Role of endothelial Notch signaling on the recruitment and education of monocyte-derived macrophages in cancer

Malignant tumor diseases are still one of the leading causes of death worldwide and epithelial ovarian cancer (EOC) is one of the deadliest gynecologic cancers for women. Nowadays, in cancer research, stromal cells within tumor microenvironment (TME) are considered to play an important role in tumor growth. Tumor-associated macrophages (TAM) are the most prominent immune cell population within the TME of EOC and play a crucial role in tumor progression and metastasis. Our group has previously described that there is frequent hyperactivation of the Notch1 signaling in endothelial cells (EC) within cancer patient samples, promoting immune cell infiltration, tumor cell (TC) transmigration, and metastasis. Ovarian cancer tissue samples showed increased infiltration of myeloid cells in the primary tumor associated with increased endothelial Notch1 activation. Therefore, the aim of this thesis was to better understand the mechanism behind the recruitment and activation of myeloid cells mediated by the Notch1 signaling in EC. To evaluate the role of the endothelial Notch signaling cascade in a mouse model, an EC-specific inducible gain-of-function model (ecNICD mice) overexpressing the intracellular Notch1 domain (N1ICD) was used. In the corresponding loss-of-function model, an EC-specific inducible deletion of the transcription factor RBPJ (RbpjiΔEC mice) was performed. In addition, two different tumor models were used. The experiments showed that loss of Notch signaling in EC leads to a significant reduction of monocyte-derived macrophages in a subcutaneous tumor model with increased vessel density. To validate the reduced myeloid cell infiltration in an angiogenesis-independent cancer model, tumor progression and myeloid cell infiltration were examined in a metastatic EOC model. Here, increased infiltration of monocyte-derived macrophages into the peritoneum of ecNICD mice was observed, whereas significantly decreased recruitment of these cells occurred in the RbpjiΔEC model. Mechanistically, secretion of angiocrine factors regulated by endothelial Notch signaling were examined. This revealed CXCL2 as a novel canonical Notch target gene. Interestingly, CXCL2 levels in ovarian cancer patients correlate with poor prognosis as well as infiltration of myeloid cells into the TME. Moreover, TCs are able to activate infiltrating macrophages in their favor to support tumor growth and a gene expression profile of these TAMs during metastatic EOC has already been described. TAMs from RbpjiΔEC mice and littermate controls were isolated and gene expression analysis was performed using the profile of TAMs during EOC tumor growth. Consistently, macrophages isolated from RbpjiΔEC mice indicated weaker expression of the gene expression profile of TAMs compared to controls. This suggests that the TAM phenotype induced by TCs requires the presence of the transcription factor RBPJ in ECs. In addition, the number of cytotoxic T cells is increased in the peritoneal cavity of RbpjiΔEC mice compared with control mice, and moreover the T cell population was more cytotoxic. Expression of CD74 on TAMs is important for the immunosuppressive phenotype and correlates with poor prognosis in patients with ovarian cancer. In recruited TAMs from RbpjiΔEC mice, CD74 was lower expressed than in TAMs from control animals. As a consequence, reduced tumor burden was observed in RbpjiΔEC mice compared to littermate controls. In conclusion, endothelial Notch signaling plays an important role in the recruitment and activation of TAMs within the TME