The cytokine interleukin 15 (IL-15, a protein of 114 amino acids) was first discovered due to IL-2-like stimulatory actions on T cells.^1,2 The heterotrimeric IL-15 receptor comprises the β and γ chains of the IL-2 receptor, with a unique α subunit. These shared receptor subunits most likely explain the similar T cell growth factor properties of both IL-2 and IL-15. Several cell types can produce IL-15, including macrophages, dendritic cells, and intestinal epithelial cells. The discovery that enterocytes can both produce and respond to IL-15,³ and that IL-15 potently stimulates intraepithelial lymphocytes,⁴ has focused attention on its role in intestinal inflammation. IL-15 also has a number of other activities, including recruitment and activation of T cells, maintenance of T cell memory, stimulation of proliferation and immunoglobulin synthesis by B cells, natural killer (NK) cell proliferation, activation of neutrophils, and inhibition of apoptosis. Mice with a genetically disrupted IL-15 gene (“knockout”) remain healthy under specific pathogen free conditions.⁵ However, they display marked reductions in numbers of thymic and peripheral NK T cells, memory …