Induction of multi-antigen multi-stage immune responses against Plasmodium falciparum in rhesus monkeys, in the absence of antigen interference, with heterologous DNA prime/poxvirus boost immunization

Induction of antigen specific intrahepatic CD8+ T cell responses by a secreted heat shock protein based gp96-Ig-PfCA malaria vaccine

Introduction

A highly efficacious and durable vaccine against malaria is an essential tool for global malaria eradication. One of the promising strategies to develop such a vaccine is to induce robust CD8+ T cell mediated immunity against malaria liver-stage parasites.

Methods

Here we describe a novel malaria vaccine platform based on a secreted form of the heat shock protein, gp96-immunoglobulin, (gp96-Ig) to induce malaria antigen specific, memory CD8+ T cells. Gp96-Ig acts as an adjuvant to activate antigen presenting cells (APCs) and chaperone peptides/antigens to APCs for cross presentation to CD8+ T cells.

Results

Our study shows that vaccination of mice and rhesus monkeys with HEK-293 cells transfected with gp96-Ig and two well-known Plasmodium falciparum CSP and AMA1 (PfCA) vaccine candidate antigens, induces liver-infiltrating, antigen specific, memory CD8+ T cell responses. The majority of the intrahepatic CSP and AMA1 specific CD8+ T cells expressed CD69 and CXCR3, the hallmark of tissue resident memory T cells (Trm). Also, we found intrahepatic, antigen-specific memory CD8+ T cells secreting IL-2, which is relevant for maintenance of effective memory responses in the liver.

Discussion

Our novel gp96-Ig malaria vaccine strategy represents a unique approach to induce liver-homing, antigen-specific CD8+ T cells critical for Plasmodium liver-stage protection.

Reciprocal Inhibition of Immunogenic Performance in Mice of Two Potent DNA Immunogens Targeting HCV-Related Liver Cancer

Chronic HCV infection and associated liver cancer impose a heavy burden on the healthcare system. Direct acting antivirals eliminate HCV, unless it is drug resistant, and partially reverse liver disease, but they cannot cure HCV-related cancer. A possible remedy could be a multi-component immunotherapeutic vaccine targeting both HCV-infected and malignant cells, but also those not infected with HCV. To meet this need we developed a two-component DNA vaccine based on the highly conserved core protein of HCV to target HCV-infected cells, and a renowned tumor-associated antigen telomerase reverse transcriptase (TERT) based on the rat TERT, to target malignant cells. Their synthetic genes were expression-optimized, and HCV core was truncated after aa 152 (Core152opt) to delete the domain interfering with immunogenicity. Core152opt and TERT DNA were highly immunogenic in BALB/c mice, inducing IFN-γ/IL-2/TNF-α response of CD4+ and CD8+ T cells. Additionally, DNA-immunization with TERT enhanced cellular immune response against luciferase encoded by a co-delivered plasmid (Luc DNA). However, DNA-immunization with Core152opt and TERT mix resulted in abrogation of immune response against both components. A loss of bioluminescence signal after co-delivery of TERT and Luc DNA into mice indicated that TERT affects the in vivo expression of luciferase directed by the immediate early cytomegalovirus and interferon-β promoters. Panel of mutant TERT variants was created and tested for their expression effects. TERT with deleted N-terminal nucleoli localization signal and mutations abrogating telomerase activity still suppressed the IFN-β driven Luc expression, while the inactivated reverse transcriptase domain of TERT and its analogue, enzymatically active HIV-1 reverse transcriptase, exerted only weak suppressive effects, implying that suppression relied on the presence of the full-length/nearly full-length TERT, but not its enzymatic activity. The effect(s) could be due to interference of the ectopically expressed xenogeneic rat TERT with biogenesis of mRNA, ribosomes and protein translation in murine cells, affecting the expression of immunogens. HCV core can aggravate this effect, leading to early apoptosis of co-expressing cells, preventing the induction of immune response.

Analysis of the Role of TpUB05 Antigen from Theileria parva in Immune Responses to Malaria in Humans Compared to Its Homologue in Plasmodium falciparum the UB05 Antigen

Despite the amount of resources deployed and the technological advancements in molecular biology, vaccinology, immunology, genetics, and biotechnology, there are still no effective vaccines against malaria. Immunity to malaria is usually seen to be species- and/or strain-specific. However, there is a growing body of evidence suggesting the possibility of the existence of cross-strain, cross-species, and cross-genus immune responses in apicomplexans. The principle of gene conservation indicates that homologues play a similar role in closely related organisms. The homologue of UB05 in Theileria parva is TpUB05 (XP_763711.1), which has been tested and shown to be associated with protective immunity in East Coast fever. In a bid to identify potent markers of protective immunity to aid malaria vaccine development, TpUB05 was tested in malaria caused by Plasmodium falciparum. It was observed that TpUB05 was better at detecting antigen-specific antibodies in plasma compared to UB05 when tested by ELISA. The total IgG raised against TpUB05 was able to block parasitic growth in vitro more effectively than that raised against UB05. However, there was no significant difference between the two study antigens in recalling peripheral blood mononuclear cell (PBMC) memory through IFN-γ production. This study suggests, for the first time, that TpUB05 from T. parva cross-reacts with UB05 from P. falciparum and is a marker of protective immunity in malaria. Hence, TpUB05 should be considered for possible development as a potential subunit vaccine candidate against malaria.

Novel Plasmodium antigens identified via genome-based antibody screen induce protection associated with polyfunctional T cell responses

The development of vaccines against complex intracellular pathogens, such as Plasmodium spp., where protection is likely mediated by cellular immune responses, has proven elusive. The availability of whole genome, proteome and transcriptome data has the potential to advance rational vaccine development but yet there are no licensed vaccines against malaria based on antigens identified from genomic data. Here, we show that the Plasmodium yoelii orthologs of four Plasmodium falciparum proteins identified by an antibody-based genome-wide screening strategy induce a high degree of sterile infection-blocking protection against sporozoite challenge in a stringent rodent malaria model. Protection increased in multi-antigen formulations. Importantly, protection was highly correlated with the induction of multifunctional triple-positive T cells expressing high amounts of IFN-γ, IL-2 and TNF. These data demonstrate that antigens identified by serological screening are targets of multifunctional cellular immune responses that correlate with protection. Our results provide experimental validation for the concept of rational vaccine design from genomic sequence data.

Enhancing protective immunity to malaria with a highly immunogenic virus-like particle vaccine

The leading malaria vaccine in development is the circumsporozoite protein (CSP)-based particle vaccine, RTS,S, which targets the pre-erythrocytic stage of Plasmodium falciparum infection. It induces modest levels of protective efficacy, thought to be mediated primarily by CSP-specific antibodies. We aimed to enhance vaccine efficacy by generating a more immunogenic CSP-based particle vaccine and therefore developed a next-generation RTS,S-like vaccine, called R21. The major improvement is that in contrast to RTS,S, R21 particles are formed from a single CSP-hepatitis B surface antigen (HBsAg) fusion protein, and this leads to a vaccine composed of a much higher proportion of CSP than in RTS,S. We demonstrate that in BALB/c mice R21 is immunogenic at very low doses and when administered with the adjuvants Abisco-100 and Matrix-M it elicits sterile protection against transgenic sporozoite challenge. Concurrent induction of potent cellular and humoral immune responses was also achieved by combining R21 with TRAP-based viral vectors and protective efficacy was significantly enhanced. In addition, in contrast to RTS,S, only a minimal antibody response to the HBsAg carrier was induced. These studies identify an anti-sporozoite vaccine component that may improve upon the current leading malaria vaccine RTS,S. R21 is now under evaluation in Phase 1/2a clinical trials.

Recombinant Mip-PilE-FlaA dominant epitopes vaccine candidate against Legionella pneumophila

Legionella pneumophila is the main causative agent of Legionnaires' disease, which is a severe multi-system disease with pneumonia as the primary manifestation. We designed a recombinant Mip-PilE-FlaA dominant epitopes vaccine against Legionella pneumophila to prevent the disease and evaluated its immunogenicity and protective immunity. The protein structures of Mip, PilE and FlaA were analyzed using a computer, and the gene sequences of the dominant epitopes of the three proteins were selected to construct and optimize the vaccine. The optimized mip, pilE, flaA and recombinant mip-pilE-flaA gene sequences were cloned, expressed and purified. The purified proteins were used as dominant epitopes vaccines to immunize BALB/c mice and determine the protective immunity and immunogenicity of these purified proteins. The identification confirmed that the recombinant mip-pilE-flaA was successfully cloned and expressed. ELISA revealed that the Mip-PilE-FlaA group produced the highest IgG response, and this protein may considerably improve the production of some cytokines in BALB/c mice. Histopathology analyses of lungs from mice immunized with Mip-PilE-FlaA revealed a certain protective effect. Our work demonstrated that the recombinant dominant epitopes of Mip-PilE-FlaA exhibited strong immunogenicity and immune protection, and this protein may be an efficient epitopes vaccine candidate against Legionella pneumophila.

Assessment of the Plasmodium falciparum Preerythrocytic Antigen UIS3 as a Potential Candidate for a Malaria Vaccine

Efforts are under way to improve the efficacy of subunit malaria vaccines through assessments of new adjuvants, vaccination platforms, and antigens. In this study, we further assessed the Plasmodium falciparum antigen upregulated in infective sporozoites 3 (PfUIS3) as a vaccine candidate. PfUIS3 was expressed in the viral vectors chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) and used to immunize mice in a prime-boost regimen. We previously demonstrated that this regimen could provide partial protection against challenge with chimeric P. berghei parasites expressing PfUIS3. We now show that ChAd63-MVA PfUIS3 can also provide partial cross-species protection against challenge with wild-type P. berghei parasites. We also show that PfUIS3-specific cellular memory responses could be recalled in human volunteers exposed to P. falciparum parasites in a controlled human malaria infection study. When ChAd63-MVA PfUIS3 was coadministered with the vaccine candidate P. falciparum thrombospondin-related adhesion protein (PfTRAP) expressed in the ChAd63-MVA system, there was no significant change in immunogenicity to either vaccine. However, when mice were challenged with double chimeric P. berghei-P. falciparum parasites expressing both PfUIS3 and PfTRAP, vaccine efficacy was improved to 100% sterile protection. This synergistic effect was evident only when the two vaccines were mixed and administered at the same site. We have therefore demonstrated that vaccination with PfUIS3 can induce a consistent delay in patent parasitemia across mouse strains and against chimeric parasites expressing PfUIS3 as well as wild-type P. berghei; when this vaccine is combined with another partially protective regimen (ChAd63-MVA PfTRAP), complete protection is induced.

Differential T-cell responses to a chimeric Plasmodium falciparum antigen; UB05-09, correlates with acquired immunity to malaria

The development of a sterilizing and cost-effective vaccine against malaria remains a major problem despite recent advances. In this study, it is demonstrated that two antigens of P. falciparum UB05, UB09 and their chimera UB05-09 can serve as protective immunity markers by eliciting higher T-cell responses in malaria semi-immune subjects (SIS) than in frequently sick subjects (FSS) and could be used to distinguish these two groups. UB05, UB09 and UB05-09 were cloned, expressed in E. coli, purified and used to stimulate PBMCs isolated from 63 subjects in a malaria endemic area, for IFN-γ production, which was measured by the ELISpot assay. The polymorphism of UB09 gene in the malaria infected population was also studied by PCR/sequencing of the gene in P. falciparum field isolates. All three antigens were preferentially recognized by PBMCs from SIS. IFN-γ production induced by these antigens correlated with the absence of fever and parasitaemia. UB09 was shown to be relatively well-conserved in nature. It is concluded that UB05, UB09 and the chimera UB05-09 posses T-cell epitopes that are associated with protection against malaria and could thus be used to distinguish SIS from FSS eventhough acute infection with malaria has been shown to reduce cytokine production in some studies. Further investigations of these antigens as potential diagnostic and/or vaccine candidates for malaria are indicated.

Strategic Priming with Multiple Antigens can Yield Memory Cell Phenotypes Optimized for Infection with Mycobacterium tuberculosis: A Computational Study

Lack of an effective vaccine results in 9 million new cases of tuberculosis (TB) every year and 1.8 million deaths worldwide. Although many infants are vaccinated at birth with BCG (an attenuated M. bovis), this does not prevent infection or development of TB after childhood. Immune responses necessary for prevention of infection or disease are still unknown, making development of effective vaccines against TB challenging. Several new vaccines are ready for human clinical trials, but these trials are difficult and expensive; especially challenging is determining the appropriate cellular response necessary for protection. The magnitude of an immune response is likely key to generating a successful vaccine. Characteristics such as numbers of central memory (CM) and effector memory (EM) T cells responsive to a diverse set of epitopes are also correlated with protection. Promising vaccines against TB contain mycobacterial subunit antigens (Ag) present during both active and latent infection. We hypothesize that protection against different key immunodominant antigens could require a vaccine that produces different levels of EM and CM for each Ag-specific memory population. We created a computational model to explore EM and CM values, and their ratio, within what we term Memory Design Space. Our model captures events involved in T cell priming within lymph nodes and tracks their circulation through blood to peripheral tissues. We used the model to test whether multiple Ag-specific memory cell populations could be generated with distinct locations within Memory Design Space at a specific time point post vaccination. Boosting can further shift memory populations to memory cell ratios unreachable by initial priming events. By strategically varying antigen load, properties of cellular interactions within the LN, and delivery parameters (e.g., number of boosts) of multi-subunit vaccines, we can generate multiple Ag-specific memory populations that cover a wide range of Memory Design Space. Given a set of desired characteristics for Ag-specific memory populations, we can use our model as a tool to predict vaccine formulations that will generate those populations.

The evolution of poxvirus vaccines

After Edward Jenner established human vaccination over 200 years ago, attenuated poxviruses became key players to contain the deadliest virus of its own family: Variola virus (VARV), the causative agent of smallpox. Cowpox virus (CPXV) and horsepox virus (HSPV) were extensively used to this end, passaged in cattle and humans until the appearance of vaccinia virus (VACV), which was used in the final campaigns aimed to eradicate the disease, an endeavor that was accomplished by the World Health Organization (WHO) in 1980. Ever since, naturally evolved strains used for vaccination were introduced into research laboratories where VACV and other poxviruses with improved safety profiles were generated. Recombinant DNA technology along with the DNA genome features of this virus family allowed the generation of vaccines against heterologous diseases, and the specific insertion and deletion of poxvirus genes generated an even broader spectrum of modified viruses with new properties that increase their immunogenicity and safety profile as vaccine vectors. In this review, we highlight the evolution of poxvirus vaccines, from first generation to the current status, pointing out how different vaccines have emerged and approaches that are being followed up in the development of more rational vaccines against a wide range of diseases.

Baculovirus-vectored multistage Plasmodium vivax vaccine induces both protective and transmission-blocking immunities against transgenic rodent malaria parasites

A multistage malaria vaccine targeting the pre-erythrocytic and sexual stages of Plasmodium could effectively protect individuals against infection from mosquito bites and provide transmission-blocking (TB) activity against the sexual stages of the parasite, respectively. This strategy could help prevent malaria infections in individuals and, on a larger scale, prevent malaria transmission in communities of endemicity. Here, we describe the development of a multistage Plasmodium vivax vaccine which simultaneously expresses P. vivax circumsporozoite protein (PvCSP) and P25 (Pvs25) protein of this species as a fusion protein, thereby acting as a pre-erythrocytic vaccine and a TB vaccine, respectively. A new-concept vaccine platform based on the baculovirus dual-expression system (BDES) was evaluated. The BDES-Pvs25-PvCSP vaccine displayed correct folding of the Pvs25-PvCSP fusion protein on the viral envelope and was highly expressed upon transduction of mammalian cells in vitro. This vaccine induced high levels of antibodies to Pvs25 and PvCSP and elicited protective (43%) and TB (82%) efficacies against transgenic P. berghei parasites expressing the corresponding P. vivax antigens in mice. Our data indicate that our BDES, which functions as both a subunit and DNA vaccine, can offer a promising multistage vaccine capable of delivering a potent antimalarial pre-erythrocytic and TB response via a single immunization regimen.

Differential T-cell responses of semi-immune and susceptible malaria subjects to in silico predicted and synthetic peptides of Plasmodium falciparum

Malaria remains a public health hazard in tropical countries as a consequence of the rise and spread of drug and insecticide resistances; hence the need for a vaccine with widespread application. Protective immunity to malaria is known to be mediated by both antibody and cellular immune responses, though characterization of the latter has been less extensive. The aim of the present investigation was to identify novel T-cell epitopes that may contribute to naturally acquired immune responses against malaria. Using the Microsoft software, Epitome™ T-cell peptide epitopes on 19 Plasmodium falciparum proteins in the Plasmodium Database (www.plasmodb.org.PlasmoDB 9.0) were predicted in-silico. The peptides were synthesized and used to stimulate peripheral blood mononuclear cells (PBMCs) in 14 semi-immune and 21 malaria susceptible subjects for interferon-gamma (IFN-γ) production ex-vivo. The level of IFN-γ production, a marker of T-cell responses, was measured by ELISPOT assay in semi-immune subjects (SIS) and frequently sick subjects (FSS) from an endemic zone with perennial malaria transmission. Of the 19 proteins studied, 17 yielded 27 pools (189 peptides), which were reactive with the subjects' PBMCs when tested for IFN-γ production, taking a stimulation index (SI) of ≥2 as a cutoff point for a positive response. There were 10 reactive peptide pools (constituting eight protein loci) with an SI of 10 or greater. Of the 19 proteins studied, two were known vaccine candidates (MSP-8 and SSP2/TRAP), which reacted both with SIS and FSS. Similarly the hypothetical proteins (PFF1030w, PFE0795c, PFD0880w, PFC0065c and PF10_0052) also reacted strongly with both SIS and FSS making them attractive for further characterization as mediators of protective immunity and/or pathogenesis.

Conserved immunogens in prime-boost strategies for the next-generation HIV-1 vaccines

INTRODUCTION

Effective vaccines are the best solution for stopping the spread of HIV/AIDS and other infectious diseases. Their development and in-depth understanding of pathogen-host interactions rely on technological advances.

AREAS COVERED

Rational vaccine development can be effectively approached by conceptual separation of, on one hand, design of immunogens from improving their presentation to the immune system and, on the other, induction of antibodies from induction of killer CD8(+) T cells. The biggest roadblock for many vaccines is the pathogens' variability. This is best tackled by focusing both antibodies and T cells on the functionally most conserved regions of proteins common to many variants, including escape mutants. For vectored vaccines, these 'universal' subunit immunogens are most efficiently delivered using heterologous prime-boost regimens, which can be further optimised by adjuvantation and route of delivery.

EXPERT OPINION

Development of vaccines against human diseases has many features in common. Acceleration of vaccine discovery depends on basic research and new technologies. Novel strategies should be safely, but rapidly tested in humans. While out-of-the-box thinking is important, vaccine success largely depends on incremental advances best achieved through small, systematic, iterative clinical studies. Failures are inevitable, but the end rewards are huge. The future will be exciting.

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.

Multivalent fusion protein vaccine for lymphatic filariasis

Lymphatic filariasis affects approximately 3% of the whole world population. Mass drug administration is currently the major control strategy to eradicate this infection from endemic regions by year 2020. Combination drug treatments are highly efficient in controlling the infection. However, there are no effective vaccines available for human or animal lymphatic filariasis despite the identification of several subunit vaccines. Lymphatic filariasis parasites are multicellular organisms and potentially use multiple mechanisms to survive in the host. Therefore, there is a need to combine two or more vaccine candidate antigens to achieve the desired effect. In this study we combined three well characterized vaccine antigens of Brugia malayi, heat shock protein 12.6 (HSP12.6), Abundant Larval transcript-2 (ALT-2) and tetraspanin large extra cellular loop (TSP-LEL) as a multivalent fusion vaccine. Putative immune individuals carry circulating antibodies against all three antigens. Depletion of these antigen specific antibodies from the sera samples removed the ability of the sera to participate in the killing of B. malayi L3 in an antibody dependent cellular cytotoxicity (ADCC) mechanism. Vaccination trials in mice with a bivalent [HSP12.6+ALT-2 (HA), HSP12.6+TSP-LEL (HT) or TSP-LEL+ALT-2 (TA)] or trivalent [HSP12.6+ALT-2+TSP-LEL (HAT)] vaccines using DNA, protein or heterologous prime boost regimen showed that trivalent HAT vaccine either as protein alone or as heterologous prime boost vaccine could confer significant protection (95%) against B. malayi L3 challenge. Immune correlates of protection suggest a Th1/Th2 bias. These finding suggests that the trivalent HAT fusion protein is a promising prophylactic vaccine against lymphatic filariasis infection in human.

Protective CD8+ T lymphocytes in primates immunized with malaria sporozoites

Live attenuated malaria vaccines are more potent than the recombinant protein, bacterial or viral platform vaccines that have been tested, and an attenuated sporozoite vaccine against falciparum malaria is being developed for humans. In mice, attenuated malaria sporozoite vaccines induce CD8⁺ T cells that kill parasites developing in the liver. We were curious to know if CD8⁺ T cells were also important in protecting primates against malaria. We immunized 9 rhesus monkeys with radiation attenuated Plasmodium knowlesi sporozoites, and found that 5 did not develop blood stage infections after challenge with live sporozoites. We then injected 4 of these protected monkeys with cM-T807, a monoclonal antibody to the CD8 molecule which depletes T cells. The fifth monkey received equivalent doses of normal IgG. In 3 of the 4 monkeys receiving cM-T807 circulating CD8⁺ T cells were profoundly depleted. When re-challenged with live sporozoites all 3 of these depleted animals developed blood stage malaria. The fourth monkey receiving cM-T807 retained many circulating CD8⁺ T cells. This monkey, and the vaccinated monkey receiving normal IgG, did not develop blood stage malaria at re-challenge with live sporozoites. Animals were treated with antimalarial drugs and rested for 4 months. During this interval CD8⁺ T cells re-appeared in the circulation of the depleted monkeys. When all vaccinated animals received a third challenge with live sporozoites, all 5 monkeys were once again protected and did not develop blood stage malaria infections. These data indicate that CD8⁺ T cells are important effector cells protecting monkeys against malaria sporozoite infection. We believe that malaria vaccines which induce effector CD8+ T cells in humans will have the best chance of protecting against malaria.

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.

Combining liver- and blood-stage malaria viral-vectored vaccines: investigating mechanisms of CD8+ T cell interference

Replication-deficient adenovirus and modified vaccinia virus Ankara (MVA) vectors expressing single pre-erythrocytic or blood-stage Plasmodium falciparum Ags have entered clinical testing using a heterologous prime-boost immunization approach. In this study, we investigated the utility of the same immunization regimen when combining viral vectored vaccines expressing the 42-kDa C terminus of the blood-stage Ag merozoite surface protein 1 and the pre-erythrocytic Ag circumsporozoite protein in the Plasmodium yoelii mouse model. We find that vaccine coadministration leads to maintained Ab responses and efficacy against blood-stage infection, but reduced secondary CD8(+) T cell responses against both Ags and efficacy against liver-stage infection. CD8(+) T cell interference can be minimized by coadministering the MVA vaccines at separate sites, resulting in enhanced liver-stage efficacy in mice immunized against both Ags compared with just one. CD8(+) T cell interference (following MVA coadministration as a mixture) may be caused partly by a lack of physiologic space for high-magnitude responses against multiple Ags, but is not caused by competition for presentation of Ag on MHC class I molecules, nor is it due to restricted T cell access to APCs presenting both Ags. Instead, enhanced killing of peptide-pulsed cells is observed in mice possessing pre-existing T cells against two Ags compared with just one, suggesting that priming against multiple Ags may in part reduce the potency of multiantigen MVA vectors to stimulate secondary CD8(+) T cell responses. These data have important implications for the development of a multistage or multicomponent viral vectored malaria vaccine for use in humans.

Measuring naturally acquired immune responses to candidate malaria vaccine antigens in Ghanaian adults

BACKGROUND

To prepare field sites for malaria vaccine trials, it is important to determine baseline antibody and T cell responses to candidate malaria vaccine antigens. Assessing T cell responses is especially challenging, given genetic restriction, low responses observed in endemic areas, their variability over time, potential suppression by parasitaemia and the intrinsic variability of the assays.

METHODS

In Part A of this study, antibody titres were measured in adults from urban and rural communities in Ghana to recombinant Plasmodium falciparum CSP, SSP2/TRAP, LSA1, EXP1, MSP1, MSP3 and EBA175 by ELISA, and to sporozoites and infected erythrocytes by IFA. Positive ELISA responses were determined using two methods. T cell responses to defined CD8 or CD4 T cell epitopes from CSP, SSP2/TRAP, LSA1 and EXP1 were measured by ex vivo IFN-γ ELISpot assays using HLA-matched Class I- and DR-restricted synthetic peptides. In Part B, the reproducibility of the ELISpot assay to CSP and AMA1 was measured by repeating assays of individual samples using peptide pools and low, medium or high stringency criteria for defining positive responses, and by comparing samples collected two weeks apart.

RESULTS

In Part A, positive antibody responses varied widely from 17%-100%, according to the antigen and statistical method, with blood stage antigens showing more frequent and higher magnitude responses. ELISA titres were higher in rural subjects, while IFA titres and the frequencies and magnitudes of ex vivo ELISpot activities were similar in both communities. DR-restricted peptides showed stronger responses than Class I-restricted peptides. In Part B, the most stringent statistical criteria gave the fewest, and the least stringent the most positive responses, with reproducibility slightly higher using the least stringent method when assays were repeated. Results varied significantly between the two-week time-points for many participants.

CONCLUSIONS

All participants were positive for at least one malaria protein by ELISA, with results dependent on the criteria for positivity. Likewise, ELISpot responses varied among participants, but were relatively reproducible by the three methods tested, especially the least stringent, when assays were repeated. However, results often differed between samples taken two weeks apart, indicating significant biological variability over short intervals.

The RTS,S malaria vaccine

RTS,S is the most advanced candidate vaccine against human malaria. During its remarkable journey from conception and design in the early 1980s to the multicenter Phase 3 trial currently underway across sub-Saharan Africa, RTS,S has overcome tremendous challenges and disproved established vaccine paradigms. In the last several years, Phase 2 studies conducted in infants and children in endemic areas have established the efficacy of RTS,S for reducing morbidity due to clinical malaria. If the results are realized in the Phase 3 trial, the chances for licensure in the near future appear high. Such progress is all the more remarkable given our lack of clear understanding regarding how the vaccine activates the human immune system, the immune correlates of protection or the mechanism whereby a vaccine targeting sporozoites and liver stage parasites can reduce the clinical disease associated with parasitemia. These unanswered questions pose important challenges to be addressed in the quest to understand the protection afforded by RTS,S and to build a more efficacious second generation vaccine against malaria. This review will focus on current knowledge about the protective efficacy of RTS,S and what we have learned regarding its impact on the human immune system.

DNA vaccines: a rational design against parasitic diseases

Parasitic diseases are one of the most devastating causes of morbidity and mortality worldwide. Although immunization against these infections would be an ideal solution, the development of effective vaccines has been hampered by specific challenges posed by parasitic pathogens. Plasmid-based DNA vaccines may prove to be promising immunization tools in this area because vectors can be designed to integrate several antigens from different stages of the parasite life cycle or different subspecies; vaccines, formulations and immunization protocols can be tuned to match the immune response that offers protective immunity; and DNA vaccination is an affordable platform for developing countries. Partial and full protective immunity have been reported following DNA vaccination against the most significant parasitic diseases in the world.

Nanotechnology solutions for mucosal immunization

The current prevalence of infectious diseases in many developing regions of the world is a serious burden, impacting both the general health as well as economic growth of these communities. Additionally, treatment with conventional medication becomes increasingly challenging due to emergence of new and drug resistant strains jeopardizing the progress made in recent years towards control and elimination of certain types of infectious diseases. Thus, from a public health perspective, prevention such as through immunization by vaccination, which has proven to be most effective, might be the best alternative to prevent and combat infectious diseases in these regions. To achieve this, development of wide-scale immunization programs become necessary including vaccines that can easily and widely be distributed, stored and administered. Mucosal vaccines offer great potential since they can be administered via oral or intranasal delivery route which does not require trained personnel, avoids the use of needles and improves overall patient compliance and acceptance. However, it necessitates the implementation of specific immunization strategies to improve their efficacy. Application of nanotechnology to design and create particle mediated delivery systems that can efficiently encapsulate vaccine components for protection of the sensitive payload, target the mucosal immune system and incorporate mucosal adjuvants maximizing immune response is key strategy to improve the effectiveness of mucosal vaccines.

Evaluation of the safety and immunogenicity of Plasmodium falciparum apical membrane antigen 1, merozoite surface protein 1 or RTS,S vaccines with adjuvant system AS02A administered alone or concurrently in rhesus monkeys

In an effort to broaden the immune response induced by the RTS,S/AS02(A),vaccine, we have evaluated the immunogenicity of the RTS,S antigen when combined with MSP1(42) and with AMA1, antigens derived from the asexual blood stage. The objectives of this study were (i) to determine whether MSP1(42) and AMA1 vaccines formulated with the AS02(A) Adjuvant System were safe and immunogenic in the rhesus monkey model; (ii) to investigate whether MSP1(42) or AMA1 induced immune interference to each other, or to RTS,S, when added singly or in combinations at a single injection site; (iii) in the event of immune interference, to determine if this could be reduced when antigens were administered at separate sites. We found that MSP1(42) and AMA1 were safe and immunogenic, eliciting antibodies, and Th1 and Th2 responses using IFN-gamma and IL-5 as markers. When malaria antigens were delivered together in one formulation, MSP1(42) and RTS,S reduced AMA1-specific antibody responses as measured by ELISA however, only MSP1(42) lowered parasite growth inhibitory activity of anti-AMA1 antibodies as measured by in vitro growth inhibition assay. Unlike RTS,S, MSP1(42) significantly reduced AMA1 IFN-gamma and IL-5 responses. MSP1(42) suppression of AMA1 IFN-gamma responses was not seen in animals receiving RTS,S+AMA1+MSP1(42) suggesting that RTS,S restored IFN-gamma responses. Conversely, AMA1 had no effect on MSP1(42) antibody and IFN-gamma and IL-5 responses. Neither AMA1 alone or combined with MSP1(42) affected RTS,S antibody or IFN-gamma and IL-5 responses. Immune interference by MSP1(42) on AMA1 antibody responses was also evident when AMA1, MSP1(42) and RTS,S were administered concurrently at separate sites. These results suggest that immune interference may be complex and should be considered for the design of multi-antigen, multi-stage vaccines against malaria.

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.