Human T-cell leukemia-lymphoma virus (HTLV) type-2, together with HTLV-1, belongs to the HTLV-bovine leukemia virus group of the Oncovirinae family; this virus was firs identified in a T-cell line derived from a patient with hairy cell leukemia (1). In spite of the fact that HTLV-1 and HTLV- 2 are highly related, they differ with regard to pathogenicity and cellular tropism. In a considerable percentage of patients, HTLV-1 is the etiologic agent of acute T-cell leukemia and lymphoma, and infection has been associated with the development of tropical spastic paraparesis (TSP) and myelopathy (2-3). In contrast, the correlation between HTLV-2 infection and hematopoietic malignancies is still controversial (4-7). However, HTLV-2 infection of T cells in vitro results in spontaneous cell proliferation and transformation (6,8, 9). In addition, HTLV-2 can be found in some patients with neurodegenerative and lymphoproliferative disorders and in a significant fraction (up to 10%) of intravenousdrug users, frequently co-infected with human immunodeficiency virus (HIV)-1. HTLV-1 enters CD4+ and CD8+ T cells, whereas HTLV-2 infects preferentially CD8+ T cells, though in patients with high proviral load both viruses can also integrate in monocytes and B cells (10-13).
HTLV infection leads to several immune dysfunctions, including spontaneous proliferation of T-cells, which has been attributed to the effect of the trans -regulatory protein Tax, encoded by the px viral gene (14). In fact, beside the action on the viral LTR to enhance viral RNA transcription, Tax is also capable of trans-activating heterologous eukaryotic promoters of genes involved in T-cell activation and proliferation, including a large array of cytokines. Further studies have shown that antibodies to Tax are often found in the serum of infected individuals (15) and a cell-mediated immune response directed towards the Tax protein is detected in a large proportion of HTLV-associated myelopathy (HAM) and TSP patients. It has been reported that HTLV-1 infected cells express a wide spectrum of chemokines and that Tax is capable of inducing the expression of a number of chemokine genes (16).
Recently our research group has observed that HTLV-2 directly acts on CD34+ hematopoietic precursor by protecting them from apoptosis and increasing their survival potential (17). It is accepted that this mitogenic effect is strongly dependent on the infected cellular host environment and that the viral envelope proteins can directly influence the homeostasis of human blood progenitors. It has been also shown that HTLV-2 promotes the secretion of CC-chemokines, in particular MIP-1a, in primary cell cultures obtained from infected subjects (18). Therefore the HTLV-2/ CD34+ cell interaction offers a unique model for study the effects of the viral stimulus on specific cellular targets. The aim of our study is to analyze the biological effects of HTLV on CD34+ hematopoietics precursors, either directly isolates from the peripheral blood of healthy individuals, or in the IL-3 dependent TF-1 cell line.
We have previously reported that the Mo strain of HTLV-2, when derived from T cells (CMo), unlike when derived from B cells (BMo), rescued TF-1 and bone marrow-derived CD34+ primary cells from apoptosis induced by IL-3 deprivation (17).
In this study we have shown that HTLV-2 induced survival and proliferation of the CD34+ TF-1 cell line in the absence of IL-3 or other growth factors. These effects were observed only when HTLV-2 was derived from T cells (CMo), but not from B cells (BMo). In addition, we demonstrated that the contact between CD34+ Tf-1 cells with HTLV-2 can induces the JAK/STAT activation, in particular STAT 5, and that HTLV-2 is able to induce the secretion of GM-CSF, INF-gamma and SCF. Actually the STAT activation induced by HTLV-2 comes from GM-CSF and INF-gamma. These results indicate that HTLV-2 interaction with CD34+ precursorc cells may lead to the expression of cytokines that, by inducing autocrine activation of STATs, may influence the host's regenerative capacity and immune response to HTLV-2 (19).
HTLV-1 and -2 are mostly T lymphotropic, showing an in vivo preferential targeting to CD4 and CD8 T-cells, respectively. However in some patients it has been recently demonstrated that both HTLV-1 and HTLV-2 can infect non-T cell populations, including monocytes and B-cells (10, 20).
In addition, we have reported that HTLV-2 up-regulates telomerase activity in hematopoietic precursor cells, thereby increasing their replicative potential and survival (17).
These observations clearly suggest that HTLV virions are capable of influencing directly the pathways controlling survival and growth in CD34+ hematopoietic precursors and that HTLVs/CD34+ cells interaction is providing a unique model to study the effects of viral stimuli on specific cellular targets. The possible elucidation of these interactions is likely to shed light on the mechanisms leading to rearrangements of host cell gene expression and to deregulation on normal cellular processes.
Our recent studies have demonstrated that: (i) HTLV-2 has a mitogenic effect on CD34+ hematopoietic precursors; (ii) this effect does not depend on cell infection; (iii) and it is inversely correlated to the presence and number of bound class II HLA molecules on its envelope during viral budding (17). Although HTLV-2 has been described many years ago, both the viral ligand and the cellular receptor remain unknown. So, we will first try to characterize the HLA-derived viral structures involved in the binding of HTLV-2 with CD34+ cells and to identify the specific factors which are protecting CD34+ TF-1 cells from apoptosis.
Regulation of apoptosis is a complex process involving a number of gene products including the survival factor Bcl-2, which has been found to be frequently deregulated in human cancer. Telomerase activation has been proposed to be a critical event in the immortalization of human cells. The presence of telomerase allows the cell to proliferate. However it does not induce proliferation and increasing its expression does not result in changes typically associated with malignant transformation. The restoration of telomerase is accompanied by ectopic expression of the hTERT telomerase catalytic subunit which is the limiting factor of telomerase activation (21). The mechanism by which telomerase is expressed in somatic cells appears to involve the transcriptional regulation of the hTERTgene, that may also occur through direct interaction of oncogene products, as c-Myc, with the promoter region of the gene (22). A recent study demonstrated the wide occurrence of telomerase activation and Bcl-2 deregulation in human cancer cells and a linkage between Bcl-2 deregulation and telomerase activity (23). Recently, it has been reported that cells infected in vitro by HTLV-1 and HTLV-2 have elevated expression of Bcl-2 and also of Bcl-XL, an other important death antagonist (29). Bcl-XL promoter is transactivated by Tax protein of HTLV-1 and HTLV-2 in human T-cells whereas HTLV-2 Tax has a reduced activity. Importantly, Bcl-XL expression is also markedly increased in leukemic cells obtained from ATLL patients, and its expression appears to correlate with the severity of the disease (24). As in other human cancers, these data support the notion that aberrant expression of Bcl-XL may increase the survival of virus-infected cells as well as their resistance to apoptotic signals, thereby contributing to HTLV-1-induced leukemogenesis. However, other studies indicate that Bcl-2 and Bcl-XL might function independently and that, in certain cell types, the two proteins show different potencies in controlling apoptosis (25). We have recently reported that telomerase activity is significantly increased in peripheral blood CD34+ cells exposed to T-cell derived HTLV-2 virion and that this increase is correlated to Bcl-2 expression (26).