Poxvirus vectors as HIV/AIDS vaccines in humans

The RV144 phase III clinical trial with the combination of the poxvirus vector ALVAC and the HIV gp120 protein has taught us that a vaccine against HIV/AIDS is possible but further improvements are still needed. Although the HIV protective effect of RV144 was modest (31.2%), these encouraging results reinforce the use of poxvirus vectors as HIV/AIDS vaccine candidates. In this review we focus on the prophylactic clinical studies thus far performed with the more widely studied poxvirus vectors, ALVAC, MVA, NYVAC and fowlpox expressing HIV antigens. We describe the characteristics of each vector administered either alone or in combination with other vectors, with emphasis on the immune parameters evaluated in healthy volunteers, percentage of responders and triggering of humoral and T cell responses. Some of these immunogens induced broad, polyfunctional and long-lasting CD4(+) and CD8(+) T cell responses to HIV-1 antigens in most volunteers, with preference for effector memory T cells, and neutralizing antibodies, immune parameters that might be relevant in protection. Finally, we consider improvements in immunogenicity of the poxvirus vectors by the selective deletion of viral immunomodulatory genes and insertion of host range genes in the poxvirus genome. Overall, the poxvirus vectors have proven to be excellent HIV/AIDS vaccine candidates, with distinct behavior among them, and the future implementation will be dictated by their optimized immune profile in clinical trials.

Live attenuated rubella viral vectors stably express HIV and SIV vaccine antigens while reaching high titers

Live attenuated viruses make potent and effective vaccines. Despite the urgent need for an HIV vaccine, this approach has not been feasible, since it has not been possible to attenuate the virus reliably and guarantee vaccine safety. Instead, live viral vectors have been proposed that could present HIV vaccine antigens in the most immunogenic way, in the context of an active infection. We have adapted the rubella vaccine strain RA27/3 as a vector to express HIV and SIV antigens, and tested the effect of insert size and composition on vector stability and viral titer. We have identified an acceptor site in the rubella nonstructural gene region, where foreign genes can be expressed as a fusion protein with the nonstructural protein P150 without affecting essential viral functions. The inserts were expressed as early genes of rubella, under control of the rubella genomic promoter. At this site, HIV and SIV antigens were expressed stably for at least seven passages, as the rubella vectors reached high titers. Rubella readily infects rhesus macaques, and these animals will provide an ideal model for testing the new vectors for replication in vivo, immunogenicity, and protection against SIV or SHIV challenge.

Novel multiplexed HIV/simian immunodeficiency virus antibody detection assay

Like most emerging infectious disease viruses, HIV is also of zoonotic origin. To assess the risk for cross-species transmission of simian immunodeficiency viruses (SIVs) from nonhuman primates to humans in the Democratic Republic of Congo, we collected 330 samples derived from nonhuman primate bushmeat at 3 remote forest sites. SIV prevalences were estimated by using a novel high-throughput assay that included 34 HIV and SIV antigens in a single well. Overall, 19% of nonhuman primate bushmeat was infected with SIVs, and new SIV lineages were identified. Highest SIV prevalences were seen in red-tailed guenons (25%) and Tshuapa red colobus monkeys (24%), representing the most common hunted primate species, thus increasing the likelihood for cross-species transmission. Additional studies are needed to determine whether other SIVs crossed the species barrier. With the newly developed assay, large-scale screening against many antigens is now easier and faster.

[HIV-1 assembly is initiated by p17 matrix protein]

HIV-1 matrix protein (MA) is multifunctional structural protein located on N-terminus of Gag precursor p55 and responsible for its transport to plasma membrane, the site of virus assembly. Here, it has been shown that MA is cleaved from Gag precursor at early stage of the virus infection and participates in virus assembly. MA is transported into the nuclei wherein it associates with viral RNA (vRNA). The MA-vRNA complex is transported to plasma membrane. Mutant MA which lost its membranotropic signal does not reach plasma membrane and MA-vRNA complex remains in the nuclei and cytoskeleton. Thus, MA seems to deliver vRNA from the nuclei to plasma membrane through cytoskeleton initiating virus assembly.

Coordinate linkage of HIV evolution reveals regions of immunological vulnerability

Cellular immune control of HIV is mediated, in part, by induction of single amino acid mutations that reduce viral fitness, but compensatory mutations limit this effect. Here, we sought to determine if higher order constraints on viral evolution exist, because some coordinately linked combinations of mutations may hurt viability. Immune targeting of multiple sites in such a multidimensionally conserved region might render the virus particularly vulnerable, because viable escape pathways would be greatly restricted. We analyzed available HIV sequences using a method from physics to reveal distinct groups of amino acids whose mutations are collectively coordinated ("HIV sectors"). From the standpoint of mutations at individual sites, one such group in Gag is as conserved as other collectively coevolving groups of sites in Gag. However, it exhibits higher order conservation indicating constraints on the viability of viral strains with multiple mutations. Mapping amino acids from this group onto protein structures shows that combined mutations likely destabilize multiprotein structural interactions critical for viral function. Persons who durably control HIV without medications preferentially target the sector in Gag predicted to be most vulnerable. By sequencing circulating viruses from these individuals, we find that individual mutations occur with similar frequency in this sector as in other targeted Gag sectors. However, multiple mutations within this sector are very rare, indicating previously unrecognized multidimensional constraints on HIV evolution. Targeting such regions with higher order evolutionary constraints provides a novel approach to immunogen design for a vaccine against HIV and other rapidly mutating viruses.

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.

T-cell immune responses against Env from CRF12_BF and subtype B HIV-1 show high clade-specificity that can be overridden by multiclade immunizations

Background

The extreme genetic diversity of the human immunodeficiency virus type 1 (HIV-1) poses a daunting challenge to the generation of an effective AIDS vaccine. In Argentina, the epidemic is characterized by the high prevalence of infections caused by subtype B and BF variants. The aim of this study was to characterize in mice the immunogenic and antigenic properties of the Env protein from CRF12_BF in comparison with clade B, employing prime-boost schemes with the combination of recombinant DNA and vaccinia virus (VV) vectors.

Methodology/Principal Findings

As determined by ELISPOT from splenocytes of animals immunized with either EnvBF or EnvB antigens, the majority of the cellular responses to Env were found to be clade-specific. A detailed peptide mapping of the responses reveal that when there is cross-reactivity, there are no amino acid changes in the peptide sequence or were minimal and located at the peptide ends. In those cases, analysis of T cell polifunctionality and affinity indicated no differences with respect to the cellular responses found against the original homologous sequence.

Significantly, application of a mixed immunization combining both clades (B and BF) induced a broader cellular response, in which the majority of the peptides targeted after the single clade vaccinations generated a positive response. In this group we could also find significant cellular and humoral responses against the whole gp120 protein from subtype B.

Conclusions/Significance

This work has characterized for the first time the immunogenic peptides of certain EnvBF regions, involved in T cell responses. It provides evidence that to improve immune responses to HIV there is a need to combine Env antigens from different clades, highlighting the convenience of the inclusion of BF antigens in future vaccines for geographic regions where these HIV variants circulate.

Conflicting selection pressures on T-cell epitopes in HIV-1 subtype B

Analysis of population-level polymorphism in eight coding genes of human immunodeficiency virus type 1 (HIV-1) subtype B revealed evidence not only of past purifying selection, but also of abundant slightly deleterious nonsynonymous variants subject to ongoing purifying selection. Both CD4 and CTL epitopes showed an excess of nonsynonymous variants that were singletons (occurring in just one sequence) in our dataset. Overall, median gene diversities at polymorphic nonsynonymous sites were highest at sites located in neither CD4 nor CTL epitopes, while polymorphic nonsynonymous sites in CD4 epitopes revealed the lowest median gene diversity. Our results support the hypothesis that there is an evolutionary conflict between immune escape and functional constraint on epitopes recognized by host T-cells, and suggest that amino acid sequences of CD4 epitopes are subject to particularly strong functional constraint.

[Peptide mapping of H-2d restricted T-cell epitope against six antigens of HIV-1 subtype B'/C by ELISPOT assay]

The purpose is to screen and identify the specific H-2d restricted T-cell epitopes. These epitopes are used to investigate the cellular immune response of BALB/c (H-2d) mice immunized with a HIV-1 vaccine which expresses six antigens of gp160, gag, pol, rev, tat and nef of HIV subtype B'/C. A replicating DNA vaccine and a non-replicating recombinant vaccinia virus vector, both expressing the six antigens mentioned above, were used to immune BALB/c (H-2d) mice in a prime-boost regiment. The six peptide libraries of HIV B'/C corresponding respectively to the six complete antigens were pooled according to a designed matrix format and used to test for IFN-gamma production from splenocytes of immunized mice by an enzyme-linked immunospot (IFN-gamma ELISPOT) assay. The ELISPOT data indicated that two of seven Gag-specific T-cell epitope peptides were identified to be the novel epitopes. One of three Pol-specific T-cell epitope is unreported. One novel epitope was confirmed in two gp160-specific T-cell epitope peptides. One Nef-specific T-cell epitope was identified. Three Tat-specific T-cell epitope peptides were continuous sequences in Tat peptide library and all contained either complete or partial sequence reported. Rev-specific T-cell epitope was not be found. The specific T-cell epitopes (H-2d restricted) were identified by IFN-7 ELISPOT assay, which could be used to detect the cellular immune response of BALB/c mice immunized with the HIV-1 vaccine expressing six antigens of gp160, gag, pol, rev, tat and nef of HIV subtype B'/C.

A versatile vector for the production of pseudotyped viruses expressing gp120 antigens from different clades of primary HIV-1 isolates

A novel HIV-1 Env expression vector (SF162-Z) was developed by introducing two new cloning sites on the backbone of an existing vector that produces a full length Env from HIV-1 SF162 isolate. These sites facilitate the swapping of the gp120 portion of the SF162 Env with matching gp120 antigens from HIV-1 isolates of different genetic clades. Final production of functional pseudotyped viruses will express chimeric Env antigens, including gp41 of the parental SF162 and gp120 from other primary isolates. This system is useful for testing the neutralizing sensitivity of partial env gene products frequently identified in viral quasi species in patients infected with HIV or when only partial gp120 gene products are available.

Characterization of immune responses induced by combined clade-A HIV-1 recombinant adenovectors in mice

BACKGROUND

Numerous evidences indicate that in some HIV-1 positive patients, the humoral and cellular immune responses are induced against HIV-1 proteins and this is inversely related to the progress of infection.

OBJECTIVE

The aim of this study was the evaluation of the Adenovectors containing HIV genes in induction of immune responses in mice.

METHODS

The HIV-1 genes including gag p24, rev, nef and exon-1 of tat were amplified from HIV-1 RNA (clade-A). The cDNA of each gene was cloned into a transfer vector. The transfer vector was then co-transformed into E. coli strain BJ5183 together with pAdenovector ∆E1/E3. The recombinant adenoviral construct was transfected into QBI-293A cells. Recombinant viruses were purified and titrated on 293 cell plates. Expression of transgenes was evaluated using western blotting. Then 10(12) viral particles were injected into 15 groups of 5 mice and all patterns of combination of these 4 HIV-1 genes were evaluated. After 2 weeks, humoral and cellular immune responses were evaluated using ELISA, cell proliferation and ELISpot (IL-2, IL-4 and IFN-γ) assays, consecutively.

RESULTS

It was demonstrated that each gene was expressed. The response targets were mostly toward Th1, though several Th2 responses were also observed. Single injection in our study induced a good cellular response but the humoral responses were not as strong as the cellular ones.

CONCLUSION

Considering and comparing all results and evaluating the various possible interactions revealed that simultaneous injection of tat and gag has enhanced the humoral and cellular responses.

[Molecular epidemiology of HIV-1 strains in the Moscow Region]

The Moscow Region is one of the HIV-1-affected subjects of the Russian Federation; there were 34613 HIV-1-infected subjects as of October 31, 2009. To characterize the molecular epidemiology of HIV-1 in the Moscow Region, the investigators obtained and studied HIV-1 variants from 61 infected subjects of the region, who were major risk groups: intravenous drug users (IDUs) and hetero- and homosexually infected persons. Genetic analysis of HIV-1 variants was carried out by sequencing the gag genes (729 nucleotides in length, including full-length protein p17 and partial p24) andlor env (270 nucleotides in length, V3 region) with further phylogenetic analysis. The findings demonstrated that HIV-1 subtype A variants are dominant in the Moscow Region and detectable in 93.5% of IDUs and 100% of heterosexually infected persons. Phylogenetically (and accordingly epidemiologically) unrelated HIV-1 subtype B strains were revealed in 4 patients, including 2 IDUs.

Immunogenic profiling in mice of a HIV/AIDS vaccine candidate (MVA-B) expressing four HIV-1 antigens and potentiation by specific gene deletions

Background

The immune parameters of HIV/AIDS vaccine candidates that might be relevant in protection against HIV-1 infection are still undefined. The highly attenuated poxvirus strain MVA is one of the most promising vectors to be use as HIV-1 vaccine. We have previously described a recombinant MVA expressing HIV-1 Env, Gag, Pol and Nef antigens from clade B (referred as MVA-B), that induced HIV-1-specific immune responses in different animal models and gene signatures in human dendritic cells (DCs) with immunoregulatory function.

Methodology/Principal Findings

In an effort to characterize in more detail the immunogenic profile of MVA-B and to improve its immunogenicity we have generated a new vector lacking two genes (A41L and B16R), known to counteract host immune responses by blocking the action of CC-chemokines and of interleukin 1β, respectively (referred as MVA-B ΔA41L/ΔB16R). A DNA prime/MVA boost immunization protocol was used to compare the adaptive and memory HIV-1 specific immune responses induced in mice by the parental MVA-B and by the double deletion mutant MVA-B ΔA41L/ΔB16R. Flow cytometry analysis revealed that both vectors triggered HIV-1-specific CD4⁺ and CD8⁺ T cells, with the CD8⁺ T-cell compartment responsible for >91.9% of the total HIV-1 responses in both immunization groups. However, MVA-B ΔA41L/ΔB16R enhanced the magnitude and polyfunctionality of the HIV-1-specific CD4⁺ and CD8⁺ T-cell immune responses. HIV-1-specific CD4⁺ T-cell responses were polyfunctional and preferentially Env-specific in both immunization groups. Significantly, while MVA-B induced preferentially Env-specific CD8⁺ T-cell responses, MVA-B ΔA41L/ΔB16R induced more GPN-specific CD8⁺ T-cell responses, with an enhanced polyfunctional pattern. Both vectors were capable of producing similar levels of antibodies against Env.

Conclusions/Significance

These findings revealed that MVA-B and MVA-B ΔA41L/ΔB16R induced in mice robust, polyfunctional and durable T-cell responses to HIV-1 antigens, but the double deletion mutant showed enhanced magnitude and quality of HIV-1 adaptive and memory responses. Our observations are relevant in the immune evaluation of MVA-B and on improvements of MVA vectors as HIV-1 vaccines.

HIV antigen incorporation within adenovirus hexon hypervariable 2 for a novel HIV vaccine approach

Adenoviral (Ad) vectors have been used for a variety of vaccine applications including cancer and infectious diseases. Traditionally, Ad-based vaccines are designed to express antigens through transgene expression of a given antigen. However, in some cases these conventional Ad-based vaccines have had sub-optimal clinical results. These sub-optimal results are attributed in part to pre-existing Ad serotype 5 (Ad5) immunity. In order to circumvent the need for antigen expression via transgene incorporation, the “antigen capsid-incorporation” strategy has been developed and used for Ad-based vaccine development in the context of a few diseases. This strategy embodies the incorporation of antigenic peptides within the capsid structure of viral vectors. The major capsid protein hexon has been utilized for these capsid incorporation strategies due to hexon's natural role in the generation of anti-Ad immune response and its numerical representation within the Ad virion. Using this strategy, we have developed the means to incorporate heterologous peptide epitopes specifically within the major surface-exposed domains of the Ad capsid protein hexon. Our study herein focuses on generation of multivalent vaccine vectors presenting HIV antigens within the Ad capsid protein hexon, as well as expressing an HIV antigen as a transgene. These novel vectors utilize HVR2 as an incorporation site for a twenty-four amino acid region of the HIV membrane proximal ectodomain region (MPER), derived from HIV glycoprotein gp41 (gp41). Our study herein illustrates that our multivalent anti-HIV vectors elicit a cellular anti-HIV response. Furthermore, vaccinations with these vectors, which present HIV antigens at HVR2, elicit a HIV epitope-specific humoral immune response.

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.

The HIV-1 matrix protein p17 activates the transcription factors c-Myc and CREB in human B cells

The human immunodeficiency virus matrix protein p17 plays a critical role in many steps of the virus life cycle. In addition, p17 displays biological activities outside infected cells. Indeed, virus-neutralizing antibodies against p17 in plasma of infected patients correlate with slower disease progression, and p17 has been shown to interact with an as yet unidentified cell surface receptor expressed on peripheral blood B cells. The present study investigated intracellular signaling pathways triggered following this interaction. Using protein/DNA arrays, we show that p17 increases phosphorylation and the DNA-binding activity of CREB and c-Myc through the time- and dose-dependent activation of the cAMP/PKA and MEK/ERK signaling pathways. Interestingly, we found that both signaling pathways are synergistically activated upon co-stimulation through the CD19 receptor. As both CREB and c-Myc are involved in the regulation of cell proliferation, differentiation, and survival, our findings might suggest a potential mechanism of B cell lymphomagenesis during HIV-1 infection.

Adaptation of HIV-1 to human leukocyte antigen class I

The rapid and extensive spread of the human immunodeficiency virus (HIV) epidemic provides a rare opportunity to witness host-pathogen co-evolution involving humans. A focal point is the interaction between genes encoding human leukocyte antigen (HLA) and those encoding HIV proteins. HLA molecules present fragments (epitopes) of HIV proteins on the surface of infected cells to enable immune recognition and killing by CD8(+) T cells; particular HLA molecules, such as HLA-B*57, HLA-B*27 and HLA-B*51, are more likely to mediate successful control of HIV infection. Mutation within these epitopes can allow viral escape from CD8(+) T-cell recognition. Here we analysed viral sequences and HLA alleles from >2,800 subjects, drawn from 9 distinct study cohorts spanning 5 continents. Initial analysis of the HLA-B*51-restricted epitope, TAFTIPSI (reverse transcriptase residues 128-135), showed a strong correlation between the frequency of the escape mutation I135X and HLA-B*51 prevalence in the 9 study cohorts (P = 0.0001). Extending these analyses to incorporate other well-defined CD8(+) T-cell epitopes, including those restricted by HLA-B*57 and HLA-B*27, showed that the frequency of these epitope variants (n = 14) was consistently correlated with the prevalence of the restricting HLA allele in the different cohorts (together, P < 0.0001), demonstrating strong evidence of HIV adaptation to HLA at a population level. This process of viral adaptation may dismantle the well-established HLA associations with control of HIV infection that are linked to the availability of key epitopes, and highlights the challenge for a vaccine to keep pace with the changing immunological landscape presented by HIV.

Frequent toggling between alternative amino acids is driven by selection in HIV-1

Host immune responses against infectious pathogens exert strong selective pressures favouring the emergence of escape mutations that prevent immune recognition. Escape mutations within or flanking functionally conserved epitopes can occur at a significant cost to the pathogen in terms of its ability to replicate effectively. Such mutations come under selective pressure to revert to the wild type in hosts that do not mount an immune response against the epitope. Amino acid positions exhibiting this pattern of escape and reversion are of interest because they tend to coincide with immune responses that control pathogen replication effectively. We have used a probabilistic model of protein coding sequence evolution to detect sites in HIV-1 exhibiting a pattern of rapid escape and reversion. Our model is designed to detect sites that toggle between a wild type amino acid, which is susceptible to a specific immune response, and amino acids with lower replicative fitness that evade immune recognition. Through simulation, we show that this model has significantly greater power to detect selection involving immune escape and reversion than standard models of diversifying selection, which are sensitive to an overall increased rate of non-synonymous substitution. Applied to alignments of HIV-1 protein coding sequences, the model of immune escape and reversion detects a significantly greater number of adaptively evolving sites in env and nef. In all genes tested, the model provides a significantly better description of adaptively evolving sites than standard models of diversifying selection. Several of the sites detected are corroborated by association between Human Leukocyte Antigen (HLA) and viral sequence polymorphisms. Overall, there is evidence for a large number of sites in HIV-1 evolving under strong selective pressure, but exhibiting low sequence diversity. A phylogenetic model designed to detect rapid toggling between wild type and escape amino acids identifies a larger number of adaptively evolving sites in HIV-1, and can in some cases correctly identify the amino acid that is susceptible to the immune response.

Viruses, such as HIV, are able to evade host immune responses through escape mutations, yet sometimes they do so at a cost. This cost is the reduction in the ability of the virus to replicate, and thus selective pressure exists for a virus to revert to its original state in the absence of the host immune response that caused the initial escape mutation. This pattern of escape and reversion typically occurs when viruses are transmitted between individuals with different immune responses. We develop a phylogenetic model of immune escape and reversion and provide evidence that it outperforms existing models for the detection of selective pressure associated with host immune responses. Finally, we demonstrate that amino acid toggling is a pervasive process in HIV-1 evolution, such that many of the positions in the virus that evolve rapidly, under the influence of positive Darwinian selection, nonetheless display quite low sequence diversity. This highlights the limitations of HIV-1 evolution, and sites such as these are potentially good targets for HIV-1 vaccines.

High-level expression of human immunodeficiency virus antigens from the tobacco and tomato plastid genomes

Transgene expression from the plant's plastid genome represents a promising strategy in molecular farming because of the plastid's potential to accumulate foreign proteins to high levels and the increased biosafety provided by the maternal mode of organelle inheritance. In this article, we explore the potential of transplastomic plants to produce human immunodeficiency virus (HIV) antigens as potential components of an acquired immunodeficiency syndrome (AIDS) vaccine. It is shown that the HIV antigens p24 (the major target of T-cell-mediated immune responses in HIV-positive individuals) and Nef can be expressed to high levels in plastids of tobacco, a non-food crop, and tomato, a food crop with an edible fruit. Optimized p24-Nef fusion gene cassettes trigger antigen protein accumulation to up to approximately 40% of the plant's total protein, demonstrating the great potential of transgenic plastids to produce AIDS vaccine components at low cost and high yield.

High-level expression of human immunodeficiency virus antigens from the tobacco and tomato plastid genomes

Transgene expression from the plant's plastid genome represents a promising strategy in molecular farming because of the plastid's potential to accumulate foreign proteins to high levels and the increased biosafety provided by the maternal mode of organelle inheritance. In this article, we explore the potential of transplastomic plants to produce human immunodeficiency virus (HIV) antigens as potential components of an acquired immunodeficiency syndrome (AIDS) vaccine. It is shown that the HIV antigens p24 (the major target of T-cell-mediated immune responses in HIV-positive individuals) and Nef can be expressed to high levels in plastids of tobacco, a non-food crop, and tomato, a food crop with an edible fruit. Optimized p24-Nef fusion gene cassettes trigger antigen protein accumulation to up to approximately 40% of the plant's total protein, demonstrating the great potential of transgenic plastids to produce AIDS vaccine components at low cost and high yield.

Advantageous Features of Plant-based Systems for the Development of HIV Vaccines

Plants have recently become an attractive option for the production of recombinant proteins. Plant-based systems can be used to produce many classes of foreign proteins including candidate vaccine antigens. The selected antigen can be purified from plant material prior to delivery by the preferred route, or alternatively delivered orally in edible plant material that has been processed to give a homogeneous and stable product. Several plant species have been used to express a wide range of vaccine candidates with tobacco, potato and corn being particularly favored. Corn seed is especially well suited to various food processing technologies that generate dry homogeneous material suitable for extended storage and refrigeration-free transport and distribution. Many antigens have been expressed in corn and assessed for efficacy in trials with generally positive results. Candidate HIV vaccines are particularly good targets for plant-based oral delivery since there is a great need for an easily distributed affordable vaccine that could be administered without injection and induce strong mucosal immune responses. As a first step in evaluating plant expression technology with a relevant antigen that might easily be tested in an animal system, we expressed the SIV major surface glycoprotein gp130 (analogous to HIV gp120) in corn seed. Expression levels were achieved that are compatible with conducting oral delivery trials in animals.

Dynamics of immune escape during HIV/SIV infection

Several studies have shown that cytotoxic T lymphocytes (CTLs) play an important role in controlling HIV/SIV infection. Notably, the observation of escape mutants suggests a selective pressure induced by the CTL response. However, it remains difficult to assess the definite role of the cellular immune response. We devise a computational model of HIV/SIV infection having a broad cellular immune response targeting different viral epitopes. The CTL clones are stimulated by viral antigen and interact with the virus population through cytotoxic killing of infected cells. Consequently, the virus population reacts through the acquisition of CTL escape mutations. Our model provides realistic virus dynamics and describes several experimental observations. We postulate that inter-clonal competition and immunodominance may be critical factors determining the sequential emergence of escapes. We show that even though the total killing induced by the CTL response can be high, escape rates against a single CTL clone are often slow and difficult to estimate from infrequent sequence measurements. Finally, our simulations show that a higher degree of immunodominance leads to more frequent escape with a reduced control of viral replication but a substantially impaired replicative capacity of the virus. This result suggests two strategies for vaccine design: Vaccines inducing a broad CTL response should decrease the viral load, whereas vaccines stimulating a narrow but dominant CTL response are likely to induce escape but may dramatically reduce the replicative capacity of the virus.

As a result of their high mutation rate, HIV and its counterpart SIV in non-human primates can evade recognition by the host immune response through the generation of viral variants, the so-called escape mutants. This avoidance of cytotoxic T lymphocyte (CTL) mediated killing seems to be one of the major reasons why virus replication is not controlled effectively. However, it remains difficult to investigate the critical properties of the dynamics of immune escape. To this end, we developed a new computational model of HIV/SIV infection consisting of several CTL clones that can recognize specific parts of viral proteins, i.e., epitopes. The simulations allow us to follow the dynamics of immune escape in detail and help to interpret longitudinal data of HIV/SIV infections. Interestingly, changing the relative sizes of the CTL clones leads to a different evolution of the virus. Instead of reducing the number of infected cells, an alternative strategy of vaccine design could be to reduce the replicative capacity of the virus that might have implications for disease progression.

Expression of HIV-1 antigens in plants as potential subunit vaccines

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) has infected more than 40 million people worldwide, mainly in sub-Saharan Africa. The high prevalence of HIV-1 subtype C in southern Africa necessitates the development of cheap, effective vaccines. One means of production is the use of plants, for which a number of different techniques have been successfully developed. HIV-1 Pr55Gag is a promising HIV-1 vaccine candidate: we compared the expression of this and a truncated Gag (p17/p24) and the p24 capsid subunit in Nicotiana spp. using transgenic plants and transient expression via Agrobacterium tumefaciens and recombinant tobamovirus vectors. We also investigated the influence of subcellular localisation of recombinant protein to the chloroplast and the endoplasmic reticulum (ER) on protein yield. We partially purified a selected vaccine candidate and tested its stimulation of a humoral and cellular immune response in mice.

RESULTS

Both transient and transgenic expression of the HIV antigens were successful, although expression of Pr55Gag was low in all systems; however, the Agrobacterium-mediated transient expression of p24 and p17/p24 yielded best, to more than 1 mg p24/kg fresh weight. Chloroplast targeted protein levels were highest in transient and transgenic expression of p24 and p17/p24. The transiently-expressed p17/p24 was not immunogenic in mice as a homologous vaccine, but it significantly boosted a humoral and T cell immune response primed by a gag DNA vaccine, pTHGagC.

CONCLUSION

Transient agroinfiltration was best for expression of all of the recombinant proteins tested, and p24 and p17/p24 were expressed at much higher levels than Pr55Gag. Our results highlight the usefulness of plastid signal peptides in enhancing the production of recombinant proteins meant for use as vaccines. The p17/p24 protein effectively boosted T cell and humoral responses in mice primed by the DNA vaccine pTHGagC, showing that this plant-produced protein has potential for use as a vaccine.