No abstract

Severe Plasmodium falciparum malaria is characterized by excessive sequestration of infected and uninfected erythrocytes in the microvasculature of the affected organ. Rosetting, the adhesion of P. falciparum–infected erythrocytes to uninfected erythrocytes is a virulent parasite phenotype associated with the occurrence of severe malaria. Here we report on the identification by single-cell reverse transcriptase PCR and cDNA cloning of the adhesive ligand P. falciparum erythrocyte membrane protein 1 (PfEMP1). Rosetting PfEMP1 contains clusters of glycosaminoglycan-binding motifs. A recombinant fusion protein (Duffy binding-like 1–glutathione S transferase; Duffy binding-like-1–GST) was found to adhere directly to normal erythrocytes, disrupt naturally formed rosettes, block rosette reformation, and bind to a heparin-Sepharose matrix. The adhesive interactions could be inhibited with heparan sulfate or enzymes that remove heparan sulfate from the cell surface whereas other enzymes or similar glycosaminoglycans of a like negative charge did not affect the binding. PfEMP1 is suggested to be the rosetting ligand and heparan sulfate, or a heparan sulfate–like molecule, the receptor both for PfEMP1 binding and naturally formed erythrocyte rosettes.

SummaryHerein we describe an assay that was developed to quantitate the binding of normal red blood cells (RBC), labeled with carboxy fluorescein diacetate (C-FDA), to rosetting Plasmodium falciparum-infected RBC . The binding of RBC obtained from various animal species or humans to different strains or clones of rosetting P. falciparum-infected RBC was studied . A strain-specific preference of rosetting was observed for either blood group A/AB or B/AB RBC for all parasites tested . The higher affinity of rosette binding of blood group A, B, or AB vs. O RBC was reflected in larger rosettes when a given parasite was grown in RBC of the preferred blood group. The small size of the rosettes formed when P. falciparum was grown in blood group O RBC may be the in vitro correlate of the relative protection against cerebral malaria afforded by belonging to blood group O rather than to blood group A or B. Rosettes of a blood group A-preferring parasite could be completely disrupted by heparin only when grown in blood group O or B RBC, but not when grown in blood group A RBC . Similarly, the rosettes of a blood group B-preferring parasite could be more easily disrupted by heparin when grown in blood group O or A RBC than when grown in blood group B RBC. Several different saccharides inhibited rosetting of group O RBC, including two monosaccharides that are basic components of heparin . The rosetting of the same parasites grown in blood group A or B RBC was less sensitive to heparin and was specifically inhibited only by the terminal mono-and trisaccharides of the A and the B blood group antigens, the H disaccharide, and fucose. Our results suggest that rosetting is mediated by multiple lectin-like interactions, the usage of which rely on the parasite phenotype and whether the receptors are present on the host cell or not .

The malaria parasite Plasmodium falciparum utilizes molecules present on the surface of uninfected red blood cells (RBC) for rosette formation, and a dependency on ABO antigens has been previously shown. In this study, the antirosetting effect of immune sera was related to the blood group of the infected human host. Sera from malaria-immune blood group A (or B) individuals were less prone to disrupt rosettes from clinical isolates of blood group A (or B) patients than to disrupt rosettes from isolates of blood group O patients. All fresh clinical isolates and laboratory strains exhibited distinct ABO blood group preferences, indicating that utilization of blood group antigens is a general feature of P. falciparum rosetting. Soluble A antigen strongly inhibited rosette formation when the parasite was cultivated in A RBC, while inhibition by glycosaminoglycans decreased. Furthermore, a soluble A antigen conjugate bound to the cell surface of parasitized RBC. Selective enzymatic digestion of blood group A antigen from the uninfected RBC surfaces totally abolished the preference of the parasite to form rosettes with these RBC, but rosettes could still form. Altogether, present data suggest an important role for A and B antigens as coreceptors in P. falciparum rosetting.Although erythrocytes have traditionally been considered relatively inert containers of hemoglobin, mounting evidence suggests that they in fact bear numerous surface molecules that are active in microbial attachment processes (22). The asexual stages of the malaria parasite Plasmodium falciparum utilize molecules on the surface of uninfected red blood cells (RBC) for rosette formation, i.e., binding of parasite-infected RBC (pRBC) to uninfected RBC. Rosetting has been associated with the occurrence of severe malaria, i.e., cerebral malaria and anemia (5,14,17,18). The P. falciparum erythrocyte membrane protein 1 (PfEMP1), a member of a family of highmolecular-weight polypeptides encoded by the var genes (4, 20, 21), has been identified as a rosetting ligand (8,19). Various host receptors on the surface of uninfected RBC have been proposed, including ABO blood group antigens (7), CD36 (11), CD35 (19), and heparan sulfate (HS) or HS-like molecules (3,8). Still, little is known about the relative contribution of different receptors in rosetting and their prevalence in nature. Previous studies have implicated the ABO blood group type in rosetting (1, 18, 24) and strain-specific blood group preferences for rosette formation have been described previously (7).In this study, we examine the utilization of ABO blood group antigens for rosette formation in laboratory strains and fresh clinical isolates. We assess the antirosetting effect of hyperimmune sera in relation to the ABO blood group type and examine the relationship of the A antigen to the rosetting phenotype. MATERIALS AND METHODSPatients, parasites, and sera. Two P. falciparum strains, FCR3S1 (a blood group A-preferring line [7] cloned by limiting dilution from FCR3S, which originated from the FCR s...