gms | German Medical Science

Objective: Glioblastoma multiforme is one of the most malignant brain tumors. This cancer is characterized by high vascularization, strong invasiveness and a large amount of infiltrating microglia/macrophages. However, the function of the microglia/macrophage cell population in the tumor context is not clarified. The aim of the present study was to determine the microglia-tumor-interaction, focusing on the pro-angiogenic activity of microglia and their influence on tumor vascularization and progression.

Method: GL261 tumor cells were implanted intracranially into syngeneic BL6/J mice. Brain sections were analyzed for different parameters by immunofluorescence stainings. In addition, microglia/macrophages of native and tumor-bearing mice were isolated by MACS technology. The purified CD11b+ cells were used for RNA extraction and Realtime-PCR of pro-angiogenic factors subsequently, as well as for tube formation assays in co-culture with brain endothelial cells in vitro. Furthermore, microglia depletion was realized by the local application of ganciclovir to brains of transgenic CD11b-HSVTK mice following inoculation of tumor cells. The growth of tumors was investigated with MRI.

Results: Microglia/macrophages accumulate in the tumor area and their absolute number was duplicated on day 21 after glioma cell implantation. Moreover, we found a defined interaction of microglia/macrophages with tumor blood vessels and the percentage of association correlates with the angiogenic activity in glioma. The treatment of CD11b-HSVTK mice with ganciclovir led to the depletion of the myeloid cell fraction. Following the reduction of microglia/macrophages in transgenic mice, the vessel density was halved as well as tumor volumes were decreased by approximately 30%. In addition, the microglia/macrophages isolated from tumor-bearing mice were able to stabilize tube formation of brain endothelial cells in vitro. Further characterization of the microglia/macrophage cell population revealed their pro-angiogenic potential by expressing a variety of factors involved in angiogenesis (e.g. VEGF, CXCL2, Hif1alpha).

Conclusions: Our data clearly demonstrate that microglia/macrophages affect the induction and/or stabilization of tumor vascularization based on their distribution in the tumor tissue as well as the release of angiogenic molecules for endothelial cell mobilization. All in all, we suggest microglia/macrophages as an additional cell population in gliomas with modulatory capability for tumor angiogenesis.