Tumoral and Systemic Immune Correlates of Response to Concurrent Pembrolizumab and Chemoradiotherapy in Patients with Resected High-Risk Head and Neck Squamous Cell Carcinoma

Purpose: Patients with pathologically high-risk, human papillomavirus–negative squamous cell carcinoma of the head and neck (HNSCC) recur frequently despite adjuvant cisplatin radiotherapy (CRT). As CRT can upregulate PD-1/L1 immune checkpoints, adding pembrolizumab may reverse treatment-induced immunosuppression. NRG-HN003 was a phase I trial assessing the safety and recommended phase II schedule of adjuvant pembrolizumab with CRT. Here, we report exploratory immune and genomic correlates of disease-free survival (DFS). Patients and Methods: Thirty-four patients received pembrolizumab with CRT. PD-L1 expression was quantified by combined positive score (CPS), and spatial localization of PD-L1+ cells was assessed by multispectral imaging of baseline tumors. Disruptive TP53 mutations were evaluated by whole-exome sequencing. Soluble serum biomarkers were evaluated by Luminex, and circulating immune subsets were profiled by spectral flow cytometry at baseline and after treatment. Results: Patients with PD-L1 CPS ≥20 showed numerically lower DFS than those with CPS <20; however, higher densities of PD-L1+ stromal cells were associated with more favorable outcomes. Disruptive TP53 mutations were not a significant negative prognostic factor. Among soluble markers at baseline, elevated serum arginase-1 was associated with inferior DFS, whereas higher levels of GM-CSF, IL-5 and nectin-2 were associated with more favorable outcomes. In circulating immune cells, higher baseline frequencies of CD8+ granzyme K+ T cells, as well as higher posttreatment frequencies of CD8+ CD39+ and regulatory CD4+ T cells, were associated with improved DFS. Increased effector and central memory T cell populations, especially after treatment, also showed favorable associations with DFS. Conclusions: These findings highlight multiple potential immunologic correlates of pembrolizumab with CRT in high-risk HNSCC, supporting further evaluation and validation in larger, adequately powered trials.