Adhesive small bowel obstruction remains a common problem for surgeons. After surgery, platelet aggregation contributes to coagulation cascade and fibrin clot formation. With clotting, fibrin degradation is simultaneously enhanced, driven by tissue plasminogen activator–mediated cleavage of plasminogen to form plasmin. The aim of this study was to investigate the cellular events and proteolytic responses that surround plasminogen activator inhibitor (PAI‐1; Serpine1 ) inhibition of postoperative adhesion. Peritoneal adhesion was induced by gauze deposition in the abdominal cavity in C57BL/6 mice and those that were deficient in fibrinolytic factors, such as Plat ⁻ / ⁻ and Serpine1 ⁻ / ⁻ . In addition, C57BL/6 mice were treated with the novel PAI‐1 inhibitor, TM5275. Some animals were treated with clodronate to deplete macrophages. Epidermal growth factor (EGF) experiments were performed to understand the role of macrophages and how EGF contributes to adhesion. In the early phase of adhesive small bowel obstruction, increased PAI‐1 activity was observed in the peritoneal cavity. Genetic and pharmacologic PAI‐1 inhibition prevented progression of adhesion and increased circulating plasmin. Whereas Serpine1 ⁻ / ⁻ mice showed intra‐abdominal bleeding, mice that were treated with TM5275 did not. Mechanistically, PAI‐1, in combination with tissue plasminogen activator, served as a chemoattractant for macrophages that, in turn, secreted EGF and up‐regulated the receptor, HER1, on peritoneal mesothelial cells, which led to PAI‐1 secretion, further fueling the vicious cycle of impaired fibrinolysis at the adhesive site. Controlled inhibition of PAI‐1 not only enhanced activation of the fibrinolytic system, but also prevented recruitment of EGF‐secreting macrophages. Pharmacologic PAI‐1 inhibition ameliorated adhesion formation in a macrophage‐dependent manner.—Honjo, K., Munakata, S., Tashiro, Y., Salama, Y., Shimazu, H., Eiamboonsert, S., Dhahri, D., Ichimura, A., Dan, T., Miyata, T., Takeda, K., Sakamoto, K., Hattori, K., Heissig, B. Plasminogen activator inhibitor‐1 regulates macrophage‐dependent postoperative adhesion by enhancing EGF‐HER1 signaling in mice. FASEB J. 31, 2625–2637 (2017). www.fasebj.org

Macrophage activation syndrome (MAS) is a life-threatening disorder characterized by a cytokine storm and multiorgan dysfunction due to excessive immune activation. Although abnormalities of coagulation and fibrinolysis are major components of MAS, the role of the fibrinolytic system and its key player, plasmin, in the development of MAS remains to be solved. We established a murine model of fulminant MAS by repeated injections of Toll-like receptor-9 (TLR-9) agonist and d-galactosamine (DG) in immunocompetent mice. We found plasmin was excessively activated during the progression of fulminant MAS in mice. Genetic and pharmacological inhibition of plasmin counteracted MAS-associated lethality and other related symptoms. We show that plasmin regulates the influx of inflammatory cells and the production of inflammatory cytokines/chemokines. Collectively, our findings identify plasmin as a decisive checkpoint in the inflammatory response during MAS and a potential novel therapeutic target for MAS. Publisher9s Note: There is an Inside Blood Commentary on this article in this issue. Key Points Plasminogen/plasmin is excessively activated in the murine model of fulminant MAS. The genetic or pharmacological inhibition of plasminogen/plasmin counteracted a cytokine storm and tissue damage in fulminant MAS.