CD4-Induced Conformational Changes in the Human Immunodeficiency Virus Type 1 gp120 Glycoprotein: Consequences for Virus Entry and Neutralization

Human immunodeficiency virus type 1 (HIV-1) entry into target cells involves sequential binding of the gp120 exterior envelope glycoprotein to CD4 and to specific chemokine receptors. Soluble CD4 (sCD4) is thought to mimic membrane-anchored CD4, and its binding alters the conformation of the HIV-1 envelope glycoproteins. Two cross-competing monoclonal antibodies, 17b and CG10, that recognize CD4-inducible gp120 epitopes and that block gp120-chemokine receptor binding were used to investigate the nature and functional significance of gp120 conformational changes initiated by CD4 binding. Envelope glycoproteins derived from both T-cell line-adapted and primary HIV-1 isolates exhibited increased binding of the 17b antibody in the presence of sCD4. CD4-induced exposure of the 17b epitope on the oligomeric envelope glycoprotein complex occurred over a wide range of temperatures and involved movement of the gp120 V1/V2 variable loops. Amino acid changes that reduced the efficiency of 17b epitope exposure following CD4 binding invariably compromised the ability of the HIV-1 envelope glycoproteins to form syncytia or to support virus entry. Comparison of the CD4 dependence and neutralization efficiencies of the 17b and CG10 antibodies suggested that the epitopes for these antibodies are minimally accessible following attachment of gp120 to cell surface CD4. These results underscore the functional importance of these CD4-induced changes in gp120 conformation and illustrate viral strategies for sequestering chemokine receptor-binding regions from the humoral immune response.     

Loss of a single N-linked glycan allows CD4-independent human immunodeficiency virus type 1 infection by altering the position of the gp120 V1/V2 variable loops

The gp120 envelope glycoprotein of primary human immunodeficiency virus type 1 (HIV-1) promotes virus entry by sequentially binding CD4 and the CCR5 chemokine receptor on the target cell. Previously, we adapted a primary HIV-1 isolate, ADA, to replicate in CD4-negative canine cells expressing human CCR5. The gp120 changes responsible for CD4-independent replication were limited to the V2 loop-V1/V2 stem. Here we show that elimination of a single glycosylation site at asparagine 197 in the V1/V2 stem is sufficient for CD4-independent gp120 binding to CCR5 and for HIV-1 entry into CD4-negative cells expressing CCR5. Deletion of the V1/V2 loops also allowed CD4-independent viral entry and gp120 binding to CCR5. The binding of the wild-type ADA gp120 to CCR5 was less dependent upon CD4 at 4 degrees C than at 37 degrees C. In the absence of the V1/V2 loops, neither removal of the N-linked carbohydrate at asparagine 197 nor lowering of the temperature increased the CD4-independent phenotypes. A CCR5-binding conformation of gp120, achieved by CD4 interaction or by modification of temperature, glycosylation, or variable loops, was preferentially recognized by the monoclonal antibody 48d. These results suggest that the CCR5-binding region of gp120 is occluded by the V1/V2 variable loops, the position of which can be modulated by temperature, CD4 binding, or an N-linked glycan in the V1/V2 stem.     

A novel human antibody against human immunodeficiency virus type 1 gp120 is V1, V2, and V3 loop dependent and helps delimit the epitope of the broadly neutralizing antibody immunoglobulin G1 b12

The V1/V2 and V3 loops are proximal to the CD4 binding site (CD4bs) of human immunodeficiency virus type 1 (HIV-1) gp120 and undergo conformational change upon CD4 receptor engagement by the HIV-1 envelope spike. Nearly all of the reported monoclonal antibodies (MAbs) against the CD4bs exhibit a very limited capacity to neutralize HIV-1. However, one such human MAb, immunoglobulin G1 (IgG1) b12, is uniquely able to neutralize primary isolates across subtypes with considerable potency. The molecular basis for the anti-HIV-1 activity of b12 is not fully understood but is relevant to vaccine design. Here we describe a novel human MAb, 4KG5, whose binding to monomeric gp120 is moderately enhanced by IgG1 b12. In sharp contrast, 4KG5 binding to gp120 is inhibited by soluble CD4 (sCD4) and by all other (n = 14) anti-CD4bs MAbs tested. 4KG5 is unable to recognize gp120 in which either V1, V2, or V3 has been deleted, and MAbs against the V2 or V3 loops inhibit the binding of 4KG5 to gp120. Moreover, 4KG5 is able to inhibit the binding of the CD4-induced MAbs 17b and X5 in the absence of sCD4, whereas 17b and X5 only weakly inhibit the binding of 4KG5 to gp120. Mutagenesis of gp120 provides further evidence of a discontinuous epitope of 4KG5 that is formed by the V1/V2 loop, the V3 loop, and a portion of the bridging sheet (C4). 4KG5 was isolated as a single-chain Fv from a phage display library constructed from the bone marrow of an HIV-1-seropositive subject (FDA2) whose serum neutralizes HIV-1 across subtypes. Despite its source, we observed no significant neutralization with 4KG5 against the autologous (R2) virus and several other strains of HIV-1. The results suggest a model in which antibody access to the CD4bs on the envelope spike of HIV-1 is restricted by the orientation and/or dynamics of the V1/V2 and V3 loops, and b12 avoids these restrictions.     

Adaptation of a CCR5-Using, Primary Human Immunodeficiency Virus Type 1 Isolate for CD4-Independent Replication

The gp120 envelope glycoprotein of the human immunodeficiency virus type 1 (HIV-1) promotes virus entry by sequentially binding CD4 and chemokine receptors on the target cell. Primary, clinical HIV-1 isolates require interaction with CD4 to allow gp120 to bind the CCR5 chemokine receptor efficiently. We adapted a primary HIV-1 isolate, ADA, to replicate in CD4-negative canine cells expressing human CCR5. The gp120 changes responsible for the adaptation were limited to alteration of glycosylation addition sites in the V2 loop-V1-V2 stem. The gp120 glycoproteins of the adapted viruses bound CCR5 directly, without prior interaction with CD4. Thus, a major function of CD4 binding in the entry of primary HIV-1 isolates can be bypassed by changes in the gp120 V1-V2 elements, which allow the envelope glycoproteins to assume a conformation competent for CCR5 binding.     

Thermodynamics of binding of a low-molecular-weight CD4 mimetic to HIV-1 gp120

NBD-556 and the chemically and structurally similar NBD-557 are two low-molecular weight compounds that reportedly block the interaction between the HIV-1 envelope glycoprotein gp120 and its receptor, CD4. NBD-556 binds to gp120 with a binding affinity of 2.7 x 10(5) M(-1) (K(d) = 3.7 muM) in a process characterized by a large favorable change in enthalpy partially compensated by a large unfavorable entropy change, a thermodynamic signature similar to that observed for binding of sCD4 to gp120. NBD-556 binding is associated with a large structuring of the gp120 molecule, as also demonstrated by CD spectroscopy. NBD-556, like CD4, activates the binding of gp120 to the HIV-1 coreceptor, CCR5, and to the 17b monoclonal antibody, which recognizes the coreceptor binding site of gp120. NBD-556 stimulates HIV-1 infection of CD4-negative, CCR5-expressing cells. The thermodynamic signature of the binding of NBD-556 to gp120 is very different from that of another viral entry inhibitor, BMS-378806. Whereas NBD-556 binds gp120 with a large favorable enthalpy and compensating unfavorable entropy changes, BMS-378806 does so with a small binding enthalpy change in a mostly entropy-driven process. NBD-556 is a competitive inhibitor of sCD4 and elicits a similar structuring of the coreceptor binding site, whereas BMS-378806 does not compete with sCD4 and does not induce coreceptor binding. These studies demonstrate that low-molecular-weight compounds can induce conformational changes in the HIV-1 gp120 glycoprotein similar to those observed upon CD4 binding, revealing distinct strategies for inhibiting the function of the HIV-1 gp120 envelope glycoprotein. Furthermore, competitive and noncompetitive compounds have characteristic thermodynamic signatures that can be used to guide the design of more potent and effective viral entry inhibitors.     

Synergistic inhibition of human immunodeficiency virus type 1 envelope glycoprotein-mediated cell fusion and infection by an antibody to CD4 domain 2 in combination with anti-gp120 antibodies.

Antibodies to several epitopes of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41) can synergize in inhibiting HIV-1 infection. In the present study we tested the ability of a monoclonal antibody (MAb), 5A8, which interacts with CD4 domain 2, and other CD4-specific MAbs to synergize with antibodies against gp120. We have previously found that 5A8 inhibits HIV-1 entry without interfering with gp120 binding to CD4, presumably by affecting a postbinding membrane fusion event. Because antibodies to the gp120 V3 loop also affect post-CD4-gp120-binding events, 5A8 was first tested in combination with anti-V3 loop antibodies for possible synergy. The anti-V3 loop antibodies 0.5 beta, NEA-9205, and 110.5 acted synergistically with 5A8 in inhibiting syncytium formation between gp120-gp41- and CD4-expressing cells. A human MAb to an epitope of gp120 involved in CD4 binding, IAM 120-1B1, and another anti-CD4 binding site antibody, PC39.13, also exerted synergistic effects in combination with 5A8. Similarly, an antibody against the gp120 binding site on CD4, 6H10, acted synergistically with an anti-V3 loop antibody, NEA-9205. However, a control anti-CD4 antibody, OKT4, which does not significantly inhibit syncytium formation alone, produced only an additive effect when combined with NEA-9205. Serum from HIV-1-infected individuals, which presumably contains antibodies to the V3 loop and the CD4 binding site, exhibited a strong synergistic effect with 5A8 in inhibiting infection by a patient HIV-1 isolate (0104B) and in blocking syncytium formation. These results indicate that therapeutics based on antibodies affecting both non-gp120 binding and gp120 binding epitopes of the target receptor molecule, CD4, could be efficient in patients who already contain anti-gp120 antibodies and could also be used to enhance passive immunization against HIV-1 in combination with anti-gp120 antibodies.     

Lineage-Specific Differences between Human and Simian Immunodeficiency Virus Regulation of gp120 Trimer Association and CD4 Binding

Metastable conformations of the gp120 and gp41 envelope glycoproteins of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) must be maintained in the unliganded state of the envelope glycoprotein trimer. Binding of gp120 to the primary receptor, CD4, triggers the transition to an open conformation of the trimer, promoting interaction with the CCR5 chemokine receptor and ultimately leading to gp41-mediated virus-cell membrane fusion and entry. Topological layers in the gp120 inner domain contribute to gp120-trimer association in the unliganded state and to CD4 binding. Here we describe similarities and differences between HIV-1 and SIVmac gp120. In both viruses, the gp120 N/C termini and the inner domain β-sandwich and layer 2 support the noncovalent association of gp120 with the envelope glycoprotein trimer. Layer 1 of the SIVmac gp120 inner domain contributes more to trimer association than the corresponding region of HIV-1 gp120. On the other hand, layer 1 plays an important role in stabilizing the CD4-bound conformation of HIV-1 but not SIVmac gp120 and thus contributes to HIV-1 binding to CD4. In SIVmac, CD4 binding is instead enhanced by tryptophan 375, which fills the Phe 43 cavity of gp120. Activation of SIVmac by soluble CD4 is dependent on tryptophan 375 and on layer 1 residues that determine a tight association of gp120 with the trimer. Distinct biological requirements for CD4 usage have resulted in lineage-specific differences in the HIV-1 and SIV gp120 structures that modulate trimer association and CD4 binding.     

Mode of action for linear peptide inhibitors of HIV-1 gp120 interactions

The linear peptide 12p1 (RINNIPWSEAMM) was previously isolated from a phage display library and was found to inhibit interaction of HIV-1 gp120 with both CD4 and a CCR5 surrogate, mAb 17b [Ferrer, M., and Harrison, S. (1999) J. Virol. 73, 5795-5802]. In this work, we investigated the mechanism that leads to this dual inhibition of gp120 binding. We found that there is a direct interaction of 12p1 with gp120, which occurs with a binding stoichiometry of 1:1. The peptide inhibits binding of monomeric YU2 gp120 to both sCD4 and 17b at IC(50) values of 1.1 and 1.6 microM, respectively. The 12p1 peptide also inhibited the binding of these ligands to trimeric envelope glycoproteins, blocked the binding of gp120 to the native coreceptor CCR5, and specifically inhibited HIV-1 infection of target cells in vitro. Analyses of sCD4 saturation of monomeric gp120 in the presence or absence of a fixed concentration of peptide suggest that 12p1 suppression of CD4 binding to gp120 is due to allosteric inhibitory effects rather than competitive inhibition of CD4 binding. Using a panel of gp120 mutants that exhibit weakened inhibition by 12p1, the putative binding site of the peptide was mapped to a region immediately adjacent to, but distinguishable from, the CD4 binding footprint. In the case of the peptide, the effects of single-12p1 residue substitutions and various peptide truncations indicate that the side chain of Trp7 and other structural elements of 12p1 are critical for gp120 binding or efficient inhibition of binding of a ligand to gp120. Finally, 12p1 was unable to inhibit binding of sCD4 to a gp120 mutant that is believed to resemble the CD4-induced conformation of gp120. These results suggest that 12p1 preferentially binds gp120 prior to engagement of CD4; binding of the peptide to gp120 limits the interaction with ligands (CD4 and CCR5) that are generally crucial for viral entry. More importantly, these results indicate that 12p1 binds to a unique site that may prove to be a prototypic target for novel CD4-gp120 inhibitors.     

HIV-1 GP120-mediated immune suppression and lymphocyte destruction in the absence of viral infection

The magnitude of immunologic defects observed in HIV-1-infected individuals before the development of overt AIDS is disproportionately high in comparison to the levels of infectious virus in these patients--suggesting that factors other than direct virus-induced cytopathology may be involved. With this in mind, we investigated the immunologic consequences of the interaction between purified HIV-1 gp120 and the CD4 molecules expressed by uncommitted as well as Ag-specific lymphocytes. HIV-1 gp120 exhibited a dose-dependent immunosuppressive effect on: 1) Ag-driven proliferation of cloned CD4+ lymphocytes, 2) OKT3-driven proliferation of cloned CD4+ lymphocytes, and 3) cytolytic activity of CD4+, EBV-specific CTL. Thus, HIV-1 gp120 can, in a manner similar to OKT4A antibodies, suppress T cell activation and the expression of cytolytic activities through its interaction with CD4. Additionally, activated CD4+ lymphoblasts can be rendered susceptible to immune cytolysis by virtue of their binding of purified gp120. This "targeting" of activated lymphoblasts can occur with levels of gp120 far below that which is needed to saturate all OKT4A-defined CD4 epitopes. Adsorbed gp120 could be demonstrated on the surface of these cells for up to 12 h, a sufficient time for interaction with host cytolytic elements. The data from these in vitro modeling experiments highlight one of many potential mechanisms of HIV-1 induced immunosuppression and lymphocyte destruction that can occur in the absence of infectious virus and that is based on the unique interaction between HIV-1 gp120 and its cellular receptor, CD4.     

A functional, discontinuous HIV-1 gp120 C3/C4 domain-derived, branched, synthetic peptide that binds to CD4 and inhibits MIP-1alpha chemokine binding.

This paper describes a branched synthetic peptide [3.7] that incorporates sequence discontinuous residues of HIV-1 gp120 constant regions. The approach was to bring together residues of gp120 known to interact with human cell membranes such that the peptide could fold to mimic the native molecule. The peptide incorporates elements of both the conserved CD4 and CCR5 binding sites. The 3.7 peptide, which cannot be produced by conventional genetic engineering methods, is recognized by antiserum raised to native gp120. The peptide also binds to CD4 and competitively inhibits binding of QS4120 an antibody directed against the CDR2 region of CD4. When preincubated with the CD4+ve MM6 macrophage cell line, which expresses mRNA for the CCR3 and CCR5 chemokine receptors, both 3.7 and gp120 inhibit binding of the chemokine MIP-1alpha. The peptide also inhibits infection of primary macrophages by M-tropic HIV-1. Thus, 3.7 is a prototype candidate peptide for a vaccine against HIV-1 and represents a novel approach to the rational design of peptides that can mimic complex sequence discontinuous ligand binding sites of clinically relevant proteins.     

Conformational and structural features of HIV-1 gp120 underlying the dual receptor antagonism by cross-reactive neutralizing antibody m18

We investigated the interaction between cross-reactive HIV-1 neutralizing human monoclonal antibody m18 and HIV-1YU-2 gp120 in an effort to understand how this antibody inhibits the entry of virus into cells. m18 binds to gp120 with high affinity (KD≈5 nM) as measured by surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC). SPR analysis further showed that m18 inhibits interactions of gp120 with both soluble CD4 and CD4-induced antibodies that have epitopes overlapping the coreceptor binding site. This dual receptor site antagonism, which occurs with equal potency for both inhibition effects, argues that m18 is not functioning as a mimic of CD4, in spite of the presence of a putative CD4-like loop formed by HCDR3 in the antibody. Consistent with this view, m18 was found to interact with gp120 in the presence of saturating concentrations of a CD4-mimicking small molecule gp120 inhibitor, suggesting that m18 does not require unoccupied CD4 Phe43 binding cavity residues of gp120. Thermodynamic analysis of the m18-gp120 interaction suggests that m18 stabilizes a conformation of gp120 that is unique from and less structured than the CD4-stabilized conformation. Conformational mutants of gp120 were studied for their impact on m18 interaction. Mutations known to disrupt the coreceptor binding region and to lead to complete suppression of 17b binding had minimal effects on m18 binding. This argues that energetically important epitopes for m18 binding lie outside the disrupted bridging sheet region used for 17b and coreceptor binding. In contrast, mutations in the CD4 region strongly affected m18 binding. Overall, the results obtained in this work argue that m18, rather than mimicking CD4 directly, suppresses both receptor binding site functions of HIV-1 gp120 by stabilizing a nonproductive conformation of the envelope protein. These results can be related to prior findings about the importance of conformational entrapment as a common mode of action for neutralizing CD4bs antibodies, with differences mainly in epitope utilization and the extent of gp120 structuring.     

Strain specificity and binding affinity requirements of neutralizing monoclonal antibodies to the C4 domain of gp120 from human immunodeficiency virus type 1.

The binding properties of seven CD4-blocking monoclonal antibodies raised against recombinant gp120 of human immunodeficiency virus type 1 strain MN (HIV-1MN) and two CD4-blocking monoclonal antibodies to recombinant envelope glycoproteins gp120 and gp160 of substrain IIIB of HIVLAI were analyzed. With a panel of recombinant gp120s from seven diverse HIV-1 isolates, eight of the nine antibodies were found to be strain specific and one was broadly cross-reactive. Epitope mapping revealed that all nine antibodies bound to epitopes located in the fourth conserved domain (C4) of gp120. Within this region, three distinct epitopes could be identified: two were polymorphic between HIV-1 strains, and one was highly conserved. Studies with synthetic peptides demonstrated that the conserved epitope, recognized by antibody 13H8, was located between residues 431 and 439. Site-directed mutagenesis of gp120 demonstrated that residue 429 and/or 432 was critical for the binding of the seven antibodies to gp120 from HIV-1MN. Similarly, residues 423 and 429 were essential for the binding of monoclonal antibody 5C2 raised against gp120 from HIV-1IIIB. The amino acids located at positions 423 and 429 were found to vary between strains of HIV-1 as well as between molecular clones derived from the MN and LAI isolates of HIV-1. Polymorphism at these positions prevented the binding of virus-neutralizing monoclonal antibodies and raised the possibility that HIV-1 neutralization serotypes may be defined on the basis of C4 domain sequences. Analysis of the binding characteristics of the CD4-blocking antibodies demonstrated that their virus-neutralizing activity was directly proportional to their gp120-binding affinity. These studies account for the strain specificity of antibodies to the C4 domain of gp120 and demonstrate for the first time that antibodies to this region can be as effective as those directed to the principal neutralizing determinant (V3 domain) in neutralizing HIV-1 infectivity.     

The V3 region of the envelope glycoprotein of human immunodeficiency virus type 1 binds sulfated polysaccharides and CD4-derived synthetic peptides.

gp120 is the envelope glycoprotein found on the surface of human immunodeficiency virus type 1 (HIV-1), and it binds to human cell surface CD4 receptors to initiate the HIV-1 infection process. It is now well-established that synthetic peptides from the V3 region on gp120 elicit antibodies that block HIV-1 infection and HIV-1-mediated cell fusion. Here we show that synthetic peptides derived from similar V3 regions of several isolates of HIV-1 bind [3H]heparin, and we also demonstrate that [3H]heparin binds to recombinant gp120 IIIB. The binding could be blocked by unlabeled heparin, dextran sulfate, and by a highly anionic benzylated synthetic peptide derived from human CD4 (amino acids 81-92). The nonbenzylated peptides from the same region were considerably less active. Unlabeled heparin, dextran sulfate, and the CD4-derived peptides were able to compete with the binding of soluble gp120 to immobilized antibodies against fragments of the V3 from isolate IIIB, but they had no effect on the binding of gp120 to anti-peptide antibodies targeted against another unrelated region of gp120. Biotin conjugated to the benzylated CD4-peptide bound to gp120 and was blocked from this binding by anti-V3 antibodies. These results indicate that the three materials that have been demonstrated by others to block HIV-1 infection in vitro, sulfated polysaccharides, certain CD4-derived synthetic peptides, and anti-V3 antibodies, may be acting through a common mechanism that includes binding to the V3 region of gp120 on HIV-1.     

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

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

Evidence by peptide mapping that the region CD4(81-92) is involved in gp120/CD4 interaction leading to HIV infection and HIV-induced syncytium formation.

Peptide fragments of the CD4 molecule were compared in their ability to 1) inhibit CD4-dependent HIV-induced cell fusion; 2) inhibit CD4-dependent HIV infection in vitro; and 3) block gp120 envelope glycoprotein binding to CD4. Peptides from the region CD4(81-92), although inactive when underivatized, were equipotent inhibitors of CD4-dependent virus infection, cell fusion, and CD4/gp120 binding when derivatized via benzylation and acetylation. Peptides of identical chemical composition, but altered sequence and derivatization pattern that blocked gp120 binding to either CD4-positive cells or solubilized CD4, also blocked infection and fusion with similar potencies. Those that did not block gp120/CD4 interaction were also inactive in HIV-1 infection and cell fusion assays. No other peptide fragments of the CD4 molecule inhibited fusion, infection, or CD4/gp120 interaction. The peptide CD4(23-56), derived from a region of CD4 implicated in binding of CD4 antibodies that neutralize HIV infection and cell fusion, had no effect on CD4-dependent cell fusion, HIV-1 infection, or CD4/gp120 binding, but did reverse OKT4A and anti-Leu 3a blockade of gp120 binding to CD4. These data provide evidence that the 81-92 region of CD4 is directly involved in gp120 binding leading to CD4-dependent HIV infection and syncytium formation. Previous observations with structural mutants of CD4 suggest that the CDR2-homologous region of CD4 is also involved, either directly or indirectly, in binding of gp120 to CD4. The CDR2- and CDR3-like domains of CD4 may both contribute to the binding of the HIV envelope necessary for HIV-1 infection and HIV-1-induced cell fusion.     

Identification and Characterization of a Broadly Cross-Reactive HIV-1 Human Monoclonal Antibody That Binds to Both gp120 and gp41

Identification of broadly cross-reactive HIV-1-neutralizing antibodies (bnAbs) may assist vaccine immunogen design. Here we report a novel human monoclonal antibody (mAb), designated m43, which co-targets the gp120 and gp41 subunits of the HIV-1 envelope glycoprotein (Env). M43 bound to recombinant gp140 s from various primary isolates, to membrane-associated Envs on transfected cells and HIV-1 infected cells, as well as to recombinant gp120 s and gp41 fusion intermediate structures containing N-trimer structure, but did not bind to denatured recombinant gp140 s and the CD4 binding site (CD4bs) mutant, gp120 D368R, suggesting that the m43 epitope is conformational and overlaps the CD4bs on gp120 and the N-trimer structure on gp41. M43 neutralized 34% of the HIV-1 primary isolates from different clades and all the SHIVs tested in assays based on infection of peripheral blood mononuclear cells (PBMCs) by replication-competent virus, but was less potent in cell line-based pseudovirus assays. In contrast to CD4, m43 did not induce Env conformational changes upon binding leading to exposure of the coreceptor binding site, enhanced binding of mAbs 2F5 and 4E10 specific for the membrane proximal external region (MPER) of gp41 Envs, or increased gp120 shedding. The overall modest neutralization activity of m43 is likely due to the limited binding of m43 to functional Envs which could be increased by antibody engineering if needed. M43 may represent a new class of bnAbs targeting conformational epitopes overlapping structures on both gp120 and gp41. Its novel epitope and possibly new mechanism(s) of neutralization could helpdesign improved vaccine immunogens and candidate therapeutics.     

Introducing metallocene into a triazole peptide conjugate reduces its off-rate and enhances its affinity and antiviral potency for HIV-1 gp120

In this work, we identified a high affinity and potency metallocene-containing triazole peptide conjugate that suppresses the interactions of HIV-1 envelope gp120 at both its CD4 and co-receptor binding sites. The ferrocene-peptide conjugate, HNG-156, was formed by an on-resin copper-catalysed [2+3] cycloaddition reaction. Surface plasmon resonance interaction analysis revealed that, compared to a previously reported phenyl-containing triazole conjugate HNG-105 (105), peptide 156 had a higher direct binding affinity for several subtypes of HIV-1 gp120 due mainly to the decreased dissociation rate of the conjugate-gp120 complex. The ferrocene triazole conjugate bound to gp120 of both clade A (92UG037-08) and clade B (YU-2 and SF162) virus subtypes with nanomolar KD in direct binding and inhibited the binding of gp120 to soluble CD4 and to antibodies that bind to HIV-1YU-2 gp120 at both the CD4 binding site and CD4-induced binding sites. HNG-156 showed a close-to nanomolar IC50 for inhibiting cell infection by HIV-1BaL whole virus. The dual receptor site antagonist activity and potency of HNG-156 make it a promising viral envelope inhibitor lead for developing anti-HIV-1 treatments.     

Fusion proteins of HIV-1 envelope glycoprotein gp120 with CD4-induced antibodies showed enhanced binding to CD4 and CD4 binding site antibodies

Development of successful AIDS vaccine immunogens continues to be a major challenge. One of the mechanisms by which HIV-1 evades antibody-mediated neutralizing responses is the remarkable conformational flexibility of its envelope glycoprotein (Env) gp120. Some recombinant gp120s do not preserve their conformations on gp140s and functional viral spikes, and exhibit decreased recognition by CD4 and neutralizing antibodies. CD4 binding induces conformational changes in gp120 leading to exposure of the coreceptor-binding site (CoRbs). In this study, we test our hypothesis that CD4-induced (CD4i) antibodies, which target the CoRbs, could also induce conformational changes in gp120 leading to better exposed conserved neutralizing antibody epitopes including the CD4-binding site (CD4bs). We found that a mixture of CD4i antibodies with gp120 only weakly enhanced CD4 binding. However, such interactions in single-chain fusion proteins resulted in gp120 conformations which bound to CD4 and CD4bs antibodies better than the original or mutagenically stabilized gp120s. Moreover, the two molecules in the fusion proteins synergized with each other in neutralizing HIV-1. Therefore, fusion proteins of gp120 with CD4i antibodies could have potential as components of HIV-1 vaccines and inhibitors of HIV-1 entry, and could be used as reagents to explore the conformational flexibility of gp120 and mechanisms of entry and immune evasion.     

HIV-1 attachment: another look

HIV-1 attachment to host cells is generally considered to take place via high-affinity binding between CD4 and gp120. However, the binding of virion-associated gp120 to cellular CD4 is often weak, and most cell types that are permissive for HIV-1 infection express little CD4. Thus, other interactions between the virion and the cell surface could dominate the attachment process.