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.本文提供供在治療疾病之方法中使用的IL-21突變蛋白及包含其之融合蛋白。本文亦提供相關結合物、核酸、載體、宿主細胞、醫藥組合物及套組。本發明提供製造該等IL-21突變蛋白及包含其之融合蛋白的方法,以及治療有需要之個體的方法。進一步提供PD-1抗原結合蛋白。