Human T-cell lymphotrophic virus type-I tax gene induces secretion of human macrophage inflammatory protein-1alpha

HTLV-1 Tax Tug-of-War: Cellular Senescence and Death or Cellular Transformation

Human T cell leukemia virus type 1 (HTLV-1) is a retrovirus associated with a lymphoproliferative disease known as adult T cell leukemia/lymphoma (ATLL). HTLV-1 infection efficiently transforms human T cells in vivo and in vitro. The virus does not transduce a proto-oncogene, nor does it integrate into tumor-promoting genomic sites. Instead, HTLV-1 uses a random mutagenesis model, resulting in cellular transformation. Expression of the viral protein Tax is critical for the immortalization of infected cells by targeting specific cellular signaling pathways. However, Tax is highly immunogenic and represents the main target for the elimination of virally infected cells by host cytotoxic T cells (CTLs). In addition, Tax expression in naïve cells induces pro-apoptotic signals and has been associated with the induction of non-replicative cellular senescence. This review will explore these conundrums and discuss the mechanisms used by the Tax viral oncoprotein to influence life-and-death cellular decisions and affect HTLV-1 pathogenesis.

The Role of Chemokines in the Pathogenesis of HTLV-1

Human T cell leukemia virus type 1 (HTLV-1) is a human retrovirus that is associated with two main diseases: HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and adult T cell leukemia/lymphoma (ATL). Chemokines are highly specialized groups of cytokines that play important roles in organizing, trafficking, homing, and in the migration of immune cells to the bone marrow, lymphoid organs and sites of infection and inflammation. Aberrant expression or function of chemokines, or their receptors, has been linked to the protection against or susceptibility to specific infectious diseases, as well as increased the risk of autoimmune diseases and malignancy. Chemokines and their receptors participate in pathogenesis of HTLV-1 associated diseases from inflammation in the central nervous system (CNS) which occurs in cases of HAM/TSP to T cell immortalization and tissue infiltration observed in ATL patients. Chemokines represent viable effective prognostic biomarkers for HTLV-1-associated diseases which provide the early identification of high-risk, treatment possibilities and high-yielding clinical trials. This review focuses on the emerging roles of these molecules in the outcome of HTLV-1-associated diseases.

Expression of parathyroid hormone-related protein during immortalization of human peripheral blood mononuclear cells by HTLV-1: implications for transformation

BACKGROUND

Adult T-cell leukemia/lymphoma (ATLL) is initiated by infection with human T-lymphotropic virus type-1 (HTLV-1); however, additional host factors are also required for T-cell transformation and development of ATLL. The HTLV-1 Tax protein plays an important role in the transformation of T-cells although the exact mechanisms remain unclear. Parathyroid hormone-related protein (PTHrP) plays an important role in the pathogenesis of humoral hypercalcemia of malignancy (HHM) that occurs in the majority of ATLL patients. However, PTHrP is also up-regulated in HTLV-1-carriers and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients without hypercalcemia, indicating that PTHrP is expressed before transformation of T-cells. The expression of PTHrP and the PTH/PTHrP receptor during immortalization or transformation of lymphocytes by HTLV-1 has not been investigated.

RESULTS

We report that PTHrP was up-regulated during immortalization of lymphocytes from peripheral blood mononuclear cells by HTLV-1 infection in long-term co-culture assays. There was preferential utilization of the PTHrP-P2 promoter in the immortalized cells compared to the HTLV-1-transformed MT-2 cells. PTHrP expression did not correlate temporally with expression of HTLV-1 tax. HTLV-1 infection up-regulated the PTHrP receptor (PTH1R) in lymphocytes indicating a potential autocrine role for PTHrP. Furthermore, co-transfection of HTLV-1 expression plasmids and PTHrP P2/P3-promoter luciferase reporter plasmids demonstrated that HTLV-1 up-regulated PTHrP expression only mildly, indicating that other cellular factors and/or events are required for the very high PTHrP expression observed in ATLL cells. We also report that macrophage inflammatory protein-1alpha (MIP-1alpha), a cellular gene known to play an important role in the pathogenesis of HHM in ATLL patients, was highly expressed during early HTLV-1 infection indicating that, unlike PTHrP, its expression was enhanced due to activation of lymphocytes by HTLV-1 infection.

CONCLUSION

These data demonstrate that PTHrP and its receptor are up-regulated specifically during immortalization of T-lymphocytes by HTLV-1 infection and may facilitate the transformation process.

The role of calcium/calmodulin-dependent protein kinase cascade on MIP-1alpha gene expression of ATL cells

OBJECTIVE

Adult T-cell leukemia (ATL) is a mature CD4(+) T-cell malignancy caused by infection with human T-lymphotrophic virus type-1 and is associated with a marked hypercalcemia in many patients. Recently, it has been proposed that macrophage inflammatory protein-1alpha (MIP-1alpha) is the clinical hallmark of hypercalcemia in ATL. In this study, we investigated the effect of extracellular calcium on MIP-1alpha secretion in ATL cells and the role of Ca(2+)/calmodulin (CaM)-dependent protein kinase (CaM-K) cascade in transcriptional activation of MIP-1alpha.

MATERIALS AND METHODS

MIP-1alpha protein levels in the culture supernatant collected from ATL cells were measured by enzyme-linked immunosorbent assay. Reporter plasmid containing the MIP-1alpha promoter was transfected to ATL cells, and the promoter activity was measured by luciferase assay.

RESULTS

The addition of calcium to the culture medium enhanced the secretion of MIP-1alpha from ATL cells, which was inhibited by the CaM-KK inhibitor. The transfection of CaM-KIV stimulated MIP-1alpha promoter activity, and the upstream kinase CaM-KK enhanced the stimulatory effect of CaM-KIV on the promoter activity. Mutation in the cyclic adenosine 5' monophosphate response element (CRE) within the MIP-1alpha promoter significantly reduced the effect of CaM-KIV, and CRE mutant promoter activity was not significantly enhanced by the addition of calcium to the culture medium as compared to wild-type promoter activity.

CONCLUSION

Hypercalcemia enhances MIP-1alpha secretion in ATL cells, and this mechanism requires the involvement of CaM-KK/CaM-KIV cascade through the CRE. These findings raise a possibility that the inhibitory effect of CaM-KK/CaM-KIV cascade may be a potential therapeutic target for ATL.

High production of RANTES and MIP-1α in the tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM)

Human T cell lymphotropic virus type 1 (HTLV-1) infection is associated with progressive neurological disorders and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). The pathogenesis of TSP/HAM is considered as immune mediated, involving cytotoxic T cell (CTL) responses to a number of viral proteins and notably the regulation protein Tax. T CD8+ cells produce beta-chemokines, which are important in the anti-viral response. In the present study, we have analyzed the CC chemokines (RANTES, MIP-1beta and MIP-1alpha) production in retrovirus-infected subjects. A total of 191 subjects were studied: 52 healthy controls, 72 asymptomatic HTLV-1-infected carriers and 67 TSP/HAM patients. Peripheral blood mononuclear cells were maintained in the presence or absence of PHA, and supernatant fluids were assayed using EIA. MIP-1beta concentration was not significantly different across groups, but RANTES and MIP-1alpha concentrations showed significant differences when the three groups were compared. In TSP/HAM patients, the increase in the production of chemokines may lead to a recruitment of pro-inflammatory factors, contributing to the membrane's myelin damage.

Modulation of dendritic cell maturation and function by the Tax protein of human T cell leukemia virus type 1

Human T cell leukemia virus type 1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is characterized by the generation of an intense CTL cell response directed against the viral transactivator protein Tax. In addition, patients diagnosed with HAM/TSP exhibit rapid activation and maturation of dendritic cells (DC), likely contributing to the robust, Tax-specific CTL response. In this study, extracellular Tax has been shown to induce maturation and functional alterations in human monocyte-derived DC, critical observations being confirmed in freshly isolated myeloid DC. Tax was shown to promote the production of proinflammatory cytokines and chemokines involved in the DC activation process in a dose- and time-dependent manner. Furthermore, Tax induced the expression of DC activation (CD40, CD80, and CD86) and maturation (CD83) markers and enhanced the T cell proliferation capability of DC. Heat inactivation of Tax resulted in abrogation of these effects, indicating a requirement for the native structure of Tax, which was found to bind efficiently to the DC membrane and was internalized within a few hours, suggesting that extracellular Tax may possess an intracellular mechanism of action subsequent to entry. Finally, inhibitors of cellular signaling pathways, NF-kappaB, protein kinase, tyrosine kinase, and phospholipase C, were shown to inhibit Tax-mediated DC activation. This is the first study reporting the immunomodulatory effects of extracellular Tax in the DC compartment. These results suggest that DC, once exposed to Tax by uptake from the extracellular environment, can undergo activation, providing constant antigen presentation and costimulation to T cells, leading to the intense T cell proliferation and inflammatory responses underlying HAM/TSP.

Use of human antigen presenting cell gene array profiling to examine the effect of human T-cell leukemia virus type 1 Tax on primary human dendritic cells

Human T-cell leukemia virus type 1 (HTLV-1) is etiologically linked to adult T-cell leukemia and a progressive demyelinating disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). One of the most striking features of the immune response in HAM/TSP centers on the expansion of HTLV-1-specific CD8(+) cytotoxic T lymphocyte (CTL) compartment in the peripheral blood and cerebrospinal fluid. More than 90% of the HTLV-1-specific CTLs are directed against the viral Tax (11-19) peptide implying that Tax is available for immune recognition by antigen presenting cells, such as dendritic cells (DCs). DCs obtained from HAM/TSP patients have been shown to be infected with HTLV-1 and exhibit rapid maturation. Therefore, we hypothesized that presentation of Tax peptides by activated DCs to naIve CD8(+) T cells may play an important role in the induction of a Tax-specific CTL response and neurologic dysfunction. In this study, a pathway-specific antigen presenting cell gene array was used to study transcriptional changes induced by exposure of monocyte-derived DCs to extracellular HTLV-1 Tax protein. Approximately 100 genes were differentially expressed including genes encoding toll-like receptors, cell surface receptors, proteins involved in antigen uptake and presentation and adhesion molecules. The differential regulation of chemokines and cytokines characteristic of functional DC activation was also observed by the gene array analyses. Furthermore, the expression pattern of signal transduction genes was also significantly altered. These results have suggested that Tax-mediated DC gene regulation might play a critical role in cellular activation and the mechanisms resulting in HTLV-1-induced disease.

Autocrine role of macrophage inflammatory protein-1 beta in human T-cell lymphotropic virus type-I tax-transfected Jurkat T-cells

The interaction of human T-cell lymphotropic virus I (HTLV-I) tax gene with host cell factors plays an important role in the maintenance of the transformed state. There have been numerous reports that have demonstrated the role of tax in transactivating several cytokines. In this study, we show that upon mitogen stimulation, macrophage inflammatory protein-1 beta (MIP-1beta) is expressed and secreted in tax-transfected Jurkat cells. Furthermore, expression of CC-chemokine receptor-5 (CCR5) mRNA in these cells suggests an autocrine role for MIP-1beta in HTLV-I-infected T-cells. These results, coupled with our earlier observations, demonstrate the influence that extracellular Tax protein might have on modulating host chemokines for proliferation and transformation of uninfected cells.

Interleukin-8 expression in AIDS-associated lymphoma B-cell lines

Interleukin 8 (IL-8), a member of the CXC subfamily of chemokines, is a potent inflammatory cytokine produced by many cell types in response to several stimuli. In an attempt to determine whether human B-cell IL-8 functions as an autocrine growth factor, a wide panel of B-cell lines derived from patients with AIDS-associated B-cell lymphomas (AABCL) (n = 5) and from non-AABCLs (n = 8) was studied for expression of IL-8, IL-8 Receptor type A (IL-8R), and secretion of IL-8 protein. Using RT-PCR and Northern Blot analysis, we were able to observe IL-8 expression ubiquitously. However, IL-8R expression was seen only in EBV negative (4 out of 7) B-cell lines. EBV and HIV-1 activated B-cell line; HBL-1, was the major secretor of IL-8. Our results demonstrate that IL-8 is expressed in malignant B-cell phenotypes that correspond to a narrow window in the B-cell differentiation pathway (pre-B, early-B, and intermediate-B) as well as in normal CD19-enriched B-cells. Furthermore, IL-8 autocrine loops were not evident since IL-8R was detected only in cell lines that did not secrete IL-8 protein.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

Human T-cell lymphotropic virus type-I tax gene induces interleukin-8 secretion by autocrine mechanism and has No effect on interleukin-16 in transfected Jurkat cells

Human T-cell lymphotropic virus I (HTLV-I) Tax has been shown to transactivate several cellular genes. In this study, we show that interleukin-8 (IL-8) is expressed and secreted in tax-transfected Jurkat cells that were further augmented by mitogen stimulation. Expression of high-affinity IL-8-R (Type A) mRNA in these cells suggests an autocrine role for this chemokine in HTLV-I-infected T-cells. However, interleukin-16 (IL-16) mRNA expression or protein secretion was not significantly modulated either constitutively or even upon mitogen stimulation in these tax-transfected cells.