Adhesion of Plasmodium falciparum-infected erythrocytes to hyaluronic acid in placental malaria

Abstract

Infection with Plasmodium falciparum during pregnancy leads to the accumulation of parasite-infected erythrocytes in the placenta, and is associated with excess perinatal mortality, premature delivery and intrauterine growth retardation in the infant, as well as increased maternal mortality and morbidity. P. falciparum can adhere to specific receptors on host cells, an important virulence factor enabling parasites to accumulate in various organs. We report here that most P. falciparum isolates from infected placentae can bind to hyaluronic acid, a newly discovered receptor for parasite adhesion that is present on the placental lining. In laboratory isolates selected for specific high-level adhesion, binding to hyaluronic acid could be inhibited by dodecamer or larger oligosaccharide fragments or polysaccharides, treatment of immobilized receptor with hyaluronidase, or treatment of infected erythrocytes with trypsin. In vitro flow-based assays demonstrated that high levels of adhesion occurred at low wall shear stress, conditions thought to prevail in the placenta. Our findings indicate that adhesion to hyaluronic acid is involved in mediating placental parasite accumulation, thus changing the present understanding of the mechanisms of placental infection, with implications for the development of therapeutic and preventative interventions.

Fig. 1

Receptor-specific adhesion of P. falciparum isolates. a, The proportion of isolates collected from the placenta (n = 15), peripheral blood of pregnant women (maternal; n = 14) and peripheral blood of children (n = 15) that bound to immobilized purified HA (■), CSA (□) and/or CD36 (hatched bars) at a level of ≥ 10 IEs bound/mm². b, The average density of adhesion of placental isolates to immobilized HA, CSA or CD36. Data represent mean (± s.e.m.) number of bound IEs per mm².

Fig. 2

Adhesion of P. falciparum-infected erythrocytes to HA and CSA. a, Adhesion of IEs is abolished by pre-treatment of immobilized HA (■) and CSA (□) with testicular hyaluronidase but not heparinase. Treatment with chondroitinase ABC reduces adhesion to CSA to a much greater degree than to HA. b, Soluble HA (■) but not colominic acid (□) added to IE suspensions before the adhesion assays competitively inhibits binding to immobilized HA in a concentration-dependent manner. There is some inhibition with soluble CSA (◆). c, Adhesion of IEs to immobilized HA (■) is effectively inhibited by oligosaccharide fragments of HA of dodecamer (12-mer) size or larger. There is no substantial inhibition of adhesion to CSA (□) (only the results for the hexadecamer (16-mer) fragments are shown). d, Surface proteins on intact IEs were cleaved with trypsin before adhesion to each of HA and CSA was tested. Trypsin treatment abolishes adhesion to immobilized HA (■) but not to CSA (□). All data represent the proportion of bound IEs expressed as a percentage of control (mean ± s.e.m. for multiple experiments).

Fig. 3

P. falciparum-infected erythrocytes adhere to immobilized HA under flow conditions. a, Adhesion of IEs from flow at increasing wall shear stresses. Data represent number of bound IEs per mm² per 1 × 10⁷ IEs perfused. b, The proportion of IEs remaining adherent at increasing levels of wall shear stress (expressed as a percentage of IEs initially adherent at 0.05 Pa). All data represent mean ± s.e.m. for multiple experiments.