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

Human immunodeficiency virus type 1 Vpu protein induces rapid degradation of CD4

CD4 is an integral membrane glycoprotein which is known as the human immunodeficiency virus (HIV) receptor for infection of human cells. The protein is synthesized in the endoplasmic reticulum (ER) and subsequently transported to the cell surface via the Golgi complex. HIV infection of CD4+ cells leads to downmodulation of cell surface CD4, due at least in part to the formation of stable intracellular complexes between CD4 and the HIV type 1 (HIV-1) Env precursor polyprotein gp160. This process "traps" both proteins in the ER, leading to reduced surface expression of CD4 and reduced processing of gp160 to gp120 and gp41. We have recently demonstrated that the presence of the HIV-1-encoded integral membrane protein Vpu can reduce the formation of Env-CD4 complexes, resulting in increased gp160 processing and decreased CD4 stability. We have studied the effect of Vpu on CD4 stability and found that Vpu induces rapid degradation of CD4, reducing the half-life of CD4 from 6 h to 12 min. By using a CD4-binding mutant of gp160, we were able to show that this Vpu-induced degradation of CD4 requires retention of CD4 in the ER, which is normally accomplished through its binding to gp160. The involvement of gp160 in the induction of CD4 degradation is restricted to its function as a CD4 trap, since, in the absence of Env, an ER retention mutant of CD4, as well as wild-type CD4 in cultures treated with brefeldin A, a drug that blocks transport of proteins from the ER, is degraded in the presence of Vpu.

Relationship of the human immunodeficiency virus type 1 gp120 third variable loop to a component of the CD4 binding site in the fourth conserved region

Neutralizing antibodies that recognize the human immunodeficiency virus gp120 exterior envelope glycoprotein and are directed against either the third variable (V3) loop or conserved, discontinuous epitopes overlapping the CD4 binding region have been described. Here we report several observations that suggest a structural relationship between the V3 loop and amino acids in the fourth conserved (C4) gp120 region that constitute part of the CD4 binding site and the conserved neutralization epitopes. Treatment of the gp120 glycoprotein with ionic detergents resulted in a V3 loop-dependent masking of both linear C4 epitopes and discontinuous neutralization epitopes overlapping the CD4 binding site. Increased recognition of the native gp120 glycoprotein by an anti-V3 loop monoclonal antibody, 9284, resulted from from single amino acid changes either in the base of the V3 loop or in the gp120 C4 region. These amino acid changes also resulted in increased exposure of conserved epitopes overlapping the CD4 binding region. The replication-competent subset of these mutants exhibited increased sensitivity to neutralization by antibody 9284 and anti-CD4 binding site antibodies. The implied relationship of the V3 loop, which mediates post-receptor binding steps in virus entry, and components of the CD4 binding region may be important for the interaction of these functional gp120 domains and for the observed cooperativity of neutralizing antibodies directed against these regions.

Human immunodeficiency virus-infected monocyte-derived macrophages express surface gp120 and fuse with CD4 lymphoid cells in vitro: a possible mechanism of T lymphocyte depletion in vivo

Monocyte-derived macrophages (MDM) infected in vitro with a macrophage-tropic strain of human immunodeficiency virus (HIV) fused with uninfected, CD4-expressing T lymphoblastoid cells, but not with a subclone of these cells lacking surface CD4. Infected MDM also fused with uninfected autologous and heterologous MDM. Recombinant soluble CD4 protein (rsCD4) (10 micrograms/ml) and full-length recombinant glycosylated gp120 (20 micrograms/ml) each inhibited fusion by 94-99%; the inhibition was dose-dependent. The N-terminal portion of gp120 did not inhibit syncytium formation. Fusion was also inhibited by a monoclonal antibody to an epitope which binds gp120 (S3.5), but not by antibody to an epitope not involved in gp120 binding (OKT4). HIV-infected MDM specifically bound fluorescein-conjugated rsCD4, and virus could be visualized budding from the surface of these cells. HIV-infected MDM express viral gp120 on their surface and fuse with CD4-bearing cells in a fashion similar to lymphoid cells. Macrophages may contribute to CD4 lymphocyte depletion in vivo by this fusion mechanism.

A CD4-derived peptide carrier blocks acute HIV-1 infection in vitro and binds to gp120 in the presence of Walter-Reed stage 1-6 HIV+ sera

A peptide containing amino acid residues 41-84 of the CD4 molecule was synthesized and coupled through a thioether bond to human serum albumin. This conjugate bound to gp120 with an affinity that was half that of CD4 and blocked the HIV infection in vitro with an efficacy tenfold lower than that of CD4. More importantly, the CD4 peptide-human serum albumin conjugate could bind to gp120 in the presence of HIV+ sera from 18 Walter Reed stage 1-6 patients.

Restoration of cell surface CD4 expression in human immunodeficiency virus type 1-infected cells by treatment with a Tat antagonist

Productive infection of T lymphocytes with human immunodeficiency virus type 1 (HIV-1) is accompanied by a diminution of surface CD4 receptors. Treatment of chronically HIV-1-infected CD4-negative T cells in vitro with the Tat antagonist Ro 5-3335 resulted in a drug dose-dependent decrease in virus protein production and a reciprocal increase in surface CD4 display. The drug-treated cells remained viable, showed significantly reduced levels of the full-length and spliced HIV-1 mRNAs as detected by Northern (RNA) blot hybridization, and maintained integrated HIV-1 DNA. In immunoprecipitation studies with drug-treated cells, the levels of free 55-kDa CD4 protein increased and gp160 complexed with CD4 decreased in amount. These results show for the first time that certain cytopathogenic effects of chronic HIV-1 infection can be reversed by suppressing virus expression.

Phase I study of continuous-infusion soluble CD4 as a single agent and in combination with oral dideoxyinosine therapy in children with symptomatic human immunodeficiency virus infection

To determine the safety and pharmacokinetics of recombinant soluble CD4 (sCD4) administered by continuous intravenous infusion to children with symptomatic human immunodeficiency virus type 1 infection, we conducted a phase I study at the National Cancer Institute. Three dose levels of sCD4 were evaluated: 100, 300, and 1000 micrograms/kg per day. After an initial 12 weeks of treatment with sCD4 alone, dideoxyinosine at a dose of 90 mg/m2 every 8 hours was added and subjects were observed for an additional 12 weeks. Combination therapy was continued in patients in whom it was well tolerated. In addition to toxicity and pharmacokinetic monitoring, surrogate markers of antiviral activity were evaluated. Eleven children were enrolled in the study. During the 12 weeks of treatment with sCD4 alone, and during subsequent sCD4 plus dideoxyinosine combination therapy, no significant toxic reaction attributable to sCD4 or dideoxyinosine was encountered. Low-level anti-CD4 antibodies developed in two patients. Steady-state sCD4 levels increased proportionately at higher doses. The CD4 cell counts and serum p24 antigen levels did not provide evidence of antiviral activity. We conclude that sCD4 was well tolerated at doses up to 1000 micrograms/kg per day when administered by continuous intravenous infusion; however, evidence of in vivo antiviral activity was not observed in this study.

Dissociation of unintegrated viral DNA accumulation from single-cell lysis induced by human immunodeficiency virus type 1

Acute cytopathic retroviral infections are accompanied by the accumulation, due to superinfection, of large amounts of unintegrated viral DNA in the cells. The cytopathic effects of human immunodeficiency virus type 1 (HIV-1) infection are specific for cells that express the CD4 viral receptor and consist of syncytium formation and single-cell lysis. Here we investigated the relationship between superinfection and single-cell lysis by HIV-1. Antiviral agents were added to C8166 or Jurkat lymphocytes after HIV-1 infection had occurred. Treatment with azidothymidine or a neutralizing anti-gp120 monoclonal antibody reduced or eliminated, respectively, the formation of unintegrated viral DNA but did not inhibit single-cell killing. Furthermore, in the infected Jurkat cells, the levels of unintegrated viral DNA peaked several days before significant single-cell lysis was observed. Essentially complete superinfection resistance was established before the occurrence of single-cell killing. These results demonstrate that single-cell lysis by HIV-1 can be dissociated from superinfection and unintegrated viral DNA accumulation. These results also indicate that single-cell killing may involve envelope glycoprotein-receptor interactions not accessible to the exterior of the cell.

CD4 and its role in infection of rabbit cell lines by human immunodeficiency virus type 1

Human CD4 (HuCD4) is the principal receptor for human immunodeficiency virus type 1 (HIV-1) in human cell infection. Susceptibility of rabbit cell lines to infection with HIV-1 raised questions concerning whether a CD4 homolog serves as HIV-1 receptor on rabbit cells. Sequence comparisons of rabbit CD4 (RbCD4) cloned from a rabbit thymus cDNA library showed that 6 of the 18 residues implicated in HIV-1 binding by CD4 differ between the human and rabbit proteins. No correlation between RbCD4 expression by rabbit cell lines and their ability to support HIV-1 infection was seen. Transfection of RbCD4-negative, HTLV-I-transformed cell lines with HuCD4 significantly enhanced HIV-1 infectivity, suggesting that these lines lack a receptor present on other RbCD4-negative lines that produce high levels of p24 in their native state. Inhibition of HIV-1 infection with soluble HuCD4 was demonstrated for all rabbit lines tested, but complete inhibition was obtained only with a rabbit T-cell line expressing RbCD4 and with HuCD4 transfectants. The results suggest that HIV-1 infection of the RbCD4-positive line proceeds through a receptor similar to HuCD4 but that an additional receptor or receptors may serve this purpose in RbCD4-negative lines.

Sulfated polyester interactions with the CD4 molecule and with the third variable loop domain (v3) of gp120 are chemically distinct

Sulfated polyesters (SP) that inhibit HIV infection interact with both the gp120 binding region of CD4 molecules and with the v3 domain loop of gp120 molecules (gp120/v3) but the contributions of these interactions to the inhibition of HIV env-mediated fusion are presently unclear. In order to characterize the molecular mechanisms by which SP inhibit HIV env-mediated fusion, we studied the effect of SP treatment on env-mediated fusion of CD4+ cells driven by recombinant vaccinia virus (vPE-16) and on the binding of anti-HIV MAbs to cellular gp120 or purified, rgp120. SP were more effective than neutralizing anti-gp120/v3 MAbs in inhibiting env-mediated fusion. In addition, SP interacted with the v3 loop of gp120 to inhibit the binding of the neutralizing MAb 9284 but not the binding of 9305, a neutralizing anti-gp120/v3 MAb that binds to an adjacent epitope. Because SP are polyanions, we compared the chemical properties of the SP-gp120/v3 and SP-CD4 interactions. Whereas the ability of SP to inhibit the binding of MAb 9284 and rgp120 was relatively independent of NaCl concentrations, the ability of SP to interfere with rCD4-rgp120 binding depended on the NaCl concentration and was maximal at low NaCl concentrations. In addition, the SP-gp120 interaction was found to be reversible, in contrast to the SP-rCD4 interaction which was previously shown to be relatively irreversible at low salt. These data are consistent with the notions that the interaction of SP with CD4 is primarily electrostatic, but that the interaction of SP with gp120 has complex characteristics that implicate a role for protein conformation.

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.

Detection of receptor-ligand interactions using surface plasmon resonance: model studies employing the HIV-1 gp120/CD4 interaction

Surface plasmon resonance (SPR), a label-free, real time optical detection principle, has been investigated for its potential to detect and quantitate macromolecular ligand-ligate interactions. As model systems, the interactions of the HIV-1 envelope glycoprotein, gp120, and the monoclonal antibody L-71, with a soluble form of the T-cell receptor CD4 (sCD4), were investigated. In an effort to demonstrate potential analytical applications of this technology, operational characteristics of the SPR instrumentation (BIAcore, Pharmacia) including stability of the sensing surface and reproducibility in the measurement of such macromolecular interactions were investigated. In addition, the ability to detect and quantitate sCD4 directly from unfractionated cell culture supernatants, such as Streptomyces lividans, was investigated. The results demonstrate that SPR has potential in quantitating macromolecular interactions in both purified and crude samples and that the reproducibility in, and sensitivity of, such determinations is comparable to other techniques.

Identification of envelope V3 loop as the major determinant of CD4 neutralization sensitivity of HIV-1

Laboratory isolates of human immunodeficiency virus type-1 (HIV-1) such as HTLV-IIIB are generally T cell line-tropic and highly sensitive to neutralization by soluble CD4 (sCD4), a potential antiviral agent that is undergoing clinical trial. However, many primary HIV-1 isolates are macrophage-tropic and sCD4-resistant. Envelope V3 loop sequences derived from primary HIV-1 isolates were sufficient to confer on HTLV-IIIB not only the tissue tropism but also the degree of sCD4 neutralization resistance characteristic of their HIV-1 strains of origin. Single amino acid changes in the V3 loop enhanced sCD4 resistance by up to tenfold. These observations suggest that the tissue tropism and sCD4 neutralization sensitivity of HIV-1 isolates are regulated by similar mechanisms.

Infection of Macaca nemestrina by human immunodeficiency virus type-1

After observations that Macaca nemestrina were exceptionally susceptible to simian immunodeficiency virus and human immunodeficiency virus type-2 (HIV-2), studies of HIV-1 replication were initiated. Several strains of HIV-1, including a recent patient isolate, replicated in vitro in peripheral blood mononuclear cells (PBMCs) and in CD4-positive M. nemestrina lymphocytes in a CD4-dependent fashion. Eight animals were subsequently inoculated with either cell-associated or cell-free suspensions of HIV-1. All animals had HIV-1 isolated by cocultivation, had HIV-1 DNA in their PBMCs as shown by polymerase chain reaction, and experienced sustained seroconversion to a broad spectrum of HIV-1 proteins. Macaca nemestrina is an animal model of HIV-1 infections that provides opportunities for evaluating the pathogenesis of acute HIV-1 replication and candidate vaccines and therapies.

HIV interactions with CD4: a continuum of conformations and consequences

Here, Lee Eiden and Jeffrey Lifson present a model for HIV envelope glycoprotein-CD4 interactions that attempts to reconcile recent, seemingly conflicting, structural, biochemical and biological observations. Central to this model is the involvement of both the CDR2-like and CDR3-like domains of CD4 in the interaction with gp120, leading to a conformational change and dissociation of gp120 from the gp120-gp41 complex.

Anti-AIDS drug development: challenges and strategies

A myriad of chemical derivatives has been shown to inhibit in vitro replication of the AIDS virus at concentrations that are nontoxic to the host cells. The majority of these agents acts by either (i) inhibiting enzymes such as reverse transcriptase (RT), protease, or glucosidase, (ii) arresting expression of genes or gene products, or (iii) inhibiting viral processes such as giant cell (syncytia) formation or viral binding to the target cell. The nucleoside RT inhibitors are the most widely studied agents at both the preclinical and the clinical levels. Their inability to cure AIDS has stimulated the discovery of several novel nonnucleoside RT inhibitors, possessing varied structures and demonstrating activity at nanomolar concentrations. These agents demonstrate a unique mode of binding to RT and show a high specificity for HIV-1. Protease inhibitors, soluble CD4 derivatives, oligonucleotides, and many anionic derivatives also demonstrate potent anti-HIV-1 activities. These derivatives possess mechanisms of action different to the nucleosides and exhibit selectivity as exemplified by their high in vitro therapeutic indices. This article discusses the structural parameters that govern activity in these agents, the pros and cons regarding the development of these compounds as putative anti-AIDS agents, and the future promise of searching for newer agents directed at novel targets to inhibit the AIDS virus.

Anti-CD4 anti-idiotype antibodies in volunteers immunized with rgp160 of HIV-1 or infected with HIV-1

We examined the sera of volunteers vaccinated with recombinant gp160 of human immunodeficiency virus type 1 (HIV-1) and control volunteers for the presence of anti-(anti-gp160 idiotype) antibodies which antigenically mimic gp160 and, therefore, bind to CD4 on human cells. Anti-CD4 antibodies were detected in the sera of 3 of 5 rgp160 recipients and 1 of 5 controls by indirect immunofluorescence using CD4-transfected HeLa cells or enzyme-linked immunosorbent assay (ELISA) using recombinant soluble CD4 as the solid phase. The control volunteer who was positive subsequently developed antibodies to HIV-1 by Western blot analysis. The anti-CD4 antibodies detected in the sera of the rgp160 vaccinees and the control volunteer appeared to be anti-idiotypic in nature, reacting with a paratope expressed on goat anti-gp160 antibodies but not on antibodies from normal goat serum. Binding to either transfected CD4+ HeLa cells or blotted anti-gp160 serum could be inhibited by preincubating the anti-CD4 serum with soluble CD4, or preincubating the cells or blotted anti-gp160 serum with recombinant gp160. Anti-CD4 antibodies were initially detectable only after the antibody response to gp160 began to decrease in the vaccinees, and the HIV-1-infected volunteer mounted a detectable anti-HIV-1 antibody response only after a decline in the anti-CD4 antibodies in his serum. These data strongly suggest that anti-CD4 antibodies which are anti-idiotypic to a paratope expressed on anti-gp160 antibodies are generated in response to both vaccination with rgp160 and infection with HIV-1.(ABSTRACT TRUNCATED AT 250 WORDS)

CPF-DD is an inhibitor of infection by human immunodeficiency virus and other enveloped viruses in vitro

The initial step in the infection cycle of human immunodeficiency virus type 1 (HIV-1) involves binding of its surface glycoprotein gp 120 to the T lymphocyte CD4 antigen. CPF-DD is a low molecular weight inhibitor of HIV infectivity that inhibits gp 120 binding to CD4 in vitro (Finberg et al., Science 249, 287-291, 1990). We find, however, that the actions of CPF-DD are not limited to its ability to interfere with gp 120-CD4 binding; its predominant action is to remove the viral envelope from the underlying core. Subsequently the virions disintegrate. Most enveloped viruses tested were inhibited by CPF-DD, but the infectivity of noneneloped viruses was unaffected or only slightly reduced.

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.

Recombinant glycoprotein 120 of human immunodeficiency virus is a potent interferon inducer

Cells infected with human immunodeficiency virus (HIV) induce antiviral activity in peripheral blood mononuclear cells (PBMC) from healthy donors. This activity is neutralized by anti-interferon-alpha antibody and partially destroyed at pH 2. Previous studies with enriched cell populations and monoclonal antibodies suggest that B lymphocytes are the main IFN-producing cells, and that both CD4 and HLA class II antigens are essential for IFN induction. Since the initial event of HIV infection of CD4+ cells is the interaction of the virus coat glycoprotein gp120 with CD4 molecule, we investigated whether gp120 is responsible for IFN induction. Using PBMC and recombinant gp120 obtained from a baculovirus expression system, dose-dependent induction of antiviral activity was observed with titers approaching 10(3) IU/ml. This induction was blocked in the presence of antibody to gp120. The antiviral activity was characterized as IFN-alpha by neutralization with IFN alpha-specific antibody. Preincubation of PBMC with anti-CD4 or the presence of soluble CD4 during incubation inhibited IFN induction, indicating that interaction of gp120 with cell-associated CD4 is responsible for this induction. Neither lymphoproliferation nor interleukin-2 (IL-2) production was observed during IFN induction. However, class G immunoglobulin secretion was enhanced by gp120, indicating that B cells are direct or indirect targets of gp120 stimulation in this experimental system. Since gp120 is shed from HIV-infected cells and occurs in the serum of acquired immunodeficiency syndrome (AIDS) patients, our data suggest that this glycoprotein is responsible for the induction of endogenous IFN and the polyclonal activation of B cells both of which are observed in AIDS patients.

Monoclonal antibodies to the C4 region of human immunodeficiency virus type 1 gp120: use in topological analysis of a CD4 binding site

We have raised antisera and monoclonal antibodies (MAbs) to the C4 region of HIV-1 gp120, using an antigen chimaera of poliovirus as immunogen. These MAbs and sera, together with MAbs to the same region raised by other methods, fall into three groups defined by their abilities to bind to recombinant gp120 and/or the immunogenic peptide. In some cases, the amino acids recognized by the MAbs have been identified by pep-scan and by solution phase peptide inhibition of binding to recombinant gp120. Our results indicate that the amino acids WQEVGKAMYA are exposed on the surface of recombinant gp120. Antibodies to these amino acids on recombinant gp120 compete for soluble CD4 binding in vitro, but only weakly neutralize HIV.

Thermodynamic and kinetic analysis of sCD4 binding to HIV-1 virions and of gp120 dissociation

Kinetic and thermodynamic aspects of the binding of sCD4 to intact virions of human immunodeficiency virus type 1 (HIV-1 RF), and of the subsequent induction of gp120 dissociation were studied. sCD4 binding to virions at 4 and 37 degrees C is half-maximal at approximately 40 and 10 nM, respectively. The transition between low-affinity and high-affinity binding of sCD4 to virions occurs over a narrow temperature range between 20 and 25 degrees C. Shedding of gp120 from virions after sCD4 binding is also temperature dependent, being initiated above approximately 20 degrees C. The minimum temperatures for the sCD4 affinity transition and gp120 shedding are, therefore, similar and we suggest how the two processes might be related mechanistically.

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

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

Human trophoblast cells express CD4 and are permissive for productive infection with HIV-1

The European collaborative study of HIV-infected pregnant women in Europe now indicates a 13% risk of fetal HIV infection (originally thought to be about 30%, and possibly higher in some countries). Several reports suggest trans-placental passage. However, the detailed mechanisms associated with such vertical transmission have not yet been clarified. We have examined the possibility that HIV enters placental tissue from maternal blood via binding to CD4 and Fc receptors (FcR) at the trophoblast level, allowing intraplacental infection. Here we report the detection of several FcR with distinct localization in the placental villus as well as CD4 surface expression on human trophoblast cells. In addition, we show that trophoblastic cells interact specifically with the gp120/gp160 viral envelope protein. By their tissue localization, these receptors could be responsible for the entry of HIV into the fetal placental cells. Furthermore, purified placental cells can be directly infected by HIV in vitro, and the infection is inhibited by soluble CD4. This suggests a crucial role of the CD4 receptor but an additional way of entry cannot be excluded. Such an in vitro model may be suitable for further studies concerning placental HIV transmission and its prevention.

Crystal structure of a soluble form of the human T cell coreceptor CD8 at 2.6 A resolution

A secreted fragment of the extracellular portion of human CD8 alpha has been expressed in CHO cells, and a deglycosylated and proteolyzed form of this fragment has been crystallized. We report here the crystal structure of this fragment as refined at 2.6 A resolution. The structure was solved by molecular replacement using a superposition of ten variable domains from immunoglobulin light chains as the search model. Only the N-terminal 114 amino acids of CD8 alpha are visible in the electron density maps. The domain formed by these residues possesses a fold typical of immunoglobulin variable domains and associates to form Fv-like homodimers.

Human immunodeficiency virus type 2 envelope glycoprotein: differential CD4 interactions of soluble gp120 versus the assembled envelope complex

Utilizing a recombinant vaccinia expression system, we investigated the biological properties and CD4 receptor interactions of the envelope glycoproteins of a noncytopathic human immunodeficiency virus type 2 strain, termed HIV-2/ST, and a highly cytopathic variant derived from it. The efficiency and host cell range of syncytium formation by the recombinant glycoproteins of both viruses were highly restricted compared to those of prototypic strains of HIV (HIV-2/ROD or HIV-1/IIIB). However, the glycoprotein of cytopathic but not wild-type ST generated numerous large syncytia in the human T-cell line Sup T1 from which it was derived. A single cell line (Molt 4 clone 8) was permissive to fusion by both wild-type and cytopathic ST envelopes, but only the glycoprotein of cytopathic ST could be inhibited with a soluble form of the viral receptor CD4 (sCD4). While these results indicated major differences in the envelope glycoprotein-CD4 receptor interactions of wild-type versus cytopathic ST, direct and competition binding assays utilizing soluble external glycoprotein (SU) and sCD4 surprisingly revealed equivalent low binding affinity for both viruses. From these experiments we conclude that relevant biological properties (e.g., CD4 binding, cytopathic potential, and sCD4 neutralization) of HIV viruses which differ in their pathogenic potential are reflected in the sCD4 interactions of the assembled native envelope complex (as on cell or virion surfaces) but not the soluble SU glycoprotein.

Characterization and anti-HIV properties of CD 4-coated red blood cells

Entry of HIV into its target cells requires its binding to the CD 4 molecule, the HIV receptor. Blocking this initial step of HIV life cycle is a potential target for the design of anti-HIV drugs. Soluble recombinant CD 4 efficiently blocks HIV infection in vitro and is the least toxic anti-HIV drug in humans. However, this molecule has a short half-life in vivo, and poorly neutralizes fresh isolates of HIV-1. Modifications of soluble CD 4 have been constructed, aimed at increasing its half life and other anti-viral properties. We have previously described an efficient method to cross-link soluble CD 4 to human red blood cells. We show here that these cells are uniformly coated as observed by immunofluorescence staining. Each of 8 different anti-CD 4 monoclonal antibodies stained these cells indicating that the epitopes recognized by these antibodies are correctly exposed at the cell membrane. These CD 4-expressing RBC inhibit specifically and in a dose dependent manner the HIV binding to, and infection of, CD 4+ target cells. On a CD 4 molar basis, this inhibition is approximately 15 times more efficient with CD 4-coated red blood cells than with soluble CD 4.

Mononuclear phagocytes as targets, tissue reservoirs, and immunoregulatory cells in human immunodeficiency virus disease

We have presented evidence in this review for the following: 1. Macrophages are likely the first cell infected by HIV. Studies document recovery of HIV into macrophages in the early stages of infection in which virus isolation in T cells is unsuccessful and detectable levels of antibodies against HIV are absent. 2. Macrophages are major tissue reservoirs for HIV during all stages of infection. Unlike the lytic infection of T cells, many HIV-infected macrophages show little or no virus-induced cytopathic effects. HIV-infected macrophages persist in tissue for extended periods of time (months) with large numbers of infectious particles contained within intracytoplasmic vacuoles. 3. Macrophages are a vector for the spread of infection to different tissues within the patient and between individuals. Several studies suggest a "Trojan horse" role for HIV-infected macrophages in dissemination of infectious particles. The predominant cell in most bodily fluids (alveolar fluid, colostrum, semen, vaginal secretions) is the macrophage. In semen, for example, the numbers of macrophages exceed those of lymphocytes by more than 20-fold (Wolf and Anderson 1988). 4. Macrophages are major regulatory cells that control the pace and intensity of disease progression in HIV infection. Macrophage secretory products are implicated in the pathogenesis of CNS disease and in control of viral latency in HIV-infected T cells. This litany of events in which macrophages participate in HIV infection in man parallels similar observations in such animal lentivirus infections as visna-maedi or caprine arthritis-encephalitis viruses. HIV interacts with monocytes differently than with T cells. Understanding this interaction may more clearly define both the pathogenesis of HIV disease and strategies for therapeutic intervention.

Investigations into the mechanism by which sulfated polysaccharides inhibit HIV infection in vitro

Sulfated polysaccharides have been shown to inhibit human immunodeficiency virus (HIV) infection in vitro. Dextrin sulfate, fucoidan, and dextran sulfate fail to neutralize virions directly, but interact with target cells to inhibit virus entry. Ionic interactions of sulfated polyanions with oppositely charged cell surface components, including CD4, have been assumed to be the inhibitory mechanism. It is shown that the sulfated polysaccharides inhibit infection of both CD4+ and CD4- cell lines by HIV and also that they inhibit HTLV-1 and, to a lesser extent, the simian retrovirus, MPMV, which use receptors other than CD4. One binding site for radiolabeled fucoidan on the surface of human T cells is an 18 kD protein, but its significance is not yet clear.

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.

Correlations between the in vitro and in vivo activity of anti-HIV agents: implications for future drug development

Some 10 years after the first recognition of acquired immunodeficiency syndrome (AIDS) as a new syndrome, we have identified a number of molecular targets to interrupt the replicative cycle of human immunodeficiency virus (HIV), the causative agent. A number of dideoxynucleosides have been identified as having anti-HIV activity in vitro, and several of these have been found to have clinical activity in patients. In contrast, while a number of agents have been found to block viral binding to the target cell in vitro, these agents have generally not shown clear-cut evidence of clinical activity. Agents which act at a variety of steps in the HIV replicative cycle are now under development, and it is likely that we will have an increased armamentarium to fight this disease in the near future.

The auto-regulation model: a unified concept of how HIV regulates its infectivity, pathogenesis and persistence

The life cycle of HIV can be divided into two distinct stages: intracellular and extracellular. The prevailing view is that the intracellular stage provides the only locus for regulating the virus in response to physiologic stimuli. Such regulation is accomplished by modulating the rates of transcription, translation and viral assembly. The extracellular stage consists of physical processes such as diffusion, adhesion and penetration of cells by viral particles. These latter processes are commonly thought to be "automatic" and not subject to regulation. For the past several years, we have developed means of more carefully measuring and characterizing the extracellular stage of HIV infection, and we have obtained evidence indicating that novel regulatory processes do, in fact, take place during this extracellular stage. We believe that this extracellular regulation permits HIV to adapt to a wide range of physiologic cell densities, to maintain persistent but slow growing infection, and to defeat the protective activity of humoral blockers. The overall purpose of this review is to consider our evidence for this hypothesis.

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

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

The fibroblast growth factor receptor is not required for herpes simplex virus type 1 infection

The early events mediating herpes simplex virus type 1 (HSV-1) infection include virion attachment to cell surface heparan sulfates and subsequent penetration. Recent evidence has suggested that the high-affinity fibroblast growth factor (FGF) receptor mediates HSV-1 entry. This report presents three lines of experimental evidence showing that the high-affinity FGF receptor is not required for HSV-1 infection. First, rat L6 myoblasts lacking FGF receptors were as susceptible to HSV-1 infection as L6 cells genetically engineered to express the FGF receptor. Second, a soluble FGF receptor fragment that inhibited FGF binding and receptor activation did not inhibit HSV-1 infection. Finally, basic FGF (but not acidic FGF) inhibited HSV-1 infection in L6 cells lacking FGF receptors, presumably by blocking cell surface heparan sulfates also required for HSV-1 infection. These results show that the high-affinity FGF receptor is not required for HSV-1 infection but instead that specific low-affinity basic FGF binding sites are used for HSV-1 infection.

Construction of a recombinant bacterial human CD4 expression system producing a bioactive CD4 molecule

The CD4 protein expressed on helper T lymphocytes is a restriction element for major histocompatibility class II immune responses. This molecule is also used by the human immunodeficiency virus as its specific cellular receptor facilitating binding of virus to cells. As soluble forms of CD4 inhibit HIV infection in tissue culture, attention has focused on this molecule. Bacterially produced CD4 would facilitate studies of the biology of the CD4 molecule. However, bacterially expressed CD4 must be refolded for assumption of its interaction with conformationally dependent anti-CD4 monoclonal antibodies as well as the HIV-1 envelope protein gp120. We report here the engineering of an external domain construct of the CD4 gene into a novel expression vector containing the nucleotide sequence encoding the pelB leader peptide of Erwinia carotovara (pDABL), to facilitate correct folding of CD4 in bacteria. Monoclonal antibodies specific for important conformational epitopes of the CD4 molecule were able to bind bacterial colonies containing the pDABL/CD4 vector but not colonies with vector alone. Importantly, recombinant gp120 produced in baculovirus bound specifically to bacterial colonies expressing the CD4 recombinant molecule. This system presents a simple screening mechanism for molecules that bind to the external domain of the CD4 glycoprotein. Vectors such as pDABL will also facilitate the production of large amounts of biologically active proteins in bacteria.

Human immunodeficiency virus type 1 Vpu protein regulates the formation of intracellular gp160-CD4 complexes

Intracellular transport and processing of the human immunodeficiency virus type 1 (HIV-1) envelope precursor glycoprotein, gp160, proceeds via the endoplasmic reticulum and Golgi complex and involves proteolytic processing of gp160 into the mature virion components, gp120 and gp41. We found that coexpression of gp160 and human CD4 in HeLa cells severely impaired gp120 production due to the formation of intracellular gp160-CD4 complexes. This CD4-mediated inhibition of gp160 processing was alleviated by coexpression of the HIV-1-encoded Vpu protein. The coexpression of Vpu and CD4 in the presence of gp160 resulted in increased degradation of CD4. Although the precise mechanism(s) responsible for the Vpu effect is presently unclear, our findings suggest that Vpu may destabilize intracellular gp160-CD4 complexes.

Strategies in the treatment of AIDS and related diseases: the lessons of cancer chemotherapy

The replication cycle of human immunodeficiency virus type 1 (HIV-1) consists of four distinct stages, each of which can be targeted for specific antiviral chemotherapy. The stages are (1) the attachment of virus to the CD4 receptor at the cell surface; (2) the uncoating of viral nucleic acid and its conversion via viral reverse transcriptase activity to DNA; (3) cellular multiplication, accompanied by the replication of integrated proviral DNA and production of viral RNA and proteins; and (4) the assembly and liberation of progeny virus from the cell and the potential reinitiation of the replication cycle in previously uninfected cells. Since each of these steps represents a potential target for anti-HIV chemotherapy, it is apparent that the rationale for the use of antiviral drugs is not dissimilar from the manner in which antineoplastic agents are targeted to specific stages in the replication cycle of tumor cells. As in the case of anticancer chemotherapy, it is hoped that combinations of drugs, which act against different steps in the viral replication cycle, might have synergistic potential. AZT or zidovudine is the most widely used drug to date to impede the replication of HIV-1; it is significant that this compound was designed initially with anticancer chemotherapy in mind. Although AZT therapy has been reasonably successful, this drug has had important toxic side effects. As in the case of many cancer chemotherapeutic agents, drug resistance to AZT is likely to be an important problem, and there have been several reports of the isolation of drug-resistant variants of HIV-1.

Phase 1 study of recombinant human CD4-immunoglobulin G therapy of patients with AIDS and AIDS-related complex

The safety and pharmacokinetics of recombinant CD4-immunoglobulin G (rCD4-IgG) were evaluated in a phase 1 study with dose escalation. A total of 16 patients, 6 with AIDS and 10 with AIDS-related complex, were evaluated at two university-affiliated hospital clinics. rCD4-IgG was administered once weekly for 12 weeks to four patients each at doses of 0.03, 0.1, 0.3, and 1.0 mg/kg of body weight. Dosing was intravenous for two patients in the 1.0-mg/kg dose group and intramuscular for the remaining patients. Dosing was intravenous for two patients in the 1.0-mg/kg dose group and intramuscular for the remaining patients. Pharmacokinetic, toxicity, and immunologic variables were monitored with all patients. Administration of rCD4-IgG was well tolerated, with no important clinical or immunologic toxicities noted. No subjects required dose reduction or discontinuation of therapy due to toxicity. No consistent changes were seen in human immunodeficiency virus antigen levels in serum or CD4 lymphocyte populations. The volume of distribution was small, and compared with that of rCD4, the half-life of the hybrid molecule was markedly prolonged following intramuscular or intravenous administration. The rate and extent of absorption following intramuscular dosing were variable. Intramuscular administration of rCD4-IgG appears to be inferior to intravenous dosing from a pharmacokinetic standpoint, with lower peak concentrations and variable absorption. After intravenous administration, peak concentrations of rCD4-IgG in serum (20 to 24 micrograms/ml) that have shown antiviral activity in vitro against more sensitive clinical isolates of human immunodeficiency virus were achieved. The peak concentrations in serum after intramuscular administration were below these levels. Treatment with rCD4-IgG was well tolerated at the doses administered to patients in this study but did not result in significant changes in CD4 lymphocyte counts or p24 antigen levels in serum.

Soluble CD4 and CD8 in the peripheral blood of patients with multiple sclerosis and HTLV-1-associated myelopathy

We evaluated the presence of soluble (s) CD4 and sCD8, released from activated T cells, in the sera of patients with multiple sclerosis (MS) and human T lymphotropic virus type 1 (HTLV-1)-associated myelopathy (HAM) using an enzyme-linked immunosorbent assay (ELISA). In addition, peripheral blood T cell subsets in patients with MS and HAM were analyzed by single and two color flow cytometry. The serum level of sCD8 was significantly elevated in MS patients as compared with controls (p less than 0.001). Sera from patients with an exacerbation of acute relapsing MS showed a higher sCD8 level than the patients in remission or controls (p less than 0.01 and p less than 0.001, respectively). The serum levels of both sCD4 and sCD8 were also significantly elevated in patients with HAM (p less than 0.001 and p less than 0.001, respectively). In addition, a significantly increased serum level of soluble interleukin-2 receptor (sIL-2R) was found in patients with HAM as compared with that of controls (p less than 0.001). These observations suggest that CD8 cells may be activated in the peripheral blood of patients with MS and sCD8 may be related to clinical activity, but that both CD4 and CD8 cells may be activated in the peripheral blood of patients with HAM.

Binding to CD4 of synthetic peptides patterned on the principal neutralizing domain of the HIV-1 envelope protein

The interaction between the viral envelope protein gp120 and the cellular surface antigen CD4 is a key event in HIV-1 infection. Reciprocal high affinity binding sites have been located in the first domain of CD4 and in the carboxy-terminal region of gp120, respectively. Upon infection, the membranes of the target cells fuse; sites of CD4 and gp120, distinct from their high affinity binding sites, play a role in the post-binding events leading to syncytia formation. We have studied the interactions of CD4 with gp120 and gp120-derived peptides using an in vitro assay based on immobilized recombinant soluble CD4 (sCD4). In this system CD4 binds to recombinant soluble gp120 and to anti-receptor peptides derived from the high affinity CD4-binding site of gp120, as well as to peptides corresponding to the principal neutralizing domain (PND) of the envelope protein, i.e., to the domain required for HIV-1-mediated syncytium formation. Competition experiments performed using epitope-specific mAbs and a variety of peptides indicated that PND-derived peptides are specifically recognized by a CD4 site adjacent to, but distinct from, the high affinity gp120-binding site of CD4. Synthetic peptides patterned on the PND of different viral isolates were retained onto sCD4-based affinity columns at different extent; some of the structural requirements for binding were analyzed. Studies performed on CD4+ T-cells showed that PND-derived peptides also interact with CD4 in its native membrane-bound conformation. These results indicate that a direct contact takes place between CD4 and the gp120 domain participating in HIV-induced syncytia formation.

Recombinant human soluble CD4 does not inhibit immune function in cynomolgus monkeys

Recombinant soluble CD4 (sT4) has been shown to inhibit infectivity of HIV. Because of the role CD4 plays in the interaction of T-helper lymphocytes and cells bearing MHC Class II antigens, a potential adverse effect of therapy with sT4 is interference with lymphocyte function. To address this issue, we studied the effects of sT4 on mitogen-mediated blastogenesis, mixed lymphocyte reactions, and delayed type hypersensitivity reactions (DTH) in cynomolgus monkeys. We found no evidence of sT4-mediated suppression on the in vitro response to concanavalin A, phytohemagglutinin or pokeweed mitogen in 2-way mixed lymphocyte reactions, either when sT4 was added to the cultures or when cells were obtained 3 hr after drug administration from animals that received up to 100 mg/kg as an intravenous bolus. Furthermore, we also found no effect of sT4 on lymphocyte subsets or on the ability of monkeys to respond to dinitrochlorobenzene (DNCB)-mediated DTH. Because of the high degree of conservation of CD4 and MHC Class II antigens across the macaque-human barrier, these data suggest that soluble CD4-like molecules are unlikely to be immunosuppressive in humans.

A single amino acid substitution in a common African allele of the CD4 molecule ablates binding of the monoclonal antibody, OKT4

The CD4 molecule is a relatively non-polymorphic 55 kDa glycoprotein expressed on a subset of T lymphocytes. A common African allele of CD4 has been identified by non-reactivity with the monoclonal antibody, OKT4. The genetic basis for the OKT4- polymorphism of CD4 is unknown. In the present paper, the structure of the CD4 molecule from an homozygous CD4OKT4- individual was characterized at the molecular level. The size of the CD4OKT4- protein and mRNA were indistinguishable from those of the OKT4+ allele. The polymerase chain reaction (PCR) was used to map the structure of CD4OKT4- cDNAs by amplifying overlapping DNA segments and to obtain partial nucleotide sequence after asymmetric amplification. PCR was then used to clone CD4OKT4- cDNAs spanning the coding region of the entire, mature CD4 protein by amplification of two overlapping segments followed by PCR recombination. The nucleotide sequence of CD4OKT4- cDNA clones revealed a G----A transition at bp 867 encoding an arginine----tryptophan substitution at amino acid 240 relative to CD4OKT4+. Expression of a CD4OKT4- cDNA containing only this transition, confirmed that the arginine----tryptophan substitution at amino acid 240 ablates the binding of the mAb OKT4. A positively charged amino acid residue at this position is found in chimpanzee, rhesus macaque, mouse and rat CD4 suggesting that this mutation may confer unique functional properties to the CD4OKT4- protein.

Rat monoclonal antibodies to nonoverlapping epitopes of human immunodeficiency virus type 1 gp120 block CD4 binding in vitro

Monoclonal antibodies (MAbs) to a recombinant form of the envelope glycoprotein gp120 of human immunodeficiency virus type 1 (HIV-1 IIIB) were raised in rats and screened for their ability to block recombinant gp120 binding to recombinant, soluble CD4 (sCD4) in vitro. Four such MAbs were identified and characterised. Each MAb bound strongly to gp120 from eight widely divergent HIV-1 strains from the United States and Africa. Two MAbs were mapped to the fourth conserved (C4) region of gp120, whereas the other two recognised an as yet undefined, conformationally sensitive epitope. MAbs to the latter epitope were the more potent in blocking the gp120-sCD4 interaction. None of the MAbs, however, had potent neutralising activity.