Molecular Identity Changes of Tumor-Associated Macrophages and Microglia after MRgFUS induced BBB Opening in a Mouse Glioblastoma Model

An orthotopically xenografted mouse GL26 glioma model (Ccr2 RFP/wt -Cx3cr1 GFP/wt ) was used to evaluate the effect of transient, focal opening of the Blood Brain Barrier (BBB) on the composition of tumor-associated macrophages and microglia (TAMs). Opening of the BBB was induced by Magnetic Resonance Imaging (MRI)-guided focused ultrasound (MRgFUS) combined with systemically-administered microbubbles. CX3CR1-GFP cells and CCR2-RFP cells in brain tumors and in peritumoral tissue were quantified utilizing cell counting in fluroscent, microscopic images. Tumors in animals treated with a single session of MRgFUS did not show significant changes in CX3CR1-GFP or CCR2-RFP cell numbers when compared to tumors in animals not receiving FUS. However, tumors that received two or three sessions of MRgFUS showed significantly increased amounts of both CX3CR1-GFP and CCR2-RFP cells.The effect of MRgFUS on immune cell compositon of tumors and the brain parenchyma was also characterized and quantified utilizing flow cytometry. Glioma implantation resulted in increased amounts of monocytes, blood-derived macrophages, and microglia-derived macrophages in the brain parenchyma. Tumors administered MRgFUS showed increased numbers of monocytes and blood-derived macrophages. In addition, MRgFUS-treated tumors exhibited more CD80+CD206- cells in monocytes, blood-derived macrophages, microglia, and microglia-derived macrophages, and fewer CD80-CD206+ cells in monocytes and microglia. This signature is indicative of a shift toward a more pro-inflammatory polarization. In summary, transient, focal opening of the BBB using MRgFUS combined with microbubbles can activate the homing and differentiation of monocytes, and induce a shift towards a more proinflammatory status of the immune environment in glioblastoma.