Fibrin deposition following bile duct injury limits fibrosis through an αMβ2-dependent mechanism

Coagulation cascade activation and fibrin deposits have been implicated or observed in diverse forms of liver damage. Given that fibrin amplifies pathological inflammation in several diseases through the integrin receptor alpha(M)beta(2), we tested the hypothesis that disruption of the fibrin(ogen)-alpha(M)beta(2) interaction in Fib gamma(390-396A) mice would reduce hepatic inflammation and fibrosis in an experimental setting of chemical liver injury. Contrary to our hypothesis, a-naphthylisothiocyanate (ANIT)-induced liver fibrosis increased in Fib gamma(390-396A) mice, whereas inflammatory cytokine expression and hepatic necrosis were similar to ANIT-challenged wild-type (WT) mice. Increased fibrosis in Fib gamma(390-396A) mice appeared to be independent of coagulation factor 13 (FXIII) transglutaminase, as ANIT challenge in FXIII-deficient mice resulted in a distinct pathological phenotype characterized by increased hepatic necrosis. Rather, bile duct proliferation underpinned the increased fibrosis in ANIT-exposed Fib gamma(390-396A) mice. The mechanism of fibrin-mediated fibrosis was linked to interferon (IFN)gamma induction of inducible nitric oxide synthase (iNOS), a gene linked to bile duct hyperplasia and liver fibrosis. Expression of iNOS messenger RNA was significantly increased in livers of ANIT-exposed Fib gamma(390-396A) mice. Fibrin(ogen)-alpha(M)beta(2) interaction inhibited iNOS induction in macrophages stimulated with IFN gamma in vitro and ANIT-challenged IFN gamma-deficient mice had reduced iNOS induction, bile duct hyperplasia, and liver fibrosis. Further, ANIT-induced iNOS expression, liver fibrosis, and bile duct hyperplasia were significantly reduced in WT mice administered leukadherin-1, a small molecule that allosterically enhances alpha(M)beta(2)-dependent cell adhesion to fibrin. These studies characterize a novel mechanism where by the fibrin(ogen)-integrin-alpha(M)beta(2) interaction reduces biliary fibrosis and suggests a novel putative therapeutic target for this difficult-to-treat fibrotic disease.