Enhancement of SIV infection with soluble receptor molecules

Core structure of gp41 from the HIV envelope glycoprotein

The envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) consists of a complex of gp120 and gp41. gp120 determines viral tropism by binding to target-cell receptors, while gp41 mediates fusion between viral and cellular membranes. Previous studies identified an alpha-helical domain within gp41 composed of a trimer of two interacting peptides. The crystal structure of this complex, composed of the peptides N36 and C34, is a six-helical bundle. Three N36 helices form an interior, parallel coiled-coil trimer, while three C34 helices pack in an oblique, antiparallel manner into highly conserved, hydrophobic grooves on the surface of this trimer. This structure shows striking similarity to the low-pH-induced conformation of influenza hemagglutinin and likely represents the core of fusion-active gp41. Avenues for the design/discovery of small-molecule inhibitors of HIV infection are directly suggested by this structure.

Inhibition of human immunodeficiency virus replication by the sulfonated stilbene dye resobene

The anti-HIV sulfonated dye, resobene, was found to be a potent inhibitor of the attachment of HIV to target cells, the fusion of envelope- and CD4-expressing cells, and the cell-to-cell transmission of virus. Resobene inhibited the infection of phenotypically distinct, established human cell lines and fresh human peripheral blood lymphocytes and macrophages by laboratory-derived isolates of human immunodeficiency virus type 1 (HIV-1) and type 2 (HIV-2), and a panel of biologically diverse primary clinical isolates, including syncytium-inducing and non-syncytium-inducing viruses and strains representative of the various virus clades found worldwide. The compound was also active against all drug-resistant virus isolates tested. Cell-based and biochemical mechanism of action studies demonstrated that the compound inhibits the attachment of infectious virus and fusion of virus-infected cells to uninfected target cells by binding to the cationic V3 loop of the envelope glycoprotein. Resobene effectively inhibited the infection of cell populations which do and do not express cell surface CD4. Resobene prevented infection of the cervical epithelial cell line ME180, suggesting the compound may effectively act as a topical microbicide to prevent the sexual transmission of HIV.

CD4-dependent, antibody-sensitive interactions between HIV-1 and its co-receptor CCR-5

The beta-chemokine receptor CCR-5 is an essential co-factor for fusion of HIV-1 strains of the non-syncytium-inducing (NSI) phenotype with CD4+ T-cells. The primary binding site for human immunodeficiency virus (HIV)-1 is the CD4 molecule, and the interaction is mediated by the viral surface glycoprotein gp120 (refs 6, 7). The mechanism of CCR-5 function during HIV-1 entry has not been defined, but we have shown previously that its beta-chemokine ligands prevent HIV-1 from fusing with the cell. We therefore investigated whether CCR-5 acts as a second binding site for HIV-1 simultaneously with or subsequent to the interaction between gp120 and CD4. We used a competition assay based on gp120 inhibition of the binding of the CCR-5 ligand, macrophage inflammatory protein (MIP)-1beta, to its receptor on activated CD4+ T cells or CCR-5-positive CD4- cells. We conclude that CD4 binding, although not absolutely necessary for the gp120-CCR-5 interaction, greatly increases its efficiency. Neutralizing monoclonal antibodies against several sites on gp120, including the V3 loop and CD4-induced epitopes, inhibited the interaction of gp120 with CCR-5, without affecting gp120-CD4 binding. Interference with HIV-1 binding to one or both of its receptors (CD4 and CCR-5) may be an important mechanism of virus neutralization.

N-butyldeoxynojirimycin-mediated inhibition of human immunodeficiency virus entry correlates with impaired gp120 shedding and gp41 exposure

The alpha-glucosidase inhibitor N-butyldeoxynojirimycin (NB-DNJ) is an inhibitor of human immunodeficiency virus (HIV) replication and HIV-induced syncytium formation in vitro. Although an NB-DNJ-mediated change in viral envelope N-glycan composition inhibits HIV entry at the level of post-CD4 binding, the exact mechanism of inhibition remains to be established. In this study we have examined the effects of NB-DNJ on virion envelope composition and CD4-induced gp120 shedding and gp41 exposure. Virion composition analysis revealed an NB-DNJ-mediated reduction of 15% in overall virion envelope glycoprotein content and a reduction of 26% in the proteolytic maturation of virion gp160. Taken together, these two effects resulted in a reduction of approximately 40% in virion gp120 content. CD4-induced shedding of gp120 from the surfaces of envelope-transfected Cos cells was undetectable when gp120 was expressed in the presence of NB-DNJ. Similarly, the shedding of virion-associated gp120 was reduced 7.4-fold. CD4-induced exposure of cryptic gp41 epitopes on the surfaces of HIV-expressing ACH-2 cells was also greatly impaired, and the exposure of virion-associated gp41 epitopes was reduced 4.0-fold. Finally, CD4-induced increases in the binding of antibodies to the V3 loop of ACH-2-cell-expressed envelope glycoproteins were reduced 25-fold when the glycoproteins were expressed in the presence of NB-DNJ. These results suggest that the NB-DNJ-mediated retention of glycosylated N-glycans inhibits HIV entry by a combined effect of a reduction in virion gp120 content and a qualitative defect within the remaining gp120, preventing it from undergoing conformational changes after CD4 binding.

SIVagm infection of its natural African green monkey host

Possible reasons for the apathogenicity of simian immunodeficiency virus (SIVagm) in its natural African green monkey (AGM) host were investigated. In most respects, the SIVagm/AGM system was shown to resemble human immunodeficiency virus type 1 (HIV-1) infection of humans. AGMs were shown to respond to infection with immune responses similar to those seen in HIV-1-infected humans, with no obvious controlling mechanism observed. The rate of SIVagm in vivo variability was likewise shown to be consistent with that described for HIV-1. Similarly, the level of infection in the peripheral blood was reminiscent of the level in asymptomatic HIV-1-infected patients, although never reaching the levels associated with AIDS. Some potentially important differences were, however, observed. Like humans, AGM CD8+ cells secrete a factor able to suppress SIVagm (and HIV-1) replication but unlike humans, AGMs have a very high percentage of CD8+ lymphocytes in circulation. Also, unlike humans during the asymptomatic stages of infection, AGM lymph nodes do not seem to act as a reservoir for SIVagm and the lymph node structure is not affected. Whether these phenomena are causative or incidental to the state of apathogenicity is the subject of further investigations.

The value of primate models for studying human immunodeficiency virus pathogenesis

Research on human immunodeficiency virus (HIV) infection is compromised by the obvious limitation in having for study only virus-infected individuals or those exposed to the virus. Steps involved in transmission or pathogenesis require planned experimentation. The identification of animal models of acquired immunodeficiency syndrome (AIDS) has therefore been helpful for evaluating phases of HIV pathogenesis. Of the seven subgenera of lentiviruses now recognized, two share the characteristics with HIV of a T cell tropism and the associated loss of CD4+ cells in the host associated with disease: the feline immunodeficiency virus (FIV) and the simian immunodeficiency virus (SIV) (Table 1). The other animal lentiviruses grow best in macrophages and their infection generally reflects clinical sequellae of infection of this cell type. This review addresses those features of SIV, HIV, and SHIV infections of non-human primates that illustrate the importance of the animal models of AIDS.

Soluble CD4 induces the binding of human immunodeficiency virus type 1 to cells via the V3 loop of glycoprotein 120 and specific sites in glycoprotein 41

We have previously reported that incubation of human immunodeficiency virus type 1 (HIV-1) at 4 degrees C with soluble CD4 (sCD4) does not block but increases the binding of virions to CD4-positive H9 cells. In this study, we investigated the mechanism of this effect. It appears that sCD4 can induce the binding of HIV-1IIIB to CD4-negative human cells and to H9 cells with downregulated expression of CD4 at both 4 and 37 degrees C. The binding is proportional to the amount of sCD4 associated with virions, and requires the presence of heparan sulfate proteoglycans on the surface of cells. Monoclonal antibody (MAb) 9284 directed at an epitope overlapping with a putative heparin binding motif in the V3 loop of gp120 almost completely blocked the sCD4-induced binding of virions, while MAbs recognizing other sites of V2 or V3 loops had no effect. The binding of sCD4-coated virions to cells was also inhibited by MAbs 50-69 and 98-6 directed at extracellular epitopes of gp41, whose exposure is increased on binding of sCD4 to virions. Therefore, sCD4 potentiates the binding of HIV-1IIIB virions to cells by inducing conformational changes that enable envelope gp120 and gp41 to interact with cell surface components other than the CD4 receptor.

Neutralizing antibodies to human immunodeficiency virus type-1 gp120 induce envelope glycoprotein subunit dissociation

The spectrum of the anti-human immunodeficiency virus (HIV) neutralizing immune response has been analyzed by the production and characterization of monoclonal antibodies (mAbs) to the viral envelope glycoproteins, gp41 and gp120. Little is known, however, about the neutralization mechanism of these antibodies. Here we show that the binding of a group of neutralizing mAbs that react with regions of the gp120 molecule associated with and including the V2 and V3 loops, the C4 domain and supporting structures, induce the dissociation of gp120 from gp41 on cells infected with the T cell line-adapted HIV-1 molecular clone Hx10. Similar to soluble receptor-induced dissociation of gp120 from gp41, the antibody-induced dissociation is dose- and time-dependent. By contrast, mAbs binding to discontinuous epitopes overlapping the CD4 binding site do not induce gp120 dissociation, implying that mAb induced conformational changes in gp120 are epitope specific, and that HIV neutralization probably involves several mechanisms.

Quantitative analysis of serum neutralization of human immunodeficiency virus type 1 from subtypes A, B, C, D, E, F, and I: lack of direct correlation between neutralization serotypes and genetic subtypes and evidence for prevalent serum-dependent infectivity enhancement

Human immunodeficiency virus type 1 (HIV-1) M group strains have been assigned to date to nine distinct genetic subtypes, designated A through I, according to phylogenetic analyses of nucleotide sequences of their env or gag genes. Whether there is any relationship between phylogenetic subtypes and the neutralization serotypes is not clear, yet defining the nature of any such relationship by mathematical means would be of major importance for the development of globally effective HIV-1 vaccines. We have therefore developed a quantitative method to analyze serum neutralization of HIV-1 isolates and to identify HIV-1 neutralization serotypes. This method involves calculations of the neutralization index, N(i), a newly defined parameter derived from plots generated from in vitro neutralization assays, calculations of pairwise serum-virus vector distances, and cluster analyses. We have applied this approach to analyze three independent neutralization matrices involving primary HIV-1 strains and sera from genetic subtypes A, B, C, D, E, F, and I. Detailed serum and HIV-1 isolate cluster analyses have shown that in general, the identified neutralization serotypes do not directly correlate with HIV-1 genetic subtypes. These results suggest that neutralization serotypes do not during natural HIV-1 infection are not governed by antibodies directed against simple epitopes within gp120 monomers. A significant proportion (28%) of 1,213 combinations of sera and HIV-1 isolates caused serum-dependent infectivity enhancement [negative N(i) values] rather than neutralization. We also noted that negative N(i) values tended to correlate better with certain HIV-1 isolates rather than with HIV-1-positive sera. Syncytium-inducing variants of HIV-1 were slightly more likely than non-syncytium-inducing variants to undergo serum-dependent infectivity enhancement, although the latter variants could clearly be susceptible to enhancement.

A trimeric structural domain of the HIV-1 transmembrane glycoprotein

Infection with HIV-1 is initiated by fusion of cellular and viral membranes. The gp41 subunit of the HIV-1 envelope plays a major role in this process, but the structure of gp41 is unknown. We have identified a stable, proteinase-resistant structure comprising two peptides, N-51 and C-43, derived from a recombinant protein fragment of the gp41 ectodomain. In isolation, N-51 is predominantly aggregated and C-43 is unfolded. When mixed, however, these peptides associate to form a stable, alpha-helical, discrete trimer of heterodimers. Proteolysis experiments indicate that the relative orientation of the N-51 and C-43 helices in the complex is antiparallel. We propose that N-51 forms an interior, parallel, homotrimeric, coiled-coil core, against which three C-43 helices pack in an antiparallel fashion. We suggest that this alpha-helical, trimeric complex is the core of the fusion-competent state of the HIV-1 envelope.

Involvement of the V1/V2 variable loop structure in the exposure of human immunodeficiency virus type 1 gp120 epitopes induced by receptor binding

The binding of human immunodeficiency virus type 1 (HIV-1) to the cellular receptor CD4 has been suggested to induce conformational changes in the viral envelope glycoproteins that promote virus entry. Conserved, discontinuous epitopes on the HIV-1 gp120 glycoprotein recognized by the 17b, 48d, and A32 antibodies are preferentially exposed upon the binding of soluble CD4 (sCD4). The binding of the 17b and 48d antibodies to the gp120 glycoprotein can also be enhanced by the binding of the A32 antibody. Here we constructed HIV-1 gp120 mutants in which the variable segments of the V1/V2 and V3 structures were deleted, individually or in combination, while the 17b, 48d, and A32 epitopes were retained. The effects of the variable loop deletions on the function of the HIV-1 envelope glycoproteins and on the exposure of epitopes induced by sCD4 or A32 binding to the monomeric gp120 glycoprotein were examined. The variable-loop-deleted envelope glycoproteins were able to mediate virus entry, albeit at lower efficiencies than those of the wild-type glycoproteins. Thus, the V1/V2 and V3 variable sequences contribute to the efficiency of HIV-1 entry but are not absolutely required for the process. Neither the V1/V2 nor V3 loops were necessary for the increase in exposure of the 17b/48d epitopes induced by binding of the A32 monoclonal antibody. By contrast, induction of the 17b, 48d, and A32 epitopes by sCD4 binding apparently involves a movement of the V1/V2 loops, which in the absence of CD4 partially mask these epitopes on the native gp120 monomer. The results obtained with a mutant glycoprotein containing a deletion of the V1 loop alone indicated that the contribution of the V2 loop to these phenomena was more significant than that of the V1 sequences. These results suggest that the V1/V2 loops, which have been previously implicated in CD4-modulated, postattachment steps in HIV-1 entry, contribute to CD4-induced gp120 conformational changes detected by the 17b, 48d, and A32 antibodies.

Replicative function and neutralization sensitivity of envelope glycoproteins from primary and T-cell line-passaged human immunodeficiency virus type 1 isolates

The structure, replicative properties, and sensitivity to neutralization by soluble CD4 and monoclonal antibodies were examined for molecularly cloned envelope glycoproteins derived from human immunodeficiency virus type 1 (HIV-1) viruses either isolated directly from patients or passaged in T-cell lines. Complementation of virus entry into peripheral blood mononuclear cell targets by primary patient envelope glycoproteins exhibited efficiencies ranging from that observed for the HXBc2 envelope glycoproteins, which are derived from a T-cell line-passaged virus, to approximately fivefold-lower values. The ability of the envelope glycoproteins to complement virus entry roughly correlated with sensitivity to neutralization by soluble CD4. Laboratory-adapted viruses were sensitive to neutralization by monoclonal antibodies directed against the CD4-binding site and the third variable (V3) loop of the gp120 glycoprotein. By comparison, viruses with envelope glycoproteins from primary patient isolates exhibited decreased sensitivity to neutralization by these monoclonal antibodies; for these viruses, neutralization sensitivity correlated with replicative ability. Subinhibitory concentrations of soluble CD4 and a CD4-binding site-directed antibody significantly enhanced the entry of viruses containing envelope glycoproteins from some primary patient isolates. The sensitivity of viruses containing the different envelope glycoproteins to neutralization by soluble CD4 or monoclonal antibodies could be predicted by assays dependent on the binding of the inhibitory molecule to the oligomeric envelope glycoprotein complex but less well by assays measuring binding to the monomeric gp120 glycoprotein. These results indicate that the intrinsic structure of the oligomeric envelope glycoprotein complex of primary HIV-1 isolates, while often less than optimal with respect to the mediation of early events in virus replication, allows a relative degree of resistance to neutralizing antibodies. The interplay of selective forces for higher virus replication efficiency and resistance to neutralizing antibodies could explain the temporal course described for the in vivo emergence of HIV-1 isolates with differing phenotypes.

Adaptation of human immunodeficiency virus type 1 to cells expressing a binding-deficient CD4 mutant (lysine 46 to aspartic acid)

Human immunodeficiency virus (HIV-1) was adapted to replicate efficiently in cells expressing an altered form of the CD4 viral receptor. The mutant CD4 (46 K/D) contained a single amino acid change (lysine 46 to aspartic acid) in the CDR2 loop of domain 1, which results in a 15-fold reduction in affinity for the viral gp120 glycoprotein. The ability of the adapted virus to replicate in CD4 46 K/D-expressing cells was independently enhanced by single amino acid changes in the V2 variable loop, the V3 variable loop, and the fourth conserved (C4) region of the gp120 glycoprotein. Combinations of these amino acids in the same envelope glycoprotein resulted in additive enhancement of virus replication in cells expressing the CD4 46 K/D molecule. In cells expressing the wild-type CD4 glycoproteins, the same V2 and V3 residue changes also increased the efficiency of replication of a virus exhibiting decreased receptor-binding ability due to an amino acid change (aspartic acid 368 to glutamic acid) in the gp120 glycoprotein. In neither instance did the adaptive changes restore the binding ability of the monomeric gp120 glycoprotein or the oligomeric envelope glycoprotein complex for the mutant or wild-type CD4 glycoproteins, respectively. Thus, particular conformations of the gp120 V2 and V3 variable loops and of the C4 region allow postreceptor binding events in the membrane fusion process to occur in the context of less than optimal receptor binding. These results suggest that the fusion-related functions of the V2, V3, and C4 regions of gp120 are modulated by CD4 binding.

Heterologous HIV-2 challenge of rhesus monkeys immunized with recombinant vaccinia viruses and purified recombinant HIV-2 proteins

In an attempt to analyse the role of anti-envelope immunity in the protection of rhesus monkeys against an HIV-2 intravenous challenge, rhesus macaques were immunized twice with recombinant HIV-2 ROD vaccinia viruses (10(8) p.f.u. each) at days 0 and 30, followed by booster injections of purified HIV-2 proteins at months 8, 9, 15 and 27. One group of five macaques was immunized with the Gag, Pol, Vif and Nef antigens, whereas a second group received the same antigens with the addition of HIV-2 Env protein. Eight months after the last boost, the animals were challenged by intravenous injection of 100 AID50 of a monkey PBMC-grown stock of HIV-2 SBL. None of the animals was protected in spite of high humoral immune responses on day of challenge as determined by ELISA and Western Blot assays.

Determinants of human immunodeficiency virus type 1 entry in the CDR2 loop of the CD4 glycoprotein

Various roles for the viral receptor, CD4, have been proposed in facilitating human immunodeficiency virus type 1 (HIV-1) entry, including virion binding to the target cell and the induction of conformational changes in the viral envelope glycoproteins required for the membrane fusion reaction. Here, we compare the structural requirements in the CDR2-like loop of CD4 domain 1, the major contact site of the gp120 envelope glycoprotein, for gp120 binding and virus entry. For every CD4 mutant examined, the level of cell surface expression and the gp120 binding affinity were sufficient to explain the relative ability to function as a viral receptor. The decrease in relative infectibility associated with decreased gp120 binding affinity was more pronounced at lower cell surface CD4 concentrations. These results imply that both receptor density and affinity determine the efficiency of HIV-1 entry and that specific structures in the CD4 residues examined are probably not required for HIV-1 entry functions other than gp120 binding.

Potent and specific inhibition of HIV envelope-mediated cell fusion and virus binding by G quartet-forming oligonucleotide (ISIS 5320)

We have previously reported identification of a phosphorothioate oligonucleotide TTGGGGTT (ISIS 5320) as a potent inhibitor of HIV infection in vitro. The oligonucleotide forms a parallel-stranded, tetrameric guanosine quartet (G-quartet) structure that specifically binds to the HIV envelope glycoprotein (gp120) and inhibits both cell-to-cell and virus-to-cell infection at submicromolar concentrations. In the current study we demonstrate that the tetramer inhibits the infection of laboratory-derived isolates of HIV-1 and HIV-2 in a variety of phenotypically distinct, established human cell lines and a panel of biologically diverse clinical isolates in fresh human peripheral blood lymphocytes and macrophages. The compound was also active against all drug-resistant virus isolates tested. In combination with AZT, ISIS 5320 exhibits additive to slightly synergistic anti-HIV activity. Cell-based mechanism of action studies demonstrate that the compound inhibits the binding of infectious virus and virus-infected cells to uninfected target cells by binding to the cationic V3 loop of the envelope glycoprotein. The G-quartet structure is a potential candidate for use in anti-HIV chemotherapy.

Biological, molecular, and structural analysis of a cytopathic variant from a molecularly cloned simian immunodeficiency virus

Some isolates of simian immunodeficiency virus (SIV) have been shown to infect Sup-T1 cells with slow kinetics and in the absence of cytopathic effects, including cell fusion or CD4 down-modulation (J. A. Hoxie, B. S. Haggarty, S. Bonser, J. Rackowski, H. Shan, and P. Kanki, J. Virol. 62:2557-2568, 1988). In the present study, we describe the isolation and characterization of a SIVmac variant, derived from the BK28 infectious molecular clone, that became highly cytopathic for Sup-T1 cells. This variant, termed CP-MAC, exhibited a number of differences from BK28, including (i) an altered tropism which largely restricted its host range to Sup-T1 cells, (ii) the ability to induce cell fusion and CD4 down-modulation, and (iii) a highly stable interaction of its external (SU) and transmembrane (TM) envelope glycoproteins. In addition, a marked increase in the level of surface envelope glycoproteins was observed both on CP-MAC-infected cells and on virions. The CP-MAC env gene was PCR amplified from infected cells, and sequence analysis identified five amino acid changes in SU and six in TM compared with BK28. The introduction of these changes into BK28 was shown to fully reconstitute the biological and morphological properties of CP-MAC. The limited number of mutations in CP-MAC should enable the molecular determinants to be more precisely defined and help to identify the underlying mechanisms responsible for the striking biological and structural alterations exhibited by this virus.

gp120-independent fusion mediated by the human immunodeficiency virus type 1 gp41 envelope glycoprotein: a reassessment

In a natural context, membrane fusion mediated by the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins involves both the exterior envelope glycoprotein (gp120) and the transmembrane glycoprotein (gp41). Perez et al. (J. Virol. 66:4134-4143, 1992) reported that a mutant HIV-1 envelope glycoprotein containing only the signal peptide and carboxyl terminus of the gp120 exterior glycoprotein fused to the complete gp41 glycoprotein was properly cleaved and that the resultant gp41 glycoprotein was able to induce the fusion of even CD4-negative cells. In the studies reported herein, mutant proteins identical or similar to those studied by Perez et al. lacked detectable cell fusion activity. The proteolytic processing of these proteins was very inefficient, and one processed product identified by Perez et al. as the authentic gp41 glycoprotein was shown to contain carboxyl-terminal gp120 sequences. Furthermore, no fusion activity was observed for gp41 glycoproteins exposed after shedding of the gp120 glycoprotein by soluble CD4. Thus, evidence supporting a gp120-independent cell fusion activity for the HIV-1 gp41 glycoprotein is currently lacking.

Conformational changes induced in the envelope glycoproteins of the human and simian immunodeficiency viruses by soluble receptor binding

We have investigated the molecular basis of biological differences observed among cell line-adapted isolates of the human immunodeficiency virus types 1 and 2 (HIV-1 and HIV-2) and the simian immunodeficiency virus (SIV) in response to receptor binding by using a soluble form of CD4 (sCD4) as a receptor mimic. We find that sCD4 binds to the envelope glycoproteins of all of the HIV-1 isolates tested with affinities within a threefold range, whereas those of the HIV-2 and SIV isolates have relative affinities for sCD4 two- to eightfold lower than those of HIV-1. Treatment of infected cells with sCD4 induced the dissociation of gp120 from gp41 and increased the exposure of a cryptic gp41 epitope on all of the HIV-1 isolates. By contrast, neither dissociation of the outer envelope glycoprotein nor increased exposure of the transmembrane glycoprotein was observed when sCD4 bound to HIV-2- or SIV-infected cells. Moreover, immunoprecipitation with sCD4 resulted in the coprecipitation of the surface and transmembrane glycoproteins from virions of the HIV-2 and SIV isolates, whereas the surface envelope glycoprotein alone was precipitated from HIV-1. However, treatment of HIV-1-, HIV-2-, and SIV-infected cells with sCD4 did result in an increase in exposure of their V2 and V3 loops, as detected by enhanced antibody reactivity. This demonstrates that receptor binding to the outer envelope glycoprotein induces certain conformational changes which are common to all of these viruses and others which are restricted to cell line-passaged isolates of HIV-1.

Human and simian immunodeficiency viruses: virus-receptor interactions

The major cellular receptor for the primate immunodeficiency viruses is the CD4 molecule. As well as mediating virion attachment to the cell surface, CD4 is thought to activate the viral fusion pathway. CD4 is not, however, sufficient for viral entry; other molecules are probably involved, and in certain circumstances these may substitute for CD4. Viral tropism and cytopathogenicity are also influenced by receptor interactions.

Relative affinity of the human parainfluenza virus type 3 hemagglutinin-neuraminidase for sialic acid correlates with virus-induced fusion activity

The ability of enveloped viruses to cause disease depends on their ability to enter the host cell via membrane fusion events. An understanding of these early events in infection, crucial for the design of methods of blocking infection, is needed for viruses that mediate membrane fusion at neutral pH, such as paramyxoviruses and human immunodeficiency virus. Sialic acid is the receptor for the human parainfluenza virus type 3 (HPF3) hemagglutinin-neuraminidase (HN) glycoprotein, the molecule responsible for binding of the virus to cell surfaces. In order for the fusion protein (F) of HPF3 to promote membrane fusion, the HN must interact with its receptor. In the present report, two variants of HPF3 with increased fusion-promoting phenotypes were selected and used to study the function of the HN glycoprotein in membrane fusion. Increased fusogenicity correlated with single amino acid changes in the HN protein that resulted in increased binding of the variant viruses to the sialic acid receptor. These results suggest that the avidity of binding of the HN protein to its receptor regulates the level of F protein-mediated fusion and begin to define one role of the receptor-binding protein of a paramyxovirus in the membrane fusion process.

Neutralization of HIV-1 and inhibition of HIV-1-induced syncytia by 1,8-naphthalimide photoactive compound

The antiviral property of a newly designed class of 1,8-naphthalimide photochemical compounds was investigated. One such photoactive compound, 1,14-bis-(N-hexyl-3'-bromo-1,8'-naphthalimide-4'-yl)-1,4,11,14- tetraazatetradecane-5,10-dione (diED66Br), when activated to an excited state by visible light (420 nm), effectively neutralized the in vitro infectivity of human immunodeficiency virus (HIV-1). Light-activated diED66Br also inhibited syncytium formation induced by cells infected with HIV-1. Nonactivated diED66Br was completely ineffective. The neutralizing and syncytium-inhibiting doses of activated diED66Br had no effect on normal human peripheral blood mononuclear cells. Radioimmunoprecipitation analysis indicated that diED66Br neutralizing activity resulted primarily from its ability to inhibit the binding of HIV-1 envelope glycoprotein gp120 to the CD4 cellular receptors. Although the exact molecular mechanism of viral neutralization by diED66Br has not been elucidated, its ability to neutralize HIV-1 infectivity and to inhibit syncytium formation supports further investigations of this photochemical as a potential therapeutic treatment of HIV-1 infection.

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.

A review of feline infectious peritonitis virus: molecular biology, immunopathogenesis, clinical aspects, and vaccination

Feline infectious peritonitis (FIP) has been an elusive and frustrating problem for veterinary practitioners and cat breeders for many years. Over the last several years, reports have begun to elucidate aspects of the molecular biology of the causal virus (FIPV). These papers complement a rapidly growing base of knowledge concerning the molecular organization and replication of coronaviruses in general. The fascinating immunopathogenesis of FIPV infection and the virus' interaction with macrophages has also been the subject of several recent papers. It is now clear that FIPV may be of interest to scientists other than veterinary virologists since its pathogenesis may provide a useful model system for other viruses whose infectivity is enhanced in the presence of virus-specific antibody. With these advances and the recent release of the first commercially-available FIPV vaccine, it is appropriate to review what is known about the organization and replication of coronaviruses and the pathogenesis of FIPV infection.

Adaptation of two primary human immunodeficiency virus type 1 isolates to growth in transformed T cell lines correlates with alterations in the responses of their envelope glycoproteins to soluble CD4

Two sCD4-resistant, primary viruses (P-08 and P-17) were compared with two sCD4-sensitive, T cell line-adapted variants (C-08 and C-17) for their biochemical responses to sCD4. At 37 degrees C, neither primary virus shed gp120 within 8 hr at sCD4 concentrations of up to 500 nM, whereas C-08 and C-17 lost gp120 within minutes of addition of 5-10 nM sCD4. At 4 degrees C, however, P-17 and C-17 shed gp120 at similar rates in response to the same sCD4 concentration. Irrespective of the temperature, gp120 dissociation from both P-17 and C-17 was inhibited by CD4 MAbs 6H10 and 5A8, the latter of which blocks events subsequent to sCD4 binding. Binding of sCD4 to P-17 was greater at 4 degrees C than at 37 degrees C, whereas the converse was found for C-17. Consistent with this, P-17 was neutralized much more potently by sCD4 at 4 degrees C than at 37 degrees C, whereas C-17 was slightly more sensitive to sCD4 at 37 degrees C than at 4 degrees C. Resistance to neutralization by sCD4 is probably determined by kinetic parameters. We suggest that the acquisition of sCD4 neutralization sensitivity and the above biochemical responses to sCD4 are coincidental to the process by which some primary viruses adapt to growth in transformed T cells. Sequence data indicate that there are a limited number of amino acid differences between the Env glycoproteins of the primary viruses and their T cell line-adapted counterparts; the significance of the individual changes is under investigation, but both pairs of viruses have amino acid substitutions in a region of gp41 thought to contact gp120.

Effects of amino acid changes in the extracellular domain of the human immunodeficiency virus type 1 gp41 envelope glycoprotein

Changes were introduced into conserved amino acids within the ectodomain of the human immunodeficiency virus type 1 (HIV-1) gp41 transmembrane envelope glycoprotein. The effect of these changes on the structure and function of the HIV-1 envelope glycoproteins was examined. The gp41 glycoprotein contains an amino-terminal fusion peptide (residues 512 to 527) and a disulfide loop near the middle of the extracellular domain (residues 598 to 604). Mutations affecting the hydrophobic sequences between these two regions resulted in two phenotypes. Some changes in amino acids 528 to 562 resulted in a loss of the noncovalent association between gp41 and the gp120 exterior glycoprotein. Amino acid changes in other parts of the gp41 glycoprotein (residues 608 and 628) also resulted in subunit dissociation. Some changes affecting amino acids 568 to 596 resulted in envelope glycoproteins partially or completely defective in mediating membrane fusion. Syncytium formation was more sensitive than virus entry to these changes. Changes in several amino acids from 647 to 675 resulted in higher-than-wild-type syncytium-forming ability. One of these amino acid changes affecting tryptophan 666 resulted in escape from neutralization by an anti-gp41 human monoclonal antibody, 2F5. These results contribute to an understanding of the functional regions of the HIV-1 gp41 ectodomain.

International collaboration comparing neutralization and binding assays for monoclonal antibodies to simian immunodeficiency virus

Thirteen laboratories characterized a coded panel of 10 MAbs to SIVmac251 envelope protein in a collaboration organized by the National Institute of Allergy and Infectious Diseases (NIAID). The MAbs were examined against SIV isolates in neutralization and radioimmune precipitation, immunoblot, enzyme-linked immunosorbent, and radioimmune assays. Although laboratories employed diverse neutralization assays that varied in sensitivity there was agreement on the relative ability of the MAbs to neutralize SIVmac251. Additionally, even though the quantity of any single MAb required to neutralize SIVmac251 varied between laboratories, there was agreement on the rank-order strength fo the five neutralizing MAbs. Based on the data from this study, the MAbs were classified according to their neutralization potential as high efficiency (MAb concentration, < 5 micrograms/ml), low efficiency (MAb concentration, 5-100 micrograms/ml), or nonneutralizing (MAb concentration, > 100 micrograms/ml). The MAbs could be assigned to four serological groups based on ability to cross-neutralize and bind different SIV isolates. The distinction between groups I, II, and III were based on the limited neutralization data obtained with the sooty mangabey isolate.

Simian immunodeficiency viruses from central and western Africa: evidence for a new species-specific lentivirus in tantalus monkeys

Although up to 50% of African green monkeys (AGMs) are infected by simian immunodeficiency viruses (SIV) in their natural habitat, they remain asymptomatic carriers of these lentiviruses. They provide an attractive model to study not only the origin but also the link among genetic variation, host-virus adaptation, and pathogenicity of primate lentiviruses. SIVagm have been isolated from three species of AGM: the vervet (Cercopithecus pygerythrus), the grivet (Cercopithecus aethiops), and the sabaeus (Cercopithecus sabaeus) monkey. We studied four new SIVagm isolates from a fourth AGM species, the tantalus monkey (Cercopithecus tantalus), caught in the Central African Republic, and four new isolates from feral sabaeus monkeys from Senegal. Antigenic properties and partial env sequences were used to evaluate the diversity among these isolates. Alignment of env sequences in SIVagm isolated from tantalus and sabaeus monkeys permitted detailed mapping of the variable and conserved domains in the external glycoprotein. Genetic distances indicated that SIVagm isolates from tantalus monkeys are the most divergent among SIVagm in feral AGMs in Africa. The fact that AGMs are infected by four distinct lentiviruses, each specific for a single AGM species, supports the hypothesis of a coevolution of these viruses and their natural hosts and suggests that SIV transmission is a rare event among separated AGM species in the wild.

Synergistic interaction between ligands binding to the CD4 binding site and V3 domain of human immunodeficiency virus type I gp120

We demonstrate that soluble CD4 (sCD4) or a monoclonal antibody (mAb), 39.13g, binding to a conformational epitope of gp120 involved in CD4 binding, and mAbs binding to the V3 domain of gp120, can synergistically neutralize human immunodeficiency virus type I (HIV-1). In contrast, a neutralizing mAb binding to a linear epitope within the CD4 binding domain was unable to exert a synergistic effect in combination with V3 mAbs, suggesting that synergism is dependent on ligands binding to the critical, discontinuous, gp120 residues constituting the CD4 binding site. A number of V3 mAbs showed increased binding to virion gp120 in the presence of sCD4, suggesting a mechanism for the synergistic neutralization. This effect was not observed with recombinant or detergent solubilized viral gp120, suggesting that the oligomeric structure of gp120 on viral particles affects V3 epitope exposure. This hypothesis is supported by the ability of two new V3 mAbs, 8/38c and 8/64b, to only neutralize HIV-1 in the presence of sCD4 or mAb 39.13g; binding studies demonstrate that these mAbs only bind to virion gp120 in the presence of sCD4. Thus, V3 epitope exposure is modulated by the interaction of virion gp120 with ligands specific for the CD4 binding domain and results in enhanced antibody-mediated neutralization.

Strong association of simian immunodeficiency virus (SIVagm) envelope glycoprotein heterodimers: possible role in receptor-mediated activation

Soluble forms of a human cell-surface molecule expressed on T lymphocytes (CD4) neutralize diverse strains of both human (HIV) and simian (SIV) immunodeficiency viruses through the induction of envelope shedding and direct competition with cellular CD4 for virus binding. However, we have previously shown that sCD4 enhances infection of simian immunodeficiency viruses from African green monkeys (SIVagm) and have theorized that this enhancement is due to the induction of conformational changes leading to viral fusion (receptor-mediated activation). In this report, we compared the relative association of the envelope glycoproteins of SIVagm with HIV type 1 (HIV-1) in order to determine if a more stable association of SIVagm envelope glycoproteins might account for the differential effects of sCD4 on the infectious process. Monospecific antisera to each of the SIVagm glycoproteins were generated and used to detect stable heterodimers by radioimmunoprecipitation. Standard solubilization buffers containing both ionic and nonionic detergents or saturating concentrations of sCD4 failed to disrupt SIVagm gp120 interactions with the transmembrane protein, gp36, whereas HIV-1 heterodimers were easily dissociated. Higher concentrations of SDS (1%) were necessary to disrupt the SIVagm envelope complexes demonstrating the existence of strong noncovalent interactions between these membrane glycoproteins. In addition, morphometric analysis by electron microscopy revealed that the linear density of SIVagm spikes was stable and resisted shedding when virus was incubated with sCD4 whereas a significant decrease in linear spike density was noted for HIV-1. Based on our original hypothesis, the strong association of SIVagm glycoprotein spikes during soluble receptor binding may allow for highly stable conformational intermediates important for viral fusion, while neutralization of HIV-1 by sCD4 results from less stable envelope associations.

Conserved structural features in the interaction between retroviral surface and transmembrane glycoproteins?

Among the retroviruses, the surface (SU) and transmembrane (TM) glycoproteins of lentiviruses are linked exclusively by noncovalent bonds. For some C-type retroviruses, however, a small proportion of the SU proteins has been shown to be linked to their TM proteins by a disulfide bond, with the remainder being noncovalently associated. A region near the carboxyl terminus of the HIV-1 SU glycoprotein has been implicated in contacting the TM glycoprotein. Computer modelling indicates that this region of divergent lentivirus and oncovirus SU glycoproteins forms a structurally conserved "pocket" which could accommodate a "knob"-like protrusion formed by an immunodominant region in the TM protein containing the CxxxxxC (lentiviruses) or CxxxxxxCC (C- and D-type viruses) motif. An anti-idiotypic monoclonal antibody, raised against a monoclonal antibody reacting with a sequence in the "pocket" of HIV-1 gp120, was found to bind to synthetic peptides close to the CxxxxxC motif. It is suggested that part of the SU-TM linkage mechanism for the lentiviruses and oncoviruses is a 'knob and socket' structure and that the interaction between SU and TM proteins is similar in one region for lentiviruses and C-type as well as D-type viruses. The conserved knob and socket linkage may be relevant to a mechanism for viral-cell membrane fusion that is broadly common to all of these retroviruses.

Multimeric CD4 binding exhibited by human and simian immunodeficiency virus envelope protein dimers

The envelope (Env) glycoproteins of human and simian immunodeficiency viruses (HIV and SIV) form noncovalently associated oligomers which mediate virus binding to the cell surface and fusion between the viral envelope and plasma membrane. A high-affinity interaction with CD4 is a critical step in this process. In this report, we show that Env protein dimers, but not monomers, can bind two CD4 molecules simultaneously. Multimeric CD4 binding may have important implications for Env protein-CD4 avidity, CD4-induced release of gp120, and subunit-subunit cooperativity during virus membrane fusion as well as for therapeutic strategies.

Lack of correlation between soluble CD4-induced shedding of the human immunodeficiency virus type 1 exterior envelope glycoprotein and subsequent membrane fusion events

The noncovalent association of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) is disrupted by soluble CD4 binding, resulting in shedding of the gp120 exterior envelope glycoprotein. This observation has led to the speculation that interaction of gp120 with the CD4 receptor triggers shedding of the exterior envelope glycoprotein, allowing exposure of gp41 domains necessary for membrane fusion steps involved in virus entry or syncytium formation. To test this hypothesis, a set of HIV-1 envelope glycoprotein mutants were used to examine the relationship of soluble CD4-induced shedding of the gp120 glycoprotein to envelope glycoprotein function in syncytium formation and virus entry. All mutants with a threefold or greater reduction in CD4-binding ability exhibited marked decreases in gp120 shedding in response to soluble CD4, even though several of these mutants exhibited significant levels of envelope glycoprotein function. Conversely, most fusion-defective mutants with wild-type gp120-CD4 binding affinity, including those with changes in the V3 loop, efficiently shed gp120 following soluble CD4 binding. Thus, soluble CD4-induced shedding of gp120 is not a generally useful marker for conformational changes in the HIV-1 envelope glycoproteins necessary for the virus entry or syncytium formation processes. Some gp120 mutants, despite being expressed on the cell surface and capable of efficiently binding soluble CD4, exhibited decreased gp120 shedding. These mutants were still sensitive to neutralization by soluble CD4, indicating that, for envelope glycoproteins exhibiting high affinity for soluble CD4, competitive inhibition may be more important than gp120 shedding for the antiviral effect.

A monoclonal antibody to CD4 domain 2 blocks soluble CD4-induced conformational changes in the envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) and HIV-1 infection of CD4+ cells

The murine monoclonal antibody (MAb) 5A8, which is reactive with domain 2 of CD4, blocks human immunodeficiency virus type 1 (HIV-1) infection and syncytium formation of CD4+ cells (L. C. Burkly, D. Olson, R. Shapiro, G. Winkler, J. J. Rosa, D. W. Thomas, C. Williams, and P. Chisholm, J. Immunol., in press). Here we show that, in contrast to the CD4 domain 1 MAb 6H10, 5A8 and its Fab fragment do not block soluble CD4 (sCD4) binding to virions, whereas they do inhibit sCD4-induced exposure of cryptic epitopes on gp41 and dissociation of gp120 from virions. Two other MAbs, OKT4 and L120, which are reactive with domains 3 and 4 of CD4, have little or no effect on HIV-1 infection, syncytium formation, or sCD4-induced conformational changes in the envelope glycoproteins. The mechanisms of action of 5A8 and 6H10 can be further distinguished in syncytium inhibition assays: 6H10 blocks competitively, while 5A8 does not. We opine that 5A8 blocks HIV-1 infection and fusion by interfering with conformational changes in gp120/gp41 and/or CD4 that are necessary for virus-cell fusion.

Human immunodeficiency virus type 2 infection and fusion of CD4-negative human cell lines: induction and enhancement by soluble CD4

We describe human immunodeficiency type 2 (HIV-2) strains which induce cell-to-cell fusion and infect certain CD4- human cell lines. Soluble CD4 (sCD4) induces or enhances fusion by most HIV-2 strains tested. Soluble CD4-immunoglobulin G chimeras and conjugates of sCD4 and antibody to the third domain of CD4 block HIV-2 fusion of CD4- cells. We conclude that HIV-2 can enter CD4- cells via an alternative cell surface receptor to CD4. While some strains entered efficiently, others retained a dependency on an interaction with sCD4 to initiate changes in the virion envelope required for membrane fusion.

Mapping genetic determinants for human immunodeficiency virus type 1 resistance to soluble CD4

Neutralization of human immunodeficiency virus type 1 (HIV-1) infection with soluble CD4 (sCD4) can be achieved over a broad range of concentrations for different virus strains. Laboratory virus strains passaged in transformed T-cell lines are typically sensitive to sCD4 neutralization, whereas primary virus isolates require over 100-fold-higher sCD4 concentrations. Using recombinant viruses generated from a laboratory strain, HIV-1NL4-3, and a primary macrophagetropic strain, HIV-1JR-FL, we mapped a region of gp120 important for determining sensitivity to sCD4 neutralization. This same region has previously been defined as important for macrophage and transformed T-cell line tropism and includes the V3 neutralization domain but does not include regions of gp120 that have been shown to be most important for CD4 binding.

Human immunodeficiency virus type 1 entry into T cells: more-rapid escape from an anti-V3 loop than from an antireceptor antibody

The entry of human immunodeficiency virus type 1 into two T-cell lines has been analyzed to determine the relative time courses with which virus entry can be blocked (i) by washing, (ii) by adding a monoclonal antibody to the V3 loop of gp120 that neutralizes without blocking CD4 binding (0.5 beta), or (iii) by adding an antireceptor monoclonal antibody that competes for virus binding (leu3a). During entry into C8166 cells, 50% escape from the wash as well as the anti-V3 loop antibody required 20 min, whereas 50% escape from the leu3a block required 45 minutes. In contrast, during entry into H9 cells, 50% escape from the wash block required 50 min, 50% escape from the anti-V3 loop antibody required 110 min, and 50% escape from the antireceptor antibody required 190 min. These results demonstrate that the times required for entering virus to escape each of the blocks were cell type specific. They also demonstrate that V3 loop-dependent steps occur relatively early in entry and suggest that binding of gp120 to CD4 is important for late as well as early steps in human immunodeficiency virus type 1 entry.

Resistance of primary isolates of human immunodeficiency virus type 1 to neutralization by soluble CD4 is not due to lower affinity with the viral envelope glycoprotein gp120

Recombinant soluble CD4 (rsCD4) has potent antiviral activity against cell line-adapted isolates of the human immunodeficiency virus type 1 (HIV-1) but low activity toward HIV-1 primary isolates from patients. A simple hypothesis proposed to explain this discrepancy, which questions the therapeutic utility of soluble CD4-based approaches, is that the major envelope glycoprotein, gp120, of patient virus has lower affinity for CD4 than does gp120 from laboratory viruses. To test this hypothesis, we have produced pairs of low- and high-passage HIV-1 isolates which, depending on culture passage history, display dramatically different sensitivities to neutralization by rsCD4. Here, we present evidence that the HIV-1 major envelope glycoprotein cDNAs cloned from one such isolate pair show only minor differences in their deduced gp120 primary structures, and these occur outside regions previously shown to be involved in CD4 interactions. In addition, recombinant gp120 from a low-passage rsCD4-resistant patient virus binds rsCD4 with high affinity, equal to that previously measured for recombinant gp120 from high-passage cell line-adapted virus isolates. These data indicate that differences in CD4-gp120 affinity do not account for rsCD4 resistance in HIV-1 recently isolated from patients.

Virions of primary human immunodeficiency virus type 1 isolates resistant to soluble CD4 (sCD4) neutralization differ in sCD4 binding and glycoprotein gp120 retention from sCD4-sensitive isolates

Primary isolates of human immunodeficiency virus type 1 (HIV-1) are much less sensitive to neutralization by soluble CD4 (sCD4) and sCD4-immunoglobulin (Ig) chimeras (CD4-IgG) than are HIV-1 strains adapted to growth in cell culture. We demonstrated that there are significant reductions (10- to 30-fold) in the binding of sCD4 and CD4-IgG to intact virions of five primary isolates compared with sCD4-sensitive, cell culture-adapted isolates RF and IIIB. However, soluble envelope glycoproteins (gp120) derived from the primary isolate virions, directly by detergent solubilization or indirectly by recombinant DNA technology, differed in affinity from RF and IIIB gp120 by only one- to threefold. The reduced binding of sCD4 to these primary isolate virions must therefore be a consequence of the tertiary or quaternary structure of the envelope glycoproteins in their native, oligomeric form on the viral surface. In addition, the rate and extent of sCD4-induced gp120 shedding from these primary isolates was lower than that from RF. We suggest that reduced sCD4 binding and increased gp120 retention together account for the relative resistance of these primary isolates to neutralization by sCD4 and CD4-IgG and that virions of different HIV-1 isolates vary both in the mechanism of sCD4 binding and in subsequent conformational changes in their envelope glycoproteins.

Recombinant CD4-selected human immunodeficiency virus type 1 variants with reduced gp120 affinity for CD4 and increased cell fusion capacity

Variants of molecularly cloned human immunodeficiency virus type 1 (HIV-1) were analyzed following selection for the ability to replicate after exposure to soluble, recombinant CD4 protein (rCD4). Two variants, 4/1 and 16/2, show 8-fold and 16-fold reduced sensitivity to rCD4 neutralization yet remain as sensitive as the parental wild-type (wt) virus to neutralization by rCD4-immunoglobulin G (IgG) chimeric molecules and to inhibition of cellular infection by anti-CD4 antibody. The 4/1 variant is more cytopathic, with faster cell fusion and replication kinetics than the wt virus. The gp120s derived from the 4/1 and 16/2 variants have 3-fold and 30-fold reduced binding affinities to rCD4, respectively. The 4/1 variant exhibits diminished shedding of virion gp120 induced by rCD4. The binding of and neutralization by V3 loop antibodies and other anti-gp120 antibodies is reduced for 4/1 but not for 16/2. Sequence analysis revealed a codon change at amino acid residue 435 in the C4 region of the gp120 of 16/2. This accounts for its rCD4 insensitivity, since the insertion of this mutation in the wt gp120 yields the same phenotype. The 4/1 variant has a codon change in the V3 region of gp120 (amino acid 311), which accounts for its reduced sensitivity to some neutralizing antibodies but not to rCD4. The ready selection of rCD4-resistant variants has obvious relevance for rCD4-based therapeutic stratagems.

Evaluation of anti-human immunodeficiency virus effect of recombinant CD4-immunoglobulin in vitro: a good candidate for AIDS treatment

CD4 molecule, a surface marker of helper T lymphocytes, interacts with gp120 of human immunodeficiency virus (HIV) with a high affinity and, hence, serves as a virus receptor. Soluble chimeric CD4-immunoglobulin (Ig) possesses anti-HIV activity due to its binding activity to gp120. Furthermore, this recombinant molecule has unique Ig-like properties representing Fc receptor-binding activity and a long half-life in vivo. In this report we have thoroughly evaluated the effect of this compound on HIV infection using different in vitro systems. Treatment with 4 micrograms/ml of recombinant CD4-Ig after infection completely blocked the HIV-specific cytopathic effect, antigen expression, and virus release in MT-4 cells, a human T cell line which is highly susceptible to HIV. Similarly, this molecule blocked the HTLV-III/B and YU-1 strains of HIV infection in peripheral blood mononuclear cells even at 1 microgram/ml. Pretreatment of the Fc receptor-positive cell line U937 with this reagent resulted not in enhancement but again in blocking of HIV infection. About 95% of HIV infection was inhibited in U937 cells when cells were treated with this compound at the time of exposure to HIV. Recombinant-CD4-Ig also completely inhibited HIV-induced syncytia formation between MOLT-4 and MOLT-4/HIV and resulting virus release at 8 and 2 micrograms/ml, respectively. Due to its stability and long half-life, this compound could be a promising therapeutic agent against HIV infection.

Conformational changes induced in the human immunodeficiency virus envelope glycoprotein by soluble CD4 binding

The human immunodeficiency virus (HIV) binds to the surface of T lymphocytes and other cells of the immune system via a high affinity interaction between CD4 and the HIV outer envelope glycoprotein, gp120. By analogy with certain other enveloped viruses, receptor binding by HIV may be followed by exposure of the hydrophobic NH2 terminus of its transmembrane glycoprotein, gp41, and fusion of the virus and cell membranes. A similar sequence of events is thought to take place between HIV-infected and uninfected CD4+ cells, resulting in their fusion to form syncytia. In this study, we have used a soluble, recombinant form of CD4 (sCD4) to model events taking place after receptor binding by the HIV envelope glycoproteins. We demonstrate that the complexing of sCD4 with gp120 induces conformational changes within envelope glycoprotein oligomers. This was measured on HIV-1-infected cells by the increased binding of antibodies to the gp120/V3 loops, and on the surface of virions by increased cleavage of this loop by an exogenous proteinase. At 37 degrees C, these conformational changes are coordinate with the dissociation of gp120/sCD4 complexes from gp41, and the increased exposure of gp41 epitopes. At 4 degrees C, gp120 dissociation from the cell surface does not occur, but increased exposure of both gp120/V3 and gp41 epitopes is detected. We propose that these events occurring after CD4 binding are integral components of the membrane fusion reaction between HIV or HIV-infected cells and CD4+ cells.

CD4: Its structure, role in immune function and AIDS pathogenesis, and potential as a pharmacological target

CD4 is critical for the development and function of the CD4+ subset of T cells and also subserves as the receptor for the human immunodeficiency viruses. Reports in the past year clarify the role and the molecular interactions of CD4 in these events. Determination of the structure of an extracellular fragment of CD4 reveals novel variations of the immunoglobulin fold and provides an atomic framework for interpretation of its interactions with MHC class II molecules and with gp120, the external envelope glycoprotein of the human immunodeficiency virus.

CD4-gp120 interactions

The three-dimensional structure of the binding domain of the CD4 molecule has been determined and extensive mutational analyses of the respective binding sites on gp120 and CD4 have been completed. The consequences of gp120-CD4 binding with respect to secondary changes in the virion, or the cell, that may be required for infection or that may interfere with cellular function are current active areas of investigation.