Interleukin 15 (IL-15) is a four-alpha helix bundle cytokine with many similar properties to IL-2, with which it shares components of its receptor. The IL-15 receptor is a heterotrimeric receptor composed of IL-15Ra, the high-affinity receptor for IL-15, as well as IL-2/15Rb (CD122) and common gamma chain (CD132). IL-15 binds to IL-15Rα receptor and can then be presented in trans to IL-2/15Rb and common gamma chain on other cells. Trans-presentation is thought to be the major mechanism by which IL-15-mediated responses occur in mice, although may not be necessary in humans (Castillo et al.). The cytoplasmic domains of IL-2/15Rb and common gamma chain mediate signaling to activate JAK/STAT and PI3K pathways. IL-15 supports the survival and proliferation of naive CD4+ and CD8+ T cells, and promotes homeostasis of memory T cells. IL-15 also promotes the survival and differentiation of NK cells and regulates their cytolytic activity (Ma et al.).