Study of the adherence of Plasmodium falciparum infected red blood cells in human placenta and structural requirements for binding

In pregnant women infected with Plasmodium falciparum, the infected red blood cells (IRBCs) selectively accumulate in the intervillous spaces of placenta, leading to poor fetal outcome and severe health complications in the mother. Although chondroitin 4-sulfate is known to mediate IRBC adherence to placenta, the natural receptor has not been identified. In the present study, the chondroitin sulfate proteoglycans (CSPGs) of human placenta were purified, structurally characterized, and examined for binding of (IRBCs). The structural requirements for the adherence and the minimum chondroitin 4-sulfate (C4S) structural motif that supports IRBC adherence was also studied. The data indicate that the placenta contains three distinct types of CSPGs; significant quantities of uniquely low sulfated, extracellular CSPGs localized in the intervillous spaces, minor amounts of two cell-associated CSPGs, and major amounts of dermatan sulfate-like CSPGs of the fibrous tissue. Adhesion studies and localization patterns of various CSPGs indicate that low sulfated CSPGs of the intervillous spaces are the receptors for placental IRBC adherence. Partially sulfated C4Ss with varying sulfate contents were prepared by solvolytic desulfation of a fully sulfated C4S. These and other nonmodified C4Ss with different proportions of 4-, 6-, and nonsulfated disaccharide repeats, were analyzed for inhibition of IRBC adherence to the placental CSPG. C4Ss containing 30–50% 4-sulfated and 50–70% nonsulfated disaccharide repeats efficiently inhibited IRBC adherence; C6S had no inhibitory activity. Oligosaccharides of varying sizes were prepared by the partial depolymerization of C4Ss containing varying levels of 4-sulfation, and their ability to inhibit the IRBC adherence was studied. Oligosaccharides with six or more disaccharide repeats inhibited IRBC adherence to the same level as that of intact C4Ss, indicating that a dodecasaccharide is the minimum structural motif required for optimal IRBC adherence. Of the C4S dodecasaccharides, only those with 2 or 3 sulfate groups per molecule showed maximum IRBC inhibition. Identification and characterization of CSPGs that support the adherence of IRBCs in the human placenta, and definition of the structural requirements involved in the binding are crucial for development of therapeutic strategies for maternal malaria.