Resistance to HIV-1 entry inhibitors

Resistance-testing technology has been incorporated into the standard of care for human immunodeficiency virus type 1 (HIV-1) infection and therapy with protease and reverse transcriptase inhibitors. Inhibitors of HIV-1 entry represent an emerging mode of antiretroviral therapy, and HIV-1 entry inhibitors encompass three mechanistically distinct classes of agents known as attachment inhibitors, coreceptor inhibitors, and fusion inhibitors. Each class of agent has demonstrated promise in controlled clinical trials, and understanding the determinants and evolution of viral resistance will be critical for the optimal development and deployment of these new treatment classes. Advances in resistance-testing technologies have paralleled the development of HIV-1 entry inhibitor therapies, and the available data support the notion that attachment, coreceptor and fusion inhibitors offer complementary modes of therapy and distinct resistance profiles.

Human immunodeficiency virus type 1 entry inhibitors PRO 542 and T-20 are potently synergistic in blocking virus-cell and cell-cell fusion

Human immunodeficiency virus type 1 (HIV-1) entry proceeds via a cascade of events that afford promising targets for therapy. PRO 542 neutralizes HIV-1 by blocking its attachment to CD4 cells, and T-20 blocks gp41-mediated fusion. Both drugs have shown promise in phase 1/2 clinical trials. Here, the drugs were tested individually and in combination in preclinical models of HIV-1 infection, and inhibition data were analyzed for cooperativity by using the combination index method. Synergistic inhibition of virus-cell and cell-cell fusion was observed for phenotypically diverse viruses for a broad range of drug concentrations, often resulting in > or = 10-fold dose reductions in vitro. Additional mechanism-of-action studies probed the molecular basis of the synergies. The markedly enhanced activity observed for the PRO 542:T-20 combination indicates that the multistep nature of HIV-1 entry leaves the virus particularly vulnerable to combinations of entry inhibitors. These findings provide a strong rationale for evaluating combinations of these promising agents for therapy in vivo.

Potent, broad-spectrum inhibition of human immunodeficiency virus type 1 by the CCR5 monoclonal antibody PRO 140

CCR5 serves as a requisite fusion coreceptor for clinically relevant strains of human immunodeficiency virus type 1 (HIV-1) and provides a promising target for antiviral therapy. However, no study to date has examined whether monoclonal antibodies, small molecules, or other nonchemokine agents possess broad-spectrum activity against the major genetic subtypes of HIV-1. PRO 140 (PA14) is an anti-CCR5 monoclonal antibody that potently inhibits HIV-1 entry at concentrations that do not affect CCR5's chemokine receptor activity. In this study, PRO 140 was tested against a panel of primary HIV-1 isolates selected for their genotypic and geographic diversity. In quantitative assays of viral infectivity, PRO 140 was compared with RANTES, a natural CCR5 ligand that can inhibit HIV-1 entry by receptor downregulation as well as receptor blockade. Despite their divergent mechanisms of action and binding epitopes on CCR5, low nanomolar concentrations of both PRO 140 and RANTES inhibited infection of primary peripheral blood mononuclear cells (PBMC) by all CCR5-using (R5) viruses tested. This is consistent with there being a highly restricted pattern of CCR5 usage by R5 viruses. In addition, a panel of 25 subtype C South African R5 viruses were broadly inhibited by PRO 140, RANTES, and TAK-779, although approximately 30-fold-higher concentrations of the last compound were required. Interestingly, significant inhibition of a dualtropic subtype C virus was also observed. Whereas PRO 140 potently inhibited HIV-1 replication in both PBMC and primary macrophages, RANTES exhibited limited antiviral activity in macrophage cultures. Thus CCR5-targeting agents such as PRO 140 can demonstrate potent and genetic-subtype-independent anti-HIV-1 activity.

Recombinant CD4-IgG2 in human immunodeficiency virus type 1-infected children: phase 1/2 study. The Pediatric AIDS Clinical Trials Group Protocol 351 Study Team

The use of recombinant CD4-IgG2 in pediatric human immunodeficiency virus type 1 (HIV-1) infection was evaluated by single and multidose intravenous infusions in 18 children in a phase 1/2 study. The study drug was well tolerated, and dose proportionality was observed in terms of area under time-concentration curve and peak serum concentration. Acute decreases of >0.7 log(10) copies/mL in serum HIV-1 RNA concentration were seen in 4 of the 6 children treated with 4 weekly 10 mg/kg doses. At 14 days after treatment, 3 children had sustained reductions in serum HIV-1 RNA; the other children had rebounded to baseline levels or above. By 28 days after therapy, the peak HIV-1 cellular infectious units was reduced in all 6 children, including the 2 who had experienced an earlier transient increase in values. Thus, recombinant CD4-IgG2 treatment of HIV-1-infected children appears to be well tolerated and capable of reducing HIV-1 burden.

Single-dose safety, pharmacology, and antiviral activity of the human immunodeficiency virus (HIV) type 1 entry inhibitor PRO 542 in HIV-infected adults

PRO 542 (CD4-IgG2) is a recombinant antibody-like fusion protein wherein the Fv portions of both the heavy and light chains of human IgG2 have been replaced with the D1D2 domains of human CD4. Unlike monovalent and divalent CD4-based proteins, tetravalent PRO 542 potently neutralizes diverse primary human immunodeficiency virus (HIV) type 1 isolates. In this phase 1 study, the first evaluation of this compound in humans, HIV-infected adults were treated with a single intravenous infusion of PRO 542 at doses of 0.2-10 mg/kg. PRO 542 was well tolerated, and no dose-limiting toxicities were identified. Area under the concentration-time curve, and peak serum concentrations increased linearly with dose, and a terminal serum half-life of 3-4 days was observed. No patient developed antibodies to PRO 542. Preliminary evidence of antiviral activity was observed as reductions in both plasma HIV RNA and plasma viremia. Sustained antiviral effects may be achieved with repeat dosing with PRO 542.

A recombinant human immunodeficiency virus type 1 envelope glycoprotein complex stabilized by an intermolecular disulfide bond between the gp120 and gp41 subunits is an antigenic mimic of the trimeric virion-associated structure

The few antibodies that can potently neutralize human immunodeficiency virus type 1 (HIV-1) recognize the limited number of envelope glycoprotein epitopes exposed on infectious virions. These native envelope glycoprotein complexes comprise three gp120 subunits noncovalently and weakly associated with three gp41 moieties. The individual subunits induce neutralizing antibodies inefficiently but raise many nonneutralizing antibodies. Consequently, recombinant envelope glycoproteins do not elicit strong antiviral antibody responses, particularly against primary HIV-1 isolates. To try to develop recombinant proteins that are better antigenic mimics of the native envelope glycoprotein complex, we have introduced a disulfide bond between the C-terminal region of gp120 and the immunodominant segment of the gp41 ectodomain. The resulting gp140 protein is processed efficiently, producing a properly folded envelope glycoprotein complex. The association of gp120 with gp41 is now stabilized by the supplementary intermolecular disulfide bond, which forms with approximately 50% efficiency. The gp140 protein has antigenic properties which resemble those of the virion-associated complex. This type of gp140 protein may be worth evaluating for immunogenicity as a component of a multivalent HIV-1 vaccine.

Differential inhibition of human immunodeficiency virus type 1 fusion, gp120 binding, and CC-chemokine activity by monoclonal antibodies to CCR5

The CC-chemokine receptor CCR5 mediates fusion and entry of the most commonly transmitted human immunodeficiency virus type 1 (HIV-1) strains. We have isolated six new anti-CCR5 murine monoclonal antibodies (MAbs), designated PA8, PA9, PA10, PA11, PA12, and PA14. A panel of CCR5 alanine point mutants was used to map the epitopes of these MAbs and the previously described MAb 2D7 to specific amino acid residues in the N terminus and/or second extracellular loop regions of CCR5. This structural information was correlated with the MAbs' abilities to inhibit (i) HIV-1 entry, (ii) HIV-1 envelope glycoprotein-mediated membrane fusion, (iii) gp120 binding to CCR5, and (iv) CC-chemokine activity. Surprisingly, there was no correlation between the ability of a MAb to inhibit HIV-1 fusion-entry and its ability to inhibit either the binding of a gp120-soluble CD4 complex to CCR5 or CC-chemokine activity. MAbs PA9 to PA12, whose epitopes include residues in the CCR5 N terminus, strongly inhibited gp120 binding but only moderately inhibited HIV-1 fusion and entry and had no effect on RANTES-induced calcium mobilization. MAbs PA14 and 2D7, the most potent inhibitors of HIV-1 entry and fusion, were less effective at inhibiting gp120 binding and were variably potent at inhibiting RANTES-induced signaling. With respect to inhibiting HIV-1 entry and fusion, PA12 but not PA14 was potently synergistic when used in combination with 2D7, RANTES, and CD4-immunoglobulin G2, which inhibits HIV-1 attachment. The data support a model wherein HIV-1 entry occurs in three stages: receptor (CD4) binding, coreceptor (CCR5) binding, and coreceptor-mediated membrane fusion. The antibodies described will be useful for further dissecting these events.

CD4-immunoglobulin G2 protects Hu-PBL-SCID mice against challenge by primary human immunodeficiency virus type 1 isolates

CD4-immunoglobulin G2 (IgG2) is a fusion protein comprising human IgG2 in which the Fv portions of both heavy and light chains have been replaced by the V1 and V2 domains of human CD4. Previous studies found that CD4-IgG2 potently neutralizes a broad range of primary human immunodeficiency virus type 1 (HIV-1) isolates in vitro and ex vivo. The current report demonstrates that CD4-IgG2 protects against infection by primary isolates of HIV-1 in vivo, using the hu-PBL-SCID mouse model. Passive administration of 10 mg of CD4-IgG2 per kg of body weight protected all animals against subsequent challenge with 10 mouse infectious doses of the laboratory-adapted T-cell-tropic isolate HIV-1(LAI), while 50 mg of CD4-IgG2 per kg protected four of five mice against the primary isolates HIV-1(JR-CSF) and HIV-1(AD6). In contrast, a polyclonal HIV-1 Ig fraction exhibited partial protection against HIV-1(LAI) at 150 mg/kg but no significant protection against the primary HIV-1 isolates. The results demonstrate that CD4-IgG2 effectively neutralizes primary HIV-1 isolates in vivo and can prevent the initiation of infection by these viruses.

Amino-terminal substitutions in the CCR5 coreceptor impair gp120 binding and human immunodeficiency virus type 1 entry

The CC-chemokine receptor CCR5 is required for the efficient fusion of macrophage (M)-tropic human immunodeficiency virus type 1 (HIV-1) strains with the plasma membrane of CD4+ cells and interacts directly with the viral surface glycoprotein gp120. Although receptor chimera studies have provided useful information, the domains of CCR5 that function for HIV-1 entry, including the site of gp120 interaction, have not been unambiguously identified. Here, we use site-directed, alanine-scanning mutagenesis of CCR5 to show that substitutions of the negatively charged aspartic acid residues at positions 2 and 11 (D2A and D11A) and a glutamic acid residue at position 18 (E18A), individually or in combination, impair or abolish CCR5-mediated HIV-1 entry for the ADA and JR-FL M-tropic strains and the DH123 dual-tropic strain. These mutations also impair Env-mediated membrane fusion and the gp120-CCR5 interaction. Of these three residues, only D11 is necessary for CC-chemokine-mediated inhibition of HIV-1 entry, which is, however, also dependent on other extracellular CCR5 residues. Thus, the gp120 and CC-chemokine binding sites on CCR5 are only partially overlapping, and the former site requires negatively charged residues in the amino-terminal CCR5 domain.

AMD3100, a small molecule inhibitor of HIV-1 entry via the CXCR4 co-receptor

The bicyclam AMD3100 (formula weight 830) blocks HIV-1 entry and membrane fusion via the CXCR4 co-receptor, but not via CCR5. AMD3100 prevents monoclonal antibody 12G5 from binding to CXCR4, but has no effect on binding of monoclonal antibody 2D7 to CCR5. It also inhibits binding of the CXC-chemokine, SDF-1alpha, to CXCR4 and subsequent signal transduction, but does not itself cause signaling and has no effect on RANTES signaling via CCR5. Thus, AMD3100 prevents CXCR4 functioning as both a HIV-1 co-receptor and a CXC-chemokine receptor. Development of small molecule inhibitors of HIV-1 entry is feasible.

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.

Human immunodeficiency virus type 1 membrane fusion mediated by a laboratory-adapted strain and a primary isolate analyzed by resonance energy transfer

Previous studies of human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein-mediated membrane fusion have focused on laboratory-adapted T-lymphotropic strains of the virus. The goal of this study was to characterize membrane fusion mediated by a primary HIV-1 isolate in comparison with a laboratory-adapted strain. To this end, a new fusion assay was developed on the basis of the principle of resonance energy transfer, using HeLa cells stably transfected with gp120/gp41 from the T-lymphotropic isolate HIV-1LA1 or the macrophage-tropic primary isolate HIV-1JR-FL. These cells fused with CD4+ target cell lines with a tropism mirroring that of infection by the two viruses. Of particular note, HeLa cells expressing HIV-1JR-FL gp120/gp41 fused only with PM1 cells, a clonal derivative of HUT 78, and not with other T-cell or macrophage cell lines. These results demonstrate that the envelope glycoproteins of these strains play a major role in mediating viral tropism. Despite significant differences exhibited by HIV-1JR-FL and HIV-1LAI in terms of tropism and sensitivity to neutralization by CD4-based proteins, the present study found that membrane fusion mediated by the envelope glycoproteins of these viruses had remarkably similar properties. In particular, the degree and kinetics of membrane fusion were similar, fusion occurred at neutral pH and was dependent on the presence of divalent cations. Inhibition of HIV-1JR-FL envelope glycoprotein-mediated membrane fusion by soluble CD4 and CD4-IgG2 occurred at concentrations similar to those required to neutralize this virus. Interestingly, higher concentrations of these agents were required to inhibit HIV-1LAI envelope glycoprotein-mediated membrane fusion, in contrast to the greater sensitivity of HIV-1LAI virions to neutralization by soluble CD4 and CD4-IgG2. This finding suggests that the mechanisms of fusion inhibition and neutralization of HIV-1 are distinct.

HIV-1 entry into CD4+ cells is mediated by the chemokine receptor CC-CKR-5

The beta-chemokines MIP-1alpha, MIP-1beta and RANTES inhibit infection of CD4+ T cells by primary, non-syncytium-inducing (NSI) HIV-1 strains at the virus entry stage, and also block env-mediated cell-cell membrane fusion. CD4+ T cells from some HIV-1-exposed uninfected individuals cannot fuse with NSI HIV-1 strains and secrete high levels of beta-chemokines. Expression of the beta-chemokine receptor CC-CKR-5 in CD4+, non-permissive human and non-human cells renders them susceptible to infection by NSI strains, and allows env-mediated membrane fusion. CC-CKR-5 is a second receptor for NSI primary viruses.

Effective ex vivo neutralization of human immunodeficiency virus type 1 in plasma by recombinant immunoglobulin molecules

We tested the ability of human monoclonal antibodies (immunoglobulin G1b12 [IgG1b12] and 19b) and CD4-based molecules (CD4-IgG2 and soluble CD4 [sCD4]) to neutralize human immunodeficiency virus type 1 directly from the plasma of seropositive donors in an ex vivo neutralization assay. IgG1b12 and CD4-IgG2, at concentrations from 1 to 25 micrograms/ml, were found to be effective at reducing the HIV-1 titer in most plasma samples. When viruses recovered from plasma samples were expanded to produce virus stocks, no correlation between the neutralization sensitivities to IgG1b12 and CD4-IgG2 of the in vitro passaged stocks and those of the ex vivo neutralizations performed directly on the plasma was observed. These differences could be due to changes in neutralization sensitivity that occur after one passage of the virus in vitro, or they could be related to the presence of complement or antibodies in the plasma. Furthermore, differences in expression of adhesion molecules on plasma-derived and phytohemagglutinin-activated peripheral blood mononuclear cell-derived viruses could be involved. These studies suggest that IgG1b12 and CD4-IgG2 have broad and potent neutralizing activity in both in vitro and ex vivo neutralization assays and should be considered for use as potential immunoprophylactic or therapeutic agents.

Cross-clade neutralization of primary isolates of human immunodeficiency virus type 1 by human monoclonal antibodies and tetrameric CD4-IgG

We have tested three human monoclonal antibodies (MAbs) IgG1b12, 2G12, and 2F5) to the envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1), and a tetrameric CD4-IgG molecule (CD4-IgG2), for the ability to neutralize primary HIV-1 isolates from the genetic clades A through F and from group O. Each of the reagents broadly and potently neutralized B-clade isolates. The 2F5 MAb and the CD4-IgG2 molecule also neutralized strains from outside the B clade, with the same breadth and potency that they showed against B-clade strains. The other two MAbs were able to neutralize a significant proportion of strains from outside the B clade, although there was a reduction in their efficacy compared with their activity against B-clade isolates. Neutralization of isolates by 2F5 correlated with their possession of the LDKW motif in a segment of gp41 near the membrane-spanning domain. The other two MAbs and CD4-IgG2 recognize discontinuous binding sites on gp120, and so no comparison between genetic sequence and virus neutralization was possible. Our data show that a vaccine based on the induction of humoral immunity that is broadly active across the genetic clades is not impossible if immunogens that express the epitopes for MAbs such as 2F5, 2G12, and IgG1b12 in immunogenic configurations can be created. Furthermore, if the three MAbs and CD4-IgG2 produce clinical benefit in immunotherapeutic trials in the United States or Europe, they may also do so elsewhere in the world.

Increase in sensitivity to soluble CD4 by primary HIV type 1 isolates after passage through C8166 cells: association with sequence differences in the first constant (C1) region of glycoprotein 120

Primary isolates of human immunodeficiency virus type 1 (HIV-1) were obtained by coculture of peripheral blood lymphocytes (PBLs) from HIV-1-infected people with PBLs from uninfected donors. These viral stocks tend to be resistant to neutralization/inactivation by soluble CD4 (sCD4). When these stocks were passed through the T cell line C8166, virus stocks emerged that were sensitive to sCD4. Pre- and post-C8166 stocks maintained their sCD4-resistant and -sensitive phenotypes, respectively, with further passage in PBLs. Pre- and post-C8166 stocks were biologically cloned by two cycles of limiting dilution. The majority (14 of 17) of pre-C8166 clones were sCD4 resistant, and, conversely, the majority of post-C8166 clones (11 of 12) were sensitive to sCD4. Nucleotide and predicted amino acid sequence analysis in the env (gp120) region revealed a limited number of differences between the clones. The only differences that sorted with biological phenotype were in the first constant (C1) region of gp120. Adaptation to growth in C8166 cells and conversion from the sCD4-resistant to the sCD4-sensitive phenotype represent the emergence to prominence of viral species in the pre-C8166 stock that have a replication advantage in C8166 coincident with increased sensitivity to sCD4.

Synergistic inhibition of HIV-1 envelope-mediated cell fusion by CD4-based molecules in combination with antibodies to gp120 or gp41

CD4-based molecules were tested in combination with HIV-1-neutralizing antibodies directed against the V3 loop of gp120 or against gp41, for inhibition of HIV-1 envelope-mediated cell fusion. A virus-free cell fusion assay was developed, using Chinese hamster ovary cells that stably express HIV-1 gp120/gp41. These cells were incubated with dilutions of CD4-based molecules, antibodies, or mixtures of both, then overlaid with C8166 CD4+ T cells. Syncytia were counted and the degree of inhibition of cell fusion was determined. Synergy, additivity, or antagonism was calculated by the combination index (CI) method. The CD4-based molecules included soluble human CD4 as well as fusion proteins composed of CD4 linked to human immunoglobulin gamma 1 or gamma 2 heavy chains. Combinations of CD4-based molecules and monoclonal or polyclonal anti-V3 loop antibodies were synergistic in blocking HIV-1 envelope-mediated cell fusion (CI = 0.21-0.91 at 95% inhibition). Synergy was also observed with combinations of the CD4-based molecules and a broadly neutralizing anti-gp41 monoclonal antibody (2F5) (CI = 0.29-0.65 at 95% inhibition). These results demonstrate that molecules inhibiting HIV attachment act synergistically with molecules inhibiting HIV-1 fusion. The results suggest that CD4-based therapeutics would be more effective in patients with naturally occurring anti-V3 loop or anti-gp41 antibodies. In addition, there may be an advantage in coadministering CD4-based molecules and antibodies that block fusion, especially broadly neutralizing anti-gp41 antibodies, as a combination therapy for HIV-1 infections.

Two mechanisms of soluble CD4 (sCD4)-mediated inhibition of human immunodeficiency virus type 1 (HIV-1) infectivity and their relation to primary HIV-1 isolates with reduced sensitivity to sCD4

Two assays for measuring inhibition of human immunodeficiency virus type 1 (HIV-1) infection by soluble CD4 (sCD4) are described. Experiments in which sCD4, HIV-1, and cell concentrations and sequence of combination, noninfectious/infectious particle ratio, and temperature were varied produced results that support the conclusion that sCD4 inhibits HIV-1 infection by two mechanisms: reversible blockage of receptor binding and irreversible inactivation of infectivity. Fresh isolates obtained from HIV-1-infected persons were tested in both assays and found to be more resistant to both mechanisms of sCD4-mediated inhibition than multiply passaged laboratory strains. Binding studies revealed similar affinities for sCD4 in detergent lysates of sensitive and resistant strains at both 4 and 37 degrees C. The avidity of intact virions for sCD4 was lower at 4 than at 37 degrees C, and in the presence of excess sCD4, less sCD4 was bound at 4 than at 37 degrees C. The avidity differences were similar for fresh isolates and laboratory strains. However, fresh isolates were more resistant to sCD4-induced shedding of envelope glycoprotein gp120 from intact virions than was the laboratory strain. Relative resistance to sCD4 by certain isolates does not represent a lower intrinsic affinity of their envelope for sCD4 or a lower capacity for sCD4 binding. Rather, an event that occurs after binding may account for the differences. This postbinding event or feature may be determined by regions of the envelope outside the CD4 binding site.

Simple assay to screen for inhibitors of interaction between the human immunodeficiency virus envelope glycoprotein and its cellular receptor, CD4

The binding of the human immunodeficiency virus envelope glycoprotein gp120 to the CD4 molecule is the initial step in the viral replicative cycle. This interaction is therefore an important target for therapeutic intervention for the treatment of human immunodeficiency virus infection. We designed an enzyme-linked immunosorbent assay which detects the interaction between recombinant soluble forms of CD4 and gp160. This assay could be used as an initial screen of libraries of synthetic chemical compounds and natural products.

Analysis of synergism/antagonism between HIV-1 antibody-positive human sera and soluble CD4 in blocking HIV-1 binding and infectivity

We tested human immunodeficiency virus type 1 (HIV-1) antibody-positive human sera and sCD4, alone and in combination, for synergistic, additive, or antagonistic effects on blocking of HIV binding and infectivity. Data were analyzed by an application of the median effect principle derived from the law of mass action. This allows the assessment of synergism/antagonism at any desired level of effect. Using three assays (whole virus binding to CD4 cells, neutralization of HIV infectivity, and binding of purified gp120 to solid-phase sCD4), we generally observed additive effects or slight synergism between antibody and sCD4 in inhibiting gp120-CD4 interaction. We used a fourth assay to measure the irreversible inactivation of HIV infectivity by sCD4, a property that can also be mediated by antibody but with considerably less potency than sCD4. The reduction in HIV infectivity mediated by mixtures of sCD4 and antibody was always equal to or greater than the arithmetic sum of the reductions by either agent alone. The relevant antiviral effects of sCD4 and anti-HIV sera may include reversible blockage of receptor binding, irreversible inactivation of HIV infectivity, and in the case of antibody, additional reactions that are independent of receptor binding. Although predictions concerning the in vivo situation are speculative, we find no evidence in vitro for antagonism between sCD4 and antibody with respect to the net effect of the two in blocking HIV binding and infectivity.

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.

Penetration of CD4 T cells by HIV-1. The CD4 receptor does not internalize with HIV, and CD4-related signal transduction events are not required for entry

Receptor binding of HIV to the CD4 molecule is required for efficient infection of T cells, but the post-binding steps that result in penetration of HIV are not well understood. CD4 is induced to internalize upon T cell activation, and mAb to CD4 modify signal transduction and T cell activation as does HIV in some systems. It is not known whether HIV binding triggers CD4 endocytosis or whether signal transduction events are required for penetration. Selected inhibitors of signal transduction were evaluated for their effects on penetration using two assays that are dependent on penetration. After short term exposure to inhibitor and HIV, cells were analyzed for reverse-transcribed HIV DNA (DNA amplification assay), or productive infection is monitored (infectivity assay). Viral penetration was tested in the presence of H7 (protein kinase C inhibition), EGTA (extracellular Ca2+ chelation), cyclosporine A (inhibition of Ca2+/calmodulin-dependent activation), or pertussis toxin (inhibition of G protein function). All agents were used at concentrations that were inhibitory for their respective signal transduction pathways. None of the inhibitors affected viral penetration. We tracked the CD4 molecule with fluorescent probes that do not interfere with HIV binding in a system where CD4 T cells were saturated with HIV and the penetration event was relatively synchronized. Under conditions where detection of CD4 was more sensitive than the detection of HIV, HIV internalization was readily detected but CD4 internalization was not.

Penetration of CD4 T cells by HIV-1. The CD4 receptor does not internalize with HIV, and CD4-related signal transduction events are not required for entry

Receptor binding of HIV to the CD4 molecule is required for efficient infection of T cells, but the post-binding steps that result in penetration of HIV are not well understood. CD4 is induced to internalize upon T cell activation, and mAb to CD4 modify signal transduction and T cell activation as does HIV in some systems. It is not known whether HIV binding triggers CD4 endocytosis or whether signal transduction events are required for penetration. Selected inhibitors of signal transduction were evaluated for their effects on penetration using two assays that are dependent on penetration. After short term exposure to inhibitor and HIV, cells were analyzed for reverse-transcribed HIV DNA (DNA amplification assay), or productive infection is monitored (infectivity assay). Viral penetration was tested in the presence of H7 (protein kinase C inhibition), EGTA (extracellular Ca2+ chelation), cyclosporine A (inhibition of Ca2+/calmodulin-dependent activation), or pertussis toxin (inhibition of G protein function). All agents were used at concentrations that were inhibitory for their respective signal transduction pathways. None of the inhibitors affected viral penetration. We tracked the CD4 molecule with fluorescent probes that do not interfere with HIV binding in a system where CD4 T cells were saturated with HIV and the penetration event was relatively synchronized. Under conditions where detection of CD4 was more sensitive than the detection of HIV, HIV internalization was readily detected but CD4 internalization was not.

Structural features of CD4 required for binding to HIV

A soluble form of the human CD4 glycoprotein (sCD4), the cellular receptor for human HIV, was treated with various physical, chemical, and enzymic regimens and tested over a range of concentrations for its capacity to inhibit the binding of HIV to CD4+ T cells. Reduction of disulfide bonds and alkylation in denaturing buffer (8 M urea) destroyed the inhibitory activity of sCD4, whereas reduction and alkylation in PBS had no effect. Derivatization or digestion of carbohydrate groups by periodate oxidation or by glycolytic enzyme digestion did not affect sCD4 inhibitory capacity. Digestion with trypsin or endoproteinase Glu-C destroyed activity. A limited digestion of sCD4 with endoproteinase Glu-C resulted in a mixture of fragments, however, and the mixture had inhibitory activity equivalent to that of intact sCD4. Within this mixture, a fragment of 23 kDa was identified that binds to HIV. Although sCD4 can be digested to yield fully active fragments, the requirement for intrachain disulfide bonding indicates that the minimum sized portion of CD4 that will retain full affinity for HIV will have to be formulated with a proper tertiary structure.

Structural features of CD4 required for binding to HIV

A soluble form of the human CD4 glycoprotein (sCD4), the cellular receptor for human HIV, was treated with various physical, chemical, and enzymic regimens and tested over a range of concentrations for its capacity to inhibit the binding of HIV to CD4+ T cells. Reduction of disulfide bonds and alkylation in denaturing buffer (8 M urea) destroyed the inhibitory activity of sCD4, whereas reduction and alkylation in PBS had no effect. Derivatization or digestion of carbohydrate groups by periodate oxidation or by glycolytic enzyme digestion did not affect sCD4 inhibitory capacity. Digestion with trypsin or endoproteinase Glu-C destroyed activity. A limited digestion of sCD4 with endoproteinase Glu-C resulted in a mixture of fragments, however, and the mixture had inhibitory activity equivalent to that of intact sCD4. Within this mixture, a fragment of 23 kDa was identified that binds to HIV. Although sCD4 can be digested to yield fully active fragments, the requirement for intrachain disulfide bonding indicates that the minimum sized portion of CD4 that will retain full affinity for HIV will have to be formulated with a proper tertiary structure.

Soluble CD4 blocks the infectivity of diverse strains of HIV and SIV for T cells and monocytes but not for brain and muscle cells

The CD4 antigen has been subverted as a receptor by the human and simian immunodeficiency viruses (HIV-1, HIV-2 and SIV). Several groups have reported that recombinant, soluble forms of the CD4 molecule (sCD4) block the infection of T lymphocytes by HIV-1, as CD4 binds the HIV envelope glycoprotein, gp120, with high affinity. We now report that sCD4 blocks diverse strains of HIV-1, HIV-2 and SIV, but is less effective for HIV-2. The blocking effect is apparent even after adsorption of virions to CD4 cells. Soluble CD4 prevents HIV infection of T-lymphocytic and myelomonocytic cell lines, but neither sCD4 nor anti-CD4 antibodies inhibit infection of glioma and rhabdomyosarcoma cell lines.

HIV infection does not require endocytosis of its receptor, CD4

The T cell surface molecule CD4 interacts with class II MHC molecules on the surface of target cells as well as with the envelope glycoprotein of human immunodeficiency virus (HIV). Internalization of CD4 molecules is observed after exposure of CD4+ T cells to either phorbol esters or appropriate antigen-bearing target cells. To determine whether HIV entry proceeds via receptor-mediated endocytosis or direct viral fusion with the cell membrane, we have constructed two mutants in the cytoplasmic domain of the CD4 protein that severely impair the ability of CD4 molecules to undergo endocytosis. Quantitative infectivity studies reveal that HeLa cell lines expressing wild-type or mutant CD4 molecules are equally susceptible to HIV infection. In addition, HIV binding does not lead to CD4 endocytosis. These studies indicate that although the CD4 molecule can be internalized, HIV entry proceeds via direct fusion of the viral envelope with the cell membrane.

Infection of B lymphocytes by the human immunodeficiency virus and their susceptibility to cytotoxic cells

The T4 molecule (CD4) is an important component of the human immunodeficiency virus (HIV) receptor. As yet, no other component has been demonstrated. We report here that two cell lines, a B lymphoblastoid cell line (Gupta) and a glial cell line (HEB) derived from human embryonal brain tissue, are productively infectable with two distinct isolates of HIV as judged by electron microscopy and immunological and virological studies. These two cell lines do not display detectable surface CD4 glycoprotein. However, using S1 nuclease analysis, we have found that both cell lines do express low levels of CD4 mRNA. Neither of them produced syncytia formation upon HIV infection, a recognized feature of HIV-infected cells strongly expressing the CD4 glycoprotein. It is conceivable that the CD4 mRNA is translated, resulting in meager surface expression of CD4 molecules undetectable by conventional techniques. Therefore, infection with HIV may be one of the most sensitive methods of demonstrating low levels of CD4 expression by human cells. Furthermore, HIV-infected Gupta cells have here been shown to be more susceptible to the lytic activity of natural killer (NK) cells than their uninfected counterparts. These phenomena may be important for pathogenesis of HIV-associated disorders.

A soluble form of CD4 (T4) protein inhibits AIDS virus infection

CD4 (T4) is a glycoprotein of relative molecular mass 55,000 (Mr 55K) on the surface of T lymphocytes which is thought to interact with class II MHC (major histocompatibility complex) molecules, mediating efficient association of helper T cells with antigen-bearing targets. The CD4 protein is also the receptor for HIV, a T-lymphotropic RNA virus responsible for the human acquired immune deficiency syndrome (AIDS) (refs 4-7). To define the mechanisms of interaction of CD4 with the surface of antigen-presenting cells and with HIV, we have isolated the CD4 gene and expressed this gene in several different cellular environments. Here we describe an efficient expression system in which a recombinant, soluble form of CD4 (sCD4) is secreted into tissue culture supernatants. This sCD4 retains the structural and biological properties of CD4 on the cell surface, binds to the envelope glycoprotein (gp110) of HIV and inhibits the binding of virus to CD4+ lymphocytes, resulting in a striking inhibition of virus infectivity.

Structure and expression of the human and mouse T4 genes

The T4 molecule may serve as a T-cell receptor recognizing molecules on the surface of specific target cells and also serves as the receptor for the human immunodeficiency virus. To define the mechanisms of interaction of T4 with the surface of antigen-presenting cells as well as with human immunodeficiency virus, we have further analyzed the sequence, structure, and expression of the human and mouse T4 genes. T4 consists of an extracellular segment comprised of a leader sequence followed by four tandem variable-joining (VJ)-like domains, a transmembrane domain, and a cytoplasmic segment. The structural domains of the T4 protein deduced from amino acid sequence are precisely reflected in the intron-exon organization of the gene. Analysis of the expression of the T4 gene indicates that T4 RNA is expressed not only in T lymphocytes, but in B cells, macrophages, and granulocytes. T4 is also expressed in a developmentally regulated manner in specific regions of the brain. It is, therefore, possible that T4 plays a more general role in mediating cell recognition events that are not restricted to the cellular immune response.

The T4 gene encodes the AIDS virus receptor and is expressed in the immune system and the brain

The isolation of clones encoding the human surface protein T4, and the expression of the T4 gene in new cellular environments, have enabled us to examine the role of this protein in the pathogenesis of AIDS. Our studies support a mechanism of AIDS virus infection that initially involves the specific interaction of the AIDS virus with T4 molecules on the cell surface. This association can be demonstrated on T4+ transformed T and B lymphocytes as well as epithelial cells. Furthermore, the presence of T4 on the surface of all human cells examined is sufficient to render these cells susceptible to AIDS virus infection. Our data suggest that the T4-AIDS virus complex is then internalized by receptor-mediated endocytosis. Finally, we find that the T4 gene is expressed in the brain as well as in lymphoid cells, providing an explanation for the dual neurotropic and lymphotropic character of the AIDS virus. In this manner, a T lymphocyte surface protein important in mediating effector cell-target cell interactions has been exploited by a human retrovirus to specifically target the AIDS virus to populations of T4+ cells.

The gene encoding the T-cell surface protein T4 is located on human chromosome 12

The surface glycoproteins T4 and T8 define functionally distinct populations of T lymphocytes. We have obtained cDNA and genomic clones encoding the T4 molecule and used these as probes to determine the chromosomal location of this gene. Genomic blotting experiments, along with in situ hybridization analyses, indicate that the T4 gene resides on the short arm of human chromosome 12, at region p12-pter. Thus, the T4 gene is not linked to any known member of the immunoglobulin gene family, including its counterpart gene, T8, which resides on human chromosome 2 immediately distal to the immunoglobulin kappa locus.

The T4 glycoprotein is a cell-surface receptor for the AIDS virus

Taken together, our studies suggest a mechanism of AIDS virus infection that initially involves the specific association of the AIDS virus with T4 molecules on the cell surface. This association does not require additional T-cell-specific molecules and can be demonstrated on both B lymphocytes and epithelial cell lines. The T4-AIDS virus complex is likely to be internalized in endosomes via receptor-mediated endocytosis. The virus can then fuse with the vacuolar membrane, releasing the viral nucleocapsid into the cytoplasm to undergo uncoating. Viral replication does not appear to require the environment of a T lymphocyte because active infection is also observed in human T4+ B lymphocytes and epithelial cell lines. Moreover, the T4 gene is expressed in the brain as well as in lymphocytes, providing an explanation for the dual neurotropic and lymphotropic character of the virus. In this manner, a T-lymphocyte surface protein thought to be important in mediating effector cell-target cell interactions has been exploited by a human lymphotropic virus to target the AIDS virus specifically to populations of T4+ cells.

The isolation and nucleotide sequence of a cDNA encoding the T cell surface protein T4: a new member of the immunoglobulin gene family

The surface glycoproteins T4 and T8 define different functional subsets of T lymphocytes and may act as recognition molecules mediating appropriate interactions between the T cell and its target. Previously we employed gene transfer and subtractive hybridization to isolate a T8 cDNA; now we have isolated and sequenced a cDNA clone encoding the T4 molecule. The deduced protein sequence reveals that T4 is an integral membrane protein that shares significant amino acid and structural homologies with members of the immunoglobulin supergene family. The overall structure of T4 consists of an N-terminal variable (V)-like domain, a joining (J)-like region, a third extracellular domain, a membrane-spanning region homologous to class II MHC beta-chains, and a highly charged cytoplasmic domain. Comparison of the protein sequences deduced from the T4 and T8 cDNAs reveals structural similarities consistent with their postulated role as recognition molecules, as well as differences suggesting that the two proteins recognize different structures on the target cell.

The isolation and sequence of the gene encoding T8: a molecule defining functional classes of T lymphocytes

The T cell surface glycoproteins T4 and T8 are thought to mediate efficient cell-cell interactions in the immune system and in this way may be responsible for the appropriate targeting of subpopulations of T cells. We have used gene transfer combined with subtractive hybridization to isolate both cDNA and functional genomic clones encoding the T8 protein. The sequence of the cDNA reveals that T8 is a transmembrane protein with an N-terminal domain which shares significant homology to immunoglobulin variable region light chains. This immunoglobulin-like structure is likely to be important in the function of T8 during differentiation and in the course of the immune response.