NF-kappaB-repressing factor inhibits elongation of human immunodeficiency virus type 1 transcription by DRB sensitivity-inducing factor

Human immunodeficiency virus type 1 (HIV-1) is able to establish a latent infection during which the integrated provirus remains transcriptionally silent. In response to specific stimuli, the HIV-1 long terminal repeat (LTR) is highly activated, enhancing both transcriptional initiation and elongation. Here, we have identified a specific binding sequence of the nuclear NF-kappaB-repressing factor (NRF) within the HIV-1 LTR. The aim of this work was to define the role of NRF in regulating the LTR. Our data show that the endogenous NRF is required for transcriptional activation of the HIV-1 LTR in stimulated cells. In unstimulated cells, however, NRF inhibits HIV-1 LTR activity at the level of transcription elongation. Binding of NRF to the LTR in unstimulated cells prevents recruitment of elongation factor DRB sensitivity-inducing factor and formation of processive elongation complexes by hyperphosphorylated RNA polymerase II. Our data suggest that NRF interrupts the regulatory coupling of LTR binding factors and transcription elongation events. This inhibitory mechanism might contribute to transcriptional quiescence of integrated HIV-1 provirus.

Differential regulation of human immunodeficiency virus type 2 and simian immunodeficiency virus promoter activity

Promoter activity of the HIV-1 long terminal repeat (LTR) is largely dependent on intact NF-kB and SpI binding sites in the U3 region. In contrast, upstream LTR sequences allow efficient simian immunodeficiency virus (SIVmac) transcription in the absence of the core enhancer promoter region. In the present study, we investigated whether the regulation of HIV-2 Rod LTR activity is more reminiscent of HIV-1 having the same host or of SIVmac239 belonging to the same phylogenetic group. Viral promoter activity was studied in the context of the integrated provirus using both single cycle assays with pseudotyped luciferase reporter viruses and replication-competent HIV-2 LTR mutants. Our results demonstrate that intact SpI binding sites are important for both HIV-2 and SIVmac LTR activity in T cells and monocyte-derived macrophages. In contrast, deletion of the NF-kB binding site or of upstream regulatory sequences impaired HIV-2 Rod LTR activity but had little effect on SIVmac239 promoter function. Thus, similar to HIV-1, regulation of HIV-2 LTR promoter activity shows a low degree of functional redundancy possibly suggesting a specific adaptation to the human host.

Alterations in HIV-1 LTR promoter activity during AIDS progression

HIV-1 variants evolving in AIDS patients frequently show increased replicative capacity compared to those present during early asymptomatic infection. It is known that late stage HIV-1 variants often show an expanded coreceptor tropism and altered Nef function. In the present study we investigated whether enhanced HIV-1 LTR promoter activity might also evolve during disease progression. Our results demonstrate increased LTR promoter activity after AIDS progression in 3 of 12 HIV-1-infected individuals studied. Further analysis revealed that multiple alterations in the U3 core-enhancer and in the transactivation-response (TAR) region seem to be responsible for the enhanced functional activity. Our findings show that in a subset of HIV-1-infected individuals enhanced LTR transcription contributes to the increased replicative potential of late stage virus isolates and might accelerate disease progression.

The most frequent naturally occurring length polymorphism in the HIV-1 LTR has little effect on proviral transcription and viral replication

About 38% of primary HIV-1 long terminal repeats (LTRs) contain an insertion (consensus: 5prime prime or minute-ACYGCTGA-3prime prime or minute), termed the most frequent naturally occurring length polymorphism (MFNLP). The MFNLP binds several transcription factors and might affect HIV-1 replication and disease progression in infected individuals. However, its relevance for proviral transcription and for HIV-1 replication in primary cells is unclear. We utilized HIV-1 NL4-3 LTR variants to investigate the effect of the MFNLP on 5prime prime or minuteLTR transcriptional activity in various cell types. Notably, viral promoter activity was studied in primary cells in the context of the integrated provirus, using both single cycle assays with pseudotyped Luciferase reporter viruses and replication-competent HIV-1 mutants. Our results demonstrate that the presence, absence, or duplication of the 5prime prime or minute-ACYGCTGA-3prime prime or minute motif has little effect on viral promoter activity in T cell lines, peripheral blood mononuclear cells (PBMC), and monocyte-derived macrophages (MDM). Furthermore, all HIV-1 LTR variants showed efficient induction upon stimulation with TPA and/or ionomycin and replicated with comparable efficiency in a human T cell line and in PBMC. Thus, the MFNLP does not significantly affect HIV-1 5prime prime or minuteLTR transcriptional activity and viral replication in primary cells, suggesting that this common sequence variation has little impact on the clinical course of HIV-1 infection.

DC-SIGN interactions with human immunodeficiency virus: virus binding and transfer are dissociable functions

The C-type lectins DC-SIGN and DC-SIGNR capture and transfer human immunodeficiency virus (HIV) to susceptible cells, although the underlying mechanism is unclear. Here we show that DC-SIGN/DC-SIGNR-mediated HIV transmission involves dissociable binding and transfer steps, indicating that efficient virus transmission is not simply due to tethering of virus to the cell surface.

DC-SIGN interactions with human immunodeficiency virus type 1 and 2 and simian immunodeficiency virus

Dendritic cells (DCs) efficiently bind and transmit human immunodeficiency virus (HIV) to cocultured T cells and so may play an important role in HIV transmission. DC-SIGN, a novel C-type lectin that is expressed in DCs, has recently been shown to bind R5 HIV type 1 (HIV-1) strains and a laboratory-adapted X4 strain. To characterize the interaction of DC-SIGN with primate lentiviruses, we investigated the structural determinants of DC-SIGN required for virus binding and transmission to permissive cells. We constructed a panel of DC-SIGN mutants and established conditions which allowed comparable cell surface expression of all mutants. We found that R5, X4, and R5X4 HIV-1 isolates as well as simian immunodeficiency and HIV-2 strains bound to DC-SIGN and could be transmitted to CD4/coreceptor-positive cell types. DC-SIGN contains a single N-linked carbohydrate chain that is important for efficient cell surface expression but is not required for DC-SIGN-mediated virus binding and transmission. In contrast, C-terminal deletions removing either the lectin binding domain or the repeat region abrogated DC-SIGN function. Trypsin-EDTA treatment inhibited DC-SIGN mediated infection, indicating that virus was maintained at the surface of the DC-SIGN-expressing cells used in this study. Finally, quantitative fluorescence-activated cell sorting analysis of AU1-tagged DC-SIGN revealed that the efficiency of virus transmission was strongly affected by variations in DC-SIGN expression levels. Thus, variations in DC-SIGN expression levels on DCs could greatly affect the susceptibility of human individuals to HIV infection.