Role of Fibrinogen-Like Protein 2 Prothrombinase/Fibroleukin in Experimental and Human Allograft Rejection

Abstract

Immune coagulation is a major contributor to the pathogenesis of xenograft rejection, viral-induced hepatocellular injury and cytokine-induced fetal loss syndrome. In this study, we investigated the contribution of the novel gene product, fibrinogen-like protein 2 (fgl2) prothrombinase, in mediating immune injury in experimental and human acute allograft rejection. Using a mouse heterotopic cardiac transplant model, mouse fgl2(mfgl2)/fibroleukin mRNA transcripts and protein were highly expressed in macrophages, CD4- and CD8-positive T lymphocytes, and endothelial cells in rejecting cardiac allografts in association with deposits of fibrin. Although mfgl2-deficient mice rejected allografts at similar rates to littermate controls, survival of grafts from mfgl2-deficient mice were prolonged and deposition of intravascular fibrin was diminished. Treatment of wild-type mice with a neutralizing anti-fgl2 Ab ameliorated histological evidence for allorejection and intravascular fibrin deposition, and resulted in an increase in graft survival. To address further the relevance of fgl2 in acute allograft rejection, we examined kidney biopsies from patients who had undergone renal transplantation. Human fgl2 mRNA transcripts and protein were markedly expressed mainly in renal tubule cells, infiltrating lymphoid cells including macrophages, CD8+ T cells, mature B cells (plasma cells), and endothelial cells. Dual staining showed fibrin deposition was localized mainly to blood vessels, in the glomerulus and interstitium and the lumen of tubules, and occurred in association with human fgl2 expression. These data collectively suggest that fgl2 accounts for the fibrin deposition seen in both experimental and human allograft rejection and provide a rationale for targeting fgl2 as adjunctive therapy to treat allograft rejection.

Targeted Deletion of fgl2 Leads to Impaired Regulatory T Cell Activity and Development of Autoimmune Glomerulonephritis

“…Furthermore, cardiac grafts from fgl2 Ϫ/Ϫ mice were resistance to thrombosis, suggesting a role for endothelial cell fgl2 expression in the fibrin deposition associated with allo-and xenotransplantation rejection. In contrast, Ning et al (13) and Hancock et al (16) showed that fgl2 Ϫ/Ϫ mice rejected fully mismatched allografts, supporting the contention that the endothelium, but not leukocytes, maybe more important for fibrin deposition in allotransplant rejection.…”

“…These proteins have been shown to exhibit coagulant and angiogenic activity, as well as immunoregulatory activity (1,2,30). Similar to other members of the fibrinogen superfamily, our laboratory has recently discovered (19) that FGL2 inhibits murine DC maturation and adaptive T cell immune responses, in addition to its role in innate immunity through its ability to activate the coagulation system (7)(8)(9)(10)(11)(12)(13)(14)(15).…”

“…FGL2 was first cloned from human CTLs, and its protein structure deduced from the nucleotide sequence showed a high degree of homology to fibrinogen ␤ and ␥ subunits (3). FGL2 has been shown to play an important role in innate immunity as an immune coagulant expressed by activated reticuloendothelial cells (macrophages and endothelial cells) (4 -6) and has been implicated in the pathogenesis of several inflammatory disorders, including viral hepatitis (7)(8)(9)(10), allo-and xenograft rejection (11)(12)(13), and cytokine induced fetal loss (14,15). In a previous report (13), we showed that Ab to FGL2 ameliorated allograft rejection.…”

The novel CD4+CD25+ regulatory T cell effector molecule fibrinogen-like protein 2 contributes to the outcome of murine fulminant viral hepatitis

“…Plates were coated and incubated overnight with 2 g/mL monoclonal anti-FGL2 (6H12) (IgG1) as a capture antibody. Plasma samples (50 L) were added to each well, and after a 1-hour incubation at 37°C and three washes with Tris-Tween buffered saline, the wells were incubated with 2 g/mL polyclonal rabbit anti-FGL2 antibody 15 for 2 hours at 37°C. The plate was washed again, and polyclonal anti-FGL2 binding was detected with a secondary horseradish peroxidase-conjugated anti-rabbit antibody.…”

The FGL2‐FcγRIIB pathway: A novel mechanism leading to immunosuppression

“…Predicted as a type II transmembrane glycoprotein [2], cell membraneassociated FGL2 was shown to exhibit novel prothrombinase activity when associated with cell membranes/phospholipid vesicles [3,4], which has been implicated in the pathogenesis of experimental and human viral-induced fulminant hepatitis [2], cytokine-induced fetal loss syndrome [5], allograft [6] and xenograft rejection [7]. The procoagulant activity was shown to depend on the serine 89 at the linear N terminal domain of FGL2 [4].…”