New gorilla adenovirus vaccine vectors induce potent immune responses and protection in a mouse malaria model

Background

A DNA-human Ad5 (HuAd5) prime-boost malaria vaccine has been shown to protect volunteers against a controlled human malaria infection. The potency of this vaccine, however, appeared to be affected by the presence of pre-existing immunity against the HuAd5 vector. Since HuAd5 seroprevalence is very high in malaria-endemic areas of the world, HuAd5 may not be the most appropriate malaria vaccine vector. This report describes the evaluation of the seroprevalence, immunogenicity and efficacy of three newly identified gorilla adenoviruses, GC44, GC45 and GC46, as potential malaria vaccine vectors.

Results

The seroprevalence of GC44, GC45 and GC46 is very low, and the three vectors are not efficiently neutralized by human sera from Kenya and Ghana, two countries where malaria is endemic. In mice, a single administration of GC44, GC45 and GC46 vectors expressing a murine malaria gene, Plasmodium yoelii circumsporozoite protein (PyCSP), induced robust PyCSP-specific T cell and antibody responses that were at least as high as a comparable HuAd5-PyCSP vector. Efficacy studies in a murine malaria model indicated that a prime-boost regimen with DNA-PyCSP and GC-PyCSP vectors can protect mice against a malaria challenge. Moreover, these studies indicated that a DNA-GC46-PyCSP vaccine regimen was significantly more efficacious than a DNA-HuAd5-PyCSP regimen.

Conclusion

These data suggest that these gorilla-based adenovectors have key performance characteristics for an effective malaria vaccine. The superior performance of GC46 over HuAd5 highlights its potential for clinical development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1911-z) contains supplementary material, which is available to authorized users.

Development of replication-deficient adenovirus malaria vaccines

INTRODUCTION

Malaria remains a major threat to endemic populations and travelers, including military personnel to these areas. A malaria vaccine is feasible, as radiation attenuated sporozoites induce nearly 100% efficacy. Areas covered: This review covers current malaria clinical trials using adenoviruses and pre-clinical research. Heterologous prime-boost regimens, including replication-deficient human adenovirus 5 (HuAd5) carrying malaria antigens, are efficacious. However, efficacy appears to be adversely affected by pre-existing anti-HuAd5 antibodies. Current strategies focus on replacing HuAd5 with rarer human adenoviruses or adenoviruses isolated from non-human primates (NHPs). The chimpanzee adenovirus ChAd63 is undergoing evaluation in clinical trials including infants in malaria-endemic areas. Key antigens have been identified and are being used alone, in combination, or with protein subunit vaccines. Gorilla adenoviruses carrying malaria antigens are also currently being evaluated in preclinical models. These replacement adenovirus vectors will be successfully used to develop vaccines against malaria, as well as other infectious diseases. Expert commentary: Simplified prime-boost single shot regimens, dry-coated live vector vaccines or silicon microneedle arrays could be developed for malaria or other vaccines. Replacement vectors with similar or superior immunogenicity have rapidly advanced, and several are now in extensive Phase 2 and beyond in malaria as well as other diseases, notably Ebola.

Controlled Human Malaria Infection (CHMI) differentially affects cell-mediated and antibody responses to CSP and AMA1 induced by adenovirus vaccines with and without DNA-priming

We have previously shown that a DNA-prime followed by an adenovirus-5 boost vaccine containing CSP and AMA1 (DNA/Ad) successfully protected 4 of 15 subjects to controlled human malaria infection (CHMI). However, the adenovirus-5 vaccine alone (AdCA) failed to induce protection despite eliciting cellular responses that were often higher than those induced by DNA/Ad. Here we determined the effect of CHMI on pre-CHMI cellular and antibody responses against CSP and AMA1 expressed as fold-changes in activities. Generally, in the DNA/Ad trial, CHMI caused pre-CHMI ELISpot IFN-γ and CD8+ T cell IFN-γ responses of the protected subjects to fall but among non-protected subjects, CHMI caused rises of pre-CHMI ELISpot IFN-γ but falls of CD8+ T cell IFN-γ responses. In contrast in the AdCA trial, CHMI caused both pre-CHMI ELISpot IFN-γ and CD8+ T cell IFN-γ responses of the AdCA subjects to fall. We suggest that the falls in activities are due to migration of peripheral CD8+ T cells to the liver in response to developing liver stage parasites, and this fall, in the DNA/Ad trial, is masked in ELISpot responses of the non-protected subjects by rises in other immune cell types. In addition, CHMI caused falls in antibody activities of protected subjects, but rises in non-protected subjects in both trials to CSP, and dramatically in the AdCA trial to AMA1, reaching 380 μg/ml that is probably due to boosting by transient blood stage infection before chloroquine treatment. Taken together, these results further define differences in cellular responses between DNA/Ad and AdCA trials, and suggest that natural transmission may boost responses induced by these malaria vaccines especially when protection is not achieved.

Discovery of Novel Plasmodium falciparum Pre-Erythrocytic Antigens for Vaccine Development

Background

Nearly 100% protection against malaria infection can be achieved in humans by immunization with P. falciparum radiation-attenuated sporozoites (RAS). Although it is thought that protection is mediated by T cell and antibody responses, only a few of the many pre-erythrocytic (sporozoite and liver stage) antigens that are targeted by these responses have been identified.

Methodology

Twenty seven P. falciparum pre-erythrocytic antigens were selected using bioinformatics analysis and expression databases and were expressed in a wheat germ cell-free protein expression system. Recombinant proteins were recognized by plasma from RAS-immunized subjects, and 21 induced detectable antibody responses in mice and rabbit and sera from these immunized animals were used to characterize these antigens. All 21 proteins localized to the sporozoite: five localized to the surface, seven localized to the micronemes, cytoplasm, endoplasmic reticulum or nucleus, two localized to the surface and cytoplasm, and seven remain undetermined. PBMC from RAS-immunized volunteers elicited positive ex vivo or cultured ELISpot responses against peptides from 20 of the 21 antigens.

Conclusions

These T cell and antibody responses support our approach of using reagents from RAS-immunized subjects to screen potential vaccine antigens, and have led to the identification of a panel of novel P. falciparum antigens. These results provide evidence to further evaluate these antigens as vaccine candidates.

Trial Registration

ClinicalTrials.gov NCT00870987 ClinicalTrials.gov NCT00392015

Sterile immunity to malaria after DNA prime/adenovirus boost immunization is associated with effector memory CD8+T cells targeting AMA1 class I epitopes

Background

Fifteen volunteers were immunized with three doses of plasmid DNA encoding P. falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1) and boosted with human adenovirus-5 (Ad) expressing the same antigens (DNA/Ad). Four volunteers (27%) demonstrated sterile immunity to controlled human malaria infection and, overall, protection was statistically significantly associated with ELISpot and CD8+ T cell IFN-γ activities to AMA1 but not CSP. DNA priming was required for protection, as 18 additional subjects immunized with Ad alone (AdCA) did not develop sterile protection.

Methodology/Principal Findings

We sought to identify correlates of protection, recognizing that DNA-priming may induce different responses than AdCA alone. Among protected volunteers, two and three had higher ELISpot and CD8+ T cell IFN-γ responses to CSP and AMA1, respectively, than non-protected volunteers. Unexpectedly, non-protected volunteers in the AdCA trial showed ELISpot and CD8+ T cell IFN-γ responses to AMA1 equal to or higher than the protected volunteers. T cell functionality assessed by intracellular cytokine staining for IFN-γ, TNF-α and IL-2 likewise did not distinguish protected from non-protected volunteers across both trials. However, three of the four protected volunteers showed higher effector to central memory CD8+ T cell ratios to AMA1, and one of these to CSP, than non-protected volunteers for both antigens. These responses were focused on discrete regions of CSP and AMA1. Class I epitopes restricted by A*03 or B*58 supertypes within these regions of AMA1 strongly recalled responses in three of four protected volunteers. We hypothesize that vaccine-induced effector memory CD8+ T cells recognizing a single class I epitope can confer sterile immunity to P. falciparum in humans.

Conclusions/Significance

We suggest that better understanding of which epitopes within malaria antigens can confer sterile immunity and design of vaccine approaches that elicit responses to these epitopes will increase the potency of next generation gene-based vaccines.

Human adenovirus 5-vectored Plasmodium falciparum NMRC-M3V-Ad-PfCA vaccine encoding CSP and AMA1 is safe, well-tolerated and immunogenic but does not protect against controlled human malaria infection

Background: In a prior study, a DNA prime / adenovirus boost vaccine (DNA/Ad) expressing P. falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1) (NMRC-M3V-D/Ad-PfCA Vaccine) induced 27% protection against controlled human malaria infection (CHMI). To investigate the contribution of DNA priming, we tested the efficacy of adenovirus vaccine alone (NMRC-M3V-Ad-PfCA ) in a Phase 1 clinical trial.

Methodology/Principal Findings: The regimen was a single intramuscular injection with two non-replicating human serotype 5 adenovectors encoding CSP and AMA1, respectively. One x 10¹⁰ particle units of each construct were combined prior to administration. The regimen was safe and well-tolerated. Four weeks later, 18 study subjects received P. falciparum CHMI administered by mosquito bite. None were fully protected although one showed delayed onset of parasitemia. Antibody responses were low, with geometric mean CSP ELISA titer of 381 (range < 50–1626) and AMA1 ELISA of 4.95 µg/mL (range 0.2–38). Summed ex vivo IFN-γ ELISpot responses to overlapping peptides were robust, with geometric mean spot forming cells/million peripheral blood mononuclear cells [sfc/m] for CSP of 273 (range 38–2550) and for AMA1 of 1303 (range 435–4594). CD4+ and CD8+ T cell IFN-γ responses to CSP were positive by flow cytometry in 25% and 56% of the research subjects, respectively, and to AMA1 in 94% and 100%, respectively.

Significance: In contrast to DNA/Ad, Ad alone did not protect against CHMI despite inducing broad, cell-mediated immunity, indicating that DNA priming is required for protection by the adenovirus-vectored vaccine. ClinicalTrials.gov Identifier: NCT00392015.

DNA prime/Adenovirus boost malaria vaccine encoding P. falciparum CSP and AMA1 induces sterile protection associated with cell-mediated immunity

Background

Gene-based vaccination using prime/boost regimens protects animals and humans against malaria, inducing cell-mediated responses that in animal models target liver stage malaria parasites. We tested a DNA prime/adenovirus boost malaria vaccine in a Phase 1 clinical trial with controlled human malaria infection.

Methodology/Principal Findings

The vaccine regimen was three monthly doses of two DNA plasmids (DNA) followed four months later by a single boost with two non-replicating human serotype 5 adenovirus vectors (Ad). The constructs encoded genes expressing P. falciparum circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1). The regimen was safe and well-tolerated, with mostly mild adverse events that occurred at the site of injection. Only one AE (diarrhea), possibly related to immunization, was severe (Grade 3), preventing daily activities. Four weeks after the Ad boost, 15 study subjects were challenged with P. falciparum sporozoites by mosquito bite, and four (27%) were sterilely protected. Antibody responses by ELISA rose after Ad boost but were low (CSP geometric mean titer 210, range 44–817; AMA1 geometric mean micrograms/milliliter 11.9, range 1.5–102) and were not associated with protection. Ex vivo IFN-γ ELISpot responses after Ad boost were modest (CSP geometric mean spot forming cells/million peripheral blood mononuclear cells 86, range 13–408; AMA1 348, range 88–1270) and were highest in three protected subjects. ELISpot responses to AMA1 were significantly associated with protection (p = 0.019). Flow cytometry identified predominant IFN-γ mono-secreting CD8+ T cell responses in three protected subjects. No subjects with high pre-existing anti-Ad5 neutralizing antibodies were protected but the association was not statistically significant.

Significance

The DNA/Ad regimen provided the highest sterile immunity achieved against malaria following immunization with a gene-based subunit vaccine (27%). Protection was associated with cell-mediated immunity to AMA1, with CSP probably contributing. Substituting a low seroprevalence vector for Ad5 and supplementing CSP/AMA1 with additional antigens may improve protection.

Trial Registration

ClinicalTrials.govNCT00870987.

Clinical trial in healthy malaria-naïve adults to evaluate the safety, tolerability, immunogenicity and efficacy of MuStDO5, a five-gene, sporozoite/hepatic stage Plasmodium falciparum DNA vaccine combined with escalating dose human GM-CSF DNA

When introduced in the 1990s, immunization with DNA plasmids was considered potentially revolutionary for vaccine development, particularly for vaccines intended to induce protective CD8 T cell responses against multiple antigens. We conducted, in 1997−1998, the first clinical trial in healthy humans of a DNA vaccine, a single plasmid encoding Plasmodium falciparum circumsporozoite protein (PfCSP), as an initial step toward developing a multi-antigen malaria vaccine targeting the liver stages of the parasite. As the next step, we conducted in 2000–2001 a clinical trial of a five-plasmid mixture called MuStDO5 encoding pre-erythrocytic antigens PfCSP, PfSSP2/TRAP, PfEXP1, PfLSA1 and PfLSA3. Thirty-two, malaria-naïve, adult volunteers were enrolled sequentially into four cohorts receiving a mixture of 500 μg of each plasmid plus escalating doses (0, 20, 100 or 500 μg) of a sixth plasmid encoding human granulocyte macrophage-colony stimulating factor (hGM-CSF). Three doses of each formulation were administered intramuscularly by needle-less jet injection at 0, 4 and 8 weeks, and each cohort had controlled human malaria infection administered by five mosquito bites 18 d later. The vaccine was safe and well-tolerated, inducing moderate antigen-specific, MHC-restricted T cell interferon-γ responses but no antibodies. Although no volunteers were protected, T cell responses were boosted post malaria challenge. This trial demonstrated the MuStDO5 DNA and hGM-CSF plasmids to be safe and modestly immunogenic for T cell responses. It also laid the foundation for priming with DNA plasmids and boosting with recombinant viruses, an approach known for nearly 15 y to enhance the immunogenicity and protective efficacy of DNA vaccines.

Modification of Ad5 hexon hypervariable regions circumvents pre-existing Ad5 neutralizing antibodies and induces protective immune responses

The development of an effective malaria vaccine is a high global health priority. Vaccine vectors based on adenovirus type 5 are capable of generating robust and protective T cell and antibody responses in animal models and are currently being evaluated in clinical trials for HIV and malaria. They appear to be more effective in terms of inducing antigen-specific immune responses as compared with non-Ad5 serotype vectors. However, the high prevalence of neutralizing antibodies to Ad5 in the human population, particularly in the developing world, has the potential to limit the effectiveness of Ad5-based vaccines. We have generated novel Ad5-based vectors that precisely replace the hexon hypervariable regions with those derived from Ad43, a subgroup D serotype with low prevalence of neutralizing antibody in humans. We have demonstrated that these hexon-modified adenovectors are not neutralized efficiently by Ad5 neutralizing antibodies in vitro using sera from mice, rabbits and human volunteers. We have also generated hexon-modified adenovectors that express a rodent malaria parasite antigen, PyCSP, and demonstrated that they are as immunogenic as an unmodified vector. Furthermore, in contrast to the unmodified vector, the hexon-modified adenovectors induced robust T cell responses in mice with high levels of Ad5 neutralizing antibody. We also show that the hexon-modified vector can be combined with unmodified Ad5 vector in prime-boost regimens to induce protective responses in mice. Our data establish that these hexon-modified vectors are highly immunogenic even in the presence of pre-existing anti-adenovirus antibodies. These hexon-modified adenovectors may have advantages in sub-Saharan Africa where there is a high prevalence of Ad5 neutralizing antibody in the population.

Adenovirus 5-vectored P. falciparum vaccine expressing CSP and AMA1. Part A: safety and immunogenicity in seronegative adults

Background

Models of immunity to malaria indicate the importance of CD8+ T cell responses for targeting intrahepatic stages and antibodies for targeting sporozoite and blood stages. We designed a multistage adenovirus 5 (Ad5)-vectored Plasmodium falciparum malaria vaccine, aiming to induce both types of responses in humans, that was tested for safety and immunogenicity in a Phase 1 dose escalation trial in Ad5-seronegative volunteers.

Methodology/Principal Findings

The NMRC-M3V-Ad-PfCA vaccine combines two adenovectors encoding circumsporozoite protein (CSP) and apical membrane antigen-1 (AMA1). Group 1 (n = 6) healthy volunteers received one intramuscular injection of 2×10∧10 particle units (1×10∧10 each construct) and Group 2 (n = 6) a five-fold higher dose. Transient, mild to moderate adverse events were more pronounced with the higher dose. ELISpot responses to CSP and AMA1 peaked at 1 month, were higher in the low dose (geomean CSP = 422, AMA1 = 862 spot forming cells/million) than in the high dose (CSP = 154, p = 0.049, AMA1 = 423, p = 0.045) group and were still positive at 12 months in a number of volunteers. ELISpot depletion assays identified dependence on CD4+ or on both CD4+ and CD8+ T cells, with few responses dependent only on CD8+ T cells. Intracellular cytokine staining detected stronger CD8+ than CD4+ T cell IFN-γ responses (CSP p = 0.0001, AMA1 p = 0.003), but similar frequencies of multifunctional CD4+ and CD8+ T cells secreting two or more of IFN-γ, TNF-α or IL-2. Median fluorescence intensities were 7–10 fold higher in triple than single secreting cells. Antibody responses were low but trended higher in the high dose group and did not inhibit growth of cultured P. falciparum blood stage parasites.

Significance

As found in other trials, adenovectored vaccines appeared safe and well-tolerated at doses up to 1×10∧11 particle units. This is the first demonstration in humans of a malaria vaccine eliciting strong CD8+ T cell IFN-γ responses.

Trial Registration

ClinicalTrials.govNCT00392015

Adenovirus-5-vectored P. falciparum vaccine expressing CSP and AMA1. Part B: safety, immunogenicity and protective efficacy of the CSP component

BACKGROUND

A protective malaria vaccine will likely need to elicit both cell-mediated and antibody responses. As adenovirus vaccine vectors induce both these responses in humans, a Phase 1/2a clinical trial was conducted to evaluate the efficacy of an adenovirus serotype 5-vectored malaria vaccine against sporozoite challenge.

METHODOLOGY/PRINCIPAL FINDINGS

NMRC-MV-Ad-PfC is an adenovirus vector encoding the Plasmodium falciparum 3D7 circumsporozoite protein (CSP). It is one component of a two-component vaccine NMRC-M3V-Ad-PfCA consisting of one adenovector encoding CSP and one encoding apical membrane antigen-1 (AMA1) that was evaluated for safety and immunogenicity in an earlier study (see companion paper, Sedegah et al). Fourteen Ad5 seropositive or negative adults received two doses of NMRC-MV-Ad-PfC sixteen weeks apart, at 1 x 1010 particle units per dose. The vaccine was safe and well tolerated. All volunteers developed positive ELISpot responses by 28 days after the first immunization (geometric mean 272 spot forming cells/million[sfc/m]) that declined during the following 16 weeks and increased after the second dose to levels that in most cases were less than the initial peak (geometric mean 119 sfc/m). CD8+ predominated over CD4+ responses, as in the first clinical trial. Antibody responses were poor and like ELISpot responses increased after the second immunization but did not exceed the initial peak. Pre-existing neutralizing antibodies (NAb) to Ad5 did not affect the immunogenicity of the first dose, but the fold increase in NAb induced by the first dose was significantly associated with poorer antibody responses after the second dose, while ELISpot responses remained unaffected. When challenged by the bite of P. falciparum-infected mosquitoes, two of 11 volunteers showed a delay in the time to patency compared to infectivity controls, but no volunteers were sterilely protected.

SIGNIFICANCE

The NMRC-MV-Ad-PfC vaccine expressing CSP was safe and well tolerated given as two doses, but did not provide sterile protection.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00392015.

Identification of two new protective pre-erythrocytic malaria vaccine antigen candidates

Background

Despite years of effort, a licensed malaria vaccine is not yet available. One of the obstacles facing the development of a malaria vaccine is the extensive heterogeneity of many of the current malaria vaccine antigens. To counteract this antigenic diversity, an effective malaria vaccine may need to elicit an immune response against multiple malaria antigens, thereby limiting the negative impact of variability in any one antigen. Since most of the malaria vaccine antigens that have been evaluated in people have not elicited a protective immune response, there is a need to identify additional protective antigens. In this study, the efficacy of three pre-erythrocytic stage malaria antigens was evaluated in a Plasmodium yoelii/mouse protection model.

Methods

Mice were immunized with plasmid DNA and vaccinia virus vectors that expressed one, two or all three P. yoelii vaccine antigens. The immunized mice were challenged with 300 P. yoelii sporozoites and evaluated for subsequent infection.

Results

Vaccines that expressed any one of the three antigens did not protect a high percentage of mice against a P. yoelii challenge. However, vaccines that expressed all three antigens protected a higher percentage of mice than a vaccine that expressed PyCSP, the most efficacious malaria vaccine antigen. Dissection of the multi-antigen vaccine indicated that protection was primarily associated with two of the three P. yoelii antigens. The protection elicited by a vaccine expressing these two antigens exceeded the sum of the protection elicited by the single antigen vaccines, suggesting a potential synergistic interaction.

Conclusions

This work identifies two promising malaria vaccine antigen candidates and suggests that a multi-antigen vaccine may be more efficacious than a single antigen vaccine.

Sterile protection against Plasmodium knowlesi in rhesus monkeys from a malaria vaccine: comparison of heterologous prime boost strategies

Using newer vaccine platforms which have been effective against malaria in rodent models, we tested five immunization regimens against Plasmodium knowlesi in rhesus monkeys. All vaccines included the same four P. knowlesi antigens: the pre-erythrocytic antigens CSP, SSP2, and erythrocytic antigens AMA1, MSP1. We used four vaccine platforms for prime or boost vaccinations: plasmids (DNA), alphavirus replicons (VRP), attenuated adenovirus serotype 5 (Ad), or attenuated poxvirus (Pox). These four platforms combined to produce five different prime/boost vaccine regimens: Pox alone, VRP/Pox, VRP/Ad, Ad/Pox, and DNA/Pox. Five rhesus monkeys were immunized with each regimen, and five Control monkeys received a mock vaccination. The time to complete vaccinations was 420 days. All monkeys were challenged twice with 100 P. knowlesi sporozoites given IV. The first challenge was given 12 days after the last vaccination, and the monkeys receiving the DNA/Pox vaccine were the best protected, with 3/5 monkeys sterilely protected and 1/5 monkeys that self-cured its parasitemia. There was no protection in monkeys that received Pox malaria vaccine alone without previous priming. The second sporozoite challenge was given 4 months after the first. All 4 monkeys that were protected in the first challenge developed malaria in the second challenge. DNA, VRP and Ad5 vaccines all primed monkeys for strong immune responses after the Pox boost. We discuss the high level but short duration of protection in this experiment and the possible benefits of the long interval between prime and boost.

Expression of vaccinia E3L and K3L genes by a novel recombinant canarypox HIV vaccine vector enhances HIV-1 pseudovirion production and inhibits apoptosis in human cells

Poxviruses that are attenuated for growth in human cells provide a safe means of HIV antigen expression and are capable of eliciting HIV-specific immune responses, including CD8+ cytotoxic T-lymphocyte (CTL) responses. HIV-1 antigen expression in human cells by attenuated poxvirus vectors may be limited by interferon-mediated host defense mechanisms. To enhance HIV antigen expression in human cells, the vaccinia virus E3L and K3L genes were inserted into a canarypox vector that expresses HIV-1 Gag, Env, and a Nef/Pol polyepitope string. E3L and K3L markedly reduced the activation of the double-stranded RNA-dependent protein kinase, PKR, and led to a significant reduction in apoptosis in HeLa cells. Production and release of HIV-1 antigen in the form of pseudovirions was enhanced in both duration and magnitude by this vector modification. The addition of immunomodulatory genes to attenuated poxviruses represents a novel strategy for enhancing antigen production by live vector HIV vaccine candidates.

Cytotoxic T lymphocyte responses to vaccinia virus antigens but not HIV-1 subtype E envelope protein seen in HIV-1 seronegative Thais

The HIV-1 prime boost phase I/II vaccine trial using a recombinant canarypox vector, vCP1521, containing subtype E env (gp120), and subtype B env (gp41), gag and protease has started in Thailand. We have demonstrated that although 4 from 15 human immunodeficiency virus type 1 (HIV-1) seronegative Individuals showed cytotoxic T lymphocyte (CTL) responses to vaccinia virus antigens, none of them showed specific CTL responses to subtype E Env after in vitro stimulation. This preliminary study suggests that specific CTL responses to subtype E envelope detected in HIV-1 seronegative Individuals after vaccination should be considered as specific responses to the immunization.

CD8+ lymphocyte antiviral activity in monkeys immunized with SIV recombinant poxvirus vaccines: potential role in vaccine efficacy

Protection against intravenous simian immunodeficiency virus (SIV) challenge was assessed in rhesus macaques after immunization with a highly attenuated vaccinia (NYVAC)-SIV recombinant. One-third of vaccinated animals controlled viral infection and progressed to disease more slowly than control animals (Benson J, et al.: J Virol 1998;72:4170). However, this protection was not associated with neutralizing antibodies, cytotoxic T lymphocytes, or helper T cell responses. To explore other potential correlates of protection, we examined CD8+ T cell antiviral activity in macaques vaccinated with NYVAC-SIV, with or without added cytokine adjuvants, and in controls receiving only IL-12 or IL-12 plus IL-2. Before immunization, naive macaques exhibited a broad range of CD8+ T cell antiviral activity. Nevertheless, in the course of immunization, the vaccinated macaques as a group developed increased CD8+ T cell antiviral activity while the controls remained stable. Infectious SIV exposure also increased antiviral activity. Prechallenge antiviral activity levels of vaccinated macaques were not sufficient to prevent SIV transmission or control viral replication during acute infection. However, vaccinated animals consistently exhibited reduced viral loads postchallenge compared with controls. Moreover, high suppressive activity 8 weeks postchallenge, at which time the viremia set point was established, was significantly correlated with reduced viral load and slow disease progression. Prechallenge antiviral activity influenced this result, as decreased viremia and slow progressor status were more apparent in macaques with high suppressive activity both pre- and postchallenge. Our data demonstrate the impact of CD8+ antiviral activity on viral replication and disease progression, and suggest that vaccine designs able to elicit high levels of this activity will contribute significantly to protective efficacy.

Recombinant vaccine-induced protection against the highly pathogenic simian immunodeficiency virus SIV(mac251): dependence on route of challenge exposure

Vaccine protection from infection and/or disease induced by highly pathogenic simian immunodeficiency virus (SIV) strain SIV(mac251) in the rhesus macaque model is a challenging task. Thus far, the only approach that has been reported to protect a fraction of macaques from infection following intravenous challenge with SIV(mac251) was the use of a live attenuated SIV vaccine. In the present study, the gag, pol, and env genes of SIV(K6W) were expressed in the NYVAC vector, a genetically engineered derivative of the vaccinia virus Copenhagen strain that displays a highly attenuated phenotype in humans. In addition, the genes for the alpha and beta chains of interleukin-12 (IL-12), as well as the IL-2 gene, were expressed in separate NYVAC vectors and inoculated intramuscularly, in conjunction with or separate from the NYVAC-SIV vaccine, in 40 macaques. The overall cytotoxic T-lymphocyte (CTL) response was greater, at the expense of proliferative and humoral responses, in animals immunized with NYVAC-SIV and NYVAC-IL-12 than in animals immunized with the NYVAC-SIV vaccine alone. At the end of the immunization regimen, half of the animals were challenged with SIV(mac251) by the intravenous route and the other half were exposed to SIV(mac251) intrarectally. Significantly, five of the eleven vaccinees exposed mucosally to SIV(mac251) showed a transient peak of viremia 1 week after viral challenge and subsequently appeared to clear viral infection. In contrast, all 12 animals inoculated intravenously became infected, but 5 to 6 months after viral challenge, 4 animals were able to control viral expression and appeared to progress to disease more slowly than control animals. Protection did not appear to be associated with any of the measured immunological parameters. Further modulation of immune responses by coadministration of NYVAC-cytokine recombinants did not appear to influence the outcome of viral challenge. The fact that the NYVAC-SIV recombinant vaccine appears to be effective per se in the animal model that best mirrors human AIDS supports the idea that the development of a highly attenuated poxvirus-based vaccine candidate can be a valuable approach to significantly decrease the spread of human immunodeficiency virus (HIV) infection by the mucosal route.

Immunogenicity and protective efficacy of a human immunodeficiency virus type 2 recombinant canarypox (ALVAC) vaccine candidate in cynomolgus monkeys

The efficacy of a recombinant human immunodeficiency virus (HIV) type 2 canarypox (ALVAC HIV-2) vaccine candidate given alone or in combination with HIV-2 envelope gp125 or HIV-2 V3 synthetic peptides was investigated in 14 cynomolgus monkeys. High antibody titers to HIV-2 gp125 were demonstrated in monkeys given booster immunizations with gp125. Neutralizing antibody titers were low (< or = 20) in all monkeys except 2. Significant lymphocyte proliferative responses to killed HIV-2 virions were observed in monkeys given booster immunizations with gp125. HIV-2-specific cytotoxic T lymphocytes were demonstrated prior to viral challenge in 3 of 12 monkeys. After challenge with homologous cell-free HIV-2 propagated in monkey cells, 4 of 10 monkeys immunized with ALVAC HIV-2 plus HIV-2 gp125 or V3 peptides were protected, as determined by negative virus isolation and polymerase chain reaction for viral DNA. Four monkeys immunized with ALVAC HIV-2 alone were not protected. All 12 control monkeys became infected. There was no correlation between the immunologic parameters studied and protection against infection in the vaccinated monkeys.

Highly attenuated HIV type 2 recombinant poxviruses, but not HIV-2 recombinant Salmonella vaccines, induce long-lasting protection in rhesus macaques

Immunization schemes employing priming with vector-based vaccine candidates followed by subunit booster administrations have been explored and shown to have merit in the human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus systems. In this study, we have assessed the priming capacity of highly attenuated poxvirus vector (NYVAC and ALVAC)-based HIV-2 recombinants, as well as Salmonella typhimurium HIV-2 recombinants in rhesus macaques. ALVAC- and NYVAC-based vaccine candidates expressing the HIV-2 gag, pol, and env genes or NYVAC-based recombinants expressing either gp160 or gp120 were used to immunize rhesus macaques in combination protocols with alum-adjuvanted HIV-2 rgp160. Following intravenous challenge exposure with 100 infectious doses of the HIV-2SBL6669 parental virus genotype mixture, seven of eight animals were protected from infection. The seven protected animals were rechallenged 6 months postprimary challenge, without additional booster inoculations, with the same dose of the HIV-2SBL6669 parental virus. Five of the seven animals remained protected against HIV-2 infection at 6 months following the second challenge. In contrast, oral immunization with recombinant Salmonella expressing the HIV-2 gag and the gp120 portion of the envelope either alone or in combination with alum-adjuvanted rgp160 failed to confer protection. These results suggest that the NYVAC- and ALVAC-based recombinants may confer long-lasting protection and that these two highly attenuated poxvirus vaccine vectors may represent promising candidates for developing an acquired immunodeficiency syndrome vaccine.

Fusogenic segments of bovine leukemia virus and simian immunodeficiency virus are interchangeable and mediate fusion by means of oblique insertion in the lipid bilayer of their target cells

Modified bovine leukemia virus (BLV) glycoproteins were expressed by using vaccinia virus recombinants, and their fusogenic capacities were examined by a syncytia-formation assay. This analysis indicates that (i) both BLV envelope glycoproteins gp51 and gp30 are necessary for cell fusion; (ii) insertion of the N-terminal segment of gp30 (fusion peptide) into the lipid bilayer in an oblique orientation, as predicted by computer conformational analysis, results in fusogenic capacities higher than insertion in a perpendicular or parallel orientation; and (iii) replacement of the BLV fusion peptide with its simian immunodeficiency virus counterpart does not modify the fusogenic capacity of the BLV glycoprotein.

Recombinant vaccinia virus expression of the bovine leukaemia virus envelope gene and protection of immunized sheep against infection

The bovine leukaemia virus (BLV) envelope gene encoding extracellular glycoprotein gp51 and transmembrane glycoprotein gp30 was cloned into the HA locus of vaccinia virus (Copenhagen strain), downstream of the vaccinia virus early-late promoter, H6, or a triple promoter element consisting of the promoter for the vaccinia virus H6 gene, the promoter for the cowpox virus A-type inclusion (ATI) gene and the promoter for the vaccinia virus HA gene. Inoculation of rabbits or sheep with the recombinant vaccinia virus coding for gp51 and gp30 or an uncleaved env precursor induced neutralizing antibodies to BLV. These antibodies competed with monoclonal antibodies directed against gp51 epitopes F, G, and H previously shown to be of crucial importance for BLV infection. Seven out of eight sheep vaccinated with the vaccinia recombinants resisted a drastic challenge (1.5 x 10(3) sheep infectious doses) with BLV-infected lymphocytes. These results show that vaccination with BLV env vaccinia recombinants protects sheep against infection with extremely high doses of BLV-infected heterologous lymphocytes.

Vaccinia virus host range genes

A gene encoding an 18-kDa polypeptide (ORF C7L) located in the vaccinia virus HindIII C fragment was shown to be functionally equivalent to previously described host range gene (ORF K1L) spanning the HindIII K/M fragment junction. Either C7L or K1L host range gene is necessary and sufficient by itself to allow replication of vaccinia virus on human cells. Deletion of both C7L and K1L genes from the wild-type vaccinia genome is required to derive a virus deficient for replication on human cells. Further, an ORF encoding a 77-kDa polypeptide derived from cowpox (CP77kDa) and previously shown to allow vaccinia to overcome the restriction for replication on Chinese hamster ovary cells could substitute for the vaccinia host range genes C7L and K1L in permitting replication of the virus on human cells. Additionally, the three unique host range genes C7L, K1L, and CP77kDa were functionally equivalent for vaccinia replication on pig kidney cells, but not on rabbit kidney cells.

Cloning and expression of foreign genes in vaccinia virus, using a host range selection system

A simple selection system has been developed for the cloning and expression of open reading frames in vaccinia virus. The selection system is based on a conditional lethal (host range) mutant of vaccinia virus. A deletion mutant of the vaccinia virus WR strain was generated by insertion of the neomycin resistance gene from transposon Tn5 and selection with the antibiotic G418. This deletion recombinant, vP293, lacked approximately 21.7 kilobases of DNA beginning 3.8 kilobases from the left end of the genome, vP293, was capable of plaquing on primary chicken embryo fibroblasts and two monkey cell lines (BSC-40 and Vero) but was defective in replication in the human cell line MRC-5. Insertion of the host range gene K1L into vP293 restored the ability to grow on MRC-5 cells. A series of plasmids were constructed which in addition to the K1L gene contained a vaccinia virus early-late promoter, H6, followed by a unique polylinker sequence, translational initiation and termination signals, and an early transcription termination signal. These plasmids, pHES1 through 4, allowed for rapid single-step cloning and expression of any open reading frame when recombined in vivo with vP293 and scored for growth on MRC-5 cells.