Tracking T Cells in Time and Space to Unlock Personalized Immunotherapy in Childhood Cancer

By combining deep single-cell T cell recpetor (TCR) sequencing with longitudinal sampling, our study establishes a comprehensive framework to identify, track, and functionally validate tumor-reactive T cells in a low-mutation pediatric cancer. We show that, unlike adult tumors dominated by bystander immunity, the immature pediatric immune system is naturally enriched for tumor-reactive clonotypes, enabling unusually high validation rates of functional TCRs from both the tumor microenvironment and peripheral blood. Spatiotemporal tracking reveals that immune checkpoint inhibition induces not only transient clonal expansion but also durable T cell memory, with therapy-expanded clonotypes persisting in circulation for over a year. Importantly, blood-derived T cells emerge as a powerful and accessible resource for TCR discovery, capturing dynamic immune responses that extend beyond the static snapshot of the primary tumor. Functional assays confirmed that both tumor-resident and ICI-induced circulating TCRs recognize patient-derived tumor cells, supporting their use in personalized TCR-T cell therapy. Together, our findings reposition T cell dynamics as a robust biomarker of immunotherapy efficacy and provide a practical roadmap for adaptive, personalized TCR-T cell therapy strategies in childhood cancers, where conventional mutation-based biomarkers fail.