Characterization of an IL-2 dependent human T-cell leukemia-virus type-I (HTLV-I) infected cell-line - a system for studying HTLV-I mediated transformation

The retrovirus Human T cell Leukemia Virus type I (HTLV-I) is the causative agent of Adult T cell Leukemia Lymphoma (ATLL) and is associated with HTLV-I Myelopathy. HTLV-I mediated transformation of CD4(+) T cells, during the course of ATLL, is poorly understood. It has been suggested that HTLV-I is responsible for the immortalization of infected cells, but transformation is dependent on secondary events. To investigate this hypothesis, we have isolated an HTLV-I infected T cell line that is dependent on IL-2 for growth in tissue culture. Further, a subclone of this cell line that is able to grow in the absence of IL-2 has been isolated. Both cell lines have identical TCR chain rearrangements and cell surface markers. Each,cell line produces viral mRNAs and proteins. Finally, both of these cell lines are sensitive to rapamycin and cyclosporin A regardless of the presence of IL-2. We propose that this system will provide a unique opportunity to study transformation to IL-2 independence in HTLV-I infected cells.

Attenuation of human immunodeficiency virus type 1 cytopathic effect by a mutation affecting the transmembrane envelope glycoprotein

The cytopathic effects of human immunodeficiency virus type 1 (HIV-1) infection are specific for cells that express the CD4 viral receptor and consist of syncytium formation and single-cell lysis. Here we report that a mutation (517A) affecting the amino terminus of the HIV-1 gp41 transmembrane envelope glycoprotein resulted in a virus that was markedly less cytopathic than was wild-type HIV-1. In systems in which cell-to-cell transmission of HIV-1 occurred, the replication ability of the 517A virus was comparable with that of the wild-type virus. Even though the levels of viral protein expression, virion production, and interaction of the envelope glycoproteins with CD4 were similar for the 517A and wild-type viruses, both syncytium formation and single-cell lysis were attenuated for the 517A mutant virus. These results demonstrate that an envelope glycoprotein region important for mediating post-receptor binding events in cell membrane fusion is important for the induction of cytopathic effects by HIV-1. These results also indicate that levels of HIV-1 viral proteins or viral particles produced in infected cells are in themselves not sufficient to induce cytopathic effects.

Identification of individual human immunodeficiency virus type 1 gp120 amino acids important for CD4 receptor binding

The binding of the CD4 receptor by the human immunodeficiency virus type 1 gp120 exterior envelope glycoprotein is important for virus entry and cytopathic effect. To investigate the CD4-binding region of the gp120 glycoprotein, we altered gp120 amino acids, excluding cysteines, that are conserved among the primate immunodeficiency viruses utilizing the CD4 receptor. Changes in two hydrophobic regions (Thr-257 in conserved region 2 and Trp-427 in conserved region 4) and two hydrophilic regions (Asp-368 and Glu-370 in conserved region 3 and Asp-457 in conserved region 4) resulted in significant reductions in CD4 binding. For most of the mutations affecting these residues, the observed effects on CD4 binding did not apparently result from global conformational disruption of the gp120 molecule, as assessed by measurements of precursor processing, subunit association, and monoclonal antibody recognition. The two hydrophilic regions exhibit a strong propensity for beta-turn formation, are predicted to act as efficient B-cell epitopes, and are located adjacent to hypervariable, glycosylated regions. This study defines a small number of gp120 residues important for CD4 binding, some of which might constitute attractive targets for immunologic intervention.

Changes in the transmembrane region of the human immunodeficiency virus type 1 gp41 envelope glycoprotein affect membrane fusion

The charged amino acids near or within the membrane-spanning region of the human immunodeficiency virus type 1 gp41 envelope glycoprotein were altered. Two mutants were defective for syncytium formation and virus replication even though levels of envelope glycoproteins on the cell or virion surface and CD4 binding were comparable to those of the wild-type proteins. Thus, in addition to anchoring the envelope glycoproteins, sequences proximal to the membrane-spanning gp41 region are important for the membrane fusion process.

Expression of enzymatically active reverse transcriptase of simian immunodeficiency virus in bacteria: sensitivity to nucleotide analogue inhibitors

A fragment of the SIVmac251 pol gene was expressed in Escherichia coli as a trpE fusion protein. Analysis of extracts from bacteria containing this expression plasmid revealed the presence of a reverse transcriptase activity dependent on Mg2+ as divalent cation and active on both poly(rA).oligo(dT) and poly(rC.oligo(dG) templates. In comparative studies, the SIV and HIV-1 reverse transcriptases expressed in bacteria displayed very similar high sensitivities to the chain terminator inhibitors AZTTP and ddTTP. The reverse transcriptase of Moloney murine leukemia virus and the DNA polymerase of E. coli were both more resistant to ddTTP, and the E. coli enzyme was significantly more resistant to AZTTP.

Rapid complementation assays measuring replicative potential of human immunodeficiency virus type 1 envelope glycoprotein mutants

Rapid assays which measure the ability of mutant human immunodeficiency virus type 1 envelope glycoproteins to mediate cell-free and/or cell-to-cell transmission of virus are described. By using these assays, envelope glycoprotein mutants with varying degrees of syncytium-forming ability were tested for ability to complement viral replication in trans. As expected, mutants that dramatically affect association of the gp120-gp41 envelope subunits, CD4 binding, or membrane fusion were unable to form syncytia or to support cell-free or cell-to-cell transmission. Surprisingly, some membrane fusion-defective mutants significantly attenuated in syncytium-forming ability were able to complement viral replication. Conversely, mutations in the carboxyl terminus of gp41 transmembrane glycoprotein, although not affecting syncytium-forming ability, significantly attenuated both forms of virus transmission. These results indicate that syncytium formation is not sufficient for cell-to-cell transmission of human immunodeficiency virus type 1. Furthermore, virus transmission appears to be less sensitive to inhibition of membrane fusion than is syncytium formation.

Identification of a sequence required for efficient packaging of human immunodeficiency virus type 1 RNA into virions

Sequences required for efficient packaging of human immunodeficiency virus type 1 (HIV-1) genome RNA into virus particles were identified. Deletion of 19 base pairs between the 5' long terminal repeat and the gag gene initiation codon of HIV-1 resulted in a virus markedly attenuated for replication in human T lymphocytes. The mutant virus was characterized by nearly wild-type ability to encode viral proteins and to produce virion particles. The mutant virions exhibited a significant reduction in the content of HIV-1-specific RNA. These results identify an important component of the HIV-1 packaging signal.

Antibodies to CD4 in individuals infected with human immunodeficiency virus type 1

The attachment of human immunodeficiency virus type 1 (HIV-1) to target cells is mediated by a specific interaction between the viral envelope glycoprotein (gp120) and the CD4 receptor. Here we report that approximately 10% of HIV-1-infected individuals produce antibodies that recognize the extracellular portion of the CD4 molecule. Carboxyl-terminal deletions of CD4 that do not affect HIV-1 gp120 binding eliminate recognition of CD4 by patient antisera. In contrast, mutations in the amino-terminal domain of CD4 that attenuate HIV-1 gp120 binding do not diminish CD4 recognition by patient antisera. These results suggest that HIV-1 infection can generate antibodies directed against a region of the viral receptor distinct from the virus-binding domain.

Mutational analysis of the 5′ non-coding region of human immunodeficiency virus type 1: effects of secondary structure on translation

The first 111 nt from the 5' end of human immunodeficiency virus type 1 (HIV-1) mRNAs are shown to have a strong inhibitory effect on the translation of mRNA in in vitro translation extracts as well as in Xenopus oocytes. Mutations in the sequence of the 5' untranslated region (UTR) designed to disrupt predicted secondary structure of this region relieve the inhibition. Inhibition is restored by mutations that reconstruct the predicted secondary structure. The accessibility of the 5'-terminal cap structure was also found to be increased by some of these mutations. We conclude that secondary structure in the 5' UTR of HIV-1 mRNAs and resultant inaccessibility of the cap structure is responsible for the inhibition of translation. The implications of these findings for the understanding of the life cycle of HIV-1 are discussed.

Cellular transcription factors and regulation of IL-2 receptor gene expression by HTLV-I tax gene product

Expression of the interleukin-2 receptor (IL-2R alpha) gene is activated by the transcriptional activator protein, Tax (previously referred to as the tat gene product), encoded by the human T-cell leukemia virus (HTLV-I). Multiple protein binding sites for specific DNA-protein interactions were identified over the upstream IL-2R alpha transcriptional regulatory sequences. However, only one region, which includes the sequence motif GGGGAATCTCCC, was required for activation by both the tax gene product and mitogenic stimulation. Remarkably, this sequence also bound the nuclear factor NF kappa B, which is important for induction of kappa-immunoglobulin gene expression. A model is presented whereby regulation of cellular gene expression by the HTLV-I tax gene product occurs via an indirect mechanism that may involve a post-translational modification of preexistent cellular transcription factors.

The art gene product of human immunodeficiency virus is required for replication

A mutation that alters the initiation codon of the art gene of the human immunodeficiency virus renders the virus replication defective. The replication defect of this mutation can be complemented in trans by the art gene product. These findings indicate that the art protein is essential for replication and, as such, represents a good target for development of new antiviral agents that could be useful in the control of human immunodeficiency virus infection.

Functional regions of the envelope glycoprotein of human immunodeficiency virus type 1

The envelope of the human immunodeficiency virus type 1 (HIV-1) plays a central role in the process of virus entry into the host cell and in the cytopathicity of the virus for lymphocytes bearing the CD4 molecule. Mutations that affect the ability of the envelope glycoprotein to form syncytia in CD4+ cells can be divided into five groups: those that decrease the binding of the envelope protein to the CD4 molecule, those that prevent a post-binding fusion reaction, those that disrupt the anchorage of the envelope glycoprotein in the membrane, those that affect the association of the two subunits of the envelope glycoprotein, and those that affect post-translational proteolytic processing of the envelope precursor protein. These findings provide a functional model of the HIV envelope glycoprotein.

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)

A second post-transcriptional trans-activator gene required for HTLV-III replication

Evidence is provided for the existence of a seventh gene in the genome of human T-lymphotropic virus type III/lymphadenopathy-associated virus. The gene is necessary for replication and acts post-transcriptionally to relieve negative regulation of the messenger RNA for the virion capsid and envelope proteins. These observations suggest mechanisms for regulating both the latent and lytic phases of the virus life cycle.

Replicative and cytopathic potential of HTLV-III/LAV with sor gene deletions

The genome of the human T-lymphotropic virus type III (HTLV-III/LAV) has the potential to encode at least three polypeptides in addition to those encoded by the gag, pol, and env genes. In this study, the product of the sor (short open reading frame) region, which overlaps the 3' end of the pol gene, was found to be a protein with a molecular weight of 23,000. An assay was developed for testing the ability of cloned HTLV-III proviruses to produce viruses cytopathic for T4+ lymphocytes. In the cell line used, C8166, neither the HTLV-III sor gene product nor the complete 3'-orf gene product were necessary for the replication or cytopathic effects of the HTLV-III.

Location of the trans-activating region on the genome of human T-cell lymphotropic virus type III

The retrovirus involved in acquired immune deficiency syndrome (HTLV-III/LAV) contains a region that is necessary for stimulation of gene expression directed by the viral long terminal repeat. This region is located between nucleotides 5365 and 5607, immediately 5' to the envelope gene. A doubly-spliced message containing this region could encode an 86-amino acid protein with structural features similar to those of nucleic acid-binding proteins.

A transcriptional activator protein encoded by the x-lor region of the human T-cell leukemia virus

Human T-cell leukemia viruses type I and II (HTLV-I and -II) exhibit several features characteristic of this retroviral family: the presence of an x-lor gene encoding a nuclear protein, transformation properties suggesting the involvement of a virus-associated trans-acting factor, and transcriptional trans-activation of the long terminal repeat (LTR) in infected cells. In the study described here the HTL x-lor products, in the absence of other viral proteins, were able to activate gene expression in trans directed by HTLV LTR. The regulation of the expression of particular genes in trans by HTLV x-lor products suggests that they play a role in viral replication and possibly in transformation of T lymphocytes.

Complete nucleotide sequence of the AIDS virus, HTLV-III

The complete nucleotide sequence of two human T-cell leukaemia type III (HTLV-III) proviral DNAs each have four long open reading frames, the first two corresponding to the gag and pol genes. The fourth open reading frame encodes two functional polypeptides, a large precursor of the major envelope glycoprotein and a smaller protein derived from the 3'-terminus long open reading frame analogous to the long open reading frame (lor) product of HTLV-I and -II.

Serological cross-reactivity between envelope gene products of type I and type II human T-cell leukemia virus

People exposed to type I human T-cell leukemia virus (HTLV-I) develop antibodies to an antigen at the surface of virus-infected cells, designated human T-cell leukemia virus membrane antigen (HTLV-MA). In an earlier study, we demonstrated that the major component of HTLV-MA is gp61, a glycoprotein encoded by the HTLV env gene. In the current study, we found that human antibodies that react with HTLV-MA on cells infected with HTLV-I react equally well with HTLV-MA on C3-44/MO, a target cell infected with type II HTLV. A glycoprotein with an approximate size of 67 kDa, gp67, was identified in C3-44/MO using immunoprecipitation and NaDodSO4/PAGE analysis. The positions of serine and cysteine residues were determined in the amino terminus of gp67 by radiolabel sequencing analysis. Comparison with the amino acid sequence deduced from the primary nucleotide sequence of HTLV-IIMO virus reveals that gp67 is also encoded, at least in part, by the env gene. The gp67 of HTLV-IIMO, like the env gene product of HTLV-ICR, gp61, is recognized both by antibodies from a HTLV-IIMO-infected patient with a variant form of hairy cell leukemia, and by antibodies from patients with HTLV-I-associated adult T-cell leukemia/lymphoma. These results indicate that, despite the divergence between HTLV-I and HTLV-II, the major env gene products of the two types of HTLV are conserved to the degree that they are serologically cross-reactive.

Molecular cloning of a highly leukemogenic, ecotropic retrovirus from an AKR mouse

SL3-3 is a leukemogenic, ecotropic retrovirus produced by a T-cell line derived from a spontaneous lymphoma of an AKR mouse. We have isolated a molecular clone of its DNA provirus from infected NIH 3T3 fibroblasts. Cloned proviral DNA produced infectious virus upon transfection onto NIH 3T3 cells. Virus derived by transfection induced lymphomas at high frequency in AKR/J, C3H(f)/Bi, CBA/J, and NFS/N mice. Heteroduplex and RNase T1 fingerprinting analyses showed that the genomes of SL3-3 and the non-leukemogenic virus, Akv, contain no major substitutions relative to one another and differ by only a few base changes. These results unambiguously show that SL3-3 is a highly leukemogenic virus and that major rearrangements of the genome relative to Akv are not required for virulence.

Nucleotide sequence of the Akv env gene

The sequence of 2,191 nucleotides encoding the env gene of murine retrovirus Akv was determined by using a molecular clone of the Akv provirus. Deduction of the encoded amino acid sequence showed that a single open reading frame encodes a 638-amino acid precursor to gp70 and p15E. In addition, there is a typical leader sequence preceding the amino terminus of gp70. The locations of potential glycosylation sites and other structural features indicate that the entire gp70 molecule and most of p15E are located on the outer side of the membrane. Internal cleavage of the env precursor to generate gp70 and p15E occurs immediately adjacent to several basic amino acids at the carboxyl terminus of gp70. This cleavage generates a region of 42 uncharged, relatively hydrophobic amino acids at the amino terminus of p15E, which is located in a position analogous to the hydrophobic membrane fusion sequence of influenza virus hemagglutinin. The mature polypeptides are predicted to associate with the membrane via a region of 30 uncharged, mostly hydrophobic amino acids located near the carboxyl terminus of p15E. Distal to this membrane association region is a sequence of 35 amino acids at the carboxyl terminus of the env precursor, which is predicted to be located on the inner side of the membrane. By analogy to Moloney murine leukemia virus, a proteolytic cleavage in this region removes the terminal 19 amino acids, thus generating the carboxyl terminus of p15E. This leaves 15 amino acids at the carboxyl terminus of p15E on the inner side of the membrane in a position to interact with virion cores during budding. The precise location and order of the large RNase T(1)-resistant oligonucleotides in the env region were determined and compared with those from several leukemogenic viruses of AKR origin. This permitted a determination of how the differences in the leukemogenic viruses affect the primary structure of the env gene products.

Mechanism of induction of RNA tumor viruses by halogenated pyrimidines

Frome these studies on JLS V-9 cells, a number of conclusions can be drawn about the mechanism of MuLV induction by halogenated pyrimidines. The compounds can induce virus from otherwise healthy cells as long as deoxycytidine is present along with the inducing agent. The compounds must be present during the S phase of the cell cycle and must be incorporated into DNA in order to induce virus (Teich et al. 1973). Only one strand of DNA need be substituted by BrdU or IdU in order to induce virus, because a one-hour period of incorporation leads to induction. From these results it is possible to construct a model for how halogenated pyrimidines are able to induce viruses from otherwise uninfected cells. Because the critical period for the incorporation of the compound is a restricted segment of the S phase of the cell, there would appear to be a critical segment of the genetic information of the cell which, when substituted with BrdU or IdU, leads to a transcriptional derepression. Presumably the critical segment of DNA is either a controlling element of the integrated provirus or it is a separate gene which controls the expression of the integrated provirus. Whichever is true, these results strongly imply that the search for specific repressors of the segments of mammalian DNA is likely to be successful and that RNA tumor viruses may offer a system in which such repression systems can be identified and investigated.