Expression of HIV env gene in a human T cell line for a rapid and quantifiable cell fusion assay

Substitutions in the glycoprotein (GP) of the Candid#1 vaccine strain of Junin virus increase dependence on human transferrin receptor 1 for entry and destabilize the metastable conformation of GP

Candid#1 (Cd1) is an attenuated vaccine strain of Junin virus, the causative agent of Argentine hemorrhagic fever. Although several substitutions are present in Cd1, their importance for attenuation has not been established. We functionally characterized the substitutions present in the Cd1 glycoprotein (GP) and identified F427I in the transmembrane domain of the GP2 subunit as reducing infectivity in a reconstituted viral system. We further showed that this phenotype derives from the destabilization of the GP metastable conformation. Lastly, we identified an increased dependence of Cd1 GP on human transferrin receptor type 1 (hTfR-1) for entry, which may affect the tropism of the attenuated strain in vivo.

HIV-1-mediated syncytium formation promotes cell-to-cell transfer of Tax protein and HTLV-I gene expression

An important increase in luciferase activity was detected following co-culture of Jurkat T cells transiently transfected with an HTLV-I-LTR-driven reporter construct with HIV-1- and HTLV-I-infected cells. Production of infectious HTLV-I and expression of the HTLV-I envelope were not required for this HIV-1-dependent induction while it was severely hampered by anti-gp120 and anti-CD4 antibodies. The HTLV-I Tax protein and the TRE1 repeats were found to be necessary for the HIV-1-mediated enhancement of HTLV-I LTR activity in the co-culture assay. As these results suggested triple fusion events involving all three cell types and the intracellular transfer of Tax, we labelled each cell line with a distinct fluorescent probe. Through confocal microscopy, a number of resulting syncytia and cell clusters were indeed observed to be positive for all three probes. We are proposing a model in which HIV-1-mediated syncytium formation between HIV-1- and HTLV-I-infected cells and uninfected T cells forms a "bridge" or "tunnel" through which Tax from the HTLV-I-infected cells can diffuse and activate HTLV-I-LTR transcription.

Mechanism of feline immunodeficiency virus envelope glycoprotein-mediated fusion

Feline immunodeficiency virus (FIV) shares remarkable homology to primate lentiviruses, human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV). The process of lentiviral env glycoprotein-mediated fusion of membranes is essential for viral entry and syncytia formation. A detailed understanding of this phenomenon has helped identify new targets for antiviral drug development. Using a model based on syncytia formation between FIV env-expressing cells and a feline CD4+ T cell line we have studied the mechanism of FIV env-mediated fusion. Using this model we show that FIV env-mediated fusion mechanism and kinetics are similar to HIV env. Syncytia formation could be blocked by CXCR4 antagonist AMD3100, establishing the importance of this receptor in FIV gp120 binding. Interestingly, CXCR4 alone was not sufficient to allow fusion by a primary isolate of FIV, as env glycoprotein from FIV-NCSU(1) failed to induce syncytia in several feline cell lines expressing CXCR4. Syncytia formation could be inhibited at a post-CXCR4 binding step by synthetic peptide T1971, which inhibits interaction of heptad repeat regions of gp41 and formation of the hairpin structure. Finally, using site-directed mutagenesis, we also show that a conserved tryptophan-rich region in the membrane proximal ectodomain of gp41 is critical for fusion, possibly at steps post hairpin structure formation.

Automated quantitation of cell-mediated HIV type 1 infection of human syncytiotrophoblast cells by fluorescence in situ hybridization and laser scanning cytometry

Infection of human placental syncytiotrophoblast cells with HIV requires direct contact with infected leukocytes. In vitro investigations into mechanisms regulating placental HIV transmission and into the development of therapeutic interventions have been hampered by difficulties inherent in quantitating HIV levels in cocultures of infected lymphocytes and adherent multinucleated syncytiotrophoblast cells. Here, we have used fluorescence in situ hybridization (FISH) for the direct detection of HIV-1 RNA within syncytiotrophoblast cells combined with laser scanning cytometry (LSC) to quantitate HIV levels exclusively in the syncytiotrophoblast cells. HIV-1-infected lymphocytic MOLT-4 cells were cocultured with primary human syncytiotrophoblast cells. Lymphocytic cells were identified with an anti-vimentin antibody and Cy5. HIV RNA was localized by in situ hybridization, using a digoxigenin-labeled riboprobe detected by Oregon Green, and nuclei were stained with 7-aminoactinomycin D. The three-color cocultures were analyzed by LSC to remove unwanted cell populations and quantitate HIV expression levels. The total HIV RNA level (green fluorescence integral) in each colony was normalized for cell size by dividing by the total DNA content (red fluorescence integral). The nuclear-normalized fluorescence integral was 2.3 times higher in infected cocultures than in uninfected cultures. When cocultures were incubated with 10 microM AZT, the green/red fluorescence integral value was significantly lower than that of cocultures incubated in the absence of AZT, corresponding to a 78% reduction in fluorescence. Laser scanning cytometry can be used to quantitate cell-mediated HIV infection in syncytiotrophoblast cells and should allow drug assessment studies and studies aimed at understanding the mechanism of virus entry into trophoblast cells to be carried out.

Fluorescence-based quantitative methods for detecting human immunodeficiency virus type 1-induced syncytia

Cell fusion induced by human immunodeficiency virus type 1 (HIV-1) is usually assessed by counting multinucleated giant cells (syncytia) visualized by light microscopy. Currently used methods do not allow quantification of syncytia, nor do they estimate the number of cells involved in cell fusion. We describe two fluorescence-based methods for the detection and quantification of HIV-1-induced in vitro syncytium formation. The lymphoblastoid cell lines MT-2 and SupT1 were infected with syncytium-inducing (SI) HIV-1 isolates. Syncytia were detected by DNA staining with propidium iodide using flow cytometry to determine cell size or by two-color cytoplasmic staining of infected cell populations by using fluorescence microscopy. Both methods were able to detect and quantify HIV-induced syncytia. The methods could distinguish between SI and non-SI HIV isolates and could be used with at least two separate types of CD4(+) T-cell lines. Small syncytia can be readily identified by the two-color cytoplasmic staining method. Both methods were also shown to be useful for evaluating antiretroviral compounds, as demonstrated by the accurate assessment of HIV inhibition by azidothymidine (zidovudine), dideoxycytidine (zalcytibine), and hydroxyurea. These fluorescence-based assays allow a rapid and practical method for measuring HIV replication and anti-HIV activity of potential inhibitory compounds.

New reporter cell lines to study macrophage-tropic HIV envelope protein-mediated cell-cell fusion

The infection of human cells by HIV-1 virus can be mimicked by a fusion process between cells expressing the HIV envelope protein (Env) and cells expressing both human CD4 (huCD4) and appropriate human chemokine receptors. In this study, a macrophage-tropic (M-tropic) HIV cell-cell fusion assay was established that utilized huCD4, human CCR5 (huCCR5), and HIV ADAgpl60 as fusion components and a Gal4/VP16-activated luciferase as a reporter system. By combining CHO cells expressing huCD4 and huCCR5 with CHO cells expressing HIV ADAgpl60, a 300-fold increase in luciferase activity could be elicited relative to control. No luciferase activity was detected when HXB2gpl60 (T-tropic) was used instead of ADAgpl60 (M-tropic) as the fusion partner in the assay. Addition of anti-huCD4 (RPA-T4) or anti-huCCR5 (2D7) monoclonal antibodies in the assay significantly inhibited the fusion event; in contrast, an anti-CXCR4 (12G5) monoclonal antibody had little effect, indicating that the fusion assay was huCD4 and huCCR5 dependent. The cell-cell fusion occurred in a time-dependent manner; the maximum luciferase activity was detected about 8 hr after mixing the cells. The fusion events could also be monitored by another reporter system in which Gal4/VP16 activated green fluorescent protein (GFP) was used as the reporter instead of luciferase. In combination with fluorescence microscopy, the GFP reporter system allowed visualization of the fusion events in real time. Compared with previously described HIV fusion models, this system has several advantages, including simplicity, sensitivity, and the ability to allow continuous monitoring of the HIV cell-cell fusion event. Finally, this cell-cell fusion system is easily adapted to study other HIV fusion events.

CD40-Mediated induction of CD4 and CXCR4 on B lymphocytes correlates with restricted susceptibility to human immunodeficiency virus type 1 infection: potential role of B lymphocytes as a viral reservoir

Human immunodeficiency virus type 1 (HIV-1) replicates primarily in lymphoid tissues where it has ready access to activated immune competent cells. We used one of the major pathways of immune activation, namely, CD40-CD40L interactions, to study the infectability of B lymphocytes isolated from peripheral blood mononuclear cells. Highly enriched populations of B lymphocytes generated in the presence of interleukin-4 and oligomeric soluble CD40L upregulated costimulatory and activation markers, as well as HIV-1 receptors CD4 and CXCR4, but not CCR5. By using single-round competent luciferase viruses complemented with either amphotropic or HIV-derived envelopes, we found a direct correlation between upregulation of HIV-1 receptors and the susceptibility of the B lymphocytes to infection with dual-tropic and T-tropic strains of HIV-1; in contrast, cells were resistant to M-tropic strains of HIV-1. HIV-1 envelope-mediated infection was completely abolished with either an anti-CD4 monoclonal antibody or a peptide known to directly block CXCR4 usage and partially blocked with stromal cell-derived factor 1, all of which had no effect on the entry of virus pseudotyped with amphotropic envelope. Full virus replication kinetics confirmed that infection depends on CXCR4 usage. Furthermore, productive cycles of virus replication occurred rapidly yet under most conditions, without the appearance of syncytia. Thus, an activated immunological environment may induce the expression of HIV-1 receptors on B lymphocytes, priming them for infection with selective strains of HIV-1 and allowing them to serve as a potential viral reservoir.

Modulation of human immunodeficiency virus type 1-induced syncytium formation by the conformational state of LFA-1 determined by a new luciferase-based syncytium quantitative assay

The ICAM-1/LFA-1 interaction has been clearly demonstrated to play an active role in syncytium formation induced by human immunodeficiency virus type 1 (HIV-1). Since it is known that a high-affinity state of LFA-1 for ICAM-1 can be induced through conformational change, such a high-affinity state may also contribute to the process of syncytium formation. In this study, we have investigated the involvement of the conformational status of LFA-1 in HIV-1-dependent syncytium formation by using the anti-LFA-1 antibody NKI-L16, which is known to activate the high-affinity state. Initial visual observations by light microscopy indeed suggested that the addition of the NKI-L16 antibody led to bigger and more numerous syncytia when different cell lines were tested. To further analyze this NKI-L16-dependent increment of syncytium formation in a quantitative assay, a new luciferase-based assay was developed by using a T-cell line containing an HIV-1 long terminal repeat (LTR)-driven luciferase construct (1G5) in coincubation with an HIV-1-positive cell line (J1.1). Upon fusion, the viral Tat protein could diffuse to the 1G5 cells, leading to a transcriptional increase of the HIV-1 LTR-driven luciferase gene. Initial evaluation of this assay showed a good correlation between the level of syncytium formation determined by microscopic observation and the level of measured luciferase activity. In addition, this assay showed a greater induction of enzymatic activity correlating with syncytium formation in comparison to a similar incubation with the HeLa-CD4-LTR-beta-gal indicator cell line. By using this test, NKI-L16 treatment of 1G5/J1.1 cells led to a three- to sevenfold increase in HIV-1 LTR-driven luciferase activity. The syncytium-dependent luciferase activity in NKI-L16-treated cells could be blocked by classical syncytium inhibitors such as soluble CD4, anti-CD4, and anti-gp120 antibodies. Inhibition could also be observed with specific blocking agents for the chemokine receptor CXCR4, as well as with soluble ICAM-1, anti-LFA-1, anti-ICAM-1, and anti-ICAM-2 blocking antibodies, indicating the requirement for the LFA-1/ICAM interaction. Treatment of peripheral blood mononuclear cells with NKI-L16 resulted in a higher level of syncytium formation in the presence of the cell line J1.1. Conversely, when PBMCs were infected with two different syncytium-inducing HIV-1 primary isolates, coincubation with NKI-L16-pretreated 1G5 cells led to higher levels of luciferase activity for both virus isolates. Our results therefore show for the first time a direct role for the LFA-1 high-affinity state in virus-mediated syncytium formation. Based on the demonstration that an increase in ICAM-1 binding is induced by T-cell activation, these data suggest an in vivo involvement of the high-affinity state of LFA-1 in HIV-1-induced syncytium formation. Moreover, syncytia might preferentially occur in lymph nodes, since this microenvironment harbors a high proportion of activated T cells.

Interference of HIV-1 Nef in the sphingomyelin transduction pathway activated by tumour necrosis factor-alpha in human glial cells

OBJECTIVE

The HIV-1 nef gene product, thought to interact with mediators of cell signalling, is overexpressed during the restricted HIV-1 infection of human astrocytes. This infection can be reactivated following exposure to tumour necrosis factor (TNF)-alpha. We examined the possibility that Nef alters the TNF-alpha-induced cell signalling in astroglioma cells through the sphingomyelin pathway.

METHODS

Sphingomyelinase activation by TNF-alpha was analysed in U251MG glial cells constitutively expressing Nef and compared with U251MG cells stably transfected with the expression vector alone. The consequent effect on the cellular proliferative response and induction of nuclear factor NF-kappa B and AP-1 binding activities were examined.

RESULTS

A marked enhancement in the levels of ceramide, a product of the sphingomyelin hydrolysis, was observed in U251MG-Nef upon stimulation with TNF-alpha. In contrast, ceramide levels in control cells were barely increased under similar conditions. A concomitant reduction of sphingomyelin level occurred in U251MG-Nef cells. In addition, the reduced survival rate of U251MG cells resulting from TNF-alpha activation was prevented in the presence of Nef. Furthermore, electrophoretic mobility shift assays indicated that nef expression inhibits AP-1 activation without altering the induction of NF-kappa B.

CONCLUSION

These results strongly suggest that nef expression in U251MG cells modulates the sphingomyelinase signalling pathway triggered by TNF-alpha, thus leading to important modifications in the activation and proliferation of glial cells. They also provide new insights to explain the widespread reactive astrogliosis observed in AIDS-associated neuropathological disorders.

Postbinding events mediated by human immunodeficiency virus type 1 are sensitive to modifications in the D4-transmembrane linker region of CD4

Evidence from both structural and functional studies of the CD4 molecule suggests that several domains, including the transmembrane (TM) domain and the adjoining extracellular region (D4-TM linker), contribute to the post-gp12O-binding events leading to human immunodeficiency virus-mediated membrane fusion. To investigate such a role in syncytium formation and cell-free infectivity, we generated several deletion and substitution mutations in the TM and D4-TM linker regions of the CD4 molecule. We found that while the TM domain of CD4 was dispensable for cell-cell and virus-cell interactions, modifications in the D4-TM linker led to perturbations in both processes. Deletion of the five amino acid residues linking D4 to the TM domain resulted in a delayed and reduced capacity to form syncytia, whereas replacement of the residues with the heterologous sequence from the CD8 molecule restored the kinetic profile to wild-type CD4 levels. On the other hand, both mutants of the CD4 D4-TM linker demonstrated delayed cell-free human immunodeficiency virus type 1 infectivity profiles. The defective fusion capacity may be linked to structural perturbations identified with anti-CD4 monoclonal antibodies in the D1-D2 interface and D3 domain of the deletion mutant yet absent in D1 and D4. While all cells were found to bind comparable levels of gp120, both D4-TM linker mutants appeared to induce a decrease in the V3 loop exposure of bound gp120. This underexposure may explain the delays in cell-free infectivities observed for both of these mutants. Together, these findings confirm a role for regions of the CD4 molecule located outside D1 in post-gp120-binding events and suggest that the D4-TM interface contributes to the conformational changes that direct the fusion process.