS induced iNOS expression in microglial cells, indicating M1 polarization. Furthermore, IFN-γ significantly increased the percentage of M1-polarized microglia expressing iNOS. Surprisingly, when these M1-polarized microglia were treated with either IL-6 or other anti-inflammatory cytokines, they returned to their non-polarized state, as demonstrated by significantly reduced expression of iNOS. Taken together, these results demonstrate differential responses of microglial cells to mediators present in dissimilar microenvironments.

Dysregulated Microglial Cell Activation and Proliferation Following Repeated Antigen Stimulation

Upon reactivation of quiescent neurotropic viruses antigen (Ag)-specific brain resident-memory CD8+ T-cells (bT_RM) may respond to de novo-produced viral Ag through the rapid release of IFN-γ, which drives subsequent interferon-stimulated gene expression in surrounding microglia. Through this mechanism, a small number of adaptive bT_RM may amplify responses to viral reactivation leading to an organ-wide innate protective state. Over time, this brain-wide innate immune activation likely has cumulative neurotoxic and neurocognitive consequences. We have previously shown that HIV-1 p24 Ag-specific bT_RM persist within the murine brain using a heterologous prime-CNS boost strategy. In response to Ag restimulation, these bT_RM display rapid and robust recall responses, which subsequently activate glial cells. In this study, we hypothesized that repeated challenges to viral antigen (Ag) (modeling repeated episodes of viral reactivation) culminate in prolonged reactive gliosis and exacerbated neurotoxicity. To address this question, mice were first immunized with adenovirus vectors expressing the HIV p24 capsid protein, followed by a CNS-boost using Pr55Gag/Env virus-like particles (HIV-VLPs). Following the establishment of the bT_RM population [>30 days (d)], prime-CNS boost animals were then subjected to in vivo challenge, as well as re-challenge (at 14 d post-challenge), using the immunodominant HIV-1 AI9 CD8+ T-cell epitope peptide. In these studies, Ag re-challenge resulted in prolonged expression of microglial activation markers and an increased proliferative response, longer than the challenge group. This continued expression of MHCII and PD-L1 (activation markers), as well as Ki67 (proliferative marker), was observed at 7, 14, and 30 days post-AI9 re-challenge. Additionally, in vivo re-challenge resulted in continued production of inducible nitric oxide synthase (iNOS) with elevated levels observed at 7, 14 and 30 days post re-challenge. Interestingly, iNOS expression was significantly lower among challenged animals when compared to re-challenged groups. Furthermore, in vivo specific Ag re-challenge produced lower levels of arginase (Arg)-1 when compared with the challenged group. Taken together, these results indicate that repeated Ag-specific stimulation of adaptive immune responses leads to cumulative dysregulated microglial cell activation.

Impairment in neurocognitive function following experimental neonatal guinea pig cytomegalovirus infection

BACKGROUND

Cytomegalovirus (CMV) is a leading infectious cause of neurologic deficits, both in the settings of congenital and perinatal infection, but few animal models exist to study neurodevelopmental outcomes. This study examined the impact of neonatal guinea pig CMV (GPCMV) infection on spatial learning and memory in a Morris water maze (MWM) model.

METHODS

Newborn pups were challenged intraperitoneally (i.p.) with a pathogenic red fluorescent protein-tagged GPCMV, or sham inoculated. On days 15-19 post infection (p.i.), pups were tested in the MWM. Viral loads were measured in blood and tissue by quantitative PCR (qPCR), and brain samples collected at necropsy were examined by histology and immunohistochemistry.

RESULTS

Viremia (DNAemia) was detected at day 3 p.i. in 7/8 challenged animals. End-organ dissemination was observed, by qPCR, in the lung, liver, and spleen. CD4-positive (CD4+) and CD8-positive (CD8+) T cell infiltrates were present in brains of challenged animals, particularly in periventricular and hippocampal regions. Reactive gliosis and microglial nodules were observed. Statistically significant spatial learning and memory deficits were observed by MWM, particularly for total maze distance traveled (p < 0.0001).

CONCLUSION

Neonatal GPCMV infection in guinea pigs results in cognitive defects demonstrable by the MWM. This neonatal guinea pig challenge model can be exploited for studying antiviral interventions.

IMPACT

CMV impairs neonatal neurocognition and memory in the setting of postnatal infection. The MWM can be used to examine memory and learning in a guinea pig model of neonatal CMV infection. CD4+ and CD8+ T cells infiltrate the brain following neonatal CMV challenge. This article demonstrates that the MWM can be used to evaluate memory and learning after neonatal GPCMV challenge. The guinea pig can be used to examine central nervous system pathology caused by neonatal CMV infection and this attribute may facilitate the study of vaccines and antivirals.

Programmed death ligand-1 induction restrains the cytotoxic T lymphocyte response against microglia

Microglial cells are the main reservoir for HIV-1 within the brain and potential exists for negative immune checkpoint blockade therapies to purge this viral reservoir. Here, we investigated cytolytic responses of CD8⁺ T lymphocytes against microglia loaded with peptide epitopes. Initially, flow cytometric analysis demonstrated efficient killing of HIV-1 p24 AI9 or YI9 peptide-loaded splenocytes in MHC-matched recipients. Cytolytic killing of microglia was first demonstrated using ovalbumin (OVA) as a model antigen for in vitro cytotoxic T lymphocyte (CTL) assays. Peptide-loaded primary microglia obtained from programmed death ligand (PD-L) 1 knockout (KO) animals showed significantly more killing than cells from wild-type (WT) animals when co-cultured with activated CD8⁺ T-cells isolated from rAd5-OVA primed animals. Moreover, when peptide loaded-microglial cells from WT animals were treated with neutralizing α-PD-L1 Ab, significantly more killing was observed compared to either untreated or IgG isotype-treated cells. Most importantly, significantly increased in vivo killing of HIV-1 p24 YI9 peptide-loaded microglia from PD-L1 KO animals, as well as AI9 peptide-loaded BALB/c microglial cells treated with α-PD-L1, was observed within brains of rAd5-p24 primed-CNS boosted C57BL/6 or BALB/c mice, respectively. Finally, ex vivo responses of brain CD8⁺ T-cells in response to AI9 stimulation showed significantly increased IFN-γ and IL-2 production when treated with α-PD-1 Abs. Greater proliferation of CD8+ T-cells from the brain was also observed following blockade. Taken together, these studies demonstrate that PD-L1 induction on microglia restrains CTL responses and indicate that immune checkpoint blockade targeting this pathway may be beneficial in clearing viral brain reservoirs.

Recall Responses from Brain-Resident Memory CD8+ T Cells (bTRM) Induce Reactive Gliosis

HIV-associated neurocognitive disorders (HAND) persist even during effective combination antiretroviral therapy (cART). Although the cause of HAND is unknown, studies link chronic immune activation, neuroinflammation, and cerebrospinal fluid viral escape to disease progression. In this study, we tested the hypothesis that specific, recall immune responses from brain-resident memory T cells (bT_RM) could activate glia and induce neurotoxic mediators. To address this question, we developed a heterologous prime-central nervous system (CNS) boost strategy in mice. We observed that the murine brain became populated with long-lived CD8⁺ bT_RM, some being specific for an immunodominant Gag epitope. Recall stimulation using HIV-1 AI9 peptide administered in vivo resulted in microglia displaying elevated levels of major histocompatibility complex class II and programmed death-ligand 1, and demonstrating tissue-wide reactive gliosis. Immunostaining further confirmed this glial activation. Taken together, these results indicate that specific, adaptive recall responses from bT_RM can induce reactive gliosis and production of neurotoxic mediators.

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Heterologous prime-CNS boost induced HIV-1-specific bT_RM, which persisted long term

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Recall responses from HIV-specific bT_RM induced tissue-wide reactive gliosis

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bT_RM induced-reactive gliosis likely has cumulative neurotoxic consequences

Glial Cell Expression of PD-L1

The programmed death (PD)-1/PD-L1 pathway is a well-recognized negative immune checkpoint that results in functional inhibition of T-cells. Microglia, the brain-resident immune cells are vital for pathogen detection and initiation of neuroimmune responses. Moreover, microglial cells and astrocytes govern the activity of brain-infiltrating antiviral T-cells through upregulation of PD-L1 expression. While T-cell suppressive responses within brain are undoubtedly beneficial to the host, preventing cytotoxic damage to this vital organ, establishment of a prolonged anti-inflammatory milieu may simultaneously lead to deficiencies in viral clearance. An immune checkpoint blockade targeting the PD-1: PD-L1 (B7-H1; CD274) axis has revolutionized contemporary treatment for a variety of cancers. However, the therapeutic potential of PD1: PD-L1 blockade therapies targeting viral brain reservoirs remains to be determined. For these reasons, it is key to understand both the detrimental and protective functions of this signaling pathway within the brain. This review highlights how glial cells use PD-L1 expression to modulate T-cell effector function and limit detrimental bystander damage, while still retaining an effective defense of the brain.

Brain-Resident T Cells Following Viral Infection

Activated CD8⁺ lymphocytes infiltrate the brain in response to many viral infections; where some remain stationed long term as memory T cells. Brain-resident memory T cells (bT_RM) are positioned to impart immediate defense against recurrent or reactivated infection. The cytokine and chemokine milieu present within a tissue is critical for T_RM generation and retention; and reciprocal interactions exist between brain-resident glia and bT_RM. High concentrations of TGF-β are found within brain and this cytokine has been shown to induce CD103 (integrin αeβ7) expression. The majority of T cells persisting within brain express CD103, which aids in retention through interaction with E-cadherin. Likewise, cytokines produced by T cells also modulate microglia. The anti-inflammatory cytokine IL-4 has been shown to preferentially polarize microglial cells toward an M2 phenotype, with a corresponding increase in E-cadherin expression. These findings demonstrate that the brain microenvironment, both during and following inflammation, prominently contributes to the role of CD103 in T cell persistence. Further evidence shows that microglia, and astrocytes, upregulate programmed death (PD) ligand 1 during neuroinflammation, likely to limit neuropathology, and the PD-1: PD-L1 pathway also aids in bT_RM generation and retention. Upon reactivation of quiescent neurotropic viruses, bT_RM may respond to small amounts of de novo-produced viral antigen by rapidly releasing IFN-γ, resulting in interferon-stimulated gene expression in surrounding glia, thereby amplifying activation of a small number of adaptive immune cells into an organ-wide innate antiviral response. While advantageous from an antiviral perspective; over time, recall response-driven, organ-wide innate immune activation likely has cumulative neurotoxic and neurocognitive consequences.

Reactive glia promote development of CD103+ CD69+ CD8+ T-cells through programmed cell death-ligand 1 (PD-L1)

INTRODUCTION

Previous work from our laboratory has demonstrated in vivo persistence of CD103⁺ CD69⁺ brain resident memory CD8⁺ T-cells (bT_RM ) following viral infection, and that the PD-1: PD-L1 pathway promotes development of these T_RM cells within the brain. Although glial cells express low basal levels of PD-L1, its expression is upregulated upon IFN-γ-treatment, and they have been shown to modulate antiviral T-cell effector responses through the PD-1: PD-L1 pathway.

METHODS

We performed flow cytometric analysis of cells from co-cultures of mixed glia and CD8⁺ T-cells obtained from wild type mice to investigate the role of glial cells in the development of bT_RM .

RESULTS

In this study, we show that interactions between reactive glia and anti-CD3 Ab-stimulated CD8⁺ T-cells promote development of CD103⁺ CD69⁺ CD8⁺ T-cells through engagement of the PD-1: PD-L1 pathway. These studies used co-cultures of primary murine glial cells obtained from WT animals along with CD8⁺ T-cells obtained from either WT or PD-1 KO mice. We found that αCD3 Ab-stimulated CD8⁺ T-cells from WT animals increased expression of CD103 and CD69 when co-cultured with primary murine glial cells. In contrast, significantly reduced expression of CD103 and CD69 was observed using CD8⁺ T-cells from PD-1 KO mice. We also observed that reactive glia promoted high levels of CD127, a marker of memory precursor effector cells (MPEC), on CD69⁺ CD8⁺ T-cells, which promotes development of T_RM cells. Interestingly, results obtained using T-cells from PD-1 KO animals showed significantly reduced expression of CD127 on CD69⁺ CD8⁺ cells. Additionally, blocking of glial PD-L1 resulted in decreased expression of CD103, along with reduced CD127 on CD69⁺ CD8⁺ T-cells.

CONCLUSIONS

Taken together, these results demonstrate a role for activated glia in promoting development of bT_RM through the PD-1: PD-L1 pathway.

Nitrosative damage during retrovirus infection-induced neuropathic pain

BACKGROUND

Peripheral neuropathy is currently the most common neurological complication in HIV-infected individuals, occurring in 35-50% of patients undergoing combination anti-retroviral therapy. Data have shown that distal symmetric polyneuropathy develops in mice by 6 weeks following infection with the LP-BM5 retrovirus mixture. Previous work from our laboratory has demonstrated that glial cells modulate antiviral T-cell effector responses through the programmed death (PD)-1: PD-L1 pathway, thereby limiting the deleterious consequences of unrestrained neuroinflammation.

METHODS

Using the MouseMet electronic von Frey system, we assessed hind-paw mechanical hypersensitivity in LP-BM5-infected wild-type (WT) and PD-1 KO animals. Using multi-color flow cytometry, we quantitatively assessed cellular infiltration and microglial activation. Using real-time RT-PCR, we assessed viral load, expression of IFN-γ, iNOS, and MHC class II. Using western blotting, we measured protein nitrosylation within the lumbar spinal cord (LSC) and dorsal root ganglion (DRG). Histochemical staining was performed to analyze the presence of CD3, ionized calcium binding adaptor molecule (Iba)-1, MHCII, nitrotyrosine, isolectin B4 (IB4) binding, and neurofilament 200 (NF200). Statistical analyses were carried out using graphpad prism.

RESULTS

Hind-paw mechanical hypersensitivity observed in LP-BM5-infected animals was associated with significantly increased lymphocyte infiltration into the spinal cord and DRG. We also observed elevated expression of IFN-γ (in LSC and DRG) and MHC II (on resident microglia in LSC). We detected elevated levels of 3-nitrotyrosine within the LSC and DRG of LP-BM5-infected animals, an indicator of nitric oxide (NO)-induced protein damage. Moreover, we observed 3-nitrotyrosine in both small (IB4+) and large (NF200+) DRG sensory neurons. Additionally, infected PD-1 KO animals displayed significantly greater mechanical hypersensitivity than WT or uninfected mice at 4 weeks post-infection (p.i.). Accelerated onset of hind-paw hypersensitivity in PD-1 KO animals was associated with significantly increased infiltration of CD4+ and CD8+ T lymphocytes, macrophages, and microglial activation at early time points. Importantly, we also observed elevated levels of 3-nitrotyrosine and iNOS in infected PD-1 KO animals when compared with WT animals.

CONCLUSIONS

Results reported here connect peripheral immune cell infiltration and reactive gliosis with nitrosative damage. These data may help elucidate how retroviral infection-induced neuroinflammatory networks contribute to nerve damage and neuropathic pain.

The PD-1: PD-L1 pathway promotes development of brain-resident memory T cells following acute viral encephalitis

Background

Previous work from our laboratory has demonstrated that during acute viral brain infection, glial cells modulate antiviral T cell effector responses through the PD-1: PD-L1 pathway, thereby limiting the deleterious consequences of unrestrained neuroinflammation. Here, we evaluated the PD-1: PD-L1 pathway in development of brain-resident memory T cells (bT_RM) following murine cytomegalovirus (MCMV) infection.

Methods

Flow cytometric analysis of immune cells was performed at 7, 14, and 30 days post-infection (dpi) to assess the shift of brain-infiltrating CD8⁺ T cell populations from short-lived effector cells (SLEC) to memory precursor effector cells (MPEC), as well as generation of bT_RMs.

Results

In wild-type (WT) animals, we observed a switch in the phenotype of brain-infiltrating CD8⁺ T cell populations from KLRG1⁺ CD127⁻ (SLEC) to KLRG1⁻ CD127⁺ (MPEC) during transition from acute through chronic phases of infection. At 14 and 30 dpi, the majority of CD8⁺ T cells expressed CD127, a marker of memory cells. In contrast, fewer CD8⁺ T cells expressed CD127 within brains of infected, PD-L1 knockout (KO) animals. Notably, in WT mice, a large population of CD8⁺ T cells was phenotyped as CD103⁺ CD69⁺, markers of bT_RM, and differences were observed in the numbers of these cells when compared to PD-L1 KOs. Immunohistochemical studies revealed that brain-resident CD103⁺ bT_RM cells were localized to the parenchyma. Higher frequencies of CXCR3 were also observed among WT animals in contrast to PD-L1 KOs.

Conclusions

Taken together, our results indicate that bT_RMs are present within the CNS following viral infection and the PD-1: PD-L1 pathway plays a role in the generation of this brain-resident population.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-017-0860-3) contains supplementary material, which is available to authorized users.

Modulation of Microglial Cell Fcγ Receptor Expression Following Viral Brain Infection

Fcγ receptors (FcγRs) for IgG couple innate and adaptive immunity through activation of effector cells by antigen-antibody complexes. We investigated relative levels of activating and inhibitory FcγRs on brain-resident microglia following murine cytomegalovirus (MCMV) infection. Flow cytometric analysis of microglial cells obtained from infected brain tissue demonstrated that activating FcγRs were expressed maximally at 5 d post-infection (dpi), while the inhibitory receptor (FcγRIIB) remained highly elevated during both acute and chronic phases of infection. The highly induced expression of activating FcγRIV during the acute phase of infection was also noteworthy. Furthermore, in vitro analysis using cultured primary microglia demonstrated the role of interferon (IFN)γ and interleukin (IL)-4 in polarizing these cells towards a M1 or M2 phenotype, respectively. Microglial cell-polarization correlated with maximal expression of either FcγRIV or FcγRIIB following stimulation with IFNγ or IL-4, respectively. Finally, we observed a significant delay in polarization of microglia towards an M2 phenotype in the absence of FcγRs in MCMV-infected Fcer1g and FcgR2b knockout mice. These studies demonstrate that neuro-inflammation following viral infection increases expression of activating FcγRs on M1-polarized microglia. In contrast, expression of the inhibitory FcγRIIB receptor promotes M2-polarization in order to shut-down deleterious immune responses and limit bystander brain damage.

Glial cell activation, recruitment, and survival of B-lineage cells following MCMV brain infection

Background

Chemokines produced by reactive glia drive migration of immune cells and previous studies from our laboratory have demonstrated that CD19⁺ B cells infiltrate the brain. In this study, in vivo and in vitro experiments investigated the role of reactive glial cells in recruitment and survival of B-lineage cells in response to (murine cytomegalovirus) MCMV infection.

Methods

Flow cytometric analysis was used to assess chemokine receptor expression on brain-infiltrating B cells. Real-time RT-PCR and ELISA were used to measure chemokine levels. Dual-immunohistochemical staining was used to co-localize chemokine production by reactive glia. Primary glial cell cultures and migration assays were used to examine chemokine-mediated recruitment. Astrocyte: B cell co-cultures were used to investigate survival and proliferation.

Results

The chemokine receptors CXCR3, CXCR5, CCR5, and CCR7 were detected on CD19⁺ cells isolated from the brain during MCMV infection. In particular, CXCR3 was found to be elevated on an increasing number of cells over the time course of infection, and it was the primary chemokine receptor expressed at 60 days post infection Quite different expression kinetics were observed for CXCR5, CCR5, and CCR7, which were elevated on the highest number of cells early during infection and decreased by 14, 30, and 60 days post infection Correspondingly, elevated levels of CXCL9, CXCL10, and CXCL13, as well as CCL5, were found within the brains of infected animals, and only low levels of CCL3 and CCL19 were detected. Differential expression of CXCL9/CXCL10 and CXCL13 between microglia and astrocytes was apparent, and B cells moved towards supernatants from MCMV-infected microglia, but not astrocytes. Pretreatment with neutralizing Abs to CXCL9 and CXCL10 inhibited this migration. In contrast, neutralizing Abs to the ligand of CXCR5 (i.e., CXCL13) did not significantly block chemotaxis. Proliferation of brain-infiltrating B cells was detected at 7 days post infection and persisted through the latest time tested (60 days post infection). Finally, astrocytes produce BAFF (B cell activating factor of the TNF family) and promote proliferation of B cells via cell-to-cell contact.

Conclusions

CXCR3 is the primary chemokine receptor on CD19⁺ B cells persisting within the brain, and migration to microglial cell supernatants is mediated through this receptor. Correspondingly, microglial cells produce CXCL9 and CXCL10, but not CXCL13. Reactive astrocytes promote B cell proliferation.

Tregs Modulate Lymphocyte Proliferation, Activation, and Resident-Memory T-Cell Accumulation within the Brain during MCMV Infection

Accumulation and retention of regulatory T-cells (Tregs) has been reported within post viral-encephalitic brains, however, the full extent to which these cells modulate neuroinflammation is yet to be elucidated. Here, we used Foxp3-DTR (diphtheria toxin receptor) knock-in transgenic mice, which upon administration of low dose diphtheria toxin (DTx) results in specific deletion of Tregs. We investigated the proliferation status of various immune cell subtypes within inflamed central nervous system (CNS) tissue. Depletion of Tregs resulted in increased proliferation of both CD8⁺ and CD4⁺ T-cell subsets within the brain at 14 d post infection (dpi) when compared to Treg-sufficient animals. At 30 dpi, while proliferation of CD8⁺ T-cells was controlled within brains of both Treg-depleted and undepleted mice, proliferation of CD4⁺ T-cells remained significantly enhanced with DTx-treatment. Previous studies have demonstrated that Treg numbers within the brain rebound following DTx treatment to even higher numbers than in untreated animals. Despite this rebound, CD8⁺ and CD4⁺ T-cells proliferated at a higher rate when compared to that of Treg-sufficient mice, thus maintaining sustained neuroinflammation. Furthermore, at 30 dpi we found the majority of CD8⁺ T-cells were CD127^hi KLRG1^- indicating that the cells were long lived memory precursor cells. These cells showed marked elevation of CD103 expression, a marker of tissue resident-memory T-cells (T_RM) in the CNS, in untreated animals when compared to DTx-treated animals suggesting that generation of T_RM is impaired upon Treg depletion. Moreover, the effector function of T_RM as indicated by granzyme B production in response to peptide re-stimulation was found to be more potent in Treg-sufficient animals. Taken together, our findings demonstrate that Tregs limit neuroinflammatory responses to viral infection by controlling cell proliferation and may direct a larger proportion of lymphocytes within the brain to be maintained as T_RM cells.

Chronic reactive gliosis following regulatory T cell depletion during acute MCMV encephalitis

Long-term, persistent central nervous system inflammation is commonly seen following brain infection. Using a murine model of viral encephalitis (murine cytomegalovirus, MCMV) we have previously shown that post-encephalitic brains are maintained in an inflammatory state consisting of glial cell reactivity, retention of brain-infiltrating tissue-resident memory CD8⁺ T-cells, and long-term persistence of antibody-producing cells of the B-lineage. Here, we report that this neuroinflammation occurs concomitantly with accumulation and retention of immunosuppressive regulatory T-cells (Tregs), and is exacerbated following their ablation. However, the extent to which these Tregs function to control neuroimmune activation following MCMV encephalitis is unknown. In this study, we used Foxp3-diphtheria toxin receptor-GFP (Foxp3-DTR-GFP) transgenic mice, which upon administration of low-dose diphtheria toxin (DTx) results in the specific depletion of Tregs, to investigate their function. We found treatment with DTx during the acute phase of viral brain infection (0-4 dpi) resulted in depletion of Tregs from the brain, exacerbation of encephalitis (i.e., increased presence of CD4⁺ and CD8⁺ T-cells), and chronic reactive phenotypes of resident glial cells (i.e., elevated MHC Class II as well as PD-L1 levels, sustained microgliosis, and increased glial fibrillary acidic protein (GFAP) expression on astrocytes) versus untreated, infected animals. This chronic proinflammatory environment was associated with reduced cognitive performance in spatial learning and memory tasks (Barnes Maze) by convalescent animals. These data demonstrate that chronic glial cell activation, unremitting post-encephalitic neuroinflammation, and its associated long-term neurological sequelae in response to viral brain infection are modulated by the immunoregulatory properties of Tregs. GLIA 2015;63:1982-1996.

Infiltrating regulatory B cells control neuroinflammation following viral brain infection

Previous studies have demonstrated the existence of a subset of B lymphocytes, regulatory B cells (Bregs), which modulate immune function. In this study, in vivo and in vitro experiments were undertaken to elucidate the role of these Bregs in controlling neuroinflammation following viral brain infection. We used multicolor flow cytometry to phenotype lymphocyte subpopulations infiltrating the brain, along with in vitro cocultures to assess their anti-inflammatory and immunoregulatory roles. This distinctive subset of CD19(+)CD1d(hi)CD5(+) B cells was found to infiltrate the brains of chronically infected animals, reaching highest levels at the latest time point tested (30 d postinfection). B cell-deficient Jh(-/-) mice were found to develop exacerbated neuroimmune responses as measured by enhanced accumulation and/or retention of CD8(+) T cells within the brain, as well as increased levels of microglial activation (MHC class II). Conversely, levels of Foxp3(+) regulatory T cells were found to be significantly lower in Jh(-/-) mice when compared with wild-type (Wt) animals. Further experiments showed that in vitro-generated IL-10-secreting Bregs (B10) were able to inhibit cytokine responses from microglia following stimulation with viral Ags. These in vitro-generated B10 cells were also found to promote proliferation of regulatory T cells in coculture studies. Finally, gain-of-function experiments demonstrated that reconstitution of Wt B cells into Jh(-/-) mice restored neuroimmune responses to levels exhibited by infected Wt mice. Taken together, these results demonstrate that Bregs modulate T lymphocyte as well as microglial cell responses within the infected brain and promote CD4(+)Foxp3(+) T cell proliferation in vitro.

Glial cells suppress postencephalitic CD8+ T lymphocytes through PD-L1

Engagement of the programmed death (PD)−1 receptor on activated cells by its ligand (PD‐L1) is a mechanism for suppression of activated T‐lymphocytes. Microglia, the resident inflammatory cells of the brain, are important for pathogen detection and initiation of innate immunity, however, a novel role for these cells as immune regulators has also emerged. PD‐L1 on microglia has been shown to negatively regulate T‐cell activation in models of multiple sclerosis and acute viral encephalitis. In this study, we investigated the role of glial cell PD‐L1 in controlling encephalitogenic CD8⁺ T‐lymphocytes, which infiltrate the brain to manage viral infection, but remain to produce chronic neuroinflammation. Using a model of chronic neuroinflammation following murine cytomegalovirus (MCMV)‐induced encephalitis, we found that CD8⁺ T‐cells persisting within the brain expressed PD‐1. Conversely, activated microglia expressed PD‐L1. In vitro, primary murine microglia, which express low basal levels of PD‐L1, upregulated the co‐inhibitory ligand on IFN‐γ‐treatment. Blockade of the PD‐1: PD‐L1 pathway in microglial: CD8⁺ T‐cell co‐cultures increased T‐cell IFN‐γ and interleukin (IL)−2 production. We observed a similar phenomenon following blockade of this co‐inhibitory pathway in astrocyte: CD8⁺ T‐cell co‐cultures. Using ex vivo cultures of brain leukocytes, including microglia and CD8⁺ T‐cells, obtained from mice with MCMV‐induced chronic neuroinflammation, we found that neutralization of either PD‐1 or PD‐L1 increased IFN‐γ production from virus‐specific CD8⁺ T‐cells stimulated with MCMV IE1_168–176 peptide. These data demonstrate that microglia and astrocytes control antiviral T‐cell responses and suggest a therapeutic potential of PD1: PD‐L1 modulation to manage the deleterious consequences of uncontrolled neuroinflammation. GLIA 2014;62:1582–1594

Microglia and astrocytes exert regulatory control over T‐cells during chronic neuroinflammation following viral brain infection.

Post-encephalitic glial cells express PD‐L1 and suppress persistent CD8 T‐cells via the PD‐1: PD‐L1 inhibitory pathway.

T-cell reconstitution during murine acquired immunodeficiency syndrome (MAIDS) produces neuroinflammation and mortality in animals harboring opportunistic viral brain infection

BACKGROUND

Highly active antiretroviral therapy (HAART) restores inflammatory immune responses in AIDS patients which may unmask previous subclinical infections or paradoxically exacerbate symptoms of opportunistic infections. In resource-poor settings, 25% of patients receiving HAART may develop CNS-related immune reconstitution inflammatory syndrome (IRIS). Here we describe a reliable mouse model to study underlying immunopathological mechanisms of CNS-IRIS.

METHODS

Utilizing our HSV brain infection model and mice with MAIDS, we investigated the effect of immune reconstitution on MAIDS mice harboring opportunistic viral brain infection. Using multi-color flow cytometry, we quantitatively measured the cellular infiltrate and microglial activation.

RESULTS

Infection with the LP-BM5 retroviral mixture was found to confer susceptibility to herpes simplex virus (HSV)-1 brain infection to normally-resistant C57BL/6 mice. Increased susceptibility to brain infection was due to severe immunodeficiency at 8 wks p.i. and a marked increase in programmed death-1 (PD-1) expression on CD4+ and CD8+ T-cells. Both T-cell loss and opportunistic brain infection were associated with high level PD-1 expression because PD-1-knockout mice infected with LP-BM5 did not exhibit lymphopenia and retained resistance to HSV-1. In addition, HSV-infection of MAIDS mice stimulated peripheral immune cell infiltration into the brain and its ensuing microglial activation. Interestingly, while opportunistic herpes virus brain infection of C57BL/6 MAIDS mice was not itself lethal, when T-cell immunity was reconstituted through adoptive transfer of virus-specific CD3+ T-cells, it resulted in significant mortality among recipients. This immune reconstitution-induced mortality was associated with exacerbated neuroinflammation, as determined by MHC class II expression on resident microglia and elevated levels of Th1 cytokines in the brain.

CONCLUSIONS

Taken together, these results indicate development of an immune reconstitution disease within the central nervous system (CNS-IRD). Experimental immune reconstitution disease of the CNS using T-cell repopulation of lymphopenic murine hosts harboring opportunistic brain infections may help elucidate neuroimmunoregulatory networks that produce CNS-IRIS in patients initiating HAART.

Modulation of experimental herpes encephalitis-associated neurotoxicity through sulforaphane treatment

Reactive oxygen species (ROS) produced by brain-infiltrating macrophages and neutrophils, as well as resident microglia, are pivotal to pathogen clearance during viral brain infection. However, unchecked free radical generation is also responsible for damage to and cytotoxicity of critical host tissue bystander to primary infection. These unwanted effects of excessive ROS are combated by local cellular production of antioxidant enzymes, including heme oxygenase-1 (HO-1) and glutathione peroxidase 1 (Gpx1). In this study, we showed that experimental murine herpes encephalitis triggered robust ROS production, as well as an opposing upregulation of the antioxidants HO-1 and Gpx1. This antioxidant response was insufficient to prevent tissue damage, neurotoxicity, and mortality associated with viral brain infection. Previous studies corroborate our data supporting astrocytes as the major antioxidant producer in brain cell cultures exposed to HSV-1 stimulated microglia. We hypothesized that stimulating opposing antioxidative responses in astrocytes, as well as neurons, would mitigate the effects of ROS-mediated neurotoxicity both in vitro and during viral brain infection in vivo. Here, we demonstrate that the addition of sulforaphane, a potent stimulator of antioxidant responses, enhanced HO-1 and Gpx1 expression in astrocytes through the activation of nuclear factor-E2-related factor 2 (Nrf2). Additionally, sulforaphane treatment was found to be effective in reducing neurotoxicity associated with HSV-stimulated microglial ROS production. Finally, intraperitoneal injections of sulforaphane into mice during active HSV infection reduced neuroinflammation via a decrease in brain-infiltrating leukocytes, macrophage- and neutrophil-produced ROS, and MHCII-positive, activated microglia. These data support a key role for astrocyte-produced antioxidants in modulating oxidative stress and neuronal damage in response to viral infection.

Persistent humoral immune responses in the CNS limit recovery of reactivated murine cytomegalovirus

Background

Experimental infection of the mouse brain with murine CMV (MCMV) elicits neuroimmune responses that terminate acute infection while simultaneously preventing extensive bystander damage. Previous studies have determined that CD8⁺ T lymphocytes are required to restrict acute, productive MCMV infection within the central nervous system (CNS). In this study, we investigated the contribution of humoral immune responses in control of MCMV brain infection.

Methodology/Principal Findings

Utilizing our MCMV brain infection model, we investigated B-lymphocyte-lineage cells and assessed their role in controlling the recovery of reactivated virus from latently infected brain tissue. Brain infiltrating leukocytes were first phenotyped using markers indicative of B-lymphocytes and plasma cells. Results obtained during these studies showed a steady increase in the recruitment of B-lymphocyte-lineage cells into the brain throughout the time-course of viral infection. Further, MCMV-specific antibody secreting cells (ASC) were detected within the infiltrating leukocyte population using an ELISPOT assay. Immunohistochemical studies of brain sections revealed co-localization of CD138⁺ cells with either IgG or IgM. Additional immunohistochemical staining for MCMV early antigen 1 (E1, m112–113), a reported marker of viral latency in neurons, confirmed its expression in the brain during latent infection. Finally, using B-cell deficient (Jh^−/−) mice we demonstrated that B-lymphocytes control recovery of reactivated virus from latently-infected brain tissue. A significantly higher rate of reactivated virus was recovered from the brains of Jh^−/− mice when compared to Wt animals.

Conclusion

Taken together, these results demonstrate that MCMV infection triggers accumulation and persistence of B-lymphocyte-lineage cells within the brain, which produce antibodies and play a significant role in controlling reactivated virus.

Reactive oxygen species drive herpes simplex virus (HSV)-1-induced proinflammatory cytokine production by murine microglia

Background

Production of reactive oxygen species (ROS) and proinflammatory cytokines by microglial cells in response to viral brain infection contributes to both pathogen clearance and neuronal damage. In the present study, we examined the effect of herpes simplex virus (HSV)-1-induced, NADPH oxidase-derived ROS in activating mitogen-activated protein kinases (MAPKs) as well as driving cytokine and chemokine expression in primary murine microglia.

Methods

Oxidation of 2', 7'-dichlorodihydrofluorescin diacetate (H₂DCFDA) was used to measure production of intracellular ROS in microglial cell cultures following viral infection. Virus-induced cytokine and chemokine mRNA and protein levels were assessed using real-time RT-PCR and ELISA, respectively. Virus-induced phosphorylation of microglial p38 and p44/42 (ERK1/2) MAPKs was visualized using Western Blot, and levels of phospho-p38 were quantified using Fast Activated Cell-based ELISA (FACE assay). Diphenyleneiodonium (DPI) and apocynin (APO), inhibitors of NADPH oxidases, were used to investigate the role of virus-induced ROS in MAPK activation and cytokine, as well as chemokine, production.

Results

Levels of intracellular ROS were found to be highly elevated in primary murine microglial cells following infection with HSV and the majority of this virus-induced ROS was blocked following DPI and APO treatment. Correspondingly, inhibition of NADPH oxidase also decreased virus-induced proinflammatory cytokine and chemokine production. In addition, microglial p38 and p44/42 MAPKs were found to be phosphorylated in response to viral infection and this activation was also blocked by inhibitors of NADPH oxidase. Finally, inhibition of either of these ROS-induced signaling pathways suppressed cytokine (TNF-α and IL-1β) production, while chemokine (CCL2 and CXCL10) induction pathways were sensitive to inhibition of p38, but not ERK1/2 MAPK.

Conclusions

Data presented herein demonstrate that HSV infection induces proinflammatory responses in microglia through NADPH oxidase-dependent ROS and the activation of MAPKs.

Cytomegalovirus-induced sensorineural hearing loss with persistent cochlear inflammation in neonatal mice

Congenital cytomegalovirus (CMV) infection is the leading cause of sensorineural hearing loss (SNHL) in children. During murine (M)CMV-induced encephalitis, the immune response is important for both the control of viral dissemination and the clearance of virus from the brain. While the importance of CMV-induced SNHL has been described, the mechanisms surrounding its pathogenesis and the role of inflammatory responses remain unclear. This study presents a neonatal mouse model of profound SNHL in which MCMV preferentially infected both cochlear perilymphatic epithelial cells and spiral ganglion neurons. Interestingly, MCMV infection induced cochlear hair cell death by 21 days post-infection, despite a clear lack of direct infection of hair cells and the complete clearance of the virus from the cochlea by 14 dpi. Flow cytometric, immunohistochemical, and quantitative PCR analysis of MCMV-infected cochlea revealed a robust and chronic inflammatory response, including a prolonged increase in reactive oxygen species production by infiltrating macrophages. These data support a pivotal role for inflammation during MCMV-induced SNHL.

Murine cytomegalovirus infection of neural stem cells alters neurogenesis in the developing brain

Background

Congenital cytomegalovirus (CMV) brain infection causes serious neuro-developmental sequelae including: mental retardation, cerebral palsy, and sensorineural hearing loss. But, the mechanisms of injury and pathogenesis to the fetal brain are not completely understood. The present study addresses potential pathogenic mechanisms by which this virus injures the CNS using a neonatal mouse model that mirrors congenital brain infection. This investigation focused on, analysis of cell types infected with mouse cytomegalovirus (MCMV) and the pattern of injury to the developing brain.

Methodology/Principal Findings

We used our MCMV infection model and a multi-color flow cytometry approach to quantify the effect of viral infection on the developing brain, identifying specific target cells and the consequent effect on neurogenesis. In this study, we show that neural stem cells (NSCs) and neuronal precursor cells are the principal target cells for MCMV in the developing brain. In addition, viral infection was demonstrated to cause a loss of NSCs expressing CD133 and nestin. We also showed that infection of neonates leads to subsequent abnormal brain development as indicated by loss of CD24(hi) cells that incorporated BrdU. This neonatal brain infection was also associated with altered expression of Oct4, a multipotency marker; as well as down regulation of the neurotrophins BDNF and NT3, which are essential to regulate the birth and differentiation of neurons during normal brain development. Finally, we report decreased expression of doublecortin, a marker to identify young neurons, following viral brain infection.

Conclusions

MCMV brain infection of newborn mice causes significant loss of NSCs, decreased proliferation of neuronal precursor cells, and marked loss of young neurons.

Reduced lymphocyte infiltration during cytomegalovirus brain infection of interleukin-10-deficient mice

Interleukin (IL)-10 deficiency results in highly elevated levels of interferon (IFN)-gamma, as well as the IFN-gamma-inducible chemokines CXCL9 and CXCL10 within murine cytomegalovirus (MCMV)-infected brains. To test the hypothesis that these elevated chemokine levels would result in enhanced brain infiltration, we compared immune cell infiltration in response to MCMV brain infection between wild-type and IL-10 knockout (KO) mice. Longitudinal analysis following adoptive transfer of cells from beta-actin-luciferase transgenic wild-type mice showed maximal brain infiltration by peripheral immune cells occurred at 5 days post infection. Although the overall percentage of CD45(hi) cells infiltrating the brain was not altered by IL-10 deficiency, paradoxically, despite elevated chemokine levels, reduced T lymphocyte (CD8+) and natural killer (NK) (CD49b+) cell infiltration into the brain was observed in IL-10-deficient animals. This decreased lymphocyte infiltration was associated with elevated levels of the lymph node homing receptor L-selectin/CD62L on CD8+ T cells. Lymph node cells obtained from MCMV-infected mice deficient in IL-10 also displayed reduced migration towards CXCL10 when compared to wild-type animals. Taken together, these data show that despite elevated chemokine levels, absence of IL-10 results in reduced lymphocyte infiltration into MCMV-infected brains.

Hemin inhibits NO production by IL-1β-stimulated human astrocytes through induction of heme oxygenase-1 and reduction of p38 MAPK activation

Background

Heme oxygenase (HO)-1 has been shown to attenuate oxidative injury and reduce apoptosis. HO-1 can be induced by various stimuli released during cellular injury, such as heme. Deleterious free heme is degraded by HO-1 to carbon monoxide, iron and biliverdin, which have potent anti-oxidant and anti-inflammatory properties. In this study, we tested the hypothesis that upregulation of HO-1 would inhibit production of the free radical (NO) by interlukin (IL)-1β-activated human astrocytes.

Methods

To measure NO production, inducible NO synthase (iNOS), HO-1 expression and mitogen-activated protein (MAP) kinase activation we used hemin as an HO-1 inducer and tin protoporphyrin (SnPP) IX as an inhibitor of HO-1 activity in human astrocyte cultures prior to IL-1β exposure. Transfection of astrocyte cultures was performed using a pLEX expression vector carrying the human HO-1 sequence prior to IL-1β treatment. Supernatants of astrocyte cultures pretreated with inhibitors of p38 MAPK or MEK1/2 prior to IL-1β exposure were collected for NO assay.

Results

IL-1β treatment of astrocytes alone induced undetectable amounts of HO-1 protein by western blot. However, HO-1 mRNA expression was modestly up-regulated in response to IL-1β stimulation. Pretreatment with hemin alone substantially induced both HO-1 mRNA and protein expression, and HO-1 mRNA expression was further enhanced when hemin was combined with IL-1β treatment. In contrast, IL-1β-induced iNOS mRNA expression and NO production were markedly inhibited by hemin treatment. When pretreated with SnPP, the inhibitory effect of hemin on IL-1β-induced NO production and iNOS expression was reversed, suggesting the involvement of HO-1. IL-1β-induced p38 MAPK activation, which is known to be required for NO production, was also down-regulated by hemin.

Conclusion

These findings support the hypothesis that up-regulation of HO-1 in astrocytes is associated with down-regulation of iNOS expression and thereby NO production, an effect that involves the p38 MAPK signaling pathway, which suggests that this glial cell response could play an important protective role against oxidative stress in the brain.

Preferential sensitivity of human dopaminergic neurons to gp120-induced oxidative damage

The dopamine (DA)-rich midbrain is known to be a key target of human immunodeficiency virus (HIV)-1. Studies of simian immunodeficiency virus (SIV)-induced neuropathogenesis recently established that there is a major disruption within the nigrostriatal dopaminergic system characterized by marked depletion of dopaminergic neurons, microglial cell activation, and reactive astrocytes. Using a human mesencephalic neuronal/glial culture model, which contains dopaminergic neurons, microglia, and astrocytes, experiments were performed to characterize the damage to dopaminergic neurons induced by HIV-1 gp120. Functional impairment was assessed by DA uptake, and neurotoxicity was measured by apoptosis and oxidative damage. Through the use of this mesencephalic neuronal/glial culture model, we were able to identify the relative sensitivity of dopaminergic neurons to gp120-induced damage, manifested as reduced function (decreased DA uptake), morphological changes, and reduced viability. We also showed that gp120-induced oxidative damage is involved in this neuropathogenic process.

Chronic cortical and subcortical pathology with associated neurological deficits ensuing experimental herpes encephalitis

Long-term neurological sequela is common among herpes simplex encephalitis (HSE) survivors. Animal models for HSE are used to investigate mechanisms of acute disease, but little has been done to model chronic manifestations of HSE. The current study presents a detailed, systematic analysis of chronic neuropathology, including characterization of topography and sequential progression of degenerative lesions and inflammation. Subsequent to intranasal HSV-1 infection, inflammatory responses that were temporally and spatially distinct persisted in infected cortical and brain stem regions. Neutrophils were present exclusively within the olfactory bulb and brain stem regions during the acute phase of infection, while the chronic inflammation was marked by plasma cells, lymphocytes and activated microglia. The chronic lymphocytic infiltrate, cytokine production, and activated microglia were associated with the loss of cortical neuropile in the entorhinal cortex and hippocampus. Animals surviving the acute infection showed a spectrum of chronic lesions from decreased brain volume, neuronal loss, activated astrocytes, and glial scar formation to severe atrophy and cavitations of the cortex. These lesions were also associated with severe spatial memory deficits in surviving animals. Taken together, this model can be utilized to further investigate the mechanisms of neurological defects that follow in the wake of HSE.

Prolonged microglial cell activation and lymphocyte infiltration following experimental herpes encephalitis

Experimental murine herpes simplex virus (HSV)-1 brain infection stimulates microglial cell-driven proinflammatory chemokine production which precedes the presence of brain-infiltrating systemic immune cells. In the present study, we investigated the phenotypes and infiltration kinetics of leukocyte trafficking into HSV-infected murine brains. Using real-time bioluminescence imaging, the infiltration of luciferase-positive splenocytes, transferred via tail vein injection into the brains of HSV-infected animals, was followed over an 18-day time course. Flow cytometric analysis of brain-infiltrating leukocytes at 5, 8, 14, and 30 days postinfection (d.p.i.), was performed to assess their phenotype. A predominantly macrophage (CD45(high)CD11b(+)Ly6C(high)) and neutrophil (CD45(high)CD11b(+)Ly6G(+)) infiltration was seen early during infection, with elevated levels of TNF-alpha mRNA expression. By 14 d.p.i., the phenotypic profile shifted to a predominantly lymphocytic (CD45(high)CD3(+)) infiltrate. This lymphocyte infiltrate was detected until 30 d.p.i., when infectious virus could not be recovered, with CD8(+) and CD4(+) T cells present at a 3:1 ratio, respectively. This T lymphocyte infiltration paralleled increased IFN-gamma mRNA expression in the brain. Activation of resident microglia (CD45(int)CD11b(+)) was also detected until 30 d.p.i., as assessed by MHC class II expression. Activated microglial cells were further identified as the predominant source of IL-1beta. In addition, infected mice given primed immunocytes at 4 d.p.i. showed a significant increase in mortality. Taken together, these results demonstrate that intranasal infection results in early macrophage and neutrophil infiltration into the brain followed by prolonged microglial activation and T lymphocyte retention. Similar prolonged neuroimmune activation may contribute to the neuropathological sequelae observed in herpes encephalitis patients.

Cytomegalovirus infection and interferon-gamma modulate major histocompatibility complex class I expression on neural stem cells

Cytomegalovirus (CMV) is the leading transmittable cause of congenital brain abnormalities in children and infection results in fatal ventriculoencephalitis in advanced acquired immunodeficiency syndrome (AIDS) patients. Pathology associated with CMV brain infection is seen predominantly in the periventricular region, an area known to harbor neural stem cells (NSCs). In the present study, using an adult model of murine CMV brain infection, the authors demonstrated that nestin-positive NSCs in the subventricular zone are susceptible to murine CMV infection. Furthermore, primary NSC cultures supported productive murine CMV replication with a 1000-fold increase in viral titers by 5 days post infection (d.p.i). Previous studies from the authors' laboratory demonstrated that CD8 lymphocytes were essential in protecting the brain against murine CMV infection. In the present study, the authors found that interferon (IFN)-gamma treatment increased the expression of major histocompatibility complex (MHC) class I on NSCs. Viral infection, on the other hand, inhibited this IFN-gamma-induced MHC up-regulation. In addition to increasing MHC class I expression, IFN-gamma (but not tumor necrosis factor [TNF]-alpha, interleukin [IL]-1 beta, or IL-10) also suppressed NSC proliferation in vitro. This decrease in proliferation was not accompanied by apoptosis or extracellular release of cellular lactate dehydrogenase (LDH), suggesting that the effects were not due to direct cytotoxicity. These studies demonstrate that NSCs are susceptible to murine CMV infection and inflammatory mediators, such as IFN-gamma, alter cellular characteristics which may have an impact on their reparative functions.

Microglia are the major cellular source of inducible nitric oxide synthase during experimental herpes encephalitis

Although production of reactive nitrogen and reactive oxygen species (RNS and ROS) is a component of innate defense against viral infection, their overproduction in the brain may also lead to deleterious consequences. To investigate potential immunopathologic roles of oxidative stress during herpes encephalitis, the authors examined the expression kinetics of inducible nitric oxide synthase (iNOS) as well as heme oxygenase-1 (HO-1), a marker of oxidative stress, and evaluated infection-induced oxidative brain damage. Results from these studies showed that both iNOS and HO-1 gene expression were highly elevated in the brain within 7 days post infection (d.p.i.) and remained elevated through 21 d.p.i. Real-time bioluminescence imaging of HO-1 promoter-luciferase transgenic mice confirmed HO-1 promoter activity in the brains of HSV-1-infected animals within 3 d.p.i., which peaked between 5 and 7 d.p.i. Immunohistochemical staining for both 3-nitrotyrosine and 8-hydroxydeoxyguanosine (8-OH-dG), as well as quantitative assessment of 8-isoprostane levels, demonstrated the presence of viral infection-induced oxidative brain damage. In addition, when brain leukocytes obtained from animals with experimental herpes encephalitis were sorted using fluorescence-activated cell sorting (FACS) and the individual cell populations analyzed, CD45(int)/CD11b(+) resident microglia were found to be the major cellular source of iNOS expression.

Histoplasma capsulatum yeast phase-specific protein Yps3p induces Toll-like receptor 2 signaling

Histoplasma capsulatum is a common cause of fungal infection in certain geographic areas, and although most infections are asymptomatic, it is capable of causing histoplasmosis, a disseminated, life-threatening disease, especially in immunocompromised individuals. A deeper understanding of this host-pathogen interaction is needed to develop novel therapeutic strategies to counter lethal infection. Although several lines of evidence suggest that this fungus is neurotropic in HIV patients, little is known about the immunobiology of Histoplasma infection in the central nervous system [CNS]. The goal of the present study was to understand the innate neuroimmune mechanisms that recognize H. capsulatum during the initial stages of infection. Using a 293T stable cell line expressing murine Toll-like receptor 2 [TLR2], we show here that TLR2 recognizes H. capsulatum cell wall protein Yps3p and induces the activation of NF-κB. In further experiments, we tested the ability of Yps3p to induce signaling from TLR2 in primary microglial cells, the resident brain macrophages of the CNS. Our data show that H. capsulatum Yps3p induced TLR2 signaling in wild-type microglia, but not in microglia isolated from TLR2 KO mice, confirming that Yps3p is a ligand for TLR2. Furthermore, Yps3p-induced TLR2 signaling was suppressed by vaccinia virus-encoded TLR inhibitors. This is the first demonstration of a fungal protein serving as a TLR ligand and mediating signaling in primary brain cells.

Dysregulated interferon-gamma responses during lethal cytomegalovirus brain infection of IL-10-deficient mice

Murine cytomegalovirus (MCMV) brain infection induces a transient increase in chemokine production, which precedes the infiltration of CD3(+) lymphocytes. In this study, we hypothesized that an absence of anti-inflammatory cytokines would result in sustained proinflammatory neuroimmune responses. Direct intracerebroventricular injection of MCMV into IL-10 knockout (KO) mice produced an unexpected result: while wild-type animals controlled MCMV, the infection was lethal in IL-10 KO animals. Identical infection of IL-4 KO animals did not produce lethal disease. To further characterize the role of IL-10, infected brain tissue from both wild-type and IL-10 KO animals was assessed for cytokine and chemokine levels, as well as viral gene expression. These data show vastly elevated levels of interferon (IFN)-gamma, and the IFN-gamma-inducible chemokines CXCL9 and CXCL10, as well as IL-6 in brain homogenates obtained from IL-10 KO animals. However, MCMV viral load, glycoprotein B mRNA levels, and titers of infectious virus were similar in both IL-10 KO and wild-type animals. Separation of cells isolated from murine brain tissue into distinct populations using FACS, along with subsequent quantitative RT real-time PCR, showed that brain-infiltrating CD45(hi)/CD11b(-) and CD45(hi)/CD11b(int) were the cellular source of IL-10 in the brain. Taken together, these data demonstrate that MCMV brain infection of IL-10-deficient mice causes lethal disease, which occurs in the presence of a dysregulated IFN-gamma-mediated neuroimmune response.

Human Neural Precursor Cells Express Functional κ-Opioid Receptors

Neural stem cells (NSCs) play an important role in the developing as well as adult brain. NSCs have been shown to migrate toward sites of injury in the brain and to participate in the process of brain repair. Like NSCs, cultured human neural precursor cells (NPCs) are self-renewing, multipotent cells capable of differentiating into neurons, astrocytes, and oligodendrocytes and of migrating toward chemotactic stimuli. Cellular and environmental factors are important for NPC proliferation and migration. Expression of kappa-opioid receptors (KORs) and mu-opioid receptors (MORs) in murine embryonic stem cells and of MORs and delta-opioid receptors in rodent neuronal precursors, as well as hippocampal progenitors has been reported by other investigators. In this study, we demonstrated robust expression of KORs in highly enriched (>90% nestin-positive) human fetal brain-derived NPCs. We found that KOR ligands, dynorphin(1-17) and trans-3,4-dichloro-N-methyl-N[2-(1-pyrolidinyl)cyclohexyl] benzeneacetamide methanesulfonate (U50,488) but not dynorphin(2-17), stimulated proliferation and migration of NPCs in a concentration-dependent manner. NPC proliferation was maximally stimulated at 10(-14) M dynorphin(1-17) and 10(-12) M U50,488. The KOR selective antagonist, nor-binaltorphimine, partially blocked the migratory and proliferative effects of KOR agonists supporting, at least in part, the involvement of a KOR-related mechanism. As has been described for rodent P19 embryonal carcinoma stem cells, retinoic acid treatment markedly suppressed KOR mRNA expression in human NPCs. Taken together, the results of this study suggest that activation of KORs alters functional properties of NPCs/NSCs that are relevant to human brain development and repair.

Cocaine alters proliferation, migration, and differentiation of human fetal brain-derived neural precursor cells

Maternal use of cocaine during pregnancy is associated with sustained morphological brain abnormalities and sustained cognitive deficits in the offspring. Here, we use a cell culture model of highly enriched human fetal brain-derived neural precursor cells (NPCs) to assess the effects of cocaine treatment on their proliferation, migration, and differentiation. Our data show that cocaine treatment markedly inhibited the proliferation of NPCs, a phenomenon that was associated with cell cycle arrest, possibly because of increased expression of the cyclin-dependent kinase inhibitor p21. In addition, treatment of NPCs with cocaine inhibited their migratory response to CXCL12 (stromal cell-derived factor-1alpha), a finding that correlated with cocaine-induced down-regulation of CXCR4 on NPCs. Finally, these data demonstrated that NPCs exposed to cocaine underwent differentiation into cells expressing neuronal markers that was associated with an inhibition of SOX2 (SRY-related HMG-box gene 2), a transcription factor that inhibits NPC differentiation. Taken together, these results point to several cellular mechanisms whereby exposure of human neural stem cells to cocaine in utero could contribute to subsequent neurodevelopmental and neurocognitive deficits.

Cocaine-induced HIV-1 expression in microglia involves sigma-1 receptors and transforming growth factor-beta1

The neuropharmacological properties of cocaine are known to be associated with the activation of sigma-1 receptors. Cocaine also has been shown to alter both cytokine production and HIV-1 expression in mononuclear phagocytes, including microglial cells. This study tested the hypothesis that sigma-1 receptors and transforming growth factor (TGF)-beta1 are involved in cocaine-induced up-regulation of HIV-1 expression in microglial cell cultures. Treatment of microglial cells with cocaine resulted in a concentration-dependent increase in viral expression assessed by measurement of p24 antigen levels in culture supernatants. This cocaine-mediated stimulation of HIV-1 expression was blocked by treatment of microglia with inhibitors of sigma-1 receptors (BD1047) and TGF-beta1 (SB-431542 and anti-TGF-beta1 antibodies). Microglia were also shown to constitutively express sigma-1 receptor mRNA. Thus, the results of this study support the notion that neuroimmunopharmacological properties of cocaine involve sigma-1 receptors and cytokines.

Microglial cells initiate vigorous yet non-protective immune responses during HSV-1 brain infection

Central nervous system (CNS) infection with herpes simplex virus (HSV)-1 triggers neuroinflammatory responses leading to peripheral immune cell infiltration into the brain. Previous in vitro studies from our laboratory, using primary human brain cells, implicated microglia as the cellular source of infection-induced chemokines, such as CXC ligand 10 (CXCL10) and CC ligand 2 (CCL2). Here, we evaluated the role of microglial cells in HSV-induced neuroimmune responses using an in vivo murine model of herpes encephalitis. Data obtained during this study demonstrated robust levels of CXCL10, CCL2 and CXCL9 detectable in the brains of infected BALB/c mice between 5 and 8 days post-infection (p.i.). Microglial cells were identified as a source of this HSV-induced chemokine production. Additional experiments established that induction of these immune mediators preceded the presence of CD3, CD4, CD8, and CD45 mRNA in the brain, and immunohistochemical analysis confirmed the presence of infiltrating CD3(+) cells. Further analysis suggested that microglia-derived chemokines drive peripheral immune cell chemotaxis, as antibodies to CXCL10 and CCL2 blocked the migration of murine splenocytes toward HSV-infected microglia by approximately 59.3+/-4.1% and 17.5+/-1.4%, respectively. Taken together, these results demonstrate that a vigorous microglia-driven cascade of pro-inflammatory immune responses is not sufficient to protect susceptible mice from HSV-1 brain infection.

Neural precursor cell susceptibility to human cytomegalovirus diverges along glial or neuronal differentiation pathways

Cytomegalovirus (CMV) is a major cause of congenital brain disease, and its neuropathogenesis may be related to viral infection of rapidly dividing, susceptible neural precursor cells (NPCs). In the present study, we evaluated the susceptibility of human fetal brain-derived NPCs (nestin(+), A2B5(+), CD133(+)) to infection with CMV. Data derived from these studies demonstrated that undifferentiated NPCs supported productive viral replication. After differentiation in the presence of serum, a treatment that promotes development of an astroglial cell phenotype (GFAP(+), nestin(-), A2B5(-)), viral expression was retained. However, differentiation of NPCs in medium containing platelet-derived growth factor and brain-derived neurotropic factor, conditions that support the development of neurons (Tuj-1(+), nestin(-), A2B5(-)), resulted in reduced viral expression, with corresponding decreased CMV major immediate-early promoter (MIEP) activity relative to undifferentiated cells. Further experiments showed that cellular differentiation into a neuronal phenotype was associated with elevated levels of various CCAAT/enhancer binding protein beta (C/EBP)-beta isoforms, which suppressed MIEP activity in cotransfected NPCs. Taken together, these data demonstrate that the susceptibility of primary human NPCs to CMV is retained concomitantly with differentiation into glial cells but is actively repressed following differentiation into neurons.

Differential apoptotic signaling in primary glial cells infected with herpes simplex virus 1

Microglial cells and astrocytes are glial cell types that perform distinct functions and generate innate immune responses to counter invading pathogens. Herpes simplex virus 1 (HSV-1) is a neurotropic virus that is capable of causing severe, necrotizing encephalitis. HSV-1 infects both of these glial cell types. Microglial cells undergo an abortive infection, yet respond to viral infection by inducing a burst of proinflammatory cytokine and chemokine production. Following this cytokine burst, they rapidly succumb to cell death. In contrast, astrocytes do permit productive viral replication, but do not generate these same innate immune mediators. Although apoptosis has been implicated in a number of acute and chronic neurological disorders, little is known about apoptosis during viral encephalitis. In the present study, the authors investigated the effect of HSV-1 infection on cell survival and studied the mechanisms of cell-death in virus-infected, primary murine glial cells. The authors report that although apoptosis occurred rapidly in microglia, it was delayed during productive infection of astrocytes. Furthermore, microarray studies revealed significant variations in the expression of apoptotic genes between these two types of glial cells, indicating crucial differences in signaling pathways. Intrinsic as well as extrinsic pathways of apoptosis were found to be activated in both glial cell types. Specifically, genes involved in the tumor necrosis factor (TNF) signaling pathway were predominantly up-regulated in microglia, whereas genes of the Fas pathway were induced during HSV infection of astrocytes.

Cutting edge: TLR2-mediated proinflammatory cytokine and chemokine production by microglial cells in response to herpes simplex virus

Recent studies indicate that TLRs are critical in generating innate immune responses during infection with HSV-1. In this study, we investigated the role of TLR2 signaling in regulating the production of neuroimmune mediators by examining cytokine and chemokine expression using primary microglial cells obtained from TLR2-/- as well as wild-type mice. Data presented here demonstrate that TLR2 signaling is required for the production of proinflammatory cytokines and chemokines: TNF-alpha, IL-1beta, IL-6, IL-12, CCL7, CCL8, CCL9, CXCL1, CXCL2, CXCL4, and CXCL5. CXCL9 and CXCL10 were also induced by HSV, but their production was not dependent upon TLR2 signaling. Because TLR2-/- mice display significantly reduced mortality and diminished neuroinflammation in response to brain infection with HSV, the TLR2-dependent cytokines identified here might function as key players influencing viral neuropathogenesis.

Anti-HIV-1 activity of propolis in CD4(+) lymphocyte and microglial cell cultures

An urgent need for additional agents to treat human immunodeficiency virus type 1 (HIV-1) infection led us to assess the anti-HIV-1 activity of the natural product propolis in CD4(+) lymphocytes and microglial cell cultures. Propolis inhibited viral expression in a concentration-dependent manner (maximal suppression of 85 and 98% was observed at 66.6 microg/ml propolis in CD4(+) and microglial cell cultures, respectively). Similar anti-HIV-1 activity was observed with propolis samples from several geographic regions. The mechanism of propolis antiviral property in CD4(+) lymphocytes appeared to involve, in part, inhibition of viral entry. While propolis had an additive antiviral effect on the reverse transcriptase inhibitor zidovudine, it had no noticeable effect on the protease inhibitor indinavir. The results of this in vitro study support the need for clinical trials of propolis or one or more of its components in the treatment of HIV-1 infection.

TNF- -induced chemokine production and apoptosis in human neural precursor cells

Recent studies have shown that proinflammatory cytokines damage rodent neural precursor cells (NPCs), a source of self-renewing, multipotent cells that play an important role in the developing as well as adult brain. In this study, the effects of tumor necrosis factor alpha (TNF-alpha) on cytokine and chemokine production by human NPCs (>98% nestin- and >90% A2B5-positive), obtained from 6- to 8-week-old fetal brain specimens, were evaluated. NPCs stimulated with this proinflammatory cytokine were found to produce abundant amounts of the chemokines monocyte chemoattractant protein 1 (MCP-1)/CC chemokine ligand 2 (CCL2) and interferon-inducible protein 10 (IP-10)/CXC chemokine ligand 10 (CXCL10) in a time- and concentration-dependent manner. TNF-alpha treatment also induced NPC apoptosis. Receptors for TNF [TNFRI (p55) and TNFRII (p75)] mRNA were constitutively expressed on NPCs. However, only TNFRI was involved in TNF-alpha-induced chemokine production and apoptosis by NPCs, as anti-TNFRI but not anti-TNFRII antibodies blocked the stimulatory effect. TNF-alpha treatment induced p38 mitogen-activated protein kinase (MAPK) phosphorylation in NPCs, and SB202190, an inhibitor of p38 MAPK, blocked TNF-alpha-induced chemokine production. Thus, this study demonstrated that NPCs constitutively express receptors for TNF-alpha, which when activated, trigger via a p38 MAPK signaling pathway production of two chemokines, MCP-1/CCL2 and IP-10/CXCL10, which are involved in infectious and inflammatory diseases of the brain.

T cell-mediated restriction of intracerebral murine cytomegalovirus infection displays dependence upon perforin but not interferon-gamma

The authors have previously reported that adoptive transfer of splenocytes suppresses murine cytomegalovirus (MCMV) brain infection following intracerebroventricular injection of immunodeficient mice and that depletion of Thy 1.2⁺ T lymphocytes abolishes this suppressive effect. Here the authors report that splenocytes depleted of CD4⁺ T lymphocytes prior to adoptive transfer retained their ability to control viral expression in the brain. In sharp contrast, depletion of the CD8⁺ T-cell population prior to transfer abolished the suppressive effect, with sixfold greater expression in the brain than when undepleted splenocytes were used. The authors went on to examine the contributions of cytokine- and perforin-mediated mechanisms in controlling MCMV brain infection using splenocytes from major histocompatibility (MHC)-matched IFN-γ-knockout (GKO), and perforin-knockout (PKO) mice. When used in adoptive transfer studies, splenocytes from GKO mice controlled viral expression; however, cells from PKO mice could not control reporter gene expression or viral DNA replication in brain tissues. The authors have previously reported that the levels of the T-cell chemoattractant CXCL10 are highly elevated in the brains of MCMV-infected mice. Here the authors found that the receptor for this ligand, CXCR3, was not essential in mediating the suppressive effects of adoptive transfer. These data indicate that peripheral CD8⁺ T cells control MCMV brain infection through a perforin-mediated mechanism and that neither IFN-γ nor CXCR3 play a critical role in this neuroprotective response.

Morphine potentiates HIV-1 gp120-induced neuronal apoptosis

To investigate the effect of opiates on human immunodeficiency virus type 1 (HIV-1)-related neuronal apoptosis, primary human fetal neuronal/glial cell cultures were exposed to gp120(IIIB) in the absence and the presence of morphine. Although morphine by itself had little effect on neuronal survival, the combination of morphine (>/=10(-7) mol/L) and gp120(IIIB) (1 nmol/L) significantly increased neuronal apoptosis. The mechanism whereby morphine potentiates gp120(IIIB)-induced neuronal apoptosis appears to involve activation of the p38 mitogen-activated protein kinase intracellular signaling pathway and microglial cells dispersed within the neuronal/glial cell cultures. These results provide additional insight into the molecular basis whereby opiate abuse could promote HIV-1-associated dementia.

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.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.

High-level expression of functional chemokine receptor CXCR4 on human neural precursor cells

Neural precursor cells (NPCs) are self-renewing, multipotent progenitors that give rise to neurons, astrocytes and oligodendrocytes in the central nervous system (CNS). Fetal NPCs have attracted attention for their potential use in studying normal CNS development. Several studies of rodent neural progenitors have suggested that chemokines and their receptors are involved in directing NPC migration during CNS development. In this study, we established a consistent system to culture human NPCs and examined the expression of chemokine receptors on these cells. NPCs were found to express the markers nestin and CD133 and to differentiate into neurons, astrocytes and oligodendrocytes at the clonal level. Flow cytometry and RNase protection assay (RPA) indicated that NPCs express high levels of CXCR4 and low levels of several other chemokine receptors. When examined using a chemotaxis assay, NPCs were able to respond to CXCL12/SDF-1alpha, a ligand of CXCR4. Treatment with anti-CXCR4 antibody or HIV-1 gp120 abolished the migratory response of NPCs towards CXCL12/SDF-1alpha. These findings suggest that CXCR4 may play a significant role in directing NPC migration during CNS development.

Intracerebral infection with murine cytomegalovirus induces CXCL10 and is restricted by adoptive transfer of splenocytes

The brain's intrinsic immune system consists of glial cells that produce cytokines and chemokines in response to stimulation with cytomegalovirus (CMV). The present experiments were undertaken to determine whether this intrinsic glial cell response alone is sufficient to control CMV infection of the central nervous system (CNS) or whether effector cells from the somatic immune system are also required. Following stereotactic, intracerebroventricular (icv), injection of murine cytomegalovirus (MCMV) into immunocompetent (C.B-17) mice, viral spread in the brain was limited to the cells of the ventricular walls and the infection was resolved by 10 days post infection (p.i.). In contrast, icv infection of immunodeficient (C.B-17 SCID/bg) mice resulted in viral spread from the ventricles throughout the brain parenchyma and these mice succumbed to lethal disease. Adoptive transfer of total splenocytes from major histocompatibility complex (MHC)-matched, MCMV-primed animals restricted intracerebral viral infection to the periventricular cells and reduced levels of reporter gene expression from the viral genome. Peripheral immune cell transfer also protected immunodeficient animals from lethal disease. Depletion of Thy 1.2(+) cells from MCMV-primed splenocytes abolished the protective effect of adoptive transfer. Viral expression was found to be fourfold greater in the brains of animals given Thy 1.2-depleted splenocytes than from those receiving total undepleted cells. As MCMV infection proceeded in the brains of immunodeficient mice, levels of the T-cell chemoattractants CXCL10 and CCL2 remained elevated, whereas CXCL10 levels waned in the brains of animals receiving transferred splenocytes. Taken together, these results demonstrate the ability of T lymphocytes to restrict intracerebral viral spread and indicate that intrinsic glial cell responses alone are insufficient to control MCMV brain infection.