TRERF1 and graft versus host disease: Combined with the result in our study that the promoting effects of hUC-EVs-ATO on M2 macrophage polarization were strengthened when macrophages were stimulated with RAPA, it is possible that RAPA may be an attractive approach to strengthen the immunosuppressive function of hUC-EVs-ATO, contributing negatively to GVHD development.