Cytoadherence of erythrocytes invaded by Plasmodium falciparum: quantitative contact-probing of a human malaria receptor.

Cytoadherence of red blood cells (RBCs) invaded by Plasmodium falciparum parasites is an important contributor to the sequestration of RBCs, causing reduced microcirculatory flow associated with fatal malaria syndromes. The phenomenon involves a parasite-derived variant antigen, the P. falciparum erythrocyte membrane protein 1 (PfEMP1), and several human host receptors, such as chondroitin sulfate A (CSA), which has been explicitly implicated in placental malaria. Elucidating the molecular mechanisms of cytoadherence requires quantitative evaluation, under physiologically relevant conditions, of the specific receptor-ligand interactions associated with pathological states of cell-cell adhesion. Such quantitative studies have not been reported thus far for P. falciparum malaria under conditions of febrile temperatures that accompany malarial infections. In this study, single RBCs infected with P. falciparum parasites (CSA binding phenotype) in the trophozoite stage were engaged in mechanical contact with the surface of surrogate cells specifically expressing CSA, so as to quantify cytoadherence to human syncytiotrophoblasts in a controlled manner. From these measurements, a mean rupture force of 43pN was estimated for the CSA-PfEMP1 complex at 37°C. Experiments carried out at febrile temperature showed a noticeable decrease in CSA-PfEMP1 rupture force (by about 23% at 41°C and about 20% after a 40°C heat treatment), in association with an increased binding frequency. The decrease in rupture force points to a weakened receptor-ligand complex after exposure to febrile temperature, while the rise in binding frequency suggests an additional display of nonspecific binding molecules on the RBC surface. The present work establishes a robust experimental method for the quantitative assessment of cytoadherence of diseased cells with specific molecule-mediated binding.