The invention provides an isolated cell that recombinantly expresses (i) a mammalian interleukin (IL-15), and (ii) a soluble form of a mammalian interleukin-15 receptor alpha (IL-15Ra), a mammalian IL-15Ra, a polypeptide comprising a mammalian IL-15Ra and a first heterologous molecule, or a polypeptide comprising a soluble form of a mammalian IL-15Ra and a second heterologous molecule, wherein the cell expresses at least 150 ng/million cells of mammalian IL-15 per day when cultured in serum-free media. The invention further provides a method of recombinantly producing (i) human IL-15 and (ii) human IL-15Ra or a polypeptide comprising a human IL-15Ra and a heterologous molecule.
