No abstract

Brain lesions of cerebral malaria (CM) are characterized by a sequestration of Plasmodium falciparumparasitized red blood cells (PRBC) and platelets within brain microvessels, as well as by blood-brain barrier (BBB) disruption. In the present study, we evaluated the possibility that PRBC and platelets induce functional alterations in brain endothelium. In a human brain endothelial cell line, named HBEC-5i, exhibiting most of the features demanded for a pathophysiological study of BBB, tumor necrosis factor (TNF) or lymphotoxin ␣ (LT-␣) reduced transendothelial electrical resistance (TEER), enhanced the permeability to 70-kDa dextran, and increased the release of microparticles, a recently described indicator of disease severity in CM patients. In vitro cocultures showed that platelets or PRBC can have a direct cytotoxic effect on activated, but not on resting, HBEC-5i cells. Platelet binding was required, as platelet supernatant had no effect. Furthermore, platelets potentiated the cytotoxicity of PRBC for TNF-or LT-␣-activated HBEC-5i cells when they were added prior to these cells on the endothelial monolayers. This effect was not observed when platelets were added after PRBC. Both permeability and TEER were strongly affected, and the apoptosis rate of HBEC-5i cells was dramatically increased. These findings provide insights into the mechanisms by which platelets can be deleterious to the brain endothelium during CM.

Platelets have recently been shown to accumulate in brain microvessels of patients with cerebral malaria and to modulate the binding of Plasmodium falciparum-infected red cells to human brain endothelium in vitro. In the present study we used a platelet-endothelial cell coculture model to investigate the mechanisms by which platelets modify the function of human brain microvascular endothelial cells (HBEC). Platelets were found to have a proapoptotic effect on TNF-activated HBEC, and this was contact-dependent, as inhibiting platelet binding prevented endothelial cell killing. We also showed that the supernatants of thrombin-activated platelets killed TNF-stimulated HBEC and that TGF-β1 was the main molecule involved in endothelial cell death, because its inhibition completely abrogated the activated-platelet supernatant effect. Our data illustrate another aspect of the duality of TGF-β1 in malaria and may provide new insights into the pathogenesis of cerebral malaria.