Induction of the chemokines interleukin-8 and IP-10 by human immunodeficiency virus type 1 tat in astrocytes

Human immunodeficiency virus type 1 Nef protein mediates neural cell death: a neurotoxic role for IP-10

HIV-1 Nef is expressed in astrocytes, but a contribution to neuropathogenesis and the development of HIV-associated dementia (HAD) remains uncertain. To determine the neuropathogenic actions of the HIV-1 Nef protein, the brain-derived (YU-2) and blood-derived (NL4-3) Nef proteins were expressed in neural cells using an alphavirus vector, which resulted in astrocyte death (P < 0.001). Supernatants from Nef-expressing astrocytes also caused neuronal death, suggesting the release of neurotoxic molecules by astrocytes. Analysis of pro-inflammatory gene induction in astrocytes expressing Nef revealed increased IP-10 mRNA expression (4000-fold) that was Nef sequence dependent. Recombinant IP-10 caused selective cell death in neurons (P < 0.001) but not astrocytes, and the cytotoxicity of supernatant from astrocytes expressing Nef YU-2 was blocked by an antibody directed against the chemokine receptor CXCR3 (P < 0.001). SCID/NOD mice implanted with a Nef YU-2-expressing vector displayed abnormal motor behavior (P < 0.05), neuroinflammation, and neuronal loss relative to controls. Analysis of mRNA levels in brains from patients with HAD also revealed increased expression of IP-10 (P < 0.05), which was confirmed by immunoreactivity detected principally in astrocytes. Phylogenetic and protein structure analyses of Nef sequences derived from HIV/AIDS patients with and without HAD suggested viral evolution toward a neurotropic Nef protein. These results indicate that HIV-1 Nef contributes to neuropathogenesis by directly causing astrocyte death together with indirect neuronal death through the cytotoxic actions of IP-10 on neurons. Furthermore, Nef molecular diversity was evident in brain tissue among patients with neurological disease and which may influence IP-10 production by astrocytes.

HIV-infection of the central nervous system: the tightrope walk of innate immunity

Infection of the central nervous system (CNS) by HIV is a frequent and sometimes very early event in the course of HIV pathogenesis. Possible consequences are diverse symptoms of neurological dysfunction, but also the establishment of a lifelong latent viral reservoir in the brain. Whereas in the periphery innate and adaptive immunity are equal partners, the blood-brain barrier (BBB) with its restricted access of peripheral immune effectors shifts this balance in favour of the local innate immunity. Four main elements of cerebral innate immunity are discussed in the present article, including two cell types with immunological functions and two soluble immune systems: (1) the stimulation of microglial cells as the predominant brain-resident immune cell and the main local reservoir for the virus; (2) the reaction of astrocytes in response to viral infection; (3) the activation of the local complement system as important soluble immune cascade; and (4) the role of chemokines and cytokines which help to conduct and cross-link the interplay between the different immune elements. These components of the cerebral innate immunity do not act separately from each other but form a functional immunity network. A dual role of these components with both harmful and protective effects further enhances the complexity of the mutual interactions.

Cytokine and chemokine expression in the central nervous system associated with protective cell-mediated immunity againstCryptococcus neoformans

Cryptococcus neoformans is a yeast that causes cryptococcosis, a life-threatening disease that develops following inhalation and dissemination of the organisms. C. neoformans has a predilection for the central nervous system (CNS) and mortality is most frequently associated with meningoencephalitis. Susceptibility to cryptococcosis is increased in patients with deficiencies in cell-mediated immunity (CMI). Because cryptococcal CNS infections are associated with mortality and diagnosis of cryptococcosis is often not made until after dissemination to the CNS, a better understanding of host defense mechanisms against C. neoformans in the CNS is needed to design improved therapies for immunocompromised individuals suffering from cryptococcosis. Using a mouse model, we previously described a protective cell-mediated immune response induced in the periphery that limited the growth of C. neoformans in the CNS. In the current investigation, we examined cytokine and chemokine expression in the CNS to identify factors important in achieving protective immunity. We observed increased expression of IL-1beta, TNF-alpha, IFN-gamma, MCP-1, RANTES, and IP-10 in C. neoformans-infected brains of immune mice compared to control mice suggesting that these cytokines and chemokines are associated with the protective immune response. Furthermore, the Th1-type cytokines TNF-alpha and IFN-gamma, but not the Th2 cytokines IL-4 and IL-5, were secreted at significantly higher levels in C. neoformans-infected brains of immune mice compared to control mice. Our results demonstrate that cytokines and chemokines associated with CMI are produced following infection in the CNS of immunized mice, and the expression of these factors correlates with protection against C. neoformans in the CNS.

Acute exposure to CXC chemokine ligand 10, but not its chronic astroglial production, alters synaptic plasticity in mouse hippocampal slices

Brain levels of CXC chemokine ligand 10 (CXCL10) are elevated in a number of neuropathological conditions. To determine its impact on neuronal function, we measured synaptic transmission and plasticity in hippocampal slices prepared from transgenic (TG) mice with chronic astroglial production of CXCL10. We also tested the acute effect of recombinant CXCL10 applied to slices from normal C57Bl/6J mice, CXCL10 TG mice and CXCR3 knock out (KO) mice. Chronic production of CXCL10 did not alter synaptic plasticity. By contrast, exogenous CXCL10 (10 ng/ml) significantly inhibited long-term potentiation (LTP) in slices from normal C57Bl/6J mice and CXCL10 TG. The effect was probably receptor-mediated because CXCL10-induced inhibition of LTP was not observed in CXCR3 KO mice. Our findings suggest that acute exposure to CXCL10 alters synaptic plasticity via CXCR3 in mouse hippocampus.

CpG oligodeoxynucleotides directly induce CXCR3 chemokines in human B cells

CpG oligodeoxynucleotides (CpG ODN) are known to elicit Th1 immune responses via TLR9. However, the precise mechanisms through which B cells are involved in this phenomenon are not fully understood. We investigated the effect of CpG ODN on the induction of Th1-chemoattractant CXCR3 chemokines, IP-10, Mig, and I-TAC, in B cells. Cells from the RPMI 8226 human B cell line and human peripheral B cells were stimulated with three distinct classes of CpG ODN. As a result, CXCR3 chemokines were strongly up-regulated by CpG-B and CpG-C, but only weakly by CpG-A. Though CXCR3 chemokines are known to be induced by IFNs, blocking mAbs against IFN receptors did not inhibit their induction by CpG-B. Induction of CXCR3 chemokines was blocked by two NF-kappaB inhibitors and a p38 inhibitor. These results strongly suggest that CXCR3 chemokines are directly induced by CpG ODN via NF-kappaB- and p38-dependent pathways in human B cells.

Chemokines and glial cells: a complex network in the central nervous system

Chemokines are small secreted proteins that are essential for the recruitment and activation of specific leukocyte subsets at sites of inflammation and for the development and homeostasis of lymphoid and nonlymphoid tissues. During the past decade, chemokines and their receptors have also emerged as key signaling molecules in neuroinflammatory processes and in the development and functioning of the central nervous system. Neurons and glial cells, including astrocytes, oligodendrocytes, and microglia, have been identified as cellular sources and/or targets of chemokines produced in the central nervous system in physiological and pathological conditions. In this article, we provide an update of chemokines and chemokine receptors expressed by glial cells focusing on their biological functions and implications in neurological diseases.

Induction of C chemokine XCL1 (lymphotactin/single C motif-1 alpha/activation-induced, T cell-derived and chemokine-related cytokine) expression by HIV-1 Tat protein

HIV-1 Tat has been proposed as a key agent in many AIDS-related disorders, including HIV-1-associated neurological diseases. We have recently shown that Tat expression induces a significant increase in T lymphocytes in the brains of Tat transgenic mice. The CNS infiltration of T lymphocytes has been noted in AIDS patients. In the present study using this unique genetic system we attempted to understand the underlying mechanisms of Tat expression-induced infiltration of T lymphocytes by examining chemokine expression. RNase protection assay revealed that in addition to CCL2 (monocyte chemoattractant protein-1), CCL3 (macrophage inflammatory protein-1alpha (MIP-1alpha)), CCL4 (MIP-1beta), CCL5 (RANTES), CXCL2 (MIP-2), and CXCL10 (inducing protein-10), XCL1 (lymphotactin/single C motif-1alpha/activation-induced, T cell-derived and chemokine-related cytokine) was identified to be up-regulated by Tat expression. XCL1 is a C chemokine and plays a specific and important role in tissue-specific recruitment of T lymphocytes. Thus, we further determined the relationship between Tat and XCL1 expression. Tat-induced XCL1 expression was further confirmed by XCL1-specific RT-PCR and ELISA. Combined in situ hybridization and immunohistochemical staining identified astrocytes, monocytes, and macrophages/microglia as XCL1-producing cells in vivo. Using human astrocytes, U87.MG cells, as an in vitro model, activation of XCL1 expression was positively correlated with Tat expression. Moreover, the XCL1 promoter-driven reporter gene assay showed that Tat-induced XCL1 expression occurred at the transcriptional level. Taken together, these results demonstrate that Tat directly trans-activated XCL1 expression and suggest potential roles of Tat-induced XCL1 expression in the CNS infiltration of T lymphocytes during HIV-1 infection and subsequent HIV-1-induced neurological diseases.

Expression of the CXCR3 ligand I-TAC by hepatocytes in chronic hepatitis C and its correlation with hepatic inflammation

The factors that regulate lymphocyte traffic in chronic hepatitis C (CHC) are not completely defined. Interferon (IFN)-inducible T cell alpha chemoattractant (I-TAC) is a relatively new member of the CXCR3 chemokine ligand family that selectively recruits activated T cells to sites of inflammation. To determine if I-TAC plays a role in CHC, we investigated I-TAC expression in hepatitis C virus (HCV)-infected liver biopsy material. I-TAC messenger RNA (mRNA) levels were significantly increased in HCV-infected liver compared with normal liver, which correlated with both portal and lobular inflammation. I-TAC expression was localized to hepatocytes throughout the liver lobule, with those in close proximity to active areas of inflammation expressing the highest concentration of I-TAC. In vitro, I-TAC mRNA and protein expression was inducible in Huh-7 cells following either IFN-alpha or -gamma stimulation and synergistically with tumor necrosis factor (TNF)-alpha. Furthermore, transfection of Huh-7 cells with either poly(I:C) or HCV RNA representing the HCV subgenomic replicon induced I-TAC mRNA expression. HCV replication was also found to modulate I-TAC expression, with stimulation of Huh-7 cells harboring either the HCV subgenomic or genomic replicon showing significantly increased synergistic effects compared with those previously seen in Huh-7 cells alone with IFN-gamma and TNF-alpha. In conclusion, these results suggest I-TAC, one of the most potent chemoattractants for activated T cells, is produced by hepatocytes in the HCV-infected liver and plays an important role in T cell recruitment and ultimately the pathogenesis of CHC.

T-cell and neuronal apoptosis in HIV infection: implications for therapeutic intervention

The pathogenesis of HIV infection involves the selective loss of CD4+ T cells contributing to immune deficiency. Although loss of T cells leading to immune dysfunction in HIV infection is mediated in part by viral infection, there is a much larger effect on noninfected T cells undergoing apoptosis in response to activation stimuli. In the subset of patients with HIV dementia complex, neuronal injury, loss, and apoptosis are observed. Viral proteins, gp120 and Tat, exhibit proapoptotic activities when applied to T cell and neuronal cultures by direct and indirect mechanisms. The pathways leading to cell death involve the activation of one or more death receptor pathways (i.e., TNF-alpha, Fas, and TRAIL receptors), chemokine receptor signaling, cytokine dysregulation, caspase activation, calcium mobilization, and loss of mitochondrial membrane potential. In this review, the mechanisms involved in T-cell and neuronal apoptosis, as well as antiapoptotic pathways potentially amenable to therapeutic application, are discussed.

Induction of chemokines in human astrocytes by picornavirus infection requires activation of both AP-1 and NF-kappa B

Infection with different picornaviruses can cause meningitis/encephalitis in humans and experimental animals. To investigate the mechanisms of such inflammatory diseases, potential chemokine gene activation in human astrocytes was investigated following infection with Theiler's murine encephalomyelitis virus (TMEV), coxsackievirus B3 (CVB3), or coxsackievirus B4 (CVB4). We report that all these viruses are potent inducers for the expression of interleukin‐8 (IL‐8) and monocyte chemoattractant protein‐1 (MCP‐1) genes in primary human astrocytes, as well as in an established astrocyte cell line (U‐373MG). Further studies indicated that both activator protein‐1 (AP‐1) and NF‐κB transcription factors are required in the activation of chemokine genes in human astrocytes infected with various picornaviruses. Interestingly, the pattern of activated chemokine genes in human astrocytes is quite restricted compared to that in mouse astrocytes infected with the same viruses, suggesting species differences in gene activation. This may result in potential differences in the pathogenic outcome in each species. © 2003 Wiley‐Liss, Inc.

Caveolin-1 knockdown by small interfering RNA suppresses responses to the chemokine monocyte chemoattractant protein-1 by human astrocytes

Astrocytes regulate the integrity of the blood-brain barrier and influence inflammatory processes in the central nervous system. The pro-inflammatory chemokine monocyte chemoattractant protein-1 (MCP-1), which is both released by and stimulates astrocytes, is thought to play a crucial role in both these activities. Because astrocytes have been shown to possess caveolae, vesicular structures that participate in intracellular transport and signal transduction events, we reasoned that expression of the major structural protein of these organelles, caveolin-1, might feature critically in the cellular responses to MCP-1. To test this hypothesis, caveolin-1 level was "knocked down" in human astrocyte cultures by using a small interfering RNA approach. This method resulted in efficient (>90% loss) and specific knockdown of caveolin-1 expression while sparring glial fibrillary acidic protein as well as several other proteins involved in endocytosis. Astrocytes suffering caveolin-1 loss showed diminished ability to down-modulate and internalize the MCP-1 receptor (CCR2) in response to exposure to this chemokine and also demonstrated significantly reduced capacity to undergo chemotaxis and calcium flux when MCP-1-stimulated. The results highlight a potentially prominent role for caveolae and/or caveolin-1 in mediating astrocyte responses to MCP-1, a feature that might significantly dictate the progression of inflammatory events at the blood-brain barrier.

Expression of the chemokine IP-10 (CXCL10) by hepatocytes in chronic hepatitis C virus infection correlates with histological severity and lobular inflammation

The factors influencing lymphocyte trafficking to the liver lobule during chronic hepaititis C virus (HCV) infection are currently not well defined. Interferon-gamma-inducible protein 10 (IP-10), a chemokine that recruits activated T lymphocytes, has recently been shown by in situ hybridization to be expressed in the liver during chronic HCV infection. This study sought to define the cellular source of IP-10 in the liver by immunohistochemistry, to examine the expression of its receptor, CXCR3, on T lymphocytes isolated from blood and liver tissue, and to correlate IP-10 expression with the histological markers of inflammation and fibrosis. IP-10 was expressed by hepatocytes but not by other cell types within the liver, and the most intense immunoreactivity was evident in the areas of lobular inflammation. The IP-10 receptor was expressed on a significantly higher proportion of T lymphocytes in the liver compared with blood. CD8 T lymphocytes, which predominate in the liver lobule, were almost uniformly CXCR3-positive. The expression of IP-10 mRNA correlated with lobular necroinflammatory activity but not with inflammation or fibrosis in the portal tracts. These findings suggest that IP-10 may be induced by HCV within hepatocytes and may be important in the pathogenesis of chronic HCV infection, as recruitment of inflammatory cells into the lobule is an important predictor of disease progression.

MCP-1 (CCL2) protects human neurons and astrocytes from NMDA or HIV-tat-induced apoptosis

Acquired immunodeficiency syndrome (AIDS)-associated dementia is often characterized by chronic inflammation, with infected macrophage infiltration of the CNS resulting in the production of human immunodeficiency virus type 1 (HIV-1) products, including tat, and neurotoxins that contribute to neuronal loss. In addition to their established role in leukocyte recruitment and activation, we identified an additional role for chemokines in the CNS. Monocyte chemoattractant protein-1 (MCP-1 or CCL2) and regulated upon activation normal T cell expressed and secreted (RANTES) were found to protect mixed cultures of human neurons and astrocytes from tat or NMDA-induced apoptosis. Neuronal and astrocytic apoptosis in these cultures was significantly inhibited by co-treatment with MCP-1 or RANTES but not IP-10. The protective effect of RANTES was blocked by antibodies to MCP-1, indicating that RANTES protection is mediated by the induction of MCP-1. The NMDA blocker, MK801, also abolished the toxic effects of both tat and NMDA. Tat or NMDA treatment of mixed cultures for 24 h resulted in increased extracellular glutamate ([Glu]e) and NMDA receptor 1 (NMDAR1) expression, potential contributors to apoptosis. Co-treatment with MCP-1 inhibited tat and NMDA-induced increases in [Glu]e and NMDAR1, and also reduced the levels and number of neurons containing intracellular tat. These data indicate that MCP-1 may play a novel role as a protective agent against the toxic effects of glutamate and tat.

Neuronal injury regulates fractalkine: relevance for HIV-1 associated dementia

Fractalkine (FKN), a chemokine highly expressed in the central nervous system, participates in inflammatory responses operative in many brain disorders including HIV-1 associated dementia (HAD). In this report, HIV-1 progeny virions and pro-inflammatory products led to FKN production associated with neuronal injury and apoptosis. FKN was produced by neurons and astrocytes; but differentially produced by the two cell types. Laboratory tests paralleled those in infected people where cerebrospinal fluid FKN levels in HIV-1 infected cognitively impaired (n=16) patients were found to be increased when compared to infected patients without cognitive impairment (n=8, P=0.0345). These results demonstrate a possible role of FKN in HAD pathogenesis.

Neuropathologies in transgenic mice expressing human immunodeficiency virus type 1 Tat protein under the regulation of the astrocyte-specific glial fibrillary acidic protein promoter and doxycycline

The human immunodeficiency virus type 1 (HIV-1) Tat protein is a key pathogenic factor in a variety of acquired immune deficiency syndrome (AIDS)-associated disorders. A number of studies have documented the neurotoxic property of Tat protein, and Tat has therefore been proposed to contribute to AIDS-associated neurological diseases. Nevertheless, the bulk of these studies are performed in in vitro neuronal cultures without taking into account the intricate cell-cell interaction in the brain, or by injection of recombinant Tat protein into the brain, which may cause secondary stress or damage to the brain. To gain a better understanding of the roles of Tat protein in HIV-1 neuropathogenesis, we attempted to establish a transgenic mouse model in which Tat expression was regulated by both the astrocyte-specific glial fibrillary acidic protein promoter and a doxycycline (Dox)-inducible promoter. In the present study, we characterized the phenotypic and neuropathogenic features of these mice. Both in vitro and in vivo assays confirmed that Tat expression occurred exclusively in astrocytes and was Dox-dependent. Tat expression in the brain caused failure to thrive, hunched gesture, tremor, ataxia, and slow cognitive and motor movement, seizures, and premature death. Neuropathologies of these mice were characterized by breakdown of cerebellum and cortex, brain edema, astrocytosis, degeneration of neuronal dendrites, neuronal apoptosis, and increased infiltration of activated monocytes and T lymphocytes. These results together demonstrate that Tat expression in the absence of HIV-1 infection is sufficient to cause neuropathologies similar to most of those noted in the brain of AIDS patients, and provide the first evidence in the context of a whole organism to support a critical role of Tat protein in HIV-1 neuropathogenesis. More importantly, our data suggest that the Dox inducible, brain-targeted Tat transgenic mice offer an in vivo model for delineating the molecular mechanisms of Tat neurotoxicity and for developing therapeutic strategies for treating HIV-associated neurological disorders.

CXCL10 production from cytomegalovirus-stimulated microglia is regulated by both human and viral interleukin-10

Glial cells orchestrate immunocyte recruitment to focal areas of viral infection within the brain and synchronize immune cell functions through a regulated network of cytokines and chemokines. Since recruitment of T lymphocytes plays a critical role in resolving cytomegalovirus (CMV) infection, we investigated the production of a T-cell chemoattractant, CXCL10 (gamma interferon-inducible protein 10) in response to viral infection of human glial cells. Infection with CMV was found to elicit the production of CXCL10 from primary microglial cells but not from astrocytes. This CXCL10 expression was not dependent on secondary protein synthesis but did require the phosphorylation of p38 mitogen-activated protein (MAP) kinase. In addition, migration of activated lymphocytes toward supernatants from CMV-stimulated microglial cells was partially suppressed by anti-CXCL10 antibodies. Since regulation of central nervous system inflammation is essential to allow viral clearance without immunopathology, microglial cells were then treated with anti-inflammatory cytokines. CMV-induced CXCL10 production from microglial cells was suppressed following treatment with interleukin-10 (IL-10) and IL-4 but not following treatment with transforming growth factor beta. The IL-10-mediated inhibition of CXCL10 production was associated with decreased CMV-induced NF-kappa B activation but not decreased p38 MAP kinase phosphorylation. Finally, CMV infection of fully permissive astrocytes resulted in mRNA expression for the viral homologue to human IL-10 (i.e., cmvIL-10 [UL111a]) in its spliced form and conditioned medium from CMV-infected astrocytes inhibited virus-induced CXCL10 production from microglial cells through the IL-10 receptor. These findings present yet another mechanism through which CMV may subvert host immune responses.

Chloroquine induces activation of nuclear factor-kappaB and subsequent expression of pro-inflammatory cytokines by human astroglial cells

Chloroquine, an antimalarial lysosomotropic base, is known for its anti-inflammatory effects and therefore used for treatment of autoimmune diseases. Given its anti-inflammatory effects, it has been under clinical trials to modify neurodegenerative processes. In this study, we examined whether chloroquine has an anti-inflammatory effect in the CNS by determining the in vitro effects of chloroquine on LPS-induced expression of cytokines by glial cells. We observed that (i) chloroquine augmented LPS-induced expression of pro-inflammatory cytokines such as lymphotoxin (LT)-beta, tumor necrosis factor (TNF)-alpha, interleukin (IL)-1alpha, IL-1beta and IL-6 in human astroglial cells, while the same treatment suppressed LPS-induced expression of cytokines in monocytic and microglial cells; (ii) chloroquine alone induced expression of pro-inflammatory cytokines in a dose- and time-dependent manner in astroglial cells; (iii) other lysosomotropic agents such as ammonium chloride and bafilomycin A1 had minimal effects on cytokine expression; and (iv) chloroquine induced the activation of nuclear factor-kappa B in astroglial cells, which is a required component of chloroquine induction of cytokines. These results suggest that chloroquine may evoke either anti- or pro-inflammatory responses in the CNS depending on the cellular context.

The C-X-C chemokine IP-10 stimulates HIV-1 replication

Chemokines play critical roles in HIV-1 infection, serving both to modulate viral replication and to recruit target cells to sites of infection. Interferon-gamma-inducible protein 10 (IP-10/CXCL10) is a C-X-C chemokine that acts specifically upon activated T cells and macrophages and attracts T cells into the cerebrospinal fluid (CSF) in HIV-associated neurological disease. We now demonstrate that IP-10 stimulates HIV-1 replication in monocyte-derived macrophages and peripheral blood lymphocytes. We further demonstrate that neutralization of endogenous IP-10 or blocking the function of its receptor, CXCR3, reduces HIV-1 replication in these same cells. Therefore, blocking the interaction between IP-10 and CXCR3 represents a possible new target for anti-retroviral therapy.

HIV-Tat protein induces oxidative and inflammatory pathways in brain endothelium

Impaired function of the brain vasculature might contribute to the development of HIV-associated dementia. For example, injury or dysfunction of brain microvascular endothelial cells (BMEC) can lead to the breakdown of the blood-brain barrier (BBB) and thus allow accelerated entry of the HIV-1 virus into the CNS. Mechanisms of injury to BMEC during HIV-1 infection are not fully understood, but the viral gene product Tat may be, at least in part, responsible for this effect. Tat can be released from infected perivascular macrophages in the CNS of patients with AIDS, and thus BMEC can be directly exposed to high concentrations of this protein. To study oxidative and inflammatory mechanisms associated with Tat-induced toxicity, BMEC were exposed to increasing doses of Tat1-72, and markers of oxidative stress, as well as redox-responsive transcription factors such as nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1), were measured. Tat1-72 treatment markedly increased cellular oxidative stress, decreased levels of intracellular glutathione and activated DNA binding activity and transactivation of NF-kappaB and AP-1. To determine if Tat1-72 can stimulate inflammatory responses in brain endothelium in vivo, expression of monocyte chemoattractant protein-1 (MCP-1), an NF-kappaB and AP-1-dependent chemokine, was studied in brain tissue in mice injected with Tat1-72 into the right hippocampus. Tat1-72 markedly elevated the MCP-1 mRNA levels in brain tissue. In addition, a double immunohistochemistry study revealed that MCP-1 protein was markedly overexpressed on brain vascular endothelium. These data indicate that Tat1-72 can induce redox-related inflammatory responses both in in vitro and in vivo environments. These changes can directly lead to disruption of the BBB. Thus, Tat can play an important role in the development of detrimental vascular changes in the brains of HIV-infected patients.

Glial cell responses to herpesvirus infections: role in defense and immunopathogenesis

Glial cells can respond to herpesvirus infections through the production of cytokines and chemokines. Although specific interactions between resident glia and lymphocytes that infiltrate the infected brain remain to be defined, the presence of T cell chemotactic signals in microglial cell supernatants following infection with cytomegalovirus or herpes simplex virus has led to the concept that chemokines initiate a cascade of neuroimmune responses that result in defense of the brain against herpesviruses. While chemokines may play a defensive role by attracting T cells into the brain, aberrant accumulation of lymphocytes may also induce brain damage. Host defense mechanisms must balance control of herpesvirus spread with associated undesirable immunopathologic effects. A growing body of evidence suggests that through complex networks of chemokines and cytokines produced in response to herpesvirus infection, glial cells orchestrate a cascade of events that result in successful defense of or damage to the brain.

Fractalkine (CX3CL1) and brain inflammation: Implications for HIV-1-associated dementia

Leukocyte migration and activation play an important role in immune surveillance and the pathogenesis of a variety of neurodegenerative disorders, including human immunodeficiency virus (HIV)-1-associated dementia (HAD). A novel chemokine named fractalkine (FKN, CX3CL1), which exists in both membrane-anchored and soluble isoforms, has been proposed to participate in the generation and progression of inflammatory brain disorders. Upon binding to the CX3C receptor one (CX3CR1), FKN induces adhesion, chemoattraction, and activation of leukocytes, including brain macrophages and microglia (MP). Constitutively expressed in the central nervous system (CNS), mainly by neurons, FKN is up-regulated and released in response to proinflammatory stimuli. Importantly, FKN is up-regulated in the brain tissue and cerebrospinal fluid (CSF) of HAD patients. Together, these observations suggest that FKN and its receptor have a unique role in regulating the neuroinflammatory events underlying disease. This review will examine how FKN contributes to the recruitment and activation of CX3CR1-expressing MP, which are critical events in the neuropathogenesis of HAD.

Human immunodeficiency virus (HIV) proteins in neuropathogenesis of HIV dementia

Human immunodeficiency virus (HIV) infection of the nervous system is unique when compared with other viral encephalitides. Neuronal cell loss occurs in the absence of neuronal infection. Viral proteins, termed "virotoxins," are released from the infected glial cells that initiate a cascade of positive feedback loops by activating uninfected microglial cells and astrocytes. These activated cells release a variety of toxic substances that result in neuronal dysfunction and cell loss. The virotoxins act by a hit and run phenomenon. Thus, a transient exposure to the proteins initiates the neurotoxic cascade. High concentrations of these proteins likely occur in tight extracellular spaces where they may cause direct neurotoxicity as well. The emerging concepts in viral protein-induced neurotoxicity are reviewed as are the neurotoxic potential of each protein. Future therapeutic strategies must target common mechanisms such as oxidative stress and dysregulation of intracellular calcium involved in virotoxin-mediated neurotoxicity.

Interleukin-8 and growth-regulated oncogene alpha mediate angiogenesis in Kaposi's sarcoma

The development of the complex neoplasm Kaposi's sarcoma is dependent on infection with the Kaposi's sarcoma-associated herpesvirus (KSHV) and appears to be greatly enhanced by cytokines and human immunodeficiency virus type 1 (HIV-1) Tat. Interleukin-8 (IL-8) and growth-regulated oncogene alpha (GRO-alpha) are chemokines involved in chemoattraction, neovascularization, and stimulation of HIV-1 replication. We have previously demonstrated that production of GRO-alpha is stimulated by exposure of monocyte-derived macrophages (MDM) to HIV-1. Here we show that exposure of MDM to HIV-1, viral Tat, or viral gp120 leads to a substantial increase in IL-8 production. We also demonstrate that IL-8 and GRO-alpha are induced by KSHV infection of endothelial cells and are crucial to the angiogenic phenotype developed by KSHV-infected endothelial cells in cell culture and upon implantation into SCID mice. Thus, the three known etiological factors in Kaposi's sarcoma pathogenesis-KSHV, HIV-1 Tat, and cellular growth factors-might be linked, in part, through induction of IL-8 and GRO-alpha.

Essential role of p38 mitogen-activated protein kinase in contact hypersensitivity

The present study was designed to elucidate the role of p38 mitogen-activated protein kinase (p38) in the pathogenesis of inflammation, using a mouse contact hypersensitivity (CHS) model induced by 2,4-dinitro-1-fluorobenzene (DNFB). Ear swelling was induced by challenge with DNFB, accompanied by infiltration of mononuclear cells, neutrophils, and eosinophils and a marked increase in mRNA levels of cytokines such as interleukin (IL)-2, interferon (IFN)-gamma, IL-4, IL-5, IL-1beta, IL-18, and tumor necrosis factor-alpha in the challenged ear skin. Both ear swelling and the number of infiltrated cells in DNFB-challenged ear skin were significantly inhibited by treatment with SB202190, a p38 inhibitor. Furthermore, the DNFB-induced expression of all cytokines except IL-4 was significantly inhibited by treatment with SB202190. Ribonuclease protection assay revealed that the mRNA levels of chemokines such as IP-10 and MCP-1 in ear skin were markedly increased at 24 h after challenge with DNFB. The induction of these chemokines was significantly inhibited by treatment with SB202190. In p38alpha +/- mice, both ear swelling and infiltration of cells induced by DNFB were reduced compared with those in wild-type mice. However, induction of cytokines by DNFB was also observed in p38alpha +/- mice, although the induction of IFN-gamma, IL-5, and IL-18 was typically reduced compared with that in wild-type mice. Challenge with DNFB slightly induced IP-10 and MCP-1 mRNA in p38alpha +/- mice, with weaker signals than those in SB202190-treated wild-type mice. These results suggest that p38 plays a key role in CHS and is an important target for the treatment of CHS.

Gene expression profile of HIV-1 Tat expressing cells: a close interplay between proliferative and differentiation signals

BACKGROUND

Expression profiling holds great promise for rapid host genome functional analysis. It is plausible that host expression profiling in an infection could serve as a universal phenotype in virally infected cells. Here, we describe the effect of one of the most critical viral activators, Tat, in HIV-1 infected and Tat expressing cells. We utilized microarray analysis from uninfected, latently HIV-1 infected cells, as well as cells that express Tat, to decipher some of the cellular changes associated with this viral activator.

RESULTS

Utilizing uninfected, HIV-1 latently infected cells, and Tat expressing cells, we observed that most of the cellular host genes in Tat expressing cells were down-regulated. The down-regulation in Tat expressing cells is most apparent on cellular receptors that have intrinsic receptor tyrosine kinase (RTK) activity and signal transduction members that mediate RTK function, including Ras-Raf-MEK pathway. Co-activators of transcription, such as p300/CBP and SRC-1, which mediate gene expression related to hormone receptor genes, were also found to be down-regulated. Down-regulation of receptors may allow latent HIV-1 infected cells to either hide from the immune system or avoid extracellular differentiation signals. Some of the genes that were up-regulated included co-receptors for HIV-1 entry, translation machinery, and cell cycle regulatory proteins.

CONCLUSIONS

We have demonstrated, through a microarray approach, that HIV-1 Tat is able to regulate many cellular genes that are involved in cell signaling, translation and ultimately control the host proliferative and differentiation signals.

Expression of stromal cell-derived factor 1alpha protein in HIV encephalitis

Analysis of the patterns of stromal cell-derived factor 1alpha (SDF-1alpha) expression in the brains from HIV-positive patients suggests that in neuronal cells, SDF-1alpha might play a role in neuroprotection and neurite extension in response to HIV infection. In all cases analyzed, SDF-1alpha immunoreactivity was primarily present in astroglial cells. Patients with HIV encephalitis (HIVE) showed intense somato-dendritic neuronal SDF-1alpha immunoreactivity, while HIVE negative patients with neurodegeneration had a significant decrease in neuronal SDF-1alpha immunoreactivity. Neuronal cells treated with SDF-1alpha displayed increased neurite outgrowth. Similarly, neurons treated with HIV-Tat, which induced SDF-1alpha expression, also showed neurite outgrowth. Tat-mediated neurite outgrowth was blocked by anti-SDF-1alpha antibody. These results suggest that SDF-1alpha may play a role in the neuronal response to HIV in the brains of AIDS patients.

Mechanistic aspects of HIV-1 reverse transcription initiation

During reverse transcription, the positive-strand HIV-1 RNA genome is converted into a double-stranded DNA copy which can be permanently integrated into the host cell genome. Recent analyses show that HIV-1 reverse transcription is a highly regulated process. The initiation reaction can be distinguished from a subsequent elongation reaction carried out by a reverse transcription complex composed of (at least) heterodimeric reverse transcriptase, cellular tRNA(lys3) and HIV-1 genomic RNA sequences. In addition, viral factors including Tat, Nef, Vif, Vpr, IN and NCp7, cellular proteins, and TAR RNA and other RNA stem-loop structures appear to influence this complex and contribute to the efficiency of the initiation reaction. As viral resistance to many antiretroviral compounds is a continuing problem, understanding the ways in which these factors influence the reverse transcription complex will likely lead to novel antiretroviral strategies.

HIV-1 gp120 and chemokine activation of Pyk2 and mitogen-activated protein kinases in primary macrophages mediated by calcium-dependent, pertussis toxin-insensitive chemokine receptor signaling

Human immunodeficiency virus type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry. It was recently demonstrated that HIV-1 glycoprotein 120 (gp120) elevated calcium and activated several ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. This study shows that chemokine receptor engagement by both CCR5-dependent (R5) and CXCR4-dependent (X4) gp120 led to rapid phosphorylation of the focal adhesion-related tyrosine kinase Pyk2 in macrophages. Pyk2 phosphorylation was also induced by macrophage inflammatory protein-1beta (MIP-1beta) and stromal cell-derived factor-1alpha, chemokine ligands for CCR5 and CXCR4. Activation was blocked by EGTA and by a potent blocker of calcium release-activated Ca++ (CRAC) channels, but was insensitive to pertussis toxin (PTX), implicating CRAC-mediated extracellular Ca++ influx but not Galpha(i) protein-dependent mechanisms. Coreceptor engagement by gp120 and chemokines also activated 2 members of the mitogen-activated protein kinase (MAPK) superfamily, c-Jun amino-terminal kinase/stress-activated protein kinase and p38 MAPK. Furthermore, gp120-stimulated macrophages secreted the chemokines monocyte chemotactic protein-1 and MIP-1beta in a manner that was dependent on MAPK activation. Thus, the gp120 signaling cascade in macrophages includes coreceptor binding, PTX-insensitive signal transduction, ionic signaling including Ca++ influx, and activation of Pyk2 and MAPK pathways, and leads to secretion of inflammatory mediators. HIV-1 Env signaling through these pathways may contribute to dysregulation of uninfected macrophage functions, new target cell recruitment, or modulation of macrophage infection.

The regulation of alpha chemokines during HIV-1 infection and leukocyte activation: relevance for HIV-1-associated dementia

Cellular immunity against human immunodeficiency virus type 1 (HIV-1)-infected brain macrophages serves to prevent productive viral replication in the nervous system. Inevitably, during advanced disease, this antiretroviral response breaks down. This could occur through virus-induced dysregulation of lymphocyte trafficking. Thus, we studied the production of non-ELR-containing alpha-chemokines and their receptor (CXCR3) expression in relevant virus target cells. Macrophages, lymphocytes, and astrocytes secreted alpha-chemokines after HIV-1 infection and/or immune activation. Lymphocyte CXCR3-mediated chemotactic responses were operative. In all, alpha-chemokine-mediated T cell migration continued after HIV-1 infection and the neuroinflammatory events operative during productive viral replication in brain.

Immune function of astrocytes

Astrocytes are the major glial cell within the central nervous system (CNS) and have a number of important physiological properties related to CNS homeostasis. The aspect of astrocyte biology addressed in this review article is the astrocyte as an immunocompetent cell within the brain. The capacity of astrocytes to express class II major histocompatibility complex (MHC) antigens and costimulatory molecules (B7 and CD40) that are critical for antigen presentation and T-cell activation are discussed. The functional role of astrocytes as immune effector cells and how this may influence aspects of inflammation and immune reactivity within the brain follows, emphasizing the involvement of astrocytes in promoting Th2 responses. The ability of astrocytes to produce a wide array of chemokines and cytokines is discussed, with an emphasis on the immunological properties of these mediators. The significance of astrocytic antigen presentation and chemokine/cytokine production to neurological diseases with an immunological component is described.

Interferon-independent, human immunodeficiency virus type 1 gp120-mediated induction of CXCL10/IP-10 gene expression by astrocytes in vivo and in vitro

The CXC chemokine gamma interferon (IFN-gamma)-inducible protein CXCL10/IP-10 is markedly elevated in cerebrospinal fluid and brain of individuals infected with human immunodeficiency virus type 1 (HIV-1) and is implicated in the pathogenesis of HIV-associated dementia (HAD). To explore the possible role of CXCL10/IP-10 in HAD, we examined the expression of this and other chemokines in the central nervous system (CNS) of transgenic mice with astrocyte-targeted expression of HIV gp120 under the control of the glial fibrillary acidic protein (GFAP) promoter, a murine model for HIV-1 encephalopathy. Compared with wild-type controls, CNS expression of the CC chemokine gene CCL2/MCP-1 and the CXC chemokine genes CXCL10/IP-10 and CXCL9/Mig was induced in the GFAP-HIV gp120 mice. CXCL10/IP-10 RNA expression was increased most and overlapped the expression of the transgene-encoded HIV gp120 gene. Astrocytes and to a lesser extent microglia were identified as the major cellular sites for CXCL10/IP-10 gene expression. There was no detectable expression of any class of IFN or their responsive genes. In astrocyte cultures, soluble recombinant HIV gp120 protein was capable of directly inducing CXCL10/IP-10 gene expression a process that was independent of STAT1. These findings highlight a novel IFN- and STAT1-independent mechanism for the regulation of CXCL10/IP-10 expression and directly link expression of HIV gp120 to the induction of CXCL10/IP-10 that is found in HIV infection of the CNS. Finally, one function of IP-10 expression may be the recruitment of leukocytes to the CNS, since the brain of GFAP-HIV gp120 mice had increased numbers of CD3(+) T cells that were found in close proximity to sites of CXCL10/IP-10 RNA expression.

Robust expression of TNF-alpha, IL-1beta, RANTES, and IP-10 by human microglial cells during nonproductive infection with herpes simplex virus

Cytokine (TNF-alpha/beta, IL-1beta, IL-6, IL-18, IL-10, and IFN-alpha/beta/gamma) and chemokine (IL-8, IP-10, MCP-1, MIP-1alpha/beta, and RANTES) production during herpes simplex virus (HSV) 1 infection of human brain cells was examined. Primary astrocytes as well as neurons were found to support HSV replication, but neither of these fully permissive cell types produced cytokines or chemokines in response to HSV. In contrast, microglia did not support extensive viral replication; however, ICP4 was detected by immunochemical staining, demonstrating these cells were infected. Late viral protein (nucleocapsid antigen) was detected in <10% of infected microglial cells. Microglia responded to nonpermissive viral infection by producing considerable amounts of TNF-alpha, IL-1beta, IP-10, and RANTES, together with smaller amounts of IL-6, IL-8, and MIP-1alpha as detected by RPA and ELISA. Surprisingly, no interferons (alpha, beta, or gamma) were detected in response to viral infection. Pretreatment of fully permissive astrocytes with TNF-alpha prior to infection with HSV was found to dramatically inhibit replication, resulting in a 14-fold reduction of viral titer. In contrast, pretreatment of astrocytes with IL-1beta had little effect on viral replication. When added to neuronal cultures, exogenous TNF-alpha or IL-1beta did not suppress subsequent HSV replication. Exogenously added IP-10 inhibited HSV replication in neurons (with a 32-fold reduction in viral titer), however, similar IP-10 treatment did not affect viral replication in astrocytes. These results suggest that IP-10 possesses direct antiviral activity in neurons and support a role for microglia in both antiviral defense of the brain as well as amplification of immune responses during neuroinflammation.

CXC chemokine receptor 4 expression and function in human astroglioma cells

Chemokines constitute a superfamily of proteins that function as chemoattractants and activators of leukocytes. Astrocytes, the major glial cell type in the CNS, are a source of chemokines within the diseased brain. Specifically, we have shown that primary human astrocytes and human astroglioma cell lines produce the CXC chemokines IFN-gamma-inducible protein-10 and IL-8 and the CC chemokines monocyte chemoattractant protein-1 and RANTES in response to stimuli such as TNF-alpha, IL-1beta, and IFN-gamma. In this study, we investigated chemokine receptor expression and function on human astroglioma cells. Enhancement of CXC chemokine receptor 4 (CXCR4) mRNA expression was observed upon treatment with the cytokines TNF-alpha and IL-1beta. The peak of CXCR4 expression in response to TNF-alpha and IL-1beta was 8 and 4 h, respectively. CXCR4 protein expression was also enhanced upon treatment with TNF-alpha and IL-1beta (2- to 3-fold). To study the functional relevance of CXCR4 expression, stable astroglioma transfectants expressing high levels of CXCR4 were generated. Stimulation of cells with the ligand for CXCR4, stromal cell-derived factor-1alpha (SDF-1alpha), resulted in an elevation in intracellular Ca(2+) concentration and activation of the mitogen-activated protein kinase cascade, specifically, extracellular signal-regulated kinase 2 (ERK2) mitogen-activated protein kinase. Of most interest, SDF-1alpha treatment induced expression of the chemokines monocyte chemoattractant protein-1, IL-8, and IFN-gamma-inducible protein-10. SDF-1alpha-induced chemokine expression was abrogated upon inclusion of U0126, a pharmacological inhibitor of ERK1/2, indicating that the ERK signaling cascade is involved in this response. Collectively, these data suggest that CXCR4-mediated signaling pathways in astroglioma cells may be another mechanism for these cells to express chemokines involved in angiogenesis and inflammation.