Broadly neutralizing antibodies targeted to the membrane-proximal external region of human immunodeficiency virus type 1 glycoprotein gp41

Binding of the 2F5 monoclonal antibody to native and fusion-intermediate forms of human immunodeficiency virus type 1 gp41: implications for fusion-inducing conformational changes

We investigated how the broadly neutralizing monoclonal antibody 2F5 affects the human immunodeficiency virus type 1 envelope glycoprotein as it undergoes receptor-induced conformational changes and show that 2F5 binds both native and fusion-intermediate conformations, suggesting inhibition of a late step in virus entry. We also demonstrate conformational changes in the C heptad of gp41.

Immunoprophylaxis to Prevent Mother-to-Child Transmission of HIV-1

Antiretroviral therapy can profoundly reduce the risk of mother-to-child transmission (MTCT) of HIV, but the drugs have a relatively short half-life and should thus be administered throughout breast-feeding to optimally prevent postnatal infection of the infant. The potential toxicities and the development of resistance may limit the long-term efficacy of antiretroviral prophylaxis, and a safe and effective active/passive immunoprophylaxis regimen, begun at birth, and potentially overlapping with interpartum or neonatal chemoprophylaxis, would pose an attractive alternative. This review draws on data presented at the Ghent Workshop on prevention of breast milk transmission and on selected issues from a workshop specifically relating to immunoprophylaxis held in Seattle in October 2002. This purpose of this review is to address the scientific rationale for the development of passive (antibody) and active (vaccine) immunization strategies for prevention of MTCT. Data regarding currently or imminently available passive and active immunoprophylaxis products are reviewed for their potential use in neonatal trials within the coming 1-2 years.

Broadly cross-reactive HIV neutralizing human monoclonal antibody Fab selected by sequential antigen panning of a phage display library

Identification of broadly cross-reactive human monoclonal antibodies (mAbs) has major implications for development of vaccines, inhibitors and research tools. Here we describe a sequential antigen panning (SAP) methodology that may facilitate the selection of such antibodies. An HIV-specific antibody Fab (m18) was selected from a human Fab phage-display library by SAP against several recombinant soluble HIV envelope glycoproteins (Envs) and Env-sCD4 complexes. This Fab bound to a variety of recombinant soluble Envs (gp140s) from primary HIV isolates representing different clades, and inhibited cell fusion and virus entry mediated by Envs of primary HIV isolates. The methodology and the results may have implications for development of HIV vaccines and inhibitors, as well as for identification of antibodies to conserved epitopes on rapidly mutating viruses and cells.

A single injection of recombinant measles virus vaccines expressing human immunodeficiency virus (HIV) type 1 clade B envelope glycoproteins induces neutralizing antibodies and cellular immune responses to HIV

The anchored and secreted forms of the human immunodeficiency virus type 1 (HIV-1) 89.6 envelope glycoprotein, either complete or after deletion of the V3 loop, were expressed in a cloned attenuated measles virus (MV) vector. The recombinant viruses grew as efficiently as the parental virus and expressed high levels of the HIV protein. Expression was stable during serial passages. The immunogenicity of these recombinant vectors was tested in mice susceptible to MV and in macaques. High titers of antibodies to both MV and HIV-Env were obtained after a single injection in susceptible mice. These antibodies neutralized homologous SHIV89.6p virus, as well as several heterologous HIV-1 primary isolates. A gp160 mutant in which the V3 loop was deleted induced antibodies that neutralized heterologous viruses more efficiently than antibodies induced by the native envelope protein. A high level of CD8+ and CD4+ cells specific for HIV gp120 was also detected in MV-susceptible mice. Furthermore, recombinant MV was able to raise immune responses against HIV in mice and macaques with a preexisting anti-MV immunity. Therefore, recombinant MV vaccines inducing anti-HIV neutralizing antibodies and specific T lymphocytes responses deserve to be tested as a candidate AIDS vaccine.

Binding of High-Mannose-Type Oligosaccharides and Synthetic Oligomannose Clusters to Human Antibody 2G12

Human antibody 2G12 broadly neutralizes human immunodeficiency virus type 1 (HIV-1) isolates and shows protective activity against viral challenge in animal models. Previous mutational analysis suggested that 2G12 recognized a novel cluster of high-mannose type oligosaccharides on HIV-1 gp120. To explore the carbohydrate antigen for HIV-1 vaccine design, we have studied the binding of 2G12 to an array of HIV-1 high-mannose type oligosaccharides by competitive ELISAs and found that Man9GlcNAc is 210- and 74-fold more effective than Man5GlcNAc and Man6GlcNAc in binding to 2G12. The results establish that the larger high-mannose oligosaccharide on HIV-1 is the favorable subunit for 2G12 recognition. To mimic the putative epitope of 2G12, we have created scaffold-based multivalent Man9 clusters and found that the galactose-scaffolded bi-, tri-, and tetra-valent Man9 clusters are 7-, 22-, and 73-fold more effective in binding to 2G12 than the monomeric Man9GlcNAc2Asn. The experimental data shed light on further structural optimization of epitope mimics for developing a carbohydrate-based HIV-1 vaccine.

The mapping and reconstitution of a conformational discontinuous B-cell epitope of HIV-1

A method for the discovery of the structure of conformational discontinuous epitopes of monoclonal antibodies (mAbs) is described. The mAb is used to select specific phages from combinatorial phage-display peptide libraries that in turn are used as an epitope-defining database that is applied via a novel computer algorithm to analyze the crystalline structure of the original antigen. The algorithm is based on the following: (1) Most contacts between a mAb and an antigen are through side-chain atoms of the residues. (2) In the three-dimensional structure of a protein, amino acid residues remote in linear sequence can juxtapose to one another through folding. (3) Tandem amino acid residues of the selected phage-displayed peptides can represent pairs of juxtaposed amino acid residues of the antigen. (4) Contact residues of the epitope are accessible to the antigen surface. (5) The most frequent tandem pairs of amino acid residues in the selected phage-displayed peptides can reflect pairs of juxtaposed amino acid residues of the epitope. Application of the algorithm enabled prediction of epitopes. On the basis of these predictions, segments of an antigen were used to reconstitute an antigenic epitope mimetic that was recognized by its original mAb.

Assorted mutations in the envelope gene of simian immunodeficiency virus lead to loss of neutralization resistance against antibodies representing a broad spectrum of specificities

Simian immunodeficiency virus (SIV) of macaques isolate SIVmac239 is highly resistant to neutralization by polyclonal antisera or monoclonal antibodies, a property that it shares with most primary isolates of human immunodeficiency virus type 1 (HIV-1). This resistance is important for the ability of the virus to persist at high levels in vivo. To explore the physical features of the viral envelope complex that contribute to the neutralization-resistant phenotype, we examined a panel of SIVmac239 derivatives for sensitivity to neutralization by a large collection of monoclonal antibodies (MAbs). These MAbs recognize both linear and conformational epitopes throughout the viral envelope proteins. The variant viruses included three derivatives of SIVmac239 with substitutions in specific N-linked glycosylation sites of gp120 and a fourth variant that lacked the 100 amino acids that encompass the V1 and V2 loops. Also included in this study was SIVmac316, a variant of SIVmac239 with distributed mutations in env that confer significantly increased replicative capacity in tissue macrophages. These viruses were chosen to represent a broad range of neutralization sensitivities based on susceptibility to pooled, SIV-positive plasma. All three of these very different kinds of mutations (amino acid substitutions, elimination of N-glycan attachment sites, and a 100-amino-acid deletion spanning variable loops V1 and V2) dramatically increased sensitivity to neutralization by MAbs from multiple competition groups. Thus, the mutations did not simply expose localized epitopes but rather conferred global increases in neutralization sensitivity. The removal of specific N-glycan attachment sites from V1 and V2 led to increased sensitivity to neutralization by antibodies recognizing epitopes from both within and outside of the V1-V2 sequence. Surprisingly, while most of the mutations that gave rise to increased sensitivity were located in the N-terminal half of gp120 (surface subunit [SU]), the greatest increases in sensitivity were to MAbs recognizing the C-terminal half of gp120 or the ectodomain of gp41 (transmembrane subunit [TM]). This reagent set and information should now be useful for defining the physical, structural, thermodynamic, and kinetic factors that influence relative sensitivity to antibody-mediated neutralization.

Structural analysis of the epitope of the anti-HIV antibody 2F5 sheds light into its mechanism of neutralization and HIV fusion

Inhibition of human immunodeficiency virus (HIV) fusion with the host cell has emerged as a viable therapeutic strategy, and rational design of inhibitors and vaccines, interfering with this process, is a prime target for antiviral research. To advance our knowledge of the structural biology of HIV fusion, we have studied the membrane-proximal region of the fusogenic envelope subunit gp41, which includes the epitope ELDKWA of the broadly neutralizing human antibody 2F5. The structural evidence available for this region is contradictory, with some studies suggesting an overall helical conformation, while the X-ray structure of the ELDKWAS peptide bound to the antibody shows it folded in a type I beta turn. We used a two-step strategy: Firstly, by a competition binding assay, we identified the proper boundaries of the domain recognized by 2F5, which we found considerably larger than the ELDKWAS hexapeptide. Secondly, we studied the structure of the resulting 13 amino acid residue peptide by collecting NMR data and analyzing them by our previously developed statistical method (NAMFIS). Our study revealed that the increase in binding affinity goes in parallel with stabilization of specific local and global conformational propensities, absent from the shorter epitope. When compounded with the available biological evidence, our structural analysis allows us to propose a specific role for the membrane-proximal region during HIV fusion, in terms of a conformational transition between the turn and the helical structure. At the same time, our hypothesis offers a structural explanation for the mechanism of neutralization of mAb 2F5.

Tyrosine sulfation of human antibodies contributes to recognition of the CCR5 binding region of HIV-1 gp120

Sulfated tyrosines at the amino terminus of the principal HIV-1 coreceptor CCR5 play a critical role in its ability to bind the HIV-1 envelope glycoprotein gp120 and mediate HIV-1 infection. Here, we show that a number of human antibodies directed against gp120 are tyrosine sulfated at their antigen binding sites. Like that of CCR5, antibody association with gp120 is dependent on sulfate moieties, enhanced by CD4, and inhibited by sulfated CCR5-derived peptides. Most of these antibodies preferentially associate with gp120 molecules of CCR5-utilizing (R5) isolates and neutralize primary R5 isolates more efficiently than laboratory-adapted isolates. These studies identify a distinct subset of CD4-induced HIV-1 neutralizing antibodies that closely emulate CCR5 and demonstrate that tyrosine sulfation can contribute to the potency and diversity of the human humoral response.

Antibody domain exchange is an immunological solution to carbohydrate cluster recognition

Human antibody 2G12 neutralizes a broad range of human immunodeficiency virus type 1 (HIV-1) isolates by binding an unusually dense cluster of carbohydrate moieties on the "silent" face of the gp120 envelope glycoprotein. Crystal structures of Fab 2G12 and its complexes with the disaccharide Manalpha1-2Man and with the oligosaccharide Man9GlcNAc2 revealed that two Fabs assemble into an interlocked VH domain-swapped dimer. Further biochemical, biophysical, and mutagenesis data strongly support a Fab-dimerized antibody as the prevalent form that recognizes gp120. The extraordinary configuration of this antibody provides an extended surface, with newly described binding sites, for multivalent interaction with a conserved cluster of oligomannose type sugars on the surface of gp120. The unique interdigitation of Fab domains within an antibody uncovers a previously unappreciated mechanism for high-affinity recognition of carbohydrate or other repeating epitopes on cell or microbial surfaces.

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.

Molecular features of the broadly neutralizing immunoglobulin G1 b12 required for recognition of human immunodeficiency virus type 1 gp120

IgG1 b12 is a broadly neutralizing antibody against human immunodeficiency virus type 1 (HIV-1). The epitope recognized by b12 overlaps the CD4 receptor-binding site (CD4bs) on gp120 and has been a target for vaccine design. Determination of the three-dimensional structure of immunoglobulin G1 (IgG1) b12 allowed modeling of the b12-gp120 interaction in which the protruding third complementarity-determining region (CDR) of the heavy chain (H3) was crucial for antibody binding. In the present study, extensive mutational analysis of the antigen-binding site of Fab b12 was carried out to investigate the validity of the model and to identify residues important for gp120 recognition and, by inference, key to the anti-HIV-1 activity of IgG1 b12. In all, 50 mutations were tested: 40 in H3, 4 each in H2 and L1, and 2 in L3. The results suggest that the interaction of gp120 with H3 of b12 is crucially dependent not only on a Trp residue at the apex of the H3 loop but also on a number of residues at the base of the loop. The arrangement of these residues, including aromatic side chains and side chains that hydrogen bond across the base of the loop, may rigidify H3 for penetration of the recessed CD4-binding cavity. The results further emphasize the importance to gp120 binding of a Tyr residue at the apex of the H2 loop that forms a second finger-like structure and a number of Arg residues in L1 that form a positively charged, shelf-like structure. In general, the data are consistent with the b12-gp120 interaction model previously proposed. At the gene level, somatic mutation is seen to be crucial for the generation of many of the structural features described. The Fab b12 mutants were also tested against the b12 epitope-mimic peptide B2.1, and the reactivity profile had many similarities but also significant differences from that observed for gp120. The paratope map of b12 may facilitate the design of molecules that are able to elicit b12-like activities.

Redox-triggered infection by disulfide-shackled human immunodeficiency virus type 1 pseudovirions

We previously described a human immunodeficiency virus type 1 (HIV-1) envelope mutant that introduces a disulfide bridge between the gp120 surface proteins and gp41 transmembrane proteins (J. M. Binley, R. W. Sanders, B. Clas, N. Schuelke, A. Master, Y. Guo, F. Kajumo, D. J. Anselma, P. J. Maddon, W. C. Olson, and J. P. Moore, J. Virol. 74:627-643, 2000). Here we produced pseudovirions bearing the mutant envelope and a reporter gene to examine the mutant's infectious properties. These pseudovirions attach to cells expressing CD4 and coreceptor but infect only when triggered with reducing agent, implying that gp120-gp41 dissociation is necessary for infection. Further studies suggested that virus entry was arrested after CD4 and coreceptor engagement. By measuring the activities of various entry inhibitors against the arrested intermediate, we found that gp120-targeting inhibitors typically act prior to virus attachment, whereas gp41 inhibitors are able to act postattachment. Unexpectedly, a significant fraction of antibodies in HIV-1-positive sera neutralized virus postattachment, suggesting that downstream fusion events and structures figure prominently in the host immune response. Overall, this disulfide-shackled virus is a unique tool with potential utility in vaccine design, drug discovery, and elucidation of the HIV-1 entry process.

Neutralizing antibodies against conserved domains of p15E of porcine endogenous retroviruses: basis for a vaccine for xenotransplantation?

Porcine xenotransplants may offer a potential solution to the problem posed by the limited supply of allotransplants. However, xenotransplantation may be associated with the risk of transmission of microorganisms, in particular of porcine endogenous retroviruses (PERVs) that are an integral part of the porcine genome and able to infect human cells in vitro. Possible strategies to prevent virus transmission include the development of PERV knockout animals or of effective vaccines. When antisera prepared against the main structural proteins of PERV were screened, a goat antiserum against the recombinant ectodomain of the transmembrane envelope protein p15E was found to neutralize PERV infectivity. Epitope mapping using overlapping peptides revealed two epitopes, E1 (GPQQLEK) and E2 (FEGWFN). These sequences are identical for all PERVs and are highly conserved among all gammaretroviruses. Interestingly, antibodies isolated from AIDS patients and specific for sequences of HIV-1 partially homologous with E2 (Mab4E10, LWNWFN) or located in close proximity to E2 (Mab2F5, ELDKWA) are known to neutralize several strains of HIV-1. It is the first report showing epitope mapping of gammaretrovirus-specific neutralizing antibodies and demonstrating similarity to corresponding epitopes in HIV. These domains of the transmembrane proteins of different retroviruses are an effective target for neutralizing antibodies and may be a useful antigen to create an antiretroviral vaccine.

Antiviral activity and conformational features of an octapeptide derived from the membrane-proximal ectodomain of the feline immunodeficiency virus transmembrane glycoprotein

Feline immunodeficiency virus (FIV) provides a valuable animal model by which criteria for lentivirus control strategies can be tested. Previous studies have shown that a 20-mer synthetic peptide of the membrane-proximal ectodomain of FIV transmembrane glycoprotein, designated peptide 59, potently inhibited the growth of tissue culture-adapted FIV in feline fibroblastoid CrFK cells. In the present report we describe the potential of this peptide to inhibit the replication of primary FIV isolates in lymphoid cells. Because antiviral activity of peptide 59 was found to map to a short segment containing three conserved Trp residues, further analyses focused on a derivative of eight amino acids ((770)W-I(777)), designated C8. Peptide C8 activity was found to be dependent on conservation of the Trp motif, to be removed from solution by FIV absorbed onto substrate cells, and to be blocked by a peptide derived from the N-terminal portion of FIV transmembrane glycoprotein. Structural studies showed that peptide C8 possesses a conformational propensity highly uncommon for peptides of its size, which may account for its considerable antiviral potency in spite of small size.

The prefusogenic intermediate of HIV-1 gp41 contains exposed C-peptide regions

The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is composed of a complex between the surface subunit gp120, which binds to cellular receptors, and the transmembrane subunit gp41. Upon activation of the envelope glycoprotein by cellular receptors, gp41 undergoes conformational changes that mediate fusion of the viral and cellular membranes. Prior to formation of a fusogenic "trimer-of-hairpins" structure, gp41 transiently adopts a prefusogenic conformation whose structural features are poorly understood. An important approach toward understanding structural conformations of gp41 during HIV-1 entry has been to analyze the structural targets of gp41 inhibitors. We have constructed epitope-tagged versions of 5-Helix, a designed protein that binds to the C-peptide region of gp41 and inhibits HIV-1 membrane fusion. Using these 5-Helix variants, we examined which conformation of gp41 is the target of 5-Helix. We find that although 5-Helix binds poorly to native gp41, it binds strongly to gp41 activated by interaction of the envelope protein with either soluble CD4 or membrane-bound cellular receptors. This preferential interaction with activated gp41 results in the accumulation of 5-Helix on the surface of activated cells. These results strongly suggest that the gp41 prefusogenic intermediate is the target of 5-Helix and that this intermediate has a remarkably "open" structure, with exposed C-peptide regions. These results provide important structural information about this intermediate that should facilitate the development of HIV-1 entry inhibitors and may lead to new vaccine strategies.

Post-exposure prophylaxis with human monoclonal antibodies prevented SHIV89.6P infection or disease in neonatal macaques

BACKGROUND

The majority of infants infected through maternal transmission acquire the virus during birth or postpartum through breastfeeding: mucosal exposure is considered to be a major route of infection.

OBJECTIVES

To develop passive immunization with human neutralizing monoclonal antibodies (mAbs) against mother-to-child transmission of HIV during delivery and through breastfeeding.

DESIGN

An oral challenge model in newborn rhesus macaques mimicked peri- and postpartum virus transmission.

METHODS

Neonatal rhesus macaques were challenged orally with the highly pathogenic, chimeric simian-human immunodeficiency virus SHIV89.6P and given post-exposure prophylaxis with a quadruple combination of neutralizing human mAbs, IgG1b12, 2G12, 2F5, and 4E10, directed against conserved epitopes of HIV envelope glycoproteins. Control animals were virus challenged but left untreated. All infants were followed prospectively for signs of viremia and immunodeficiency.

RESULTS

Two out of four macaque infants treated with neutralizing mAbs showed no evidence of infection; the other two maintained normal CD4 T cell counts. In contrast, all control animals became highly viremic and had profound CD4 T cell losses; three out of four died from AIDS within 1.5-6 weeks of the challenge.

CONCLUSIONS

Passive immunization with this quadruple neutralizing mAbs combination may represent a promising approach to prevent peri- and postnatal HIV transmission. Furthermore, the epitopes recognized by the four neutralizing mAbs are key determinants to achieve complete protection and represent important targets against which to develop active, antibody-response-based AIDS vaccines.

Primary African HIV clade A and D isolates: effective cross-clade neutralization with a quadruple combination of human monoclonal antibodies raised against clade B

We investigated the ability of several human neutralizing monoclonal antibodies (nmAbs), originally raised against human immunodeficiency virus (HIV) clade B isolates, to neutralize primary clade A and D isolates as single agents and in combinations. All four primary HIV clade A isolates and five primary HIV clade D isolates tested were neutralized >99% by the quadruple combination of nmAbs IgG1b12, 2G12, 2F5, and 4E10. These mAbs recognize conserved epitopes on HIV-1 envelope (Env), resulting in strong cross-clade neutralization. Previously, we showed synergistic neutralization of primary HIV-1 clade C isolates in vitro by the same nMAb combination. We and others also showed neutralization of primary HIV clade B strains. Together, our data show that the quadruple combination of mAbs effectively neutralized primary HIV clade A, B, C, and D isolates.

Pre-transmembrane sequence of Ebola glycoprotein. Interfacial hydrophobicity distribution and interaction with membranes

The membrane-interacting domain that precedes the transmembrane anchor of Ebola glycoprotein has been characterized. This aromatic-rich region is predicted to bind the membrane interface adopting an alpha-helical structure. Peptides representing either the Ebola glycoprotein pre-transmembrane sequence, or a 'scrambled' control with a different hydrophobic-at-interface moment, have been studied. Insertion into lipid monolayers, changes in intrinsic fluorescence and in infrared spectra demonstrated that only the wild-type peptide bound the interface under equilibrium conditions and adopted an alpha-helical conformation. The presence of the raft-associated lipid sphingomyelin did not affect membrane insertion, but it stimulated highly the membrane-destabilizing capacity of the pre-transmembrane sequence. A parallel study of the effects of the viral sequence and of melittin suggests that Ebola glycoprotein pre-transmembrane sequence might target membranes inherently prone to destabilization by lytic peptides.

Nonneutralizing antibodies to the CD4-binding site on the gp120 subunit of human immunodeficiency virus type 1 do not interfere with the activity of a neutralizing antibody against the same site

We have investigated whether nonneutralizing monoclonal antibodies (MAbs) to the gp120 subunit of the envelope glycoprotein (Env) complex of human immunodeficiency virus type 1 (HIV-1) can interfere with HIV-1 neutralization by another anti-gp120 MAb. We used neutralizing (b12) and nonneutralizing (205-42-15, 204-43-1, 205-46-9) MAbs to the epitope cluster overlapping the CD4-binding site (CD4BS) on gp120. All the MAbs, neutralizing or otherwise, cross-competed for binding to monomeric gp120, indicating the close topological proximity of their epitopes. However, the nonneutralizing CD4BS MAbs did not interfere with the neutralization activity of MAb b12. In contrast, in a binding assay using oligomeric Env expressed on the surface of Env-transfected cells, the nonneutralizing MAbs did partially compete with b12 for Env binding. The surface of Env-transfected cells contains two categories of binding site for CD4BS MAbs. One type of site is recognized by both b12 and nonneutralizing CD4BS MAbs; the other is recognized by only b12. Binding assays for Env-gp120 interactions based on the use of monomeric gp120 or Env-transfected cells do not predict the outcome of HIV-1 neutralization assays, and they should therefore be used only with caution when gauging the properties of anti-Env MAbs.

Do not underestimate the power of antibodies--lessons from adoptive transfer of antibodies against HIV

Successes for neutralizing antibodies (nAbs) against the human immunodeficiency virus (HIV) include potent cross-clade neutralization of primary virus isolates by human neutralizing monoclonal antibodies (nmAbs) targeting conserved envelope epitopes. Furthermore, passively administered combinations of human nmAbs prevented infection in primates, indicating that epitopes recognized by such nmAbs are key determinants for protection. Lastly, in the absence of CD8+ T cells, nAbs may act as a second line of defense during chronic infection. Taken together, these results argue for generating nAb response-based prophylactic and/or therapeutic AIDS vaccines. We suggest that the epitopes identified by passive immunization represent excellent targets for the rational design of nAb response-based AIDS vaccines.

Antigenic properties of the human immunodeficiency virus transmembrane glycoprotein during cell-cell fusion

Human immunodeficiency virus (HIV) entry is triggered by interactions between a pair of heptad repeats in the gp41 ectodomain, which convert a prehairpin gp41 trimer into a fusogenic three-hairpin bundle. Here we examined the disposition and antigenic nature of these structures during the HIV-mediated fusion of HeLa cells expressing either HIV(HXB2) envelope (Env cells) or CXCR4 and CD4 (target cells). Cell-cell fusion, indicated by cytoplasmic dye transfer, was allowed to progress for various lengths of time and then arrested. Fusion intermediates were then examined for reactivity with various monoclonal antibodies (MAbs) against immunogenic cluster I and cluster II epitopes in the gp41 ectodomain. All of these MAbs produced similar staining patterns indicative of reactivity with prehairpin gp41 intermediates or related structures. MAb staining was seen on Env cells only upon exposure to soluble CD4, CD4-positive, coreceptor-negative cells, or stromal cell-derived factor-treated target cells. In the fusion system, the MAbs reacted with the interfaces of attached Env and target cells within 10 min of coculture. MAb reactivity colocalized with the formation of gp120-CD4-coreceptor tricomplexes after longer periods of coculture, although reactivity was absent on cells exhibiting cytoplasmic dye transfer. Notably, the MAbs were unable to inhibit fusion even when allowed to react with soluble-CD4-triggered or temperature-arrested antigens prior to initiation of the fusion process. In comparison, a broadly neutralizing antibody, 2F5, which recognizes gp41 antigens in the HIV envelope spike, was immunoreactive with free Env cells and Env-target cell clusters but not with fused cells. Notably, exposure of the 2F5 epitope required temperature-dependent elements of the HIV envelope structure, as MAb binding occurred only above 19 degrees C. Overall, these results demonstrate that immunogenic epitopes, both neutralizing and nonneutralizing, are accessible on gp41 antigens prior to membrane fusion. The 2F5 epitope appears to depend on temperature-dependent elements on prefusion antigens, whereas cluster I and cluster II epitopes are displayed by transient gp41 structures. Such findings have important implications for HIV vaccine approaches based on gp41 intermediates.

[Present situation regarding development of an HIV vaccine]

The AIDS epidemic continues to advance, and the development of a preventive HIV vaccine has become a major objective for scientific research. An effective vaccine against this virus is not available and complete protection still has not been achieved in animal models. In this review the major challenges related to the development of a vaccine against HIV are analyzed, particularly the mechanisms involved in viral escape from the immune response, and the results obtained with the various therapeutic and preventive vaccine prototypes are summarized. Finally, the social, economic and health aspects related to research on HIV vaccines and the current controversy around the performance of clinical trials with these agents is discussed.

Stabilization of the soluble, cleaved, trimeric form of the envelope glycoprotein complex of human immunodeficiency virus type 1

The envelope glycoprotein (Env) complex of human immunodeficiency virus type 1 has evolved a structure that is minimally immunogenic while retaining its natural function of receptor-mediated virus-cell fusion. The Env complex is trimeric; its six individual subunits (three gp120 and three gp41 subunits) are associated by relatively weak, noncovalent interactions. The induction of neutralizing antibodies after vaccination with individual Env subunits has proven very difficult, probably because they are inadequate mimics of the native complex. Our hypothesis is that a stable form of the Env complex, perhaps with additional modifications to rationally alter its antigenic structure, may be a better immunogen than the individual subunits. A soluble form of Env, SOS gp140, can be made that has gp120 stably linked to the gp41 ectodomain by an intermolecular disulfide bond. This protein is fully cleaved at the proteolysis site between gp120 and gp41. However, the gp41-gp41 interactions in SOS gp140 are too weak to maintain the protein in a trimeric configuration. Consequently, purified SOS gp140 is a monomer (N. Schülke, M. S. Vesanen, R. W. Sanders, P. Zhu, D. J. Anselma, A. R. Villa, P. W. H. I. Parren, J. M. Binley, K. H. Roux, P. J. Maddon, J. P. Moore, and W. C. Olson, J. Virol. 76:7760-7776, 2002). Here we describe modifications of SOS gp140 that increase its trimer stability. A variant SOS gp140, designated SOSIP gp140, contains an isoleucine-to-proline substitution at position 559 in the N-terminal heptad repeat region of gp41. This protein is fully cleaved, has favorable antigenic properties, and is predominantly trimeric. SOSIP gp140 trimers are noncovalently associated and can be partially purified by gel filtration chromatography. These gp140 trimers are dissociated into monomers by anionic detergents or heat but are relatively resistant to nonionic detergents, high salt concentrations, or exposure to a mildly acidic pH. SOSIP gp140 should be a useful reagent for structural and immunogenicity studies.

Phage display technology: clinical applications and recent innovations

Phage display is a molecular diversity technology that allows the presentation of large peptide and protein libraries on the surface of filamentous phage. Phage display libraries permit the selection of peptides and proteins, including antibodies, with high affinity and specificity for almost any target. A crucial advantage of this technology is the direct link that exists between the experimental phenotype and its encapsulated genotype, which allows the evolution of the selected binders into optimized molecules. Phage display facilitates engineering of antibodies with regard to their size, valency, affinity, and effector functions. The selection of antibodies and peptides from libraries displayed on the surface of filamentous phage has proven significant for routine isolation of peptides and antibodies for diagnostic and therapeutic applications. This review serves as an introduction to phage display, antibody engineering, the development of phage-displayed peptides and antibody fragments into viable diagnostic reagents, and recent trends in display technology.

Oligomeric and conformational properties of a proteolytically mature, disulfide-stabilized human immunodeficiency virus type 1 gp140 envelope glycoprotein

We describe the further properties of a protein, designated SOS gp140, wherein the association of the gp120 and gp41 subunits of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein is stabilized by an intersubunit disulfide bond. HIV-1(JR-FL) SOS gp140, proteolytically uncleaved gp140 (gp140(UNC)), and gp120 were expressed in stably transfected Chinese hamster ovary cells and analyzed for antigenic and structural properties before and after purification. Compared with gp140(UNC), SOS gp140 reacted more strongly in surface plasmon resonance and radioimmunoprecipitation assays with the neutralizing monoclonal antibodies (MAbs) 2G12 (anti-gp120), 2F5 (anti-gp41), and 17b (to a CD4-induced epitope that overlaps the CCR5-binding site). In contrast, gp140(UNC) displayed the greater reactivity with nonneutralizing anti-gp120 and anti-gp41 MAbs. Immunoelectron microscopy studies suggested a model for SOS gp140 wherein the gp41 ectodomain (gp41(ECTO)) occludes the "nonneutralizing" face of gp120, consistent with the antigenic properties of this protein. We also report the application of Blue Native polyacrylamide gel electrophoresis (BN-PAGE), a high-resolution molecular sizing method, to the study of viral envelope proteins. BN-PAGE and other biophysical studies demonstrated that SOS gp140 was monomeric, whereas gp140(UNC) comprised a mixture of noncovalently associated and disulfide-linked dimers, trimers, and tetramers. The oligomeric and conformational properties of SOS gp140 and gp140(UNC) were largely unaffected by purification. An uncleaved gp140 protein containing the SOS cysteine mutations (SOS gp140(UNC)) was also oligomeric. Surprisingly, variable-loop-deleted SOS gp140 proteins were expressed (although not yet purified) as cleaved, noncovalently associated oligomers that were significantly more stable than the full-length protein. Overall, our findings have relevance for rational vaccine design.

Neutralizing antibodies against HIV -- back in the major leagues?

The past few months have seen encouraging successes for neutralizing antibodies against HIV; human monoclonal antibodies targeting conserved HIV envelope epitopes potently neutralized primary virus isolates, including strains of different clades. In primates, passive immunization with combinations containing human monoclonal antibodies completely prevented infection, even after mucosal virus challenges. Epitopes recognized by the protective monoclonal antibodies are important determinants for protection and provide a rational basis for AIDS vaccine development.

HIV-1 gp41 envelope residues 650-685 exposed on native virus act as a lectin to bind epithelial cell galactosyl ceramide

The initial step in the interaction between human immunodeficiency virus (HIV-1) and epithelial cells is the binding of HIV-1 envelope glycoproteins to the epithelial cell galactosyl ceramide (GalCer). Here we show that HIV-1 envelope gp41 residues 650-685 bind GalCer in a galactose-specific manner. The gp41 residues that display this lectin activity are highly conserved among HIV-1 isolates and constitute three regions: residues 650-661, which encompass a charged helix; residues 662-667, referred to as the conserved epitope ELDKWA, the epitope recognized by antibodies that neutralize HIV-1 entry in epithelial and CD4(+)-mononucleated cells; and residues 668-685, a hydrophobic Trp-rich sequence that stabilizes the structure of the galactose binding site. Similar to other galactose-specific lectins, the gp41 lectin site is active only as an oligomer. Finally the orientation of the galactose toward the gp41 lectin site appears to be controlled by the lipid microenvironment of the epithelial membrane. From the experimental data we construct a theoretical model of the interaction between gp41 and GalCer based on thermodynamic considerations. This model integrates the dynamics and the spatial organization of the viral envelope glycoproteins, GalCer organized in raft microdomains in the apical region of the epithelial cell membrane and the interfacial water. Characterization of the minimal sequence and structure of gp41 in direct interaction with GalCer may help unravel the still unknown immunogenic determinant able to elicit antibodies against ELDKWA and target of one of the rare neutralizing antibodies against gp41.

The broadly neutralizing anti-human immunodeficiency virus type 1 antibody 2G12 recognizes a cluster of alpha1-->2 mannose residues on the outer face of gp120

2G12 is a broadly neutralizing human monoclonal antibody against human immunodeficiency virus type-1 (HIV-1) that has previously been shown to bind to a carbohydrate-dependent epitope on gp120. Here, site-directed mutagenesis and carbohydrate analysis were used to define further the 2G12 epitope. Extensive alanine scanning mutagenesis showed that elimination of the N-linked carbohydrate attachment sequences associated with residues N295, N332, N339, N386, and N392 by N-->A substitution produced significant decreases in 2G12 binding affinity to gp120(JR-CSF). Further mutagenesis suggested that the glycans at N339 and N386 were not critical for 2G12 binding to gp120(JR-CSF). Comparison of the sequences of isolates neutralized by 2G12 was also consistent with a lesser role for glycans attached at these positions. The mutagenesis studies provided no convincing evidence for the involvement of gp120 amino acid side chains in 2G12 binding. Antibody binding was inhibited when gp120 was treated with Aspergillus saitoi mannosidase, Jack Bean mannosidase, or endoglycosidase H, indicating that Man(alpha)1-->2Man-linked sugars of oligomannose glycans on gp120 are required for 2G12 binding. Consistent with this finding, the binding of 2G12 to gp120 could be inhibited by monomeric mannose but not by galactose, glucose, or N-acetylglucosamine. The inability of 2G12 to bind to gp120 produced in the presence of the glucose analogue N-butyl-deoxynojirimycin similarly implicated Man(alpha)1-->2Man-linked sugars in 2G12 binding. Competition experiments between 2G12 and the lectin cyanovirin for binding to gp120 showed that 2G12 only interacts with a subset of available Man(alpha)1-->2Man-linked sugars. Consideration of all the data, together with inspection of a molecular model of gp120, suggests that the most likely epitope for 2G12 is formed from mannose residues contributed by the glycans attached to N295 and N332, with the other glycans playing an indirect role in maintaining epitope conformation.

Genetic evidence that interhelical packing interactions in the gp41 core are critical for transition of the human immunodeficiency virus type 1 envelope glycoprotein to the fusion-active state

The envelope glycoprotein complex (gp120-gp41) of human immunodeficiency virus type 1 (HIV-1) promotes the fusion of viral and cellular membranes through formation of the fusion-active six-helix bundle in the gp41 ectodomain. This gp41 core structure consists of three C-terminal helices packed in an antiparallel manner into hydrophobic grooves on the surface of the N-terminal trimeric coiled coil. Alanine mutations that destabilize the N- and C-terminal interhelical packing interactions also reduce viral infectivity. Here we show that viruses bearing these mutations exhibit a marked potentiation of inhibition by peptides that make up the gp41 core. By contrast, these viruses are unchanged in their sensitivities to soluble CD4, the CXCR4 coreceptor ligand SDF-1alpha, and human anti-HIV immunoglobulin, reagents that impact the initial, receptor-induced conformational changes in the envelope glycoprotein. Our results support the notion that these alanine mutations specifically affect the conformational transition to the fusion-active gp41 structure. The mutations also increase viral sensitivity to the gp41-directed monoclonal antibody 2F5, suggesting that this broadly neutralizing antibody may also interfere with this transition. The conformational activation of the HIV-1 envelope glycoprotein likely represents a viable target for vaccine and antiviral drug development.

Sphingomyelin and cholesterol promote HIV-1 gp41 pretransmembrane sequence surface aggregation and membrane restructuring

The interfacial sequence DKWASLWNWFNITNWLWYIK, preceding the transmembrane anchor of gp41 glycoprotein subunit, has been shown to be essential for fusion activity and incorporation into virions. HIV(c), a peptide representing this region, formed lytic pores in liposomes composed of the main lipids occurring in the human immunodeficiency virus, type 1 (HIV-1), envelope, i.e. 1-palmitoyl-2-oleoylphosphatidylcholine (POPC):sphingomyelin (SPM):cholesterol (Chol) (1:1:1 mole ratio), at low (>1:10,000) peptide-to-lipid mole ratio, and promoted the mixing of vesicular lipids at >1:1000 peptide-to-lipid mole ratios. Inclusion of SPM or Chol in POPC membranes had different effects. Whereas SPM sustained pore formation, Chol promoted fusion activity. Even if partitioning into membranes was not affected in the absence of both SPM and Chol, HIV(c) had virtually no effect on POPC vesicles. Conditions described to disturb occurrence of lateral separation of phases in these systems reproduced the high peptide-dose requirements for leakage as found in pure POPC vesicles and inhibited fusion. Surface aggregation assays using rhodamine-labeled peptides demonstrated that SPM and Chol promoted HIV(c) self-aggregation in membranes. Employing head-group fluorescent phospholipid analogs in planar supported lipid layers, we were able to discern HIV(c) clusters associated to ordered domains. Our results support the notion that the pretransmembrane sequence may participate in the clustering of gp41 monomers within the HIV-1 envelope, and in bilayer architecture destabilization at the loci of fusion.

Broadly cross-reactive HIV-1-neutralizing human monoclonal Fab selected for binding to gp120-CD4-CCR5 complexes

HIV-1 entry into cells involves formation of a complex between gp120 of the viral envelope glycoprotein (Env), a receptor (CD4), and a coreceptor, typically CCR5. Here we provide evidence that purified gp120(JR-FL)-CD4-CCR5 complexes exhibit an epitope recognized by a Fab (X5) obtained by selection of a phage display library from a seropositive donor with a relatively high broadly neutralizing serum antibody titer against an immobilized form of the trimolecular complex. X5 bound with high (nM) affinity to a variety of Envs, including primary isolates from different clades and Envs with deleted variable loops (V1, -2, -3). Its binding was significantly increased by CD4 and slightly enhanced by CCR5. X5 inhibited infection of peripheral blood mononuclear cells by a selection of representative HIV-1 primary isolates from clades A, B, C, D, E, F, and G with an efficiency comparable to that of the broadly neutralizing antibody IgG1 b12. Furthermore, X5 inhibited cell fusion mediated by Envs from R5, X4, and R5X4 viruses. Of the five broadly cross-reactive HIV-1-neutralizing human monoclonal antibodies known to date, X5 is the only one that exhibits increased binding to gp120 complexed with receptors. These findings suggest that X5 could possibly be used as entry inhibitor alone or in combination with other antiretroviral drugs for the treatment of HIV-1-infected individuals, provide evidence for the existence of conserved receptor-inducible gp120 epitopes that can serve as targets for potent broadly cross-reactive neutralizing antibodies in HIV-1-infected patients, and have important conceptual and practical implications for the development of vaccines and inhibitors.

Passive immunization with human neutralizing monoclonal antibodies: correlates of protective immunity against HIV

Passive immunization with synergistic combinations of human monoclonal antibodies (mAbs) directed against conserved epitopes of the human immunodeficiency virus (HIV) envelope completely protected 13 out of 16 rhesus monkeys challenged intravenously or orally with chimeric simian-human immunodeficiency virus (SHIV) strains; partial protection was seen in another two. A high degree of protection was seen among orally challenged neonates. Thus, we propose that passive immunization with synergistic combinations of neutralizing human mAbs may be effective in preventing maternal HIV transmission when given as post-exposure prophylaxis at birth and as prophylaxis against milk-borne transmission. Because we only used mAbs with well-defined epitope specificities, our studies also yield key information for designing AIDS vaccines: the correlates of immune protection. Vaccine strategies that can evoke antibody responses to epitopes recognized by the mAbs used in our primate studies could be important components of successful AIDS vaccines.

Mutations that destabilize the gp41 core are determinants for stabilizing the simian immunodeficiency virus-CPmac envelope glycoprotein complex

The human and simian immunodeficiency viruses (HIV and SIV) envelope glycoprotein consists of a trimer of two noncovalently and weakly associated subunits, gp120 and gp41. Upon binding of gp120 to cellular receptors, this labile native envelope complex undergoes conformational changes, resulting in a stable trimer-of-hairpins structure in gp41. Formation of the hairpin structure is thought to mediate membrane fusion by placing the viral and cellular membranes in close proximity. An in vitro-derived variant of SIVmac251, denoted CPmac, has acquired an unusually stable virion-associated gp120-gp41 complex. This unique phenotype is conferred by five amino acid substitutions in the gp41 ectodomain. Here we characterize the structural and physicochemical properties of the N40(L6)C38 model of the CPmac gp41 core. The 1.7-A resolution crystal structure of N40(L6)C38 is very similar to the six-helix bundle structure present in the parent SIVmac251 gp41. In both structures, three N40 peptides form a central three-stranded coiled coil, and three C38 peptides pack in an antiparallel orientation into hydrophobic grooves on the coiled-coil surface. Thermal unfolding studies show that the CPmac mutations destabilize the SIVmac251 six-helix bundle by 15 kJ/mol. Our results suggest that the formation of the gp41 trimer-of-hairpins structure is thermodynamically coupled to the conformational stability of the native envelope glycoprotein and raise the intriguing possibility that introduction of mutations to destabilize the six-helix bundle may lead to the stabilization of the trimeric gp120-gp41 complex. This study suggests a potential strategy for the production of stably folded envelope protein immunogens for HIV vaccine development.

The evolving role of information technology in the drug discovery process

Information technologies for chemical structure prediction, heterogeneous database access, pattern discovery, and systems and molecular modeling have evolved to become core components of the modern drug discovery process. As this evolution continues, the balance between in silico modeling and 'wet' chemistry will continue to shift and it might eventually be possible to step through the discovery pipeline without the aid of traditional laboratory techniques. Rapid advances in the industrialization of gene sequencing combined with databases of protein sequence and structure have created a target-rich but lead-poor environment. During the next decade, newer information technologies that facilitate the molecular modeling of drug-target interactions are likely to shift this balance towards molecular-based personalized medicine -- the ultimate goal of the drug discovery process.

A potent cross-clade neutralizing human monoclonal antibody against a novel epitope on gp41 of human immunodeficiency virus type 1

We have established a panel of human monoclonal antibodies against human immunodeficiency virus type 1 (HIV-1). The antibodies 2F5 and 2G12 have been identified to be two of the most potently in vitro neutralizing antibodies against HIV-1. Here we report on a further antibody, 4E10, of similar in vitro neutralizing potency. 4E10 binds to a novel epitope C terminal of the ELDKWA sequence recognized by 2F5, which has been so far the only described broadly neutralizing anti-gp41 antibody. Both 4E10 and 2F5 bind only weakly to infected cells compared with gp120-specific 2G12 and polyclonal anti-HIV-1 immunoglobulin (HIVIG), but show potent in vitro neutralizing properties. 4E10 neutralizes potently not only tissue culture-adapted strains but also primary isolates of different clades, including A, B, C, D, and E. Viruses that were found to be resistant to 2F5 were neutralized by 4E10 and vice versa; none of the tested isolates was resistant to both anti-gp41 antibodies. This confirms that the region recognized by 2F5 and 4E10 is essential for viral infectivity and may be important for vaccine design. Moreover, our results suggest that 4E10 should be further investigated for passive anti-HIV immunotherapy.

Effector function activities of a panel of mutants of a broadly neutralizing antibody against human immunodeficiency virus type 1

The human antibody immunoglobulin G1 (IgG1) b12 neutralizes a broad range of human immunodeficiency virus-type 1 (HIV-1) isolates in vitro and is able to protect against viral challenge in animal models. Neutralization of free virus, which is an antiviral activity of antibody that generally does not require the antibody Fc fragment, likely plays an important role in the protection observed. The role of Fc-mediated effector functions, which may reduce infection by inducing phagocytosis and lysis of virions and infected cells, however, is less clear. To investigate this role, we constructed a panel of IgG1 b12 mutants with point mutations in the second domain of the antibody heavy chain constant region (CH2). These mutations, as expected, did not affect gp120 binding or HIV-1 neutralization. IgG1 b12 mediated strong antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) of HIV-1-infected cells, but these activities were reduced or abrogated for the antibody mutants. Two mutants were of particular interest. K322A showed a twofold reduction in FcgammaR binding affinity and ADCC, while C1q binding and CDC were abolished. A double mutant (L234A, L235A) did not bind either FcgammaR or C1q, and both ADCC and CDC functions were abolished. In this study, we confirmed that K322 forms part of the C1q binding site in human IgG1 and plays an important role in the molecular interactions leading to complement activation. Less expectedly, we demonstrate that the lower hinge region in human IgG1 has a strong modulating effect on C1q binding and CDC. The b12 mutants K322A and L234A, L235A are useful tools for dissecting the in vivo roles of ADCC and CDC in the anti-HIV-1 activity of neutralizing antibodies.

Neutralization synergy of human immunodeficiency virus type 1 primary isolates by cocktails of broadly neutralizing antibodies

Several reports have described the existence of synergy between neutralizing monoclonal antibodies (MAbs) against human immunodeficiency virus type 1 (HIV-1). Synergy between human MAbs b12, 2G12, 2F5, and 4E10 in neutralization of primary isolates is of particular interest. Neutralization synergy of these MAbs, however, has not been studied extensively, and the mechanism of synergy remains unclear. We investigated neutralization synergy among this human antibody set by using the classical approach of titrating antibodies mixed at a fixed ratio as well as by an alternative, variable ratio approach in which the neutralization curve of one MAb is assessed in the presence and absence of a fixed, weakly neutralizing concentration of a second antibody. The advantage of this second approach is that it does not require mathematical analysis to establish synergy. No neutralization enhancement of any of the MAb combinations tested was detected for the T-cell-line-adapted molecular HIV-1 clone HxB2 using both assay formats. Studies of primary isolates (89.6, SF162, and JR-CSF) showed neutralization synergy which was relatively weak, with a maximum of two- to fourfold enhancement between antibody pairs, thereby increasing neutralization titers about 10-fold in triple and quadruple antibody combinations. Analysis of b12 and 2G12 binding to oligomeric envelope glycoprotein by using flow cytometry failed to demonstrate cooperativity in binding between these two antibodies. The mechanism by which these antibodies synergize is, therefore, not yet understood. The results lend some support to the notion that an HIV-1 vaccine that elicits moderate neutralizing antibodies to multiple epitopes may be more effective than hereto supposed, although considerable caution in extrapolating to a vaccine situation is required.