Mouse FGL2 Alexa Fluor® 700-conjugated Antibody

FGL2 (fibrinogen-like protein 2), also called fibroleukin, is a 64-70 kDa secreted glycoprotein of the Fibrinogen-like superfamily. It has prothrombinase activity and also promotes T regulatory (T_reg) activity (1-6). The mouse FGL2 gene encodes a 432 amino acid (aa) protein that contains a 19 aa signal sequence and a 413 aa mature sequence with a coiled-coil region and a fibronectin C-terminal homology domain or FRED (1, 2). A 260-280 aa FGL2 complex is thought to be a tetramer formed by covalent disulfide linkage of dimers that are asoociated via coiled-coil interactions (2, 3). Mature mouse FGL2 shares 91% aa identity with rat FGL2, and 77-80% aa identity with human, equine, porcine, bovine and canine FGL2. FGL2 appears to have two modes of action. One mode involves its prothrombinase activity, which requires calcium and acidic phospholipids (4). This mode is thought to be active during hepatitis viral infections when FGL2, produced by macrophages in response to IFN-gamma, induces hepatic apoptosis and fibrin deposition (7). In addition, FGL2 produced by endothelial cells in response to TNF-alpha within cardiac xenografts or allografts promotes coagulation during acute vascular rejection (7-9). A second mode of action involves soluble (not phospholipid-associated) FGL2 and is independent of prothrombinase activity (2). Soluble FGL2 is required for T_reg function, and directly suppresses DC, T, and B cell immune reactivity; consequently, some FGL2-deficent mice develop autoimmune glomerulonephritis (5, 6). In vitro, soluble FGL2 can skew T cell polarization toward Th2 and inhibit proliferation of stimulated T cells and maturation of DC (6). In pregnancy, fetal trophoblast cells secrete FGL2. The immune suppressive mode of FGL2 may prevent early fetal loss; however, the procoagulant mode is thought to mediate infection-triggered abortion (10).