Safety, immunogenicity and efficacy of a pre-erythrocytic malaria candidate vaccine, ICC-1132 formulated in Seppic ISA 720

Immunoinformatics-guided design of a multi-valent vaccine against Rotavirus and Norovirus (ChRNV22)

Rotavirus (RV) and Norovirus (NV) are the main viral etiologic agents of acute gastroenteritis (AG), a serious pediatric condition associated with significant death rates and long-term complications. Anti-RV vaccination has been proved efficient in the reduction of severe AG worldwide, however, the available vaccines are all attenuated and have suboptimal efficiencies in developing countries, where AG leads to substantial disease burden. On the other hand, no NV vaccine has been licensed so far. Therefore, we used immunoinformatics tools to develop a multi-epitope vaccine (ChRNV22) to prevent severe AG by RV and NV. Epitopes were predicted against 17 prevalent genotypes of four structural proteins (NV's VP1, RV's VP4, VP6 and VP7), and then assembled in a chimeric protein, with two small adjuvant sequences (tetanus toxin P2 epitope and a conserved sequence of RV's enterotoxin, NSP4). Simulations of the immune response and interactions with immune receptors indicated the immunogenic properties of ChRNV22, including a Th1-biased response. In silico search for putative host-homologous, allergenic and toxic regions also indicated the vaccine safety. In summary, we developed a multi-epitope vaccine against different NV and RV genotypes that seems promising for the prevention of severe AG, which will be further assessed by in vivo tests.

Controlled Human Infection Models To Accelerate Vaccine Development

The timelines for developing vaccines against infectious diseases are lengthy, and often vaccines that reach the stage of large phase 3 field trials fail to provide the desired level of protective efficacy. The application of controlled human challenge models of infection and disease at the appropriate stages of development could accelerate development of candidate vaccines and, in fact, has done so successfully in some limited cases. Human challenge models could potentially be used to gather critical information on pathogenesis, inform strain selection for vaccines, explore cross-protective immunity, identify immune correlates of protection and mechanisms of protection induced by infection or evoked by candidate vaccines, guide decisions on appropriate trial endpoints, and evaluate vaccine efficacy. We prepared this report to motivate fellow scientists to exploit the potential capacity of controlled human challenge experiments to advance vaccine development. In this review, we considered available challenge models for 17 infectious diseases in the context of the public health importance of each disease, the diversity and pathogenesis of the causative organisms, the vaccine candidates under development, and each model's capacity to evaluate them and identify correlates of protective immunity. Our broad assessment indicated that human challenge models have not yet reached their full potential to support the development of vaccines against infectious diseases. On the basis of our review, however, we believe that describing an ideal challenge model is possible, as is further developing existing and future challenge models.

A Plasmodium falciparum ATP binding cassette transporter is essential for liver stage entry into schizogony

Plasmodium sporozoites invade hepatocytes and transform into liver stages within a parasitophorous vacuole (PV). The parasites then grow and replicate their genome to form exoerythrocytic merozoites that infect red blood cells. We report that the human malaria parasite Plasmodium falciparum (Pf) expresses a C-type ATP-binding cassette transporter, Pf ABCC2, which marks the transition from invasive sporozoite to intrahepatocytic early liver stage. Using a humanized mouse infection model, we show that Pf ABCC2 localizes to the parasite plasma membrane in early and mid-liver stage parasites but is not detectable in late liver stages. Pf abcc2^— sporozoites invade hepatocytes, form a PV, and transform into liver stage trophozoites but cannot transition to exoerythrocytic schizogony and fail to transition to blood stage infection. Thus, Pf ABCC2 is an expression marker for early phases of parasite liver infection and plays an essential role in the successful initiation of liver stage replication.

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Pf ABCC2 expression marks the transition from sporozoite to early liver stage

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Pf ABCC2 localizes to the early and mid-liver stage plasma membrane

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Pf ABCC2 is critical for initiation of exoerythrocytic schizogony

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Pf abcc2^– liver stages fail to transition to blood stage infection

Reproducibility of malaria sporozoite challenge model in humans for evaluating efficacy of vaccines and drugs: a systematic review

BACKGROUND

The development of novel malaria vaccines and antimalarial drugs is limited partly by emerging challenges to conduct field trials in malaria endemic areas, including unknown effects of existing immunity and a reported fall in malaria incidence. As a result, Controlled Human Malaria Infection (CHMI) has become an important approach for accelerated development of malarial vaccines and drugs. We conducted a systematic review of the literature to establish aggregate evidence on the reproducibility of a malaria sporozoite challenge model.

METHODS

A systematic review of research articles published between 1990 and 2018 on efficacy testing of malaria vaccines and drugs using sporozoite challenge and sporozoite infectivity studies was conducted using Pubmed, Scopus, Embase and Cochrane Library, ClinicalTrials.gov and Trialtrove. The inclusion criteria were randomized and non-randomized, controlled or open-label trials using P. falciparum or P. vivax sporozoite challenges. The data were extracted from articles using standardized data extraction forms and descriptive analysis was performed for evidence synthesis. The endpoints considered were infectivity, prepatent period, parasitemia and safety of sporozoite challenge.

RESULTS

Seventy CHMI trials conducted with a total of 2329 adult healthy volunteers were used for analysis. CHMI was induced by bites of mosquitoes infected with P. falciparum or P. vivax in 52 trials and by direct venous inoculation of P. falciparum sporozoites (PfSPZ challenge) in 18 trials. Inoculation with P. falciparum-infected mosquitoes produced 100% infectivity in 40 studies and the mean/median prepatent period assessed by thick blood smear (TBS) microscopy was ≤ 12 days in 24 studies. On the other hand, out of 12 infectivity studies conducted using PfSPZ challenge, 100% infection rate was reproduced in 9 studies with a mean or median prepatent period of 11 to 15.3 days as assessed by TBS and 6.8 to 12.6 days by PCR. The safety profile of P. falciparum and P.vivax CHMI was characterized by consistent features of malaria infection.

CONCLUSION

There is ample evidence on consistency of P. falciparum CHMI models in terms of infectivity and safety endpoints, which supports applicability of CHMI in vaccine and drug development. PfSPZ challenge appears more feasible for African trials based on current evidence of safety and efficacy.

Diversify and Conquer: The Vaccine Escapism of Plasmodium falciparum

Over the last century, a great deal of effort and resources have been poured into the development of vaccines to protect against malaria, particularly targeting the most widely spread and deadly species of the human-infecting parasites: Plasmodium falciparum. Many of the known proteins the parasite uses to invade human cells have been tested as vaccine candidates. However, precisely because of the importance and immune visibility of these proteins, they tend to be very diverse, and in many cases redundant, which limits their efficacy in vaccine development. With the advent of genomics and constantly improving sequencing technologies, an increasingly clear picture is emerging of the vast genomic diversity of parasites from different geographic areas. This diversity is distributed throughout the genome and includes most of the vaccine candidates tested so far, playing an important role in the low efficacy achieved. Genomics is a powerful tool to search for genes that comply with the most desirable attributes of vaccine targets, allowing us to evaluate function, immunogenicity and also diversity in the worldwide parasite populations. Even predicting how this diversity might evolve and spread in the future becomes possible, and can inform novel vaccine efforts.

A replication-competent late liver stage-attenuated human malaria parasite

Whole-sporozoite vaccines engender sterilizing immunity against malaria in animal models and importantly, in humans. Gene editing allows for the removal of specific parasite genes, enabling generation of genetically attenuated parasite (GAP) strains for vaccination. Using rodent malaria parasites, we have previously shown that late liver stage-arresting replication-competent (LARC) GAPs confer superior protection when compared with early liver stage-arresting replication-deficient GAPs and radiation-attenuated sporozoites. However, generating a LARC GAP in the human malaria parasite Plasmodium falciparum (P. falciparum) has been challenging. Here, we report the generation and characterization of a likely unprecedented P. falciparum LARC GAP generated by targeted gene deletion of the Mei2 gene: P. falciparum mei2-. Robust exoerythrocytic schizogony with extensive cell growth and DNA replication was observed for P. falciparum mei2- liver stages in human liver-chimeric mice. However, P. falciparum mei2- liver stages failed to complete development and did not form infectious exoerythrocytic merozoites, thereby preventing their transition to asexual blood stage infection. Therefore, P. falciparum mei2- is a replication-competent, attenuated human malaria parasite strain with potentially increased potency, useful for vaccination to protect against P. falciparum malaria infection.

Towards functional antibody-based vaccines to prevent pre-erythrocytic malaria infection

INTRODUCTION

An effective malaria vaccine would be considered a milestone of modern medicine, yet has so far eluded research and development efforts. This can be attributed to the extreme complexity of the malaria parasites, presenting with a multi-stage life cycle, high genome complexity and the parasite's sophisticated immune evasion measures, particularly antigenic variation during pathogenic blood stage infection. However, the pre-erythrocytic (PE) early infection forms of the parasite exhibit relatively invariant proteomes, and are attractive vaccine targets as they offer multiple points of immune system attack. Areas covered: We cover the current state of and roadblocks to the development of an effective, antibody-based PE vaccine, including current vaccine candidates, limited biological knowledge, genetic heterogeneity, parasite complexity, and suboptimal preclinical models as well as the power of early stage clinical models. Expert commentary: PE vaccines will need to elicit broad and durable immunity to prevent infection. This could be achievable if recent innovations in studying the parasites' infection biology, rational vaccine selection and design as well as adjuvant formulation are combined in a synergistic and multipronged approach. Improved preclinical assays as well as the iterative testing of vaccine candidates in controlled human malaria infection trials will further accelerate this effort.

P. falciparum and P. vivax Epitope-Focused VLPs Elicit Sterile Immunity to Blood Stage Infections

In order to design P. falciparum preerythrocytic vaccine candidates, a library of circumsporozoite (CS) T and B cell epitopes displayed on the woodchuck hepatitis virus core antigen (WHcAg) VLP platform was produced. To test the protective efficacy of the WHcAg-CS VLPs, hybrid CS P. berghei/P. falciparum (Pb/Pf) sporozoites were used to challenge immunized mice. VLPs carrying 1 or 2 different CS repeat B cell epitopes and 3 VLPs carrying different CS non-repeat B cell epitopes elicited high levels of anti-insert antibodies (Abs). Whereas, VLPs carrying CS repeat B cell epitopes conferred 98% protection of the liver against a 10,000 Pb/Pf sporozoite challenge, VLPs carrying the CS non-repeat B cell eptiopes were minimally-to-non-protective. One-to-three CS-specific CD4/CD8 T cell sites were also fused to VLPs, which primed CS-specific as well as WHcAg-specific T cells. However, a VLP carrying only the 3 T cell domains failed to protect against a sporozoite challenge, indicating a requirement for anti-CS repeat Abs. A VLP carrying 2 CS repeat B cell epitopes and 3 CS T cell sites in alum adjuvant elicited high titer anti-CS Abs (endpoint dilution titer >1x10⁶) and provided 80–100% protection against blood stage malaria. Using a similar strategy, VLPs were constructed carrying P. vivax CS repeat B cell epitopes (WHc-Pv-78), which elicited high levels of anti-CS Abs and conferred 99% protection of the liver against a 10,000 Pb/Pv sporozoite challenge and elicited sterile immunity to blood stage infection. These results indicate that immunization with epitope-focused VLPs carrying selected B and T cell epitopes from the P. falciparum and P. vivax CS proteins can elicit sterile immunity against blood stage malaria. Hybrid WHcAg-CS VLPs could provide the basis for a bivalent P. falciparum/P. vivax malaria vaccine.

Malaria DNA vaccine gp96NTD-CSP elicits both CSP-specific antibody and CD8(+) T cell response

It is ideal for the pre-erythrocytic stage subunit vaccine to induce both CSP-specific antibody and CD8(+) T cell response. Here, we designed a novel malaria DNA vaccine gp96NTD-CSP, which was constructed by fusing the full-length of CSP with the N-terminal domain of gp96 that deleted the endoplasmic reticulum-localized motif KDEL, and investigated its protective efficacy. We found that the fusion protein gp96NTD-CSP was mainly distributed on the surface of eukaryotic cells after transfection and could be sensed as a "danger signal" by the host immune system. Interestingly, both liver parasite burden and parasitemia in mice immunized with gp96NTD-CSP were significantly lower than those in the mice immunized either with gp96NTD, CSP, or gp96NTD-SYVPSAEQI, which was constructed by fusing the CSP-specific CD8(+) T cell epitope with the N-terminal domain of gp96 deleted with KDEL. Consistently, both the level of CSP-specific antibody and the frequency of IFN-γ secreted-CSP-specific CD8(+) T cells were much higher in mice immunized with gp96NTD-CSP than those in the mice immunized either with gp96NTD, CSP, or gp96NTD-SYVPSAEQI. Our results suggest that the malaria DNA vaccine gp96NTD-CSP could induce both humoral and cellular immune responses, which is attributed to the adjuvant effect of gp96NTD and full-length CSP.

Increased sample volume and use of quantitative reverse-transcription PCR can improve prediction of liver-to-blood inoculum size in controlled human malaria infection studies

Background

Controlled human malaria infection (CHMI) studies increasingly rely on nucleic acid test (NAT) methods to detect and quantify parasites in the blood of infected participants. The lower limits of detection and quantification vary amongst the assays used throughout the world, which may affect the ability of mathematical models to accurately estimate the liver-to-blood inoculum (LBI) values that are used to judge the efficacy of pre-erythrocytic vaccine and drug candidates.

Methods

Samples were collected around the time of onset of pre-patent parasitaemia from subjects who enrolled in two different CHMI clinical trials. Blood samples were tested for Plasmodium falciparum 18S rRNA and/or rDNA targets by different NAT methods and results were compared. Methods included an ultrasensitive, large volume modification of an established quantitative reverse transcription PCR (qRT-PCR) assay that achieves detection of as little as one parasite/mL of whole blood.

Results

Large volume qRT-PCR at the University of Washington was the most sensitive test and generated quantifiable data more often than any other NAT methodology. Standard quantitative PCR (qPCR) performed at the University of Oxford and standard volume qRT-PCR performed at the University of Washington were less sensitive than the large volume qRT-PCR, especially at 6.5 days after CHMI. In these trials, the proportion of participants for whom LBI could be accurately quantified using parasite density value greater than or equal to the lower limit of quantification was increased. A greater improvement would be expected in trials in which numerous subjects receive a lower LBI or low dose challenge.

Conclusions

Standard qPCR and qRT-PCR methods with analytical sensitivities of ~20 parasites/mL probably suffice for most CHMI purposes, but the newly developed large volume qRT-PCR may be able to answer specific questions when more analytical sensitivity is required.

Targeted immunomodulation using antigen-conjugated nanoparticles

The growing prevalence of nanotechnology in the fields of biology, medicine, and the pharmaceutical industry is confounded by the relatively small amount of data on the impact of these materials on the immune system. In addition to concerns surrounding the potential toxicity of nanoparticle (NP)-based delivery systems, there is also a demand for a better understanding of the mechanisms governing interactions of NPs with the immune system. Nanoparticles can be tailored to suppress, enhance, or subvert recognition by the immune system. This 'targeted immunomodulation' can be achieved by delivery of unmodified particles, or by modifying particles to deliver drugs, proteins/peptides, or genes to a specific site. In order to elicit the desired, beneficial immune response, considerations should be made at every step of the design process: the NP platform itself, ligands, and other modifiers, the delivery route, and the immune cells that will encounter the conjugated NPs can all impact host immune responses.

Protection against lethal enterovirus 71 challenge in mice by a recombinant vaccine candidate containing a broadly cross-neutralizing epitope within the VP2 EF loop

Human enterovirus 71 (EV71) is the main causative agent of hand, foot, and mouth disease (HFMD) and is associated with several severe neurological complications in the Asia-Pacific region. Here, we evaluated that while passive transfer of neutralizing monoclonal antibody (nMAb) against the VP2 protein protect against lethal EV71 infection in BALB/c mice. Protective nMAb were mapped to residues 141-155 of VP2 by peptide ELISA. High-resolution structural analysis showed that the epitope is part of the VP2 EF loop, which is the “puff” region that forms the “southern rim” of the canyon. Moreover, a three-dimensional structural characterization for the puff region with prior neutralizing epitopes and receptor-binding sites that can serve to inform vaccine strategies. Interestingly, using hepatitis B virus core protein (HBc) as a carrier, we demonstrated that the cross-neutralizing EV71 antibodies were induced, and the VP2 epitope immunized mice serum also conferred 100% in vivo passive protection. The mechanism of in vivo protection conferred by VP2 nMAb is in part attributed to the in vitro neutralizing titer and ability to bind authentic viral particles. Importantly, the anti-VP2(aa141-155) antibodies could inhibit the binding of human serum to EV71 virions showed that the VP2 epitope is immunodominant. Collectively, our results suggest that a broad-spectrum vaccine strategy targeting the high-affinity epitope of VP2 EF loop may elicits effective immune responses against EV71 infection.

CD8+ T Cell Responses to Plasmodium and Intracellular Parasites

Parasitic protozoa are major threats to human health affecting millions of people around the world. Control of these infections by the host immune system relies on a myriad of immunological mechanisms that includes both humoral and cellular immunity. CD8⁺ T cells contribute to the control of these parasitic infections in both animals and humans. Here, we will focus on the CD8⁺ T cell response against a subset of these protozoa: Plasmodium, Toxoplasma gondii, Leishmania and Trypanosoma cruzi, with an emphasis on experimental rodent systems. It is evident a complex interaction occurs between CD8⁺ T cells and the invading protozoa. A detailed understanding of how CD8⁺ T cells mediate protection should provide the basis for the development of effective vaccines that prevent and control infections by these parasites.

Combination vaccines: synergistic simultaneous induction of antibody and T-cell immunity

Vaccines have traditionally been designed to induce antibody responses and have been licensed on their capacity to induce high titers of circulating antibody to the pathogen. With our increased knowledge of host-pathogen interactions, it became apparent that induction of the cellular arm of the immune response is crucial to the efficacy of vaccines against intracellular pathogens and for providing appropriate help for antibody induction. Diverging strategies emerged that concentrate on developing candidate vaccines that solely induce either cellular or humoral responses. As most microbes reside at some point in the infectious cycle in the extracellular as well as intracellular space, and there is interplay between antibody and T cells, it is now apparent that both arms of immunity are essential to effectively control and eliminate the infection. It is, therefore, necessary to develop vaccines that can effectively induce a broad adaptive immune response. For vaccines targeted at diseases of the developing world, such as HIV, tuberculosis and malaria, it is imperative that these vaccines are simple to deliver and cost effective, that is,that optimum T-cell and antibody immunity is achieved with the minimum number of vaccinations. Combination vaccines, where an antibody-inducing subunit protein vaccine is coadministered with a T-cell-inducing poxvirus-based vaccine fulfill these requirements and induce sterile immunity to pathogen challenge.

Hepatitis B core-based virus-like particles to present heterologous epitopes

Since the first effort to recombinantly express the hepatitis B core protein (HBc) in bacteria, the remarkable virion-like structure has fuelled interest in unraveling the structural and antigenic properties of this protein. Initial studies proved HBc virus-like particles to possess strong immunogenic properties, which can be conveyed to linked antigens. More than 35 years later, numerous studies have been performed using HBc as a carrier protein for antigens derived from over a dozen different pathogens and diseases. In this review, the authors highlight the intriguing features of HBc as carrier and antigen, illustrated by some examples and experimental results that underscore the value of HBc as an antigen-presenting platform. Two of these HBc fusions, targeting influenza A and malaria, have even progressed into clinical testing. In the future, the HBc-based virus-like particles platform will probably continue to be used for the display of poorly immunogenic antigens, mainly because virus-like particle formation by HBc capsomers is compatible with nearly any available recombinant gene expression system.

Determining the validity of hospital laboratory reference intervals for healthy young adults participating in early clinical trials of candidate vaccines

This was a retrospective study to determine the validity of institutional reference intervals for interpreting biochemistry and hematology results in healthy adults in the context of clinical trials of preventive vaccines. An example population of 974 healthy adults participating in clinical trials at the Jenner Institute, Oxford, UK, between 1999 and 2009 was studied. Methods for calculating the central 95% ranges and determining the coefficients of within person variation were demonstrated. Recommendations have been made as to how these data can be usefully applied to the interpretation of blood results in healthy adult subjects for the purposes of clinical trial inclusion decisions and post-vaccination safety monitoring.

Malaria vaccine adjuvants: latest update and challenges in preclinical and clinical research

There is no malaria vaccine currently available, and the most advanced candidate has recently reported a modest 30% efficacy against clinical malaria. Although many efforts have been dedicated to achieve this goal, the research was mainly directed to identify antigenic targets. Nevertheless, the latest progresses on understanding how immune system works and the data recovered from vaccination studies have conferred to the vaccine formulation its deserved relevance. Additionally to the antigen nature, the manner in which it is presented (delivery adjuvants) as well as the immunostimulatory effect of the formulation components (immunostimulants) modulates the immune response elicited. Protective immunity against malaria requires the induction of humoral, antibody-dependent cellular inhibition (ADCI) and effector and memory cell responses. This review summarizes the status of adjuvants that have been or are being employed in the malaria vaccine development, focusing on the pharmaceutical and immunological aspects, as well as on their immunization outcomings at clinical and preclinical stages.

Comparison of clinical and parasitological data from controlled human malaria infection trials

BACKGROUND

Exposing healthy human volunteers to Plasmodium falciparum-infected mosquitoes is an accepted tool to evaluate preliminary efficacy of malaria vaccines. To accommodate the demand of the malaria vaccine pipeline, controlled infections are carried out in an increasing number of centers worldwide. We assessed their safety and reproducibility.

METHODS

We reviewed safety and parasitological data from 128 malaria-naïve subjects participating in controlled malaria infection trials conducted at the University of Oxford, UK, and the Radboud University Nijmegen Medical Center, The Netherlands. Results were compared to a report from the US Military Malaria Vaccine Program.

RESULTS

We show that controlled human malaria infection trials are safe and demonstrate a consistent safety profile with minor differences in the frequencies of arthralgia, fatigue, chills and fever between institutions. But prepatent periods show significant variation. Detailed analysis of Q-PCR data reveals highly synchronous blood stage parasite growth and multiplication rates.

CONCLUSIONS

Procedural differences can lead to some variation in safety profile and parasite kinetics between institutions. Further harmonization and standardization of protocols will be useful for wider adoption of these cost-effective small-scale efficacy trials. Nevertheless, parasite growth rates are highly reproducible, illustrating the robustness of controlled infections as a valid tool for malaria vaccine development.

Real-time quantitative reverse transcription PCR for monitoring of blood-stage Plasmodium falciparum infections in malaria human challenge trials

To detect pre-patent parasitemia, we developed a real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) for the asexual 18S ribosomal RNA (rRNAs) of Plasmodium falciparum. Total nucleic acids extracted from whole blood were combined with control RNA and tested by qRT-PCR. The assay quantified > 98.7% of parasite-containing samples to ±0.5 log(10) parasites/mL of the nominal value without false positives. The analytical sensitivity was ≥ 20 parasites/mL. The coefficient of variation was 0.6% and 1.8% within runs and 1.6% and 4.0% between runs for high and low parasitemia specimens, respectively. Using this assay, we determined that A-type 18S rRNAs are stably expressed at 1 × 10(4) copies per ring-stage parasite. When used to monitor experimental P. falciparum infection of human volunteers, the assay detected blood-stage infections 3.7 days earlier on average than thick blood smears. This validated, internally controlled qRT-PCR method also uses a small (50 μL) sample volume requiring minimal pre-analytical handling, making it useful for clinical trials.

A review of malaria vaccine clinical projects based on the WHO rainbow table

Development and Phase 3 testing of the most advanced malaria vaccine, RTS,S/AS01, indicates that malaria vaccine R&D is moving into a new phase. Field trials of several research malaria vaccines have also confirmed that it is possible to impact the host-parasite relationship through vaccine-induced immune responses to multiple antigenic targets using different platforms. Other approaches have been appropriately tested but turned out to be disappointing after clinical evaluation.

As the malaria community considers the potential role of a first-generation malaria vaccine in malaria control efforts, it is an apposite time to carefully document terminated and ongoing malaria vaccine research projects so that lessons learned can be applied to increase the chances of success for second-generation malaria vaccines over the next 10 years.

The most comprehensive resource of malaria vaccine projects is a spreadsheet compiled by WHO thanks to the input from funding agencies, sponsors and investigators worldwide. This spreadsheet, available from WHO's website, is known as "the rainbow table". By summarizing the published and some unpublished information available for each project on the rainbow table, the most comprehensive review of malaria vaccine projects to be published in the last several years is provided below.

A phase 1 trial of MSP2-C1, a blood-stage malaria vaccine containing 2 isoforms of MSP2 formulated with Montanide® ISA 720

Background

In a previous Phase 1/2b malaria vaccine trial testing the 3D7 isoform of the malaria vaccine candidate Merozoite surface protein 2 (MSP2), parasite densities in children were reduced by 62%. However, breakthrough parasitemias were disproportionately of the alternate dimorphic form of MSP2, the FC27 genotype. We therefore undertook a dose-escalating, double-blinded, placebo-controlled Phase 1 trial in healthy, malaria-naïve adults of MSP2-C1, a vaccine containing recombinant forms of the two families of msp2 alleles, 3D7 and FC27 (EcMSP2-3D7 and EcMSP2-FC27), formulated in equal amounts with Montanide® ISA 720 as a water-in-oil emulsion.

Methodology/Principal Findings

The trial was designed to include three dose cohorts (10, 40, and 80 µg), each with twelve subjects receiving the vaccine and three control subjects receiving Montanide® ISA 720 adjuvant emulsion alone, in a schedule of three doses at 12-week intervals. Due to unexpected local reactogenicity and concern regarding vaccine stability, the trial was terminated after the second immunisation of the cohort receiving the 40 µg dose; no subjects received the 80 µg dose. Immunization induced significant IgG responses to both isoforms of MSP2 in the 10 µg and 40 µg dose cohorts, with antibody levels by ELISA higher in the 40 µg cohort. Vaccine-induced antibodies recognised native protein by Western blots of parasite protein extracts and by immunofluorescence microscopy. Although the induced anti-MSP2 antibodies did not directly inhibit parasite growth in vitro, IgG from the majority of individuals tested caused significant antibody-dependent cellular inhibition (ADCI) of parasite growth.

Conclusions/Significance

As the majority of subjects vaccinated with MSP2-C1 developed an antibody responses to both forms of MSP2, and that these antibodies mediated ADCI provide further support for MSP2 as a malaria vaccine candidate. However, in view of the reactogenicity of this formulation, further clinical development of MSP2-C1 will require formulation of MSP2 in an alternative adjuvant.

Trial Registration

Australian New Zealand Clinical Trials Registry 12607000552482

Experimental human challenge infections can accelerate clinical malaria vaccine development

Malaria is one of the most frequently occurring infectious diseases worldwide, with almost 1 million deaths and an estimated 243 million clinical cases annually. Several candidate malaria vaccines have reached Phase IIb clinical trials, but results have often been disappointing. As an alternative to these Phase IIb field trials, the efficacy of candidate malaria vaccines can first be assessed through the deliberate exposure of participants to the bites of infectious mosquitoes (sporozoite challenge) or to an inoculum of blood-stage parasites (blood-stage challenge). With an increasing number of malaria vaccine candidates being developed, should human malaria challenge models be more widely used to reduce cost and time investments? This article reviews previous experience with both the sporozoite and blood-stage human malaria challenge models and provides future perspectives for these models in malaria vaccine development.

Advances and challenges in malaria vaccine development

Malaria caused by Plasmodium falciparum remains a major public health threat, especially among children and pregnant women in Africa. An effective malaria vaccine would be a valuable tool to reduce the disease burden and could contribute to elimination of malaria in some regions of the world. Current malaria vaccine candidates are directed against human and mosquito stages of the parasite life cycle, but thus far, relatively few proteins have been studied for potential vaccine development. The most advanced vaccine candidate, RTS,S, conferred partial protection against malaria in phase II clinical trials and is currently being evaluated in a phase III trial in Africa. New vaccine targets need to be identified to improve the chances of developing a highly effective malaria vaccine. A better understanding of the mechanisms of naturally acquired immunity to malaria may lead to insights for vaccine development.

Reducing empiricism in malaria vaccine design

Gains in the control of malaria and the promising progress of a malaria vaccine that is partly efficacious do not reduce the need for a high-efficacy vaccine in the longer term. Evidence supports the feasibility of developing a highly efficacious malaria vaccine. However, design of candidate malaria vaccines remains empirical and is necessarily based on many unproven assumptions because much of the knowledge needed to design vaccines and to predict efficacy is not available. Data to inform key questions of vaccine science might allow the design of vaccines to progress to a less empirical stage, for example through availability of assay results associated with vaccine efficacy. We discuss six strategic gaps in knowledge that contribute to empiricism in the design of vaccines. Comparative evaluation, assay and model standardisation, greater sharing of information, collaboration and coordination between groups, and rigorous evaluation of existing datasets are steps that can be taken to enable reductions in empiricism over time.

Use of hepadnavirus core proteins as vaccine platforms

The first virus-like particle to be tested for use as a vaccine carrier was based on the hepatitis B virus nucleocapsid protein. This viral subunit, while not infectious on its own, is a 36-nm particle that is highly immunogenic during a natural infection. The self-assembly and high degree of immunogenicity is maintained when expressed as a recombinant protein and, moreover, can confer a high degree of immunogenicity on foreign antigens linked to the particle, either chemically or genetically. This review describes the current state of the hepadnaviral core protein as a vaccine carrier.

Phase 1 safety and immunogenicity trial of the Plasmodium falciparum blood-stage malaria vaccine AMA1-C1/ISA 720 in Australian adults

A Phase 1 trial was conducted in malaria-naïve adults to evaluate the recombinant protein vaccine apical membrane antigen 1-Combination 1 (AMA1-C1) formulated in Montanide ISA 720 (SEPPIC, France), a water-in-oil adjuvant. Vaccinations were halted early due to a formulation issue unrelated to stability or potency. Twenty-four subjects (12 in each group) were enrolled and received 5 or 20 microg protein at 0 and 3 months and four subjects were enrolled and received one vaccination of 80 microg protein. After first vaccination, nearly all subjects experienced mild to moderate local reactions and six experienced delayed local reactions occurring at Day 9 or later. After the second vaccination, three subjects experienced transient grade 3 (severe) local reactions; the remainder experienced grade 1 or 2 local reactions. All related systemic reactogenicity was grade 1 or 2, except one instance of grade 3 malaise. Anti-AMA1-C1 antibody responses were dose dependent and seen following each vaccination, with mean antibody levels 2-3 fold higher in the 20 microg group compared to the 5 microg group at most time points. In vitro growth-inhibitory activity was a function of the anti-AMA1 antibody titer. AMA1-C1 formulated in ISA 720 is immunogenic in malaria-naïve Australian adults. It is reasonably tolerated, though some transient, severe, and late local reactions are seen.

Enhanced immunogenicity of Plasmodium falciparum peptide vaccines using a topical adjuvant containing a potent synthetic Toll-like receptor 7 agonist, imiquimod

Plasmodium sporozoites injected into the skin by malaria-infected mosquitoes can be effectively targeted by antibodies that block parasite invasion of host hepatocytes and thus prevent the subsequent development of blood stage infections responsible for clinical disease. Malaria subunit vaccines require potent adjuvants, as they lack known pathogen-associated molecular patterns found in attenuated viral or bacterial vaccines that function as Toll-like receptor (TLR) agonists to stimulate dendritic cells and initiate strong adaptive immune responses. A synthetic TLR7 agonist, imiquimod, which is FDA approved for topical treatment of various skin conditions, can function as a potent adjuvant for eliciting T-cell responses to intracellular pathogens and model protein antigens. In the current studies, the topical application of imiquimod at the site of subcutaneously injected Plasmodium falciparum circumsporozoite (CS) peptides elicited strong parasite-specific humoral immunity that protected against challenge with transgenic rodent parasites that express P. falciparum CS repeats. In addition, injection of a simple linear peptide followed by topical imiquimod elicited strong Th1 CD4(+) T-cell responses, as well as high antibody titers. The correlation of high anti-repeat antibody titers with resistance to sporozoite challenge in vivo and in vitro supports use of this topical TLR7 agonist adjuvant to elicit protective humoral immunity. The safety, simplicity, and economic advantages of a topical synthetic TLR7 agonist adjuvant also apply to other vaccines requiring high antibody titers, such as malaria asexual or sexual blood stage antigens to prevent red blood cell invasion and block transmission to the mosquito vector, and to vaccines to other extracellular pathogens.

The coming of age of virus-like particle vaccines

Virus-like particles are supra-molecular assemblages, usually icosahedral or rod-like structures. They incorporate key immunologic features of viruses which include repetitive surfaces, particulate structures and induction of innate immunity through activation of pathogen-associated molecular-pattern recognition receptors. They carry no replicative genetic information and can be produced recombinantly in large scale. Virus-like particles thus represent a safe and effective vaccine platform for inducing potent B- and T-cell responses. In addition to being effective vaccines against the corresponding virus from which they are derived, virus-like particles can also be used to present foreign epitopes to the immune system. This can be achieved by genetic fusion or chemical conjugation. This technological innovation has greatly broadened the scope of their use, from immunizing against microbial pathogens to immunotherapy for chronic diseases. Towards this end, virus-like particles have been used to induce autoantibodies to disease-associated self-molecules involved in chronic diseases, such as hypertension and Alzheimer's disease. The recognition of the potent immunogenicity and commercial potential for virus-like particles has greatly accelerated research and development activities. During the last decade, two prophylactic virus-like particle vaccines have been registered for human use, while another 12 vaccines entered clinical development.

Various carrier system(s)- mediated genetic vaccination strategies against malaria

The introduction of vaccine technology has facilitated an unprecedented multiantigen approach to develop an effective vaccine against complex pathogens, such as Plasmodium spp., that cause severe malaria. The capacity of multisubunit DNA vaccines encoding different stage Plasmodium antigens to induce CD8(+) cytotoxic T lymphocytes and IFN-gamma responses in mice, monkeys and humans has been observed. Moreover, genetic vaccination may be multi-immune (i.e., capable of eliciting more than one type of immune response, including cell-mediated and humoral). In the case of malaria parasites, a cytotoxic T-lymphocyte response is categorically needed against the intracellular hepatocyte stage while a humoral response, with antibodies targeted against antigens from all stages of the life cycle, is also needed. Therefore, the key to success for any DNA-based therapy is to design a vector able to serve as a safe and efficient delivery system. This has encouraged the development of nonviral DNA-mediated gene-transfer techniques, such as liposomes, virosomes, microspheres and nanoparticles. Efficient and relatively safe DNA transfection using lipoplexes makes them an appealing alternative to be explored for gene delivery. In addition, liposome-entrapped DNA has been shown to enhance the potency of DNA vaccines, possibly by facilitating uptake of the plasmid by antigen-presenting cells. Another recent technology using cationic lipids has been deployed and has generated substantial interest in this approach to gene transfer. This review comprises various aspects that could be decisive in the formulation of efficient and stable carrier system(s) for the development of malaria vaccines.

Phase I trial of an alhydrogel adjuvanted hepatitis B core virus-like particle containing epitopes of Plasmodium falciparum circumsporozoite protein

The objectives of this non-randomized, non-blinded, dose-escalating Phase I clinical trial were to assess the safety, reactogenicity and immunogenicity of ICC-1132 formulated with Alhydrogel (aluminum hydroxide) in 51 healthy, malaria-naive adults aged 18 to 45 years. ICC-1132 (Malariavax) is a recombinant, virus-like particle malaria vaccine comprised of hepatitis core antigen engineered to express the central repeat regions from Plasmodium falciparum circumsporozoite protein containing an immunodominant B [(NANP)(3)] epitope, an HLA-restricted CD4 (NANPNVDPNANP) epitope and a universal T cell epitope (T*) (amino acids 326-345, NF54 isolate). We assessed an Alhydrogel (aluminum hydroxide)-adjuvanted vaccine formulation at three ICC-1132 dose levels, each injected intramuscularly (1.0 mL) on study days 0, 56 and 168. A saline vaccine formulation was found to be unstable after prolonged storage and this formulation was subsequently removed from the study. Thirty-two volunteers were followed for one year. Local and systemic adverse clinical events were measured and immune responses to P. falciparum and hepatitis B virus core antigens were determined utilizing the following assays: IgG and IgM ELISA, indirect immunofluorescence against P. falciparum sporozoites, circumsporozoite precipitin (CSP) and transgenic sporozoite neutralization assays. Cellular responses were measured by proliferation and IL-2 assays. Local and systemic reactions were similarly mild and well tolerated between dose cohorts. Depending on the ICC-1132 vaccine concentration, 95 to 100% of volunteers developed antibody responses to the ICC-1132 immunogen and HBc after two injections; however, only 29-75% and 29-63% of volunteers, respectively, developed malaria-specific responses measured by the malaria repeat synthetic peptide ELISA and IFA; 2 of 8 volunteers had positive reactions in the CSP assay. Maximal transgenic sporozoite neutralization assay inhibition was 54%. Forty-seven to seventy-five percent demonstrated T cell proliferation in response to ICC-1132 or to recombinant circumsporozoite protein (rCS) NF-54 isolate. This candidate malaria vaccine was well tolerated, but the vaccine formulation was poorly immunogenic. The vaccine may benefit from a more powerful adjuvant to improve immunogenicity.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00587249.

Virus-like particles: flexible platforms for vaccine development

Virus-like particles (VLPs) consist of viral structural proteins that, when overexpressed, spontaneously self-assemble into particles that are antigenically indistinguishable from infectious virus or subviral particles. VLPs can be considered as dense, repetitive arrays of one or more protein subunits with properties that are highly advantageous for use as stand-alone vaccines or as vaccine platforms. This review discusses the development of VLP-based platform technologies for vaccines against pathogens, as well as nontraditional targets such as self-antigens involved in chronic diseases.

Combination of protein and viral vaccines induces potent cellular and humoral immune responses and enhanced protection from murine malaria challenge

The search for an efficacious vaccine against malaria is ongoing, and it is now widely believed that to confer protection a vaccine must induce very strong cellular and humoral immunity concurrently. We studied the immune response in mice immunized with the recombinant viral vaccines fowlpox strain FP9 and modified virus Ankara (MVA), a protein vaccine (CV-1866), or a combination of the two; all vaccines express parts of the same preerythrocytic malaria antigen, the Plasmodium berghei circumsporozoite protein (CSP). Mice were then challenged with P. berghei sporozoites to determine the protective efficacies of different vaccine regimens. Two immunizations with the protein vaccine CV-1866, based on the hepatitis B core antigen particle, induced strong humoral immunity to the repeat region of CSP that was weakly protective against sporozoite challenge. Prime-boost with the viral vector vaccines, FP9 followed by MVA, induced strong T-cell immunity to the CD8+ epitope Pb9 and partially protected animals from challenge. Physically mixing CV-1866 with FP9 or MVA and then immunizing with the resultant combinations in a prime-boost regimen induced both cellular and humoral immunity and afforded substantially higher levels of protection (combination, 90%) than either vaccine alone (CV-1866, 12%; FP9/MVA, 37%). For diseases such as malaria in which different potent immune responses are required to protect against different stages, using combinations of partially effective vaccines may offer a more rapid route to achieving deployable levels of efficacy than individual vaccine strategies.

Immunogenicity in rhesus of the Plasmodium vivax mosquito stage antigen Pvs25H with Alhydrogel and Montanide ISA 720

Pvs25 is an ookinete surface protein from Plasmodium vivax that is the target of transmission-blocking antibodies. Two immunogenicity trials in rhesus monkeys with a recombinant form of the protein, Pvs25H, were undertaken. Monkeys were vaccinated with Pvs25H adsorbed to Alhydrogel or emulsified in Montanide ISA 720 at 0, 4 and 27 weeks (study 1) or in Montanide ISA 720 at 0 and 18 weeks (study 2) with 1.5 or 15 microg Pvs25H in 0.1 or 0.5 mL of emulsion (four combinations). Immunogenicity was assessed by ELISA and by membrane-feeding experiments using P. vivax-infected blood from human volunteers (studies 1 and 2) or from chimpanzees (study 1). Both vaccine trials generated antibodies that blocked transmission of P. vivax to mosquitoes. Antibody titres and transmission blocking were higher with Montanide ISA 720 than with Alhydrogel in the first trial and with the 15 microg Pvs25H/0.5 mL ISA 720 combination in the second trial.

Economic and practical challenges to the formulation of vaccines against endemic infectious diseases such as malaria

Herein, we analyze in general the current vaccine market and identify potential factors driving and modulating supply and demand for vaccines. An emphasis is placed on changes in regulation in the last 20 years which have led to increased indirect costs of production, and which can create a barrier against the timely use of technological advances to reduce direct costs. Other defining industry characteristics, such as firm numbers and sizes, cost and pricing strategies, nature extent and impact of Government involvement and international regulation are noted. These considerations, far from being removed from basic vaccine research, influence its ability to achieve aims that can be then progressed into effective vaccine products. We discuss specifically the development of particulate vaccines against malaria, a major lethal disease and health problem prevalent in Africa, including some key economic and methodological challenges and opportunities. We note some practical issues blocking the development of effective particulate vaccines for the Third World, mainly driven by the regulatory spiral noted above.

Developmental biology of sporozoite-host interactions in Plasmodium falciparum malaria: implications for vaccine design

The Plasmodium falciparum sporozoite infects different types of cells in a mosquito's salivary glands and human epithelial and Kuppfer cells and hepatocytes. These become differentiated later on, transforming themselves into the invasive red blood cell form, the merozoite. The ability of sporozoites to interact with different types of cells requires a wide variety of mechanisms allowing them to survive in both hosts: mobility, receptor-ligand interactions with different cellular receptors, and transformation and development into other invasive parasite forms, which are vitally important for parasite survival. Sporozoite complexity is reflected in the large quantity of proteins that can be expressed. Some of them have been extensively studied, such as CSP, TRAP, STARP, LSA-1, LSA-3, SALSA, SPECT1, SPECT2, MAEBL, and SPATR, due to their importance in infection and their potential use as vaccines. Our work has been focused on the search for the molecular mechanisms of parasite-host cellular receptor-ligand interactions by identifying amino acid sequences and the critical binding residues from these proteins relevant to parasite invasion. Once such sequences have been identified, it will be possible to modify them to induce a strong immune response against P. falciparum in the experimental Aotus monkey model. This all leads towards developing multistage, multicomponent, subunit-based vaccines that will be effective in eradicating or controlling malaria caused by P. falciparum.

A DNA prime-modified vaccinia virus ankara boost vaccine encoding thrombospondin-related adhesion protein but not circumsporozoite protein partially protects healthy malaria-naive adults against Plasmodium falciparum sporozoite challenge

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.

Thick blood film examination for Plasmodium falciparum malaria has reduced sensitivity and underestimates parasite density

BACKGROUND

Thick blood films are routinely used to diagnose Plasmodium falciparum malaria. Here, they were used to diagnose volunteers exposed to experimental malaria challenge.

METHODS

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.

RESULTS

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.

CONCLUSION

It appears large numbers of parasites are lost during staining. This limits their sensitivity, and leads to erroneous estimates of parasite density.

Vaccines for preventing malaria (pre-erythrocytic)

BACKGROUND

Vaccines against all stages of the malaria parasite are in development, mainly for Plasmodium falciparum, which causes the most serious form of malaria. Pre-erythrocytic vaccines act to prevent or delay a malaria attack by attacking the sporozoite and liver stages before the parasite reaches the bloodstream.

OBJECTIVES

To assess the efficacy and safety of pre-erythrocytic malaria vaccines against any type of human malaria.

SEARCH STRATEGY

In March 2006, we searched the Cochrane Infectious Diseases Group Specialized Register, CENTRAL (The Cochrane Library 2006, Issue 1), MEDLINE, EMBASE, LILACS, and the Science Citation Index. We also searched conference proceedings and reference lists of articles, and contacted organizations and researchers in the field.

SELECTION CRITERIA

Randomized controlled trials comparing pre-erythrocytic vaccines with placebo, control vaccine, or routine antimalarial control measures in people of any age receiving an artificial challenge or natural exposure to malaria infection.

DATA COLLECTION AND ANALYSIS

Both authors independently assessed trial quality and extracted data. Results of meta-analyses were expressed as relative risks with 95% confidence intervals (CI) using an intention-to-treat analysis.

MAIN RESULTS

Nine safety and efficacy trials, and two safety trials, with over 3000 participants were included. In semi-immune children, RTS,S vaccine reduced clinical episodes of malaria by 26% (95% CI 13% to 37%) and severe malaria by 58% (95% CI 15% to 79%) for up to 18 months. Prevalence of parasitaemia was also reduced by 26% (95% CI 11% to 38%) at six months after immunization. RTS,S also reduced clinical malaria episodes by 63% (95% CI 18% to 83%) in semi-immune adult men in the second year of follow up after a booster dose. No severe adverse events were judged to be related to RTS,S vaccine, although the frequencies of injection site pain, swelling, arm motion limitation, headache, and malaise were increased in the vaccine groups. There was no evidence for effect of the CS-NANP vaccines (307 participants, 3 trials), CS102 peptide vaccine (14 participants, 1 trial), or the ME-TRAP vaccine (372 participants, 1 trial).

AUTHORS' CONCLUSIONS

RTS,S vaccine was effective in preventing a significant number of clinical malaria episodes, including good protection against severe malaria in children for 18 months. No severe adverse events were attributable to the vaccine. Progression of this vaccine towards licensing is justified while efforts to increase its efficacy continue. The other vaccines do not look promising and further research is a priority.

A review of human vaccine research and development: malaria

The last several years have seen significant progress in the development of vaccines against malaria. Most recently, proof-of-concept of vaccine-induced protection from malaria infection and disease was demonstrated in African children. Pursued by various groups and on many fronts, several other candidate vaccines are in early clinical trials. Yet, despite the optimism and promise, an effective malaria vaccine is not yet available, in part because of the lack of understanding of the types of immune responses needed for protection, added to the difficulty of identifying, selecting and producing the appropriate protective antigens from a parasite with a genome of well over five thousand genes and to the frequent need to enhance the immunogenicity of purified antigens through the use of novel adjuvants or delivery systems. Insufficient clinical trial capacity and normative research functions such as local ethical committee reviews also contribute to slow down the development process. This article attempts to summarize the state of the art of malaria vaccine development.

Innate Immune Responses to Human Malaria: Heterogeneous Cytokine Responses to Blood-Stage Plasmodium falciparum Correlate with Parasitological and Clinical Outcomes

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.

Glutamate-rich protein (GLURP) induces antibodies that inhibit in vitro growth of Plasmodium falciparum in a phase 1 malaria vaccine trial

The glutamate-rich protein (GLURP) of P. falciparum is the target of cytophilic antibodies which are significantly associated with protection against clinical malaria. A phase 1 clinical trial was conducted in healthy adult volunteers with the long synthetic peptide (LSP) GLURP(85-213) combined with either Aluminum Hydroxide (Alum, 18 volunteers) or Montanide ISA 720 (ISA, 18 volunteers) as adjuvants. Immunizations with 10, 30 or 100 microg GLURP(85-213) were administered subcutaneously at days 0, 30, and 120. Adverse events occurred more frequently with increasing dosage of GLURP(85-213) LSP and were more prevalent in the ISA group. Serious vaccine-related adverse events were not observed. The vaccine induced dose-dependent cellular and humoral immune responses, with high levels of (mainly cytophilic IgG1) antibodies that recognize parasites by immunofluorescence (IFA). Plasma samples collected 30 days after the last immunization induced a dose-dependent inhibition of parasite growth in vitro in the presence of monocytes. In conclusion, immunizations with GLURP(85-213) LSP formulations induce adverse events but can be administered safely, generating antibodies with capacity to mediate growth-inhibitory activity against P. falciparum in vitro.

Advances in vaccine development against the pre-erythrocytic stage of Plasmodium falciparum malaria

With approximately 2.4 billion people at risk, Plasmodium falciparum malaria infection caused by an infectious bite of an Anopheles mosquito continues to be a major cause of mortality and morbidity, mainly in the tropics and subtropics. Measures to control the mosquito vector on a broad scale are expensive and need to be maintained continuously. The rapid emergence of parasite strains that are resistant to affordable drugs highlights the urgent need for a cheap and effective vaccine. Candidate vaccines that have been developed to date target different stages of the parasite life cycle. This review describes the recent advances in the development of a vaccine that aims to terminate the infection at its first stage in the liver. The candidate vaccines that are currently under clinical evaluation are introduced and the results from recent trials discussed. The review aims to explain the immunologic challenges a successful vaccine has to meet, as well as the different strategies that are currently employed in an attempt to induce a protective immune response. Furthermore, an outline of available options to be tested in the near future will be presented.

Safety and immunogenicity of the malaria candidate vaccines FP9 CS and MVA CS in adult Gambian men

We assessed the safety and immunogenicity of prime-boost vectors encoding the Plasmodium falciparum circumsporozoite (CS) protein expressed either in the attenuated fowl-pox virus (FP9) or modified vaccinia virus Ankara (MVA). Thirty-two adult Gambians in groups of four to eight received one, two or three doses of FP9 CS and/or MVA CS. No serious adverse event was observed following vaccination. The most immunogenic regimen was two doses of FP9 followed by a single dose of MVA 4 weeks later (an average of 1000 IFN-gamma spot forming units/million PBMCs). This level of effector T-cell responses appears higher than that seen in previously reported studies of CS-based candidate malaria vaccines.

Safety and immunogenicity of the Plasmodium falciparum merozoite surface protein-3 long synthetic peptide (MSP3-LSP) malaria vaccine in healthy, semi-immune adult males in Burkina Faso, West Africa

The merozoite surface protein-3 long synthetic peptide (MSP3-LSP) comprises the amino acid sequence 186-276 of the Plasmodium falciparum protein MSP3. It is currently in development as an erythrocytic stage (blood stage) malaria vaccine candidate. We report here the first data on the safety, reactogenicity and immunogenicity of three doses of MSP3-LSP, adjuvanted with aluminium hydroxide, in healthy male adults living in a malaria endemic area.

METHODS

A phase 1b single-blind controlled trial was performed in the village of Balonghin in Burkina Faso. Thirty male volunteers aged 18-40 years were randomised to receive either three doses of 30 microg MSP3-LSP or 0.5 ml of tetanus toxoid vaccine. The second and third vaccine doses were given 28 and 112 days after the first dose. We followed participants for 1 year.

RESULTS

There were no serious adverse events in either vaccine group. In both groups participants reported local reactions at the site of injection when compared to an earlier trial in European volunteers. Only one systemic adverse event (tachycardia) was identified which occurred immediately after the first vaccination in one individual receiving MSP3-LSP. No clinically significant biological abnormalities following vaccination were observed. Humoral immune responses (IgG, IgG subclasses, IgM) to MSP3-LSP peptide were similar in the two groups following vaccination. Some cell-mediated immune responses appeared to differ between the two vaccine groups. After the second dose of MSP3-LSP, there appeared to be a marked increase in the lymphocyte proliferation index and IFN-gamma in response to stimulation with MSP3-LSP.

CONCLUSION

These data suggest that three doses of 30 microg MSP3-LSP when administered subcutaneously on days 0, 28 and 112 are well-tolerated by adult males previously exposed to natural P. falciparum infection. They also suggest that MSP3-LSP is able to stimulate an enhanced cell-mediated immune response in individuals with some degree of preexisting immunity.

Pre-erythrocytic malaria vaccines: towards greater efficacy

The complex life cycle of the malaria parasite Plasmodium falciparum provides many options for vaccine design. Several new types of vaccine are now being evaluated in clinical trials. Recently, two vaccine candidates that target the pre-erythrocytic stages of the malaria life cycle - a protein particle vaccine with a powerful adjuvant and a prime-boost viral-vector vaccine - have entered Phase II clinical trials in the field and the first has shown partial efficacy in preventing malarial disease in African children. This Review focuses on the potential immunological basis for the encouraging partial protection induced by these vaccines, and it considers ways for developing more effective malaria vaccines.

Merozoite surface protein 1 of Plasmodium vivax induces a protective response against Plasmodium cynomolgi challenge in rhesus monkeys

The 42-kDa fragment of the merozoite surface protein 1 (MSP-1(42)) is a leading candidate for the development of a vaccine to control malaria. We previously reported a method for the production of Plasmodium vivax MSP-1(42) (PvMSP-1(42)) as a soluble protein (S. Dutta, L. W. Ware, A. Barbosa, C. F. Ockenhouse, and D. E. Lanar, Infect. Immun. 69:5464-5470, 2001). We report here a process to manufacture the same PvMSP-1(42) protein but as an insoluble inclusion body-derived protein which was then refolded in vitro. We compared the immunogenicity and protective efficacy of the soluble and refolded forms of PvMSP-1(42) protein by using a heterologous but closely related P. cynomolgi-rhesus monkey challenge model. As comparative controls we also expressed, purified, and immunized rhesus with the soluble and refolded forms of the P. cynomolgi MSP-1(42) (PcMSP-1(42)) proteins. All proteins induced equally high-titer, cross-reacting antibodies. Upon challenge with P. cynomolgi, none of the MSP-1(42)-vaccinated groups demonstrated sterile protection or a delay in the prepatent period. However, following an initial rise in parasitemia, all MSP-1-vaccinated animals had significantly lower parasite burdens as indicated by lower cumulative parasitemia, lower peak parasitemia, lower secondary peak parasitemia, and lower average daily parasitemia compared to the adjuvant control group (P < 0.05). Except the soluble PcMSP-1(42) group, monkeys in all other groups had fewer numbers of days with parasitemia of >10,000 parasites mm(-3). Interestingly, there was no significant difference in the level of partial protection observed in the homologous and heterologous groups in this challenge model. The soluble and refolded forms of PcMSP-1(42) and PvMSP-1(42) proteins also appeared to have a similar partially protective effect.