Vaccines against the blood stages of falciparum malaria

Immunological bases of increased susceptibility to invasive nontyphoidal Salmonella infection in children with malaria and anaemia

Malaria and anaemia are key underlying factors for iNTS disease in African children. Knowledge of clinical and epidemiological risk-factors for iNTS disease has not been paralleled by an in-depth knowledge of the immunobiology of the disease. Herein, we review human and animal studies on mechanisms of increased susceptibility to iNTS in children.

Development of designed site-directed pseudopeptide-peptido-mimetic immunogens as novel minimal subunit-vaccine candidates for malaria

Synthetic vaccines constitute the most promising tools for controlling and preventing infectious diseases. When synthetic immunogens are designed from the pathogen native sequences, these are normally poorly immunogenic and do not induce protection, as demonstrated in our research. After attempting many synthetic strategies for improving the immunogenicity properties of these sequences, the approach consisting of identifying high binding motifs present in those, and then performing specific changes on amino-acids belonging to such motifs, has proven to be a workable strategy. In addition, other strategies consisting of chemically introducing non-natural constraints to the backbone topology of the molecule and modifying the α-carbon asymmetry are becoming valuable tools to be considered in this pursuit. Non-natural structural constraints to the peptide backbone can be achieved by introducing peptide bond isosters such as reduced amides, partially retro or retro-inverso modifications or even including urea motifs. The second can be obtained by strategically replacing L-amino-acids with their enantiomeric forms for obtaining both structurally site-directed designed immunogens as potential vaccine candidates and their Ig structural molecular images, both having immuno-therapeutic effects for preventing and controlling malaria.

Immunity to recombinant plasmodium falciparum merozoite surface protein 1 (MSP1): protection in Aotus nancymai monkeys strongly correlates with anti-MSP1 antibody titer and in vitro parasite-inhibitory activity

A number of malarial blood-stage candidate vaccines are currently being tested in human clinical trials, but our understanding of the relationship between clinical immunity and data obtained from in vitro assays remains inadequate. An in vitro assay which could reliably predict protective immunity in vivo would facilitate vaccine development. Merozoite surface protein1 (MSP1) is a leading blood-stage malaria vaccine candidate, and anti-MSP1 antibodies from individuals that are clinically immune to malaria inhibit the invasion of Plasmodium merozoites into erythrocytes in vitro. Using expression in Escherichia coli and subsequent refolding, we have produced two allelic forms of MSP1(42) (FVO and 3D7). Aotus nancymai monkeys were immunized with MSP1(42)-FVO, MSP1(42)-3D7, or a combination of FVO and 3D7 allelic forms, (MSP1(42)-C1) and were subsequently challenged with Plasmodium falciparum FVO parasites. Sera obtained prior to challenge were tested by standardized enzyme-linked immunosorbent assay (ELISA) to determine antibody titer, and immunoglobulin G (IgG) fractions were also obtained from the same sera; the IgG fractions were tested in an in vitro growth inhibition (GI) assay to evaluate biological activity of the antibodies. Regardless of the immunogen used, all monkeys that had >200,000 ELISA units against MSP1(42)-FVO antigen before challenge controlled their infections. By contrast, all monkeys whose purified IgGs gave <60% inhibition activity in an in vitro GI assay with P. falciparum FVO required treatment for high parasitemia after challenge. There is a strong correlation between ELISA units (Spearman rank correlation of greater than 0.75) or GI activity (Spearman rank correlation of greater than 0.70) and protective immunity judged by various parameters (e.g., cumulative parasitemia or day of patency). These data indicate that, in this monkey model, the ELISA and GI assay values can significantly predict protective immunity induced by a blood-stage vaccine, and they support the use of these assays as part of evaluation of human clinical trials of MSP1-based vaccines.

Immunogenicity of C-terminus of Plasmodium falciparum merozoite surface protein 1 expressed as a non-glycosylated polypeptide in yeast

The C-terminal region of the merozoite surface protein 1 (MSP119) is one of the most promising vaccine candidates against the erythrocytic forms of malaria. In the present study, a gene encoding Plasmodium falciparum MSP119 was expressed in yeast Pichia pastoris. A non-glycosylated form of the recombinant protein MSP119 was purified from culture medium. This recombinant protein maintains its antigenicity. Significant immune responses were seen in C57BL/6 mice after the second immunization. Moreover, the specific antibodies recognized the native antigens of P. falciparum. The prevailing isotypes of immunoglobulin (Ig) G associated with immunization were IgG1, IgG2a and IgG2b. The antibodies isolated from mouse sera immunized with MSP119 can inhibit parasite growth in vitro. Based on these immunological studies, we concluded that MSP119 deserves further evaluation in pre-clinical immunizations against P. falciparum.

posttranslational modification of recombinant Plasmodium falciparum apical membrane antigen 1: impact on functional immune responses to a malaria vaccine candidate

Recombinant apical membrane antigen 1 (AMA1) is a leading vaccine candidate for Plasmodium falciparum malaria, as antibodies against recombinant P. falciparum AMA1 (PfAMA1) interrupt merozoite invasion into erythrocytes. In order to investigate the role of posttranslational modification in modulating the functional immune response to recombinant AMA1, two separate alleles of PfAMA1 (FVO and 3D7), in which native N-glycosylation sites have been mutated, were produced using Escherichia coli and a Pichia pastoris expression system. Recombinant Pichia pastoris AMA1-FVO (PpAMA1-FVO) and PpAMA1-3D7 are O-linked glycosylated, and 45% of PpAMA1-3D7 is nicked, though all four recombinant molecules react with conformation-specific monoclonal antibodies. To address the immunological effect of O-linked glycosylation, we compared the immunogenicity of E. coli AMA1-FVO (EcAMA1-FVO) and PpAMA1-FVO antigens, since both molecules are intact. The effect of antigen nicking was then investigated by comparing the immunogenicity of EcAMA1-3D7 and PpAMA1-3D7. Our data demonstrate that there is no significant difference in the rabbit antibody titer elicited towards EcAMA1-FVO and PpAMA1-FVO or to EcAMA1-3D7 and PpAMA1-3D7. Furthermore, we have demonstrated that recombinant AMA1 (FVO or 3D7), whether expressed and refolded from E. coli or produced from the Pichia expression system, is equivalent and mimics the functionality of the native protein in in vitro growth inhibition assay experiments. We conclude that in the case of recombinant AMA1, the E. coli- and P. pastoris-derived antigens are immunologically and functionally equivalent and are unaffected by the posttranslational modification resulting from expression in these two systems.

Induction of strain-transcending immunity against Plasmodium chabaudi adami malaria with a multiepitope DNA vaccine

A major goal of current malaria vaccine programs is to develop multivalent vaccines that will protect humans against the many heterologous malaria strains that circulate in endemic areas. We describe a multiepitope DNA vaccine, derived from a genomic Plasmodium chabaudi adami DS DNA expression library of 30,000 plasmids, which induces strain-transcending immunity in mice against challenge with P. c. adami DK. Segregation of this library and DNA sequence analysis identified vaccine subpools encoding open reading frames (ORFs)/peptides of >9 amino acids [aa] (the V9+ pool, 303 plasmids) and >50 aa (V50+ pool, 56 plasmids), respectively. The V9+ and V50+ plasmid vaccine subpools significantly cross-protected mice against heterologous P. c. adami DK challenge, and protection correlated with the induction of both specific gamma interferon production by splenic cells and opsonizing antibodies. Bioinformatic analysis showed that 22 of the V50+ ORFs were polypeptides conserved among three or more Plasmodium spp., 13 of which are predicted hypothetical proteins. Twenty-nine of these ORFs are orthologues of predicted Plasmodium falciparum sequences known to be expressed in the blood stage, suggesting that this vaccine pool encodes multiple blood-stage antigens. The results have implications for malaria vaccine design by providing proof-of-principle that significant strain-transcending immunity can be induced using multiepitope blood-stage DNA vaccines and suggest that both cellular responses and opsonizing antibodies are necessary for optimal protection against P. c. adami.

Progress in DNA-based heterologous prime-boost immunization strategies for malaria

An effective vaccine against malaria is urgently required to relieve the immense human suffering and mortality caused by this parasite. A successful subunit vaccine against the liver stage of malaria will require the induction of high levels of protective T cells. Despite success in small animal models, DNA vaccines fail to induce strong cellular immune responses in humans. However, DNA vaccines can induce a T-cell response that can be strongly boosted by recombinant viral vectors. We have evaluated this heterologous prime-boost approach using the Plasmodium berghei mouse model for immunogenicity and protective efficacy against malaria challenge using combinations of plasmid DNA, recombinant modified vaccinia virus Ankara, fowlpox virus, and non-replicating adenovirus. We have proceeded to test immunogenicity and efficacy of successful heterologous prime-boost vaccines in phase I/IIa trials in malaria naïve subjects in the UK and in semi-immune individuals in The Gambia. In these clinical trials, remarkably high levels of effector T-cell responses have been induced and significant protection documented in a human sporozoite challenge model. We summarize the preclinical design and development of these heterologous prime-boost vaccines and discuss the encouraging results that have been observed in vaccinated humans.

Members of the merozoite surface protein 7 family with similar expression patterns differ in ability to protect against Plasmodium yoelii malaria

Previously, we described the isolation of the Plasmodium yoelii sequence-related molecules P. yoelii MSP-7 (merozoite surface protein 7) and P. yoelii MSRP-2 (MSP-7-related protein 2) by their ability to interact with the amino-terminal end of P. yoelii MSP-1 in a yeast two-hybrid system. One of these molecules was the homologue of Plasmodium falciparum MSP-7, which was biochemically isolated as part of the shed MSP-1 complex. In the present study, with antibodies directed against recombinant proteins, immunoprecipitation analyses of the rodent system demonstrated that both P. yoelii MSP-7 and P. yoelii MSRP-2 could be isolated from parasite lysates and from parasite culture supernatants. Immunofluorescence studies colocalized P. yoelii MSP-7 and P. yoelii MSRP-2 with the amino-terminal portion of MSP-1 and with each other on the surface of schizonts. Immunization with P. yoelii MSRP-2 but not P. yoelii MSP-7 protected mice against a lethal infection with P. yoelii strain 17XL. These results establish that both P. yoelii MSP-7 and P. yoelii MSRP-2 are expressed on the surface of merozoites and released from the parasite and that P. yoelii MSRP-2 may be the target of a protective immune response.

The immune response to Plasmodium falciparum malaria

Malaria is still a major cause of severe disease which is responsible for millions of deaths, mostly in children under 5 years old, in tropical countries, especially sub-Saharan Africa. Complications of severe anaemia and cerebral malaria are thought to be the major cause of morbidity and mortality but recent evidence suggests that the host's immunological response could also contribute to the pathophysiology of the disease in human beings. Intensive studies of the immune response to malaria parasites in human beings have provided a wealth of information about the cells and cytokines implicated in the pathophysiology of survival and fatal outcome in severe infections. This review focuses on the pivotal role of macrophages and other important cellular effectors, molecules, and cytokines involved in the activation of the immune response at the different stages of human falciparum malaria. Our understanding of the putative mechanisms by which cytokines may mediate beneficial and harmful effects, through activation of phagocytic cells, could help to develop new treatment strategies, regardless of the emergence of parasite multidrug resistance.

Safety, tolerability and immunogenicity of new formulations of the Plasmodium falciparum malaria peptide vaccine SPf66 combined with the immunological adjuvant QS-21

SPf66 is a synthetic malaria peptide vaccine, which has been widely tested in combination with aluminium hydroxide (alum) as the adjuvant. Since this formulation is weakly immunogenic, we sought to improve its immunogenicity by using the saponin adjuvant QS-21. SPf66/QS-21 vaccines were evaluated for safety, tolerability and immunogenicity in healthy adults. The vaccines were found to be safe in 87/89 (97.8%) volunteers studied. However, two individuals developed severe vaccine allergy following the third dose of 1/3 SPf66/QS-21 formulations tested. Vaccine formulations containing QS-21 induced a 45- to over 200-fold increase in anti-SPf66 IgG titres over the alum formulation after the second and third doses, respectively. Anti-SPf66 antibody from some subjects reacted against asexual blood stage parasites, as demonstrated by immunofluorescence and immunoblotting. Antibody responses generated by the QS-21 formulations were of longer duration compared to those evoked by the alum formulation. While SPf66/alum has been found to induce only CD4+ T cell response, the QS-21 formulations exhibited the potential to also elicit SPf66-specific CD8+ responses. These observations demonstrate that the use of QS-21 can substantially enhance the immunogenicity of peptide vaccines, such as SPf66.

Are trials in New World monkeys on the critical path for blood-stage malaria vaccine development?

The development of malaria blood-stage vaccines is gathering momentum: there are several new funding initiatives, one multiantigen formulation is currently being tested and at least one other blood-stage vaccine is expected to begin trials in 2001. However, there is no consensus over the best way to select which form of an antigen to take into clinical testing. There is thus a danger that less-effective vaccines might be tested in the field in the order of their availability, rather than merit. Here, we argue that first proving efficacy in the New World monkey challenge model would accelerate development.

Structural conformers produced during malaria vaccine production in yeast

A recombinant protein expression system based on Saccharomyces cerevisiae has been used to express malarial vaccine candidate antigens. The antigens so produced have been used in three Phase 1 clinical trials and numerous preclinical non-human primate trials. Further Phase I trials are planned using these candidate vaccine antigens. These molecules were identified as attractive candidates for antimalarial vaccines, as they are all surface-exposed at some stage in the parasite's life cycle. They all share an unusual structural feature: epidermal growth factor (EGF)-like motifs. When these proteins are expressed in our S. cerevisiae expression system, they are produced as a series of stable structural conformers, each with a different disulphide bonding pattern. This leads to both biochemical and, more importantly, antigenic differences between the conformers (e.g. presence or absence of an antibody B cell epitope). These findings have important ramifications for other EGF-domain-containing proteins expressed in S. cerevisiae, or for proteins which contain other cysteine-folding motifs not normally expressed by this organism, both for vaccine production or for research/reagent purposes.

The immunology of malaria infection

As global malaria mortality increases the urgency for vaccine development, analysis of immune responses in naturally exposed populations is providing clues to the nature of protective immunity. Recently, sophisticated immune evasion strategies adopted by the parasite have been analysed at the molecular level. More immunogenic vaccination strategies have been identified, providing renewed optimism that effective malaria control through vaccination should be feasible.