Polymorphism in two merozoite surface proteins of Plasmodium falciparum isolates from Gabon

Abstract

Background: Plasmodium falciparum antigenic diversity and polymorphism confuses the issue of antimalarial vaccine development. Merozoite surface protein (MSP)-1 and -2 are two highly polymorphic vaccine candidates. Characterisation of their precise polymorphism in endemic regions may facilitate the design of an effective vaccine.

Methods: Isolates obtained in 52 Gabonese children presenting with uncomplicated malaria were genotyped by nested-PCR of msp-1 block 2, and msp-2 block 3, to analyze both parasite population polymorphism and clone fluctuations.

Results: Twenty-five and 19 different alleles were respectively obtained for msp-1 and msp-2 loci, the RO33 family of msp-1 being poorly polymorphic. Four cases of non-random distribution of alleles were reported of the FC27, and/or 3D7 families of msp-2. All but two isolates were composed of more than one genotype, and the multiplicity of infection (MOI) was 4.0. Neither parasite density nor age was related to MOI. Clone fluctuations were studied for ten subjects who were sampled again at reappearance of parasites in blood. Disappearance and reappearance of alleles were observed following treatment, suggesting difficulties in assessing polymorphism and in distinguishing reinfection from recrudescence.

Conclusion: P. falciparum polymorphism is extensive in Southeast Gabon, and most of infections are composed of multiple clones. The fluctuation of clones contributes to parasite diversity.

Figure 1

Mean number of genotypes in merozoite surface protein (msp)-1 and -2 polymorphic families of Plasmodium falciparum collected in the blood of 52 children, 0 to 95 months old, presenting with a malaria attack in Gabon, by age.

Figure 2

Examples of clonal fluctuations in successive blood samples between Day 0 and Day 28. The different clonal fluctuation figures were classified by type a, b, or c. Type a: one or several of the initial allele(s) transiently vanished, and were present again at the final point. Type b: new alleles appeared at intermediate and/or final points, although some of the initial alleles may be still present. Type c: alleles observed at intermediate and final points were all present at D0.