IB-05 * TUMOR-PROCESSED OSTEOPONTIN/SPP1 SHAPES MICROGLIA POLARIZATION AND IMMUNE MICROENVIRONMENT OF GLIOMA

Malignant gliomas attract resident microglia and macrophages and re-program these cells into pro-invasive, immunosuppressive cells. It results in formation of tumor supportive microenvironment and evasion of antitumor responses. The analysis of marker genes in microglia and macrophages infiltrating experimental gliomas indicates different kinetics and functions of various myeloid subpopulations during glioma progression. Signals responsible for recruitment and polarization of tumor infiltrating myeloid cells are poorly known. Proteomic analysis of glioma secretome revealed a presence of osteopontin (SPP1) and lactadherin (MGF-E8) in microglia-activating fractions. Both proteins stimulated microglia via αvβ3 integrin signaling that results in activation of PI-3K/Akt and FAK, enhancement of microglial migration, phagocytosis and transcriptional responses. Inhibition of ligand-integrin interactions with short interfering peptides abolished pro-invasive polarization in glioma-microglia co-cultures. We found that SPP1, highly overexpressed in glioma cells, was specifically processed by sequential thrombin and metalloproteinase-dependent cleavage. This type of processing results in losing SPP1 pro-inflammatory activity leaving intact its pro-tumorigenic activity and does not occur in non-transformed cells. Knockdown of SPP1 in glioma cells strongly reduced growth of intracranial gliomas. The number of infiltrating microglia/macrophages (Iba1+) was not affected, but amoeboid transformation, expression of the alternative phenotype markers and angiogenesis were reduced in SPP1-depleted gliomas. Interestingly, infiltrating Iba1+ cells did not undergo pro-invasive polarization. Moreover, SPP1-depleted tumors were infiltrated with T cytotoxic lymphocytes, while accumulation of T regulatory cells was significantly reduced. The expression of SPP1 is up-regulated in human glioblastoma and inversely correlated with patient's survival. Our findings demonstrate that glioma-derived and specifically processed osteopontin/SPP1 polarizes glioma-infiltrating microglia and contributes to transformation of tumor microenvironment. This defines osteopontin/SPP1 a new biomarker and target for glioma therapy, and shows that targeting glioma-microglia interactions within tumor microenvironment is a promising strategy to fight these tumors. Studies were supported by a grant 2012/04/A/NZ3/00630 from the National Science Center.