HIV-1 evades antibody-mediated neutralization through conformational masking of receptor-binding sites

Antigenic conservation and immunogenicity of the HIV coreceptor binding site

Immunogenic, broadly reactive epitopes of the HIV-1 envelope glycoprotein could serve as important targets of the adaptive humoral immune response in natural infection and, potentially, as components of an acquired immune deficiency syndrome vaccine. However, variability in exposed epitopes and a combination of highly effective envelope-cloaking strategies have made the identification of such epitopes problematic. Here, we show that the chemokine coreceptor binding site of HIV-1 from clade A, B, C, D, F, G, and H and circulating recombinant form (CRF)01, CRF02, and CRF11, elicits high titers of CD4-induced (CD4i) antibody during natural human infection and that these antibodies bind and neutralize viruses as divergent as HIV-2 in the presence of soluble CD4 (sCD4). 178 out of 189 (94%) HIV-1–infected patients had CD4i antibodies that neutralized sCD4-pretreated HIV-2 in titers (50% inhibitory concentration) as high as 1:143,000. CD4i monoclonal antibodies elicited by HIV-1 infection also neutralized HIV-2 pretreated with sCD4, and polyclonal antibodies from HIV-1–infected humans competed specifically with such monoclonal antibodies for binding. In vivo, variants of HIV-1 with spontaneously exposed coreceptor binding surfaces were detected in human plasma; these viruses were neutralized directly by CD4i antibodies. Despite remarkable evolutionary diversity among primate lentiviruses, functional constraints on receptor binding create opportunities for broad humoral immune recognition, which in turn serves to constrain the viral quasispecies.

The challenges of HIV vaccine development and testing

A vaccine against HIV remains the best hope for bringing the epidemic under control. An intensive global effort is underway to develop such a vaccine; however, the challenges are considerable. Several new vaccine technologies that have been developed and shown promise in animal models are now being tested in early phase safety trials in humans. Because there is no laboratory assay that will predict whether an HIV vaccine can protect humans from infection, clinical trials involving thousands of volunteers will need to be conducted to determine the efficacy of HIV vaccines. These trials need to take place in the developing countries that bear the burden of the epidemic, requiring a substantial amount of infrastructure development and capacity building.

Current situation in the development of a preventive HIV vaccine

The uncontrolled progression of the aids epidemic has made the development of an efficacious human immunodeficiency virus (HIV) vaccine a major objective of scientific research. No effective preventive vaccine against HIV is currently available and sterilizing immunity has not yet been achieved in animal models. This review analyses the major challenges in developing an aids vaccine, in particular the mechanisms involved in viral escape from the immune response, and summarizes the results obtained with the different prototypes of therapeutic and preventive vaccines. Finally, social, economic and healthcare aspects of research into HIV vaccines and current controversies regarding the development of clinical trials are discussed.

Situación actual en el desarrollo de una vacuna preventiva frente al VIH

El avance de la epidemia de sida ha convertido la obtención de una vacuna eficaz frente al virus de la inmunodeficiencia humana (VIH) como un objetivo científico prioritario. En el momento actual no disponemos de una vacuna preventiva frente a la infección por el VIH y en ningún modelo animal se ha conseguido la protección frente a la infección. En esta revisión se analizan las dificultades existentes en el desarrollo de una vacuna contra el sida, en especial los mecanismos de escape viral a la respuesta inmunitaria y se describen los prototipos de vacunas preventivas y terapéuticas en desarrollo y los resultados obtenidos. Por otra parte se sitúa esta investigación en el contexto sanitario, económico y social de la pandemia de sida y se analizan las polémicas actualmente planteadas en el desarrollo de ensayos clínicos con los diferentes tipos de vacunas.

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Biologic and genetic characterization of a panel of 60 human immunodeficiency virus type 1 isolates, representing clades A, B, C, D, CRF01_AE, and CRF02_AG, for the development and assessment of candidate vaccines

A critical priority for human immunodeficiency virus type 1 (HIV-1) vaccine development is standardization of reagents and assays for evaluation of immune responses elicited by candidate vaccines. To provide a panel of viral reagents from multiple vaccine trial sites, 60 international HIV-1 isolates were expanded in peripheral blood mononuclear cells and characterized both genetically and biologically. Ten isolates each from clades A, B, C, and D and 10 isolates each from CRF01_AE and CRF02_AG were prepared from individuals whose HIV-1 infection was evaluated by complete genome sequencing. The main criterion for selection was that the candidate isolate was pure clade or pure circulating recombinant. After expansion in culture, the complete envelope (gp160) of each isolate was verified by sequencing. The 50% tissue culture infectious dose and p24 antigen concentration for each viral stock were determined; no correlation between these two biologic parameters was found. Syncytium formation in MT-2 cells and CCR5 or CXCR4 coreceptor usage were determined for all isolates. Isolates were also screened for neutralization by soluble CD4, a cocktail of monoclonal antibodies, and a pool of HIV-1-positive patient sera. The panel consists of 49 nonsyncytium-inducing isolates that use CCR5 as a major coreceptor and 11 syncytium-inducing isolates that use only CXCR4 or both coreceptors. Neutralization profiles suggest that the panel contains both neutralization-sensitive and -resistant isolates. This collection of HIV-1 isolates represents the six major globally prevalent strains, is exceptionally large and well characterized, and provides an important resource for standardization of immunogenicity assessment in HIV-1 vaccine trials.

Discovery of small-molecule human immunodeficiency virus type 1 entry inhibitors that target the gp120-binding domain of CD4

The interaction between human immunodeficiency virus type 1 (HIV-1) gp120 and the CD4 receptor is highly specific and involves relatively small contact surfaces on both proteins according to crystal structure analysis. This molecularly conserved interaction presents an excellent opportunity for antiviral targeting. Here we report a group of pentavalent antimony-containing small molecule compounds, NSC 13778 (molecular weight, 319) and its analogs, which exert a potent anti-HIV activity. These compounds block the entry of X4-, R5-, and X4/R5-tropic HIV-1 strains into CD4(+) cells but show little or no activity in CD4-negative cells or against vesicular stomatitis virus-G pseudotyped virions. The compounds compete with gp120 for binding to CD4: either immobilized on a solid phase (soluble CD4) or on the T-cell surface (native CD4 receptor) as determined by a competitive gp120 capture enzyme-linked immunosorbent assay or flow cytometry. NSC 13778 binds to an N-terminal two-domain CD4 protein, D1/D2 CD4, immobilized on a surface plasmon resonance sensor chip, and dose dependently reduces the emission intensity of intrinsic tryptophan fluorescence of D1/D2 CD4, which contains two of the three tryptophan residues in the gp120-binding domain. Furthermore, T cells incubated with the compounds alone show decreased reactivity to anti-CD4 monoclonal antibodies known to recognize the gp120-binding site. In contrast to gp120-binders that inhibit gp120-CD4 interaction by binding to gp120, these compounds appear to disrupt gp120-CD4 contact by targeting the specific gp120-binding domain of CD4. NSC 13778 may represent a prototype of a new class of HIV-1 entry inhibitors that can break into the gp120-CD4 interface and mask the gp120-binding site on the CD4 molecules, effectively repelling incoming virions.

Cryptic nature of a conserved, CD4-inducible V3 loop neutralization epitope in the native envelope glycoprotein oligomer of CCR5-restricted, but not CXCR4-using, primary human immunodeficiency virus type 1 strains

The external subunit of the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env), gp120, contains conserved regions that mediate sequential interactions with two cellular receptor molecules, CD4 and a chemokine receptor, most commonly CCR5 or CXCR4. However, antibody accessibility to such regions is hindered by diverse protective mechanisms, including shielding by variable loops, conformational flexibility and extensive glycosylation. For the conserved neutralization epitopes hitherto described, antibody accessibility is reportedly unrelated to the viral coreceptor usage phenotype. Here, we characterize a novel, conserved gp120 neutralization epitope, recognized by a murine monoclonal antibody (MAb), D19, which is differentially accessible in the native HIV-1 Env according to its coreceptor specificity. The D19 epitope is contained within the third variable (V3) domain of gp120 and is distinct from those recognized by other V3-specific MAbs. To study the reactivity of MAb D19 with the native oligomeric Env, we generated a panel of PM1 cells persistently infected with diverse primary HIV-1 strains. The D19 epitope was conserved in the majority (23/29; 79.3%) of the subtype-B strains tested, as well as in selected strains from other genetic subtypes. Strikingly, in CCR5-restricted (R5) isolates, the D19 epitope was invariably cryptic, although it could be exposed by addition of soluble CD4 (sCD4); epitope masking was dependent on the native oligomeric structure of Env, since it was not observed with the corresponding monomeric gp120 molecules. By contrast, in CXCR4-using strains (X4 and R5X4), the epitope was constitutively accessible. In accordance with these results, R5 isolates were resistant to neutralization by MAb D19, becoming sensitive only upon addition of sCD4, whereas CXCR4-using isolates were neutralized regardless of the presence of sCD4. Other V3 epitopes examined did not display a similar divergence in accessibility based on coreceptor usage phenotype. These results provide the first evidence of a correlation between HIV-1 biological phenotype and neutralization sensitivity, raising the possibility that the in vivo evolution of HIV-1 coreceptor usage may be influenced by the selective pressure of specific host antibodies.

Immunology of scorpion toxins and perspectives for generation of anti-venom vaccines

Scorpions and other venomous animals contain concentrates of biologically active substances developed to block vital physiological and biochemical functions of the victims. These have contrasting human health concerns, provide important pharmacological raw material and pose a serious threat to human life and health in tropical and subtropical regions. Because only occasional and minor quantities of venom are introduced into the human organism with a scorpion sting and their mortal effect is an acute phenomenon these substances are unknown to the immune defense system and thus no immunity has appeared against them during evolution. Antidotes prepared from animal anti-sera are effective against some species of scorpions but depend on the manufacturer and the availability of product to the medical community. Although significant progress has been made in immunological studies of certain groups of toxins, few centers are dedicated to this research. Information is still insufficient to generate a comprehensive picture of the subject and to propose vaccines against venoms. A novel approach based on mimotopes selected from phage-displayed random peptide libraries show potential to impel further progress of toxin immunological studies and to provide putative vaccine resources. In this report we revise the "state of the art" in the field.

Variants of human immunodeficiency virus type 1 that efficiently use CCR5 lacking the tyrosine-sulfated amino terminus have adaptive mutations in gp120, including loss of a functional N-glycan

By selecting the R5 human immunodeficiency virus type 1 (HIV-1) strain JR-CSF for efficient use of a CCR5 coreceptor with a badly damaged amino terminus [i.e., CCR5(Y14N)], we previously isolated variants that weakly utilize CCR5(Delta18), a low-affinity mutant lacking the normal tyrosine sulfate-containing amino-terminal region of the coreceptor. These previously isolated HIV-1(JR-CSF) variants contained adaptive mutations situated exclusively in the V3 loop of their gp120 envelope glycoproteins. We now have weaned the virus from all dependency on the CCR5 amino terminus by performing additional selections with HeLa-CD4 cells that express only a low concentration of CCR5(Delta18). The adapted variants had additional mutations in their V3 loops, as well as one in the V2 stem (S193N) and four alternative mutations in the V4 loop that eliminated the same N-linked oligosaccharide from position N403. Assays using pseudotyped viruses suggested that these new gp120 mutations all made strong contributions to use of CCR5(Delta18) by accelerating a rate-limiting CCR5-dependent conformational change in gp41 rather than by increasing viral affinity for this damaged coreceptor. Consistent with this interpretation, loss of the V4 N-glycan at position N403 also enhanced HIV-1 use of a different low-affinity CCR5 coreceptor with a mutation in extracellular loop 2 (ECL2) [i.e., CCR5(G163R)], whereas the double mutant CCR5(Delta18,G163R) was inactive. We conclude that loss of the N-glycan at position N403 helps to convert the HIV-1 envelope into a hair-trigger form that no longer requires strong interactions with both the CCR5 amino terminus and ECL2 but efficiently uses either site alone. These results demonstrate a novel functional role for a gp120 N-linked oligosaccharide and a high degree of adaptability in coreceptor usage by HIV-1.

Design, expression, and immunogenicity of a soluble HIV trimeric envelope fragment adopting a prefusion gp41 configuration

The human immunodeficiency virus-1 (HIV-1) envelope glycoprotein (Env) is comprised of non-covalently associated gp120/gp41 subunits that form trimeric spikes on the virion surface. Upon binding to host cells, Env undergoes a series of structural transitions, leading to gp41 rearrangement necessary for fusion of viral and host membranes. Until now, the prefusion state of gp41 ectodomain (e-gp41) has eluded molecular and structural analysis, and thus assessment of the potential of such an e-gp41 conformer to elicit neutralizing antibodies has not been possible. Considering the importance of gp120 amino (C1) and carboxyl (C5) segments in the association with e-gp41, we hypothesize that these regions are sufficient to maintain e-gp41 in a prefusion state. Based on the available gp120 atomic structure, we designed several truncated gp140 variants by including the C1 and C5 regions of gp120 in a gp41 ectodomain fragment. After iterative cycles of protein design, expression and characterization, we obtained a variant truncated at Lys(665) that stably folds as an elongated trimer under physiologic conditions. Several independent biochemical/biophysical analyses strongly suggest that this mini-Env adopts a prefusion e-gp41 configuration that is strikingly distinct from the postfusion trimer-of-hairpin structure. Interestingly, this prefusion mini-Env, lacking the fragment containing the 2F5/4E10 neutralizing monoclonal antibody binding sites, displays no detectable HIV-neutralizing epitopes when employed as an immunogen in rabbits. The result of this immunogenicity study has important implications for HIV-1 vaccine design efforts. Moreover, this engineered mini-Env protein should facilitate three-dimensional structural studies of the prefusion e-gp41 and serve to guide future attempts at pharmacologic and immunologic intervention of HIV-1.

Targeted lysis of HIV-infected cells by natural killer cells armed and triggered by a recombinant immunoglobulin fusion protein: implications for immunotherapy

Natural killer (NK) cells play an important role in both innate and adaptive antiviral immune responses. The adaptive response typically requires that virus-specific antibodies decorate infected cells which then direct NK cell lysis through a CD16 mediated process termed antibody-dependent cellular cytotoxicity (ADCC). In this report, we employ a highly polymerized chimeric IgG1/IgA immunoglobulin (Ig) fusion protein that, by virtue of its capacity to extensively crosslink CD16, activates NK cells while directing the lysis of infected target cells. We employ HIV as a model system, and demonstrate that freshly isolated NK cells preloaded with an HIV gp120-specific chimeric IgG1/IgA fusion protein efficiently lyse HIV-infected target cells at picomolar concentrations. NK cells pre-armed in this manner retain the capacity to kill targets over an extended period of time. This strategy may have application to other disease states including various viral infections and cancers.

Engineering immunogenic consensus T helper epitopes for a cross-clade HIV vaccine

Developing a vaccine that will stimulate broad HIV-specific T cell responses is difficult because of the variability in HIV T cell epitope sequences, which is in turn due to the high mutation rate and consequent strain diversity of HIV-1. We used a new Class II version of the EpiMatrix T cell epitope-mapping tool and Conservatrix to select highly conserved and promiscuous Class II HLA-restricted T cell epitopes from a database of 18,313 HIV-1 env sequences. Criteria for selection were: (1) number of HIV-1 strains represented as measured by Conservatrix; (2) EpiMatrix score; and (3) promiscuity (number of unique MHC motifs contained in the peptide). Using another vaccine design tool called the EpiAssembler, a new set of overlapping, conserved and immunogenic HIV-1 peptides were engineered creating extended "immunogenic consensus" sequences. Each overlapping 9-mer of the 20-23 amino acid long immunogenic consensus peptides was conserved in a large number (range 893-2254) of individual HIV-1 strains, although the novel peptides were not representative of any single strain of HIV. We synthesized nine representative peptides. T helper cell responses to the peptides were evaluated by ELISpot (gamma-interferon) assay, using peripheral blood monocytes (PBMC) obtained from 34 healthy long term non-progressor (LT) or moderate-progressor (MP) donors (median years infected = 8.88, median CD4 T cells = 595, median VL = 1044). Nine peptides were tested, of which eight were confirmed in ELISpot assays using PBMC from the LT/MP subjects. These epitopes were ranked by Conservation and EpiMatrix score 1, 2, 3, 5, 7, 11, and 14 out of the set of 9 original peptides. Five of these peptides were selected for inclusion in an epitope-driven cross-clade HIV-1 vaccine (the GAIA vaccine). These data confirm the utility of bioinformatics tools to select and construct novel "immunogenic consensus sequence" T cell epitopes for a globally relevant vaccine against HIV.

Evidence for a polytopic form of the E1 envelope glycoprotein of Hepatitis C virus

The polyprotein precursor of the Hepatitis C virus (HCV) contains multiple membrane-spanning domains that define the membrane topology and subsequent maturation of the viral structural proteins. In order to examine the biogenesis of the E1-E2 heterodimeric complex, we inserted an affinity tag (S-peptide) at specific locations within the envelope glycoproteins. In particular, and based on the prediction that the E1 glycoprotein may be able to assume a polytopic topology containing two membrane-spanning domains, we inserted the affinity tag within a putative cytoplasmic loop of the E1 glycoprotein. The HCV structural polyprotein containing this tag (at amino acids 295/296) was highly expressed and able to form a properly processed and noncovalently associated E1-E2 complex. This complex was bound by murine and conformation-dependent human monoclonal antibodies (MAbs) comparably to the native untagged complex. In addition, MAb recognition was retained upon reconstituting the tagged E1-E2 complex in lipid membrane as topologically constrained proteoliposomes. Our findings are consistent with the model of a topologically flexible E1 glycoprotein that is able to adopt a polytopic form. This form of the E1-E2 complex may be important in the HCV life cycle and in pathogenesis.

Structure of an unliganded simian immunodeficiency virus gp120 core

Envelope glycoproteins of human and simian immunodeficiency virus (HIV and SIV) undergo a series of conformational changes when they interact with receptor (CD4) and co-receptor on the surface of a potential host cell, leading ultimately to fusion of viral and cellular membranes. Structures of fragments of gp120 and gp41 from the envelope protein are known, in conformations corresponding to their post-attachment and postfusion states, respectively. We report the crystal structure, at 4 A resolution, of a fully glycosylated SIV gp120 core, in a conformation representing its prefusion state, before interaction with CD4. Parts of the protein have a markedly different organization than they do in the CD4-bound state. Comparison of the unliganded and CD4-bound structures leads to a model for events that accompany receptor engagement of an envelope glycoprotein trimer. The two conformations of gp120 also present distinct antigenic surfaces. We identify the binding site for a compound that inhibits viral entry.

Characterization of gp120 and its single-chain derivatives, gp120-CD4D12 and gp120-M9: implications for targeting the CD4i epitope in human immunodeficiency virus vaccine design

Single-chain derivatives of JRFL gp120 linked to the first two domains of human CD4 (gp120-CD4D12) or to the CD4 miniprotein analog CD4M9 (gp120-M9), have been constructed. Biacore studies revealed that gp120-CD4D12 and gp120-M9 bound to antibody 17b with dissociation constants of 0.8 and 25 nM, respectively, at pH 7.0, while gp120 alone did not bind. The binding of gp120-CD4D12 to 17b is not affected by the addition of excess soluble CD4D12, while the binding of gp120-M9 is enhanced. This finding indicates that the M9 component of the single chain interacts relatively weakly with gp120 and can be displaced by soluble CD4D12. Immunogenicity studies of gp120, gp120-CD4D12, and gp120-M9 were carried out with guinea pigs. All three molecules were highly immunogenic. The resulting antisera were examined for neutralizing activities against various human immunodeficiency virus type 1 isolates. Broadly neutralizing activity was observed only with sera generated against gp120-CD4D12. These antisera were depleted of anti-CD4D12 antibodies by being passed over a column containing immobilized CD4D12. The depleted sera showed a loss of broadly neutralizing activity. Sera that were affinity purified over a column containing immobilized gp120-M9 also lacked such neutralizing activity. This finding suggests that the broadly neutralizing response observed is exclusively due to anti-CD4 antibodies. Competition experiments showed that only antisera generated against gp120-CD4D12 competed with the CD4i antibody 17b and that this activity was not affected by depletion of anti-CD4 antibodies. The data indicate that although antibodies targeting the CD4i epitope were generated by the gp120-CD4D12 immunogen, these antibodies were nonneutralizing.

Analysis of the neutralizing antibody response elicited in rabbits by repeated inoculation with trimeric HIV-1 envelope glycoproteins

The elicitation of broadly neutralizing antibodies directed against the human immunodeficiency virus type 1 (HIV-1) envelope glycoproteins, gp120 and gp41, remains a major challenge. Attempts to utilize monomeric gp120 as an immunogen to elicit high titers of neutralizing antibodies have been disappointing. Envelope glycoprotein constructs that better reflect the trimeric structure of the functional envelope spike have exhibited improved immunogenicity compared with monomeric gp120. We have described soluble gp140 ectodomain constructs with a heterologous trimerization motif; these have previously been shown to elicit antibodies in mice that were able to neutralize a number of HIV-1 isolates, among them primary isolate viruses. Recently, solid-phase proteoliposomes retaining the envelope glycoproteins as trimeric spikes in a physiologic membrane setting have been described. Here, we compare the immunogenic properties of these two trimeric envelope glycoprotein formulations and monomeric gp120 in rabbits. Both trimeric envelope glycoprotein preparations generated neutralizing antibodies more effectively than gp120. In contrast to monomeric gp120, the trimeric envelope glycoproteins elicited neutralizing antibodies with some breadth of neutralization. Furthermore, repeated boosting with the soluble trimeric formulations resulted in an increase in potency that allowed neutralization of a subset of neutralization-resistant HIV-1 primary isolates. We demonstrate that the neutralization is concentration-dependent, is mediated by serum IgG and that the major portion of the neutralizing activity is not directed against the gp120 V3 loop. Thus, mimics of the trimeric envelope glycoprotein spike described here elicit HIV-1-neutralizing antibodies that could contribute to a protective immune response and provide platforms for further modifications to improve the efficiency of this process.

Strong cellular and humoral anti-HIV Env immune responses induced by a heterologous rhabdoviral prime-boost approach

Recombinant rhabdovirus vectors expressing human immunodeficiency virus (HIV) and/or simian immunodeficiency virus (SIV) proteins have been shown to induce strong immune responses in mice and rhesus macaques. However, the finding that such responses protect rhesus macaques from AIDS-like disease but not from infection indicates that further improvements for these vectors are needed. Here, we designed a prime-boost schedule consisting of a rabies virus (RV) vaccine strain and a recombinant vesicular stomatitis virus (VSV) both expressing HIV Envelope (Env). Mice were primed and boosted with the two vaccine vehicles by different routes and in different combinations. Mucosal and systemic humoral responses were assessed using enzyme linked immunosorbent assay (ELISA) while the cellular immune response was determined by an IFN-gamma ELISPOT assay. We found that an immunization combination of RV and VSV elicited the highest titers of anti-Env antibodies and the greatest amount of Env-specific IFN-gamma secreting cells pre- and post-challenge with a recombinant vaccinia virus expressing HIV(89.6) Env. Furthermore, intramuscular immunization did not induce antigen-specific mucosal antibodies while intranasal inoculation stimulated vector-specific IgA antibodies in vaginal washings and serum. Our results show that it is feasible to elicit robust cellular and humoral anti-HIV responses using two different live attenuated Rhabdovirus vectors to sequentially prime and boost.

V3: HIV's switch-hitter

The third variable region, V3, of the gp120 surface envelope glycoprotein is an approximately 35-residue-long, frequently glycosylated, highly variable, disulfide-bonded structure that has a major influence on HIV-1 tropism. Thus the sequence of V3, directly or indirectly, can determine which coreceptor (CCR5 or CXCR4) is used to trigger the fusion potential of the Env complex, and hence which cells the virus can infect. V3 also influences HIV-1's sensitivity to, and ability to escape from, entry inhibitors that are being developed as antiviral drugs. For some strains, V3 is a prominent target for HIV-1 neutralizing antibodies (NAbs); indeed, for many years it was considered to be the "principal neutralization determinant" (PND). Some efforts to use V3 as a vaccine target continue to this day, despite disappointing progress over more than a decade. Recent findings on the structure, function, antigenicity, and immunogenicity of V3 cast new doubts on the value of this vaccine approach. Here, we review recent advances in the understanding of V3 as a determinant of viral tropism, and discuss how this new knowledge may inform the development of HIV-1 drugs and vaccines.

Contribution of N-Linked Glycans to the Conformation and Function of Intercellular Adhesion Molecules (ICAMs)

The crystal structures of the glycosylated N-terminal two domains of ICAM-1 and ICAM-2 provided a framework for understanding the role of glycosylation in the structure and function of intercellular adhesion molecules (ICAMs). The most conserved glycans were less flexible in the structures, interacting with protein residues and contributing to receptor folding and expression. The first N-linked glycan in ICAM-2 contacts an exposed tryptophan residue, defining a conserved glycan-W motif critical for the conformation of the integrin binding domain. The absence of this motif in human ICAM-1 exposes regions used in receptor dimerization and rhinovirus recognition. Experiments with soluble molecules having the N-terminal two domains of human ICAMs identified glycans of the high mannose type N-linked to the second domain of the dendritic cell-specific ICAM-grabbing nonintegrin lectin-ligands ICAM-2 and ICAM-3. About 40% of those receptor molecules bear endoglycosidase H sensitive glycans responsible of the lectin binding activity. High mannose glycans were absent in ICAM-1, which did not bind to the lectin, but they appeared in ICAM-1 mutants with additional N-linked glycosylation and lectin binding activity. N-Linked glycosylation regulate both conformation and immune related functions of ICAM receptors.

MVA as a vector for vaccines against HIV-1

A vaccine against HIV Type 1 (HIV-1) is urgently needed. Modified vaccinia virus Ankara is an attenuated smallpox vaccine which can be adapted to express HIV-1 antigens. In this review, we discuss the features which make modified vaccinia virus Ankara an attractive vector for genetic vaccines and have put it, together with several other recombinant viral vectors, at the forefront of HIV-1 vaccine development. Many candidate vaccines including those vectored by modified vaccinia virus Ankara are now entering human trials, the results of which will become available in the coming years.

Immunogenicity of HIV type 1 gp120 CD4 binding site phage mimotopes

The conserved domain of the CD4 binding site (CD4bs) on the human immunodeficiency virus type 1 (HIV- 1) envelope represents a potential target for vaccine development. Here we describe selection of peptide mimotopes by panning a phage peptide library on the HIV-1 CD4bs-specific, broadly neutralizing anti-HIV-1 monoclonal antibody, IgG(1) b12. We identified an initial consensus sequence for IgG1 b12 binding (M/VThetaSD, where Theta represents an aromatic amino acid). A molecular evolution approach, using second- and third-generation libraries, led us to identify a refined consensus sequence (GLLVWSDEL). The resulting IgG1 b12 phage mimotopes compete with gp160 for the IgG1 b12 antigen-binding site, but the phage coat protein (pIII) may play an important structural role, since both free peptides and KLH-conjugated peptides have no detectable binding activity. Mice immunized with IgG1 b12 phage mimotopes elicited a weak but persistent humoral response directed against the HIV-1 envelope. An antibody fragment was isolated from the antibody repertoires of these animals. It is noteworthy that while it has a relatively low affinity for HIV-1 gp160, the antibody targets an epitope that overlaps with that of IgG1 b12. Our data therefore suggest that engineered IgG1 b12 mimotopes share immunogenic features with the CD4bs. However, these peptidic structures will require further improvement in order to generate broad specificity neutralizing antibodies like IgG1 b12.

Characterization of the outer domain of the gp120 glycoprotein from human immunodeficiency virus type 1

The core of the gp120 glycoprotein from human immunodeficiency virus type 1 (HIV-1) is comprised of three major structural domains: the outer domain, the inner domain, and the bridging sheet. The outer domain is exposed on the HIV-1 envelope glycoprotein trimer and contains binding surfaces for neutralizing antibodies such as 2G12, immunoglobulin G1b12, and anti-V3 antibodies. We expressed the outer domain of HIV-1(YU2) gp120 as an independent protein, termed OD1. OD1 efficiently bound 2G12 and a large number of anti-V3 antibodies, indicating its structural integrity. Immunochemical studies with OD1 indicated that antibody responses against the outer domain of the HIV-1 gp120 envelope glycoprotein are rare in HIV-1-infected human sera that potently neutralize the virus. Surprisingly, such outer-domain-directed antibody responses are commonly elicited by immunization with recombinant monomeric gp120. Immunization with soluble, stabilized HIV-1 envelope glycoprotein trimers elicited antibody responses that more closely resembled those in the sera of HIV-1-infected individuals. These results underscore the qualitatively different humoral immune responses elicited during natural infection and after gp120 vaccination and help to explain the failure of gp120 as an effective vaccine.

DNA vaccines against human immunodeficiency virus type 1

Development of a vaccine against human immunodeficiency virus type 1 (HIV-1) is the main hope for controlling the acquired immunodeficiency syndrome pandemic. An ideal HIV vaccine should induce neutralizing antibodies, CD4+ helper T cells, and CD8+ cytotoxic T cells. While the induction of broadly neutralizing antibodies remains a highly challenging goal, there are a number of technologies capable of inducing potent cell-mediated responses in animal models, which are now starting to be tested in humans. Naked DNA immunization is one of them. This review focuses on the stimulation of HIV-specific T cells and discusses in the context of the current 'state-of-art' of DNA vaccines, the areas where this technology might assist either alone or as a part of more complex vaccine formulations in the HIV vaccine development.

Comprehensive cross-clade neutralization analysis of a panel of anti-human immunodeficiency virus type 1 monoclonal antibodies

Broadly neutralizing monoclonal antibodies (MAbs) are potentially important tools in human immunodeficiency virus type 1 (HIV-1) vaccine design. A few rare MAbs have been intensively studied, but we still have a limited appreciation of their neutralization breadth. Using a pseudovirus assay, we evaluated MAbs from clade B-infected donors and a clade B HIV(+) plasma against 93 viruses from diverse backgrounds. Anti-gp120 MAbs exhibited greater activity against clade B than non-B viruses, whereas anti-gp41 MAbs exhibited broad interclade activity. Unexpectedly, MAb 4E10 (directed against the C terminus of the gp41 ectodomain) neutralized all 90 viruses with moderate potency. MAb 2F5 (directed against an epitope adjacent to that of 4E10) neutralized 67% of isolates, but none from clade C. Anti-gp120 MAb b12 (directed against an epitope overlapping the CD4 binding site) neutralized 50% of viruses, including some from almost every clade. 2G12 (directed against a high-mannose epitope on gp120) neutralized 41% of the viruses, but none from clades C or E. MAbs to the gp120 V3 loop, including 447-52D, neutralized a subset of clade B viruses (up to 45%) but infrequently neutralized other clades ( Collapse

Key Words Collapse

MESH Headings

• AIDS Vaccines/immunology
• Antibodies, Monoclonal/immunology
• HIV Antibodies/immunology
• HIV Envelope Protein gp120/immunology
• HIV Envelope Protein gp41/immunology
• HIV-1/immunology
• Humans
• Neutralization Tests
• Protein Conformation

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Grants

• R01 HL059725 NHLBI NIH HHS
• R37 AI033292 NIAID NIH HHS
• R01 AI033292 NIAID NIH HHS
• HL 59725 NHLBI NIH HHS
• P30 AI027742 NIAID NIH HHS
• AI 27742 NIAID NIH HHS
• AI33292 NIAID NIH HHS
• R01 AI093278 NIAID NIH HHS

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Affiliation(s)

James M Binley IMM2, Department of Immunology, The Scripps Research Institute, 10550 N. Torrey Pines Rd., La Jolla, CA 92037, USA
Terri Wrin
Bette Korber
Michael B Zwick
Meng Wang
Colombe Chappey
Gabriela Stiegler
Renate Kunert
Susan Zolla-Pazner
Hermann Katinger
Christos J Petropoulos
Dennis R Burton

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Collaborators Collapse

Kinetics of protective antibodies are determined by the viral surface antigen

Delayed and weak virus neutralizing antibody (nAb) responses represent a hallmark correlating not only with the establishment of persistent infection but also with unsuccessful vaccine development. Using a reverse genetic approach, we evaluated possible underlying mechanisms in 2 widely studied viral infection models. Swapping the glycoproteins (GPs) of lymphocytic choriomeningitis virus (LCMV, naturally persisting, noncytolytic, inefficient nAb inducer) and vesicular stomatitis virus (VSV, nonpersisting, cytolytic, potent nAb inducer) transferred the only target of nAb's from either virus to the other. We analyzed the nAb response to each of the 2 recombinant and parent viruses in infected mice and found that nAb kinetics were solely determined by the viral surface GP and not by the virus backbone. Moreover, the slowly and poorly nAb-triggering LCMV virion was a potent immunogenic matrix for the more antigenic VSV-GP. These findings indicate that the viral GP determines nAb kinetics largely independently of the specific viral infection context. They further suggest that structural features of viral GPs or coevolutionary adaptation of the virus's GP to the host's naive B cell repertoire, or both, may critically limit nAb kinetics and improvement of vaccine efficacy.

Potency of HIV-1 envelope glycoprotein gp120 antibodies to inhibit the interaction of DC-SIGN with HIV-1 gp120

The interaction of DC-SIGN with gp120 provides an attractive target for intervention of HIV-1 transmission. Here, we have investigated the potency of gp120 antibodies to inhibit the DC-SIGN-gp120 interaction. We demonstrate that although the V3 loop is not essential for DC-SIGN binding, antibodies against the V3 loop partially inhibit DC-SIGN binding, suggesting that these antibodies sterically hinder DC-SIGN binding to gp120. Polyclonal antibodies raised against non-glycosylated gp120 inhibited both low and high avidity DC-SIGN-gp120 interactions in contrast to polyclonal antibodies raised against glycosylated gp120. Thus, glycans present on gp120 may prevent the generation of antibodies that block the DC-SIGN-gp120 interactions. Moreover, the polyclonal antibodies against non-glycosylated gp120 efficiently inhibited HIV-1 capture by both DC-SIGN transfectants and immature dendritic cells. Therefore, non-glycosylated gp120 may be an attractive immunogen to elicit gp120 antibodies that block the binding to DC-SIGN. Furthermore, we demonstrate that DC-SIGN binding to gp120 enhanced CD4 binding, suggesting that DC-SIGN induces conformational changes in gp120, which may provide new targets for neutralizing antibodies.

Structure and mechanistic analysis of the anti-human immunodeficiency virus type 1 antibody 2F5 in complex with its gp41 epitope

The membrane-proximal region of the ectodomain of the gp41 envelope glycoprotein of human immunodeficiency virus type 1 (HIV-1) is the target of three of the five broadly neutralizing anti-HIV-1 antibodies thus far isolated. We have determined crystal structures of the antigen-binding fragment for one of these antibodies, 2F5, in complex with 7-mer, 11-mer, and 17-mer peptides of the gp41 membrane-proximal region, at 2.0-, 2.1-, and 2.2-A resolutions, respectively. The structures reveal an extended gp41 conformation, which stretches over 30 A in length. Contacts are made with five complementarity-determining regions of the antibody as well as with nonpolymorphic regions. Only one exclusive charged face of the gp41 epitope is bound by 2F5, while the nonbound face, which is hydrophobic, may be hidden due to occlusion by other portions of the ectodomain. The structures reveal that the 2F5 antibody is uniquely built to bind to an epitope that is proximal to a membrane surface and in a manner mostly unaffected by large-scale steric hindrance. Biochemical studies with proteoliposomes confirm the importance of lipid membrane and hydrophobic context in the binding of 2F5 as well as in the binding of 4E10, another broadly neutralizing antibody that recognizes the membrane-proximal region of gp41. Based on these structural and biochemical results, immunization strategies for eliciting 2F5- and 4E10-like broadly neutralizing anti-HIV-1 antibodies are proposed.

Entropy calculation of HIV-1 Env gp120, its receptor CD4, and their complex: an analysis of configurational entropy changes upon complexation

The HIV-1 gp120/CD4 interaction shows a large, unprecedented entropy/enthalpy compensation, with the capacity to fine-tune recognition over a broad range of affinity. The intermolecular interaction involves stable hydrophobic contacts with a unique protruding CD4-Phe43 structure surrounded by an intermolecular hydrogen-bond network that covers the hemisphere of the CD4 D1 domain. We have applied a heuristic formula based on the covariance matrix of atom-positional fluctuations to assess the configurational entropy of the gp120/CD4 complex at different levels. The system was dissected into various subsets of atoms to evaluate the entropic contributions of different functional elements. By combining the trajectories of the free and complex forms, further insight into the conformational sampling was extracted. Despite the limited sampling time of 10 ns, the theoretically derived changes in configurational entropy are in fair agreement with the experimentally determined data. The simultaneous evaluation of different interaction modes through a decomposition approach is only feasible with the knowledge of the atomic trajectory of the system. The configurational entropy analysis in terms of combined trajectories presented here shall potentially provide accurate estimations of thermodynamic properties of biomolecules given sufficient sampling of conformational space.

Correlates of immune protection in HIV-1 infection: what we know, what we don't know, what we should know

The field of vaccinology began in ignorance of how protection was instilled in vaccine recipients. Today, a greater knowledge of immunology allows us to better understand what is being stimulated by various vaccines that leads to their protective effects: that is, their correlates of protection. Here we describe what is known about the correlates of protection for existing vaccines against a range of different viral diseases and discuss the correlates of protection against disease during natural infection with HIV-1. We will also discuss why it is important to design phase 3 clinical trials of HIV vaccines to determine the correlates of protection for each individual vaccine.

Determinants within gp120 and gp41 contribute to CD4 independence of SIV Envs

Entry of simian immunodeficiency virus (SIV) into cells is mediated by binding of the viral envelope (Env) glycoprotein to cellular CD4 and chemokine receptor molecules. Interaction of the Env gp120 subunit with CD4 induces conformational changes that result in exposure of a conserved coreceptor binding site. The chemokine receptor CCR5 is the major coreceptor used for SIV entry. Many SIV Envs have the ability to bind directly to CCR5 in the absence of CD4, and CD4-independent SIVs have been shown to exhibit macrophage tropism, enhanced neutralization sensitivity, and reduced pathogenicity in nonhuman primates. SIVmac239 is a pathogenic, T-tropic, neutralization-resistant virus which encodes a CD4-dependent Env. By contrast, the SIVmac316 virus, which differs from 239 in Env by only eight amino acid substitutions and a gp41 cytoplasmic domain truncation, exhibits macrophage tropism in vitro, attenuated pathogenesis, neutralization sensitivity, and CD4-independent entry. We mapped the residues contributing to CD4-independent entry to substitutions at position 165 in the V1/V2 region of gp120 and position 573 of gp41. We find that substitution of both residues in replication-competent SIVmac239 virus results in gain of CD4 independence and enhanced neutralization sensitivity. By contrast, the converse substitutions placed in the background of SIVmac316 resulted in loss of CD4 independence and decreased neutralization sensitivity. Thus, as few as two amino acid changes can have dramatic effects on SIV Env phenotype.

Inhibition of hepatitis C virus-like particle binding to target cells by antiviral antibodies in acute and chronic hepatitis C

Hepatitis C virus (HCV) is a leading cause of chronic viral hepatitis worldwide. The study of antibody-mediated virus neutralization has been hampered by the lack of an efficient and high-throughput cell culture system for the study of virus neutralization. The HCV structural proteins have been shown to assemble into noninfectious HCV-like particles (HCV-LPs). Similar to serum-derived virions, HCV-LPs bind and enter human hepatocytes and hepatoma cell lines. In this study, we developed an HCV-LP-based model system for a systematic functional analysis of antiviral antibodies from patients with acute or chronic hepatitis C. We demonstrate that cellular HCV-LP binding was specifically inhibited by antiviral antibodies from patients with acute or chronic hepatitis C in a dose-dependent manner. Using a library of homologous overlapping envelope peptides covering the entire HCV envelope, we identified an epitope in the N-terminal E2 region (SQKIQLVNTNGSWHI; amino acid positions 408 to 422) as one target of human antiviral antibodies inhibiting cellular particle binding. Using a large panel of serum samples from patients with acute and chronic hepatitis C, we demonstrated that the presence of antibodies with inhibition of binding activity was not associated with viral clearance. In conclusion, antibody-mediated inhibition of cellular HCV-LP binding represents a convenient system for the functional characterization of human anti-HCV antibodies, allowing the mapping of envelope neutralization epitopes targeted by naturally occurring antiviral antibodies.

Evolution of two amino acid positions governing broad neutralization resistance in a strain of feline immunodeficiency virus over 7 years of persistence in cats

Fresh isolates of lentiviruses are characterized by an outstanding resistance to antibody-mediated neutralization. By investigating the changes that occurred in a neutralization-sensitive tissue culture-adapted strain of feline immunodeficiency virus after it was reinoculated into cats, a previous study had identified two amino acid positions of the surface glycoprotein (residues 481 and 557) which govern broad neutralization resistance (BNR) in this virus. By extending the follow-up of six independently evolving in vivo variants of such virus for up to 92 months, we now show that the changes at the two BNR-governing positions not only were remarkably stereotyped but also became fixed in an ordered sequential fashion with the duration of in vivo infection. In one variant, the two positions were also seen to slowly alternate at determining BNR. Evidence that evolution at the BNR-governing positions was accompanied, and possibly driven, by changes in the antigenic makeup of the viral surface brought about by the mutations at such positions is also presented.

Immunofocusing: antigen engineering to promote the induction of HIV-neutralizing antibodies

Understanding how neutralizing antibodies recognize HIV could aid the design of antigens that induce protective antibodies upon immunization. There have been several advances in this area of AIDS vaccine research, including structural elucidation of the core of two gp120 envelope glycoproteins and, more recently, determination of the structures of the broadly HIV-1-neutralizing antibodies b12, 2G12 and 2F5. The structures have enabled features governing the molecular interaction of each antibody with its epitope to be explored. For the antibody b12, this has resulted in the design of novel, tailored antigens aimed at inducing similar antibodies with equivalent neutralizing properties. This template-based approach to immunogen design shows promise as a means to engineer innovative AIDS vaccine candidates.

Essential role for virus-neutralizing antibodies in sterilizing immunity against Friend retrovirus infection

The current experiments use the Friend retrovirus model to demonstrate that vaccine-primed B cells are essential for sterilizing immunity, and the results indicate that the requisite function of these cells is the production of virus-neutralizing antibodies rather than priming or reactivation of T cells. B cell-deficient mice were poorly protected by vaccination, but adoptive transfer experiments showed that the T cells from B cell-deficient mice were primed as well as those from wild-type mice. Furthermore, passive transfer of virus-neutralizing antibodies completely compensated for B cell deficiency. The presence of virus-neutralizing antibodies at the time of infection was crucial for vaccine efficacy. Interestingly, virus-neutralizing antibodies worked synergistically with vaccine-primed T cells to provide a level of protection many orders of magnitude greater than either antibodies or immune T cells alone. Nonneutralizing antibodies also contributed to protection and acted cooperatively with neutralizing antibodies to reduce infection levels. These results emphasize the importance of inducing both T cell responses and virus-neutralizing antibody responses for effective retroviral vaccine protection.

New simian immunodeficiency virus infecting De Brazza's monkeys (Cercopithecus neglectus): evidence for a cercopithecus monkey virus clade

Nearly complete sequences of simian immunodeficiency viruses (SIVs) infecting 18 different nonhuman primate species in sub-Saharan Africa have now been reported; yet, our understanding of the origins, evolutionary history, and geographic distribution of these viruses still remains fragmentary. Here, we report the molecular characterization of a lentivirus (SIVdeb) naturally infecting De Brazza's monkeys (Cercopithecus neglectus). Complete SIVdeb genomes (9,158 and 9227 bp in length) were amplified from uncultured blood mononuclear cell DNA of two wild-caught De Brazza's monkeys from Cameroon. In addition, partial pol sequences (650 bp) were amplified from four offspring of De Brazza's monkeys originally caught in the wild in Uganda. Full-length (9068 bp) and partial pol (650 bp) SIVsyk sequences were also amplified from Sykes's monkeys (Cercopithecus albogularis) from Kenya. Analysis of these sequences identified a new SIV clade (SIVdeb), which differed from previously characterized SIVs at 40 to 50% of sites in Pol protein sequences. The viruses most closely related to SIVdeb were SIVsyk and members of the SIVgsn/SIVmus/SIVmon group of viruses infecting greater spot-nosed monkeys (Cercopithecus nictitans), mustached monkeys (Cercopithecus cephus), and mona monkeys (Cercopithecus mona), respectively. In phylogenetic trees of concatenated protein sequences, SIVdeb, SIVsyk, and SIVgsn/SIVmus/SIVmon clustered together, and this relationship was highly significant in all major coding regions. Members of this virus group also shared the same number of cysteine residues in their extracellular envelope glycoprotein and a high-affinity AIP1 binding site (YPD/SL) in their p6 Gag protein, as well as a unique transactivation response element in their viral long terminal repeat; however, SIVdeb and SIVsyk, unlike SIVgsn, SIVmon, and SIVmus, did not encode a vpu gene. These data indicate that De Brazza's monkeys are naturally infected with SIVdeb, that this infection is prevalent in different areas of the species' habitat, and that geographically diverse SIVdeb strains cluster in a single virus group. The consistent clustering of SIVdeb with SIVsyk and the SIVmon/SIVmus/SIVgsn group also suggests that these viruses have evolved from a common ancestor that likely infected a Cercopithecus host in the distant past. The vpu gene appears to have been acquired by a subset of these Cercopithecus viruses after the divergence of SIVdeb and SIVsyk.

Human immunodeficiency virus type 1 evades T-helper responses by exploiting antibodies that suppress antigen processing

T-helper responses are important for controlling chronic viral infections, yet T-helper responses specific to human immunodeficiency virus type 1 (HIV-1), particularly to envelope glycoproteins, are lacking in the vast majority of HIV-infected individuals. It was previously shown that the presence of antibodies to the CD4-binding domain (CD4bd) of HIV-1 glycoprotein 120 (gp120) prevents T-helper responses to gp120, but their suppressive mechanisms were undefined (C. E. Hioe et al., J. Virol. 75:10950-10957, 2001). The present study demonstrates that gp120, when complexed to anti-CD4bd antibodies, becomes more resistant to proteolysis by lysosomal enzymes from antigen-presenting cells such that peptide epitopes are not released and presented efficiently by major histocompatibility complex class II molecules to gp120-specific CD4 T cells. Antibodies to other gp120 regions do not confer this effect. Thus, HIV may evade anti-viral T-helper responses by inducing and exploiting antibodies that conceal the virus envelope antigens from T cells.

Prospects for an AIDS vaccine: three big questions, no easy answers

The unremitting devastation created by the AIDS pandemic will probably only be controlled when a vaccine is developed that is safe, effective, affordable, and simple enough to permit implementation in developing countries where the impact of AIDS is most severe. Although formidable practical, political, economic, social, and ethical challenges face the AIDS vaccine development effort, the most fundamental challenges now reside at the level of the basic biology of HIV-1 infection and pathogenesis. Of these biological considerations, three questions loom especially large: can we design immunogens that will elicit neutralising antibodies that are reactive against a wide variety of primary HIV isolates; will vaccine-elicited cytotoxic T cells be fundamentally better at controlling HIV-1 replication and ameliorating disease progression than those responses that arise during natural HIV infection; and to what extent will the tremendous global genetic diversity of HIV-1 compromise the breadth of vaccine-elicited protective immunity and the overall effectiveness of an AIDS vaccine? Although these are three exceptionally challenging questions, they are now being approached with clear hypotheses whose testing is being facilitated by an ever-improving array of technologies for vaccine design and immunological characterisation. The extent to which the field of AIDS vaccine research can now come together to answer these questions in the best coordinated, most efficient manner will probably be an important determinant of how and when an effective AIDS vaccine will be developed.

Engineering RENTA, a DNA prime-MVA boost HIV vaccine tailored for Eastern and Central Africa

For the development of human immunodeficiency virus type 1 (HIV-1) vaccines, traditional approaches inducing virus-neutralizing antibodies have so far failed. Thus the effort is now focused on elicitation of cellular immunity. We are currently testing in clinical trials in the United Kingdom and East Africa a T-cell vaccine consisting of HIV-1 clade A Gag-derived immunogen HIVA delivered in a prime-boost regimen by a DNA plasmid and modified vaccinia virus Ankara (MVA). Here, we describe engineering and preclinical development of a second immunogen RENTA, which will be used in combination with the present vaccine in a four-component DNA/HIVA-RENTA prime-MVA/HIVA-RENTA boost formulation. RENTA is a fusion protein derived from consensus HIV clade A sequences of Tat, reverse transcriptase, Nef and gp41. We inactivated the natural biological activities of the HIV components and confirmed immunogenicities of the pTHr.RENTA and MVA.RENTA vaccines in mice. Furthermore, we demonstrated in mice and rhesus monkeys broadening of HIVA-elicited T-cell responses by a parallel induction of HIVA- and RENTA-specific responses recognizing multiple HIV epitopes.

Mechanisms of HIV-1 escape from immune responses and antiretroviral drugs

Despite the fact that HIV-1 induces vigorous antiviral immune responses, viral replication is never completely controlled in infected individuals. Recent studies have provided insight into the mechanisms by which focused immune pressure directed at particular B or T cell epitopes leads to the rapid appearance of escape mutations. Even if anti-HIV-1 immune responses could be enhanced to the point where they inhibit viral replication to the same extent as certain combinations of antiretroviral drugs, eradication would be unlikely because of the persistence of the virus in an extremely stable latent reservoir in resting memory CD4(+) T cells.

Atomic insight into the CD4 binding-induced conformational changes in HIV-1 gp120

The entry of HIV-1 into a target cell requires gp120 and receptor CD4 as well as coreceptor CCR5/CXCR4 recognition events associated with conformational changes of the involved proteins. The binding of CD4 to gp120 is the initiation step of the whole process involving structural rearrangements that are crucial for subsequent pathways. Despite the wealth of knowledge about the gp120/CD4 interactions, details of the conformational changes occurring at this stage remain elusive. We have performed molecular dynamics simulations in explicit solvent based on the gp120/CD4/CD4i crystal structure in conjunction with modeled V3 and V4 loops to gain insight into the dynamics of the binding process. Three differentiated interaction modes between CD4 and gp120 were found, which involve electrostatics, hydrogen bond and van der Waals networks. A "binding funnel" model is proposed based on the dynamical nature of the binding interface together with a CD4-attraction gradient centered in gp120 at the CD4-Phe43-binding cavity. Distinct dynamical behaviors of free and CD4-bound gp120 were monitored, which likely represent the ground and pre-fusogenic states, respectively. The transition between these states revealed concerted motions in gp120 leading to: i) loop contractions around the CD4-Phe43-insertion cavity; ii) stabilization of the four-stranded "bridging sheet" structure; and iii) translocation and clustering of the V3 loop and the bridging sheet leading to the formation of the coreceptor binding site. Our results provide new insight into the dynamic of the underlying molecular recognition mechanism that complements the biochemical and structural studies.

Oligosaccharide and Glycoprotein Microarrays as Tools in HIV Glycobiology

Defining HIV envelope glycoprotein interactions with host factors or binding partners advances our understanding of the infectious process and provides a basis for the design of vaccines and agents that interfere with HIV entry. Here we employ carbohydrate and glycoprotein microarrays to analyze glycan-dependent gp120-protein interactions. In concert with new linking chemistries and synthetic methods, the carbohydrate arrays combine the advantages of microarray technology with the flexibility and precision afforded by organic synthesis. With these microarrays, we individually and competitively determined the binding profiles of five gp120 binding proteins, established the carbohydrate structural requirements for these interactions, and identified a potential strategy for HIV vaccine development.

Development of a homology model for clade A human immunodeficiency virus type 1 gp120 to localize temporal substitutions arising in recently infected women

The virus population transmitted by a human immunodeficiency virus type 1 (HIV-1) infected individual undergoes restriction and subsequent diversification in the new host. However, in contrast to men, who have limited virus diversity at seroconversion, there is measurable diversity in viral envelope gene sequences in women infected with clade A HIV-1. In this study, virus sequence diversity in three unrelated, clade A infected women preceding and shortly after seroconversion was evaluated. It was demonstrated that there is measurable evolution of envelope gene sequences over this time interval. Furthermore, in each of the three individuals, amino acid substitutions arose at five or six positions in sequences derived at or shortly after seroconversion relative to sequences obtained from the seronegative sample. Presented here is a model of clade A gp120 to determine the location of substitutions that appeared as the virus population became established in three clade A HIV-1 infected women.

Immunogenicity of constrained monoclonal antibody A32-human immunodeficiency virus (HIV) Env gp120 complexes compared to that of recombinant HIV type 1 gp120 envelope glycoproteins

One strategy for the generation of broadly reactive neutralizing antibodies (NA) against human immunodeficiency virus type 1 (HIV-1) primary isolates is to use immunogens that have constrained HIV-1 envelope gp120 conformations reflective of triggered envelope on the surface of virions. A major change in gp120 following binding to CD4 is the enhanced exposure of the CCR5 binding site. One inducer of CCR5 binding site epitopes on gp120 is the human anti-gp120 monoclonal antibody, A32. We have made cross-linked A32-rgp120(89.6) and A32-rgp120(BaL) complexes and have compared their immunogenicities to those of uncomplexed recombinant gp120(BaL) (rgp120(BaL)) and rgp120(89.6). A32-rgp120(89.6) and A32-rgp120(BaL) complexes had stable induced CCR5 binding site expression compared to that of uncomplexed rgp120s. However, the A32-rgp120 complexes had similar capacities in guinea pigs for induction of NA against HIV-1 primary isolates versus that of rgp120 alone. A32-rgp120(89.6) induced antibodies that neutralized 6 out of 11 HIV-1 isolates, while rgp120(89.6) alone induced antibodies that neutralized 4 out of 11 HIV-1 isolates. A32-rgp120(BaL) complexes induced antibodies that neutralized 4 out of 14 HIV-1 isolates while, surprisingly, non-cross-linked rgp120(BaL) induced antibodies that neutralized 9 out of 14 (64%) HIV-1 isolates. Thus, stable enhanced expression of the coreceptor binding site on constrained gp120 is not sufficient for inducing broadly neutralizing anti-HIV-1 NA. Moreover, the ability of HIV-1 rgp120(BaL) to induce antibodies that neutralized approximately 60% of subtype B HIV-1 isolates warrants consideration of using HIV-1 BaL as a starting point for immunogen design for subtype B HIV-1 experimental immunogens.

The V1/V2 domain of gp120 is a global regulator of the sensitivity of primary human immunodeficiency virus type 1 isolates to neutralization by antibodies commonly induced upon infection

A major problem hampering the development of an effective vaccine against human immunodeficiency virus type 1 (HIV-1) is the resistance of many primary viral isolates to antibody-mediated neutralization. To identify factors responsible for this resistance, determinants of the large differences in neutralization sensitivities of HIV-1 pseudotyped with Env proteins derived from two prototypic clade B primary isolates were mapped. SF162 Env pseudotypes were neutralized very potently by a panel of sera from HIV-infected individuals, while JR-FL Env pseudotypes were neutralized by only a small fraction of these sera. This differential sensitivity to neutralization was also observed for a number of monoclonal antibodies (MAbs) directed against sites in the V2, V3, and CD4 binding domains, despite often similar binding affinities of these MAbs towards the two soluble rgp120s. The neutralization phenotypes were switched for chimeric Envs in which the V1/V2 domains of these two sequences were exchanged, indicating that the V1/V2 region regulated the overall neutralization sensitivity of these Envs. These results suggested that the inherent neutralization resistance of JR-FL, and presumably of related primary isolates, is to a great extent mediated by gp120 V1/V2 domain structure rather than by sequence variations at the target sites. Three MAbs (immunoglobulin G-b12, 2G12, and 2F5) previously reported to possess broad neutralizing activity for primary HIV-1 isolates neutralized JR-FL virus at least as well as SF162 virus and were not significantly affected by the V1/V2 domain exchanges. The rare antibodies capable of neutralizing a broad range of primary isolates thus appeared to be targeted to exceptional epitopes that are not sensitive to V1/V2 domain regulation of neutralization sensitivity.