Cerebral malaria in mice: demonstration of cytoadherence of infected red blood cells and microrheologic correlates.

To understand the microcirculatory events during cerebral malaria, we have studied the lethal strain of rodent Plasmodia, Plasmodium yoelii 17XL, originally described by Yoeli and Hargreaves in 1974. The virulence of P. yoelii 17XL is caused by intravascular sequestration of infected red blood cells (IRBCs), especially in the brain vessels and capillaries. This mouse model resembles human P. falciparum infection more closely than P. berghei ANKA infection since it shows little, if any, inflammation of the brain. In vivo microcirculatory studies on cytoadherence of IRBCs were performed using the cremaster muscle preparation, which is an easily accessible vasculature for intravital observations. Ex vivo assay of cytoadherence was carried out in the artificially perfused mesocecum preparation of the rat. The results in either preparation demonstrated cytoadherence of IRBCs that was restricted to postcapillary venules. Furthermore, the in vivo measurements showed the prevalence of cytoadherence in small-diameter (< 40 microns) venules in accordance with the local wall shear rates. The parasitized animals demonstrated significantly reduced red blood cell velocities and wall shear rates in the small-diameter postcapillary venules of the cremaster. The relationship between cytoadherence and venular wall shear rates was also reflected in the inverse correlation between the number of adhered cells and the venular diameter in the ex vivo mesocecum preparation. In the ex vivo preparation, cytoadherence of IRBCs was accompanied by a higher peripheral resistance. Transmission electron microscopy of the cremaster muscle and brain tissues showed a tight association of IRBCs with the endothelium of small venules. These observations demonstrate that cytoadherence of P. yoelii 17XL-infected mouse red blood cells is very similar to that of P. falciparum-infected cells. Thus, this model should allow a detailed analysis of the molecular mechanisms involved in the generation of cerebral malaria by cytoadherence of the infected red blood cells to the vascular endothelium.