IMMU-33. NEOANTIGEN-SPECIFIC T CELLS CAN INFILTRATE IDH-MUTANT LOWER GRADE GLIOMAS AND PERSIST IN THE PERIPHERAL BLOOD

The prospect of using immunotherapy for IDH-mutant LGGs has been daunting given the immune-poor microenvironment and low mutational burden. We hypothesized that LGG-targeting T cells might still be present at low frequency and with limited regional infiltration into the tumor. To improve sensitivity, we combined high-density multi-region tumor sampling with high-throughput neoantigen-T cell screening for a patient with WHO Grade II diffuse astrocytoma who eventually progressed, at second recurrence, to anaplastic astrocytoma. We performed maximal-anatomic sampling from 10 distinct regions of the tumor at the initial resection, as well as single sampling at first recurrence, for exome-based prediction of clonal and subclonal expressed neoantigens, RNAseq-based estimation of regional immune cell composition, and T cell receptor (TCR) beta deep sequencing. Based on our predictions, we then generated a barcoded library of patient-specific peptide-HLA multimers loaded with predicted neopeptides. Using this library, neoantigen-specific CD8 T cells were captured and isolated from patient peripheral blood and subjected to single cell TCR sequencing. We screened patient-derived peripheral blood drawn two years after initial resection and identified five T cell clones recognizing three LGG neoepitopes. Two neoepitopes were derived from truncal, tumor-wide mutations, including a truncating splice-site mutation in TP53 and a missense mutation in MRPL46. Each of these neoepitopes were recognized by two distinct TCRs, consistent with TCR convergence. A third neoepitope was derived from a subclonal MRPL46 mutation. Using the TCR beta sequence as a molecular barcode, TCRs specific to the 2 clonal neoepitopes, but not the subclonal neoepitope, were detectable in the glioma and blood at initial resection, as well as in the recurrent glioma. In summary, we demonstrate the existence and persistence of neoantigen-targeting T cells within the blood and tumor of an IDH-mutant LGG patient. These findings suggest a feasible methodology to develop personalized T cell-based immunotherapies for IDH-mutant LGGs.