Levels of plasma immunoglobulin G with specificity against the cysteine-rich interdomain regions of a semiconserved Plasmodium falciparum erythrocyte membrane protein 1, VAR4, predict protection against malarial anemia and febrile episodes

Abstract

Antibodies to variant surface antigen have been implicated as mediators of malaria immunity in studies measuring immunoglobulin G (IgG) binding to infected erythrocytes. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important target for these antibodies, but no study has directly linked the presence of PfEMP1 antibodies in children to protection. We measured plasma IgG levels to the cysteine-rich interdomain region 1alpha (CIDR1alpha) of VAR4 (VAR4-CIDR1alpha), a member of a semiconserved PfEMP1 subfamily, by enzyme-linked immunosorbent assay in 561 Tanzanian individuals, who were monitored clinically for 7 months. The participants resided in Mkokola (a high-transmission village where malaria is holoendemic) or Kwamasimba (a moderate-transmission village). For comparison, plasma IgG levels to two merozoite surface protein 1 (MSP1) constructs, MSP1-19 and MSP1 block 2, and a control CIDR1 domain were measured. VAR4-CIDR1alpha antibodies were acquired at an earlier age in Mkokola than in Kwamasimba, but after the age of 10 years the levels were comparable in the two villages. After controlling for age and other covariates, the risk of having anemia at enrollment was reduced in VAR4-CIDR1alpha responders for Mkokola (adjusted odds ratio [AOR], 0.49; 95% confidence interval [CI], 0.29 to 0.88; P = 0.016) and Kwamasimba (AOR, 0.33; 95% CI, 0.16 to 0.68; P = 0.003) villages. The risk of developing malaria fever was reduced among individuals with a measurable VAR4-CIDR1alpha response from Mkokola village (AOR, 0.51; 95% CI, 0.29 to 0.89; P = 0.018) but not in Kwamasimba. Antibody levels to the MSP1 constructs and the control CIDR1alpha domain were not associated with morbidity protection. These data strengthen the concept of developing vaccines based on PfEMP1.

FIG. 1.

Age-specific prevalence and density of P. falciparum in individuals aged 0 to 59 years from two Tanzanian villages situated in areas of high transmission (Mkokola [A]) and moderate transmission (Kwamasimba [B]). Error bars represent geometric means of the P. falciparum density with 95% confidence intervals. Note that the scale of P. falciparum density in panel A differs from that in panel B. Lines with filled circles represent the prevalence of P. falciparum.

FIG. 2.

Age-specific hemoglobin levels, anemia prevalence, and incidence of febrile malaria episodes in two villages characterized by different P. falciparum transmission intensities. (A) Hemoglobin levels and prevalence of anemia. Box plots illustrate medians with 25th and 75th percentiles and whiskers for 10th and 90th percentiles, including outliers (filled or empty circles), of hemoglobin levels. Shown are filled and empty box plots for Mkokola and Kwamasimba, respectively. Solid lines with filled squares and triangles represent the proportions with anemia for Mkokola and Kwamasimba, respectively. (B) Age-specific incidence rates and 95% confidence intervals of febrile malaria episodes per person years at risk (PYAR). Filled and empty columns represent Mkokola and Kwamasimba, respectively.

FIG. 3.

Age-specific IgG responses to recombinant surface antigens in two villages characterized by different P. falciparum transmission intensities. Panels A, B, and C represent VAR4-CIDR1α, MSP1-19, and MSP1-BL2, respectively. Box plots illustrate medians with 25th and 75th percentiles and whiskers for 10th and 90th percentiles, including outliers (filled or empty circles), of IgG levels to each of the surface antigens. Filled and empty box plots for Mkokola and Kwamasimba, respectively, are shown. Solid lines with filled squares and triangles represent the proportion of responders to each of the surface antigens in Mkokola and Kwamasimba, respectively.