Soluble fusion molecules multimeric IL-15 and methods for making and using the same

A complex of soluble fusion proteins comprising at least two soluble fusion proteins wherein the first fusion protein comprises (a) a biologically active first polypeptide covalently linked to (b) polypeptide of interleukin-15 (IL-15) or a functional fragment thereof; and the second fusion protein comprises (c) a second biologically active peptide covalently linked to (d) polypeptide receptor interleukin-15 α (IL-15Rα) soluble or a functional fragment thereof in which one or both of the proteins first and second melt further comprise an immunoglobulin Fc domain or a functional fragment thereof; where the domain of IL-15 of the first fusion protein binds to the domain of IL-15Rα soluble second fusion protein to form a complex of soluble fusion proteins, and wherein the IL-15 is a IL-15 variant having a substitution of N for D at position 72 of the sequence of mature IL-15 and wherein the IL-15Rα comprises the sushi domain of IL-15Rα human.