Intravenously Delivered Allogeneic Mesenchymal Stem Cells Bidirectionally Regulate Inflammation and Induce Neurotrophic Effects in Distal Middle Cerebral Artery Occlusion Rats Within the First 7 Days After Stroke

Abstract

Background/Aims: Neurotrophic effects and immunosuppression are the main therapeutic mechanisms of mesenchymal stem cells (MSCs) in stroke treatment. Neurotrophins are produced by graft cells, host neurons, astrocytes, and even microglia/macrophages. Meanwhile, MSCs can increase inflammation if they are not sufficiently induced by pro-inflammatory cytokines. We examined whether intravenously transplanted bone marrow MSCs (BM-MSCs) increase inflammation in distal middle cerebral artery occlusion (dMCAO) rats, how long the increased inflammation effect persists for, and what the final therapeutic outcomes will be. We also tested the neurotrophic role of BM-MSCs and attempted to identify the neurotrophin-producing cells. Methods: At 1 h after dMCAO was performed on Sprague-Dawley rats, allogeneic BM-MSCs were transplanted intravenously. The infarct volume was examined by Tetrazolium Red staining at 2 days (day 2), and the behavioral tests (cylinder test and grid walking test) were performed at 2, 4 (day 4) and 7 days (day 7) after transplantation. The concentrations of inflammation related cytokines and neurotrophins in the ischemic cortex, ipsilateral striatum, and serum, were measured using ELISA at days 2-7. The cell source of neurotrophins was observed by immunohistochemistry. Results: The transplanted cells were mainly found in the infarct border zone (IBZ) of the brain. Infarct volume was reduced and behavioral outcomes were improved at 2 days after ischemia. In the striatum and circulation, BM-MSC transplantation increased inflammation at day 2 and decreased it at day 7. At days 2-7, insulin-like growth factor-1 (IGF-1) and brain-derived neurotrophic factor (BDNF) concentrations in the ischemic core of the cortex were significantly higher in the BM-MSC group than in the ischemia vehicle group. IGF-1 and BDNF were derived mainly from host microglia/macrophages in the ischemic core, and transplanted cells in the IBZ. At day 2, BM-MSC transplantation significantly increased the number of IGF-1⁺CD68⁺ and BDNF⁺Iba-1⁺ double positive cells in the ischemic core cortex. Conclusions: Although increased inflammation, BM-MSCs were still beneficial to dMCAO recovery at day 2. The immunopromoting effect of MSCs was transient and shifted to an immunosuppressive action at day 7. The neurotrophic factors IGF-1 and BDNF, which were mainly derived from transplanted BM-MSCs and host microglia/macrophages, contributed to the therapeutic effects from day 2 to day 7.