Rouleaux-forming serum proteins are involved in the rosetting of Plasmodium falciparum-infected erythrocytes

Excessive sequestration of Plasmodium falciparum-infected (pRBC) and uninfe cted erythrocytes (RBC) in the microvasculature, cytoadherence, and resetti ng, have been suggested to be correlated with the development of cerebral m alaria. P, falciparum erythrocyte membrane protein-1 (PfEMP1) is the parasi te-derived adhesin which mediates resetting. Herein we show that serum prot eins are crucial for the rosette formation of four strains of parasites (FC R3S1, TM284, TM180, and R29), whereas the rosettes of a fifth strain (DD2) are serum independent. Some parasites, e.g., FCR3S1, can be depleted of all rosettes by washes in heparin and Na citrate and none of the rosettes rema in when the parasite is grown in foetal calf serum or ALBUMAX. Rosettes of other parasites are less sensitive; e.g., 20% of TM180 and R29 and 70% of T M284 rosettes still prevail after cultivation. A serum fraction generated b y ion-exchange chromatography and poly-ethylene-glycol precipitation restor ed 50% of FCR3S1 and approx 40 to 100% of TM180 rosettes. In FCR3S1, antibo dies to fibrinogen reverted the effect of the serum fraction and stained fi brinogen bound to the pRBC surface in transmission electron microscopy. Nor mal, nonimmune IgM and/or IgG was also found attached to the pRBC of the fo ur serum-dependent strains as seen by surface immunofluorescens. Our result s suggest that serum proteins, known to participate in rouleaux formation o f normal erythrocytes, produce stable rosettes in conjunction with the rece ntly identified parasite-derived resetting ligand PfEMP1. (C) 1999 Academic Press.