Quantitative measurement of fusion between human immunodeficiency virus and cultured cells using membrane fluorescence dequenching

Targeted gene delivery systems for T-cell engineering

T lymphocytes are indispensable for the host systems of defense against pathogens, tumors, and environmental threats. The therapeutic potential of harnessing the cytotoxic properties of T lymphocytes for antigen-specific cell elimination is both evident and efficacious. Genetically engineered T-cells, such as those employed in CAR-T and TCR-T cell therapies, have demonstrated significant clinical benefits in treating cancer and autoimmune disorders. However, the current landscape of T-cell genetic engineering is dominated by strategies that necessitate in vitro T-cell isolation and modification, which introduce complexity and prolong the development timeline of T-cell based immunotherapies. This review explores the complexities of gene delivery systems designed for T cells, covering both viral and nonviral vectors. Viral vectors are known for their high transduction efficiency, yet they face significant limitations, such as potential immunogenicity and the complexities involved in large-scale production. Nonviral vectors, conversely, offer a safer profile and the potential for scalable manufacturing, yet they often struggle with lower transduction efficiency. The pursuit of gene delivery systems that can achieve targeted gene transfer to T cell without the need for isolation represents a significant advancement in the field. This review assesses the design principles and current research progress of such systems, highlighting the potential for in vivo gene modification therapies that could revolutionize T-cell based treatments. By providing a comprehensive analysis of these systems, we aim to contribute valuable insights into the future development of T-cell immunotherapy.

Understanding Retroviral Life Cycle and its Genomic RNA Packaging

Members of the family Retroviridae are important animal and human pathogens. Being obligate parasites, their replication involves a series of steps during which the virus hijacks the cellular machinery. Additionally, many of the steps of retrovirus replication are unique among viruses, including reverse transcription, integration, and specific packaging of their genomic RNA (gRNA) as a dimer. Progress in retrovirology has helped identify several molecular mechanisms involved in each of these steps, but many are still unknown or remain controversial. This review summarizes our present understanding of the molecular mechanisms involved in various stages of retrovirus replication. Furthermore, it provides a comprehensive analysis of our current understanding of how different retroviruses package their gRNA into the assembling virions. RNA packaging in retroviruses holds a special interest because of the uniqueness of packaging a dimeric genome. Dimerization and packaging are highly regulated and interlinked events, critical for the virus to decide whether its unspliced RNA will be packaged as a "genome" or translated into proteins. Finally, some of the outstanding areas of exploration in the field of RNA packaging are highlighted, such as the role of epitranscriptomics, heterogeneity of transcript start sites, and the necessity of functional polyA sequences. An in-depth knowledge of mechanisms that interplay between viral and cellular factors during virus replication is critical in understanding not only the virus life cycle, but also its pathogenesis, and development of new antiretroviral compounds, vaccines, as well as retroviral-based vectors for human gene therapy.

Viral vector platforms within the gene therapy landscape

Throughout its 40-year history, the field of gene therapy has been marked by many transitions. It has seen great strides in combating human disease, has given hope to patients and families with limited treatment options, but has also been subject to many setbacks. Treatment of patients with this class of investigational drugs has resulted in severe adverse effects and, even in rare cases, death. At the heart of this dichotomous field are the viral-based vectors, the delivery vehicles that have allowed researchers and clinicians to develop powerful drug platforms, and have radically changed the face of medicine. Within the past 5 years, the gene therapy field has seen a wave of drugs based on viral vectors that have gained regulatory approval that come in a variety of designs and purposes. These modalities range from vector-based cancer therapies, to treating monogenic diseases with life-altering outcomes. At present, the three key vector strategies are based on adenoviruses, adeno-associated viruses, and lentiviruses. They have led the way in preclinical and clinical successes in the past two decades. However, despite these successes, many challenges still limit these approaches from attaining their full potential. To review the viral vector-based gene therapy landscape, we focus on these three highly regarded vector platforms and describe mechanisms of action and their roles in treating human disease.

Anionic Liposomes Inhibit Human Immunodeficiency Virus Type 1 (HIV-1) Infectivity in CD4+ A3.01 and H9 Cells

Immunodeficiency viruses undergo fusion with liposomes containing anionic phospholipids (Larsen etal., 1990). We have investigated the effect of liposomes composed of cardiolipin, phosphatidylserine or phosphatidylinositol, on the infectivity of three strains of HIV-1 in A3.01 and H9 cells, measured by p24 (gag) production in the medium. The infectivity of HIV-1 in A3.01 or H9 cells was inhibited by the presence of cardiolipin liposomes during a 2 h infection period, with IC50's of 23.0, 4.8, and 5.0 μM phospholipid, respectively, for the different strains. Liposomes composed of phosphatidylserine or phosphatidylinositol were ineffective under similar conditions. However, prolonged pre-incubation of the virus with these liposomes also inhibited infectivity. Inhibition of virus binding to cells could not account for the inhibition of infectivity. We propose that the fusion products of HIV-1 and anionic liposomes are impaired in their ability to fuse with the plasma membrane.

Fluorosomes: fluorescent virus-like nanoparticles that represent a convenient tool to visualize receptor-ligand interactions

Viruses are the smallest life forms and parasitize on many eukaryotic organisms, including humans. Consequently, the study of viruses and viral diseases has had an enormous impact on diverse fields of biology and medicine. Due to their often pathogenic properties, viruses have not only had a strong impact on the development of immune cells but also on shaping entire immune mechanisms in their hosts. In order to better characterize virus-specific surface receptors, pathways of virus entry and the mechanisms of virus assembly, diverse methods to visualize virus particles themselves have been developed in the past decades. Apart from characterization of virus-specific mechanisms, fluorescent virus particles also serve as valuable platforms to study receptor-ligand interactions. Along those lines the authors have developed non-infectious virus-like nanoparticles (VNP), which can be decorated with immune receptors of choice and used for probing receptor-ligand interactions, an especially interesting application in the field of basic but also applied immunology research. To be able to better trace receptor-decorated VNP the authors have developed technology to introduce fluorescent proteins into such particles and henceforth termed them fluorosomes (FS). Since VNP are assembled in a simple expression system relying on HEK-293 cells, gene-products of interest can be assembled in a simple and straightforward fashion—one of the reasons why the authors like to call fluorosomes ‘the poor-man's staining tool’. Within this review article an overview on virus particle assembly, chemical and recombinant methods of virus particle labeling and examples on how FS can be applied as sensors to monitor receptor-ligand interactions on leukocytes are given.

Inhibitory effect of chondroitin sulfate type E on the binding step of human T-cell leukemia virus type 1

Cell surface heparan sulfate proteoglycans (HSPGs) are involved in the binding and entry of human T-cell leukemia virus type 1 (HTLV-1) into host cells, while sulfated polysaccharides such as heparin inhibit HTLV-1 infection. Chondroitin sulfate proteoglycans (CSPGs) are classified as another major type of proteoglycans. Here, we examined the effect of four types of chondroitin sulfate (CS) on HTLV-1 infection. Accordingly, a human T cell line, MOLT-4, was inoculated with cell-free HTLV-1 in the presence or absence of soluble CS, and the synthesis of reverse-transcribed HTLV-1 DNA within cells 20 h after inoculation was detected using polymerase chain reaction (PCR). Among the four types of CS (A, C, D, and E), the E type (CSE), which was derived from the squid cartilage, exhibited anti-HTLV-1 activity. Furthermore, we observed that CSE directly interacted with recombinant HTLV-1 envelope (Env) proteins and inhibited the binding of HTLV-1 virions to MOLT-4 cells, indicating that the interaction between Env and CSE plays a significant role in its anti-HTLV-1 activity. In addition, CSE inhibited syncytium formation that was induced by HTLV-1-producing cells. When CSE was mixed with the synthetic fusion inhibitor peptide corresponding to the ectodomain of the Env transmembrane subunit (TM) gp21, the HTLV-1 infection was further inhibited when compared with the inhibitory effect of each compound alone. Thus, further elucidation of the in vitro antiviral mechanism of CSE shown in this study will lead to the development of CSE-like molecules for the entry inhibition of HTLV-1.

Betaretroviral envelope subunits are noncovalently associated and restricted to the mammalian class

The structure of the transmembrane subunit (TM) of the retroviral envelope glycoprotein (Env) is highly conserved among most retrovirus genera and includes a pair of cysteines that forms an intramolecular disulfide loop within the ectodomain. Alpha-, gamma-, and deltaretroviruses have a third cysteine, adjacent to the loop, which forms a disulfide bond between TM and the surface subunit (SU) of Env, while lentiviruses, which have noncovalently associated subunits, lack this third cysteine. The Betaretrovirus genus includes Jaagsiekte sheep retrovirus (JSRV) and mouse mammary tumor virus (MMTV), as well as many endogenous retroviruses. Envelope subunit association had not been characterized in the betaretroviruses, but lack of a third cysteine in the TM ectodomain suggested noncovalently associated subunits. We tested the Env proteins of JSRV and MMTV, as well as human endogenous retrovirus K (HERV-K)108--a betaretrovirus-like human endogenous retrovirus--for intersubunit bonding and found that, as in the lentiviruses, the Env subunits lack an intersubunit disulfide bond. Since these results suggest that the number of cysteines in the TM loop region readily distinguishes between covalent and noncovalent structure, we surveyed endogenous retroviral TM sequences in the genomes of vertebrates represented in public databases and found that (i) retroviruses with noncovalently associated subunits have been present during all of anthropoid evolution and (ii) the noncovalent env motif is limited to mammals, while the covalent type is found among five vertebrate classes. We discuss implications of these findings for retroviral evolution, cross-species transmissions, and recombination events involving the env gene.

Novel, rapid assay for measuring entry of diverse enveloped viruses, including HIV and rabies

Entry is the first and essential step in virus replication and is a target for therapeutic intervention. However, current knowledge on entry mechanism for the majority of viruses is poor, partly due to lack of a simple, sensitive and accurate entry assay that can be applied to diverse viruses. To overcome this obstacle, a novel contents-mixing-based virus entry assay is described that can be broadly applied to many enveloped viruses. By fusing firefly luciferase to the HIV Nef protein, luciferase was directly packaged into HIV particles pseudotyped with envelope proteins of diverse viruses including HIV, rabies and others. Upon cell entry, the luciferase-fusion protein was released into the cell cytoplasm, reacted with its substrates and was detected by light emission. The assay was validated by demonstrating its versatility in measuring virus entry. Entry was detected much more rapidly (in real-time) with higher sensitivity (a multiplicity of infection <0.1 gives a robust signal) and lower background (signal/noise ration >1000) than other comparable assays. In addition to its utility in studying virus entry mechanisms, the assay will aid in screening potential entry/fusion inhibitors and in diagnosis of virus infections.

Rapid and sensitive detection of retrovirus entry by using a novel luciferase-based content-mixing assay

We describe a novel assay that permits measurement of entry of murine leukemia virus and pseudotypes with greater sensitivity and more rapidly than previously possible. To achieve this, we encapsulated a sensitive reporter enzyme, luciferase, directly into fully infectious, intact viral particles. The enzyme is specifically targeted to the viral lumen, as a C-terminal fusion on the viral envelope protein. Only when the incorporated luciferase is released from the viral lumen and gains access to its substrates is light emitted and readily detected. When cells are perfused with luciferin, quantitative measurements of entry can be made in real time on live cells. Uniquely, the amount of cell-bound virus can be determined in the same assay by addition of detergent to expose the luciferase. We demonstrate that virus carrying a mutation in the fusion peptide binds normally to cells but is unable to infect them and gives no entry signal. Using this assay, we show that inhibitors of endosomal acidification inhibit signal from vesicular stomatitis virus pseudotypes but not murine leukemia virus, consistent with a pH-independent mode of entry for the latter virus. Additionally, the fusion kinetics are rapid, with a half-life of 25 min after a delay of 10 to 15 min. The future use of this assay will permit a detailed examination of the entry mechanism of viruses and provide a convenient platform to discover novel entry inhibitors. The design also permits packaging of potential therapeutic protein cargoes into functional virus particles and their specific delivery to cellular targets.

Low pH is required for avian sarcoma and leukosis virus Env-induced hemifusion and fusion pore formation but not for pore growth

Binding of avian sarcoma and leukosis virus (ASLV) to its cognate receptor on the cell surface causes conformational changes in its envelope protein (Env). It is currently debated whether low pH is required for ASLV infection. To elucidate the role of low pH, we studied the association between ASLV subgroup B (ASLV-B) and liposomes and fusion between effector cells expressing Env from ASLV-A and ASLV-B and target cells expressing cognate receptors. Neither EnvA nor EnvB promoted cell-cell fusion at neutral pH, but lowering the pH resulted in quick and extensive fusion. As expected for a low-pH-triggered reaction, fusion was a steep function of pH. Steps that required low pH were identified. Binding a soluble form of the receptor caused ASLV-B to hydrophobically associate with liposome membranes at neutral pH, indicating that low pH is not required for insertion of Env's fusion peptides into membranes. But both cell-cell hemifusion and fusion pore formation were pH dependent. It is proposed that fusion peptide insertion stabilizes the conformation of ASLV Env into a form that can be acted upon by low pH. At this point, but not before, low pH can induce fusion and is in fact required for fusion to occur. However, low pH is no longer necessary after formation of the initial fusion pore: pore enlargement does not require low pH.

A point mutation in the binding subunit of a retroviral envelope protein arrests virus entry at hemifusion

The transmembrane subunits of viral envelope proteins are thought to perform all of the functions required for membrane fusion during entry of enveloped viruses. However, changes in a conserved SPHQ motif near the N terminus of the receptor binding subunit of a murine leukemia virus (MLV) envelope protein block infection and induction of cell-cell fusion but not receptor binding. Here we report evidence that a histidine-to-arginine change at position 8 (H8R) in the SPHQ motif of Moloney MLV blocks infection by arresting virus-cell fusion at the hemifusion state. In cell-cell fusion assays, H8R envelope protein induced mixing of membrane outer leaflet lipids but did not lead to content mixing, a finding indicative of fusion pore formation. Kinetic studies of virus-cell fusion showed that lipid mixing of H8R virus membranes begins much later than for wild-type virus. The length of the delay in lipid mixing decreased upon addition of two second-site changes that increase H8R virus infection to 100-fold less than the wild-type virus. Finally, chlorpromazine, dibucaine, and trifluoperazine, agents that induce pores in an arrested hemifusion state, rescued infection by H8R virus to within 2.5-fold of the level of wild-type virus infection and cell-cell fusion to half that mediated by wild-type envelope protein. We interpret these results to indicate that fusion progressed to the hemifusion intermediate but fusion pore formation was inhibited. These results establish that membrane fusion of Moloney MLV occurs via a hemifusion intermediate. We also interpret these findings as evidence that histidine 8 is a key switch-point residue between the receptor-induced conformation changes that expose fusion peptide and those that lead to six-helix bundle formation.

Polyanions--a lost chance in the fight against HIV and other virus diseases?

Polyanions are known to exhibit potent antiviral activity in vitro, and may represent future therapeutic agents. This review summarizes literature reports, pertinent to anionic polymers as antiviral agents. The in vitro antiviral effects of numerous polyanionic compounds (sulphated polysaccharides, negatively charged serum albumin and milk proteins, synthetic sulphated polymers, polymerized anionic surfactants and polyphosphates) are described. This class of antiviral agent exhibits several unique properties that are not shared by other presently known antiviral agents: (i) a remarkable broad-spectrum antiviral activity against HIV-1, HIV-2 and a series of other enveloped viruses; (ii) the ability to inhibit syncytium formation between HIV-infected and normal CD4 T lymphocytes, a mechanism that drastically enhances HIV infectivity; and (iii) a low induction of viral drug-resistance. There is increasing evidence that polyanions interfere with the fusion process, a vital step in the viral replication cycle. The inhibition of virus-cell fusion appears to be the source of the antiviral activity of polyanions. In vivo, the pharmacological properties of polyanions result in a low bioavailability of the drugs to their viral targets, and hence a poor antiviral activity in vivo. It is suggested that polyanions must be used in combination with drug delivery systems in order to become therapeutically useful antiviral agents. Some drug delivery systems are briefly discussed.

Structures and mechanisms in flavivirus fusion

This chapter focuses on the work carried out with tick-borne encephalitis (TBE) virus, the structurally best characterized of the flaviviruses. The data is related to those obtained with other flaviviruses, which are assumed to have a conserved structural organization, and compare the characteristics of flavivirus fusion to those of other enveloped viruses. Fusion proteins from several different virus families, including Orthomyxoviridae , Paramyxoviridae , Retroviridae , and Filoviridae have been shown to exhibit striking structural similarities; they all use a common mechanism for inducing membrane fusion, and the same general model applies to all of these cases. The flavivirus genome is a positive-stranded RNA molecule consisting of a single, long open reading frame of more than 10,000 nucleotides flanked by noncoding regions at the 5′ and 3′ ends. The fusion properties of flaviviruses have been investigated using several different assay systems, including virus-induced cell–cell fusion and virus–liposome fusion. All of these studies indicate that flaviviruses require an acidic pH for fusion, consistent with their proposed mode of entry.

Use of a gp120 binding assay to dissect the requirements and kinetics of human immunodeficiency virus fusion events

Binding of the extracellular subunit of human immunodeficiency type 1 (HIV-1) envelope (Env) glycoprotein (gp120) to CD4 triggers the induction or exposure of a highly conserved coreceptor binding site in gp120 that helps mediate membrane fusion. Characterizing the structural features involved in gp120-coreceptor binding and the conditions under which binding occurs is important for understanding the fusion process, the evolution of pathogenic strains in vivo, the identification of novel anti-HIV compounds, and the development of HIV vaccines that utilize triggered structures of Env. Here we use the kinetics of interaction between CCR5 and gp120 to understand temporal and structural changes that occur during viral fusion. Using saturation binding and homologous competition analysis, we estimated the K(d) of interaction between CCR5 and gp120 from the macrophage tropic HIV-1 strain JRFL to be 4 nM. Unlike Env-mediated fusion, gp120 binding to CCR5 did not require divalent cations or elevated temperatures. Binding was not significantly affected by the pH of binding, G-protein coupling of CCR5, or partial gp120 deglycosylation. Oligomeric, uncleaved JRFL gp140 failed to bind CCR5 despite its ability to bind CD4 and monoclonal antibody 17b, suggesting that the uncleaved ectodomain of gp41 interferes with full exposure of the chemokine receptor binding site. Exposure of the chemokine receptor binding site on gp120 could be induced rapidly by CD4, but exposure of this site was lost upon CD4 dissociation from gp120, indicating that the conformational changes in gp120 induced by CD4 binding are fully reversible. The functional gp120-soluble CD4 complex was remarkably stable over time and temperature ranges, offering the possibility that complexes in which the highly conserved coreceptor binding site in gp120 is exposed can be used for vaccine development.

Acidic pH enhancement of the fusion of Newcastle disease virus with cultured cells

Fusion of the lentogenic strain "Clone 30" of Newcastle disease virus (NDV) with the cell line COS-7 has been studied. Fusion was monitored using the octadecylrhodamine B chloride dequenching assay [Hoekstra, D., de Boer, T., Klappe, K. and Wilschut, J. (1984). Biochemistry 23, 5675-5681]. In the present work, fusion of NDV with COS-7 cells was found to occur in a time- and temperature-dependent fashion. Significant dequenching of the probe occurred at temperatures higher than 28 degrees C. A 20-fold excess of unlabeled virus inhibited fusion by about 53% compared with the control, whereas 62% inhibition of fusion was obtained after digestion of viral glycoproteins with trypsin. The data are discussed in terms of the nonfusion transfer of the probe. In addition, preincubation of cells with 50 mM ammonium chloride or 0.1% sodium azide prevented NDV from fusing with COS-7 cells by about 30% in comparison with the control. The cytopathic effect of NDV infection in cell culture in the presence of ammonium chloride was reduced compared with control. Moreover, viral preincubation at pH 5 yielded a mild inhibition of fusogenic activity. Our results suggest that NDV may use the endocytic pathway as a complementary way of entering cells by direct fusion with the plasma membrane.

Membrane interactions of the synthetic N-terminal peptide of HIV-1 gp41 and its structural analogs

Structural and functional studies assessed the membrane actions of the N terminus of HIV-1 glycoprotein 41000 (gp41). Earlier site-directed mutagenesis has shown that key amino acid changes in this gp41 domain inhibit viral infection and syncytia formation. Here, a synthetic peptide corresponding to the N terminus of gp41 (FP; 23 residues, 519-541), and also FP analogs (FP520V/E with Val-->Glu at residue 520; FP527L/R with Leu-->Arg at 527; FP529F/Y with Phe-->Tyr at 529; and FPCLP1 with FP truncated at 525) incorporating these modifications were prepared. When added to human erythrocytes at physiologic pH, the lytic and aggregating activities of the FP analogs were much reduced over those with the wild-type FP. With resealed human erythrocyte ghosts, the lipid-mixing activities of the FP analogs were also substantially depressed over that with the wild-type FP. Combined with results from earlier studies, theoretical calculations using hydrophobic moment plot analysis and physical experiments using circular dichroism and Fourier transform infrared spectroscopy indicate that the diminished lysis and fusion noted for FP analogs may be due to altered peptide-membrane lipid interactions. These data confirm that the N-terminal gp41 domain plays critical roles in the cytolysis and fusion underlying HIV-cell infection.

Probe transfer with and without membrane fusion in a fluorescence fusion assay

An analysis of the R18 fusion assay was made during the fusion of the Sendai virus with erythrocyte ghosts. The increase in R18 fluorescence, reflecting the interaction process, was evaluated in terms of the different processes that in principle may contribute to this increase, that is, monomeric probe transfer, hemifusion, and complete fusion. To this end, the kinetics of the R18-labeled lipid mixing were compared to those obtained with an assay in which the fusion-monitoring probe, eosin-maleimide, was attached to the viral surface proteins. The experiments relied on the use of native and fusion-inactive viruses and studies involving viral and target membranes that were modified by the incorporation of the lysophospholipid. The total dequenching signal detected in the R18 assay consists of components from probe transferred without fusion and from fusion itself. At 37 degrees C, the initial rate of dequenching (within two minutes) was predominately from the probe diluted by fusion with little contribution from transfer. The dequenching signal due to the probe transfer without fusion occurred at temperatures as low as 10 degrees C and increased linearly with time. Complete fusion started at about 20-25 degrees C and increased sharply at 30 degrees C. The extent of hemifusion was deduced from the total R18 dequenching data and those of the eosin-maleimide labeled protein dilution method for the limiting cases; the analysis indicates that hemifusion started at about 15 degrees C and increased over the range 20-25 degrees C. The initial rate of dequenching of the R18 assay measured within 2 min gives an accurate measure of membrane fusion above 30 degrees C.

Transfer of host T-cell membrane HLA-DR and CD25 to target cells by human retroviruses

Many enveloped viruses incorporate host membrane proteins, some of which remain functionally active and significantly affect viral phenotype. We investigated whether human retroviruses can transfer host membrane proteins to target cells. Following incubation with HTLV-I, HLA-DR and CD25 were detected on up to 70% of HPB-ALL cells. Similarly, HLA-DR and CD25 were also detected on cells following incubation with HIV-1. Cyclohexamide or azidothymidine (AZT) had no effect on detection, indicating that binding of virus or infection did not induce expression of these proteins. Detection of host proteins on target cells depended on binding as well as fusion of virus to the cell membrane, indicating that these proteins were inserted into target cell membranes. Virions also transferred host proteins to peripheral blood mononuclear cells (PBMCs). This aberrant transfer of T-cell activation proteins by HIV or HTLV may alter the state of activation or proliferation of target cells and contribute to the immunodeficiencies associated with infection by these viruses.

Interaction of the influenza hemagglutinin fusion peptide with lipid bilayers: area expansion and permeation

Fusion is a crucial event in the infection of animal cells by enveloped viruses (e.g., HIV or influenza). Viral fusion is mediated by glycoproteins, spanning the viral envelope, which attach to a membrane surface and induce fusion of the viral envelope to the cellular membrane. Influenza fusion protein (hemagglutinin) contains an amino-terminal segment critical to fusion, referred to as the fusion peptide. We show here that the native fusion peptide (wt-20) of hemagglutinin destabilizes membranes formed of 99% 1 -stearoyl-2-oleoylphosphatidylcholine (SOPC). The first step in destabilization is rapid insertion of the peptide into the membrane, in which membrane area increases by as much as 11% in just seconds. We visualized and quantified the area expansion by using optical video microscopy combined with micropipette aspiration. This rapid membrane area expansion is followed by the formation of membrane defects in the size range of 0.5 nm, and results in membrane rupture. Both the rate of area increase and maximum area increase are significantly higher at a pH near 5.0 compared to pH 7.0. These results suggest that enhanced membrane insertion of wt-20 and accompanying area expansion at pH 5.0 are responsible for the relatively greater lytic activity at this pH. We show that a deletion of the N-terminal glycine of wt-20 results in a lack of area expansion or membrane perturbation at pH 5.0.

A human IgG1 (b12) specific for the CD4 binding site of HIV-1 neutralizes by inhibiting the virus fusion entry process, but b12 Fab neutralizes by inhibiting a postfusion event

The human b12 IgG1, specific for the CD4 binding site of the gp120 of HIV-1, was prepared by recombinant DNA technology. It had a high neutralization rate constant (-3.5 x 10(5) M(-1) sec(-1)), although this is about 10-fold less than the values for the best poliovirus or influenza A virus MAbs. The recombinant b12 Fab neutralized well, with about one-tenth of the activity of b12 IgG. The mechanisms by which b12 IgG1 and its Fab neutralize HIV-1 IIIB on C8166 cells have been investigated. Neither inhibited attachment of virus to the target cell as judged by FACS, immunofluorescence, and ELISA data. This was controlled using MAb F105, another human IgG1, that did neutralize by inhibiting attachment under our conditions. The interactions of b12 IgG- and Fab-neutralized virions with target cells were compared with those of nonneutralized virus using a number of different techniques (fluorescence dequenching of R18-labeled virions, immunofluorescence of virion gp41 and p24 antigens, and acquisition of resistance to removal of virions from the cell by protease). These and the inhibition of HIV-1-mediated cell-cell fusion all demonstrated that b12 IgG neutralized by inhibiting the primary fusion-uncoating mechanism. However, the interactions of b12 Fab-neutralized and nonneutralized virions with C8166 cells were indistinguishable. Thus b12 Fab did not inhibit fusion uncoating, and by inference inhibited a stage of infection that occurs after the entry of the virion core into the cytoplasm. It is therefore possible that b12 IgG kills HIV-1 twice over, by fusion-inhibition and by inhibiting the postentry event proposed for the Fab. The mechanism of neutralization of b12 Fab and of other MAbs that neutralize in a similar way and why b12 Fab and IgG neutralize by different mechanisms are discussed.

Fusion of bovine leukemia virus with target cells monitored by R18 fluorescence and PCR assays

PCR and R18 fluorescence dequenching assays have been combined to monitor the kinetics of fusion of bovine leukemia virus with target cells (CC81, OVK, or Raji). Antibodies raised against gp51 allow us to demonstrate that not only the hydrophobic N-terminal domain of the transmembrane glycoprotein gp30 but also specific domains of gp51 (amino acids 39 to 103) are involved in bovine leukemia virus-cell fusion.

Synthesis of full-length viral DNA in CD4-positive membrane vesicles exposed to HIV-1. A model for studies of early stages of the hiv-1 life cycle

CD4-positive membrane vesicles (MV) were isolated under isotonic conditions from human T lymphoblastoid cells MT-2 and CEM and tested for their ability to support reverse transcription of viral RNA upon exposure to human immunodeficiency virus, type 1 (HIV-1). MV contained cytoplasms as confirmed by the presence of mitochondrial DNA but were devoid of chromosomal DNA. Virus binding and vesicle lysis assays revealed that 4-19% (depending upon virus dose) of MV-bound HIV-1 entered the vesicles. HIV-1 internalized in MV was able to initiate and complete viral DNA synthesis as determined by the detection of products of reverse transcription using polymerase chain reaction amplification of viral DNA using regions present in early (strong stop) transcripts and full-length double-stranded molecules. Viral DNA was undetectable in MV exposed to HIV-1 at 0 degrees C, in MV exposed to UV-inactivated virus at 37 degrees C, or after exposure to intact virus at 37 degrees C in the presence of reverse transcriptase inhibitors 2',3'-dideoxycytidine and a tetrahydroimidazo[4,5,1-jk](1,4)-benzodiazepin-2-(1H)-thione derivative, indicating that viral DNA detected in HIV-1-exposed MV was synthesized de novo. Kinetic studies revealed that HIV-1 DNA synthesis in MV was very rapid; full-length viral DNA was detected within 15 min of exposure at 37 degrees C, and the DNA levels increased 90-fold after 1 h and declined thereafter. Strong stop viral DNA was 10-fold more abundant than full-length DNA after 1 h at 37 degrees C, indicating that 10% of input viral genomes are fully transcribed in MV within this time frame. This system preserves the critical features of intact CD4-bearing cells to permit studies of HIV-1 entry, uncoating, and reverse transcription of viral RNA.

CD4 confers susceptibility to human immunodeficiency virus type 1 infection in a rat fibroblast cell line

A new model system is delineated that will enable study of CD4 cofactors and gp120 binding proteins other than CD4. We have previously described a nontransformed rat fibroblast cell line that can efficiently produce HIV-1 upon transfection with an HIV-1 infectious clone, in contrast to other nonhuman mammalian cell lines tested. In the present study we analyzed the susceptibility to HIV-1 infection of Rat2 cells expressing the human CD4 protein. We have used the mammalian expression vector pKS286, in which HIV-1 LTR drives the expression of the CD4 gene. We show that the Rat2 cell line, expressing the human CD4 (Rat2/CD4), is susceptible to fusion with and infection by HIV-1. The virus produced by the Rat2/CD4 cells was infectious. CD4 expression in the Rat2/CD4 was down-regulated over time, similarly to HIV-1 expression in HIV-1-transfected Rat2 cells. Transfection of the Rat2/CD4 cells with a tat expression vector reestablished the CD4 expression on the surface of those cells, as expected. We conclude that the expression of a gp120 binding protein on the Rat2 cells' surface suffices to render the Rat2 cells susceptible to HIV-1 infection.

Characterization of CD4-gp120 activation intermediates during human immunodeficiency virus type 1 syncytium formation

The mechanism by which cells expressing HIV envelope glycoproteins progress from binding CD4+ cells to syncytia formation is not entirely understood. The purpose of these investigations was to use physical and biochemical tools (temperature shifts, soluble CD4, protease inhibitors, and a battery of anti-CD4 monoclonal antibodies) to isolate discrete steps during syncytia formation. Previously (Fu et al., J Virol 1993;67:3818), we found that preincubation of cells stably expressing HIV-1 gp 160 (TF228.1.16) with CD4+ SupT1 cells at 16 degrees C, a temperature that is nonpermissive for syncytia formation, resulted in an increased rate of syncytia formation when the cocultures were shifted to the syncytia-permissive temperature of 37 degrees C. We have since found that syncytia formation is further enhanced by shifting the cocultures from 16 to 4 degrees C prior to incubation at 37 degrees C. Together, these data suggest that two discrete states, which we term the first and second activation intermediates (FAI and SAI), are involved in syncytia formation. We have found that acquisition of the FAI (by preincubation at 16 degree C) is sensitive to some serine protease inhibitors (PI), soluble CD4 (sCD4), shedding of gp120, and anti-CD4 monoclonal antibodies (MAb) directed toward the CDR-1/2 and CDR-3 regions of domain 1 on CD4. Expression of the FAI (formation of syncytia by shifting from 16 to 37 degrees C) remains sensitive to sCD4, shedding of gp120, and MAb directed toward CDR-1/2 but is less sensitive to MAb that bind CDR-3 and is insensitive to PI. Similarly, acquisition of the SAI (shifting cocultures from 16 to 4 degrees C), is sensitive to sCD4, shedding of gp120, and MAb directed toward CDR-1/2. In contrast, expression of the SAI (shifting cocultures from 16 to 4 to 37 degrees C) is sensitive only to MAb directed toward CDR-1/2 and cannot be blocked by sCD4, shedding of gp120, or PI. These data allow us to propose that syncytia formation, mediated by HIV-1 envelope glycoproteins, proceeds by a multistep cascade.

Infection by retroviral vectors outside of their host range in the presence of replication-defective adenovirus

Retrovirus infection is normally limited to cells within a specific host range which express a cognate receptor that is recognized by the product of the env gene. We describe retrovirus infection of cells outside of their normal host range when the infection is performed in the presence of a replication-defective adenovirus (dl312). In the presence of adenovirus, several different ecotropic vectors are shown to infect human cell lines (HeLa and PLC/PRF), and a xenotropic vector is shown to infect murine cells (NIH 3T3). Infectivity is demonstrated by 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal) staining, selection with G418 for neomycin resistance, and PCR identification of the provirus in infected cells. Infectivity is quantitatively dependent upon both the concentration of adenovirus (10(6) to 10(8) PFU/ml) and the concentration of retrovirus. Infection requires the simultaneous presence of adenovirus in the retrovirus infection medium and is not stimulated by preincubation and removal of adenovirus from the cells before retrovirus infection. The presence of adenovirus is shown to enhance the uptake of fluorescently labeled retrovirus particles into cells outside of their normal host range, demonstrating that the adenovirus enhances viral entry into cells in the absence of the recognized cognate receptor. This observation suggests new opportunities for developing safe retroviral vectors for gene therapy and new mechanisms for the pathogenesis of retroviral disease.

Increase in soluble CD4 binding to and CD4-induced dissociation of gp120 from virions correlates with infectivity of human immunodeficiency virus type 1

Mutations in the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins gp120 and gp41, previously shown to confer an enhanced replicative capacity and broadened host range to the ELI1 strain of HIV-1, were analyzed for their biochemical effects on envelope structure and function. The tendency of purified virions to release their extracellular gp120 component, either spontaneously or after interacting with soluble CD4 (CD4-induced shedding) was assessed. A single amino acid substitution in part of the CD4 binding site of gp120 (Gly-427 to Arg) enhanced both spontaneous and CD4-induced shedding of gp120 from virions, while a single change in the fusogenic region of gp41 (Met-7 to Val) affected only CD4-induced shedding. Although each codon change alone conferred increased growth ability, virus with both mutations exhibited the most rapid replication kinetics. In addition, when both of these mutations were present, virions had the highest tendency to shed gp120, both spontaneously and after exposure to soluble CD4. Analysis of CD4 binding to virion-associated gp120 showed that the changes in both gp120 and gp41 contributed to increased binding. These results demonstrated that the increased replicative capacity of the ELI variants in human CD4+ cell lines was associated with altered physical and functional properties of the virion envelope glycoproteins.

Characterization of stable Chinese hamster ovary cells expressing wild-type, secreted, and glycosylphosphatidylinositol-anchored human immunodeficiency virus type 1 envelope glycoprotein

We generated Chinese hamster ovary cell lines that stably express wild-type, secreted, and glycosylphosphatidylinositol (GPI)-anchored envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1). The cells expressing wild-type Env (WT cells) express both the precursor gp160 and the mature gp120/gp41 and readily form large syncytia when cocultivated with CD4+ human cells. The cells expressing secreted Env (SEC cells) release 140-kDa precursor and mature 120-kDa envelope glycoproteins into the supernatants. The cells expressing GPI-anchored Env (PI cells) express both 140-kDa precursor and mature gp120/gp41 envelope glycoproteins, which can be released from the cell surface by treatment with phosphatidylinositol-specific phospholipase C (PI-PLC). Both the secreted and PI-PLC-released envelope glycoproteins form oligomers that can be detected on nonreducing sodium dodecyl sulfate-polyacrylamide gels. In contrast to the WT cells, the SEC and PI cells do not form syncytia when cocultivated with CD4+ human cells. The availability of cells producing water-soluble oligomers of HIV-1 Env should facilitate studies of envelope glycoprotein structure and function. The WT cells, which readily induce syncytia with CD4+ cells, provide a convenient system for assessing potential fusion inhibitors and for studying the fusion mechanism of the HIV Env glycoprotein.

A comparative study of the effect of dextran sulfate on the fusion and the in vitro replication of influenza A and B, Semliki Forest, vesicular stomatitis, rabies, Sendai, and mumps virus

The effect of dextran sulfate on the fusion of a series of enveloped viruses, bearing specifically different fusion proteins, was investigated. The fusion with model- and with biological membranes was monitored by an R18 fluorescence-dequenching fusion assay. Dextran sulfate strongly suppresses the fusion of orthomxyo- (influenza A (H1N1 and H3N2 subtypes) and influenza B), of toga- (Semliki Forest virus), and of rhabdoviruses (vesicular stomatitis and rabies virus). The fusion of the paramyxo-viruses Sendai and mumps was not significantly affected by the anionic polysaccharide. The response to dextran sulfate was virus-specific, and identical for the different members of one virusfamily, bearing the same fusion protein. It was shown that dextran sulfate attaches with high affinity to the viruses studied, but not to erythrocytes. The anionic polymer appears to attach to the fusion epitope of the viral membrane. The inhibition of virus replication in vitro shows a remarkable correlation with the observed anti-fusion effects of dextran sulfate.

Physicochemical dissociation of CD4-mediated syncytium formation and shedding of human immunodeficiency virus type 1 gp120

The mechanism of CD4-mediated fusion via activated human immunodeficiency virus type 1 (HIV-1) gp41 and the biological significance of soluble CD4 (sCD4)-induced shedding of gp120 are poorly understood. The purpose of these investigations was to determine whether shedding of gp120 led to fusion activation or inactivation. BJAB cells (TF228.1.16) stably expressing HIV-1 envelope glycoproteins (the gp120-gp41 complex) were used to examine the effects of pH and temperature on sCD4-induced shedding of gp120 and on cell-to-cell fusion (syncytium formation) with CD4+ SupT1 cells. sCD4-induced shedding of gp120 was maximal at pH 4.5 to 5.5 and did not occur at pH 8.5. At physiologic pH, sCD4-induced shedding of gp120 occurred at 22, 37, and 40 degrees C but neither at 16 nor 4 degrees C. In contrast, syncytia formed at pH 8.5 (maximally at pH 7.5) but not at pH 4.5 to 5.5. At pH 7.5, syncytia formed at 37 and 40 degrees C but not at 22, 16, or 4 degrees C. Preincubation of cocultures of TF228.1.16 and SupT1 cells at 4, 16, or 22 degrees C before the shift to 37 degrees C resulted in similar, increased, or decreased syncytium formation, respectively, compared with the control. Furthermore, an activated intermediate of CD4-gp120-gp41 ternary complex may form at 16 degrees C; this intermediate rapidly executes fusion upon a shift to 37 degrees C but readily decays upon a shift to the shedding-permissive but fusion-nonpermissive temperature of 22 degrees C. These physicochemical data indicate that shedding of HIV-1 gp120 is not an integral step in the fusion cascade and that CD4 may inactivate the fusion complex in a process analogous to sCD4-induced shedding of gp120.

Human immunodeficiency virus type 1 (HIV-1) fusion with model membranes: kinetic analysis and the role of lipid composition, pH and divalent cations

The kinetics and extent of HIV-1 fusion with model membranes was studied. HIV-1 was labeled with octadecyl rhodamine B chloride, and fusion was monitored continuously as the dilution of the probe into target membranes. The results were analyzed by a mass action model which yielded good simulations and predictions for the kinetics and final extents of fluorescence increase. The model determined the percent of virions capable of fusing and rate constants of fusion, aggregation and dissociation. Ultrastructural analysis of the virus and reaction products by electron microscopy also provided evidence of HIV-1 fusion with membranes lacking CD4. HIV-1 fusion activity depends on the target membrane lipid composition according to the sequence: cardiolipin (CL) > > phosphatidylinositol > CL/dioleoylphosphatidylcholine (DOPC) (3:7), phosphatidic acid > phosphatidylserine (PS), PS/cholesterol (2:1) > PS/PC (1:1), PS/phosphatidylethanolamine (1:1) > DOPC, erythrocyte ghosts. Reduction of pH from 7.5 generally enhances the rate and extent of HIV-1 fusion. Physiologically relevant concentrations of calcium stimulate HIV-1 fusion with several liposome compositions and with erythrocyte ghost membranes. The fusion products of HIV-1 with liposomes consist of a single virus and several liposomes. The mass action analysis revealed that, compared to intact virions, the fusion products show a striking reduction in the fusion rate constant. Like influenza and Sendai viruses, HIV-1 fusion with membranes containing its own envelope glycoprotein(s) is strongly inhibited. Unlike these viruses, HIV-1 fusion is promoted by physiological levels of calcium. HIV-1 fusion with liposomes is qualitatively similar to simian immunodeficiency virus fusion.

Saturable attachment sites for polyhedron-derived baculovirus on insect cells and evidence for entry via direct membrane fusion

This research provides the first evidence for specific receptor binding of polyhedron-derived baculovirus (PDV) to host cells and to lepidopteran brush border membrane vesicles (BBMV) and demonstration of entry via a nonendocytotic pathway involving direct membrane fusion. The technique of fluorescence-activated cell sorting analysis was used to investigate the specificity of binding between the PDV phenotype of Lymantria dispar nuclear polyhedrosis virus (LdNPV) and host membranes. Fluorescein isothiocyanate-labeled PDV was found to bind in a saturable manner to the gypsy moth cell line IPLB-LdEIta and to L. dispar BBMV. The IPLB-LdEIta cell line was found to possess approximately 10(6) PDV-specific receptor sites per cell. Excess levels of unlabeled PDV were highly efficient in competing with fluorescein isothiocyanate-labeled PDV for limited receptor sites, further supporting the specificity of the interaction. Major reductions in virus binding (as high as 70%) after protease treatment of cells indicated that a protein receptor is involved. A fluorescence dequenching assay of membrane fusion with octadecyl rhodamine B (R18)-labeled PDV was used to identify PDV fusion to host cells and BBMV. Direct membrane fusion of PDV occurred at 27 degrees C to both target membranes as well as at 4 degrees C at approximately 55% of the levels achieved at 27 degrees C. Viral fusion to BBMV occurred throughout the pH range of 4 to 11, with dramatically increased fusion levels (threefold) under the alkaline conditions normal for lepidopteran larval midguts. Treatment of cells with chloroquine, a lysosomotropic agent, did not significantly affect PDV fusion to cells or infectivity in tissue culture assays.

CD4 molecules with a diversity of mutations encompassing the CDR3 region efficiently support human immunodeficiency virus type 1 envelope glycoprotein-mediated cell fusion

The third complementarity-determining region (CDR3) within domain 1 of the human CD4 molecule has been suggested to play a critical role in membrane fusion mediated by the interaction of CD4 with the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein. To analyze in detail the role of CDR3 and adjacent regions in the fusion process, we used cassette mutagenesis to construct a panel of 30 site-directed mutations between residues 79 and 96 of the full-length CD4 molecule. The mutant proteins were transiently expressed by using recombinant vaccinia virus vectors and were analyzed for cell surface expression, recombinant gp120-binding activity, and overall structural integrity as assessed by reactivity with a battery of anti-CD4 monoclonal antibodies. Cells expressing the CD4 mutants were assayed for their ability to form syncytia when mixed with cells expressing the HIV-1 envelope glycoprotein. Surprisingly in view of published data from others, most of the mutations had little effect on syncytium-forming activity. Normal fusion was observed in 21 mutants, including substitution of human residues 85 to 95 with the corresponding sequences from either chimpanzee, rhesus, or mouse CD4; a panel of Ser-Arg double insertions after each residue from 86 to 91; and a number of other charge, hydrophobic, and proline substitutions and insertions within this region. The nine mutants that showed impaired fusion all displayed defective gp120 binding and disruption of overall structural integrity. In further contrast with results of other workers, we observed that transformant human cell lines expressing native chimpanzee or rhesus CD4 efficiently formed syncytia when mixed with cells expressing the HIV-1 envelope glycoprotein. These data refute the conclusion that certain mutations in the CDR3 region of CD4 abolish cell fusion activity, and they suggest that a wide variety of sequences can be functionally tolerated in this region, including those from highly divergent mammalian species. Syncytium formation mediated by several of the CDR3 mutants was partially or completely resistant to inhibition by the CDR3-directed monoclonal antibody L71, suggesting that the corresponding epitope is not directly involved in the fusion process.(ABSTRACT TRUNCATED AT 400 WORDS)

Inactivation of PR8 influenza virus through the octadecylrhodamine B chloride membrane marker

The octadecylrhodamine B chloride (R18) membrane marker was incorporated into PR8 influenza viruses and virus receptor (GD1a-) containing small unilamellar vesicles (SUV). Both were tested in a fusion/lipid transfer assay [Wunderli-Allenspach, H., & Ott, S. (1990) Biochemistry 29, 1990-1997] to find out whether incorporation into artificial and biological membranes yields equivalent results. The R18 assay is based on incorporation of quenched concentrations of the label into donor membranes and monitoring of the dequenching upon its dilution into unlabeled acceptors. With PR8 viruses and R18-labeled SUV, a fast, hemagglutinin-specific fusion takes place at pH 5.3, independently of the initial quenching. At neutral pH, a slow, nonspecific R18 transfer occurs. Both processes follow second-order kinetics. Upon incubation of R18-labeled PR8 viruses with unlabeled SUV or LUV in neutral buffer, transfer is also found. At pH 5.3, a complex dequenching curve best described with superposition of two second-order functions was encountered: a fast, hemagglutinin-specific component and a slow, nonspecific component. A decreasing proportion of the fast fusion was found with increasing initial quenching of labeled virus. Extrapolation showed that full fusion activity is obtained only with low initial quenching (20-30%). The gradual inactivation of the virus by increasing amounts of R18 was confirmed with biological assays (e.g., infectivity). The R18 surface density is much higher in viruses than in liposomes to obtain the same initial quenching. Analysis with the Stern-Volmer plot revealed that R18 monomers and dimers contribute to the quenching in labeled PR8 viruses, whereas only dimers determine the quench curve in liposomes.

ph-dependent hydrophobicity profile of hemagglutinin of influenza virus and its possible relevance in virus fusion

The hydropathy profile of hemagglutinin (HA) subunits HA1 and HA2 of influenza virus X31 and A/PR 8/34 is analyzed at different pH. At neutral pH (7.4) pronounced hydrophobic sequences of HA correspond to the N-terminus and the transmembrane spanning sequence of HA2. At pH 5.0 where influenza virus is known to fuse with biological membranes several hydrophobic sequences in the ectodomain exist which are comparable in both the hydrophobicity and length of the N-terminus of HA2. It is suggested that these hydrophobic stretches are important for the fusion complex, in addition to the N-terminal site of HA2.

Virus-cell membrane fusion does not predict efficient infection of alveolar macrophages by human immunodeficiency virus type 1 (HIV-1)

Alveolar macrophages (AM) are the principal target cells for HIV-1 in lung tissue. To investigate the mechanisms of HIV-1 infection and efficient replication in these cells we isolated AM from 14 HIV-1 negative donors and exposed them to two virus isolates, either N1T, which replicates well in T lymphocytic and monocytic cell lines, or ADA, a monocytotropic virus. Membrane fluorescence dequenching assays demonstrated that HIV-1/N1T fuses efficiently with AM plasma membranes at neutral pH and that this interaction requires cellular CD4. Despite efficient fusion, AM from eight of 14 donors were not susceptible to productive infection with N1T. In contrast, ADA replicated in all AM populations tested. Soluble CD4 blocked infection of AM by either N1T or ADA, indicating that, like membrane fusion, entry of infectious virus requires an interaction with cellular CD4. Analysis of HIV-1 DNA accumulation in infected cells by enzymatic amplification revealed that productive infection by ADA correlated with a high HIV-1 DNA copy number and abortive infection by N1T was characterized by little or no stable cDNA. These studies suggest that the differences between the two HIV-1 strains studied in their ability to replicate in AM reside in phases of the virus life cycle that follow virus-cell fusion.

Kinetics of HIV-1 interactions with sCD4 and CD4+ cells: Implications for inhibition of virus infection and initial steps of virus entry into cells

The mechanisms of human immunodeficiency virus (HIV-1) entry into CD4+ cells and HIV-1 inactivation by sCD4 were studied by analyzing the kinetics of inhibition of viral infection by sCD4 and the kinetics of fusion of CD4+ cells with intact virions labeled with the lipid fluorophore octadecylrhodamine (R18). sCD4 inhibited HIV-1 infection much more effectively when preincubated with virus prior to interaction with CD4+ cells than when mixed simultaneously with virions and cells. The kinetics of inhibition of infection was much slower at 4 degrees and at low sCD4 concentrations than at 37 degrees and at high sCD4 concentrations. In the absence of sCD4, attachment of virus to cells leading to productive infection occurred within 10-30 min. Fusion of the virions with cells started after a 1-2 min lag time and was complete within 15 min. In high-density cell suspensions (5 x 10(7) cells/ml), even very high sCD4 concentrations (100 micrograms/ml) failed to block viral infection during simultaneous mixing of cells, sCD4 and HIV-1. We conclude that the kinetics of sCD4-virus interaction and the competition of sCD4 with the cell surface associated CD4 for the virus are crucial factors in the inhibition of HIV-1 infection by sCD4. These results provide insight into mechanisms of viral penetration into cells and should be considered when designing new approaches for AIDS therapy.

Kinetics of soluble CD4 binding to cells expressing human immunodeficiency virus type 1 envelope glycoprotein

The high-affinity interaction between the envelope glycoprotein (gp120-gp41) of the human immunodeficiency virus type 1 and its receptor, CD4, is important for viral entry into cells and therapeutical approaches based on the soluble form of CD4 (sCD4). Using flow cytometry, we studied the kinetics of binding of sCD4 to gp120-gp41 expressed on the cell surface. sCD4 binding was dependent on sCD4 concentration and temperature and exhibited bimolecular reaction kinetics. Binding was very slow at low sCD4 concentrations (below 0.2 micrograms/ml) and low temperatures (below 13 degrees C) but increased sharply with increasing temperature. The rate constant for association at 37 degrees C (1.5 x 10(5) M-1 s-1) was 14-fold higher than at 4 degrees C, but the affinity of sCD4 to membrane-bound gp120-gp41 was not significantly affected. The activation energy at higher temperatures (28 to 37 degrees C) was less than at lower temperatures (4 to 13 degrees C). After long periods of incubation, we observed a decrease of surface-bound sCD4 and gp120, even at low temperatures, which was attributed to sCD4-induced shedding of gp120. The rate of gp120 shedding was much lower than the rate of sCD4 binding and was dependent on sCD4 concentration and temperature. The finding that sCD4 binding is slow, especially at low sCD4 concentrations, can be of critical importance for efficient blocking of viral infection by sCD4 and should be considered when designing new protocols in the therapy of AIDS patients.

Initial stages of HIV-1 envelope glycoprotein-mediated cell fusion monitored by a new assay based on redistribution of fluorescent dyes

Membrane fusion is an essential step in the infection of permissive cells with human immunodeficiency virus (HIV). Infected cells frequently fuse with each other, and then progress to form multinucleated giant cells (syncytia). To gain insight into mechanisms of HIV env-mediated membrane fusion, we developed a new assay for studying the initial events. The assay is based on the redistribution of fluorescent markers between membranes and cytoplasm of adjacent cells examined by means of fluorescence video microscopy. Membrane fusion between HIV-1 envelope glycoprotein (gp120/41) expressing effector cells and CD4+ target cells was observed 90 min after the association of cells, whereas the first syncytia only became apparent after 5 h. Moreover, membrane fusion events were observed under conditions where no syncytia were detected, for example, when the effector:target cell ratio was greater than 100:1, or less than 1:100. A significant number of cells with fused membranes were not involved in the syncytia. In order to determine whether quantitative differences in receptor expression might influence the extent of membrane fusion, we used laboratory-selected variants of CEM cells that differ in their expression of CD4. We found that CD4 is required on the target membrane for HIV env-mediated membrane fusion, but its extent is only partially dependent on CD4 surface concentration. The ability of those CEM variants to take part in HIV env-mediated membrane fusion did not correlate with their capacity to form syncytia. These findings indicate that additional steps are needed to form syncytia after membrane fusion.

Stimulation of glycoprotein gp120 dissociation from the envelope glycoprotein complex of human immunodeficiency virus type 1 by soluble CD4 and CD4 peptide derivatives: implications for the role of the complementarity-determining region 3-like region in membrane fusion

We have used a recombinant vaccinia virus vector encoding the envelope glycoprotein of human immunodeficiency virus type 1 to study receptor-induced structural changes related to membrane fusion. A truncated soluble form of human CD4 (sCD4) was found to stimulate dissociation of the external subunit (gp120) from the envelope glycoprotein complex of human immunodeficiency virus type 1 expressed at the cell surface. sCD4 stimulation of gp120 release was time- and concentration-dependent and was associated with specific binding of sCD4 to gp120. Synthetic peptide derivatives corresponding to residues 81-92 of human CD4 (overlapping the complementarity-determining region 3-like region) inhibited cell-cell fusion mediated by the interaction between recombinant vaccinia-encoded CD4 and human immunodeficiency virus envelope glycoprotein. These peptide derivatives also stimulated gp120 release from the envelope glycoprotein complex. An analogous peptide derivative from chimpanzee CD4 (containing a single Glu----Gly substitution at the position corresponding to CD4 residue 87) was considerably less active at inhibition of cell-cell fusion and stimulation of gp120 release, consistent with the known inhibitory effect of this substitution on the ability of membrane-associated CD4 to mediate cell fusion. These results suggest that the sCD4-induced release of gp120 reflects postbinding structural changes in the envelope glycoprotein complex involved in membrane fusion, with the complementarity-determining region 3-like region playing a critical role.

Several CD4 domains can play a role in human immunodeficiency virus infection in cells

The human immunodefiency virus (HIV) uses the human CD4 glycoprotein as a receptor for infection of susceptible cells. Cells expressing a series of mutated forms of the CD4 gene have shown a variability in their ability to support replication of three HIV type 1 (HIV-1) and three HIV-2 strains. Moreover, when different stages of virus production were examined by a variety of assays, a consistent delay was observed in all cell lines containing CD4 mutants compared with those with intact full-length CD4. Cells expressing the CD4.415 mutant (modified at the serine 415 corresponding to a phosphorylation site of the cytoplasmic domain) showed only a minimal effect on virus replication. Cells expressing CD4.403 and CD4.401 mutants (lacking the whole cytoplasmic domain) manifested a moderate delay in production of virus progeny. The most substantial effect on HIV replication was observed in cells expressing a chimeric hybrid containing sequences corresponding to the first 177 residues of the N-terminal CD4 fused to CD8 sequences encoding the hinge, transmembrane, and cytoplasmic domains of the human CD8. Furthermore, in a cell-to-cell contact assay, fusion was absent when the CD4 proximal membrane domain was replaced by the CD8 counterpart. In addition, a strong correlation between the down-modulation of the surface CD4 and HIV expression was observed. These observations suggest that in addition to the known binding region, other domains of CD4 could play an important role in regulating HIV entry of cells.

Interaction of HIV-1 with susceptible lymphoblastoid cells. 1H NMR studies

Different strains of HIV susceptible lymphoblastoid cells have been infected by HIV-1 and examined by means of 1H NMR spectroscopy at different times after infection, taking advantage of the presence of high resolution lipid signals from the plasma membrane of tumor cells. A transient decrease in intensity of fatty acid signals, originated by changes in membrane structure, has been observed early after viral infection. Marked alterations in membrane-dependent steps of phospholipid synthesis can also be inferred by the observed transient depression in peaks from choline-based metabolites. Spectral modifications deriving from changes in lipid metabolism are also produced both in infected cells a few days after infection and in permanently infected cells. 1H NMR can, therefore, monitor structural and metabolic effects induced by HIV infection.

Full-length CD4 electroinserted in the erythrocyte membrane as a long-lived inhibitor of infection by human immunodeficiency virus

Recombinant full-length CD4 expressed in Spodoptera frugiperda 9 cells with the baculovirus system was electroinserted in erythrocyte (RBC) membranes. Of the inserted CD4, 70% was "correctly" oriented as shown by fluorescence quenching experiments with fluorescein-labeled CD4. The inserted CD4 displayed the same epitopes as the naturally occurring CD4 in human T4 cells. Double-labeling experiments (125I-CD4 and 51Cr-RBC) showed that the half-life of CD4 electroinserted in RBC membrane in rabbits was approximately 7 days. Using the fluorescence dequenching technique with octadecylrhodamine B-labeled human immunodeficiency virus (HIV)-1, we showed fusion of the HIV envelope with the plasma membrane of RBC-CD4, whereas no such fusion could be detected with RBC. The dequenching efficiency of RBC-CD4 is the same as that of CEM cells. Exposure to anti-CD4 monoclonal antibody OKT4A, which binds to the CD4 region that attaches to envelope glycoprotein gp120, caused a significant decrease in the dequenching of fluorescence. In vitro infectivity studies showed that preincubation of HIV-1 with RBC-CD4 reduced by 80-90% the appearance of HIV antigens in target cells, the amount of viral reverse transcriptase, and the amount of p24 core antigen produced by the target cells. RBC-CD4, but not RBCs, aggregated with chronically HIV-1-infected T cells and caused formation of giant cells. These data show that the RBC-CD4 reagent is relatively long lived in circulation and efficient in attaching to HIV-1 and HIV-infected cells, and thus it may have value as a therapeutic agent against AIDS.