Importance of the stem cell microenvironment for ophthalmological cell-based therapy

Cell therapy is a promising treatment for diseases that are caused by cell degeneration or death. The cells for clinical transplantation are usually obtained by culturing healthy allogeneic or exogenous tissue invitro. However, for diseases of the eye, obtaining the adequate number of cells for clinical transplantation is difficult due to the small size of tissue donors and the frequent needs of long-term amplification of cells in vitro, which results in low cell viability after transplantation. In addition, the transplanted cells often develop fibrosis or degrade and have very low survival. Embryonic stem cells(ESCs) and induced pluripotent stem cells(i PS) are also promising candidates for cell therapy. Unfortunately, the differentiation of ESCs can bring immune rejection, tumorigenicity and undesired differentiated cells, limiting its clinical application. Although i PS cells can avoid the risk of immune rejection caused by ES cell differentiation post-transplantation, the low conversion rate, the risk of tumor formation and the potentially unpredictable biological changes that could occur through genetic manipulation hinder its clinical application. Thus, the desired clinical effect of cell therapy is impaired by these factors. Recent research findings recognize that the reason for low survival of the implanted cells not only depends on the seeded cells, but also on the cell microenvironment, which determines the cell survival, proliferation and even reverse differentiation. When used for cell therapy, the transplanted cells need a specific three-dimensional structure to anchor and specific extra cellular matrix components in addition to relevant cytokine signaling to transfer the required information to support their growth. These structures present in the matrix in which the stem cells reside are known as the stem cell microenvironment. The microenvironment interaction with the stem cells provides the necessary homeostasis for cell maintenance and growth. A large number of studies suggest that to explore how to reconstruct the stem cell microenvironment and strengthen its combination with the transplanted cells are key steps to successful cell therapy. In this review, we will describe the interactions of the stem cell microenvironment with the stem cells, discuss the importance of the stem cell microenvironment for cell-based therapy in ocular diseases, and introduce the progress of stem cell-basedtherapy for ocular diseases.     

WJSC 6th Anniversary Special Issues（2）:Mesenchymal stem cells Neurotrauma and mesenchymal stem cells treatment:From experimental studies to clinical trials

Mesenchymal stem cell(MSC)therapy has attracted the attention of scientists and clinicians around the world.Basic and pre-clinical experimental studies have highlighted the positive effects of MSC treatment after spinal cord and peripheral nerve injury.These effects are believed to be due to their ability to differentiate into other cell lineages,modulate inflammatory and immunomodulatory responses,reduce cell apoptosis,secrete several neurotrophic factors and respond to tissue injury,among others.There are many pre-clinical studies on MSC treatment for spinal cord injury(SCI)and peripheral nerve injuries.However,the same is not true for clinical trials,particularly those concerned with nerve trauma,indicating the necessity of more well-constructed studies showing the benefits that cell therapy can provide for individuals suffering the consequences of nerve lesions.As for clinical trials for SCI treatment the results obtained so far are not as beneficial as those described in experimental studies.For these reasons basic and pre-clinical studies dealing with MSC therapy should emphasize the standardization of protocols that could be translated to the clinical set with consistent and positive outcomes.This review is based on pre-clinical studies and clinical trials available in the literature from 2010 until now.At the time of writing this article there were 43 and 36 pre-clinical and 19 and 1 clinical trials on injured spinal cord and peripheral nerves,respectively.     

Neural stem cell therapy for brain disease

Brain diseases, including brain tumors, neurodegenerative disorders, cerebrovascular diseases, and traumatic brain injuries, are among the major disorders influencing human health, currently with no effective therapy. Due to the low regeneration capacity of neurons, insufficient secretion of neurotrophic factors, and the aggravation of ischemia and hypoxia after nerve injury, irreversible loss of functional neurons and nerve tissue damage occurs. This damage is difficult to repair and regenerate the central nervous system after injury. Neural stem cells (NSCs) are pluripotent stem cells that only exist in the central nervous system. They have good self-renewal potential and ability to differentiate into neurons, astrocytes, and oligodendrocytes and improve the cellular microenvironment. NSC transplantation approaches have been made for various neurodegenerative disorders based on their regenerative potential. This review summarizes and discusses the characteristics of NSCs, and the advantages and effects of NSCs in the treatment of brain diseases and limitations of NSC transplantation that need to be addressed for the treatment of brain diseases in the future.     

Tissue engineering with muscle-derived stem cells

Tissue engineering and cell therapy approaches aim to take advantage of the repopulating ability and plasticity of multipotent stem cells to regenerate lost or diseased tissue. Researchers continue to investigate stem cells in mature tissues and demonstrate the potential ability of organ-specific cells to differentiate into multiple lineages. One stem cell that displays such promise is the muscle-derived stem cell (MDSC). Data supporting the existence of MDSCs have emerged as part of investigations to improve myoblast cell transplantation for the treatment of muscular dystrophies. As these efforts continue, the potential for MDSC-based therapy for other musculoskeletal injuries, as well as for cardiac and smooth muscle injuries, is currently being explored.     

Optimizing stem cells for cardiac repair: Current status and new frontiers in regenerative cardiology

Cell therapy has the potential to improve healing of ischemic heart, repopulate injured myocardium and restore cardiac function. The tremendous hope and potential of stem cell therapy is well understood, yet recent trials involving cell therapy for cardiovascular diseases have yielded mixed results with inconsistent data thereby readdressing controversies and unresolved questions regarding stem cell efficacy for ischemic cardiac disease treatment. These controversies are believed to arise by the lack of uniformity of the clinical trial methodologies, uncertainty regarding the underlying reparative mechanisms of stem cells, questions concerning the most appropriate cell population to use, the proper delivery method and timing in relation to the moment of infarction, as well as the poor stem cell survival and engraftment especially in a diseased microenvironment which is collectively acknowledged as a major hindrance to any form of cell therapy. Indeed, the microenvironment of the failing heart exhibits pathological hypoxic, oxidative and inflammatory stressors impairing the survival of transplanted cells. Therefore, in order to observe any significant therapeutic benefit there is a need to increase resilience of stem cells to death in the transplant microenvironment while preserving or better yet improving their reparative functionality. Although stem cell differentiation into cardiomyocytes has been observed in some instance, the prevailing reparative benefits are afforded through paracrine mechanisms that promote angiogenesis, cell survival, transdifferentiate host cells and modulate immune responses. Therefore, to maximize their reparative functionality, ex vivo manipulation of stem cells through physical, genetic and pharmacological means have shown promise to enable cells to thrive in the postischemic transplant microenvironment. In the present work, we will overview the current status of stem cell therapy for ischemic heart disease, discuss the most recurring cell populations employed, the mechanisms by which stem cells deliver a therapeutic benefit andstrategies that have been used to optimize and increase survival and functionality of stem cells including ex vivo preconditioning with drugs and a novel "pharmacooptimizer" as well as genetic modifications.     

Stem and progenitor cells for neurological repair: minor issues, major hurdles, and exciting opportunities for paracrine-based therapeutics

The transplantation of cultured stem and progenitor cells is a key element in the rapidly growing field of regenerative medicine. Based on their ability to rescue and/or repair injured tissue and partially restore organ function, multiple types of stem/progenitor cells have already entered into clinical trials. However, despite several decades of intense research, the goal to apply culture-expanded stem/progenitor cells in a manner that can effectively replace cells after injury has yet to be realized. Many sources of potentially useful cells are available, but something is clearly missing. In addition, recent studies suggest that paracrine effects of secreted or released factors are responsible for most of the benefits observed after cell transplantation, rather than direct cell replacement. These data call into question the need for cell transplantation for many types of therapy, in particular for acute injuries such as myocardial infarction and stroke. In this review, we examine current progress in the area of cell transplantation and minor issues and major hurdles regarding the clinical application of different cell types. We discuss the "paracrine hypothesis" for the action of transplanted stem/progenitor cells as an opportunity to identify defined combinations of biomolecules to rescue and/or repair tissues after injury. Although many of the concepts in this review will apply to multiple injury/repair systems, we will focus primarily on stem/progenitor cell-based treatments for neurological disorders and stroke.     

The application of stem cells in the treatment of ischemic diseases

Ischemia causes oxygen deprivation, cell injury and related organ dysfunction. Although ischemic injury may be local, it involves many biochemical changes in different cell types. The ability of stem cells to differentiate into different cell lineages provides the possibility of their use in treating a variety of diseases requiring tissue repair or reconstitution, such as stroke, ischemic retinopathy, myocardial infarction, ischemic disorders of the liver, ischemic renal failure, and ischemic limb dysfunction. Several cell types including embryonic stem cells, various progenitor and stem cells of hematopoietic or mesenchymal origin have been used in attempts to reconstitute injured tissue. Xenologous or autologous stem cells may be administered either through the peripheral vascular system or directly by regional injection. The stem cells are then guided to the infarct site by homing signals. Either by cell differentiation or paracrine effects, stem cells or progenitor cells participate in the reconstruction of a favorable microenvironment resulting in neovascularization and tissue regeneration that eventually improve the physiological function of organs with ischemic damage.     

The role of recipient T cells in mesenchymal stem cell-based tissue regeneration

Significant progress has been made in stem cell biology, regenerative medicine, and stem cell-based tissue engineering. Such scientific strides highlight the potential of replacing or repairing damaged tissues in congenital abnormalities, diseases, or injuries, as well as constructing functional tissue or organs in vivo. Since mesenchymal stem cells (MSCs) are capable of differentiating into bone-forming cells, they constitute an appropriate cell source to repair damaged bone tissues. In addition, the immunoregulatory property of MSCs provides a foundation for their use in treating a variety of autoimmune diseases. However, the interaction between MSCs and immune cells in cell-based tissue regeneration is largely unknown. In this review, we will discuss the current understanding of MSC-based tissue regeneration, emphasizing the role of the immune microenvironment in bone regeneration.     

Effects of shear stress on differentiation of stem cells into endothelial cells

Stem cell transplantation is an appealing potential therapy for vascular diseases and an indispensable key step in vascular tissue engineering. Substantial effort has been made to differentiate stem cells toward vascular cell phenotypes, including endothelial cells (ECs) and smooth muscle cells. The microenvironment of vascular cells not only contains biochemical factors that influence differentiation but also exerts hemodynamic forces, such as shear stress and cyclic strain. More recently, studies have shown that shear stress can influence the differentiation of stem cells toward ECs. A deep understanding of the responses and underlying mechanisms involved in this process is essential for clinical translation. This review highlights current data supporting the role of shear stress in stem cell differentiation into ECs. Potential mechanisms and signaling cascades for transducing shear stress into a biological signal are proposed. Further study of stem cell responses to shear stress will be necessary to apply stem cells for pharmacological applications and cardiovascular implants in the realm of regenerative medicine.     

Application of human mesenchymal and pluripotent stem cell microcarrier cultures in cellular therapy: Achievements and future direction

Mesenchymal stem cells (MSCs) have recently made significant progress with multiple clinical trials targeting modulation of immune responses, regeneration of bone, cartilage, myocardia, and diseases like Metachromatic leukodystrophy and Hurler syndrome. On the other hand, the use of human embryonic and induced pluripotent stem cells (hPSCs) in clinical trials is rather limited mainly due to safety issues. Only two clinical trials, retinal pigment epithelial transplantation and treatment of spinal cord injury were reported. Cell doses per treatment can range between 50,000 and 6 billion cells. The current 2-dimensional tissue culture platform can be used when low cell doses are needed and it becomes impractical when doses above 50 million are needed. This demand for future cell therapy has reinvigorated interests in the use of the microcarrier platform for generating stem cells in a scalable 3-dimensional manner.     

Immunobiology of mesenchymal stem cells

Mesenchymal stem cells (MSCs) can be isolated from almost all tissues and effectively expanded in vitro. Although their true in situ properties and biological functions remain to be elucidated, these in vitro expanded cells have been shown to possess potential to differentiate into specific cell lineages. It is speculated that MSCs in situ have important roles in tissue cellular homeostasis by replacing dead or dysfunctional cells. Recent studies have demonstrated that in vitro expanded MSCs of various origins have great capacity to modulate immune responses and change the progression of different inflammatory diseases. As tissue injuries are often accompanied by inflammation, inflammatory factors may provide cues to mobilize MSCs to tissue sites with damage. Before carrying out tissue repair functions, MSCs first prepare the microenvironment by modulating inflammatory processes and releasing various growth factors in response to the inflammation status. In this review, we focus on the crosstalk between MSCs and immune responses and their potential clinical applications, especially in inflammatory diseases.     

Potential advantages of acute kidney injury management by mesenchymal stem cells

Mesenchymal stem cells are currently considered as a promising tool for therapeutic application in acute kidney injury (AKI) management. AKI is characterized by acute tubular injury with rapid loss of renal function. After AKI, inflammation, oxidative stress and excessive deposition of extracellular matrix are the molecular events that ultimately cause the end-stage renal disease. Despite numerous improvement of supportive therapy, the mortality and morbidity among patients remain high. Therefore, exploring novel therapeutic options to treat AKI is mandatory. Numerous evidence in animal models has demonstrated the capability of mesenchymal stem cells (MSCs) to restore kidney function after induced kidney injury. After infusion, MSCs engraft in the injured tissue and release soluble factors and microvesicles that promote cell survival and tissue repairing. Indeed, the main mechanism of action of MSCs in tissue regeneration is the paracrine/endocrine secretion of bioactive molecules. MSCs can be isolated from several tissues, including bone marrow, adipose tissue, and blood cord; pre-treatment procedures to improve MSCs homing and their paracrine function have been also described. This review will focus on the application of cell therapy in AKI and it will summarize preclinical studies in animal models and clinical trials currently ongoing about the use of mesenchymal stem cells after AKI.     

Placenta Stem/Stromal Cell–Derived Extracellular Vesicles for Potential Use in Lung Repair

Many acute and chronic lung injuries are incurable and rank as the fourth leading cause of death globally. While stem cell treatment for lung injuries is a promising approach, there is growing evidence that the therapeutic efficacy of stem cells originates from secreted extracellular vesicles (EVs). Consequently, EVs are emerging as next‐generation therapeutics. While EVs are extensively researched for diagnostic applications, their therapeutic potential to promote tissue repair is not fully elucidated. By housing and delivering tissue‐repairing cargo, EVs refine the cellular microenvironment, modulate inflammation, and ultimately repair injury. Here, the potential use of EVs derived from two placental mesenchymal stem/stromal cell (MSC) lines is presented; a chorionic MSC line (CMSC29) and a decidual MSC cell line (DMSC23) for applications in lung diseases. Functional analyses using in vitro models of injury demonstrate that these EVs have a role in ameliorating injuries caused to lung cells. It is also shown that EVs promote repair of lung epithelial cells. This study is fundamental to advancing the field of EVs and to unlock the full potential of EVs in regenerative medicine.     

Which place for stem cell therapy in the treatment of acute radiation syndrome?

Radiation-induced (RI) tissue injuries can be caused by radiation therapy, nuclear accidents or radiological terrorism. Notwithstanding the complexity of RI pathophysiology, there are some effective approaches to treatment of both acute and chronic radiation damages. Cytokine therapy is the main strategy capable of preventing or reducing the acute radiation syndrome (ARS), and hematopoietic growth factors (GF) are particularly effective in mitigating bone marrow (BM) aplasia and stimulating hematopoietic recovery. However, first, as a consequence of RI stem and progenitor cell death, use of cytokines should be restricted to a range of intermediate radiation doses (3 to 7 Gy total body irradiation). Second, ARS is a global illness that requires treatment of damages to other tissues (epithelial, endothelial, glial, etc.), which could be achieved using pleiotropic or tissue-specific cytokines. Stem cell therapy (SCT) is a promising approach developed in the laboratory that could expand the ability to treat severe radiation injuries. Allogeneic hematopoietic stem cell transplantation (BM, mobilized peripheral blood and cord blood) transplantation has been used in radiation casualties with variable success due to limiting toxicity related to the degree of graft histocompatibility and combined injuries. Ex vivo expansion should be used to augment cord blood graft size and/or promote very immature stem cells. Autologous SCT might also be applied to radiation casualties from residual hematopoietic stem and progenitor cells (HSPC). Stem cell plasticity of different tissues such as liver or skeletal muscle, may also be used as a source of hematopoietic stem cells. Finally, other types of stem cells such as mesenchymal, endothelial stem cells or other tissue committed stem cells (TCSC), could be used for treating damages to nonhematopoietic organs.     

The use of patient-specific stem cells in different autoimmune diseases

Autoimmune diseases are developed when the immune system mistakenly attacks the body’s cells. These inflammatory disorders can be inherited or triggered by external forces, such as type 1 diabetes, which is caused by the immune system's destruction of pancreatic beta cells. So far, stem cells such as hESC and iPSC have been used to treat autoimmune disorders such as type 1 diabetes, rheumatoid arthritis (RA), multiple sclerosis (MS), and systemic lupus erythematosus (SLE), although these procedures have certain ethical concerns. On the other hand, bone marrow-derived mesenchymal stem cells (BM-MSC) are thought to be the best source of stem cells. Later, it was shown that mesenchymal stem cells produced from autologous adipose tissues have a great potential for producing huge volumes of stem cells. In-vitro and in-vivo investigations using autologous hematopoietic stem cells and autologous mesenchymal stem cells have been carried out on various rodent and human models, while clinical trials for inflammatory diseases such as multiple sclerosis and diabetes mellitus have yielded promising results. We attempted to summarise the usage of diverse stem cells in the therapy of various autoimmune disorders in this review. Shortly, we expect that the use of autologous stem cells will provide a new perspective on the treatment of autoimmune disorders.     

Lipocalin 2 enhances mesenchymal stem cell-based cell therapy in acute kidney injury rat model

Acute kidney injury (AKI) is one of the most common health-threatening diseases in the world. There is still no effective medical treatment for AKI. Recently, Mesenchymal stem cell (MSC)-based therapy has been proposed for treatment of AKI. However, the microenvironment of damaged kidney tissue is not favorable for survival of MSCs which would be used for therapeutic intervention. In this study, we genetically manipulated MSCs to up-regulate lipocalin-2 (Lcn2) and investigated whether the engineered MSCs (MSC-Lcn2) could improve cisplatin-induced AKI in a rat model. Our results revealed that up-regulation of Lcn2 in MSCs efficiently enhanced renal function. MSC Lcn2 up-regulates expression of HGF, IGF, FGF and VEGF growth factors. In addition, they reduced molecular biomarkers of kidney injury such as KIM-1 and Cystatin C, while increased the markers of proximal tubular epithelium such as AQP-1 and CK18 following cisplatin-induced AKI. Overall, here we over-expressed Lcn2, a well-known cytoprotective factor against acute ischemic renal injury, in MSCs. This not only potentiated beneficial roles of MSCs for cell therapy purposes but also suggested a new modality for treatment of AKI.     

间充质干细胞与肿瘤形成的研究进展

间充质干细胞(Mesenchymal stem cells,MSCs)具有独特的免疫调节作用、自我更新和跨胚层多向分化的潜能,存在于许多组织中并活跃地向组织损伤部位迁移,参与伤口修复。在对肿瘤的信号发生反应后,MSCs不断被招募并成为肿瘤微环境的成分。肿瘤相关MSCs(Tumor-associated MSCs, TA-MSCs)在肿瘤发生、促进、进展和转移中有重要作用。本文对MSCs在调节肿瘤细胞的存活、增殖、迁移、药物抵抗中如何发挥作用,以及MSCs对肿瘤微环境免疫状态的影响作一综述。我们强调了MSCs和其他肿瘤基质细胞之间的复杂关系,特别是炎症细胞可以改变肿瘤微环境的免疫状态,以期通过对TA-MSCs进一步的研究来取得对不同肿瘤类型和肿瘤进展不同阶段中肿瘤相关MSCs功能的更好的理解,并优化MSCs来得到更有效和安全的MSCs为基础的肿瘤治疗。MSCs已被有效用于治疗慢性炎性疾病和慢性损伤,因此,其机制方面的研究还有利于在其他疾病中合理利用MSCs从而达到疾病治疗的目的。     

Mesenchymal stem cells as carrier of the therapeutic agent in the gene therapy of blood disorders

Nonhematopoietic stem cells as a delivery platform of therapeutic useful genes have attracted widespread attention in recent years, owing to gained a long lifespan, easy separation, high proliferation, and high transfection capacity. Mesenchymal stem/stromal cells (MSCs) are the choice of the cells for gene and cell therapy due to high self-renewal capacity, high migration rate to the site of the tumor, and with immune suppressive and anti-inflammatory properties. Hence, it has a high potential of safety genetic modification of MSCs for antitumor gene expression and has paved the way for the clinical application of these cells to target the therapy of cancers and other diseases. The aim of gene therapy is targeted treatment of cancers and diseases through recovery, change, or enhancement cell performance to the sustained secretion of useful therapeutic proteins and induction expression of the functional gene in intended tissue. Recent developments in the vectors designing leading to the increase and durability of expression and improvement of the safety of the vectors that overcome a lot of problems, such as durability of expression and the host immune response. Nowadays, gene therapy approach is used by MSCs as a delivery vehicle in the preclinical and the clinical trials for the secretion of erythropoietin, recombinant antibodies, coagulation factors, cytokines, as well as angiogenic inhibitors in many blood disorders like anemia, hemophilia, and malignancies. In this study, we critically discuss the status of gene therapy by MSCs as a delivery vehicle for the treatment of blood disorders. Finally, the results of clinical trial studies are assessed, highlighting promising advantages of this emerging technology in the clinical setting.     

Mesenchymal stem cells in hematopoietic stem cell transplantation

Mesenchymal stromal/stem cells (MSC) of bone marrow (BM) origin not only provide the supportive microenvironmental niche for hematopoietic stem cells (HSC) but are capable of differentiating into various cell types of mesenchymal origin, such as bone, fat and cartilage. In vitro and in vivo data suggest that MSC have low inherent immunogenicity, modulate/suppress immunologic responses through interactions with immune cells, and home to damaged tissues to participate in regeneration processes through their diverse biologic properties. MSC derived from BM are being evaluated for a wide range of clinical applications, including disorders as diverse as myocardial infarction and newly diagnosed diabetes mellitus type 1. However, their use in HSC transplantation, either for enhancement of hematopoietic engraftment or for treatment/prevention of graft-versus-host disease, is far ahead of other indications. Ease of isolation and ex vivo expansion of MSC, combined with their intriguing immunomodulatory properties and their impressive record of safety in a wide variety of clinical trials, make these cells promising candidates for further investigation.     

A matter of life and death: self‐renewal in stem cells

If Narcissus could have self‐renewed even once on seeing his own reflection, he would have died a happy man. Stem cells, on the other hand, have an enormous capacity for self‐renewal; in other words, the ability to replicate and generate more of the same. In adult organisms, stem cells reside in specialized niches within each tissue. They replenish tissue cells that are lost during normal homeostasis, and on injury they repair damaged tissue. The ability of a stem cell to self‐renew is governed by the dynamic interaction between the intrinsic proteins it expresses and the extrinsic signals that it receives from the niche microenvironment. Understanding the mechanisms governing when to proliferate and when to differentiate is vital, not only to normal stem cell biology, but also to ageing and cancer. This review focuses on elucidating conceptually, experimentally and mechanistically, our understanding of adult stem cell self‐renewal. We use skin as a paradigm for discussing many of the salient points about this process, but also draw on the knowledge gained from these and other adult stem cell systems to delineate shared underlying principles, as well as highlight mechanistic distinctions among adult tissue stem cells. By doing so, we pinpoint important questions that still await answers.