P12.09.b smac mimetics reprograms glioblastoma immune landscape and polarizes microglia toward a pro-inflammatory anti-tumoral phenotype

Abstract BACKGROUND Glioblastomas are the most aggressive brain tumors in adult characterized by an immunosuppressive microenvironment. They are invaded by tumor-associated macrophages (TAMs) supporting tumor growth and treatment escape. One current therapeutic challenge is to restore an immunocompetent environment to improve treatment efficiency. A better knowledge of spatial distribution and function of TAMs is needed to design new promising therapeutic strategies. MATERIAL AND METHODS We used small molecules SMAC mimetics (SM) which are inhibitor of apoptosis (IAP) antagonists to decipher spatial distribution and function of TAMs. By using a syngeneic glioblastoma mouse model, we performed whole mount stainings and brain clearing to investigate immune cells quantity, and spatial organization within tumors and related to tumor vessels. We then performed molecular mass cytometry (CYTOF) to accurately phenotype the immune cells involved in SM response and gain functional data. We finally used human derived glioblastoma models (explants, immune cells sorting and tumoroids) to investigate the clinical relevance of our results. RESULTS Results showed that SM promoted immune cells CD45+ quantity and penetration within tumors including microglia. The treatment increased monocyte-derived dendritic cells, CD8EM T cells, reactive microglia while decreasing anti-inflammatory macrophages and basal microglia. Furthermore, SM treatment decreased contacts between microglia and CD8 T cells, and microglia PDL1+. Thanks to the use of human derived model, we found that CD14 is decreased upon SM treatment. By using TCGA databases, we found that low level of CD14 expression was correlated with a better prognosis. CONCLUSION SM reprogram glioblastoma immune landscape and modulate immune cell infiltration and organization within tumors. Furthermore, microglia is a key mediator in SM response. The targeting of CD14, IAP and microglia would be a promising therapeutic strategy.