The pathogenesis of Plasmodium falciparum malaria is related to the ability of parasite‑infected erythrocytes (IEs) to adhere to the vascular endothelium (cytoadhesion/sequestration) or to surrounding uninfected erythrocytes (rosetting). Both processes are mediated by the expression of members of the clonally variant PfEMP1 parasite protein family on the surface of the IEs. Recent evidence obtained with laboratory-adapted clones indicates that P. falciparum can exploit human serum factors, such as IgM and α 2 ‑macroglobulin (α 2 M), to increase the avidity of PfEMP1-mediated binding to host receptors, as well as to evade host PfEMP1-specific immune responses. It has remained unclear whether PfEMP1 variants present in field isolates share these characteristics, and whether they are associated with clinical malaria severity. These issues were investigated here.
Children between 1-12 years reporting with P. falciparum malaria to Hohoe Municipal Hospital, Ghana were enrolled in the study. Parasites from children with uncomplicated (UM) and severe malaria (SM) were collected. Binding of α 2 M and IgM from non-immune individuals to erythrocytes infected by P. falciparum isolates from 34 children (UM and SM) were analysed by flow cytometry. Rosetting in the presence of IgM or α 2 M was also evaluated. Experimental results were related to the clinical presentation of the patients.
Clinical data from 108 children classified as UM (n=54) and SM cases (n=54) were analysed. Prostration, severe malaria anaemia, and hyperparasitaemia were the most frequent complications. Three children were diagnosed with cerebral malaria, and one child died. Parasite isolates from 34 children were analysed. Most of the field isolates bound non‑immune IgM (33/34), whereas the α 2 M-binding was less common (23/34) and mostly low. Binding of both non-immune IgM and α 2 M was higher in IEs from children with SM than from UM. In combination, IgM and α 2 M supported rosette formation at levels similar to that observed in the presence of 10% human serum.
The results support the hypothesis that binding of non-immune IgM and/or α 2 M to IEs facilitates rosette formation and contributes to P. falciparum malaria severity.