Malaria vaccines

Asymptomatic Plasmodium vivax parasitaemia in the low-transmission setting: the role for a population-based transmission-blocking vaccine for malaria elimination

Plasmodium vivax remains an important cause of morbidity and mortality across the Americas, Horn of Africa, East and South East Asia. Control of transmission has been hampered by emergence of chloroquine resistance and several intrinsic characteristics of infection including asymptomatic carriage, challenges with diagnosis, difficulty eradicating the carrier state and early gametocyte appearance. Complex human-parasite-vector immunological interactions may facilitate onward infection of mosquitoes. Given these challenges, new therapies are being explored including the development of transmission to mosquito blocking vaccines. Herein, the case supporting the need for transmission-blocking vaccines to augment control of P. vivax parasite transmission and explore factors that are limiting eradication efforts is discussed.

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

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

Recent developments in malaria vaccinology

The development of a highly effective malaria vaccine remains a key goal to aid in the control and eventual eradication of this devastating parasitic disease. The field has made huge strides in recent years, with the first-generation vaccine RTS,S showing modest efficacy in a Phase III clinical trial. The updated 2030 Malaria Vaccine Technology Roadmap calls for a second generation vaccine to achieve 75% efficacy over two years for both Plasmodium falciparum and Plasmodium vivax, and for a vaccine that can prevent malaria transmission. Whole-parasite immunisation approaches and combinations of pre-erythrocytic subunit vaccines are now reporting high-level efficacy, whilst exciting new approaches to the development of blood-stage and transmission-blocking vaccine subunit components are entering clinical development. The development of a highly effective multi-component multi-stage subunit vaccine now appears to be a realistic ambition. This review will cover these recent developments in malaria vaccinology.

Detection of human malaria using recombinant Plasmodium knowlesi merozoire surface protein-1 (MSP-1₁₉) expressed in Escherichia coli

Malaria remains one of the world's most important infectious diseases and is responsible for enormous mortality and morbidity. Human infection with Plasmodium knowlesi is widely distributed in Southeast Asia. Merozoite surface protein-1₁₉ (MSP-1₁₉), which plays an important role in protective immunity against asexual blood stage malaria parasites, appears as a leading immunogenic antigen of Plasmodium sp. We evaluated the sensitivity and specificity of recombinant P. knowlesi MSP-1₁₉ (rMSP-1₁₉) for detection of malarial infection. rMSP-1₁₉ was expressed in Escherichia coli expression system and the purified rMSP-1₁₉ was evaluated with malaria, non-malaria and healthy human serum samples (n = 215) in immunoblots. The sensitivity of rMSP-1₁₉ for detection of P. knowlesi, Plasmodium falciparum, Plasmodium  vivax and Plasmodium  ovale infection was 95.5%, 75.0%, 85.7% and 100%, respectively. rMSP-1₁₉ did not react with all the non-malaria and healthy donor sera, which represents 100% specificity. The rMSP-1₁₉ could be used as a potential antigen in serodiagnosis of malarial infection in humans.

Application of a Scalable Plant Transient Gene Expression Platform for Malaria Vaccine Development

Despite decades of intensive research efforts there is currently no vaccine that provides sustained sterile immunity against malaria. In this context, a large number of targets from the different stages of the Plasmodium falciparum life cycle have been evaluated as vaccine candidates. None of these candidates has fulfilled expectations, and as long as we lack a single target that induces strain-transcending protective immune responses, combining key antigens from different life cycle stages seems to be the most promising route toward the development of efficacious malaria vaccines. After the identification of potential targets using approaches such as omics-based technology and reverse immunology, the rapid expression, purification, and characterization of these proteins, as well as the generation and analysis of fusion constructs combining different promising antigens or antigen domains before committing to expensive and time consuming clinical development, represents one of the bottlenecks in the vaccine development pipeline. The production of recombinant proteins by transient gene expression in plants is a robust and versatile alternative to cell-based microbial and eukaryotic production platforms. The transfection of plant tissues and/or whole plants using Agrobacterium tumefaciens offers a low technical entry barrier, low costs, and a high degree of flexibility embedded within a rapid and scalable workflow. Recombinant proteins can easily be targeted to different subcellular compartments according to their physicochemical requirements, including post-translational modifications, to ensure optimal yields of high quality product, and to support simple and economical downstream processing. Here, we demonstrate the use of a plant transient expression platform based on transfection with A. tumefaciens as essential component of a malaria vaccine development workflow involving screens for expression, solubility, and stability using fluorescent fusion proteins. Our results have been implemented for the evidence-based iterative design and expression of vaccine candidates combining suitable P. falciparum antigen domains. The antigens were also produced, purified, and characterized in further studies by taking advantage of the scalability of this platform.

Malaria vaccine candidate antigen targeting the pre-erythrocytic stage of Plasmodium falciparum produced at high level in plants

Plants have emerged as low-cost production platforms suitable for vaccines targeting poverty-related diseases. Besides functional efficacy, the stability, yield, and purification process determine the production costs of a vaccine and thereby the feasibility of plant-based production. We describe high-level plant production and functional characterization of a malaria vaccine candidate targeting the pre-erythrocytic stage of Plasmodium falciparum. CCT, a fusion protein composed of three sporozoite antigens (P. falciparum cell traversal protein for ookinetes and sporozoites [PfCelTOS], P. falciparum circumsporozoite protein [PfCSP], and P. falciparum thrombospondin-related adhesive protein [PfTRAP]), was transiently expressed by agroinfiltration in Nicotiana benthamiana leaves, accumulated to levels up to 2 mg/g fresh leaf weight (FLW), was thermostable up to 80°C and could be purified to >95% using a simple two-step procedure. Reactivity of sera from malaria semi-immune donors indicated the immunogenic conformation of the purified fusion protein consisting of PfCelTOS, PfCSP_TSR, PfTRAP_TSR domains (CCT) protein. Total IgG from the CCT-specific mouse immune sera specifically recognized P. falciparum sporozoites in immunofluorescence assays and induced up to 35% inhibition in hepatocyte invasion assays. Featuring domains from three promising sporozoite antigens with different roles (attachment and cell traversal) in the hepatocyte invasion process, CCT has the potential to elicit broader immune responses against the pre-erythrocytic stage of P. falciparum and represents an interesting new candidate, also as a component of multi-stage, multi-subunit malaria vaccine cocktails.

Genetic diversity in human malarial parasites of Khyber Agency Pakistan

Malaria is wide spread in poor world and its burden has been assessed by the enumeration of malarial parasites in blood of patients. This study was designed to find a relationship between social structure, and spread of malaria in Khyber agency. The average parasite density was 2050 parasite/μl in Khyber Agency. Due to economic and social setup most of the people have habit of sleeping in open air thus playing role in high malaria prevalence and Plasmodium vivax remains the prevalent species. Genetic study performed on 110 Blood samples showed less genetic diversity for both Plasmodium vivax and Plasmodium falciparum. Eight alleles were distinguished both for Pvmsp 3α and Pvmsp 3β in total of 20 and 39 amplified samples of P. vivax respectively. Out of 17 samples amplified for P. falciparum 11 showed genotype K1 and 10 for MAD at Pfmsp-1 while 14 alleles were identified for 3D7/1C and two for FC27 of corresponding families of Pfmsp-2 gene. This shows that Plasmodium parasites are not genetically diverse in Khyber agency.

Platform for Plasmodium vivax vaccine discovery and development

Plasmodium vivax is the most prevalent malaria parasite on the American continent. It generates a global burden of 80-100 million cases annually and represents a tremendous public health problem, particularly in the American and Asian continents. A malaria vaccine would be considered the most cost-effective measure against this vector-borne disease and it would contribute to a reduction in malaria cases and to eventual eradication. Although significant progress has been achieved in the search for Plasmodium falciparum antigens that could be used in a vaccine, limited progress has been made in the search for P. vivax components that might be eligible for vaccine development. This is primarily due to the lack of in vitro cultures to serve as an antigen source and to inadequate funding. While the most advanced P. falciparum vaccine candidate is currently being tested in Phase III trials in Africa, the most advanced P. vivax candidates have only advanced to Phase I trials. Herein, we describe the overall strategy and progress in P. vivax vaccine research, from antigen discovery to preclinical and clinical development and we discuss the regional potential of Latin America to develop a comprehensive platform for vaccine development.

Genetic diversity in merozoite surface protein (MSP)-1 and MSP-2 genes of Plasmodium falciparum in a major endemic region of Iran

Merozoite surface protein-1 (MSP-1) and merozoite surface protein-2 (MSP-2) were used to develop vaccines and to investigate the genetic diversity in Plasmodium falciparum malaria in Iran. Nested polymerase chain reaction amplification was used to determine polymorphisms of block 2 of the MSP-1 and the central domain of MSP-2 genes. A total of 67 microscopically positive P. falciparum infected individuals from a major endemic region, southeast Iran, were included in this trial. Nine alleles of MSP-1 and 11 alleles of MSP-2 were identified. The results showed that amplified product from these surface antigen genes varied in size and there was specific pattern for each isolate. Besides, regarding this pattern, 23 multiple infections with at least 2 alleles were observed. While the endemic regions of malaria in Iran is classified in low to moderate group, but extensive polymorphism was observed for each marker and the MSP-2 central repeat was the most diverse that could be considered in designing malaria vaccine.

Overproduction of Pichia pastoris or Plasmodium falciparum protein disulfide isomerase affects expression, folding and O-linked glycosylation of a malaria vaccine candidate expressed in P. pastoris

Production of recombinant malaria proteins in the methylotrophic yeast Pichia pastoris has been difficult due to constraints in transcription, translation and/or post-translation controls. Use of codon-optimized genes has resolved many of the transcriptional controls; however, efforts to overcome translational and post-translational modifications involving disulfide bond formation and glycosylation have been mostly restricted to knocking-out putative N-linked glycosylation sites. We report now on the effect of overproduction of P. pastoris protein disulfide isomerase (PpPDI) and Plasmodium falciparum (PfPDI) on production of a disulfide-rich P. falciparum transmission-blocking vaccine candidate, Pfs25. Pfs25 is expressed in P. pastoris as two isoforms (A and B); the A form has been selected for Phase I human studies. Overproduction of PpPDI in the P. pastoris Pfs25 production clone markedly enhanced the expression level of Pfs25(A) and (B) by 3-fold, while overproduction of PfPDI increased the proportion of Pfs25(A) compared to (B). The resultant Pfs25 products were purified and fully characterized biochemically. In addition to differences in production levels, the mass spectra of PpPDI-Pfs25(A) compared to Pfs25(A) and PfPDI-Pfs25(A) were different due to the pattern and level of O-linked glycosylation. The overproduction of PpPDI or PfPDI provides new platforms for expression of disulfide-rich malaria proteins.

A novel method for development of malaria vaccines using full-length cDNA libraries

We describe a novel method to screen malaria DNA vaccine candidates using a full-length cDNA library and a murine malaria infection model. For the development of effective malaria vaccines, much effort has been made with meager success. The completion of genome sequencing of Plasmodium falciparum has provided invaluable information for achieving this goal. We have been studying full-length cDNA libraries of malaria parasites as a part of genome analysis. Mice vaccinated with a DNA vaccine consisting of 2000 pooled clones showed significantly prolonged survival after challenge infection. In addition, spleen cells of vaccinated mice produced augmented levels of IL-2 and IFN-gamma when incubated with the crude parasite antigens, indicating that cellular immunity plays an important role in the protection. This approach will not only form the basis for development of malaria vaccines but will also be applicable to other parasites and pathogenic microorganisms.

DEVELOPMENT AND REGULATION OF CELL-MEDIATED IMMUNE RESPONSES TO THE BLOOD STAGES OF MALARIA: Implications for Vaccine Research

The immune response to the malaria parasite is complex and poorly understood. Although antibodies and T cells can control parasite growth in model systems, natural immunity to malaria in regions of high endemicity takes several years to develop. Variation and polymorphism of antibody target antigens are known to impede immune responses, but these factors alone cannot account for the slow acquisition of immunity. In human and animal model systems, cell-mediated responses can control parasite growth effectively, but such responses are regulated by parasite load via direct effects on dendritic cells and possibly on T and B cells as well. Furthermore, high parasite load is associated with pathology, and cell-mediated responses may also harm the host. Inflammatory cytokines have been implicated in the pathogenesis of cerebral malaria, anemia, weight loss, and respiratory distress in malaria. Immunity without pathology requires rapid parasite clearance, effective regulation of the inflammatory anti-parasite effects of cellular responses, and the eventual development of a repertoire of antibodies effective against multiple strains. Data suggest that this may be hastened by exposure to malaria antigens in low dose, leading to augmented cellular immunity and rapid parasite clearance.

The immunological challenge to developing a vaccine to the blood stages of malaria parasites

Twenty-one years after malaria antigens were first cloned, a vaccine still appears to be a long way off. There have been periods of great excitement, and in model systems, subunit vaccine homologs can induce robust protection. However, significant challenges exist concerning antigenic variation and polymorphism, immunological non-responsiveness to individual vaccine antigens, parasite-induced apoptosis of immune effector and memory cells, and immune deviation as a result of maternal immunity and alterations of dendritic cell function. Novel approaches will be required. This review addresses some of the approaches that might present malaria antigens in a way designed to induce superior immune responses or that target novel conserved epitopes. Cell-mediated immunity, acting independently of antibody, may exert potent anti-parasite effects, and identification of multiple target antigens/epitopes could lead to the development of vaccines with profound efficacy.

Malaria vaccine developments

Large gains in the reduction of malaria mortality in the early 20th century were lost in subsequent decades. Malaria now kills 2-3 million people yearly. Implementation of malaria control technologies such as insecticide-treated bednets and chemotherapy could reduce mortality substantially, but an effective malaria vaccine is also needed. Advances in vaccine technology and immunology are being used to develop malaria subunit vaccines. Novel approaches that might yield effective vaccines for other diseases are being evaluated first in malaria. We describe progress in malaria vaccine development in the past 5 years: reasons for cautious optimism, the type of vaccine that might realistically be expected, and how the process could be hastened. Although exact predictions are not possible, if sufficient funding were mobilised, a deployable, effective malaria vaccine is a realistic medium-term to long-term goal.

DNA vaccines and their application against parasites--promise, limitations and potential solutions

DNA or nucleic acid vaccines are being evaluated for efficacy against a range of parasitic diseases. Data from studies in rodent model systems have provided proof of principle that DNA vaccines are effective at inducing both humoral and T cell responses to a variety of candidate vaccine antigens. In particular, the induction of potent cellular responses often gives DNA vaccination an immunological advantage over subunit protein vaccination. Protection against parasite challenge has been demonstrated in a number of systems. However, application of parasite DNA vaccines in large animals including ruminants, primates and humans has been compromised by the relative lack of immune responsiveness to the vaccines, but the reasons for this hyporesponsiveness are not clear. Here, we review DNA vaccines against protozoan parasites, in particular vaccines for malaria, and the use of genomic approaches such as expression library immunization to generate novel vaccines. The application of DNA vaccines in ruminants is reviewed. We discuss some of the approaches being evaluated to improve responsiveness in large animals including the use of cytokines as adjuvants, targeting molecules as delivery ligands, electroporation and CpG oligonucleotides.

Utilization of genomic sequence information to develop malaria vaccines

Recent advances in the fields of genomics, proteomics and molecular immunology offer tremendous opportunities for the development of novel interventions against public health threats, including malaria. However, there is currently no algorithm that can effectively identify the targets of protective T cell or antibody responses from genomic data. Furthermore, the identification of antigens that will stimulate the most effective immunity against the target pathogen is problematic, particularly if the genome is large. Malaria is an attractive model for the development and validation of approaches to translate genomic information to vaccine development because of the critical need for effective anti-malarial interventions and because the Plasmodium parasite is a complex multistage pathogen targeted by multiple immune responses. Sterile protective immunity can be achieved by immunization with radiation-attenuated sporozoites, and anti-disease immunity can be induced in residents in malaria-endemic areas. However, the 23 Mb Plasmodium falciparum genome encodes more than 5300 proteins, each of which is a potential target of protective immune responses. The current generation of subunit vaccines is based on a single or few antigens and therefore might elicit too narrow a breadth of response. We are working towards the development of a new generation vaccine based on the presumption that duplicating the protection induced by the whole organism may require a vaccine nearly as complex as the organism itself. Here, we present our strategy to exploit the genomic sequence of P. falciparum for malaria vaccine development.

Progress in the development of recombinant and synthetic blood-stage malaria vaccines

The use of asexual blood-stage proteins as malaria vaccines is strongly supported by experimental data directly implicating antibodies induced by these antigens in parasite clearance and protection from re-challenge. The selection of blood-stage antigens is based on their ability to interfere with the pathogenesis of clinical malaria by reducing parasitemias. These vaccines could complement other vaccines aimed at preventing infection, such as those targeted at pre-erythrocytic or mosquito stages of the parasite. Asexual blood-stage vaccines may reduce disease by blockade of red blood cell invasion, inhibition of parasite growth in red cells or interference in cytoadherence of infected red cells. Clearance of blood-stage parasites is dependent primarily on antibody-mediated mechanisms, but CD4 T cells may also play an important role in help for B cells and probably have a direct effector function in the clearance of blood-stage parasites. Since asexual blood-stage parasites reside within erythrocytes, they are accessible to immune clearance mechanisms only for a short time, which imposes special requirements on vaccines. For example, immunity that induces high titers of antibody will be required. Antigenic variation and extensive polymorphism of malarial proteins also needs to be addressed. Several recombinant antigens derived from blood-stage proteins have moved beyond basic research and are now poised for phase I trials in endemic countries. In this review we discuss the state of asexual blood-stage vaccines, focusing on recombinant antigens from Plasmodium falciparum. The significance of polymorphism and antigenic variation, the relevance of parasite immune evasion mechanisms, the need for reliable measures of successful intervention and new adjuvants are reviewed. Results from trials of asexual blood stage vaccine that support the continued effort to develop these antigens as key ingredients of multicomponent,multistage malaria vaccines are documented.

The malaria vaccine development program in Papua New Guinea

Through a collaborative project led by the Papua New Guinea Institute of Medical Research (PNGIMR), Papua New Guinea has a significant role in the global effort to develop a malaria vaccine, ensuring that the malaria patterns in Asia and the Pacific region are considered in vaccine development strategies. Some of the major perspectives and achievements of the program are discussed here, one of the most successful being the trial of Combination B, a vaccine comprising three asexual blood-stage proteins [merozoite surface protein (MSP)1, MSP2 and ring-infected erythrocyte surface antigen (RESA)], which led to a considerable reduction of parasite density in the immunized children.

Malaria vaccines: where are we and where are we going?

Malaria is still killing over one million people each year and its incidence is increasing. The need for an effective vaccine is greater than ever. A major difficulty with vaccine research is that the malaria parasite presents thousands of antigens to the human immune system that vary throughout its life cycle. Identifying those that may prove to be vaccine targets is complicated and time consuming. Most vaccines are targeted at individual stages of the malaria life cycle, although it is likely that only the development of a multistage vaccine will offer complete protection to both visitors to, and residents of, a malaria-endemic area. With the development of a successful vaccine other issues such as cost, distribution, education, and compliance will have to be addressed. This review describes some of the current vaccine candidates for immunising against malaria.