Transplantation of mesenchymal stem cells (MSCs) offers an interesting adjuvant approach next to thrombolysis for treatment of ischemic stroke. However, MSCs are not integrated into residing neural networks but act indirectly, inducing neuroprotection and promoting neuroregeneration. Although the mechanisms by which MSCs act are still elusive, recent evidence has suggested that extracellular vesicles (EVs) might be responsible for MSC-induced effects under physiological and pathological conditions. The present study has demonstrated that EVs are not inferior to MSCs in a rodent stroke model. EVs induce long-term neuroprotection, promote neuroregeneration and neurological recovery, and modulate peripheral post-stroke immune responses. Also, because EVs are well-tolerated in humans, as previously reported, the administration of EVs under clinical settings might set the path for a novel and innovative therapeutic stroke concept without the putative side effects attached to stem cell transplantation.

Neuronal damage in autoimmune neuroinflammation is the correlate for long-term disability in multiple sclerosis (MS) patients. Here, we investigated the role of immune cells in neuronal damage processes in animal models of MS by monitoring experimental autoimmune encephalomyelitis (EAE) by using twophoton microscopy of living anaesthetized mice. In the brainstem, we detected sustained interaction between immune and neuronal cells, particularly during disease peak. Direct interaction of myelin oligodendrocyte glycoprotein (MOG)-specific Th17 and neuronal cells in demyelinating lesions was associated with extensive axonal damage. By combining confocal, electron, and intravital microscopy, we showed that these contacts remarkably resembled immune synapses or kinapses, albeit with the absence of potential T cell receptor engagement. Th17 cells induced severe, localized, and partially reversible fluctuation in neuronal intracellular Ca 2+ concentration as an early sign of neuronal damage. These results highlight the central role of the Th17 cell effector phenotype for neuronal dysfunction in chronic neuroinflammation.

Neuronal injury from ischemic stroke is aggravated by invading peripheral immune cells. Early infiltrates of neutrophil granulocytes and T-cells influence the outcome of stroke. So far, however, neither the timing nor the cellular dynamics of neutrophil entry, its consequences for the invaded brain area, or the relative importance of T-cells has been extensively studied in an intravital setting. Here, we have used intravital two-photon microscopy to document neutrophils and brain-resident microglia in mice after induction of experimental stroke. We demonstrated that neutrophils immediately rolled, firmly adhered, and transmigrated at sites of endothelial activation in stroke-affected brain areas. The ensuing neutrophil invasion was associated with local blood-brain barrier breakdown and infarct formation. Brain-resident microglia recognized both endothelial damage and neutrophil invasion. In a cooperative manner, they formed cytoplasmic processes to physically shield activated endothelia and trap infiltrating neutrophils. Interestingly, the systemic blockade of very-late-antigen-4 immediately and very effectively inhibited the endothelial interaction and brain entry of neutrophils. This treatment thereby strongly reduced the ischemic tissue injury and effectively protected the mice from stroke-associated behavioral impairment. Behavioral preservation was also equally well achieved with the antibody-mediated depletion of myeloid cells or specifically neutrophils. In contrast, T-cell depletion more effectively reduced the infarct volume without improving the behavioral performance. Thus, neutrophil invasion of the ischemic brain is rapid, massive, and a key mediator of functional impairment, while peripheral T-cells promote brain damage. Acutely depleting T-cells and inhibiting brain infiltration of neutrophils might, therefore, be a powerful early stroke treatment.