Plasmodium falciparum population dynamics in a cohort of pregnant women in Senegal

Abstract

Background: Pregnant women acquire protective antibodies that cross-react with geographically diverse placental Plasmodium falciparum isolates, suggesting that surface molecules expressed on infected erythrocytes by pregnancy-associated malaria (PAM) parasites have conserved epitopes and, that designing a PAM vaccine may be envisaged. VAR2CSA is the main candidate for a pregnancy malaria vaccine, but vaccine development may be complicated by its sequence polymorphism.

Methods: The dynamics of P. falciparum genotypes during pregnancy in 32 women in relation to VAR2CSA polymorphism and immunity was determined. The polymorphism of the msp2 gene and five microsatellites was analysed in consecutive parasite isolates, and the DBL5epsilon + Interdomain 5 (Id5) part of the var2csa gene of the corresponding samples was cloned and sequenced to measure variation.

Results: In primigravidae, the multiplicity of infection in the placenta was associated with occurrence of low birth weight babies. Some parasite genotypes were able to persist over several weeks and, still be present in the placenta at delivery particularly when the host anti-VAR2CSA antibody level was low. Comparison of diversity among genotyping markers confirmed that some PAM parasites may harbour more than one var2csa gene copy in their genome.

Conclusions: Host immunity to VAR2CSA influences the parasite dynamics during pregnancy, suggesting that the acquisition of protective immunity requires pre-exposure to a limited number of parasite variants. Presence of highly conserved residues in surface-exposed areas of the VAR2CSA immunodominant DBL5epsilon domain, suggest its potential in inducing antibodies with broad reactivity.

Figure 1

Dynamics of parasite populations during pregnancy. Two kinds of patterns (A and B) are described. P1, P2 represent positive parasitaemia from peripheral blood collected during the pregnancy, Pla represents placental blood collected at delivery. msp2/microsatellites genotypes are represented by squares. For one pattern, the various colors of the square represent a distinct msp2/microsatellite genotype. The different DBL5ε+Id5 sequence types are represented by different colored circles.

Figure 2

Distribution of Birth Weight according to parity and to diversity of Plasmodium falciparum placental infection. Diversity was determined by msp2/microsatellites genotyping.

Figure 3

Phylogenetic relationship between different VAR2CSA DBL5ε sequences. Bayesian inference tree of 172 DBL5ε sequences from PAM isolates and 17 DBL5ε database sequences available from Genbank (MTS1, M24, WR80, PC40, P13, FCR3, IT4var4, V1 S, Indo, 3D7, KMWII, T2C6, Mcvar6, HB3-1, HB3-2, PC49, P13). Multiple and distinct DBL5ε sequences from monoallelic isolates are shown in colored circles.

Figure 4

DBL5ε variable positions. A. Positions where all residues have a conservation of <70% in the alignment of 172 sequences from pregnant women are mapped in red on a DBL5ε structure model [36]. B. The same mapped positions as A, where the model is rotated 180 degrees, showing the variable sites mostly located in the S3 region of the DBL structure.

Figure 5

DBL5ε conserved and immunogenic positions. A. Positive selection sites mapped in the DBL5ε structure model are colored in red. In orange are shown sites which correspond to differential positions associated to parity and birth weight in the alignment of the 172 sequences from pregnant women. In purple is shown the position that is both under positive selection pressure and correlated with birth weight and parity. B. Similar to A, but rotated 180 degrees. C. Epitopes identified previously, in blue are shown the conserved residues, and in green, the variable residues, predicted to be targeted by surface reactive antibodies[51].