CD4 and graft versus host disease: Beyond just better survival and reconstitution, CD4+ Tregs were necessary early post-transplant for GVHD prevention by PTCy in both xenogeneic and MHC-matched HCT models (21, 22), and thymically-derived natural CD4+Foxp3+ Tregs, rather than peripherally induced CD4+Foxp3+ Tregs, appeared to be playing the major role in the murine MHC-matched HCT models (22).