Results obtained from completed and on-going clinical studies indicate huge therapeutic potential of stem cell-based therapy in the treatment of degenerative, autoimmune and genetic disorders. However, clinical application of stem cells raises numerous ethical and safety concerns.

There is growing evidence that mesenchymal stem cell (MSC)-based immunosuppression was mainly attributed to the effects of MSC-derived extracellular vesicles (MSC-EVs). MSC-EVs are enriched with MSC-sourced bioactive molecules (messenger RNA (mRNA), microRNAs (miRNAs), cytokines, chemokines, immunomodulatory factors) that regulate phenotype, function and homing of immune cells. In this review article we emphasized current knowledge regarding molecular mechanisms responsible for the therapeutic effects of MSC-EVs in attenuation of autoimmune and inflammatory diseases. We described the disease-specific cellular targets of MSC-EVs and defined MSC-sourced molecules, which were responsible for MSC-EV-based immunosuppression. Results obtained in a large number of experimental studies revealed that both local and systemic administration of MSC-EVs efficiently suppressed detrimental immune response in inflamed tissues and promoted survival and regeneration of injured parenchymal cells. MSC-EVs-based anti-inflammatory effects were relied on the delivery of immunoregulatory miRNAs and immunomodulatory proteins in inflammatory immune cells (M1 macrophages, dendritic cells (DCs), CD4+Th1 and Th17 cells), enabling their phenotypic conversion into immunosuppressive M2 macrophages, tolerogenic DCs and T regulatory cells. Additionally, through the delivery of mRNAs and miRNAs, MSC-EVs activated autophagy and/or inhibited apoptosis, necrosis and oxidative stress in injured hepatocytes, neurons, retinal cells, lung, gut and renal epithelial cells, promoting their survival and regeneration.

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Local, controlled induction of angiogenesis remains a challenge that limits tissue engineering approaches to replace or restore diseased tissues. We present a new class of bioactive synthetic hydrogel matrices based on poly(ethylene glycol) (PEG) and synthetic peptides that exploits the activity of vascular endothelial growth factor (VEGF) alongside the base matrix functionality for cellular ingrowth, that is, induction of cell adhesion by pendant RGD‐containing peptides and provision of cell‐mediated remodeling by cross‐linking matrix metalloproteinase substrate peptides. By using a Michael‐type addition reaction, we incorporated variants of VEGF121 and VEGF165 covalently within the matrix, available for cells as they invade and locally remodel the material. The functionality of the matrix‐conjugated VEGF was preserved and was critical for in vitro endothelial cell survival and migration within the matrix environment. Consistent with a scheme of locally restricted availability of VEGF, grafting of these VEGF‐modified hydrogel matrices atop the chick chorioallontoic membrane evoked strong new blood vessel formation precisely at the area of graft‐membrane contact. When implanted subcutaneously in rats, these VEGF‐containing matrices were completely remodeled into native, vascularized tissue. This type of synthetic, biointeractive matrix with integrated angiogenic growth factor activity, presented and released only upon local cellular demand, could become highly useful in a number of clinical healing applications of local therapeutic angiogenesis.

Mesenchymal stem cell (MSC)-sourced secretome, defined as the set of MSC-derived bioactive factors (soluble proteins, nucleic acids, lipids and extracellular vesicles), showed therapeutic effects similar to those observed after transplantation of MSCs. MSC-derived secretome may bypass many side effects of MSC-based therapy, including unwanted differentiation of engrafted MSCs. In contrast to MSCs which had to be expanded in culture to reach optimal cell number for transplantation, MSC-sourced secretome is immediately available for treatment of acute conditions, including fulminant hepatitis, cerebral ischemia and myocardial infarction. Additionally, MSC-derived secretome could be massively produced from commercially available cell lines avoiding invasive cell collection procedure. In this review article we emphasized molecular and cellular mechanisms that were responsible for beneficial effects of MSC-derived secretomes in the treatment of degenerative and inflammatory diseases of hepatobiliary, respiratory, musculoskeletal, gastrointestinal, cardiovascular and nervous system. Results obtained in a large number of studies suggested that administration of MSC-derived secretomes represents a new, cell-free therapeutic approach for attenuation of inflammatory and degenerative diseases. Therapeutic effects of MSC-sourced secretomes relied on their capacity to deliver genetic material, growth and immunomodulatory factors to the target cells enabling activation of anti-apoptotic and pro-survival pathways that resulted in tissue repair and regeneration.

Formation of the yolk sac vascular system and its connection to the embryonic circulation is crucial for embryo survival in both mammals and birds. Most mice with mutations in genes involved in vascular development die because of a failure to establish this circulatory loop. Surprisingly,formation of yolk sac arteries and veins has not been well described in the recent literature. Using time-lapse video-microscopy, we have studied arterial-venous differentiation in the yolk sac of chick embryos. Immediately after the onset of perfusion, the yolk sac exhibits a posterior arterial and an anterior venous pole, which are connected to each other by cis-cis endothelial interactions. To form the paired and interlaced arterial-venous pattern characteristic of mature yolk sac vessels, small caliber vessels of the arterial domain are selectively disconnected from the growing arterial tree and subsequently reconnected to the venous system, implying that endothelial plasticity is needed to fashion normal growth of veins. Arterial-venous differentiation and patterning are controlled by hemodynamic forces, as shown by flow manipulation and in situ hybridization with arterial markers ephrinB2 and neuropilin 1, which show that expression of both mRNAs is not genetically determined but plastic and regulated by flow. In vivo application of ephrinB2 or EphB4 in the developing yolk sac failed to produce any morphological effects. By contrast, ephrinB2 and EphB4 application in the allantois of older embryos resulted in the rapid formation of arterial-venous shunts. In conclusion, we show that flow shapes the global patterning of the arterial tree and regulates the activation of the arterial markers ephrinB2 and neuropilin 1.