The immune checkpoint PD-1 and its ligand PD-L1 are involved in the induction of immunological tolerance of solid tumors including oral squamous cell carcinoma (OSCC). The aim of the study was to establish the clinical and prognostic significance of PD-L1 in OSCC.

Tissue microarrays of 125 resected OSCC were stained with two different commercially available PD-L1 antibodies (clones E1L3N and 22C3), alongside PD-1 immunostaining.

PD-L1 expression in more than 10% of tumor cells was associated with poorer survival, and established as a clinically relevant cut-off point. This relevant PD-L1 expression was detected in 10% to 15% OSCC specimens depending on the anti-PD-L1 antibody, and showed an inverse correlation with tobacco and alcohol consumption. We consistently found that PD-L1 expression was associated with tumor recurrence and lower disease-specific survival. Multivariate analysis further revealed that neck node metastasis (HR 2.304; P = 0.009) and tumor PD-L1 expression (HR 2.571; P = 0.01) were significant independent factors for poor prognosis.

PD-L1 expression in more than 10% of tumor cells was a significant and independent factor of poor prognosis in OSCC.

PD-L1 expression in more than 10% of tumor cells was consistently established as a clinically relevant cut-off point by using two different antibodies. Remarkably, PD-L1 expression emerges as an independent poor prognosis marker in patients with OSCC.