IL15 and lymphopenia: Taken together, our data demonstrate that irradiation-induced lymphopenia promotes adoptive Te cell survival via IL-15 signaling the STAT5/Bcl-2 pathway, and enhances T-cell memory formation via IL-15 activation of the FOXO/Eomes pathway for memory programming and the ULK1/Atg7 autophagy pathway and via IL-15 activation of the mitochondrial biogenesis for maintenance of their memory metabolism.