Protective Antibodies against Placental Malaria and Poor Outcomes during Pregnancy, Benin

Abstract

Placental malaria is caused by Plasmodium falciparum-infected erythrocytes that bind to placental tissue. Binding is mediated by VAR2CSA, a parasite antigen coded by the var gene, which interacts with chondroitin sulfate A (CSA). Consequences include maternal anemia and fetal growth retardation. Antibody-mediated immunity to placental malaria is acquired during successive pregnancies, but the target of VAR2CSA-specific protective antibodies is unclear. We assessed VAR2CSA-specific antibodies in pregnant women and analyzed their relationships with protection against placental infection, preterm birth, and low birthweight. Antibody responses to the N-terminal region of VAR2CSA during early pregnancy were associated with reduced risks for infections and low birthweight. Among women infected during pregnancy, an increase in CSA binding inhibition was associated with reduced risks for placental infection, preterm birth, and low birthweight. These data suggest that antibodies against VAR2CSA N-terminal region mediate immunity to placental malaria and associated outcomes. Our results validate current vaccine development efforts with VAR2CSA N-terminal constructs.

Keywords: Benin; Plasmodium falciparum; VAR2CSA; antibodies; erythrocytes; malaria; outcomes; parasites; placental malaria; pregnancy.

Figure 1

Antibody levels at study inclusion and delivery, by parity, against placental malaria in pregnant women, Benin. A) Apical membrane antigen 1 (AMA-1); B–F) Duffy binding-like (DBL) antigen; G) Full-length ectodomain of variant surface antigen 2 chondroitin sulfate (FV2); H) Variant surface antigen (VSA). Solid circles indicate medians for inclusion, solid squares indicate medians for delivery, and error bars indicate interquartile ranges. AU, absorbance units; rMFI, relative median fluorescence intensity. *Parity dependence at inclusion (p<0.05 by Fisher exact test). †Parity dependence at delivery (p<0.05 by Fisher exact test). ‡Decrease between inclusion and delivery (p<0.05 by paired Wilcoxon test). §Increase between inclusion and delivery (p<0.05 by paired Wilcoxon test).

Figure 2

Antibody levels at study inclusion and delivery, by parasitemia during pregnancy, against placental malaria in pregnant women, Benin. A) Apical membrane antigen 1 (AMA-1); B–F) Duffy binding-like (DBL) antigen; G) Full-length ectodomain of variant surface antigen 2 chondroitin sulfate (FV2); H) Variant surface antigen (VSA). Solid circles indicate medians for inclusion, solid squares indicate medians for delivery, and error bars indicate interquartile ranges, and error bars indicate interquartile ranges. AU, absorbance units; rMFI, relative median fluorescence intensity. *Significantly higher in women with parasitemia during pregnancy (p<0.05 by Fisher exact test). †Decrease between inclusion and delivery (p<0.05 by paired Wilcoxon test). ‡Increase between inclusion and delivery (p<0.05 by paired Wilcoxon test).

Figure 3

Binding inhibition profile of plasma from pregnant women against placental malaria, Benin. Plasma binding inhibitory capacity according to parity (n = 109 primigravidae and 573 multigravidae) (A) and to parasitemia during follow-up (B) (n = 384 women with undetected parasitemia, 115 with parasitemia detected at study inclusion, and 183 with parasitemia detected after inclusion). A) Binding inhibitory capacity was significantly higher at inclusion in multigravidae than in primigravidae and increased at delivery compared with that at inclusion in both groups (all p<0.05). B) Significant increase between inclusion and delivery and a higher level at delivery in women with documented parasitemia during pregnancy (p<0.05, by Fisher exact test). Horizontal lines indicate medians, boxes indicate interquartile ranges, and error bars indicate ranges. IRBCs, infected red blood cells; AU, absorbance units.

Figure 4

Binding inhibitory capacity of plasma, by adverse outcomes, in pregnant women with documented Plasmodium falciparum infection during follow-up, Benin. Binding inhibition was assessed according to adverse outcomes in the subgroup of women who had ≥1 parasitemia documented between study inclusion and delivery. A) Placental infection (52 infected placentas and 214 uninfected placentas). B) Low birthweight (LBW) (36 with LBW and 254 without LBW). C) Preterm birth (29 preterm and 269 not preterm). Horizontal lines indicate medians, boxes indicate interquartile ranges, and error bars indicate ranges. Plasma binding inhibitory capacity was significantly higher at delivery in women without adverse outcomes (p<0.05, by Fisher exact test), and the increase between inclusion and delivery was also significant (p<0.05, by paired-Wilcoxon test). No associations were observed at inclusion. IRBCs, infected red blood cells.