Antibodies to variant surface antigens of Plasmodium falciparum-infected erythrocytes and adhesion inhibitory antibodies are associated with placental malaria and have overlapping and distinct targets

Abstract

We measured antibodies to chondroitin sulfate A (CSA)-binding and placental Plasmodium falciparum-infected red blood cells (PRBCs) among pregnant women with or without placental malaria. Immunoglobulin G to PRBC surface antigens was rare in uninfected primigravidae (3.7%), more prevalent in infected primigravidae (70%; P<.001), and common in infected (77%) and uninfected (83%) multigravidae. Similar patterns were seen for agglutinating antibodies, and antibodies were similar among women with past or active placental infection. PRBC adhesion to CSA was inhibited 60% by serum from infected primigravidae but 24% by serum from uninfected primigravidae (P=.025), whereas infection did not alter adhesion inhibition by multigravidae (77% inhibition)[corrected]. There was substantial heterogeneity in antibody type and levels. Antibodies did not correlate with parasite density or pregnancy outcome. Comparisons between antibodies suggest that adhesion-inhibitory antibodies and those to PRBC variant antigens have distinct and overlapping epitopes, may be acquired independently, and have different roles in immunity.

Figure 1

Associations between antibodies to the surface of chondroitin sulfate A (CSA)-binding Plasmodium falciparum and gravidity and placental infection. A, Proportion (±SE) of samples that tested positive for agglutination with CS2 among matched women with or without placental infection at term and men; P < .01, infected vs. uninfected groups. B, Proportion (±SE) of samples positive for agglutination with CS2 among matched women with active infection or past placental infection at term and men; differences between infected and past infection groups were not statistically significant. C, Proportion of samples (±SE), obtained at delivery, having detectable IgG binding among infected and uninfected primigravidae (PG) and multigravidae (MG); P < .001, for difference among PG samples. Samples with a mean fluorescence intensity value greater than the mean ±3 SD of nonagglutinating men’s samples were classified as positive for IgG to CS2 P. falciparum-infected red blood cells. D, Levels of the IgG binding (fluorescence intensity) of individual samples obtained at delivery from infected and uninfected PG, MG, and men. Horizontal lines represent the mean values; P < .01, infected vs. uninfected PG and MG vs. men. E, Proportion (±SE) of serum samples from smear-negative PG and MG at midpregnancy and men that were positive by agglutination using either placental isolates or the CSA-binding parasite isolate CS2; P < .05, for difference. F, Proportion (±SE) of samples obtained from women at term that were positive for agglutination using isolate E8B among pregnant women with active infection, no infection, or past placental infection, and men; differences were not statistically significant. SG, secundigravidae.

Figure 2

Associations between the inhibition of Plasmodium falciparum-infected red blood cell (PRBC) adhesion to chondroitin sulfate A (CSA) and gravidity and placental infection. A, The effect on adhesion of CS2 by samples from infected and uninfected primigravidae (PG) and multigravidae (MG) at term and men. Horizontal lines represent the median value; P < .05, infected vs. uninfected PG and MG or infected PG vs. men. B, Adhesion (mean + SE of 3 experiments) of CS2 in the presence of pooled samples from infected and uninfected PG, secundigravidae (SG), and MG at term or men; P < .01, infected vs. uninfected PG and SG or MG vs. men. C, Adhesion (mean ± SE of 3 experiments) of CS2 in the presence of pooled samples from PG and MG at term with active or past infection or men; P < .01, for the difference among PG and SG or MG vs. men. D, Adhesion (mean ± SE) of placental isolates in the presence of samples from smear-negative PG or MG at midpregnancy or men, tested individually; P < .05, for differences.

Figure 3

The relationship between different measures of antibodies to the surface of CS2 Plasmodium falciparum-infected red blood cells (PRBCs). A, The effect on adhesion to chondroitin sulfate A (CSA) by samples from pregnant women that were positive or negative in parasite agglutination assays; P < .001, for differences. B, The relationship between the effect on adhesion to CSA and IgG binding (fluorescence intensity by flow cytometry) among samples from pregnant women (r = 0.50; P < .001). C, Levels of IgG binding of individual samples from infected and uninfected pregnant women that were either positive or negative in parasite cell-agglutination assays; P < .001, for differences.

Figure 4

Relationship between antibodies to CS2 Plasmodium falciparum-infected red blood cells (PRBCs) and placental parasitemia. A, Relationship between level of adhesion of CS2 PRBCs to chondroitin sulfate A (CSA) in the presence of samples obtained from women at delivery and the placental parasitemia of each donor (P = .652). B, Relationship between IgG binding (mean fluorescence intensity [MFI] by flow cytometry) to CS2 PRBCs in samples obtained from women at delivery and the placental parasitemia of the donor (P = .33). Placental parasitemias were determined by histological examination.