Mitochondrial Transfer via Tunneling Nanotubes is an Important Mechanism by Which Mesenchymal Stem Cells Enhance Macrophage Phagocytosis in the In Vitro and In Vivo Models of ARDS

The Necrobiology of Mesenchymal Stromal Cells Affects Therapeutic Efficacy

Rapid progress is occurring in understanding the mechanisms underlying mesenchymal stromal cell (MSC)-based cell therapies (MSCT). However, the results of clinical trials, while demonstrating safety, have been varied in regard to efficacy. Recent data from different groups have shown profound and significant influences of the host inflammatory environment on MSCs delivered systemically or through organ-specific routes, for example intratracheal, with subsequent actions on potential MSC efficacies. Intriguingly in some models, it appears that dead or dying cells or subcellular particles derived from them, may contribute to therapeutic efficacy, at least in some circumstances. Thus, the broad cellular changes that accompany MSC death, autophagy, pre-apoptotic function, or indeed the host response to these processes may be essential to therapeutic efficacy. In this review, we summarize the existing literature concerning the necrobiology of MSCs and the available evidence that MSCs undergo autophagy, apoptosis, transfer mitochondria, or release subcellular particles with effector function in pathologic or inflammatory in vivo environments. Advances in understanding the role of immune effector cells in cell therapy, especially macrophages, suggest that the reprogramming of immunity associated with MSCT has a weighty influence on therapeutic efficacy. If correct, these data suggest novel approaches to enhancing the beneficial actions of MSCs that will vary with the inflammatory nature of different disease targets and may influence the choice between autologous or allogeneic or even xenogeneic cells as therapeutics.

Mitochondrial Dysfunction and Diabetes: Is Mitochondrial Transfer a Friend or Foe?

Obesity, insulin resistance and type 2 diabetes are accompanied by a variety of systemic and tissue-specific metabolic defects, including inflammation, oxidative and endoplasmic reticulum stress, lipotoxicity, and mitochondrial dysfunction. Over the past 30 years, association studies and genetic manipulations, as well as lifestyle and pharmacological invention studies, have reported contrasting findings on the presence or physiological importance of mitochondrial dysfunction in the context of obesity and insulin resistance. It is still unclear if targeting mitochondrial function is a feasible therapeutic approach for the treatment of insulin resistance and glucose homeostasis. Interestingly, recent studies suggest that intact mitochondria, mitochondrial DNA, or other mitochondrial factors (proteins, lipids, miRNA) are found in the circulation, and that metabolic tissues secrete exosomes containing mitochondrial cargo. While this phenomenon has been investigated primarily in the context of cancer and a variety of inflammatory states, little is known about the importance of exosomal mitochondrial transfer in obesity and diabetes. We will discuss recent evidence suggesting that (1) tissues with mitochondrial dysfunction shed their mitochondria within exosomes, and that these exosomes impair the recipient's cell metabolic status, and that on the other hand, (2) physiologically healthy tissues can shed mitochondria to improve the metabolic status of recipient cells. In this context the determination of whether mitochondrial transfer in obesity and diabetes is a friend or foe requires further studies.

Mesenchymal stem cells and their mitochondrial transfer: a double-edged sword

Mitochondrial dysfunction has been linked to many diseases including organ degeneration and cancer. Mesenchymal stem cells/stromal cells (MSCs) provide a valuable source for stem cell-based therapy and represent an emerging therapeutic approach for tissue regeneration. Increasing evidence suggests that MSCs can directly donate mitochondria to recover from cell injury and rescue mitochondrial damage-provoked tissue degeneration. Meanwhile, cancer cells and cancer stromal cells also cross-talk through mitochondrial exchange to regulate cancer metastasis. This review summarizes the research on MSCs and their mitochondrial transfer. It provides an overview of the biology, function, niches and signaling that play a role in tissue repair. It also highlights the pathologies of cancer growth and metastasis linked to mitochondrial exchange between cancer cells and surrounding stromal cells. It becomes evident that the function of MSC mitochondrial transfer is a double-edged sword. MSC mitochondrial transfer may be a pharmaceutical target for tissue repair and cancer therapy.

Mesenchymal stem cell therapy targeting mitochondrial dysfunction in acute kidney injury

Mitochondria take part in a network of cellular processes that regulate cell homeostasis. Defects in mitochondrial function are key pathophysiological changes during acute kidney injury (AKI). Mesenchymal stem cells (MSCs) have shown promising regenerative effects in experimental AKI models, but the specific mechanism is still unclear. Some studies have demonstrated that MSCs are able to target mitochondrial dysfunction during AKI. In this review, we summarize these articles, providing an integral and updated view of MSC therapy targeting mitochondrial dysfunction during AKI, which is aimed at promoting the therapeutic effect of MSCs in AKI patients.

Human Bone Marrow Mesenchymal Stem Cells Rescue Endothelial Cells Experiencing Chemotherapy Stress by Mitochondrial Transfer Via Tunneling Nanotubes

Tunneling nanotubes (TNTs) are newly discovered tubular structures between two distant cells that facilitate the intercellular exchange of signals and components. Recent reports show that mesenchymal stem cells (MSCs) can rescue injured target cells and promote recovery from a variety of stresses via TNT-mediated mitochondrial transfer. In this study, we explored how TNTs form between bone marrow MSCs and endothelial cells (ECs) by using a human umbilical cord vein endothelial cell (HUVEC) model. TNT formation between MSCs and HUVECs could be induced by treating HUVECs with cytarabine (Ara-C), and human bone marrow mesenchymal stem cells (hBMMSCs) could transfer mitochondria to injured HUVECs through TNTs. Mitochondrial transfer from hBMMSCs to HUVECs via TNTs rescued the injured HUVECs by reducing apoptosis, promoting proliferation and restoring the transmembrane migration ability as well as the capillary angiogenic capacity of HUVECs. This study provides novel insights into the cell-cell communication between MSCs and ECs and supports the results of prior studies indicating that ECs promote hematopoietic regeneration. An improved understanding of MSC-EC cross-talk will promote the development of MSC-directed strategies for improving EC function and hematopoietic system regeneration following myelosuppressive and myeloablative injuries.

Mitochondrial bioenergetics and pulmonary dysfunction: Current progress and future directions

This review summarizes current understanding of mitochondrial bioenergetic dysfunction applicable to mechanisms of lung diseases and outlines challenges and future directions in this rapidly emerging field. Although the role of mitochondria extends beyond the term of cellular "powerhouse", energy generation remains the most fundamental function of these organelles. It is not counterintuitive to propose that intact energy supply is important for favorable cellular fate following pulmonary insult. In this review, the discussion of mitochondrial dysfunction focuses on those molecular mechanisms that alter cellular bioenergetics in the lungs: (a) inhibition of mitochondrial respiratory chain, (b) mitochondrial leak and uncoupling, (c) alteration of mitochondrial Ca²⁺ handling, (d) mitochondrial production of reactive oxygen species and self-oxidation. The discussed lung diseases were selected according to their pathological nature and relevance to pediatrics: Acute lung injury (ALI), defined as acute parenchymal lung disease associated with cellular demise and inflammation (Acute Respiratory Distress Syndrome, ARDS, Pneumonia), alveolar developmental failure (Bronchopulmonary Dysplasia, BPD or chronic lung disease in premature infants), obstructive airway diseases (Bronchial asthma) and vascular remodeling affecting pulmonary circulation (Pulmonary Hypertension, PH). The analysis highlights primary mechanisms of mitochondrial bioenergetic dysfunction contributing to the disease-specific pulmonary insufficiency and proposes potential therapeutic targets.

Mesenchymal Stromal Cell Exosomes Ameliorate Experimental Bronchopulmonary Dysplasia and Restore Lung Function through Macrophage Immunomodulation

RATIONALE

Mesenchymal stem/stromal cell (MSC) therapies have shown promise in preclinical models of pathologies relevant to newborn medicine, such as bronchopulmonary dysplasia (BPD). We have reported that the therapeutic capacity of MSCs is comprised in their secretome, and demonstrated that the therapeutic vectors are exosomes produced by MSCs (MSC-exos).

OBJECTIVES

To assess efficacy of MSC-exo treatment in a preclinical model of BPD and to investigate mechanisms underlying MSC-exo therapeutic action.

METHODS

Exosomes were isolated from media conditioned by human MSC cultures. Newborn mice were exposed to hyperoxia (HYRX; 75% O₂), treated with exosomes on Postnatal Day (PN) 4 and returned to room air on PN7. Treated animals and appropriate controls were harvested on PN7, -14, or -42 for assessment of pulmonary parameters.

MEASUREMENTS AND MAIN RESULTS

HYRX-exposed mice presented with pronounced alveolar simplification, fibrosis, and pulmonary vascular remodeling, which was effectively ameliorated by MSC-exo treatment. Pulmonary function tests and assessment of pulmonary hypertension showed functional improvements after MSC-exo treatment. Lung mRNA sequencing demonstrated that MSC-exo treatment induced pleiotropic effects on gene expression associated with HYRX-induced inflammation and immune responses. MSC-exos modulate the macrophage phenotype fulcrum, suppressing the proinflammatory "M1" state and augmenting an antiinflammatory "M2-like" state, both in vitro and in vivo.

CONCLUSIONS

MSC-exo treatment blunts HYRX-associated inflammation and alters the hyperoxic lung transcriptome. This results in alleviation of HYRX-induced BPD, improvement of lung function, decrease in fibrosis and pulmonary vascular remodeling, and amelioration of pulmonary hypertension. The MSC-exo mechanism of action is associated with modulation of lung macrophage phenotype.

Hypercapnic acidosis induces mitochondrial dysfunction and impairs the ability of mesenchymal stem cells to promote distal lung epithelial repair

Acute respiratory distress syndrome (ARDS) is a devastating disorder characterized by diffuse inflammation and edema formation. The main management strategy, low tidal volume ventilation, can be associated with the development of hypercapnic acidosis (HCA). Mesenchymal stem cells (MSCs) are a promising therapeutic candidate currently in early-phase clinical trials. The effects of HCA on the alveolar epithelium and capillary endothelium are not well established. The therapeutic efficacy of MSCs has never been reported in HCA. In the present study, we evaluated the effects of HCA on inflammatory response and reparative potential of the primary human small airway epithelial and lung microvasculature endothelial cells as well as on the capacity of bone marrow-derived MSCs to promote wound healing in vitro. We demonstrate that HCA attenuates the inflammatory response and reparative potential of primary human small airway epithelium and capillary endothelium and induces mitochondrial dysfunction. It was found that MSCs promote lung epithelial wound repair via the transfer of functional mitochondria; however, this proreparative effect of MSCs was lost in the setting of HCA. Therefore, HCA may adversely impact recovery from ARDS at the cellular level, whereas MSCs may not be therapeutically beneficial in patients with ARDS who develop HCA.-Fergie, N., Todd, N., McClements, L., McAuley, D., O'Kane, C., Krasnodembskaya, A. Hypercapnic acidosis induces mitochondrial dysfunction and impairs the ability of mesenchymal stem cells to promote distal lung epithelial repair.

Mitochondria transfer from mesenchymal stem cells structurally and functionally repairs renal proximal tubular epithelial cells in diabetic nephropathy in vivo

The underlying therapeutic mechanism of renal tubular epithelium repair of diabetic nephropathy (DN) by bone marrow-derived mesenchymal stem cells (BM-MSCs) has not been fully elucidated. Recently, mitochondria (Mt) transfer was reported as a novel action of BM-MSCs to rescue injured cells. We investigated Mt transfer from systemically administered BM-MSCs to renal proximal tubular epithelial cells (PTECs) in streptozotocin (STZ)-induced diabetic animals. BM-MSCs also transferred their Mt to impaired PTECs when co-cultured in vitro, which suppressed apoptosis of impaired PTECs. Additionally, BM-MSC-derived isolated Mt enhanced the expression of mitochondrial superoxide dismutase 2 and Bcl-2 expression and inhibited reactive oxygen species (ROS) production in vitro. Isolated Mt also inhibited nuclear translocation of PGC-1α and restored the expression of megalin and SGLT2 under high glucose condition (HG) in PTECs. Moreover, isolated Mt directly injected under the renal capsule of STZ rats improved the cellular morphology of STZ-PTECs, and the structure of the tubular basement membrane and brush border in vivo. This study is the first to show Mt transfer from systemically administered BM-MSCs to damaged PTECs in vivo, and the first to investigate mechanisms underlying the potential therapeutic effects of Mt transfer from BM-MSCs in DN.

Exosomes in Allergic Airway Diseases

PURPOSE OF REVIEW

This review will cover what is known regarding exosomes and allergy, and furthermore discuss novel mechanism of exosome-mediated immune modulation and metabolic regulation via the transfer of mitochondria.

RECENT FINDINGS

Exosomes are nano-sized extracellular vesicles (EVs) derived from the endosome that play a direct role in governing physiological and pathological conditions by transferring bioactive cargo such as proteins, enzymes, nucleic acids (miRNA, mRNA, DNA), and metabolites. Recent evidence suggest that exosomes may signal in autocrine but, most importantly, in paracrine and endocrine manner, being taken up by neighboring cells or carried to distant sites. Exosomes also mediate immunogenic responses, such as antigen presentation and inflammation. In asthma and allergy, exosomes facilitate cross-talk between immune and epithelial cells, and drive site-specific inflammation through the generation of pro-inflammatory mediators like leukotrienes. Recent studies suggest that myeloid cell-generated exosomes transfer mitochondria to lymphocytes. Exosomes are nano-sized mediators of the immune system which can modulate responses through antigen presentation, and the transfer of pro- and anti-inflammatory mediators. In addition to conventional mechanisms of immune modulation, exosomes may act as a novel courier of functional mitochondria that is capable of modulating the recipient cells bioenergetics, resulting in altered cellular responses. The transfer of mitochondria and modulation of bioenergetics may result in immune activation or dampening depending on the context.

Mitochondrial Transfer from Bone Marrow Mesenchymal Stem Cells to Motor Neurons in Spinal Cord Injury Rats via Gap Junction

Recent studies have demonstrated that bone marrow mesenchymal stem cells (BMSCs) protect the injured neurons of spinal cord injury (SCI) from apoptosis while the underlying mechanism of the protective effect of BMSCs remains unclear. In this study, we found the transfer of mitochondria from BMSCs to injured motor neurons and detected the functional improvement after transplanting. Methods: Primary rat BMSCs were co-cultured with oxygen-glucose deprivation (OGD) injured VSC4.1 motor neurons or primary cortical neurons. FACS analysis was used to detect the transfer of mitochondria from BMSCs to neurons. The bioenergetics profiling of neurons was detected by Extracellular Flux Analysis. Cell viability and apoptosis were also measured. BMSCs and isolated mitochondria were transplanted into SCI rats. TdT-mediated dUTP nick end labelling staining was used to detect apoptotic neurons in the ventral horn. Immunohistochemistry and Western blotting were used to measure protein expression. Re-myelination was examined by transmission electron microscope. BBB scores were used to assess locomotor function. Results: MitoTracker-Red labelled mitochondria of BMSCs could be transferred to the OGD injured neurons. The gap junction intercellular communication (GJIC) potentiator retinoid acid increased the quantity of mitochondria transfer from BMSCs to neurons, while GJIC inhibitor 18β glycyrrhetinic acid decreased mitochondria transfer. Internalization of mitochondria improved the bioenergetics profile, decreased apoptosis and promoted cell survival in post-OGD motor neurons. Furthermore, both transplantation of mitochondria and BMSCs to the injured spinal cord improved locomotor functional recovery in SCI rats. Conclusions: To our knowledge, this is the first evidence that BMSCs protect against SCI through GJIC to transfer mitochondrial to the injured neurons. Our findings suggested a new therapy strategy of mitochondria transfer for the patients with SCI.

The chaperone ERp29 is required for tunneling nanotube formation by stabilizing MSec

Tunneling nanotubes (TNTs) are membrane conduits that mediate long-distance intercellular cross-talk in several organisms and play vital roles during development, pathogenic transmission, and cancer metastasis. However, the molecular mechanisms of TNT formation and function remain poorly understood. The protein MSec (also known as TNFα-induced protein 2 (TNFAIP2) and B94) is essential for TNT formation in multiple cell types. Here, using affinity protein purification, mass spectrometric identification, and confocal immunofluorescence microscopy assays, we found that MSec interacts with the endoplasmic reticulum (ER) chaperone ERp29. siRNA-mediated ERp29 depletion in mammalian cells significantly reduces TNT formation, whereas its overexpression induces TNT formation, but in a strictly MSec-dependent manner. ERp29 stabilized MSec protein levels, but not its mRNA levels, and the chaperone activity of ERp29 was required for maintaining MSec protein stability. Subcellular ER fractionation and subsequent limited proteolytic treatment suggested that MSec is associated with the outer surface of the ER. The ERp29-MSec interaction appeared to require the presence of other bridging protein(s), perhaps triggered by post-translational modification of ERp29. Our study implicates MSec as a target of ERp29 and reveals an indispensable role for the ER in TNT formation, suggesting new modalities for regulating TNT numbers in cells and tissues.

Platelet lysate outperforms FCS and human serum for co-culture of primary human macrophages and hMSCs

In vitro co-cultures of different primary human cell types are pivotal for the testing and evaluation of biomaterials under conditions that are closer to the human in vivo situation. Especially co-cultures of macrophages and mesenchymal stem cells (MSCs) are of interest, as they are both present and involved in tissue regeneration and inflammatory reactions and play crucial roles in the immediate inflammatory reactions and the onset of regenerative processes, thus reflecting the decisive early phase of biomaterial contact with the host. A co-culture system of these cell types might thus allow for the assessment of the biocompatibility of biomaterials. The establishment of such a co-culture is challenging due to the different in vitro cell culture conditions. For human macrophages, medium is usually supplemented with human serum (hS), whereas hMSC culture is mostly performed using fetal calf serum (FCS), and these conditions are disadvantageous for the respective other cell type. We demonstrate that human platelet lysate (hPL) can replace hS in macrophage cultivation and appears to be the best option for co-cultivation of human macrophages with hMSCs. In contrast to FCS and hS, hPL maintained the phenotype of both cell types, comparable to that of their respective standard culture serum, as well as the percentage of each cell population. Moreover, the expression profile and phagocytosis activity of macrophages was similar to hS.

Forms of extracellular mitochondria and their impact in health

Mitochondria play an important role as an intracellular energy plant and signaling organelle. However, mitochondria also exist outside cells where they could mediate cell-to-cell communication, repair and serve as an activator of the immune response. Their effects depend on the mitochondrial state or the form in which it is present, either as a whole functional structure as fragments or only as mitochondrial DNA. Herein, we provide evidence of why extracellular mitochondria and their varying forms are considered regenerative factors or pro-inflammatory activators. Understanding these aspects will provide the base of their use in therapy or as a biomarker of disease severity and prognosis.

Pathogenesis of Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a syndrome of acute respiratory failure caused by noncardiogenic pulmonary edema. Despite five decades of basic and clinical research, there is still no effective pharmacotherapy for this condition and the treatment remains primarily supportive. It is critical to study the molecular and physiologic mechanisms that cause ARDS to improve our understanding of this syndrome and reduce mortality. The goal of this review is to describe our current understanding of the pathogenesis and pathophysiology of ARDS. First, we will describe how pulmonary edema fluid accumulates in ARDS due to lung inflammation and increased alveolar endothelial and epithelial permeabilities. Next, we will review how pulmonary edema fluid is normally cleared in the uninjured lung, and describe how these pathways are disrupted in ARDS. Finally, we will explain how clinical trials and preclinical studies of novel therapeutic agents have further refined our understanding of this condition, highlighting, in particular, the study of mesenchymal stromal cells in the treatment of ARDS.

Clinical Application of Stem/Stromal Cells in COPD

Chronic obstructive pulmonary disease (COPD) is a progressive life-threatening disease that is significantly increasing in prevalence and is predicted to become the third leading cause of death worldwide by 2030. At present, there are no true curative treatments that can stop the progression of the disease, and new therapeutic strategies are desperately needed. Advances in cell-based therapies provide a platform for the development of new therapeutic approaches in severe lung diseases such as COPD. At present, a lot of focus is on mesenchymal stem (stromal) cell (MSC)-based therapies, mainly due to their immunomodulatory properties. Despite increasing number of preclinical studies demonstrating that systemic MSC administration can prevent or treat experimental COPD and emphysema, clinical studies have not been able to reproduce the preclinical results and to date no efficacy or significantly improved lung function or quality of life has been observed in COPD patients. Importantly, the completed appropriately conducted clinical trials uniformly demonstrate that MSC treatment in COPD patients is well tolerated and no toxicities have been observed. All clinical trials performed so far, have been phase I/II studies, underpowered for the detection of potential efficacy. There are several challenges ahead for this field such as standardized isolation and culture procedures to obtain a cell product with high quality and reproducibility, administration strategies, improvement of methods to measure outcomes, and development of potency assays. Moreover, COPD is a complex pathology with a diverse spectrum of clinical phenotypes, and therefore it is essential to develop methods to select the subpopulation of patients that is most likely to potentially respond to MSC administration. In this chapter, we will discuss the current state of the art of MSC-based cell therapy for COPD and the hurdles that need to be overcome.

Mitochondrial transfer from mesenchymal stem cells to neural stem cells protects against the neurotoxic effects of cisplatin

Mesenchymal stem cells (MSCs) transfer healthy mitochondria to damaged acceptor cells via actin-based intercellular structures. In this study, we tested the hypothesis that MSCs transfer mitochondria to neural stem cells (NSCs) to protect NSCs against the neurotoxic effects of cisplatin treatment. Our results show that MSCs donate mitochondria to NSCs damaged in vitro by cisplatin. Transfer of healthy MSC-derived mitochondria decreases cisplatin-induced NSC death. Moreover, mitochondrial transfer from MSCs to NSCs reverses the cisplatin-induced decrease in mitochondrial membrane potential. Blocking the formation of actin-based intercellular structures inhibited the transfer of mitochondria to NSCs and abrogated the positive effects of MSCs on NSC survival. Conversely, overexpression of the mitochondrial motor protein Rho-GTPase 1 (Miro1) in MSCs increased mitochondrial transfer and further improved survival of cisplatin-treated NSCs.

In vivo, MSC administration prevented the loss of DCX+ neural progenitor cells in the subventricular zone and hippocampal dentate gyrus which occurs as a result of cisplatin treatment. We propose mitochondrial transfer as one of the mechanisms via which MSCs exert their therapeutic regenerative effects after cisplatin treatment.

Concise Review: Intercellular Communication Via Organelle Transfer in the Biology and Therapeutic Applications of Stem Cells

The therapeutic potential of stem cell-based therapies may be largely dependent on the ability of stem cells to modulate host cells rather than on their differentiation into host tissues. Within the last decade, there has been considerable interest in the intercellular communication mediated by the transfer of cytoplasmic material and organelles between cells. Numerous studies have shown that mitochondria and lysosomes are transported between cells by various mechanisms, such as tunneling nanotubes, microvesicles, and cellular fusion. This review will focus on the known instances of organelle transfer between stem cells and differentiated cells, what effects it has on recipient cells and how organelle transfer is regulated. Stem Cells 2019;37:14-25.

Human tissue-specific MSCs demonstrate differential mitochondria transfer abilities that may determine their regenerative abilities

BACKGROUND

Recent studies have demonstrated mesenchymal stem cells (MSCs) as effective mitochondrial donors with therapeutic success in multiple experimental models of human disease. MSCs obtained from different tissue sources such as bone marrow (BM), adipose (AD), dental pulp (DP), and Wharton's jelly (WJ) are routinely used in clinical trials with no known study of their mitochondrial donor capacity. Here, we show for the first time that MSCs derived from different tissue sources have different mitochondrial donor properties and that this is correlated with their intrinsic respiratory states.

METHODS

MitoTracker^®-labeled MSCs were co-cultured with Cell Trace-labeled U87-MG cells or rat cardiomyocytes. Mitochondrial transfer abilities of MSCs were assessed by using flow cytometry analysis and fluorescence imaging. Mitochondrial reactive oxygen species (mtROS) levels were analyzed by using MitoSOX red-based staining, and mitochondrial respiration parameters were analyzed by using a Seahorse XF Analyzer.

RESULTS

AD-MSCs and BM-MSCs displayed higher mitochondrial transfer than DP-MSCs and WJ-MSCs. Counterintuitively, DP-MSCs and WJ-MSCs were more effective in suppressing mtROS levels in stressed recipient cells than AD-MSCs or BM-MSCs. Interestingly, the oxygen consumption rates and intrinsic mitochondrial respiration parameters like ATP levels, basal and maximal respiration, and mitochondrial DNA copy number in donor MSCs showed a highly significant inverse correlation with their mitochondrial donation.

CONCLUSIONS

We find that there are intrinsic differences in the mitochondrial respiration, donation capacity, and therapeutic efficacy among MSCs of different tissue origin. MSCs with high mitochondrial respiration capacities are associated with lower mitochondrial transfer but more effective suppression of mtROS in stressed recipient cells. This is most compatible with a model where recipient cells optimally regulate mitochondrial transfer such that they take more mitochondria from MSCs with lower mitochondrial function. Furthermore, it appears to be advantageous to use MSCs such as DP-MSCs or WJ-MSCs with higher mitochondrial respiratory abilities that achieved better therapeutic effect with lower mitochondrial transfer in our study. This opens up a new direction in stem cell therapeutics.

Therapeutic use of mesenchymal stem cell-derived extracellular vesicles in acute lung injury

Acute respiratory distress syndrome is a major cause of respiratory failure in critically ill patients. Despite extensive research into its pathophysiology, mortality remains high. No effective pharmacotherapy exists. Based largely on numerous preclinical animal studies, administration of mesenchymal stem or stromal cell (MSC) as a therapeutic for acute lung injury (ALI) holds great promise, and Phase I and II clinical trials are currently under way internationally. However, concern for the use of stem cells, specifically the risk of iatrogenic tumor formation, as well as the prohibitive cost of production, storage, and distribution of cells in bone marrow transplant facilities, may limit access to this lifesaving therapy. Accumulating evidence now suggest that novel stem cell-derived therapies, including MSC-conditioned medium and extracellular vesicles (EVs) released from MSCs, might constitute compelling alternatives. The current review summarizes the preclinical studies testing MSC EVs as treatment for ALI and other inflammatory lung diseases. While certain logistic obstacles limit the clinical applications of MSC-conditioned medium such as the volume required for treatment and lack of standardization of what constitutes the components of conditioned medium, the therapeutic application of MSC EVs remains promising, primarily due to ability of EVs to maintain the functional phenotype of the parent cell. However, utilization of MSC EVs will require large-scale production and standardization concerning identification, characterization, and quantification.

Direct Observation of Tunneling Nanotubes within Human Mesenchymal Stem Cell Spheroids

Tunneling nanotubes (TNTs) play an important role in cell-cell communication. TNTs have been predominantly reported among cells in monolayer culture. Using various imaging modalities, including scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM), this work reports the finding of TNTs between cells within human mesenchymal stem cell (MSC) spheroids. TNTs visualized by SEM are consistent in size and geometry with those observed in cellular monolayer culture. LSCM imaging of living spheroids confirms the presence of F-actin filaments within the TNTs, which are known to maintain nanotube integrity. In addition, LSCM revealed the distribution of F-actin fibers across the entire spheroid body instead of being confined within individual cells. Intracellular material transport by TNTs was tested in MSC spheroids treated with cytochalasin D (CytoD), a known actin polymerization inhibitor for disrupting TNT formation. CytoD treatment decreased the transport of cytosolic material by at least four-fold compared to untreated spheroids. To the best of our knowledge, this work represents the first direct observation of TNTs within MSC spheroids. These findings offer new physical insight into cellular interactions within spheroids, providing structural information for increasing interests in spheroid-based cell therapy.

Safety and Efficacy of Adult Stem Cell Therapy for Acute Myocardial Infarction and Ischemic Heart Failure (SafeCell Heart): A Systematic Review and Meta-Analysis

Preclinical and clinical evidence suggests that mesenchymal stem cells (MSCs) may be beneficial in treating both acute myocardial infarction (AMI) and ischemic heart failure (IHF). However, the safety profile and efficacy of MSC therapy is not well‐known. We conducted a systematic review of clinical trials that evaluated the safety or efficacy of MSCs for AMI or IHF. Embase, PubMed/Medline, and Cochrane Central Register of Controlled Trials were searched from inception to September 27, 2017. Studies that examined the use of MSCs administered to adults with AMI or IHF were eligible. The Cochrane risk of bias tool was used to assess bias of included studies. The primary outcome was safety assessed by adverse events and the secondary outcome was efficacy which was assessed by mortality and left ventricular ejection fraction (LVEF). A total of 668 citations were reviewed and 23 studies met eligibility criteria. Of these, 11 studies evaluated AMI and 12 studies evaluated IHF. There was no association between MSCs and acute adverse events. There was a significant improvement in overall LVEF in patients who received MSCs (SMD 0.73, 95% CI 0.24–1.21). No significant difference in mortality was noted (Peto OR 0.68, 95% CI 0.38–1.22). Results from our systematic review suggest that MSC therapy for ischemic heart disease appears to be safe. There is a need for a well‐designed adequately powered randomized control trial (with rigorous adverse event reporting and evaluations of cardiac function) to further establish a clear risk‐benefit profile of MSCs. Stem Cells Translational Medicine2018;7:857–866

Cell therapy in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is driven by a severe pro-inflammatory response resulting in lung damage, impaired gas exchange and severe respiratory failure. There is no specific treatment that effectively improves outcome in ARDS. However, in recent years, cell therapy has shown great promise in preclinical ARDS studies. A wide range of cells have been identified as potential candidates for use, among these are mesenchymal stromal cells (MSCs), which are adult multi-lineage cells that can modulate the immune response and enhance repair of damaged tissue. The therapeutic potential of MSC therapy for sepsis and ARDS has been demonstrated in multiple in vivo models. The therapeutic effect of these cells seems to be due to two different mechanisms; direct cellular interaction, and paracrine release of different soluble products such as extracellular vesicles (EVs)/exosomes. Different approaches have also been studied to enhance the therapeutic effect of these cells, such as the over-expression of anti-inflammatory or pro-reparative molecules. Several clinical trials (phase I and II) have already shown safety of MSCs in ARDS and other diseases. However, several translational issues still need to be addressed, such as the large-scale production of cells, and their potentiality and variability, before the therapeutic potential of stem cells therapies can be realized.

Intercellular Communication in Tumor Biology: A Role for Mitochondrial Transfer

Intercellular communication between cancer cells and other cells in the tumor microenvironment plays a defining role in tumor development. Tumors contain infiltrates of stromal cells and immune cells that can either promote or inhibit tumor growth, depending on the cytokine/chemokine milieu of the tumor microenvironment and their effect on cell activation status. Recent research has shown that stromal cells can also affect tumor growth through the donation of mitochondria to respiration-deficient tumor cells, restoring normal respiration. Nuclear and mitochondrial DNA mutations affecting mitochondrial respiration lead to some level of respiratory incompetence, forcing cells to generate more energy by glycolysis. Highly glycolytic cancer cells tend to be very aggressive and invasive with poor patient prognosis. However, purely glycolytic cancer cells devoid of mitochondrial DNA cannot form tumors unless they acquire mitochondrial DNA from adjacent cells. This perspective article will address this apparent conundrum of highly glycolytic cells and cover aspects of intercellular communication between tumor cells and cells of the microenvironment with particular emphasis on intercellular mitochondrial transfer.

Genetic Modification of Mesenchymal Stem Cells Overexpressing Angiotensin II Type 2 Receptor Increases Cell Migration to Injured Lung in LPS-Induced Acute Lung Injury Mice

Although mesenchymal stem cells (MSCs) transplantation has been shown to promote the lung respiration in acute lung injury (ALI) in vivo, its overall restorative capacity appears to be restricted mainly because of low retention in the injured lung. Angiotensin II (Ang II) are upregulated in the injured lung. Our previous study showed that Ang II increased MSCs migration via Ang II type 2 receptor (AT2R). To determine the effect of AT2R in MSCs on their cell migration after systemic injection in ALI mice, a human AT2R expressing lentiviral vector and a lentivirus vector carrying AT2R shRNA were constructed and introduced into human bone marrow MSCs. A mouse model of lipopolysaccharide‐induced ALI was used to investigate the migration of AT2R‐regulated MSCs and the therapeutic potential in vivo. Overexpression of AT2R dramatically increased Ang II‐enhanced human bone marrow MSC migration in vitro. Moreover, MSC‐AT2R accumulated in the damaged lung tissue at significantly higher levels than control MSCs 24 and 72 hours after systematic MSC transplantation in ALI mice. Furthermore, MSC‐AT2R‐injected ALI mice exhibited a significant reduction of pulmonary vascular permeability and improved the lung histopathology and had additional anti‐inflammatory effects. In contrast, there were less lung retention in MSC‐ShAT2R‐injected ALI mice compared with MSC‐Shcontrol after transplantation. Thus, MSC‐ShAT2R‐injected group exhibited a significant increase of pulmonary vascular permeability and resulted in a deteriorative lung inflammation. Our results demonstrate that overexpression of AT2R enhance the migration of MSCs in ALI mice and may provide a new therapeutic strategy for ALI. Stem Cells Translational Medicine2018;7:721–730

Clinical-grade mesenchymal stem cells derived from umbilical cord improve septic shock in pigs

BACKGROUND

Septic shock is the leading cause of death in intensive care units. The pathophysiological complexity of this syndrome contributes to an absence of specific treatment. Several preclinical studies in murine models of septic shock have shown improvements to organ injury and survival after administration of mesenchymal stem cells (MSCs). To better mimic a clinical approach in humans, we investigated the impact of randomized controlled double-blind administration of clinical-grade umbilical cord-derived MSCs to a relevant pig model of septic shock.

METHODS

Septic shock was induced by fecal peritonitis in 12 male domestic pigs. Animals were resuscitated by an experienced intensivist including fluid administration and vasopressors. Four hours after the induction of peritonitis, pigs were randomized to receive intravenous injection of thawed umbilical cord-derived MSCs (UCMSC) (1 × 106 UCMSCs/kg diluted in 75 mL hydroxyethyl starch (HES), (n = 6) or placebo (HES alone, n = 6). Researchers were double-blinded to the treatment administered. Hemodynamic parameters were continuously recorded. Gas exchange, acid-base status, organ function, and plasma cytokine concentrations were assessed at regular intervals until 24 h after the onset of peritonitis when animals were sacrificed under anesthesia.

RESULTS

Peritonitis induced profound hypotension, hyperlactatemia, and multiple organ failure. These disorders were significantly attenuated when animals were treated with UCMSCs. In particular, cardiovascular failure was attenuated, as attested by a better mean arterial pressure and reduced lactatemia, despite lower norepinephrine requirements. As such, UCMSCs improved survival in this very severe model (60% survival vs. 0% at 24 h).

CONCLUSION

UCMSCs administration is beneficial in this pig model of polymicrobial septic shock.

Specialized Intercellular Communications via Cytonemes and Nanotubes

In recent years, thin membrane protrusions such as cytonemes and tunneling nanotubes have emerged as a novel mechanism of intercellular communication. Protrusion-based cellular interactions allow for specific communication between participating cells and have a distinct spectrum of advantages compared to secretion- and diffusion-based intercellular communication. Identification of protrusion-based signaling in diverse systems suggests that this mechanism is a ubiquitous and prevailing means of communication employed by many cell types. Moreover, accumulating evidence indicates that protrusion-based intercellular communication is often involved in pathogenesis, including cancers and infections. Here we review our current understanding of protrusion-based intercellular communication.

The role of metabolism and tunneling nanotube-mediated intercellular mitochondria exchange in cancer drug resistance

Intercellular communications play a major role in tissue homeostasis. In pathologies such as cancer, cellular interactions within the tumor microenvironment (TME) contribute to tumor progression and resistance to therapy. Tunneling nanotubes (TNTs) are newly discovered long-range intercellular connections that allow the exchange between cells of various cargos, ranging from ions to whole organelles such as mitochondria. TNT-transferred mitochondria were shown to change the metabolism and functional properties of recipient cells as reported for both normal and cancer cells. Metabolic plasticity is now considered a hallmark of cancer as it notably plays a pivotal role in drug resistance. The acquisition of cancer drug resistance was also associated to TNT-mediated mitochondria transfer, a finding that relates to the role of mitochondria as a hub for many metabolic pathways. In this review, we first give a brief overview of the various mechanisms of drug resistance and of the cellular communication means at play in the TME, with a special focus on the recently discovered TNTs. We further describe recent studies highlighting the role of the TNT-transferred mitochondria in acquired cancer cell drug resistance. We also present how changes in metabolic pathways, including glycolysis, pentose phosphate and lipid metabolism, are linked to cancer cell resistance to therapy. Finally, we provide examples of novel therapeutic strategies targeting mitochondria and cell metabolism as a way to circumvent cancer cell drug resistance.

Bone Marrow-Derived Mesenchymal Stem Cells Exert Diverse Effects on Different Macrophage Subsets

Mesenchymal stem cells (MSCs) and their secreted molecules have shown great potential for tissue regeneration and the treatment of inflammation and autoimmune diseases. However, they can also be associated with therapeutic failure or even side effects. Possible causes for this could include the state of the stem cells themselves and the influence of the local microenvironment, wherein macrophages play important roles. As such, we utilized conditioned medium from bone marrow-derived MSCs (MSC-CM) and studied its effect on different macrophage subsets. Effects on macrophage proliferation, apoptosis, polarization, and phagocytosis were determined, and it was discovered that MSC-CM had no significant effect on macrophage proliferation but inhibited M0 macrophage apoptosis and marginally induced M1 macrophage apoptosis. MSC-CM was shown to reduce CD80 expression on the surface of M1 macrophages. Moreover, it promoted and inhibited CD163 expression on the surface of M0 and M1 macrophages, respectively. However, MSC-CM tended to initially promote CD163 expression on M2 macrophages but inhibited expression of this marker after additional incubation time. Unlike MSCs, MSC-CM had no significant effect on the expression of TNF-α and IL-10 in macrophages. Thus, the effect of MSC-CM on different types of macrophages is different, and after stem cells are implanted, their effects on the local immune microenvironment are closely related to the original immune status of the implantation site. Therefore, we suggest that when utilizing stem cells for therapeutics, the immune status of the treatment site should be fully elucidated.

Protective Effects of Endothelial Progenitor Cell-Derived Extracellular Mitochondria in Brain Endothelium

Endothelial progenitor cells (EPCs) have been pursued as a potential cellular therapy for stroke and central nervous system injury. However, their underlying mechanisms remain to be fully defined. Recent experimental studies suggest that mitochondria may be released and transferred between cells. In this proof-of-concept study, we asked whether beneficial effects of EPCs may partly involve a mitochondrial phenomenon as well. First, EPC-derived conditioned medium was collected and divided into supernatant and particle fractions after centrifugation. Electron microscopy, Western blots, and flow cytometry showed that EPCs were able to release mitochondria. ATP and oxygen consumption assays suggested that these extracellular mitochondria may still be functionally viable. Confocal microscopy confirmed that EPC-derived extracellular mitochondria can be incorporated into normal brain endothelial cells. Adding EPC particles to brain endothelial cells promoted angiogenesis and decreased the permeability of brain endothelial cells. Next, we asked whether EPC-derived mitochondria may be protective. As expected, oxygen-glucose deprivation (OGD) increased brain endothelial permeability. Adding EPC-derived mitochondria particles to the damaged brain endothelium increased levels of mitochondrial protein TOM40, mitochondrial DNA copy number, and intracellular ATP. Along with these indirect markers of mitochondrial transfer, endothelial tightness was also restored after OGD. Taken together, these findings suggest that EPCs may support brain endothelial energetics, barrier integrity, and angiogenic function partly through extracellular mitochondrial transfer. Stem Cells 2018;36:1404-1410.

Exosomal transfer of mitochondria from airway myeloid-derived regulatory cells to T cells

Chronic inflammation involving both innate and adaptive immune cells is implicated in the pathogenesis of asthma. Intercellular communication is essential for driving and resolving inflammatory responses in asthma. Emerging studies suggest that extracellular vesicles (EVs) including exosomes facilitate this process. In this report, we have used a range of approaches to show that EVs contain markers of mitochondria derived from donor cells which are capable of sustaining a membrane potential. Further, we propose that these participate in intercellular communication within the airways of human subjects with asthma. Bronchoalveolar lavage fluid of both healthy volunteers and asthmatics contain EVs with encapsulated mitochondria; however, the % HLA-DR⁺ EVs containing mitochondria and the levels of mitochondrial DNA within EVs were significantly higher in asthmatics. Furthermore, mitochondria are present in exosomes derived from the pro-inflammatory HLA-DR⁺ subsets of airway myeloid-derived regulatory cells (MDRCs), which are known regulators of T cell responses in asthma. Exosomes tagged with MitoTracker Green, or derived from MDRCs transduced with CellLight Mitochondrial GFP were found in recipient peripheral T cells using a co-culture system, supporting direct exosome-mediated cell-cell transfer. Importantly, exosomally transferred mitochondria co-localize with the mitochondrial network and generate reactive oxygen species within recipient T cells. These findings support a potential novel mechanism of cell-cell communication involving exosomal transfer of mitochondria and the bioenergetic and/or redox regulation of target cells.

Mesenchymal stem cells enhance NOX2-dependent reactive oxygen species production and bacterial killing in macrophages during sepsis

Human mesenchymal stem/stromal cells (MSCs) have been reported to produce an M2-like, alternatively activated phenotype in macrophages. In addition, MSCs mediate effective bacterial clearance in pre-clinical sepsis models. Thus, MSCs have a paradoxical antimicrobial and anti-inflammatory response that is not understood.Here, we studied the phenotypic and functional response of monocyte-derived human macrophages to MSC exposure in vitroMSCs induced two distinct, coexistent phenotypes: M2-like macrophages (generally elongated morphology, CD163⁺, acute phagosomal acidification, low NOX2 expression and limited phagosomal superoxide production) and M1-like macrophages characterised by high levels of phagosomal superoxide production. Enhanced phagosomal reactive oxygen species production was also observed in alveolar macrophages from a rodent model of pneumonia-induced sepsis. The production of M1-like macrophages was dependent on prostaglandin E₂ and phosphatidylinositol 3-kinase. MSCs enhanced human macrophage phagocytosis of unopsonised bacteria and enhanced bacterial killing compared with untreated macrophages. Bacterial killing was significantly reduced by blockade of NOX2 using diphenyleneiodonium, suggesting that M1-like cells are primarily responsible for this effect. MSCs also enhanced phagocytosis and polarisation of M1-like macrophages derived from patients with severe sepsis.The enhanced antimicrobial capacity (M1-like) and inflammation resolving phenotype (M2-like) may account for the paradoxical effect of these cells in sepsis in vivo.

Stem cell-derived mitochondria transplantation: a novel strategy and the challenges for the treatment of tissue injury

Damage of mitochondria in the initial period of tissue injury aggravates the severity of injury. Restoration of mitochondria dysfunction and mitochondrial-based therapeutics represent a potentially effective therapeutic strategy. Recently, mitochondrial transfer from stem cells has been demonstrated to play a significant role in rescuing injured tissues. The possible mechanisms of mitochondria released from stem cells, the pathways of mitochondria transfer between the donor stem cells and recipient cells, and the internalization of mitochondria into recipient cells are discussed. Moreover, a novel strategy for tissue injury based on the concept of stem cell-derived mitochondrial transplantation is pointed out, and the advantages and challenges are summarized.

Mesenchymal Stromal Cells and Their Extracellular Vesicles Enhance the Anti-Inflammatory Phenotype of Regulatory Macrophages by Downregulating the Production of Interleukin (IL)-23 and IL-22

Resolution-phase macrophage population orchestrates active dampening of the inflammation by secreting anti-inflammatory and proresolving products including interleukin (IL)-10 and lipid mediators (LMs). We investigated the effects of both human bone marrow-derived mesenchymal stromal cells (MSCs) and MSC-derived extracellular vesicles (MSC-EVs) on mature human regulatory macrophages (Mregs). The cytokines and LMs were determined from cell culture media of Mregs cultivated with MSCs and MSC-EVs. In addition, the alterations in the expression of cell surface markers and the phagocytic ability of Mregs were investigated. Our novel findings indicate that both MSC coculture and MSC-EVs downregulated the production of IL-23 and IL-22 enhancing the anti-inflammatory phenotype of Mregs and amplifying proresolving properties. The levels of prostaglandin E2 (PGE2) were substantially upregulated in MSC coculture media, which may endorse proresolving LM class switching. In addition, our results manifest, for the first time, that MSC-EVs mediate the Mreg phenotype change via PGE2. These data suggest that both human MSC and MSC-EVs may potentiate tolerance-promoting proresolving phenotype of human Mregs.

Regenerative abilities of mesenchymal stem cells through mitochondrial transfer

The past decade has witnessed an upsurge in studies demonstrating mitochondrial transfer as one of the emerging mechanisms through which mesenchymal stem cells (MSCs) can regenerate and repair damaged cells or tissues. It has been found to play a critical role in healing several diseases related to brain injury, cardiac myopathies, muscle sepsis, lung disorders and acute respiratory disorders. Several studies have shown that various mechanisms are involved in mitochondrial transfer that includes tunnel tube formation, micro vesicle formation, gap junctions, cell fusion and others modes of transfer. Few studies have investigated the mechanisms that contribute to mitochondrial transfer, primarily comprising of signaling pathways involved in tunnel tube formation that facilitates tunnel tube formation for movement of mitochondria from one cell to another. Various stress signals such as release of damaged mitochondria, mtDNA and mitochondrial products along with elevated reactive oxygen species levels trigger the transfer of mitochondria from MSCs to recipient cells. However, extensive cell signaling pathways that lead to mitochondrial transfer from healthy cells are still under investigation and the changes that contribute to restoration of mitochondrial bioenergetics in recipient cells remain largely elusive. In this review, we have discussed the phenomenon of mitochondrial transfer from MSCs to neighboring stressed cells, and how this aids in cellular repair and regeneration of different organs such as lung, heart, eye, brain and kidney. The potential scope of mitochondrial transfer in providing novel therapeutic strategies for treatment of various pathophysiological conditions has also been discussed.

Polymer Functionalization of Isolated Mitochondria for Cellular Transplantation and Metabolic Phenotype Alteration

Aberrant mitochondrial energy transfer underlies prevalent chronic health conditions, including cancer, cardiovascular, and neurodegenerative diseases. Mitochondrial transplantation represents an innovative strategy aimed at restoring favorable metabolic phenotypes in cells with dysfunctional energy metabolism. While promising, significant barriers to in vivo translation of this approach abound, including limited cellular uptake and recognition of mitochondria as foreign. The objective is to functionalize isolated mitochondria with a biocompatible polymer to enhance cellular transplantation and eventual in vivo applications. Herein, it is demonstrated that grafting of a polymer conjugate composed of dextran with triphenylphosphonium onto isolated mitochondria protects the organelles and facilitates cellular internalization compared with uncoated mitochondria. Importantly, mitochondrial transplantation into cancer and cardiovascular cells has profound effects on respiration, mediating a shift toward improved oxidative phosphorylation, and reduced glycolysis. These findings represent the first demonstration of polymer functionalization of isolated mitochondria, highlighting a viable strategy for enabling clinical applications of mitochondrial transplantation.

Cornea-Derived Mesenchymal Stromal Cells Therapeutically Modulate Macrophage Immunophenotype and Angiogenic Function

Macrophages are crucial drivers of inflammatory corneal neovascularization and thus are potential targets for immunomodulatory therapies. We hypothesized that therapeutic use of cornea-derived mesenchymal stromal cells (cMSCs) may alter the function of macrophages. We found that cMSCs can modulate the phenotype and angiogenic function of macrophages. In vitro, cMSCs induce apoptosis of macrophages while preferentially promoting a distinct CD14^hi CD16^hi CD163^hi CD206^hi immunophenotype that has significantly reduced angiogenic effects based on in vitro angiogenesis assays. In vivo, application of cMSCs to murine corneas after injury leads to reduced macrophage infiltration and higher expression of CD206 in macrophages. Macrophages cocultured ("educated") by cMSCs express significantly higher levels of anti-angiogenic and anti-inflammatory factors compared with control macrophages. In vivo, injured corneas treated with cMSC-educated macrophages demonstrate significantly less neovascularization compared with corneas treated with control macrophages. Knocking down the expression of pigment epithelial derived factor (PEDF) in cMSCs significantly abrogates its modulating effects on macrophages, as shown by the reduced rate of apoptosis, decreased expression of sFLT-1/PEDF, and increased expression of vascular endothelial growth factor-A in the cocultured macrophages. Similarly, cMSCs isolated from PEDF knockout mice are less effective compared with wild-type cMSCs at inhibiting macrophage infiltration when applied to wild-type corneas after injury. Overall, these results demonstrate that cMSCs therapeutically suppress the angiogenic capacity of macrophages and highlight the role of cMSC secreted PEDF in the modulation of macrophage phenotype and function. Stem Cells 2018;36:775-784.

Tunneling Nanotubes: Intimate Communication between Myeloid Cells

Tunneling nanotubes (TNT) are dynamic connections between cells, which represent a novel route for cell-to-cell communication. A growing body of evidence points TNT towards a role for intercellular exchanges of signals, molecules, organelles, and pathogens, involving them in a diverse array of functions. TNT form among several cell types, including neuronal cells, epithelial cells, and almost all immune cells. In myeloid cells (e.g., macrophages, dendritic cells, and osteoclasts), intercellular communication via TNT contributes to their differentiation and immune functions. Importantly, TNT enable myeloid cells to communicate with a targeted neighboring or distant cell, as well as with other cell types, therefore creating a complex variety of cellular exchanges. TNT also contribute to pathogen spread as they serve as “corridors” from a cell to another. Herein, we addressed the complexity of the definition and in vitro characterization of TNT in innate immune cells, the different processes involved in their formation, and their relevance in vivo. We also assess our current understanding of how TNT participate in immune surveillance and the spread of pathogens, with a particular interest for HIV-1. Overall, despite recent progress in this growing research field, we highlight that further investigation is needed to better unveil the role of TNT in both physiological and pathological conditions.

The promise of mesenchymal stem cell therapy for acute respiratory distress syndrome

This review describes the current state of the science on mesenchymal stem cell (MSC) treatment for acute lung injury (ALI). The general characteristics, regenerative potential, and mechanism of action of MSCs are first presented. Next, particular emphasis is placed on the application of MSCs for the treatment of acute respiratory distress syndrome (ARDS) in preclinical and clinical studies. Finally, we discuss current challenges and future directions in the field presented from a clinician-researcher perspective. The objective of this work is to provide the readership with a current review of the literature discussing the hurdles and overall promise of MSCs as therapeutic interventions for the treatment of ARDS.

"Good things come in small packages": application of exosome-based therapeutics in neonatal lung injury

Infants born at very low gestational age contribute disproportionately to neonatal morbidity and mortality. Advancements in antenatal steroid therapies and surfactant replacement have favored the survival of infants with ever-more immature lungs. Despite such advances in medical care, cardiopulmonary and neurological impairment prevail in constituting the major adverse outcomes for neonatal intensive care unit survivors. With no single effective therapy for either the prevention or treatment of such neonatal disorders, the need for new tools to treat and reduce risk of further complications associated with extreme preterm birth is urgent. Mesenchymal stem/stromal cell (MSC)-based approaches have shown promise in numerous experimental models of lung injury relevant to neonatology. Recent studies have highlighted that the therapeutic potential of MSCs is harnessed in their secretome, and that the therapeutic vector therein is represented by the exosomes released by MSCs. In this review, we summarize the development and significance of stem cell-based therapies for neonatal diseases, focusing on preclinical models of neonatal lung injury. We emphasize the development of MSC exosome-based therapeutics and comment on the challenges in bringing these promising interventions to clinic.

Strategies to enhance paracrine potency of transplanted mesenchymal stem cells in intractable neonatal disorders

Mesenchymal stem cell (MSC) transplantation represents the next breakthrough in the treatment of currently intractable and devastating neonatal disorders with complex multifactorial etiologies, including bronchopulmonary dysplasia, hypoxic ischemic encephalopathy, and intraventricular hemorrhage. Absent engraftment and direct differentiation of transplanted MSCs, and the "hit-and-run" therapeutic effects of these MSCs suggest that their pleiotropic protection might be attributable to paracrine activity via the secretion of various biologic factors rather than to regenerative activity. The transplanted MSCs, therefore, exert their therapeutic effects not by acting as "stem cells," but rather by acting as "paracrine factors factory." The MSCs sense the microenvironment of the injury site and secrete various paracrine factors that serve several reparative functions, including antiapoptotic, anti-inflammatory, antioxidative, antifibrotic, and/or antibacterial effects in response to environmental cues to enhance regeneration of the damaged tissue. Therefore, the therapeutic efficacy of MSCs might be dependent on their paracrine potency. In this review, we focus on recent investigations that elucidate the specifically regulated paracrine mechanisms of MSCs by injury type and discuss potential strategies to enhance paracrine potency, and thus therapeutic efficacy, of transplanted MSCs, including determining the appropriate source and preconditioning strategy for MSCs and the route and timing of their administration.

Alveolar injury and regeneration following deletion of ABCA3

Adaptation to air breathing after birth is dependent upon the synthesis and secretion of pulmonary surfactant by alveolar type 2 (AT2) cells. Surfactant, a complex mixture of phospholipids and proteins, is secreted into the alveolus, where it reduces collapsing forces at the air-liquid interface to maintain lung volumes during the ventilatory cycle. ABCA3, an ATP-dependent Walker domain containing transport protein, is required for surfactant synthesis and lung function at birth. Mutations in ABCA3 cause severe surfactant deficiency and respiratory failure in newborn infants. We conditionally deleted the Abca3 gene in AT2 cells in the mature mouse lung. Loss of ABCA3 caused alveolar cell injury and respiratory failure. ABCA3-related lung dysfunction was associated with surfactant deficiency, inflammation, and alveolar-capillary leak. Extensive but incomplete deletion of ABCA3 caused alveolar injury and inflammation, and it initiated proliferation of progenitor cells, restoring ABCA3 expression, lung structure, and function. M2-like macrophages were recruited to sites of AT2 cell proliferation during the regenerative process and were present in lung tissue from patients with severe lung disease caused by mutations in ABCA3. The remarkable and selective regeneration of ABCA3-sufficient AT2 progenitor cells provides plausible approaches for future correction of ABCA3 and other genetic disorders associated with surfactant deficiency and acute interstitial lung disease.

Mesenchymal Stromal Cells Modulate Macrophages in Clinically Relevant Lung Injury Models by Extracellular Vesicle Mitochondrial Transfer

RATIONALE

Acute respiratory distress syndrome (ARDS) remains a major cause of respiratory failure in critically ill patients. Mesenchymal stromal cells (MSCs) are a promising candidate for a cell-based therapy. However, the mechanisms of MSCs' effects in ARDS are not well understood. In this study, we focused on the paracrine effect of MSCs on macrophage polarization and the role of extracellular vesicle (EV)-mediated mitochondrial transfer.

OBJECTIVES

To determine the effects of human MSCs on macrophage function in the ARDS environment and to elucidate the mechanisms of these effects.

METHODS

Human monocyte-derived macrophages (MDMs) were studied in noncontact coculture with human MSCs when stimulated with LPS or bronchoalveolar lavage fluid (BALF) from patients with ARDS. Murine alveolar macrophages (AMs) were cultured ex vivo with/without human MSC-derived EVs before adoptive transfer to LPS-injured mice.

MEASUREMENTS AND MAIN RESULTS

MSCs suppressed cytokine production, increased M2 macrophage marker expression, and augmented phagocytic capacity of human MDMs stimulated with LPS or ARDS BALF. These effects were partially mediated by CD44-expressing EVs. Adoptive transfer of AMs pretreated with MSC-derived EVs reduced inflammation and lung injury in LPS-injured mice. Inhibition of oxidative phosphorylation in MDMs prevented the modulatory effects of MSCs. Generating dysfunctional mitochondria in MSCs using rhodamine 6G pretreatment also abrogated these effects.

CONCLUSIONS

In the ARDS environment, MSCs promote an antiinflammatory and highly phagocytic macrophage phenotype through EV-mediated mitochondrial transfer. MSC-induced changes in macrophage phenotype critically depend on enhancement of macrophage oxidative phosphorylation. AMs treated with MSC-derived EVs ameliorate lung injury in vivo.

Stem Cell–based Therapies for Sepsis

Sepsis is a life-threatening syndrome resulting in shock and organ dysfunction stemming from a microbial infection. Sepsis has a mortality of 40% and is implicated in half of all in-hospital deaths. The host immune response to microbial infection is critical, with early-phase sepsis characterized by a hyperinflammatory immune response, whereas the later phase of sepsis is often complicated by suppression. Sepsis has no treatment, and management remains supportive.

Stem cells constitute exciting potential therapeutic agents for sepsis. In this review, we examine the rationale for stem cells in sepsis, focusing on mesenchymal stem/stromal cells, which currently demonstrate the greatest therapeutic promise. We examine the preclinical evidence base and evaluate potential mechanisms of action of these cells that are important in the setting of sepsis. We discuss early-phase clinical trials and critically appraise translational barriers to the use of mesenchymal stem/stromal cells in patients with sepsis.