**4.2 Microenvironment immunomodulation**

MSCs have proven to regulate the immune response through cell-to-cell contact and by the secretion of soluble mediators including cytokines, prostaglandins, enzymes, and proapoptotic and antiapoptotic molecules [27, 37–39]. Different studies involving MSC transplantation in exacerbated immune response models such as peritonitis and ulcerative colitis ameliorate inflammation by reducing the expression levels of pro-inflammatory cytokines such as interleukin-1 beta, interleukin-12, interleukin-6, and tumor necrosis factor-α (TNFα). In addition, these cells exert a decrease of the classical phenotype M1 marker and an increase of the alternative phenotype M2, as well as a marked macrophage reprogramming from M1 to M2 [17, 40–42]. Moreover, MSCs can suppress T cell activation and proliferation by downregulating the expression of costimulatory molecules on the surface of dendritic cells [43], interleukin-10, transforming growth factor-B (TGFβ), nitric oxide, and indoleamine 2,3-dioxygenase enzyme production in response to inflammation as well as interleukin-2 absorption [37, 44, 45]. Similarly, B cell activation can be disturbed, and the regulatory B cell phenotype can be promoted [46, 47].

#### **4.3 Tissue repairing and regeneration**

MSCs participate in repairing many tissues, mostly by the secretion of TGFβ and vascular endothelial growth factor (VEGF) to promote angiogenesis [48], extracellular matrix remodeling, and reduction of the scar formation in chronic wounds [49, 50]. Similarly, in pathologies where gliosis, demyelination, and neuroinflammation occur, MSCs have shown neuroprotective activities such as vascular stabilization and angiogenesis by tight junction protein expression [51], neuronal suppression apoptosis [52, 53], glia hypertrophy prevention [54], and promotion of myelinization by the activation of oligodendrocyte precursor cells [53, 55]. Additionally, MSCs promote synaptic transmission [56], neurite outgrowth, and axonal sprouting mostly by excretion of trophic factors including brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) [12, 57]. SCI models motor skills are increased [58] inclusively; bladder and erectile dysfunction improvement have been reported [59, 60].

#### **4.4 Multipotential capacity**

Originally it was believed that MSCs could exclusively differentiate into cells from the mesodermal lineage [23, 24]; however, in the last 20 years many authors have proven that a proper microenvironment can promote greater plasticity and that MSCs from different sources can differentiate into dermal, neural, or glial cells in vitro and in vivo when they are exposed to neurotrophic factors and specific cytokines [61–63]. Smooth muscle and endothelial cells derived from MSCs can be detected and improve heart functions in ischemic myocardium models [64]. Also, skin, articular cartilage, and bone regeneration have been reported, but mostly when MSCs are combined with natural and artificial scaffolds or when genetically modified [65–67]. In order to achieve CNS regeneration, different sources of MSCs and culture methods have been tested in murine SCI models; however, transplants have demonstrated to improve functional recovery by differentiation into neurons, astrocytes, but mostly oligodendrocytes [54, 55, 58].

### **4.5 Exosomes as mechanisms for cell-to-cell communication and drug delivery vehicles**

Exosomes are extracellular vesicles released by many cells, including MSCs. Their length is between 30 and 100nm, and they can bind to cells through receptor and ligand interaction or by fusion with the target cell membrane to deliver high amounts of cytokines, growth factors, microRNAs, and mRNAs capable of modifying peptide and protein synthesis [60]. Thus, the derived MSC exosomes could be the most attractive therapy in SCI models since neuronal differentiation from MSCs remains poor [58, 68] and several studies show that the regenerative and anti-inflammatory mechanisms are mostly mediated by paracrine factors [27, 36, 69–71]. Furthermore, MSC exosomes are natural drug delivery vehicles that can be modified and produced in high quantities [28, 72, 73]. MSCs have proven to be safe in many different preclinical studies; however, clinical trials involving exosomes in SCI therapy are not yet recruited due to the fact that optimal MSC culture conditions and protocols for exosome isolation is still to be established (www.clinicaltrials.gov).
