Interleukin-15 (IL-15) is a pro-inflammatory cytokine, a glycoprotein of 14-15 kD. Constitutive expression has been reported in various cells and tissues including monocytes and macrophages, fibroblasts, keratinocytes and dendritic cells (Waldmann and Tagaya, 1999; Fehniger and Caligiuri, 2001).