We show here that the Plasmodium falciparum isolate FCR3 does not express the ring-infected erythrocyte surface antigen (RESA). This is because the 5' end of the RESA gene has been inverted and partly deleted and a telomere has been added to it. We propose a model to explain these events.
To understand the molecular mechanisms that lead to sequestration of red blood cells infected with mature stages of Plasmodium falciparum and to examine the relevance of earlier studies on adherence properties of laboratory-derived P falciparum parasites to the natural parasite population, we analyzed Gambian and Tanzanian isolates for in vitro cytoadherence and antibody-mediated microagglutination. Eighteen cryopreserved isolates of ring-stage parasites were cultured for 20 to 30 hours in vitro, in the patients original erythrocytes, to the trophozoite and schizont stage. All parasites were positive in the microagglutination assay with at least one of four African hyperimmune sera. In a rosetting assay, only 2 of the 18 isolates were strongly positive (35% and 41% of parasitized erythrocytes with more than two uninfected cells bound). Thirteen isolates showed either intermediate (5% to 18%) or low (less than 5%) rosetting while three isolates did not form rosettes. Infected cell-binding of the different isolates to immobilized CD36 or thrombospondin, or C32 melanoma cells correlated with the percentage of mature parasites in the blood samples (r = .932 for CD36, r = .946 for thrombospondin, and r = .881 for C32 melanoma cells). There was a high correlation between binding to CD36 and thrombospondin (r = .982). The extent of infected cell rosetting with uninfected cells in these blood samples was not correlated with these other receptor properties. We also observed coexpression of rosetting and cytoadherence receptors on the same parasitized erythrocytes.
We have recently shown that rosetting of Plasmodium falciparum (MC R+ line)-infected erythrocytes (parasitized red blood cells [PRBCs]) with uninfected erythrocytes (RBCs) is blocked by coating of the RBCs with anti-CD36 monoclonal antibodies (MoAbs; Handunnetti et al, Blood 80:2097, 1992). Adult RBCs have previously been considered negative for CD36. However, using fluorescence-activated cell sorter analysis with the anti-CD36 MoAbs 8A6, OKM5, and OKM8, which reverse rosetting, we consistently detect CD36 on the majority of normal adult RBCs. Absorption of the MoAb solutions with CD36-transfected Chinese hamster ovary (CHO-CD36) cells removed the reactivity against both CHO-CD36 cells and RBCs, whereas absorption with CHO cells had no effect. By comparison with staining for glycophorin A, LFA-3, and CR1, the level of expression of CD36 appeared to be low. Nevertheless, normal RBCs were capable of adhering to plastic coated with anti-CD36 MoAbs. RBCs from one African malaria patient were identified as deficient in CD36 and these RBCs did not rosette with the patient's own P falciparum PRBCs, even though these PRBCs were capable of rosetting with RBCs from a normal donor in a CD36-dependent manner. Therefore, the level of expression of CD36 on normal RBCs is sufficient to be important in cell adherence, and may have a biologic role in normal individuals as well as in the pathology of P falciparum malaria.
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