LCL-specific CD4⁺ T-cell recognition of HLA class II–matched BL lines. (A) EBV-positive lines and (B) EBV-positive and -negative lines. T cells (10⁴ cells/well) were cocultured overnight with standard numbers (5 × 10⁴ cells/well) of LCLs and BL lines whose EBV status and matching for the relevant HLA class II–restricting allele are shown. In each case, the lines that had been preincubated either with medium alone or in the presence of the relevant HLA class II–blocking mAb (as described in the legend for Figure 2). All responses are shown as the mean − 1 SD of triplicate wells in which IFNγ release into the supernatant medium was determined by ELISA (in ng/mL). Results are representative of those seen on 3 occasions of testing.