Abstract
VEGF is an important factor in tumor vascularization and used as target for anti-angiogenic treatment strategies in glioma. In this study, we demonstrate for the first time that VEGF is a modulator of the innate immune response with suppressive effects on the immunologic and pro-angiogenic function of microglia/macrophages in a glioblastoma rodent model. High level of VEGF led to threefold enlarged tumor volumes and a pronounced remodeling of the vascular structure along with a reduced infiltration of microglia/macrophages by approximately 50%. Remaining microglia/macrophages showed an enhanced rate of apoptosis as well as significant downregulation of the VEGF-receptor, VEGFR2, and others such as CXCR4. Consequently, we determined a substantially impaired migration of these microglia/macrophages to VEGF and SDF1α in vitro. Furthermore, we observed an increased presentation of the surface molecules MHCI and MHCII on microglia/macrophages from VEGF-overexpressing gliomas that are essential for activation of the adaptive immune system. In contrast, the expression of pro-inflammatory and suppressive cytokines, associated with the innate immune response, were mainly downregulated. Remarkably, the abundance of VEGF provoked less accumulation of microglia/macrophages within the perivascular niche and concomitantly reduced the release of pro-angiogenic factors, like VEGF, suggesting a possible regulatory feedback mechanism. Thus, the quantity of VEGF in the glioma microenvironment seems to be crucial for the participation of microglia/macrophages on tumor progression and should be considered for developing novel therapeutic approaches.