P02.03.B GLIOBLASTOMA CELL-DERIVED APLN SHOWS DIFFERENTIAL EFFECTS ON MYELOID CELL MIGRATION

Abstract BACKGROUND The rapid progression of glioblastoma (GBM) is fostered by GBM-induced immunosuppression and by tumor-associated myeloid cells (TAMs), which massively accumulate in GBM and support glioma expansion. Hence, signaling-pathways controlling the tumor tropism of myeloid cells or immune-regulation of TAMs constitute a potential therapeutic target for GBM. We previously found that the angiogenic factor apelin (APLN) signals through its G-protein coupled receptor APLNR to regulate neo-vascularization and tumor cell invasion. When screening for APLNR co-regulated genes in the TCGA database they correlated best with lymphocyte-mediated immune responses. MATERIAL AND METHODS To study the role of APLN-APLNR-signaling in GBM-immune cell interaction we used different mouse GBM models and analyzed them for TAM by immunohistochemistry. Primary GBM and murine myeloid cells (BV2 and J774) were investigated in spheroid invasion or transwell migration assays. APLIN/APLNR expression was assessed by qPCR, immunohistochemistry and FACS and functional studies were performed using APLN knock-down (AKD) cells and APLN knock out (aplnKO) mice. RESULTS Analyzing xenografts of different human GBM cells we found a direct correlation of APLN with TAM cell density in the tumor area and the invasive zone. We detected APLN expression in primary GBM cells and APLNR upregulation in Iba1-postitive TAMs and murine myeloid cell cultures allowing for direct interactions. Indeed, BV2 microglia reacted chemotactic to the smallest functional APLNR-agonistic peptide apelin-13 while J774 macrophages showed no response. Interestingly, using conditioned medium of AKD GBM cells variable response to endogenous APLN were obtained with microglia showing reduced and macrophages increased mobility. In vivo, complete depletion of APLN in aplnKO mice further reduced TAM-density, while intracerebral apelin-13 infusion restored their accumulation. CONCLUSION Altogether, we propose that APLN-APLNR signaling is involved in the inflammatory response to tumor cells mediating the specific myeloid cell composition in GBM. We suggest that blockade of APLN-APLNR signaling mediates broad anti-tumor effects by inhibiting angiogenesis, GBM cell invasion and immune suppression.