An isolated cell that recombinantly expresses: (i) a human interleukin 15 (IL-15), and (ii) a soluble form of a human interleukin 15 receptor alpha (IL-15Ra), or a polypeptide comprising a soluble form of a human IL-15Ra and a heterologous molecule, wherein the cell expresses at least 150 ng/million cells of human IL-15 per day when cultured in serum-free media, wherein the human IL-15: (a) has an amino acid sequence that is at least 95% identical to the amino acid sequence consisting of amino acid residues 49 to 162 of SEQ ID NO: 1; (b) is encoded by a nucleic acid sequence that is at least 80% identical to SEQ ID NO:2; (c) is encoded by a nucleic acid sequence that is at least 80% identical to the nucleic acid sequence consisting of nucleic acid residues 145 to 489 of SEQ ID NO:2; (d) has the amino acid sequence consisting of amino acid residues 49 to 162 of SEQ ID NO:1; (e) is encoded by SEQ ID NO:2; (f) is encoded by the nucleic acid sequence consisting of nucleic acid residues 145 to 489 of SEQ ID NO:2; (g) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1257 of SEQ ID NO:9; (h) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1218 of SEQ ID NO:11; or (i) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1164 of SEQ ID NO:17, and wherein the soluble form of human IL-15Ra: (a) has an amino acid sequence that is at least 95% identical to the amino acid sequence consisting of amino acid residues 31 to 205 of SEQ ID NO: 4; (b) is encoded by a nucleic acid sequence that is at least 80% identical to SEQ ID NO:6; (c) is encoded by a nucleic acid sequence that is at least 80% identical to the nucleic acid sequence consisting of nucleic acid residues 91 to 615 of SEQ ID NO:6; (d) has the amino acid sequence consisting of amino acid residues 31 to 205 of SEQ ID NO: 4; (e) is encoded by SEQ ID NO:6; or (f) is encoded by a nucleic acid sequence consisting of nucleic acid residues 91 to 615 of SEQ ID NO:6. (i) a human interleukin 15 (IL-15), and (ii) a soluble form of a human interleukin 15 receptor alpha (IL-15Ra), or a polypeptide comprising a soluble form of a human IL-15Ra and a heterologous molecule, wherein the cell expresses at least 150 ng/million cells of human IL-15 per day when cultured in serum-free media, wherein the human IL-15: (a) has an amino acid sequence that is at least 95% identical to the amino acid sequence consisting of amino acid residues 49 to 162 of SEQ ID NO: 1; (b) is encoded by a nucleic acid sequence that is at least 80% identical to SEQ ID NO:2; (c) is encoded by a nucleic acid sequence that is at least 80% identical to the nucleic acid sequence consisting of nucleic acid residues 145 to 489 of SEQ ID NO:2; (d) has the amino acid sequence consisting of amino acid residues 49 to 162 of SEQ ID NO:1; (e) is encoded by SEQ ID NO:2; (f) is encoded by the nucleic acid sequence consisting of nucleic acid residues 145 to 489 of SEQ ID NO:2; (g) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1257 of SEQ ID NO:9; (h) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1218 of SEQ ID NO:11; or (i) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1164 of SEQ ID NO:17, and wherein the soluble form of human IL-15Ra: (a) has an amino acid sequence that is at least 95% identical to the amino acid sequence consisting of amino acid residues 31 to 205 of SEQ ID NO: 4; (b) is encoded by a nucleic acid sequence that is at least 80% identical to SEQ ID NO:6; (c) is encoded by a nucleic acid sequence that is at least 80% identical to the nucleic acid sequence consisting of nucleic acid residues 91 to 615 of SEQ ID NO:6; (d) has the amino acid sequence consisting of amino acid residues 31 to 205 of SEQ ID NO: 4; (e) is encoded by SEQ ID NO:6; or (f) is encoded by a nucleic acid sequence consisting of nucleic acid residues 91 to 615 of SEQ ID NO:6. (a) has an amino acid sequence that is at least 95% identical to the amino acid sequence consisting of amino acid residues 49 to 162 of SEQ ID NO: 1; (b) is encoded by a nucleic acid sequence that is at least 80% identical to SEQ ID NO:2; (c) is encoded by a nucleic acid sequence that is at least 80% identical to the nucleic acid sequence consisting of nucleic acid residues 145 to 489 of SEQ ID NO:2; (d) has the amino acid sequence consisting of amino acid residues 49 to 162 of SEQ ID NO:1; (e) is encoded by SEQ ID NO:2; (f) is encoded by the nucleic acid sequence consisting of nucleic acid residues 145 to 489 of SEQ ID NO:2; (g) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1257 of SEQ ID NO:9; (h) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1218 of SEQ ID NO:11; or (i) is encoded by the nucleic acid sequence consisting of nucleic acid residues 769 to 1164 of SEQ ID NO:17, and (a) has an amino acid sequence that is at least 95% identical to the amino acid sequence consisting of amino acid residues 31 to 205 of SEQ ID NO: 4; (b) is encoded by a nucleic acid sequence that is at least 80% identical to SEQ ID NO:6; (c) is encoded by a nucleic acid sequence that is at least 80% identical to the nucleic acid sequence consisting of nucleic acid residues 91 to 615 of SEQ ID NO:6; (d) has the amino acid sequence consisting of amino acid residues 31 to 205 of SEQ ID NO: 4; (e) is encoded by SEQ ID NO:6; or (f) is encoded by a nucleic acid sequence consisting of nucleic acid residues 91 to 615 of SEQ ID NO:6.