CTLA-4 Y218* mutation leads to protein instability. (A–C) (A) Representative histogram showing MFI of CTLA-4 levels in Jurkat cells with WT or Y218* CTLA-4 treated with CHX to block protein translation, Baf to block lysosome acidification, or untreated. Jurkat cells lacking CTLA-4 (−CTLA4) shown as negative control. MFI quantified in B, and fold change quantified in C; the y-axis in C shows fold change relative to the untreated condition. The mutant panel uses a different scale due to higher fold-change values compared to WT. Fold-change values in the statistical comparison bar represent relative differences between the indicated conditions; repeated measures one-way ANOVA test. N = 5 independent repeats. **P < 0.001, ***P < 0.001, and ****P < 0.0001; ns: not significant; number represents fold change.