Role of Fas–FasL in inflammatory diseases

Abstract

Fas ligand (FasL) induces programmed cell death, or ‘apoptosis’, in cells expressing its cognate receptor, Fas (CD95/APO-1). There is evidence that FasL precludes inflammatory reactions from sites of ‘immune privilege’ by triggering Fas-mediated apoptosis of infiltrating pro-inflammatory cells. The ability of FasL to impair immune responses is being pursued as a possible means of protecting tissue transplants from immunological rejection, and therapeutic promise has been reported in some experiments. However, FasL is becoming an enigmatic molecule, exhibiting pro-inflammatory activity independently of its ability to mediate immune downregulation. FasL can recruit and activate neutrophils and macrophages in some experimental situations. Triggering of Fas in some cell types has been shown to upregulate expression of certain pro-inflammatory cytokines and chemokines, providing an unexpected link between apoptosis and inflammation. FasL appears to contribute to the destruction of Fas-sensitive end-organ cells during inflammation. This appears to occur in two ways: (1) direct killing by cytotoxic immune effector cells expressing FasL; or (2) autocrine cell suicide of end-organ cells that upregulate their own FasL in the inflammatory context. Depending on the condition, or the site of inflammation, either or both mechanisms may occur. Prevention of Fas-mediated end-organ apoptosis and enhancement of Fas-mediated apoptosis of inflammatory cells are emerging as potential anti-inflammatory therapeutic goals.