Oishi, Masahiro

Abstract Glioblastoma is the most common malignant primary brain tumor and is universally fatal. Treatment involves maximal safe resection with adjuvant chemoradiation. Poor prognosis in glioblastoma is multifactorial, and local and systemic immunosuppression is an important factor. Extracellular vesicles (EVs) are small, membrane-bound particles released by cells into the external environment. They play a key role in intercellular communication. In glioblastomas, EVs regulate immunosuppression through induction of myeloid-derived suppressor cells (MDSCs) in a process dependent on the PD-1/PD-L1 and IL-6 axes. Primary cilia are microtubule-based organelles that play crucial roles in various cellular processes and signal transduction cascades. We recently showed that primary cilia modulate IL-6 expression and consequently, MDSC induction. Comparative proteomic analysis of EVs derived from human glioblastoma cells devoid of cilia showed loss of ICAM-1 expression compared to control glioblastoma cells. ICAM-1 (Intercellular Adhesion Molecule-1) is a cell surface glycoprotein that mediates cell-cell adhesion and immune responses. In this study, we showed that depletion of primary cilia in human glioblastoma cells result in increased PD-L1 but loss of IL-6 and ICAM-1. RNAi-meidated loss of ICAM-1 was synergistic with primary cilia depletion in preventing MDSC inductionn normal human monocytes exposed to glioblastoma-derived EVs. Exogenous IFN-g was sufficient to restore IL-6 levels and MDSC induction despite loss of primary cilia and ICAM-1 suggesting that primary cilia act upstream of IFN-g, IL-6 and ICAM-1 in glioblastoma-mediated MDSC induction. This study is the first to connect primary cilia signal transduction to IL-6 and ICAM-1 expression and to MDSC induction. The evidence suggests a potential parallel primary cilia regulatory cascade independent of the PD-L1 axis that controls tumor-mediated immunosuppression. These results may inform novel therapeutic strategies aimed at increasing efficacy of immunotherapy in the glioblastoma and other neoplastic processes.

Abstract Glioblastoma is the most common primary malignant brain tumor. Despite maximal safe resection, followed by chemoradiation, median survival is 15 months, and 5-year survival is 6.8%. Efficacy of immunotherapy is limited by glioblastoma-mediated immunosuppression. Extracellular vesicles (EVs) are a heterogeneous group of small membrane-bound particles that are released from all cell types. Glioblastoma-derived EVs regulate the tumor microenvironment and tumor progression in a paracrine manner. They also regulate glioblastoma-mediated systemic immunosuppression in an IL6- and PDL1-dependent manner by inducing myeloid-derived suppressor cells (MDSCs) ultimately resulting in T cell depletion. Primary cilia are ubiquitous microtubule-based organelles that project from the mother centriole and are present on glioblastoma cells. Inhibition of ciliogenesis–through knockdown of KIF3A, IFT88, or ARL13B (structural ciliary proteins required for ciliogenesis)–abolishes IL6 induction in response to chemoradiation and significantly reduces the induction of MDSCs in normal human donor monocytes exposed to glioblastoma-derived EVs. Inhibition of ciliogenesis does not affect the biophysical properties of EVs but our data instead suggest significant alterations in EV packaging and content in response to ciliary loss. These data suggest a novel role for primary cilia in packaging of EV contents and connects these organelles with tumor EV-mediated immunosuppression in glioblastomas. These findings have potential implications for improving the efficacy of immunotherapy in glioblastoma–a universally fatal disease.