Heterogeneity of PD-L1 expression on tumor cells and tumor-infiltrating immune cells between the primary tumor and matched lymph node metastases in head and neck squamous cell carcinomas

Background: The role of immune checkpoint inhibition in treating head and neck squamous cell carcinoma (HNSCC) is expanding, yet response rates to PD-L1 therapy remain inconsistent and generally poor. Although several studies have examined heterogeneous intratumoral PD-L1 expression, the disparity in response to PD-L1 therapy between primary tumors and their associated lymph node metastases remains unclear. Methods: Primary tumor samples and two matching lymph node metastases were obtained from a cohort of 50 patients and immunohistochemically stained with a PD-L1 antibody. PD-L1 expression, assessed using the combined positive score (CPS) and tumor proportion score (TPS), and immune infiltration, measured with an immunoreactive score (IRS), were compared between the primary tumor and lymph node metastases. These measures were then correlated with other histopathological and clinical features. Results: PD-L1 expression, evaluated by CPS and TPS, showed no significant differences between the primary tumor and matched lymph node metastases. However, 18% (CPS) respectively 4% (TPS) of patients exhibited clinically relevant discordant CPS and TPS values between their primary tumor and lymph node metastases with respect to the cutoff level for FDA and EMA approval. CPS and TPS values were not influenced by primary tumor subsite or HPV status. Conversely, immune infiltration measured by IRS was significantly affected by primary tumor subsite location. Both HPV tumor status and primary tumor subsite were statistically significantly associated with overall survival. Conclusion: Our study emphasizes the heterogeneity in PD-L1 expression in HNSCC patients and provides potential explanations for the heterogeneous response of primary tumors and lymph node metastases to immune checkpoint therapy as observed in recent clinical trials. These findings underscore the need for a more comprehensive understanding of PD-L1 expression in head and neck cancer and delineate a rational for re-biopsies including lymph node metastases if therapeutic cut-offs for Anti-PD-1 therapy are initially not met.