Cytotoxicity Mediated by the Fas Ligand (FasL)-activated Apoptotic Pathway in Stem Cells

Whereas it is now clear that human bone marrow stromal cells (BMSCs) can be immunosuppressive and escape cytotoxic lymphocytes (CTLs) in vitro and in vivo, the mechanisms of this phenomenon remain controversial. Here, we test the hypothesis that BMSCs suppress immune responses by Fas-mediated apoptosis of activated lymphocytes and find both Fas and FasL expression by primary BMSCs. Jurkat cells or activated lymphocytes were each killed by BMSCs after 72 h of co-incubation. In comparison, the cytotoxic effect of BMSCs on non-activated lymphocytes and on caspase-8(−/−) Jurkat cells was extremely low. Fas/Fc fusion protein strongly inhibited BMSC-induced lymphocyte apoptosis. Although we detected a high level of Fas expression in BMSCs, stimulation of Fas with anti-Fas antibody did not result in the expected BMSC apoptosis, regardless of concentration, suggesting a disruption of the Fas activation pathway. Thus BMSCs may have an endogenous mechanism to evade Fas-mediated apoptosis. Cumulatively, these data provide a parallel between adult stem/progenitor cells and cancer cells, consistent with the idea that stem/progenitor cells can use FasL to prevent lymphocyte attack by inducing lymphocyte apoptosis during the regeneration of injured tissues.Human bone marrow stromal cells (BMSCs)² (also referred to as mesenchymal stem cells (MSCs)) (1) contain a subset of multipotent, non-hematopoietic stem/progenitor cells. BMSCs can differentiate into hematopoiesis-supporting stromal tissue, adipocytes, osteoblasts, and chondrocytes (2, 3). In addition, they may be able to transdifferentiate into hepatocytes, myocytes, neuroectodermal cells, and endothelial cells, (4–6) although proof of such differentiation is not definitive to date. BMSCs have immunosuppressive potential, as recently demonstrated in both in vitro (7) and in vivo (8, 9) systems, including clinical studies (10, 11). However, the mechanisms by which BMSCs suppress immune responses are unresolved. Soluble factor-mediated immunosuppressive effects are beginning to come to light, (10, 12), and in addition there are as yet unexplained effects of cell-to-cell contact.In the present study, we hypothesize that BMSC-mediated cytotoxicity of lymphocytes involves the FasL-activated apoptotic machinery. FasL is a type II transmembrane protein belonging to the tumor necrosis factor (TNF) family. FasL interacts with its receptor, Fas (CD95/APO-1) and triggers a cascade of subcellular events culminating in apoptotic cell death. FasL and Fas are key regulators of apoptosis in the immune system. In addition, FasL is expressed by cells in immune-privileged sites, such as cancer cells, neurons, eyes, cytotrophoblasts of the placenta, and reproductive organs (13–17). In neurons, FasL expression specifically protects against T cell-mediated cytotoxicity (16).The discovery that FasL is also expressed by a variety of tumor cells raises the possibility that FasL may mediate immune privilege in human tumors (18). Activated T cells expressing Fas are sensitive to Fas-mediated apoptosis. Thus, up-regulation of FasL expression by tumor cells may enable tumorigenesis by targeting apoptosis in infiltrating lymphocytes. In the present work, we show that BMSCs can mediate immunosuppressive activity by FasL-induced killing of activated lymphocytes. Thus, BMSCs have properties of immune-privileged cells.