The present relates to interleukin 15 (IL-15) antagonists and uses thereof, in particular for the treatment of autoimmune diseases and inflammatory diseases. In particular, the present invention relates to an IL-15 mutant polypeptide having the amino acid sequence as set forth in SEQ ID NO:1 wherein the leucine residue at position 45 is substituted by an aspartic acid residue, the asparagines residue at position 65 is substituted by a lysine residue and the leucine residue at position 69 is substituted by an arginine residue.