Expression of PD-L1 on the surface of neoplastic MCs in SM. (A-B) KIT⁺/CD34⁻ BM MCs obtained from 3 patients with ISM (A) and 3 with advanced SM (2 with MCL, 1 with ASM) (B) were stained with antibodies against PD-1 (CD279) (left subpanels) or PD-L1 (CD274) (right subpanels) by multicolor flow cytometry as described in “Materials and methods.” Antibody staining is shown in gray. The isotype-matched control is also shown (red lines and open histograms). (C) The MCL-derived human MC lines HMC-1.1 (KIT D816V⁻) (left subpanel) and HMC-1.2 (KIT D816V⁺) (right subpanel) were stained with antibodies against PD-1 (top subpanels) or PD-L1 (bottom subpanels).