Fas ligand (FasL) is increased in several immune-mediated diseases, including acute graft versus host disease (aGVHD), a donor cell-mediated disorder post-hematopoietic stem cell transplantation (post-HSCT). In this disease, FasL is involved in T-cell mediated damage to host tissues. However, the role of its expression on donor non T-cells has, so far, never been addressed. Using a well-established CD4- and CD8-mediated GVHD murine model, we found that precocious gut damage and mice mortality are increased with graft of donor T- and B-depleted bone marrow cells (TBD-BM) devoid of FasL as compared with their wild type counterparts. Interestingly, serum levels of both soluble FasL (s-FasL) and of IL-18 are drastically reduced in the recipients of FasL-deficient grafts indicating that s-FasL stems from donor BM-derived cells. In addition, the correlation between the concentrations of these two cytokines suggests that IL-18 production arises through a s-FasL-driven mechanism. These data highlight the importance of FasL-dependent production in IL-18 production and in mitigating aGVHD. Overall, our data reveal the functional duality of FasL according to its source.

To better understand the stoichiometry of CD95L required to trigger apoptotic and non-apoptotic signals, we generated several CD95L concatemers from dimer to hexamer conjugatedviaa flexible link (GGGGS)2. These ligands reveal that although the hexameric structure is the best stoichiometry to trigger cell death, a dimer is sufficient to induce the apoptotic response in CD95-sensitive Jurkat cells. Interestingly, only trimeric and hexameric forms can implement a potent Ca2+response, suggesting that while CD95 aggregation controls the implementation of the apoptotic signal, both aggregation and conformation are required to implement the Ca2+pathway.