Longitudinal liquid biopsy identifies an early predictive biomarker of immune checkpoint blockade response in head and neck squamous cell carcinoma

Immune checkpoint inhibition (ICI) has emerged as a pivotal therapy for head and neck squamous cell carcinoma (HNSCC), yet the development of predictive biomarkers to guide its clinical application has significantly lagged. Current biomarkers such as tumor mutational burden and PD-L1 fail to capture systemic immune dynamics that may better reflect host immune fitness and the coordinated immune response driving durable tumor control. We hypothesized that early treatment-induced changes in circulating immune repertoires could provide a dynamic, non-invasive readout of ICI responsiveness. Using a time-resolved, multi-omic approach in a murine HNSCC model, we characterized peripheral immune responses to anti-PD-1 across defined pre- and on-treatment timepoints. Single-cell transcriptomics and paired T/B cell receptor analyses revealed an early and robust, but transient, expansion of effector memory T and B cell repertoires in responders, preceding tumor regression. Temporal changes in effector T and B cell abundance, clonality, and gene expression strongly predicted immunotherapy response, with early on-treatment timepoints emerging as the optimal window for assessing treatment response. These dynamic immune features informed a composite transcriptional signature, Liquid Biomarker for Immuno-Oncology (LiBIO), derived from effector T and B cell programs that accurately predicts ICI response in independent human HNSCC cohorts and outperforms contemporary biomarkers such as PD-L1 combined positive score and tumor mutational burden. LiBIO further generalizes to melanoma, non-small cell lung cancer, and breast cancer without retraining, suggesting that early peripheral immune dynamics capture conserved features of effective antitumor immunity across cancer types. Collectively, these findings support the premise that antitumor immune responses initiated regionally in tumor-draining lymphatics give rise to transient, stereotyped peripheral immune responses that precede successful primary tumor control, and that peripheral immune events can serve as a foundation for liquid biomarker discovery. This innovative approach represents a paradigm shift from static, tumor-centric biomarkers to dynamic monitoring of host immunity, enabling real-time treatment adaptation during the critical early window of immune activation.