CTIM-01. INCREASED BLOOD–TUMOR TCR REPERTOIRE OVERLAP CORRELATES WITH FAVORABLE CLINICAL RESPONSES IN CHILDREN WITH RECURRENT MEDULLOBLASTOMA TREATED WITH ADOPTIVE CELL TRANSFER THERAPY

Abstract BACKGROUND Adoptive cellular therapy (ACT), which involves the infusion of tumor-specific lymphocytes, has shown clinical promise across a range of malignancies. However, monitoring the in vivo dynamics of transferred T cells remains a major challenge, particularly in recurrent medulloblastoma, where the tumor-infiltrating lymphocyte (TIL) repertoire is often highly complex and heterogeneous. METHODS To enable precise tracking of tumor-reactive T cells, we developed a customized T-cell receptor (TCR) sequencing protocol using RNA extracted directly from tumor tissue. This method employs a 5’-RACE-like strategy to capture full-length TCR transcripts, with first-strand cDNA synthesis primed by TCR-specific oligonucleotides to ensure efficient and unbiased amplification. RESULTS TCR sequencing revealed marked intra-tumoral clonal expansion, with dominant clonotypes comprising 1.1% to 51% of the T cell repertoire (mean: 16.1%). In 17 patients, the 25 most abundant T cell clones represented 27% to 73% of the TIL population. To investigate the kinetics of these tumor-associated T cells in peripheral blood, we analyzed samples collected at four time points: baseline (pre-treatment), day 0 (post-lymphodepletion, pre-vaccination), day 14 (following ACT and the first dendritic cell [DC]-based vaccine), and day 28 (post-second DC vaccine, loaded with autologous total tumor RNA). Patients with favorable clinical responses exhibited significantly greater blood–tumor TCR repertoire overlap, with up to 150 shared clonotypes detectable post-vaccination. Importantly, these overlapping clonotypes correlated with prolonged overall survival. Transcriptional profiling of shared T cells further confirmed their functional competence, demonstrating cytotoxicity, cytokine production, and proliferative potential. CONCLUSIONS Our findings demonstrate that increased overlap between blood and tumor TCR repertoires is associated with favorable clinical responses to ACT in pediatric medulloblastoma. These shared clonotypes likely represent circulating tumor-reactive T cells capable of mounting durable systemic immune responses. TCR repertoire analysis may serve as a predictive biomarker for ACT efficacy and inform personalized immunotherapeutic strategies in pediatric brain tumors.