Interleukin 15 (IL-15) is a glycosylated 14-15 kDa cytokine with structural similarity to IL-2. Like IL-2, IL-15 binds to and signals through the IL-2/IL-15 beta chain (CD122) and the common gamma chain (gamma-C, CD132). IL-15 is constitutively expressed by a large number of cell types and tissues, including monocytes, macrophages, dendritic cells, keratinocytes, fibroblasts, and nerve cells. IL-15 up-regulation plays a central role in the development of several autoimmune or chronic inflammatory disorders. This cytokine induces cell proliferation of natural killer cells; cells of the innate immune system whose principal role is to kill virally infected cells. IL-15 has been shown to enhance the anti-tumor immunity of CD8+ T cells in pre-clinical models.