It was previously shown that plasmin activates human peripheral monocytes in terms of lipid mediator release and chemotactic migration. Here it is demonstrated that plasmin induces proinflammatory cytokine release and tissue factor (TF) expression by monocytes. Plasmin 0.043 to 1.43 CTA U/mL, but not active site-blocked plasmin, triggered concentration-dependent expression of mRNA for interleukin-1alpha (IL-1alpha), IL-1beta, tumor necrosis factor-alpha (TNF-alpha), and TF with maximum responses after 4 hours. Plasmin-mediated mRNA expression was inhibited in a concentration-dependent manner by the lysine analogue trans-4-(aminomethyl)cyclohexane-1-carboxylic acid (t-AMCA). Increases in mRNA levels were followed by concentration- and time-dependent release of IL-1alpha, IL-1beta and TNF-alpha and by TF expression on monocyte surfaces. Neither cytokines nor TF could be detected when monocytes were preincubated with actinomycin D or cycloheximide. Electrophoretic mobility shift assays indicated plasmin-induced activation of NF-kappaB; DNA-binding complexes were composed of p50, p65, and c-Rel, as shown by supershift experiments. Nuclear translocation of NF-kappaB/Rel proteins coincided with IkappaBalpha degradation. At variance with endotoxic lipopolysaccharide, plasmin elicited the rapid degradation of another cytoplasmic NF-kappaB inhibitor, p105. Proteolysis of NF-kappaB inhibitors was apparently due to transient activation of IkappaB kinase (IKK) beta that reached maximum activity at 1 hour after plasmin stimulation. In addition, AP-1 binding was increased in plasmin-treated monocytes, with most complexes composed of JunD, c-Fos, and FosB. These findings further substantiate the role of plasmin as a proinflammatory activator of human monocytes and reveal an important new link between the plasminogen-plasmin system and inflammation. (Blood. 2001;97:3941-3950)