Microglial activation state exerts a biphasic influence on brain endothelial cell proliferation by regulating the balance of TNF and TGF-β1

Background: Studies of cerebral ischemia and other neuroinflammatory states have demonstrated a strong association between new vessel formation and microglial recruitment and activation, raising the possibility that microglia may be involved in promoting angiogenesis. As endothelial cell proliferation is a fundamental early step in angiogenesis, the aim of this study was to test this hypothesis by examining the influence of microglial secreted factors on brain endothelial cell (BEC) proliferation using BrdU incorporation. Methods: Primary cultures of mouse BEC, microglia and astrocytes were used in this study. Proliferation of BEC was examined by BrdU incorporation. ELISA was used to quantify TNF and TGF-beta 1 levels within cell culture supernatants. Results: Microglia regulated BEC proliferation in a biphasic manner; microglia conditioned medium (MG-CM) from resting microglia inhibited, while that from activated microglia promoted BEC proliferation. A screen of microglial cytokines revealed that BEC proliferation was inhibited by TGF-beta 1, but promoted by TNF. ELISA showed that TNF and TGF-beta 1 were both present in MG-CM, and that while TGF-beta 1 dominated in resting MG-CM, TNF levels were massively increased in activated MG-CM, shifting the balance in favor of TNF. Antibody-blocking studies revealed that the influence of MG-CM to inhibit or promote BEC proliferation was largely attributable to the cytokines TGF-beta 1 and TNF, respectively. Conclusion: This data suggests that microglial activation state might be an important determinant of cerebral angiogenesis; inhibiting BEC proliferation and neovascularization in the normal central nervous system (CNS), but stimulating the growth of new capillaries under neuroinflammatory conditions.