Fas Pulls the Trigger on Psoriasis

Fas/FasL signaling is best known for induction of apoptosis. However, there is an alternate pathway of Fas signaling that induces inflammatory cytokines, particularly tumor necrosis factor (TNF)-α and interleukin (IL)-8. This pathway is prominent in cells that express high levels of anti-apoptotic molecules such as Bcl-xL. Because TNF-α is central to the pathogenesis of psoriasis and psoriatic epidermis has a low apoptotic index with high expression of Bcl-xL, we hypothesized that inflammatory Fas signaling mediates induction of psoriasis by activated lymphocytes. Noninvolved skin from psoriasis patients was grafted to beige-severe combined immunodeficiency mice, and psoriasis was induced by injection of FasL-positive autologous natural killer cells that were activated by IL-2. Induction of psoriasis was inhibited by injection of a blocking anti-Fas (ZB4) or anti-FasL (4A5) antibody on days 3 and 10 after natural killer cell injection. Anti-Fas monoclonal antibody significantly reduced cell proliferation (Ki-67) and epidermal thickness, with inhibition of epidermal expression of TNF-α, IL-15, HLA-DR, and ICAM-1. Fas/FasL signaling is an essential early event in the induction of psoriasis by activated lymphocytes and is necessary for induction of key inflammatory cytokines including TNF-α and IL-15.