Radiation bone marrow (BM)chimeric recipients with fully engrafted donor-derived IFN-γR–deficient APCs had less indirect allostimulatory capacity, resulting in attenuated activation of host-alloantigen–specific donor lymphocytes from the delayed donor lymphocyte infusion (DLI) and development of GVHD. Here, IFNGR1 is linked to graft versus host disease.