Adhesion of normal and Plasmodium falciparum ring–infected erythrocytes to endothelial cells and the placenta involves the rhoptry-derived ring surface protein-2

Recent findings have challenged the current view of Plasmodium falciparum (P falciparum) blood-stage biology by demonstrating the cytoadhesion of early ring-stage–infected erythrocytes (rIEs) to host endothelial cells and placental syncytiotrophoblasts. The adhesion of rIEs was observed only in parasites that bind to the placenta via chondroitin sulfate A (CSA). In this work, a panel of mouse monoclonal antibodies (mAbs) that specifically inhibit cytoadhesion of rIEs but not of mature IEs was generated The previously described ring surface protein 2 (RSP-2), a 42-kDa protein, was identified as the target of the ring-stage–specific mAbs. Time course surface fluorescence experiments revealed a short overlap (approximately 4 hours) of expression between RSP-2 and P falciparum erythrocyte membrane protein 1 (PfEMP1). Their consecutive expression enables IEs to adhere to endothelial cells during the entire blood-stage cycle. During this study, a new phenotype was detected in parasite cultures, the adhesion of normal erythrocytes (nEs) to endothelial cells. All adherent nEs were coated with RSP-2. Immunolocalization studies show that RSP-2 is a rhoptry-derived protein that is discharged onto the erythrocyte membrane during contact with merozoites. Our results identify RSP-2 as a key molecule in sequestration of young blood-stage forms and nEs to endothelial cells.