Human immunodeficiency virus type 1 gag-pol frameshifting is dependent on downstream mRNA secondary structure: demonstration by expression in vivo.

The human immunodeficiency virus type 1 (HIV-1) Gag-Pol fusion polyprotein is produced via ribosomal frameshifting. Previous studies in vitro and in Saccharomyces cerevisiae have argued against a significant role for RNA secondary structure 3' of the shift site, in contrast with other systems, in which such structure has been shown to be required. Here we show, by expressing the HIV-1 gag-pol domain in cultured vertebrate cells, that a stem-loop structure 3' of the HIV-1 shift site is indeed important for wild-type levels of frameshifting in vivo.     

Phenotypically Vif- human immunodeficiency virus type 1 is produced by chronically infected restrictive cells.

The permissivity of CD4+ transformed T cells for the replication of human immunodeficiency virus type 1 (HIV-1) vif mutants varies widely between different cell lines. Mutant vif-negative viruses propagate normally in permissive CD4+ cell lines but are unable to establish a productive infection in restrictive cell lines such as H9. As a consequence, elucidation of the function of Vif has been considerably hampered by the inherent difficulty in obtaining a stable source of authentically replication-defective vif-negative viral particles produced by restrictive cells. vif-negative, vpr-negative HIV-1 strain NDK stock, produced by the permissive SupT1 cell line, was used to infect restrictive H9 cells. By using a high multiplicity, infection of H9 cells was achieved, leading to persistent production of viral particles displaying a dramatically reduced infectious virus titer when measured in a single-cycle infectivity assay. Although these viral particles were unable to further propagate in H9 cells, they could replicate normally in CEM and SupT1 cells. Comparison of unprocessed and processed Gag proteins in the persistently produced vif-negative viral particles revealed no defect in the processing of polypeptide precursors, with no inversion of the Pr55gag/p24 ratio. In addition, there was no defect in Env incorporation for the vif-negative viral particles. Despite their apparently normal protein content, these particles were morphologically abnormal when examined by transmission electron microscopy, displaying a previously described abnormally condensed nucleoid. Chronically infected restrictive cell lines producing stable levels of phenotypically vif-negative HIV-1 particles could prove particularly useful in further studies on the function of Vif in the virus life cycle.     

Requirement of active human immunodeficiency virus type 1 integrase enzyme for productive infection of human T-lymphoid cells.

The human immunodeficiency virus type 1 (HIV-1) integrase enzyme exhibits significant amino acid sequence conservation with integrase proteins of other retroviruses. We introduced specific amino acid substitutions at a number of the conserved residue positions of recombinant HIV-1 integrase. Some of these substitutions resulted in proteins which were not able to be purified in the same manner as the wild-type enzyme, and these were not studied further. The remaining mutant enzymes were assessed for their abilities to perform functions characteristic of the integrase protein. These included specific removal of the terminal dinucleotides from oligonucleotide substrates representative of the viral U5-long terminal repeat, nonspecific cleavage of oligonucleotide substrates, and mediation of the strand transfer (integration) reaction. Substitution at position 43, within the protein's zinc finger motif region, resulted in an enzyme with reduced specificity for cleavage of the terminal dinucleotide. In addition, a double substitution of aspartic acid and glutamine for valine and glutamic acid, respectively, at positions 151 and 152 within the D,D(35)E motif region rendered the integrase protein inactive for all of its functions. The introduction of this double substitution into an infectious HIV-1 provirus yielded a mutant virus that was incapable of productively infecting human T-lymphoid cells in culture.     

Analysis of natural variants of the human immunodeficiency virus type 1 gag-pol frameshift stem-loop structure

Human immunodeficiency virus type 1 uses ribosomal frameshifting for translation of the Gag-Pol polyprotein. Frameshift activities are thought to be tightly regulated. Analysis of gag p1 sequences from 270 plasma virions identified in 64% of the samples the occurrence of polymorphism that could lead to changes in thermodynamic stability of the stem-loop. Expression in Saccharomyces cerevisiae of p1-beta-galactosidase fusion proteins from 10 representative natural stem-loop variants and three laboratory mutant constructs (predicted the thermodynamic stability [Delta G degrees] ranging from -23.0 to -4.3 kcal/mol) identified a reduction in frameshift activity of 13 to 67% compared with constructs with the wild-type stem-loop (Delta G degrees, -23.5 kcal/mol). Viruses carrying stem-loops associated with greater than 60% reductions in frameshift activity presented profound defects in viral replication. In contrast, viruses with stem-loop structures associated with 16 to 42% reductions in frameshift efficiency displayed no significant viral replication deficit.     

Unintegrated human immunodeficiency virus type 1 DNA in chronically infected cell lines is not correlated with surface CD4 expression.

Using a polymerase chain reaction-based assay on total cell lysates, we have detected unintegrated human immunodeficiency virus type 1 (HIV-1) DNA in chronically infected T-lymphocytic (ACH-2, J1) and promyelocytic (OM-10.1) cell lines. Treatment with 3'-azido-3'-deoxythymidine (AZT) or soluble CD4 inhibited accumulation of unintegrated viral DNA about 10-fold within 72 h; removal of AZT permitted recovery to pretreatment levels within 72 h. Our results indicate that unintegrated HIV-1 DNA is unstable in these cell lines and originates from a continuous process of reinfection. OM-10.1 cells had relatively high levels of surface CD4 by flow cytometry and high levels of unintegrated viral DNA by polymerase chain reaction. ACH-2 cells had very low levels of both surface CD4 and unintegrated viral DNA. However, J1 cells, with surface CD4 below the level of detection of flow cytometry had a high level of unintegrated viral DNA similar to that of OM-10.1 cells. This implies that the number of CD4 receptors is not rate limiting for reinfection.     

A review on architecture of the gag-pol ribosomal frameshifting RNA in human immunodeficiency virus: a variability survey of virus genotypes

Programmed ‘-1’ ribosomal frameshifting is necessary for expressing the pol gene overlapped from a gag of human immunodeficiency virus. A viral RNA structure that requires base pairing across the overlapping sequence region suggests a mechanism of regulating ribosome and helicase traffic during expression. To get precise roles of an element around the frameshift site, a review on architecture of the frameshifting RNA is performed in combination of reported information with augments of a representative set of 19 viral samples. In spite of a different length for the viral RNAs, a canonical comparison on the element sequence allocation is performed for viewing variability associations between virus genotypes. Additionally, recent and historical insights recognized in frameshifting regulation are looked back as for indel and single nucleotide polymorphism of RNA. As specially noted, structural changes at a frameshift site, the spacer sequence, and a three-helix junction element, as well as two Watson–Crick base pairs near a bulge and a C–G pair close a loop, are the most vital strategies for the virus frameshifting regulations. All of structural changes, which are dependent upon specific sequence variations, facilitate an elucidation about the RNA element conformation-dependent mechanism for frameshifting. These facts on disrupting base pair interactions also allow solving the problem of competition between ribosome and helicase on a same RNA template, common to single-stranded RNA viruses. In a broad perspective, each new insight of frameshifting regulation in the competition systems introduced by the RNA element construct changes will offer a compelling target for antiviral therapy.     

The Vif and Gag proteins of human immunodeficiency virus type 1 colocalize in infected human T cells.

The Vif protein of human immunodeficiency virus type 1 (HIV-1) and other lentiviruses is required for efficient replication in primary cells and certain immortalized cell lines in vitro and, in all likelihood, for the establishment of pathogenic infections in vivo. Current hypotheses concerning Vif's mechanism of action posit that it operates in virus-expressing cells during virion assembly, budding, or maturation such that released virions are modified in a manner that enables them to undergo productive infection in subsequent viral challenges. To gain further insight into the mechanism of action of lentivirus Vif proteins, we have performed a variety of in situ localization and biochemical fractionation studies using cells in which Vif is essential for efficient replication. Double-label immunofluorescence analyses of cells productively infected with HIV-1 or feline immunodeficiency virus revealed dramatic patterns of colocalization between Vif and the virally encoded Gag proteins. Subcellular fractionations of human T cells expressing HIV-1 Vif performed in the absence of any detergent demonstrated that greater than 90% of Vif is associated with cellular membranes. Additional purification using a continuous density gradient indicated that the majority of the membrane-bound Vif copurifies with the plasma membrane. Taken together, these observations suggest that lentivirus Vif and Gag proteins colocalize at the plasma membrane as virion assembly and budding take place. As a result, Vif is able to exert its modulatory effect(s) on these late steps of the virus life cycle.     

Integration is not necessary for expression of human immunodeficiency virus type 1 protein products.

A common feature in the life cycle of cytocidal retroviruses, including human immunodeficiency virus type 1 (HIV-1), is the accumulation of large amounts of unintegrated viral DNA. As yet, the role of unintegrated viral DNA in the cytopathogenesis of cytocidal retrovirus infections remains unresolved. HIV-1 mutants which were deleted in the integrase/endonuclease gene and which were unable to establish an integrated form of the virus were constructed. Despite an inability to integrate, these mutants were fully competent templates for HIV-1 core and envelope antigen production. HIV-1 antigen could be detected in the supernatants of lymphocyte cultures infected with HIV-1 integrase mutants. However, an inability to rescue infectious virus from these cultures indicated that HIV-1 integration was required for the production of infectious HIV-1. On the basis of the ability of unintegrated HIV-1 DNA to serve as a template for HIV-1 antigen production, it is plausible that unintegrated viral DNA can contribute to the HIV-1 antigen pool during HIV-1 replication.     

Fate of the human immunodeficiency virus type 1 provirus in infected cells: a role for vpr.

We investigated the fate of human immunodeficiency virus type 1 (HIV-1) viral DNA in infected peripheral blood lymphocytes and immortalized T-cell lines by using a replication-defective HIV-1. We observed that integrated HIV-1 DNA and viral gene expression decrease over time. A frameshift mutation in vpr resulted in maintenance of the HIV-1 provirus and stable persistence of viral expression. Transfection of vpr together with the neomycin resistance gene in the absence of other viral genes decreased the formation of geneticin-resistant colonies, indicating either a cytotoxic or a cytostatic effect upon cells. Therefore, maintenance of HIV-1 infection within an infected proliferating population is due to two competing processes, the rate of viral spread and the degree of cell growth inhibition and/or death induced by Vpr.     

Requirements for incorporation of Pr160gag-pol from human immunodeficiency virus type 1 into virus-like particles.

The roles of the human immunodeficiency virus precursor polyproteins Pr55gag and Pr160gag-pol in viral core assembly were studied in CMT3-COS cells. To do this, the precursors were expressed separately by using a simian virus 40 late replacement vector system described previously. Consistent with previously published data, our results show that the Pr55gag precursor, when expressed alone, was able to form particles which had an immature morphology and that particle formation required the presence of a myristate addition signal at the amino terminus of the precursor. In contrast, the Pr160gag-pol precursor was not able to form particles when expressed alone, although it still underwent proteolytic processing. Coexpression of the two precursor polyproteins from separate vectors in the same cell resulted in processing of the Pr55gag in trans by the protease embedded in Pr160gag-pol and the formation of virus-like particles containing the products of both precursors. Proteolytic processing occurred independently of the presence of a functional myristate addition signal on either precursor. On the other hand, removal of myristate from one or the other precursor had nonreciprocal effects on virus particle formation. Cells expressing Pr55gag lacking myristate and Pr160gag-pol containing it did not produce particles. Cells expressing a myristylated Pr55gag and unmyristylated Pr160gag-pol still produced virus-like particles which contained nearly normal amounts of Pr160gag-pol. The results suggest that the incorporation of Pr160gag-pol into particles is largely determined by intermolecular protein-protein interactions between the two precursor polypeptides.     

Simian immunodeficiency virus RNA is efficiently encapsidated by human immunodeficiency virus type 1 particles.

Packaging of retroviral RNA is attained through the specific recognition of a cis-acting encapsidation site (located near the 5' end of the viral RNA) by components of the Gag precursor protein. Human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) are two lentiviruses that lack apparent sequence similarity in their putative encapsidation regions. We used SIV vectors to determine whether HIV-1 particles can recognize the SIV encapsidation site and functionally propagate SIV nucleic acid. SIV nucleic acid was replicated by HIV-1 proteins. Thus, efficient lentivirus pseudotyping can take place at the RNA level. Direct examination of the RNA contents of virus particles indicated that encapsidation of this heterologous RNA is efficient. Characterization of deletion mutants in the untranslated leader region of SIV RNA indicates that only a very short region at the 5' end of the SIV RNA is needed for packaging. Comparison of this region with the corresponding region of HIV-1 reveals that both are marked by secondary structures that are likely to be similar. Thus, it is likely that a similar higher-order RNA structure is required for encapsidation.     

Development of AIDS in a chimpanzee infected with human immunodeficiency virus type 1.

The condition of a chimpanzee (C499) infected with three different isolates of human immunodeficiency virus type 1 (HIV-1) for over 10 years progressed to AIDS. Disease development in this animal was characterized by (i) a decline in CD4+ cells over the last 3 years; (ii) an increase in viral loads in plasma; (iii) the presence of a virus, termed HIV-1JC, which is cytopathic for chimpanzee peripheral blood mononuclear cells; and (iv) the presence of an opportunistic infection and blood dyscrasias. Genetic analysis of the V1-V2 region of the envelope gene of HIV-1JC showed that the virus present in C499 was significantly divergent from all inoculating viruses (> or = 16% divergent at the amino acid level) and was suggestive of a large quasispecies. Blood from C499 transfused into an uninfected chimpanzee (C455) induced a rapid and sustained CD4+-cell decline in the latter animal, concomitant with high plasma viral loads. These results show that HIV-1 can induce AIDS in chimpanzees and suggest that long-term passage of HIV-1 in chimpanzees can result in the development of a more pathogenic virus.     

Prevalence of GB virus type C in urban Americans infected with human immunodeficiency virus type 1

Human T-lymphotropic virus type 1 (HTLV-1) and HTLV-2 were among the first human retroviruses discovered in the early 1980's. The International Retrovirology Association is an organized effort that fostered the efforts of scientists and clinicians to form interdisciplinary groups to study this group of retroviruses and their related diseases. The Association promotes excellent science, patient education, and fosters the training of young scientists to promote "bench-to-bedside" research. The International Conference on Human Retrovirology: HTLV and Related Viruses sponsored by the Association supports clinicians and researchers in the exchange of research findings and stimulation of new research directions. This years conference will be held from June 22 to 25, in Montego Bay, Jamaica http://www.htlvconference.org.jm/. Since its inception in 1988, these conferences have provided a highly interactive forum for the global community of HTLV scientists. This is of particular importance as HTLV research enters its third decade and a new generation of scientists takes over this important work. Many of the scientists attending the meeting will be from developing countries where HTLV is endemic, consistent with the history of international collaborations that have characterized HTLV research. The International Conference on Human Retrovirology provides a unique opportunity for researchers of all disciplines interested in HTLV infections to meet their peers and to address the questions facing clinicians and scientists who study retroviruses, like HTLV.     

TAR-independent replication of human immunodeficiency virus type 1 in glial cells.

The molecular mechanisms involved in the replication of human immunodeficiency virus type 1 (HIV-1) may differ in various cell types and with various exogenous stimuli. Astrocytic glial cells, which can support HIV-1 replication in cell cultures and may be infected in vivo, are demonstrated to provide a cellular milieu in which TAR mutant HIV-1 viruses may replicate. Using transfections of various TAR mutant HIV-1 proviral constructs, we demonstrate TAR-independent replication in unstimulated astrocytic cells. We further demonstrate, using viral constructs with mutations in the tat gene and in the nuclear factor kappa B (NF-kappa B)-binding sites (enhancer) of the HIV-1 long terminal repeat, that TAR-independent HIV-1 replication in astrocytic cells requires both intact NF-kappa B moiety-binding motifs in the HIV-1 long terminal repeat and Tat expression. We measured HIV-1 p24 antigen production, syncytium formation, and levels and patterns of viral RNA expression by Northern (RNA) blotting to characterize TAR-independent HIV-1 expression in astrocytic glial cells. This alternative regulatory pathway of TAR-independent, Tat-responsive viral production may be important in certain cell types for therapies which seek to perturb Tat-TAR binding as a strategy to interrupt the viral lytic cycle.     

CD63 is not required for production of infectious human immunodeficiency virus type 1 in human macrophages

During the assembly of human immunodeficiency virus type 1 (HIV-1) particles, the tetraspanin CD63 can be incorporated into the viral membrane. Indeed, cell surface tetraspanin microdomains that include CD63 have been proposed as sites for virus release. In addition, antibodies against CD63 can inhibit HIV infection of macrophages. In this cell type, HIV assembles into intracellularly sequestered plasma membrane domains that contain several other tetraspanins, including CD81, CD9, and CD53. CD63 is recruited to this domain following HIV infection. Together, these observations suggest that CD63 may have some function in the assembly of infectious virus particles and/or the infectivity of assembled virions. Here we have used RNA interference to knock down CD63 expression in monocyte-derived primary macrophages. We show that in the absence of CD63, HIV assembly is quantitatively comparable to that seen in CD63-expressing macrophages and that virus assembly occurs on compartments positive for CD81, CD9, and CD53. Moreover, the infectivity of macrophage-derived virus is unaffected by the loss of CD63. Together, our results indicate that at least in tissue culture, CD63 expression is not required for either the production or the infectivity of HIV-1.