IL-15 (114 amino acids) is a cytokine that regulates T and natural killer cell activation and proliferation. It has a predicted molecular mass of approximately 12.5 kDa. Human IL-15 shares approximately 97% and 73% amino acid sequence identity with simian and mouse IL-15, respectively. Both human and simian IL-15 are active on mouse cells. IL-15 was initially isolated from the simian kidney epithelial cell line CV1/EBNA. It has also been isolated from mouse and human cell sources. The cytokines IL-15 and IL2 share many biological properties and stimulatory activities (T, B, and NK cells). Both IL-15 and IL-2 stimulate mouse CTLL2 cells. In activated peripheral blood T lymphocytes, IL-2 is highly expressed but the expression of IL-15 is not detectable. There is no sequence homology between IL-15 and IL-2, though computer modeling indicates both possess a four alpha helical bundle structure. IL-15 competes for binding sites with IL-2, as both IL2 and IL-15 stimulate the growth of cells through the IL-2 receptor. IL-15 mRNA is expressed in many cell types and tissues including adherent peripheral blood mononuclear cells, fibroblasts, and epithelial cells, monocytes, placenta, and skeletal muscle.

Isoform IL-15-S48AA: Secreted. Isoform IL-15-S21AA: Cytoplasm. Nucleus. Note: IL-15-S21AA is not secreted, but rather is stored intracellularly, appearing in the nucleus and cytoplasmic components.