The mitogenic activity of human T-cell leukemia virus type I is T-cell associated and requires the CD2/LFA-3 activation pathway

Human T cell leukemia virus type 1 infection drives spontaneous proliferation of natural killer cells

Most human T cell leukemia virus type 1 (HTLV-1) infected subjects remain asymptomatic throughout their lives, with a few individuals developing HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP) or adult T cell leukemia. Lymphocytes from about half of HTLV-1 infected subjects spontaneously proliferate in vitro, and how this phenomenon relates to symptomatic disease outcome and viral burden is poorly understood. Spontaneous proliferation was measured in lymphocyte subsets, and these findings were correlated with HTLV-1 proviral load and Tax expression in PBMCs. We found that in addition to previously described vigorous CD8+ T cell spontaneous proliferation, natural killer (NK) cells spontaneously proliferated to a similar high level, resulting in expansion of CD56-expressing NK cells. Spontaneous NK cell proliferation positively correlated with HTLV-1 proviral load but not with Tax expression or the presence of HAM/TSP. The strongest correlate with clinical outcome in this cohort was the ability of cells to express Tax, while HTLV-1 proviral load was more closely related to spontaneous NK cell proliferation. These results demonstrate that spontaneous proliferation, Tax expression, and proviral load are inter-related but not equivalent, and that spontaneous lymphocyte proliferation is not restricted to T cells, the targets of HTLV-1 infection.

The SCHOOL of nature: IV. Learning from viruses

During the co-evolution of viruses and their hosts, the latter have equipped themselves with an elaborate immune system to defend themselves from the invading viruses. In order to establish a successful infection, replicate and persist in the host, viruses have evolved numerous strategies to counter and evade host antiviral immune responses as well as exploit them for productive viral replication. These strategies include those that modulate signaling mediated by cell surface receptors. Despite tremendous advancement in recent years, the exact molecular mechanisms underlying these critical points in viral pathogenesis remain unknown. In this work, based on a novel platform of receptor signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) platform, I suggest specific mechanisms used by different viruses such as human immunodeficiency virus (HIV), cytomegalovirus (CMV), severe acute respiratory syndrome coronavirus, human herpesvirus 6 and others, to modulate receptor signaling. I also use the example of HIV and CMV to illustrate how two unrelated enveloped viruses use a similar SCHOOL mechanism to modulate the host immune response mediated by two functionally different receptors: T cell antigen receptor and natural killer cell receptor, NKp30. This suggests that it is very likely that similar general mechanisms can be or are used by other viral and possibly non-viral pathogens. Learning from viruses how to target cell surface receptors not only helps us understand viral strategies to escape from the host immune surveillance, but also provides novel avenues in rational drug design and the development of new therapies for immune disorders.

[CCR4, HTLV-1 infection, and ATL oncogenesis]

Adult T-cell leukemia (ATL) is a malignancy of mature CD4+ T cells that is etiologically associated with the infection of human T-cell leukemia virus type 1 (HTLV-1), an exogenous human retrovirus. Previously, we have shown that leukemic cells of most ATL patients express CCR4, a chemokine receptor known to be selectively expressed by T cell subsets such as Th2 cells, skin-homing memory/effector T cells, and regulatory T cells. Therefore, the expression of CCR4 suggests that ATL cells are mostly derived from one of these T cell subsets. We have also shown that Tax, the HTLV-1-encoded potent transcriptional activator, strongly induces the expression of CCL22, a CCR4 ligand, which promotes the cell-dependent transmission of HTLV-1 from HTLV-1-infected T cells to CCR4+ target T cells by inducing close cell-to-cell interactions. We have also shown that ATL cells aberrantly express the AP-1 family member Fra-2 which, by forming the heterodimer with JunD, potently induces the expression of not only CCR4 but also the genes such as c-Myb, MDM2 and Bcl-6, the well-known proto-oncogenes. Thus, Fra-2 is a novel oncogene of ATL, and CCR4 may be regarded as a useful tumor marker of ATL.

Viral pathogenesis, modulation of immune receptor signaling and treatment

During the co-evolution of viruses and their hosts, the latter have equipped themselves with an elaborate immune system to defend themselves from the invading viruses. In order to establish a successful infection, replicate and persist in the host, viruses have evolved numerous strategies to counter and evade host antiviral immune responses as well as exploit them for productive viral replication. These strategies include those that target immune receptor transmembrane signaling. Uncovering the exact molecular mechanisms underlying these critical points in viral pathogenesis will not only help us understand strategies used by viruses to escape from the host immune surveillance but also reveal new therapeutic targets for antiviral as well as immunomodulatory therapy. In this chapter, based on our current understanding of transmembrane signal transduction mediated by multichain immune recognition receptors (MIRRs) and the results of sequence analysis, we discuss the MIRR-targetingviral strategies of immune evasion and suggest their possible mechanisms that, in turn, reveal new points of antiviral intervention. We also show how two unrelated enveloped viruses, human immunodeficiency virus and human cytomegalovirus, use a similar mechanism to modulate the host immune response mediated by two functionally different MIRRs-T-cell antigen receptor and natural killer cell receptor, NKp30. This suggests that it is very likely that similar general mechanisms can be or are used by other viral and possibly nonviral pathogens.

Envelope is a major viral determinant of the distinct in vitro cellular transformation tropism of human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2

Human T-cell leukemia virus type 1 (HTLV-1) and HTLV-2 are related deltaretroviruses but are distinct in their disease-inducing capacity. These viruses can infect a variety of cell types, but only T lymphocytes become transformed, which is defined in vitro as showing indefinite interleukin-2-independent growth. Studies have indicated that HTLV-1 has a preferential tropism for CD4+ T cells in vivo and is associated with the development of leukemia and neurological disease. Conversely, the in vivo T-cell tropism of HTLV-2 is less clear, although it appears that CD8+ T cells preferentially harbor the provirus, with only a few cases of disease association. The difference in T-cell transformation tropism has been confirmed in vitro as shown by the preferential transformation of CD4+ T cells by HTLV-1 versus the transformation of CD8+ T cells by HTLV-2. Our previous studies showed that Tax and overlapping Rex do not confer the distinct T-cell transformation tropisms between HTLV-1 and HTLV-2. Therefore, for this study HTLV-1 and HTLV-2 recombinants were generated to assess the contribution of LTR and env sequences in T-cell transformation tropism. Both sets of proviral recombinants expressed p19 Gag following transfection into cells. Furthermore, recombinant viruses were replication competent and had the capacity to transform T lymphocytes. Our data showed that exchange of the env gene resulted in altered T-cell transformation tropism compared to wild-type virus, while exchange of long terminal repeat sequences had no significant effect. HTLV-2/Env1 preferentially transformed CD4+ T cells similarly to wild-type HTLV-1 (wtHTLV-1), whereas HTLV-1/Env2 had a transformation tropism similar to that of wtHTLV-2 (CD8+ T cells). These results indicate that env is a major viral determinant for HTLV T-cell transformation tropism in vitro and provides strong evidence implicating its contribution to the distinct pathogenesis resulting from HTLV-1 versus HTLV-2 infections.

Epidemiological and clinical interaction between HTLV-1 and Strongyloides stercoralis

Strongyloides stercoralis is the most common human parasitic nematode that is able to complete a life cycle and proliferate within its host. The majority of patients with strongyloidiasis have an asymptomatic infection or mild disease. However, when autoinfection occurs, a high number of infecting larvae can gain access to the bloodstream by penetrating the colonic mucosa leading to a severe hyperinfection and the development of disseminated strongyloidiasis. The human T cell lymphotropic virus type 1 (HTLV-1) predominantly infects T cells and induces spontaneous lymphocyte proliferation and secretion of high levels of type 1 cytokines. Strongyloides stercoralis patients with HTLV-1 co-infection have a modified immunological responses against parasite antigens and co-infection has clinical implications for strongyloidiasis. The high production of IFN-gamma observed in patients co-infected with HTLV-1 and Strongyloides stercoralis decreases the production of IL-4, IL-5, IL-13 and IgE, molecules that participate in the host defence mechanism against helminths. Moreover, there is a decrease in the efficacy of treatment of Strongyloides stercoralis in patients co-infected with HTLV-1. Alterations in the immune response against Strongyloides stercoralis and the decrease in the efficacy of anti-parasitic drugs are responsible for the increased prevalence of Strongyloides stercoralis among HTLV-1 infected subjects and make HTLV-1 infection the most important risk factor for disseminated strongyloidiasis.

Blockade of chemokine-induced signalling inhibits CCR5-dependent HIV infection in vitro without blocking gp120/CCR5 interaction

BACKGROUND

Cellular infection with human immunodeficiency virus (HIV) both in vitro and in vivo requires a member of the chemokine receptor family to act as a co-receptor for viral entry. However, it is presently unclear to what extent the interaction of HIV proteins with chemokine receptors generates intracellular signals that are important for productive infection.

RESULTS

In this study we have used a recently described family of chemokine inhibitors, termed BSCIs, which specifically block chemokine-induced chemotaxis without affecting chemokine ligands binding to their receptors. The BSCI termed Peptide 3 strongly inhibited CCR5 mediated HIV infection of THP-1 cells (83 +/- 7% inhibition assayed by immunofluoresence staining), but had no effect on gp120 binding to CCR5. Peptide 3 did not affect CXCR4-dependent infection of Jurkat T cells.

CONCLUSION

These observations suggest that, in some cases, intracellular signals generated by the chemokine coreceptor may be required for a productive HIV infection.

HIV, 'an evolving species'. Roles of cellular activation and co-infections

Each small variation of the genome of a species can be preserved if it is useful for the survival of the species in a given environment. Within this framework, the finality of the biological cycle of HIV consists in a search for harmony (biological coherence) with man, which is to say a stable condition. Cellular activation appears to be the strategy developed by HIV in order to achieve this coherence. The price of this strategy is the AIDS. The first contact between HIV and immune system appears to determine the subsequent clinical outcome and the future of HIV. Lymphocytic activation varies during the course of the vital cycle of HIV. For each individual, this lymphocytic activation depends on both the HLA repertoire acquired during thymic ontogenesis and the antigenic experience before and after HIV infection. Thus intercurrent infections alter the immune condition of the organism and influence the outcome of HIV. We described a synthetic analysis of the effects of HIV on the surface protein expression and the cellular activation pathways which should provide insights in the evolutionary relationship between HIV and man and should permit to do a more physiological therapeutic approach.

Regulation of the cell-surface expression of an HTLV-I binding protein in human T cells during immune activation

Little is known about the requirements for human T-cell leukemia virus type I (HTLV-I) entry, including the identity of the cellular receptor(s). Recently, we have generated an HTLV-I surface glycoprotein (SU) immunoadhesin, HTSU-IgG, which binds specifically to cell-surface protein(s) critical for HTLV-I-mediated entry in cell lines. Here, expression of the HTLV-I SU binding protein on primary cells of the immune system was examined. The immunoadhesin specifically bound to adult T cells, B cells, NK cells, and macrophages. Cell stimulation dramatically increased the amount of binding, with the highest levels of binding on CD4(+) and CD8(+) T cells. Naive (CD45RA(high), CD62L(high)) CD4(+) T cells derived from cord blood cells, in contrast to other primary cells and all cell lines examined, bound no detectable HTLV-I SU. However, following stimulation, the level of HTSU-IgG binding was rapidly induced (fewer than 6 hours), reaching the level of binding seen on adult CD4(+) T cells by 72 hours. In contrast to HTLV-I virions, the soluble HTSU-IgG did not effect T-cell activation or proliferation. When incubated with human peripheral blood mononuclear cells in a mixed leukocyte reaction, HTSU-IgG inhibited proliferation at less than 1 ng/mL. These results indicate that cell-surface expression of the HTLV SU binding protein is up-regulated during in vitro activation and suggest a role for the HTLV-I SU binding proteins in the immunobiology of CD4(+) T cells.

Molecular biology and pathogenesis of the human T-cell leukaemia/lymphotropic virus Type-1 (HTLV-1)

Retroviruses are associated with a variety of diseases, including immunological and neurological disorders, and various forms of cancer. In humans, the Human T-cell Leukaemia/Lymphotropic virus type 1 (HTLV-1), which belongs to the Oncovirus family, is the aetiological agent of two diverse diseases: Adult T-cell leukaemia/lymphoma (ATLL) (Poiesz et al. 1980; Hinuma et al. 1981; Yoshida et al. 1982), as well as the neurological disorder tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM) (Gessain et al. 1985; Rodgers-Johnson et al. 1985; Osame et al. 1986). HTLV-1 is the only human retrovirus known to be the aetiological agent of cancer. A genetically related virus, HTLV-2, has been identified and isolated (Kalyanaraman et al. 1982). However, there has been no demonstration of a definitive aetiological role for HTLV-2 in human disease to date. Simian T-cell lymphotropic viruses types 1 and 2 (STLV-1 and -2) and bovine leukaemia virus (BLV) have also been classified in same group, Oncoviridae, based upon their similarities in genetic sequence and structure to HTLV-1 and -2 (Burny et al. 1988; Dekaban et al. 1995; Slattery et al. 1999). This article will focus on HTLV-1, reviewing its discovery, molecular biology, and its role in disease pathogenesis.

CD4 T lymphocyte activation in BLV-induced persistent B lymphocytosis in cattle

Bovine leukemia virus (BLV) is an oncogenic retrovirus in the human T cell leukemia virus family. BLV infects B lymphocytes and induces a nonmalignant persistent lymphocytosis (PL) and leukemia/lymphoma in cattle. There is evidence that CD4 T lymphocytes are activated during BLV infection and promote the development of PL. How CD4 T lymphocytes are activated by BLV infection is not known. We observed that CD4 T lymphocytes from PL cattle proliferated in the presence of autologous, irradiated peripheral blood mononuclear cells (PBMC), whereas no proliferation occurred in cell cultures from BLV-infected non-PL cattle. Proliferation required direct contact with metabolically active irradiated PBMC but was not associated with viral protein expression or inhibited by antibodies to BLV. Unexpectedly, B lymphocytes alone failed to account for the irradiated PBMC stimulation of CD4 T lymphocytes. These observations and the magnitude of the proliferative response suggest that activation is polyclonal and involves mechanisms other than BLV antigen-specific stimulation.

HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) pathogenesis hypothesis. A shift of homologous peptides pairs, central nervous system (CNS)/HTLF-1, HTLV-1/thymus, thymus/CNS, in a thymus-like CNS environment, underlies the pathogenesis of HAM/TSP

Determinants shared by thymus, brain and HTLV-1 induce lymphocytic neurotropism and demyelinization in HAM/TSP, within the framework thymus-like brain environment. The disease evolves in two phases. The first phase of the disease would be dependent on CD4 T-lymphocytes specific for thymic autoantigens, reactivated by viral antigens homologous to thymus and CNS autoantigens. During this phase, demyelinization could be due initially to a stop in the synthesis of myelin following an altered expression of adhesion proteins at the surface of oligodendrocytes and neurons. The second phase, which covers the inflammatory and chronic character of the disease, would be dependent, on the one hand, on CD8 T-lymphocytes specific for viral peptides, and on the other hand, on CD8 T-lymphocytes specific for peptides arising from the cell-proteases induced progressive proteolysis of protein components from the myelin layers and other protein components of the CNS. Non-specific inflammatory and non-inflammatory cytokines keep the activation going of the different cellular types. The thoracic spinal cord cell-location specificity would be linked to a peptidic coherence between HTLV-1 (significant agent), thymus and thoracic spinal cord antigens, genetically peculiar to HAM/TSP patients.

Mechanisms of T-cell activation by human T-cell lymphotropic virus type I

The interactions between human T-cell lymphotropic virus type I (HTLV-I) and the cellular immune system can be divided into viral interference with functions of the infected host T cell and the subsequent interactions between the infected T cell and the cellular immune system. HTLV-I-mediated activation of the infected host T cell is induced primarily by the viral protein Tax, which influences transcriptional activation, signal transduction pathways, cell cycle control, and apoptosis. These properties of Tax may well explain the ability of HTLV-I to immortalize T cells. It is not clear, though, how HTLV-I induces T-cell transformation (interleukin-2 [IL-2] independence). Recent evidence suggests that Tax may promote the G1- to S-phase transition, although this may involve additional proteins. A role for other viral proteins that may constitutively activate the IL-2 receptor pathway has also been suggested. By virtue of their activated state, HTLV-I-infected T cells can nonspecifically activate resting, uninfected T cells via virus-mediated upregulation of adhesion molecules. This may favor viral dissemination. Moreover, the induction of a remarkably high frequency of antiviral CD8(+) T cells does not appear to eliminate the infection. Indeed, individuals with a high frequency of virus-specific CD8(+) T cells have a high viral load, indicating a state of chronic immune system stimulation. Thus, while an activated immune system is needed to eradicate the infection, the spread of the HTLV-I is also accelerated under these conditions. A detailed knowledge of the molecular interactions between virus-specific CD8(+) T cells and immunodominant viral epitopes holds promise for the development of specific antiviral therapy.

Marked suppression of T cells by a benzothiophene derivative in patients with human T-lymphotropic virus type I-associated myelopathy/tropical spastic paraparesis

In a search for new anti-autoimmune agents that selectively suppress activation of autoreactive T cells, one such agent, 5-methyl-3-(1-methylethoxy)benzo[b]thiophene-2-carboxamide (CI-959-A), was found to be effective. This compound, which is known to suppress tumor necrosis factor alpha (TNF-alpha)-induced CD54 expression, inhibited the primary proliferative response of the T cell to antigen (Ag)-presenting cells (APCs) including allogenic dendritic cells (DCs), autologous Epstein-Barr virus-infected B cells, and human T lymphotropic virus type I (HTLV-I)-infected T cells. Autoreactive T cells from patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP) spontaneously proliferate in vitro, and their activation is reported to be associated with CD54 expression. The spontaneous proliferation of T cells from patients with HAM/TSP was entirely blocked by CI-959-A. However, in this study, the T-cell proliferation in 15 patients with HAM/TSP was found to depend more extensively on major histocompatibility complex (MHC) class II and CD86 than on CD54 Ags. Since most important APCs for the development of HAM/TSP are DCs and HTLV-I-infected T cells, the effect of CI-959-A on DC generation and on the expression of surface molecules on activated T cells is examined. CI-959-A suppressed recombinant granulocyte-macrophage colony stimulating factor (GM-CSF)- and recombinant interleukin-4-dependent differentiation of DCs from monocytes and inhibited the expression of CD54 and, more extensively, MHC class II and CD86 Ags. CI-959-A showed little toxicity toward lymphoma or HTLV-I-infected T-cell lines or toward monocytes and cultured DCs. These results suggest that CI-959-A might be a potent anti-HAM/TSP agent.

Transcriptional activation of the intercellular adhesion molecule 1 (CD54) gene by human T lymphotropic virus types I and II Tax is mediated through a palindromic response element

In vitro infection of T cells with human T lymphotropic virus types I and II (HTLV-I and HTLV-II) resulted in constitutive expression of ICAM-1. Higher levels of ICAM-1 mRNA were expressed in HTLV-transformed cell lines (MT-2, MoT, C8166) when compared with uninfected T cell lines (A301). We demonstrate that this activation is conferred through a site on the ICAM-1 promoter that is activated in trans by the Tax protein of HTLV-I and HTLV-II. Enhanced promoter activity was detected when the ICAM-1 construct (-1162/+1) was transfected into HTLV-I-infected (MT-2), HTLV-II-infected (MoT, AI 1050), or an HTLV-I Tax-only-expressing (C8166) cell line as compared to the uninfected T cell line (A3.01). Cotransfection of the uninfected T cell line A3.01 with the ICAM construct along with Tax-I and Tax-II expression plasmid also resulted in increased promoter activity. Furthermore, experiments with deletion constructs of the ICAM-1 promoter region indicated that a region between -88 and -53 bp relative to the transcription start site is sufficient for Tax-inducible CAT expression. This segment includes an 11-bp palindromic segment (TTTCCGGGAAA) that has homology with the IFN-gamma and IL-6 response element. An 11-bp segment containing this regulatory region proved to be sufficient to confer Tax-I and Tax-II inducibility on a heterologous promoter (TK-CAT). Taken together these findings indicate that constitutive expression of ICAM-1 by HTLV-infected cells is influenced by the viral trans-activator protein Tax. This increased expression of ICAM-1 in response to the Tax protein may play an important role in the lymphoproliferation associated with HTLV infection.

Isolation of sooty mangabey simian T-cell leukemia virus type I [STLV-I(sm)] and characterization of a mangabey T-cell line coinfected with STLV-I(sm) and simian immunodeficiency virus SIVsmmPBj14

It has been postulated that dual infections of humans with human immunodeficiency virus (HIV) and human T-cell leukemia/lymphotropic virus (HTLV) may potentiate disease progression. Counterparts of both of these pathogenic human retroviruses have been identified in various simian species indigenous to Asia and Africa, including sooty mangabey monkeys (Cercocebus atys). Using peripheral blood mononuclear cells (PBMC) from a mangabey naturally infected with both SIV and STLV-I, T-cell lines were established and maintained continuously for more than 3 years; these cell lines harbored only a newly identified mangabey STLV-I(sm) or both STLV-I(sm) and the acutely lethal variant SIVsmmPBj14. The dually infected cell line (FEd-P14) was established by de novo infection of mangabey PBMC with SIVsmmPBj14. This cell line was characterized by multiple assays which showed that structural proteins encoded by both viruses were produced in large quantities, but that the predominant viral glycoprotein on the cell surface was the STLV-I(sm) Env. Unusual interactions of the two retroviral glycoproteins were suggested by the formation of syncytia between Raji and the FEd-P14 cells, but not between Raji and simian cells infected with only one retrovirus or human cells infected with HTLV-I. The STLV-I(sm) strain obtained from the sooty mangabey was transmitted to normal macaque and mangabey PBMC and was shown to be unique by sequencing of the entire env gene. STLV-I(sm) from this African species was more closely related to "cosmopolitan" HTLV-I strains than to the prototypic STLV-I from an Asian pig-tailed macaque. In vitro and in vivo studies of STLV-I(sm) and SIVsmm, both isolated from a naturally infected mangabey monkey, may provide insight into disease induction and manifestations associated with coinfection by their human counterparts.

Signaling via the CD2 receptor enhances HTLV-1 replication in T lymphocytes

Human T lymphotropic virus type 1 (HTLV-1) is considered the etiologic agent of adult T cell leukemia/lymphoma and several chronic progressive immune-mediated diseases. Approximately 1-4% of infected individuals develop disease, generally decades following infection. Increased proviral transcription, mediated by the viral 40-kDa trans-activating protein, Tax, has been implicated in the pathogenesis of HTLV-1-associated diseases. Since the HTLV-1 promoter contains sequences responsive to cyclic AMP and protein kinase C, we hypothesized that lymphocyte activation signals initiated through the TCR/CD3 complex or CD2 receptor promote viral replication in HTLV-1-infected lymphocytes. We demonstrate that mAbs directed against the CD2, but not the CD3 receptor increase viral p24 capsid protein 1.5- to 5.7-fold in CD2/CD3+ HTLV-1-infected cell culture supernatants. Northern blot analysis demonstrated a 2.5- to 4-fold increase in all species of viral mRNA following CD2 cross-linking of OSP2/4 cells, an immortalized HTLV-1 cell line. Consistent with transcriptional regulation, reporter gene activity increased approximately 11-fold in CD2-stimulated Jurkat T cells cotransfected with a Tax-expressing plasmid and a CAT reporter gene construct under control of the HTLV-1 promoter. These data suggest a possible physiologic mechanism, whereby CD2-mediated cell adhesion and lymphocyte activation may promote viral transcription in infected lymphocytes.

Human T-cell leukemia viruses: epidemiology, biology, and pathogenesis

The human T-cell lymphotropic viruses type I and type II are closely related human retroviruses that have similar biological properties, genetic organization and tropism for T lymphocytes. Along with the simian T-cell lymphoma virus type I, they define the group of retroviruses known as the primate T-cell leukemia/lymphoma viruses. Initially identified in 1980, the human T-cell lymphotropic virus type I has been implicated as the etiologic agent of adult T-cell leukemia/lymphoma and of a degenerative neurologic disorder known as tropical spastic paraparesis or human T-cell lymphotropic virus type I-associated myelopathy. The intriguing link between human T-cell lymphotropic virus type, T-cell malignancy, and a totally unrelated and non-overlapping neurological disorder suggests divergent and unique pathogenetic mechanisms. This review will address the epidemiology, molecular biology, and pathogenesis of human T-cell leukemia viruses.

HTLV-I-infected T cells activate autologous CD4+ T cells susceptible to HTLV-I infection in a costimulatory molecule-dependent fashion

A vigorous production of human T lymphotropic virus type I (HTLV-I)-infected CD4+ T cells is closely associated with the development of adult T cell leukemia (ATL) and neurological disease. However, the immunological mechanisms leading to generation of the HTLV-I-infected cells are not fully clarified. The modulation of CD80 and CD86 expression on the HTLV-I-infected cells and its physiological role in the interaction of infected CD4+ T cells with uninfected CD4+ T cells was examined. The HTLV-I-infected CD4+ T cell lines established from ATL patients and normal donors by infecting their CD4+ T cells with the virus expressed CD80, CD86, and HLA-DR, and induced a proliferation of autologous and allogenic CD4+ T cells. While the CD4+ T cells stimulated with the autologous HTLV-I-infected cells for 7 days expressed CD80 and CD86 but not HTLV-I gene products, they expressed HTLV-I gag antigen after 4 weeks. The interaction of HTLV-I-infected and -uninfected CD4+ T cells was profoundly suppressed by a combination of CD80 and CD86 monoclonal antibodies. These results suggest that the induction of CD80 and CD86 on HTLV-I-infected CD4+ T cells participates actively in the generation of the virus-infected progenitor cells.

Differential transactivation of the intercellular adhesion molecule 1 gene promoter by Tax1 and Tax2 of human T-cell leukemia viruses

Previously, we showed that surface expression of intercellular adhesion molecule 1 (ICAM-1) was strongly upregulated in T cells carrying proviral human T-cell leukemia virus type 1 (HTLV-1) and that the viral transactivator protein Tax1 was capable of inducing the ICAM-1 gene. To determine the responsive elements in the human ICAM-1 gene promoter, a reporter construct in which the 5'-flanking 4.4-kb region of the ICAM-1 gene was linked to the promoterless chloramphenicol acetyltransferase (CAT) gene was cotransfected with expression vectors for Tax1 and Tax2, both of which were separately confirmed to be potent transactivators of the HTLV-1 long terminal repeat (LTR). Tax1 strongly activated the ICAM-1 promoter in all the cell lines tested: three T-cell lines (Jurkat, MOLT-4, and CEM), one monocytoid cell line (U937), and HeLa. Unexpectedly, Tax2 activated the ICAM-1 promoter only in HeLa. By deletion and mutation analyses of the 1.3-kb 5'-flanking region, we found that Tax1 transactivated the ICAM-1 promoter mainly via a cyclic AMP-responsive element (CRE)-like site at -630 to -624 in the Jurkat T-cell line and via an NF-kappaB site at -185 to -177 and an SP-1 site at -59 to -54 in HeLa. On the other hand, Tax2 was totally inactive on the ICAM-1 promoter in Jurkat but transactivated the promoter via the NF-kappaB site at -185 to -177 in HeLa. Gel mobility shift assays demonstrated proteins specifically binding to the CRE-like site at -630 to -624 in Tax1-expressing T-cell lines. Stable expression of Tax1 but not Tax2 in Jurkat subclones enhanced the surface expression of ICAM-1. The differential ability of Tax1 and Tax2 in transactivation of the ICAM-1 gene may be related to the differential pathogenicity of HTLV-1 and HTLV-2.

T helper cell activation and human retroviral pathogenesis

T helper (Th) cells are of central importance in regulating many critical immune effector mechanisms. The profile of cytokines produced by Th cells correlates with the type of effector cells induced during the immune response to foreign antigen. Th1 cells induce the cell-mediated immune response, while Th2 cells drive antibody production. Th cells are the preferential targets of human retroviruses. Infections with human T-cell leukemia virus (HTLV) or human immunodeficiency virus (HIV) result in the expansion of Th cells by the action of HTLV (adult T-cell leukemia) or the progressive loss of T cells by the action of HIV (AIDS). Both retrovirus infections impart a high-level activation state in the host immune cells as well as systemically. However, diverging responses to this activation state have contrasting effects on the Th-cell population. In HIV infection, Th-cell loss has been attributed to several mechanisms, including a selective elimination of cells by apoptosis. The induction of apoptosis in HIV infection is complex, with many different pathways able to induce cell death. In contrast, infection of Th cells with HTLV-1 affords the cell a protective advantage against apoptosis. This advantage may allow the cell to escape immune surveillance, providing the opportunity for the development of Th-cell cancer. In this review, we will discuss the impact of Th-cell activation and general immune activation on human retrovirus expression with a focus upon Th-cell function and the progression to disease.

The tax gene of human T-cell leukemia virus type 2 is essential for transformation of human T lymphocytes

The mechanism of human T-cell leukemia virus (HTLV)-mediated transformation and induction of malignancy is unknown; however, several studies have implicated the viral gene product, Tax. Conclusive evidence for the role of Tax in the HTLV malignant process has been impeded by the inability to mutate tax in the context of an infectious virus and dissociate viral replication from cellular transformation. To circumvent this problem we constructed a mutant of HTLV type 2 (HTLV-2) that replicates by a Tax-independent mechanism. For these studies, the Tax response element in the viral long terminal repeat was replaced with the cytomegalovirus immediate-early promoter enhancer (C-enh). Transcription of the chimeric HTLV-2 (HTLVC-enh) was efficiently directed by this heterologous promoter. Also, the chimeric virus transformed primary human T lymphocytes with an efficiency similar to that of wild-type HTLV-2. A tax-knockout virus, termed HTLVC-enhDeltaTax, was constructed to directly assess the importance of Tax in cellular transformation. Transfection and infection studies indicated that HTLVC-enhDeltaTax was replication competent; however, HTLVC-enhDeltaTax failed to transform primary human T lymphocytes. We conclude that Tax is essential for HTLV-mediated transformation of human T lymphocytes. Furthermore, this chimeric HTLV, that replicates in the absence of Tax, should facilitate studies to determine the precise mechanism of T-lymphocyte transformation by HTLV.

Stimulation of the CD2 receptor pathway induces apoptosis in human T lymphotropic virus type I-infected cell lines

We demonstrate that CD2 receptor engagement, but not CD3 crosslinking, induces apoptosis in lymphocytes transformed by human T-cell lymphotrophic virus type I (HTLV-I). Mitogenic pairs of anti-CD2 monoclonal antibodies inhibited [3H]thymidine incorporation from 25 to 62% in CD2+ HTLV-I-infected lymphocytes. This inhibition was associated with a 20-40% reduction in cell number and viability over a 3-day period, morphologic evidence of apoptosis, and irreversible DNA fragmentation. While cyclosporin A abrogated CD2-mediated proliferation in peripheral blood mononuclear cells, it had no effect on CD2-induced apoptosis in the HTLV-I-infected cell lines. Since HTLV-I is mitogenic to resting lymphocytes through CD2 activation pathways, these results suggest that HTLV-I-infected lymphocytes are primed for apoptosis following additional CD2 stimulation. This CD2-mediated apoptosis might be a factor in immune regulation of HTLV-I-associated diseases or might offer a novel adjunctive approach to treatment.

The HTLV-I envelope glycoproteins: structure and functions

The human T-cell lymphotropic virus type I (HTLV-I) envelope has a structural organization shared by all retroviral envelopes, which contain two mature viral glycoproteins deriving from a common precursor: an external surface protein (SU), associated with a transmembrane protein (TM) responsible for anchoring the SU-TM complex at the cell surface or in the viral envelope. Our understanding of the tertiary structure of these proteins is extremely poor. The intracellular maturation follows the normal cellular secretory pathway, resulting in expression of the mature glycoproteins at the cell surface. The five potential N-glycosylation sites are glycosylated. Most mutations artificially introduced into the glycoproteins result in loss of function, mostly due to abnormal intracellular maturation. This probably indicates a very compact structure of these proteins, where the entire structure is involved in correct conformation. Studies using neutralizing antibodies or mutagenesis have defined functional domains in the SU protein, which is responsible for receptor binding. These domains occur throughout the SU glycoprotein. Sequence analysis of the HTLV-I TM predicts a structure, and probably functions, similar to other retrovirus TMs: involvement of this glycoprotein in the different oligomerization steps leading to a fusogenic SU-TM complex and in the fusion process itself. These features remain to be proven, and it is not yet understood why the free HTLV-I viral particle is not infectious.

HTLV-I-induced T-cell activation

Infection by the human T-cell lymphotropic virus type I (HTLV-I) causes T-cell activation by at least two separate mechanisms. One mechanism involves activation of the T cells harboring the virus and is exemplified by in vivo infected nonimmortalized T-cell clones that display a prolonged state of activation. This HTLV-I-induced T-cell activation is inhibited by rapamycin, a drug that inhibits p70 S6-kinase and blocks cell cycle in G1, but is not inhibited by FK506 or cyclosporin A, both of which inhibit interleukin-2 (IL-2) production. The phenotype of this pathway is consistent with an hyperactive IL-2R pathway or CD28 pathway, indicating that HTLV-I may contribute a costimulatory signal to the infected T cell. As a separate mechanism, HTLV-I-infected T cells can induce activation of uninfected T cells via T-T-cell interaction mediated by the LFA-3-CD2 pathway. This may induce IL-2 production from the uninfected T cells, leading to a more generalized activation of the immune system that potentially could provide a basis for some of the diseases associated with HTLV-I. Moreover, this THTLV-I-T-cell interaction could explain the spontaneous proliferation observed in patients with HTLV-I-associated myelopathy/tropical spastic paraparesis.

Infection with human T-lymphotropic virus types I and II results in alterations of cellular receptors, including the up-modulation of T-cell counterreceptors CD40, CD54, and CD80 (B7-1)

To examine the phenotypic alterations associated with human T-lymphotropic virus types I and II (HTLV-I and -II) infection, long-term cell lines (n = 12 HTLV-I cell lines; n = 11 HTLV-II cell lines; n = 6 virus-negative cell lines) were analyzed for the cell surface expression of various lineage markers (i.e., myeloid, progenitor, and leukocyte), integrin receptors, and receptor-counterreceptor (R-CR) pairs responsible for cellular activation. As expected, all cell lines expressed the markers characterizing the leukocyte lineage (CD43, CD44, and CD53). Of the progenitor-myeloid markers examined (CD9, CD13, CD33, CD34, and CD63), only the percent expression of CD9 was significantly increased on HTLV-I and -II-infected cell lines as compared with that on virus-negative cell lines. Analysis of the beta 1 integrin subfamily (CD29, CD49b, CD49d, CD49e, and CD49f) showed no significant change, except that CD49e was significantly decreased on the HTLV-infected cell lines. For the beta 2 integrin subfamily, the cell surface density was increased for CD18 and CD11a, while the CD11c molecule was expressed exclusively on the HTLV-I- and HTLV-II-infected cell lines. Analysis of several R-CR pairs (CD2-CD58, CD45RO-CD22, CD5-CD72, CD11a-CD54, gp39-CD40, and CD28-CD80) demonstrated that comparable levels of expression of the Rs (CD2, CD45RO, CD5, and CD28) and of some of the CRs (CD58, CD22, and CD72) were in all cell lines; however, CD54, CD40, and CD80 were expressed constitutively on the HTLV-I- and HTLV-II-infected cell lines. Functionally, the expression of these R-CR pairs did not appear to affect the autologous proliferation since monoclonal antibodies to these R-CR pairs were not able to inhibit proliferation of the infected cell lines. Taken together, our results indicate that HTLV-I and -II can modulate the expression of several T-cell activation molecules and CRs normally expressed on alternate cell types.

Human T lymphocyte virus 1 from a leukemogenic cell line mediates in vivo and in vitro lymphocyte apoptosis

HTLV-1 is implicated in the development of diverse diseases. However, most HTLV-1-infected individuals remain asymptomatic. How HTLV-1 infection leads to disparate consequences remains a mystery, despite extensive investigation of HTLV-1 isolates from infected individuals. As in human infection, experimental HTLV-1 infection in rabbits is generally benign, although HTLV-1-infected rabbit T cell lines that mediate lethal leukemia-like disease have been reported. We report here that thymuses from mature outbred rabbits inoculated with a lethal leukemia-like disease have been reported. We report here that thymuses from mature outbred rabbits inoculated with a lethal HTLV-1 T cell line (RH/K34) showed morphological and biochemical evidence of apoptosis, whereas thymuses from rabbits inoculated with nonlethal HTLV-1 T cell lines showed no signs of apoptosis. Exposure of rabbit or human lymphocytes to purified virus from RH/K34 caused rapid induction of apoptosis, providing an in vitro correlate to the pathogenic effects. By contrast, virus isolated from a nonlethal cell line mediated dose-dependent lymphocyte proliferation. These data implicate lymphocyte apoptosis as a potential mechanism by which the lethal HTLV-1 cell line causes fulminant disease and provide a means to identify factors contributing to HTLV-1 disease. Results from this HTLV-1 infection model can provide insight into variations in HTLV-1 pathogenicity in human infection.

Induction of ICAM-1 and LFA-3 by Tax1 of human T-cell leukemia virus type 1 and mechanism of down-regulation of ICAM-1 or LFA-1 in adult-T-cell-leukemia cell lines

The present study was undertaken to determine the role of HTLV-I TaxI in the up-regulation of ICAM-I and LFA-3 in human T cells transformed with HTLV-I and the mechanism of down-regulation of ICAM-I and LFA-I in ATL-derived cell lines. Induction of TaxI in a human T-cell line Jurkat carrying the TaxI gene under the metallothionein promoter led to increases in mRNA and surface expression of ICAM-I. The response of LFA-3 to TaxI induction was, on the other hand, relatively slow and weak, and might be indirect. Transactivation of the ICAM-I promoter by TaxI was further shown by co-transfection of a CAT reporter construct with the ICAM-I promoter and a plasmid expressing TaxI. The mechanism of down-regulation of ICAM-I or LFA-I in 4 ATL cell lines was next examined. ICAM-I mRNA was quite low in MT-I, but no genomic changes were found. The CAT reporter with the ICAM-I promoter was inactive in MT-I. Finally, combined treatment of MT-I with 5-azacytidine and IFN-gamma induced re-expression of ICAM-I. Collectively, (a) transcriptional factor(s) necessary for expression of ICAM-I gene may be repressed in MT-I through DNA methylation. Three other ATL cell lines (TL-OmI, H582, HuT102) were found to have little mRNA for the LFA-I beta chain (CD18). H582 and HuT102 were also negative for the LFA-I alpha chain (CDIIa) mRNA. No genomic changes were found, and a CAT reporter gene with the CD18 promoter was inactive in the 3 of them, again suggesting lack of (a) transcriptional factor(s) necessary for CD18 expression.

Inhibition of apoptosis in T cells expressing human T cell leukemia virus type I Tax

This study set out to determine whether T cell dysfunction associated with HTLV-I led to increased sensitivity of infected cells to apoptosis or, owing to their potential to develop ATL, if infected cells would become resistant to this process. To test this hypothesis we utilized the monoclonal antibody anti-APO-1, which has been demonstrated to induce apoptosis in human T cells. Human T cell lines expressing HTLV-I showed reduced susceptibility to anti-APO-1-induced apoptosis despite expression of high levels of cell surface APO-1. Cell-free supernatant of the Tax-expressing cell line C8166 and heat-inactivated supernatant of the HTLV-I-producing cell line MT2 transferred increased resistance to anti-APO-1 to susceptible Jurkat T cells. Susceptible T cells transfected with an HTLV-I Tax-expressing vector or treated with soluble Tax protein became less susceptible to anti-APO-1-induced cell death. Furthermore, primary human lymphocytes treated with soluble Tax were less susceptible to apoptosis induced by anti-APO-1. The protective effect of Tax in T cell lines and primary human lymphocytes was reversed by the addition of anti-Tax antibodies. Anti-APO-1-induced apoptosis was also found to be inhibited in Jurkat cells by the induction of protein kinase C (PKC) with 12-O-tetradecanoylphorbol-13-acetate (TPA). Resistance to apoptosis conferred by HTLV-I Tax and an active PKC pathway may be factors contributing to the survival of dysregulated HTLV-I-infected T cells prone to the development of adult T cell leukemia.

Human T-cell leukemia virus type I-induced proliferation of human immature CD2+CD3- thymocytes

The mitogenic activity of human T-cell leukemia virus type I (HTLV-I) is triggering the proliferation of human resting T lymphocytes through the induction of the interleukin-2 (IL-2)/IL-2 receptor autocrine loop. This HTLV-I-induced proliferation was found to be mainly mediated by the CD2 T-cell antigen, which is first expressed on double-negative lymphoid precursors after colonization of the thymus. Thus, immature thymocytes express the CD2 antigen before that of the CD3-TCR complex. We therefore investigated the responsiveness of these CD2+CD3- immature thymocytes and compared it with that of unseparated thymocytes, containing a majority of the CD2+CD3+ mature thymocytes, and that of the CD2-CD3- prothymocytes. Both immature and unseparated thymocytes were incorporating [3H]thymidine in response to the virus, provided that they were cultivated in the presence of submitogenic doses of phytohemagglutinin. In contrast, the prothymocytes did not proliferate. Downmodulation of the CD2 molecule by incubating unseparated and immature thymocytes with a single anti-CD2 monoclonal antibody inhibited the proliferative response to HTLV-I. These results clearly underline that the expression of the CD2 molecule is exclusively required in mediating the proliferative response to the synergistic effect of phytohemagglutinin and HTLV-I. Immature thymocytes treated with a pair of anti-CD2 monoclonal antibodies were shown to proliferate in response to HTLV-I, even in the absence of exogenous IL-2. We further verified that the proliferation of human thymocytes is consecutive to the expression of IL-2 receptors and the synthesis of IL-2. These observations provide evidence that the mitogenic stimulus delivered by HTLV-I is more efficient than that provided by other conventional mitogenic stimuli, which are unable to trigger the synthesis of endogenous IL-2. Collectively, these results show that the mitogenic activity of HTLV-I is able to trigger the proliferation of cells which are at an early stage of T-cell development. They might therefore represent target cells in which HTLV-I infection could favor the initiation of the multistep lymphoproliferative process leading to adult T-cell leukemia.