Addition of chemotherapy to radiotherapy promotes progenitor-exhausted CD8⁺ T-cell clonal dominance in head and neck cancer.

Concomitant chemoradiotherapy (CRT) is a standard-of-care treatment for unresectable locally advanced head and neck squamous cell carcinoma (LA-SCCHN), but its immune effects, particularly relative to radiotherapy (RT) alone, remain incompletely understood. Using syngeneic murine models, we integrated Nr4a3-Tocky reporter analysis with single-cell transcriptomics and T cell receptor clonotyping to comprehensively profile intratumoural CD8⁺ T-cell responses following RT and CRT. We show that CRT uniquely drives robust clonal expansion of antigen-engaged CD8⁺ T-cells and biases differentiation toward progenitor-exhausted (TPEX) states, whereas RT favours terminal exhaustion (TEX). Single-cell analyses further reveal preferential clonal dominance within TPEX populations following CRT. In peripheral blood mononuclear cells from patients treated with CRT, TEX populations predominated early after treatment, followed by delayed expansion of circulating TPEX-like populations at 3 months, suggesting phased recovery of progenitor-like T-cell states. Together, these findings demonstrate that CRT and RT induce distinct CD8⁺ T-cell differentiation trajectories and clonal architectures, providing insight into treatment-induced immune remodelling in LA-SCCHN.