Adherent leukocytes capture sickle erythrocytes in an in vitro flow model of vaso-occlusion

Recent in vivo studies suggest that adherent leukocytes bind RBCs and contribute to the microvascular pathology that characterizes sickle cell disease (SCD). A parallel-plate flow assay was used: to investigate the capture of RBCs by adherent neutrophils, monocytes, and T-lymphocytes; to examine whether RBC capture is elevated in patients with SCD; and to determine whether hydroxyurea (HU) therapy affects these interactions. Four measures of cell-cell adhesion were used: adhesion of leukocytes to TNF-alpha-treated human umbilical vein endothelial cells (HUVECs), percent of adherent leukocytes that captured RBCs, number of RBCs captured per interacting leukocyte, and duration of RBC capture. Leukocyte subpopulations from sickle patients were more adherent to activated ECs and captured more RBCs per interacting leukocyte than the corresponding subpopulations from healthy controls. While HU did not affect leukocyte adhesion to activated ECs, it reduced the proportion of adherent leukocytes that captured RBCs, as well as the number of RBCs captured per neutrophil. T-lymphocytes demonstrated elevated adhesion in all measures, and may be the leukocyte subpopulation whose behavior is most altered in SCD. Our findings suggest that neutrophils, monocytes, and T-lymphocytes could all be involved in adhesive interactions with autologous RBCs in patients with SCD.