Longitudinal monitoring of microglial/ macrophage activation in ischemic rat brain using Iba-1-specific nanoparticle enhanced MRI

After a stroke, the brain’s resident immune cells, the microglia/macrophages, become activated with one phenotype predominating over another in a time-dependent manner. Microglia rapidly develop a pro-inflammatory phenotype in response to acute stroke injury; meanwhile, activation of microglia also present reparative and anti-inflammatory roles through a regulatory/homeostatic phenotype, which facilitates recovery. Previous studies have shown that the position of the active microglia/macrophages determined via dynamic analyses in relation to the infarct in the ischemic brain at different times is an important feature of these different phenotypes. To develop treatment approaches that will both suppress the harmful effects of microglial/macrophage activation while still maintaining neurovascular remodeling and repair, it is necessary to understand the dynamics in vivo of stroke-induced activation of cerebral microglial/ macrophages.

To determine the spatiotemporal distribution of microglial/macrophage activation in the brain of a living rat, University of New Mexico Health Sciences Center scientists:  Professor Laurel Sillerud, Dr. Yirong Yang, Lisa Yang, Kelsey Duval, Jeffrey Thompson, and Professor Yi Yang used nanoparticle-enhanced magnetic resonance imaging (MRI) to visualize neuroinflammation occurring in the living brain after an ischemic stroke. The original research article is published in the Journal of Cerebral Blood Flow & Metabolism¹.  Up to four weeks post-stroke, MRI with the aid of superparamagnetic iron–platinum (FePt) nanoparticles, containing surface anti-Iba-1 antibodies specific for activated microglia/macrophages, revealed the positions and amounts of microglia/macrophages using T₂*-weighted imaging. The team found that the nanoparticles accumulated in the regions of the brain (Figure 1) that coincided with areas where active microglia/macrophages were detected by post-mortem immunohistochemistry.

They also observed a good correlation in morphological and distributive dynamic changes between the Fe⁺-cells and the Iba-1⁺-microglia/macrophages. The microglial/macrophage activation and phenotypic changes that were assessed by post-mortem immunohistochemistry over the four weeks post-stroke were found to parallel the spatiotemporal changes of nanoparticle-induced image contrast detected by T₂*-weighted MRI. Using these techniques, the research team concluded that the maximum level of microglial/macrophage activation occurred 7- days post-stroke; the activation then diminished after two weeks and continued to fall at four weeks.  Professor Sillerud said that it was “now possible to essentially count the microglial cells responsible for stroke-induced neuroinflammation in the living brain.”

In this molecular imaging study, the authors were able to successfully demonstrate the enhancement of MRI using anti-Iba-1conjugated FePt nanoparticles and to visualize the longitudinal time-course and spatial distribution of the inflammation induced by stroke and to show the precise and localized changes in microglial/macrophage activation. Their results provide evidence that the progression and treatment of stroke in living animals and the dynamic development of the associated neuroinflammation can be monitored non-invasively using this novel application of targeted nanoparticles. It will be interesting in the future to modify the FePt nanoparticles and adapt them to develop a theranostic agent where both detection and therapeutic molecules can be combined for better stroke therapy.