IL-15 is an immunomodulating cytokine that stimulates the proliferation of T lymphocytes and shares many biological properties with IL-2. IL-15 exerts its biological activites primarily on T cells. It is also essential in the development, survival and activation of NK cells. Increased expression of IL-15 has been implicated with rheumatoid arthritis, inflammatory bowel disease and diseases affiliated with retroviruses HIV and HTLV-I. Human IL-15 is biologically active on mouse cells as measured by the dose-dependent stimulation of the proliferation of mouse CTLL cells. Recombinant human IL-15 is a 12.9 kDa protein consisting of 114 amino acid residues.