The abnormal adherence of sickle erythrocytes to endothelial cells (EC) has been hypothesized to play a role in the initiation of vaso-occlusion in sickle cell anemia. Erythrocyte/endothelial cell interactions were studied under controlled flow conditions for normal (AA), homozygous sickle cell (SS), sickle trait (AS), mechanically injured normal, and "high reticulocyte control" red blood cells (RBC). Human umbilical vein endothelial cells grown to confluence on glass slides formed the base of a parallel plate flow chamber into which RBC suspensions were perfused at a constant flow rate, producing a wall shear stress of 1 dyne/cm('2). Adhesion was monitored using video microscopy, and the number of adherent RBC was determined at ten-minute intervals during a wash-out period. Results indicate that SS RBC were more adherent than AA RBC. Mechanically injured (sheared) RBC were also more adherent than control normal cells, but less adherent than SS RBC. AS RBC did not differ significantly in their adhesive properties from normal RBC. Less dense (younger) RBC were more adherent to EC than dense (older) cells for normal, SS and "high reticulocyte control" RBC. The average velocity of individual SS RBC in the region near the EC monolayer was approximately half that of AA RBC at the same bulk volumetric flow rate, as determined using image analysis techniques. The influence of several factors on the adherence of normal and sickle cells to endothelial cells was examined: (1) increasing shear rates resulted in decreased adhesion, (2) pretreating EC with a chemotactic agent had little effect on adherence properties, (3) treating RBC with pentoxifylline diminished the adherence of sickle RBC, but had no effect on AA RBC and (4) suspending RBC in a protein free suspending medium did not affect the demonstration of adherence differences between SS and AA RBC. These findings suggest that the increased adhesion of sickle RBC is at least partially related to the increased numbers of young RBC present. Increased adherence of young cells to the EC lining vessel walls could contribute to microvascular occlusion by lengthening vascular transit times of other sickle cells. Pentoxifylline may aid in the treatment of sickle cell disease by decreasing these obstructive RBC-EC interactions that may play a role in vaso-occlusion
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