Inflammation and Stem Cell Therapy for Stroke

Ischemic stroke is a leading cause of death and disability worldwide. Currently, there is no treatment that can promote recovery in the chronic phase. It has been shown that neurogenesis occurs in ischemic striatum in rodents and probably also in humans. Moreover, blood-borne macrophages have been found to enhance spontaneous post- stroke recovery in mice. These findings have suggested potential new targets to improve functional restoration after stroke. In this thesis, we first showed that inflammation without neuronal loss is sufficient to trigger striatal neurogenesis comparable to that after stroke, indicating that inflammation might be the main inducer of post-stroke striatal neurogenesis. Using microarray on sorted microglia from subventricular zone (SVZ) and striatum, several factors were identified that potentially could regulate different steps of striatal neurogenesis after stroke. Some of the identified factors have previously been reported to regulate neural stem/progenitor cells (NSPC) proliferation or differentiation. We examined in some detail one factor, Cxcl13, and found that it promotes neuroblasts migration in vitro. Next, we provided evidence that monocyte-derived macrophages (MDM) can take the choroid plexus (CP)-cerebrospinal fluid (CSF) route for infiltration into the brain after cortical stroke. We found that in vitro-derived anti-inflammatory (M2-like) MDM delivered into CSF migrate into ischemic cortex, maintain their M2-like phenotype, and most importantly, improve recovery of motor and cognitive function in stroke-subjected mice without influencing infarct volume. These findings highlight the crucial role of inflammatory cells, such as microglia and macrophages, in post-stroke cellular plasticity and functional recovery. We also explored another approach for cell delivery into the brain using human induced pluripotent stem cells (iPSC)-derived long-term neuroepithelial-like stem (lt-NES) cells. Following our previous findings that transplantation of these cells and their derivatives promotes post-stroke motor function recovery, we showed that strokeinfluences the migration and axonal projection pattern of iPSC-derived lt-NES cells implanted adjacent to the neurogenic SVZ. These data indicate that the occurrence of ischemic injury strongly affects crucial parameters in the behavior of transplanted neural progenitors, which will be important to consider in a potential, future clinical translation. Finally, by combining immunoelectron microscopy, rabies virus-based trans-synaptic tracing, in vivo electrophysiological recordings and optogenetic techniques, we for the first time showed that neurons derived from transplanted iPSC-derived lt-NES cells receive functional synaptic inputs from host neurons located in the appropriate brain areas, e.g. ventral thalamus, after stroke. We demonstrated that tactile stimulation of nose and paws can activate or inhibit spontaneous activity in grafted neurons, providing evidence that they can become incorporated into injured cortical circuitry. Since we have found that transplanted M2-like MDM promote post-stroke recovery, possibly by modulating neuronal circuit plasticity, it seems highly warranted to examine whether delivery of M2-like MDM would further enhance the integration of neurons generated from grafted iPSC-derived lt-NES cells in the stroke model. Taken together, our findings raise the possibility that modulation of inflammatory mechanisms, delivery of M2-like MDM and transplantation of neurons generated from iPSC-derived lt-NES cells might become of value in future therapeutic approaches for improved functional recovery in stroke patients.