The invention provides soluble fusion protein complexes having at least two soluble fusion proteins. The first fusion protein is a biologically active polypeptide covalently linked to an interleukin-15 (IL-15) polypeptide or a functional fragment thereof. The second fusion protein is a second biologically active polypeptide covalently linked to a soluble interleukin- 15 receptor alpha (IL-15Ra) polypeptide or a functional fragment thereof. In the complexes of the invention, one or both of the first and second fusion proteins further includes an immunoglobulin Fc domain or a functional fragment thereof； and the first fusion protein binds to the soluble IL-15Ra domain of the second fusion protein to form a soluble fusion protein complex. The invention further provides methods for making and using the complexes of the invention.
