IMMU-41. H3.3K27M MUTATION-DERIVED NOVEL NEOANTIGEN – CHARACTERIZATION OF THE HLA-A2-BINDING EPITOPE AND A SPECIFIC T CELL RECEPTOR FOR DEVELOPMENT OF T CELL-BASED IMMUNOTHERAPY

The median overall survival for children with diffuse intrinsic pontine glioma (DIPG) is less than a year. The majority of diffuse midline gliomas, including over 70% of DIPG, harbor an amino-acid substitution from lysine (K) to methionine (M) at position 27 of histone 3 variant 3.3. As predicted by the NetMHC3.4 peptide binding algorithm, a competitive binding inhibition assay demonstrated that 10-mer peptide encompassing the K27M mutation (H3.3K27M peptide hereafter), but not the corresponding non-mutant peptide, has an excellent binding affinity to human leukocyte antigen (HLA)-A2. Importantly, mass spectrometry analysis of H3.3K27M+ glioma cells demonstrated that the H3.3K27M peptide is naturally processed and presented by HLA class I. H3.3K27M-specific CD8+ T cell responses were detected in patients´ peripheral blood mononuclear cells by interferon-γ ELISPOT assay. Repeated stimulation of HLAA2+ CD8+ T cells with the H3.3K27M peptide led to establishment of H3.3K27M-reactive CD8+ T-cell clones. From one clone with excellent binding to the HLA-A2-H3.3K27M tetramer, cDNA for T cell receptor (TCR) α and β chains were cloned into a retroviral vector. Human HLA-A2+ T-cells transduced with the TCR efficiently killed HLA-A2+ H3.3K27M+ glioma cells in an antigen- and HLA-specific manner. Adoptive transfer of TCR-transduced T-cells significantly suppressed the progression of orthotopic xenografts in mice. Furthermore, alanine-scanning assays demonstrated that there are no known human proteins that share the set of key amino acid residues required for recognition by the TCR, strongly suggesting that the TCR could be safely used in patients without causing off-target toxicity. These data provide us with a strong basis for developing novel immunotherapy strategies targeting this epitope. We have initiated a pilot study evaluating the safety and preliminary clinical activities of the H3.3K27M peptide vaccine in HLAA2+ pediatric patients with H3.3K27M+ glioma though Pacific Pediatric Neuro-Oncology Consortium (PNOC-007).