Recent studies in our laboratory, as well as others, have suggested that fibrin can regulate cell function in vitro and likely control inflammation in vivo by acting as a potent cell activator. This has led us to hypothesize that during tissue and vascular injury, fibrin can enhance leukocyte recruitment by inducing vascular endothelial cell expression of leukocyte chemotactic factors. To begin to test this hypothesis, we developed an in vitro model of in situ fibrin polymerization on human umbilical vein endothelial cell culture (HUVEC) and determined the ability of fibrin to induce HUVEC expression of the potent leukocyte chemotactic factor interleukin-8 (IL-8). Our initial studies showed that fibrin induced IL-8 expression in a time- and dose-dependent fashion. Fibrin-induced IL-8 expression in HUVEC could be seen as early as 2 hours post-fibrin stimulation. Additionally, fibrin concentrations as low as 30 microg/mL stimulated a detectable level of IL-8 antigen expression from HUVEC. We also showed that this fibrin induced IL-8 had the identical molecular weight and similar antigenic identity as recombinant and monocyte derived IL-8. Northern blot analysis showed that the IL-8 antigen increase seen in fibrin treated HUVEC was due to fibrin induced elevation of steady state mRNA expression in HUVEC. These data clearly support our hypothesis that fibrin is a potent vascular endothelial cell (VEC) activator that can directly contribute to leukocyte recruitment and activation by inducing leukocyte chemotactic factor expression from VEC.