IL-15 is produced by a variety of cell types within and outside of the immune system, including dendritic cells, monocytes and macrophages, stromal cells, endothelial cells, and epithelial cells [41] , and pathogenic stimuli are known to induce this expression above constitutive levels [17, 42, 43, 44] . Although dendritic cells are known to be an important source of IL-15 at day 6 post influenza infection [27] , it is unclear whether DCs or other cell types produce IL-15 to facilitate specific immune responses to influenza. For example, lung epithelial cells constitutively express IL-15 and IL-15Ra [45] , and neutrophils and macrophages can be a major source of IL-15 that is produced during a variety of lung inflammatory diseases including sarcoidosis, tuberculosis, bronchitis, and asthma [46] . Following influenza infection, the IL-15producing cell type(s) is/are still unknown, but influenza-induced expression of IL-15 is clearly an important regulator of the NK and CD8 T cell responses to this virus [19] .