Acquisition of invasion-inhibitory antibodies specific for the 19-kDa fragment of merozoite surface protein 1 in a transmigrant population requires multiple infections

Antibodies against the 19 kDa C-terminal fragment of merozoite surface protein 1 (MSP1(19)) are a major component of the invasion-inhibitory response in individuals immune to malaria. We report here the acquisition of MSP1(19)-specific invasion-inhibitory antibodies in a group of transmigrants who experienced their sequential malaria infections during settlement in an area of Indonesia where malaria is highly endemic. We used 2 transgenic Plasmodium falciparum parasite lines that expressed either endogenous MSP1(19) or the homologous region from P. chabaudi to measure the MSP1(19)-specific invasion-inhibitory antibodies. The results revealed that the acquisition of MSP1(19)-specific invasion-inhibitory antibodies required 2 or more P. falciparum infections. In contrast, enzyme-linked immunosorbent assays on the same serum samples showed that MSP1(19)-specific antibodies are present after the first malaria infection. This delay in the acquisition of functional antibodies by residents of areas where malaria is endemic is consistent with the observation that multiple malaria infections are required before clinical immunity is acquired.