We also demonstrate that transferred T-cells with prolonged survival in irradiated IL-15 sufficient B6 mice increase phosphorylation of STAT5 and up-regulate expression of Bcl-2, when compared to those in irradiated IL-15 deficient mice, further arguing that irradiation-induced lymphopenia promotes T-cell survival via IL-15 stimulation of the STAT5/Bcl-2 pathway. Here, IL15 is linked to lymphopenia.