Allelic Diversity and Naturally Acquired Allele-Specific Antibody Responses to Plasmodium falciparum Apical Membrane Antigen 1 in Kenya

Abstract

Although Plasmodium falciparum apical membrane antigen 1 (AMA1) is a leading malaria vaccine candidate, extensive allelic diversity may compromise its vaccine potential. We have previously shown that naturally acquired antibodies to AMA1 were associated with protection from clinical malaria in this Kenyan population. To assess the impact of allelic diversity on naturally acquired immunity, we first sequenced the ectodomainencoding region of P. falciparum ama1 from subjects with asymptomatic, mild, and severe malaria and measured allele frequency distributions. We then measured antibodies to three allelic AMA1 proteins (AMA1_3D7, AMA1_FVO, and AMA1_HB3) and used competition enzyme-linked immunosorbent assays (ELISAs) to analyze allele-specific antibodies. Seventy-eight unique haplotypes were identified from 129 alleles sampled. No clustering of allelic haplotypes with disease severity or year of sampling was observed. Differences in nucleotide frequencies in clinical (severe plus mild malaria) versus asymptomatic infections were observed at 16 polymorphic positions. Allele frequency distributions were indicative of balancing selection, with the strongest signature being identified in domain III (Tajima's D ‫؍‬ 2.51; P < 0.05). Antibody reactivities to each of the three allelic AMA1 proteins were highly correlated (P < 0.001 for all pairwise comparisons). Although antibodies to conserved epitopes were abundant, 48% of selected children with anti-AMA1 IgG (n ‫؍‬ 106) had detectable reactivity to allele-specific epitopes as determined by a competition ELISA. Antibodies to both conserved and allele-specific epitopes in AMA1 may contribute to clinical protection.

Distinct patterns of diversity, population structure and evolution in the AMA1 genes of sympatric Plasmodium falciparum and Plasmodium vivax populations of Papua New Guinea from an area of similarly high transmission

“…However, the results of the present study are analogous to those obtained for 372 global Pv AMA1 sequences, the largest and most comprehensive analysis of worldwide Pv AMA1 diversity performed to date, and suggest that DI is an immunodominant region of Pv AMA1 [51]. That nucleotide diversity and signatures of immune selection for Pf AMA1 are highest within DI and DIII, has also been reported following analysis of African P. falciparum sequences [36,86]. It has been proposed that strong signatures of selection within Pf AMA1 DIII may be due to T-cell responses against allele-specific epitopes [36,87,88].…”

Overcoming Allelic Specificity by Immunization with Five Allelic Forms of Plasmodium falciparum Apical Membrane Antigen 1

“…In addition, other polymorphic sites in domain I (amino acid residues 172, 207, 224, 242, and 283) and domain III (amino acid residues 439, 485, and 496) were also found to be important. Since studies with field parasites showed a strong balancing selection in domain III and not only in domain I (7,9,10), it is reasonable to suggest that there are also key polymorphic sites in domain III. This study and others have made it clear that not all of the amino acid polymorphisms contribute equally to the growth-inhibitory activity.…”

“…While a study by Triglia et al has shown that AMA1 is an essential protein for erythrocyte invasion (6), the amino acid polymorphism in the AMA1 protein has hampered AMA1 vaccine development. Indeed, there is evidence of balancing selection in domains I and III in field parasites from Nigeria (7), Papua New Guinea (8), Thailand (9), and Kenya (10). We and other investigators have conducted multiple phase 1 trials of AMA1 using AMA1-3D7, AMA1-FVO, or a mixture of both (AMA1-C1), and the antibodies induced by the vaccines showed strain-specific functional activities, as judged by the in vitro growth inhibition assay (GIA; also referred to as the invasion inhibition assay [IIA]) (11)(12)(13)(14).…”

Malaria Immunoepidemiology in Low Transmission: Correlation of Infecting Genotype and Immune Response to Domains of Plasmodium falciparum Merozoite Surface Protein 3

“…Allelespecific responses are frequently detected using antibody depletion experiments, where antibodies that recognize one antigen are depleted from a serum sample by multiple incubations with that antigen before the presence of antibodies that recognize a different antigen variant is detected. Such studies have been extremely successful, with important implications for vaccine candidates such as AMA1 (23,24), MSP1 (14,34), and MSP3 (4,22,25). However, they are not always possible in all studies because of sample volume limitations, particularly when multiple different variants of each antigen are used in separate competition experiments on the same serum sample.…”