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 activities primarily on T cells.  It is also essential in the development, survival and activation of NK cells.  Increased expression of IL-15 has been linked to 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.