Rosette formation by Plasmodium vivax.

In contrast to Plasmodium falciparum, infections with P. vivax are seldom fatal. Red blood cells containing mature forms of P. falciparum sequester in the microvasculature of vital organs, and adhere to vascular endothelium (cytoadherence) and to uninfected red cells (rosetting). Rosetting of P. falciparum has been associated with the lethal syndrome of cerebral malaria. We have studied the rosetting properties of red blood cells infected with P. vivax obtained from adults with acute malaria in Thailand. Of 35 parasite isolates studied, 25 (71%) showed rosetting with a mean proportion of 41% of infected red cells (SD 34%, range 14-100%). Rosetting of P. vivax was related to maturation of the parasite; only cells containing parasites with visible malaria pigment rosetted. Rosetting and parasitaemia were not correlated. However, unlike P. falciparum, cells infected with P. vivax did not adhere to human umbilical vein endothelial cells, to C32 melanoma cells, to platelets, or to purified adhesion receptor molecule CD36. These findings suggest that thrombocytopenia in vivax malaria is not related to platelet-red cell attachment, and that rosetting alone is insufficient to cause the syndrome of cerebral malaria

Rosetting in Plasmodium vivax : a cytoadhesion phenotype associated with anaemia

BACKGROUND: Plasmodium vivax can potentially lead to life-threatening episodes but the mechanisms underlying severe disease remain poorly defined. Cytoadhesion of infected erythrocytes may contribute to P. vivax sequestration and organ injury although its physiological impact is still unknown. Here, we aimed to describe clinically-relevant cytoadhesive phenotypes of P. vivax isolates. METHODOLOGYPRINCIPAL FINDINGS: Rosetting and adhesion to CSA, CD36, ICAM1, placental and brain cryosections were determined in P. vivax peripheral isolates from 12 pregnant women, 24 non-pregnant women and 23 men from Manaus (Brazil). P. falciparum co-infection was excluded by PCR and P. vivax isolates were genotyped by assessing the size polymorphism of microsatellites ms2, ms20 and msp1F3 through capillary electrophoresis of PCR products. P. vivax monoinfection was confirmed by PCR in 59 isolates, with 50 (85%) of them being single-clone infections. One P. vivax haplotype was more frequently found among pregnant women (33%) than in non-pregnant women (0%) and men (4%; p = 0.010). Rosetting was observed in 64% of the isolates, adhesion to CSA in 15%, to ICAM1 in 12% and to placental cryosections in 9%, being similar among pregnant and non-pregnant groups. Intensity of rosetting was higher among anaemic individuals compared to non-anaemic (p = 0.010) and decreased with increasing haematocrit (p = 0.033) and haemoglobin levels (p = 0.015). CONCLUSIONSSIGNIFICANCE: P. vivax peripheral isolates from pregnant women do not exhibit a prominent adhesion to CSA, although other parasite phenotypes still unknown may increase the propagation of certain P. vivax clones observed among pregnant hosts. Rosetting is a frequent cytoadhesive phenotype in P. vivax infections that may contribute to the development of anaemi