Inhibition of binding of malaria-infected erythrocytes by a tetradecasaccharide fraction from chondroitin sulfate A

Abstract

Adherence of parasite-infected erythrocytes (IEs) to the microvascular endothelium of various organs, a process known as sequestration, is a feature of Plasmodium falciparum malaria. This event is mediated by specific adhesive interactions between parasite proteins, expressed on the surface of IEs, and host molecules. P. falciparum IEs can bind to purified chondroitin sulfate A (CS-A), to the proteoglycan thrombomodulin through CS-A side chains, and to CS-A present on the surface of brain and lung endothelial cells and placental syncytiotrophoblasts. In order to identify structural characteristics of CS-A important for binding, oligosaccharide fragments ranging in size from 2 to 20 monosaccharide units were isolated from CS-A and CS-C, following controlled chondroitin lyase digestion, and used as competitive inhibitors of IE binding to immobilized ligands. Inhibition of binding to CS-A was highly dependent on molecular size: a CS-A tetradecasaccharide fraction was the minimum length able to almost completely inhibit binding. The effect was dose dependent and similar to that of the parent polysaccharide, and the same degree of inhibition was not found with the CS-C oligosaccharides. There was no effect on binding of IEs to other ligands, e.g., CD36 and intercellular adhesion molecule 1. Hexadeca- and octadecasaccharide fractions of CS-A were required for maximum inhibition of binding to thrombomodulin. Analyses of oligosaccharide fractions and polysaccharides by electrospray mass spectrometry and high-performance liquid chromatography suggest that the differences between the activities of CS-A and CS-C oligosaccharides can be attributed to differences in sulfate content and sulfation pattern and that iduronic acid is not involved in IE binding.

FIG. 1

Inhibition of binding of CS2 parasites to immobilized CS-A by oligosaccharides (di- to eicosamers) and polysaccharides from CS-A and CS-C at a concentration of 90 μg/ml. Values are means (± standard errors) for quadruplicate experiments.

FIG. 2

Inhibition of binding of CS2 parasites to immobilized CS-A at increasing concentrations of oligosaccharides and polysaccharide. Values are means ± standard errors for triplicate experiments.

FIG. 3

Inhibition of binding of CS2 to immobilized recombinant human TM by CS-A oligosaccharides and polysaccharide at a concentration of 90 μg/ml. Values are means ± standard errors for duplicate experiments.

FIG. 4

Effect on binding of parasite lines FAF-EA8 and FAF6 to ICAM-1 and CD36, respectively, and of CS2 to CS-A by CS-A oligosaccharides and polysaccharide at a concentration of 90 μg/ml. Values are means ± standard errors for duplicate experiments.