Moving ahead an HIV vaccine: to neutralize or not, a key HIV vaccine question

A MUC16 IgG Binding Activity Selects for a Restricted Subset of IgG Enriched for Certain Simian Immunodeficiency Virus Epitope Specificities

We have recently shown that MUC16, a component of the glycocalyx of some mucosal barriers, has elevated binding to the G0 glycoform of the Fc portion of IgG. Therefore, IgG from patients chronically infected with human immunodeficiency virus (HIV), who typically exhibit increased amounts of G0 glycoforms, showed increased MUC16 binding compared to uninfected controls. Using the rhesus macaque simian immunodeficiency virus SIVmac251 model, we can compare plasma antibodies before and after chronic infection. We find increased binding of IgG to MUC16 after chronic SIV infection. Antibodies isolated for tight association with MUC16 (MUC16-eluted antibodies) show reduced FcγR engagement and antibody-dependent cellular cytotoxicity (ADCC) activity. The glycosylation profile of these IgGs was consistent with a decrease in FcγR engagement and subsequent ADCC effector function, as they contain a decrease in afucosylated bisecting glycoforms that preferentially bind FcγRs. Testing of the SIV antigen specificity of IgG from SIV-infected macaques revealed that the MUC16-eluted antibodies were enriched for certain specific epitopes, including regions of gp41 and gp120. This enrichment of specific antigen responses for fucosylated bisecting glycoforms and the subsequent association with MUC16 suggests that the immune response has the potential to direct specific epitope responses to localize to the glycocalyx through interaction with this specific mucin.IMPORTANCE Understanding how antibodies are distributed in the mucosal environment is valuable for developing a vaccine to block HIV infection. Here, we study an IgG binding activity in MUC16, potentially representing a new IgG effector function that would concentrate certain antibodies within the glycocalyx to trap pathogens before they can reach the underlying columnar epithelial barriers. These studies reveal that rhesus macaque IgG responses during chronic SIV infection generate increased antibodies that bind MUC16, and interestingly, these MUC16-tethered antibodies are enriched for binding to certain antigens. Therefore, it may be possible to direct HIV vaccine-generated responses to associate with MUC16 and enhance the antibody's ability to mediate immune exclusion by trapping virions within the glycocalyx and preventing the virus from reaching immune target cells within the mucosa. This concept will ultimately have to be tested in the rhesus macaque model, which is shown here to have MUC16-targeted antigen responses.

Predicting first traversal times for virions and nanoparticles in mucus with slowed diffusion

Particle-tracking experiments focusing on virions or nanoparticles in mucus have measured mean-square displacements and reported diffusion coefficients that are orders of magnitude smaller than the diffusion coefficients of such particles in water. Accurate description of this subdiffusion is important to properly estimate the likelihood of virions traversing the mucus boundary layer and infecting cells in the epithelium. However, there are several candidate models for diffusion that can fit experimental measurements of mean-square displacements. We show that these models yield very different estimates for the time taken for subdiffusive virions to traverse through a mucus layer. We explain why fits of subdiffusive mean-square displacements to standard diffusion models may be misleading. Relevant to human immunodeficiency virus infection, using computational methods for fractional subdiffusion, we show that subdiffusion in normal acidic mucus provides a more effective barrier against infection than previously thought. By contrast, the neutralization of the mucus by alkaline semen, after sexual intercourse, allows virions to cross the mucus layer and reach the epithelium in a short timeframe. The computed barrier protection from fractional subdiffusion is some orders of magnitude greater than that derived by fitting standard models of diffusion to subdiffusive data.

Cross-Neutralization Activity of Single-Chain Variable Fragment (scFv) Derived from Anti-V3 Monoclonal Antibodies Mediated by Post-Attachment Binding

The V3 loop in the envelope (Env) of HIV-1 is one of the major targets of neutralizing antibodies. However, this antigen is hidden inside the Env trimer in most isolates and is fully exposed only during CD4-gp120 interaction. Thus, primary HIV-1 isolates are relatively resistant to anti-V3 antibodies because IgG is too large to access the V3 loop. To overcome this obstacle, we constructed single-chain variable fragments (scFvs) from anti-V3 monoclonal antibodies 0.5γ, 5G2, and 16G6. Enhanced neutralization by 0.5γ and 5G2 scFvs was observed in strains resistant to their IgG counterparts. Neutralization coverage by 0.5γ scFv reached up to 90% of the tested viruses (tier 2 and 3 classes). The temperature-regulated neutralization assay revealed that extensive cross-neutralization of 0.5γ scFv can be explained by post-attachment neutralization. Neutralization assay involving viruses carrying an inter-subunit disulfide bond (SOS virus) showed that the neutralization-susceptible timeframe after attachment was 60 to 120 min. These results indicate that the scFvs efficiently access the V3 loop and subsequently neutralize HIV-1, even after virus attachment to the target cells. Based on its broad and potent neutralizing activity, further development of anti-V3 scFv for therapeutic and preventive strategies is warranted.

HIV Envelope Trimer Specific Immune Response Is Influenced by Different Adjuvant Formulations and Heterologous Prime-Boost

The development of a preventive vaccine against human immunodeficiency virus (HIV-1) infection is the most efficient method to control the epidemic. The ultimate goal is to develop a vaccine able to induce specific neutralizing, non-neutralizing antibodies and cellular mediated immunity (CMI). Humoral and CMI responses can be directed to glycoproteins that are normally presented as a trimeric spike on the virus surface (gp140). Despite safer, subunit vaccines are normally less immunogenic/effective and need to be delivered together with an adjuvant. The choice of a suitable adjuvant can induce effective humoral and CMI that utterly lead to full protection against disease. In this report, we established a hierarchy of adjuvant potency on humoral and CMI when admixed with the recombinant HIV gp140 trimer. We show that vaccination with gp140 in the presence of different adjuvants can induce high-affinity antibodies, follicular helper T cells and germinal center B cells. The data show that poly (I:C) is the most potent adjuvant to induce specific CMI responses evidenced by IFN-γ production and CD4⁺/CD8⁺ T cell proliferation. Furthermore, we demonstrate that combining some adjuvants like MPL plus Alum and MPL plus MDP exert additive effects that impact on the magnitude and quality of humoral responses while mixing MDP with poly (I:C) or with R848 had no impact on total IgG titers but highly impact IgG subclass. In addition, heterologous DNA prime- protein boost yielded higher IgG titers when compare to DNA alone and improved the quality of humoral response when compare to protein immunization as evidenced by IgG1/IgG2a ratio. The results presented in this paper highlight the importance of selecting the correct adjuvant-antigen combination to potentiate desired cells for optimal stimulation.

Short Communication: Exploring Antibody Potential as Prophylactic/Therapeutic Strategies for Prevention of Early Mucosal HIV-1 Infection

Mucosal tissues are the predominant sites for genital HIV-1 transmission. We investigated the mechanisms by which broadly neutralizing antibodies (bNAbs) inhibit HIV-1 replication in a coculture model including primary mucosal dendritic cells (DCs), such as Langerhans cells, interstitial dendritic cells, and CD4(+) T lymphocytes. We show that bNAbs efficiently prevent HIV-1 infection by inhibiting HIV-1 transmission to CD4(+) T lymphocytes. This inhibition of cell-to-cell transmission was observed with equal potency as the inhibition of cell-free infection of primary CD4(+) T lymphocytes. In addition, a decrease in HIV-1 replication in DCs and the induction of DC maturation were detected. This additional inhibition was Fc mediated as it was blocked by the use of specific anti-FcγR monoclonal Abs. The DC maturation by bNAbs during HIV transmission may contribute to mucosal protection. Therefore, multiple antibody-mediated inhibitory functions should be combined for the improvement of future preventive/therapeutic strategies to cure HIV.

Short Communication: Virion Aggregation by Neutralizing and Nonneutralizing Antibodies to the HIV-1 Envelope Glycoprotein

We used dynamic light scattering to detect aggregation of HIV-1 virions by antibodies (IgG) to the viral envelope glycoprotein (Env). Virions of different strains were inactivated by 2,2'-dithiodipyridine (AT-2), a procedure that abrogates infectivity but preserves the native antigenic structure of Env. Neutralizing antibodies directed to a V3-base- and glycan-dependent epitope on gp120 and to the apex of the Env trimer, as well as nonneutralizing antibodies to the epitope cluster I on the gp41-ectodomain, aggregated virions, but in markedly narrow concentration ranges. In contrast, the neutralizing antibody 2G12, which is specific for a composite glycan-dependent epitope on gp120 and functionally monovalent because of its unusual domain-swap structure, was nonaggregating. These results have potentially complex implications for vaccine development.

A High Throughput Protein Microarray Approach to Classify HIV Monoclonal Antibodies and Variant Antigens

In recent years, high throughput discovery of human recombinant monoclonal antibodies (mAbs) has been applied to greatly advance our understanding of the specificity, and functional activity of antibodies against HIV. Thousands of antibodies have been generated and screened in functional neutralization assays, and antibodies associated with cross-strain neutralization and passive protection in primates, have been identified. To facilitate this type of discovery, a high throughput-screening tool is needed to accurately classify mAbs, and their antigen targets. In this study, we analyzed and evaluated a prototype microarray chip comprised of the HIV-1 recombinant proteins gp140, gp120, gp41, and several membrane proximal external region peptides. The protein microarray analysis of 11 HIV-1 envelope-specific mAbs revealed diverse binding affinities and specificities across clades. Half maximal effective concentrations, generated by our chip analysis, correlated significantly (P<0.0001) with concentrations from ELISA binding measurements. Polyclonal immune responses in plasma samples from HIV-1 infected subjects exhibited different binding patterns, and reactivity against printed proteins. Examining the totality of the specificity of the humoral response in this way reveals the exquisite diversity, and specificity of the humoral response to HIV.

Modulation of SIV and HIV DNA vaccine immunity by Fas-FasL signaling

Signaling through the Fas/Apo-1/CD95 death receptor is known to affect virus-specific cell-mediated immune (CMI) responses. We tested whether modulating the Fas-apoptotic pathway can enhance immune responses to DNA vaccination or lymphocytic choriomeningitis virus (LCMV) infection. Mice were electroporated with plasmids expressing a variety of pro- or anti-apoptotic molecules related to Fas signaling and then either LCMV-infected or injected with plasmid DNA expressing SIV or HIV antigens. Whereas Fas or FasL knockout mice had improved CMI, down-regulation of Fas or FasL by shRNA or antibody failed to improve CMI and was accompanied by increases in regulatory T cells (Treg). Two “adjuvant” plasmids were discovered that significantly enhanced plasmid immunizations. The adjuvant effects of Fas-associated death domain (FADD) and of cellular FLICE-inhibitory protein (cFLIP) were consistently accompanied by increased effector memory T lymphocytes and increased T cell proliferation. This adjuvant effect was also observed when comparing murine infections with LCMV-Armstrong and its persisting variant LCMV-Clone 13. LCMV-Armstrong was cleared in 100% of mice nine days after infection, while LCMV-Clone 13 persisted in all mice. However, half of the mice pre-electroporated with FADD or cFLIP plasmids were able to clear LCMV-Clone 13 by day nine, and, in the case of cFLIP, increased viral clearance was accompanied by higher CMI. Our studies imply that molecules in the Fas pathway are likely to affect a number of events in addition to the apoptosis of cells involved in immunity.

Antibody to the gp120 V1/V2 loops and CD4+ and CD8+ T cell responses in protection from SIVmac251 vaginal acquisition and persistent viremia

The human papillomavirus pseudovirions (HPV-PsVs) approach is an effective gene-delivery system that can prime or boost an immune response in the vaginal tract of nonhuman primates and mice. Intravaginal vaccination with HPV-PsVs expressing SIV genes, combined with an i.m. gp120 protein injection, induced humoral and cellular SIV-specific responses in macaques. Priming systemic immune responses with i.m. immunization with ALVAC-SIV vaccines, followed by intravaginal HPV-PsV-SIV/gp120 boosting, expanded and/or recruited T cells in the female genital tract. Using a stringent repeated low-dose intravaginal challenge with the highly pathogenic SIVmac251, we show that although these regimens did not demonstrate significant protection from virus acquisition, they provided control of viremia in a number of animals. High-avidity Ab responses to the envelope gp120 V1/V2 region correlated with delayed SIVmac251 acquisition, whereas virus levels in mucosal tissues were inversely correlated with antienvelope CD4(+) T cell responses. CD8(+) T cell depletion in animals with controlled viremia caused an increase in tissue virus load in some animals, suggesting a role for CD8(+) T cells in virus control. This study highlights the importance of CD8(+) cells and antienvelope CD4(+) T cells in curtailing virus replication and antienvelope V1/V2 Abs in preventing SIVmac251 acquisition.

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.

Live simian immunodeficiency virus vaccine correlate of protection: local antibody production and concentration on the path of virus entry

We sought design principles for a vaccine to prevent HIV transmission to women by identifying correlates of protection conferred by a highly effective live attenuated SIV vaccine in the rhesus macaque animal model. We show that SIVmac239Δnef vaccination recruits plasma cells and induces ectopic lymphoid follicle formation beneath the mucosal epithelium in the rhesus macaque female reproductive tract. The plasma cells and ectopic follicles produce IgG Abs reactive with viral envelope glycoprotein gp41 trimers, and these Abs are concentrated on the path of virus entry by the neonatal FcR in cervical reserve epithelium and in vaginal epithelium. This local Ab production and delivery system correlated spatially and temporally with the maturation of local protection against high-dose pathogenic SIV vaginal challenge. Thus, designing vaccines to elicit production and concentration of Abs at mucosal frontlines could aid in the development of an effective vaccine to protect women against HIV-1.

Fcgbp - A Potential Viral Trap in RV144

Years of extensive research have yielded much knowledge in many aspects of HIV-1 infection, treatments, and education. However, without a vaccine, the number of people infected worldwide continues to grow. The partial success of the Thai RV144 vaccine trial provides hope that a method of protection is indeed possible. Understanding the mechanism behind the protection is critical if we hope to achieve our goal of inhibiting new infections of HIV-1. We hypothesize that the Fc of IgG binding protein (Fcgbp) is associated with the protection observed in the RV144 vaccine trial. It has the ability to trap viral-antibody complexes in the mucosa by binding the Fc of IgG to Fcgbp. This property could be used in the form of a microbicide containing antibodies to a variety of HIV-1 epitopes to prevent sexual transmission of HIV-1. The aim of this paper is to stimulate further research into Fcgbp and its role in innate immunity.

Vaccine-induced HIV-1 envelope gp120 constant region 1-specific antibodies expose a CD4-inducible epitope and block the interaction of HIV-1 gp140 with galactosylceramide

Mucosal epithelial cell surface galactosylceramide (Galcer) has been postulated to be a receptor for HIV-1 envelope (Env) interactions with mucosal epithelial cells. Disruption of the HIV-1 Env interaction with such alternate receptors could be one strategy to prevent HIV-1 entry through the mucosal barrier. To study antibody modulation of HIV-1 Env-Galcer interactions, we used Galcer-containing liposomes to assess whether natural- and vaccine-induced monoclonal antibodies can block HIV-1 Env binding to Galcer. HIV-1 Env gp140 proteins bound to Galcer liposomes with Kds (dissociation constants) in the nanomolar range. Several HIV-1 ALVAC/AIDSVAX vaccinee-derived monoclonal antibodies (MAbs) specific for the gp120 first constant (C1) region blocked Galcer binding of a transmitted/founder HIV-1 Env gp140. Among the C1-specific MAbs that showed Galcer blocking, the antibody-dependent cellular cytotoxicity-mediating CH38 IgG and its natural IgA isotype were the most potent blocking antibodies. C1-specific IgG monoclonal antibodies that blocked Env binding to Galcer induced upregulation of the gp120 CD4-inducible (CD4i) epitope bound by MAb 17B, demonstrating that a conformational change in gp120 may be required for Galcer blocking. However, the MAb 17B itself did not block Env-Galcer binding, suggesting that the C1 antibody-induced gp120 conformational changes resulted in alteration in a Galcer binding site distant from the CD4i 17B MAb binding site.

IMPORTANCE

Galactosyl ceramide, a glycosphingolipid, has been postulated to be a receptor for the HIV-1 envelope glycoprotein (Env) interaction with mucosal epithelial cells. Here, we have mimicked this interaction by using an artificial membrane containing synthetic Galcer and recombinant HIV-1 Env proteins to identify antibodies that would block the HIV-1 Env-Galcer interaction. Our study revealed that a class of vaccine-induced human antibodies potently blocks HIV-1 Env-Galcer binding by perturbing the HIV-1 Env conformation.

Nonneutralizing functional antibodies: a new "old" paradigm for HIV vaccines

Animal and human data from various viral infections and vaccine studies suggest that nonneutralizing antibodies (nNAb) without neutralizing activity in vitro may play an important role in protection against viral infection in vivo. This was illustrated by the recent human immunodeficiency virus (HIV) RV144 vaccine efficacy trial, which demonstrated that HIV-specific IgG-mediated nNAb directed against the V2 loop of HIV type 1 envelope (Env) were inversely correlated with risk for HIV acquisition, while Env-specific plasma IgA-mediated antibodies were directly correlated with risk. However, tier 1 NAb in the subset of responders with a low level of plasma Env-specific IgA correlated with decreased risk. Nonhuman primate simian immunodeficiency virus (SIV) and simian-human immunodeficiency virus (SHIV) challenge studies suggest that Env-mediated antibodies are essential and sufficient for protection. A comparison of immune responses generated in human efficacy trials reveals subtle differences in the fine specificities of the antibody responses, in particular in HIV-specific IgG subclasses. The underlying mechanisms that may have contributed to protection against HIV acquisition in humans, although not fully understood, are possibly mediated by antibody-dependent cell-mediated cytotoxicity (ADCC) and/or other nonneutralizing humoral effector functions, such as antibody-mediated phagocytosis. The presence of such functional nNAb in mucosal tissues and cervico-vaginal and rectal secretions challenges the paradigm that NAb are the predominant immune response conferring protection, although this does not negate the desirability of evoking neutralizing antibodies through vaccination. Instead, NAb and nNAb should be looked upon as complementary or synergistic humoral effector functions. Several HIV vaccine clinical trials to study these antibody responses in various prime-boost modalities in the systemic and mucosal compartments are ongoing. The induction of high-frequency HIV-specific functional nNAb at high titers may represent an attractive hypothesis-testing strategy in future HIV vaccine efficacy trials.

Capacity for infectious HIV-1 virion capture differs by envelope antibody specificity

Antibody capacity to recognize infectious virus is a prerequisite of many antiviral functions. We determined the infectious virion capture index (IVCI) of different antibody specificities. Whereas broadly neutralizing antibodies (bNAbs), except for an MPER bNAb, selectively captured infectious virions, non-bNAbs and mucosal human immunodeficiency virus type 1 (HIV-1)-positive IgG captured subsets of both infectious and noninfectious virions. Infectious virion capture was additive with a mixture of antibodies, providing proof of concept for vaccine-induced antibodies that together have improved capacity to recognize infectious virions.

A global approach to HIV-1 vaccine development

A global human immunodeficiency virus-1 (HIV-1) vaccine will have to elicit immune responses capable of providing protection against a tremendous diversity of HIV-1 variants. In this review, we first describe the current state of the HIV-1 vaccine field, outlining the immune responses that are desired in a global HIV-1 vaccine. In particular, we emphasize the likely importance of Env-specific neutralizing and non-neutralizing antibodies for protection against HIV-1 acquisition and the likely importance of effector Gag-specific T lymphocytes for virologic control. We then highlight four strategies for developing a global HIV-1 vaccine. The first approach is to design specific vaccines for each geographic region that include antigens tailor-made to match local circulating HIV-1 strains. The second approach is to design a vaccine that will elicit Env-specific antibodies capable of broadly neutralizing all HIV-1 subtypes. The third approach is to design a vaccine that will elicit cellular immune responses that are focused on highly conserved HIV-1 sequences. The fourth approach is to design a vaccine to elicit highly diverse HIV-1-specific responses. Finally, we emphasize the importance of conducting clinical efficacy trials as the only way to determine which strategies will provide optimal protection against HIV-1 in humans.

A "prime-pull" vaccine strategy has a modest effect on local and systemic antibody responses to HIV gp140 in mice

One potential strategy for the prevention of HIV infection is to induce virus specific mucosal antibody that can act as an immune barrier to prevent transmission. The mucosal application of chemokines after immunisation, termed “prime-pull”, has been shown to recruit T cells to mucosal sites. We wished to determine whether this strategy could be used to increase B cells and antibody in the vaginal mucosa following immunisation with an HIV antigen. BALB/c mice were immunised intranasally with trimeric gp140 prior to vaginal application of the chemokine CCL28 or the synthetic TLR4 ligand MPLA, without antigen six days later. There was no increase in vaginal IgA, IgG or B cells following the application of CCL28, however vaginal application of MPLA led to a significant boost in antigen specific vaginal IgA. Follow up studies to investigate the effect of the timing of the “pull” stimulation demonstrated that when given 14 days after the initial immunisation MPLA significantly increased systemic antibody responses. We speculate that this may be due to residual inflammation prior to re-immunisation. Overall we conclude that in contrast to the previously observed effect on T cells, the use of “prime-pull” has only a modest effect on B cells and antibody.

Advances in antiviral vaccine development

Antiviral vaccines have been the most successful biomedical intervention for preventing epidemic viral disease. Vaccination for smallpox in humans and rinderpest in cattle was the basis for disease eradication, and recent progress in polio eradication is promising. Although early vaccines were developed empirically by passage in live animals or eggs, more recent vaccines have been developed because of the advent of new technologies, particularly cell culture and molecular biology. Recent technological advances in gene delivery and expression, nanoparticles, protein manufacturing, and adjuvants have created the potential for new vaccine platforms that may provide solutions for vaccines against viral pathogens for which no interventions currently exist. In addition, the technological convergence of human monoclonal antibody isolation, structural biology, and high-throughput sequencing is providing new opportunities for atomic-level immunogen design. Selection of human monoclonal antibodies can identify immunodominant antigenic sites associated with neutralization and provide reagents for stabilizing and solving the structure of viral surface proteins. Understanding the structural basis for neutralization can guide selection of vaccine targets. Deep sequencing of the antibody repertoire and defining the ontogeny of the desired antibody responses can reveal the junctional recombination and somatic mutation requirements for B-cell recognition and affinity maturation. Collectively, this information will provide new strategic approaches for selecting vaccine antigens, formulations, and regimens. Moreover, it creates the potential for rational vaccine design and establishing a catalogue of vaccine technology platforms that would be effective against any given family or class of viral pathogens and improve our readiness to address new emerging viral threats.

A brief history of the global effort to develop a preventive HIV vaccine

Soon after HIV was discovered as the cause of AIDS in 1983-1984, there was an expectation that a preventive vaccine would be rapidly developed. In trying to achieve that goal, three successive scientific paradigms have been explored: induction of neutralizing antibodies, induction of cell mediated immunity, and exploration of combination approaches and novel concepts. Although major progress has been made in understanding the scientific basis for HIV vaccine development, efficacy trials have been critical in moving the field forward. In 2009, the field was reinvigorated with the modest results obtained from the RV144 trial conducted in Thailand. Here, we review those vaccine development efforts, with an emphasis on events that occurred during the earlier years. The goal is to provide younger generations of scientists with information and inspiration to continue the search for an HIV vaccine.

HIV-1 subtype C superinfected individuals mount low autologous neutralizing antibody responses prior to intrasubtype superinfection

Background

The potential role of antibodies in protection against intra-subtype HIV-1 superinfection remains to be understood. We compared the early neutralizing antibody (NAb) responses in three individuals, who were superinfected within one year of primary infection, to ten matched non-superinfected controls from a Zambian cohort of subtype C transmission cases. Sequence analysis of single genome amplified full-length envs from a previous study showed limited diversification in the individuals who became superinfected with the same HIV-1 subtype within year one post-seroconversion. We hypothesized that this reflected a blunted NAb response, which may have made these individuals more susceptible to superinfection.

Results

Neutralization assays showed that autologous plasma NAb responses to the earliest, and in some cases transmitted/founder, virus were delayed and had low to undetectable titers in all three superinfected individuals prior to superinfection. In contrast, NAbs with a median IC50 titer of 1896 were detected as early as three months post-seroconversion in non-superinfected controls. Early plasma NAbs in all subjects showed limited but variable levels of heterologous neutralization breadth. Superinfected individuals also exhibited a trend toward lower levels of gp120- and V1V2-specific IgG binding antibodies but higher gp120-specific plasma IgA binding antibodies.

Conclusions

These data suggest that the lack of development of IgG antibodies, as reflected in autologous NAbs as well as gp120 and V1V2 binding antibodies to the primary infection virus, combined with potentially competing, non-protective IgA antibodies, may increase susceptibility to superinfection in the context of settings where a single HIV-1 subtype predominates.

A mouse model for HIV-1 entry

Passive transfer of neutralizing antibodies against HIV-1 can prevent infection in macaques and seems to delay HIV-1 rebound in humans. Anti-HIV antibodies are therefore of great interest for vaccine design. However, the basis for their in vivo activity has been difficult to evaluate systematically because of a paucity of small animal models for HIV infection. Here we report a genetically humanized mouse model that incorporates a luciferase reporter for rapid quantitation of HIV entry. An antibody's ability to block viral entry in this in vivo model is a function of its bioavailability, direct neutralizing activity, and effector functions.

Cell-cell transmission enables HIV-1 to evade inhibition by potent CD4bs directed antibodies

HIV is known to spread efficiently both in a cell-free state and from cell to cell, however the relative importance of the cell-cell transmission mode in natural infection has not yet been resolved. Likewise to what extent cell-cell transmission is vulnerable to inhibition by neutralizing antibodies and entry inhibitors remains to be determined. Here we report on neutralizing antibody activity during cell-cell transmission using specifically tailored experimental strategies which enable unambiguous discrimination between the two transmission routes. We demonstrate that the activity of neutralizing monoclonal antibodies (mAbs) and entry inhibitors during cell-cell transmission varies depending on their mode of action. While gp41 directed agents remain active, CD4 binding site (CD4bs) directed inhibitors, including the potent neutralizing mAb VRC01, dramatically lose potency during cell-cell transmission. This implies that CD4bs mAbs act preferentially through blocking free virus transmission, while still allowing HIV to spread through cell-cell contacts. Thus providing a plausible explanation for how HIV maintains infectivity and rapidly escapes potent and broadly active CD4bs directed antibody responses in vivo.

HIV is known to spread both in a cell-free state and from cell to cell, however the relative importance of the cell-cell transmission mode in natural infection has not yet been resolved. Design of vaccines attempt to inhibit HIV entry into target cells as do engineered entry inhibitors used as therapeutics. While these agents are known to block the entry of cell-free HIV particles into cells, to what extent cell-cell transmission is vulnerable to such inhibition is unclear. Here we report that the activity of neutralizing antibodies and inhibitors during cell-cell transmission varies depending on their mode of action. A prominent class of neutralizing antibodies directed to the CD4 binding site on the virus envelope very efficiently blocks binding of the virus to its primary receptor on target cells, the CD4 molecule. These types of antibodies are elicited in natural infection and once isolated from infected individuals have shown to be highly potent. Why HIV still replicates in the presence of such potent antibodies remains unclear. Here we show that these CD4 binding site antibodies are dramatically less potent inhibitors of cell-cell transmission, and therefore act preferentially by blocking free virus transmission while allowing HIV to spread through cell-cell contact.

Inclusion of a CRF01_AE HIV envelope protein boost with a DNA/MVA prime-boost vaccine: Impact on humoral and cellular immunogenicity and viral load reduction after SHIV-E challenge

The current study assessed the immunogenicity and protective efficacy of various prime-boost vaccine regimens in rhesus macaques using combinations of recombinant DNA (rDNA), recombinant MVA (rMVA), and subunit gp140 protein. The rDNA and rMVA vectors were constructed to express Env from HIV-1 subtype CRF01_AE and Gag-Pol from CRF01_AE or SIVmac 239. One of the rMVAs, MVA/CMDR, has been recently tested in humans. Immunizations were administered at months 0 and 1 (prime) and months 3 and 6 (boost). After priming, HIV env-specific serum IgG was detected in monkeys receiving gp140 alone or rMVA but not in those receiving rDNA. Titers were enhanced in these groups after boosting either with gp140 alone or with rMVA plus gp140. The groups that received the rDNA prime developed env-specific IgG after boosting with rMVA with or without gp140. HIV Env-specific serum IgG binding antibodies were elicited more frequently and of higher titer, and breadth of neutralizing antibodies was increased with the inclusion of the subunit Env boost. T cell responses were measured by tetramer binding to Gag p11c in Mamu-A*01 macaques, and by IFN-γ ELISPOT assay to SIV-Gag. T cell responses were induced after vaccination with the highest responses seen in macaques immunized with rDNA and rMVA. Macaques were challenged intravenously with a novel SHIV-E virus (SIVmac239 Gag-Pol with an HIV-1 subtype E-Env CAR402). Post challenge with SHIV-E, antibody titers were boosted in all groups and peaked at 4 weeks. Robust T cell responses were seen in all groups post challenge and in macaques immunized with rDNA and rMVA a clear boosting of responses was seen. A greater than two-log drop in RNA copies/ml at peak viremia and earlier set point was achieved in macaques primed with rDNA, and boosted with rMVA/SHIV-AE plus gp140. Post challenge viremia in macaques immunized with other regimens was not significantly different to that of controls. These results demonstrate that a gp140 subunit and inclusion of SIV Gag-Pol may be critical for control of SHIV post challenge.