Fas-ligand enhances hematopoietic cell engraftment through abrogation of alloimmune responses and non-immunogenic interactions

¹ Frankel Laboratory, Center for Stem Cell Research, Department of Pediatric Hematology-Oncology, Schneider Children's Medical Center of Israel, Petach Tikva, Israel
² Institute for Cellular Therapeutics and Department of Microbiology and Immunology, University of Louisville, Louisville, Kentucky
³ Department of Surgery, Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel

^* To whom correspondence should be addressed. E-mail: anadir{at}012.net.il.

	Abstract

Early after transplantation, donor lineage-negative bone marrow cells (lin^- BMC) constitutively upregulated their expression of Fas-ligand (FasL), suggesting an involvement of the Fas/FasL axis in engraftment. Following the observation of impaired engraftment in the presence of a dysfunctional Fas/FasL axis in FasL-defective (gld) donors or Fas-defective (lpr) recipients, we expressed a non-cleavable FasL chimeric protein on the surface of donor lin- BMC. Despite a short life span of the protein in vivo, expression of FasL protein on the surface of all the donor lin- BMC improved the efficiency of engraftment two-fold. The FasL-coated donor cells efficiently blunted the host alloimmune responses in primary recipients and retained their hematopoietic reconstituting potential in secondary transplants. Surprisingly, FasL protein improved the efficiency of engraftment in syngeneic transplants. The deficient engraftment in lpr recipients was not reversed in chimeric mice with Fas^- stroma and Fas⁺ BMC, demonstrating that the host marrow stroma was also a target of donor cell FasL. Hematopoietic stem and progenitor cells are insensitive to Fas-mediated apoptosis, but can exploit the constitutive expression of FasL to exert potent veto activities in the early stages of the engraftment process. Manipulation of the donor cells using ectopic FasL protein accentuated the immunogenic and non-immunogenic interactions between the donor cells and the host, alleviating the requirement for a megadose of transplanted cells to achieve a potent veto effect.