Inhibition of chondroitin-4-sulfate-specific adhesion of Plasmodium falciparum-infected erythrocytes by sulfated polysaccharides

Abstract

Adhesion of Plasmodium falciparum-infected erythrocytes to placental chondroitin 4-sulfate (CSA) has been linked to the severe disease outcome of pregnancy-associated malaria. Soluble polysaccharides that release mature-stage parasitized erythrocytes into the peripheral circulation may help elucidate these interactions and have the potential to aid in developing therapeutic strategies. We have screened a panel of 11 sulfated polysaccharides for their capacities to inhibit adhesion of infected erythrocytes to CSA expressed on CHO-K1 cells and ex vivo human placental tissue. Two carrageenans and a cellulose sulfate (CS10) were able to inhibit adhesion to CSA and to cause already bound infected erythrocytes to de-adhere in a dose-dependent manner. CS10, like CSA and in contrast to all other compounds tested, remained bound to infected erythrocytes after washing and continued to inhibit binding. Both carrageenans and CS10 inhibited adhesion to placental tissue. Although highly sulfated dextran sulfate can inhibit CSA-mediated adhesion to CHO cells, this polysaccharide amplified adhesion to placental tissue severalfold, demonstrating the importance of evaluating inhibitory compounds in systems as close to in vivo as possible. Interestingly, and in contrast to all other compounds tested, which had a random distribution of sulfate groups, CS10 exhibited a clustered sulfate pattern along the polymer chain, similar to that of the undersulfated placental CSA preferred by placental-tissue-binding infected erythrocytes. Therefore, the specific anti-adhesive capacity observed here seems to depend not only on the degree of charge and sulfation but also on a particular pattern of sulfation.

FIG. 1.

Adhesion of FCR3csa-infected erythrocytes to CHO-K1 cells. Varying concentrations of test substances were used either to inhibit binding (░⃞) or to cause already bound infected erythrocytes to de-adhere (⧫). Mean percentages of binding (± standard deviations) compared to that of matched PBS controls (100% binding) for three independent experiments are shown.

FIG. 2.

Inhibition of adhesion of FCR3csa-infected erythrocytes to ex vivo placental cryosections. (A) Effect of 100 μg/ml of each substance on adhesion of infected erythrocytes to sets of three consecutive placental cryosections. Mean percentages of binding to the syncytiotrophoblast compared to that of controls (± standard deviations) for four experiments are shown. (B) Giemsa-stained placental cryosections (magnification, ×1,000) showing adherent FCR3csa-infected erythrocytes (left) and accumulation of FCR3csa-infected and uninfected erythrocytes in the presence of 100 μg/ml 500-kDa dextran sulfate (right).

FIG. 3.

Adhesion of FCR3csa-infected erythrocytes to CHO-K1 cells after preincubation with polysaccharides and extensive washing. FCR3csa-infected erythrocytes were preincubated with varying concentrations of test compounds, washed, and allowed to adhere to CHO-K1 cells. Mean percentages of binding (± standard deviations) compared to that of matched PBS controls (100% binding) for three independent experiments are shown.