Increased monocyte adhesion to the endothelial lining of blood vessels by cytokine-inducible adhesion proteins is a crucial event in inflammatory processes. Moreover, adherence is known to induce cytokine gene expression, suggesting a possible positive feedback mechanism. Therefore, we determined whether monocyte adhesion to endothelial cells (ECs) amplifies their adhesion by inducing intercellular adhesion molecule 1 (ICAM-l), and whether such positive feedback mechanism could be mediated by secretion of interleukin-1 (IL-1). Using monocyte-EC couples obtained after monocyte adhesion to ECs, and methods of quantitative polymerase chain reaction and immunofluorescence flow cytometry, we showed a biphasic increase of ICAM-I mRNA content (2 and 16 hours) and a time-dependent increase of cell surface expression of ICAM-1, mainly on ECs, and couple adhesiveness, after monocyte adhesion to ECs. Anti-ICAM-1 monoclonal antibody inhibited 63% of the enhancement of adhesiveness induced on monocyte-EC couples by previous monocyte adhesion, suggesting that monocyte adhesion to ECs induces an increase of couple adhesiveness which is partially dependent on the ICAM-I pathway. The early ICAM-1 mRNA induction was associated with a fast induction of IL-l p mRNA and a 7.7-fold increase in IL-1 p protein in supernatant. However, 30% of this 2-hour ICAM-1 mRNA peak was abolished by recombinant soluble human IL-1 receptor, suggesting that the early ICAM-1 overexpression was partially mediated by IL-lp, and could be induced directly by adherence. The second ICAM-1 mRNA peak was accompanied by a marked increase in IL-I p mRNA and protein secretion (2.6 n g h l ) . The binding to ICAM-1 did not appear to directly stimulate IL-1 p synthesis. These results indicate that monocyte adhesion to endothelial cells appears to stimulate their own recruitment via induction of ICAM-1 thereby constituting a self-perpetuating positive feedback System.