Temporal regulation of viral and cellular gene expression during human T-lymphotropic virus type I-mediated lymphocyte immortalization

Non-clinical safety assessment of novel drug modalities: Genome safety perspectives on viral-, nuclease- and nucleotide-based gene therapies

Gene therapies have emerged as promising treatments for various conditions including inherited diseases as well as cancer. Ensuring their safe clinical application requires the development of appropriate safety testing strategies. Several guidelines have been provided by health authorities to address these concerns. These guidelines state that non-clinical testing should be carried out on a case-by-case basis depending on the modality. This review focuses on the genome safety assessment of frequently used gene therapy modalities, namely Adeno Associated Viruses (AAVs), Lentiviruses, designer nucleases and mRNAs. Important safety considerations for these modalities, amongst others, are vector integrations into the patient genome (insertional mutagenesis) and off-target editing. Taking into account the constraints of in vivo studies, health authorities endorse the development of novel approach methodologies (NAMs), which are innovative in vitro strategies for genotoxicity testing. This review provides an overview of NAMs applied to viral and CRISPR/Cas9 safety, including next generation sequencing-based methods for integration site analysis and off-target editing. Additionally, NAMs to evaluate the oncogenicity risk arising from unwanted genomic modifications are discussed. Thus, a range of promising techniques are available to support the safe development of gene therapies. Thorough validation, comparisons and correlations with clinical outcomes are essential to identify the most reliable safety testing strategies. By providing a comprehensive overview of these NAMs, this review aims to contribute to a better understanding of the genome safety perspectives of gene therapies.

Characteristics of Interleukin 2 and Interleukin 4 Dependent T Cell Lines Infected with HTLV-1 in Vitro

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) is a lymphotropic retrovirus. Cells infected with HTLV-1 in vitro, when maintained in interleukin-2 (IL-2) can be immortalized, remaining for a long time strictly dependent on IL-2 addition. In this study we have compared the effect of interleukin-4 (IL-4) and interleukin-2 on HTLV-1 infection of cord blood or normal adult mononuclear cells. The results showed that either cultures of cord blood or normal adult T cells are susceptible to HTLV-1 infection in presence of IL-4 as well as IL-2. Moreover HTLV-1 infected cells in the presence of IL-4 survived only for a limited length of time in culture, while those grown in IL-2 showed the characteristics of immortalized cell lines. Moreover the profile of cytokine production showed a different pattern in HTLV-1 infected cell lines maintained in IL-4 or IL-2. This suggests that the lymphokines differently modulate retrovirus infection in vitro.

Fine tuning of the temporal expression of HTLV-1 and HTLV-2

Human T-cell leukemia virus types 1 and 2 (HTLV-1 and HTLV-2) are delta retroviruses that share a common overall genetic organization, splicing pattern, and ability to infect and immortalize T-cells in vitro. However, HTLV-1 and HTLV-2 exhibit a clearly distinct pathogenic potential in infected patients. To find clues to the possible viral determinants of the biology of these viruses, recent studies investigated the timing of expression and the intracellular compartmentalization of viral transcripts in ex-vivo samples from infected patients. Results of these studies revealed a common overall pattern of expression of HTLV-1 and -2 with a two-phase kinetics of expression and a nuclear accumulation of minus-strand transcripts. Studies in cells transfected with HTLV-1 molecular clones demonstrated the strict Rex-dependency of this "two-phase" kinetics. These studies also highlighted interesting differences in the relative abundance of transcripts encoding the Tax and Rex regulatory proteins, and that of the accessory proteins controlling Rex expression and function, thus suggesting a potential basis for the different pathobiology of the two viruses.

Imaging spontaneous tumorigenesis: inflammation precedes development of peripheral NK tumors

Early events in tumor development are spontaneous, microscopic, and affected by the microenvironment. We developed a mouse model of spontaneous lymphoma in which malignant transformation is coupled with light emission that can be detected noninvasively using bioluminescent imaging. The human T-cell leukemia virus (HTLV) type 1 transcriptional transactivator Tax is an oncogene sufficient to produce lymphoma in transgenic animal models. Using the granzyme B promoter to restrict Tax expression to the mature natural killer (NK)/T-cell compartment, we have reproduced many elements of HTLV-associated adult T-cell leukemia/lymphoma. Tax activates signaling cascades associated with transformation, inflammation, and tumorigenesis. Here, we report that Tax-mediated activation of luciferase in long terminal repeat-luciferase (LTR-LUC) mice serves as a reporter for imaging these processes in vivo. Using bioluminescent imaging (BLI), we discovered that microscopic intraepithelial lesions precede the onset of peripheral subcutaneous tumors, tumorigenesis progresses through early reversible stages, and Tax is sufficient for inducing tumors. Based on these findings, we propose that Tax expression in activated lymphocytes initiates a cascade of events that leads to NK/T cell recruitment, activation, and transformation. The use of BLI expands our ability to interrogate the role of Tax in tumorigenesis in vivo and has made the association of inflammation with tumor initiation amenable for study.

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.

Human T-cell lymphotropic virus type 1 p12I enhances interleukin-2 production during T-cell activation

Human T-cell lymphotropic virus type 1 (HTLV-1) causes adult T-cell leukemia/lymphoma (ATLL) and a variety of lymphoproliferative disorders. The early virus-cell interactions that determine a productive infection remain unclear. However, it is well recognized that T-cell activation is required for effective retroviral integration into the host cell genome and subsequent viral replication. The HTLV-1 pX open reading frame I encoding protein, p12(I), is critical for the virus to establish persistent infection in vivo and for infection in quiescent primary lymphocytes in vitro. p12(I) localizes in the endoplasmic reticulum (ER) and cis-Golgi apparatus, increases intracellular calcium and activates nuclear factor of activated T cells (NFAT)-mediated transcription. To clarify the function of p12(I), we tested the production of IL-2 from Jurkat T cells and peripheral blood mononuclear cells (PBMC) expressing p12(I). Lentiviral vector expressed p12(I) in Jurkat T cells enhanced interleukin-2 (IL-2) production in a calcium pathway-dependent manner during T-cell receptor (TCR) stimulation. Expression of p12(I) also induced higher NFAT-mediated reporter gene activities during TCR stimulation in Jurkat T cells. In contrast, p12 expression in PBMC elicited increased IL-2 production in the presence of phorbal ester stimulation, but not during TCR stimulation. Finally, the requirement of ER localization for p12(I)-mediated NFAT activation was demonstrated and two positive regions and two negative regions in p12(I) were identified for the activation of this transcription factor by using p12(I) truncation mutants. These results are the first to indicate that HTLV-1, an etiologic agent associated with lymphoproliferative diseases, uses a conserved accessory protein to induce T-cell activation, an antecedent to efficient viral infection.

Efficacy of 3'-azido 3'deoxythymidine (AZT) in preventing HTLV-1 transmission to human cord blood mononuclear cells

The present study investigated the effect of 3'-azido 3'deoxythymidine (AZT) treatment on in vitro infection of human cord blood mononuclear cells (CBMCs) exposed to HTLV-1 by cocultivation with the MT-2 cell line. Cultures of CBMCs were grown in IL-2 and were either left untreated or were treated with concentrations of AZT ranging from 0.0078 to 32 microM. HTLV-1-infected cultures were monitored at different times of culture by evaluating proliferation activity, cell growth and the presence and expression of HTLV-1 genes. Results showed that untreated cultures infected with HTLV-1 were able to grow for several weeks, while those treated with AZT at 0.03 microM or higher concentrations were limited in their growth capacity. Moreover, the addition of AZT at the moment of infection significantly inhibited cell proliferation in a dose-dependent fashion. In the presence of AZT, detection of proviral DNA and, more remarkably, viral RNA expression were clearly reduced. In addition, treatment with AZT resulted in a noticeable decrease in Tax protein expression. Using treatment with relatively low doses of AZT, effective in exerting an antiviral action, cytotoxicity on CBMCs was not observed, whereas higher doses induced apoptosis in uninfected CBMCs. These data show that CBMCs are protected by AZT against HTLV-1 transmission even at low, non-toxic doses.

Proliferation response to interleukin-2 and Jak/Stat activation of T cells immortalized by human T-cell lymphotropic virus type 1 is independent of open reading frame I expression

Human T-cell lymphotropic virus type 1 (HTLV-1), a complex retrovirus, encodes a hydrophobic 12-kD protein from pX open reading frame (ORF) I that localizes to cellular endomembranes and contains four minimal SH3 binding motifs (PXXP). We have demonstrated the importance of ORF I expression in the establishment of infection and hypothesize that p12(I) has a role in T-cell activation. In this study, we tested interleukin-2 (IL-2) receptor expression, IL-2-mediated proliferation, and Jak/Stat activation in T-cell lines immortalized with either wild-type or ORF I mutant clones of HTLV-1. All cell lines exhibited typical patterns of T-cell markers and maintained mutation fidelity. No significant differences between cell lines were observed in IL-2 receptor chain (alpha, beta, or gamma(c)) expression, in IL-2-mediated proliferation, or in IL-2-induced phosphorylated forms of Stat3, Stat5, Jak1, or Jak3. The expression of ORF I is more likely to play a role in early HTLV-1 infection, such as in the activation of quiescent T cells in vivo.

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.

Repression of Transcription From the Human T-Cell Leukemia Virus Type I Long Terminal Repeat and Cellular Gene Promoters by Wild-Type p53

Human T-cell leukemia virus type-I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL) transforms human T cells both in vivo and in vitro. However, the long latency period between infection and development of ATL, as well as the small fraction of the infected population that actually develops this disease, suggest that factors in addition to the virus are involved in its pathogenesis. Mutation of tumor suppressor gene p53 has been found in both HTLV-I–transformed T-cell lines and ATL cases at relatively low frequency. However, increasing evidence supports p53 functional impairment in HTLV-I–transformed T cells. Tax, the major transactivator of HTLV-I, is critical for the initial events involved in transformation. We have considered the possibility that p53 may regulate transcription of viral and cellular genes important for viral replication and transformation. Inactivation of p53 function might then permit constitutive expression of these viral and cellular genes. We have investigated the effects of wild-type and mutant p53 on Tax-mediated activation of the HTLV-I long terminal repeat (LTR) and the promoters of several cellular genes including the interleukin (IL)-1α, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF ), and IL-2 receptor α chain gene. Jurkat, HuT78, and U937 cells were cotransfected with plasmids containing a chloramphenicol acetyltransferase (CAT ) reporter gene under viral or cellular promoter control and the Tax expression vector, in addition to vectors for a wild-type or mutant p53. Wild-type p53 is a potent repressor of viral and cellular activation by Tax. Mutations within p53 severely inhibit this downregulation. We also show that wild-type p53 suppresses transcription from the HTLV-I LTR in Jurkat-Tax, a T-cell line stably expressing Tax, and MT-2, a HTLV-I–transformed T-cell line. Wild-type, but not mutant, p53 interfered with the binding of TATA-binding protein (TBP) to the TATA motif of the HTLV-I LTR. These results suggest that p53 inactivation may lead to upregulation of viral and cellular genes and may also be important for establishment of productive viral infection and development of ATL.

Repression of Transcription From the Human T-Cell Leukemia Virus Type I Long Terminal Repeat and Cellular Gene Promoters by Wild-Type p53

Human T-cell leukemia virus type-I (HTLV-I), the etiologic agent of adult T-cell leukemia (ATL) transforms human T cells both in vivo and in vitro. However, the long latency period between infection and development of ATL, as well as the small fraction of the infected population that actually develops this disease, suggest that factors in addition to the virus are involved in its pathogenesis. Mutation of tumor suppressor gene p53 has been found in both HTLV-I–transformed T-cell lines and ATL cases at relatively low frequency. However, increasing evidence supports p53 functional impairment in HTLV-I–transformed T cells. Tax, the major transactivator of HTLV-I, is critical for the initial events involved in transformation. We have considered the possibility that p53 may regulate transcription of viral and cellular genes important for viral replication and transformation. Inactivation of p53 function might then permit constitutive expression of these viral and cellular genes. We have investigated the effects of wild-type and mutant p53 on Tax-mediated activation of the HTLV-I long terminal repeat (LTR) and the promoters of several cellular genes including the interleukin (IL)-1α, IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF ), and IL-2 receptor α chain gene. Jurkat, HuT78, and U937 cells were cotransfected with plasmids containing a chloramphenicol acetyltransferase (CAT ) reporter gene under viral or cellular promoter control and the Tax expression vector, in addition to vectors for a wild-type or mutant p53. Wild-type p53 is a potent repressor of viral and cellular activation by Tax. Mutations within p53 severely inhibit this downregulation. We also show that wild-type p53 suppresses transcription from the HTLV-I LTR in Jurkat-Tax, a T-cell line stably expressing Tax, and MT-2, a HTLV-I–transformed T-cell line. Wild-type, but not mutant, p53 interfered with the binding of TATA-binding protein (TBP) to the TATA motif of the HTLV-I LTR. These results suggest that p53 inactivation may lead to upregulation of viral and cellular genes and may also be important for establishment of productive viral infection and development of ATL.

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.

Physical and functional interaction between the human T-cell lymphotropic virus type 1 Tax1 protein and the CCAAT binding protein NF-Y

Tax1, a potent activator of human T-cell lymphotropic virus type 1 (HTLV-1) transcription, has been shown to modulate expression of many cellular genes. Tax1 does not bind DNA directly but regulates transcription through protein-protein interactions with sequence-specific transcription factors. Using the yeast two-hybrid system to screen for proteins which interact with Tax1, we isolated the B subunit of the CCAAT binding protein NF-Y from a HeLa cDNA library. The interaction of Tax1 with NF-YB was specific in that NF-YB did not interact with a variety of other transcription factors, including human immunodeficiency virus Tat, human papillomavirus E6, and Bicoid, or with the M7 (amino acids 29CP-AS) Tax1 mutant. However, NF-YB did interact with the C-terminal Tax1 mutants M22 (130TL-AS) and M47 (319LL-RS). We also show that in vitro-translated NF-YB specifically bound to a glutathione S-transferase-Tax1 fusion protein. Further, Tax1 coimmunoprecipitated with NF-Y from nuclear extracts of HTLV-1-transformed cells, providing evidence for in vivo interaction of Tax1 and NF-YB. We further demonstrate that Tax1 specifically activated the NF-Y-responsive DQbeta promoter, as well as a minimal promoter which contains only the Y-box element. In addition, mutation of the Y-box element alone abrogated Tax1-mediated activation. Taken together, these data indicate that Tax1 interacts with NF-Y through the B subunit and that this interaction results in activation of the major histocompatibility complex class II promoter. Through activation of this and other NF-Y driven promoters, the Tax1-NF-Y interaction may play a critical role in causing cellular transformation and HTLV-1 pathogenesis.

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.

In vitro CD4+ lymphocyte transformation and infection in a rabbit model with a molecular clone of human T-cell lymphotrophic virus type 1

We transfected human and rabbit peripheral blood mononuclear cells (PBMC) with the ACH molecular clone of human T-cell lymphotropic virus type 1 (HTLV-1) to study its in vitro and in vivo properties. PBMC transfected with ACH were shown to transfer infection to naive PBMC. ACH transformed rabbit PBMC, as indicated by interleukin-2-independent proliferation of a transfectant culture. This transformant culture was shown by flow cytometric analysis to be a CD4+ CD25+ T-lymphocyte population containing, as determined by Southern blot analysis, at least three integrated HTLV-1 proviral copies. HTLV-1 infection was produced in rabbits inoculated with ACH-transfected, irradiated PBMC. Inoculated rabbits seroconverted to positivity for antibodies against HTLV-1 and had steady or rising HTLV-1 enzyme-linked immunosorbent assay antibody titers. Western blot (immunoblot) analysis revealed sustained seroconversion of rabbits to positivity for antibodies against all major viral antigenic determinants. Infection of rabbits was further demonstrated by antigen capture assay of p24 in PBMC and lymph node cultures and PCR amplification of proviral sequences from PBMC. These data suggest that ACH, like wild-type HTLV-1, infects and transforms primary CD4+ T lymphocytes and is infectious in vivo. This clone will facilitate investigations into the role of viral genes on biological properties of HTLV-1 in vitro and in vivo.

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.

Cellular transformation by human T-cell leukemia virus type I

Human T-cell leukemia virus type I (HTLV-I) is an oncogenic retrovirus that was isolated in 1980 from a patient with adult T-cell leukemia. From the numerous experiments using infected patient T-cells, transgenic mice and tissue culture transformation assays, the Tax protein has been determined to be the transforming component of HTLV-I. Tax-mediated transformation is linked to its ability to transcriptionally regulate the expression of cellular genes involved in growth and proliferation. Ultimately, unregulated and continued activation of these important growth modulating genes by Tax leads to transformation.

Human T-cell lymphotropic virus type 1 Tax mediates enhanced transcription in CD4+ T lymphocytes

Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia/lymphoma and is associated with a variety of immunoregulatory disorders. HTLV-1 has been shown to bind to and infect a variety of hematopoietic and nonhematopoietic cells. However, both in vivo and in vitro, the provirus is mostly detected in and preferentially transforms CD4+ T cells. The molecular mechanism that determines the CD4+ T-cell tropism of HTLV-1 has not been determined. Using cocultures of purified CD4+ and CD8+ T cells with an HTLV-1 producing cell line, we measured viral transcription by using Northern (RNA) blot analysis, protein production by using a p24 antigen capture assay and flow cytometric analysis for viral envelope, and proviral integration by using DNA slot blot analysis. We further measured HTLV-1 long terminal repeat-directed transcription in purified CD4+ and CD8+ T cells by using transient transfection assays and in vitro transcription. We demonstrate a higher rate of viral transcription in primary CD4+ T cells than in CD8+ T cells. HTLV-1 protein production was 5- to 25-fold greater in CD4+ cocultures and mRNA levels were 5-fold greater in these cultures than in the CD8+ cocultures. Transient transfection and in vitro transcription indicated a modest increase in basal transcription in CD4+ T cells, whereas there was a 20-fold increase in reporter gene activity in CD4+ T cells cotransfected with tax. These data suggest that unique or activated transcription factors, particularly Tax-responsive factors in CD4+ T cells, recognize regulatory sequences within the HTLV-1 long terminal repeat, and this mediates the observed enhanced viral transcription and ultimately the cell tropism and leukemogenic potential of the virus.

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.

Transgenic mouse models for HTLV-I infection

Human T-cell leukemia virus type I (HTLV-I) was the first human retrovirus isolated and is responsible for at least one form of human leukemia. The pathogenic mechanism(s) whereby HTLV transforms T lymphocytes in vivo is(are) obscure due to its long-term latency and the lack of practical representative animal models. The tax gene of HTLV-I has been implicated in this transformation process because of its ability to transactivate several cellular genes associated with T-cell replication and activation. Here, transgenic mouse models are discussed that express the Tax protein of HTLV-I and provide insights into its role in the cellular transformation process.

Development of leukemia in mice transgenic for the tax gene of human T-cell leukemia virus type I

The human T-cell leukemia virus type I Tax protein trans-activates several cellular genes implicated in T-cell replication and activation. To investigate its leukemogenic potential, Tax was targeted to the mature T-lymphocyte compartment in transgenic mice by using the human granzyme B promoter. These mice developed large granular lymphocytic leukemia, demonstrating that expression of Tax in the lymphocyte compartment is sufficient for the development of leukemia. Furthermore, these observations suggest that human T-cell leukemia virus infection may be involved in the development of large granular lymphocytic 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.

Protective effect of interferon beta on human T cell leukaemia virus type I infection of CD4+ T cells isolated from human cord blood

The present study shows the effect of human interferon beta (IFN beta) on the susceptibility of highly purified cord blood CD4+ T cells to infection with the human T cell leukaemia virus type I (HTLV-I). Unfractionated cord blood mononuclear cells (CBMC), or a separated CD4+ T cell subpopulation (CBCD4) were exposed to HTLV-I by cocultivation with a chronically infected virus-donor cell line. The results show that presence of proviral DNA as well as virus transcription was markedly reduced by IFN beta in both populations, indicating that this cytokine protects not only unfractionated CBMC but also purified CBCD4 cells from virus infection. Moreover IFN beta treatment caused 60%-80% inhibition of virus expression in CBCD4, assayed as the presence of virus core protein p19. This study demonstrates that IFN beta is able to inhibit HTLV-I infection of CBMC through a mechanism that does not necessarily involve cell-mediated natural or antigen-dependent immunity afforded by CBMC subpopulations distinct from targets of HTLV-I infection. Therefore it is reasonable to conclude that IFN beta has a direct protective effect on CBCD4, through induction of antiviral resistance/activity in target cells.

Expression of complement receptor 2 (CR2) on HTLV-1-infected lymphocytes and transformed cell lines

The expression of complement receptor 2 (CR2) has been demonstrated in established HTLV-1-transformed cell lines and in 12 studied de novo infected peripheral blood lymphocytes cultures, using 2 HTLV-1 sources. The simultaneous detection of CR2 and HTLV-1 antigens in both co-cultivated and supernatant-infected peripheral blood lymphocytes suggest that the increased CR2 expression is in tandem with the increasing HTLV-1 antigen expression. CR2 up-regulation seen during polyclonal activation is presumably in response to a viral protein, although a cellular factor has not been ruled out. Increasing CR2 expression during early infection suggests its possible involvement in selection or development of subsequent transformation events. Variable levels of CR2 in immortalized cell lines argue against its obligate expression of function in the maintenance of the transformed state. The expression of CR2 in cellular activation of T cells may be stage restricted. This study also expands the cellular distribution for CR2.

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

The presence of a high number of activated T cells in the bloodstream and spontaneous proliferation of peripheral blood mononuclear cells in vitro are striking characteristics of human T-cell leukemia virus type I (HTLV-I) infection. The HTLV-I regulatory protein Tax and the envelope protein gp46 have been implicated in mediating the activation process. In this study, HTLV-I-producing cell lines and purified virus from the cell lines were examined for the ability to activate peripheral blood lymphocytes (PBLs) and Jurkat cells. Antisera and monoclonal antibodies against several cellular adhesion proteins involved in T-cell activation and against viral proteins were used to identify which molecules may be participating in the activation process. First, neither virus from a T-cell line, MT2, nor virus produced from the human osteosarcoma cell line HOS/PL was able to induce PBLs to proliferate. In contrast, both fixed and irradiated HTLV-I-producing T-cell lines induced proliferation of PBLs; HOS/PL cells did not activate PBLs. Second, HTLV-I-positive T-cell lines were capable of activating interleukin-2 mRNA expression in Jurkat cells. Induction of interleukin-2 expression was inhibited by anti-CD2 and anti-lymphocyte function-associated antigen 3 (LFA-3) monoclonal antibodies but not anti-human leukocyte antigen-DR, anti-CD4, anti-LFA-1, or anti-intercellular adhesion molecule 1. Similar results were obtained with PBLs as the responder cells. Furthermore, monoclonal antibodies and antisera against various regions of the HTLV-I envelope proteins gp46 and gp21 as well as p40tax did not block activation. These data indicate that HTLV-I viral particles are not intrinsically mitogenic and that infection of target T cells is not necessary for activation. Instead, the mitogenic activity is restricted to virus-producing T cells, requires cell-to-cell contact, and may be mediated through the LFA-3/CD2 activation pathway.

Proliferative response of Tax1-transduced primary human T cells to anti-CD3 antibody stimulation by an interleukin-2-independent pathway

The growth properties of human T-cell leukemia virus Tax1-transduced primary human T cells derived from peripheral blood lymphocytes were compared with those of the same subset of T cells transduced with a control vector. Tax1-transduced T cells exhibited slightly elevated responsiveness to externally added interleukin-2 (IL-2) and a markedly higher proliferative response to stimulation with anti-CD3 antibody. The proliferation after anti-CD3 antibody stimulation was mainly via an IL-2-independent pathway. Therefore, some other mechanism than the previously proposed IL-2 autocrine model seems to be involved in the process of deregulation of T-cell proliferation by Tax1. Moreover, Tax1-transduced T cells have continued to proliferate in medium containing IL-2 long after control T cells ceased to grow, and so they are considered to be immortalized.

Combined treatments with interferon (alpha,beta) plus PGA1 to control early infection with HTLV-I in primary cord blood-derived mononuclear cells

Interferon (IFN) alpha and beta can activate an antiviral and immunomodulating response in primary cord blood-derived mononuclear cells (CBMC) exposed to infection with Human T-cell Leukemia Virus type I (HTLV-I), resulting in partial inhibition of early infection in vitro. On the other hand, PGA1, a PGE1-derived cyclopentenone prostaglandin, can inhibit in vitro the proliferation of virus-infected CBMC, preventing the emergence of the potentially transformed clone. In order to achieve a complete control of HTLV-I infection in this experimental model, we evaluated whether the antiviral activity of IFNs and the antiproliferative activity of PGA1 could be preserved in a combination therapy scheme. Recipient CBMC were treated with IFN alpha or beta (1000 IU/ml) at the onset of the co-culture with lethally irradiated virus-donor MT-2 cells, followed by multiple treatments with PGA1 (4 micrograms/ml every 4 days, starting on day 0) for 6 weeks post infection (p.i.). In PGA1-treated co-cultures the percentage of virus-positive CBMC was constantly doubled during culture time as well as the amount of viral transcripts and p19 virus core protein production were increased. The antiviral effects of IFNs, resulting in about a 50% reduction of the percentage of virus-positive CBMC and consequently in a partial inhibition of virus expression (HTLV-I transcription and p19 production) until 4 weeks p.i., were suppressed by multiple PGA1 treatments. However, the antiproliferative effect of PGA1 was enforced in IFN-treated co-cultures, leading to earlier control of proliferation of virus-infected cells. Interestingly, infection of CBMC with HTLV-I was associated with persistent expression of 70 kDa heat shock protein (HSP70), for at least 4 weeks p.i. IFNs and PGA1 showed antagonistic effects on HSP70 production in infected CBMC. In fact, production of HSP70 was suppressed (or prevented) in IFN-treated co-cultures, tested 2 and 4 weeks p.i. The fact that the expression of HSP70 is apparently suppressed (or prevented) by IFN treatment is surprising, since expression of this protein family has been associated with antiviral immunity. PGA1 could totally reverse the IFN-mediated suppression of HSP70 expression in these co-cultures. It is presently unclear whether HSP70 expression is directly involved in the control of proliferation exerted by PGA1 against virus-infected CBMC or is an epiphenomenon associated with inhibition of cell growth.

Over expression of insulin-like growth factor receptor type-I in T-cell lines infected with human T-lymphotropic virus types-I and -II

To determine if aberrant expression of tyrosine kinase growth factor receptors may be related to the cell transformation capabilities of human T-lymphotropic viruses (HTLVs), we examined the expression of the epidermal growth factor receptor (EGF-R), insulin receptor (INS-R), and insulin-like growth factor receptor type-I (IGFR-I) in cell lines infected with HTLV type I (MT-2, HuT-102) and HTLV type II (Mo-T). Levels of mRNA transcripts for IGFR-I were significantly higher in both MT-2, HuT-102 (HTLV-I) and Mo-T (HTLV-II) cell lines than in uninfected cell lines (HuT-78, Jurkat); no detectable levels of EGF-R or INS-R mRNA transcript were observed in HTLV-infected or uninfected cell lines. Southern blot analysis demonstrated that no amplification or rearrangement of the IGFR-I gene occurred in either the MT-2 or Mo-T cell line. Flow cytometry analysis demonstrated that while IGFR-I protein was constitutively expressed on the cell surface in both MT-2 and Mo-T cell lines, neither EGF-R nor INS-R proteins could be detected. Ligand binding studies with MT-2 and Mo-T cell lines demonstrating binding of 125I insulin-like growth factor type-1 (IGF-I) in a dose-dependent manner and this response could be inhibited by increasing concentrations of cold IGF-I. These data demonstrate that deregulated expression of functional IGFR-I, the regular component of the growth control machinery of normal cells, may contribute to cellular proliferation and eventual transformation in HTLV-I- and HTLV-II-infected cell lines.