I n an attempt to understand the regulatory mechanisms governing passage of neutrophils from the vascular bed to the interstitial tissue, we analyzed the effect of the pleiotropic monokines interleukin 1 (IL-1) and tumor necrosis factor (TNF) on transendothelial neutrophil traffic. Short-time preincubation of human umbilical vein endothelial cell (HUVE) monolayers with IL-1 and TNF led to an impressive time- and dose-dependent increase of endothelial cell-associated neutrophils when working in a full plasma system on petri dishes. Electron microscopic analysis revealed junctional penetration of monolayers by neutrophils. More quantitatively, when using a monolayer-on-filter-system, priming led to a severalfold increase in complete layer passage occurring in the absence of an external chemotactic gradient. Direct comparison with an upside-down modification of the system together with data demonstrating the vectorial behavior of such migration revealed that IL-1-stimulated transendothelial neutrophil traffic is polarized. The described enhancement of neutrophil transendothelial passage was found to be a unique feature of IL-1/TNF-activated HUVE since HUVE-dependent transmigration potentiation was not observed as a consequence of mere neutrophil attachment to endothelial cells (e.g., induced by Fc-mediated adherence of PMN to HUVE). IL-1 acts selectively on endothelial cells as demonstrated by total inhibition of its effect by actinomycin D. Moreover, IL-1 does not induce HUVE monolayers to secrete a chemotaxin, and the neutrophil passage guiding principle is removable from the HUVE surface by short trypsin exposure. Congruent results were obtained with human adult arterial as well as saphenous vein endothelial cells. As shown by blockade of neutrophil migration with pertussis toxin, IL-1- and TNF-inducible transendothelial migration can be dissected into an initial anchoring step, which is succeeded by active neutrophil migration, possibly along a putative endothelial membrane-bound gradient.