Molecular analysis of Plasmodium vivax relapses using the MSP1 molecule as a genetic marker.

Plasmodium vivax has hepatocytic dormant stages, hypnozoites, that cause relapses. This work compared paired isolates from primary attacks and relapses obtained from 10 individuals in Brazil using the merozoite surface protein 1 gene, PvMSP1, as a genetic marker. Four samples from primary attacks contained genetically mixed parasites harboring the 2 major PvMSP1 allelic forms. PCR revealed the presence of these 2 forms in the relapse parasites of 2 patients, demonstrating that the activation of hypnozoites is not clonal. DNA sequences from paired primary/relapse samples demonstrated that the parasites from the primary attack are identical to those in relapse samples in which the same allele forms were detected in both infections. Studies on the naturally acquired humoral immune responses of these patients against a recombinant protein expressing the C-terminus PvMSP1 demonstrated an increase in the titers, affinity maturation, and predominance of the IgG1 subclass during the relapse.