Engineered il-21 variants and methods of use thereof

A fusion protein comprising, optionally from N-terminus to C-terminus:

• (a) optionally a His-tag;
• (b) a HSA;
• (c) a linker peptide, and
• (d) an engineered IL-21 polypeptide.

A method of improving the stability of a protein (e.g., cytokine), comprising

• (a) providing a 3D structure of the protein (e.g., cytokine),
• (b) measuring distance of the Calpha atoms of one or more of amino acid residues in the 3D structure;
• and
• (c) selecting two amino acid residues from the one or more amino acid residues, wherein the Calpha atoms of the two selected amino acid residues are within 3-7 angstroms.

A method of screening a cytokine variant with an improved anti-tumor efficacy, comprising

• (a) providing a 3D structure of the cytokine,
• (b) measuring distance of the Calpha atoms of one or more of amino acid residues in the 3D structure;
• and
• (c) selecting two amino acid residues from the one or more amino acid residues, wherein the Calpha atoms of the two selected amino acid residues are within 3-7 angstroms.