Antibodies to variant antigens on the surfaces of infected erythrocytes are associated with protection from malaria in Ghanaian children

Abstract

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a variant antigen expressed on the surface of infected erythrocytes. Each parasite genome contains about 40 PfEMP1 genes, but only 1 PfEMP1 gene is expressed at a given time. PfEMP1 serves as a parasite-sequestering ligand to endothelial cells and enables the parasites to avoid splenic passage. PfEMP1 antibodies may protect from disease by inhibiting sequestration, thus facilitating the destruction of infected erythrocytes in the spleen. In this study, we have measured antibodies in Ghanaian children to a conserved region of PfEMP1 by enzyme-linked immunosorbent assay and antibodies to variant molecules on erythrocytes infected with field isolates of P. falciparum by flow cytometry. Based on close clinical monitoring, the children were grouped into those who did (susceptible) and those who did not (protected) have malaria during the season. The prevalences of antibodies to both the conserved PfEMP1 peptide and the variant epitopes were greater than 50%, and the levels of immunoglobulin G (IgG) correlated with age. The levels of antibodies to both the conserved peptide and the variant epitopes were higher in protected than in susceptible children. After correcting for the effect of age, the levels of IgG to variant antigens on a Sudanese and a Ghanaian parasite isolate remained significantly higher in protected than in susceptible children. Thus, the levels of IgG to variant antigens expressed on the surface of infected erythrocytes correlated with protection from clinical malaria. In contrast, the levels of IgG to a peptide derived from a conserved part of PfEMP1 did not correlate with protection from malaria.

FIG. 1

(A) Prevalence and 95% confidence interval (C.I.) of IgG antibody to variant antigens on the surface of erythrocytes infected with Ghanaian parasite isolates (L73, L50, and Busua) or Sudanese isolates (Sd-2D3 and G12) and to a peptide corresponding to a conserved part of PfEMP1 in premalaria season plasma from Ghanaian children. (B) Levels (median units and 95% confidence interval) of IgG to variant antigens on parasites from Ghana or Sudan in premalaria season plasma samples from Ghanaian children. Open bars indicate malaria-susceptible individuals, and hatched bars represent individuals protected from malaria.

FIG. 2

(A to D) Age-stratified (3 to 5, 6 to 9, and 10 to 15 years) plasma antibody levels to variant antigens on the surface of erythrocytes infected with parasite isolates L50, Busua, or G12 and to the conserved PfEMP1 peptide in Ghanaian children classified as susceptible (open bars labeled mal) to malaria or protected (hatched bars labeled no mal). (E to H) Age-adjusted IgG levels in susceptible and protected children.

FIG. 3

IgG levels in plasma to variant antigens on erythrocytes infected with parasites isolate L50 (A) or Busua (B) in pre- and post-malaria transmission samples from Ghanaian children, who had (open bars) or did not have (hatched bars) malaria.