DIPG-32. NEOANTIGEN HEAT SHOCK PROTEIN VACCINE, RHSC-DIPGVAX, IS ASSOCIATED WITH INCREASED VACCINE-INDUCED B CELLS AND BCR/TCR REPERTOIRE DIVERSITY IN PEDIATRIC PATIENTS WITH DIFFUSE MIDLINE GLIOMAS

Abstract

Abstract BACKGROUND Diffuse midline gliomas (DMG/DIPG) remain a devastating class of tumors with limited treatment options. Cancer vaccines educate the immune system to tumor neoantigens to facilitate immune-mediated tumor destruction. DMGs have high-frequency, co-occurring neoantigens and are therefore a relevant target for activation of anti-tumor immunity via vaccine. To this end, we have initiated a clinical trial (NCT04943848) to determine if a neoantigen peptide-based vaccine with recombinant human heat-shock constitutive protein 70 and QS-21 adjuvant can promote anti-tumor immunity against DMG/DIPG. NCT04943848 is ongoing and now enrolling subsequent arms with rHSC-DIPGVax plus checkpoint blockade. METHODS Whole blood was collected before the first vaccine cycle (C1D1). Vaccine doses were administered every 2 weeks and blood was collected prior to the third or fifth vaccine cycle (C3D1 & C5D1). Peripheral blood mononuclear cells (PBMCs) were isolated and stored at -160°C. PBMCs from each patient, pre- and post-vaccine, were stained with human T-cell, myeloid, and B-cell antibody panels before spectral flow cytometry, single-cell RNA sequencing, and genomic DNA isolation for T- and B-cell receptor (TCR/BCR) sequencing. Spectral flow cytometry data was analyzed via FlowJo v10. TCR/BCR sequencing was processed via MiXCR and Immunarch. RESULTS Following vaccination, there were higher percentages of two mature CD20+CD19+IgM+ B cell populations, including a 4-1BBL+CD86+ population previously shown to induce robust CD8+ T-cell activation in adults with glioblastoma. Interestingly, given the recent discovery of TIM3’s importance in DIPG, we also identified TIM3-expressing CD4+ and CD8+ T cell populations. Finally, TCR/BCR sequencing demonstrated increased receptor repertoire diversity after vaccination, which is indicative of the vaccine shaping adaptive immunity. CONCLUSIONS Peripheral immune populations and TCR/BCR sequences change during vaccination with rHSC-DIPGVax. Ongoing analyses will determine the specificity of the patient immune response to vaccine antigens and correlate changes in immune populations to clinical course.