Therapeutic stress triggers tumor STAT1 acetylation to disarm immunotherapy.

Sequential cancer therapy presents a critical challenge, as the impact of prior treatments on immunotherapy remains unclear. Here, we demonstrate that therapeutic stress from prolonged cetuximab exposure induces tumor-intrinsic resistance to immune checkpoint blockade (ICB) in head and neck squamous cell carcinoma (HNSCC). In a multicenter analysis, extended cetuximab treatment correlates with poor ICB response and survival. Mechanistically, chronic therapeutic stress provokes an initial inflammatory response that transitions into immune resistance. A previously unknown post-translational modification, STAT1 lysine 637 acetylation, serves as the molecular switch driving this process. Triggered by treatment-induced tumor necrosis factor alpha (TNF-α), this acetylation impairs STAT1 dimerization and transcriptional activity, while treatment-induced interferon (IFN)-β promotes STAT1 phosphorylation at tyrosine 701 and subsequent degradation. These modifications disrupt tumor IFN-γ responsiveness. Importantly, STAT1 acetylation in pre-treatment tumor samples predicts ICB efficacy, underscoring its potential as a clinically relevant biomarker for guiding immunotherapy decisions.