Provided herein are IL-21 muteins and fusion proteins comprising the same for use in methods of treating a disease. Related conjugates, nucleic acids, vectors, host cells, pharmaceutical compositions and kits are also provided herein. Methods of making the IL-21 muteins and fusion proteins comprising the same, as well as methods of treating a subject in need thereof, are provided by the present disclosure. Further provided are PD-1 antigen-binding proteins.