The present invention relates to a therapeutic polypeptide and methods for its creation and use for modulating an immune response in a
host organism in need thereof. In particular, the invention relates to the administration to an
organism in need thereof, of an effective amount of a pre-coupled polypeptide complex comprising a
lymphokine polypeptide portion, for example IL-15 (SEQ ID NO: 5, 6), IL-2 (SEQ ID NO: 10, 12) or combinations of both, and an
interleukin receptor polypeptide portion, for example IL-15Ra (SEQ ID NO: 7, 8), IL-2Ra (SEQ ID NO: 9, 11) or combinations of both, for augmenting the
immune system in, for example,
cancer, SCID, AIDS, or
vaccination; or inhibiting the
immune system in, for example,
rheumatoid arthritis, or Lupus. The therapeutic complex of the invention surprisingly demonstrates increased half-life, and
efficacy in vivo.