HIV-specific antibodies but not t-cell responses are associated with protection in seronegative partners of HIV-1-infected individuals in Cambodia

Baseline host determinants of robust human HIV-1 vaccine-induced immune responses: A meta-analysis of 26 vaccine regimens

BACKGROUND

The identification of baseline host determinants that associate with robust HIV-1 vaccine-induced immune responses could aid HIV-1 vaccine development. We aimed to assess both the collective and relative performance of baseline characteristics in classifying individual participants in nine different Phase 1-2 HIV-1 vaccine clinical trials (26 vaccine regimens, conducted in Africa and in the Americas) as High HIV-1 vaccine responders.

METHODS

This was a meta-analysis of individual participant data, with studies chosen based on participant-level (vs. study-level summary) data availability within the HIV-1 Vaccine Trials Network. We assessed the performance of 25 baseline characteristics (demographics, safety haematological measurements, vital signs, assay background measurements) and estimated the relative importance of each characteristic in classifying 831 participants as High (defined as within the top 25th percentile among positive responders or above the assay upper limit of quantification) versus Non-High responders. Immune response outcomes included HIV-1-specific serum IgG binding antibodies and Env-specific CD4+ T-cell responses assessed two weeks post-last dose, all measured at central HVTN laboratories. Three variable importance approaches based on SuperLearner ensemble machine learning were considered.

FINDINGS

Overall, 30.1%, 50.5%, 36.2%, and 13.9% of participants were categorized as High responders for gp120 IgG, gp140 IgG, gp41 IgG, and Env-specific CD4+ T-cell vaccine-induced responses, respectively. When including all baseline characteristics, moderate performance was achieved for the classification of High responder status for the binding antibody responses, with cross-validated areas under the ROC curve (CV-AUC) of 0.72 (95% CI: 0.68, 0.76) for gp120 IgG, 0.73 (0.69, 0.76) for gp140 IgG, and 0.67 (95% CI: 0.63, 0.72) for gp41 IgG. In contrast, the collection of all baseline characteristics yielded little improvement over chance for predicting High Env-specific CD4+ T-cell responses [CV-AUC: 0.53 (0.48, 0.58)]. While estimated variable importance patterns differed across the three approaches, female sex assigned at birth, lower height, and higher total white blood cell count emerged as significant predictors of High responder status across multiple immune response outcomes using Approach 1. Of these three baseline variables, total white blood cell count ranked highly across all three approaches for predicting vaccine-induced gp41 and gp140 High responder status.

INTERPRETATION

The identified features should be studied further in pursuit of intervention strategies to improve vaccine responses and may be adjusted for in analyses of immune response data to enhance statistical power.

FUNDING

National Institute of Allergy and Infectious Diseases (UM1AI068635 to YH, UM1AI068614 to GDT, UM1AI068618 to MJM, and UM1 AI069511 to MCK), the Duke CFAR P30 AI064518 to GDT, and National Institute of Dental and Craniofacial Research (R01DE027245 to JJK). This work was also supported by the Bill and Melinda Gates Foundation. The content is solely the responsibility of the authors and does not necessarily represent the official views of any of the funding sources.

A high mucosal blocking score is associated with HIV protection

BACKGROUND

Early steps of HIV infection are mediated by the binding of the envelope to mucosal receptors as α4β7 and the C-type lectins DC-SIGN and langerin. Previously Env-specific B-cell responses have been reported in highly exposed seronegative individuals (HESN).

METHOD

Here, we studied gp120-specific antibodies ability to block HIV interaction with α4β7, DC-SIGN and/or langerinin HESN. New cell-based assays were developed to analyze whether antibodies that can alter gp120 binding to α4β7, DC-SIGN and/or langerin are induced in HESN. A mucosal blocking score (MBS) was defined based on the ability of antibodies to interfere with gp120/α4β7, gp120/DC-SIGN, and gp120/langerin binding. A new MBS was evaluated in a cohort of 86 HESN individuals and compared with HIV+ patients or HIV- unexposed healthy individuals.

RESULTS

Antibodies reducing gp120 binding to both α4β7 and DC-SIGN were present in HESN serum but also in mucosal secretions, whereas antibodies from HIV+ patients facilitated gp120 binding to DC-SIGN. Any correlation was observed between MBS and the capacity of antibodies to neutralize infection of α4β7 CD4+ T cells with primary isolates.

CONCLUSIONS

MBS is significantly associated with protection in HESN and might reflect altered HIV spreading to mucosal-associated lymphoid tissues.

Identification of HIV gp41-specific antibodies that mediate killing of infected cells

Antibodies that mediate killing of HIV-infected cells through antibody-dependent cellular cytotoxicity (ADCC) have been implicated in protection from HIV infection and disease progression. Despite these observations, these types of HIV antibodies are understudied compared to neutralizing antibodies. Here we describe four monoclonal antibodies (mAbs) obtained from one individual that target the HIV transmembrane protein, gp41, and mediate ADCC activity. These four mAbs arose from independent B cell lineages suggesting that in this individual, multiple B cell responses were induced by the gp41 antigen. Competition and phage peptide display mapping experiments suggested that two of the mAbs target epitopes in the cysteine loop that are highly conserved and a common target of HIV gp41-specific antibodies. The amino acid sequences that bind these mAbs are overlapping but distinct. The two other mAbs were competed by mAbs that target the C-terminal heptad repeat (CHR) and the fusion peptide proximal region (FPPR) and appear to both target a similar unique conformational epitope. These gp41-specific mAbs mediated killing of infected cells that express high levels of Env due to either pre-treatment with interferon or deletion of vpu to increase levels of BST-2/Tetherin. They also mediate killing of target cells coated with various forms of the gp41 protein, including full-length gp41, gp41 ectodomain or a mimetic of the gp41 stump. Unlike many ADCC mAbs that target HIV gp120, these gp41-mAbs are not dependent on Env structural changes associated with membrane-bound CD4 interaction. Overall, the characterization of these four new mAbs that target gp41 and mediate ADCC provides evidence for diverse gp41 B cell lineages with overlapping but distinct epitopes within an individual. Such antibodies that can target various forms of envelope protein could represent a common response to a relatively conserved HIV epitope for a vaccine.

Mucosal antibody responses to vaccines targeting SIV protease cleavage sites or full-length Gag and Env proteins in Mauritian cynomolgus macaques

HIV mutates rapidly and infects CD4⁺ T cells, especially when they are activated. A vaccine targeting conserved, essential viral elements while limiting CD4⁺ T cell activation could be effective. Learning from natural immunity observed in a group of highly HIV-1 exposed seronegative Kenyan female sex workers, we are testing a novel candidate HIV vaccine targeting the 12 viral protease cleavage sites (PCSs) (the PCS vaccine), in comparison with a vaccine targeting full-length Gag and Env (the Gag/Env vaccine) in a Mauritian cynomolgus macaque/SIV model. In this study we evaluated these vaccines for induction of mucosal antibodies to SIV immunogens at the female genital tract. Bio-Plex and Western blot analyses of cervicovaginal lavage samples showed that both the PCS and Gag/Env vaccines can elicit mucosal IgG antibody responses to SIV immunogens. Significantly higher increase of anti-PCS antibodies was induced by the PCS vaccine than by the Gag/Env vaccine (p<0.0001). The effect of the mucosal antibody responses in protection from repeated low dose pathogenic SIVmac251 challenges is being evaluated.

Evaluation of Different Parameters of Humoral and Cellular Immune Responses in HIV Serodiscordant Heterosexual Couples: Humoral Response Potentially Implicated in Modulating Transmission Rates

As the HIV/AIDS pandemic still progresses, understanding the mechanisms governing viral transmission as well as protection from HIV acquisition is fundamental. In this context, cohorts of HIV serodiscordant heterosexual couples (SDC) represent a unique tool. The present study was aimed to evaluate specific parameters of innate, cellular and humoral immune responses in SDC. Specifically, plasma levels of cytokines and chemokines, HIV-specific T-cell responses, gp120-specific IgG and IgA antibodies, and HIV-specific antibody-dependent cellular cytotoxicity (ADCC) activity were assessed in nine HIV-exposed seronegative individuals (ESN) and their corresponding HIV seropositive partners (HIV⁺-P), in eighteen chronically infected HIV subjects (C), nine chronically infected subjects known to be HIV transmitters (CT) and ten healthy HIV⁻ donors (HD). Very low magnitude HIV-specific cellular responses were found in two out of six ESN. Interestingly, HIV⁺-P had the highest ADCC magnitude, the lowest IgA levels and the highest IgG/IgA ratio, all compared to CT. Positive correlations between CD4⁺ T-cell counts and both IgG/IgA ratios and %ADCC killing uniquely distinguished HIV⁺-P. Additionally, evidence of IgA interference with ADCC responses from HIV⁺-P and CT is provided. These data suggest for the first time a potential role of ADCC and/or gp120-specific IgG/IgA balance in modulating heterosexual transmission. In sum, this study provides key information to understand the host factors that influence viral transmission, which should be considered in both the development of prophylactic vaccines and novel immunotherapies for HIV-1 infection.

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The evaluation of different immune parameters in HIV serodiscordant couples helped identify factors shaping transmission.

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Innate and cellular immune responses were apparently not involved in this scenario.

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HIV-specific ADCC, IgA titer and IgG/IgA balance were identified as factors involved in modulating viral transmission.

The existence of individuals that remain HIV negative despite being repeatedly exposed to the virus has long been described. To date, only homozygosis for a 32-base pair deletion in the ccr5 gene has been consistently shown to be a determinant of HIV resistance. Still, subjects bearing the WT ccr5 gene have also been described as resistant or less susceptible to HIV. Thus, other mechanisms must be involved in this phenomenon. The results presented here postulate ADCC and IgG/IgA ratio as potential mechanisms involved in modulating HIV transmission in the context of serodiscordant couples and inspire further investigations.

Breadth and magnitude of antigen-specific antibody responses in the control of plasma viremia in simian immunodeficiency virus infected macaques

Background

Increasing evidence suggests an unexpected potential for non-neutralizing antibodies to prevent HIV infection. Consequently, identification of functional linear B-cell epitopes for HIV are important for developing preventative and therapeutic strategies. We therefore explored the role of antigen-specific immune responses in controlling plasma viremia in SIV infected rhesus macaques.

Methods

Thirteen rhesus macaques were inoculated either intravaginally or intrarectally with SIV_MAC251. Peripheral blood CD4+ T-cells were quantified. Plasma was examined for viremia, antigen specific IgG, IgA and IgM binding responses and neutralizing antibodies. Regions containing binding epitopes for antigen-specific IgG, IgM and IgA responses were determined, and the minimum size of linear Envelope epitope responsible for binding antibodies was identified.

Results

The presence of neutralizing antibodies did not correlate the outcome of the disease. In a few SIV-infected macaques, antigen-specific IgG and IgM responses in plasma correlated with decreased plasma viremia. Early induction and the breadth of antigen-specific IgG responses were found to be significantly correlated with the control of plasma viral load. Immunoglobulin classes share similar functional linear B-cell epitopes. SIV-specific linear envelope B-cell epitopes were found to be 12 amino-acids in length.

Conclusions

Early induction of combination of peptide-specific IgG responses were found to be responsible for the control of plasma viral load and indicative of disease outcome in SIV-infected rhesus macaques and might be important for the development of therapeutic strategies for control or prevention of HIV/AIDS.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-016-0652-x) contains supplementary material, which is available to authorized users.

A HIV-Tat/C4-binding protein chimera encoded by a DNA vaccine is highly immunogenic and contains acute EcoHIV infection in mice

DNA vaccines are cost-effective to manufacture on a global scale and Tat-based DNA vaccines have yielded protective outcomes in preclinical and clinical models of human immunodeficiency virus (HIV), highlighting the potential of such vaccines. However, Tat-based DNA vaccines have been poorly immunogenic, and despite the administration of multiple doses and/or the addition of adjuvants, these vaccines are not in general use. In this study, we improved Tat immunogenicity by fusing it with the oligomerisation domain of a chimeric C4-binding protein (C4b-p), termed IMX313, resulting in Tat heptamerisation and linked Tat to the leader sequence of tissue plasminogen activator (TPA) to ensure that the bulk of heptamerised Tat is secreted. Mice vaccinated with secreted Tat fused to IMX313 (pVAX-sTat-IMX313) developed higher titres of Tat-specific serum IgG, mucosal sIgA and cell-mediated immune (CMI) responses, and showed superior control of EcoHIV infection, a surrogate murine HIV challenge model, compared with animals vaccinated with other test vaccines. Given the crucial contribution of Tat to HIV-1 pathogenesis and the precedent of Tat-based DNA vaccines in conferring some level of protection in animal models, we believe that the virologic control demonstrated with this novel multimerised Tat vaccine highlights the promise of this vaccine candidate for humans.

Induction of Potent and Long-Lived Antibody and Cellular Immune Responses in the Genitorectal Mucosa Could be the Critical Determinant of HIV Vaccine Efficacy

The field of HIV prevention has indeed progressed in leaps and bounds, but with major limitations of the current prevention and treatment options, the world remains desperate for an HIV vaccine. Sadly, this continues to be elusive, because more than 30 years since its discovery there is no licensed HIV vaccine. Research aiming to define immunological biomarkers to accurately predict vaccine efficacy have focused mainly on systemic immune responses, and as such, studies defining correlates of protection in the genitorectal mucosa, the primary target site for HIV entry and seeding are sparse. Clearly, difficulties in sampling and analysis of mucosal specimens, as well as their limited size have been a major deterrent in characterizing the type (mucosal antibodies, cytokines, chemokines, or CTL), threshold (magnitude, depth, and breadth) and viral inhibitory capacity of HIV-1-specific immune responses in the genitorectal mucosa, where they are needed to immediately block HIV acquisition and arrest subsequent virus dissemination. Nevertheless, a few studies document the existence of HIV-specific immune responses in the genitorectal mucosa of HIV-infected aviremic and viremic controllers, as well as in highly exposed persistently seronegative (HEPS) individuals with natural resistance to HIV-1. Some of these responses strongly correlate with protection from HIV acquisition and/or disease progression, thus providing significant clues of the ideal components of an efficacious HIV vaccine. In this study, we provide an overview of the key features of protective immune responses found in HEPS, elite and viremic controllers, and discuss how these can be achieved through mucosal immunization. Inevitably, HIV vaccine development research will have to consider strategies that elicit potent antibody and cellular immune responses within the genitorectal mucosa or induction of systemic immune cells with an inherent potential to home and persist at mucosal sites of HIV entry.

IgM Repertoire Biodiversity is Reduced in HIV-1 Infection and Systemic Lupus Erythematosus

Background: HIV-1 infection or systemic lupus erythematosus (SLE) disrupt B cell homeostasis, reduce memory B cells, and impair function of IgG and IgM antibodies.

Objective: To determine how disturbances in B cell populations producing polyclonal antibodies relate to the IgM repertoire, the IgM transcriptome in health and disease was explored at the complementarity determining region 3 (CDRH3) sequence level.

Methods: 454-deep pyrosequencing in combination with a novel analysis pipeline was applied to define populations of IGHM CDRH3 sequences based on absence or presence of somatic hypermutations (SHM) in peripheral blood B cells.

Results: HIV or SLE subjects have reduced biodiversity within their IGHM transcriptome compared to healthy subjects, mainly due to a significant decrease in the number of unique combinations of alleles, although recombination machinery was intact. While major differences between sequences without or with SHM occurred among all groups, IGHD and IGHJ allele use, CDRH3 length distribution, or generation of SHM were similar among study cohorts. Antiretroviral therapy failed to normalize IGHM biodiversity in HIV-infected individuals. All subjects had a low frequency of allelic combinations within the IGHM repertoire similar to known broadly neutralizing HIV-1 antibodies.

Conclusion: Polyclonal expansion would decrease overall IgM biodiversity independent of other mechanisms for development of the B cell repertoire. Applying deep sequencing as a strategy to follow development of the IgM repertoire in health and disease provides a novel molecular assessment of multiple points along the B cell differentiation pathway that is highly sensitive for detecting perturbations within the repertoire at the population level.

Secretory IgA specific for MPER can protect from HIV-1 infection in vitro

This preliminary study was performed to assess the neutralization activity of anti-MPER-secretory IgA (SIgA) from parotid saliva in 88 HIV+ patients, in five exposed seronegative (ESN) and in five seronegative volunteers. 48.9% of the HIV+ patients and 100% of ESN patients showed antigp41 SIgA. 25.6% of anti-MPER SIgA from HIV+ patients and 100% from ESN patients neutralized efficiently HIV-1 primary isolate SF162 (IC 50). The neutralization activity is maintained for much lower concentrations in ESN than in HIV+ secretions.

HIV exposed seronegative individuals show antibodies specifically recognizing native HIV envelope glycoprotein

BACKGROUND

Susceptibility to HIV transmission by sexual intercourse has been associated with cellular anti-HIV responses. We aimed to also evaluate potential systemic humoral responses against HIV in a group of HIV-exposed seronegative individuals (HESN) in stable relationship with HIV-infected partners.

METHODS

We recruited 27 serodiscordant couples. HESN were classified according to HIV exposure into very low/low and moderate/high risk. Plasma from HESN and HIV partners were tested for neutralizing capacity and for the recognition of cell-surface expressed and recombinant forms of HIV envelope glycoproteins (Env). Healthy individuals (healthy control, n=11) were used as controls.

RESULTS

Recognition of cell-surface expressed Env by both immunoglobulin (Ig)G and IgA was higher in plasma samples from HESN than in healthy controls (P=0.0062 and P=0.0144, respectively). IgG binding to Env was significantly increased in HESN after unmasking CD4-induced epitopes (P=0.001), suggesting a wide range of targeted epitopes. Remarkably, ELISA assays using trimeric gp140 or monomeric gp120 failed to detect significant differences in reactivity between groups. Neutralization analysis showed residual activity in only three HESN samples (11%), whereas 70% of HIV-infected partners showed neutralizing activity. Although anti-Env humoral responses were found in 85% of HESN, their magnitude was not associated with the estimated level of exposure or the detection of HIV-specific cellular immune responses.

CONCLUSION

A high proportion of HESN show detectable plasma IgG or IgA recognizing different exposed and cryptic Env native epitopes unrelated to neutralizing capacity. Therefore, low but persistent HIV exposure induces new virus-specific systemic humoral responses or boosts preexisting natural antibodies.

The gp41 epitope, QARVLAVERY, is highly conserved and a potent inducer of IgA that neutralizes HIV-1 and inhibits viral transcytosis

Mucosal surfaces are the predominant site of human immunodeficiency virus (HIV)-1 transmission. For prophylactic approaches to effectively prevent HIV infection and subsequent dissemination, the induction of mucosally relevant protective immunity will be critical. Here, we have characterized the antibody (Ab) response generated by a highly conserved gp41epitope, QARVLAVERY, in an optimized immunization model that elicits potent epitope-specific Abs in the serum, vaginal washes, and fecal secretions of immunized mice. Our results show that QARVLAVERY is indeed a potent inducer of IgA and importantly, QARVLAVERY-specific IgA was effective in neutralizing HIV and inhibiting viral transcytosis. Intriguingly, QARVLAVERY also generated an approximate 1:1 ratio of IgG:IgA in the serum of immunized mice, independent of the delivery regimen and produced early systemic IgA, even before IgG. In light of the significantly high IgA induction by QARVLAVERY and the functionality of epitope-specific Abs in the inhibition of HIV infection and transcytosis, QARVLAVERY is an attractive epitope to be considered in mucosal vaccination strategies against HIV.

HIV-specific functional antibody responses in breast milk mirror those in plasma and are primarily mediated by IgG antibodies

Despite months of mucosal virus exposure, the majority of breastfed infants born to HIV-infected mothers do not become infected, raising the possibility that immune factors in milk inhibit mucosal transmission of HIV. HIV Envelope (Env)-specific antibodies are present in the milk of HIV-infected mothers, but little is known about their virus-specific functions. In this study, HIV Env-specific antibody binding, autologous and heterologous virus neutralization, and antibody-dependent cell cytotoxicity (ADCC) responses were measured in the milk and plasma of 41 HIV-infected lactating women. Although IgA is the predominant antibody isotype in milk, HIV Env-specific IgG responses were higher in magnitude than HIV Env-specific IgA responses in milk. The concentrations of anti-HIV gp120 IgG in milk and plasma were directly correlated (r = 0.75; P < 0.0001), yet the response in milk was 2 logarithm units lower than in plasma. Similarly, heterologous virus neutralization (r = 0.39; P = 0.010) and ADCC activity (r = 0.64; P < 0.0001) in milk were directly correlated with that in the systemic compartment but were 2 log units lower in magnitude. Autologous neutralization was rarely detected in milk. Milk heterologous virus neutralization titers correlated with HIV gp120 Env-binding IgG responses but not with IgA responses (r = 0.71 and P < 0.0001, and r = 0.17 and P = 0.30). Moreover, IgGs purified from milk and plasma had equal neutralizing potencies against a tier 1 virus (r = 0.65; P < 0.0001), whereas only 1 out of 35 tested non-IgG milk fractions had detectable neutralization. These results suggest that plasma-derived IgG antibodies mediate the majority of the low-level HIV neutralization and ADCC activity in breast milk.

Scarcity or absence of humoral immune responses in the plasma and cervicovaginal lavage fluids of heavily HIV-1-exposed but persistently seronegative women

To address an existing controversy concerning the presence of HIV-1-specific antibodies of the IgA isotype in the female genital tract secretions of highly-exposed but persistently seronegative (HEPSN) women, 41 samples of plasma and cervicovaginal lavage (CVL) fluid were distributed to six laboratories for their blinded evaluation using ELISA with 10 different HIV-1 antigens, chemiluminescence-enhanced Western blots (ECL-WB), and virus neutralization. HIV-specific IgG or IgA antibodies in plasma samples from HEPSN women were absent or detectable only at low levels. In CVL, 11/41 samples displayed low levels of reactivity in ELISA against certain antigens. However, only one sample was positive in two of five laboratories. All but one CVL sample yielded negative results when analyzed by ECL-WB. Viral neutralizing activity was either absent or inconsistently detected in plasma and CVL. Plasma and CVL samples from 26 HIV-1-infected women were used as positive controls. Irrespective of the assays and antigens used, the results generated in all laboratories displayed remarkable concordance in the detection of HIV-1-specific antibodies of the IgG isotype. In contrast, IgA antibodies to HIV-1 antigens were not detected with consistency, and where present, IgA antibodies were at markedly lower levels than IgG. Although HIV-neutralizing activity was detected in plasma of all HIV-1-infected women, only a few of their CVL samples displayed such activity. In conclusion, frequent HIV-1 sexual exposure does not stimulate uniformly detectable mucosal or systemic HIV-1-specific responses, as convincingly documented in the present blindly performed study using a broad variety of immunological assays. Although HIV-1-infection leads to vigorous IgG responses in plasma and CVL, it does not stimulate sustained IgA responses in either fluid.

Evidence for the innate immune response as a correlate of protection in human immunodeficiency virus (HIV)-1 highly exposed seronegative subjects (HESN)

The description of highly exposed individuals who remain seronegative (HESN) despite repeated exposure to human immunodeficiency virus (HIV)-1 has heightened interest in identifying potential mechanisms of HIV-1 resistance. HIV-specific humoral and T cell-mediated responses have been identified routinely in HESN subjects, although it remains unknown if these responses are a definitive cause of protection or merely a marker for exposure. Approximately half of HESN lack any detectible HIV-specific adaptive immune responses, suggesting that other mechanisms of protection from HIV-1 infection also probably exist. In support of the innate immune response as a mechanism of resistance, increased natural killer (NK) cell activity has been correlated with protection from infection in several high-risk cohorts of HESN subjects, including intravenous drug users, HIV-1 discordant couples and perinatally exposed infants. Inheritance of protective NK KIR3DL1(high) and KIR3DS1 receptor alleles have also been observed to be over-represented in a high-risk cohort of HESN intravenous drug users and HESN partners of HIV-1-infected subjects. Other intrinsic mechanisms of innate immune protection correlated with resistance in HESN subjects include heightened dendritic cell responses and increased secretion of anti-viral factors such as β-chemokines, small anti-viral factors and defensins. This review will highlight the most current evidence in HESN subjects supporting the role of epithelial microenvironment and the innate immune system in sustaining resistance against HIV-1 infection. We will argue that as a front-line defence the innate immune response determines the threshold of infectivity that HIV-1 must overcome to establish a productive infection.

Molecular characterization of the cervical and systemic B-cell repertoire: Unique, yet overlapping, immune compartments of an HIV-1 resistant individual

The cervical mucosa of women who are highly exposed to HIV-1, yet remain persistently seronegative (HEPS), presents a unique opportunity to study the dynamics of an immune compartment potentially capable of preventing HIV-1 infection. Herein, we provide a detailed characterization of the immunoglobulin repertoire of cervical and systemic B cells from one such HEPS individual from Nairobi, Kenya. Analysis was done on 512 VH sequences that were RT-PCR amplified from B cells in a paired sample from the cervix and peripheral blood. The VH3 and DH repertoire of class switched cervical B cells differs significantly from that of systemic B cells indicating that the cervical environment affects local B cell populations and hence VH gene expression. Six networks of clonally related, heavily mutated B cells were identified that spanned the systemic and cervical B cell compartments. Analysis of somatic mutations suggests this is likely the result of systemic, class switched B cells homing to the cervical mucosa. Multiple networks of somatically mutated V-gene sequences, unique to the cervical mucosa, were also identified. This supports the notion that site specific responses occur and have unique regulation of tolerance and recruitment into local memory or blast B cell compartments. We conclude that while the nature of the cervical environment shapes the local B cell repertoire, the infusion of post germinal center B cells to the human cervix is a common occurrence, and represents a means by which systemic immunization could provide the local antibodies necessary to prevent HIV-1 at the site of initial contact.

Acute mucosal pathogenesis of feline immunodeficiency virus is independent of viral dose in vaginally infected cats

BACKGROUND

The mucosal pathogenesis of HIV has been shown to be an important feature of infection and disease progression. HIV-1 infection causes depletion of intestinal lamina propria CD4+ T cells (LPL), therefore, intestinal CD4+ T cell preservation may be a useful correlate of protection in evaluating vaccine candidates. Vaccine studies employing the cat/FIV and macaque/SIV models frequently use high doses of parenterally administered challenge virus to ensure high plasma viremia in control animals. However, it is unclear if loss of mucosal T cells would occur regardless of initial viral inoculum dose. The objective of this study was to determine the acute effect of viral dose on mucosal leukocytes and associated innate and adaptive immune responses.

RESULTS

Cats were vaginally inoculated with a high, middle or low dose of cell-associated and cell-free FIV. PBMC, serum and plasma were assessed every two weeks with tissues assessed eight weeks following infection. We found that irrespective of mucosally administered viral dose, FIV infection was induced in all cats. However, viremia was present in only half of the cats, and viral dose was unrelated to the development of viremia. Importantly, regardless of viral dose, all cats experienced significant losses of intestinal CD4+ LPL and CD8+ intraepithelial lymphocytes (IEL). Innate immune responses by CD56+CD3- NK cells correlated with aviremia and apparent occult infection but did not protect mucosal T cells. CD4+ and CD8+ T cells in viremic cats were more likely to produce cytokines in response to Gag stimulation, whereas aviremic cats T cells tended to produce cytokines in response to Env stimulation. However, while cell-mediated immune responses in aviremic cats may have helped reduce viral replication, they could not be correlated to the levels of viremia. Robust production of anti-FIV antibodies was positively correlated with the magnitude of viremia.

CONCLUSIONS

Our results indicate that mucosal immune pathogenesis could be used as a rapid indicator of vaccine success or failure when combined with a physiologically relevant low dose mucosal challenge. We also show that innate immune responses may play an important role in controlling viral replication following acute mucosal infection, which has not been previously identified.

Antibody-Mediated Fcγ Receptor-Based Mechanisms of HIV Inhibition: Recent Findings and New Vaccination Strategies

The HIV/AIDS pandemic is one of the most devastating pandemics worldwide. Today, the major route of infection by HIV is sexual transmission. One of the most promising strategies for vaccination against HIV sexual infection is the development of a mucosal vaccine, which should be able to induce strong local and systemic protective immunity. It is believed that both humoral and cellular immune responses are needed for inducing a sterilizing protection against HIV. Recently, passive administration of monoclonal neutralizing antibodies in macaques infected by vaginal challenge demonstrated a crucial role of FcγRs in the protection afforded by these antibodies. This questioned about the role of innate and adaptive immune functions, including ADCC, ADCVI, phagocytosis of opsonized HIV particles and the production of inflammatory cytokines and chemokines, in the mechanism of HIV inhibition in vivo. Other monoclonal antibodies - non-neutralizing inhibitory antibodies - which recognize immunogenic epitopes, have been shown to display potent FcγRs-dependent inhibition of HIV replication in vitro. The potential role of these antibodies in protection against sexual transmission of HIV and their biological relevance for the development of an HIV vaccine therefore need to be determined. This review highlights the potential role of FcγRs-mediated innate and adaptive immune functions in the mechanism of HIV protection.

Sexual transmission of HIV-1

HIV-1 transmission occurs in a limited number of ways all of which are preventable. Overall, the risk of HIV-1 transmission following a single sexual exposure is low especially in comparison with other sexually transmitted infections (STIs); with estimates of the average probability of male to female HIV-1 transmission only 0.0005-0.0026 per coital act. The risk of acquiring HIV-1 from a single contact varies enormously and is dependant upon the infectiousness of the HIV-1 positive individual and the susceptibility to HIV-1 of their sexual partner. An understanding of the determinants of HIV-1 transmission is important not only to assess the infection risk to an individual when exposed to the virus (e.g. to determine the provision of post exposure prophylaxis), but also to make accurate predictions on the potential spread of HIV-1 infection in a population and to direct appropriate targeted prevention strategies. In this review article we summarise the current literature on the major worldwide source of HIV-1 acquisition, sexual transmission. This article forms part of a special issue of Antiviral Research marking the 25th anniversary of antiretroviral drug discovery and development, Vol 85, issue 1, 2010.

HIV-1 gp41-specific monoclonal mucosal IgAs derived from highly exposed but IgG-seronegative individuals block HIV-1 epithelial transcytosis and neutralize CD4(+) cell infection: an IgA gene and functional analysis

AIDS is mainly a sexually transmitted disease, and accordingly, mucosal tissues are the primary sites of natural human immunodeficiency virus type-1 (HIV-1) transmission. Mucosal immunoglobulin A (IgA) antibody specific for HIV-1 envelope gp41 subunit is one correlate of protection in individuals who are highly sexually exposed to HIV-1 but remain persistently IgG seronegative (HEPS). Understanding these peculiar IgAs at the gene and functional level is possible only with monoclonal IgAs. We have constructed a mucosal Fab IgA library from HEPS and have characterized a series of HIV-1 IgAs specific for gp41 that, in vitro, are transcytosis-blocking and infection-neutralizing. Characterization of their IgA genes shows that Fab specific for the gp41 membrane-proximal region harbors a long heavy-chain CDR3 loop (CDRH3) similar to the two broadly neutralizing IgG monoclonal antibodies, 2F5 and 4E10. Furthermore, the selected Fab IgA shows extensive somatic mutations that cluster in the CDR regions, indicating that affinity maturation due to an antigen-driven process had occurred in HEPS individuals, presumably upon multiple exposures to HIV. This analysis of HEPS monoclonal IgA gives a unique opportunity to correlate an antibody function (resistance to a pathogen in vivo) with an antibody gene. Such neutralizing monoclonal IgAs could be used in microbicide formulation.

What does the structure-function relationship of the HIV-1 Tat protein teach us about developing an AIDS vaccine?

The human immunodeficiency virus type 1 (HIV-1) trans-activator of transcription protein Tat is an important factor in viral pathogenesis. In addition to its function as the key trans-activator of viral transcription, Tat is also secreted by the infected cell and taken up by neighboring cells where it has an effect both on infected and uninfected cells. In this review we will focus on the relationship between the structure of the Tat protein and its function as a secreted factor. To this end we will summarize some of the exogenous functions of Tat that have been implicated in HIV-1 pathogenesis and the impact of structural variations and viral subtype variants of Tat on those functions. Finally, since in some patients the presence of Tat-specific antibodies or CTL frequencies are associated with slow or non-progression to AIDS, we will also discuss the role of Tat as a potential vaccine candidate, the advances made in this field, and the importance of using a Tat protein capable of eliciting a protective or therapeutic immune response to viral challenge.

HIV/AIDS vaccines: a need for new concepts?

HIV vaccine research is at a crossroads carefully contemplating on the next path. The unexpected results of the Merck vaccine trial, while providing a stunning blow to a field in dire need of a protective vaccine, has also raised several fundamental questions regarding the candidate immunogen itself, preexisting immunity to vaccine vectors, surrogate assays and animal models used for assessing preclinical protective responses, as well as relevant endpoints to be measured in a clinical trial. As a result, the research community is faced with the daunting task of identifying novel vaccine concepts and products to continue the search. This review highlights and addresses some of the scientific and practical concerns.

Cervical HIV-specific IgA in a population of commercial sex workers correlates with repeated exposure but not resistance to HIV

We conducted a comprehensive cross-sectional analysis of total and HIV-specific cervical antibody levels in HIV-1-resistant, uninfected, and infected women in order to examine the role of HIV-specific antibody responses in the female genital tract and examine the effect on antibody levels of various epidemiologic factors in this population. Cervical lavages were collected from 272 subjects of the Pumwani commercial sex worker cohort. Total and HIV-specific genital tract IgA and IgG levels were measured using an ELISA and correlated with behavioral and demographic factors. No significant difference was seen between cervical HIV-specific IgA levels in infected, uninfected, and resistant individuals, nor were any correlations between cervical HIV-specific IgA and neutralization capacity or viral shedding seen. We did, however, note increased HIV-specific IgA in HIV-negative women with four or more clients per day, and decreased HIV-specific IgA in both long-term nonprogressors and long-term survivors. These results show that there is not a strong cohort-wide correlation between HIV-specific cervical IgA levels and resistance to infection by HIV-1 as previously believed, but there is a correlation between exposure to HIV and HIV-specific cervical IgA. Our findings do not preclude the possibility that functional differences in the cervical IgA of HEPS women may play a role in resistance, but argue that HIV-specific responses may not be a universal protective factor. They also indicate that resistance to HIV is a complex condition related to more factors than exposure. Further studies of correlates of immune protection in these individuals would be beneficial to the field.

Not just sheer luck! Immune correlates of protection against HIV-1 infection

Susceptibility to HIV infection is widely different among individuals, and it is known that individuals can be identified who are repeatedly exposed to HIV but in whom neither infection nor disease are seen. The possibility that sheer luck is not the only determinant of this phenomenon begun to be considered in 1989 when it was reported that T cell responses to HIV proteins could be detected in antibody-negative sexual partners of known HIV-positive men. In this review, we will summarize the body of knowledge that stemmed from that first observation.

Mucosal HIV-binding antibody and neutralizing activity in high-risk HIV-uninfected female participants in a trial of HIV-vaccine efficacy

BACKGROUND

This study investigated gp120-binding antibody and neutralizing activity, at the gingival- and cervical-mucosal levels, in response to a bivalent gp120 candidate vaccine.

METHODS

Women who met the study's inclusion criteria for documented high-risk behaviors participated in a nested substudy of the multicenter phase 3 trial of human immunodeficiency virus (HIV)-vaccine efficacy, VAX004. Gingival, cervicovaginal lavage, and plasma specimens were collected at 6-month intervals for 3 years. Binding-antibody and neutralizing-activity assays quantified the presence of anti-HIV activity in mucosal specimens.

RESULTS

Vaccine recipients were more likely than placebo recipients to have IgG binding antibodies in all 3 compartments tested and to have only IgA binding antibody in plasma (P<.0001). The relationship between vaccine and cervicovaginal IgG achieved significance (odds ratio [OR], 6.6 [P=.01]) but was weakened by the presence of cervicovaginal leukocytes. There was no relationship between immunization and the presence of neutralizing activity, in either bivariate or multivariate modeling (OR, 6.0 [P=.29]).

CONCLUSIONS

Vaccination is associated with the presence of both gp120-binding IgG in all compartments and plasma IgA but not with neutralizing activity. There is a role for the measurement of mucosal immunity in response to candidate vaccines and, in particular, for a determination of HIV-specific neutralizing antibodies.

Comparison of immunogenicity between codon optimized HIV-1 Thailand subtype B gp140 and gp145 vaccines

HIV-1 pandemic posed an unprecedented challenge to the global health and it is believed that an effective vaccine will be the final solution against HIV-1. HIV-1 envelope is the primary immunogen in developing neutralization antibody based HIV vaccine. To define the suitable Env derived immunogen, we systemically compared the immunogenicity of gp140 and gp145 in a DNA vaccination alone and a prime-boost modalities. Two DNA vaccines and two recombinant Tiantan vaccinia vaccines (rTTV) were constructed for vaccination of female Balb/c mice. Elispot assay was used to read out the T cell immunity and ELISA assay was used to quantify antibody immunity. PLL (poly-L-leucine)-ELISA assay was used in linear antibody epitope mapping. Mice primed with gp145 tended to elicit more Env-specific T cells responses than those primed with gp140, significant difference was observed in DNA immunization alone. The ultimate T cell responses in prime-boost regimen tend to be determined mainly by the priming efficacy. Linear antibody epitope mapping displayed that sera raised by gp145 priming were vigorously reactive to more peptides than that by gp140. Our data demonstrated HIV-1 Thailand B-derived gp145 may raise higher T-cell responses and broader linear peptide-specific antibody responses than gp140 does. However, it remains to be determined that how these observations are relevant to the neutralization of antibody activities.

Structure and function of the HIV envelope glycoprotein as entry mediator, vaccine immunogen, and target for inhibitors

This chapter discusses the advances of the envelope glycoprotein (Env) structure as related to the interactions of conserved Env structures with receptor molecules and antibodies with implications for the design of vaccine immunogens and inhibitors. The human immunodeficiency virus (HIV) Env binds to cell surface–associated receptor (CD4) and coreceptor (CCR5 or CXCR4) by one of its two non-covalently associated subunits, gp120. The induced conformational changes activate the other subunit (gp41), which causes the fusion of the viral with the plasma cell membranes resulting in the delivery of the viral genome into the cell and the initiation of the infection cycle. As the only HIV protein exposed to the environment, the Env is also a major immunogen to which neutralizing antibodies are directed and a target that is relatively easy to access by inhibitors. A fundamental problem in the development of effective vaccines and inhibitors against HIV is the rapid generation of alterations at high levels of expression during long chronic infection and the resulting significant heterogeneity of the Env. The preservation of the Env function as an entry mediator and limitations on size and expression impose restrictions on its variability and lead to the existence of conserved structures.