A principal target of human immunity to malaria identified by molecular population genetic and immunological analyses

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Abstract
New strategies are required to identify the most important targets of protective immunity in complex eukaryotic pathogens. Natural selection maintains allelic variation in some antigens of the malaria parasite Plasmodium falciparum1,2,3. Analysis of allele frequency distributions could identify the loci under most intense selection4,5,6,7. The merozoite surface protein 1 (Msp1) is the most-abundant surface component on the erythrocyte-invading stage of P. falciparum8,9,10. Immunization with whole Msp1 has protected monkeys completely against homologous11 and partially against non-homologous12 parasite strains. The single-copy msp1 gene, of about 5 kilobases, has highly divergent alleles13 with stable frequencies in endemic populations14,15. To identify the region of msp1 under strongest selection to maintain alleles within populations, we studied multiple intragenic sequence loci in populations in different regions of Africa and Southeast Asia. On both continents, the locus with the lowest inter-population variance in allele frequencies was block 2, indicating selection in this part of the gene. To test the hypothesis of immune selection, we undertook a large prospective longitudinal cohort study. This demonstrated that serum IgG antibodies against each of the two most frequent allelic types of block 2 of the protein were strongly associated with protection from P. falciparum malaria.