IMMU-27. INHIBITION OF THE EXTRACELLULAR MATRIX PROTEIN EFEMP1/FIBULIN-3 REDUCES IMMUNOSUPPRESIVE SIGNALING IN TUMOR STEM CELLS AND INCREASES MACROPHAGE ACTIVATION AGAINST GLIOBLASTOMA

Abstract Glioblastoma tumors remain a formidable challenge for immune-based treatments because of their molecular heterogeneity, poor immunogenicity, and growth in the largely isolated and immunosuppresive neural environment. As the tumor grows, GBM cells change the composition and architecture of the neural extracellular matrix (ECM), affecting the mobility, survival, and function of immune cells such as tumor-associated microglia and infiltrated macrophages (TAMs). We have previously described the unique expression of the ECM protein EFEMP1/fibulin-3 in GBM compared to normal brain and demonstrated that this secreted protein promotes growth and resistance of the GBM stem cell (GSC) population. Here, we describe a novel immunomodulatory role of fibulin-3 and the immuno-boosting effects of targeting this ECM protein. Mice carrying fibulin-3-deficient intracranial tumors showed increased myeloid infiltration and reduced expression of TAM pro-tumoral markers (Arginase, CD206) compared to controls. The opposite was observed in orthotopic tumors overexpressing fibulin-3. In silico dataset analysis revealed positive correlation of fibulin-3 with an immunosuppresive signature in GBM, which was validated in GBM stem cells (GSCs). Accordingly, expression of fibulin-3 by GSCs promoted anti-inflammatory polarization of co-cultured macrophages. We further demonstrated that fibulin-3 regulated the expression of immunosuppresive signals (CSF-1, TGFbeta) and the innate immune checkpoint CD47 in GSCs via autocrine activation of NF-kB signaling. Immunosuppresive signals in GSCs were downregulated by knockdown of fibulin-3 or inhibition of this protein with an anti-fibulin-3 antibody (mAb428.2). Genetic or antibody-mediated targeting of fibulin-3 in GSCs co-cultured with macrophages (U937 and THP1 lines) increased macrophage phagocytosis and reduced the viability of the tumor cells. Furthermore, local delivery of mAb428.2 in mice carrying intracranial GBM resulted in increased infiltration of TAMs expressing pro-inflammatory markers and reducing tumor viability. Our findings show that anti-fibulin-3 approaches, which impact the tumor ECM, can diminish immunosuppression in GBM and boost innate immune responses against the tumor.