Inhibition of the extracellular matrix protein fibulin-3 reduces immunosuppressive signaling in tumor stem cells and increases macrophage activation against glioblastoma

Glioblastoma tumors remain a formidable challenge for immune-based treatments because of their molecular heterogeneity, poor immunogenicity, and growth in the largely isolated and immunosuppressive 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 the growth 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 of clinical datasets revealed positive correlation of fibulin-3 with an immunosuppressive signature, which was validated in GSCs and in vivo. We further demonstrated that fibulin-3 regulates the expression of immunosuppressive signals (CSF-1, TGF{beta}) and the innate immune checkpoint CD47 in GSCs via autocrine activation of NF-{kappa}B signaling. Accordingly, immunosuppressive signals were downregulated in GSCs by knockdown of fibulin-3 or inhibition of this protein with an anti-fibulin-3 antibody. Co-culture of GBM cells with syngeneic macrophage lines or primary macrophages in presence of anti-fibulin-3 antibody increased macrophage phagocytosis and antibody-dependent killing of the tumor cells, Furthermore, locoregional delivery of anti-fibulin-3 antibody in mice carrying intracranial GBM increased the infiltration of TAMs expressing pro-inflammatory markers, reducing tumor viability. Our findings show that anti-fibulin-3 approaches, which impact the pericellular ECM surrounding tumor cells and TAMs, can diminish immunosuppression in GBM and boost innate immune responses against the tumor.