PD-L1 pitfalls: Emphasizing the importance of membranous localization and correlation with tumor cell and macrophage distributions

Use of the 22C3 anti–programmed death‐ligand 1 antibody to determine programmed death‐ligand 1 expression in cytology samples obtained from non–small cell lung cancer patients

“…It should also be emphasized that careful interpretation of PD‐L1 staining in cytology samples is warranted because of the presence of immune cells, particularly macrophages, which are found in large numbers in pleural effusions. If these samples are evaluated without reference to a hematoxylin and eosin slide or, in some cases, complementary stains to assess immune cells such as macrophages or lymphocytes, the PD‐L1 staining might be misinterpreted as positive tumor cell PD‐L1 expression, even though tumor cells adjacent to the macrophage infiltrate have no PD‐L1 expression, as shown in Supporting Figure 6 (see online supporting information) and previously reported . In addition, although it is necessary to have ≥100 viable tumor cells present in tissue specimen slides to determine the percentage of PD‐L1–stained cells, there is no established minimum requirement for the number of tumor cells in cytology specimens.…”

“…If these samples are evaluated without reference to a hematoxylin and eosin slide or, in some cases, complementary stains to assess immune cells such as macrophages or lymphocytes, the PD-L1 staining might be misinterpreted as positive tumor cell PD-L1 expression, even though tumor cells adjacent to the macrophage infiltrate have no PD-L1 expression, as shown in Supporting Figure 6 (see online supporting information) and previously reported. 15 In addition, although it is necessary to have 100 viable tumor cells present in tissue specimen slides to determine the percentage of PD-L1-stained cells, 8 there is no established minimum requirement for the number of tumor cells in cytology specimens. PD-L1 staining and quantitation should be technically feasible in principle as long as appropriate protocols and quality-control measures are being used.…”