Abstract 1706: Inhibition of fibulin-3 reverses macrophage polarization in glioblastoma and increases anti-tumor inflammatory responses

Abstract Glioblastomas (GBMs) are the most common primary tumors of the Central Nervous System and one of the most aggressive and difficult to treat forms of cancer. Our laboratory was the first to identify the extracellular protein fibulin-3 (EFEMP1) in GBMs and demonstrate the pro-tumoral functions of this protein in solid tumors. We have recently developed a function-blocking antibody ("mAb428.2") against fibulin-3, which inhibits the enhancing effect of this protein on ADAM17 activity and NFkB signaling (Nandhu et al., Clin Cancer Res, 2017). Using mAb428.2 we observed a significant anti-tumor effect and prolongation of survival in nude mice carrying patient-derived GBM xenografts implanted subQ or intracranially (antibody treatment: 8x 30 mg/kg, q24h, IV). Interestingly, reduction of tumor growth during mAb428.2 treatment was accompanied with significant tumor necrosis and increased infiltration of Iba1-positive tumor-associated macrophages (TAMs) both in subQ and orthotopic tumors. Remarkably, TAMs in mAb428.2-treated GBMs showed very low co-expression of Arginase-I, a bona-fide marker of M2 ("tumor promoting") TAM polarization. Further analysis of mouse-specific gene expression in the tumor mass and in recovered macrophages (CD11b-positive cells) showed that TAMs in mAb428.2-treated GBMs had significantly increased expression of inflammatory cytokines (IFN-gamma, IL-1b, IL-10) and downregulation of M2 markers (CD163, CD206, ARG1), suggesting that our anti-fibulin-3 treatment prevented the expected M2 polarization of TAMs. Accordingly, mAb428.2-treated tumors showed drastically decreased expression of CSF-1a, an M2-inducing cytokine that may be regulated by fibulin-3. Co-culture experiments of PMA-activated U937 macrophages with GBM stem-like cells did not affect tumor cell viability when a control IgG1 was added to the cultures. However, addition of mAb428.2 caused significant decrease of GBM cell viability, suggesting that anti-fibulin-3 treatment enables TAMs to attack tumor cells. Taken together, our results strongly suggest that anti-fibulin-3 and possibly other anti-ECM approaches may induce a marked inflammatory reaction driven by innate immune cells, potentiating anti-tumor therapy. Citation Format: Sharon L. Longo, Prajna Behera, Mariano S. Viapiano, Mohan Sobhana Nandhu. Inhibition of fibulin-3 reverses macrophage polarization in glioblastoma and increases anti-tumor inflammatory responses [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1706.