Molecular Identity Changes of Tumor-Associated Macrophages and Microglia After Magnetic Resonance Imaging-Guided Focused Ultrasound-Induced Blood-Brain Barrier Opening in a Mouse Glioblastoma Model.

Authors: Zhang Y, Wang J, Ghobadi SN, Zhou H, Huang A, Gerosa M, Hou Q, Keunen O, Golebiewska A, Habte FG, Grant GA, Paulmurugan R, Lee KS, Wintermark M

An orthotopically allografted mouse GL26 glioma model (Ccr2<sup>RFP/wt</sup>-Cx3cr1<sup>GFP/wt</sup>) 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). BBB opening was induced by magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS) combined with microbubbles. CX3CR1-GFP cells and CCR2-RFP cells in brain tumors were quantified in microscopic images. Tumors in animals treated with a single session of MRgFUS did not exhibit significant changes in cell numbers when compared with tumors in animals not receiving FUS. However, tumors that received two or three sessions of MRgFUS had significantly increased amounts of both CX3CR1-GFP and CCR2-RFP cells. The effect of MRgFUS on immune cell composition was also characterized and quantified using flow cytometry. Glioma implantation resulted in increased amounts of lymphocytes, monocytes and neutrophils in the brain parenchyma. Tumors administered MRgFUS exhibited increased numbers of monocytes and monocyte-derived TAMs. In addition, MRgFUS-treated tumors exhibited more CD80+ cells in monocytes and microglia. 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 toward a more pro-inflammatory status of the immune environment in glioblastoma.

Introduction

Purpose Drug delivery with BBB opening

Study Objective To evaluate how transient, focal opening of the blood–brain barrier by MRI-guided focused ultrasound (MRgFUS) with microbubbles alters the composition and activation state of tumor-associated macrophages and microglia in a mouse glioma model.

Animal model / Human subject Mouse (orthotopic GL26 glioma model; Ccr2 RFP/wt - Cx3cr1 GFP/wt); age 8-10 week old; sex not specified

Disease model glioblastoma

MRI or image guidance method MRI-guided focused ultrasound (MRgFUS)

Outcomes and Safety

Summary of Outcomes Transient focal BBB opening with MRI-guided focused ultrasound (MRgFUS) combined with microbubbles increased recruitment and differentiation of monocytes into tumor-associated macrophages and shifted the glioma immune microenvironment toward a more proinflammatory (CD80+) state; this effect was significant after two or three MRgFUS sessions but not after a single session.

Duration of biological effect 2 days

Safety-related matter The paper does not report any safety concerns or adverse effects associated with MRgFUS; no adverse events are mentioned. Tumors treated with a single MRgFUS session did not show significant changes in cell numbers compared to controls.

Brain Region

Visualization unavailable

Ultrasound Parameters

Ultrasound instrument MR-compatible, pre-focused, eight-element annular array, 1.5-MHz transducer

FUS Frequency 1.5 MHz

FUS Pressure 0.5 MPa

FUS Mode pulsed

Pulse duration 20 ms

Duration of a single FUS session 90 s

Focal Characteristics Focal depth: None; Focal length: None; Aperture size: None

Treatment frequency Both single and multiple sessions

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