Antibodies against full-length Tat protein and some low-molecular-weight Tat-peptides correlate with low or undetectable viral load in HIV-1 seropositive patients

Role of HIV-1 Tat Protein Interactions with Host Receptors in HIV Infection and Pathogenesis

Each time the virus starts a new round of expression/replication, even under effective antiretroviral therapy (ART), the transactivator of viral transcription Tat is one of the first HIV-1 protein to be produced, as it is strictly required for HIV replication and spreading. At this stage, most of the Tat protein exits infected cells, accumulates in the extracellular matrix and exerts profound effects on both the virus and neighbor cells, mostly of the innate and adaptive immune systems. Through these effects, extracellular Tat contributes to the acquisition of infection, spreading and progression to AIDS in untreated patients, or to non-AIDS co-morbidities in ART-treated individuals, who experience inflammation and immune activation despite virus suppression. Here, we review the role of extracellular Tat in both the virus life cycle and on cells of the innate and adaptive immune system, and we provide epidemiological and experimental evidence of the importance of targeting Tat to block residual HIV expression and replication. Finally, we briefly review vaccine studies showing that a therapeutic Tat vaccine intensifies ART, while its inclusion in a preventative vaccine may blunt escape from neutralizing antibodies and block early events in HIV acquisition.

HIV-1 therapeutic vaccines in clinical development to intensify or replace antiretroviral therapy: the promising results of the Tat vaccine

INTRODUCTION

Upon the introduction of the combination antiretroviral therapy (cART), HIV infection has become a chronic disease. However, cART is unable to eradicate the virus and fails to restore the CD4 counts in about 30% of the treated individuals. Furthermore, treatment is life-long, and it does not protect from morbidities typically observed in the elderly. Therapeutic vaccines represent the most cost-effective intervention to intensify or replace cART.

AREAS COVERED

Here, we briefly discuss the obstacles to the development and evaluation of the efficacy of therapeutic vaccines and review recent approaches evaluated in clinical trials.

EXPERT OPINION

Although vaccines were generally safe and immunogenic, evidence of efficacy was negligible or marginal in most trials. A notable exception is the therapeutic Tat vaccine approach showing promising results of cART intensification, with CD4 T-cell increase and proviral load reduction beyond those afforded by cART alone. Rationale and evidence in support of choosing Tat as the vaccine target are thoroughly discussed.

New insights into pathogenesis point to HIV-1 Tat as a key vaccine target

Despite over 30 years of enormous effort and progress in the field, no preventative and/or therapeutic vaccines against human immunodeficiency virus (HIV) are available. Here, we briefly summarize the vaccine strategies and vaccine candidates that in recent years advanced to efficacy trials with mostly unsatisfactory results. Next, we discuss a novel and somewhat contrarian approach based on biological and epidemiological evidence, which led us to choose the HIV protein Tat for the development of preventive and therapeutic HIV vaccines. Toward this goal, we review here the role of Tat in the virus life cycle as well as experimental and epidemiological evidence supporting its key role in the natural history of HIV infection and comorbidities. We then discuss the preclinical and clinical development of a Tat therapeutic vaccine, which, by improving the functionality and homeostasis of the immune system and by reducing the viral reservoir in virologically suppressed vaccinees, helps to establish key determinants for intensification of combination antiretroviral therapy (cART) and a functional cure. Future developments and potential applications of the Tat therapeutic vaccine are also discussed, as well as the rationale for its use in preventative strategies. We hope this contribution will lead to a reconsideration of the current paradigms for the development of HIV/AIDS vaccines, with a focus on targeting of viral proteins with key roles in HIV pathogenesis.

Use of a Novel Peptide Welding Technology Platform for the Development of B- and T-Cell Epitope-Based Vaccines

Peptide vaccines incorporating B- and T-cell epitopes have shown promise in the context of various cancers and infections. These vaccines are relatively simple to manufacture, but more immunogenic formulations are considered a priority. We developed tetrabranched derivatives for this purpose based on a novel peptide welding technology (PWT). PWTs provide molecular scaffolds for the efficient synthesis of ultrapure peptide dendrimers, which allow the delivery of multiple ligands within a single macromolecular structure. Peptide vaccines incorporating T-cell epitopes derived from melanoma and B-cell epitopes derived from human immunodeficiency virus, synthesized using this approach, elicited primary immune responses in vitro and in vivo. Subcutaneous administration of the B-cell epitope-based vaccines also elicited more potent humoral responses than subcutaneous administration of the corresponding peptides alone. Highly immunogenic peptide epitope-based vaccines can therefore be generated quickly and easily using a novel PWT.

Evaluation of HIV-1 Regulatory and Structural Proteins as Antigen Candidate in Mice and Humans

BACKGROUND

The diagnosis of HIV infection is important among different groups. Moreover, combination antiretroviral therapy is used to treat HIV-1, but it cannot eradicate the infection. Thus, the development of therapeutic vaccines, along with antiretroviral therapy, is recommended. This study evaluates the values of four HIV proteins as antigen candidates in therapeutic vaccine design as well as a possible diagnostic marker for HIV infection in humans.

METHODS

In this study, the HIV-1 Tat and Rev regulatory proteins and structural Gp120 and p24 proteins were generated in E. coli expression system. Their immunogenicity was evaluated in BALB/ c mice using homologous and heterologous prime/boost strategies. Moreover, the detection of anti- HIV IgG antibodies against these recombinant proteins was assessed in untreated (Naïve/ HIV-infected), treated, and drug-resistant patients compared to the healthy (control) group as a possible diagnostic marker for HIV infection.

RESULTS

In humans, our results showed that among HIV-1 proteins, anti-Gp120 antibody was not detected in treated individuals compared to the healthy (control) group. The levels of anti-Gp120 antibody were significantly different between the treated group and Naïve as well as drug-resistant subjects. Moreover, the level of anti-p24 antibody was significantly lower in the treated group than the Naive group. In mice, the results of immunization indicated that the Rev antigen could significantly induce IgG2a, IgG2b, and IFN-γ secretion aimed at Th1 response as well as Granzyme B generation as CTL activity in comparison with other antigens. Furthermore, the heterologous DNA prime/ protein boost regimen was more potent than the homologous regimen for stimulation of cellular immunity.

CONCLUSION

Briefly, the levels of both anti-Gp120 and anti-p24 antibodies can be considered for the diagnosis of the HIV-infected individuals in different groups compared to the healthy group. Moreover, among four recombinant proteins, Rev elicited Th1 cellular immunity and CTL activity in mice as an antigen candidate in therapeutic vaccine development.

Anti-Tat immunity defines CD4+ T-cell dynamics in people living with HIV on long-term cART

BACKGROUND

Low-level HIV viremia originating from virus reactivation in HIV reservoirs is often present in cART treated individuals and represents a persisting source of immune stimulation associated with sub-optimal recovery of CD4⁺ T cells. The HIV-1 Tat protein is released in the extracellular milieu and activates immune cells and latent HIV, leading to virus production and release. However, the relation of anti-Tat immunity with residual viremia, persistent immune activation and CD4⁺ T-cell dynamics has not yet been defined.

METHODS

Volunteers enrolled in a 3-year longitudinal observational study were stratified by residual viremia, Tat serostatus and frequency of anti-Tat cellular immune responses. The impact of anti-Tat immunity on low-level viremia, persistent immune activation and CD4⁺ T-cell recovery was investigated by test for partitions, longitudinal regression analysis for repeated measures and generalized estimating equations.

FINDINGS

Anti-Tat immunity is significantly associated with higher nadir CD4⁺ T-cell numbers, control of low-level viremia and long-lasting CD4⁺ T-cell recovery, but not with decreased immune activation. In adjusted analysis, the extent of CD4⁺ T-cell restoration reflects the interplay among Tat immunity, residual viremia and immunological determinants including CD8⁺ T cells and B cells. Anti-Env immunity was not related to CD4⁺ T-cell recovery.

INTERPRETATION

Therapeutic approaches aiming at reinforcing anti-Tat immunity should be investigated to improve immune reconstitution in people living with HIV on long-term cART.

TRIAL REGISTRATION

ISS OBS T-002 ClinicalTrials.gov identifier: NCT01024556 FUNDING: Italian Ministry of Health, special project on the Development of a vaccine against HIV based on the Tat protein and Ricerca Corrente 2019/2020.

Physicochemical studies on the structural stability of the HIV-1 vaccine candidate recombinant Tat protein

HIV-1 transactivator of transcription protein is one of the most promising AIDS vaccine candidates and plays central roles in the virus life cycle and pathogenesis. Understanding structural properties of vaccine candidate antigens leads to rational design of vaccines which improves their presentation to immune system and facilitates their manufacturing and storage. This study aims to investigate structural properties and stability of one variant of HIV-1 Tat recombinant protein using different spectroscopic, electrophoretic, and microscopic methods. Therefore, after the gene transformation, protein expression was optimized in E. coli cells and the C-terminal His₆-tagged protein was purified using Ni-NTA resin. The structural stability of the pure protein was then investigated under different conditions including pH, Zn²⁺ ions, thermal and chemical stress. Acidic and alkaline pHs affects spectroscopic properties of the vaccine in different ways. The structure unfolding experiment shows relatively poor stability of the zinc-free protein sample compared to the ion-containing one. According to the quenching experiment and also thermal stability study results, the protein has attained more structural compactness in the presence of Zn²⁺. Secondary structure of the protein is mainly disordered and didn't significantly affect under various conditions. Finally, different degrees of oligomerization and aggregation were found under physiological conditions.

An electrochemical label-free DNA impedimetric sensor with AuNP-modified glass fiber/carbonaceous electrode for the detection of HIV-1 DNA

In this study, a highly sensitive, electrochemical, and label-free DNA impedimetric sensor was developed using carbonized glass fiber-coal tar pitch (GF-CTP) electrodes supported with gold nanoparticles (AuNPs) for the detection of HIV-1 gene. Thiol-modified GF-CTP electrodes were prepared using amine crosslinking chemistry and AuNPs were self-assembled obtaining highly conductive nanoelectrodes, GF-CTP-ATP-Au. All steps of electrode modifications were characterized using electrochemical, spectroscopic, and microscopic methods. GF-CTP-ATP-Au electrode was then modified with a capture DNA probe (C-ssDNA) and optimized with a target DNA probe in terms of hybridization time and temperature between 30 and 180 min and 20-50 °C, respectively. Finally, the analytic performance of the developed ssDNA biosensor was evaluated using electrochemical impedance spectroscopy. The calibration of the sensor was obtained between 0.1 pM and 10 nM analyte working range. The limit of detection was calculated using signal to noise ratio of 3 (S/N = 3) as 13 fM. Moreover, interference results for two noncomplementary DNA probes were also tested to demonstrate non-specific ssDNA interactions. An electrochemical label-free DNA impedimetric sensor was successfully developed using a novel GF-CTP-ATP-Au electrode. This study suggests that highly sensitive DNA-based biosensors can be developed using relatively low-cost carbonaceous materials.

Tat-Based Therapies as an Adjuvant for an HIV-1 Functional Cure

The human immunodeficiency virus type 1 (HIV) establishes a chronic infection that can be well controlled, but not cured, by combined antiretroviral therapy (cART). Interventions have been explored to accomplish a functional cure, meaning that a patient remains infected but HIV is undetectable in the blood, with the aim of allowing patients to live without cART. Tat, the viral transactivator of transcription protein, plays a critical role in controlling HIV transcription, latency, and viral rebound following the interruption of cART treatment. Therefore, a logical approach for controlling HIV would be to block Tat. Tackling Tat with inhibitors has been a difficult task, but some recent discoveries hold promise. Two anti-HIV proteins, Nullbasic (a mutant of Tat) and HT1 (a fusion of HEXIM1 and Tat functional domains) inhibit viral transcription by interfering with the interaction of Tat and cellular factors. Two small molecules, didehydro-cortistatin A (dCA) and triptolide, inhibit Tat by different mechanisms: dCA through direct binding and triptolide through enhanced proteasomal degradation. Finally, two Tat-based vaccines under development elicit Tat-neutralizing antibodies. These vaccines have increased the levels of CD4⁺ cells and reduced viral loads in HIV-infected people, suggesting that the new vaccines are therapeutic. This review summarizes recent developments of anti-Tat agents and how they could contribute to a functional cure for HIV.

HIV therapeutic vaccines aimed at intensifying combination antiretroviral therapy

Introduction: Although successful at suppressing HIV replication, combination antiretroviral therapy (cART) only partially restores immune functions and fails to reduce the latent HIV reservoir, thus requiring novel interventions for its intensification.Areas covered: Here are reviewed therapeutic vaccine candidates that are being developed to this goal. Among them, the Tat vaccine has been shown to promote immune restoration, including CD4+ T-cell recovery in low immunological responders, and to reduce the virus reservoirs well beyond what achieved with long-term suppressive cART.Expert opinion: The authors propose the Tat vaccine as a promising vaccine candidate for cART intensification toward HIV reservoirs depletion, functional cure, and eradication strategies, suggesting that targeting a key protein in the virus life cycle is pivotal to success.

Anti-Tat Immunity in HIV-1 Infection: Effects of Naturally Occurring and Vaccine-Induced Antibodies Against Tat on the Course of the Disease

HIV-1 Tat is an essential protein in the virus life cycle, which is required for virus gene expression and replication. Most Tat that is produced during infection is released extracellularly and it plays a key role in HIV pathogenesis, including residual disease upon combination antiretroviral therapy (cART). Here, we review epidemiological and experimental evidence showing that antibodies against HIV-1 Tat, infrequently occurring in natural infection, play a protective role against disease progression, and that vaccine targeting Tat can intensify cART. In fact, Tat vaccination of subjects on suppressive cART in Italy and South Africa promoted immune restoration, including CD4+ T-cell increase in low immunological responders, and a reduction of proviral DNA even after six years of cART, when both CD4+ T-cell gain and DNA decay have reached a plateau. Of note, DNA decay was predicted by the neutralization of Tat-mediated entry of Env into dendritic cells by anti-Tat antibodies, which were cross-clade binding and neutralizing. Anti-Tat cellular immunity also contributed to the DNA decay. Based on these data, we propose the Tat therapeutic vaccine as a pathogenesis-driven intervention that effectively intensifies cART and it may lead to a functional cure, providing new perspectives and opportunities also for prevention and virus eradication strategies.

Continued Decay of HIV Proviral DNA Upon Vaccination With HIV-1 Tat of Subjects on Long-Term ART: An 8-Year Follow-Up Study

Introduction: Tat, a key HIV virulence protein, has been targeted for the development of a therapeutic vaccine aimed at cART intensification. Results from phase II clinical trials in Italy (ISS T-002) and South Africa (ISS T-003) indicated that Tat vaccination promotes increases of CD4⁺ T-cells and return to immune homeostasis while reducing the virus reservoir in chronically cART-treated patients. Here we present data of 92 vaccinees (59% of total vaccinees) enrolled in the ISS T-002 8-year extended follow-up study (ISS T-002 EF-UP, ClinicalTrials.gov NCT02118168). Results: Anti-Tat antibodies (Abs) induced upon vaccination persisted for the entire follow-up in 34/92 (37%) vaccinees, particularly when all 3 Ab classes (A/G/M) were present (66% of vaccinees), as most frequently observed with Tat 30 μg regimens. CD4⁺ T cells increased above study-entry levels reaching a stable plateau at year 5 post-vaccination, with the highest increase (165 cells/μL) in the Tat 30 μg, 3 × regimen. CD4⁺ T-cell increase occurred even in subjects with CD4⁺ nadir ≤ 250 cells/uL and in poor immunological responders and was associated with a concomitant increase of the CD4⁺/CD8⁺ T-cell ratio, a prognostic marker of morbidity/mortality inversely related to HIV reservoir size. Proviral DNA load decreased over time, with a half-life of 2 years and an estimated 90% reduction at year 8 in the Tat 30 μg, 3 × group. In multivariate analysis the kinetic and amplitude of both CD4⁺ T-cell increase and proviral DNA reduction were fastest and highest in subjects with all 3 anti-Tat Ab classes and in the 30 μg, 3 × group, irrespective of drug regimens (NNRTI/NRTI vs. PI). HIV proviral DNA changes from baseline were inversely related to CD4⁺/CD8⁺ T-cell ratio and CD4⁺ T-cell changes, and directly related to the changes of CD8⁺ T cells. Further, HIV DNA decay kinetics were inversely related to the frequency and levels of intermittent viremia. Finally, Tat vaccination was similarly effective irrespective of the individual immunological status or HIV reservoir size at study entry. Conclusions: Tat immunization induces progressive immune restoration and reduction of virus reservoirs above levels reached with long-term cART, and may represent an optimal vaccine candidate for cART intensification toward HIV reservoirs depletion, functional cure, and eradication strategies.

"cART intensification by the HIV-1 Tat B clade vaccine: progress to phase III efficacy studies"

INTRODUCTION

In spite of its success at suppressing HIV replication, combination antiretroviral therapy (cART) only partially reduces immune dysregulation and loss of immune functions. These cART-unmet needs appear to be due to persistent virus replication and cell-to-cell transmission in reservoirs, and are causes of increased patients' morbidity and mortality. Up to now, therapeutic interventions aimed at cART-intensification by attacking the virus reservoir have failed.

AREAS COVERED

We briefly review the rationale and clinical development of Tat therapeutic vaccine in cART-treated subjects in Italy and South Africa (SA). Vaccination with clade-B Tat induced cross-clade neutralizing antibodies, immune restoration, including CD4⁺ T cell increase particularly in low immunological responders, and reduction of proviral DNA. Phase III efficacy trials in SA are planned both in adult and pediatric populations.

EXPERT COMMENTARY

We propose the Tat therapeutic vaccine as a pathogenesis-driven intervention that effectively intensifies cART and may lead to a functional cure and provide new perspectives for prevention and virus eradication strategies.

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.

Association between different anti-Tat antibody isotypes and HIV disease progression: data from an African cohort

BACKGROUND

The presence of IgG and IgM against Tat, an HIV protein important for viral replication and immune dysfunction, is associated with slow disease progression in clade B HIV-infected individuals. However, although Tat activities strictly depend on the viral clade, our knowledge about the importance of anti-Tat antibodies in non-clade B HIV infection is poor. The objective of this study was to investigate the association of different anti-Tat antibody isotypes with disease progression in non-clade B HIV-infected subjects and to study the relationship between anti-Tat humoral responses and immunological abnormalities.

METHODS

Anti-clade B and -clade C Tat IgG, IgM and IgA titers were assessed in serum samples from 96 cART-naïve subjects with chronic HIV infection from Mbeya, Tanzania, and associated with CD4(+) T cell count, plasma viremia and CD4(+) and CD8(+) T cell phenotypes.

RESULTS

Anti-Tat IgM were preferentially detected in chronic HIV-infected subjects with low T cell activation (p-value = 0.03) and correlated with higher CD4(+) T cell counts and lower viral loads irrespective of the duration of infection (p-value = 0.019 and p-value = 0.037 respectively). Conversely, anti-Tat IgA were preferentially detected in individuals with low CD4(+) T cell counts and high viral load (p-value = 0.02 and p-value < 0.001 respectively). The simultaneous presence of anti-Tat IgG and IgM protected from fast CD4(+) T cell decline (p-value < 0.01) and accumulation of CD38(+)HLADR(+)CD8(+) T cells (p- value = 0.029).

CONCLUSIONS

Anti-Tat IgG alone are not protective in non-clade B infected subjects, unless concomitant with IgM, suggesting a protective role of persistent anti-Tat IgM irrespective of the infecting clade.

Effects of different routes of administration on the immunogenicity of the Tat protein and a Tat-derived peptide

The use of the Tat protein of HIV in vaccines against AIDS showed promising results in primate and human studies. To characterize the impact of the administration route on the induction of humoral responses at systemic and mucosal levels, we compared intradermal, intramuscular and mucosal immunizations with Tat and a Tat-derived peptide. Mice were immunized with the Tat protein by different routes and the titer and isotype of anti-Tat antibodies were assessed in serum and mucosal lavages. Intramuscular and intradermal administrations showed comparable immunogenicity, while the mucosal administration was unable to induce IgM in serum and IgG at mucosal sites but showed superior immunogenicity in terms of IgA induction. Anti-Tat antibodies were also obtained upon vaccination with the immunodominant Tat 1-20 peptide which was, however, less immunogenic than the whole Tat protein.

HIV-1 Tat biosensor: Current development and trends for early detection strategies

Human immunodeficiency virus (HIV) has infected almost 35 million people worldwide. Various tests have been developed to detect the presence of HIV during the early stages of the disease in order to reduce the risk of transmission to other humans. The HIV-1 Tat protein is one of the proteins present in HIV that are released abundantly approximately 2-4 weeks after infection. In this review, we have outlined various strategies for detecting the Tat protein, which helps transcribe the virus and enhances replication. Detection strategies presented include immunoassays, biosensors and gene expression, which utilize antibodies or aptamers as common probes to sense the presence of Tat. Alternatively, measuring the levels of gene transcription is a direct method of analysing the HIV gene to confirm the presence of Tat. By detection of the Tat protein, virus transmission can be detected in high-risk individuals in the early stages of the disease to reduce the risk of an HIV pandemic.

HIV-Specific Antibody Responses in HIV-Infected Patients: From a Monoclonal to a Polyclonal View

HIV infections represent a major global health threat, affecting more than 35 million individuals worldwide. High infection rates and problems associated with lifelong antiretroviral treatment emphasize the need for the development of prophylactic and therapeutic immune intervention strategies. It is conceivable that insights for the design of new immunogens capable of eliciting protective immune responses may come from the analysis of HIV-specific antibody responses in infected patients. Using sophisticated technologies, several monoclonal neutralizing antibodies were isolated from HIV-infected individuals. However, the majority of polyclonal antibody responses found in infected patients are nonneutralizing. Comprehensive analyses of the molecular targets of HIV-specific antibody responses identified that during natural infection antibodies are mainly misdirected towards gp120 epitopes outside of the CD4-binding site and against regions and proteins that are not exposed on the surface of the virus. We therefore argue that vaccines aiming to induce protective responses should include engineered immunogens, which are capable of focusing the immune response towards protective epitopes.

Development of a novel AIDS vaccine: the HIV-1 transactivator of transcription protein vaccine

INTRODUCTION

Classical approaches aimed at targeting the HIV-1 envelope as well as other structural viral proteins have largely failed. The HIV-1 transactivator of transcription (Tat) is a key HIV virulence factor, which plays pivotal roles in virus gene expression, replication, transmission and disease progression. Notably, anti-Tat Abs are uncommon in natural infection and, when present, correlate with the asymptomatic state and lead to lower or no disease progression. Hence, targeting Tat represents a pathogenesis-driven intervention.

AREAS COVERED

Here, we review the rationale and the translational development of a therapeutic vaccine targeting the Tat protein. Preclinical and Phase I studies, Phase II trials with Tat in anti-Tat Ab-negative, virologically suppressed highly active antiretroviral therapy-treated subjects in Italy and South Africa were conducted. The results indicate that Tat-induced immune responses are necessary to restore immune homeostasis, to block the replenishment and to reduce the size of the viral reservoir. Additionally, they may help in establishing key parameters for highly active antiretroviral therapy intensification and a functional cure.

EXPERT OPINION

We propose the therapeutic setting as the most feasible to speed up the testing and comparison of preventative vaccine candidates, as the distinction lies in the use of the vaccine in uninfected versus infected subjects and not in the vaccine formulation.

The grafting of universal T-helper epitopes enhances immunogenicity of HIV-1 Tat concurrently improving its safety profile

Extracellular Tat (eTat) plays an important role in HIV-1 pathogenesis. The presence of anti-Tat antibodies is negatively correlated with disease progression, hence making Tat a potential vaccine candidate. The cytotoxicity and moderate immunogenicity of Tat however remain impediments for developing Tat-based vaccines. Here, we report a novel strategy to concurrently enhance the immunogenicity and safety profile of Tat. The grafting of universal helper T-lymphocyte (HTL) epitopes, Pan DR Epitope (PADRE) and Pol₇₁₁ into the cysteine rich domain (CRD) and the basic domain (BD) abolished the transactivation potential of the Tat protein. The HTL-Tat proteins elicited a significantly higher titer of antibodies as compared to the wild-type Tat in BALB/c mice. While the N-terminal epitope remained immunodominant in HTL-Tat immunizations, an additional epitope in exon-2 was recognized with comparable magnitude suggesting a broader immune recognition. Additionally, the HTL-Tat proteins induced cross-reactive antibodies of high avidity that efficiently neutralized exogenous Tat, thus blocking the activation of a Tat-defective provirus. With advantages such as presentation of multiple B-cell epitopes, enhanced antibody response and importantly, transactivation-deficient Tat protein, this approach has potential application for the generation of Tat-based HIV/AIDS vaccines.

Biocompatible anionic polymeric microspheres as priming delivery system for effetive HIV/AIDS Tat-based vaccines

Here we describe a prime-boost regimen of vaccination in Macaca fascicularis that combines priming with novel anionic microspheres designed to deliver the biologically active HIV-1 Tat protein and boosting with Tat in Alum. This regimen of immunization modulated the IgG subclass profile and elicited a balanced Th1-Th2 type of humoral and cellular responses. Remarkably, following intravenous challenge with SHIV89.6P_cy243, vaccinees significantly blunted acute viremia, as compared to control monkeys, and this control was associated with significantly lower CD4⁺ T cell depletion rate during the acute phase of infection and higher ability to resume the CD4⁺ T cell counts in the post-acute and chronic phases of infection. The long lasting control of viremia was associated with the persistence of high titers anti-Tat antibodies whose profile clearly distinguished vaccinees in controllers and viremics. Controllers, as opposed to vaccinated and viremic cynos, exhibited significantly higher pre-challenge antibody responses to peptides spanning the glutamine-rich and the RGD-integrin-binding regions of Tat. Finally, among vaccinees, titers of anti-Tat IgG1, IgG3 and IgG4 subclasses had a significant association with control of viremia in the acute and post-acute phases of infection. Altogether these findings indicate that the Tat/H1D/Alum regimen of immunization holds promise for next generation vaccines with Tat protein or other proteins for which maintenance of the native conformation and activity are critical for optimal immunogenicity. Our results also provide novel information on the role of anti-Tat responses in the prevention of HIV pathogenesis and for the design of new vaccine candidates.

Surface-bound Tat inhibits antigen-specific CD8+ T-cell activation in an integrin-dependent manner

OBJECTIVE

The identification of still unrevealed mechanisms affecting the anti-HIV CD8 T-cell response in HIV-1 infection.

DESIGN

Starting from the observation that anti-Tat immunization is associated with improved CD8 T-cell immunity, we developed both in-vitro and ex-vivo assays to characterize the effects of extra-cellular Tat on the adaptive CD8 T-cell response.

METHODS

The effects of Tat on CD8 T-cell activation were assayed using CD8 T-cell clones specific for either cellular (MART-1) or viral (HIV-1 Nef) antigens, and HIV-1 Gag-specific CD8 T cells from HIV-1 patients.

RESULTS

The interaction between CD8 T lymphocytes and immobilized Tat, but not its soluble form, inhibits peptide-specific CD8 T-lymphocyte activation. The inhibition does not depend on Tat trans-activation activity, but on the interaction of the Tat RGD domain with α5β1 and αvβ3 integrins. Impaired CD8 T-cell activation was also observed in cocultures of CD8 T cells with HIV-1-infected cells. Anti-Tat Abs abrogate the inhibitory effect, consistently with the evidence that extracellular Tat accumulates on the cell membrane of virus-producing cells. The Tat-induced inhibition of cell activation associates with increased apoptosis of CD8 T cells. Finally, the inhibition of cell activation also takes place in Gag-specific CD8 T lymphocytes from HIV-1-infected patients.

CONCLUSION

Our results support the idea that CD8 T-cell apoptosis induced by surface-bound extracellular Tat can contribute to the dysregulation of the CD8 T-cell adaptive response against HIV as well as other pathogens present in AIDS patients.

Challenges in HIV Vaccine Research for Treatment and Prevention

Many attempts have been made or are ongoing for HIV prevention and HIV cure. Many successes are in the list, particularly for HIV drugs, recently proposed also for prevention. However, no eradication of infection has been achieved so far with any drug. Further, a residual immune dysregulation associated to chronic immune activation and incomplete restoration of B and T cell subsets, together with HIV DNA persistence in reservoirs, are still unmet needs of the highly active antiretroviral therapy, causing novel “non-AIDS related” diseases that account for a higher risk of death even in virologically suppressed patients. These “ART unmet needs” represent a problem, which is expected to increase by ART roll out. Further, in countries such as South Africa, where six millions of individuals are infected, ART appears unable to contain the epidemics. Regretfully, all the attempts at developing a preventative vaccine have been largely disappointing. However, recent therapeutic immunization strategies have opened new avenues for HIV treatment, which might be exploitable also for preventative vaccine approaches. For example, immunization strategies aimed at targeting key viral products responsible of virus transmission, activation, and maintenance of virus reservoirs may intensify drug efficacy and lead to a functional cure providing new perspectives also for prevention and future virus eradication strategies. However, this approach imposes new challenges to the scientific community, vaccine developers, and regulatory bodies, such as the identification of novel immunological and virological biomarkers to assess efficacy end-points, taking advantage from the natural history of infection and exploiting lessons from former trials. This review will focus first on recent advancement of therapeutic strategies, then on the progresses made in preventative approaches, discussing concepts, and problems for the way ahead for the development of vaccines for HIV treatment and prevention.

IL-7 receptor recovery on CD8 T-cells isolated from HIV+ patients is inhibited by the HIV Tat protein

Expression of the IL-7 receptor α-chain (CD127) is decreased on CD8 T-cells in HIV infected patients and partially recovers in those receiving antiretroviral therapy with sustained viral suppression. We have shown that soluble HIV Tat protein down regulates CD127 expression on CD8 T-cells isolated from healthy HIV-negative individuals. Tat is taken up by CD8 T-cells via endocytosis, exits the endosome and then translocates to the inner leaflet of the cell membrane where it binds to the cytoplasmic tail of CD127 inducing receptor internalization and degradation by the proteasome. This down regulation of CD127 by Tat results in impaired CD8 T-cell function. Interestingly, suppression of CD127 by Tat is reversible and requires the continual presence of Tat in the culture media. We thus questioned whether the low IL-7 receptor expression evident on CD8 T-cells in HIV+ patients was similarly reversible and if suppression of the receptor could be maintained ex vivo by Tat protein alone. We show here that when CD8 T-cells isolated from HIV+ patients are incubated alone in fresh medium, low CD127 expression on the cell surface recovers to normal levels. This recovery of CD127, however, is completely inhibited by the addition of HIV Tat protein to the culture media. This study then provides evidence that soluble factor(s) are responsible for low CD127 expression on circulating CD8 T-cells in HIV+ individuals and further implicates Tat in suppressing this receptor essential to CD8 T-cell proliferation and function.

HIV-1 Tat C phosphorylates VE-cadherin complex and increases human brain microvascular endothelial cell permeability

Background

Human brain microvascular endothelial cells (hBMVECs) are integral part of the blood brain barrier. Post-translational modifications of adherens junction proteins regulate the permeability of human brain microvascular endothelial cells. Pro-inflammatory signals can induce tyrosine phosphorylation of adherens junction proteins. The primary objective of this work is to provide a molecular model; how the HIV-1 Tat protein can compromise the BBB integrity and eventually lead to neurological consequences. We exposed hBMVECs to recombinant HIV-1 clade C Tat protein to study the effect of HIV-1 Tat C on permeability of hBMVECs. Trans-endothelial electrical resistance and fluorescent dye migration assay have been used to check the permeability of hBMVECs. DCFDA staining has been used for intracellular reactive oxygen species (ROS) detection. Western blotting has been used to study the expression levels and co-immunoprecipitation has been used to study the interactions among adherens junction proteins.

Results

HIV-1 Tat C protein induced NOX2 and NOX4 expression level and increased intracellular ROS level. Redox-sensitive kinase; PYK2 activation led to increased tyrosine phosphorylation of VE-cadherin and β-catenin, leading to disruption of junctional assembly. The dissociation of tyrosine phosphatases VE-PTP and SHP2 from cadherin complex resulted into increased tyrosine phosphorylation of VE-cadherin and β-catenin in HIV-1 Tat C treated hBMVECs.

Conclusion

Unrestricted phosphorylation of junctional proteins in hBMVECs, in response to HIV-1 Tat C protein; leads to the disruption of junctional complexes and increased endothelial permeability.

The presence of anti-Tat antibodies in HIV-infected individuals is associated with containment of CD4+ T-cell decay and viral load, and with delay of disease progression: results of a 3-year cohort study

Background

Tat is a key HIV-1 virulence factor, which plays pivotal roles in virus gene expression, replication, transmission and disease progression. After release, extracellular Tat accumulates in tissues and exerts effects on both the virus and the immune system, promoting immune activation and virus spreading while disabling the host immune defense. In particular, Tat binds Env spikes on virus particles forming a virus entry complex, which favors infection of dendritic cells and efficient transmission to T cells via RGD-binding integrins. Tat also shields the CCR5-binding sites of Env rendering ineffective virus neutralization by anti-Env antibodies (Abs). This is reversed by the anti-Tat Abs present in natural infection or induced by vaccination.

Findings

Here we present the results of a cohort study, showing that the presence of anti-Tat Abs in asymptomatic and treatment-naïve HIV-infected subjects is associated with containment of CD4⁺ T-cell loss and viral load and with a delay of disease progression. In fact, no subjects with high anti-Tat Ab titers initiated antiretroviral therapy during the three years of follow-up. In contrast, no significant effects were seen for anti-Env and anti-Gag Abs. The increase of anti-Env Ab titers was associated with a reduced risk of starting therapy only in the presence of anti-Tat Abs, suggesting an effect of combined anti-Tat and anti-Env Abs on the Tat/Env virus entry complex and on virus neutralization.

Conclusions

Anti-Tat immunity may help delay HIV disease progression, thus, targeting Tat may offer a novel therapeutic intervention to postpone antiretroviral treatment or to increase its efficacy.

Induction of antibodies and T cell responses by a recombinant influenza virus carrying an HIV-1 TatΔ51-59 protein in mice

Recombinant influenza viruses hold promise as vectors for vaccines to prevent transmission of mucosal pathogens. In this study, we generated a recombinant WSN/TatΔ(51-59) virus in which Tat protein lacking residues 51 to 59 of the basic domain was inserted into the N-terminus of the hemagglutinin (HA) of A/WSN/33 virus. The TatΔ(51-59) insertion into the viral HA caused a 2-log reduction in viral titers in cell culture, compared with the parental A/WSN/33 virus, and severely affected virus replication in vivo. Nevertheless, Tat-specific antibodies and T cell responses were elicited upon a single intranasal immunization of BALB/c mice with WSN/TatΔ(51-59) virus. Moreover, Tat-specific immune responses were also detected following vaccine administration via the vaginal route. These data provide further evidence that moderately large HIV antigens can be delivered by chimeric HA constructs and elicit specific immune responses, thus increasing the options for the potential use of recombinant influenza viruses, and their derivatives, for prophylactic and therapeutic vaccines.

HIV-1 Tat protein induces viral internalization through Env-mediated interactions in dose-dependent manner

OBJECTIVE

To study the dose-dependent manner of HIV-1 Tat-induced effects on viral replication, internalization and spread, and to directly observe these effects on soluble Env immunogens and virus-like particles.

DESIGN

In order to determine the manner through which Tat affects viral replication, we incubated cells, virions and soluble Env spikes with Tat at different concentrations, and directly visualized the effects of such incubation.

METHODS

Cell-based infectivity assays were carried out to assay Tat dose-dependency of viral infectivity. Transmission electron microscopy of virus-like particles and soluble gp140 immunogens incubated with Tat at various concentrations was performed to directly observe Tat-induced effects.

RESULTS

Treating virus with exogenous Tat increased infectivity in a dose-dependent manner. In the presence of anti-Tat antibodies, virus replication and spread were repressed, postulating Tat contributions to disease progression. When CXCR4 coreceptors were blocked, Tat treatment overcame the inhibition relative to absence of Tat treatment. Similarly, syncytium formation between chronically infected and uninfected target cells was also increased by exogenous Tat treatment. Inhibiting the CD4 receptor for virus entry abolished syncytium formation and Tat treatment was unable to overcome CD4 dependency. We show that Tat reduces virus infectivity at higher Tat concentrations through Env interactions resulting in viral aggregation.

CONCLUSION

Treating virions or chronically infected cells with exogenous Tat could enhance virus infectivity and spread through coreceptor tropism switch or through another undetermined mechanism. The aggregation potential of Tat suggests a mechanism of negative-feedback regulation of viral replication, providing another regulative function to control viral replication.

Novel biopanning strategy to identify epitopes associated with vaccine protection

Identifying immune correlates of protection is important to develop vaccines against infectious diseases. We designed a novel, universally applicable strategy to profile the antibody (Ab) repertoire of protected vaccine recipients, using recombinant phages encoding random peptide libraries. The new approach, termed "protection-linked (PL) biopanning," probes the Ab paratopes of protected vaccinees versus those with vaccine failure. As proof of concept, we screened plasma samples from vaccinated rhesus macaques (RMs) that had completely resisted multiple mucosal challenges with R5-tropic simian-human immunodeficiency viruses (SHIVs). The animals had been immunized with a multicomponent vaccine (multimeric HIV-1 gp160, HIV-1 Tat, and SIV Gag-Pol particles). After PL biopanning, we analyzed the phagotopes selected for amino acid homologies; in addition to the expected Env mimotopes, one recurring motif reflected the neutralizing Ab epitope at the N terminus (NT) of HIV-1 Tat. Subsequent binding and functional assays indicated that anti-Tat NT Abs were present only in completely or partially protected RMs; peak viremia of the latter was inversely correlated with anti-Tat NT Ab titers. In contrast, highly viremic, unvaccinated controls did not develop detectable Abs against the same epitope. Based upon the protective effect observed in vivo, we suggest that Tat should be included in multicomponent HIV-1 vaccines. Our data highlight the power of the new PL-biopanning strategy to identify Ab responses with significant association to vaccine protection, regardless of the mechanism(s) or targets of the protective Abs. PL biopanning is also unbiased with regard to pathogens or disease model, making it a universal tool.

HIV-1 tat promotes integrin-mediated HIV transmission to dendritic cells by binding Env spikes and competes neutralization by anti-HIV antibodies

Use of Env in HIV vaccine development has been disappointing. Here we show that, in the presence of a biologically active Tat subunit vaccine, a trimeric Env protein prevents in monkeys virus spread from the portal of entry to regional lymph nodes. This appears to be due to specific interactions between Tat and Env spikes that form a novel virus entry complex favoring R5 or X4 virus entry and productive infection of dendritic cells (DCs) via an integrin-mediated pathway. These Tat effects do not require Tat-transactivation activity and are blocked by anti-integrin antibodies (Abs). Productive DC infection promoted by Tat is associated with a highly efficient virus transmission to T cells. In the Tat/Env complex the cysteine-rich region of Tat engages the Env V3 loop, whereas the Tat RGD sequence remains free and directs the virus to integrins present on DCs. V2 loop deletion, which unshields the CCR5 binding region of Env, increases Tat/Env complex stability. Of note, binding of Tat to Env abolishes neutralization of Env entry or infection of DCs by anti-HIV sera lacking anti-Tat Abs, which are seldom present in natural infection. This is reversed, and neutralization further enhanced, by HIV sera containing anti-Tat Abs such as those from asymptomatic or Tat-vaccinated patients, or by sera from the Tat/Env vaccinated monkeys. Thus, both anti-Tat and anti-Env Abs are required for efficient HIV neutralization. These data suggest that the Tat/Env interaction increases HIV acquisition and spreading, as a mechanism evolved by the virus to escape anti-Env neutralizing Abs. This may explain the low effectiveness of Env-based vaccines, which are also unlikely to elicit Abs against new Env epitopes exposed by the Tat/Env interaction. As Tat also binds Envs from different clades, new vaccine strategies should exploit the Tat/Env interaction for both preventative and therapeutic interventions.

Cannabinoids inhibit migration of microglial-like cells to the HIV protein Tat

Microglia are a population of macrophage-like cells in the central nervous system (CNS) which, upon infection by the human immunodeficiency virus (HIV), secrete a plethora of inflammatory factors, including the virus-specified trans-activating protein Tat. Tat has been implicated in HIV neuropathogenesis since it elicits chemokines, cytokines, and a chemotactic response from microglia. It also harbors a β-chemokine receptor binding motif, articulating a mode by which it acts as a migration stimulus. Since select cannabinoids have anti-inflammatory properties, cross the blood-brain barrier, and target specific receptors, they have potential to serve as agents for dampening untoward neuroimmune responses. The aim of this study was to investigate the effect of select cannabinoids on the migration of microglial-like cells toward Tat. Using a mouse BV-2 microglial-like cell model, it was demonstrated that the exogenous cannabinoids Delta-9-tetrahydrocannabinol (THC) and CP55940 exerted a concentration-related reduction in the migration of BV-2 cells towards Tat. A similar inhibitory response was obtained when the endogenous cannabinoid 2-arachidonoylglycerol (2-AG) was used. The CB(2) receptor (CB2R) antagonist SR144528, but not the CB(1) receptor (CB1R) antagonist SR141716A, blocked this inhibition of migration. Similarly, CB2R knockdown with small interfering RNA reversed the cannabinoid-mediated inhibition. In addition, the level of the β-chemokine receptor CCR-3 was reduced and its intracellular compartmentation was altered. These results indicate that cannabinoid-mediated inhibition of BV-2 microglial-like cell migration to Tat is linked functionally to the CB2R. Furthermore, the results indicate that activation of the CB2R leads to altered expression and compartmentation of the β-chemokine receptor CCR-3.

Human immunodeficiency virus (HIV) immunopathogenesis and vaccine development: a review

The development of a safe, effective and globally affordable HIV vaccine offers the best hope for the future control of the HIV-1 pandemic. Since 1987, scores of candidate HIV-1 vaccines have been developed which elicited varying degrees of protective responses in nonhuman primate models, including DNA vaccines, subunit vaccines, live vectored recombinant vaccines and various prime-boost combinations. Four of these candidate vaccines have been tested for efficacy in human volunteers, but, to the exception of the recent RV144 Phase III trial in Thailand, which elicited a modest but statistically significant level of protection against infection, none has shown efficacy in preventing HIV-1 infection or in controlling virus replication and delaying progression of disease in humans. Protection against infection was observed in the RV144 trial, but intensive research is needed to try to understand the protective immune mechanisms at stake. Building-up on the results of the RV144 trial and deciphering what possibly are the immune correlates of protection are the top research priorities of the moment, which will certainly accelerate the development of an highly effective vaccine that could be used in conjunction with other HIV prevention and treatment strategies. This article reviews the state of the art of HIV vaccine development and discusses the formidable scientific challenges met in this endeavor, in the context of a better understanding of the immunopathogenesis of the disease.

HIV-1 Tat-induced platelet activation and release of CD154 contribute to HIV-1-associated autoimmune thrombocytopenia

BACKGROUND

Enhanced platelet activation in human immunodeficiency virus (HIV)-1-infected patients has been reported and shown to strongly correlate with plasma viral load. Activated platelets are known to express and to release a variety of proteins that can modulate the immune system. Specifically, platelet-derived CD154 has been shown to be directly involved in the development of autoimmune thrombocytopenia (ITP). The mechanism by which HIV-1 infection leads to platelet activation and the effect of this activation on the development of HIV-1 ITP, however, is not fully understood.

OBJECTIVE

We have investigated the effect of HIV-1 Trans activating factor (Tat) on platelet activation.

RESULTS

We report that HIV-1 Tat directly interacts with platelets and induces platelet activation resulting in platelet micro-particle release. This activation by Tat requires the chemokine receptor CCR3 and β3-integrin expression on platelets, as well as calcium flux. Tat-induced activation of platelets releases platelet CD154, an immune modulator. Enhanced B-cell activity is found in mouse spleen B cells co-cultured with platelets treated with Tat in vitro. An early antibody response against adenovirus is found in Tat-injected mouse immunized with adenovirus, suggesting an enhanced immune response in vivo.

CONCLUSIONS

We have described a role of Tat-induced platelet activation in the modulation of the immune system, with implications for the development of HIV-1-associated thrombocytopenia.

IgG subclass profiles in infected HIV type 1 controllers and chronic progressors and in uninfected recipients of Env vaccines

We have studied IgG subclass responses to the HIV-1 proteins gp120, gp41, p24, and Tat in individuals who control their infection without using antiretroviral drugs (HIV-1 controllers; HC) or who progress to disease (chronic progressors; CP). We also measured IgG subclass titers to gp120 in vaccinated individuals. In all cases, the IgG1 subclass dominated the overall response to each antigen. The only IgG titer that differed significantly between the HC and CP groups was to the p24 Gag protein, which was higher in the HC group. IgG1 titers to both p24 and gp120 were significantly higher in the HC group, and IgG3 anti-gp120 antibodies, although rare, were detected more frequently in that group than in CP. Overall, significantly more patients had IgG2 antibodies to gp120 than to gp41. Antibodies to other IgG subclasses were infrequent and their frequency or titers did not differ between the two patient groups. Anti-gp41 and anti-Tat responses also did not correlate with immune control, and anti-Tat antibodies were infrequently detected. Although we found isotypic differences in IgG responses to HIV-1 antigens among vaccinees and the HC and CP individuals, there were no indications of differential T(H)1:T(H)2 polarization between the different groups.

HIV-1 Tat-based vaccines: an overview and perspectives in the field of HIV/AIDS vaccine development

The HIV epidemic continues to represent one of the major problems worldwide, particularly in the Asia and Sub-Saharan regions of the world, with social and economical devastating effects. Although antiretroviral drugs have had a dramatically beneficial impact on HIV-infected individuals that have access to treatment, it has had a negligible impact on the global epidemic. Hence, the inexorable spreading of the HIV pandemic and the increasing deaths from AIDS, especially in developing countries, underscore the urgency for an effective vaccine against HIV/AIDS. However, the generation of such a vaccine has turned out to be extremely challenging. Here we provide an overview on the rationale for the use of non-structural HIV proteins, such as the Tat protein, alone or in combination with other HIV early and late structural HIV antigens, as novel, promising preventative and therapeutic HIV/AIDS vaccine strategies.

A universal anti-HIV-1 Tat epitope vaccine that is fully synthetic and self-adjuvanting

Circulating HIV-1 Tat protein is essential for maintenance of the chronic HIV replication that predicates both HIV transmission and clinical progression to AIDS/death. A synthetic anti-Tat epitope vaccine (TUTI-16) was designed to induce neutralizing antibodies to Tat and, hopefully, provide immunological control of HIV replication. TUTI-16 is composed of (1) a conserved Tat B cell epitope (Tat 4-12), rendered universal by introducing known variant amino acids at variable positions 7, 9 and 12 during solid phase synthesis, (2) a promiscuous T helper sequence and (3) a lipopeptide toll-like receptor 2 (TLR2) agonist. TUTI-16 induced high titer antibodies against all 8 known variants of the Tat epitope.

HIV-1 Tat-specific IgG antibodies in high-responders target a B-cell epitope in the cysteine-rich domain and block extracellular Tat efficiently

Tat, an important regulatory protein of HIV-1, has been implicated in HIV-related pathogenesis. Immune responses to Tat, although underrepresented, confer protection against disease progression, in natural infection and experimental immunization, making Tat an attractive vaccine candidate. Information on immune responses to Tat from India which has the second largest HIV incidence has been lacking. Here we report a cross-sectional study evaluating the humoral response to Tat from a large number of samples from two southern states of India. 14% of the seropositive (63/447) and 4.6% of seronegative samples (7/150) harbored Tat-reactive antibodies. A significant number of the seropositive samples contained high levels of anti-Tat antibodies (31/447) which demonstrated class-switch to IgG1 and bound to Tat with high avidity. Cross-reactivity analysis showed that these antibodies interacted with Tat from different clades with variable degree with the highest interaction with subtype-AE and the least with subtype-B Tat. Importantly, a B-cell epitope in the cysteine-rich domain was found to be the most immunodominant one and antibodies interacting with this epitope blocked extracellular Tat efficiently. To the best of our knowledge this is the first report on immune responses to Tat from Indian populations and the data presented here could significantly contribute to HIV Tat vaccine design.

Priming with a very low dose of DNA complexed with cationic block copolymers followed by protein boost elicits broad and long-lasting antigen-specific humoral and cellular responses in mice

Cationic block copolymers spontaneously assemble via electrostatic interactions with DNA molecules in aqueous solution giving rise to micellar structures that protect the DNA from enzymatic degradation both in vitro and in vivo. In addition, we have previously shown that they are safe, not immunogenic and greatly increased antigen-specific CTL responses following six intramuscular inoculations of a very low dose (1microg) of the vaccine DNA as compared to naked DNA. Nevertheless, they failed to elicit detectable humoral responses against the antigen. To gain further insight in the potential application of this technology, here we show that a shorter immunization protocol based on two DNA intramuscular inoculations of 1microg of DNA delivered by these copolymers and a protein boost elicits in mice broad (both humoral and cellular) and long-lasting responses and increases the antigen-specific Th1-type T cell responses and CTLs as compared to priming with naked DNA. These results indicate that cationic block copolymers represent a promising adjuvant and delivery technology for DNA vaccination strategies aimed at combating intracellular pathogens.

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.

Contribution of nonneutralizing vaccine-elicited antibody activities to improved protective efficacy in rhesus macaques immunized with Tat/Env compared with multigenic vaccines

Previously, chronic-phase protection against SHIV(89.6P) challenge was significantly greater in macaques primed with replicating adenovirus type 5 host range mutant (Ad5hr) recombinants encoding HIVtat and env and boosted with Tat and Env protein compared with macaques primed with multigenic adenovirus recombinants (HIVtat, HIVenv, SIVgag, SIVnef) and boosted with Tat, Env, and Nef proteins. The greater protection was correlated with Tat- and Env-binding Abs. Because the macaques lacked SHIV(89.6P)-neutralizing activity prechallenge, we investigated whether Ab-dependent cellular cytotoxicity (ADCC) and Ab-dependent cell-mediated viral inhibition (ADCVI) might exert a protective effect. We clearly show that Tat can serve as an ADCC target, although the Tat-specific activity elicited did not correlate with better protection. However, Env-specific ADCC activity was consistently higher in the Tat/Env group, with sustained cell killing postchallenge exhibited at higher levels (p < 0.00001) for a longer duration (p = 0.0002) compared with the multigenic group. ADCVI was similarly higher in the Tat/Env group and significantly correlated with reduced acute-phase viremia at wk 2 and 4 postchallenge (p = 0.046 and 0.011, respectively). Viral-specific IgG and IgA Abs in mucosal secretions were elicited but did not influence the outcome of the i.v. SHIV(89.6P) challenge. The higher ADCC and ADCVI activities seen in the Tat/Env group provide a plausible mechanism responsible for the greater chronic-phase protection. Because Tat is known to enhance cell-mediated immunity to coadministered Ags, further studies should explore its impact on Ab induction so that it may be optimally incorporated into HIV vaccine regimens.

Comparative study of Tat vaccine regimens in Mauritian cynomolgus and Indian rhesus macaques: influence of Mauritian MHC haplotypes on susceptibility/resistance to SHIV(89.6P) infection

Protection afforded by HIV Tat-based vaccines has differed in Indian rhesus and Mauritian cynomolgus macaques. We evaluated native Tat and Ad-HIVtat priming/Tat-boosting regimens in both species. Both vaccines were immunogenic. Only the Ad-tat regimen modestly reduced acute viremia in rhesus macaques after SHIV(89.6P) challenge. Confounding variables uncovered in Mauritian macaques included significant associations of susceptibility to infection with MHC class IB and class II H2 and H5 haplotypes, and resistance to infection with class IB haplotypes H3 and H6. Although protection here was limited, Tat-based vaccines incorporating other HIV components have shown greater efficacy. Combination strategies should be further explored.

Increasing the humoral immunogenic properties of the HIV-1 Tat protein using a ligand-stabilizing strategy

Tat is regarded as an attractive target for the development of an AIDS vaccine. However, works suggest that Tat is a poorly immunogenic protein and therefore we attempted to increase its immunogenic potency. As we observed that Tat is highly sensitive to enzymatic degradation in vitro we tried to make it less susceptible to proteolysis using ligands. We complexed Tat101 with various sulfated sugars and observed that some of these ligands made the protein more resistant to proteolysis and more immunogenic. In a more thorough study, we observed that a low-molecular-weight heparin fragment, called Hep6000, altered both the cell-binding capacity and transactivating activity of Tat101, suggesting that this sulfated polysaccharide can make the protein less toxic. Sera raised against Tat101 and Tat101/Hep6000 similarly bound mainly to the N-terminal region of the protein, indicating that formation of the complex does not alter the B-cell immunodominant region. Anti-Tat101/Hep6000 antisera neutralized the transactivating activity of Tat101 more efficiently than anti-Tat101 antisera. Altogether, these results indicate that stabilization of Tat101 using sulfated sugars increases its immunogenicity and might be of value in increasing its vaccine efficacy.

Tat mutations in an African cohort that do not prevent transactivation but change its immunogenic properties

Humoral responses against extra-cellular HIV-1 Tat may be beneficial as Tat has been implicated in the viral pathogenesis associated with HIV-1 disease progression. We determined the levels of anti-Tat IgG in sera of HIV-1 seropositive individuals from the Rural Clinical Cohort in Uganda using nine different Tat proteins representative of the major subtypes presently accounting for 97% of infections worldwide. We observed the presence of anti-Tat IgG able to react against the various subtypes tested, although none cross-reacted against all nine variants. We show that 46.25% of seropositive patients were able to recognise at least one Tat variant with 1:1000 sera dilution. We also show that the C terminus of Tat is the most variable region and an important epitope that might explain the limitation of cross-recognition of Tat antibodies regarding Tat variants. This study shows in seropositive patients that Tat can tolerate mutations without modification of its primary function but with changes in its immunogenic properties. These findings should be considered when designing Tat-based HIV-1 vaccines.

A replication-competent adenovirus-human immunodeficiency virus (Ad-HIV) tat and Ad-HIV env priming/Tat and envelope protein boosting regimen elicits enhanced protective efficacy against simian/human immunodeficiency virus SHIV89.6P challenge in rhesus macaques

We previously demonstrated that replication-competent adenovirus (Ad)-simian immunodeficiency virus (SIV) recombinant prime/protein boost regimens elicit potent immunogenicity and strong, durable protection of rhesus macaques against SIV(mac251). Additionally, native Tat vaccines have conferred strong protection against simian/human immunodeficiency virus SHIV(89.6P) challenge of cynomolgus monkeys, while native, inactivated, or vectored Tat vaccines have failed to elicit similar protective efficacy in rhesus macaques. Here we asked if priming rhesus macaques with replicating Ad-human immunodeficiency virus (HIV) tat and boosting with the Tat protein would elicit protection against SHIV(89.6P). We also evaluated a Tat/Env regimen, adding an Ad-HIV env recombinant and envelope protein boost to test whether envelope antibodies would augment acute-phase protection. Further, expecting cellular immunity to enhance chronic viremia control, we tested a multigenic group: Ad-HIV tat, -HIV env, -SIV gag, and -SIV nef recombinants and Tat, Env, and Nef proteins. All regimens were immunogenic. A hierarchy was observed in enzyme-linked immunospot responses (with the strongest response for Env, followed by Gag, followed by Nef, followed by Tat) and antibody titers (with the highest titer for Env, followed by Tat, followed by Nef, followed by Gag). Following intravenous SHIV(89.6P) challenge, all macaques became infected. Compared to controls, no protection was seen in the Tat-only group, confirming previous reports for rhesus macaques. However, the multigenic group blunted acute viremia by approximately 1 log (P = 0.017), and both the multigenic and Tat/Env groups reduced chronic viremia by 3 and 4 logs, respectively, compared to controls (multigenic, P = 0.0003; Tat/Env, P < 0.0001). The strikingly greater reduction in the Tat/Env group than in the multigenic group (P = 0.014) was correlated with Tat and Env binding antibodies. Since prechallenge anti-Env antibodies lacked SHIV(89.6P)-neutralizing activity, other functional anti-Env and anti-Tat activities are under investigation, as is a possible synergy between the Tat and Env immunogens.