A vaccine for HIV type 1: the antibody perspective

Antiviral neutralizing antibodies: from in vitro to in vivo activity

Neutralizing antibodies (nAbs) are being increasingly used as passive antiviral reagents in prophylactic and therapeutic modalities and to guide viral vaccine design. In vivo, nAbs can mediate antiviral functions through several mechanisms, including neutralization, which is defined by in vitro assays in which nAbs block viral entry to target cells, and antibody effector functions, which are defined by in vitro assays that evaluate nAbs against viruses and infected cells in the presence of effector systems. Interpreting in vivo results in terms of these in vitro assays is challenging but important in choosing optimal passive antibody and vaccine strategies. Here, I review findings from many different viruses and conclude that, although some generalizations are possible, deciphering the relative contributions of different antiviral mechanisms to the in vivo efficacy of antibodies currently requires consideration of individual antibody-virus interactions.

Antigen pressure from two founder viruses induces multiple insertions at a single antibody position to generate broadly neutralizing HIV antibodies

Vaccination strategies aimed at maturing broadly neutralizing antibodies (bnAbs) from naïve precursors are hindered by unusual features that characterize these Abs, including insertions and deletions (indels). Longitudinal studies of natural HIV infection cases shed light on the complex processes underlying bnAb development and have suggested a role for superinfection as a potential enhancer of neutralization breadth. Here we describe the development of a potent bnAb lineage that was elicited by two founder viruses to inform vaccine design. The V3-glycan targeting bnAb lineage (PC39-1) was isolated from subtype C-infected IAVI Protocol C elite neutralizer, donor PC39, and is defined by the presence of multiple independent insertions in CDRH1 that range from 1-11 amino acids in length. Memory B cell members of this lineage are predominantly atypical in phenotype yet also span the class-switched and antibody-secreting cell compartments. Development of neutralization breadth occurred concomitantly with extensive recombination between founder viruses before each virus separated into two distinct population "arms" that evolved independently to escape the PC39-1 lineage. Ab crystal structures show an extended CDRH1 that can help stabilize the CDRH3. Overall, these findings suggest that early exposure of the humoral system to multiple related Env molecules could promote the induction of bnAbs by focusing Ab responses to conserved epitopes.

Mining for humoral correlates of HIV control and latent reservoir size

While antiretroviral therapy (ART) has effectively revolutionized HIV care, the virus is never fully eliminated. Instead, immune dysfunction, driven by persistent non-specific immune activation, ensues and progressively leads to premature immunologic aging. Current biomarkers monitoring immunologic changes encompass generic inflammatory biomarkers, that may also change with other infections or disease states, precluding the antigen-specific monitoring of HIV-infection associated changes in disease. Given our growing appreciation of the significant changes in qualitative and quantitative properties of disease-specific antibodies in HIV infection, we used a systems approach to explore humoral profiles associated with HIV control. We found that HIV-specific antibody profiles diverge by spontaneous control of HIV, treatment status, viral load and reservoir size. Specifically, HIV-specific antibody profiles representative of changes in viral load were largely quantitative, reflected by differential HIV-specific antibody levels and Fc-receptor binding. Conversely, HIV-specific antibody features that tracked with reservoir size exhibited a combination of quantitative and qualitative changes marked by more distinct subclass selection profiles and unique HIV-specific Fc-glycans. Our analyses suggest that HIV-specific antibody Fc-profiles provide antigen-specific resolution on both cell free and cell-associated viral loads, pointing to potentially novel biomarkers to monitor reservoir activity.

Current combination antiretroviral therapy (ART) regimens have reversed the death sentence once associated with an HIV diagnosis. However, the virus is never fully eliminated. Rather, latently infected cells with integrated virus (latent reservoir) persist and the virus rebounds rapidly upon discontinuation of therapy. Further, even for those on ART, immune dysfunction, driven by persistent non-specific immune activation, ensues and progressively leads to premature immunologic aging. Current biomarkers monitoring these changes are non-specific–they focus on generic inflammatory changes that may also track with other infections or disease states. In this manuscript, we used an unbiased analytical systems approach to identify antigen-specific biomarkers of HIV disease state/treatment status, active viremia and the latent reservoir. By virtue of them being antigen-specific, these are robust context-specific biomarkers of HIV disease progression, viremia and reservoir size. Our framework highlights the strength of using systems approaches in identifying humoral biomarkers, and can be used in other contexts to identify antigen-specific correlates of infectious disease outcome.

Passive immunotherapy of viral infections: 'super-antibodies' enter the fray

Antibodies have been used for over a century prophylactically and, less often, therapeutically against viruses.

'Super-antibodies' — a new generation of highly potent and/or broadly cross-reactive human monoclonal antibodies — offer new opportunities for prophylaxis and therapy of viral infections.

Super-antibodies are typically generated infrequently and/or in a limited number of individuals during natural infections.

Isolation of these antibodies has primarily been achieved by large-scale screening for suitable donors and new single B cell approaches to human monoclonal antibody generation.

Super-antibodies may offer the possibility of treating multiple viruses of a given family with a single reagent. They are also valuable templates for rational vaccine design.

The great potency of super-antibodies has many advantages for practical development as therapeutic reagents. These advantages can be enhanced by a variety of antibody engineering technologies.

So-called super-antibodies are highly potent, broadly reactive antiviral antibodies that offer promise for the treatment of various chronic and emerging viruses. This Review describes how recent technological advances led to their isolation from rare, infected individuals and their development for the prevention and treatment of various viral infections.

Antibodies have been used for more than 100 years in the therapy of infectious diseases, but a new generation of highly potent and/or broadly cross-reactive human monoclonal antibodies (sometimes referred to as 'super-antibodies') offers new opportunities for intervention. The isolation of these antibodies, most of which are rarely induced in human infections, has primarily been achieved by large-scale screening for suitable donors and new single B cell approaches to human monoclonal antibody generation. Engineering the antibodies to improve half-life and effector functions has further augmented their in vivo activity in some cases. Super-antibodies offer promise for the prophylaxis and therapy of infections with a range of viruses, including those that are highly antigenically variable and those that are newly emerging or that have pandemic potential. The next few years will be decisive in the realization of the promise of super-antibodies.

Sensitivity of immune response quality to influenza helix 190 antigen structure displayed on a modular virus-like particle

Biomolecular engineering enables synthesis of improved proteins through synergistic fusion of modules from unrelated biomolecules. Modularization of peptide antigen from an unrelated pathogen for presentation on a modular virus-like particle (VLP) represents a new and promising approach to synthesize safe and efficacious vaccines. Addressing a key knowledge gap in modular VLP engineering, this study investigates the underlying fundamentals affecting the ability of induced antibodies to recognize the native pathogen. Specifically, this quality of immune response is correlated to the peptide antigen module structure. We modularized a helical peptide antigen element, helix 190 (H190) from the influenza hemagglutinin (HA) receptor binding region, for presentation on murine polyomavirus VLP, using two strategies aimed to promote H190 helicity on the VLP. In the first strategy, H190 was flanked by GCN4 structure-promoting elements within the antigen module; in the second, dual H190 copies were arrayed as tandem repeats in the module. Molecular dynamics simulation predicted that tandem repeat arraying would minimize secondary structural deviation of modularized H190 from its native conformation. In vivo testing supported this finding, showing that although both modularization strategies conferred high H190-specific immunogenicity, tandem repeat arraying of H190 led to a strikingly higher immune response quality, as measured by ability to generate antibodies recognizing a recombinant HA domain and split influenza virion. These findings provide new insights into the rational engineering of VLP vaccines, and could ultimately enable safe and efficacious vaccine design as an alternative to conventional approaches necessitating pathogen cultivation.

Structure based sequence analysis & epitope prediction of gp41 HIV1 envelope glycoprotein isolated in Pakistan

BACKGROUND

Gp41 is an envelope glycoprotein of human immune deficiency virus (HIV). HIV viral glycoprotein gp41, present in complex with gp120, assists the viral entry into host cell. Over eighty thousands individuals are HIV infected in Pakistan which makes about 0.2% of 38.6 million infected patients worldwide. Hence, HIV gp41 protein sequences isolated in Pakistan were analyzed for the CD4 and CD8 T cells binding epitopes.

RESULTS

Immunoinformatics tools were applied for the study of variant region of HIV gp41envelope protein. The protein nature was analyzed using freely accessible computational software. About 90 gp41 sequences of Pakistani origin were aligned and variable and conserved regions were found. Four segments were found to be conserved in gp41 viral protein. A method was developed, involving the secondary structure, surface accessibility, hydrophobicity, antigenicity and molecular docking for the prediction and location of epitopes in the viral glycoprotein. Some highly conserved CD4 and CD8 binding epitopes were also found using multiple parameters. The predicted continuous epitopes mostly fall in the conserved region of 1-12; 14-22 and 25-46 and can be used as effective vaccine candidates.

CONCLUSIONS

The study revealed potential HIV subtype a derived cytotoxic T cell (CTL) epitopes from viral proteome of Pakistani origin. The conserved epitopes are very useful for the diagnosis of the HIV 1 subtype a. This study will also help scientists to promote research for vaccine development against HIV 1 subtype a, isolated in Pakistan.

Vaccine protection against simian immunodeficiency virus in monkeys using recombinant gamma-2 herpesvirus

Recombinant strains of replication-competent rhesus monkey rhadinovirus (RRV) were constructed in which strong promoter/enhancer elements were used to drive expression of simian immunodeficiency virus (SIV) Env or Gag or a Rev-Tat-Nef fusion protein. Cultured rhesus monkey fibroblasts infected with each recombinant strain were shown to express the expected protein. Three RRV-negative and two RRV-positive rhesus monkeys were inoculated intravenously with a mixture of these three recombinant RRVs. Expression of SIV Gag was readily detected in lymph node biopsy specimens taken at 3 weeks postimmunization. Impressive anti-SIV cellular immune responses were elicited on the basis of major histocompatibility complex (MHC) tetramer staining and gamma interferon enzyme-linked immunospot (ELISPOT) assays. Responses were much greater in magnitude in the monkeys that were initially RRV negative but were still readily detected in the two monkeys that were naturally infected with RRV at the time of immunization. By 3 weeks postimmunization, responses measured by MHC tetramer staining in the two Mamu-A*01(+) RRV-negative monkeys reached 9.3% and 13.1% of all CD8(+) T cells in peripheral blood to the Gag CM9 epitope and 2.3% and 7.3% of all CD8(+) T cells in peripheral blood to the Tat SL8 epitope. Virus-specific CD8(+) T cell responses persisted at high levels up to the time of challenge at 18 weeks postimmunization, and responding cells maintained an effector memory phenotype. Despite the ability of the RRVenv recombinant to express high levels of Env in cultured cells, and despite the appearance of strong anti-RRV antibody responses in immunized monkeys, anti-Env antibody responses were below our ability to detect them. Immunized monkeys, together with three unimmunized controls, were challenged intravenously with 10 monkey infectious doses of SIVmac239. All five immunized monkeys and all three controls became infected with SIV, but peak viral loads were 1.2 to 3.0 log(10) units lower and chronic-phase viral loads were 1.0 to 3.0 log(10) units lower in immunized animals than the geometric mean of unimmunized controls. These differences were statistically significant. Anti-Env antibody responses following challenge indicated an anamnestic response in the vaccinated monkeys. These findings further demonstrate the potential of recombinant herpesviruses as preventive vaccines for AIDS. We hypothesize that this live, replication-competent, persistent herpesvirus vector could match, or come close to matching, live attenuated strains of SIV in the degree of protection if the difficulty with elicitation of anti-Env antibody responses can be overcome.

Structural basis of the cross-reactivity of genetically related human anti-HIV-1 mAbs: implications for design of V3-based immunogens

Human monoclonal antibodies 447-52D and 537-10D, both coded by the VH3 gene and specific for the third variable region (V3) of the HIV-1 gp120, were found to share antigen-binding structural elements including an elongated CDR H3 forming main-chain interactions with the N terminus of the V3 crown. However, water-mediated hydrogen bonds and a unique cation-pi sandwich stacking allow 447-52D to be broadly reactive with V3 containing both the GPGR and GPGQ crown motifs, while the deeper binding pocket and a buried Glu in the binding site of 537-10D limit its reactivity to only V3 containing the GPGR motif. Our results suggest that the design of immunogens for anti-V3 antibodies should avoid the Arg at the V3 crown, as GPGR-containing epitopes appear to select for B cells making antibodies of narrower specificity than V3 that carry Gln at this position.

Human immunodeficiency virus type 1 elite neutralizers: individuals with broad and potent neutralizing activity identified by using a high-throughput neutralization assay together with an analytical selection algorithm

The development of a rapid and efficient system to identify human immunodeficiency virus type 1 (HIV-1)-infected individuals with broad and potent HIV-1-specific neutralizing antibody responses is an important step toward the discovery of critical neutralization targets for rational AIDS vaccine design. In this study, samples from HIV-1-infected volunteers from diverse epidemiological regions were screened for neutralization responses using pseudovirus panels composed of clades A, B, C, and D and circulating recombinant forms (CRFs). Initially, 463 serum and plasma samples from Australia, Rwanda, Uganda, the United Kingdom, and Zambia were screened to explore neutralization patterns and selection ranking algorithms. Samples were identified that neutralized representative isolates from at least four clade/CRF groups with titers above prespecified thresholds and ranked based on a weighted average of their log-transformed neutralization titers. Linear regression methods selected a five-pseudovirus subset, representing clades A, B, and C and one CRF01_AE, that could identify top-ranking samples with 50% inhibitory concentration (IC(50)) neutralization titers of >or=100 to multiple isolates within at least four clade groups. This reduced panel was then used to screen 1,234 new samples from the Ivory Coast, Kenya, South Africa, Thailand, and the United States, and 1% were identified as elite neutralizers. Elite activity is defined as the ability to neutralize, on average, more than one pseudovirus at an IC(50) titer of 300 within a clade group and across at least four clade groups. These elite neutralizers provide promising starting material for the isolation of broadly neutralizing monoclonal antibodies to assist in HIV-1 vaccine design.

Net positive charge of HIV-1 CRF01_AE V3 sequence regulates viral sensitivity to humoral immunity

The third variable region (V3) of the human immunodeficiency virus type 1 (HIV-1) envelope gp120 subunit participates in determination of viral infection coreceptor tropism and host humoral immune responses. Positive charge of the V3 plays a key role in determining viral coreceptor tropism. Here, we examined by bioinformatics, experimental, and protein modelling approaches whether the net positive charge of V3 sequence regulates viral sensitivity to humoral immunity. We chose HIV-1 CRF01_AE strain as a model virus to address the question. Diversity analyses using CRF01_AE V3 sequences from 37 countries during 1984 and 2005 (n = 1361) revealed that reduction in the V3's net positive charge makes V3 less variable due to limited positive selection. Consistently, neutralization assay using CRF01_AE V3 recombinant viruses (n = 30) showed that the reduction in the V3's net positive charge rendered HIV-1 less sensitive to neutralization by the blood anti-V3 antibodies. The especially neutralization resistant V3 sequences were the particular subset of the CCR5-tropic V3 sequences with net positive charges of +2 to +4. Molecular dynamics simulation of the gp120 monomers showed that the V3's net positive charge regulates the V3 configuration. This and reported gp120 structural data predict a less-exposed V3 with a reduced net positive charge in the native gp120 trimer context. Taken together, these data suggest a key role of the V3's net positive charge in the immunological escape and coreceptor tropism evolution of HIV-1 CRF01_AE in vivo. The findings have molecular implications for the adaptive evolution and vaccine design of HIV-1.

Importance of the V1/V2 loop region of simian-human immunodeficiency virus envelope glycoprotein gp120 in determining the strain specificity of the neutralizing antibody response

Plasma samples from individuals infected with human immunodeficiency virus type 1 (HIV-1) are known to be highly strain specific in their ability to neutralize HIV-1 infectivity. Such plasma samples exhibit significant neutralizing activity against autologous HIV-1 isolates but typically exhibit little or no activity against heterologous strains, although some cross-neutralizing activity can develop late in infection. Monkeys infected with the simian-human immunodeficiency virus (SHIV) clone DH12 generated antibodies that neutralized SHIV DH12, but not SHIV KB9. Conversely, antibodies from monkeys infected with the SHIV clone KB9 neutralized SHIV KB9, but not SHIV DH12. To investigate the role of the variable loops of the HIV-1 envelope glycoprotein gp120 in determining this strain specificity, variable loops 1 and 2 (V1/V2), V3, or V4 were exchanged individually or in combination between SHIV DH12 and SHIV KB9. Despite the fact that both parental viruses exhibited significant infectivity and good replication in the cell lines examined, 3 of the 10 variable-loop chimeras exhibited such poor infectivity that they could not be used further for neutralization assays. These results indicate that a variable loop that is functional in the context of one particular envelope background will not necessarily function within another. The remaining seven replication-competent chimeras allowed unambiguous assignment of the sequences principally responsible for the strain specificity of the neutralizing activity present in SHIV-positive plasma. Exchange of the V1/V2 loop sequences conferred a dominant loss of sensitivity to neutralization by autologous plasma and a gain of sensitivity to neutralization by heterologous plasma. Substitution of V3 or V4 had little or no effect on the sensitivity to neutralization. These data demonstrate that the V1/V2 region of HIV-1 gp120 is principally responsible for the strain specificity of the neutralizing antibody response in monkeys infected with these prototypic SHIVs.

A glycoconjugate antigen based on the recognition motif of a broadly neutralizing human immunodeficiency virus antibody, 2G12, is immunogenic but elicits antibodies unable to bind to the self glycans of gp120

The glycan shield of human immunodeficiency virus type 1 (HIV-1) gp120 contributes to viral evasion from humoral immune responses. However, the shield is recognized by the HIV-1 broadly neutralizing antibody (Ab), 2G12, at a relatively conserved cluster of oligomannose glycans. The discovery of 2G12 raises the possibility that a carbohydrate immunogen may be developed that could elicit 2G12-like neutralizing Abs and contribute to an AIDS vaccine. We have previously dissected the fine specificity of 2G12 and reported that the synthetic tetramannoside (Man(4)) that corresponds to the D1 arm of Man(9)GlcNAc(2) inhibits 2G12 binding to gp120 as efficiently as Man(9)GlcNAc(2) itself, indicating the potential use of Man(4) as a building block for creating immunogens. Here, we describe the development of neoglycoconjugates displaying variable copy numbers of Man(4) on bovine serum albumin (BSA) molecules by conjugation to Lys residues. The increased valency enhances the apparent affinity of 2G12 for Man(4) up to a limit which is achieved at approximately 10 copies per BSA molecule, beyond which no further enhancement is observed. Immunization of rabbits with BSA-(Man(4))(14) elicits significant serum Ab titers to Man(4). However, these Abs are unable to bind gp120. Further analysis reveals that the elicited Abs bind a variety of unbranched and, to a lesser extent, branched Man(9) derivatives but not natural N-linked oligomannose containing the chitobiose core. These results suggest that Abs can be readily elicited against the D1 arm; however, potential differences in the presentation of Man(4) on neoglycoconjugates, compared to glycoproteins, poses challenges for eliciting anti-mannose Abs capable of cross-reacting with gp120 and HIV-1.

Nasal immunization with a recombinant HIV gp120 and nanoemulsion adjuvant produces Th1 polarized responses and neutralizing antibodies to primary HIV type 1 isolates

Epidemiological and experimental data suggest that both robust neutralizing antibodies and potent cellular responses play important roles in controlling primary HIV-1 infection. In this study we have investigated the induction of systemic and mucosal immune responses to HIV gp120 monomer immunogen administered intranasally in a novel, oil-in-water nanoemulsion (NE) adjuvant. Mice and guinea pigs intranasally immunized by the application of recombinant HIV gp120 antigen mixed in NE demonstrated robust serum anti-gp120 IgG, as well as bronchial, vaginal, and serum anti-gp120 IgA in mice. The serum of these animals demonstrated antibodies that cross-reacted with heterologous serotypes of gp120 and had significant neutralizing activity against two clade-B laboratory strains of HIV (HIVBaL and HIVSF162) and five primary HIV-1 isolates. The analysis of gp120-specific CTL proliferation, INF-gamma induction, and prevalence of anti-gp120 IgG2 subclass antibodies indicated that nasal vaccination in NE also induced systemic, Th1-polarized cellular immune responses. This study suggests that NE should be evaluated as a mucosal adjuvant for multivalent HIV vaccines.

HIV-1 DNA vaccine efficacy is enhanced by coadministration with plasmid encoding IFN-α

Numerous strategies have been employed in an attempt to improve the immunogenicity and efficacy of nucleic acid vaccines. In the present study, the immunogenicity in the induction of humoral and cellular immune responses to HIV-1 DNA vaccine expressing a chimeric gene of gag and gp120 and the adjuvant effect of IFN-alpha on HIV-1 DNA vaccine were studied in a murine model. The DNA vaccine plasmid pVAX1-gag-gp120 and eukaryotic expression plasmid pVAX1-IFN were constructed by inserting the chimeric gene of gag and gp120 of HIV-1 and IFN-alpha into the downstream of CMV promoter of eukaryotic expression vector pVAX1, respectively. In vitro expression detected by RT-PCR and Western blotting showed that the genes of interest could be expressed in transfected HeLa cells. After BALB/c mice were immunized by three intramuscular inoculations of the HIV-1 DNA vaccine plasmids alone or in combination with IFN-alpha expression plasmids, the different levels of anti-HIV-1 humoral and cellular responses were measured comparable to the control groups immunized with pVAX1-IFN, parent plasmid pVAX1 or PBS. The percentage of CD3+CD4+ and CD3+CD8+ subgroups of spleen T lymphocytes and the specific cytotoxicity activities of splenic CTLs in the coinoculation group were significantly higher than those in the separate inoculation group, and an enhancement of antibody response was also observed in the coinoculation group compared with the separate inoculation group. Take together, coadministration of HIV-1 DNA vaccine plasmids and IFN-alpha expression plasmids can elicit stronger humoral and cellular immune responses in mice than HIV-1 DNA vaccine plasmids alone, and IFN-alpha can be an effective immunological adjuvant in DNA vaccination against HIV-1.

Modulation of human immunodeficiency virus type 1 infectivity through incorporation of tetraspanin proteins

Accumulating evidence indicates that human immunodeficiency virus type 1 (HIV-1) acquires various cellular membrane proteins in the lipid bilayer of the viral envelope membrane. Although some virion-incorporated cellular membrane proteins are known to potently affect HIV-1 infectivity, the virological functions of most virion-incorporated membrane proteins remain unclear. Among these host proteins, we found that CD63 was eliminated from the plasma membranes of HIV-1-producing T cells after activation, followed by a decrease in the amount of virion-incorporated CD63, and in contrast, an increase in the infectivity of the released virions. On the other hand, we found that CD63 at the cell surface was preferentially embedded on the membrane of released virions in an HIV-1 envelope protein (Env)-independent manner and that virion-incorporated CD63 had the potential to inhibit HIV-1 Env-mediated infection in a strain-specific manner at the postattachment entry step(s). In addition, these behaviors were commonly observed in other tetraspanin proteins, such as CD9, CD81, CD82, and CD231. However, L6 protein, whose topology is similar to that of tetraspanins but which does not belong to the tetraspanin superfamily, did not have the potential to prevent HIV-1 infection, despite its successful incorporation into the released particles. Taken together, these results suggest that tetraspanin proteins have the unique potential to modulate HIV-1 infectivity through incorporation into released HIV-1 particles, and our findings may provide a clue to undiscovered aspects of HIV-1 entry.

Carbohydrate-binding agents: a potential future cornerstone for the chemotherapy of enveloped viruses?

Carbohydrate-binding agents (CBAs) inhibit HIV-1 and it is proposed that therapy with such agents may have important implications for the future of anti-HIV therapy. Examples of CBAs include the procaryotic cyanovirin-N (CV-N), plant lectins such as HHA, GNA, NPA, CA and UDA, the monoclonal antibody 2G12 directed against a glycan-containing epitope on HIV envelope gp120, and the mannose-specific non-peptidic antibiotic Pradimicin A, which inhibits the entry of HIV-1 into its target cells. CBAs prevent not only virus infection of susceptible cells, but also inhibit syncytia formation between persistently HIV-infected cells and uninfected lymphocytes. In addition, CBAs may also prevent DC-SIGN-mediated transmission of HIV to T-lymphocytes. Therefore, CBAs qualify as potential microbicide drugs. Long-term exposure of HIV to CBAs in cell culture results in the progressive deletion of N-glycans of HIV gpl20 in an attempt of the virus to escape drug pressure. In this respect, the CBAs are endowed with a high genetic barrier. Multiple mutations at N-glycosylation sites are required before pronounced phenotypic drug resistance development becomes evident. CBA treatment of HIV may consist of a novel chemotherapeutic concept with a dual mechanism of antiviral action: a direct antiviral activity by preventing HIV entry and transmission to its target cells, and an indirect antiviral activity by forcing HIV to delete glycans in its gpl20 envelope. The latter phenomenon will result in creating 'holes' in the protective glycan shield of the HIV envelope, whereby the immune system may become triggered to produce neutralizing antibodies against previously hidden immunogenic epitopes of gp120. If this concept can be proven in in vivo, low-molecular-weight non-peptidic CBAs such as Pradimycin A may become the cornerstone for the efficient treatment of infections of those viruses that require a glycosylated envelope (that is, HIV, but also hepatitis C virus) for entry into its target cells. In addition, influenza virus and coronavirus infections may also qualify to be treated by CBAs.

Resistance of HIV-1 to the broadly HIV-1-neutralizing, anti-carbohydrate antibody 2G12

The 2G12 mAb inhibits the infection of HIV-1 laboratory-adapted viruses at 50% inhibitory concentrations (IC(50)) ranging from 0.02 to 0.2 microg/ml when evaluated in different cell-types. However, isolates from various HIV-1 subtypes (such as clade C, D, A/E, F and group O) were not inhibited by 2G12 mAb (IC(50) >20 microg/ml). 2G12 mAb pressure in HIV-1 IIIB- and NL4.3-infected T cell cultures selected for resistant viruses containing only few (1 to 3 N-glycosylation) deletions in gp120. The 2G12-resistant viruses keep their full sensitivity to various mannose-specific lectins and other known HIV entry inhibitors. Moreover, we observed that the NL4.3-2G12-resistant virus, with the N295K mutation in gp120, became significantly more sensitive to several mannose-specific lectins. This is, to our knowledge, the first report showing that a resistant virus generated in vitro against a neutralizing mAb and containing a mutation in gp120, has increased sensitivity to another class of HIV entry inhibitors.

Impact of V2 mutations on escape from a potent neutralizing anti-V3 monoclonal antibody during in vitro selection of a primary human immunodeficiency virus type 1 isolate

KD-247, a humanized monoclonal antibody to an epitope of gp120-V3 tip, has potent cross-neutralizing activity against subtype B primary human immunodeficiency virus type 1 (HIV-1) isolates. To assess how KD-247 escape mutants can be generated, we induced escape variants by exposing bulked primary R5 virus, MOKW, to increasing concentrations of KD-247 in vitro. In the presence of relatively low concentrations of KD-247, viruses with two amino acid mutations (R166K/D167N) in V2 expanded, and under high KD-247 pressure, a V3 tip substitution (P313L) emerged in addition to the V2 mutations. However, a virus with a V2 175P mutation dominated during passaging in the absence of KD-247. Using domain swapping analysis, we demonstrated that the V2 mutations and the P313L mutation in V3 contribute to partial and complete resistance phenotypes against KD-247, respectively. To identify the V2 mutation responsible for the resistance to KD-247, we constructed pseudoviruses with single or double amino acid mutations in V2 and measured their sensitivity to neutralization. Interestingly, the neutralization phenotypes were switched, so that amino acid residue 175 (Pro or Leu) located in the center of V2 was exchanged, indicating that the amino acid at position 175 has a crucial role, dramatically changing the Env oligomeric state on the membrane surface and affecting the neutralization phenotype against not only anti-V3 antibody but also recombinant soluble CD4. These data suggested that HIV-1 can escape from anti-V3 antibody attack by changing the conformation of the functional envelope oligomer by acquiring mutations in the V2 region in environments with relatively low antibody concentrations.

Computational prediction of the cross-reactive neutralizing epitope corresponding to the [corrected] monclonal [corrected] antibody b12 specific for HIV-1 gp120

Backtracking from antibodies to their corresponding epitopes is a rational approach for vaccine design. Here we apply such a reverse immunological strategy for mapping the cross-reactive neutralizing epitope corresponding to the monoclonal antibody (mAb) b12 specific for HIV-1 gp120. b12 was used to screen a combinatorial phage display random peptide library and nineteen 12mer cysteine-looped peptides were affinity purified. These were used as input for analysis with the predictive algorithm Mapitope. Based on the input panel of peptides and the antigen's atomic structure, Mapitope predicts candidate epitopes on the surface of the antigen. Two major clusters were predicted as candidate b12 epitopes. These could be discriminated by a series of experiments, which included point mutagenesis of selected residues and binding assays. Moreover, the prediction of the b12 epitope was further strengthened by comparison with additional predictions for two competing antibodies, b6 and m14. Finally, support of our prediction was obtained in view of the fact that b12, m14, and b6 were found to compete against mAb 17b binding to gp120. The b12 epitope is predicted to consist of four peptide segments of gp120 (residues V254-T257, D368-F376, E381-Y384, and I420-I424), which lie at the periphery of the CD4 binding site.

Building collaborative networks for HIV/AIDS vaccine development: the AVIP experience

The need for an effective HIV/AIDS vaccine is imperative to halt a pandemic that involves more than 40 million individuals worldwide as of 2005 and is causing enormous socio-economic losses, especially in developing countries (DC). The overall failure of more than two decades of HIV vaccine research justifies the demands for a concerted effort for the rapid development of new and efficacious vaccines against HIV/AIDS. In this context, building international collaborative networks is a must for speeding up scientific research and optimizing the use of funding in a synergistic fashion, as resources for HIV/AIDS are limited and do not involve most of the biggest Pharmas that are more interested in drug discovery. The AIDS Vaccine Integrated Project (AVIP) consortium is an example of synergistic partnership of international European Union and DC experts with a common research goal. AVIP is a European Commission-funded (FP-6), consortium-based, 5-year program directed to the fast development of new HIV/AIDS vaccine candidates to be tested in phase I clinical trials in Europe for future advancement to phase II/III testing in DC. To ensure their rapid development, AVIP novel combined vaccines include both regulatory and structural HIV antigens, which have already been tested, as single components, in phase I clinical trials. In particular, such combination vaccines may be superior to earlier vaccine candidates, the vast majority of which are based only on either structural or regulatory HIV products. In fact, the generation of immune responses to both types of viral antigens expressed either early (regulatory products) or late (structural products) during the viral life cycle can maximize immune targeting of both primary or chronic viral infection. Further, the rational design of combined vaccines allows exploitation of immunomodulatory functions of HIV regulatory proteins, which can improve immunity against structural vaccine components. The building of the AVIP consortium and its scientific strategy will be reviewed in this paper as an example of the establishment of a consortium regulated by a specific intellectual property agreement.

Studies on GM-CSF DNA as an adjuvant for neutralizing Ab elicited by a DNA/MVA immunodeficiency virus vaccine

Here, we use a vaccine consisting of DNA priming followed by MVA boosting in rhesus macaques to investigate the ability of GM-CSF DNA to serve as an adjuvant for the elicitation of neutralizing Ab against an HIV-1 Env. The trial used Gag, Pol, and Env sequences from SHIV-89.6 in the immunogens and a neutralization escape variant of SHIV-89.6, SHIV-89.6P, for challenge. Co-delivery of GM-CSF and vaccine DNAs enhanced the temporal appearance of neutralizing Ab and broadened the specificity of the neutralizing activity to include SHIV-89.6P. Two long-term SHIV-89.6 infections elicited neutralizing activity for SHIV-89.6 but not SHIV-89.6P. Studies on the avidity of the anti-Env antisera revealed that the GM-CSF-adjuvanted vaccine had elicited higher avidity Ab than the non-adjuvanted vaccine or the infection. The GM-CSF-adjuvanted group showed a trend towards better control of the challenge infection and had better control of re-emergent virus (P < 0.01) than the non-adjuvanted group.

Sequential immunization with V3 peptides from primary human immunodeficiency virus type 1 produces cross-neutralizing antibodies against primary isolates with a matching narrow-neutralization sequence motif

An antibody response capable of neutralizing not only homologous but also heterologous forms of the CXCR4-tropic human immunodeficiency virus type 1 (HIV-1) MNp and CCR5-tropic primary isolate HIV-1 JR-CSF was achieved through sequential immunization with a combination of synthetic peptides representing HIV-1 Env V3 sequences from field and laboratory HIV-1 clade B isolates. In contrast, repeated immunization with a single V3 peptide generated antibodies that neutralized only type-specific laboratory-adapted homologous viruses. To determine whether the cross-neutralization response could be attributed to a cross-reactive antibody in the immunized animals, we isolated a monoclonal antibody, C25, which neutralized the heterologous primary viruses of HIV-1 clade B. Furthermore, we generated a humanized monoclonal antibody, KD-247, by transferring the genes of the complementary determining region of C25 into genes of the human V region of the antibody. KD-247 bound with high affinity to the "PGR" motif within the HIV-1 Env V3 tip region, and, among the established reference antibodies, it most effectively neutralized primary HIV-1 field isolates possessing the matching neutralization sequence motif, suggesting its promise for clinical applications involving passive immunizations. These results demonstrate that sequential immunization with B-cell epitope peptides may contribute to a humoral immune-based HIV vaccine strategy. Indeed, they help lay the groundwork for the development of HIV-1 vaccine strategies that use sequential immunization with biologically relevant peptides to overcome difficulties associated with otherwise poorly immunogenic epitopes.

DNA prime/protein boost immunization against HIV clade C: safety and immunogenicity in mice

The induction of both cellular and humoral immunity is an important goal for vaccine development against HIV. As a step towards the development of an efficacious vaccine against HIV clade C, the world's most prevalent strain, a combination DNA prime/protein boost immunization strategy was tested. A DNA expression vector was prepared encoding a codon-optimized env gene derived from a pediatric HIV clade C isolate, 1084i. Mice were immunized with HIV1084i env-encoding DNA, then boosted with homologous 1084i gp160. HIV1084i Env-specific T-cell responses were induced with DNA vaccination alone, but the strongest cellular immune responses were seen after boosting with gp160. Immunization with gp160 alone induced high-titer antibodies but required two inoculations. In contrast, high-titer antibodies were seen after a single 1084i gp160 boost in DNA-primed animals. All animals given gp160 inoculations, whether DNA primed or not, developed neutralizing antibodies reactive with HIV1084i and a macaque-passaged simian/human immunodeficiency construct, SHIV-1084ip. The results demonstrate the utility of this DNA prime/protein boost approach to generate cellular immunity, as well as neutralizing antibodies, against HIV clade C env antigens.

Carbohydrate-binding Agents Cause Deletions of Highly Conserved Glycosylation Sites in HIV GP120

Mannose-binding proteins derived from several plants (i.e. Hippeastrum hybrid and Galanthus nivalis agglutinin) or prokaryotes (i.e. cyanovirin-N) inhibit human immunodeficiency virus (HIV) replication and select for drug-resistant viruses that show profound deletion of N-glycosylation sites in the GP120 envelope (Balzarini, J., Van Laethem, K., Hatse, S., Vermeire, K., De Clercq, E., Peumans, W., Van Damme, E., Vandamme, A.-M., Bolmstedt, A., and Schols, D. (2004) J. Virol. 78, 10617-10627; Balzarini, J., Van Laethem, K., Hatse, S., Froeyen, M., Van Damme, E., Bolmstedt, A., Peumans, W., De Clercq, E., and Schols, D. (2005) Mol. Pharmacol. 67, 1556-1565). Here we demonstrated that the N-acetylglucosamine-binding protein from Urtica dioica (UDA) prevents HIV entry and eventually selects for viruses in which conserved N-glycosylation sites in GP120 were deleted. In contrast to the mannose-binding proteins, which have a 50-100-fold decreased antiviral activity against the UDA-exposed mutant viruses, UDA has decreased anti-HIV activity to a very limited extent, even against those mutant virus strains that lack at least 9 of 22 ( approximately 40%) glycosylation sites in their GP120 envelope. Therefore, UDA represents the prototype of a new conceptual class of carbohydrate-binding agents with an unusually specific and targeted drug resistance profile. It forces HIV to escape drug pressure by deleting the indispensable glycans on its GP120, thereby obligatorily exposing previously hidden immunogenic epitopes on its envelope.

Targeting the glycans of gp120: a novel approach aimed at the Achilles heel of HIV

The development of drug resistance in HIV compromises the long-term efficacy of current therapies. Furthermore, vaccine development faces huge problems, mainly because of the low antigenicity and immunogenicity of the HIV envelope glycoprotein gp120 and the efficient hiding of highly immunogenic epitopes by its glycans. There is evidence that mutant HIV strains containing glycosylation site deletions trigger the production of specific neutralising antibodies to previously hidden gp120 epitopes. I present a hypothesis that development of resistance against drugs that target the glycans on gp120 would result in a marked enhancement of neutralisation of HIV by the immune system--ie, drugs directed against the carbohydrate component of gp120 will select for mutant virus strains that progressively gain deletions in the glycosylation sites of gp120. Previously hidden epitopes would then be uncovered, and the virus will become highly susceptible to markedly increased immunological neutralisation. I believe this novel approach may become an entirely new therapeutic concept that exploits the high mutation rate of HIV and allows drug therapy to act in concert with a triggered immune response to suppress HIV more efficiently. Moreover, this approach could be applied to treat other chronic infections by viruses that contain a glycosylated envelope (eg, hepatitis B and C).

Human immunodeficiency virus type 1 variants isolated from single plasma samples display a wide spectrum of neutralization sensitivity

Individuals infected with human immunodeficiency virus type 1 (HIV-1) harbor a mixture of viral variants with different sequences and in some instances with different phenotypic properties. Major and rapid fluctuations in the proportion of viral variants coexisting in an infected individual can be observed under strong pharmacological and immune selective pressure. Because of the short half-life of HIV-infected cells and of HIV virions in the blood, plasma virus populations are highly relevant to HIV evolution in the face of these selective pressures. Here we analyzed the sensitivity to antibody-mediated neutralization of viral variants coexisting in the plasma virus populations of two infected patients. For each patient, several replication-competent viral clones were constructed that carry primary envelope gene sequences obtained from a single plasma sample. Viral clones differed in their tropism and replicative capacity and in the number and positions of glycosylation sites in the envelope glycoproteins. Viruses were tested against heterologous and autologous sera obtained at different time points. Interestingly, we found that viral variants coexisting in each plasma sample were highly heterogeneous in terms of sensitivity to neutralization. The order of sensitivity depended on the serum used and was not associated with virus tropism. The neutralization potency of sera increased with the duration of the infection for both autologous and heterologous neutralization.

Resistance of human immunodeficiency virus type 1 to the high-mannose binding agents cyanovirin N and concanavalin A

Due to the biological significance of the carbohydrate component of the human immunodeficiency virus type 1 (HIV-1) glycoproteins in viral pathogenesis, the glycosylation step constitutes an attractive target for anti-HIV therapy. Cyanovirin N (CV-N), which specifically targets the high-mannose (HM) glycans on gp120, has been identified as a potent HIV-1 entry inhibitor. Concanavalin A (ConA) represents another mannose-binding lectin, although it has a lower specificity for HM glycans than that of CV-N. For the present study, we selected CV-N- and ConA-resistant HIV-1 strains in the presence of CV-N and ConA, respectively. Both resistant strains exhibited a variety of mutations eliminating N-linked glycans within gp120. Strains resistant to CV-N or ConA displayed high levels of cross-resistance towards one another. The N-glycan at position 302 was eliminated in both of the lectin-resistant strains. However, the elimination of this glycan alone by site-directed mutagenesis was not sufficient to render HIV-1 resistant to CV-N or ConA, suggesting that HIV-1 needs to mutate several N-glycans to become resistant to these lectins. Both strains also demonstrated clear cross-resistance towards the carbohydrate-dependent monoclonal antibody 2G12. In contrast, the selected strains did not show a reduced susceptibility towards the nonlectin entry inhibitors AMD3100 and enfuvirtide or towards reverse transcriptase or protease inhibitors. Recombination of the mutated gp160 genes of the strains resistant to CV-N or ConA into a wild-type background fully reproduced the (cross-)resistance profiles of the originally selected strains, pointing to the impact of the N-glycan mutations on the phenotypic resistance profiles of both selected strains.

The host environment drives HIV-1 fitness

Viral fitness is defined by the ability of an individual genotype to produce infectious progeny in a specific environment. For HIV the environment is never constant but rather fluctuates in time and space. For instance, environmental factors that determine viral fitness during transmission from host to host are different to the pressures from either cytotoxic T-lymphocytes (CTLs) or antiviral drugs. Consequently, viral fitness is highly dependent on the environment and the accurate determination of this value therefore depends strongly on the chosen environmental setting. This review describes how the host environment imposes selective pressures on the virus that shape its genotype and fitness. The most important environments that the virus encounters throughout its life cycle and during natural infection are discussed. In order of appearance, CTLs are discussed, followed by neutralising antibodies and antiretroviral drug treatment. It then goes on to describe receptor molecules that mediate viral entry and intracellular restriction factors, which represent selective pressures that are present directly from the start of a natural infection. It concludes by discussing the complexity of viral fitness and how an accurate measure of viral fitness eventually may, for example, contribute to the improvement of antiretroviral therapy or help in the formulation of an optimal vaccination strategy.

Assays for the assessment of neutralizing antibody activities against Severe Acute Respiratory Syndrome (SARS) associated coronavirus (SCV)

Accurate assessment of neutralizing antibody activities is important either for patients infected with Severe Acute Respiratory Syndrome (SARS) or for animals and volunteers immunized with the experimental vaccines against the SARS associated coronavirus (SCV). However, the current assay based on the cytopathic effect (CPE) which has been frequently cited in literature has several limitations. The CPE assay relies on the visual observation on the damage of SCV infected target cells under a microscope. It is subjected to observer variations and it is difficult to generate a quantitative determination of neutralizing activities based on the level of CPE. In the current study, we established the utility of two additional assays to measure the neutralizing activities against SCV: the plaque reduction (PR) and the neutral red staining (NRS) assays. The PR assay described in this study was modified from the traditional viral plaque reduction assay by using an improved crystal staining method to achieve better plague formation in SCV infected Vero E6 cells. The NRS neutralization assay was adopted from a similar system used for detecting neutralizing antibody responses against human immunodeficiency virus type 1 (HIV-1). In this assay, the protective effect of neutralizing antibodies was determined by the cell viability which is measured by the uptake of neutral red dye at A₅₄₀. The neutralizing antibody titers can be easily determined with either of the two new assays. In this report, we described the utility of these two new neutralization assays in measuring the neutralizing activities against SCV infection from rabbit sera immunized with various forms of spike protein of SCV.

Proteins that bind high-mannose sugars of the HIV envelope

A broad range of proteins bind high-mannose carbohydrates found on the surface of the envelope protein gp120 of the human immunodeficiency virus and thus interfere with the viral life cycle, providing a potential new way of controlling HIV infection. These proteins interact with the carbohydrate moieties in different ways. A group of them interacts as typical C-type lectins via a Ca2+ ion. Another group interacts with specific single, terminal sugars, without the help of a metal cation. A third group is involved in more intimate interactions, with multiple carbohydrate rings and no metal ion. Finally, there is a group of lectins for which the interaction mode has not yet been elucidated. This review summarizes, principally from a structural point of view, the current state of knowledge about these high-mannose binding proteins and their mode of sugar binding.

Vaccine strategies for infectious diseases

Infectious diseases are the major cause of death worldwide; in developing countries such diseases are responsible for nearly half the burden of premature death and disability. Therefore, the need for the development of new vaccine strategies aimed at preventing or limiting disease is extremely urgent. Important successes have been achieved against some infectious diseases that were once endemic or, even, epidemic (e.g., polio, smallpox, diptheria). Advances in our knowledge of the pathogenesis and immune correlates of protections are needed to develop novel vaccinal approaches to diseases such as hepatitis C, AIDS and malaria. In this review we will analyse the biological problems associated with the prevention of development and/or improvement of vaccine strategies for infectious diseases, focusing on the difficulties facing the creation of new effective vaccines for HIV infection and malaria.

Studies in macaques on cross-clade T cell responses elicited by a DNA/MVA AIDS vaccine, better conservation of CD8 than CD4 T cell responses

One of the unknowns faced by an HIV/AIDS vaccine is the ability of a single clade vaccine to protect against the multiple genetic subtypes and recombinant forms of HIV-1 present in the current pandemic. Here, we use a macaque model to investigate the ability of our clade B vaccine that consists of DNA priming and modified vaccinia Ankara (MVA) virus boosting to elicit T cell responses that recognize an A/G recombinant of HIV-1. To test for cross-reactive T cells, intracellular cytokine staining was conducted using five pools of Gag and six pools of Env peptides representing B or A/G sequences. Studies using the peptide pools revealed essentially complete conservation of the CD8 response but only approximately 50% conservation of the CD4 response. Thus, the ability of an HIV vaccine for one clade to protect against other clades may be more limited by the ability to provide CD4 T cell help than the ability to elicit CD8 effector functions.

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.

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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Evolutionary dynamics of the glycan shield of the human immunodeficiency virus envelope during natural infection and implications for exposure of the 2G12 epitope

Elucidation of the kinetics of exposure of neutralizing epitopes on the envelope of human immunodeficiency virus type 1 (HIV-1) during the course of infection may provide key information about how HIV escapes the immune system or why its envelope is such a poor immunogen to induce broadly efficient neutralizing antibodies. We analyzed the kinetics of exposure of the epitopes corresponding to the broadly neutralizing human monoclonal antibodies immunoglobulin G1b12 (IgG1b12), 2G12, and 2F5 at the quasispecies level during infection. We studied the antigenicity and sequences of 94 full-length envelope clones present during primary infection and at least 4 years later in four HIV-1 clade B-infected patients. No or only minor exposure differences were observed for the 2F5 and IgG1b12 epitopes between the early and late clones. Conversely, the envelope glycoproteins of the HIV-1 quasispecies present during primary infection did not expose the 2G12 neutralizing epitope, unlike those present after several years in three of the four patients. Sequence analysis revealed major differences at potential N-linked glycosylation sites between early and late clones, particularly at positions known to be important for 2G12 binding. Our study, in natural mutants, confirms that the glycosylation sites N295, N332, and N392 are essential for 2G12 binding. This study demonstrates the relationship between the evolving "glycan shield " of HIV and the kinetics of exposure of the 2G12 epitope during the course of natural infection.

Identification and characterization of a new cross-reactive human immunodeficiency virus type 1-neutralizing human monoclonal antibody

The identification and characterization of new human monoclonal antibodies (hMAbs) able to neutralize primary human immunodeficiency virus type 1 (HIV-1) isolates from different subtypes may help in our understanding of the mechanisms of virus entry and neutralization and in the development of entry inhibitors and vaccines. For enhanced selection of broadly cross-reactive antibodies, soluble HIV-1 envelope glycoproteins (Envs proteins) from two isolates complexed with two-domain soluble CD4 (sCD4) were alternated during panning of a phage-displayed human antibody library; these two Env proteins (89.6 and IIIB gp140s), and one additional Env (JR-FL gp120) alone and complexed with sCD4 were used for screening. An antibody with relatively long HCDR3 (17 residues), designated m14, was identified that bound to all antigens and neutralized heterologous HIV-1 isolates in multiple assay formats. Fab m14 potently neutralized selected well-characterized subtype B isolates, including JRCSF, 89.6, IIIB, and Yu2. Immunoglobulin G1 (IgG1) m14 was more potent than Fab m14 and neutralized 7 of 10 other clade B isolates; notably, although the potency was on average significantly lower than that of IgG1 b12, IgG1 m14 neutralized two of the isolates with significantly lower 50% inhibitory concentrations than did IgG1 b12. IgG1 m14 neutralized four of four selected clade C isolates with potency higher than that of IgG1 b12. It also neutralized 7 of 17 clade C isolates from southern Africa that were difficult to neutralize with other hMAbs and sCD4. IgG1 m14 neutralized four of seven primary HIV-1 isolates from other clades (A, D, E, and F) much more efficiently than did IgG1 b12; for the other three isolates, IgG b12 was much more potent. Fab m14 bound with high (nanomolar range) affinity to gp120 and gp140 from various isolates; its binding was reduced by soluble CD4 and antibodies recognizing the CD4 binding site (CD4bs) on gp120, and its footprint as defined by alanine-scanning mutagenesis overlaps that of b12. These results suggest that m14 is a novel CD4bs cross-reactive HIV-1-neutralizing antibody that exhibits a different inhibitory profile compared to the only known potent broadly neutralizing CD4bs human antibody, b12, and may have implications for our understanding of the mechanisms of immune evasion and for the development of inhibitors and vaccines.

Durable cytotoxic immune responses against gp120 elicited by recombinant SV40 vectors encoding HIV-1 gp120 +/- IL-15

Background

A vaccine that elicits durable, powerful anti-HIV immunity remains an elusive goal. In these studies we tested whether multiple treatments with viral vector-delivered HIV envelope antigen (gp120), with and without IL-15, could help to approach that goal. For this purpose, we used recombinant Tag-deleted SV40-derived vectors (rSV40s), since they do not elicit neutralizing antibody responses, and so can be given multiply without loss of transduction efficiency.

Methods

SV(gp120) carried the coding sequences for HIV-1NL4-3 Env, and SV(mIL-15) carried the cDNA for mouse IL-15. Singly, and in combination, these two vectors were given monthly to BALB/cJ mice. Cytotoxic immunity and cytotoxic memory were tested in direct cytotoxicity assays using unselected effector cells. Antibody vs. gp120 was measured in a binding assay. In both cases, targets were P815 cells that were stably transfected with gp120.

Results

Multiple injections of SV(gp120) elicited powerful anti-gp120 cytolytic activity (>70% specific lysis) by unselected spleen cells. Cells from multiply-immunized mice that were rested 1 year after their last injections still showed >60% gp120-specific lysis. Anti-gp120 antibody was first detected after 2 monthly injections of SV(gp120) and remained elevated thereafter. Adding SV(mIL-15) to the immunization regimen dramatically accelerated the development of memory cytolytic responses, with ≥ 50% specific lysis seen 1 month after two treatments. IL-15 did not alter the development of antibody responses.

Conclusions

Thus, rSV40s encoding antigens and immunostimulatory cytokines may be useful tools for priming and/or boosting immune responses against HIV.

Atomic force microscopy investigation of wild-type Moloney murine leukemia virus particles and virus particles lacking the envelope protein

Moloney murine leukemia virus (M-MuLV) lacking the gene for the envelope glycoprotein (env(-)) was produced in NIH 3T3 cells and investigated using atomic force microscopy (AFM). The particles were compared with similarly produced wild-type virions, some of which had been exposed to a monoclonal antibody against the surface component of the envelope protein (SU protein). The env(-) particles generally exhibit a distinctly different external appearance suggesting only a low density of associated proteins that have an almost fluid, mechanically unstable character. The weakly associated proteins may be host cell membrane proteins that are incorporated into the viral membrane in place of or in addition to virus envelope protein. The amount of this non-viral protein on virion surfaces appears to vary from negligible in most cases to a substantial complement in others. It seems clear that the presence of the envelope protein, in a mechanical sense, significantly strengthens and stabilizes the virion envelope. Binding of monoclonal antibody to wild-type virions indicates that some particles expose a significant amount of antigen while adjacent virions may not. This suggests that the conformation of the envelope glycoprotein or the disposition of oligosaccharides may be different among particles, on some virions exposing the specific epitope, and others little or none.

Synthesis, conformation, and immunogenicity of monosaccharide-centered multivalent HIV-1 gp41 peptides containing the sequence of DP178

Several monosaccharide-centered multivalent HIV-1 gp41 peptides containing the sequence of DP178 were synthesized. Conformational studies showed that multivalent assembly enhanced the alpha-helical content of the peptide. Therefore, 2-, 3-, or 4-alpha-helix bundles of peptide DP178 could be obtained by assembling the peptide on a suitable bi-, tri-, or tetravalent template. Immunization studies indicated that while peptide DP178 alone was poorly immunogenic, the tetravalent peptide MVP-1 raised high titers of antibodies in mice that recognize not only peptide DP178 but also the native HIV-1 glycoprotein gp41, even in the absence of a carrier protein or adjuvant. The study suggests that carbohydrate-centered multivalent peptides provide not only a model for mimicking protein alpha-helix-bundle structure, but also an effective immunogen for raising high-titer antibodies against HIV-1 envelope glycoprotein gp41.

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.

Conserved neutralizing epitopes of HIV type 1 CRF01_AE against primary isolates in long-term nonprogressors

Linear conserved B cell epitopes in envelope glycoprotein of long-term nonprogressors (LTNPs) HIV-1 CRF01_AE were determined. The envelope sequences of HIV-1 subtype E from Thailand were aligned to define consensus sequences. Then the peptides corresponding to these predicted regions were synthesized as peptides represent C1, C2, C3, C5, V2, V3, and gp41 regions. After that, the neutralizing B cell epitopes were determined by neutralized competitive assay with pool sera of typical progressor and LTNP HIV-1 CRF01_AE patients against HIV-1 CRF01_AE 24 primary isolates (PI) and laboratory strains (TCLA). We found that the strength and breadth of neutralization were greater for sera from LTNPs compared with sera from typical progressors. Peptides C1E and C2E could inhibit primary isolates but not the TCLA strain in LTNP sera. The new B cell epitopes, which were located in the C1 and C2 regions of CRF01_AE against primary HIV-1 isolates, were identified in HIV-1 CRF01_AE LTNPs. This may be important in HIV-1 vaccine development and trial.

Correlation between env V1/V2 region diversification and neutralizing antibodies during primary infection by simian immunodeficiency virus sm in rhesus macaques

Evolution of the domain encoding the V1/V2 variable region of the simian immunodeficiency virus sm (SIVsm) envelope (env) gene was analyzed in relation to route of virus challenge, virus load, and neutralizing antibody (NAb) titers during primary infection of rhesus macaques with the pathogenic SIVsmE660 isolate. In this model system animals are initially infected with multiple viruses as evidenced by the presence of multiple V1/V2 genotypic variants that could be resolved by using a heteroduplex tracking assay (HTA). Overlapping subsets of the multiple variants were established in each animal. There was no selection for the establishment of specific variants in comparing intravenous- and intrarectal-challenged macaques at week 2 postinfection, suggesting that no genotypic selection occurred at the mucosal surface. There was an initial period of significant stability of the V1/V2 variants. Macaques challenged intravenously displayed subsequent V1/V2 diversification significantly earlier than macaques challenged intrarectally and well past the initial resolution of viremia. The time when SIVsmE660-specific NAbs reached a threshold titer of 100 was significantly correlated with the timing of V1/V2 diversification, even though antibodies to the Env protein could be detected much earlier. The time when NAbs reached a titer of 400 was significantly correlated with virus load late in infection. These results show that the route of infection affects the timing of V1/V2 diversification and that this diversification is correlated with the maturation of a specific NAb response. However, prior immunization capable of priming an anamnestic Env antibody response did not accelerate V1/V2 diversification. This result suggests that diversification of the SIV env V1/V2 region is the result of a type-specific antibody response.