Proteasome-mediated digestion of HA-3^T and HA-3^M. The 25-mer HA-3 peptides were cleaved with J-Y–derived immunoproteasomes as described in “Patient, materials, and methods.” Correct C-terminal cleavage was observed for both HA-3^T (panel A, m/z 938.46⁺²) and HA-3^M (panel B, m/z 953.41⁺²). Arrows indicate the correct cleavage product. Destruction of the HA-3^T peptide could not be observed, as no peptide ion with m/z 565.32⁺² (DAVLQRDLVT) was detectable (C). The HA-3^M peptide was cleaved behind P2 of the HA-3 9-mer as indicated by the presence of DAVLQRDLVM (panel D, m/z 580.32²⁺). (E) Proteasome inhibitor PSI was able to inhibit antigen-specific lysis of HA-3⁺ EBV-BLCL HoRe significantly. Target cell lysis was restored after adding exogenous HA-3^T peptide.