CD4-derived synthetic peptide blocks the binding of HIV-1 GP120 to CD4-bearing cells and prevents HIV-1 infection

Short synthetic peptides derived from viral proteins compete with HIV gp120 for the binding to CD4 receptors

In the complex mechanism of adhesion, internalization, and infection of cells by human immunodeficiency virus (HIV) viral particles, a determinant role is played by the viral envelope glycoprotein gp120, which binds to CD4 receptors of T cells and monocytes. We tested the ability of a panel of 7- to 12-residue synthetic peptides, selected from the region 414-434 of the HIV-1 gp120, to inhibit the binding of the viral protein to CD4 receptors of cultured human lymphoid cells. The assay was based on the observation that the binding of gp120 to the receptors interferes with the binding of a specific anti-CD4 monoclonal antibody, as a result of the masking of the antibody epitope; thus, we tested whether preincubation of cells with the peptides before gp120 addition might restore the recognition of the CD4 molecule by the antibody. High expression of CD4 receptors was thus assumed as indication that the binding of the viral protein had been inhibited. Maximum activity was displayed by a 9-residue peptide located near the amino terminal end of the 414-434 fragment. In addition, several fragments deduced from other viral proteins, possessing partial amino acid sequence homology with the HIV gp120 fragment, exhibited a similar type of interaction with the CD4 receptor. All active peptides contain the Cys residue (position 423 of gp120). This residue is essential, although not sufficient, for inhibiting gp120 binding, as few other amino acid residues within the fragment play a complementary role in increasing or decreasing the inhibitory ability.

Increased association of glycoprotein 120-CD4 with HIV type 1 coreceptors in the presence of complex-enhanced anti-CD4 monoclonal antibodies

CD4-specific monoclonal antibodies (CG1, CG7, and CG8), which bind with a 5- to 10-fold higher avidity to preformed CD4-gp120 complexes than to CD4, were previously shown to recognize newly identified conformational epitopes in the D1-CDR3 region of CD4. In the current study, these and other complex-enhanced MAbs were tested in three separate assays of HIV-1 coreceptor (CXCR4/CCR5) recruitment. In these assays, the CD4-specific MAbs CG1, -7, and -8 stabilized the association of coreceptor, gp120, and CD4 in trimolecular complexes. In contrast, the gp120-specific, complex-enhanced MAbs 48d and 17b were inhibitory. These data suggest that conformational changes in the CDR3 region of CD4-D1, induced by gp120 binding, may be involved in coreceptor association and thus play a positive role in the HIV-1 cell fusion process.

Further insights on the inhibition of HIV type 1 infection in vitro by CD4-modified synthetic peptides containing phenylalanine

Phenylalanine-containing peptides from CD4 were synthesized on the basis of chemical similarity with active CD4(81-92)-benzylated peptides. Systematic replacement of amino acids of these peptides bearing the benzyl group by phenylalanine, afforded several peptides that were able to block the binding of gp120 to CD4 and to inhibit HIV-induced syncytium formation. These experiments showed that substitution of residues 81 and 85 by phenylalanine was the most important for activity. Following optimization of the length of phenylalanine-substituted peptides it was found that FYICFVED and FYICFVEDE were the most active. Their IC50 for the inhibition of syncytium formation was around 1.2-1.6 microM. This activity is at least 30 times higher than that of the parent peptide FYIFFVEDQKEEDD previously reported (Lasarte et al., J Acquir Immune Defic Syndr 1994;7:129-134). Binding competition experiments with two different anti-peptide antisera recognizing the V3 region of gp120 and FYICFVEDE, show that the active peptides bind to V3 or to a sterically near region of V3. None of the active peptides was toxic to cells in vitro. The enhanced activity and simplicity of these new phenylalanine-substituted CD4 peptides might be a good starting point for the development of mimotopes of potential use for the treatment of AIDS.

Selective effects of electrostatic changes in the CD4 CDR-3-like loop on infection by different human immunodeficiency virus type 1 isolates

The role of the CDR-3-like loop of the first domain of the CD4 molecule in infection by the human immunodeficiency virus type 1 (HIV-1) is controversial. In an attempt to determine whether the strong negative charge in the CDR-3-like loop influences HIV-1 infection we have substituted by mutagenesis negative for positively charged residues at position 87/88 and 91/92. These mutations were shown to have no obvious effect on CD4 conformation outside of the CDR-3-like loop. Infection of cells expressing the E87K/D88K substitution mutant resulted in a selective reduction in infectivity for certain HIV-1 viruses compared to cells expressing wile-type CD4. Viruses Hx10, HxB2, and MN were 4- to 13-fold less efficient at infecting the E87K/D88K mutant, whereas SF2, RF, and NDK yielded an efficiency of infection similar to, or slightly greater than, that of the wild type. To investigate the step at which infectivity was selectively reduced, we compared early events in the life cycles of Hx10 and SF2 viruses using PCR entry and gp120-binding assays. Both gp120 binding and virus entry were reduced for Hx10 on the mutant CD4-expressing cells as compared to wild-type CD4-expressing cells, whereas no difference was seen in either assay with SF2. Although relatively small in magnitude, the contribution of the CDR-3-like loop to the overall CD4-gp120 interaction may serve to modify the binding and entry of certain virus isolates.

Phorbol ester-induced down modulation of tailless CD4 receptors requires prior binding of gp120 and suggests a role for accessory molecules

The entry of human immunodeficiency virus type 1 into cells proceeds via a fusion mechanism that is initiated by binding of the viral glycoprotein gp120-gp41 to its cellular receptor CD4. Species- and tissue-specific restrictions to viral entry suggested the participation of additional membrane components in the postbinding fusion events. In a previous study (H. Golding, J. Manischewitz, L. Vujcic, R. Blumenthal, and D. Dimitrov, J. Virol. 68:1962-1968, 1994), it was found that phorbol myristate acetate (PMA) inhibits human immunodeficiency virus type 1 envelope-mediated cell fusion by inducing down modulation of an accessory component(s) in the CD4-expressing cells. The fusion inhibition was seen in a variety of cells, including T-cell transfectants expressing engineered CD4 receptors (CD4.401 and CD4.CD8) which are not susceptible to down modulation by PMA treatment. In the current study, it was found that preincubation of A2.01.CD4.401 cells with soluble monomeric gp120 for 1 h at 37 degrees C primed them for PMA-induced down modulation (up to 70%) of the tailless CD4 receptors. The gp120-priming effect was temperature dependent, and the down modulation may have occurred via clathrin-coated pits. Importantly, nonhuman cell lines expressing tailless CD4 molecules did not down modulate their CD4 receptors under the same conditions. The gp120-dependent PMA-induced down modulation of tailless CD4 receptors could be efficiently blocked by the human monoclonal antibodies 48D and 17B, which bind with increased avidity to gp120 that was previously bound to CD4 (M. Thali, J. P. Moore, C. Furman, M. Charles, D. D. Ho, J. Robinson, and J. Sodroski, J. Virol. 67:3978-3988, 1993). These findings suggest that gp120 binding to cellular CD4 receptors induces conformational changes leading to association of the gp120-CD4 complexes with accessory transmembrane molecules that are susceptible to PMA-induced down modulation and can target the virions to clathrin-coated pits.

Antiviral therapy for human immunodeficiency virus infections

Depending on the stage of their intervention with the viral replicative cycle, human immunodeficiency virus inhibitors could be divided into the following groups: (i) adsorption inhibitors (i.e., CD4 constructs, polysulfates, polysulfonates, polycarboxylates, and polyoxometalates), (ii) fusion inhibitors (i.e., plant lectins, succinylated or aconitylated albumins, and betulinic acid derivatives), (iii) uncoating inhibitors (i.e., bicyclams), (iv) reverse transcription inhibitors acting either competitively with the substrate binding site (i.e., dideoxynucleoside analogs and acyclic nucleoside phosphonates) or allosterically with a nonsubstrate binding site (i.e., non-nucleoside reverse transcriptase inhibitors), (v) integration inhibitors, (vi) DNA replication inhibitors, (vii) transcription inhibitors (i.e., antisense oligodeoxynucleotides and Tat antagonists), (viii) translation inhibitors (i.e., antisense oligodeoxynucleotides and ribozymes), (ix) maturation inhibitors (i.e., protease inhibitors, myristoylation inhibitors, and glycosylation inhibitors), and finally, (x) budding (assembly/release) inhibitors. Current knowledge, including the therapeutic potential, of these various inhibitors is discussed. In view of their potential clinical the utility, the problem of virus-drug resistance and possible strategies to circumvent this problem are also addressed.

Multibranched peptide constructs derived from the V3 loop of envelope glycoprotein gp120 inhibit human immunodeficiency virus type 1 infection through interaction with CD4

The V3 loop of the gp120 of human immunodeficiency virus type 1 (HIV-1) is assumed to be involved in HIV-1-mediated membrane fusion. V3-derived peptides have been shown either to enhance or to prevent HIV-1 infection. Multibranched peptide constructs (MBPCs) derived from the V3 North American/European consensus sequence were designed to sort out these conflicting findings. At 5 microM, MBPC1 (8-branched GPGRAF) totally, and MBPC2 ([RKSIHIGPGRAFYT]4) partially, inhibited HIV-1LAI infection, whereas the GPGRAF monomer had only a limited effect. A peptide of the entire V3 consensus loop and a control MBPC had no detectable activity. The 5 microM MBPC1 HIV-1-inhibiting concentration was not cytotoxic, nor did it alter T lymphocyte allogeneic, antigen-, or mitogen-induced reactivities, and it was about 5- to 50-fold lower (MBPC2 and MBPC1, respectively) than that resulting in 50% cell death. Analysis of MBPC immunoreactivity showed that MBPC2, but not MBPC1, strongly reacted with human HIV-1 positive sera. Only MBPC2 elicited significant antibody responses in rabbits. The V3-derived MBPCs bound to CD4+ cells, as determined by immunofluorescence analysis. The binding was inhibited either by soluble CD4 or by CD4 monoclonal antibody (mAb) MT151, which recognizes the CDR3 region of the D1 domain of CD4, but not by other CD4 mAbs Leu3a, OKT4A, Q4021, 13B8-2, 5A8, RFT4, nor by the CD26 mAb BA5. Therefore, it appears likely that MBPCs inhibit HIV-1 infection by interacting with the CDR3 region of CD4 or with a region in its vicinity.

Receptor function of CD4 structures from African green monkey and pig-tail macaque for simian immunodeficiency virus, SIVsm, SIVagm, and human immunodeficiency virus type-1

Differences in kinetics of infection, cellular tropism, and cytopathology of SIV and HIV appear to depend on both viral and host factors. We investigated the role of critical CD4 structures from African green monkeys (AGM) a natural SIV host, from pig-tailed macaques (PT) an unnatural SIV host, and from humans, as well as the role of species-specific cellular factors involved in the tropism, kinetics of infection, and cytopathic effects of several SIV and HIV-1. Critical regions of the PT macaque and AGM CD4 genes (V1, V1J1, and V1J1V2J2) were stably expressed as chimeras with the human CD4 gene in human (HeLa and 293) and macaque (CMMT) cell lines. CD4 expressing cell lines were used for infection studies with cell-free SIVsm, SIVmac, SIVsmmPBj, SIVagm, and HIV-1. Results show that both PT CD4 and AGM CD4 supported infection with comparable infection kinetics by all SIV or HIV-1 strains tested. Although structural analysis predicted a major change in secondary structure of AGM CD4/CDR-3, these structural changes did not influence the degree of syncytia formation induced by several SIV and HIV-1. However, the cell line used to express the CD4 gene appeared to be a critical determinant of infection. Thus, SlV strains did not infect human cell lines regardless of the CD4 expressed in these cells. In contrast, HIV-1 did not infect any macaque cell line. This study demonstrates that the differences in CD4 structure among different primate species are clearly not responsible for differences in SIV and HIV infection kinetics, tropism, and cytopathology. However, species-specific factor(s), presumably expressed on the cell surface, markedly influences the ability of SIV or HIV to infect cells expressing CD4.

Multiple effects of CD4 CDR3-related peptide derivatives showing anti-HIV-1 activity on HIV-1 gp120 functions

The interaction of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 with CD4 CDR3-related peptide derivatives showing anti-HIV-1 activity has been studied. Conformational changes in gp120, which could affect its interaction with CD4 and its shedding from virions, were detected by fluorescence spectrum analysis of tryptophan residues after addition of peptide representative of the CD4 CDR3-related region, but not the CD4 CDR2-related region. Interestingly, the addition of scrambled peptide, S1 (with altered amino acid sequence compared with the native CDR3-related peptide but unaltered overall composition), which we recently showed to have stronger anti-HIV-1 activity than the original CDR3-related peptide, had no effects on the conformational change in gp120 or on its interaction with CD4 and its shedding from HIV-1 virions. However, all of the CDR3-related peptides, including S1, showed blocking effects on the binding of antibodies against gp120 V3 loop and C-terminus regions. Thus, we concluded that there were at least two separable activities of the CDR3-related peptides in anti-HIV-1 activity, i.e. induction of conformational changes in gp120, which could affect its binding to CD4 and to gp41 (as observed in native CDR3-related peptides), and inactivation of V3 loop and C-terminus regions in gp120 (as observed in all of the CDR3-related peptides, including S1).

CD4-derived peptide and sulfated polysaccharides have similar mechanisms of anti-HIV activity based on electrostatic interactions with positively charged gp120 fragments

The mechanism of antiviral activity of the CD4-derived peptide 75-99 was compared with that of sulfated polysaccharides. A set of peptides representing all the high positive charge density regions of gp120 and gp41 was used to determine whether electrostatic interactions occur between these negatively charged agents and positively charged HIV envelope fragments. Synthetic peptide AZ2, amino acids 75-99 from V1 CD4, KIEDSDTYIC(Acm)-EVEDQKEEVQLLVFG, and dextran sulfate 500,000 (DS 500) were used as inhibitory agents of antibody binding in ELISA using: (1) anti-peptide rabbit antibodies; (2) sera from HIV infected persons. Peptide AZ2 and DS were both shown to block antibody binding to peptide (301-323) from the principal neutralizing domain (PND) and peptide (495-516) from the gp120 C-terminus. The blocking concns were 1-2 micrograms/ml for DS and 125-250 micrograms/ml for AZ2. The ELISA system based on rabbit anti-peptide antibodies was less sensitive than that based on positive human sera. Chemical modification of lysine epsilon-amino groups of these peptides resulted in complete failure to bind either DS or AZ2. A correlation was found between the inhibitory activities of a number of sulfated polysaccharides in a syncytium formation assay and in peptide ELISA. The mechanism of direct interactions of specific regions of gp120 with the CDR3-like region of CD4 is proposed.

A monoclonal antibody to the CDR-3 region of CD4 inhibits soluble CD4 binding to virions of human immunodeficiency virus type 1

The CDR-3 region of CD4 has been proposed to be involved in the fusion reaction between human immunodeficiency virus type 1 (HIV-1) and CD4+ cells, either at a stage involving virus binding or subsequent to virus binding. Part of the evidence for this has been the observation that monoclonal antibodies (MAbs) to CDR-3 block HIV infection potently without strongly inhibiting the binding of monomeric gp120 to CD4. Here I show that, in a system using oligomeric, virion-bound gp120, a MAb to CDR-3 resembles those to CDR-2 in that it inhibits soluble CD4 binding to virions. Consequently, ternary complexes of MAb-soluble CD4-gp120 cannot be detected with CDR-2 MAbs and are detectable only at a very low level with a CDR-3 MAb, but they clearly form when a control MAb to CD4 domain 4 is used. Although not in direct conflict with previously published data on the role of CDR-3 MAbs in the inhibition of HIV-1 infection, these experiments do not support the hypothesis that the CDR-3 region is specifically involved in virus entry at a postbinding stage.

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

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

Characterization of a unique scrambled peptide derived from the CD4 CDR3-related region which shows substantial activity for blocking HIV-1 infection

We have previously identified CD4 peptides that exhibited blocking activity on human immunodeficiency virus type 1 (HIV-1) infection, i.e. CD4(68-130) and CD4(66-92) which include the region corresponding to the third complementarity-determining region of IgG. Here we describe a unique peptide derived from CD4(66-92), altered in amino acid sequence but not in composition, which was found to have increased anti-HIV-1 activity. The acidic amino acid residues in this scrambled peptide, S1, localized at the N-terminus, while in the native peptide they clustered at the C-terminus. On the other hand, a second scrambled peptide, S2, in which the acidic amino acid residues were fully dispersed, did not show any anti-HIV-1 activity. However, we could not identify any correlation between CD4(66-92) and S1 peptides by their hydrophobic or circular dichroism spectrum analyses. The results provide insight into the mechanisms of HIV-1 gp120 and CD4 interaction and may be useful as a new approach to AIDS therapy.

CD4(81-92)-based peptide derivatives. Structural requirements for blockade of HIV infection, blockade of HIV-induced syncytium formation, and virostatic activity in vitro

CD4(81-92) peptide block human immunodeficiency virus (HIV) infection, virus-induced cell fusion, and antigen production by HIV-1-infected cells when derivatized on specific amino acid residues. An extensive series of structural variants of 1,4,5-tribenzyl-10-acetyl-CD4(81-92) were tested as anti-viral agents in an attempt to define the sequence and derivatization requirements for antiviral activity, and to maximize potency and stability for use as potential therapeutic agents. Alteration of the primary amino acid sequence of the stem compound 1,4,5-tribenzyl-CD4(81-92) diminished or abolished in parallel all three indices of anti-viral activity in a series of altered sequence compounds. Replacement of d- for l-amino acid residues at positions 1, 2, 3, 4, 5, or 6 but not position 10 decreased anti-viral potency, again with parallel effects on infection, synctium formation, and virostatic activity. Omission of the glutamine residue at position 9 did not affect anti-viral potency, while removal of the glutamic acids at position 11 and 12 resulted in virtually complete loss of biological activity. Changes in the derivatization pattern of the CD4(81-92) peptide backbone also affected anti-viral potency and efficacy. Optimal activity was obtained with benzyl residues at positions 1, 4, and 5, whereas the 1,4,7-tribenzyl-CD4(81-92) compound was without activity in all assays tested. Replacement of one of the benzyl groups with an acetamidomethyl moiety resulted in complete loss of biological activity. The previously reported (Nara et al., Proc Natl Acad Sci USA 86: 7139-7143, 1989) virostatic activity of 1,4,5-tribenzyl-10-acetyl-CD4(81-92) (peptide #18) is apparently due to acetylation, since the desacetyl stem compound shows much less virostatic activity while still possessing full anti-infective and anti-syncytial activity, and acetylation of the N-terminus rather than the lysine of 1,4,5-tribenzyl-CD4(81-92) yields a virostatic compound equipotent to peptide #18. Cyclization of the tribenzyl peptide to further conformationally restrict the molecule resulted in a compound with anti-infection, anti-syncytial, and virostatic activity at submicromolar concentrations.

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

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

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.

Synthetic peptides allow discrimination of structural features of CD4(81-92) important for HIV-1 infection versus HIV-1-induced syncytium formation

Benzylated peptides with a primary amino acid sequence corresponding to either human CD4(81-92) (#18), or chimpanzee CD4(81-92) (#18C), were equipotent inhibitors of human immunodeficiency virus type 1 (HIV-1) infection of CD4+ cells and high-affinity binding of 125I-gp120 to CD4+ cells. The chimpanzee-based CD4(81-92) peptide, however, which differs from the human peptide by a single amino acid substitution (E for G) at position 87, was considerably less potent than the human CD4(81-92)-based peptide congener to inhibit HIV-1-induced cell-cell fusion. These data suggest that a portion of the CD4 molecule contained within the sequence CD4(81-92) is involved in binding gp120 during both HIV-1 infection and HIV-1-induced syncytium formation in human cells, but that the presence of a glutamic acid at position 87 in this sequence is more critical for the CD4/gp120 interaction leading to syncytium formation than for the CD4/gp120 interaction leading to primary infection of CD4-positive cells. The region CD4(81-92) may critically contribute to CD4-mediated HIV-1 pathogenesis in humans, and its alteration might explain the lack of pathogenic sequelae of HIV-1 infection in chimpanzees.

Anti-idiotypic antibodies against anti-CD4 antibodies MT151 and OKT4A

Anti-idiotypic antibodies (Ab2) binding to the antigen-combining site of other antibodies may functionally and even structurally mimic antigen. Ab2 to antibodies directed against the lymphocyte CD4 receptor for human immunodeficiency virus type 1 (HIV-1) may mimic the receptor and therefore inhibit viral infectivity. We have produced Ab2 against monoclonal anti-CD4 receptor antibodies (Ab1). The Ab1 strongly inhibit HIV-1 binding to the receptor. Six monoclonal rat Ab2 and two polyclonal rabbit Ab2 were produced against the Ab1 MT151 and nine monoclonal Ab2 against the Ab1 OKT4A. These Ab2 bound only to Ab1 and not to a panel of nine unrelated murine monoclonal antibodies (MAbs). The Ab2 completely inhibited the binding of the homologous Ab1 to CD4-positive target cells, and recombinant soluble CD4 inhibited binding of Ab2 to Ab1. Thus, the Ab2 seemed to mimic the Ab1-binding site of the CD4 receptor, although the results of inhibition assays did not exclude steric hindrance of antibody-combining sites. However, none of the 17 Ab2 bound to gp120 of HIV-1 envelope or inhibited syncytia formation between cells infected and uninfected with HIV-1. These results suggest that the Ab2 do not mimic the HIV-1 binding site of the CD4 receptor. They further suggest that the Ab1 may not bind within the virus-binding site of the CD4 receptor.