Biological characterization of noninfectious HIV-1 particles lacking the envelope protein

HIV UTR, LTR, and Epigenetic Immunity

The duel between humans and viruses is unending. In this review, we examine the HIV RNA in the form of un-translated terminal region (UTR), the viral DNA in the form of long terminal repeat (LTR), and the immunity of human DNA in a format of epigenetic regulation. We explore the ways in which the human immune responses to invading pathogenic viral nucleic acids can inhibit HIV infection, exemplified by a chromatin vaccine (cVaccine) to elicit the immunity of our genome—epigenetic immunity towards a cure.

Pokeweed antiviral protein alters splicing of HIV-1 RNAs, resulting in reduced virus production

Processing of HIV-1 transcripts results in three populations in the cytoplasm of infected cells: full-length RNA, singly spliced, and multiply spliced RNAs. Rev, regulator of virion expression, is an essential regulatory protein of HIV-1 required for transporting unspliced and singly spliced viral transcripts from the nucleus to the cytoplasm. Export allows these RNAs to be translated and the full-length RNA to be packaged into virus particles. In our study, we investigate the activity of pokeweed antiviral protein (PAP), a glycosidase isolated from the pokeweed plant Phytolacca americana, on the processing of viral RNAs. We show that coexpression of PAP with a proviral clone alters the splicing ratio of HIV-1 RNAs. Specifically, PAP causes the accumulation of multiply spliced 2-kb RNAs at the expense of full-length 9-kb and singly spliced 4-kb RNAs. The change in splicing ratio is due to a decrease in activity of Rev. We show that PAP depurinates the rev open reading frame and that this damage to the viral RNA inhibits its translation. By decreasing Rev expression, PAP indirectly reduces the availability of full-length 9-kb RNA for packaging and translation of the encoded structural proteins required for synthesis of viral particles. The decline we observe in virus protein expression is not due to cellular toxicity as PAP did not diminish translation rate. Our results describing the reduced activity of a regulatory protein of HIV-1, with resulting change in virus mRNA ratios, provides new insight into the antiviral mechanism of PAP.

Pokeweed antiviral protein increases HIV-1 particle infectivity by activating the cellular mitogen activated protein kinase pathway

Pokeweed antiviral protein (PAP) is a plant-derived N-glycosidase that exhibits antiviral activity against several viruses. The enzyme removes purine bases from the messenger RNAs of the retroviruses Human immunodeficiency virus-1 and Human T-cell leukemia virus-1. This depurination reduces viral protein synthesis by stalling elongating ribosomes at nucleotides with a missing base. Here, we transiently expressed PAP in cells with a proviral clone of HIV-1 to examine the effect of the protein on virus production and quality. PAP reduced virus production by approximately 450-fold, as measured by p24 ELISA of media containing virions, which correlated with a substantial decline in virus protein synthesis in cells. However, particles released from PAP-expressing cells were approximately 7-fold more infectious, as determined by single-cycle infection of 1G5 cells and productive infection of MT2 cells. This increase in infectivity was not likely due to changes in the processing of HIV-1 polyproteins, RNA packaging efficiency or maturation of virus. Rather, expression of PAP activated the ERK1/2 MAPK pathway to a limited extent, resulting in increased phosphorylation of viral p17 matrix protein. The increase in infectivity of HIV-1 particles produced from PAP-expressing cells was compensated by the reduction in virus number; that is, virus production decreased upon de novo infection of cells over time. However, our findings emphasize the importance of investigating the influence of heterologous protein expression upon host cells when assessing their potential for antiviral applications.

In vitro treatment of human monocytes/macrophages with myristoylated recombinant Nef of human immunodeficiency virus type 1 leads to the activation of mitogen-activated protein kinases, IkappaB kinases, and interferon regulatory factor 3 and to the release of beta interferon

The viral protein Nef is a virulence factor that plays multiple roles during the early and late phases of human immunodeficiency virus (HIV) replication. Nef regulates the cell surface expression of critical proteins (including down-regulation of CD4 and major histocompatibility complex class I), T-cell receptor signaling, and apoptosis, inducing proapoptotic effects in uninfected bystander cells and antiapoptotic effects in infected cells. It has been proposed that Nef intersects the CD40 ligand signaling pathway in macrophages, leading to modification in the pattern of secreted factors that appear able to recruit and activate T lymphocytes, rendering them susceptible to HIV infection. There is also increasing evidence that in vitro cell treatment with Nef induces signaling effects. Exogenous Nef treatment is able to induce apoptosis in uninfected T cells, maturation in dendritic cells, and suppression of CD40-dependent immunoglobulin class switching in B cells. Previously, we reported that Nef treatment of primary human monocyte-derived macrophages (MDMs) induces a cycloheximide-independent activation of NF-kappaB and the synthesis and secretion of a set of chemokines/cytokines that activate STAT1 and STAT3. Here, we show that Nef treatment is capable of hijacking cellular signaling pathways, inducing a very rapid regulatory response in MDMs that is characterized by the rapid and transient phosphorylation of the alpha and beta subunits of the IkappaB kinase complex and of JNK, ERK1/2, and p38 mitogen-activated protein kinase family members. In addition, we have observed the activation of interferon regulatory factor 3, leading to the synthesis of beta interferon mRNA and protein, which in turn induces STAT2 phosphorylation. All of these effects require Nef myristoylation.

Human immunodeficiency virus type 1 (HIV-1) Nef activates STAT3 in primary human monocyte/macrophages through the release of soluble factors: involvement of Nef domains interacting with the cell endocytotic machinery

Increasing evidence indicates that the expression of the human immunodeficiency virus-1 (HIV-1) Nef protein significantly influences the activation state of the host cell. Here we report that Nef specifically activates STAT3 in primary human monocyte-derived macrophages (MDM). This was demonstrated by both single-cycle infection experiments driven by Vesicular Stomatitis virus glycoprotein (VSV-G) pseudotyped HIV-1 and treatment with exogenous recombinant Nef. The analysis of the effects of Nef mutants revealed that domains of the C-terminal flexible loop interacting with the cell endocytotic machinery are involved in the STAT3 activation. In particular, our data suggest that the Nef-dependent STAT3 activation relies on the targeting of Nef to the late endosome/lysosome compartment. In addition, we found that Nef activates STAT3 through a mechanism mediated by the release of soluble factor(s), including MIP-1alpha, that requires de novo protein synthesis but appears independent from the activation of src tyrosine kinases. The results presented here support the idea that the first intervention of Nef in the intracellular signaling of monocyte-macrophages could generate, by means of the release of soluble factor(s), a secondary wave of activation that could be of a potential pathogenetic significance.

Deletions in env gene of HIV-1 in AIDS patients treated with highly active antiretroviral therapy (HAART)

Many AIDS patients retain high CD4+ T-cell counts despite a significant increase in PCR viral load after varied periods of treatment on drug combination with Highly Active Antiretroviral Therapy (HAART). In order to investigate this contradictory phenomenon, we assayed for infectious HIV-1 from the plasma of such patients. Since the biological assays failed to reveal any infectious virus, we undertook molecular characterization of the plasma HIV-1 genes. These studies revealed large deletions in the env gene of the free virus, while there were no deletions in the proviral DNA obtained from the infected cells of the patients' blood. This suggests that the viral particles produced and released by the infected cells during the HAART treatment have deletions in the env gene. The deletions were large enough to produce an envelop-deficient virus, which can readily explain why it is not infectious. Such a defective virus is the most likely explanation for its failure to infect the T-cells, which in turn lead to the discordance between the high PCR viral load and stable CD4+ T cell counts.

HIV-1 Nef activates STAT1 in human monocytes/macrophages through the release of soluble factors

Monocytes/macrophages play a predominant role in the immunologic network by secreting and reacting to a wide range of soluble factors. Human immunodeficiency virus (HIV) infection leads to deep immunologic dysfunctions, also as a consequence of alterations in the pattern of cytokine release. Recent studies on in vivo models demonstrated that the expression of HIV Nef alone mimics many pathogenetic effects of HIV infection. In particular, Nef expression in monocytes/macrophages has been correlated with remarkable modifications in the pattern of secreted soluble factors, suggesting that the interaction of Nef with monocytes/macrophages plays a role in the pathogenesis of acquired immunodeficiency syndrome (AIDS). This study sought to define possible alterations in intracellular signaling induced by Nef in monocytes/macrophages. Results demonstrate that HIV-1 Nef specifically activates both alpha and beta isoforms of the signal transducer and activator of transcription 1 (STAT1). This was observed both by infecting human monocyte-derived macrophages (MDMs) with HIV-1 deletion mutants, and by exploiting the ability of MDMs to internalize soluble, recombinant Nef protein (rNef). STAT1-alpha activation occurs on phosphorylation of both C-terminal Tyr701 and Ser727 and leads to a strong binding activity. Nef-dependent STAT1 activation is followed by increased expression of both STAT1 and interferon regulatory factor-1, a transcription factor transcriptionally regulated by STAT1 activation. It was also established that Nef-induced STAT1- alpha/beta activation occurs through the secretion of soluble factors. Taken together, the results indicate that HIV-1 Nef could interfere with STAT1-governed intracellular signaling in human monocytes/macrophages.

Molecular cloning and biological characterization of full-length HIV-1 subtype C from Botswana

Human immunodeficiency virus type 1 (HIV-1) subtype C is now responsible for more than half of all HIV-1 infections in the global epidemic and for the high levels of HIV-1 prevalence in southern Africa. To facilitate studies of the biological nature and the underlying molecular determinants of this virus, we constructed eight full-length proviral clones from two asymptomatic and three AIDS patients infected with HIV-1 subtype C from Botswana. Analysis of viral lysates showed that Gag, Pol, and Env structural proteins were present in the virions. In four clones, the analysis suggested inefficient envelope glycoprotein processing. Nucleotide sequence analysis of the eight clones did not reveal frameshifts, deletions, premature truncations, or translational stop codons in any structural, regulatory, or accessory genes. None of the subtype C clones were replication competent in donor peripheral blood mononuclear cells (PBMCs), macrophages, Jurkat(tat) cells, or U87. CD4.CCR5 cells. However, infection by two clones could be rescued by complementation with a functional subtype C envelope clone, resulting in a productive infection of PBMCs, macrophages, and U87. CD4.CCR5 cells.

Human immunodeficiency virus Env-independent infection of human CD4(-) cells

CD4(-) epithelial cells covering mucosal surfaces serve as the primary barrier to prevent human immunodeficiency virus type 1 (HIV-1) infection. We used HIV-1 vectors carrying the enhanced green fluorescent protein gene as a reporter gene to demonstrate that HIV-1 can infect some CD4(-) human epithelial cell lines with low but significant efficiencies. Importantly, HIV-1 infection of these cell lines is independent of HIV-1 envelope proteins. The Env-independent infection of CD4(-) cells by HIV-1 suggests an alternative pathway for HIV-1 transmission. Even on virions bearing Env, a neutralizing antibody directed against gp120 is incapable of neutralizing the infection of these cells, thus raising potential implications for HIV-1 vaccine development.

cis expression of the F12 human immunodeficiency virus (HIV) Nef allele transforms the highly productive NL4-3 HIV type 1 to a replication-defective strain: involvement of both Env gp41 and CD4 intracytoplasmic tails

F12 human immunodeficiency virus type 1 (HIV-1) nef is a naturally occurring nef mutant cloned from the provirus of a nonproductive, nondefective, and interfering HIV-1 variant (F12-HIV). We have already shown that cells stably transfected with a vector expressing the F12-HIV nef allele do not downregulate CD4 receptors and, more peculiarly, become resistant to the replication of wild type (wt) HIV. In order to investigate the mechanism of action of such an HIV inhibition, the F12-HIV nef gene was expressed in the context of the NL4-3 HIV-1 infectious molecular clone by replacing the wt nef gene (NL4-3/chi). Through this experimental approach we established the following. First, NL4-3/chi and nef-defective (Deltanef) NL4-3 viral particles behave very similarly in terms of viral entry and HIV protein production during the first replicative cycle. Second, no viral particles were produced from cells infected with NL4-3/chi virions, whatever the multiplicity of infection used. The viral inhibition apparently occurs at level of viral assembling and/or release. Third, this block could not be relieved by in-trans expression of wt nef. Finally, NL4-3/chi reverts to a producer HIV strain when F12-HIV Nef is deprived of its myristoyl residue. Through a CD4 downregulation competition assay, we demonstrated that F12-HIV Nef protein potently inhibits the CD4 downregulation induced by wt Nef. Moreover, we observed a redistribution of CD4 receptors at the cell margin induced by F12-HIV Nef. These observations strongly suggest that F12-HIV Nef maintains the ability to interact with the intracytoplasmic tail of the CD4 receptor molecule. Remarkably, we distinguished the intracytoplasmic tails of Env gp41 and CD4 as, respectively, viral and cellular targets of the F12-HIV Nef-induced viral retention. For the first time, the inhibition of the viral life cycle by means of in-cis expression of a Nef mutant is here reported. Delineation of the F12-HIV Nef mechanism of action may offer additional approaches to interference with the propagation of HIV infection.

A mechanism of restricted human immunodeficiency virus type 1 expression in human glial cells

We characterized in detail the life cycle of human immunodeficiency virus type 1 (HIV-1) in human glioma H4/CD4 cells which stably express transfected CD4 DNA (B. Volsky, K. Sakai, M. Reddy, and D. J. Volsky, Virology 186:303-308, 1992). Infection of cloned H4/CD4 cells with the N1T strain of cell-free HIV-1 (HIV-1/N1T) was rapid and highly productive as measured by the initial expression of viral DNA, RNA, and protein, but all viral products declined to low levels by 14 days after infection. Chronically infected, virus-producing H4/CD4 cells could be obtained by cell cloning, indicating that HIV-1 DNA can integrate and remain expressed in these cells. The HIV-1 produced in H4/CD4 cells was noninfectious to glial cells, but it could be transmitted with low efficiency to CEM cells. Examination of viral protein composition by immunoprecipitation with AIDS serum or anti-gp120 antibody revealed that HIV-1/N1T-infected H4/CD4 cells produced all major viral proteins including gp160, but not gp120. Deglycosylation experiments with three different glycosidases determined that the absence of gp120 was not due to aberrant glycosylation of gp160, indicating a defect in gp160 proteolytic processing. Similar results were obtained in acutely and chronically infected H4/CD4 cells. To determine the generality of this HIV-1 replication phenotype in H4/CD4 cells, nine different viral clones were tested for replication in H4/CD4 cells by transfection. Eight were transiently productive like N1T, but one clone, NL4-3, established a long-lived productive infection in H4/CD4 cells, produced infectious progeny virus, and produced both gp160 and gp120. We conclude that for most HIV-1 strains tested, HIV-1 infection of H4/CD4 is restricted to a single cycle because of the defective processing of gp160, resulting in the absence of gp120 on progeny virus.

Molecular biology of human immunodeficiency virus type-1

Tremendous progress has been made in our understanding of the multiplication and pathogenesis of the human immunodeficiency virus, the causative agent of acquired immunodeficiency syndrome (AIDS). To block virus multiplication several targets in the life cycle of the virus have already been identified for which antiviral drugs can be developed and gene therapy can be envisaged as a possible treatment or cure of AIDS. The combination of several therapies might be needed for effective treatment. Prevention of HIV infections through effective vaccines still awaits novel, unconventional strategies.

Processing of the envelope glycoprotein gp160 in immunotoxin-resistant cell lines chronically infected with human immunodeficiency virus type 1

We describe the isolation and characterization of variant cell lines which are chronically infected with the human immunodeficiency virus (HIV) and resistant to the action of immunotoxins directed against the HIV envelope protein. These variants all produce normal levels of HIV proteins, budding virions, and the envelope protein precursor gp160. Two of the variants, 10E and 11E, contain a mutation within the env gene which results in the production of a truncated precursor and altered processing and transport of the protein to the cell surface. Variants B9 and G4 are defective in gp160 cleavage and do not efficiently transport the envelope protein to the cell surface. There are no mutations in the expressed viruses of B9 and G4. These cell lines express higher levels of CD4 protein and mRNA than H9/NL4-3. Thus, 10E, 11E, B9, and G4 have escaped immunotoxin action by downmodulating the envelope protein from their cell surfaces. None of these variants produce infectious HIV. Two other immunotoxin-resistant variants, E9-3 and 41-17, produce normal levels of gp160, efficiently transport the cleaved and processed subunits to the cell surface, and secrete infectious HIV. These studies identify alterations in gp160 processing that underscore the importance of the relationship between HIV and the cell that it infects.

Nascent human immunodeficiency virus type 1 reverse transcription occurs within an enveloped particle

Although a small amount of viral DNA has been shown to be enclosed within human immunodeficiency virus type 1 (HIV-1) virions, the majority of full-length viral DNA is formed after this virus infects target cells. Hence, we undertook investigations to identify the physical characteristics of the HIV-1 replication unit during the early events of infection. In these studies, nascent viral DNA synthesis was found to occur between 15 and 30 min after purified, DNase-treated HIV-1 virions were added to HUT 78 cells. At 1 h postinfection, a large amount of strong-stop viral DNA and some first-strand viral DNA had been synthesized. Several lines of evidence, including purification, nuclease digestion, and immunoprecipitation, indicated that these nascent viral DNAs were located within particles containing components such as reverse transcriptase and p24gag and gp120env proteins and having physical characteristics similar to those of intact virions.

An HIV-1 protease screening assay using a non-infectious proviral clone

An in-vitro assay was developed to screen for HIV-1 protease inhibitors using a non-infectious proviral clone (X19) with a deletion in the envelope gene (Ratner et al., 1991). The proviral clone, X19, was expressed transiently in the COS 7 cell line. The virus was able to replicate as assessed by the presence of p24 in the supernatants, yet the virions produced did not infect CD4 positive cells. To determine the effect of a known protease inhibitor on p24 antigen production, PD 148310 (a Ro 31-8959 analog) was added immediately after transfection of the COS 7 cells. Virus particles were produced maximally after 24 h and cell supernatants were assayed for p24 antigen production using a p24 ELISA assay. PD 148310 inhibited p24 release in a dose-dependent manner with an IC50 of 23.6 nM. Western blot analysis of the supernatants using a mouse monoclonal antibody against p24 confirmed the presence of a well-defined p24 band in the control lane. At 1000 nM of PD 148310 the p24 band was not detectable, leaving only the unprocessed p55 Gag precursor. This technique is a useful tool to screen for potential HIV-1 protease inhibitors.

In vivo infection of sheep by bovine leukemia virus mutants

Direct inoculation of a cloned bovine leukemia virus (BLV) provirus into sheep has allowed study of the viral infectivity of genetic mutants in vivo. Three BLV variants cloned from BLV-induced tumors and 12 in vitro-modified proviruses were isolated and analyzed for viral expression in cell culture. The proviruses were then inoculated into sheep in order to assess viral infectivity in vivo. Of three variants cloned from BLV-induced tumors (344, 395, and 1345), one (344) was found infectious in vivo. This particular provirus was used to engineer 12 BLV mutants. A hybrid between the 5' region of the complete but noninfectious provirus 395 and the 3' end of mutant 344 was infectious in vivo, suggesting that the tax/rex sequences were altered in virus 395. As expected, several regions of the BLV genome appeared to be essential for viral infection: the protease, pol, and env genes. Even discrete modifications in the fusion peptide located at the NH2 end of the transmembrane gp30 glycoprotein destroyed the infectious potential. In contrast, mutations and deletions in the X3 region present between the env gene and the 3' tax/rex region did not interfere with viral infection in vivo. This region of unknown function could thus be used to introduce foreign sequences. A BLV recombinant carrying a ribozyme directed against the tax/rex sequences was still infectious in vivo. Cotransfection of two noninfectious mutants carrying deletions led to infection in two of four independent injections, the infectious virus being then a recombinant between the two deletants. The experimental approach described here should help to gain insight into essential mechanisms such as in vivo viral replication, cooperation between deletants for viral infectivity, and viral superinfections. The gene products in the X3 and X4 region which are dispensable for in vivo infection could be involved in leukemogenesis, and thus proviruses deleted in these sequences could constitute the basis for a live attenuated vaccine.