Functional changes in astrocytes by human T-lymphotropic virus type-1 T-lymphocytes.

The human T-lymphotropic virus type-1 (HTLV-1) is the causative agent of a chronic progressive myelopathy (TSP/HAM) in which lesions of the central nervous system (CNS) are associated with infiltration of HTLV-1-infected T-cells. In a model that mimics the interaction between glial and T-cells, we show that transient contact with T-lymphocytes chronically infected with HTLV-1 induce profound metabolic alterations in astrocytes. Within the first week post-contact, an overall activation of astrocyte metabolism was observed as assessed by enhanced uptake of glutamate and glucose, and lactate release. In contrast, longer examination showed a reduced astrocytic accumulation of glutamate. The time course of the change in glutamate uptake was in fact biphasic. Previous observations indicated that HTLV-1 protein Tax-1 was involved in this delayed decrease, via the induction of TNF-alpha. The expression of the glial glutamate transporters, GLAST and GLT-1 decreased in parallel. These decreases in glutamate uptake and transporters' expression were associated with an imbalance in the expression of the catabolic enzymes of glutamate, GS and GDH, presumably due to Tax-1. Given the fact that impairment of glutamate management in astrocytes is able to compromise the functional integrity of neurons and oligodendrocytes, our results altogether give new insights into the physiopathology of TSP/HAM.

Astrocytic alterations induced by HTLV type 1-infected T lymphocytes: a role for Tax-1 and tumor necrosis factor alpha.

In the neurological disease associated with HTLV-1 infected T lymphocytes infiltrated within the CNS are suspected of playing a prominent role in pathogenesis via inflammatory cytokines and the viral protein Tax-1. We hypothesized that T lymphocytes initiate functional perturbation in astrocytes, resulting in neuronal alteration as glial cells have a crucial role in CNS homeostasis. In particular, astrocytes manage the steady state level of glutamate and continuously provide metabolite precursors to neurons and oligodendrocytes. Using a model system of HTLV-1-infected T cells-astrocytes interaction, we show that after contact with T cells, astrocyte acquire a phenotype typical of gliosis: secretion of proinflammatory cytokines (TNF-alpha, IL-1alpha, IL-6) and matrix metalloproteinases (MMP-9, MMP-3). The concomitant increase in the expression of MMPs and of their endogenous inhibitors (TIMP-1 and TIMP-3) suggests a perturbation in MMP/TIMP balance. This may alter the extracellular matrix and, in turn, the cell environment. At a functional level, glutamate transport and catabolism are impaired in astrocytes. A decrease in glutamate uptake is associated with downregulated expression of glutamate transporters GLAST and GLT1. The expression of astrocytic enzyme of glutamate metabolism is modified with up-regulation of glutamine synthetase and down-regulation of glutamate dehydrogenase. The involvement of Tax-1 in these alterations, directly or indirectly via TNF-alpha, is shown. Altered glutamate uptake and catabolism associated with impairment in cell connectivity via MMP/TIMP imbalance could compromise the functional integrity of the CNS in general and that of neurons and oligodendrocytes in particular.

Long-term effects of HTLV-1 on brain astrocytes: sustained expression of Tax-1 associated with synthesis of inflammatory mediators.

HTLV-1 is the causative agent of a chronic neurological disease, TSP/HAM. The persistently activated CTL response to the viral protein Tax-1 suggests the existence of persistent viral replication with continuous expression of Tax-1. Although CD4+ T-cell is the main target for HTLV-1, previous observations have indicated that the astrocyte, the major neural cell in close contact with blood vessel and thus with HTLV-1-infected T-cells infiltrating the CNS, may also be infected. We tested in vitro the hypothesis of persistent/restricted infection in human and rat astrocytes after transient contact with an infectious T-cell line (C91PL). Long-term analysis showed prolonged expression of Tax-1 in astrocytes, associated with secretion of inflammatory mediators (TNFalpha, IL1alpha, MMP-2, and MMP-9). These data suggest a possible contribution of Tax-1-expressing astrocytes to TSP/HAM pathogenesis.

Human T-cell lymphotropic virus type 1-infected T lymphocytes impair catabolism and uptake of glutamate by astrocytes via Tax-1 and tumor necrosis factor alpha.

Human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of a chronic progressive myelopathy called tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In this disease, lesions of the central nervous system (CNS) are associated with perivascular infiltration by lymphocytes. We and others have hypothesized that these T lymphocytes infiltrating the CNS may play a prominent role in TSP/HAM. Here, we show that transient contact of human or rat astrocytes with T lymphocytes chronically infected by HTLV-1 impairs some of the major functions of brain astrocytes. Uptake of extracellular glutamate by astrocytes was significantly decreased after transient contact with infected T cells, while the expression of the glial transporters GLAST and GLT-1 was decreased. In two-compartment cultures avoiding direct cell-to-cell contact, similar results were obtained, suggesting possible involvement of soluble factors, such as cytokines and the viral protein Tax-1. Recombinant Tax-1 and tumor necrosis factor alpha (TNF-alpha) decreased glutamate uptake by astrocytes. Tax-1 probably acts by inducing TNF-alpha, as the effect of Tax-1 was abolished by anti-TNF-alpha antibody. The expression of glutamate-catabolizing enzymes in astrocytes was increased for glutamine synthetase and decreased for glutamate dehydrogenase, the magnitudes of these effects being correlated with the level of Tax-1 transcripts. In conclusion, Tax-1 and cytokines produced by HTLV-1-infected T cells impair the ability of astrocytes to manage the steady-state level of glutamate, which in turn may affect neuronal and oligodendrocytic functions and survival.

T lymphocytes activated by persistent viral infection differentially modify the expression of metalloproteinases and their endogenous inhibitors, TIMPs, in human astrocytes: relevance to HTLV-I-induced neurological disease.

Activation of T lymphocytes by human pathogens is a key step in the development of immune-mediated neurologic diseases. Because of their ability to invade the CNS and their increased secretion of proinflammatory cytokines, activated CD4+ T cells are thought to play a crucial role in pathogenesis. In the present study, we examined the expression of inflammatory mediators the cytokine-induced metalloproteinases (MMP-2, -3, and -9) and their endogenous inhibitors, tissue inhibitors of metalloproteinases (TIMP-1, -2, and -3), in human astrocytes in response to activated T cells. We used a model system of CD4+ T lymphocytes activated by persistent viral infection (human T lymphotropic virus, HTLV-I) in transient contact with human astrocytes. Interaction with T cells resulted in increased production of MMP-3 and active MMP-9 in astrocytes despite increased expression of endogenous inhibitors, TIMP-1 and TIMP-3. These data suggest perturbation of the MMP/TIMP balance. These changes in MMP and TIMP expression were mediated, in part, by soluble factors (presumably cytokines) secreted by activated T cells. Integrin-mediated cell adhesion is also involved in the change in MMP level, since blockade of integrin subunits (alpha1, alpha3, alpha5, and beta1) on T cells resulted in less astrocytic MMP-9-induced expression. Interestingly, in CNS tissues from neurological HTLV-I-infected patients, MMP-9 was detected in neural cells within the perivascular space, which is infiltrated by mononuclear cells. Altogether, these data emphasize the importance of the MMP-TIMP axis in the complex interaction between the CNS and invading immune cells in the context of virally mediated T cell activation.