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To investigate the binding properties of dendritic cells (DC) to vascular endothelium, a comparative analysis was undertaken of DC, monocytes and lymphocytes isolated from the blood of 25 healthy subjects using monolayers of human umbilical vein endothelial cells as the substrate. More blood DC(mean 24% adherence) were adherent to endothelial monolayers than monocytes(mean 18%; P < 0·001) and lymphocytes (mean 12%; P < 0·001). When the monolayers were pretreated with tumour necrosis factor-alpha (TNF-α) all leucocyte populations exhibited an increased attachment, but there was still greater binding of DC (mean 37% adherence) in comparison with monocytes (mean 23%; P < 0·001) and lymphocytes (mean 18%;P < 0·001). Flow cytometric analysis revealed that in relation to monocytes and lymphocytes the DC had a higher surface expression of the adhesion molecules CD11a (P < 0·05), CD11c(P < 0·005) and CD54 (P < 0·005) but a lower prevalence of cells bearing CD49d (mean 38%; P< 0·05) and the homing receptor CD62L (mean 14%; P< 0·001). CD1a was present on 22% of DC and virtually absent from the surface of monocytes and lymphocytes. The intensity of expression of theβ1-integrins, CD49c, CD49d and CD49e was greater on DC than lymphocytes and monocytes (P < 0·05). Antibody blocking studies demonstrated that DC binding to untreated and TNF-α-treated endothelium was dependent upon the expression of CD11a, CD18 and CD49d, and the simultaneous application of anti-CD18 and anti-CD49d antibodies produced an approximate 70% inhibition of adhesion (P < 0·001). Thus, the expression of both β1- and β2-integrins contributes to the adhesive interaction between DC and endothelium.