Beyond the Percentages of PD-L1-Positive Tumor Cells: Induced Versus Constitutive PD-L1 Expression in Primary and Metastatic Head and Neck Squamous Cell Carcinoma

Anti-PD1 antibody has been approved for metastatic squamous cell carcinoma of the head and neck (SCCHN) and objective response rates of approximately 20% have been reported. Defining PD-L1 expression at ≥ 1% tumor cells as positive, PD-L1-positive tumors showed a higher response rate. However, it is unclear whether 1% is the optimal cutoff, and studies on lung cancer suggested 50% cutoff as a stronger predictive biomarker. 96 primary SCCHN from oropharynx and oral cavity and 34 corresponding metastatic lesions were typed for membranous PD-L1 expression. p16 immunohistochemistry was used as a surrogate marker for HPV status in SCCHN from the oropharynx. Fifty-two of 96 (54%) tumors were PD-L1-positive, 72% if PD-L1 expression in tumor-infiltrating immunocytes was also included as positive. Fifteen of 34 primary-metastasis tumor pairs differed in PD-L1 expression, and p16(+) cases more frequently showed PD-L1 expression in immunocytes than p16(−) cases (82 vs. 45%, p < 0.05). PD-L1-positive SCCHN showed two distinct patterns of expression. In the induced pattern of expression, PD-L1-positive tumor cells were limited to the periphery of tumor nests at the tumor–immunocyte interface, comprising < 5% of tumor cells, and were almost always associated with PD-L1-positive immunocytes. In contrast, tumors with constitutive PD-L1 expression had a higher percentage of positive tumor cells, often diffusely distributed throughout the tumor, and often were not accompanied by PD-L1-positive immunocytes. We propose that distinguishing these two biologically distinctive patterns of PD-L1 expression and typing metastatic instead of primary lesions might better predict immunotherapeutic response to anti-PD1/PD-L1 regimens beyond just the percentage of PD-L1-positive tumor cells.