Mesenchymal stem cells (MSCs) hold great potential as a regenerative therapy for stroke, leading to increased repair and functional recovery in animal models of cerebral ischaemia. While it was initially hypothesised that cell replacement was an important mechanism of action of MSCs, focus has shifted to their paracrine actions or the so called “bystander” effect. MSCs secrete a wide array of growth factors, chemokines, cytokines and extracellular vesicles, commonly referred to as the MSC secretome. There is evidence suggesting the MSC secretome can promote repair through a number of mechanisms including preventing cell apoptosis, modulating the inflammatory response and promoting endogenous repair mechanisms such as angiogenesis and neurogenesis. In this review, we will discuss the in vitro approaches currently being employed to drive the MSC secretome towards a more anti-inflammatory and regenerative phenotype. We will then examine the role of the secretome in promoting repair and improving recovery in preclinical models of cerebral ischaemia.

BackgroundInflammation is a key contributor to central nervous system (CNS) injury such as stroke, and is a major target for therapeutic intervention. Effective treatments for CNS injuries are limited and applicable to only a minority of patients. Stem cell-based therapies are increasingly considered for the treatment of CNS disease, because they can be used as in-situ regulators of inflammation, and improve tissue repair and recovery. One promising option is the use of bone marrow-derived mesenchymal stem cells (MSCs), which can secrete anti-inflammatory and trophic factors, can migrate towards inflamed and injured sites or can be implanted locally. Here we tested the hypothesis that pre-treatment with inflammatory cytokines can prime MSCs towards an anti-inflammatory and pro-trophic phenotype in vitro.MethodsHuman MSCs from three different donors were cultured in vitro and treated with inflammatory mediators as follows: interleukin (IL)-1α, IL-1β, tumour necrosis factor alpha (TNF-α) or interferon-γ. After 24 h of treatment, cell supernatants were analysed by ELISA for expression of granulocyte-colony stimulating factor (G-CSF), IL-10, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), IL-1 receptor antagonist (IL-1Ra) and vascular endothelial growth factor (VEGF). To confirm the anti-inflammatory potential of MSCs, immortalised mouse microglial BV2 cells were treated with bacterial lipopolysaccharide (LPS) and exposed to conditioned media (CM) of naïve or IL-1-primed MSCs, and levels of secreted microglial-derived inflammatory mediators including TNF-α, IL-10, G-CSF and IL-6 were measured by ELISA.ResultsUnstimulated MSCs constitutively expressed anti-inflammatory cytokines and trophic factors (IL-10, VEGF, BDNF, G-CSF, NGF and IL-1Ra). MSCs primed with IL-1α or IL-1β showed increased secretion of G-CSF, which was blocked by IL-1Ra. Furthermore, LPS-treated BV2 cells secreted less inflammatory and apoptotic markers, and showed increased secretion of the anti-inflammatory IL-10 in response to treatment with CM of IL-1-primed MSCs compared with CM of unprimed MSCs.ConclusionsOur results demonstrate that priming MSCs with IL-1 increases expression of trophic factor G-CSF through an IL-1 receptor type 1 (IL-1R1) mechanism, and induces a reduction in the secretion of inflammatory mediators in LPS-activated microglial cells. The results therefore support the potential use of preconditioning treatments of stem cells in future therapies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13287-017-0531-4) contains supplementary material, which is available to authorized users.

Cell therapy for spinal cord injury (SCI) is a promising strategy for clinical application. Both bone marrow mesenchymal stromal cells (MSCs; also known as bone marrow-derived 'mesenchymal stem cells') and olfactory ensheathing cells (OECs) have demonstrated beneficial effects following transplantation in animal models of SCI. However, due to the large number of affecting parameters that determine the therapy success and the lack of methodological consensus, the comparison of different works is difficult. Therefore, we compared the effects of MSC and OEC transplants at early or delayed time after a spinal cord contusion injury in the rat. Functional outcomes for locomotion, sensory perception and electrophysiological responses were assessed. Moreover, the grafted cells survival and the amount of cavity and spared tissue were studied. The findings indicate that grafted cells survived until 7 days post-injection, but markedly disappeared in the following 2 weeks. Despite the low survival of the cells, MSC and OEC grafts provided tissue protection after early and delayed transplantation. Nevertheless, only acute MSC grafts improved locomotion recovery in treadmill condition and electrophysiological outcomes with respect to the other injured groups. These results, together with previous works, indicate that the MSC seem a better option than OEC for treatment of contusion injuries.

BackgroundMesenchymal stem cells (MSCs) are one of the most promising candidates for the treatment of major neurological disorders. Desirable therapeutic properties of MSCs include reparative and regenerative potential but, despite their proven safety, the efficacy of MSCs remains controversial. Therefore, it is essential to optimise culture protocols to enhance the therapeutic potential of the MSC secretome. Here we aimed to: assess the increase in secretion of cytokines that may induce repair, regeneration, or immunomodulation when cultured in three dimensions; study the effect of interleukin (IL)-1 priming on two- (2D) and three-dimensional (3D) cultures of MSC; and evaluate the potential use of the modified secretome using microglial-MSC co-cultures.MethodsWe established a 3D spheroid culture of human MSCs, and compared the secretome in 2D and 3D cultures under primed (IL-1) and unprimed conditions. BV2 microglial cells were stimulated with lipopolysaccharide (LPS) and treated with spheroid conditioned media (CM) or were co-cultured with whole spheroids. Concentrations of secreted cytokines were determined by enzyme-linked immunosorbent assay (ELISA). Protein arrays were used to further evaluate the effect of IL-1 priming in 2D and 3D cultures.Results3D culture of MSCs significantly increased secretion of the IL-1 receptor antagonist (IL-1Ra), vascular endothelial growth factor (VEGF), and granulocyte-colony stimulating factor (G-CSF) compared with 2D culture, despite priming treatments with IL-1 being more effective in 2D than in 3D. The addition of CM of 3D-MSCs reduced LPS-induced tumour necrosis factor (TNF)-α secretion from BV2 cells, while the 3D spheroid co-cultured with the BV2 cells induced an increase in IL-6, but had no effect on TNF-α release. Protein arrays indicated that priming treatments trigger a more potent immune profile which is necessary to orchestrate an effective tissue repair. This effect was lost in 3D, partly because of the overexpression of IL-6.ConclusionsIncreased secretion of anti-inflammatory markers occurs when MSCs are cultured in 3D, but this specific secretome did not translate into anti-inflammatory effects on LPS-treated BV2 cells in co-culture. These data highlight the importance of optimising priming treatments and culture conditions to maximise the therapeutic potential of MSC spheroids.