HLA specificity of allo-reactive CD4+ T-cell clones. HLA restriction of all isolated allo-reactive CD4+ T-cell clones from patient 1 (n = 19) (A) and patient 2 (n = 16) (B) was analyzed using blocking moAb specific for HLA class I, HLA class II, and HLA-DR, -DQ, and -DP with IFN-γ ELISA. Each dot represents the release of IFN-γ (pg/mL) in 10-µL supernatants by a single allo-reactive CD4+ T-cell clone in response to patient EBV-LCL in the absence or presence of blocking moAb and donor EBV-LCL. All CD4+ T-cell clones showed HLA-DP–restricted recognition, as illustrated by abrogated IFN-γ production upon preincubation with moAb specific for HLA class II and HLA-DP, but not with moAb against HLA class I, HLA-DR, or HLA-DQ. (C-D) All allo-reactive HLA-DP–restricted CD4+ T-cell clones were tested for recognition of patient and donor EBV-LCL, and donor EBV-LCL retrovirally transduced with patient HLA-DPB1 and -DPA1 alleles (HLA-DPB1*0101 and -DPB1*0301 with DPA1*0201 for patient 1 [C] and HLA-DPB1*0301 with -DPA1*0103 for patient 2 [D]) with IFN-γ ELISA. HLA-DPB1–restricted CD4+ T-cell clones from patient 1 recognized either HLA-DPB1*0101 (○) or HLA-DPB1*0301 (●) transduced donor EBV-LCL. Each dot represents the release of IFN-γ (pg/mL) in 10 µL supernatant by a single CD4+ T-cell clone.