Cross-linking by tissue transglutaminase-2 alters fibrinogen-directed macrophage proinflammatory activity

Background The blood coagulation factor fibrin(ogen) can modulate inflammation by altering leukocyte activity. Analyses of fibrin(ogen)-mediated proinflammatory activity have largely focused on leukocyte integrin binding activity revealed by conversion of fibrinogen to a stabilized fibrin polymer by blood coagulation enzymes. In addition to coagulation enzymes, fibrinogen is a substrate for tissue transglutaminase-2 (TG2), a widely expressed enzyme that produces unique fibrinogen A alpha-gamma chain cross-linked products. Objectives We tested the hypothesis that TG2 dependent cross-linking alters the proinflammatory activity of surface-adhered fibrinogen. Methods Mouse bone marrow-derived macrophages (BMDMs) were cultured on tissue culture plates coated with fibrinogen or TG2-cross-linked fibrinogen (10 mu g/ml) and then stimulated with lipopolysaccharide (LPS, 1 ng/ml) or vehicle for various times. Results In the absence of LPS stimulation, TG2-cross-linked fibrin(ogen) enhanced inflammatory gene induction (e.g., Tnf alpha) compared with unmodified fibrinogen. LPS stimulation induced mitogen-activated protein kinase phosphorylation, I kappa B alpha degradation, and expression of proinflammatory cytokines (e.g., tumor necrosis factor alpha) within 60 min. This initial cellular activation was unaffected by unmodified or TG2-cross-linked fibrinogen. In contrast, LPS induction of interleukin-10 mRNA and protein and STAT3 phosphorylation was selectively attenuated by TG2-cross-linked fibrinogen, which was associated with enhanced proinflammatory cytokine secretion by LPS-stimulated BMDMs at later time points (6 and 24 h). Conclusions The results indicate that atypical cross-linking by TG2 imparts unique proinflammatory activity to surface-adhered fibrinogen. The results suggest a novel coagulation-independent mechanism controlling fibrinogen-directed macrophage activation.