HTLV x-gene product: requirement for the env methionine initiation codon

Activation of a cryptic splice site in the tax gene of HTLV-I by a single nucleotide change

We identified a T-to-C mutation 2 nucleotides (nt) upstream from the AG in a GT-AG intron between exons 2 and 3 in the human T-cell leukemia virus type I (HTLV-I) tax mRNA. This mutation resulted in the preferential usage of an alternative splice site, causing a 75-nt elongation of tax mRNA and reduced production of viral antigens. When the clone containing this T-to-C mutation was reverted to the wild-type (T) DNA sequence, normal splicing of tax mRNA ensued and viral production was restored. These results suggest that the nucleotide at the position 2nt upstream from the AG in a GT-AG intron is important for the proper splicing of the HTLV-I tax gene, although it is not considered important for splicing in eukaryotes.

HTLV-1-encoded p30II is a post-transcriptional negative regulator of viral replication

Human T-cell leukemia/lymphoma virus type 1 (HTLV-1) persists despite a vigorous virus-specific host immune response, and causes adult T-cell leukemia and lymphoma in approximately 2% of infected individuals. Here we report that HTLV-1 has evolved a genetic function to restrict its own replication by a novel post-transcriptional mechanism. The HTLV-1-encoded p30(II) is a nuclear-resident protein that binds to, and retains in the nucleus, the doubly spliced mRNA encoding the Tax and Rex proteins. Because Tex and Rex are positive regulators of viral gene expression, their inhibition by p30(II) reduces virion production. p30(II) inhibits virus expression by reducing Tax and Rex protein expression.

T-Cell Control 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. Collective evidence from in vitro studies indicates that 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, such as antigen stimulation and receptor-ligand interaction, suggesting that extracellular signals are important for HTLV-1 oncogenesis. Environmental factors such as chronic antigen stimulation may therefore be of importance, as also suggested by epidemiological data. Thus genetic and environmental factors together with the virus contribute to disease development. 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. The relevance of these laboratory findings is related to clonal T-cell proliferation and adult T-cell leukemia/lymphoma development in vivo.

Tumor-associated antigens: from discovery to immunity

There is a renewed enthusiasm for therapeutic vaccination as a viable treatment for patients with cancer. Early tumor vaccines were comprised of whole tumor cells, fragments of tumor cells, or protein lysate from tumor cells. Limited results with these approaches led investigators to begin developing the next generation of cancer vaccines based on defined tumor-associated antigens (TAAs). Defining and characterizing TAAs for human cancer, development of new approaches for identifying TAAs, and novel strategies to deliver the antigens as potent therapeutic vaccines have all been the focus of intense research in the past decade and will continue to be the focus for decades to come.

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.

Persistent activation of NF-kappaB by the tax transforming protein of HTLV-1: hijacking cellular IkappaB kinases

Biochemical coupling of transcription factor NF-kappaB to antigen and co-stimulatory receptors is required for the temporal control of T-cell proliferation. In contrast to its transitory activation during normal growth-signal transduction, NF-kappaB is constitutively deployed in T-cells transformed by the type 1 human T-cell leukemia virus (HTLV-1). This viral/host interaction is mediated by the HTLV-1-encoded Tax protein, which has potent oncogenic properties. As reviewed here, Tax activates NF-kappaB primarily via a pathway leading to the chronic phosphorylation and degradation of IkappaBalpha, a cytoplasmic inhibitor of NF-kappaB. To access this pathway, Tax associates stably with a cytokine-inducible IkappaB kinase (IKK), which contains both catalytic (IKKalpha and IKKbeta) and noncatalytic (IKKgamma) subunits. Unlike their transiently induced counterparts in cytokine-treated cells, Tax-associated forms of IKKalpha and IKKbeta are persistently activated in HTLV-1-infected T cells. Acquisition of the deregulated IKK phenotype is contingent on the presence of IKKgamma, which functions as a molecular adaptor in the assembly of pathologic Tax/IkappaB kinase complexes. These findings highlight a key mechanistic role for IKK in the Tax/NF-kappaB signaling axis and define new intracellular targets for the therapeutic control of HTLV-1-associated disease.

Mechanisms of Tax Regulation of Human T Cell Leukemia Virus Type I Gene Expression

During the last several years, the human T cell leukemia virus type I (HTLV-I) has become recognized as an important cause for public health concern throughout the world. HTLV-I is the causative agent of a variety of clinical diseases, including an aggressive lymphoproliferative disorder named adult T cell leukemia. HTLV-I induces pathogenicity in the infected host cell through the synthesis of a virally encoded protein called Tax. Expression of Tax is critical to the life cycle of the virus, as the protein greatly increases the efficiency of HTLV-I gene transcription and replication. Furthermore, Tax has been shown to deregulate the transcription of many cellular genes, leading to the hypothesis that the presence of Tax promotes unchecked growth in the HTLV-I-infected cell. The mechanism of Tax trans-activation of HTLV-I gene expression is not known. Tax does not bind directly to the Tax-responsive promoter elements of the virus, but appears to function through interaction with certain cellular DNA binding proteins, including activating transcription factor 2 and cAMP response element binding protein that recognize these sequences. This review summarizes some of the recent work in the field aimed at elucidating the mechanism of Tax trans-activation of HTLV-I gene expression. Copyright 1995 S. Karger AG, Basel

Reexamination of the coding potential of the HTLV-1 pX region

The human T-cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia (ATL) and tropical spastic paraparesis/HTLV-1-associated myelopathy (TSP/HAM). In addition to tax and rex genes eight additional open reading frames are present in the pX region of HTLV-1. The possibility that these open reading frames can encode a protein in human cell lines persistently infected by HTLV-1 or in COS-1 cells transfected with a pX expressor plasmid was investigated. The results presented here indicate that tax and rex are the only proteins detected. Moreover, antibodies against proteins in vitro translated by five of the additional open reading frames of the pX region were not found in sera of ATL and TSP/HAM patients, further indicating that those proteins are not usually made in infected cells.

Expression of alternatively spliced human T-lymphotropic virus type I pX mRNA in infected cell lines and in primary uncultured cells from patients with adult T-cell leukemia/lymphoma and healthy carriers

Although human T-cell lymphotropic virus type I (HTLV-I) is the etiologic agent of adult T-cell leukemia/lymphoma (ATL), the role of viral gene expression in the progression to and maintenance of the leukemic state in vivo is unclear because of the inability of most previous studies to readily detect HTLV-I RNA in infected individuals. By using the reverse transcriptase-polymerase chain reaction, we detected spliced messages for the HTLV-I pX regulatory genes in primary uncultured cells from ATL patients and healthy asymptomatic carriers. In addition to the expected doubly spliced pX message, three alternatively spliced mRNAs were demonstrated (pX delta 17, pX-p21rex, and pX-orfII mRNAs, where orf = open reading frame). The same splice sites were shown in the messages from uncultured ATL cells and from the HTLV-I-producing C10/MJ cell line. Alternatively spliced pX mRNAs have the potential to code for known and putative pX gene products. Among the transcripts is a monocistronic mRNA likely to code for p21rex (pX-p21rex mRNA). Since alternative splicing of HTLV-I pX mRNA can be found in primary uncultured cells, it is likely to have a functional significance in vivo. This suggests possible roles for HTLV-I gene expression in the progression to and maintenance of ATL, as well as in the phase preceding it.

Regulation of HTLV-II gene expression by Rex involves positive and negative cis-acting elements in the 5'long terminal repeat

Regulation of human T-cell leukemia virus type II (HTLV-II) gene expression by the trans-acting viral protein, Rex, is mediated through specific cis-acting sequences in the HTLV-II long terminal repeat (LTR). Augmentation of 5' LTR-linked gene expression by Rex requires two distinct cis-acting elements: one termed the "Rex-responsive element" (RxRE), which allows Rex to overcome the inhibitory effect of a second, termed the "cis-acting repressive sequences" (CRS). The HTLV-II RxRE is located between nt +91 and +317 relative to the cap site in the R/U5 region of the 5'LTR, and the HTLV-II CRS is contained within the RxRE from nt +208 to +317, which is downstream of the splice donor site, in the R/U5 region of the 5' LTR. Deletion of the CRS results in significantly increased cytoplasmic levels of LTR-linked mRNA independent of the presence of Rex. Our results show that Rex acts post-transcriptionally and induces a shift from nucleus to cytoplasm of gag/pol mRNA, the only HTLV-II mRNA that contains both the RxRE and the CRS in the 5' LTR-derived leader sequence.

Nucleolar targeting signal of human T-cell leukemia virus type I rex-encoded protein is essential for cytoplasmic accumulation of unspliced viral mRNA

The posttranscriptional regulator (rex) of human T-cell leukemia virus type I is known to be located predominantly in the cell nucleolus and to induce the accumulation of gag and env viral mRNAs. The N-terminal 19 amino acids of rex-encoded protein (Rex) has been shown to be sufficient to direct hybrid proteins to the cell nucleolus. We have studied the function of the nucleolar targeting signal (NOS) of rex by using full-length proviral DNA and mutant rex expression plasmids. Partial deletions of the NOS sequence abolished the accumulation of unspliced cytoplasmic mRNA, although the gene products of rex mutants were found in the nucleoplasm. These results indicate that NOS sequence, or nucleolar localization of Rex, is essential for Rex function.

A polyclonal CD4+ and CD8+ lymphocytosis in a patient doubly infected with HTLV-I and HIV-1: a clinical and molecular analysis

HTLV-I is associated with adult T-cell leukemia/lymphoma (ATL) characterized by monoclonal expansions of CD4+ T-lymphocytes. In this report we describe a histologically benign, polyclonal HTLV-I infection in a patient exhibiting both an absolute CD4+ and CD8+ lymphocytosis. Three T-cell lines containing integrated HTLV-I proviral copies established from this patient were initially polyclonal, but with time all grew out the same two clones as determined by analysis of their T-cell antigen receptor beta chain gene rearrangements. The patient subsequently developed pulmonary and nasopharyngeal nodules containing HTLV-I infected cells. Restriction analysis of the patient's HTLV-I provirus revealed no differences from prototype HTLV-I and the tax gene was normally expressed in vivo and in vitro. The patient's T-lymphocytosis and HTLV-I+ pulmonary tract nodules were put into a complete clinical remission by treatment with alkylating agents and steroids. Subsequently, the patient developed a severe immunodeficiency state and expired. Retrospective serologic and gene amplification assays for HIV-1 demonstrated that he had been doubly infected from the time of presentation. Postmortem analysis by polymerase chain reaction revealed the presence of both HTLV-I and HIV-1 in lymphatic tissues and the testes; HIV-1 was also detected in brain tissue.

Identification of a cis-regulatory element involved in accumulation of human T-cell leukemia virus type II genomic mRNA

The X gene products of the human T-cell leukemia viruses type I and II are thought to be involved not only in viral replication but also in mediating the expression of certain cellular genes. These X gene products are known to be translated from doubly spliced viral mRNA, while viral structural proteins, such as the gag, pol, and env gene products, are translated from unspliced or singly spliced viral mRNA. One of the X gene products of human T-cell leukemia virus type II, tax2 protein, has been shown to be responsible for transcriptional stimulation from the viral long terminal repeat. The other X gene product(s) of human T-cell leukemia virus type II, the rex2 protein(s), is located in the nuclear fraction of virus-infected cells, but its function is not known. This article reports evidence that rex2 protein(s) enhances the accumulation of unspliced viral RNA by interacting posttranscriptionally, either directly or indirectly, with a cis-regulatory element downstream from the first splice donor site in the long terminal repeat.

Transcriptional activation of homologous viral long terminal repeats by the human immunodeficiency virus type 1 or the human T-cell leukemia virus type I tat proteins occurs in the absence of de novo protein synthesis

The genomes of human retroviruses [human immunodeficiency virus type 1 (HIV-1) and human T-cell leukemia virus (HTLV-I)] encode positive trans-activator proteins, named tat. In the presence of tat, the transcriptional activity of the homologous HIV-1 or HTLV-I long terminal repeat (LTR) promoter is markedly increased. We have constructed mammalian cell lines that contain stably integrated copies of a HIV-1 or a HTLV-I LTR-chloramphenicol acetyltransferase (CAT) gene. When presynthesized HIV-1 or HTLV-I tat proteins were separately introduced into these cells in the presence of cycloheximide, we found a strong increase in the steady-state expression of the homologous viral LTR. Nuclear "run-on" assays verified that this tat-mediated enhancement, occurring in the absence of de novo cellular protein synthesis, was due to increased transcriptional initiation at the LTR promoter. We conclude that one aspect of transcriptional trans-activation of viral LTR by the HIV-1 and HTLV-I tat proteins does not require the production of new cellular proteins.

HTLV-II transactivation is regulated by the overlapping tax/rex nonstructural genes

The human T-cell leukemia virus (HTLV) types I and II have two nonstructural genes that are encoded in overlapping reading frames. One of these genes, known as tax, has been shown to encode a protein responsible for enhanced transcription (transactivation) from the viral long terminal repeats (LTRs). Genetic evidence indicates that the second nonstructural gene of HTLV-II, here designated rex, acts in trans to modulate tax gene-mediated transactivation in a concentration-dependent fashion. The rex gene may regulate the process of transactivation during the viral life cycle.

Synthesis in Escherichia coli of the HTLV-I trans-acting protein p40x

The pX gene of human T-cell leukemia virus type I (HTLV-I) encodes a protein that activates the expression of viral genes in trans. Plasmid constructs designed to express the pX gene under the control of either the temperature-inducible lambda PL promoter or the trp promoter were used to transform several Escherichia coli strains, including murein-lipoprotein and Ion mutant strains. Upon induction it was possible to detect the synthesis of a new polypeptide of approximately 40 kDa which reacted specifically with serum from an ATL patient.

Expression of a cDNA clone corresponding to the long open reading frame (XBL-I) of the bovine leukemia virus

Nucleotide sequence analysis of a cDNA clone corresponding to the XBL-I open reading-frame of bovine leukemia virus (BLV) revealed that the AUG initiation codon was located 44 bases downstream from that of the env gene and was part of the p34x mRNA splice donor. . .ATGG/GTAA at the end of the pol gene sequence. RNA from this clone was synthesized in vitro by the SP6 RNA polymerase and translated into a 34,000 mol wt protein in rabbit reticulocyte lysates. The protein (p34x) is recognized in Western blots by most sera of BLV-infected sheep and tumor-bearing cattle, by an anti-synthetic peptide rabbit serum, and by the serum of a rabbit immunized by XBL-I RNA programmed reticulocyte lysates. Both sera react with a 34,000 mol wt protein present in nuclei of BLV-infected cells.

Human protein binding to DNA sequences surrounding the human T-cell lymphotropic virus type-I long terminal repeat polyadenylation site

The long terminal repeats (LTRs) of RNA tumor viruses, including human T-cell lymphotropic virus type I (HTLV-I), contain the control elements for expression of viral genes. Sequence-specific LTR-DNA-binding proteins could regulate viral functions. To search for such proteins we have used an in vitro non-denaturing polyacrylamide gel assay, with restriction fragments of the HTLV-I LTR and nuclear protein extracts from several HTLV-I-infected cell lines and an uninfected T-cell line, H9. Four DNA-binding activities were observed, including non-specific DNA-binding activity and at least two activities (forms I and II) which bind specifically to a HinfI restriction fragment from nucleotides +181 to +334 relative to the transcription start site. DNA-binding activities I and II were partially resolved by ion-exchange chromatography and mapped by protection experiments to two 10-20-bp blocks surrounding the polyadenylation site at +221. Of the cell lines tested, form II was abundantly found in C10/MJ, and forms I and IV were also found in C91/PL, C81-66-45, MT2 and H9 cells.

Bovine leukemia virus transcription is controlled by a virus-encoded trans-acting factor and by cis-acting response elements

Bovine leukemia virus (BLV) gene expression is exquisitely regulated at multiple levels, including a transcriptional control effected by virus-encoded trans-acting factors and cis-acting target sequences. Like the human T-cell leukemia viruses type I and type II, but unlike other RNA tumor viruses, BLV contains several open reading frames at the 3' end of its genome. A subgenomic mRNA which encodes two overlapping reading frames from this region could produce proteins of 38 and 18 kilodaltons (kDa). A series of cis-trans experiments using transfected virus gene constructs in different combinations revealed that expression of the 38-kDa protein was both necessary and sufficient to activate, in trans, the BLV promoter. This activation was specific for the BLV long terminal repeat, as a variety of related retroviral promoters were not responsive to the expression of the 38-kDa protein p38(XBL). Deletion analysis and construction of chimeric promoters identified a 75-base-pair long terminal repeat region which functions like a p38(XBL)-dependent enhancer element.

Expression of the bovine leukemia virus X region in virus-infected cells

Bovine leukemia virus, like its closest relatives the human T-cell leukemia virus types I and II, contains a 1.8-kilobase X region between the env gene and the 3' long terminal repeat. In this communication, we report the detection and characterization of a subgenomic mRNA from which this X region is presumably translated. This mRNA was produced by a complex splicing mechanism which resulted in juxtaposition of the 5' end of the env gene and the two overlapping X-region open reading frames. Translation of this mRNA could yield at least two distinct proteins depending on which initiation codon is used. Detection of the protein encoded by the BLV X-region long open reading frame has been reported (N. Sagata, J. Tsuzuku-Kawamura, M. Nagayoshi-Aida, F. Shimizu, K.-I. Imagawa, and Y. Ikawa, Proc. Natl. Acad. Sci. USA 82:7879-7883, 1985). Using synthetic peptide antisera, we detected a protein encoded by the short open reading frame in virus-infected cells. The protein migrated in sodium dodecyl sulfate-polyacrylamide gels with an apparent molecular weight of 19,000. It is a nuclear phosphoprotein.

Abundant synthesis of functional human T-cell leukemia virus type I p40x protein in eucaryotic cells by using a baculovirus expression vector

The human T-cell leukemia virus type I (HTLV-I) p40x protein is a 40-kilodalton polypeptide encoded in the 3'-terminal region of the virus. This protein is responsible for positive transcriptional trans-activation of promoter elements located within the HTLV-I long terminal repeat. We introduced the protein-coding region of HTLV-I p40x into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus. After infection of the insect Spodoptera frugiperda (SF9) cell line, this recombinant strain of baculovirus produced approximately 200 mg of intact p40x protein per 2.5 X 10(8) cells. The protein was biologically active in trans-activation of an HTLV-I long terminal repeat-human beta-globin construct. Biochemical analyses of the protein suggest that the p40x polypeptide underwent posttranslational modification in these eucaryotic SF9 cells.

HTLV x gene mutants exhibit novel transcriptional regulatory phenotypes

The human T-cell leukemia viruses, HTLV-I and HTLV-II, contain a gene, termed x, with transcriptional regulatory function. The properties of the x proteins were analyzed by constructing mutant genes containing site-directed deletions and point mutations. The results demonstrate that the amino terminal 17 amino acids of the x protein constitute part of a functional domain that is critical for the transcriptional activating properties of the protein. Within this region, substitution of a leucine residue for a proline residue results in major changes in the trans-activation phenotype of the protein. The mutant HTLV-II x protein, though incapable of activating the HTLV-II long terminal repeat, will block trans-activation of the HTLV-II long terminal repeat by the wild-type protein. The altered phenotype of this mutant suggests a potential negative regulatory function of the x protein.

Replication and pathogenesis of the human T-cell leukemia/lymphotropic retroviruses

The broad outlines of mechanisms of tumorigenesis by the HTLV-I family of viruses are beginning to emerge. The viruses encode at least three genes in addition to the genes (gag, pol, and env) required for virus replication. These additional genes encoded for by the X region are likely to affect in a specific fashion the growth of lymphocytes. The tat gene appears to mimick at least part of the response of mature lymphocytes to recognition of the cognate antigen. That is, in T-lymphocytes the tatI gene seems to induce the IL-2 and IL-2 receptor genes (W. Greene et al. 1986). The alternative reading-frame proteins, pp21 and pp27, have some similarity of cellular proteins that are associated with G0 to G1 transitions and may contribute to the transformed phenotype in cooperation with the tat gene. The expression of viral genes in infected lymphocytes, the tat gene and pp21 and pp27 proteins, and possibly other viral genes (since the coding capacity of the X region is not exhausted by the tat and pp21 and pp27 proteins) may be sufficient to account for the transformation of T cells in culture. A secondary change in the infected cells in culture is not required to explain the outgrowth of cells which are clonal with respect to the site of viral genomic integration, as selection of the most rapidly growing infected cell could account for this observation. The case of infected patients is more complex. Infection of T cells with the HTLV-I or -II virus is not sufficient to produce malignant disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of p26Xb and p24Xb of human T-cell leukemia virus type II

Human T-cell leukemia virus type II (HTLV-II) isolated from a T-cell variant of hairy cell leukemia contains gag, pol and env genes as well as a fourth gene termed X, which can code three major open reading frames Xa, Xb and Xc. Proteins with molecular masses of 26 kDa (p26Xb) and 24 kDa (p24Xb) encoded by the Xb open reading frame were identified with antisera directed against synthetic peptides corresponding to the N-terminal and C-terminal amino acid sequences deduced from the structure of the Xb open reading frame. More than half the Xb products were found to be located in the nuclear fraction of HTLV-II-infected cells.

A single species of pX mRNA of human T-cell leukemia virus type I encodes trans-activator p40x and two other phosphoproteins

Human T-cell leukemia virus type I (HTLV-I) contains the pX sequence which codes for the trans-activator of the long terminal repeat (LTR) and is thus postulated to be associated with leukemogenesis in adult T-cell leukemia. Overlapping open reading frames (ORF) in the pX sequence were recently found to code for p27x-III and p21x-III by ORF III, in addition to p40x coded for by ORF IV. The mechanism of expression of these newly identified proteins and their possible association with trans-activation were studied. On transfection of an expression plasmid that contains a cDNA sequence of the pX mRNA, products from both ORFs III and IV were detected in the cells. The RNA was synthesized in vitro from the cDNA clone by SP6 RNA polymerase and translated in a rabbit reticulocyte lysate. As translation products, two proteins, p27x-III and p21x-III, were detected in addition to p40x. Elimination of the first and second ATG codons in ORF III resulted in loss of the ability to code for p27x-III and p21x-III, respectively, which indicated that the translations from these two ATG codons were independent. A mutant that lacked both ATG codons was fully active in trans-activation of chloramphenicol acetyltransferase gene expression directed by the LTR. These results indicate that a 2.1-kilobase pX mRNA of HTLV-I independently encodes three proteins, p40x, p27x-III, and p21x-III, by different ORFs and that the last two proteins are not involved in trans-activation of the unintegrated LTR.

Expression of the complete human T-cell leukemia virus type I pX coding sequence as a functional protein in Escherichia coli

Human T-cell leukemia virus type I (HTLV-I), a virus associated with adult T-cell leukemia, contains a long open reading frame (LOR) in the 3' end of its genome between the env region and the 3' long terminal repeat (LTR). This open reading frame encodes a 40-kDa protein (designated p40x) that has been implicated as a positive control element for transcription from the HTLV-I LTR in a phenomenon known as trans-activation. We now report the expression of the complete p40x coding sequence as a 40-kDa protein in Escherichia coli. The p40x protein produced in bacteria is shown, using the protoplast fusion technique, to possess biological activity by its ability to trans-activate a HTLV-I LTR-chloramphenicol acetyltransferase plasmid that is stably integrated into the genome of mouse L cells. This stimulatory activity could be detected within 2 hr after fusion, suggesting the possibility of a direct role for p40x in trans-activation of the HTLV-I LTR. The production of p40x in large quantities in E. coli, together with the rapid protoplast fusion assay for its biological activity, should facilitate the analysis of p40x mutants and the elucidation of the molecular mechanism of trans-activation.

The human interleukin-2 receptor: analysis of structure and function

Considerable information presently exists regarding the molecular, biochemical, and biological features of the human IL-2 receptor. The IL-2 receptor protein, multiple receptor mRNAs, and a single structural gene have now been identified. The important role of this receptor in normal T-cell growth is well established and its potential participation in B-cell growth and differentiation appreciated. The availability of cloned gene products for both the IL-2 receptor and IL-2 may permit the future development of novel biological agents capable of either augmenting or blunting the T-cell immune response. The intriguing interrelationship of HTLV-I and -II infection and altered IL-2 receptor expression is now being unraveled. However, the structural difference in high and low affinity receptors as well as the mechanism by which signals for T-cell growth are propagated through the high affinity receptor remain dominant, unanswered questions in the field.

Trans-activator gene of HTLV-II induces IL-2 receptor and IL-2 cellular gene expression

The human T-lymphotropic viruses types I and II (HTLV-I and -II) have been etiologically linked with certain T-cell leukemias and lymphomas that characteristically display membrane receptors for interleukin-2. The relation of these viruses to this growth factor receptor has remained unexplained. It is demonstrated here that introduction of the trans-activator (tat) gene of HTLV-II into the Jurkat T-lymphoid cell line results in the induction of both interleukin-2 receptor and interleukin-2 gene expression. The coexpression of these cellular genes may play a role in the altering T-cell growth following retroviral infection.

Molecular analysis of a deletion mutant provirus of type I human T-cell lymphotropic virus: evidence for a doubly spliced x-lor mRNA

The genome of the human T-cell leukemia/lymphotropic virus type I (HTLV-I) contains a functional gene denominated x-lor that may be important in HTLV-I transformation of human T cells. To study the role of x-lor and other HTLV-I genes in cellular transformation, we obtained a transformed nonproducer human T-cell line containing a single defective HTLV-I provirus (HTLV-I 55/PL). This 7-kilobase provirus had undergone a deletion involving the entire envelope gene and the nonconserved region. The point of the deletion corresponded to the junction of a donor splice site, located between the polymerase gene and the envelope gene (nucleotide 5183), and the acceptor site for the mRNA of the x-lor gene (nucleotide 7302). The juxtaposition of nucleotides 5182 and 7302 brings the initiating methionine codon of the envelope gene immediately 5' to the x-lor region, leaving the DNA sequence in frame for expression of a protein product. This finding suggests that a double splicing mechanism is used to express the x-lor gene, and that the defective provirus 55/PL was generated through the reverse transcription of a partially spliced mRNA. Analysis of the x-lor mRNA of other HTLV-I-transformed cell lines revealed that a double splicing process is commonly used. Furthermore, since 55/PL can be faithfully transmitted and is able to immortalize recipient T cells, we can conclude that the envelope gene is not necessary for in vitro transformation by HTLV-I.

Identification of the gene responsible for human T-cell leukaemia virus transcriptional regulation

Human T-cell leukaemia viruses (HTLVs) have genomic organization distinct from that of other replication-competent retroviruses, possessing four genes, gag, pol, env and chi. The unique fourth gene, chi (also referred to as lor), is located between env and the 3' long terminal repeat (LTR), encoding a protein of relative molecular mass 40,000 for HTLV-I and 37,000 for HTLV-II, located in the nucleus of infected cells. HTLV-I is the causative agent of adult T-cell leukaemia (ATL), a T-lymphocyte malignancy, while HTLV-II has been found associated with a T-cell variant of hairy cell leukaemia. Both viruses immortalize T cells in vitro. However, the mechanism of cellular transformation induced by HTLV is not known as there seems to be no common site of provirus integration in primary ATL cells and the virus contains no classical oncogene sequences. These observations have provoked speculation that the unique and strongly conserved chi protein (85% amino-acid homology between HTLV-I and -II) is involved in HTLV leukaemogenesis. Recent mutagenesis experiments in our laboratory have shown that the chi gene is essential for HTLV replication. It has also has been shown that the LTRs of HTLV and the related bovine leukaemia virus (BLV) are activated in trans in virus-infected cells, and, although such experiments did not directly demonstrate a role for the chi protein in transcriptional activation, it has been suggested that the chi protein is responsible for the transcriptional activation of the LTR and may be involved in cellular transformation. We have now developed a transient co-transfection assay which demonstrates that transcriptional activation of the HTLV LTR is mediated solely by the chi protein and that no other virus genes are required.

Association of the pX gene product of human T-cell leukemia virus type-I with nucleus

Human T-cell leukemia virus type I (HTLV-I) contains a unique gene pX coding for p40 chi, and this protein was suggested to activate the transcription from the LTR of HTLV. By a similar mechanism, this viral function might be involved in immortalization of T-cells and leukemogenesis in adult T-cell leukemia induced by HTLV-I. In this communication, a part of the p40 chi was found to be tightly associated with nuclei in infected cell lines by subcellular fractionation and immunofluorescence staining.

p27x-III and p21x-III, proteins encoded by the pX sequence of human T-cell leukemia virus type I

Human T-cell leukemia virus type I (HTLV-I) is an etiological agent of adult T-cell leukemia and has a unique sequence, pX, that contains four possible open reading frames, I-IV. p40x was previously identified as the gene product of frame IV (x-lor) and was suggested to mediate transcriptional trans-activation of the viral long terminal repeats. We have identified two pX gene products, p27x-III and p21x-III, encoded by frame III, which mostly overlapped frame IV. These proteins were detected with rabbit antiserum against the synthetic peptide predicted from the 3' end of frame III. p27x-III is phosphorylated in cultured cells, and the phosphoprotein (pp27x-III) is localized in nuclei; some pp27x-III was tightly bound to nuclear components. p27x-III was detected in a number of cell lines that express other viral antigens, including a cell line previously reported to express only p40x as a viral protein. The function(s) of p27x-III and p21x-III is not known, but the tight binding of pp27x-III to nuclear components suggests that it is associated with regulation of viral gene expression.

Two distinct polypeptides may be translated from a single spliced mRNA of the X genes of human T-cell leukemia and bovine leukemia viruses

Human T-cell leukemia and bovine leukemia viruses have a potential transforming gene, termed X. In addition to the major open reading frame known to encode a functional protein, the X gene harbors another short open reading frame which overlaps this major one. Both of these open reading frames are found on a single spliced X mRNA in a potentially functional form. Circumstantial evidence strongly suggests that they are both translated from the single X mRNA molecule, showing striking similarity to the translation mechanism of an adenovirus Elb gene mRNA. We note that the short open reading frame has the capability to encode a putative nuclear protein with structural features similar to those of an AIDS virus trans-acting protein.

The x gene is essential for HTLV replication

The human T-cell leukemia viruses (HTLV) are associated with T-cell malignancies in man and will transform normal human T cells in vitro. The mechanism of malignant transformation by HTLV is unknown but appears to be distinct from that of other classes of retroviruses, which induce malignant transformation through viral or cellular oncogenes. Recently a new gene, termed x, was identified in HTLV. This gene has been hypothesized to be the transforming gene of HTLV because of its conservation within the HTLV class of retroviruses. By in vitro mutagenesis of the HTLV-II x gene, it is now demonstrated that the presence of a functional x gene product is necessary for efficient HTLV transcription. Therefore, these studies provide direct evidence for an important function of the x gene in HTLV replication. The functional analogies between the x gene and transcriptional regulatory genes of some DNA viruses suggest that these viruses share similar mechanisms for cellular transformation.

Expression of the pX gene of HTLV-I: general splicing mechanism in the HTLV family

Human T-cell leukemia virus type I (HTLV-I) is an etiological agent of adult T-cell leukemia. A viral gene pX encodes for p40X and it has been proposed that this protein trans-activates the viral long terminal repeat and possibly some cellular genes; this activation may be associated with T-cell transformation. The mechanism of pX gene expression and the primary structure of p40X are now reported. Two-step splicing generates the 2.1-kilobase pX mRNA; the initiator methionine for env becomes part of the pX protein. These splicing signals are conserved among all members of the HTLV family except for the acquired immune deficiency syndrome-associated viruses.

Studies of the putative transforming protein of the type I human T-cell leukemia virus

The putative transforming protein of the type I human T-cell leukemia virus (HTLV-1) is a 40-kilodalton protein encoded by the X region and is termed p40XI. On the basis of both subcellular fractionation techniques and immunocytochemical analysis, it is now shown that p40XI is a nuclear protein with a relatively short half-life (120 minutes). It is synthesized de novo in considerable quantities in a human T-cell line infected with and transformed by the virus in vitro, and it is not packaged in detectable amounts in the extracellular virus.