[Show abstract][Hide abstract] ABSTRACT: Controlled human malaria infection is used to measure efficacy of candidate malaria vaccines before field studies are undertaken. Mathematical modelling using data from quantitative PCR (qPCR) parasitemia monitoring can discriminate between vaccine effects upon the parasite's liver and blood stages. Uncertainty regarding the most appropriate modelling method hinders interpretation of such trials.We used qPCR data from 267 Plasmodium falciparum infections to compare linear, sine-wave, and normal-cumulative-density-function models. We find that the parameters estimated by these models are closely correlated, and their predictive accuracy for omitted data points was similar. We propose that future studies include the linear model.
The Journal of Infectious Diseases 04/2013; · 5.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Vaccine development in human Plasmodium falciparum malaria has been hampered by the exceptionally high levels of CD8(+) T cells required for efficacy. Use of potently immunogenic human adenoviruses as vaccine vectors could overcome this problem, but these are limited by preexisting immunity to human adenoviruses.
From 2007 to 2010, we undertook a phase I dose and route finding study of a new malaria vaccine, a replication-incompetent chimpanzee adenovirus 63 (ChAd63) encoding the preerythrocytic insert multiple epitope thrombospondin-related adhesion protein (ME-TRAP; n = 54 vaccinees) administered alone (n = 28) or with a modified vaccinia virus Ankara (MVA) ME-TRAP booster immunization 8 weeks later (n = 26). We observed an excellent safety profile. High levels of TRAP antigen-specific CD8(+) and CD4(+) T cells, as detected by interferon γ enzyme-linked immunospot assay and flow cytometry, were induced by intramuscular ChAd63 ME-TRAP immunization at doses of 5 × 10(10) viral particles and above. Subsequent administration of MVA ME-TRAP boosted responses to exceptionally high levels, and responses were maintained for up to 30 months postvaccination.
The ChAd63 chimpanzee adenovirus vector appears safe and highly immunogenic, providing a viable alternative to human adenoviruses as vaccine vectors for human use.
The Journal of Infectious Diseases 03/2012; 205(5):772-81. · 5.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Naturally acquired immunity to Plasmodium falciparum's asexual blood stage reduces parasite multiplication at microscopically detectable densities. The effect of natural immunity on initial prepatent parasite multiplication during the period following a new infection has been uncertain, contributing to doubt regarding the utility of experimental challenge models for blood-stage vaccine trials. Here we present data revealing that parasite multiplication rates during the initial prepatent period in semi-immune Gambian adults are substantially lower than in malaria-naive participants. This supports the view that a blood-stage vaccine capable of emulating the disease-reducing effect of natural immunity could achieve a detectable effect during the prepatent period.
The Journal of Infectious Diseases 05/2011; 203(9):1337-40. · 5.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We examined the safety, immunogenicity and efficacy of a prime-boost vaccination regime involving two poxvirus malaria subunit vaccines, FP9-PP and MVA-PP, expressing the same polyprotein consisting of six pre-erythrocytic antigens from Plasmodium falciparum. Following safety assessment of single doses, 15 volunteers received a heterologous prime-boost vaccination regime and underwent malaria sporozoite challenge. The vaccines were safe but interferon-γ ELISPOT responses were low compared to other poxvirus vectors, despite targeting multiple antigens. There was no vaccine efficacy as measured by delay in time to parasitaemia. A number of possible explanations are discussed, including the very large insert size of the polyprotein transgene.
[Show abstract][Hide abstract] ABSTRACT: Vaccination against Plasmodium falciparum malaria could reduce the worldwide burden of this disease, and decrease its high mortality in children. Replication-defective recombinant adenovirus vectors carrying P. falciparum epitopes may be useful as part of a vaccine that raises cellular immunity to the pre-erythrocytic stage of malaria infection. However, existing immunity to the adenovirus vector results in antibody-mediated neutralization of the vaccine vector, and reduced vaccine immunogenicity. Our aim was to examine a population of children who are at risk from P. falciparum malaria for neutralizing immunity to replication-deficient recombinant chimpanzee adenovirus 63 vector (AdC63), compared to human adenovirus 5 vector (AdHu5). We measured 50% and 90% vector neutralization titers in 200 individual sera, taken from a cohort of children from Kenya, using a secreted alkaline phosphatase neutralization assay. We found that 23% of the children (aged 1-6 years) had high-titer neutralizing antibodies to AdHu5, and 4% had high-titer neutralizing antibodies to AdC63. Immunity to both vectors was age-dependent. Low-level neutralization of AdC63 was significantly less frequent than AdHu5 neutralization at the 90% neutralization level. We conclude that AdC63 may be a useful vector as part of a prime-boost malaria vaccine in children.
[Show abstract][Hide abstract] ABSTRACT: There is increasing interest in malaria vaccines targeting the asexual blood stage of Plasmodium falciparum. Without accepted immunologic correlates of clinical protection, challenge studies are useful for assessing the efficacy of candidate vaccines in vivo in healthy volunteers. We report a pilot study of a safe and robust challenge protocol using a blood-stage inoculum. We have applied well-validated trial endpoints and twice daily real-time quantitative polymerase chain reaction monitoring of parasitemia to blood-stage challenge, which enabled direct comparison with sporozoite challenge. We found that greater accuracy in quantification of blood-stage growth rates can be achieved with blood-stage challenge. This finding may provide greater power to detect partial efficacy of many blood-stage candidate vaccines. We discuss the potential utility of blood-stage challenge studies in accelerating malaria vaccine development.
The American journal of tropical medicine and hygiene 07/2008; 78(6):878-83. · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Previous research indicates that a combination vaccine targeting different stages of the malaria life cycle is likely to provide the most effective malaria vaccine. This trial was the first to combine two existing vaccination strategies to produce a vaccine that induces immune responses to both the pre-erythrocytic and blood stages of the P. falciparum life cycle.
This was a Phase I/IIa study of a new combination malaria vaccine FFM ME-TRAP+PEV3A. PEV3A includes peptides from both the pre-erythrocytic circumsporozoite protein and the blood-stage antigen AMA-1. This study was conducted at the Centre for Clinical Vaccinology and Tropical Medicine, University of Oxford, Oxford, UK. The participants were healthy, malaria naïve volunteers, from Oxford. The interventions were vaccination with PEV3A alone, or PEV3A+FFM ME-TRAP. The main outcome measure was protection from malaria in a sporozoite challenge model. Other outcomes included measures of parasite specific immune responses induced by either vaccine; and safety, assessed by collection of adverse event data.
We observed evidence of blood stage immunity in PEV3A vaccinated volunteers, but no volunteers were completely protected from malaria. PEV3A induced high antibody titres, and antibodies bound parasites in immunofluorescence assays. Moreover, we observed boosting of the vaccine-induced immune response by sporozoite challenge. Immune responses induced by FFM ME-TRAP were unexpectedly low. The vaccines were safe, with comparable side effect profiles to previous trials. Although there was no sterile protection two major observations support an effect of the vaccine-induced response on blood stage parasites: (i) Lower rates of parasite growth were observed in volunteers vaccinated with PEV3A compared to unvaccinated controls (p = 0.012), and this was reflected in the PCR results from PEV3A vaccinated volunteers. These showed early control of parasitaemia by some volunteers in this group. One volunteer, who received PEV3A alone, was diagnosed very late, on day 20 compared to an average of 11.8 days in unvaccinated controls. (ii). Morphologically abnormal parasites were present in the blood of all (n = 24) PEV3A vaccinated volunteers, and in only 2/6 controls (p = 0.001). We describe evidence of vaccine-induced blood stage efficacy for the first time in a sporozoite challenge study.
PLoS ONE 02/2008; 3(1):e1493. · 3.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is well documented that the density of Plasmodium in its vertebrate host modulates the physiological response induced; this in turn regulates parasite survival and transmission. It is less clear that parasite density in the mosquito regulates survival and transmission of this important pathogen. Numerous studies have described conversion rates of Plasmodium from one life stage to the next within the mosquito, yet few have considered that these rates might vary with parasite density. Here we establish infections with defined numbers of the rodent malaria parasite Plasmodium berghei to examine how parasite density at each stage of development (gametocytes; ookinetes; oocysts and sporozoites) influences development to the ensuing stage in Anopheles stephensi, and thus the delivery of infectious sporozoites to the vertebrate host. We show that every developmental transition exhibits strong density dependence, with numbers of the ensuing stages saturating at high density. We further show that when fed ookinetes at very low densities, oocyst development is facilitated by increasing ookinete number (i.e., the efficiency of ookinete-oocyst transformation follows a sigmoid relationship). We discuss how observations on this model system generate important hypotheses for the understanding of malaria biology, and how these might guide the rational analysis of interventions against the transmission of the malaria parasites of humans by their diverse vector species.
[Show abstract][Hide abstract] ABSTRACT: The feasibility of using a sensitive polymerase chain reaction (PCR) to evaluate malaria vaccines in small group sizes was tested in 102 adult Gambian volunteers who received either the malaria vaccine regimen FP9 ME-TRAP/MVA ME-TRAP or rabies vaccine. All volunteers received the antimalarial drugs primaquine and Lapdap plus artesunate to eliminate malaria parasites. Volunteers in a further group received an additional single treatment with sulfadoxine-pyrimethamine (SP) to prevent new infections. There was substantially lower T-cell immunogenicity than in previous trials with this vaccine regimen and no protection against infection in the malaria vaccine group. Using the primary endpoint of 20 parasites per mL, no difference was found in the prevalence of low-level infections in volunteers who received SP compared with those who did not, indicating that SP did not reduce the incidence of very low-density infection. However, SP markedly reduced the incidence of higher density infections. These findings support the feasibility and potential of this approach to screen pre-erythrocytic vaccines for efficacy against infection in small numbers of vaccinees in endemic areas.
The American journal of tropical medicine and hygiene 04/2007; 76(3):486-93. · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Taking advantage of a sporozoite challenge model established to evaluate the efficacy of new malaria vaccine candidates, we have explored the kinetics of systemic cytokine responses during the prepatent period of Plasmodium falciparum infection in 18 unvaccinated, previously malaria-naive subjects, using a highly sensitive, bead-based multiplex assay, and relate these data to peripheral parasite densities as measured by quantitative real-time PCR. These data are complemented with the analysis of cytokine production measured in vitro from whole blood or PBMC, stimulated with P. falciparum-infected RBC. We found considerable qualitative and quantitative interindividual variability in the innate responses, with subjects falling into three groups according to the strength of their inflammatory response. One group secreted moderate levels of IFN-gamma and IL-10, but no detectable IL-12p70. A second group produced detectable levels of circulating IL-12p70 and developed very high levels of IFN-gamma and IL-10. The third group failed to up-regulate any significant proinflammatory responses, but showed the highest levels of TGF-beta. Proinflammatory responses were associated with more rapid control of parasite growth but only at the cost of developing clinical symptoms, suggesting that the initial innate response may have far-reaching consequences on disease outcome. Furthermore, the in vitro observations on cytokine kinetics presented here, suggest that intact schizont-stage infected RBC can trigger innate responses before rupture of the infected RBC.
The Journal of Immunology 11/2006; 177(8):5736-45. · 5.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The safety, immunogenicity, and efficacy of DNA and modified vaccinia virus Ankara (MVA) prime-boost regimes were assessed by using either thrombospondin-related adhesion protein (TRAP) with a multiple-epitope string ME (ME-TRAP) or the circumsporozoite protein (CS) of Plasmodium falciparum. Sixteen healthy subjects who never had malaria (malaria-naive subjects) received two priming vaccinations with DNA, followed by one boosting immunization with MVA, with either ME-TRAP or CS as the antigen. Immunogenicity was assessed by ex vivo gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT) and antibody assay. Two weeks after the final vaccination, the subjects underwent P. falciparum sporozoite challenge, with six unvaccinated controls. The vaccines were well tolerated and immunogenic, with the DDM-ME TRAP regimen producing stronger ex vivo IFN-gamma ELISPOT responses than DDM-CS. One of eight subjects receiving the DDM-ME TRAP regimen was completely protected against malaria challenge, with this group as a whole showing significant delay to parasitemia compared to controls (P = 0.045). The peak ex vivo IFN-gamma ELISPOT response in this group correlated strongly with the number of days to parasitemia (P = 0.033). No protection was observed in the DDM-CS group. Prime-boost vaccination with DNA and MVA encoding ME-TRAP but not CS resulted in partial protection against P. falciparum sporozoite challenge in the present study.
Infection and Immunity 11/2006; 74(10):5933-42. · 4.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Heterologous prime-boost immunization with DNA and various recombinant poxviruses encoding malaria antigens is capable of inducing strong cell-mediated immune responses and partial protection in human sporozoite challenges. Here we report a series of trials assessing recombinant fowlpox virus and modified vaccinia virus Ankara encoding the Plasmodium falciparum circumsporozoite protein in various prime-boost combinations, doses, and application routes. For the first time, these vaccines were administered intramuscularly and at doses of up to 5 x 10(8) PFU. Vaccines containing this antigen proved safe and induced modest immune responses but showed no evidence of efficacy in a sporozoite challenge.
Infection and Immunity 06/2006; 74(5):2706-16. · 4.07 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: T-cell-mediated responses against the liver-stage of Plasmodium falciparum are critical for protection in the human irradiated sporozoite model and several animal models. Heterologous prime-boost approaches, employing plasmid DNA and viral vector delivery of malarial DNA sequences, have proved particularly promising for maximising T-cell-mediated protection in animal models. The T-cell responses induced by this prime-boost regime, in animals and humans, are substantially greater than the sum of the responses induced by DNA or MVA vaccines used alone, leading to the term introduced here of "synergistic" prime-boost immunisation. The insert in our first generation clinical constructs is known as multiple epitope-thrombospondin-related adhesion protein (ME-TRAP). We have performed an extensive series of phase I/II trials evaluating various prime-boost combination regimens for delivery of ME-TRAP in over 500 malaria-naïve and malaria-exposed individuals. The three delivery vectors are DNA, modified vaccinia virus Ankara (MVA) and, more recently, fowlpox strain 9 (FP9). Administration was intra-epidermal and intramuscular for DNA and intradermal for MVA and FP9. Doses of DNA ranged from 4 microg to 2mg. Doses of MVA were up to 1.5 x 10(8) plaque forming units (pfu) and of FP9, up to 1.0 x 10(8)pfu. Further trials employing bacille Calmette-Guérin (BCG) as the priming agent and MVA expressing antigen 85A of Mycobacterium tuberculosis as the boosting agent has extended the scope of synergistic prime-boost vaccination. In this review we summarise the safety, immunogenicity and efficacy results from these malaria and tuberculosis vaccine clinical trials.
[Show abstract][Hide abstract] ABSTRACT: Heterologous prime-boost immunisation with RTS,S/AS02A and the poxvirus MVA-CS was evaluated in 18 healthy malaria-naïve subjects in Oxford. Both priming with RTS,S and boosting MVA-CS, and the reverse, were found to be safe and well tolerated. T cell responses as measured by IFN-gamma ex vivo ELISPOT were induced, but the responses were low to moderate in both groups, with heterologous boosting yielding only small increments in T cell immunogenicity and no increased antibody response. Protection against 3D7 Plasmodium falciparum sporozoite challenge 4 weeks after the final vaccination was equal for both regimens at 33% (95% C.I. 4.3-77.7%), with one subject remaining fully protected on rechallenge at 5 months.
[Show abstract][Hide abstract] ABSTRACT: Thick blood films are routinely used to diagnose Plasmodium falciparum malaria. Here, they were used to diagnose volunteers exposed to experimental malaria challenge.
The frequency with which blood films were positive at given parasite densities measured by PCR were analysed. The poisson distribution was used to calculate the theoretical likelihood of diagnosis. Further in vitro studies used serial dilutions to prepare thick films from malaria cultures at known parasitaemia.
Even in expert hands, thick blood films were considerably less sensitive than might have been expected from the parasite numbers measured by quantitative PCR. In vitro work showed that thick films prepared from malaria cultures at known parasitaemia consistently underestimated parasite densities.
It appears large numbers of parasites are lost during staining. This limits their sensitivity, and leads to erroneous estimates of parasite density.
[Show abstract][Hide abstract] ABSTRACT: Understanding the regulation of immune responses is central for control of autoimmune and infectious disease. In murine models of autoimmunity and chronic inflammatory disease, potent regulatory T lymphocytes have recently been characterized. Despite an explosion of interest in these cells, their relevance to human disease has been uncertain. In a longitudinal study of malaria sporozoite infection via the natural route, we provide evidence that regulatory T cells have modifying effects on blood-stage infection in vivo in humans. Cells with the characteristics of regulatory T cells are rapidly induced following blood-stage infection and are associated with a burst of TGF-beta production, decreased proinflammatory cytokine production, and decreased antigen-specific immune responses. Both the production of TGF-beta and the presence of CD4+CD25+FOXP3+ regulatory T cells are associated with higher rates of parasite growth in vivo. P. falciparum-mediated induction of regulatory T cells may represent a parasite-specific virulence factor.
[Show abstract][Hide abstract] ABSTRACT: The demand for an effective malaria vaccine is high, with millions of people being affected by the disease every year. A large variety of potential vaccines are under investigation worldwide, and when tested in clinical trials, researchers need to extract as much data as possible from every vaccinated and control volunteer. The use of quantitative real-time polymerase chain reaction (PCR), carried out in real-time during the clinical trials of vaccines designed to act against the liver stage of the parasite's life cycle, provides more information than the gold standard method of microscopy alone and increases both safety and accuracy. PCR can detect malaria parasites in the blood up to 5 days before experienced microscopists see parasites on blood films, with a sensitivity of 20 parasites/mL blood. This PCR method has so far been used to follow 137 vaccinee and control volunteers in Phase IIa trials in Oxford and on 220 volunteer samples during a Phase IIb field trial in The Gambia.
The American journal of tropical medicine and hygiene 08/2005; 73(1):191-8. · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Malaria is a major global health problem for which an effective vaccine is required urgently. Prime-boost vaccination regimes involving plasmid DNA and recombinant modified vaccinia virus Ankara-encoding liver-stage malaria antigens have been shown to be powerfully immunogenic for T cells and capable of inducing partial protection against experimental malaria challenge in humans, manifested as a delay in time to patent parasitemia. Here, we report that substitution of plasmid DNA as the priming vector with a specific attenuated recombinant fowlpox virus, FP9, vaccine in such prime-boost regimes can elicit complete sterile protection that can last for 20 months. Protection at 20 months was associated with persisting memory but not effector T cell responses. The protective efficacy of various immunization regimes correlated with the magnitude of induced immune responses, supporting the strategy of maximizing durable T cell immunogenicity to develop more effective liver-stage vaccines against Plasmodium falciparum malaria.
Proceedings of the National Academy of Sciences 04/2005; 102(13):4836-41. · 9.81 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We calculated the number and growth rate of Plasmodium falciparum parasites emerging in recipients of candidate preerythrocytic malaria vaccines and unvaccinated control subjects undergoing mosquito-bite challenge. This was done to measure vaccine efficacy and to distinguish the effects on blood-stage multiplication from those on liver-stage parasites. Real-time polymerase chain reaction measurements of parasite densities were analyzed by nonlinear regression and mixed-effects models. Substantial reductions in numbers of liver parasites resulted from the use of 2 immunization regimens: FP9 boosted by modified virus Ankara (MVA) encoding the malaria epitope-thrombospondin-related adhesion protein insert (92% reduction) and RTS,S/AS02 used in heterologous prime-boost immunization regimens, with MVA encoding the circumsporozoite protein (97% reduction). Forty-eight-hour growth rates in blood from control subjects were not different from those in blood from any vaccination group (mean, 14.4-fold [95% confidence interval, 11-19-fold]).
The Journal of Infectious Diseases 03/2005; 191(4):619-26. · 5.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ICC-1132, a recombinant virus-like particle comprising of a modified hepatitis B core protein with a B cell (NANP) and two T cell epitopes of Plasmodium falciparum circumsporozoite protein (CSP), was administered i.m. as a single 50 microg dose in Seppic ISA 720 to 11 volunteers. Local reactogenicity and systemic side effects were acceptable with the predominant finding being mild pain at the injection site. This regimen induced anti-NANP antibodies in 10/11 and modest T cell responses. There was no evidence of protection from experimental challenge with P. falciparum sporozoites. Other formulations and/or multi-dose regimens will be required to enhance the immunogenicity and efficacy of ICC-1132.