Endothelial Rbpj is essential for the education of tumor-associated macrophages

Epithelial ovarian cancer (EOC) is one of the most lethal gynecological cancers worldwide. EOC cells educate tumor-associated macrophages (TAMs) through CD44-mediated cholesterol depletion to generate an immunosuppressive tumor microenvironment (TME). In addition, tumor cells frequently activate Notch1 receptors on endothelial cells (ECs) to facilitate metastasis. However, little is known whether the endothelium would also influence the education of recruited monocytes. Here, we report that canonical Notch signaling through RBPJ in ECs is an important player in the education of TAMs and EOC progression. Deletion of Rbpj in the endothelium of adult mice reduced infiltration of monocyte-derived macrophages into the TME of EOC and prevented the acquisition of a typical TAM gene signature. This was associated with stronger cytotoxic activity of T cells and decreased tumor burden. Mechanistically, we identified CXCL2 as a novel Notch/RBPJ target gene. This angiocrine factor regulates the expression of CD44 on monocytes and subsequent cholesterol depletion of TAMs. Bioinformatic analysis of ovarian cancer patient data showed that increased CXCL2 expression is accompanied by higher expression of CD44 and TAM education. As such, EOC cells employ the tumor endothelium to secrete CXCL2 in order to facilitate an immunosuppressive microenvironment. Samples were obteined after cell sorting of tumor-associated macrophages (TAMs) CCR2+ (CD45+, CD11b+, F4/80+, CCR2+) and TAMs CCR2- (CD45+, CD11b+, F4/80+, CCR2-) in RbpjiΔEC and control mice.