An ancient family of human endogenous retroviruses encodes a functional homolog of the HIV-1 Rev protein

The human endogenous retrovirus K (HERV-K) family of endogenous retroviruses consists of ≈50 proviral copies per haploid human genome. Herein, the HERV-Ks are shown to encode a sequence-specific nuclear RNA export factor, termed K-Rev, that is functionally analogous to the HIV-1 Rev protein. Like HIV-1 Rev, K-Rev binds to both the Crm1 nuclear export factor and to a cis-acting viral RNA target to activate nuclear export of unspliced RNAs. Surprisingly, this HERV-K RNA sequence, which is encoded within the HERV-K long terminal repeat, is also recognized by HIV-1 Rev. These data provide surprising evidence for an evolutionary link between HIV-1 and a group of endogenous retroviruses that first entered the human genome ≈30 million years ago.

HIV type-1 infection of the cotton rat (Sigmodon fulviventer and S. hispidus)

Cotton rats (Sigmodon hispidus and S. fulviventer) are susceptible to many viruses that infect humans (e.g., poliovirus, respiratory syncytial virus, influenza virus, adenovirus, and parainfluenza virus) and have been influential in developing therapeutic clinical intervention strategies for many viral infections of man. This study set out to determine whether cotton rats are susceptible to infection with HIV type 1 (HIV-1). Results indicate that HIV-1 does infect the cotton rat and S. fulviventer is more susceptible than S. hispidus. The virus was passaged from animal to animal for a total of three serial passages; but HIV replicated poorly in vivo, was only detectable as proviral DNA, and never exceeded one provirus per 1.8 × 105 cotton rat peripheral blood mononuclear cells. Infection induced a distinct and characteristic anti-HIV antibody response that, in some animals, included neutralizing antibodies, recognized all of the major HIV-1 antigens and the antibodies lasted out to 52 wk post-infection. Neonate S. fulviventer were not more susceptible to infection than adults. In vitro culture studies produced indirect evidence of viral replication by detection of viral gag gene RNA in reverse transcriptase–PCR assays on viral culture supernatants. Collectively, these results indicate that HIV-1 can replicate in a nontransgenic rodent and that this system may have potential as an animal model for HIV-1 infection if viral replication rates can be improved in vivo.

Stochastic processes strongly influence HIV-1 evolution during suboptimal protease-inhibitor therapy

It has long been assumed that HIV-1 evolution is best described by deterministic evolutionary models because of the large population size. Recently, however, it was suggested that the effective population size (Ne) may be rather small, thereby allowing chance to influence evolution, a situation best described by a stochastic evolutionary model. To gain experimental evidence supporting one of the evolutionary models, we investigated whether the development of resistance to the protease inhibitor ritonavir affected the evolution of the env gene. Sequential serum samples from five patients treated with ritonavir were used for analysis of the protease gene and the V3 domain of the env gene. Multiple reverse transcription–PCR products were cloned, sequenced, and used to construct phylogenetic trees and to calculate the genetic variation and Ne. Genotypic resistance to ritonavir developed in all five patients, but each patient displayed a unique combination of mutations, indicating a stochastic element in the development of ritonavir resistance. Furthermore, development of resistance induced clear bottleneck effects in the env gene. The mean intrasample genetic variation, which ranged from 1.2% to 5.7% before treatment, decreased significantly (P < 0.025) during treatment. In agreement with these findings, Ne was estimated to be very small (500–15,000) compared with the total HIV-1 RNA copy number. This study combines three independent observations, strong population bottlenecking, small Ne, and selection of different combinations of protease-resistance mutations, all of which indicate that HIV-1 evolution is best described by a stochastic evolutionary model.

HIV-1 and T cell dynamics after interruption of highly active antiretroviral therapy (HAART) in patients with a history of sustained viral suppression

Identifying the immunologic and virologic consequences of discontinuing antiretroviral therapy in HIV-infected patients is of major importance in developing long-term treatment strategies for patients with HIV-1 infection. We designed a trial to characterize these parameters after interruption of highly active antiretroviral therapy (HAART) in patients who had maintained prolonged viral suppression on antiretroviral drugs. Eighteen patients with CD4+ T cell counts ≥ 350 cells/μl and viral load below the limits of detection for ≥1 year while on HAART were enrolled prospectively in a trial in which HAART was discontinued. Twelve of these patients had received prior IL-2 therapy and had low frequencies of resting, latently infected CD4 cells. Viral load relapse to >50 copies/ml occurred in all 18 patients independent of prior IL-2 treatment, beginning most commonly during weeks 2–3 after cessation of HAART. The mean relapse rate constant was 0.45 (0.20 log10 copies) day−1, which was very similar to the mean viral clearance rate constant after drug resumption of 0.35 (0.15 log10 copies) day−1 (P = 0.28). One patient experienced a relapse delay to week 7. All patients except one experienced a relapse burden to >5,000 RNA copies/ml. Ex vivo labeling with BrdUrd showed that CD4 and CD8 cell turnover increased after withdrawal of HAART and correlated with viral load whereas lymphocyte turnover decreased after reinitiation of drug treatment. Virologic relapse occurs rapidly in patients who discontinue suppressive drug therapy, even in patients with a markedly diminished pool of resting, latently infected CD4+ T cells.

Antigen-driven CD4+ T cell and HIV-1 dynamics: Residual viral replication under highly active antiretroviral therapy

Antigen-induced stimulation of the immune system can generate heterogeneity in CD4+ T cell division rates capable of explaining the temporal patterns seen in the decay of HIV-1 plasma RNA levels during highly active antiretroviral therapy. Posttreatment increases in peripheral CD4+ T cell counts are consistent with a mathematical model in which host cell redistribution between lymph nodes and peripheral blood is a function of viral burden. Model fits to patient data suggest that, although therapy reduces HIV replication below replacement levels, substantial residual replication continues. This residual replication has important consequences for long-term therapy and the evolution of drug resistance and represents a challenge for future treatment strategies.

HIV-1 Tat transactivator recruits p300 and CREB-binding protein histone acetyltransferases to the viral promoter

In cells infected with HIV type 1 (HIV-1), the integrated viral promoter is present in a chromatin-bound conformation and is transcriptionally silent in the absence of stimulation. The HIV-1 Tat protein binds to a stem-loop structure at the 5′ end of viral mRNA and relieves this inhibition by inducing a remodeling of the nucleosome arrangement downstream of the transcription-initiation site. Here we show that Tat performs this activity by recruiting to the viral long terminal repeat (LTR) the transcriptional coactivator p300 and the closely related CREB-binding protein (CBP), having histone acetyltransferase (HAT) activity. Tat associates with HAT activity in human nuclear extracts and binds to p300 and CBP both in vitro and in vivo. Integrity of the basic domain of Tat is essential for this interaction. By a quantitative chromatin immunoprecipitation assay we show that the delivery of recombinant Tat induces the association of p300 and CBP with the chromosomally integrated LTR promoter. Expression of human p300 in both human and rodent cells increases the levels of Tat transactivation of the integrated LTR. These results reinforce the evidence that p300 and CBP have a pivotal function at both cellular and viral promoters and demonstrate that they also can be recruited by an RNA-targeted activator. Additionally, these findings have important implications for the understanding of the mechanisms of HIV-1 latency and reactivation.

TAK, an HIV Tat-associated kinase, is a member of the cyclin-dependent family of protein kinases and is induced by activation of peripheral blood lymphocytes and differentiation of promonocytic cell lines

We have previously identified a cellular protein kinase activity termed TAK that specifically associates with the HIV types 1 and 2 Tat proteins. TAK hyperphosphorylates the carboxyl-terminal domain of the large subunit of RNA polymerase II in vitro in a manner believed to activate transcription [Herrmann, C. H. & Rice, A. P. (1995) J. Virol. 69, 1612–1620]. We show here that the catalytic subunit of TAK is a known human kinase previously named PITALRE, which is a member of the cyclin-dependent family of proteins. We also show that TAK activity is elevated upon activation of peripheral blood mononuclear cells and peripheral blood lymphocytes and upon differentiation of U1 and U937 promonocytic cell lines to macrophages. Therefore, in HIV-infected individuals TAK may be induced in T cells following activation and in macrophages following differentiation, thus contributing to high levels of viral transcription and the escape from latency of transcriptionally silent proviruses.

The crystal structure of the Rev binding element of HIV-1 reveals novel base pairing and conformational variability

The crystal and molecular structure of an RNA duplex corresponding to the high affinity Rev protein binding element (RBE) has been determined at 2.1-Å resolution. Four unique duplexes are present in the crystal, comprising two structural variants. In each duplex, the RNA double helix consists of an annealed 12-mer and 14-mer that form an asymmetric internal loop consisting of G-G and G-A noncanonical base pairs and a flipped-out uridine. The 12-mer strand has an A-form conformation, whereas the 14-mer strand is distorted to accommodate the bulges and noncanonical base pairing. In contrast to the NMR model of the unbound RBE, an asymmetric G-G pair with N2-N7 and N1-O6 hydrogen bonding, is formed in each helix. The G-A base pairing agrees with the NMR structure in one structural variant, but forms a novel water-mediated pair in the other. A backbone flip and reorientation of the G-G base pair is required to assume the RBE conformation present in the NMR model of the complex between the RBE and the Rev peptide.

Antiretroviral resistance during successful therapy of HIV type 1 infection

HIV type 1 (HIV-1) drug resistance mutations were selected during antiretroviral therapy successfully suppressing plasma HIV-1 RNA to <50 copies/ml. New resistant mutant subpopulations were identified by clonal sequencing analyses of viruses cultured from blood cells. Drug susceptibility tests showed that biological clones of virus with the mutations acquired during successful therapy had increased resistance. Each of the five subjects with new resistant mutants had evidence of some residual virus replication during highly active antiretroviral therapy (HAART), based on transient episodes of plasma HIV-1 RNA > 50 copies/ml and virus env gene sequence changes. Each had received a suboptimal regimen before starting HAART. Antiretroviral-resistant HIV-1 can be selected from residual virus replication during HAART in the absence of sustained rebound of plasma HIV-1 RNA.

Influence of follicular dendritic cells on decay of HIV during antiretroviral therapy

Drug treatment of HIV type 1 (HIV-1) infection leads to a rapid initial decay of plasma virus followed by a slower second phase of decay. To investigate the role of HIV-1 retained on follicular dendritic cells (FDCs) in this process, we have developed and analyzed a mathematical model for HIV-1 dynamics in lymphoid tissue (LT) that includes FDCs. Analysis of clinical data using this model indicates that decay of HIV-1 during therapy may be influenced by release of FDC-associated virus. The biphasic character of viral decay can be explained by reversible multivalent binding of HIV-1 to receptors on FDCs, indicating that the second phase of decay is not necessarily caused by long-lived or latently infected cells. Furthermore, viral clearance and death of short-lived productively infected cells may be faster than previously estimated. The model, with reasonable parameter values, is consistent with kinetic measurements of viral RNA in plasma, viral RNA on FDCs, productively infected cells in LT, and CD4+ T cells in LT during therapy.

Trans-activation by human immunodeficiency virus Tat protein requires the C-terminal domain of RNA polymerase II.

Human immunodeficiency virus (HIV)-encoded trans-activator (Tat) acts through the trans-activation response element RNA stem-loop to increase greatly the processivity of RNA polymerase II. Without Tat, transcription originating from the HIV promoter is attenuated. In this study, we demonstrate that transcriptional activation by Tat in vivo and in vitro requires the C-terminal domain (CTD) of RNA polymerase II. In contrast, the CTD is not required for basal transcription and for the formation of short, attenuated transcripts. Thus, trans-activation by Tat resembles enhancer-dependent activation of transcription. These results suggest that effects of Tat on the processivity of RNA polymerase II require proteins that are associated with the CTD and may result in the phosphorylation of the CTD.

Differential inhibition of HIV-1 preintegration complexes and purified integrase protein by small molecules.

To replicate, HIV-1 must integrate a cDNA copy of the viral RNA genome into a chromosome of the host. The integration system is a promising target for antiretroviral agents, but to date no clinically useful integration inhibitors have been identified. Previous screens for integrase inhibitors have assayed inhibition of reactions containing HIV-1 integrase purified from an Escherichia coli expression system. Here we compare action of inhibitors in vitro on purified integrase and on subviral preintegration complexes (PICs) isolated from lymphoid cells infected with HIV-1. We find that many inhibitors active against purified integrase are inactive against PICs. Using PIC assays as a primary screen, we have identified three new anthraquinone inhibitors active against PICs and also against purified integrase. We propose that PIC assays are the closest in vitro match to integration in vivo and, as such, are particularly appropriate for identifying promising integration inhibitors.

New approach for inhibiting Rev function and HIV-1 production using the influenza virus NS1 protein.

The Rev protein of HIV-1, which facilitates the nuclear export of HIV-1 pre-mRNAs, has been a target for antiviral therapy. Here we describe a new strategy for inhibiting Rev function and HIV-1 replication. In contrast to previous approaches, we use a wild-type rather than a mutant Rev protein and covalently link this Rev sequence to the NS1 protein of influenza A virus, a protein that inhibits the nuclear export of mRNAs. The NS1 protein contains an RNA-binding domain mutation (RM), so that the only functional RNA-binding domain in the chimeric protein (NS1RM-Rev) is in the Rev protein sequence. In the presence of the NS1RM-Rev chimeric protein, HIV-1 pre-mRNAs were retained in, rather than exported from, the nucleus. In addition, this chimeric protein effectively inhibited Rev function in trans in transfection experiments and effectively inhibited the production of HIV-1 in tissue culture cells transfected with an infectious molecular clone of HIV-1 DNA. The inhibitory activities of the NS1RM-Rev chimera were at least equivalent to those of the Rev M10 mutant protein, which has been considered to be the prototype trans inhibitor of Rev function and is currently in phase I clinical trials for the treatment of AIDS patients. We discuss (i) the potential for increasing the inhibitory activity of NS1-Rev chimeras against HIV-1 and (ii) the need for additional studies to evaluate these chimeras for the treatment of AIDS.

A three-hybrid system to detect RNA-protein interactions in vivo.

RNA-protein interactions are pivotal in fundamental cellular processes such as translation, mRNA processing, early development, and infection by RNA viruses. However, in spite of the central importance of these interactions, few approaches are available to analyze them rapidly in vivo. We describe a yeast genetic method to detect and analyze RNA-protein interactions in which the binding of a bifunctional RNA to each of two hybrid proteins activates transcription of a reporter gene in vivo. We demonstrate that this three-hybrid system enables the rapid, phenotypic detection of specific RNA-protein interactions. As examples, we use the binding of the iron regulatory protein 1 (IRP1) to the iron response element (IRE), and of HIV trans-activator protein (Tat) to the HIV trans-activation response element (TAR) RNA sequence. The three-hybrid assay we describe relies only on the physical properties of the RNA and protein, and not on their natural biological activities; as a result, it may have broad application in the identification of RNA-binding proteins and RNAs, as well as in the detailed analysis of their interactions.

Host-parasite dynamics and outgrowth of virus containing a single K70R amino acid change in reverse transcriptase are responsible for the loss of human immunodeficiency virus type 1 RNA load suppression by zidovudine.

The association between human immunodeficiency virus type I (HIV-1) RNA load changes and the emergence of resistant virus variants was investigated in 24 HIV-1-infected asymptomatic persons during 2 years of treatment with zidovudine by sequentially measuring serum HIV-1 RNA load and the relative amounts of HIV-1 RNA containing mutations at reverse transcriptase (RT) codons 70 (K-->R), 41 (M-->L), and 215 (T-->Y/F). A mean maximum decline in RNA load occurred during the first month, followed by a resurgence between 1 and 3 months, which appeared independent of drug-resistance. Mathematical modeling suggests that this resurgence is caused by host-parasite dynamics, and thus reflects infection of the transiently increased numbers of CD4+ lymphocytes. Between 3 and 6 months of treatment, the RNA load returned to baseline values, which was associated with the emergence of virus containing a single lysine to arginine amino acid change at RT codon 70, only conferring an 8-fold reduction in susceptibility. Despite the relative loss of RNA load suppression, selection toward mutations at RT codons 215 and 41 continued. Identical patterns were observed in the mathematical model. While host-parasite dynamics and outgrowth of low-level resistant virus thus appear responsible for the loss of HIV-1 RNA load suppression, zidovudine continues to select for alternative mutations, conferring increasing levels of resistance.