IMMU-28. LOW-INTENSITY PULSED ULTRASOUND COMBINED WITH POLY-ICLC AND INTERLEUKIN-2 UNLOCKS THE IMMUNE PRIVILEGE OF THE BRAIN BY PROMOTING EPITOPE SPREADING AND ACTIVATION OF CNS-SPECIFIC NAÏVE T CELLS

Abstract Immunotherapies targeting glioma, such as chimeric antigen receptor (CAR) T cell therapies, offer new therapeutic avenues; however, their application has yielded limited success in clinical trials so far. Due to the antigenic heterogeneity of gliomas, targeting a few or several antigens still results in recurrence with “antigen-loss” tumors, indicating the need to engage the endogenous immune system to eliminate these therapy-resistant tumor cells. However, natural anatomical barriers and an immunosuppressive environment allow brain tumors to evade detection by the endogenous immune system. Low-intensity pulsed ultrasound combined with microbubbles (LIPU/MB) has recently been shown to transiently open the blood-brain barrier (BBB), enabling penetration of cells and drugs into the CNS. Using intravital two-photon imaging, two-photon microscopy, multiparametric spectral flow cytometry, and bone marrow chimera mice, alongside blood samples from glioblastoma patients treated with LIPU/MB, we observed that LIPU/MB promotes migration of CNS antigen-presenting cells from the perivascular space into the periphery. To assess the immunological relevance of LIPU/MB-mediated CNS antigen presentation to the peripheral immune system, we have developed new transgenic mice (GFAP-minigene mice) with an intact BBB expressing four immunogenic peptides under the CNS-specific GFAP promoter. In GFAP-minigene mice, LIPU/MB combined with toll-like receptor 3 agonist poly-ICLC and Interleukin-2 promoted priming, activation, and homing of naïve CNS-specific T cells into the brain parenchyma. Cytotoxic T cell activity in the brain was accompanied by MHC-I and MHC-II epitope spreading, mirrored by the occurrence of endogenous T cells in the brain, deep cervical lymph nodes, and spleen harboring other CNS-(minigene) antigen-specific T cell receptors. Collectively, our findings suggest that LIPU/MB combined with poly-ICLC can overcome inherent limitations of immunotherapy by enhancing CNS epitope spreading and activation of naïve CNS-antigen-specific T cells within the context of an intact blood-brain barrier.