IMMU-51. THE LANDSCAPE OF TUMOR INFILTRATING T CELLS (TILS) OF CHILDHOOD BRAIN TUMORS

Abstract

Abstract T lymphocytes have a unique ability to recognize a vast array of antigens prompt by an enormous T cell receptor (TCR) repertoire. Characterization of tumor-infiltrating T cells (TILs) and their TCRs is key to understand MHC-restricted anti-tumor immunity and for developing T cell centered immunotherapies such as adoptive cell therapy and tumor vaccines. In the current work, we investigated RNAseq data from nearly 1,000 pediatric brain tumor patients and performed the first large-scale comprehensive examination of the immunogenomic and TCR landscape of TILs across the spectrum of pediatric brain tumors. We show that the relative ratio between T cell diversity (clonality) and T cell abundance within each sample, represented by the clonal expansion index (CEI), is a strong predictor of prognosis across tumor types and molecular subtypes. Interestingly, we found that CEI was strongly associated with molecular subgroups of medulloblastoma but not with known genomic features of these subgroups. Analysis of single-cell (sc)RNAseq and scTCRseq data of TILs across several tumor types, validated clonal expansion of TILs which were frequently associated with effector-cytotoxic T cell phenotypes, whereas non-expanded TILs were associated mostly with CD4 T cell phenotypes. Investigation of T cell clones recognizing a common recurrent tumor-antigen across patients based on CDR3 homology and characteristics, reveals several T cell specificity groups which are tumor type restricted with defined HLA dominance. Using machine learning investigations of genomic profiles of the patients which contain these T cell specificity groups identified novel putative HLA-restricted tumor antigens shared across patients. Importantly, our framework grounded the foundations for developing a precision medicine approach of T cell centered immunotherapies. In all, our findings have major implications for understanding the interplay between T cell and tumor genomic, and for improving immunotherapy strategies for childhood brain tumors.