GVHD-mediated Vicious Cascade: Impaired Reconstitution of Immunosuppressive Donor Plasmacytoid Dendritic Cells

In patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT), severe graft-versus-host disease (GVHD) is tied to failed donor plasmacytoid DC (pDC) reconstitution. The mechanism whereby GVHD causes donor pDC defects and the precise role donor pDCs play in GVHD remain poorly understood. pDCs develop from HSCs in successive stages through Flt3 signaling-dependent differentiation, beginning as multi-potent progenitors (MPPs) and later becoming common DC progenitors. We observed that MPPs were critical for sustaining pDCs and were severely depleted during GVHD. Loss of MPPs was associated with decreased amounts of MPP-producing HSCs and mTOR activation-induced cell death of proliferating MPPs. Additionally, GM-CSF derived from alloreactive T cells subverted MPP production of pDCs. Together, GVHD and alloreactive T cells caused donor pDC defects by impairing the generation, maintenance and differentiation of HSCs and MPPs. Importantly, pDCs suppressed the proliferation and expansion of activated autologous CD4+ T cells via a type I-IFN signaling-dependent mechanism. Transfer of donor pDCs early after transplantation repressed GVHD and preserved potent GVL effects, significantly improving the survival of leukemia mice undergoing allo-HSCT. Our findings identify novel strategies that improve the recovery of donor pDCs early after allo-HSCT and thus may represent an effective therapy to control GVHD. Our findings have broad implications for other diseases in which functional DC development is impaired, such as chronic infections, autoimmunity and cancer.