2. PD-L1 immunohistochemistry

PD-L1 expression was assessed in formalin-fixed paraffin-embedded tumor samples acquired by surgical resection of each patient’s individual sample, using the PD-L1 IHC 22C3 pharmDx assay (Dako North America). All PD-L1 assays were performed at LabCorp Clinical Trials, Los Angeles (Laboratory Corporation of America, Burlington, NC), the same laboratory, using the same procedures, that was employed in the pembrolizumab clinical trials for NSCLC [21,22]. The assay is described in detail elsewhere [18,19]. Membranous PD-L1 expression on tumor cells was defined by tumor proportion scores (TPSs) of ≥ 50% and 1%-49%, respectively (Fig. 1), consistent with the cutoffs used in pembrolizumab clinical trials [21,22].

Sample images of programmed death ligand 1 (PD-L1) staining in non-small cell lung cancer. (A) PD-L1 negative (tumor proportion score [TPS] < 1%). (B) PD-L1 TPS 1%-49%. (C) PD-L1 TPS ≥ 50%. All images are at original magnification ×20, blue counterstain is hematoxylin, and PD-L1 is identified by brown chromagen.