IL15 and chronic graft versus host disease: NK cells have been traditionally recognized as possessing antitumour functions without causing GVHD.41 However, Shah NN et al reported that adoptive IL‐15/4‐1BBL‐activated NK cell infusion post‐HLA‐matched, T cell‐depleted transplantation contributed to acute GVHD.42 Therefore, the contribution of NK1 cells to the prognosis of chronic GVHD warrants further study.