HIV vaccines 1983-2003

Recombinant Mycobacterium smegmatis with a pMyong2 vector expressing Human Immunodeficiency Virus Type I Gag can induce enhanced virus-specific immune responses

Recently, we have developed a novel Mycobacterium-Escherichia coli shuttle vector system using pMyong2, which can provide an enhanced expression of heterologous genes in recombinant Mycobacterium smegmatis (rSmeg). To investigate the usefulness of rSmeg using pMyong2 in vaccine application, we vaccinated M. smegmatis with pMyong2 system expressing Human Immunodeficiency Virus Type I (HIV-1) Gag p24 antigen (rSmeg-pMyong2-p24) into mice and examined its cellular and humoral immune responses against HIV gag protein. We found that rSmeg-pMyong2-p24 expressed higher levels of Gag protein in bacteria, macrophage cell line (J774A.1) and mouse bone marrow derived dendritic cells (BMDCs) compared to rSmeg strains using two other vector systems, pAL5000 derived vector (rSmeg-pAL-p24) and the integrative plasmid, pMV306 (rSmeg-pMV306-p24). Inoculation of mice with rSmeg-pMyong2-p24 elicited more effective immunity compared to the other two rSmeg strains, as evidenced by higher levels of HIV-1 Gag-specific CD4 and CD8 T lymphocyte proliferation, interferon gamma ELISPOT cell induction, and antibody production. Furthermore, rSmeg-pMyong2-p24 showed a higher level of cytotoxic T cell response against target cells expressing Gag p24 proteins. Our data suggest that Mycobacterium-Escherichia coli shuttle vector system with pMyong2 may provide an advantage in vaccine application of rSmeg over other vector systems.

Fragments of the V1/V2 domain of HIV-1 glycoprotein 120 engineered for improved binding to the broadly neutralizing PG9 antibody

The V1/V2 domain of the HIV-1 envelope protein gp120 possesses two important epitopes: a glycan-dependent epitope recognized by the prototypic broadly neutralizing monoclonal antibody (bN-mAb), PG9, as well as an epitope recognized by non-neutralizing antibodies that has been associated with protection from HIV infection in the RV144 HIV vaccine trial. Because both of these epitopes are poorly immunogenic in the context of full length envelope proteins, immunization with properly folded and glycosylated fragments (scaffolds) represents a potential way to enhance the immune response to these specific epitopes. Previous studies showed that V1/V2 domain scaffolds could be produced from a few selected isolates, but not from many of the isolates that would be advantageous in a multivalent vaccine. In this paper, we used a protein engineering approach to improve the conformational stability and antibody binding activity of V1/V2 domain scaffolds from multiple diverse isolates, including several that were initially unable to bind the prototypic PG9 bN-mAb. Significantly, this effort required replicating both the correct glycan structure as well as the β-sheet structure required for PG9 binding. Although scaffolds incorporating the glycans required for PG9 binding (e.g., mannose-5) can be produced using glycosylation inhibitors (e.g., swainsonine), or mutant cell lines (e.g. GnTI(-) 293 HEK), these are not practical for biopharmaceutical production of proteins intended for clinical trials. In this report, we describe engineered glycopeptide scaffolds from three different clades of HIV-1 that bind PG9 with high affinity when expressed in a wildtype cell line suitable for biopharmaceutical production. The mutations that improved PG9 binding to scaffolds produced in normal cells included amino acid positions outside of the antibody contact region designed to stabilize the β-sheet and turn structures. The scaffolds produced address three major problems in HIV vaccine development: (1) improving antibody responses to poorly immunogenic epitopes in the V1/V2 domain; (2) eliminating antibody responses to highly immunogenic (decoy) epitopes outside the V1/V2 domain; and (3) enabling the production of V1/V2 scaffolds in a cell line suitable for biopharmaceutical production.

Toward a cure for HIV--Seeking effective therapeutic vaccine strategies

This review article focuses on the rationale and evaluation of therapeutic vaccines against HIV. This strategy has been developed in order to restore or restimulate HIV-specific immunity in patients treated with antiretroviral therapies. Despite the lack of good candidate vaccines against HIV, two objectives have been targeted during the past 15 years. Therapeutic immunization was first proposed to help control virus relapses during treatment interruptions. More recently, the concept of therapeutic immunization has been boosted by efforts to reach HIV remission or cure, in combination to HIV reactivating agents, to help purge HIV reservoirs in a "shock and kill" strategy. This review analyses the rationales for these strategies and the results of the most widely therapeutic vaccines designed to generate T-cell immunity, i.e. recombinant viral vectors and dendritic cell-based strategies, while extremely few strategies targeted HIV-specific Abs. Only marginal control of HIV was obtained with cellular-based strategies, suggesting that approaches targeting or using broadly neutralizing Abs, should be of benefit for future efforts of therapeutic immunization against HIV in the quest toward a cure for HIV.

HIV vaccine development: an exploratory review of the trials and tribulations

While advances have been made in some areas, more often than not attempts at vaccine development against the human immunodeficiency virus only serve to highlight gaps in our knowledge of host immunity. While numerous approaches have been explored, to control infection, an HIV vaccine will need to be able to induce production of neutralizing antibodies and a cytotoxic T cell response in order to prevent the formation of the CD4+ T cell viral reservoir. However, challenges still remain for the development of an HIV vaccine. Incomplete knowledge of host immunity lies at the core of the tribulations lying in the face of effective vaccine development.

Immune interventions in HIV infection

Immune-based therapy (IBT) interventions have found a window of opportunity within some limitations of the otherwise successful combined antiretroviral therapy (cART). Two major paradigms drove immunotherapeutic research to combat human immunodeficiency virus (HIV) infection. First, IBTs were proposed either to help restore CD4(+) T-cell counts in cases of therapeutic failures with cytokines, interleukin-2 (IL-2) or IL-7, or to better control HIV and disease progression during treatment interruptions with anti-HIV therapeutic candidate vaccines. The most widely used candidates were HIV-recombinant live vector-based alone or combined with other vaccine compounds and dendritic cell (DC) therapies. A more recent and current paradigm aims at achieving HIV cure by combining IBT with cART using either cytokines to reactivate virus production in latently infected cells and/or therapeutic immunization to boost HIV-specific immunity in a 'shock and kill' strategy. This review summarizes the rationale, hopes, and mechanisms of successes and failures of these cytokine-based and vaccine-based immune interventions. Results from these first series of IBTs have been so far somewhat disappointing in terms of clinical relevance, but have provided lessons that are discussed in light of the future combined strategies to be developed toward an HIV cure.

Suppressor of cytokine signaling 1 inhibition strategy to enhance anti-HIV vaccination

Extensive efforts aimed at stimulating immune responses by modifying HIV antigens and using various delivery systems and adjuvants have so far failed to generate promising HIV vaccines, highlighting the urgent need to explore alternative immunization approaches. Antigen-presenting cells, such as dendritic cells, play a critical role in the initiation and maintenance of immune responses against HIV infection and dendritic cells are regulated by stimulatory, as well as inhibitory signaling. Recent studies demonstrate that the suppressor of cytokine signaling 1 (SOCS1) functions as an antigen-presentation attenuator by restricting the Janus-activated kinase-signal transducers and activators of transcription and Toll-like receptor-signaling pathways. SOCS1-silenced dendritic cells produce higher levels of both T-helper 1- and 2-polarizing cytokines, broadly enhance memory HIV-specific B-cell and T-cell responses and activate natural killer cells owing to unbridled cytokine feedback signaling loops. Therefore, the inhibition of antigen-presentation attenuators represents a generally applicable and alternative strategy for enhancing the potency of various forms of prophylactic and therapeutic HIV vaccines.

Pulse HIV vaccination: feasibility for virus eradication and optimal vaccination schedule

We modify the classical virus dynamics model by incorporating an immune response with fixed or fluctuating vaccination frequencies and dosages to obtain a system of impulsive differential equations for the virus dynamics of both the wild-type and mutant strains. This model framework permits us to obtain precise conditions for the virus elimination, which are much more feasible compared with existing results, which require frequent vaccine administration with large dosage. We also consider the corresponding impulsive optimal control problem to describe when and how much of the vaccine should be administered in order to maximize levels of healthy CD4(+) T cells and immune response cells. A gradient-based optimization method is applied to obtain the optimal schedule numerically. For a case study when the CTL vaccine is administered in a period of one year, our numerical studies support the optimal vaccination schedule consisting of vaccine administration three times, with the first dosage strong (to boost the immune system), followed by a second dosage shortly after (to strengthen the immune response) and then the third and final dosage long after (to ensure the immune system can handle viruses rebound).

The nucleocapsid protein of Rift Valley fever virus is a potent human CD8+ T cell antigen and elicits memory responses

There is no licensed human vaccine currently available for Rift Valley Fever Virus (RVFV), a Category A high priority pathogen and a serious zoonotic threat. While neutralizing antibodies targeting the viral glycoproteins are protective, they appear late in the course of infection, and may not be induced in time to prevent a natural or bioterrorism-induced outbreak. Here we examined the immunogenicity of RVFV nucleocapsid (N) protein as a CD8(+) T cell antigen with the potential for inducing rapid protection after vaccination. HLA-A*0201 (A2)-restricted epitopic determinants were identified with N-specific CD8(+) T cells from eight healthy donors that were primed with dendritic cells transduced to express N, and subsequently expanded in vitro by weekly re-stimulations with monocytes pulsed with 59 15mer overlapping peptides (OLPs) across N. Two immunodominant epitopes, VT9 (VLSEWLPVT, N(121-129)) and IL9 (ILDAHSLYL, N165-173), were defined. VT9- and IL9-specific CD8(+) T cells identified by tetramer staining were cytotoxic and polyfunctional, characteristics deemed important for viral control in vivo. These peptides induced specific CD8(+) T cell responses in A2-transgenic mice, and more importantly, potent N-specific CD8(+) T cell reactivities, including VT9- and IL9-specific ones, were mounted by mice after a booster vaccination with the live attenuated RVF MP-12. Our data suggest that the RVFV N protein is a potent human T cell immunogen capable of eliciting broad, immunodominant CD8(+) T cell responses that are potentially protective. Understanding the immune responses to the nucleocapsid is central to the design of an effective RVFV vaccine irrespective of whether this viral protein is effective as a stand-alone immunogen or only in combination with other RVFV antigens.

Efficiency of pH-sensitive fusogenic polymer-modified liposomes as a vaccine carrier

The usefulness of pH-sensitive fusogenic polymer-(succinylated poly(glycidol)-(SucPG-) modified liposomes as a vaccine carrier in the induction of immune responses was evaluated. Mice were intraperitoneally immunized with ovalbumin- (OVA-) containing SucPG-modified liposomes. After immunization, significant OVA-specific antibodies were detected in the serum. When sera were analyzed for isotype distribution, OVA-specific IgG1 antibody responses were noted in mice immunized with OVA-containing polymer-unmodified liposomes, whereas immunization with OVA-containing SucPG-modified liposomes resulted in the induction of OVA-specific IgG1, IgG2a, and IgG3 Ab responses. In spleen lymphocytes from mice immunized with OVA-containing SucPG-modified liposomes, both IFN-γ-(Th1-type-) and IL-4-(Th2 type-) specific mRNA were detected. Moreover, substantial production of IFN-γ and IL-4 was demonstrated in spleen cells from OVA-containing SucPG-modified liposomes in vitro. These results suggest that the pH-sensitive fusogenic polymer-(SucPG-) modified liposomes would serve effectively as an antigen delivery vehicle for inducing Th1 and Th2 immune responses.

Conserved hydrogen bonds and water molecules in MDR HIV-1 protease substrate complexes

The success of highly active antiretroviral therapy (HAART) in anti-HIV therapy is severely compromised by the rapidly developing drug resistance. HIV-1 protease inhibitors, part of HAART, are losing their potency and efficacy in inhibiting the target. Multi-drug resistant (MDR) 769 HIV-1 protease (resistant mutations at residues 10, 36, 46, 54, 62, 63, 71, 82, 84, 90) was selected for the present study to understand the binding to its natural substrates. The nine crystal structures of MDR769 HIV-1 protease substrate hepta-peptide complexes were analyzed in order to reveal the conserved structural elements for the purpose of drug design against MDR HIV-1 protease. Our structural studies demonstrated that highly conserved hydrogen bonds between the protease and substrate peptides, together with the conserved crystallographic water molecules, played a crucial role in the substrate recognition, substrate stabilization and protease stabilization. In addition, the absence of the key flap-ligand bridging water molecule might imply a different catalytic mechanism of MDR769 HIV-1 protease compared to that of wild type (WT) HIV-1 protease.

Immunogenicity of repeated Sendai viral vector vaccination in macaques

Induction of durable cellular immune responses by vaccination is an important strategy for the control of persistent pathogen infection. Viral vectors are promising vaccine tools for eliciting antigen-specific T-cell responses. Repeated vaccination may contribute to durable memory T-cell induction, but anti-vector antibodies could be an obstacle to its efficacy. We previously developed a Sendai virus (SeV) vector vaccine and showed the potential of this vector for efficient T-cell induction in macaques. Here, we examined whether repeated SeV vector vaccination with short intervals can enhance antigen-specific CD8(+) T-cell responses. Four rhesus macaques possessing the MHC-I haplotype 90-120-Ia were immunized three times with intervals of three weeks. For the vaccination, we used replication-defective F-deleted SeV vectors inducing CD8(+) T-cell responses specific for simian immunodeficiency virus Gag(206-216) and Gag(241-249), which are dominant epitopes restricted by 90-120-Ia-derived MHC-I molecules. All four animals showed higher Gag(206-216)-specific and Gag(241-249)-specific CD8(+) T-cell responses after the third vaccination than those after the first vaccination, indicating enhancement of antigen-specific CD8(+) T-cell responses by the second/third SeV vector vaccination even with short intervals. These results suggest that repeated SeV vector vaccination can contribute to induction of efficient and durable T-cell responses.

Cellular immune responses and susceptibility to HIV-1 superinfection: a case-control study

A case-control study was performed to determine the effects of HIV-1-specific cellular immune responses on the odds of acquiring a second HIV-1 infection (superinfection). Changes in the frequency of cytokine-producing or cytolytic CD8+ or CD4+ T cells were not associated with significant alterations in the odds of superinfection, suggesting that HIV-1 specific cellular immune responses at the level induced by chronic infection do not appear to significantly contribute to protection from HIV-1 superinfection.

Prime-boost regimens with adjuvanted synthetic long peptides elicit T cells and antibodies to conserved regions of HIV-1 in macaques

OBJECTIVES

Administration of synthetic long peptides (SLPs) derived from human papillomavirus to cervical cancer patients resulted in clinical benefit correlated with expansions of tumour-specific T cells. Because vaginal mucosa is an important port of entry for HIV-1, we have explored SLP for HIV-1 vaccination. Using immunogen HIVconsv derived from the conserved regions of HIV-1, we previously showed in rhesus macaques that SLP.HIVconsv delivered as a boost increased the breath of T-cell specificities elicited by single-gene vaccines. Here, we compared and characterized the use of electroporated pSG2.HIVconsv DNA (D) and imiquimod/montanide-adjuvanted SLP.HIVconsv (S) as priming vaccines for boosting with attenuated chimpanzee adenovirus ChAdV63.HIVconsv (C) and modified vaccinia virus Ankara MVA.HIVconsv (M).

DESIGN

Prime-boost regimens of DDDCMS, DSSCMS and SSSCMS in rhesus macaques.

METHODS

Animals' blood was analysed regularly throughout the vaccination for HIV-1-specific T-cell and antibody responses.

RESULTS

We found that electroporation spares DNA dose, both SLP.HIVconsv and pSG2.HIVconsv DNA primed weakly HIVconsv-specific T cells, regimen DDDCM induced the highest frequencies of oligofunctional, proliferating CD4(+) and CD8(+) T cells, and a subsequent SLP.HIVconsv boost expanded primarily CD4(+) cells. DSS was the most efficient regimen inducing antibodies binding to regions of trimeric HIV-1 Env, which are highly conserved among the four major global clades, although no unequivocal neutralizing activity was detected.

CONCLUSION

The present results encourage evaluation of the SLP.HIVconsv vaccine modality in human volunteers along the currently trialled pSG2.HIVconsv DNA, ChAdV63.HIVconsv and MVA.HIVconsv vaccines. These results are discussed in the context of the RV144 trial outcome.

Activation of NK cells by ADCC antibodies and HIV disease progression

Antibody-dependent cellular cytotoxicity (ADCC) is of considerable interest as an immune response that may facilitate the control of HIV infection. We studied ADCC responses prospectively in a cohort of 79 HIV-positive subjects followed up for a mean of 2.3 years without antiretroviral therapy. We used a novel assay of the ability of ADCC to activate natural killer (NK) cells, either from the same HIV-positive subject or from a healthy blood donor. We found that ADCC responses to either gp140 Env protein or HIV peptide pools were common in HIV-positive subjects when NK cells from the HIV-positive subject were used but did not correlate with markers of HIV disease progression. In contrast, ADCC responses to whole gp140 Env protein were strongly associated with a slower decline in CD4 T-cell loss when healthy donor NK cells were used as effectors. Our data had implications for induction of the most effective ADCC responses by HIV vaccines.

HIV-1 glycoprotein 41 ectodomain induces activation of the CD74 protein-mediated extracellular signal-regulated kinase/mitogen-activated protein kinase pathway to enhance viral infection

Besides mediating the viral entry process, the human immunodeficiency virus (HIV-1) envelope protein gp41 can bind to many host cell components and regulate cell functions. Using a yeast two-hybrid system, we screened a human bone marrow cDNA library and identified a novel gp41-binding protein, CD74 (the MHC class II-associated invariant chain). Here, we report possible biological effects mediated by interaction between gp41 and CD74. We found that HIV-1 gp41 could bind directly to host CD74 in HIV-1-infected cells, and the peptide 6358 derived from gp41 loop region (aa 597-611) could effectively block the gp41-CD74 interaction. As a result of this binding, recombinant soluble gp41 and gp41 peptide 6358 activated the CD74-mediated ERK/MAPK pathway and significantly enhanced HIV-1 infection in vitro. Conversely, the enhancing effect could be suppressed by the recombinant CD74 extracellular domain. These results reveal a novel mechanism underlying gp41 mediation of HIV-1 infection and replication.

Intranasal Sendai viral vector vaccination is more immunogenic than intramuscular under pre-existing anti-vector antibodies

Viral vectors are promising vaccine tools for eliciting potent cellular immune responses. Pre-existing anti-vector antibodies, however, can be an obstacle to their clinical use in humans. We previously developed a Sendai virus (SeV) vector vaccine and showed the potential of this vector for efficient CD8(+) T-cell induction in macaques. Here, we investigated the immunogenicity of SeV vector vaccination in the presence of anti-SeV antibodies. We compared antigen-specific CD8(+) T-cell responses after intranasal or intramuscular immunization with a lower dose (one-tenth of that in our previous studies) of SeV vector expressing simian immunodeficiency virus Gag antigen (SeV-Gag) between naive and pre-SeV-infected cynomolgus macaques. Intranasal SeV-Gag immunization efficiently elicited Gag-specific CD8(+) T-cell responses not only in naive but also in pre-SeV-infected animals. In contrast, intramuscular SeV-Gag immunization induced Gag-specific CD8(+) T-cell responses efficiently in naive but not in pre-SeV-infected animals. These results indicate that both intranasal and intramuscular SeV administrations are equivalently immunogenic in the absence of anti-SeV antibodies, whereas intranasal SeV vaccination is more immunogenic than intramuscular in the presence of anti-SeV antibodies. It is inferred from a recent report investigating the prevalence of anti-SeV antibodies in humans that SeV-specific neutralizing titers in more than 70% of people are no more than those at the SeV-Gag vaccination in pre-SeV-infected macaques in the present study. Taken together, this study implies the potential of intranasal SeV vector vaccination to induce CD8(+) T-cell responses even in humans, suggesting a rationale for proceeding to a vaccine clinical trial using this vector.

Immunogenicity of chloroplast-derived HIV-1 p24 and a p24-Nef fusion protein following subcutaneous and oral administration in mice

High-level expression of foreign proteins in chloroplasts of transplastomic plants provides excellent opportunities for the development of oral vaccines against a range of debilitating or fatal diseases. The HIV-1 capsid protein p24 and a fusion of p24 with the negative regulatory protein Nef (p24-Nef) accumulate to ∼4% and ∼40% of the total soluble protein of leaves of transplastomic tobacco (Nicotiana tabacum L.) plants. This study has investigated the immunogenicity in mice of these two HIV-1 proteins, using cholera toxin B subunit as an adjuvant. Subcutaneous immunization with purified chloroplast-derived p24 elicited a strong antigen-specific serum IgG response, comparable to that produced by Escherichia coli-derived p24. Oral administration of a partially purified preparation of chloroplast-derived p24-Nef fusion protein, used as a booster after subcutaneous injection with either p24 or Nef, also elicited strong antigen-specific serum IgG responses. Both IgG1 and IgG2a subtypes, associated with cell-mediated Th1 and humoral Th2 responses, respectively, were found in sera after subcutaneous and oral administration. These results indicate that chloroplast-derived HIV-1 p24-Nef is a promising candidate as a component of a subunit vaccine delivered by oral boosting, after subcutaneous priming by injection of p24 and/or Nef.

Nine crystal structures determine the substrate envelope of the MDR HIV-1 protease

Under drug selection pressure, emerging mutations render HIV-1 protease drug resistant, leading to the therapy failure in anti-HIV treatment. It is known that nine substrate cleavage site peptides bind to wild type (WT) HIV-1 protease in a conserved pattern. However, how the multidrug-resistant (MDR) HIV-1 protease binds to the substrate cleavage site peptides is yet to be determined. MDR769 HIV-1 protease (resistant mutations at residues 10, 36, 46, 54, 62, 63, 71, 82, 84, and 90) was selected for present study to understand the binding to its natural substrates. MDR769 HIV-1 protease was co-crystallized with nine substrate cleavage site hepta-peptides. Crystallographic studies show that MDR769 HIV-1 protease has an expanded substrate envelope with wide open flaps. Furthermore, ligand binding energy calculations indicate weaker binding in MDR769 HIV-1 protease-substrate complexes. These results help in designing the next generation of HIV-1 protease inhibitors by targeting the MDR HIV-1 protease.

Mycobacterium bovis Bacille Calmette-Guérin as a Vaccine Vector for Global Infectious Disease Control

Mycobacterium bovis bacille Calmette-Guérin (BCG) is the only available vaccine for tuberculosis (TB). Although this vaccine is effective in controlling infantile TB, BCG-induced protective effects against pulmonary diseases in adults have not been clearly demonstrated. Recombinant BCG (rBCG) technology has been extensively applied to obtain more potent immunogenicity of this vaccine, and several candidate TB vaccines have currently reached human clinical trials. On the other hand, recent progress in the improvement of the BCG vector, such as the codon optimization strategy and combination with viral vector boost, allows us to utilize this bacterium in HIV vaccine development. In this paper, we review recent progress in rBCG-based vaccine studies that may have implications in the development of novel vaccines for controlling global infectious diseases in the near future.

Protective efficacy of serially up-ranked subdominant CD8+ T cell epitopes against virus challenges

Immunodominance in T cell responses to complex antigens like viruses is still incompletely understood. Some data indicate that the dominant responses to viruses are not necessarily the most protective, while other data imply that dominant responses are the most important. The issue is of considerable importance to the rational design of vaccines, particularly against variable escaping viruses like human immunodeficiency virus type 1 and hepatitis C virus. Here, we showed that sequential inactivation of dominant epitopes up-ranks the remaining subdominant determinants. Importantly, we demonstrated that subdominant epitopes can induce robust responses and protect against whole viruses if they are allowed at least once in the vaccination regimen to locally or temporally dominate T cell induction. Therefore, refocusing T cell immune responses away from highly variable determinants recognized during natural virus infection towards subdominant, but conserved regions is possible and merits evaluation in humans.

Dendritic cells exposed to MVA-based HIV-1 vaccine induce highly functional HIV-1-specific CD8(+) T cell responses in HIV-1-infected individuals

Currently, MVA virus vectors carrying HIV-1 genes are being developed as HIV-1/AIDS prophylactic/therapeutic vaccines. Nevertheless, little is known about the impact of these vectors on human dendritic cells (DC) and their capacity to present HIV-1 antigens to human HIV-specific T cells. This study aimed to characterize the interaction of MVA and MVA expressing the HIV-1 genes Env-Gag-Pol-Nef of clade B (referred to as MVA-B) in human monocyte-derived dendritic cells (MDDC) and the subsequent processes of HIV-1 antigen presentation and activation of memory HIV-1-specific T lymphocytes. For these purposes, we performed ex vivo assays with MDDC and autologous lymphocytes from asymptomatic HIV-infected patients. Infection of MDDC with MVA-B or MVA, at the optimal dose of 0.3 PFU/MDDC, induced by itself a moderate degree of maturation of MDDC, involving secretion of cytokines and chemokines (IL1-ra, IL-7, TNF-α, IL-6, IL-12, IL-15, IL-8, MCP-1, MIP-1α, MIP-1β, RANTES, IP-10, MIG, and IFN-α). MDDC infected with MVA or MVA-B and following a period of 48 h or 72 h of maturation were able to migrate toward CCL19 or CCL21 chemokine gradients. MVA-B infection induced apoptosis of the infected cells and the resulting apoptotic bodies were engulfed by the uninfected MDDC, which cross-presented HIV-1 antigens to autologous CD8⁺ T lymphocytes. MVA-B-infected MDDC co-cultured with autologous T lymphocytes induced a highly functional HIV-specific CD8⁺ T cell response including proliferation, secretion of IFN-γ, IL-2, TNF-α, MIP-1β, MIP-1α, RANTES and IL-6, and strong cytotoxic activity against autologous HIV-1-infected CD4⁺ T lymphocytes. These results evidence the adjuvant role of the vector itself (MVA) and support the clinical development of prophylactic and therapeutic anti-HIV vaccines based on MVA-B.

Liposome-coupled antigens are internalized by antigen-presenting cells via pinocytosis and cross-presented to CD8 T cells

We have previously demonstrated that antigens chemically coupled to the surface of liposomes consisting of unsaturated fatty acids were cross-presented by antigen-presenting cells (APCs) to CD8⁺ T cells, and that this process resulted in the induction of antigen-specific cytotoxic T lymphocytes. In the present study, the mechanism by which the liposome-coupled antigens were cross-presented to CD8⁺ T cells by APCs was investigated. Confocal laser scanning microscopic analysis demonstrated that antigens coupled to the surface of unsaturated-fatty-acid-based liposomes received processing at both MHC class I and class II compartments, while most of the antigens coupled to the surface of saturated-fatty-acid-based liposomes received processing at the class II compartment. In addition, flow cytometric analysis demonstrated that antigens coupled to the surface of unsaturated-fatty-acid-liposomes were taken up by APCs even in a 4°C environment; this was not true of saturated-fatty-acid-liposomes. When two kinds of inhibitors, dimethylamiloride (DMA) and cytochalasin B, which inhibit pinocytosis and phagocytosis by APCs, respectively, were added to the culture of APCs prior to the antigen pulse, DMA but not cytochalasin B significantly reduced uptake of liposome-coupled antigens. Further analysis of intracellular trafficking of liposomal antigens using confocal laser scanning microscopy revealed that a portion of liposome-coupled antigens taken up by APCs were delivered to the lysosome compartment. In agreement with the reduction of antigen uptake by APCs, antigen presentation by APCs was significantly inhibited by DMA, and resulted in the reduction of IFN-γ production by antigen-specific CD8⁺ T cells. These results suggest that antigens coupled to the surface of liposomes consisting of unsaturated fatty acids might be pinocytosed by APCs, loaded onto the class I MHC processing pathway, and presented to CD8⁺ T cells. Thus, these liposome-coupled antigens are expected to be applicable for the development of vaccines that induce cellular immunity.

Long peptides induce polyfunctional T cells against conserved regions of HIV-1 with superior breadth to single-gene vaccines in macaques

A novel T-cell vaccine strategy designed to deal with the enormity of HIV-1 variation is described and tested for the first time in macaques to inform and complement approaching clinical trials. T-cell immunogen HIVconsv, which directs vaccine-induced responses to the most conserved regions of the HIV-1, proteome and thus both targets diverse clades in the population and reduces the chance of escape in infected individuals, was delivered using six different vaccine modalities: plasmid DNA (D), attenuated human (A) and chimpanzee (C) adenoviruses, modified vaccinia virus Ankara (M), synthetic long peptides, and Semliki Forest virus replicons. We confirmed that the initial DDDAM regimen, which mimics one of the clinical schedules (DDDCM), is highly immunogenic in macaques. Furthermore, adjuvanted synthetic long peptides divided into sub-pools and delivered into anatomically separate sites induced T-cell responses that were markedly broader than those elicited by traditional single-open-reading-frame genetic vaccines and increased by 30% the overall response magnitude compared with DDDAM. Thus, by improving both the HIV-1-derived immunogen and vector regimen/delivery, this approach could induce stronger, broader, and theoretically more protective T-cell responses than vaccines previously used in humans.

HLA-associated immune pressure on Gag protein in CRF01_AE-infected individuals and its association with plasma viral load

Background

The human leukocyte antigen (HLA)-restricted cytotoxic T-lymphocyte (CTL) immune response is one of the major factors determining the genetic diversity of human immunodeficiency virus (HIV). There are few population-based analyses of the amino acid variations associated with the host HLA type and their clinical relevance for the Asian population. Here, we identified HLA-associated polymorphisms in the HIV-1 CRF01_AE Gag protein in infected married couples, and examined the consequences of these HLA-selected mutations after transmission to HLA-unmatched recipients.

Methodology/Principal Findings

One hundred sixteen HIV-1-infected couples were recruited at a government hospital in northern Thailand. The 1.7-kb gag gene was amplified and directly sequenced. We identified 56 associations between amino acid variations in Gag and HLA alleles. Of those amino acid variations, 35 (62.5%) were located within or adjacent to regions reported to be HIV-specific CTL epitopes restricted by the relevant HLA. Interestingly, a significant number of HLA-associated amino acid variations appear to be unique to the CRF01_AE-infected Thai population. Variations in the capsid protein (p24) had the strongest associations with the viral load and CD4 cell count. The mutation and reversion rates after transmission to a host with a different HLA environment varied considerably. The p24 T242N variant escape from B57/58 CTL had a significant impact on the HIV-1 viral load of CRF01_AE-infected patients.

Conclusions/Significance

HLA-associated amino acid mutations and the CTL selection pressures on the p24 antigen appear to have the most significant impact on HIV replication in a CRF01_AE-infected Asian population. HLA-associated mutations with a low reversion rate accumulated as a footprint in this Thai population. The novel HLA-associated mutations identified in this study encourage us to acquire more extensive information about the viral dynamics of HLA-associated amino acid polymorphisms in a given population as effective CTL vaccine targets.

High throughput T epitope mapping and vaccine development

Mapping of antigenic peptide sequences from proteins of relevant pathogens recognized by T helper (Th) and by cytolytic T lymphocytes (CTL) is crucial for vaccine development. In fact, mapping of T-cell epitopes provides useful information for the design of peptide-based vaccines and of peptide libraries to monitor specific cellular immunity in protected individuals, patients and vaccinees. Nevertheless, epitope mapping is a challenging task. In fact, large panels of overlapping peptides need to be tested with lymphocytes to identify the sequences that induce a T-cell response. Since numerous peptide panels from antigenic proteins are to be screened, lymphocytes available from human subjects are a limiting factor. To overcome this limitation, high throughput (HTP) approaches based on miniaturization and automation of T-cell assays are needed. Here we consider the most recent applications of the HTP approach to T epitope mapping. The alternative or complementary use of in silico prediction and experimental epitope definition is discussed in the context of the recent literature. The currently used methods are described with special reference to the possibility of applying the HTP concept to make epitope mapping an easier procedure in terms of time, workload, reagents, cells and overall cost.

Immunization with a Mixture of HIV Env DNA and VLP Vaccines Augments Induction of CD8 T Cell Responses

The immune response induced by immunization with HIV Env DNA and virus-like particle (VLP) vaccines was investigated. Immunization with the HIV Env DNA vaccine induced a strong CD8 T cell response but relatively weak antibody response against the HIV Env whereas immunization with VLPs induced higher levels of antibody responses but little CD8 T cell response. Interestingly, immunization with a mixture the HIV Env DNA and VLP vaccines induced enhanced CD8 T cell and antibody responses. Further, it was observed that the mixing of DNA and VLP vaccines during immunization is necessary for augmenting induction of CD8 T cell responses and such augmentation of CD8 T cell responses was also observed by mixing the HIV Env DNA vaccine with control VLPs. These results show that immunization with a mixture of DNA and VLP vaccines combines advantages of both vaccine platforms for eliciting high levels of both antibody and CD8 T cell responses.

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.

Effective simian immunodeficiency virus-specific CD8+ T cells lack an easily detectable, shared characteristic

The immune correlates of human/simian immunodeficiency virus control remain elusive. While CD8(+) T lymphocytes likely play a major role in reducing peak viremia and maintaining viral control in the chronic phase, the relative antiviral efficacy of individual virus-specific effector populations is unknown. Conventional assays measure cytokine secretion of virus-specific CD8(+) T cells after cognate peptide recognition. Cytokine secretion, however, does not always directly translate into antiviral efficacy. Recently developed suppression assays assess the efficiency of virus-specific CD8(+) T cells to control viral replication, but these assays often use cell lines or clones. We therefore designed a novel virus production assay to test the ability of freshly ex vivo-sorted simian immunodeficiency virus (SIV)-specific CD8(+) T cells to suppress viral replication from SIVmac239-infected CD4(+) T cells. Using this assay, we established an antiviral hierarchy when we compared CD8(+) T cells specific for 12 different epitopes. Antiviral efficacy was unrelated to the disease status of each animal, the protein from which the tested epitopes were derived, or the major histocompatibility complex (MHC) class I restriction of the tested epitopes. Additionally, there was no correlation with the ability to suppress viral replication and epitope avidity, epitope affinity, CD8(+) T-cell cytokine multifunctionality, the percentage of central and effector memory cell populations, or the expression of PD-1. The ability of virus-specific CD8(+) T cells to suppress viral replication therefore cannot be determined using conventional assays. Our results suggest that a single definitive correlate of immune control may not exist; rather, a successful CD8(+) T-cell response may be comprised of several factors.

HIV-1, lipid rafts, and antibodies to liposomes: implications for anti-viral-neutralizing antibodies (Review)

The human immunodeficiency virus type 1 (HIV-1) is an enveloped virus with a lipid bilayer that contains several glycoproteins that are anchored in, or closely associated with, the membrane surface. The envelope proteins have complex interactions with the lipids both on the host cells and on the target cells. The processes of budding from host cells and entry into target cells occur at sites on the plasma membrane, known as lipid rafts, that represent specialized regions that are rich in cholesterol and sphingolipids. Although the envelope glycoproteins are antigenic molecules that potentially might be used for development of broadly neutralizing antibodies in a vaccine to HIV-1, the development of such antibodies that have broad specificities against primary field isolates of virus has been largely thwarted to date by the ability of the envelope proteins to evade the immune system through various mechanisms. In this review, the interactions of HIV-1 with membrane lipids are summarized. Liposomes are commonly used as models for understanding interactions of proteins with membrane lipids; and liposomes have also been used both as carriers for vaccines, and as antigens for induction of antibodies to liposomal lipids. The possibility is proposed that liposomal lipids, or liposome-protein combinations, could be useful as antigens for inducing broadly neutralizing antibodies to HIV-1.

Impact of cytotoxic-T-lymphocyte memory induction without virus-specific CD4+ T-Cell help on control of a simian immunodeficiency virus challenge in rhesus macaques

Despite many efforts to develop AIDS vaccines eliciting virus-specific T-cell responses, whether induction of these memory T cells by vaccination before human immunodeficiency virus (HIV) exposure can actually contribute to effective T-cell responses postinfection remains unclear. In particular, induction of HIV-specific memory CD4(+) T cells may increase the target cell pool for HIV infection because the virus preferentially infects HIV-specific CD4(+) T cells. However, virus-specific CD4(+) helper T-cell responses are thought to be important for functional CD8(+) cytotoxic-T-lymphocyte (CTL) induction in HIV infection, and it has remained unknown whether HIV-specific memory CD8(+) T cells induced by vaccination without HIV-specific CD4(+) T-cell help can exert effective responses after virus exposure. Here we show the impact of CD8(+) T-cell memory induction without virus-specific CD4(+) T-cell help on the control of a simian immunodeficiency virus (SIV) challenge in rhesus macaques. We developed a prophylactic vaccine by using a Sendai virus (SeV) vector expressing a single SIV Gag(241-249) CTL epitope fused with enhanced green fluorescent protein (EGFP). Vaccination resulted in induction of SeV-EGFP-specific CD4(+) T-cell and Gag(241-249)-specific CD8(+) T-cell responses. After a SIV challenge, the vaccinees showed dominant Gag(241-249)-specific CD8(+) T-cell responses with higher effector memory frequencies in the acute phase and exhibited significantly reduced viral loads. These results demonstrate that virus-specific memory CD8(+) T cells induced by vaccination without virus-specific CD4(+) T-cell help could indeed facilitate SIV control after virus exposure, indicating the benefit of prophylactic vaccination eliciting virus-specific CTL memory with non-virus-specific CD4(+) T-cell responses for HIV control.

An African perspective on mucosal immunity and HIV-1

HIV prevention mandates an understanding of the mechanisms of mucosal immunity with attention to some unique features of the epidemic and mucosal environment in the developing world. An effective vaccine will have to induce mucosal protection against a highly diverse virus, which is equipped with a number of immune evasion strategies. Its development will require assessment of mucosal immune responses, and it will have to protect a mucosal environment where inflammation and altered immune responses are common because of the presence of other mucosal infections, such as sexually transmitted infections and parasites, and where nutritional status may also be compromised. Ideally, not only prevention methods would protect adults but also provide cover against gastrointestinal transmission through maternal milk. Prevention might also be complemented by microbicides and circumcision, two alternative approaches to mucosal protection. It seems unlikely that a single solution will work in all instances and intervention might have to act at multiple levels and be tailored to local circumstances. We review here some of the mucosal events associated with HIV infection that are most relevant in an African setting.

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.

[Role of the HIV-1 gp120 V1/V2 domains in the induction of neutralizing antibodies]

The development of a preventive vaccine against human immunodeficiency virus type-1 (HIV-1) provides hope for control of the pandemic over the coming years. Nevertheless, it is clear that one of the greatest difficulties in achieving this vaccine is the high mutation rate of the virus, which enables it to evade the host's immune response. The production of neutralizing antibodies (NAb) against the HIV-1 envelope proteins is believed to play an important role in controlling the infection and in providing effective protection following vaccination. Several studies have shown that the V1/V2 domain of the HIV-1 gp120 envelope protein is involved in viral tropism during infection, in masking conserved neutralizing epitopes, in the conformational changes occurring after coreceptor binding, and in NAb induction. Nonetheless, this domain has been poorly investigated. However, because the V1/V2 domain is highly glycosylated, numerous studies have determined the influence of carbohydrates on NAb production. The present review focuses on the importance of NAb directed against epitopes of the variable regions, mainly V1/V2, their importance in protecting against HIV-1 infection, and the role these regions play in evading the immune response. Lastly, we will discuss the importance of NAb in the search for an effective vaccine against HIV-1.

The caveolin-1 binding domain of HIV-1 glycoprotein gp41 (CBD1) contains several overlapping neutralizing epitopes

The CBD1 peptide (SLEQIWNNMTWMQWDK), corresponding to the consensus caveolin-1 binding domain in HIV-1 envelope glycoprotein gp41 (CBD1), elicits the production of antibodies that inhibit infection of primary CD4(+) T lymphocytes by various primary HIV-1 isolates. Here we show that HIV-neutralizing antibodies against CBD1 react with multiple conformational epitopes that overlap the highly conserved caveolin-1 binding motif (CBM) with the N-terminal conserved isoleucine residue. The CBM-based peptides IWNNMTWMQW and IWNNMTW when fused to a T helper epitope are immunogenic by inducing high titer CBM-specific antibodies capable of neutralizing HIV-1 infection in primary T lymphocyte cultures. Interestingly, neutralizing immune sera raised against a given peptide do not cross-react with related CBM-derived peptides, thus suggesting the existence of distinct neutralizing epitopes that probably reflect the dynamic conformational features of CBD1. In accord with this, the mixture of neutralizing immune sera raised against several CBM-derived peptides exerts a synergistic neutralizing activity against HIV-1 infection. Finally, the existence of several distinct overlapping epitopes in CBD1 is confirmed by murine monoclonal antibodies that we generated against the CBM-derived chimeric peptides. Our results indicate that CBD1- and CBM-based peptides mimic distinct dynamic conformations of CBD1, and thus such peptides could provide specific immunogens for an efficient vaccine preparation against HIV/AIDS infection.

Designed recombinant adenovirus type 5 vector induced envelope-specific CD8(+) cytotoxic T lymphocytes and cross-reactive neutralizing antibodies against human immunodeficiency virus type 1

BACKGROUND

A monoclonal antibody (mAb) 2F5 binds to the membrane-proximal external region (MPER) of the transmembrane subunit gp41 of human immunodeficiency virus type 1 (HIV-1) is known to broadly neutralize HIV-1 strains. The Adenovirus type 5 vector (Ad5) has been widely applied for HIV-1 vaccine, and hexon hypervariable region 5 (HVR5) is exposed on viral surface and easily target host immune responses against Ad5.

METHODS

We constructed a recombinant adenovirus type 5 vector (rAd5) with a 2F5-binding epitope (ELDKWA) of MPER on Ad5-HVR5. In addition, we developed rAd5 encoding the HIV-1(IIIB) envelope (Env) gene for the induction of Env-specific cellular immunity.

RESULTS

The virus titers of the constructed rAd5 were similar to that of the parental Ad5 vector. Furthermore, high-dose immunization of rAd5 induced Env-specific CD8(+) cells and high levels of anti-ELDKWA antibodies. Moreover, an in vitro HIV-1 neutralization assay indicated that ELDKWA-specific mAbs derived from rAd5-immunized mice neutralized a wide range of HIV-1 strains.

CONCLUSIONS

The present study outlines the development of an Ad5-based HIV-1 vaccine targeting the hypervariable regions of Ad5. The constructed rAd5 induced an HIV-1-specific cellular immune response and neutralizing antibodies against various strains of HIV-1 simultaneously.

Novel humanized murine models for HIV research

There are few models in which HIV pathogenesis, particularly gut-associated lymphoid tissue CD4(+) T-cell depletion, can be studied and in which potential clinical interventions against HIV disease can be evaluated. HIV cannot be studied in normal mice due to the limited species tropism of the virus. Through the pioneering efforts of many investigators, humanized mice are now routinely used to rapidly advance HIV research. It is important to recognize that not all humanized murine models are equal, and their strengths and weaknesses must be taken into consideration to obtain information that is most relevant to the human condition. This review distinguishes the major humanization protocols and highlights each model's recent contributions to HIV research, including mucosal transmission, gut-associated lymphoid tissue pathogenesis, and the evaluation of novel therapeutic and prevention approaches to potentially treat HIV disease and prevent the further spread of AIDS.

Immunogenically optimized peptides derived from natural mutants of HIV CTL epitopes and peptide combinatorial libraries

Two strategies were aimed at identifying immunogenically optimized peptides for the potential use in the formulation of an effective prophylactic or therapeutic HIV-1 vaccine. Three CTL epitopes were investigated: Gag p24(19-27) TV9, Gag p17(77-85) SL9, and RT(309-317) IV9. The first strategy derives from the hypothesis that a number of rare mutant CTL epitopes of HIV-1 may be more immunogenic than the common ones. As such, these rare mutant sequences might be highly effective in generating cross reactive anti-HIV-1 CTL responses against a range of mutant sequences. As anticipated, several rare mutant peptide sequences were identified that generated strong CTL responses against both the consensus sequences and several naturally occurring mutants in human PBL cultures primed ex vivo and in HLA-A2 transgenic mice immunized in vivo. Finally, to reach beyond the sequence diversity of the "natural" library of mutated sequences, a synthetic combinatorial peptide library was screened with a TV9 specific T-cell line; this resulted in the identification of an immunogenically optimized mimic peptide sequence that provoked highly effective CTL immune responses against TV9 and mutants. Sequence homologies between the natural mutants and synthetic mimic may provide insight into key contact positions in the MHC/TCR/peptide complex.

Analysis of flow cytometry data using an automatic processing tool

In spite of recent advances in flow cytometry technology, most cytometry data is still analyzed manually which is labor-intensive for large datasets and prone to bias and inconsistency. We designed an automatic processing tool (APT) to rapidly and consistently define and describe cell populations across large datasets. Image processing, smoothing, and clustering algorithms were used to generate an expert system that automatically reproduces the functionality of commercial manual cytometry processing tools. The algorithms were developed using a dataset collected from CMV-infected infants and combined within a graphical user interface, to create the APT. The APT was used to identify regulatory T-cells in HIV-infected adults, based on expression of FOXP3. Results from the APT were compared directly with the manual analyses of five immunologists and showed close agreement, with a concordance correlation coefficient of 0.96 (95% CI 0.91-0.98). The APT was well accepted by users and able to process around 100 data files per hour. By applying consistent criteria to all data generated by a study, the APT can provide a level of objectivity that is difficult to match using conventional manual analysis.

The CBD1 peptide corresponding to the caveolin-1 binding domain of HIV-1 glycoprotein gp41 elicits neutralizing antibodies in cynomolgus macaques when administered with the tetanus T helper epitope

CBD1 peptide (SLEQIWNNMTWMQWDK), corresponding to the consensus caveolin-1 binding domain in HIV-1 envelope glycoprotein gp41, elicits the production of antibodies that inhibit infection of primary CD4(+) T lymphocytes by various primary HIV-1 isolates. Here the immunogenicity of the CBD1 peptide was investigated in cynomolgus macaques using adjuvants that are acceptable for human use. In the first set of studies, macaques were immunized with the CBD1 peptide in association with muramyl dipeptide derivative MDP-Lys(L18) combined with the oil-in-water emulsion, MF-59. After five immunizations at 4 weeks interval, the antibody titer against the CBD1 peptide was found to be either medium, poor, weak or none, thus suggesting that the CBD1 immune response might be restricted by the major histocompatibility complex (MHC) class II molecules. In the second set of studies therefore, macaques were immunized with the CBD1 peptide in association with the 'promiscuous' T cell epitope from the tetanus toxin, either as free peptides or covalently linked with the dilysine linker using CpG ODN and Montanide ISA 51 as adjuvants. This latter immunization procedure boosted markedly the anti-CBD1 antibody response, since even the non-responders generated high-titered peptide-specific antibodies. Moreover, co-immunization of the CBD1 and the T helper epitope as free peptides seemed to be favorable for the production of neutralizing antibodies, with 50% inhibition of HIV-1 infection occurring at 300-400-fold dilution of the immune sera. Finally, neutralizing and non-neutralizing immune macaque sera could be differentiated by the profile of cross-reactivity with overlapping CBD1-related peptides in ELISA. Taken together, our results demonstrate that the CBD1 peptide is immunogenic in macaques and that an eventual MHC-restriction could be overcome by the administration with an appropriate T helper epitope.

Importance of the V1/V2 loop region of simian-human immunodeficiency virus envelope glycoprotein gp120 in determining the strain specificity of the neutralizing antibody response

Plasma samples from individuals infected with human immunodeficiency virus type 1 (HIV-1) are known to be highly strain specific in their ability to neutralize HIV-1 infectivity. Such plasma samples exhibit significant neutralizing activity against autologous HIV-1 isolates but typically exhibit little or no activity against heterologous strains, although some cross-neutralizing activity can develop late in infection. Monkeys infected with the simian-human immunodeficiency virus (SHIV) clone DH12 generated antibodies that neutralized SHIV DH12, but not SHIV KB9. Conversely, antibodies from monkeys infected with the SHIV clone KB9 neutralized SHIV KB9, but not SHIV DH12. To investigate the role of the variable loops of the HIV-1 envelope glycoprotein gp120 in determining this strain specificity, variable loops 1 and 2 (V1/V2), V3, or V4 were exchanged individually or in combination between SHIV DH12 and SHIV KB9. Despite the fact that both parental viruses exhibited significant infectivity and good replication in the cell lines examined, 3 of the 10 variable-loop chimeras exhibited such poor infectivity that they could not be used further for neutralization assays. These results indicate that a variable loop that is functional in the context of one particular envelope background will not necessarily function within another. The remaining seven replication-competent chimeras allowed unambiguous assignment of the sequences principally responsible for the strain specificity of the neutralizing activity present in SHIV-positive plasma. Exchange of the V1/V2 loop sequences conferred a dominant loss of sensitivity to neutralization by autologous plasma and a gain of sensitivity to neutralization by heterologous plasma. Substitution of V3 or V4 had little or no effect on the sensitivity to neutralization. These data demonstrate that the V1/V2 region of HIV-1 gp120 is principally responsible for the strain specificity of the neutralizing antibody response in monkeys infected with these prototypic SHIVs.

Gag-specific cytotoxic T-lymphocyte-based control of primary simian immunodeficiency virus replication in a vaccine trial

Gag-specific cytotoxic T lymphocytes (CTLs) exert strong suppressive pressure on human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) replication. However, it has remained unclear whether they can actually contain primary viral replication. Recent trials of prophylactic vaccines inducing virus-specific T-cell responses have indicated their potential to confer resistance against primary SIV replication in rhesus macaques, while the immunological determinant for this vaccine-based viral control has not been elucidated thus far. Here we present evidence implicating Gag-specific CTLs as responsible for the vaccine-based primary SIV control. Prophylactic vaccination using a Gag-expressing Sendai virus vector resulted in containment of SIVmac239 challenge in all rhesus macaques possessing the major histocompatibility complex (MHC) haplotype 90-120-Ia. In contrast, 90-120-Ia-positive vaccinees failed to contain SIVs carrying multiple gag CTL escape mutations that had been selected, at the cost of viral fitness, in SIVmac239-infected 90-120-Ia-positive macaques. These results show that Gag-specific CTL responses do play a crucial role in the control of wild-type SIVmac239 replication in vaccinees. This study implies the possibility of Gag-specific CTL-based primary HIV containment by prophylactic vaccination, although it also suggests that CTL-based AIDS vaccine efficacy may be abrogated in viral transmission between MHC-matched individuals.