In certain aspects, described herein include covalently or non-covalently bound to interleukin-15 receptor α (“IL-15Ra”) to enhance IL-15-mediated immune function. Is a periodic dosing regimen for administration to a patient of a complex containing interleukin-15 ("IL-15"). In certain aspects, a cyclic dosing regimen achieves IL-15 plasma levels above basal levels while minimizing the toxicity associated with administration of IL-15. In certain aspects, the periodic regimen is effective in the prevention, treatment and / or management of diseases in which IL-15-mediated functions are promoted (such as cancer, infection, immunodeficiency and lymphopenia) . Also described herein are purified soluble IL-15Ra, cells recombinantly expressing soluble IL-15Ra, and covalently bound or non-binding to soluble IL-15Ra. A composition comprising a covalently bound complex of IL-15. Further described herein is a host cell that recombinantly expresses an IL-15Ra derivative containing a mutation or deletion at the site of cleavage of the extracellular domain, wherein the extracellular domain and heterologous of IL-15Ra Includes IL-15Ra derivatives containing the transmembrane domain of the molecule. Furthermore, described herein is a method of proliferating, activating and / or differentiating IL-15 responsive cells, wherein IL-15 reactive cell (s) are expressed as IL-15Ra. A method comprising co-culture with a recombinantly expressed host cell for a period of time and isolating IL-15Ra-reactive cells from the host cell (s). The immune cells, IL-15 reactive cells, can be administered for the prevention, treatment and / or management of various diseases including cancer, infections, immunodeficiencies and lymphopenia. Further described herein are methods of enhancing immune function mediated by IL-15, as well as cancer in a subject in which it is beneficial to enhance immune function mediated by IL-15, etc. A method for the prevention, treatment and / or management of a disease of c
