Author(s): Kwadwo A. Kusi 1,2, Bart W. Faber 1, Alan W. Thomas 1,*, Edmond J. Remarque 1
Introduction
Malaria continues to be one of the most important human parasitic diseases, with a global estimate of about 247 million clinical cases and almost 1 million deaths annually [1]. The greater burden of the disease is caused by Plasmodium falciparum in sub-Saharan Africa, where children under 5 years old, pregnant women (mostly primigravid) and their foetuses are at the greatest risk. A cost-effective vaccine would form a powerful additional component in control strategies for malaria and a number of Plasmodium antigens expressed at different stages of the parasite's complex life cycle are currently undergoing clinical evaluation [2].
Among the candidates in clinical testing is Plasmodium falciparum Apical Membrane Antigen 1 (Pf AMA1), a protein expressed in sporozoites and in merozoites of both liver and asexual erythrocytic development stages, the vaccine-related properties of which has recently been reviewed [3]. In brief, AMA1 is a merozoite membrane protein initially located in micronemes. Around the time of merozoite release from schizonts AMA1 is translocated to the merozoite surface, where it is involved in merozoite/red cell interactions preceding invasion [4]-[6]. Anti-AMA1 antibodies can interfere with AMA1 function and prevent invasion in vitro [7]-[10], this effect requiring immunisation with correctly folded AMA1 [11], [12]. The ectodomain of AMA1, which is the vaccine target, is shed as 44 and 48 kDa alternate proteins from the merozoite surface upon RBC invasion [13]. The amino acid sequence of the ectodomain has 16 cysteine residues that are conserved in all AMA1 sequences and these form disulphide bonds that result in a structure with three distinguishable but interactive domains (reviewed in [3]).
Polymorphism in AMA1 has long been evident [14], thought to be an effect of selection exerted by host immune responses [15], [16]. Immunisation with one allele of AMA1 ectodomain induces antibodies that inhibit homologous parasite growth in vitro to a greater extent as compared to heterologous parasites [12], [17], [18]. The induction of functional antibodies has been demonstrated in a number of ways, including rodent and primate challenge/passive immunisation studies [15], [19]-[21]. In some cases, and particularly with the rodent parasite P. chabaudi , antibodies are protective against parasites expressing homologous AMA1 but not those expressing heterologous AMA1 alleles [15], [22]. Antibodies to both conserved and strain-specific Pf AMA1 antibody epitopes have been observed in malaria-exposed humans [23], [24].
About 10% of amino acid residues of AMA1 are polymorphic, and even when appearing distant in the primary structure, may cluster within the tertiary structure [25]-[27]. These polymorphic clusters occur predominantly on one surface of the AMA1 molecule, which suggests that this face is accessible to antibody at the parasite surface [28]. This points to the significance of strain-specific epitopes in eliciting protective antibodies [12], [17], [29], although conserved AMA1 epitopes are also targets for inhibitory antibodies [30], [31].
Antibodies induced by immunisation with a combination of two allelic forms of Pf AMA1 (FVO and 3D7) inhibit the in vitro growth of...