Identification of antigenically active tryptic fragments of apical membrane antigen-1 (AMA1) of Plasmodium chabaudi malaria: strategies for assembly of immunologically active peptides

Erythrocyte invasion and functionally inhibitory antibodies in Plasmodium falciparum malaria

Malaria is a disease that kills several million people every year. P. falciparum merozoites invade new erythrocytes every 48 h, causing fever, anemia and cerebral malaria. Effective immunity against malaria develops slowly and only after repeated exposure. Antibodies are an important part of this immunity. However, the antigens that mediate immunity by inducing functionally imperative antibodies have not yet been identified. This review gives an overview of the erythrocyte invasion process, which has been described to include several different antigens. Invasion inhibitory antibodies can inhibit merozoite penetration of new erythrocytes, and different methods for measurement of the presence of functionally important antibodies have been employed. ELISA, Invasion inhibition assays and ADCI are some of the methods discussed.

Apical membrane antigen 1: a malaria vaccine candidate in review

Apical membrane antigen 1 (AMA1) is a micronemal protein of apicomplexan parasites that appears to be essential during the invasion of host cells. Immune responses to Plasmodium AMA1 can have profound parasite-inhibitory effects, both as measured in vitro and in animal challenge models, suggesting AMA1 as a potential vaccine component. However, AMA1 is polymorphic, probably as a result of immune selection operating on an important target of naturally occurring immunity. The current understanding of AMA1 will be presented, particularly in relation to the vaccine potential of AMA1 and the approaches being taken towards clinical development.

Allelic polymorphisms in apical membrane antigen-1 are responsible for evasion of antibody-mediated inhibition in Plasmodium falciparum

Apical membrane antigen-1 (AMA-1) is a target of antibodies that inhibit invasion of Plasmodium falciparum into human erythrocytes and is a candidate for inclusion in a malaria vaccine. We have identified a line of P. falciparum (W2mef) less susceptible to anti-AMA1 antibodies raised to the protein from a heterologous parasite line (3D7). We have constructed transgenic P. falciparum expressing heterologous AMA-1 alleles. In vitro invasion assays show that these transgenic parasites differ from parental lines in susceptibility to inhibitory antibodies, providing direct evidence that sequence polymorphisms within AMA-1 are responsible for evasion of immune responses that inhibit parasite invasion. We also generated a parasite line that would express a chimeric AMA-1 protein, in which highly polymorphic residues within domain 1 were exchanged. Inhibition assays suggest that these residues are not sufficient for inhibition by invasion-blocking antibodies. This study is the first to use P. falciparum allelic exchange to examine the relationship between genetic diversity and susceptibility to protective antibodies. The findings have important implications for the development of an AMA-1-based malaria vaccine.

Induction of parasite growth-inhibitory antibodies by a virosomal formulation of a peptidomimetic of loop I from domain III of Plasmodium falciparum apical membrane antigen 1

Apical membrane antigen 1 (AMA-1) of Plasmodium falciparum is a leading candidate antigen for inclusion in a malaria subunit vaccine. Its ectodomain can be divided into three subdomains, each with disulfide bond-stabilized structures. Since the majority of antibodies raised against the ectodomain appear to recognize strain-specific epitopes in domain I, we attempted to develop a vaccine formulation which directs the immune response to a region that contains more conserved epitopes. Here we demonstrate that a virosomal formulation of a peptide that mimics the semiconserved loop I of domain III elicits parasite growth-inhibitory antibodies. A synthetic peptide comprising residues 446 to 490 of AMA-1 (AMA-1(446-490)) was conjugated through the N terminus to a derivative of phosphatidylethanolamine and the phosphatidylethanolamine-peptide conjugate was incorporated into immunopotentiating reconstituted influenza virosomes as a human-compatible antigen delivery system. Both cyclized and linear versions of the peptide antigen elicited antibodies which specifically bound to parasite-expressed AMA-1 in Western blotting with parasite lysates as well as in immunofluorescence assays with blood stage parasites. All 11 peptidomimetic-specific monoclonal antibodies generated were cross-reactive with parasite-expressed AMA-1. Antigen binding assays with a library of overlapping cyclic peptides covering the target sequence revealed differences in the fine specificity of these monoclonal antibodies and provided evidence that at least some of them recognized discontinuous epitopes. The two immunodominant epitopes comprised the conserved linear sequences K(459)RIKLN(464) and D(467)DEGNKKII(475). A key feature of the synthetic vaccine formulation proposed here is the display of the peptide antigen in a native-like state on the surface of the virosome.