Allelic Diversity and Antibody Recognition of Plasmodium Falciparum Merozoite Surface Protein 1 During Hypoendemic Malaria Transmission in the Brazilian Amazon Region

The polymorphic merozoite surface protein (MSP-1) of Plasmodium falciparum is a major asexual blood-stage malaria vaccine candidate. The impact of allelic diversity on recognition of MSP-1 during the immune response remains to be investigated in areas of hypoendemicity such as the Brazilian Amazon region. In this study, PCR was used to type variable regions, blocks 2, 4, and 10, of the msp-1 gene and to characterize major gene types (unique combinations of allelic types in variable blocks) in P. falciparum isolates collected across the Amazon basin over a period of 12 years. Twelve of the 24 possible gene types were found among 181 isolates, and 68 (38%) of them had more than one gene type. Temporal, but not spatial, variation was found in the distribution of MSP-1 gene types in the Amazon. Interestingly, some gene types occurred more frequently than expected from random assortment of allelic types in different blocks, as previously found in other areas of endemicity. We also compared the antibody recognition of polymorphic (block 2), dimorphic (block 6), and conserved (block 3) regions of MSP-1 in Amazonian malaria patients and clinically immune Africans, using a panel of recombinant peptides. Results were summarized as follows. (i) All blocks were targeted by naturally acquired cytophilic antibodies of the subclasses IgG1 and IgG3, but the balance between IgG1 and IgG3 depended on the subjects' cumulative exposure to malaria. (ii) The balance between IgG1 and IgG3 subclasses and the duration of antibody responses differed in relation to distinct MSP-1 peptides. (iii) Antibody responses to variable blocks 2 and 6 were predominantly type specific, but variant-specific antibodies that target isolate-specific repetitive motifs within block 2 were more frequent in Amazonian patients than in previously studied African populations.