P142

Malignant gliomas are fast-growing, heterogeneous and invasive brain tumors strongly infiltrated by non-tumor cells. Glioma attracts variety of immune cells, in particular microglia/macrophages and re-program these cells into immunosuppressive, tumor-supporting cells. Factors responsible for pro-invasive macrophage polarization and shaping tumor microenvironment in tumor-supporting manner are poorly known. We analyzed glioma secretome using proteomical approach and identified lactadherin (Mfge8) and osteopontin (Spp1) in microglia-activating fractions. Both osteopontin and lactadherin are αvβ3/αvβ5 integrin ligands able to interact with receptors present on microglia and macrophages and thus could be involved in pro-invasive polarization of microglia/macrophages. Moreover, both Spp1 and Mfge8 are overexpressed in glioma cells, but not in non-transformed astrocytes. C6 glioma cells stably expressing shRNA specific to lactadherin (shMfge8), osteopontin (shSpp1) and negative shRNA (shNeg) were implanted into striatum of Wistar rats. There was no difference in proliferation and viability of C6 glioma cells, cells stably expressing shRNA specific to lactadherin, ostopontin and negative shRNA in vitro, that demonstrates the negligible effect of autocrine production of both protein on tumor cell growth. Knockdown of Spp1 and Mfge8 resulted in significant reduction of tumor volume in rat model of glioma. Immunochemical analysis of brain sections revealed similar numbers of infiltrating microglia/macrophages (Iba1 staining), but the reduced number of ameboid, arginase 1 expressing cells in Mfge8 – depleted tumor. Treatment of endothelial cells with rhMFGE8 revealed significant effect of that protein on angiogenesis in vitro, however lactadherin-depleted tumors do not exhibit reduced blood vessel density in rat glioma model. FACS analysis showed that silencing of Spp1 does not affect total number of CD11b-positive cells, but strongly modulates microenvironment by leading to significant changes in percentage of Tc and Treg cells infiltrating tumor-bearing hemisphere. Our results suggest that glioma-derived integrin ligands are important factor in polarization of glioma infiltrating microglia/macrophages into the pro-invasive phenotype and its targeting could be a new therapeutic strategy.