Waller et al. showed that donor bone marrow precursors of plasmacytoid dendritic cells (pre-pDCs) can activate donor T cells toward Th1 immune polarization, then induce indoleamine 2,3-dioxygenase synthesis to limit GVHD by altering the balance between donor Treg and inflammatory T cells [33]. The gene discussed is IDO2; the disease is graft versus host disease.