Prognostic significance of p16 and immune cell infiltration in recurrent/metastatic head and neck squamous cell carcinoma treated with PD-1 inhibition: a national DAHANCA cohort study.

PD-1 inhibition has become an established treatment option for recurrent/metastatic head and neck squamous cell carcinoma (rmHNSCC). However, there is a clear need for improved prognostic tools. This study aimed to identify immune-related tissue biomarkers associated with overall survival (OS) or progression-free survival (PFS) in patients treated with PD-1 inhibition. This national real-world phase IV multicenter retrospective cohort study included Danish patients treated between 2017 and 2023. Pre-treatment biopsies were collected for immunohistochemical analyses. All patients were PD-L1 positive with histologically confirmed rmHNSCC treated with pembrolizumab or nivolumab monotherapy. Biomarker expression was assessed for CD4, CD8, FOXP3, CD20, CD66b, CD68, STING, cGAS, and tumor-infiltrating lymphocytes (TILs), using the median expression as the cut-off value. Formalin-fixed, paraffin-embedded tumor tissue was obtained from 263 eligible patients. Concurrent above median levels of FOXP3 and CD68 were associated with a lower risk of progression (HR: 0.47 [95 % CI: 0.33-0.67]). This interaction appeared to be driven by p16+ oropharyngeal cancers (OPC), where patients with concurrent above median levels of FOXP3 and CD68 showed a median 2-year PFS of 68 % [95 % CI: 42-86] in contrast to those with one or none of the two markers above the median level with a 2-year PFS of 3 % [95 % CI: 0-12] (p < 0.001). In this real-world cohort, a subgroup with a promising prognosis was identified. This subgroup was characterized by p16+ OPC along with concurrent above median levels of FOXP3 and CD68. PD-L1 alone showed no significant association with outcomes.