Soluble Fibrinogen-Like Protein 2/Fibroleukin Exhibits Immunosuppressive Properties: Suppressing T Cell Proliferation and Inhibiting Maturation of Bone Marrow-Derived Dendritic Cells

Abstract

Fibrinogen-like protein 2 (fgl2)/fibroleukin is a member of the fibrinogen-related protein superfamily. In addition to its established role in triggering thrombosis, it is known to be secreted by T cells. The soluble fgl2 (sfgl2) protein generated in a baculovirus expression system bound to both T cells and bone marrow-derived dendritic cells (DC) in a specific manner. sfgl2 exhibited immunomodulatory properties capable of inhibiting T cell proliferation stimulated by alloantigens, anti-CD3/anti-CD28 mAbs, and Con A in a dose-dependent manner; however, it had no inhibitory effects on CTL activity. The time- and dose-dependent inhibitory effect of sfgl2 on alloreactive T cell proliferation could be neutralized by a mAb against mouse fgl2. Polarization toward a Th2 cytokine profile with decreased production of IL-2 and IFN-γ and increased production of IL-4 and IL-10 was observed in sfgl2-treated allogeneic cultures. Exposure of immature DC to sfgl2 abrogated the expression of CD80high and MHC class IIhigh molecules and markedly inhibited NF-κB nuclear translocation, thus inhibiting their maturation. sFgl2-treated DC had an impaired ability to stimulate allogeneic T cell proliferation. Maximal inhibition of proliferation was observed when allogeneic T cells were cultured with sfgl2-treated DC and sfgl2 protein was added in the culture. These data provide the first evidence to demonstrate that sfgl2 exerts immunosuppressive effects on T cell proliferation and DC maturation.