H3.3K27M Peptide Vaccine With Nivolumab for Children With Newly Diagnosed DIPG and Other Gliomas

Inclusion Criteria: * Stratum A: • Newly diagnosed children (3-21 years old) with DIPG who are positive for the H3.3K27M mutation (positive testing in Clinical Laboratory Improvement Amendments (CLIA) laboratory) that underwent standard radiation therapy. * Stratum B: • Newly diagnosed children (3-21 years old) with diagnosis of glioma other than DIPG who are positive for the H3.3K27M mutation (positive testing in CLIA laboratory) including spinal cord gliomas that underwent standard radiation therapy. * Stratum C: * Newly diagnosed children 3-21 years of age with diagnosis of DIPG or midline glioma other than DIPG (excluding primary spinal cord gliomas) who are positive for the H3.3K27M mutation (positive testing from a CLIA or equivalent laboratory required), that underwent standard radiation therapy. The following eligibility criteria apply to strata A, B and C: * The patient must test positive for HLA-A\*02:01 (positive testing from a CLIA or equivalent laboratory required; only the HLA A\*02:01 subtype is eligible; other subtypes are excluded) * The patient must be either off systemic steroids or be on stable dose of dexamethasone or equivalent (max 0.1 mg/kg/day; maximum 4mg/day) at time of enrollment. * Patients must not have received any prior chemotherapy, immunotherapy or bone marrow transplant for the treatment of their tumor. Prior use of temozolomide during radiation at maximum of the standard pediatric dosing (defined as 90 mg/m\^2/dose continuously …