HTLV-I myelitis: isolation of virus, genomic analysis, and infection in neural cell cultures

Establishment of a novel human T-cell leukemia virus type 1 infection model using cell-free virus

The primary mode of infection by human T-cell leukemia virus type 1 (HTLV-1) is cell-to-cell transmission during contact between infected cells and target cells. Cell-free HTLV-1 infections are known to be less efficient than infections with other retroviruses, and transmission of free HTLV-1 is considered not to occur in vivo. However, it has been demonstrated that cell-free HTLV-1 virions can infect primary lymphocytes and dendritic cells in vitro, and that virions embedded in biofilms on cell membranes can contribute to transmission. The establishment of an efficient cell-free HTLV-1 infection model would be a useful tool for analyzing the replication process of HTLV-1 and the clonal expansion of infected cells. We first succeeded in obtaining supernatants with high-titer cell-free HTLV-1 using a highly efficient virus-producing cell line. The HTLV-1 virions retained the structural characteristics of retroviruses. Using this cell-free infection model, we confirmed that a variety of cell lines and primary cultured cells can be infected with HTLV-1 and demonstrated that the provirus was randomly integrated into all chromosomes in the target cells. The provirus-integrated cell lines were HTLV-1-productive. Furthermore, we demonstrated for the first time that cell-free HTLV-1 is infectious in vivo using a humanized mouse model. These results indicate that this cell-free infection model recapitulates the HTLV-1 life cycle, including entry, reverse transcription, integration into the host genome, viral replication, and secondary infection. The new cell-free HTLV-1 infection model is promising as a practical resource for studying HTLV-1 infection.IMPORTANCECo-culture of infected and target cells is frequently used for studying HTLV-1 infection. Although this method efficiently infects HTLV-1, the cell mixture is complex, and it is extremely difficult to distinguish donor infected cells from target cells. In contrast, cell-free HTLV-1 infection models allow for more strict experimental conditions. In this study, we established a novel and efficient cell-free HTLV-1 infection model. Using this model, we successfully evaluated the infectivity titers of cell-free HTLV-1 as proviral loads (copies per 100 cells) in various cell lines, primary cultured cells, and a humanized mouse model. Interestingly, the HTLV-1-associated viral biofilms played an important role in enhancing the infectivity of the cell-free infection model. This cell-free HTLV-1 infection model reproduces the replication cycle of HTLV-1 and provides a simple, powerful, and alternative tool for researching HTLV-1 infection.

Human T-cell Lymphotropic Viruses: Review and Prospects for Antiviral Therapy

The human T-cell lymphotropic viruses types I and II (HTLV-I, II) pose challenges to researchers and clinicians who seek to unveil mechanisms of viral transformation and pathogenesis. HTLV-I infection in humans is associated with a wide array of primary and secondary diseases ranging from mild immunosuppression to adult T-cell leukaemia/lymphoma and HTLV-associated myelopathy/tropical spastic paraparesis (HAM/TSP), a neurological degenerative syndrome. As retroviruses, HTLV-I and II share similar replicative cycles with human immunodeficiency virus (HIV), the causative agent of acquired immunodeficiency syndrome. However, in contrast to HIV-I which destroys CD4+ T cells, HTLV-I and II can preferentially transform a CD4+ T-cell subset to an unrestricted growth state.

HTLV-I and II, along with simian T-lymphotropic virus (STLV) and bovine leukaemia virus (BLV), form a phylogenetic group which is distinct from ungulate, non-human primate and human lentiviruses such as visna, simian immunodeficiency virus (SIV), and human immunodeficiency viruses types 1 and 2. The proviral genome of HTLV-I is flanked at the 5′ and 3′ ends by long terminal repeats (LTR) and is further subdivided into structural gag and env genes, a pro gene encoding an aspartyl protease, a pol gene which encodes reverse transcriptase and endonuclease, and the regulatory gene elements tax and rex. Regions within the LTR contain recognition sites for cellular proteins and the tax gene product that collectively promote viral expression. Tax-mediated activation of cellular genes involved in growth and differentiation is suspected to play a dominant role in the leukaemogenic process associated with HTLV-I infection. Differential rex-regulated splicing of viral message gives rise to transcripts encoding the polyprotein precursor gag-pro-pol (unspliced), envelope (single spliced), or tax/rex (doubly spliced). The 100nm HTLV virion contains an electron-dense core surrounding a divalent-single stranded DNA genome. This core is in turn enclosed by concentric shells of matrix protein and an outer lipid bilayer, the latter acquired as the virus buds from the surface of the infected cell. Envelope glycoproteins associated with the outside of this lipid bilayer can interact with viral receptors on cells and mediate virus entry. Antiviral strategies have been directed at inhibiting viral entry into cells (sulphated and non-sulphated polysaccharides, vaccines), blocking of viral replication (AZT, suramin), intracellular immunization (transdominant repression of rex), and elimination of virus infected cells (IL-2 receptor-directed toxins). Serological screening of the blood supply and curtailing breast feeding of children by HTLV-I + mothers have likely had a major impact in preventing HTLV-I infection.

Expression of glucose transporter 1 confers susceptibility to human T-cell leukemia virus envelope-mediated fusion

Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus identified and causes both adult T-cell leukemia/lymphoma and tropical spastic paraparesis/HTLV-1-associated myelopathy, among other disorders. In vitro, HTLV-1 has an extremely broad host cell tropism in that it is capable of infecting most mammalian cell types, although at the same time viral titers remain relatively low. Despite years of study, only recently has a bona fide candidate cellular receptor, glucose transporter 1 (glut-1), been identified. Although glut-1 was shown to bind specifically to the ectodomain of HTLV-1 and HTLV-2 envelope glycoproteins, which was reversible with small interfering RNA directed against glut-1, cellular susceptibility to HTLV upon expression of glut-1 was not established. Here we show that expression of glut-1 in relatively resistant MDBK cells conferred increased susceptibility to both HTLV-1- and HTLV-2-pseudotyped particles. glut-1 also markedly increased syncytium formation in MDBK cells after exposure to HTLV-1. Another assay also demonstrated HTLV-1 envelope-cell fusion in the presence of glut-1. Taken together, these results provide additional evidence that glut-1 is a receptor for HTLV.

Syncytium-inhibiting monoclonal antibodies produced against human T-cell lymphotropic virus type 1-infected cells recognize class II major histocompatibility complex molecules and block by protein crowding

Four new monoclonal antibodies (MAbs) that inhibit human T-cell lymphotropic virus type 1 (HTLV-1)-induced syncytium formation were produced by immunizing BALB/c mice with HTLV-1-infected MT2 cells. Immunoprecipitation studies and binding assays of transfected mouse cells showed that these MAbs recognize class II major histocompatibility complex (MHC) molecules. Previously produced anti-class II MHC antibodies also blocked HTLV-1-induced cell fusion. Coimmunoprecipitation and competitive MAb binding studies indicated that class II MHC molecules and HTLV-1 envelope glycoproteins are not associated in infected cells. Anti-MHC antibodies had no effect on human immunodeficiency virus type 1 (HIV-1) syncytium formation by cells coinfected with HIV-1 and HTLV-1, ruling out a generalized disruption of cell membrane function by the antibodies. High expression of MHC molecules suggested that steric effects of bound anti-MHC antibodies might explain their inhibition of HTLV-1 fusion. An anti-class I MHC antibody and a polyclonal antibody consisting of several nonblocking MAbs against other molecules bound to MT2 cells at levels similar to those of class II MHC antibodies, and they also blocked HTLV-1 syncytium formation. Dose-response experiments showed that inhibition of HTLV-1 syncytium formation correlated with levels of antibody bound to the surface of infected cells. The results show that HTLV-1 syncytium formation can be blocked by protein crowding or steric effects caused by large numbers of immunoglobulin molecules bound to the surface of infected cells and have implications for the structure of the cellular HTLV-1 receptor(s).

Effect of phospholipids on adsorption and penetration of human T-cell leukemia virus type I

We have studied the ability of some phospholipids (PLs) and phospholipases (PLases) to interfere with infection of human T-cell leukemia virus type I (HTLV-I). Plating of pseudotype of vesicular stomatitis virus (VSV) bearing envelope antigens of HTLV-I, VSV(HTLV-I), was markedly inhibited by treatment of the cells with cardiolipin (CL) after, but not before, infection. Treatment of the cells with CL after infection also inhibited the plating of VSV pseudotype of bovine leukemia virus (BLV), but scarcely affected VSV infection. Furthermore, the plating of VSV(HTLV-I) was markedly enhanced by treatment with PLCase after infection. Treatment with PLCase, however, did not affect the plating of VSV. These results were also confirmed by polymerase chain reaction (PCR): Formation of proviral DNA was inhibited when indicator cells were treated with CL after cell-free infection of HTLV-I, but not before, and enhanced when indicator cells were treated with PLCase after HTLV-I infection. These findings suggested that PLs might play a role at the early stage of HTLV-I infection.

Human T-cell lymphotropic virus type 1 (HTLV-1)-induced syncytium formation mediated by vascular cell adhesion molecule-1: evidence for involvement of cell adhesion molecules in HTLV-1 biology

While studying the potential role of vascular cell adhesion molecule-1 (VCAM-1) in infection of endothelial cells by human immunodeficiency virus (HIV), we found that VCAM-1 can mediate human T-cell lymphotropic virus type 1 (HTLV-1)-induced syncytium formation. Both expression-vector-encoded and endogenously expressed VCAM-1 supported fusion of uninfected cells with HTLV-1-infected cells. Fusion was obtained with cell lines carrying the HTLV-1 genome and expressing viral proteins but not with an HTLV-1-transformed cell line that does not express viral proteins. In clones of VCAM-1-transfected cells, the degree of syncytium formation observed directly reflected the level of VCAM-1 expression. Syncytium formation between HTLV-1-expressing cells and VCAM-1+ cells could be blocked with antiserum against HTLV-1 gp46 and with a monoclonal antibody (MAb) against VCAM-1. Fusion was not blocked by antiserum against HIV or a MAb against VLA-4, the physiological counter-receptor for VCAM-1. The results indicate that VCAM-1 can serve as an accessory molecule or potential coreceptor for HTLV-1-induced cell fusion and provide direct evidence of a role for cell adhesion molecules in the biology of HTLV-1.

Clonal analysis suggests provirus expression in a subpopulation of human malignant trophoblast cells harbouring the human T cell lymphotropic virus type-I genome

Previous studies have indicated that the villous trophoblast may be involved in intrauterine HTLV-I infection. Although the data furnished by our group (Liu et al., 1995) have demonstrated that the human trophoblast-derived malignant cell lines JAR and JEG-3 are susceptible to HTLV-I, the infection, even after thorough analysis, appeared to be limited to expression at the transcriptional level. In the present report, we sought to explore virus expression at the single cell level using eight clonally selected cell lines which were derived by limiting dilution from the previously infected parental cultures. Of the three cell lines JAR-H2, JAR-H3, and JEG-H3, all of which harboured full-length provirus, only in two (JAR-H2 and JEG-H3) were the virus-specific tax/rex and env transcripts demonstrated using RT-PCR. When compared with MT-2 cells, the detected steady-state levels of HTLV-I mRNA appeared to be lower by three orders of magnitude. Viral Tax protein displaying a typical intranuclear localization was found in 1-2% of JAR-H2 and JEG-H3 cells. Moreover, an altered phenotype characterized by multinucleated syncytia was observed in these cell cultures with the same frequency as Tax transactivator, implying a fusogenic activity of env protein. Infectious virus, however, could not be rescued from JAR-H2 or JEG-H3 clones by coculture with cord blood mononuclear cells. Our data suggest that trophoblast represents a susceptible, albeit a slightly permissive, host system for HTLV-I.

Bidirectional enhancing activities between human T cell leukemia-lymphoma virus type I and human cytomegalovirus in human term syncytiotrophoblast cells cultured in vitro

The syncytiotrophoblast layer of the human placenta has an important role in limiting transplacental viral spread from mother to fetus. Human cytomegalovirus (HCMV) is capable of establishing a latent infection in syncytiotrophoblast cells, with restriction of gene expression to immediate-early and early proteins. We analyzed the extent of replication of human T cell leukemia-lymphoma virus type I (HTLV-I) in human term syncytiotrophoblasts infected with HTLV-I alone or coinfected with HTLV-I and HCMV. Although syncytiotrophoblasts could be infected with cell-free HTLV-I, no viral protein expression was found in the singly infected cells. On the contrary, coinfection of the cells with HTLV-I and HCMV resulted in simultaneous replication of both viruses. Bidirectional enhancing activities between HTLV-I and HCMV were mediated primarily by the Tax and immediate-early proteins, respectively. The stimulatory effect of HTLV-I Tax on HCMV replication appeared to be mediated partly by tumor necrosis factor beta and transforming growth factor beta-1. We observed formation of pseudotypes with HTLV-I nucleocapsids within HCMV envelopes, whereas HCMV was not pseudotyped by HTLV-I envelopes in dually infected syncytiotrophoblast cells. Our data suggest that in vivo dual infection of syncytiotrophoblast cells with HTLV-I and HCMV may facilitate the transplacental transmission of both viruses.

Virions released from cells transfected with a molecular clone of human T-cell leukemia virus type I give rise to primary and secondary infections of T cells

The ability of molecular clones of human T-cell leukemia virus type I (HTLV-I) to direct the synthesis of infectious virions has not previously been demonstrated. An HTLV-I provirus originating from an adult T-cell leukemia patient was cloned into a plasmid vector and is designated pCS-HTLV. This molecular clone was shown to direct the synthesis of viral mRNA and proteins in transiently transfected cells; in addition, virus structural proteins were released into the culture medium. Viral proteins were assembled into virions that sedimented at a buoyant density characteristic of retrovirus particles and whose morphology was verified by electron microscopy. Virions concentrated from transiently transfected cell supernatants were incubated with primary cord blood lymphocytes or with transformed T-cell lines to establish that these particles were infectious. Expression of spliced, viral mRNAs in the T-cell cultures after both primary and secondary infections with cell-free virus revealed that pCS-HTLV encodes an infectious provirus.

HTLV-I proviral DNA amount correlates with infiltrating CD4+ lymphocytes in the spinal cord from patients with HTLV-I-associated myelopathy

A quantitative method utilizing polymerase chain reaction was employed to evaluate the amount of human T-cell leukemia virus type I (HTLV-I) proviral DNA in the affected spinal cords from patients with HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Central nervous system (CNS) tissues were obtained at post-mortem from five patients with HAM/TSP, who vary in the duration of illness from 2.5-10 years, and one patient with adult T-cell leukemia (ATL), who had leukemic cell infiltration in the CNS. The presence of HTLV-I pX and pol sequences in the CNS tissues were demonstrated in all patients examined. In HAM/TSP, the proviral DNA quantified in the thoracic cord was 0.002-2 copies per 100 tissue cells, and that in the peripheral blood mononuclear cells (PBMC) was 2-8 copies per 100 PBMC. The proviral DNA amount in the thoracic cord of the patient with ATL was 0.4 copies per 100 tissue cells. An apparent propensity for the amount of integrated HTLV-I in the thoracic cord to decrease with the disease duration in patients with HAM/TSP was observed. The decline in HTLV-I proviral DNA amount in the thoracic cord lesions was paralleled with the alteration of proportion of CD4+ T lymphocytes in patients with HAM/TSP. These findings suggest that preferential virus reservoir may be infiltrating CD4+ T lymphocytes in the spinal cord lesions of patients with HAM/TSP, and HTLV-I infection in the CNS of patients is declining with the disease duration in spite of the chronic course of neurological manifestations at least in some patients with HAM/TSP.

Human T-cell leukemia virus type I infection of monocytes and microglial cells in primary human cultures

The pathogenesis of progressive spastic paraparesis [HTLV-I-associated myelopathy/tropical spastic paraparesis (HAM/TSP)], a serious consequence of human T-cell leukemia virus type I (HTLV-I) infection, is unclear. T and B lymphocytes can be naturally infected by HTLV-I, but the susceptibility to HTLV-I infection of other cell types that could contribute to the pathogenesis of HAM/TSP has not been determined. We found that a human monocyte cell line (THP-1), primary human peripheral blood monocytes, and isolated microglial cells but not astrocytes or oligodendroglial cells derived from adult human brain were infected by HTLV-I in vitro. Infection with HTLV-I enhanced the secretion of interleukin 6 in human microglial cell-enriched cultures but did not stimulate the release of interleukin 1 from monocytes or microglial cells. Tumor necrosis factor alpha production was stimulated by HTLV-I infection of monocytes and microglial cells and could be enhanced by suboptimal amounts of lipopolysaccharide. Since both tumor necrosis factor alpha and interleukin 6 have been implicated in inflammatory demyelination and gliosis, our findings suggest that human microglial cells and monocytes infected with and activated by HTLV-I could play a role in the pathogenesis of HAM/TSP.

Coexistence of acute monoblastic leukemia and adult T-cell leukemia: possible association with HTLV-I infection in both cases?

A 64 year-old Japanese man who developed acute monoblastic leukemia during the course of adult T-cell leukemia/lymphoma (ATL) was studied. Leukemic cells in the peripheral blood and bone marrow were monoblasts positive for alpha-naphthol butyrate esterase (alpha-NBE) staining, CD11c and CD36 antigens, whereas tumor cells in the pleural effusion were ATL cells positive for CD2, CD4, CD25, CD29 and CD45RA antigens. These two malignant cells had different chromosomal abnormalities. Monoclonal integration of human T-cell leukemia virus type I (HTLV-I) proviral DNA and T-cell receptor C beta gene (TCR C beta) rearrangement were detected in the ATL cells, but not in the leukemic monoblasts. By polymerase chain reaction (PCR) in the peripheral blood mononuclear cells (CD11c+ 98%, CD2+ 4%, CD20+ 0%) not containing ATL cells, the presence of the gag region of HTLV-I was confirmed. These facts indicate that a double positive T cell (CD29+, CD45RA+) was possibly the target cell for HTLV-I infection and that HTLV-I was not directly related to the oncogenesis of the monocyte lineage in the present case, even if it did infect the monocytes. However, there is still an outside possibility that HTLV-I induced acute monoblastic leukemia indirectly.

Functional analysis of two long terminal repeats from the HTLV-I retrovirus

Human T-cell leukemia virus type I (HTLV-I) is associated with a large spectrum of clinical manifestations in man. Viral and host factors are probably involved in determining the consequences of infection. Although most of the genome of HTLV-I appears remarkably stable, considerable variation is observed in the long terminal repeat (LTR) which harbors the promoter region. So far, no correlation between specific mutations and pathogenesis has been found, and the current opinion is that sequence variations reflect the geographical origin of the isolate more than the associated pathology. To assess whether the mutations observed between two HTLV-I LTRs were functionally significant, two LTRs, which differ by ten mutations, were coupled to the highly sensitive eukaryotic luciferase-encoding reporter gene, luc, and tested by transfection in a variety of cell lines. Marked differences in promoter activity were observed in some of the cells tested, whereas in other both LTRs were equally active. This result demonstrates that the minor differences observed between two HTLV-I LTRs can affect the activity level of the promoter in some cellular environments, a result which could point to the LTR as one determinant of HTLV-I cell tropism in vivo.

Correlation between P19 presence and MHC class II expression in human fetal astroglial cells cocultured with HTLV-I donor cells

The possibility of a direct infection of human brain by HTLV-I, has been studied using an in vitro model. Human fetal astroglial cells were cocultivated with irradiated HTLV-I donor cell line MT-2, and assayed for the presence of HTLV-I core protein p19 after 1 week. Fifty-six per cent of GFAP positive astrocytes showed the viral core protein p19 and increased expression of Class II MHC antigens. Electron microscopy of astroglial cells exposed to HTLV-I revealed the presence of vacuoli-like structures containing viral core protein p19. Cell intermediate filament cytoskeleton was also disorganized. Even if this study does not provide direct evidence for virus replication inside astroglial cells, all these findings suggest that HTLV-I can indeed enter the cell and exert a cytopathic effect. Therefore the results of the present study are consistent with the hypothesis that astroglial cells could be involved in demyelination processes occurring in the HTLV-I associated neurological disorders, such as human associated myelopathy and tropical spastic paraparesis.

Tissue expression pattern directed in transgenic mice by the LTR of an HTLV-I provirus isolated from a case of tropical spastic paraparesis

Human T lymphotropic virus type I (HTLV-I) causes adult T cell leukemia/lymphoma and a chronic neurological disease called either tropical spastic paraparesis (TSP) or HTLV-I-associated myelopathy. The different outcomes of this infection could be due to both host and viral factors and it has been proposed that genetic differences could make some HTLV-I strains neurotropic. In this paper, we examined the pattern of tissue-specific expression determined by a long terminal repeat (LTR) obtained from a case of TSP. We constructed transgenic mice in which this LTR controlled the expression of the nlslacZ reporter gene. We observed that in three independent lines of transgenic mice, the reporter gene was expressed predominantly in the central nervous system (CNS), in choroid plexus, and in cells of the hippocampus and cerebellum. Our observations indicate the existence of CNS cells permissive for the expression of HTLV-I and which may be of importance in the pathogenesis of TSP.

Infection of peripheral blood mononuclear cells and cell lines by cell-free human T-cell lymphoma/leukemia virus type I

Previous studies of in vitro infection by human T-cell lymphoma/leukemia virus type I (HTLV-I) have required cocultivation of target cells with HTLV-I cell lines or vesicular stomatitis virus pseudotypes containing HTLV-I envelope proteins. We report here the development of a cell-free infection assay for HTLV-I. Target cells were incubated with purified, DNase-treated HTLV-I virions for 4 h at 37 degrees C. Target cell DNA was then analyzed for the presence of newly synthesized HTLV-I proviral DNA by the highly sensitive polymerase chain reaction. Using this assay system, we have been able to consistently detect in vitro infection of a variety of cellular targets by different HTLV-I isolates. Optimal infection required the presence of 10 micrograms of DEAE-dextran per ml. The assay was dose dependent with respect to virus input. In general, the amount of proviral DNA detected correlated with the level of HTLV-I receptors present on the surface of the target cells, as measured by fluorochrome-labelled HTLV-I binding. Finally, the specificity of the assay was confirmed by demonstrating that the cell line, L1q, a somatic cell hybrid containing human chromosome 17q, to which the gene for the HTLV-I receptor has been mapped, was susceptible to infection by HTLV-I, while the parental mouse cell line from which it was derived, LMTK-, which lacks human chromosome 17q, was not.

Effect of human T lymphotropic retrovirus-I exposure on cultured human glioma cell lines

Four different human tumor cell lines of glial origin have been exposed to a human T lymphotropic retrovirus (HTLV-I). All these cell lines were positive for the glial marker glial fibrillary acidic protein (GFAP). The presence of virus RNA was demonstrated by in situ hybridization using an HTLV-I, SStI-SStI viral insert as probe. Virus expression has been monitored through an indirect immunofluorescence assay using a monoclonal antibody against virus core protein p19. All the four glioma cell lines tested became positive for p19 after 2 weeks of co-cultivation and showed a clear alteration of GFAP expression.

In vivo cellular tropism of human T-cell leukemia virus type 1

To establish the phenotype of human T-cell leukemia virus type 1 (HTLV-1)-infected cells in peripheral blood, the polymerase chain reaction was used to detect and quantitate viral DNA in subpopulations of leukocytes obtained from patients with tropical spastic paraparesis and asymptomatic carriers. HTLV-1 could not be detected in peripheral blood mononuclear cells thoroughly depleted of T lymphocytes (E- CD3-), nor could it be detected in highly enriched populations of B lymphocytes (E- CD19+), monocytes (E- CD14+), or natural killer cells (E- CD16+). T lymphocytes were strongly positive for HTLV-1, and fractionation of this population revealed that 90 to 99% of the HTLV-1 DNA segregated with the CD4+ CD8- and CD45RO+ subsets. No difference between the cell type distribution of HTLV-1 in the asymptomatic carrier and the subjects with tropical spastic paraparesis was evident. Southern hybridization of genomic DNA prepared from the peripheral blood of HTLV-1 carriers indicated that up to 10% of circulating leukocytes may carry the HTLV-1 provirus.