Fig 1.

Inhibition of the cytotoxic activity of CTL clone by engagement of the PD-1 receptor with PD-L1 on the specific target cells. (A and B) Expression of PD-1, H-2L^d, and PD-L1 on 2C CTL clone and P815 tumor cells as well as their stable transfectant clones of PD-L1(P815/PD-L1 nos. 1–3) were analyzed by using a flow cytometry. Although not shown, none of the cells expressed PD-L2. (C) 2C cells were incubated with ⁵¹Cr-labeled P815 (○) or three independent P815/PD-L1 clones (□, ◊, ▵) in the absence or presence (▴) of 10 μg/ml rat anti-PD-L1 mAb F(ab′)₂ at varying effector-to-target (E/T) ratios for 4 h, and the specific ⁵¹Cr release was determined. The means and SE of triplicate cultures are indicated.