A single low-dose injection with anti-DLL4 in a nonhuman primate model of acute graft-vs.-host disease (aGVHD) dramatically improved post-transplant survival, providing durable protection from otherwise lethal gastrointestinal GVHD, researchers reported in the June 28, 2023, issue of Science Translational Medicine. Blocking DLL4 specifically increased the migration of beneficial regulatory T cells into the intestines, with concomitant reduction in effector T cells, which are the main culprits in aGVHD. Ultimately, these activities effectively provided protection against T-cell-mediated damage in a nonhuman macaque primate model.