IL2 and graft versus host disease: Human peripheral blood (PB) or umbilical cord blood Tregs can be expanded in vitro through T cell receptor (TCR) stimulation in the presence of IL-2.11–26 In vitro expanded human Tregs prevent transplant rejection,27, 28 transplant arteriosclerosis29 and graft versus host disease (GvHD)21, 30 in humanised mice.