Mesenchymal stem cells (MSCs) are promising candidates for adult cell therapies in regenerative medicine. To fully exert their potential, efficient homing and migration toward lesion sites play an important role. Local transplantation deposits MSC in spatial proximity to the lesion, but often requires invasive procedures. Systemic administration routes are favored, but require the targeted extravasation of the circulating MSC at the site of injury. Transplanted MSC can indeed leave the blood flow and transmigrate through the endothelial barrier, and reach the lesion site. However, the underlying processes are not completely dissolved yet. Recent in vitro and in vivo research identified some key molecules scattered light on the extravasation mechanism. This review provides a detailed overview over the current knowledge of MSC transendothelial migration. We use the leukocyte extravasation process as a role model to build a comprehensive concept of MSC egress mechanisms from the blood stream and identified relevant similarities as well as important differences between the extravasation mechanisms. Stem Cells 2017;35:1446-1460.

Rationale: Stroke is the third most common cause of death in industrialized countries. The main therapeutic target is the ischemic penumbra, potentially salvageable brain tissue that dies within the first few hours after blood flow cessation. Hence, strategies to keep the penumbra alive until reperfusion occurs are needed.Objective: To study the effect of inhaled nitric oxide on cerebral vessels and cerebral perfusion under physiological conditions and in different models of cerebral ischemia. Methods and Results:This experimental study demonstrates that inhaled nitric oxide (applied in 30% oxygen/70% air mixture) leads to the formation of nitric oxide carriers in blood that distribute throughout the body. This was ascertained by in vivo microscopy in adult mice. Although under normal conditions inhaled nitric oxide does not affect cerebral blood flow, after experimental cerebral ischemia induced by transient middle cerebral artery occlusion it selectively dilates arterioles in the ischemic penumbra, thereby increasing collateral blood flow and significantly reducing ischemic brain damage. This translates into significantly improved neurological outcome. These findings were validated in independent laboratories using two different mouse models of cerebral ischemia and in a clinically relevant large animal model of stroke. Key Words: collateral blood flow Ⅲ ischemic penumbra Ⅲ ischemic stroke Ⅲ nitric oxide inhalation E very year stroke is responsible for the death of 5.5 million people. 1 Despite its high incidence and mortality, clinical therapeutic options are still limited. 2 Research efforts to find novel treatment strategies focus primarily on rescuing the ischemic penumbra, the viable tissue surrounding the nonviable infarct core. In the penumbra, blood flow is critically reduced but still suffices to sustain neuronal integrity for several hours. The delayed nature of cell death in the penumbra leaves a unique window of opportunity for therapeutic interventions. If adequate cerebral perfusion is re-established sufficiently fast, then penumbral tissue can be effectively saved. 3 Therefore, penumbral reperfusion at the earliest possible time is the most critical factor in determining neurological outcome and in preventing mortality after stroke. 4 Conclusions:

IntroductionCerebral small vessel disease (cSVD) is one of the most prevalent neurological disorders. The progressive remodeling of brain microvessels due to arterial hypertension or other vascular risk factors causes subtle, but constant cognitive decline through to manifest dementia and substantially increases the risk for stroke. Preliminary evidence suggests the contribution of the immune system to disease initiation and progression, but a more detailed understanding is impaired by the unavailability of appropriate animal models. Here, we introduce the spontaneously hypertensive rat (SHR) as a model for early onset cSVD and unveiled substantial immune changes in conjunction with brain abnormalities that resemble clinical findings.ResultsIn contrast to age-matched normotensive Wistar Kyoto (WKY) rats, male SHR exhibited non-spatial memory deficits. Magnetic resonance imaging showed brain atrophy and a reduction of white matter volumes in SHR. Histological analyses confirmed white matter demyelination and unveiled a circumscribed blood brain barrier dysfunction in conjunction with micro- and macrogliosis in deep cortical regions. Flow cytometry and histological analyses further revealed substantial disparities in cerebral CD45high leukocyte counts and distribution patterns between SHR and WKY. SHR showed lower counts of T cells in the choroid plexus and meningeal spaces as well as decreased interleukin-10 levels in the cerebrospinal fluid. On the other hand, both T and NK cells were significantly augmented in the SHR brain microvasculature.ConclusionsOur results indicate that SHR share behavioral and neuropathological characteristics with human cSVD patients and further undergird the relevance of immune responses for the initiation and progression of cSVD.Electronic supplementary materialThe online version of this article (doi:10.1186/s40478-014-0169-8) contains supplementary material, which is available to authorized users.

IntroductionIntra-arterial cell infusion is an efficient delivery route with which to target organs such as the ischemic brain. However, adverse events including microembolisms and decreased cerebral blood flow were recently reported after intra-arterial cell delivery in rodent models, raising safety concerns. We tested the hypothesis that cell dose, infusion volume, and velocity would be related to the severity of complications after intra-arterial cell delivery.MethodsIn this study, 38 rats were subjected to a sham middle cerebral artery occlusion (sham-MCAO) procedure before being infused with allogeneic bone-marrow mesenchymal stem cells at different cell doses (0 to 1.0 × 106), infusion volumes (0.5 to 1.0 ml), and infusion times (3 to 6 minutes). An additional group (n = 4) was infused with 1.0 × 106 cells labeled with iron oxide for in vivo tracking of cells. Cells were infused through the external carotid artery under laser Doppler flowmetry monitoring 48 hours after sham-MCAO. Magnetic resonance imaging (MRI) was performed 24 hours after cell infusion to reveal cerebral embolisms or hemorrhage. Limb placing, cylinder, and open field tests were conducted to assess sensorimotor functions before the rats were perfused for histology.ResultsA cell dose-related reduction in cerebral blood flow was noted, as well as an increase in embolic events and concomitant lesion size, and sensorimotor impairment. In addition, a low infusion velocity (0.5 ml/6 minutes) was associated with high rate of complications. Lesions on MRI were confirmed with histology and corresponded to necrotic cell loss and blood-brain barrier leakage.ConclusionsParticularly cell dose but also infusion velocity contribute to complications encountered after intra-arterial cell transplantation. This should be considered before planning efficacy studies in rats and, potentially, in patients with stroke.

Almost every experimental treatment strategy using non-autologous cell, tissue or organ transplantation is tested in small and large animal models before clinical translation. Because these strategies require immunosuppression in most cases, immunosuppressive protocols are a key element in transplantation experiments. However, standard immunosuppressive protocols are often applied without detailed knowledge regarding their efficacy within the particular experimental setting and in the chosen model species. Optimization of such protocols is pertinent to the translation of experimental results to human patients and thus warrants further investigation. This review summarizes current knowledge regarding immunosuppressive drug classes as well as their dosages and application regimens with consideration of species-specific drug metabolization and side effects. It also summarizes contemporary knowledge of novel immunomodulatory strategies, such as the use of mesenchymal stem cells or antibodies. Thus, this review is intended to serve as a state-of-the-art compendium for researchers to refine applied experimental immunosuppression and immunomodulation strategies to enhance the predictive value of preclinical transplantation studies.

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As effective stroke treatment by thrombolysis is bound to a narrow time window excluding most patients, numerous experimental treatment strategies have been developed to gain new options for stroke treatment. However, all approaches using neuroprotective agents that have been successfully evaluated in rodents have subsequently failed in clinical trials. Existing large animal models are of significant scientific value, but sometimes limited by ethical drawbacks and mostly do not allow for long-term observation. In this study, we are introducing a simple, but reliable stroke model using permanent middle cerebral artery occlusion in sheep. This model allows for control of ischemic lesion size and subsequent neurofunctional impact, and it is monitored by behavioral phenotyping, magnetic resonance imaging, and positron emission tomography. Neuropathologic and (immuno)-histologic investigations showed typical ischemic lesion patterns whereas commercially available antibodies against vascular, neuronal, astroglial, and microglial antigens were feasible for ovine brain specimens. Based on absent mortality in this study and uncomplicated species-appropriate housing, long-term studies can be realized with comparatively low expenditures. This model could be used as an alternative to existing large animal models, especially for longitudinal analyses of the safety and therapeutic impact of novel therapies in the field of translational stroke research.

Cell therapies are increasingly recognized as a promising option to augment the limited therapeutic arsenal available to fight ischemic stroke. During the last two decades, cumulating preclinical evidence has indicated a substantial efficacy for most cell treatment paradigms and first clinical trials are currently underway to assess safety and feasibility in patients. However, the strong and still unmet demand for novel stroke treatment options and exciting findings reported from experimental studies may have drawn our attention away from potential side effects related to cell therapies and the ways by which they are commonly applied. This review summarizes common and less frequent adverse events that have been discovered in preclinical and clinical investigations assessing cell therapies for stroke. Such adverse events range from immunological and neoplastic complications over seizures to cell clotting and cell-induced embolism. It also describes potential complications of clinically applicable administration procedures, detrimental interactions between therapeutic cells, and the pathophysiological environment that they are placed into, as well as problems related to cell manufacturing. Virtually each therapeutic intervention comes at a certain risk for complications. Side effects do therefore not generally compromise the value of cell treatments for stroke, but underestimating such complications might severely limit therapeutic safety and efficacy of cell treatment protocols currently under development. On the other hand, a better understanding will provide opportunities to further improve existing therapeutic strategies and might help to define those circumstances, under which an optimal effect can be realized. Hence, the review eventually discusses strategies and recommendations allowing us to prevent or at least balance potential complications in order to ensure the maximum therapeutic benefit at minimum risk for stroke patients.