HLA class I restriction of the CTL activation. Optimal gp96/MART-1 complexes (40 µg/ml and 20 µg/ml) were coincubated for 24 h with 8x10⁴ HLA-A2-positive APCs (black bar) and HLA-A2-negative APCs (diagonal-hatched bar), respectively. When compared with HLA-A2-positive APCs, the presence of HLA-A2-negative APCs significantly reduced the CTL stimulation (paired t-test; P=0.0127) by 76% and 97%, respectively, and thus demonstrated the HLA restriction of the APC-dependent CTL stimulation (cross-presentation).