Programmed death-ligand 1 (PD-L1)/thyroid transcription factor-1 double immunohistochemical staining facilitates scoring of tumor PD-L1 expression in cytopathology specimens from lung adenocarcinoma patients.

Background: Immune checkpoint inhibitor therapy has revolutionized lung adenocarcinoma therapy. Treatment with antibodies against the immune checkpoint molecules programmed death-1 (PD-1) and programmed death-ligand 1 (PD-L1) can induce a durable response in a subset of patients. Immunohistochemistry characterization of tumor PD-L1 expression using either a histopathology specimen or a cytopathology specimen has been shown to correlate with treatment response. However, the current practice relies on pathologists' visual estimation of tumor PD-L1 staining, which can be variable in certain conditions. Highlighting tumor cells via double immunostaining with PD-L1 and thyroid transcription factor-1 (TTF-1) may improve estimation accuracy.

Methods: We performed PD-L1 single staining and PD-L1/TTF-1 double staining in 42 pairs of cytopathology and histopathology specimens from lung adenocarcinoma patients. An experienced pathologist visually estimated PD-L1 expression in each case and placed tumor PD-L1 expression into 1 of 3 categories: <1%, 1%-49%, or ≥50%. A medical technologist also performed estimations of the same cases based on a count of 200 tumor cells, and the results were compared.

Results: PD-L1/TTF-1 double immunohistochemistry could better identify the PD-L1-positive tumor cells in cytopathology specimens compared with PD-L1 single staining. The concordance of PD-L1 expression categorization between the pathologist's visual estimation and the medical technologist's counting was increased by double staining in cytopathology specimens (Cohen's weighted kappa: single stain, 0.784; double stain, 0.880). Double staining reduced possible error in the pathologist's visual estimation of PD-L1 expression from 9.5% to 4.8%. The benefit was not observed in histopathology specimens.

Conclusion: A simple PD-L1/TTF-1 double immunohistochemistry technique can be applied successfully to cytopathology specimens in better identifying patients who can potentially benefit from immune checkpoint blockade treatment.