Recognition of FSP26-pulsed T2 cells is blocked by mAbs. In vitro-primed CTLs (3,000 cells/well) were stimulated with 35,000 T2 cells pulsed either with FSP26 or with the control peptide P68 as targets. HLA specificity was determined by blocking HLA-A2 on T2 target cells with the anti-HLA-A2 mAb BB7.2 or the anti-MHCI mAb W6/32. The influence of blocking CD28 and CD3 on the release of IFN-gamma by T cells was tested using the CD28-2.1 and OKT3 mAbs, respectively. Cells were incubated 1 h with hybridoma supernatant before being seeded in 96-well plates. The number of IFN-gamma-releasing activated T cells (spots) for the total number of cells analyzed (10⁴) is given. As a measure of background IFN-gamma release, reactivity against control peptide P68 is shown.