Characterization of an infectious molecular clone of human T-cell leukemia virus type I

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.

The Past, Present, and Future of a Human T-Cell Leukemia Virus Type 1 Vaccine

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic human retrovirus which causes a lifelong infection. An estimated 5-10 million persons are infected with HTLV-1 worldwide - a number which is likely higher due to lack of reliable epidemiological data. Most infected individuals remain asymptomatic; however, a portion of HTLV-1-positive individuals will develop an aggressive CD4+ T-cell malignancy called adult T-cell leukemia/lymphoma (ATL), or a progressive neurodegenerative disease known as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Few treatment options exist for HAM/TSP outside of palliative care and ATL carries an especially poor prognosis given the heterogeneity of the disease and lack of effective long-term treatments. In addition, the risk of HTLV-1 disease development increases substantially if the virus is acquired early in life. Currently, there is no realistic cure for HTLV-1 infection nor any reliable measure to prevent HTLV-1-mediated disease development. The severity of HTLV-1-associated diseases (ATL, HAM/TSP) and limited treatment options highlights the need for development of a preventative vaccine or new therapeutic interventions. This review will highlight past HTLV-1 vaccine development efforts, the current molecular tools and animal models which might be useful in vaccine development, and the future possibilities of an effective HTLV-1 vaccine.

Repression of Human T-lymphotropic virus type 1 Long Terminal Repeat sense transcription by Sp1 recruitment to novel Sp1 binding sites

Human T-lymphotropic Virus type 1 (HTLV-1) infection is characterized by viral latency in the majority of infected cells and by the absence of viremia. These features are thought to be due to the repression of viral sense transcription in vivo. Here, our in silico analysis of the HTLV-1 Long Terminal Repeat (LTR) promoter nucleotide sequence revealed, in addition to the four Sp1 binding sites previously identified, the presence of two additional potential Sp1 sites within the R region. We demonstrated that the Sp1 and Sp3 transcription factors bound in vitro to these two sites and compared the binding affinity for Sp1 of all six different HTLV-1 Sp1 sites. By chromatin immunoprecipitation experiments, we showed Sp1 recruitment in vivo to the newly identified Sp1 sites. We demonstrated in the nucleosomal context of an episomal reporter vector that the Sp1 sites interfered with both the sense and antisense LTR promoter activities. Interestingly, the Sp1 sites exhibited together a repressor effect on the LTR sense transcriptional activity but had no effect on the LTR antisense activity. Thus, our results demonstrate the presence of two new functional Sp1 binding sites in the HTLV-1 LTR, which act as negative cis-regulatory elements of sense viral transcription.

HTLV-1 Tax activates HIV-1 transcription in latency models

HIV-1 latency is a major obstacle to HIV-1 eradication. Coinfection with HTLV-1 has been associated with faster progression to AIDS. HTLV-1 encodes the transactivator Tax which can activate both HTLV-1 and HIV-1 transcription. Here, we demonstrate that Tax activates HIV transcription in latent CD4⁺ T cells. Tax promotes the activation of P-TEFb, releasing CDK9 and Cyclin T1 from inactive forms, promoting transcription elongation and reactivation of latent HIV-1. Tax mutants lacking interaction with the HIV-1-LTR promoter were not able to activate P-TEFb, with no subsequent activation of latent HIV. In HIV-infected primary resting CD4⁺ T cells, Tax-1 reactivated HIV-1 transcription up to five fold, confirming these findings in an ex vivo latency model. Finally, our results confirms that HTLV-1/Tax hijacks cellular partners, promoting HIV-1 transcription, and this interaction should be further investigated in HIV-1 latency studies in patients with HIV/HTLV-1 co-infection.

Animals Models of Human T Cell Leukemia Virus Type I Leukemogenesis

Infection with human T cell leukemia virus type I (HTLV-I) causes adult T cell leukemia (ATL) in a minority of infected individuals after long periods of viral persistence. The various stages of HTLV-I infection and leukemia development are studied by using several different animal models: (1) the rabbit (and mouse) model of persistent HTLV-I infection, (2) transgenic mice to model tumorigenesis by HTLV-I specific protein expression, (3) ATL cell transfers into immune-deficient mice, and (4) infection of humanized mice with HTLV-I. After infection, virus replicates without clinical disease in rabbits and to a lesser extent in mice. Transgenic expression of both the transactivator protein (Tax) and the HTLV-I bZIP factor (HBZ) protein have provided insight into factors important in leukemia/lymphoma development. To investigate factors relating to tumor spread and tissue invasion, a number of immune-deficient mice based on the severe combined immunodeficiency (SCID) or non-obese diabetic/SCID background have been used. Inoculation of adult T cell leukemia cell (lines) leads to lymphoma with osteolytic bone lesions and to a lesser degree to leukemia development. These mice have been used extensively for the testing of anticancer drugs and virotherapy. A recent development is the use of so-called humanized mice, which, upon transfer of CD34(+)human umbilical cord stem cells, generate human lymphocytes. Infection with HTLV-I leads to leukemia/lymphoma development, thus providing an opportunity to investigate disease development with the aid of molecularly cloned viruses. However, further improvements of this mouse model, particularly in respect to the development of adaptive immune responses, are necessary.

Characterization of New Zealand White Rabbit Gut-Associated Lymphoid Tissues and Use as Viral Oncology Animal Model

Rabbits have served as a valuable animal model for the pathogenesis of various human diseases, including those related to agents that gain entry through the gastrointestinal tract such as human T cell leukemia virus type 1. However, limited information is available regarding the spatial distribution and phenotypic characterization of major rabbit leukocyte populations in mucosa-associated lymphoid tissues. Herein, we describe the spatial distribution and phenotypic characterization of leukocytes from gut-associated lymphoid tissues (GALT) from 12-week-old New Zealand White rabbits. Our data indicate that rabbits have similar distribution of leukocyte subsets as humans, both in the GALT inductive and effector sites and in mesenteric lymph nodes, spleen, and peripheral blood. GALT inductive sites, including appendix, cecal tonsil, Peyer's patches, and ileocecal plaque, had variable B cell/T cell ratios (ranging from 4.0 to 0.8) with a predominance of CD4 T cells within the T cell population in all four tissues. Intraepithelial and lamina propria compartments contained mostly T cells, with CD4 T cells predominating in the lamina propria compartment and CD8 T cells predominating in the intraepithelial compartment. Mesenteric lymph node, peripheral blood, and splenic samples contained approximately equal percentages of B cells and T cells, with a high proportion of CD4 T cells compared with CD8 T cells. Collectively, our data indicate that New Zealand White rabbits are comparable with humans throughout their GALT and support future studies that use the rabbit model to study human gut-associated disease or infectious agents that gain entry by the oral route.

BLV-CoCoMo-qPCR-2: improvements to the BLV-CoCoMo-qPCR assay for bovine leukemia virus by reducing primer degeneracy and constructing an optimal standard curve

Bovine leukemia virus (BLV) is the etiological agent of enzootic bovine leukosis, which is the most common neoplastic disease of cattle. Because BLV infection can remain clinically silent, the proviral load is an important index for estimating disease progression. CoCoMo-qPCR, an assay developed to estimate BLV proviral load, allows the highly sensitive detection of BLV originating in different countries. Here, we developed a modified version of the CoCoMo-qPCR assay, the "BLV-CoCoMo-qPCR-2" assay, which uses optimized degenerate primers. We also constructed a new plasmid standard. Finally, we used both assays to examine DNA samples from BLV-infected cattle and compared the results.

Animal Models Utilized in HTLV-1 Research

Since the isolation and discovery of human T-cell leukemia virus type 1 (HTLV-1) over 30 years ago, researchers have utilized animal models to study HTLV-1 transmission, viral persistence, virus-elicited immune responses, and HTLV-1-associated disease development (ATL, HAM/TSP). Non-human primates, rabbits, rats, and mice have all been used to help understand HTLV-1 biology and disease progression. Non-human primates offer a model system that is phylogenetically similar to humans for examining viral persistence. Viral transmission, persistence, and immune responses have been widely studied using New Zealand White rabbits. The advent of molecular clones of HTLV-1 has offered the opportunity to assess the importance of various viral genes in rabbits, non-human primates, and mice. Additionally, over-expression of viral genes using transgenic mice has helped uncover the importance of Tax and Hbz in the induction of lymphoma and other lymphocyte-mediated diseases. HTLV-1 inoculation of certain strains of rats results in histopathological features and clinical symptoms similar to that of humans with HAM/TSP. Transplantation of certain types of ATL cell lines in immunocompromised mice results in lymphoma. Recently, “humanized” mice have been used to model ATL development for the first time. Not all HTLV-1 animal models develop disease and those that do vary in consistency depending on the type of monkey, strain of rat, or even type of ATL cell line used. However, the progress made using animal models cannot be understated as it has led to insights into the mechanisms regulating viral replication, viral persistence, disease development, and, most importantly, model systems to test disease treatments.

The cellular autophagy pathway modulates human T-cell leukemia virus type 1 replication

Autophagy, a general homeostatic process for degradation of cytosolic proteins or organelles, has been reported to modulate the replication of many viruses. The role of autophagy in human T-cell leukemia virus type 1 (HTLV-1) replication has, however, been uncharacterized. Here, we report that HTLV-1 infection increases the accumulation of autophagosomes and that this accumulation increases HTLV-1 production. We found that the HTLV-1 Tax protein increases cellular autophagosome accumulation by acting to block the fusion of autophagosomes to lysosomes, preventing the degradation of the former by the latter. Interestingly, the inhibition of cellular autophagosome-lysosome fusion using bafilomycin A increased the stability of the Tax protein, suggesting that cellular degradation of Tax occurs in part through autophagy. Our current findings indicate that by interrupting the cell's autophagic process, Tax exerts a positive feedback on its own stability.

Antibody to heat shock protein 70 (HSP70) inhibits human T-cell lymphoptropic virus type I (HTLV-I) production by transformed rabbit T-cell lines

Adult T cell leukemia is a fatal malignant transformation caused by the human T-cell lymphoptropic virus type I (HTLV-I). HTLV-I is only associated with the development of this disease in a small percentage of infected individuals. Using two rabbit transformed T-cell lines; RH/K30 (asymptomatic) and RH/K34 (leukemogenic), we have investigated the expression of heat shock proteins (HSP) 90 and 70 and the role of anti-HSPs antibodies on virus production. HSPs surface expression was higher on RH/K34 than RH/K30 cells. Heat treatment of cells increased the expression of HSPs proteins and virus production; HSPs augmentation was stabilized after 12 h and virus production reached a maximum between 8 h-12 h then returned to normal level after 24 h of culture. Incubation of cells only with rabbit anti-HSP 70 antibodies prevented virus production specifically in the leukemogenic cell line. The results indicate a relationship between HSP 70 and virus production.

Visualizing spatiotemporal dynamics of apoptosis after G1 arrest by human T cell leukemia virus type 1 Tax and insights into gene expression changes using microarray-based gene expression analysis

Background

Human T cell leukemia virus type 1 (HTLV-1) Tax is a potent activator of viral and cellular gene expression that interacts with a number of cellular proteins. Many reports show that Tax is capable of regulating cell cycle progression and apoptosis both positively and negatively. However, it still remains to understand why the Tax oncoprotein induces cell cycle arrest and apoptosis, or whether Tax-induced apoptosis is dependent upon its ability to induce G₁ arrest. The present study used time-lapse imaging to explore the spatiotemporal patterns of cell cycle dynamics in Tax-expressing HeLa cells containing the fluorescent ubiquitination-based cell cycle indicator, Fucci2. A large-scale host cell gene profiling approach was also used to identify the genes involved in Tax-mediated cell signaling events related to cellular proliferation and apoptosis.

Results

Tax-expressing apoptotic cells showed a rounded morphology and detached from the culture dish after cell cycle arrest at the G₁ phase. Thus, it appears that Tax induces apoptosis through pathways identical to those involved in G₁ arrest. To elucidate the mechanism(s) by which Tax induces cell cycle arrest and apoptosis, regulation of host cellular genes by Tax was analyzed using a microarray containing approximately 18,400 human mRNA transcripts. Seventeen genes related to cell cycle regulation were identified as being up or downregulated > 2.0-fold in Tax-expressing cells. Several genes, including SMAD3, JUN, GADD45B, DUSP1 and IL8, were involved in cellular proliferation, responses to cellular stress and DNA damage, or inflammation and immune responses. Additionally, 23 pro- and anti-apoptotic genes were deregulated by Tax, including TNFAIP3, TNFRS9, BIRC3 and IL6. Furthermore, the kinetics of IL8, SMAD3, CDKN1A, GADD45A, GADD45B and IL6 expression were altered following the induction of Tax, and correlated closely with the morphological changes observed by time-lapse imaging.

Conclusions

Taken together, the results of this study permit a greater understanding of the biological events affected by HTLV-1 Tax, particularly the regulation of cellular proliferation and apoptosis. Importantly, this study is the first to demonstrate the dynamics of morphological changes during Tax-induced apoptosis after cell cycle arrest at the G₁ phase.

Molecular determinants of human T-lymphotropic virus type 1 transmission and spread

Human T-lymphotrophic virus type-1 (HTLV-1) infects approximately 15 to 20 million people worldwide, with endemic areas in Japan, the Caribbean, and Africa. The virus is spread through contact with bodily fluids containing infected cells, most often from mother to child through breast milk or via blood transfusion. After prolonged latency periods, approximately 3 to 5% of HTLV-1 infected individuals will develop either adult T-cell leukemia/lymphoma (ATL), or other lymphocyte-mediated disorders such as HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The genome of this complex retrovirus contains typical gag, pol, and env genes, but also unique nonstructural proteins encoded from the pX region. These nonstructural genes encode the Tax and Rex regulatory proteins, as well as novel proteins essential for viral spread in vivo such as, p30, p12, p13 and the antisense encoded HBZ. While progress has been made in the understanding of viral determinants of cell transformation and host immune responses, host and viral determinants of HTLV-1 transmission and spread during the early phases of infection are unclear. Improvements in the molecular tools to test these viral determinants in cellular and animal models have provided new insights into the early events of HTLV-1 infection. This review will focus on studies that test HTLV-1 determinants in context to full length infectious clones of the virus providing insights into the mechanisms of transmission and spread of HTLV-1.

BLV-CoCoMo-qPCR: Quantitation of bovine leukemia virus proviral load using the CoCoMo algorithm

Background

Bovine leukemia virus (BLV) is closely related to human T-cell leukemia virus (HTLV) and is the etiological agent of enzootic bovine leukosis, a disease characterized by a highly extended course that often involves persistent lymphocytosis and culminates in B-cell lymphomas. BLV provirus remains integrated in cellular genomes, even in the absence of detectable BLV antibodies. Therefore, to understand the mechanism of BLV-induced leukemogenesis and carry out the selection of BLV-infected animals, a detailed evaluation of changes in proviral load throughout the course of disease in BLV-infected cattle is required. The aim of this study was to develop a new quantitative real-time polymerase chain reaction (PCR) method using Coordination of Common Motifs (CoCoMo) primers to measure the proviral load of known and novel BLV variants in clinical animals.

Results

Degenerate primers were designed from 52 individual BLV long terminal repeat (LTR) sequences identified from 356 BLV sequences in GenBank using the CoCoMo algorithm, which has been developed specifically for the detection of multiple virus species. Among 72 primer sets from 49 candidate primers, the most specific primer set was selected for detection of BLV LTR by melting curve analysis after real-time PCR amplification. An internal BLV TaqMan probe was used to enhance the specificity and sensitivity of the assay, and a parallel amplification of a single-copy host gene (the bovine leukocyte antigen DRA gene) was used to normalize genomic DNA. The assay is highly specific, sensitive, quantitative and reproducible, and was able to detect BLV in a number of samples that were negative using the previously developed nested PCR assay. The assay was also highly effective in detecting BLV in cattle from a range of international locations. Finally, this assay enabled us to demonstrate that proviral load correlates not only with BLV infection capacity as assessed by syncytium formation, but also with BLV disease progression.

Conclusions

Using our newly developed BLV-CoCoMo-qPCR assay, we were able to detect a wide range of mutated BLV viruses. CoCoMo algorithm may be a useful tool to design degenerate primers for quantification of proviral load for other retroviruses including HTLV and human immunodeficiency virus type 1.

Construction and characterization of a full-length infectious simian T-cell lymphotropic virus type 3 molecular clone

Together with their simian T-cell lymphotropic virus (STLV) equivalent, human T-cell lymphotropic virus type 1 (HTLV-1), HTLV-2, and HTLV-3 form the primate T-cell lymphotropic virus (PTLV) group. Over the years, understanding the biology and pathogenesis of HTLV-1 and HTLV-2 has been widely improved by the creation of molecular clones. In contrast, so far, PTLV-3 experimental studies have been restricted to the overexpression of the tax gene using reporter assays. We have therefore decided to construct an STLV-3 molecular clone. We generated a full-length STLV-3 proviral clone (8,891 bp) by PCR amplification of overlapping fragments. This STLV-3 molecular clone was then transfected into 293T cells. Reverse transcriptase PCR experiments followed by sequence analysis of the amplified products allowed us to establish that both gag and tax/rex mRNAs were transcribed. Western blotting further demonstrated the presence of the STLV-3 p24gag protein in the cell culture supernatant from transfected cells. Transient transfection of 293T cells and of 293T-long terminal repeat-green fluorescent protein cells with the STLV-3 clone promoted syncytium formation, a hallmark of PTLV Env expression, as well as the appearance of fluorescent cells, also demonstrating that the Tax3 protein was expressed. Virus particles were visible by electron microscopy. These particles are infectious, as demonstrated by our cell-free-infection experiments with purified virions. All together, our data demonstrate that the STLV-3 molecular clone is functional and infectious. This clone will give us a unique opportunity to study in vitro the different pX transcripts and the putative presence of antisense transcripts and to evaluate the PTLV-3 pathogenicity in vivo.

Constitutive and induced activation of JAK/Stat pathway in leukemogenic and asymptomatic human T-cell lymphoptropic virus type 1 (HTLV-1) transformed rabbit cell lines

We have shown that Vav and C-cbl are activated in the leukemogenic HTLV-I transformed rabbit T cell line RH/K34 but not in the asymptomatic one RH/K30. We extended these observations and investigated the activation of JAKs (Janus Kinase) and the STATs (signal transducers and activators of transcription) pathway in these cell lines. We found that Tyk2 and Stat3 are constitutively tyrosine phosphorylated in the leukemogenic cell line. Phosphorylation of Tyk2 can be induced in RH/K30 by treatment with IL-10, interferon alpha (INFalpha) and by the supernatant of RH/K34 which contain both these cytokines. Stat3 tyrosine phosphorylation can be induced in RH/K30 by treatment with IL-10. Transfection of RL-5, a rabbit T-cell line, with the RH/K34 viral clone transiently increased the expression of serine/threonine phosphorylated Stat3. Differences were also observed on induced Stat5 phosphorylation. These results highlight the relation between the virulence of HTLV-I and the activation of the Jak/Stat pathway.

HTLV-I antisense transcripts initiating in the 3'LTR are alternatively spliced and polyadenylated

BACKGROUND

Antisense transcription in retroviruses has been suggested for both HIV-1 and HTLV-I, although the existence and coding potential of these transcripts remain controversial. Thorough characterization is required to demonstrate the existence of these transcripts and gain insight into their role in retrovirus biology.

RESULTS

This report provides the first complete characterization of an antisense retroviral transcript that encodes the previously described HTLV-I HBZ protein. In this study, we show that HBZ-encoding transcripts initiate in the 3' long terminal repeat (LTR) at several positions and consist of two alternatively spliced variants (SP1 and SP2). Expression of the most abundant HBZ spliced variant (SP1) could be detected in different HTLV-I-infected cell lines and importantly in cellular clones isolated from HTLV-I-infected patients. Polyadenylation of HBZ RNA occurred at a distance of 1450 nucleotides downstream of the HBZ stop codon in close proximity of a typical polyA signal. We have also determined that translation mostly initiates from the first exon located in the 3' LTR and that the HBZ isoform produced from the SP1 spliced variant demonstrated inhibition of Tax and c-Jun-dependent transcriptional activation.

CONCLUSION

These results conclusively demonstrate the existence of antisense transcription in retroviruses, which likely plays a role in HTLV-I-associated pathogenesis through HBZ protein synthesis.

Ataxia-telangiectasia-mutated (ATM) protein can enhance human immunodeficiency virus type 1 replication by stimulating Rev function

The ataxia-telangiectasia-mutated (ATM) kinase plays a central role in responses to various forms of DNA damage and has been suggested to facilitate human immunodeficiency virus type 1 (HIV-1) integration. Here, we describe a series of experiences that indicate that ATM can enhance HIV-1 replication by stimulating the action of the Rev viral posttranscriptional regulator. The Rev-dependent stimulation of viral late gene expression was observed with ATM-overexpressing cells, a result confirmed with a Rev-dependent reporter construct. Both parameters were also enhanced upon treatment of HeLa cells with caffeine, a xanthine that, in this cellular context, stimulates ATM activity. As well, decreased levels of virions with reduced infectivity were released by ATM knockdown cells. Notably, ATM overexpression did not stimulate the HIV-1 late gene expression within the context of Rev-independent constructs or the Rex-dependent production of capsid from human T-cell leukemia virus type 1 proviral constructs. Altogether, these results indicate that ATM can positively influence HIV-1 Rev function.

Animal models for human T-lymphotropic virus type 1 (HTLV-1) infection and transformation

Over the past 25 years, animal models of human T-lymphotropic virus type 1 (HTLV-1) infection and transformation have provided critical knowledge about viral and host factors in adult T-cell leukemia/lymphoma (ATL). The virus consistently infects rabbits, some non-human primates, and to a lesser extent rats. In addition to providing fundamental concepts in viral transmission and immune responses against HTLV-1 infection, these models have provided new information about the role of viral proteins in carcinogenesis. Mice and rats, in particular immunodeficient strains, are useful models to assess immunologic parameters mediating tumor outgrowth and therapeutic invention strategies against lymphoma. Genetically altered mice including both transgenic and knockout mice offer important models to test the role of specific viral and host genes in the development of HTLV-1-associated lymphoma. Novel approaches in genetic manipulation of both HTLV-1 and animal models are available to address the complex questions that remain about viral-mediated mechanisms of cell transformation and disease. Current progress in the understanding of the molecular events of HTLV-1 infection and transformation suggests that answers to these questions are approachable using animal models of HTLV-1-associated lymphoma.

A new sensitive and quantitative HTLV-I-mediated cell fusion assay in T cells

Similar to several other viruses, human T cell leukemia virus type I (HTLV-I) induces the formation of multinucleated giant cells (also known as syncytium) when amplified in tissue culture. These syncytia result from the fusion of infected cells with uninfected cells. Due to the intrinsic difficulty of infecting cells with cell-free HTLV-I virions, syncytium formation has become an important tool in the study of HTLV-I infection and transmission. Since most HTLV-I-based cell fusion assays rely on the use of non-T cells, the aim of this study was to optimize a new HTLV-I-induced cell fusion assay in which HTLV-I-infected T cell lines are co-cultured with T cells that have been transfected with an HTLV-I long terminal repeat (LTR) luciferase reporter construct. We demonstrate that co-culture of various HTLV-I-infected T cells with different transfected T cell lines resulted in induction of luciferase activity. Cell-to-cell contact and expression of the viral gp46 envelope protein was crucial for this induction while other cell surface proteins (including HSC70) did not have a significant effect. This quantitative assay was shown to be very sensitive. In this assay, the cell fusion-mediated activation of NF-kappaB and the HTLV-I LTR occurred through previously described Tax-dependent signaling pathways. This assay also showed that cell fusion could activate Tax-inducible cellular promoters. These results thus demonstrate that this new quantitative HTLV-I-dependent cell fusion assay is versatile, highly sensitive, and can provide an important tool to investigate cellular promoter activation and intrinsic signaling cascades that modulate cellular gene expression.

Quantification of HTLV-I proviral load in experimentally infected rabbits

BACKGROUND

Levels of proviral load in HTLV-1 infected patients correlate with clinical outcome and are reasonably prognostic. Adaptation of proviral load measurement techniques is examined here for use in an experimental rabbit model of HTLV-1 infection. Initial efforts sought to correlate proviral load with route and dose of inoculation and with clinical outcome in this model. These methods contribute to our continuing goal of using the model to test treatments that alleviate virus infection.

RESULTS

A real-time PCR assay was used to measure proviral load in blood and tissue samples from a series of rabbits infected using HTLV-1 inocula prepared as either cell-free virus particles, infected cells or blood, or by naked DNA injection. Proviral loads from asymptomatically infected rabbits showed levels corresponding to those reported for human patients with clinically silent HTLV-1 infections. Proviral load was comparably increased in 50% of experimentally infected rabbits that developed either spontaneous benign or malignant tumors while infected. Similarly elevated provirus was found in organs of rabbits with experimentally induced acute leukemia/lymphoma-like disease. Levels of provirus in organs taken at necropsy varied widely suggesting that reservoirs of infections exist in non-lymphoid organs not traditionally thought to be targets for HTLV-1.

CONCLUSION

Proviral load measurement is a valuable enhancement to the rabbit model for HTLV-1 infection providing a metric to monitor clinical status of the infected animals as well as a means for the testing of treatment to combat infection. In some cases proviral load in blood did not reflect organ proviral levels, revealing a limitation of this method for monitoring health status of HTLV-1 infected individuals.

APOBEC3G targets human T-cell leukemia virus type 1

Background

Apolipoprotein B mRNA-editing enzyme-catalytic polypeptide-like 3G (APOBEC3G) is a host cellular protein with a broad antiviral activity. It inhibits infectivitiy of a wide variety of retroviruses by deaminating deoxycytidine (dC) into deoxyuridine (dU) in newly synthesized minus strand DNA, resulting in G-to-A hypermutation of the viral plus strand DNA. To clarify the mechanism of its function, we have examined the antiviral activity of APOBEC3G on human T-cell leukemia virus type 1 (HTLV-1), the first identified human retrovirus.

Results

In this study, we have demonstrated that overexpressed as well as endogenous APOBEC3G were incorporated into HTLV-1 virions and that APOBEC3G inhibited the infection of HTLV-1. Interestingly, several inactive mutants of APOBEC3G also inhibited HTLV-1 and no G-to-A hypermutation was induced by APOBEC3G in HTLV-1 genome. Furthermore, we introduced the human immunodeficiency virus type 1 (HIV-1) vif gene into HTLV-1 producing cell line, MT-2, to antagonize APOBEC3G by reducing its intracellular expression and virion incorporation, which resulted in upregulation of the infectivity of produced viruses.

Conclusion

APOBEC3G is incorporated into HTLV-1 virions and inhibits the infection of HTLV-1 without exerting its cytidine deaminase activity. These results suggest that APOBEC3G might act on HTLV-1 through different mechanisms from that on HIV-1 and contribute to the unique features of HTLV-1 infection and transmission.

Activation of HTLV-I gene transcription by protein tyrosine phosphatase inhibitors

Human T-cell leukemia virus type I (HTLV-I) transcription generally depends on the ability of the viral Tax protein to bind the CREB transcription factor and form an active complex by recruiting CBP/p300 coactivators to the long terminal repeat (LTR). Studies have demonstrated that T-cell activating agents that stimulate CREB are potent inducers of HTLV-I transcription. Herein, we demonstrate that bpV[pic], a protein tyrosine phosphatase (PTP) inhibitor activates the HTLV-I LTR in the presence and absence of Tax expression. Optimal activation occurred at 8 h and was synergistic with forskolin or PGE(2). Infected cell lines and cells transfected with HTLV-I proviral DNA were equally responsive to the synergistic effect of bpV and forskolin on HTLV-I gene expression. Activation of the LTR by bpV[pic] was T-cell receptor-independent, but required ZAP70, calcineurin activity and functional calcium entry. Inhibition of the SHP-1 PTP was suggested to be important. Transfection experiments with a CREB dominant-negative mutant and with isolated TRE1- or CREB-responsive reporter constructs and treatment with the MDL-12,330A adenylate cyclase inhibitor all supported the involvement of a CREB/ATF family member in this bpV-dependent activation of the HTLV-I LTR, although CREB itself did not seem to be involved. Analysis of HTLV-I reporter constructs containing mutated CREB-binding sites also implied the involvement of another element in this activation. These results demonstrate for the first time a powerful effect of PTP inhibitors on HTLV-I LTR activity and suggest participation of both CREB-dependent and -independent pathways in this activation.

Construction of a full-length human T cell leukemia virus type I genome from MT-2 cells containing multiple defective proviruses using overlapping polymerase chain reaction

Human T cell leukemia virus type I (HTLV-I), the etiological agent of adult T cell leukemia, integrates into the host genome as a provirus. Multiple defective copies of the integrated provirus are often present in the host genome. For this reason it is difficult to clone the intact provirus from HTLV-I-infected cells using conventional techniques. Here, we used overlapping polymerase chain reaction (PCR) to construct a full-length provirus of HTLV-I directly from an HTLV-I-transformed cell line, MT-2, which contains multiple defective proviruses. First, four overlapping proviral HTLV-I fragments (1.4-3.9 kb each) were constructed from genomic MT-2 DNA using PCR. Next, the complete HTLV-I proviral DNA (9 kb) was generated from these fragments using asymmetric PCR and cloned into a plasmid vector. 293 T cells transfected with this plasmid produced virus-like particles, and we show that these particles are capable of infecting a human T cell line. We propose that this cloning technique constitutes a powerful tool for constructing infectious molecular clones from cells of patients infected with HTLV-I or other viruses.

Purifying Selection Masks the Mutational Flexibility of HIV-1 Reverse Transcriptase

DNA and RNA polymerases share a core architecture composed of three structurally conserved motifs: A, B, and C. Although the amino acid sequences of these motifs are highly conserved between closely related organisms, variation across broader evolutionary distances suggests that only a few residues in each motif are indispensable for polymerase function. To test this, we constructed libraries of human immunodeficiency virus type-1 (HIV-1) containing random single amino acid replacements in motif B of reverse transcriptase (RT), and we used selection in culture to assess RT function. Despite the nearly absolute constancy of motif B in vivo, virus replicating in culture tolerated a range of conservative and nonconservative substitutions at 10 of the 11 amino acid positions examined. These included residues that are invariant across all retroviral subfamilies and highly conversed in diverse retroelements. Several mutants retained wild type infectivity, and serial passage experiments revealed replacements that were neutral or even beneficial to viral fitness. In addition, a number of the selected variants exhibited altered susceptibility to the nucleoside analog inhibitors AZT and 3TC. Taken together, these data indicate that HIV-1 tolerates a range of substitutions at conserved RT residues and that selection against slightly deleterious mutations (purifying selection) in vivo masks a large repertoire of viable phenotypic variants. This mutational flexibility likely contributes to HIV-1 evolution in response to changing selection pressures in infected individuals.

Development of a novel surrogate virus for human T-cell leukemia virus type 1: inhibition of infection by osteoprotegerin

To develop a high-titer surrogate virus for human T-cell leukemia virus type 1 (HTLV-1), we generated recombinant vesicular stomatitis viruses (VSVs) in which the gene encoding the single transmembrane glycoprotein (G) was deleted. Genes encoding HTLV-1 envelope glycoproteins (HTEnv) or HTEnvG hybrid proteins were then inserted into either of two different sites in the VSV genome. The viruses also encoded a green fluorescent protein. With this surrogate virus, we found that a soluble protein, osteoprotegerin (OPG), or an OPG/Fc chimeric protein inhibited the infection of various cell lines. Our experiments indicate that this inhibition resulted from binding of heparan sulfate by OPG.

Seizing of T Cells by Human T-Cell Leukemia⧸Lymphoma Virus Type 1

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) causes neoplastic transformation of human T-cells in a small number of infected individuals several years from infection. Several viral proteins act in concert to increase the responsiveness of T-cells to extracellular stimulation, modulate proapoptotic and antiapoptotic gene signals, enhance T-cell survival, and avoid immune recognition of the infected T-cells. The virus promotes T-cell proliferation by usurping several signaling pathways central to immune T-cell function. Viral proteins modulate the downstream effects of antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation are therefore important, as also suggested by epidemiological data. The ability of a given individual to respond to specific antigens is determined genetically. Thus, genetic and environmental factors, together with the virus, contribute to disease development. As in the case of other virus-associated cancers, HTLV-1-induced leukemia/lymphoma can be prevented by avoiding viral infection or by intervention during the asymptomatic phase with approaches able to interrupt the vicious cycle of virus-induced proliferation of a subset of T-cells. This review focuses on current knowledge of the mechanisms regulating HTLV-1 replication and the T-cell pathways that are usurped by viral proteins to induce and maintain clonal proliferation of infected T-cells in vitro. The relevance of these laboratory findings will be related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Monoclonally integrated HTLV type 1 in epithelial cancers from rabbits infected with an HTLV type 1 molecular clone

In addition to T cell leukemias and lymphomas, human T cell leukemia virus type 1 (HTLV-1) infection has been associated with nonhematologic malignancies and described as the cause of one case of small-cell lung carcinoma. Infected primary epithelial cells have been isolated from sweat gland and oral mucosae of HTLV-1-infected human patients. In the present study, epithelial neoplasms developed in two rabbits experimentally infected with a molecular clone of HTLV-1 (strain K30p). Serologic detection of anti-HTLV-1 and isolation of virus from blood lymphocytes at multiple time points postinjection established a course of chronic asymptomatic infection in both. One rabbit, infected for 5.5 years after intramuscular injection of HTLV-1 DNA, developed a thymoma having features of medullary differentiation. HTLV-1 provirus was detected in both thymocytes and neoplastic epithelium isolated discretely from the thymoma by laser capture microdissection. These findings provide the first experimental evidence of HTLV-1 disease after infection by HTLV-1 DNA injection. Endometrial adenocarcinoma occurred in a second rabbit 2.5 years after its inoculation with cell-associated virus. In this second case, an epithelial cell line derived ex vivo from a metastatic lesion produced virus in culture. In tumors from each of the two rabbits, the neoplastic epithelium was infected and harbored monoclonally integrated HTLV-1 provirus. Although monoclonal provirus integration alone does not establish retroviral cause of carcinogenesis unequivocally, these and other accumulating data indicate that there may be a role for HTLV-1 in diseases associated with infection of epithelia, including some epithelial cancers.

HTLV-1 cell lines differ in constitutively activated signaling pathways that can be altered by cytokine exposure

Examination of signaling pathways used by HTLV-1-infected rabbit cell lines revealed differences between one, RH/K30, that mediates asymptomatic infection and another, RH/K34, that causes lethal experimental leukemia. Both lines are IL-2 independent; RH/K30 produces IL-4 while RH/K34 produces IL-10. Examination of the Jak/STAT (Janus kinase/signal transducer and activator of transcription) activation of the lines revealed constitutive phosphorylation of Jak1 in both STAT6 phosphorylation, not previously reported for HTLV-1 cells, was observed in RH/K30; STAT1 and STAT3 were phosphorylated in RH/K34. Treatment with cytokines altered the activation of the STAT proteins: IL-2 induced STAT5 phosphorylation in both lines. Supernatant from RH/K34 or IL-10 induced STAT3 phosphorylation in RH/K30 cells. Supernatant from RH/K30 or IL-4 induced STAT6 phosphorylation in RH/K34 cells, which could be reversed with a Jak kinase inhibitor--AG-490.

The NH2-terminal domain of the human T-cell leukemia virus type 1 capsid protein is involved in particle formation

The human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus type 1 (HTLV-1) capsid proteins (CA) display similar structures formed by two independently folded N-terminal (NTD) and C-terminal (CTD) domains. To characterize the functions harbored by the HTLV-1 CA domains in particle formation, 12 sites scattered throughout the protein were mutated. The effects of the mutations on Gag membrane binding, proteolytic processing, and virus-like particle secretion were analyzed. It appears that the NTD is the major partner of indirect or direct Gag-Gag interactions. In particular, most of the NTD mutations impaired virion morphogenesis, and no mutation located in the NTD could be fully rescued by coexpression of wild-type Gag. In contrast, the CTD seems not to be involved in Gag-Gag interactions. Nevertheless, an unknown function required for particle formation is located in the CTD. Thus, despite an overall structural similarity between the HIV-1 and HTLV-1 CA proteins, their NTDs and CTDs exhibit different functions.

Evidence for humoral and cellular reactivity against keratin and thyroglobulin in HTLV-I infected rabbits

Human T cell leukemia virus type I (HTLV-I) infection was initially associated with T cell leukemia and a progressive neurologic disease but has since been linked to an increasing number of autoimmune disorders, including Sjogren's syndrome, uveitis, and polyarthritis. A survey of serum samples from a rabbit model of HTLV-I infection revealed that all had antibodies against keratin and thyroglobulin. Sera from several infected rabbits also reacted with collagen, while antibody reactions with other autoantigens tested, including DNA, were rare and sporadic. In addition to antibodies, cellular reactivity to keratin, but not thyroglobulin, was demonstrated by cellular proliferation in presence of IL-2 and keratin. Expanded cell cultures were positive for T cell activation markers and CD8. Association of the auto-reactivity with HTLV-I infection rather than random anti-cellular responses was supported by the fact that no antikeratin or antithyroglobulin was seen in uninfected controls, including that inoculated with uninfected lymphocytes. Finding autoantibodies in rabbits infected using naked HTLV-I DNA clones provided further assurance that infection induced the autoimmune reactions detected.

Differential budding efficiencies of human T-cell leukemia virus type I (HTLV-I) Gag and Gag-Pro polyproteins from insect and mammalian cells

In this study, we examined the ability of human T-cell leukemia virus type I (HTLV-I) Gag and Gag-Pro to assemble immature virus-like particles (VLPs) and bud from insect and mammalian cells. Transmission electron microscopy of insect cells infected with a recombinant baculovirus carrying the entire gag gene revealed that Pr53(Gag) is targeted to the plasma membrane, where it extensively accumulates and forms electron-dense evaginations. However, no particles could be detected either inside the cells or in the culture supernatants. With the Gag-Pro-expressing construct, we observed HTLV-I-specific cytoplasmic proteolysis of the Gag precursor, but again no particle released in the culture supernatants. Transmission electron microscopic analysis of insect cells expressing Gag-Pro polyprotein revealed large vacuoles in the cytoplasm and no budding particles at the plasma membrane. In contrast, human immunodeficiency virus type 1 Gag polyprotein expressed in insect cells is able to release VLPs. These data showed that unlike other retroviruses, Pr53(Gag) is unable to be released as immature VLPs from insect cells. To determine whether the block in particle budding and release is due to an intrinsic property of Pr53(Gag) or the absence of essential cellular factors in insect cells, we expressed Gag and Gag-Pro polyproteins in human 293 cells. The results indicate that Pr53(Gag) and p24 capsid are released within particles into the culture supernatants of human 293 cells. We found that the myristylation of the N-terminal glycine residue is essential for Gag release. Altogether, these results strongly suggest that the proper assembly of HTLV-I particles is dependent on mammalian host cell factors.

The human T-cell leukemia virus type I (HTLV-I) X region encoded protein p13(II) interacts with cellular proteins

Interactions between the Human T-cell leukemia virus type I (HTLV-I) gene product p13(II) and cellular proteins were investigated using the yeast two-hybrid system. Variant forms of p13(II) were derived from two HTLV-I molecular clones, K30p and K34p, that differ in both virus production and in vivo and in vitro infectivity. Two nucleotide differences between the p13 from K30p (p13K30) and K34p (p13K34) result in a Trp-Arg substitution at amino acid 17 and the truncation of the 25 carboxyl-terminal residues of p13K34. A cDNA library from an HTLV-I-infected rabbit T-cell line was screened with p13K30 and p13K34 as bait. Products of two cDNA clones, C44 and C254, interacted with p13K34 but not with p13K30. Interactions were further confirmed using the GST-fusion protein coprecipitation assay. Sequence analysis of C44 and C254 cDNA clones revealed similarities to members of the nucleoside monophosphate kinase superfamily and actin-binding protein 280, respectively. Further analysis of the function of these two proteins and the consequence of their interaction with p13 may help elucidate a role for p13 in virus production, infectivity, or the pathogenesis of HTLV-I.

Definition of an amino-terminal domain of the human T-cell leukemia virus type 1 envelope surface unit that extends the fusogenic range of an ecotropic murine leukemia virus

Murine leukemia viruses (MuLV) and human T-cell leukemia viruses (HTLV) are phylogenetically highly divergent retroviruses with distinct envelope fusion properties. The MuLV envelope glycoprotein surface unit (SU) comprises a receptor-binding domain followed by a proline-rich region which modulates envelope conformational changes and fusogenicity. In contrast, the receptor-binding domain and SU organization of HTLV are undefined. Here, we describe an HTLV/MuLV envelope chimera in which the receptor-binding domain and proline-rich region of the ecotropic MuLV were replaced with the potentially corresponding domains of the HTLV-1 SU. This chimeric HTLV/MuLV envelope was processed, specifically interfered with HTLV-1 envelope-mediated fusion, and similar to MuLV envelopes, required cleavage of its cytoplasmic tail to exert significant fusogenic properties. Furthermore, the HTLV domain defined here broadened ecotropic MuLV envelope-induced fusion to human and simian cell lines.

Passage of human T-cell leukemia virus type-1 during progression to cutaneous T-cell lymphoma results in myelopathic disease in an HTLV-1 infection model

Studies comparing functional differences in human T-cell leukemia virus type 1 (HTLV-1) clones that mediate distinct outcomes in experimentally infected rabbits, resulted in a dermatopathic smoldering adult T-cell leukemia/lymphoma following chronic infection with HTLV-1 strain RH/K34. During the 3.5 years' follow-up, HTLV-1 skin disease progressed to cutaneous T-cell lymphoma. When infection was passed to several naive rabbits, progressive paraparesis due to myelopathic neurodegeneration, analogous to HTLV-associated myelopathy, resulted in one of 4 transfusion recipients. Similar proviral loads were detected in the two diseases, regardless of stage of progression or tissue compartment of infection. Complete proviral sequences obtained from the donor and affected recipient aligned identically with each other and with the inoculated virus clone. Existence of disparate pathogenic outcomes following infectious transmission further extends the analogy of using rabbits to model human infection and disease. Although the experimental outcomes shown are limited by numbers of animals affected, they mimic the infrequency of HTLV-1 disease and authenticate epidemiological evidence of virus sequence stability regardless of disease phenotype. The findings suggest that further investigation of a possible role for HTLV-1 in some forms of cutaneous T-cell lymphoma is warranted.

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.

Solution structure of the capsid protein from the human T-cell leukemia virus type-I

The solution structure of the capsid protein (CA) from the human T-cell leukemia virus type one (HTLV-I), a retrovirus that causes T-cell leukemia and HTLV-I-associated myelopathy in humans, has been determined by NMR methods. The protein consists of independent N and C-terminal domains connected by a flexible linker. The domains are structurally similar to the N-terminal "core" and C-terminal "dimerization" domains, respectively, of the human immunodeficiency virus type one (HIV-1) and equine infectious anemia virus (EIAV) capsid proteins, although several important differences exist. In particular, hydrophobic residues near the major homology region are partially buried in HTLV-I CA, which is monomeric in solution, whereas analogous residues in HIV-1 and EIAV CA project from the C-terminal domain and promote dimerization. These differences in the structure and oligomerization state of the proteins appear to be related to, and possibly controlled by, the oxidation state of conserved cysteine residues, which are reduced in HTLV-I CA but form a disulfide bond in the HIV-1 and EIAV CA crystal structures. The results are consistent with an oxidative capsid assembly mechanism, in which CA oligomerization or maturation is triggered by disulfide bo nd formation as the budding virus enters the oxidizing environment of the bloodstream.

Defective DNA repair in cells with human T-cell leukemia/bovine leukemia viruses: role of tax gene

BACKGROUND

Human T-cell leukemia virus (HTLV)/bovine leukemia virus (BLV) group retroviruses, which cause hematopoietic cancers, encode a unique protein, Tax, involved in the transformation of infected cells. Our purpose was to determine whether the mechanism by which Tax protein induces transformation in HTLV- or BLV-infected cells involves DNA damage.

METHODS

We used a micronucleus assay to measure chromosomal damage and alkali denaturation analysis to test host-cell DNA integrity in cells infected with HTLV, BLV, or simian T-lymphotropic virus or in cells transfected with the tax gene of HTLV or BLV. Controls included uninfected cells and cells infected with other oncogenic retroviruses or oncogenic DNA viruses. We used a plasmid reactivation assay to examine whether the damage might be due to the inhibition of DNA repair. To ascertain which of several repair pathways might be inhibited, chemical methods were used to selectively introduce lesions repaired by specific pathways into the reporter plasmid.

RESULTS

The presence of Tax was associated with DNA damage. HTLV- or BLV-infected or tax-transfected cells showed normal ability to repair damage induced by deoxyribonuclease I or psoralen but markedly decreased ability to repair damage induced by UV light, quercetin, or hydrogen peroxide.

CONCLUSIONS

These data suggest that the DNA repair pathway most inhibited by Tax is base-excision repair of oxidative damage. To our knowledge, this is the first report demonstrating inhibition of DNA repair by any retrovirus and suggests that this inhibition of DNA repair may contribute to the mechanism of cell transformation by the HTLV/BLV group of viruses.

Multiplex detection of four pathogenic retroviruses using molecular beacons

We describe a multiplex nucleic acid assay that identifies and determines the abundance of four different pathogenic retroviruses (HIV-1, HIV-2, and human T-lymphotrophic virus types I and II). Retroviral DNA sequences are amplified in a single, sealed tube by simultaneous PCR assays, and the resulting amplicons are detected in real time by the hybridization of four differently colored, amplicon-specific molecular beacons. The color of the fluorescence generated in the course of amplification identifies which retroviruses are present, and the number of thermal cycles required for the intensity of each color to rise significantly above background provides an accurate measure of the number of copies of each retroviral sequence that were present originally in the sample. Fewer than 10 retroviral genomes can be detected. Moreover, 10 copies of a rare retrovirus can be detected in the presence of 100, 000 copies of an abundant retrovirus. Ninety-six samples can be analyzed in 3 hr on a single plate, and the use of a closed-tube format eliminates crossover contamination. Utilizing previously well characterized clinical samples, we demonstrate that each of the pathogenic retroviruses can be identified correctly and no false positives occur. This assay enables the rapid and reliable screening of donated blood and transplantable tissues.

Cloning and expression of a human T-lymphotropic virus type 1 protein with reverse transcriptase activity

Unlike most other characterized retroviruses, there is little published information on the biochemical properties of human T-lymphotropic virus type 1 (HTLV-1) reverse transcriptase (RT). Specifically, no reports of a cloned functional RT enzyme have been published. Since the RT enzyme is an essential component of the virus, our objective was to clone, express, and purify a functional RT enzyme from HTLV-1. Our approach was to clone and express a protein of approximately 60 to 65 kDa that we hypothesized would correspond to the RT region encoded by the pol reading frame. The predicted region encoding the RT enzyme comprised nucleotides 2617 to 4312 of the HTLV-1 MT-2 isolate. A putative RT gene was obtained by PCR and was ligated into various prokaryotic expression vectors. A novel cloning approach allowed us to generate a stable clone in the prokaryotic expression vector pGEX-4T-1 and produce a recombinant protein of approximately 60 to 65 kDa. The partially purified protein displays RT activity in both amplification RT (AMP-RT) assays and traditional RT assays. This is the first report of a cloned protein from HTLV-1 which displays RT activity and is the first step in the characterization of HTLV-1 RT.

Source and route of exposure influence infectivity of a molecular clone of human T cell leukemia virus type I

Infection with human T cell leukemia virus type I (HTLV-I) is typically asymptomatic, but does result in diverse diseases ranging from adult T cell leukemia to spastic neuromyelopathy. To date, differences in HTLV-I provirus structure have not been correlated with pathogenic or asymptomatic outcome of infection. Molecular clones of HTLV-I are now available and represent a powerful tool to link virus structure to pathogenesis. Present studies to explore in vivo infectivity and pathogenicity of an HTLV-I molecular clone, K30p, have utilized the rabbit as a model system. This clone was administered to neonatal or adult rabbits by several different routes and infectivity and pathogenicity were examined. Detection of antiviral humoral immune responses, presence of provirus in tissue samples, and isolation of virus in cultures of blood lymphocytes were used to establish systemic HTLV-I infection. Intramuscular, but not nervous system, exposure to K30p HTLV-I naked DNA resulted in infection. Conversely, neural exposure to T cells that had been transfected with the K30p HTLV-I DNA consistently resulted in systemic infection. Despite detection of HTLV-I provirus in brain and spinal cord of some infected rabbits, no clinical or neuropathological changes occurred. Source and route of virus exposure played a role in infectivity, but did not influence the pathogenic outcome of HTLV-I infection.

Human and simian T-cell leukemia viruses type 2 (HTLV-2 and STLV-2(pan-p)) transform T cells independently of Jak/STAT activation

Human T-lymphotropic virus type 1 (HTLV-1) and HTLV-2 differ in pathogenicity in vivo. HTLV-1 causes leukemia and neurologic and inflammatory diseases, whereas HTLV-2 is less clearly associated with human disease. Both retroviruses transform human T cells in vitro, and transformation by HTLV-1 was found to be associated with the constitutive activation of the Jak/STAT pathway. To assess whether HTLV-2 transformation may also result in constitutive activation of the Jak/STAT pathway, six interleukin-2-independent, HTLV-2-transformed T-cell lines were analyzed for the presence of activated Jak and STAT proteins by electrophoretic mobility shift assay. In addition, the phosphorylation status of Jak and STAT proteins was assessed directly by immunoprecipitation and immunoblotting with an antiphosphotyrosine antibody. Jak/STAT proteins were not found to be constitutively activated in any of the T-cell lines infected by the type 2 human and nonhuman primate viruses, suggesting that HTLV-2 and the cognate virus simian T-lymphotropic virus type 2 from Pan paniscus transform T cells in vitro by mechanisms at least partially different from those used by HTLV-1.

Genes in the pX region of human T cell leukemia virus I influence Vav phosphorylation in T cells

Human T cell leukemia virus I (HTLV-I) causes acute leukemic disease in a low percentage of infected individuals through obscure mechanisms. Our studies compare two rabbit HTLV-I-infected T cell lines: one, RH/K34, causes lethal experimental leukemia and the other, RH/K30, mediates asymptomatic infection. We show herein that the product of the protooncogene vav is constitutively Tyr-phosphorylated in RH/K34 but not in RH/K30. A role for the retrovirus in phosphorylation of Vav was assigned by transfection experiments with molecular clones of HTLV-I derived from the two lines. The HTLV-I molecular clone from RH/K30, but not that from RH/K34, down-regulates Vav phosphorylation in a Herpesvirus ateles-transformed T cell line. Use of recombinant virus clones revealed that a pX region sequence differing by two nucleotides between the two clones mediates this down-regulation. Because Vav is involved in T cell signaling and Vav phosphorylation occurs upon activation of T cells, control of the activation state of Vav by viral proteins may relate to the leukemogenic potential of certain HTLV-I-infected cells.

The human T-cell leukemia virus type 1 Rex regulatory protein exhibits an impaired functionality in human lymphoblastoid Jurkat T cells

The Rex protein of human T-cell leukemia virus type 1 (HTLV-1) intervenes in the posttranscriptional regulation of proviral gene expression. Its binding to the Rex response element (XRE) present in the 3' long terminal repeat ensures the coordinate cytoplasmic accumulation of spliced and unspliced forms of viral messengers. Consequently, synthesis of viral structural and enzymatic proteins is strictly dependent on the Rex posttranscriptional activity. Here we report that synthesis of HTLV-1 envelope glycoproteins by Jurkat T cells could be detected only when they were regulated in a Rex-independent manner. Indeed, Jurkat T cells transfected with a Rex-dependent env expression vector (encompassing both the env and pX open reading frames) do not produce significant levels of envelope glycoproteins despite the production of significant amounts of Rex protein. The analysis of levels and distribution patterns of the unspliced env and of the singly spliced tax/rex transcripts suggests that the failure in envelope glycoprotein synthesis may be ascribed to a deficiency of Rex in mediating the nucleocytoplasmic transport of unspliced env RNAs in these cells. Furthermore, despite the synthesis of regulatory proteins, HTLV-1 structural proteins were not detected in Jurkat T cells transfected with an HTLV-1 infectious provirus. Conversely, and as expected, structural proteins were produced by Jurkat cells transfected by a human immunodeficiency virus type 1 (HIV-1) infectious provirus. This phenotype appeared to be linked to a specific dysfunction of Rex, since the functionally equivalent Rev protein of HIV-1 was shown to be fully efficient in promoting the synthesis of HTLV-1 envelope glycoproteins in Jurkat cells. Therefore, it seems likely that the block to Rex function in these lymphoblastoid T cells is determined by inefficient Rex-XRE interactions. These observations suggest that the acquisition of this Rex-deficient phenotype by in vivo-infected HTLV-1 T cells may represent a critical event in the lymphoproliferation induced by this human retrovirus, leading to leukemia.

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.

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.

Broad host range of human T-cell leukemia virus type 1 demonstrated with an improved pseudotyping system

Studies of human T-cell leukemia virus type 1 (HTLV-1) have been hampered by the difficulty of achieving high cell-free and cell-associated infectious titers. Current retroviral pseudotyping systems using the HTLV-1 envelope generate titers of less than 200 infectious particles per ml. We describe here an improved system for pseudotyping using a defective human immunodeficiency virus (HIV) type 1 genome in combination with HTLV-1 env in 293T producer cells. Introduction of additional copies of rev and treatment of cells with sodium butyrate resulted in a cell-associated titer of 10(5)/ml and cell-free titers of greater than 10(4)/ml . By using this system, we found that the host range of HTLV-1 is even greater than previously suspected. Earlier studies which assigned a chromosomal location for the HTLV-1 receptor may therefore reflect cell-to-cell variation in receptor number rather than the absolute presence or absence of a receptor. The generation of higher-titer HIV(HTLV-1) may facilitate identification of the cellular receptor and investigations of the pathophysiology of HTLV-1 infection.

Infectivity of chimeric human T-cell leukemia virus type I molecular clones assessed by naked DNA inoculation

Two human T-cell leukemia virus type I (HTLV-I) molecular clones, K30p and K34p were derived from HTLV-I-infected rabbit cell lines. K30p and K34p differ by 18 bp with changes in the long terminal repeats (LTRs) as well as in the gag, pol, and rex but not tax or env gene products. Cells transfected with clone K30p were infectious in vitro and injection of the K30p transfectants or naked K30p DNA into rabbits leads to chronic infection. In contrast, K34p did not mediate infection in vitro or in vivo, although the cell line from which it was derived is fully infectious and K34p transfectants produce intact virus particles. To localize differences involved in the ability of the clones to cause infection, six chimeric HTLV-I clones were constructed by shuffling corresponding fragments containing the substitutions in the LTRs, the gag/pol region and the rex region between K30p and K34p. Cells transfected with any of the six chimeras produced virus, but higher levels of virus were produced by cells transfected with those constructs containing the K30p rex region. Virus production was transient except in cells transfected with K30p or with a chimera consisting of the entire protein coding region of K30p flanked by K34p LTRs; only the transfectants showing persistent virus production mediated in vitro infection. In vivo infection in rabbits following intramuscular DNA injection was mediated by K30p as well as by a chimera of K30p containing the K34p rex gene. Comparisons revealed that virus production was greater and appeared earlier in rabbits injected with K30p. These data suggest that several defects in the K34p clone preclude infectivity and furthermore, provide systems to explore functions of HTLV-I genes.

Examining the molecular genetics of HTLV-I with an infectious molecular clone of the virus and permissive cell culture systems

Infectious molecular clones of HTLV-I proviruses have only recently been reported. The long wait for such provirus clones reflects the difficulties inherent in propagating HTLV-I in vitro, and thus a rigorous demonstration of infectivity has awaited improved cell culture systems and sensitive detection techniques for HTLV-I. An intact HTLV-I provirus, originating from an American ATL patient, was subcloned into a plasmid vector and was designated pCS-HTLV. Transient transfections of mammalian cells with pCS-HTLV resulted in the synthesis of viral proteins and mRNAs which were assembled into virions that had physical and morphological characteristics typical of HTLV-I particles. The ability of these virus particles to infect cells, replicate, and produce infectious progeny was demonstrated initially in short term, cell-free infection assays by monitoring the expression of specific viral mRNAs. These studies have been extended in cell culture systems that support continuous virus production. Primary T-lymphocytes have been infected either with cell-free supernatant fluids from, or by coculture with, cells transiently transfected with pCS-HTLV, giving rise to continuous, IL-2-dependent cell lines that have been in culture for >1 year. Furthermore, fetal rhesus lung cells (FRhL) were shown to be permissive for HTLV-I replication and sustained virus expression after infection with pCS-HTLV. Continuous FRhL cell lines now have been established that express various HTLV-I proviruses and mutants. These provirus clones and cell lines provide us with the means to address long-standing questions dealing with the biology of HTLV-I.