Neoepitope-Specific Vaccination Of Patients With Diffuse Midline Glioma Targeting H3K27M Induces Polyclonal B And T Cell Responses Across Diverse Hla Alleles

H3K27M, a driver mutation with T- and B-cell neoepitope characteristics, defines anaggressive subtype of diffuse glioma with poor survival. We functionally dissect the immuneresponse of two patients treated with an H3K27M peptide vaccine, one of which enteredcomplete remission. The vaccine robustly expanded class II HLA-restricted peripheralH3K27M-specific T cells. Using functional assays, we characterized 41 clonally uniqueH3K27M-reactive T cell receptors across both patients and identified critical, conserved motifsin their CDR3 regions. Using detailed HLA mapping, we further demonstrate that diverse HLA-DP, -DQ, and -DR alleles present immunogenic H3K27M epitopes. Furthermore, we identifiedand profiled H3K27M-reactive B cell receptors from activated B cells in the cerebrospinal fluid.Our results uncover the breadth of adaptive immune responses against a shared clonalneoantigen across multiple HLA allelotypes and support the use of class II-restricted peptidevaccines to stimulate tumor-specific T and B cells harboring receptors with therapeuticpotential.