Identification of the HLA-DR restriction elements. All 4 HLA-DR alleles of the patient (HLA-DRB1*0301, -DRB1*1301, -DRB3*0101, and -DRB3*0202) were isolated and retrovirally transduced into mHag⁺ EBV-LCLs lacking the appropriate HLA-DR molecules. CD4⁺ T-cell clones were tested against nontransduced and HLA-DR–transduced EBV-LCLs in IFN-γ ELISA. (A) T-cell clone 62 recognized LB-MR1-1R in the context of HLA-DRB3*0202, (B) T-cell clone 78 recognized LB-PTK2B-1T in the context of HLA-DRB3*0101, (C) T-cell clone 80 recognized LB-LY75-1K in the context of HLA-DRB1*1301, and (D) T-cell clone 87 recognized LB-MTHFD1-1Q in the context of HLA-DRB1*0301. Mean release of IFN-γ (ng/mL) in 50-μL supernatants of duplicate wells is shown.