B-cell epitopes in HIV-1 Tat and Rev proteins colocalize with T-cell epitopes and with functional domains

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.

The MHC class II cofactor HLA-DM interacts with Ig in B cells

B cells internalize extracellular Ag into endosomes using the Ig component of the BCR. In endosomes, Ag-derived peptides are loaded onto MHC class II proteins. How these pathways intersect remains unclear. We find that HLA-DM (DM), a catalyst for MHC class II peptide loading, coprecipitates with Ig in lysates from human tonsillar B cells and B cell lines. The molecules in the Ig/DM complexes have mature glycans, and the complexes colocalize with endosomal markers in intact cells. A larger fraction of Ig precipitates with DM after BCR crosslinking, implying that complexes can form when DM meets endocytosed Ig. In vitro, in the endosomal pH range, soluble DM directly binds the Ig Fab domain and increases levels of free Ag released from immune complexes. Taken together, these results argue that DM and Ig intersect in the endocytic pathway of B cells with potential functional consequences.

Production of human antibodies by in vitro immunization using a fusion protein containing the transcriptional transactivator of HIV-1

Antigen-specific activation of human B cells represents a key step for the production of monoclonal antibodies. Several approaches have been developed over the last thirty years in order to improve the process of lymphocyte activation in vitro. In the present study, we investigated whether the transcriptional transactivator (Tat) of human immunodeficiency virus, which possesses numerous biological activities, is able to trigger antibody secretion when incubated with human peripheral blood mononuclear cells. No such effect was observed when using Tat as a free protein. However, we found a significant IgM antibody production when Tat was previously fused to a double domain, called ZZ, derived from protein A of Staphylococcus aureus. The effect was also observed when the fusion protein, called ZZTat101, was incubated with purified B cells, indicating that the phenomenon does not require T-cell help. Antibody secretion was observed in the absence of cytokines that are usually used during in vitro immunization experiments, indicating that ZZTat101 provides the signals required for the initiation of the immune response. Antibody secretion was observed using a ZZTat mutant, containing only the Tat residues 22 to 57, called ZZTat22-57, indicating that this region is sufficient to initiate the immune response. In contrast, the effect was not found with a ZZTat22-57 mutant devoid of the seven Tat cysteines located between residues 22 and 37, demonstrating that these residues play a crucial role in the phenomenon. Our results pave the way to the development of a new in vitro immunization method based on antigens associated with ZZTat.

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.

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.

MH2 domain of Smad3 reduces HIV-1 Tat-induction of cytokine secretion

HIV-1 infection of the central nervous system (CNS) is associated with dysregulation of several important cytokines and chemokines, which are involved in inflammatory process. Earlier studies ascribed a critical role for Tat, a potent viral transcription activator, in this process by enhancing the expression of several immunomodulators including TGFbeta and MCP-1. Investigation of signaling pathways which are controlled by these cytokines led to identification of MH2 domain of Smad3, the downstream activator of TGFbeta pathway, as a modulator of MCP-1 promoter activity. The level of MCP-1 is increased in AIDS patients with neurologic problems, through recruitment of inflammatory cells, which can contribute to neuropathogenesis of AIDS. Therefore, we attempted to investigate the effect of MH2 on expression of MCP-1 and other immunolmodulators in CNS cells. By employing an adenovirus expression vector, we demonstrated that MH2 can decrease the levels of Tat-induced activation of MCP-1 and several other cytokines and chemokines in astrocytic cells. In addition, we showed that MH2 significantly reduced the activity of cytokines produced by cultures of adenovirus-MH2 transduced cells as measured by the transmigration of human PBMC cells. Thus, MH2 domain of Smad3 is a potential agent that may be developed as an inhibitor for the cytokine-mediated inflammatory responses in the brain and may have the potential to prevent transmigration of HIV-1-infected monocytes across the blood brain barrier in AIDS patients.

Induction of neutralizing antibodies and Th1-polarized and CD4-independent CD8+ T-cell responses following delivery of human immunodeficiency virus type 1 Tat protein by recombinant adenylate cyclase of Bordetella pertussis

HIV-Tat, a conserved protein playing a key role in the early life cycle of the human immunodeficiency virus (HIV) has been proposed as a potential AIDS vaccine. An HIV-Tat-based vaccine should elicit a broad, long-lasting, and neutralizing immune response. We have previously demonstrated that the adenylate cyclase (CyaA) from Bordetella pertussis targets dendritic cells and delivers CD8(+) and CD4(+) T-cell epitopes into the major histocompatibility complex class I and class II presentation pathways. We have also showed that CyaA induced specific and protective cytotoxic T cell responses in vivo. Here, we designed a prototype vaccine based on the HIV type 1 Tat delivered by CyaA (CyaA-E5-Tat) and tested its capacity to induce HIV-Tat-specific cellular as well as antibody responses. We showed that immunization of mice by CyaA-E5-Tat in the absence of adjuvant elicited strong and long-lasting neutralizing anti-Tat antibody responses more efficient than those obtained after immunization with Tat toxoid in aluminum hydroxide adjuvant. Analyses of the anti-Tat immunoglobulin G isotypes and the cytokine pattern showed that CyaA-E5-Tat induced a Th1-polarized immune response in contrast to the Th2-polarized immune responses obtained with the Tat toxoid. In addition, our data demonstrated that HIV-Tat-specific gamma interferon-producing CD8(+) T cells were generated after vaccination with CyaA-E5-Tat in a CD4(+) T-cell-independent manner. Based on these findings, CyaA-E5-Tat represents an attractive vaccine candidate for both preventive and therapeutic vaccination involving CyaA as an efficient nonreplicative vector for protein delivery.

The binding affinity of double-stranded RNA motifs to HIV-1 Tat protein affects transactivation and the neutralizing capacity of anti-Tat antibodies elicited after intranasal immunization

In this study we examined the hypothesis that the binding affinity of two double-stranded (ds) RNA motifs to HIV-1 Tat protein might affect transactivation and the type of anti-Tat immune responses. Using surface plasmon resonance technology we demonstrated the capacity of the poly(A):poly(U) (pA:pU) motif to bind with high affinity to a totally synthetic Tat protein and to inhibit more efficiently the Tat/transactivation response element (TAR) RNA interaction as compared to the poly(I):poly(C) (pI:pC) motif. Furthermore, the pA:pU motif was tenfold more effective in inhibiting Tat-driven transactivation than the pI:pC motif. Following intranasal immunization of mice, both dsRNA motifs enhanced the antibody (serum and mucosal) and cellular responses to Tat. However, only the serum samples of mice immunized with Tat + pI:pC inhibited Tat-driven transactivation. The profile of serum antibody subclasses together with the secreted cytokines by Tat-stimulated splenocyte cultures indicated that both dsRNA motifs favored the induction of a balanced Th1 and Th2 immune response. The demonstration in this study that two dsRNA motifs had a marked effect on Tat/TAR RNA interaction and on the neutralizing capacity of anti-Tat specific antibody responses highlights their potential for biological applications and the importance of selecting the appropriate motif as an adjuvant for vaccine design.

Tat-neutralizing versus Tat-protecting antibodies in rhesus macaques vaccinated with Tat peptides

The human immunodeficiency virus (HIV) regulatory protein Tat represents an attractive target for developing vaccine strategies. Both humoral and cellular responses against Tat might reduce disease progression by interfering with the deleterious functions of extracellularly secreted protein and by reducing viral replication. We have immunized Rhesus macaques intramuscularly and intranasally with a cocktail of three Tat peptides encompassing residues 1-20, 1-61 and 44-61 administrated in the presence of Montanide ISA 720 as adjuvant. The monkeys were challenged by the intrarectal route with 10 MID50 of SHIV BX08. All immunized macaques but one gave a good cross-reactive antibody response to Tat but the proliferative response and levels of IL-2, IFN-gamma and TNF-alpha secretion of peripheral blood mononuclear cells (PBMCs) recalled ex vivo with active Tat protein were weak. After viral challenge one peptide-vaccinated macaque only remained free of virus. The presence in the serum of vaccinated animals of neutralizing antibodies able to inhibit Tat transactivation activity or Tat-induced apoptosis was not correlated to protection.

Codon optimization of the tat antigen of human immunodeficiency virus type 1 generates strong immune responses in mice following genetic immunization

DNA vaccines have been successful in eliciting potent immune responses in mice. Their efficiency, however, is restricted in larger animals. One reason for the limited performance of the DNA vaccines is the lack of molecular strategies to enhance immune responses. Additionally, genes directly cloned from pathogenic organisms may not be efficiently translated in a heterologous host expression system as a consequence of codon bias. To evaluate the influence of codon optimization on the immune response, we elected to use the Tat antigens of human immunodeficiency virus type 1 (HIV-1) (subtype C) and HIV-2, as these viral antigens are poorly immunogenic in natural infection and in experimental immunization and they are functionally important in viral infectivity and pathogenesis. Substituting codons that are optimally used in the mammalian system, we synthetically assembled Tat genes and compared them with the wild-type counterparts in two different mouse strains. Codon-optimized Tat genes induced qualitatively and quantitatively superior immune responses as measured in a T-cell proliferation assay, enzyme-linked immunospot assay, and chromium release assay. Importantly, while the wild-type genes promoted a mixed Th1-Th2-type cytokine profile, the codon-optimized genes induced a predominantly Th1 profile. Using a pepscan strategy, we mapped an immunodominant T-helper epitope to the core and basic domains of HIV-1 Tat. We also identified cross-clade immune responses between HIV-1 subtype B and C Tat proteins mapped to this T-helper epitope. Developing molecular strategies to optimize the immunogenicity of DNA vaccines is critical for inducing strong immune responses, especially to antigens like Tat. Our identification of a highly conserved T-helper epitope in the first exon of HIV-1 Tat of subtype C and the demonstration of a cross-clade immune response between subtypes B and C are important for a more rational design of an HIV vaccine.

Identification of antigenic regions of duck hepatitis B virus core protein with antibodies elicited by DNA immunization and chronic infection

The induction of humoral response in ducks by DNA-based immunization against duck hepatitis B virus (DHBV) core protein (DHBc) was investigated. In addition, the amino acid specificity of the induced response was compared by using peptide scanning to that elicited either by protein immunization or during chronic DHBV infection. Immunization of ducks with a plasmid expressing DHBc protein led to the induction of a long-lasting antibody response able to specifically recognize viral protein in chronically infected duck livers. Peptide scanning analysis of anti-DHBc response induced during chronic DHBV infection allowed us to identify six major antigenic regions (AR1 to AR6). The reactivity spectrum of duck sera elicited by protein immunization appeared narrower and was restricted to only four of these antigenic regions in spite of higher anti-DHBc antibody titers. Interestingly, anti-DHBc antibodies induced by DNA-based immunization recognized five of six antigenic regions, and the epitope pattern was broader and more closely related to that observed in chronic viral infections. To gain more insight into the location of antigenic regions, we built a three-dimensional (3-D) model of DHBc protein based on human and duck core sequence alignment data and the HBc 3-D crystal structure. The results suggest that two identified antigenic regions (AR2, amino acids [aa] (64)T-P(84), and AR5, aa (183)A-R(210)) are located at positions on the protein surface equivalent to those of the two HBc major epitopes. Moreover, we identified another antigenic region (AR3, aa (99)I-I(112)) that was recognized by all sera from chronically infected, DNA- or protein-immunized ducks within the large 45-aa insertion in DHBc protein, suggesting that this region, which lacks HBc, is externally exposed.

Zn2+ binding to cysteine-rich domain of extracellular human immunodeficiency virus type 1 Tat protein is associated with Tat protein-induced apoptosis

The Tat protein has several functional domains, one of which is the cysteine-rich domain that is a highly conserved region in spite of the presence of many subtypes of human immunodeficiency virus type 1 (HIV-1). Although the cysteine-rich domain is a potential site for Zn(2+) binding, it is controversial whether Zn(2+) is substantially essential for the structure and activities of the Tat protein. To study the significance of Zn(2+) in the cysteine-rich domain of the Tat protein particularly released to the extracellular space, we raised the monoclonal antibody (MAb) 5A4, which has an attractive property of recognizing the Zn(2+)-binding Tat(20-41) peptide but not the apo-Tat(20-41) peptide. MAb 5A4 inhibited the trans-activation of the HIV long terminal repeat (LTR) in HeLa-CD4-LTR/beta-gal cells induced by treatment with the recombinant Tat protein, indicating that MAb 5A4 can recognize the full-length Tat protein and inhibit its trans-activity. The antibody also inhibited the apoptosis of Jurkat cells induced by treatment with the released native-Tat-protein-containing supernatant from the culture of HIV-1(JRFL)-infected cells. These results suggest that Zn(2+), whose structure is closely associated with not only the trans-activation of HIV-LTR but also the induction of apoptosis, binds to the extracellular native Tat protein. The Zn(2+)-binding cysteine-rich domain therefore can be a molecular target in the development of an anti-Tat vaccine and agents for the control of extracellular-Tat-protein-mediated pathogenesis leading to the progression of acquired immunodeficiency syndrome.

Mucosal delivery of the human immunodeficiency virus-1 Tat protein in mice elicits systemic neutralizing antibodies, cytotoxic T lymphocytes and mucosal IgA

Human immunodeficiency virus (HIV)-1 Tat protein induces protection in non-human primates upon systemic vaccination. In view of the design of mucosal vaccines against HIV-1 we studied the immune response to native Tat (aa 1-86) in mice following intranasal delivery of the protein with two mucosal adjuvants, Escherichia coli heat-labile enterotoxin (LT) and LT-R72, a non-toxic mutant of LT. Immunization with Tat and the two adjuvants induced in BALB/c but not in C57BL/6 mice high and persistent levels of serum IgG and secretory IgA in vaginal and intestinal fluids. Mice sera neutralized Tat and recognized two epitopes mapping in the regions 1-20 and 46-60. Furthermore, their splenocytes proliferated and secreted IFN-gamma and IL-6 in response to Tat. Finally, CTLs were also elicited and they recognized an epitope localized within aa 11-40 of Tat.

Specificity and effect on apoptosis of Tat antibodies from vaccinated and SHIV-infected rhesus macaques and HIV-infected individuals

Recent contributions have demonstrated that actively secreted Tat protein plays an important functional role in human immunodeficiency virus-1 (HIV-1) infection and that Tat antibodies might interfere with disease progression by blocking the protein extracellularly. In this context we have studied the recognition of several Tat mutants as well as various synthetic Tat fragments by anti-Tat monoclonal antibodies and by IgG antibodies from a large collection of slow and fast-progressor infected individuals. We have also tested the sera from simian/human immunodeficiency virus (SHIV)-infected macaques with these Tat peptides. Important differences were found between long-term non-progressors and fast-progressors, and between human and monkey sera in terms of antibody specificity. Rabbits and macaques were immunised with several Tat peptides and we found that certain antibody subsets from immunised animals recognised the cognate protein Tat and had the capacity to inhibit Tat-induced apoptosis of T cells. Such antibodies might be important for controlling Tat-induced death in cells uninfected by HIV-1.

Identification of immunodominant epitopes in inactivated Tat-vaccinated healthy and HIV-1-infected volunteers

We analyzed the epitopes and the molecular forms of Tat recognized by the antibodies raised by Tat-toxoid vaccination in both healthy and HIV-infected volunteers. Tat-toxoid-vaccinated healthy volunteer sera reacted predominantly with peptides covering amino acids 1 through 24 and 46 through 60, corresponding to the N-terminus and basic domains of Tat. In contrast, whereas all sera from vaccinated HIV-1-positive patients reacted with the N-terminus and (with a single exception) with the basic domain, most of these sera also recognized peptides encompassing distinct domains of Tat, particularly the C-terminus (79-86). The sera of vaccinated individuals recognized both monomeric and oligomeric forms of Tat 1 through 86 or of Tat 1 through 101 and also blocked the ability of cell-released extracellular Tat to transactivate the HIV-1 LTR promoter. Synthetic Tat preincubated with sera from vaccinated individuals lost its functional activity as well. This is probably because of its inability to enter the cells as a result of immune complex formation with anti-Tat IgG. These data demonstrate that Tat-toxoid vaccination induces an efficient antibody response blocking the functional activity of Tat.

Antibody responses against B-cell epitopes of the hypervariable region 1 of hepatitis C virus in self-limiting and chronic human hepatitis C followed-up using consensus peptides

A rare collection of serum samples from patients with hepatitis C virus (HCV) infection followed up from the onset of clinical symptoms was acquired. RNA corresponding to the hypervariable region 1 (HVR1) of E2 protein of HCV isolated from nine patients was reverse-transcribed, amplified, sequenced, and HVR1 amino acid sequences were deduced. These sequences and a selection of HVR1 amino acid sequences of matching HCV genotypes from protein and translated DNA sequence databanks were used to create the HVR1 amino acid consensus. The degenerated peptides mimicking N- and C-termini of the consensus were synthesized. Most (76%) of 17 patients followed up for the period from 1 week to a minimum of 7 months from the onset of acute symptoms developed antibodies reacting with peptides representing N- and/or C- termini of HVR1. Antibody recognition of the consensus HVR1 peptides indicates that the variability of HVR1 sequence on the protein level is limited with certain conserved structure(s) being untouched. A tendency was observed for a slower development of anti-HVR1 antibody response in patients developing chronic HCV, as compared to those with self-limiting HCV infection.

Identification of a peptide capable of inducing an HIV-1 Tat-specific CTL response

Although Tat-specific CTL responses are elicited in HIV-infected patients and in non-human primate models, specific CTL epitopes within Tat have not been identified. In this study, we mucosally immunized mice with recombinant, full-length Tat protein or individual Tat-specific, overlapping peptides to map putative H-2d-restricted, Tat-specific CTL epitopes. Standard chromium release assays from splenocytes of immunized animals identified a peptide (QPKTACTNC) capable of inducing Tat-specific CTL responses. This newly-identified epitope lies within a region of low sequence variability among HIV-1 subtypes, suggesting its potential use in a multicomponent AIDS vaccine.

Epitope clusters in the major outer membrane protein of Chlamydia trachomatis

Immunopathology that is caused by re-infection with Chlamydia trachomatis is very common in humans despite regular responses to multiple, often conserved, antibody and T cell epitopes. Recurrent mutations that disrupt T cell epitopes in the major outer membrane protein in clinical isolates and the reduced transcription of HLA genes by infected cells may be evidence for pathogen evasion of protective immune responses. Subunit vaccines containing recently discovered clusters of T cell epitopes in the major outer membrane protein that are presented with diverse HLA allotypes may allow widespread protective immunization while avoiding the suppression of lasting immunity that occurs by unknown mechanisms associated with infection.