Rational design of vaccines to elicit broadly neutralizing antibodies to HIV-1

HIV-1 resistance to neutralizing antibodies: Determination of antibody concentrations leading to escape mutant evolution

Broadly neutralizing antibodies against human immunodeficiency virus type 1 (HIV-1) are considered vital components of novel therapeutics and blueprints for vaccine research. Yet escape to even the most potent of these antibodies is imminent in natural infection. Measures to define antibody efficacy and prevent mutant selection are thus urgently needed. Here, we derive a mathematical framework to predict the concentration ranges for which antibody escape variants can outcompete their viral ancestors, referred to as mutant selection window (MSW). When determining the MSW, we focus on the differential efficacy of neutralizing antibodies against HIV-1 in two canonical infection routes, free-virus infection and cell-cell transmission. The latter has proven highly effective in vitro suggesting its importance for both in vivo spread as well as for escaping targeted intervention strategies. We observed a range of MSW patterns that highlight the potential of mutants to arise in both transmission pathways and over wide concentration ranges. Most importantly, we found that only when the arising mutant has both, residual sensitivity to the neutralizing antibody and reduced infectivity compared to the parental virus, antibody dosing outside of the MSW to restrict mutant selection is possible. Emergence of mutants that provide complete escape and have no considerable fitness loss cannot be prevented by adjusting antibody doses. The latter may in part explain the ubiquitous resistance to neutralizing antibodies observed in natural infection and antibody treatment. Based on our findings, combinations of antibodies targeting different epitopes should be favored for antibody-based interventions as this may render complete resistance less likely to occur and also increase chances that multiple escapes result in severe fitness loss of the virus making longer-term antibody treatment more feasible.

Competitive exclusion by autologous antibodies can prevent broad HIV-1 antibodies from arising

The past decade has seen the discovery of numerous broad and potent monoclonal antibodies against HIV type 1 (HIV-1). Eliciting these antibodies via vaccination appears to be remarkably difficult, not least because they arise late in infection and are highly mutated relative to germline antibody sequences. Here, using a computational model, we show that broad antibodies could in fact emerge earlier and be less mutated, but that they may be prevented from doing so as a result of competitive exclusion by the autologous antibody response. We further find that this competitive exclusion is weaker in infections founded by multiple distinct strains, with broadly neutralizing antibodies emerging earlier than in infections founded by a single strain. Our computational model simulates coevolving multitype virus and antibody populations. Broadly neutralizing antibodies may therefore be easier for the adaptive immune system to generate than previously thought. If less mutated broad antibodies exist, it may be possible to elicit them with a vaccine containing a mixture of diverse virus strains.

Development of an AIDS vaccine using Sendai virus vectors

Development of an effective AIDS vaccine is crucial for the control of global human immunodeficiency virus type 1 (HIV-1) prevalence. We have developed a novel AIDS vaccine using a Sendai virus (SeV) vector and investigated its efficacy in a macaque AIDS model of simian immunodeficiency virus (SIV) infection. Its immunogenicity and protective efficacy have been shown, indicating that the SeV vector is a promising delivery tool for AIDS vaccines. Here, we describe the potential of SeV vector as a vaccine antigen delivery tool to induce effective immune responses against HIV-1 infection.

Capacity of Broadly Neutralizing Antibodies to Inhibit HIV-1 Cell-Cell Transmission Is Strain- and Epitope-Dependent

An increasing number of broadly neutralizing antibodies (bnAbs) are considered leads for HIV-1 vaccine development and novel therapeutics. Here, we systematically explored the capacity of bnAbs to neutralize HIV-1 prior to and post-CD4 engagement and to block HIV-1 cell-cell transmission. Cell-cell spread is known to promote a highly efficient infection with HIV-1 which can inflict dramatic losses in neutralization potency compared to free virus infection. Selection of bnAbs that are capable of suppressing HIV irrespective of the transmission mode therefore needs to be considered to ascertain their in vivo activity in therapeutic use and vaccines. Employing assay systems that allow for unambiguous discrimination between free virus and cell-cell transmission to T cells, we probed a panel of 16 bnAbs for their activity against 11 viruses from subtypes A, B and C during both transmission modes. Over a wide range of bnAb-virus combinations tested, inhibitory activity against HIV-1 cell-cell transmission was strongly decreased compared to free virus transmission. Activity loss varied considerably between virus strains and was inversely associated with neutralization of free virus spread for V1V2- and V3-directed bnAbs. In rare bnAb-virus combinations, inhibition for both transmission modes was comparable but no bnAb potently blocked cell-cell transmission across all probed virus strains. Mathematical analysis indicated an increased probability of bnAb resistance mutations to arise in cell-cell rather than free virus spread, further highlighting the need to block this pathway. Importantly, the capacity to efficiently neutralize prior to CD4 engagement correlated with the inhibition efficacy against free virus but not cell-cell transmitted virus. Pre-CD4 attachment activity proved strongest amongst CD4bs bnAbs and varied substantially for V3 and V1V2 loop bnAbs in a strain-dependent manner. In summary, bnAb activity against divergent viruses varied depending on the transmission mode and differed depending on the window of action during the entry process, underscoring that powerful combinations of bnAbs are needed for in vivo application.

When selecting broadly neutralizing antibodies (bnAbs) for clinical application, potency and breadth against free viruses are vital, but additional features may be needed to ensure in vivo efficacy. Considering that HIV-1 can utilize free virus and cell-cell transmission to infect, the efficacy of neutralizing antibodies in vivo may depend on their ability to block both pathways. While breadth and potency of bnAbs against free viruses have been intensely studied, their precise activity during cell-cell spread remains uncertain. Our analysis of the cell-cell neutralization capacity of a large selection of bnAbs against a spectrum of HIV-1 strains revealed that while bnAbs showed an overall decreased activity during cell-cell transmission, losses varied substantially depending on bnAb and virus strain probed. Although bnAbs occasionally retained activity during cell-cell transmission for individual viruses, this ability was rare and generally not associated with a high potency against free virus spread. Notably, neutralization of free virus but not cell-cell transmission was linked with the activity of bnAbs to inhibit prior to CD4 engagement, highlighting the functional differences of the processes. Since no single bnAb combines the entire range of mechanistic features anticipated to support in vivo efficacy, our study adds further evidence that combinations of bnAbs need to be considered for human application.

Blockage of CD59 Function Restores Activities of Neutralizing and Nonneutralizing Antibodies in Triggering Antibody-Dependent Complement-Mediated Lysis of HIV-1 Virions and Provirus-Activated Latently Infected Cells

Both HIV-1 virions and infected cells use their surface regulators of complement activation (RCA) to resist antibody-dependent complement-mediated lysis (ADCML). Blockage of the biological function of RCA members, particularly CD59 (a key RCA member that controls formation of the membrane attack complex at the terminal stage of the complement activation cascades via all three activation pathways), has rendered both HIV-1 virions and infected cells sensitive to ADCML mediated by anti-Env antibodies (Abs) or sera/plasma from patients at different stages of viral infection. In the current study, we used the well-characterized anti-HIV-1 neutralizing Abs (nAbs), including 2G12, 2F5, and 4E10, and non-nAbs, including 2.2C, A32, N5-i5, and N12-i15, to investigate whether the enhancement of ADCML by blockage of CD59 function is mediated by nAbs, non-nAbs, or both. We found that all nAbs and two non-nAbs (N5-i5 and A32) strongly reacted to three HIV-1 laboratory strains (R5, X4, and R5/X4), six primary isolates, and provirus-activated ACH-2 cells examined. In contrast, two non-nAbs, 2.2C and N12-i15, reacted weakly and did not react to these targets, respectively. After blockage of CD59 function, the reactive Abs, regardless of their neutralizing activities, significantly enhanced specific ADCML of HIV-1 virions (both laboratory strains and primary isolates) and provirus-activated latently infected cells. The ADMCL efficacy positively correlated with the enzyme-linked immunosorbent assay-reactive intensity of those Abs with their targets. Thus, blockage of RCA function represents a novel approach to restore activities of both nAbs and non-nAbs in triggering ADCML of HIV-1 virions and provirus-activated latently infected cells.

IMPORTANCE

There is a renewed interest in the potential role of non-nAbs in the control of HIV-1 infection. Our data, for the first time, demonstrated that blockage of the biological function of RCA members rendered both HIV-1 virions and infected cells sensitive to ADCML mediated by not only nAbs but also non-nAbs. Our results are significant in developing novel immune-based approaches to restore the functions of nAbs and non-nAbs in the circulation of HIV-1-infected individuals to specifically target and clear HIV-1 virions and infected cells. Our data also provide new insights into the mechanisms by which HIV-1 virions and infected cells escape Ab-mediated immunity and could aid in the design and/or development of therapeutic HIV-1 vaccines. In addition, a combination of antiretroviral therapy with RCA blockage, provirus activators, and therapeutic vaccines may represent a novel approach to eliminate HIV-1 reservoirs, i.e., the infected cells harboring replication-competent proviruses and residual viremia.

The role of evolutionarily conserved germ-line DH sequence in B-1 cell development and natural antibody production

Because of N addition and variation in the site of VDJ joining, the third complementarity-determining region of the heavy chain (CDR-H3) is the most diverse component of the initial immunoglobulin antigen-binding site repertoire. A large component of the peritoneal cavity B-1 cell component is the product of fetal and perinatal B cell production. The CDR-H3 repertoire is thus depleted of N addition, which increases dependency on germ-line sequence. Cross-species comparisons have shown that DH gene sequence demonstrates conservation of amino acid preferences by reading frame. Preference for reading frame 1, which is enriched for tyrosine and glycine, is created both by rearrangement patterns and by pre-BCR and BCR selection. In previous studies, we have assessed the role of conserved DH sequence by examining peritoneal cavity B-1 cell numbers and antibody production in BALB/c mice with altered DH loci. Here, we review our finding that changes in the constraints normally imposed by germ-line-encoded amino acids within the CDR-H3 repertoire profoundly affect B-1 cell development, especially B-1a cells, and thus natural antibody immunity. Our studies suggest that both natural and somatic selection operate to create a restricted B-1 cell CDR-H3 repertoire.

Immunogens Modeling a Fusion-Intermediate Conformation of gp41 Elicit Antibodies to the Membrane Proximal External Region of the HIV Envelope Glycoprotein

The membrane proximal external region (MPER) of the gp41 subunit of the HIV-1 envelope glycoprotein (Env) contains determinants for broadly neutralizing antibodies and has remained an important focus of vaccine design. However, creating an immunogen that elicits broadly neutralizing antibodies to this region has proven difficult in part due to the relative inaccessibility of the MPER in the native conformation of Env. Here, we describe the antigenicity and immunogenicity of a panel of oligomeric gp41 immunogens designed to model a fusion-intermediate conformation of Env in order to enhance MPER exposure in a relevant conformation. The immunogens contain segments of the gp41 N- and C-heptad repeats to mimic a trapped intermediate, followed by the MPER, with variations that include different N-heptad lengths, insertion of extra epitopes, and varying C-termini. These well-characterized immunogens were evaluated in two different immunization protocols involving gp41 and gp140 proteins, gp41 and gp160 DNA primes, and different immunization schedules and adjuvants. We found that the immunogens designed to reduce extension of helical structure into the MPER elicited the highest MPER antibody binding titers, but these antibodies lacked neutralizing activity. The gp41 protein immunogens also elicited higher MPER titers than the gp140 protein immunogen. In prime-boost studies, the best MPER responses were seen in the groups that received DNA priming with gp41 vectors followed by gp41 protein boosts. Finally, although titers to the entire protein immunogen were similar in the two immunization protocols, MPER-specific titers differed, suggesting that the immunization route, schedule, dose, or adjuvant may differentially influence MPER immunogenicity. These findings inform the design of future MPER immunogens and immunization protocols.

Characterization of protective immune response elicited by a trimeric envelope protein from an Indian clade C HIV-1 isolate in rhesus macaques

BACKGROUND

Recent preclinical studies have demonstrated the use of properly folded trimeric HIV-1 envelope proteins as immunogen for eliciting protecting immune response in macaques.

METHODS

Trimeric gp145 protein of Indian clade C HIV-1 (93IN101) was characterized for antigenicity by evaluating its binding to sCD4, and several monoclonal antibodies to HIV-1 by bio-layer interferometry. Ten macaques were immunized four times with purified gp145 in adjuplex adjuvant, and serum antibodies were characterized for binding to gp145 and neutralization. Immunized macaques were subjected to weekly low-dose vaginal challenge with SHIV1157-ipEL-p for 8 weeks.

RESULTS

Env protein elicited strong antibody response in macaques. Following challenge, seven of ten immunized macaques resisted challenge, while six of eight control animals were infected.

CONCLUSIONS

Env proteins from a clade C Indian isolate can elicit protective immune response and therefore may be a candidate for inclusion in a multiclade-based HIV-1 vaccine.

Synthesis of a polymerizable, bivalent glycan mimetic of the HIV envelope spike gp120

A synthetic study on the creation of a bivalent, ROMP capable monomer has the ability to be polymerized into the corresponding neo-glycopolymer mimetic of the surface glycans on gp120 envelope spike of the HIV virus. In our approach, we have developed a new strategy for orthogonally attaching both the terminal Manα1-2Man disaccharide unit of the D1 arm of Man9GlcNAc2 of HIV gp120 and the terminal Manα1-2 unit of its D2 arm to a bivalent scaffold to produce the corresponding polymerizable monomer. The Manα1-2 saccharide moieties were assembled using a nickel catalyst, Ni(4-F-PhCN)₄(OTf)₂, to activate trihaloacetimidate donors under mild and operationally simple procedure.

Antigenicity and immunogenicity of a trimeric envelope protein from an Indian clade C HIV-1 isolate

Human immunodeficiency virus type 1 (HIV-1) isolates from India mainly belong to clade C and are quite distinct from clade C isolates from Africa in terms of their phylogenetic makeup, serotype, and sensitivity to known human broadly neutralizing monoclonal antibodies. Because many of these properties are associated with the envelope proteins of HIV-1, it is of interest to study the envelope proteins of Indian clade C isolates as part of the ongoing efforts to develop a vaccine against HIV-1. To this end, we purified trimeric uncleaved gp145 of a CCR5 tropic Indian clade C HIV-1 (93IN101) from the conditioned medium of 293 cells. The purified protein was shown to be properly folded with stable structure by circular dichroism. Conformational integrity was further demonstrated by its high affinity binding to soluble CD4, CD4 binding site antibodies such as b12 and VRC01, quaternary epitope-specific antibody PG9, and CD4-induced epitope-specific antibody 17b. Sera from rabbits immunized with gp145 elicited high titer antibodies to various domains of gp120 and neutralized a broad spectrum of clade B and clade C HIV-1 isolates. Similar to other clade B and clade C envelope immunogens, most of the Tier 1 neutralizing activity could be absorbed with the V3-specific peptide. Subsequent boosting of these rabbits with a clade B HIV-1 Bal gp145 resulted in an expanded breadth of neutralization of HIV-1 isolates. The present study strongly supports the inclusion of envelopes from Indian isolates in a future mixture of HIV-1 vaccines.

A New Scientific Paradigm may be Needed to Finally Develop an HIV Vaccine

The bulk of current HIV vaccine research is conducted within the infectious disease paradigm that has been very successful in developing vaccines against many other viral diseases. Different HIV vaccine concepts, based on the induction of neutralizing antibodies and/or cell mediated immunity, have been developed and clinically tested over the last 30 years, resulting in a few small successes and many disappointments. As new scientific knowledge is obtained, HIV vaccine concepts are constantly modified with the hope that the newly introduced tweaks (or paradigm drifts) will provide the solution to one of the most difficult challenges that modern biomedical research is confronting. Efficacy trials have been critical in guiding HIV vaccine development. However, from the five phase III efficacy trials conducted to date, only one (RV144) resulted in modest efficacy. The results from RV144 were surprising in many ways, including the identified putative correlates of protection (or risk), which did not include neutralizing antibodies or cytotoxic T-cells. The solution to the HIV vaccine challenge may very well come from approaches based on the current paradigm. However, at the same time, out-of-the-paradigm ideas should be systematically explored to complement the current efforts. New mechanisms are needed to identify and support the innovative research that will hopefully accelerate the development of an urgently needed HIV vaccine.

Selection of intracellular single-domain antibodies targeting the HIV-1 Vpr protein by cytoplasmic yeast two-hybrid system

The targeting of HIV-1 using antibodies is of high interest as molecular tools to better understand the biology of the virus or as a first step toward the design of new inhibitors targeting critical viral intracellular proteins. Small and highly stable llama-derived single-domain antibodies can often be functionally expressed as intracellular antibodies in the cytoplasm of eukaryotic cells. Using a selection method based on the Sos Recruitment System, a cytoplasmic yeast two-hybrid approach, we have isolated single-domain antibodies able to bind HIV-1 Vpr and Capside proteins in the yeast cytoplasm. One anti-Vpr single domain antibody was able to bind the HIV-1 regulatory Vpr protein in the cytoplasm of eukaryotic cells, leading to its delocalization from the nucleus to the cytoplasm. To our knowledge, this is the first description of a functional single-domain intrabody targeting HIV-1 Vpr, isolated using an in vivo cytoplasmic selection method that alleviates some limitations of the conventional yeast two-hybrid system.

Neutralisation of HIV-1 cell-cell spread by human and llama antibodies

BACKGROUND

Direct cell-cell spread of HIV-1 is a very efficient mode of viral dissemination, with increasing evidence suggesting that it may pose a considerable challenge to controlling viral replication in vivo. Much current vaccine research involves the study of broadly neutralising antibodies (bNabs) that arise during natural infection with the aims of eliciting such antibodies by vaccination or incorporating them into novel therapeutics. However, whether cell-cell spread of HIV-1 can be effectively targeted by bNabs remains unclear, and there is much interest in identifying antibodies capable of efficiently neutralising virus transmitted by cell-cell contact.

RESULTS

In this study we have tested a panel of bNAbs for inhibition of cell-cell spread, including some not previously evaluated for inhibition of this mode of HIV-1 transmission. We found that three CD4 binding site antibodies, one from an immunised llama (J3) and two isolated from HIV-1-positive patients (VRC01 and HJ16) neutralised cell-cell spread between T cells, while antibodies specific for glycan moieties (2G12, PG9, PG16) and the MPER (2F5) displayed variable efficacy. Notably, while J3 displayed a high level of potency during cell-cell spread we found that the small size of the llama heavy chain-only variable region (VHH) J3 is not required for efficient neutralisation since recombinant J3 containing a full-length human heavy chain Fc domain was significantly more potent. J3 and J3-Fc also neutralised cell-cell spread of HIV-1 from primary macrophages to CD4+ T cells.

CONCLUSIONS

In conclusion, while bNabs display variable efficacy at preventing cell-cell spread of HIV-1, we find that some CD4 binding site antibodies can inhibit this mode of HIV-1 dissemination and identify the recently described llama antibody J3 as a particularly potent inhibitor. Effective neutralisation of cell-cell spread between physiologically relevant cell types by J3 and J3-Fc supports the development of VHH J3 nanobodies for therapeutic or prophylactic applications.

Aggregate complexes of HIV-1 induced by multimeric antibodies

Background

Antibody mediated viral aggregation may impede viral transfer across mucosal surfaces by hindering viral movement in mucus, preventing transcytosis, or reducing inter-cellular penetration of epithelia thereby limiting access to susceptible mucosal CD4 T cells and dendritic cells. These functions may work together to provide effective immune exclusion of virus from mucosal tissue; however little is known about the antibody characteristics required to induce HIV aggregation. Such knowledge may be critical to the design of successful immunization strategies to facilitate viral immune exclusion at the mucosal portals of entry.

Results

The potential of neutralizing and non-neutralizing IgG and IgA monoclonals (mAbs) to induce HIV-1 aggregation was assessed by Dynamic light scattering (DLS). Although neutralizing and non-neutralizing IgG mAbs and polyclonal HIV-Ig efficiently aggregated soluble Env trimers, they were not capable of forming viral aggregates. In contrast, dimeric (but not monomeric) IgA mAbs induced stable viral aggregate populations that could be separated from uncomplexed virions. Epitope specificity influenced both the degree of aggregation and formation of higher order complexes by dIgA. IgA purified from serum of uninfected RV144 vaccine trial responders were able to efficiently opsonize viral particles in the absence of significant aggregation, reflective of monomeric IgA.

Conclusions

These results collectively demonstrate that dIgA is capable of forming stable viral aggregates providing a plausible basis for testing the effectiveness of aggregation as a potential protection mechanism at the mucosal portals of viral entry.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0078-8) contains supplementary material, which is available to authorized users.

Eliciting neutralizing antibodies with gp120 outer domain constructs based on M-group consensus sequence

One strategy being evaluated for HIV-1 vaccine development is focusing immune responses towards neutralizing epitopes on the gp120 outer domain (OD) by removing the immunodominant, but non-neutralizing, inner domain. Previous OD constructs have not elicited strong neutralizing antibodies (nAbs). We constructed two immunogens, a monomeric gp120-OD and a trimeric gp120-OD×3, based on an M group consensus sequence (MCON6). Their biochemical and immunological properties were compared with intact gp120. Results indicated better preservation of critical neutralizing epitopes on gp120-OD×3. In contrast to previous studies, our immunogens induced potent, cross-reactive nAbs in rabbits. Although nAbs primarily targeted Tier 1 viruses, they exhibited significant breadth. Epitope mapping analyses indicated that nAbs primarily targeted conserved V3 loop elements. Although the potency and breadth of nAbs were similar for all three immunogens, nAb induction kinetics indicated that gp120-OD×3 was superior to gp120-OD, suggesting that gp120-OD×3 is a promising prototype for further gp120 OD-based immunogen development.

Microdevices for examining immunological responses of single cells to HIV

More than 60 million people in the world have been diagnosed with HIV infections since the virus was recognized as the causative agent of AIDS in the 1980s. Even though more than half of the infected patients have died, effective disease treatment and prevention measures have not been established. ART (antiretroviral therapy) is the only proven HIV treatment that sustains the suppression of patient viraemia. Current routine approaches to treat HIV infections are targeted at developing vaccines that will induce humoral or cell memory immune responses. However, developing an effective vaccine has been challenging because the HIV mutates rapidly, which allows the virus to evade immune surveillances established against the previous strain. In addition, the virus is able to quickly establish a reservoir and treatment is difficult because of the general lack of knowledge about HIV immune response mechanisms. This review introduces common disease symptoms and the progression of HIV infection with a brief summary of the current treatment approaches. Different cellular immune responses against HIV are also discussed, with emphasis on a nanotechnology research that has focused on probing T-cell response to HIV infection. Furthermore, we discuss recent noteworthy nanotechnology updates on T-cell response screening that is focused on HIV infection. Finally, we review potential future treatment strategies based on the correlations between T-cell response and HIV infection.

Synthetic carbohydrate antigens for HIV vaccine design

The heavy glycosylation of HIV envelope constitutes a strong defense mechanism for the virus to evade host immune response, which accounts for a major barrier for HIV vaccine development. Nevertheless, the identification of a number of glycan-dependent broadly HIV-neutralizing antibodies from HIV-infected individuals, including 2G12, PG9, PG16, PGT121-123, PGT125-128, and PGT135, strongly suggests that the defensive viral 'glycan shield' can be important targets of vaccines. The novel glycan recognition mode exhibited by these antibodies provides new templates for immunogen design. This review highlights recent work on the characterization of the glycan-dependent epitopes of these neutralizing antibodies and recent advances in the synthesis of the relevant carbohydrate antigens for HIV vaccine design.

Design of lipid nanocapsule delivery vehicles for multivalent display of recombinant Env trimers in HIV vaccination

Immunization strategies that elicit antibodies capable of neutralizing diverse virus strains will likely be an important part of a successful vaccine against HIV. However, strategies to promote robust humoral responses against the native intact HIV envelope trimer structure are lacking. We recently developed chemically cross-linked lipid nanocapsules as carriers of molecular adjuvants and encapsulated or surface-displayed antigens, which promoted follicular helper T-cell responses and elicited high-avidity, durable antibody responses to a candidate malaria antigen. To apply this system to the delivery of HIV antigens, Env gp140 trimers with terminal his-tags (gp140T-his) were anchored to the surface of lipid nanocapsules via Ni-NTA-functionalized lipids. Initial experiments revealed that the large (409 kDa), heavily glycosylated trimers were capable of extracting fluid phase lipids from the membranes of nanocapsules. Thus, liquid-ordered and/or gel-phase lipid compositions were required to stably anchor trimers to the particle membranes. Trimer-loaded nanocapsules combined with the clinically relevant adjuvant monophosphoryl lipid A primed high-titer antibody responses in mice at antigen doses ranging from 5 μg to as low as 100 ng, whereas titers dropped more than 50-fold over the same dose range when soluble trimer was mixed with a strong oil-in-water adjuvant comparator. Nanocapsule immunization also broadened the number of distinct epitopes on the HIV trimer recognized by the antibody response. These results suggest that nanocapsules displaying HIV trimers in an oriented, multivalent presentation can promote key aspects of the humoral response against Env immunogens.

Humoral immune responses to HIV in the mucosal secretions and sera of HIV-infected women

Although sera and all external secretions contain antibodies to human immunodeficiency virus (HIV), their levels, specificity, isotypes, and relevant effector functions display a great degree of variability. Antibodies that bind HIV antigens and neutralize the virus are predominantly associated with the IgG isotype in sera and in all external secretions, even where total levels of IgG are much lower than those of IgA. Rectal fluid that contains high IgA, but low IgG levels, displayed low neutralizing activity independent of antibodies. Therefore, external secretions should be evaluated before and after selective depletion of Ig. At the systemic level, HIV-specific IgA may interfere with the effector functions of IgG, as suggested by recent studies of individuals systemically immunized with an experimental HIV vaccine. Although HIV-specific IgG and IgA antibodies may exhibit their protective activities at mucosal surfaces through interference with viral entry and local neutralization at the systemic level, such antibodies may display discordant effector functions.

Polyethyleneimine is a potent systemic adjuvant for glycoprotein antigens

Polyethyleneimine (PEI) is an organic polycation used extensively as a gene and DNA vaccine delivery reagent. Although the DNA targeting activity of PEI is well documented, its immune activating activity is not. We recently reported that PEI has robust mucosal adjuvanticity when administered intranasally with glycoprotein antigens. Here, we show that PEI has strong immune activating activity after systemic delivery. PEI administered subcutaneously with viral glycoprotein (HIV-1 gp140) enhanced antigen-specific serum IgG production in the context of mixed Th1/Th2-type immunity. PEI elicited higher titers of both antigen binding and neutralizing antibodies than alum in mice and rabbits and induced an increased proportion of antibodies reactive with native antigen. In an intraperitoneal model, PEI recruited neutrophils followed by monocytes to the site of administration and enhanced antigen uptake by antigen-presenting cells. The Th bias was modulated by PEI activation of the Nlrp3 inflammasome; however its global adjuvanticity was unchanged in Nlrp3-deficient mice. When coformulated with CpG oligodeoxynucleotides, PEI adjuvant potency was synergistically increased and biased toward a Th1-type immune profile. Taken together, these data support the use of PEI as a versatile systemic adjuvant platform with particular utility for induction of secondary structure-reactive antibodies against glycoprotein antigens.

Broadly neutralizing influenza hemagglutinin stem-specific antibody CR8020 targets residues that are prone to escape due to host selection pressure

Broadly neutralizing antibodies (bNAb) that target a conserved region of a viral antigen hold significant therapeutic promise. CR8020 is a bNAb that targets the stem region of influenza A virus (IAV) hemagglutinin (HA). CR8020 is currently being evaluated for prophylactic use against group 2 IAVs in phase II studies. Structural and computational analyses reported here indicate that CR8020 targets HA residues that are prone to antigenic drift and host selection pressure. Critically, CR8020 escape mutation is seen in certain H7N9 viruses from recent outbreaks. Furthermore, the ability of the bNAb Fc region to effectively engage activating Fcγ receptors (FCγR) is essential for antibody efficacy. In this regard, our data indicate that the membrane could sterically hinder the formation of HA-CR8020-FcγRIIa/HA-IgG-FcγRIIIa ternary complexes. Altogether, our analyses suggest that epitope mutability and accessibility to immune complex assembly are important attributes to consider when evaluating bNAb candidates for clinical development.

Immunization with hybrid proteins containing the membrane proximal external region of HIV-1

The transmembrane envelope (TM) protein gp41 of HIV-1 is an attractive target when designing a vaccine to induce neutralizing antibodies. A few broadly neutralizing antibodies (2F5, 4E10, and 10E8) that target conserved epitopes in the membrane proximal external region (MPER) of gp41 have been isolated from infected individuals. However, attempts to induce such antibodies by immunizations with gp41 and Env derivatives containing the MPER were successful only to some extent. In contrast, immunizations with the ectodomain of the TM protein p15E of different gamma retroviruses resulted in the induction of neutralizing antibodies. These sera recognized epitopes located in the MPER and in the fusion peptide proximal region (FPPR) of p15E. Based on these results, both regions of p15E were substituted with the corresponding sequences derived from gp41 of HIV-1. Thus, four different hybrid antigens were produced. One of the inserted sequences contained the epitopes of 2F5 and 4E10 in the MPER; the other corresponded to the FPPR. Vaccination of rats, guinea pigs, and a goat induced binding antibodies directed against the FPPR of gp41 and the 2F5 epitope (ELDKWA) located in the MPER. Despite the exact recognition of the 2F5 epitope, no or very weak neutralization of HIV-1NL4-3 by the immune sera was demonstrated. Nonetheless, using the strategy of hybrid proteins, antibodies targeting the desired epitope were successfully induced.

Structure-based design, synthesis and validation of CD4-mimetic small molecule inhibitors of HIV-1 entry: conversion of a viral entry agonist to an antagonist

This Account provides an overview of a multidisciplinary consortium focused on structure-based strategies to devise small molecule antagonists of HIV-1 entry into human T-cells, which if successful would hold considerable promise for the development of prophylactic modalities to prevent HIV transmission and thereby alter the course of the AIDS pandemic.

Entry of the human immunodeficiency virus (HIV) into target T-cells entails an interaction between CD4 on the host T-cell and gp120, a component of the trimeric envelope glycoprotein spike on the virion surface. The resultant interaction initiates a series of conformational changes within the envelope spike that permits binding to a chemokine receptor, formation of the gp41 fusion complex, and cell entry. A hydrophobic cavity at the CD4–gp120 interface, defined by X-ray crystallography, provided an initial site for small molecule antagonist design. This site however has evolved to facilitate viral entry. As such, the binding of prospective small molecule inhibitors within this gp120 cavity can inadvertently trigger an allosteric entry signal.

Structural characterization of the CD4–gp120 interface, which provided the foundation for small molecule structure-based inhibitor design, will be presented first. An integrated approach combining biochemical, virological, structural, computational, and synthetic studies, along with a detailed analysis of ligand binding energetics, revealed that modestly active small molecule inhibitors of HIV entry can also promote viral entry into cells lacking the CD4 receptor protein; these competitive inhibitors were termed small molecule CD4 mimetics. Related congeners were subsequently identified with both improved binding affinity and more potent viral entry inhibition. Further assessment of the affinity-enhanced small molecule CD4 mimetics demonstrated that premature initiation of conformational change within the viral envelope spike, prior to cell encounter, can lead to irreversible deactivation of viral entry machinery. Related congeners, which bind the same gp120 site, possess different propensities to elicit the allosteric response that underlies the undesired enhancement of CD4-independent viral entry.

Subsequently, key hotspots in the CD4–gp120 interface were categorized using mutagenesis and isothermal titration calorimetry according to the capacity to increase binding affinity without triggering the allosteric signal. This analysis, combined with cocrystal structures of small molecule viral entry agonists with gp120, led to the development of fully functional antagonists of HIV-1 entry. Additional structure-based design exploiting two hotspots followed by synthesis has now yielded low micromolar inhibitors of viral entry.

Virulence and pathogenesis of HIV-1 infection: an evolutionary perspective

Why some individuals develop AIDS rapidly whereas others remain healthy without treatment for many years remains a central question of HIV research. An evolutionary perspective reveals an apparent conflict between two levels of selection on the virus. On the one hand, there is rapid evolution of the virus in the host, and on the other, new observations indicate the existence of virus factors that affect the virulence of infection whose influence persists over years in infected individuals and across transmission events. Here, we review recent evidence that shows that viral genetic factors play a larger role in modulating disease severity than anticipated. We propose conceptual models that reconcile adaptive evolution at both levels of selection. Evolutionary analysis provides new insight into HIV pathogenesis.

Early phase clinical trials with human immunodeficiency virus-1 and malaria vectored vaccines in The Gambia: frontline challenges in study design and implementation

Human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS) and malaria are among the most important infectious diseases in developing countries. Existing control strategies are unlikely to curtail these diseases in the absence of efficacious vaccines. Testing of HIV and malaria vaccines candidates start with early phase trials that are increasingly being conducted in developing countries where the burden of the diseases is high. Unique challenges, which affect planning and implementation of vaccine trials according to internationally accepted standards have thus been identified. In this review, we highlight specific challenges encountered during two early phase trials of novel HIV-1 and malaria vectored vaccine candidates conducted in The Gambia and how some of these issues were pragmatically addressed. We hope our experience will be useful for key study personnel involved in day-to-day running of similar clinical trials. It may also guide future design and implementation of vaccine trials in resource-constrained settings.

Broadly neutralizing antibodies and the promise of universal vaccines: where are we now?

Recent research has provided strong support for the utility of broadly neutralizing antibodies generated against viruses, which inherently possess a high degree of antigenic variability (such as influenza virus or HIV) as a feasible means to prevent infection. Many of these antibodies share the ability to bind to highly conserved regions within the stem of the virus ‘spike’ or surface glycoprotein, in such a way that they interfere with virus entry, including membrane fusion. As a result, broadly neutralizing antibodies could be supplied to patients as a form of passive immunotherapy, as well as play a role in the design of new ‘universal’ vaccines and antiviral agents. The following article describes the most recent innovations in this exciting field.

10E8-like neutralizing antibodies against HIV-1 induced using a precisely designed conformational peptide as a vaccine prime

Recent studies have demonstrated that the membrane-proximal external region (MPER) of human immunodeficiency virus 1 (HIV-1) glycoprotein 41 contains a series of epitopes for human monoclonal antibodies, including 2F5, Z13e1, 4E10, and 10E8, which were isolated from HIV-1-infected individuals and show broad neutralizing activities. This suggests that MPER is a good target for the development of effective HIV-1 vaccines. However, many studies have shown that it is difficult to induce antibodies with similar broad neutralizing activities using MPER-based peptide antigens. Here, we report that 10E8-like neutralizing antibodies with effective anti-HIV-1 activity were readily induced using a precisely designed conformational immunogenic peptide containing the 10E8-specific epitope. This immunogenic peptide (designated T10HE) contains a 15-mer MPER-derived 10E8-specific epitope fused to T-helper-cell epitopes from tetanus toxin (tt), which showed a significantly stabilized α-helix structure after a series of modifications, including substitution with an (S)-α-(2'-pentenyl) alanine containing an olefin-bearing tether and ruthenium-catalyzed olefin metathesis, compared with the unmodified T10E peptide. The stabilized α-helix structure of T10HE did not affect its capacity to bind the 10E8 antibody, as evaluated with an enzyme linked immunosorbent assay (ELISA) and surface plasmon resonance binding assay (SPR assay). The efficacies of the T10HE and T10E epitope vaccines were evaluated after a standard vaccination procedure in which the experimental mice were primed with either the T10HE or T10E immunogen and boosted with HIV-1 JRFL pseudoviruses. Higher titers of 10E8-like antibodies were induced by T10HE than that by T10E. More importantly, the antibodies induced by T10HE showed enhanced antiviral potency against HIV-1 strains with both X4 and R5 tropism and a greater degree of broad neutralizing activity than the antibodies induced by T10E. These results indicate that a 10E8-epitope-based structure-specific peptide immunogen can elicit neutralizing antibodies when used as a vaccine prime.

Conference report: "Functional Glycomics in HIV Type 1 Vaccine Design" workshop report, Bethesda, Maryland, April 30-May 1, 2012

A vital part of the renewed hope for a vaccine against the human immunodeficiency virus (HIV-1) is based on recent studies that have highlighted major sites of HIV-1 vulnerability that could be effectively targeted by a preventive vaccine. One of these potential vulnerabilities includes the dense cluster of carbohydrates surrounding HIV-1's envelope glycoproteins gp120 and gp41, typically referred to as the "glycan shield." Recent data from several laboratories have shown that glycans on the HIV-1 envelope form key epitopes for broadly neutralizing antibodies (bNAb). Moreover, HIV-1 envelope glycans play an important role in viral transmission, antigenicity, and immunogenicity. The recent availability of novel tools and technologies has now allowed investigators to leverage glycomic structure-function relationships in the design of candidate HIV-1 vaccines. Additionally, glycans modulate the immune response, playing an essential role in Fc receptor and complement activity. To promote cross-disciplinary collaboration and promote synergistic HIV-1- glycomics research, the National Institutes of Health (NIH) cosponsored and convened a 1.5-day workshop entitled "Functional Glycomics in HIV-1 Vaccine Design." The meeting focused on the role of glycan interactions with neutralizing antibodies, the influence of immunoglobulin G (IgG) Fc receptor glycosylation, newly available glycomics technologies, and how new information on the role of glycans could be applied in HIV-1 immunogen design strategies. This report summarizes the discussions of this workshop.

Clinical use of vaginal or rectally applied microbicides in patients suffering from HIV/AIDS

Microbicides, primarily used as topical pre-exposure prophylaxis, have been proposed to prevent sexual transmission of HIV. This review covers the trends and challenges in the development of safe and effective microbicides to prevent sexual transmission of HIV Initial phases of microbicide development used such surfactants as nonoxynol-9 (N-9), C13G, and sodium lauryl sulfate, aiming to inactivate the virus. Clinical trials of microbicides based on N-9 and C31G failed to inhibit sexual transmission of HIV. On the contrary, N-9 enhanced susceptibility to sexual transmission of HIV-1. Subsequently, microbicides based on polyanions and a variety of other compounds that inhibit the binding, fusion, or entry of virus to the host cells were evaluated for their efficacy in different clinical setups. Most of these trials failed to show either safety or efficacy for prevention of HIV transmission. The next phase of microbicide development involved antiretroviral drugs. Microbicide in the form of 1% tenofovir vaginal gel when tested in a Phase IIb trial (CAPRISA 004) in a coitally dependent manner revealed that tenofovir gel users were 39% less likely to become HIV-infected compared to placebo control. However, in another trial (VOICE MTN 003), tenofovir gel used once daily in a coitally independent mode failed to show any efficacy to prevent HIV infection. Tenofovir gel is currently in a Phase III safety and efficacy trial in South Africa (FACTS 001) employing a coitally dependent dosing regimen. Further, long-acting microbicide-delivery systems (vaginal ring) for slow release of such antiretroviral drugs as dapivirine are also undergoing clinical trials. Discovering new markers as correlates of protective efficacy, novel long-acting delivery systems with improved adherence in the use of microbicides, discovering new compounds effective against a broad spectrum of HIV strains, developing multipurpose technologies incorporating additional features of efficacy against other sexually transmitted infections, and contraception will help in moving the field of microbicide development forward.

Chimeric rhinoviruses displaying MPER epitopes elicit anti-HIV neutralizing responses

Background

The development of an effective AIDS vaccine has been a formidable task, but remains a critical necessity. The well conserved membrane-proximal external region (MPER) of the HIV-1 gp41 glycoprotein is one of the crucial targets for AIDS vaccine development, as it has the necessary attribute of being able to elicit antibodies capable of neutralizing diverse isolates of HIV.

Methodology/Principle Findings

Guided by X-ray crystallography, molecular modeling, combinatorial chemistry, and powerful selection techniques, we designed and produced six combinatorial libraries of chimeric human rhinoviruses (HRV) displaying the MPER epitopes corresponding to mAbs 2F5, 4E10, and/or Z13e1, connected to an immunogenic surface loop of HRV via linkers of varying lengths and sequences. Not all libraries led to viable chimeric viruses with the desired sequences, but the combinatorial approach allowed us to examine large numbers of MPER-displaying chimeras. Among the chimeras were five that elicited antibodies capable of significantly neutralizing HIV-1 pseudoviruses from at least three subtypes, in one case leading to neutralization of 10 pseudoviruses from all six subtypes tested.

Conclusions

Optimization of these chimeras or closely related chimeras could conceivably lead to useful components of an effective AIDS vaccine. While the MPER of HIV may not be immunodominant in natural infection by HIV-1, its presence in a vaccine cocktail could provide critical breadth of protection.

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

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

A twin-cysteine motif in the V2 region of gp120 is associated with SIV envelope trimer stabilization

The V1 and V2 variable regions of the primate immunodeficiency viruses contribute to the trimer association domain of the gp120 exterior envelope glycoprotein. A pair of V2 cysteine residues at 183 and 191 (“twin cysteines”) is present in several simian immunodeficiency viruses, human immunodeficiency virus type 2 (HIV-2) and some SIV_cpz lineages, but not in HIV-1. To examine the role of this potentially disulfide-bonded twin-cysteine motif, the cysteine residues in the SIVmac239 envelope glycoproteins were individually and pairwise substituted by alanine residues. All of the twin-cysteine mutants exhibited decreases in gp120 association with the Env trimer, membrane-fusing activity, and ability to support virus entry. Thus, the twin-cysteine motif plays a role in Env trimer stabilization in SIV and may do so in HIV-2 and some SIV_cpz as well. This implies that HIV-1 lost the twin-cysteines, and may have relatively unstable Env trimers compared to SIV and HIV-2.

Development of prophylactic vaccines against HIV-1

The focus of most current HIV-1 vaccine development is on antibody-based approaches. This is because certain antibody responses correlated with protection from HIV-1 acquisition in the RV144 phase III trial, and because a series of potent and broad spectrum neutralizing antibodies have been isolated from infected individuals. Taken together, these two findings suggest ways forward to develop a neutralizing antibody-based vaccine. However, understanding of the correlates of protection from disease in HIV-1 and other infections strongly suggests that we should not ignore CTL-based research. Here we review recent progress in the field and highlight the challenges implicit in HIV-1 vaccine design and some potential solutions.

HIV-1 neutralizing antibodies: understanding nature's pathways

The development of an effective vaccine has been hindered by the enormous diversity of human immunodeficiency virus-1 (HIV-1) and its ability to escape a myriad of host immune responses. In addition, conserved vulnerable regions on the HIV-1 envelope glycoprotein are often poorly immunogenic and elicit broadly neutralizing antibody responses (BNAbs) in a minority of HIV-1-infected individuals and only after several years of infection. All of the known BNAbs demonstrate high levels of somatic mutations and often display other unusual traits, such as a long heavy chain complementarity determining region 3 (CDRH3) and autoreactivity that can be limited by host tolerance controls. Nonetheless, the demonstration that HIV-1-infected individuals can make potent BNAbs is encouraging, and recent progress in isolating such antibodies and mapping their immune pathways of development is providing new strategies for vaccination.

Synthetic glycopeptides reveal the glycan specificity of HIV-neutralizing antibodies

A new class of glycan-reactive HIV-neutralizing antibodies, including PG9 and PG16, has been recently discovered that appear to recognize novel glycopeptide epitopes on HIV-1 gp120. However, further characterization and reconstitution of the precise neutralizing epitopes are complicated by the heterogeneity of glycosylation. We report here the design, synthesis, and antigenic evaluation of novel cyclic V1V2 glycopeptides carrying defined N-linked glycans at the conserved glycosylation sites (N160 and N156/N173) derived from gp120 of two HIV-1 isolates. Antibody binding studies confirmed the necessity of a Man₅GlcNAc₂ glycan at N160 for recognition by PG9 and PG16, and further revealed a critical role of a sialylated N-glycan at the secondary site (N156/N173) in the context of glycopeptides for antibody binding. In addition to defining the glycan specificities of PG9 and PG16, the identified synthetic glycopeptides provide a valuable template for HIV-1 vaccine design.

The Antibody Response against HIV-1

Neutralizing antibodies (NAbs) typically play a key role in controlling viral infections and contribute to the protective effect of many successful vaccines. In the case of HIV-1 infection, there is compelling data in experimental animal models that NAbs can prevent HIV-1 acquisition, although there is no similar data in humans and their role in controlling established infection in humans is also limited. It is clear HIV-specific NAbs drive the evolution of the HIV-1 envelope glycoprotein within an infected individual. The virus's ability to evade immune selection may be the main reason HIV-1 NAbs exert limited control during infection. The extraordinary antigenic diversity of HIV-1 also presents formidable challenges to defining NAbs that could provide broad protection against diverse circulating HIV-1 strains. Several new potent monoclonal antibodies (MAbs) have been identified, and are beginning to yield important clues into the epitopes common to diverse HIV-1 strains. In addition, antibodies can also act in concert with effector cells to kill HIV-infected cells; this could provide another mechanism for antibody-mediated control of HIV-1 replication. Understanding the impact of antibodies on HIV-1 transmission and pathogenesis is critical to helping move forward with rational HIV-1 vaccine design.

Report of the Cent Gardes HIV Vaccine Conference: The B-cell Response to HIV. Part 1: Broadly Neutralizing Antibodies Fondation Mérieux Conference Center, Veyrier du Lac, France, 5-7 November 2012

The Cent Gardes Conference on HIV Vaccines took place on November 5-7, 2012 at the Fondation Mérieux Conference Center, Annecy, France. The aim of the meeting was to review the B cell response to human immunodeficiency virus-1 (HIV-1) infection and immunization, from broadly neutralizing antibodies (bnAbs) to non-neutralizing antibodies (n-nAbs). The production of cross-reactive bnAbs is one of the greatest challenges in HIV-1 vaccine development. In natural HIV infection, bnAbs are observed in only a minority of infected individuals and take a few years to develop. This report presents a comprehensive review of how these Abs arise, the possible role of viral evolution, and the activation and maturation requirements of B cell lines that lead to their production. Passive immunization with bnAbs provides efficient, cross-clade protection against simian/human immunodeficiency chimeric virus (SHIV) challenges in nonhuman primates. Despite many efforts to design immunogens that elicit them by active immunization, no immunogen other than HIV itself has yet been able to elicit a bnAb response. For this reason, innovative approaches are under investigation, including their production in the body through a gene delivery approach, vector immunoprophylaxis.

A Blueprint for HIV Vaccine Discovery

Despite numerous attempts over many years to develop an HIV vaccine based on classical strategies, none has convincingly succeeded to date. A number of approaches are being pursued in the field, including building upon possible efficacy indicated by the recent RV144 clinical trial, which combined two HIV vaccines. Here, we argue for an approach based, in part, on understanding the HIV envelope spike and its interaction with broadly neutralizing antibodies (bnAbs) at the molecular level and using this understanding to design immunogens as possible vaccines. BnAbs can protect against virus challenge in animal models, and many such antibodies have been isolated recently. We further propose that studies focused on how best to provide T cell help to B cells that produce bnAbs are crucial for optimal immunization strategies. The synthesis of rational immunogen design and immunization strategies, together with iterative improvements, offers great promise for advancing toward an HIV vaccine.

A gp41 MPER-specific llama VHH requires a hydrophobic CDR3 for neutralization but not for antigen recognition

The membrane proximal external region (MPER) of the HIV-1 glycoprotein gp41 is targeted by the broadly neutralizing antibodies 2F5 and 4E10. To date, no immunization regimen in animals or humans has produced HIV-1 neutralizing MPER-specific antibodies. We immunized llamas with gp41-MPER proteoliposomes and selected a MPER-specific single chain antibody (VHH), 2H10, whose epitope overlaps with that of mAb 2F5. Bi-2H10, a bivalent form of 2H10, which displayed an approximately 20-fold increased affinity compared to the monovalent 2H10, neutralized various sensitive and resistant HIV-1 strains, as well as SHIV strains in TZM-bl cells. X-ray and NMR analyses combined with mutagenesis and modeling revealed that 2H10 recognizes its gp41 epitope in a helical conformation. Notably, tryptophan 100 at the tip of the long CDR3 is not required for gp41 interaction but essential for neutralization. Thus bi-2H10 is an anti-MPER antibody generated by immunization that requires hydrophobic CDR3 determinants in addition to epitope recognition for neutralization similar to the mode of neutralization employed by mAbs 2F5 and 4E10.

Safety and immunogenicity of DNA prime and modified vaccinia ankara virus-HIV subtype C vaccine boost in healthy adults

A randomized, double-blind, placebo-controlled phase I trial was conducted in 32 HIV-uninfected healthy volunteers to assess the safety and immunogenicity of 3 doses of DNA vaccine (Advax) plus 1 dose of recombinant modified vaccinia virus Ankara (MVA) (TBC-M4) or 3 doses of TBC-M4 alone (groups A and B, respectively). Both vaccine regimens were found to be safe and well tolerated. Gamma interferon (IFN-γ) enzyme-linked immunosorbent spot (ELISPOT) assay responses were detected in 1/10 (10%) individuals in group A after three Advax primes and in 9/9 individuals (100%) after the MVA boost. In group B, IFN-γ ELISPOT responses were detected in 6/12 (50%) and 7/11 (64%) individuals after the second and third MVA vaccinations, respectively. Responses to all vaccine components, but predominantly to Env, were seen. The breadth and magnitude of the T cell response and viral inhibition were greater in group A than in group B, indicating that the quality of the T-cell response was enhanced by the DNA prime. Intracellular cytokine staining indicated that the T-cell responses were polyfunctional but were skewed toward Env with a CD4(+) phenotype. At 2 weeks after the last vaccination, HIV-specific antibody responses were detected in all (100%) group B and 1/11 (9.1%) group A vaccinees. Vaccinia virus-specific responses were detected in all (100%) group B and 2/11 (18.2%) group A vaccinees. In conclusion, HIV-specific T-cell responses were seen in the majority of volunteers in groups A and B but with a trend toward greater quality of the T-cell response in group A. Antibody responses were better in group B than in group A.

Preparation of biologically active single-chain variable antibody fragments that target the HIV-1 gp120 V3 loop

KD-247 is a humanized monoclonal antibody that targets the third hypervariable (V3) loop of gp120. It can efficiently neutralize a broad panel of clade B, but not non-clade B, HIV-1 isolates. To overcome this limitation, we are seeking to prepare genetically-engineered single-chain variable fragments (scFvs) of KD-247 that will have broader neutralizing activity against both clade B and non-clade B HIV-1 isolates. Initial attempts of optimizing the expression of KD-247 scFv have resulted in the formation of insoluble protein. Therefore, we have established purification protocols to recover, purify, and refold the KD-247 scFv from inclusion bodies. The protocol involved step-wise refolding of denatured scFv by dilution, dialysis, and on-column nickel-affinity purification. Monomeric scFv was further purified by size-exclusion chromatography. Using far UV circular dichroism (CD) spectroscopy we confirmed the expected beta-sheet profile of the refolded KD-247 scFv. Importantly, the refolded KD-247 scFv showed neutralizing activity against replication-competent HIV-1 BaL and JR-FL Env pseudotyped HIV-1, at potency comparable to that of the native full-size KD-247 antibody. Ongoing studies focus on the application of this system in generating KD-247 scFv variants with the ability to neutralize clade B and non-clade B HIV-1 isolates.

Designing a soluble near full-length HIV-1 gp41 trimer

The HIV-1 envelope spike is a trimer of heterodimers composed of an external glycoprotein gp120 and a transmembrane glycoprotein gp41. gp120 initiates virus entry by binding to host receptors, whereas gp41 mediates fusion between viral and host membranes. Although the basic pathway of HIV-1 entry has been extensively studied, the detailed mechanism is still poorly understood. Design of gp41 recombinants that mimic key intermediates is essential to elucidate the mechanism as well as to develop potent therapeutics and vaccines. Here, using molecular genetics and biochemical approaches, a series of hypotheses was tested to overcome the extreme hydrophobicity of HIV-1 gp41 and design a soluble near full-length gp41 trimer. The two long heptad repeat helices HR1 and HR2 of gp41 ectodomain were mutated to disrupt intramolecular HR1-HR2 interactions but not intermolecular HR1-HR1 interactions. This resulted in reduced aggregation and improved solubility. Attachment of a 27-amino acid foldon at the C terminus and slow refolding channeled gp41 into trimers. The trimers appear to be stabilized in a prehairpin-like structure, as evident from binding of a HR2 peptide to exposed HR1 grooves, lack of binding to hexa-helical bundle-specific NC-1 mAb, and inhibition of virus neutralization by broadly neutralizing antibodies 2F5 and 4E10. Fusion to T4 small outer capsid protein, Soc, allowed display of gp41 trimers on the phage nanoparticle. These approaches for the first time led to the design of a soluble gp41 trimer containing both the fusion peptide and the cytoplasmic domain, providing insights into the mechanism of entry and development of gp41-based HIV-1 vaccines.

CryoEM visualization of an adenovirus capsid-incorporated HIV antigen

Adenoviral (Ad) vectors show promise as platforms for vaccine applications against infectious diseases including HIV. However, the requirements for eliciting protective neutralizing antibody and cellular immune responses against HIV remain a major challenge. In a novel approach to generate 2F5- and 4E10-like antibodies, we engineered an Ad vector with the HIV membrane proximal ectodomain region (MPER) epitope displayed on the hypervariable region 2 (HVR2) of the viral hexon capsid, instead of expressed as a transgene. The structure and flexibility of MPER epitopes, and the structural context of these epitopes within viral vectors, play important roles in the induced host immune responses. In this regard, understanding the critical factors for epitope presentation would facilitate optimization strategies for developing viral vaccine vectors. Therefore we undertook a cryoEM structural study of this Ad vector, which was previously shown to elicit MPER-specific humoral immune responses. A subnanometer resolution cryoEM structure was analyzed with guided molecular dynamics simulations. Due to the arrangement of hexons within the Ad capsid, there are twelve unique environments for the inserted peptide that lead to a variety of conformations for MPER, including individual α-helices, interacting α-helices, and partially extended forms. This finding is consistent with the known conformational flexibility of MPER. The presence of an extended form, or an induced extended form, is supported by interaction of this vector with the human HIV monoclonal antibody 2F5, which recognizes 14 extended amino acids within MPER. These results demonstrate that the Ad capsid influences epitope structure, flexibility and accessibility, all of which affect the host immune response. In summary, this cryoEM structural study provided a means to visualize an epitope presented on an engineered viral vector and suggested modifications for the next generation of Ad vectors with capsid-incorporated HIV epitopes.