Antibody recognition of Plasmodium falciparum erythrocyte surface antigens in Kenya: evidence for rare and prevalent variants

Abstract

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is the name given to a family of parasite proteins that are inserted into the infected erythrocyte surface. Studies using agglutination assays have shown previously that PfEMP1 epitopes are extremely diverse. In a study in Kenya, 21 parasite isolates, including nine from children with severe malaria, were tested for agglutination by 33 pairs of plasma, 21 of which were from the corresponding children. Each plasma pair consisted of a sample taken at the time of disease (acute) and one taken 3 weeks later (convalescent). In agreement with previous studies, infection was generally followed by the induction of antibodies specific to the homologous parasite isolate. In addition however, the results show that (i) some isolates were agglutinated very frequently by heterologous plasma; (ii) unexpectedly, these frequently agglutinated isolates tended to be from individuals with severe malaria; (iii) an inverse relationship existed between the agglutination frequency of each parasite isolate in heterologous plasma and the agglutinating antibody repertoire of the homologous child at the time of disease; and (iv) A 3-month-old child apparently still carrying maternal antibodies was infected by a rarely agglutinated isolate. This child's plasma agglutinated all isolates at the time of disease, apart from the homologous isolate. These results support the idea that preexisting anti-PfEMP1 antibodies can select the variants that are expressed during a new infection and may suggest the existence of a dominant subset of PfEMP1 variants.

FIG. 1

Parasite recognition profiles. Isolates are listed along the top; labels for nine isolates from patients with severe malaria are underlined. Each isolate was tested against 33 pairs of plasma samples listed at the left. Each row of boxes shows agglutination of 21 isolates by a single patient. Plasma samples from patients with severe disease are underlined. Each pair is labeled A (acute [sampled at the time of disease]) and C (convalescent [sampled 3 weeks later]). (a) Plasma pairs corresponding to the 21 isolates. Reactions against homologous isolates lie on a diagonal running from the top left to the bottom right. (b) Twelve additional pairs of plasma samples not corresponding to cultured isolates. Patients who received a blood transfusion are indicated with arrows. The bottom row is a profile of an immune adult. For each isolate, the estimated number of clones present (multiplicity) and the age of the corresponding child is indicated.

FIG. 2

Diverse agglutination profiles of common isolates. The 49 plasma samples that agglutinated 1 to 9 of the 10 common isolates were treated as independent typing reagents and stacked vertically, with the most reactive at the top. Each row of rectangles represents agglutination of the 10 isolates by a single plasma sample. Scores are represented by shading as in Fig. 1.

FIG. 3

Induction of agglutinating antibodies. Histograms of Δ scores (the change in agglutination score between disease and convalescence) are compared between various sets of plasma pair tests (plasma-parasite combinations [see text]). (a) Comparison between plasma pair tests against homologous isolates (n = 16) and against heterologous isolates (n = 530), excluding plasma pairs from transfused children. (b) Comparison between plasma pair tests against common heterologous isolates (n = 255) and against rare heterologous isolates (n = 275), excluding plasma samples from transfused children. (c) Evidence for passive antibody transfer during blood transfusion: comparison between plasma pair tests against common heterologous isolates (n = 65) and against rare heterologous isolates (n = 77), considering only plasma pairs from transfused children.

FIG. 4

Evidence for immune selection of rare surface variants: relationship between each child’s repertoire of agglutinating antibodies at the time of disease and the agglutination frequency of that child’s parasites. Antibody repertoire = percentage of heterologous parasite isolates agglutinated by each patient’s acute plasma samples (i.e., 21 plasma samples tested against 20 isolates). Each point is labeled by the identification number of the plasma-parasite sample. Samples from patients with severe disease are shown with filled circles.