Graft-versus-host tolerance: donor T cells and “immature” host DCs are nonstimulatory. IL-10 and transforming growth factor-β are produced by the DC and IL-4 by the T cell; no costimulatory molecules are expressed by indoleamin 2,3 deoxygenase is released. This peripheral tolerance is maintained as long as there is no conditioning treatment with radiation or chemotherapy and no infection that may activate the DCs.