The late phase of post-stroke neurorepair in aged rats is reflected by MRI-based measures

Non-invasive criteria determining the progress of brain healing are especially important in aging, providing a case-specific therapeutic strategy in populations with dysregulated neurorepair mechanisms. We hypothesized that temporal evolution of magnetic resonance imaging (MRI) of T2 tissue relaxation values correlate with neurological severity scores (NS), and provide a robust indicator of healing in the aging brain after stroke. Pre-treatment of aged rats with brain-only proton irradiation was undertaken to pre-condition the inflammatory system. Irradiation was performed 10days prior to right middle cerebral artery occlusion (MCAO) for 50min (MCAO+Rad). Control rats included naïve (no ischemia, no radiation), irradiated-only (Rad), irradiated ischemic, or ischemic-only (MCAO). MRI and NS were obtained at 3, 14 and 28days post-stroke. At 28days post-stroke, immunofluorescence for visualizing blood vessels (Von Willebrand factor; vWF), neurons (neuronal nuclear antigen; NeuN), astrocytes (glial fibrillary acidic protein; GFAP), activated microglia/macrophages (ionized calcium-binding adapter molecule 1, Iba1), T-lymphocytes (CD3), phagocytes (ED1) and apoptotic cells (caspase-3) was assessed. We found a positive T2-NS correlation in irradiated, ischemic rats that corresponded to late-stage brain recovery. Late-stage brain recovery was characterized by improved neovascularization, formation of glio-vascular complexes (visualized by GFAP/vWF) and enhanced neuronal viability (by NeuN/caspase-3) in the peri-lesional zone. The immune response plateaued at the late stage of repair as evidenced by significantly decreased expression (41.7%) and distribution of phagocytes (phagocytic rim decreased 44.6%). We also found reduced infiltration of T-lymphocytes (CD3) in the brain and normalization of blood lymphocytes. The observed T2-NS correlations may provide a simple MRI-based criterion for recognition of regenerative brain transformation in aged patients following stroke. Selective activation of innate immunity and accelerated transition from pro-inflammatory to pro-healing macrophage phenotypes induced by localized brain irradiation is a potential mechanism for enhancing repair ability in the elderly.