Disclosed herein are modified interleukin 21 receptor extracellular domain (IL21R ECD) proteins comprising a modification selected from: one or more amino acid substitutions at or within 4 amino acid residues of the tryptophan at position 148 of the amino acid sequence of SEQ ID NO:1, an insertion of one or more amino acids at position 148 of the amino acid sequence of SEQ ID NO:1, or deletion of the tryptophan at position 148 of the amino acid sequence of SEQ ID NO:1. Additional substitutions are also contemplated. Also disclosed are fusion proteins comprising the modified IL21R ECD proteins, pharmaceutical compositions comprising the modified IL21R ECD proteins and fusion proteins, and methods of treatment and uses relating to the modified IL21R ECD proteins and fusion proteins.本發明揭示經修飾之介白素21受體細胞外域(IL21R ECD)蛋白質,其包含選自以下之修飾:在處於胺基酸序列SEQ ID NO:1之位置148之色胺酸的4個胺基酸殘基處或在其內的一或多個胺基酸取代;在該胺基酸序列SEQ ID NO:1之位置148處的一或多個胺基酸插入;或在該胺基酸序列SEQ ID NO:1之位置148處的該色胺酸缺失。亦涵蓋其他取代。亦揭示融合蛋白,其包含該等經修飾之IL21R ECD蛋白質;包含該等經修飾之IL21R ECD蛋白質及融合蛋白的醫藥組合物;及與該等經修飾之IL21R ECD蛋白質及融合蛋白相關之治療方法及用途。