Objectives: We aimed to explore the histopathologic characteristics of programmed death-ligand 1 (PD-L1) expression in multiple sites of head and neck squamous cell carcinoma (HNSCC). We sought to compare PD-L1 expression in HNSCC versus normal epithelium and determine the prognostic implication of PD-L1 expression in different tumor sites.
Methods: The Karmanos Cancer Institute head and neck tumor bank was assessed to analyze 29 patients with surgically-resected HNSCC. Tissue blocks from paraffin-fixed tumor specimen as well as negative epithelial margins were selected for analysis. Cored sections from tumor and negative epithelium were used to create a tissue microarray (TMA). Immunohistochemical analysis of the whole tissue sections as well as the TMA was performed using PD-L1 antibody. Sections were scored 1+ to 4+ by blinded investigators to quantify PD-L1 expression.
Results: Twenty-nine patients with surgically-resected HNSCC were analyzed, ages 40-88 (median 59). Various tumor sites were represented including oral cavity (14/29), oropharynx (7/29), and larynx (8/29). We describe the architectural pattern of PD-L1 expression in each of the major HN sites, including the importance of assessing PD-L1 expression within the basal epithelium. This is especially important within the oropharynx where strong PD-L1 expression may be seen in normal lymphatic tissue deep to the basement membrane. TMA analysis demonstrated increased levels of PD-L1 expression in tumor epithelium (2.85) vs. negative margins (1.71, p = 0.0003). PD-L1 expression was found to be increased in laryngeal carcinoma (3.71) vs. oral (2.77) and oropharyngeal HNSCC (2.14, p = 0.02). No difference was found in PD-L1 expression between patients who remained disease free (2.89) versus those who developed disease progression after treatment (2.33, p > 0.05).
Conclusions: PD-L1 expression is increased in HNSCC versus normal epithelial controls. Different subtypes of HNSCC show variable expression of PD-L1, with laryngeal SCC demonstrating significantly increased expression. The prognostic significance of PD-L1 expression remains unclear despite its growing importance as a molecular target in systemic therapy.