Expansion of circulating NKG7⁺ cytotoxic CD4⁺ T cells as a predictor of response to PD-1 blockade in recurrent and/or metastatic head and neck squamous cell carcinoma (R/M HNSCC): A prospective phase II study with translational analysis.

6043 Background: Nivolumab has demonstrated meaningful survival benefit in patients with refractory R/M HNSCC. Nevertheless, reliable predictive biomarkers remain scarce—particularly those capable of identifying long-term survivors—underscoring the need for translational studies to uncover immune correlates of durable response. In this prospective phase II study (NCT04603248), we sought to define dynamic circulating immune cell–based biomarkers predicting durable clinical outcomes with nivolumab in patients with R/M HNSCC. Methods: Patients with R/M HNSCC who had prior failure of or intolerance to platinum-based chemotherapy were treated with nivolumab (3 mg/kg) intravenously every 2 weeks until disease progression or unacceptable toxicity occurred. Clinical outcomes were correlated with single-cell transcriptomic profiles of circulating immune cells at baseline (cycle 1 day 1) and on-treatment (cycle 2 day 1). To validate the transcriptomic findings at the protein level, mass cytometry by time-of-flight (CyTOF) analysis was additionally performed. Results: A total of 48 patients were enrolled. The objective response rate was 22.9%, and the disease control rate was 62.5%. The median progression-free survival (PFS) and overall survival (OS) were 4.4 and 13.3 months, respectively. Single-cell transcriptomic analysis revealed a significant expansion of circulating NKG7⁺ cytotoxic CD4⁺ T cells in long-term responders (PFS > 48 months; n=6) compared with early progressors (PFS < 2 months; n=6) at cycle 2 day 1. T cell receptor analysis further demonstrated that nivolumab induced marked clonal expansion of these NKG7⁺ cytotoxic CD4⁺ T cells, particularly in long-term responders. Their sustained presence was confirmed in blood samples collected one year after treatment initiation in long-term responders. CyTOF analysis (n=37) revealed that expansion of NKG7⁺ cytotoxic CD4⁺ T cells at cycle 2 day 1 was significantly associated with both PFS and OS, supporting their potential role as predictive biomarkers of response to PD-1 blockade. Conclusions: Expansion and clonal amplification of circulating NKG7⁺ cytotoxic CD4⁺ T cells represent a key immune correlate of favorable outcomes with nivolumab in refractory R/M HNSCC. These findings highlight their potential as predictive biomarkers of durable response to PD-1 blockade in R/M HNSCC and implicate this immune subset as a promising target for future immunotherapeutic strategies. Clinical trial information: NCT04603248 .