Differential binding of clonal variants of Plasmodium falciparum to allelic forms of intracellular adhesion molecule 1 determined by flow adhesion assay

Abstract

Adhesion of Plasmodium falciparum-infected erythrocytes to the endothelial ligand intercellular adhesion molecule 1 (ICAM-1) has been implicated in the pathogenesis of cerebral malaria. Recently, a high-frequency coding polymorphism in the N-terminal domain of ICAM-1 (ICAM-1(Kilifi)) that is associated with susceptibility to cerebral disease in Kenya has been described. Preliminary static adhesion assays suggested that two different selected P. falciparum lines, ITO4-A4 (A4) and ItG-ICAM (ItG), have different properties of binding to the natural variant proteins ICAM-1 and ICAM-1(Kilifi). Using a flow adhesion assay system, we have confirmed differences between the two lines in binding of parasitized erythrocytes to the variant ICAM-1 proteins. Total adhesion of ItG-infected erythrocytes to ICAM-1 and ICAM-1(Kilifi) is greater than that of A4-infected erythrocytes, and erythrocytes infected by both parasite strains show reduced binding to ICAM-1(Kilifi). However, under these physiologically relevant flow conditions, we have shown differences between A4 and ItG strains in dynamic rolling behavior on ICAM-1(Kilifi). The percentage of erythrocytes infected with A4 that roll on both ICAM-1 and ICAM-1(Kilifi) is greater than that of those infected with ItG. Also, the rolling velocity of A4-infected erythrocytes on ICAM-1(Kilifi) is markedly increased compared to that on ICAM-1, in contrast to the rolling velocity of ItG-infected erythrocytes, which is similar on both proteins. These findings suggest that different parasite lines can vary in their avidity for the same host ligand, which may have important consequences for the pathophysiology of P. falciparum malaria.

FIG. 1

Differential total binding (rolling and static) of A4- and ItG-infected PBRC to ICAM-1^ref at an inflow shear stress of 0.05 Pa. Experiment and analysis were carried out as described in Materials and Methods, and data represent the means ± standard errors (error bars) of the results of 3 to 10 experiments.

FIG. 2

Effect of antibody blockade on the total adhesion (rolling and static) of A4- and ItG-infected PBRC to ICAM-1^ref at an ICAM-1 coating concentration of 17.5 μg/ml and an inflow shear stress of 0.05 Pa. Experimental details are described in Materials and Methods, and data represent the means ± standard errors (error bars) of the results of two experiments.

FIG. 3

Total adhesion (rolling and static) of A4-infected (A) and ItG-infected (B) PBRC on ICAM-1^ref and ICAM-1^Kilifi at an inflow shear stress of 0.05 Pa. Experiment and analysis were carried out as described in Materials and Methods, and data represent the means ± standard errors (error bars) of the results of 3 to 10 experiments.

FIG. 4

The percentages of A4-infected (A) and ItG-infected (B) populations that roll on ICAM-1^ref and ICAM-1^Kilifi at different concentrations and at an inflow shear stress of 0.05 Pa. ∗, no measurable adhesion (rolling or static). The experiment and analysis were carried out as described in Materials and Methods, and data represent the means ± standard errors (error bars) of the results of 3 to 10 experiments.

FIG. 5

Hypothetical representation of the binding curves of erythrocytes infected by A4 and ItG strains on ICAM-1^ref. The curves show what may happen to the total adhesion (rolling and static) when the receptor concentration and/or the flow rate is varied.