Neutrophil recruitment and leukocyte response following focused ultrasound and microbubble mediated blood-brain barrier treatments.

Authors: Poon C, Pellow C, Hynynen K

Rationale: Delivery of therapeutic agents to the brain is limited by the presence of the blood-brain barrier (BBB). An emerging strategy to temporarily and locally increase the permeability of the BBB is the use of transcranial focused ultrasound (FUS) and systematically injected microbubbles (MBs). FUS+MB BBB treatments cause an acute inflammatory response, marked by a transient upregulation of pro-inflammatory genes; however, the cellular immune response remains unknown. Methods: FUS+MB BBB treatments were monitored in real-time using two-photon fluorescence microscopy and transgenic EGFP Wistar rats, which harbour several fluorescent cell types. Leukocyte identification and counts were confirmed using magnetic resonance imaging-guided FUS+MB BBB treatments. Participation of leukocytes in reducing β-amyloid pathology following repeated FUS+MB BBB treatments was investigated in the TgCRND8 mouse model of Alzheimer's disease. Results: Intravascular leukocyte activity indicative of acute inflammation were identified, including transendothelial migration, formation of cell aggregates, and cell masses capable of perturbing blood flow. Leukocyte responses were only observed after the onset of sonication. Neutrophils were identified to be a key participating leukocyte. Significantly more neutrophils were detected in the sonicated hemisphere compared to the contralateral hemisphere, and to untreated controls. Three to five biweekly FUS+MB BBB treatments did not induce significantly more neutrophil recruitment, nor neutrophil phagocytosis of β-amyloid plaques, in TgCRND8 mice compared to untreated controls. Conclusions: This study provides evidence that the cellular aspect of the peripheral immune response triggered by FUS+MB BBB treatments begins immediately after sonication, and emphasizes the importance for further investigations to be conducted to understand leukocyte dynamics and cerebral blood flow responses to FUS+MB BBB treatments.

Introduction

Purpose Drug delivery with BBB opening

Study Objective To investigate recruitment and behavior of peripheral immune cells induced by focused ultrasound with microbubbles (FUS+MB) blood-brain barrier treatments and their contribution to β-amyloid clearance.

Animal model / Human subject Transgenic EGFP Wistar rat (Wistar, EGFP transgenic), age not reported, sex not reported; TgCRND8 mouse (Mus musculus, TgCRND8 Alzheimer's model), age not reported, sex not reported

Disease model Alzheimer's disease

MRI or image guidance method Two-photon fluorescence microscopy monitoring and magnetic resonance imaging-guided (MRI-guided) FUS

Outcomes and Safety

Summary of Outcomes Focused ultrasound with microbubbles (single-point MR-guided sonication and repeated biweekly 3–5 sonications) acutely and locally increased BBB permeability and triggered immediate neutrophil recruitment, intravascular leukocyte aggregation/occlusions and transient blood-flow perturbations, although repeated treatments did not enhance neutrophil-mediated Aβ plaque phagocytosis.

Duration of biological effect 4 h

Safety-related matter FUS+MB treatments induced an acute inflammatory response with neutrophil recruitment, intravascular cell aggregates/occlusions and transient blood-flow arrests; in one case extravascular leukocyte swarming suggested possible vascular damage and persistent capillary stalls were noted as potentially detrimental to function. The authors therefore caution that these transient vascular and immune effects represent potential adverse events and emphasize careful parameter selection and real-time monitoring to avoid tissue damage.

Neutrophil recruitment and leukocyte response following focused ultrasound and microbubble mediated blood-brain barrier treatments.

Introduction

Outcomes and Safety

Brain Region

Ultrasound Parameters