Hematopoeitic Stem Cells and Their Progenitors Critically Require Autophagy to Promote Early Engraftment Following Allogeneic Stem Cell Transplantation

Stem cell transplantation (SCT) is the only curative therapy for the majority of hematological malignancies and relies on successful engraftment of donor hematopoietic stem cells (HSC) to reconstitute the patient's hematopoiesis and immunity. Graft-versus-host disease (GVHD) and graft failure are complications of SCT that contribute to significant morbidity and mortality. Recent studies have established a role for autophagy in the long-term survival and function of HSCs and as a regulator of hematopoiesis. We investigated the contribution of autophagy to HSC in the setting of allogeneic transplantation. Using LC3-GFP bone marrow as a graft source in the well-established B6 into F1 model of GVHD, together with imaging flow cytometry, we demonstrated increased autophagy (LC3-GFP punctae) within donor HSC and progenitor cells in GVHD compared to non-GVHD recipients. To assess the contribution of autophagy to HSC development and function we used mice deficient in Atg5, an essential protein for the autophagy pathway. In primary transplants, ATG5 KO foetal liver (FL) had reduced ability to engraft lethally irradiated congenic recipient mice compared to WT FL. Furthermore, competitive transplantation of 50% ATG5 KO FL cells together with 50% WT FL cells demonstrated that ATG5 KO cells had a reduced capacity to reconstitute compared to congenic WT FL cells. Finally, to address the role of autophagy in HSC in the setting of GVHD, we transplanted WT or ATG5 KO FL ± T cells into lethally irradiated B6D2F1 recipients. At D10 post-transplant ATG5 KO recipients failed to establish tri-lineage engraftment in peripheral blood and lacked expansion of myeloid precursor cells in the BM, leading to engraftment failure and significantly reduced survival compared to WT FL recipients (50% WT vs 10% Atg5 KO p<0.0001). This early engraftment failure was confirmed to be autophagy specific using VAV^cre x Atg7^fl/fl donor mice, in which Atg7 deficiency is restricted to hematopoietic cells. Furthermore, the essential requirement for autophagy in early progenitors and HSC was confirmed using LysM^cre Atg7^fl/fl mice, which ablate Atg7 from the granulocyte-macrophage progenitor population onwards. Survival, or engraftment, in mice receiving the LysM^cre Atg7^fl/fl donor graft did not differ significantly from those receiving a WT graft. We demonstrate that autophagy is increased in the GVHD setting and that without autophagy early myeloid precursors fail to provide short term reconstitution leading to primary graft failure and mortality. Thus, intervention to increase autophagy in these cells post-transplant may improve engraftment in the clinic.