Immune focusing and enhanced neutralization induced by HIV-1 gp140 chemical cross-linking.

Experimental vaccine antigens based upon the HIV-1 envelope glycoproteins (Env) have failed to induce neutralizing antibodies (NAbs) against the majority of circulating viral strains as a result of antibody evasion mechanisms, including amino acid variability and conformational instability. A potential vaccine design strategy is to stabilize Env, thereby focusing antibody responses on constitutively exposed, conserved surfaces, such as the CD4 binding site (CD4bs). Here, we show that a largely trimeric form of soluble Env can be stably cross-linked with glutaraldehyde (GLA) without global modification of antigenicity. Cross-linking largely conserved binding of all potent broadly neutralizing antibodies (bNAbs) tested, including CD4bs-specific VRC01 and HJ16, but reduced binding of several non- or weakly neutralizing antibodies and soluble CD4 (sCD4). Adjuvanted administration of cross-linked or unmodified gp140 to rabbits generated indistinguishable total gp140-specific serum IgG binding titers. However, sera from animals receiving cross-linked gp140 showed significantly increased CD4bs-specific antibody binding compared to animals receiving unmodified gp140. Moreover, peptide mapping of sera from animals receiving cross-linked gp140 revealed increased binding to gp120 C1 and V1V2 regions. Finally, neutralization titers were significantly elevated in sera from animals receiving cross-linked gp140 rather than unmodified gp140. We conclude that cross-linking favors antigen stability, imparts antigenic modifications that selectively refocus antibody specificity and improves induction of NAbs, and might be a useful strategy for future vaccine design.

Human immunodeficiency virus type 1 neutralization is determined by epitope exposure on the gp120 oligomer.

The major target of the neutralizing antibody response to infection by the human immunodeficiency virus type 1 (HIV-1) is the outer envelope glycoprotein, gp120. The spectrum of HIV-1 neutralization specificity is currently represented by monoclonal antibodies (mAbs) that can be divided broadly into five groups. We have studied the binding of these mAbs to functional oligomeric and soluble monomeric gp120 derived from the molecular clone of a cell line-adapted isolate of HIV-1, and compared these binding properties with virus neutralization. Binding of all mAbs except those reactive with the V3 loop was much weaker to oligomeric than to monomeric gp120. This reduction in binding to oligomeric gp120 was determined mostly by a slower relative rate of association, although the dissociation rate also had some influence on relative variation in mAb affinity. Virus neutralization correlated broadly with mAb binding to the oligomeric rather than to the monomeric form of gp120, and neutralization potency was related to the estimated association rate. Thus, with the exception of the hypervariable V3 loop, regions of HIV-1 gp120 with the potential to induce a neutralization response are likely to be poorly presented for antibody recognition on the surface of cell line-adapted virions.

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

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

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

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

Structural analysis of the human immunodeficiency virus-binding domain of CD4. Epitope mapping with site-directed mutants and anti-idiotypes.

The CD4 molecule, a differentiation marker expressed primarily by T lymphocytes, plays an important role in lymphocyte activation. CD4 is also the receptor for HIV. A number of recent studies have localized the high affinity binding site of the HIV envelope glycoprotein, gp120, to the NH2-terminal (V1) domain of CD4, a region with sequence and predicted structural homology with Ig kappa chain V domains (V kappa). In this report, we show that V1 bears structural similarities with V kappa regions through detailed epitope mapping of 26 CD4 mAbs. The binding sites of these mAbs were initially defined relative to one another by crossblocking analysis and were then localized to specific domains of CD4 in blocking studies with truncated, soluble CD4 proteins. The epitopes within the V1 domain were mapped in detail with a panel of 17 substitution mutants, and the specificities of several mAbs that appear to recognize very similar epitopes were examined in crossblocking studies with anti-idiotype antibodies. The location of the epitopes is consistent with a V kappa-like structure of V1. Most of the epitopes lie within regions of predicted exposed loops. A number of these epitopes span discontinuous residues in the linear sequence that lies in close proximity in an Ig fold. Alignment of CD4 V1 with the Ig V kappa chains places these epitopes within stretches corresponding to the complimentarity-determining regions. This epitope analysis is relevant for a vaccine strategy for HIV based on anti-idiotype antibodies to CD4 mAbs and for studies with CD4 antibodies on the role of CD4 in T lymphocyte activation.

Inhibition of virus attachment to CD4+ target cells is a major mechanism of T cell line-adapted HIV-1 neutralization.

Antibody-mediated neutralization of human immunodeficiency virus type-1 (HIV-1) is thought to function by at least two distinct mechanisms: inhibition of virus-receptor binding, and interference with events after binding, such as virus-cell membrane fusion. Here we show, by the use of a novel virus-cell binding assay, that soluble CD4 and monoclonal antibodies to all confirmed glycoprotein (gp)120 neutralizing epitopes, including the CD4 binding site and the V2 and V3 loops, inhibit the adsorption of two T cell line-adapted HIV-1 viruses to CD4+ cells. A correlation between the inhibition of virus binding and virus neutralization was observed for soluble CD4 and all anti-gp120 antibodies, indicating that this is a major mechanism of HIV neutralization. By contrast, antibodies specific for regions of gp120 other than the CD4 binding site showed little or no inhibition of either soluble gp120 binding to CD4+ cells or soluble CD4 binding to HIV-infected cells, implying that this effect is specific to the virion-cell interaction. However, inhibition of HIV-1 attachment to cells is not a universal mechanism of neutralization, since an anti-gp41 antibody did not inhibit virus-cell binding at neutralizing concentrations, implying activity after virus-cell binding.

Novel anti-CD4 monoclonal antibodies separate human immunodeficiency virus infection and fusion of CD4+ cells from virus binding.

Human immunodeficiency virus (HIV) binds to cells via an interaction between CD4 and the virus envelope glycoprotein, gp120. Previous studies have localized the high affinity binding site for gp120 to the first domain of CD4, and monoclonal antibodies (mAbs) reactive with this region compete with gp120 binding and thereby block virus infectivity and syncytium formation. Despite a detailed understanding of the binding of gp120 to CD4, little is known of subsequent events leading to membrane fusion and virus entry. We describe two new mAbs reactive with the third domain of CD4 that inhibit steps subsequent to virus binding critical for HIV infectivity and cell fusion. Binding of recombinant gp120 or virus to CD4 is not inhibited by these antibodies, whereas infection and syncytium formation by a number of HIV isolates are blocked. These findings demonstrate that in addition to virus binding, CD4 may have an active role in membrane fusion.

Dissociation of gp120 from HIV-1 virions induced by soluble CD4.

The CD4 antigen is the high affinity cellular receptor for the human immunodeficiency virus type-1 (HIV-1). Binding of recombinant soluble CD4 (sCD4) or the purified V1 domain of sCD4 to the surface glycoprotein gp120 on virions resulted in rapid dissociation of gp120 from its complex with the transmembrane glycoprotein gp41. This may represent the initial stage in virus-cell and cell-cell fusion. Shedding of gp120 from virions induced by sCD4 may also contribute to the mechanism by which these soluble receptor molecules neutralize HIV-1.

Epitopes of the CD4 antigen and HIV infection.

The CD4 (or T4) surface antigen of human T lymphocytes is an important part of the receptor for the human immunodeficiency virus (HIV). After binding to the receptor, the HIV may enter the T cell and induce the formation of syncytia. In an attempt to identify the receptor site more closely, monoclonal antibodies (Mab's) to CD4 were tested for their ability to block HIV infection in a syncytium formation assay, and the CD4 epitopes so identified were mapped by antibody cross-blocking. The antibodies that showed strong inhibition of HIV fell into two main families while a third group of Mab's blocked syncytia formation weakly or not at all. Several different isolates of HIV as well as the laboratory strain CBL1 grown in CEM cells were used to induce the syncytia. The data indicate that only some epitopes of CD4 are important for virus binding and imply that the virus-binding site for CD4 is conserved in different isolates of HIV with substantially divergent env gene sequences. Preliminary studies of patients suggest that polymorphism of these epitopes does not play a role in determining susceptibility to infection.

Neutralization of HIV-1 by antibody.

The humoral immune response to HIV-1 has been extensively studied over the past few years and considerable advances have been made in understanding the dynamics and specificity of the neutralizing antibody component during and after seroconversion. Despite this, there is still no clear understanding of the role of neutralizing antibodies in controlling or preventing HIV-1 infection. Candidate vaccines have been based on immunogens designed to elicit a neutralizing response, but the recent vaccine trial failures force us to reconsider the role of neutralizing antibodies in HIV-1 infection and the type of vaccine preparation used.

HIV gp120: double lock strategy foils host defences.

The recent determination of the structure of a complex formed between the HIV-1 glycoprotein gp120, CD4 and an antibody fragment has revealed new mechanisms for viral evasion of the immune response and shed light on how the virus enters target cells. The results of this work, together with related biochemical studies, may assist in the future design of therapeutic strategies against HIV-1 infection.

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.

Human and simian immunodeficiency viruses: virus-receptor interactions.

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

Failure to demonstrate anti-lymphocytic antibody in serum of patients with AIDS.

Several studies have produced evidence for anti-lymphocytic antibodies (ALA) in AIDS. We attempted to demonstrate ALA by immunofluorescent flow cytometry. Normal human peripheral blood lymphocytes (PBL) and the T-cell line, CEM, were incubated with sera from patients with AIDS, patients with chronic HIV infection and HIV-seronegative blood donors. ALA were not detected in the AIDS sera with fluorescein isothiocyanate (FITC)-labelled rabbit anti-mu, anti-alpha or the F(ab)2 fragment of anti-human gamma. A small number of CEM cells (2%) fluoresced with either AIDS or normal serum. A larger proportion of PBL were immunofluorescent after serum treatment but there was no difference between normal and AIDS serum. We were able to detect ALA in the serum of patients with systemic lupus erythematosus with both CEM and PBL. In contrast, incubation of either CEM or PBL with some AIDS sera, and to a lesser degree normal sera, enhanced the binding of intact FITC-rabbit anti-gamma. Anti-gamma was not bound by CEM cells unexposed to human serum. The binding was blocked by rabbit immunoglobulin, demonstrable with CEM fixed in 1% formalin, and unrelated to the density of CD4 on CEM cells.

The human and simian immunodeficiency viruses HIV-1, HIV-2 and SIV interact with similar epitopes on their cellular receptor, the CD4 molecule.

The cellular receptor for HIV-1 is the leucocyte differentiation antigen, CD4. Blocking of HIV-1 infectivity can be achieved with monoclonal antibodies (MAbs) to some, but not all epitopes of this antigen. We demonstrate here, by inhibition of virus infection, blocking of syncytium formation and inhibition of pseudotype infection with a panel of CD4 MAbs, that HIV-1, HIV-2 and simian immunodeficiency virus (SIV) isolates share the same cellular receptor, the CD4 glycoprotein. It is also shown that very similar epitopes of this molecule are involved in virus binding. We infer from these data that the binding sites on these viruses are highly conserved regions, and may therefore make good targets for potential vaccines. In addition, we show that cell surface expression of CD4 is similarly modulated after infection of cell lines by all the viruses.

Human immunodeficiency viruses: neutralization and receptors.

The envelope glycoproteins of HIV, gp120 and gp41, contain epitopes recognized by neutralizing antibodies. Studies of human sera from infected individuals indicate that group-specific neutralization antigens common to most isolates of HIV-1 exist, and that some HIV-2 antisera cross-neutralize HIV-1. Neutralization epitopes for HIV-1 have been identified and mapped, including a group-specific antigen on gp41, and a type-specific antigen on gp120. Neutralization "escape" mutants have been selected in vitro with a neutralizing mab to the type-specific antigenic loop. The CD4 antigen binds HIV-1 gp120 with high affinity and acts as the receptor on human and simian T-lymphocytes and monocytes for all strains of HIV-1, HIV-2, and SIV tested. Following binding to the CD4 receptor, HIV becomes internalized by a pH-independent process. The principle binding domain for gp120 is located in the N-terminal V domain of CD4. Anti-idiotypic sera to CD4 mabs recognizing the same site weakly neutralize HIVs of many strains, and soluble, recombinant forms of CD4 strongly neutralize HIV. Neither anti-CD4 mabs nor sCD4 inhibit the low level of plating of HIV observed on tumour cells in culture of glial (brain) and muscle origin, indicating that CD4 is not essential for infection of these cell types.

Erratum to "Relevance of the antibody response against human immunodeficiency virus type 1 envelope to vaccine design".

Understanding the antibody response in HIV-1 infection is important to vaccine design. We have studied the antibody response to HIV-1 envelope at the molecular level and determined the characteristics of neutralizing and non-neutralizing antibodies. These antibodies were isolated from phage display libraries prepared from long-term seropositive asymptomatic individuals. The HIV-1 envelope is presented to the immune system in several antigenically distinct configurations: unprocessed gp160, gp120 and gp41 subunits and native envelope, each of which may be important in eliciting an antibody response in HIV-1 infection. The antibodies tested characteristically had poor affinities for native envelope as expressed on the surface of virions or infected cells, but had high affinities against non-native forms of HIV-1 envelope (viral debris). An exceptionally potent neutralizing antibody in contrast, bound native envelope with equivalent or somewhat higher affinity than this. This indicates that the antibody response in HIV-1 infection is principally elicited by viral debris rather than virions, and that these antibodies bind and neutralize viruses sub-optimally. Potential vaccines should be designed to elicit responses against native envelope.

Relevance of the antibody response against human immunodeficiency virus type 1 envelope to vaccine design.

Understanding the antibody response in HIV-1 infection is important to vaccine design. We have studied the antibody response to HIV-1 envelope at the molecular level and determined the characteristics of neutralizing and non-neutralizing antibodies. These antibodies were isolated from phage display libraries prepared from long-term seropositive asymptomatic individuals. The HIV-1 envelope is presented to the immune system in several antigenically distinct configurations: unprocessed gp160, gp120 and gp41 subunits and native envelope, each of which may be important in eliciting an antibody response in HIV-1 infection. The antibodies tested characteristically had poor affinities for native envelope as expressed on the surface of virions or infected cells, but had high affinities against non-native forms of HIV-1 envelope (viral debris). An exceptionally potent neutralizing antibody in contrast, bound native envelope with equivalent or somewhat higher affinity than this. This indicates that the antibody response in HIV-1 infection is principally elicited by viral debris rather than virions, and that these antibodies bind and neutralize viruses sub-optimally. Potential vaccines should be designed to elicit responses against native envelope.

Antibody neutralization of HIV-1 and the potential for vaccine design.

Neutralisation by antibody is, for a number of viruses, an in vitro correlate for protection in vivo. For HIV-1 this is controversial. However, the induction of a potent anti-HIV neutralising antibody response remains one of the principal goals in vaccine development. A greater knowledge of the fundamental mechanisms underlying the neutralisation process would help direct research towards suitable vaccine immunogens. The primary determinant of HIV neutralisation appears to be antibody affinity for the trimeric envelope glycoprotein spike on the virion, suggesting that epitope-specific effects are secondary and implying a single, dominant mechanism of neutralisation. Antibody interference with virion attachment to the target cell appears to be a major mechanism of neutralisation by gp120-specific antibodies. This is probably achieved both by antibody-induced dissociation of gp120 from gp41 and by direct inhibition of virus binding to receptor-coreceptor complexes. A gp41-specific antibody neutralises by interfering with post-attachment steps leading to virus membrane fusion. Recent advances in structural analyses of the HIV envelope glycoproteins coupled with data obtained from antibody mapping and neutralisation studies allow a greater understanding of Env function and its inhibition. This in turn should lead to a more rational basis for vaccine design aimed at stimulating highly effective neutralising antibodies.