PD-L1 expression predicts longer disease free survival in high risk head and neck cutaneous squamous cell carcinoma

Programmed cell death (PD-1) and its ligand (PD-L1) inhibitors have shown clinical response in many tumours. PD-L1 data are limited in head and neck cutaneous squamous cell carcinoma (HNcSCC) and no clinical trials of PD-1/PD-L1 inhibitors are published. We performed PD-L1 immunohistochemistry on 74 cases of high risk HNcSCC with 38 matched metastases and evaluated clinicopathological associations, prognostic significance and heterogeneity in matched metastases. We observed PD-L1 expression in >5% of primary tumour cells in 29 cases (39.2%), primary tumour infiltrating lymphocytes (TILs) in 40 cases (70.2%), metastatic tumour cells in 15 cases (39.5%), and metastatic TILs in 18 cases (47.4%). PD-L1 expression in >5% of primary tumour cells was associated with an inflammatory phenotype (p = 0.04), and in primary TILs with clear margins (p = 0.05). PD-L1 expression in >5% of primary tumour cells (p = 0.01), primary TILs (p = 0.001), and metastatic TILs (p = 0.02) was associated with improved disease free survival. PD-L1 expression in >5% of tumour cells was heterogeneous between primary and metastatic tumours in 13 cases (34.2%). PD-L1 expression is common in HNcSCC supporting the rationale for a clinical trial of PD-1/PD-L1 inhibitors. PD-L1 expression in tumour cells or TILs predicts longer disease free survival and demonstrates temperospatial heterogeneity.