Essay topic 2: Mesenchymal stem cells Flashcards
(12 cards)
what are mesenchymal stem cells?
Mesenchymal stem cells (MSCs) are a diverse subset of multipotent
precursors present in the stromal fraction of many adult tissues. Since
their original description, stromal cells categorized as MSCs based on trilineage (osteo-blast, adipocyte and chondrocyte) potential
in vitro have been isolated from the adherent fraction of many adult and embryonic tissues in multiple species8–11 the choice of the appropriate source depends on the type of disease and the patient’s needs. (FIG. 1).is speculated that MSCs in situ have important roles in tissue cellular homeostasis by replacing dead or dysfunctional cells. Beyond their ability to generate osteo-
blasts, adipocytes and chondrocytes in vitro 5 mesenchymal stromal cells give rise to bone
and cartilage after ectopic implantation in vivo 17,18 and have been documented to contribute to bone regeneration in animal models of genetic bone disorders19 Many
studies have further reported mesenchymal stromal cell differentiation into multiple
other cell types of mesodermal and non-mesodermal origin, including endothelial cells20 cardiomyocytes21, hepatocytes22 and neural cells23,24 The most commonly used sources of MSCs: are the Bone marrow, Fat (adipose tissue-derived MSCs) and Umbilical cord (Wharton’s Jelly)
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.
Ma et al. (2014) - for the immunology part
NOTE: The criteria that define MSCs is their ability to differentiate into adipocytes, chondrocytes and
osteoblasts.
CFU-fibroblasts
Mesenchymal stem cells (MSCs) are defined as a subset of multipotent stromal cells capable of adhering to plastic in culture and differentiating into osteoblasts, adipocytes, and chondrocytes under specific conditions. In vitro, MSCs give rise to colonies of fibroblast-like cells, known as colony-forming unit fibroblasts (CFU-Fs) with a spindle-shape morphology. These colonies are thought to arise from a heterogeneous population of multipotent progenitors located perivascularly in many tissues. CFU-Fs express markers typical of pericytes, including CD146, NG2 (CSPG4), and PDGFRβ, suggesting a perivascular origin. When cultured under appropriate conditions, individual CFU-Fs can be expanded through multiple passages while retaining their mesenchymal differentiation potential, forming the basis of what are now commonly referred to as multipotent mesenchymal stromal cells.
To standardise the identification of MSCs in vitro, the International Society for Cellular Therapy (ISCT) proposed minimal criteria. According to these guidelines, MSCs must (1) adhere to plastic under standard culture conditions, (2) possess trilineage differentiation potential into osteoblasts, adipocytes and chondrocytes, and (3) express a characteristic surface marker profile. Specifically, they are positive for CD105, CD73 and CD90, and negative for haematopoietic markers such as CD45, CD34, CD14 or CD11b, CD79α or CD19, and HLA-DR. These markers help distinguish MSCs from other stromal or blood-derived cell types in culture.
MSC-based treatments
Many studies have attempted to exploit the potential of MSCs to differentiate and thus
replace damaged resident cells, such as endothelial cells, smooth muscle cells,
cardiomyocytes or hepatocytes, and thereby promote tissue regeneration in various organs
such as the heart, kidneys and liver. In most cases, however, the rate of MSC engraftment is
poor, and engrafted MSCs tend to be short-lived, which indicates that there must be other
mechanisms by which MSCs exert their therapeutic effects. Systemic infusion of mesenchymal stromal cells has proved beneficial in different preclinical models of acute lung injury, myocardial infarction,
diabetes and multiple sclerosis, as well as renal and hepatic failure74,78 Also, in experimental animal models, such as liver cirrhosis, myocardial infarction, renal failure and neural degeneration, the success of MSC therapy does not correlate with the
efficiency of cell engraftment and replacement. Furthermore, inflammatory diseases have been effectively treated with only the culture supernatants of MSCs (the ‘MSC secretome’) containing growth factors, which indicates
that extended engraftment of MSCs is unnecessary for such therapies. Therefore, the therapeutic effects of MSCs may depend largely on the capacity of MSCs to regulate inflammation and tissue homeostasis via an arrayi
immunosuppressive
factors, cytokines, growth factors and differentiation factors.
.
MSC mode of action
As part of cell “empowerment” modus of action,
MSCs have a double effect:
- Promote tissue repair
- Regulate inflammatory response
Under the stimulation of different inflammatory cytokines at the damaged tissue sites, the
newly immigrated MSCs release a plethora of growth factors. These growth factors
orchestrate endothelial cells, fibroblasts as well as stem cells to promote tissue regeneration
and repair through enhancing angiogenesis, inhibiting leukocyte transmigration and eliciting
intrinsic progenitor cell/stem cell differentiation. One of the most significant roles played by the MSC is the prevention of over-production of inflammatory T cells and the increased production of Treg (anti-inflammatory type cells) and
the decrease in other inflammatory signals from dendritic cells and other cells. This is important to autoimmune disorders like: inflammatory bowel disease,
neurodegenerative disease, chronic arthritis, allergies…
MSCs in tissue damage repair
MSCs are believed to have
critical roles in repairing damaged tissues.18 Tissue injury is
always associated with the activation of immune/inflammatory
cells, not only macrophages and neutrophils but also adaptive
immune cells, including CD4 þ T cells, CD8 þ T cells and B
cells, which are recruited by factors from apoptotic cells,
necrotic cells, damaged microvasculature and stroma. 19,20
Meanwhile, inflammatory mediators, such as TNF-a, IL-1b,
free radicals, chemokines and leukotrienes, are often
produced by phagocytes in response to damaged cells and
spilled cell contents. 21 Thus, these inflammatory molecules
and immune cells, together with endothelial cells and
fibroblasts, orchestrate changes in the microenvironment that
result in the mobilization and differentiation of MSCs into
stromal and/or replacement of damaged tissue cells. These
MSCs can be tissue-resident or be recruited from the bone
marrow. Once MSCs have entered the
microenvironment of injured tissues, many factors, including
cytokines such as TNF-a, IL-1, IFN-g, toxins of infectious
agents and hypoxia can stimulate the release of many growth
factors by MSCs, including epidermal growth factor (EGF),
fibroblast growth factor (FGF), platelet-derived growth factor
(PDGF), transforming growth factor-b (TGF-b), vascular
endothelial growth factor (VEGF), hepatocyte growth
factor (HGF), insulin growth factor-1 (IGF-1), angiopoietin-1
(Ang-1), keratinocyte growth factor (KGF) and stromal cell-
derived factor-1 (SDF-1). 22–25 These growth factors, in turn,
promote the development of fibroblasts, endothelial cells and
tissue progenitor cells, which carried out tissue regeneration
and repair (Figure 2, Table 1).
(IDO) is upregulated, leading to the consumption of tryptophan and the accumulation of its metabolites, thereby reducing the proliferation of T cells37. W
MSCs in repairing tissue: endothelial monolayer
The barrier function of the endothelial monolayer in the
capillary bed is often broken down in damaged tissues,
allowing the release of protein-rich plasma and some
leukocytes from the blood. MSCs produce various factors,
like Ang-1, VEGF, HGF, EGF, PDGF, FGF, KGF and TGF-b,
which directly affect endothelial cells. These paracrine trophic
factors are potentially important in maintaining endothelial
integrity and promoting angiogenesis through their ability to
regulate endothelial cell proliferation and extracellular matrix
production, reduce endothelial permeability or prevent
interactions between leukocytes and endothelial cells. 26,27
Apart from angiogenesis mediated by endothelial cells, in
response to such trophic factors, fibroblasts also have
essential functions in maintaining tissue integrity and promot-
ing wound healing through their secretion of extracellular
matrix and matrix metalloproteinase. Some in vivo studies
have suggested that growth factors secreted by MSCs can be
applied to improve wound healing and recovery from
myocardial infarction.28–30
MSC function
The Multifaceted Role of MSCs in Tissue Repair and Immune Modulation
Mesenchymal stem cells (MSCs) play a crucial role in the repair of damaged tissues through both direct regeneration and modulation of the immune microenvironment. Upon tissue injury, immune and inflammatory cells such as macrophages, neutrophils, CD4⁺ and CD8⁺ T cells, and B cells are activated by signals from apoptotic and necrotic cells, as well as damaged stroma. These cells release inflammatory mediators like TNF-α, IL-1β, and reactive oxygen species, which not only amplify the immune response but also mobilize MSCs from the bone marrow or activate tissue-resident MSCs.
MSCs play a key role in preserving endothelial barrier function, which is often compromised in damaged tissues, leading to increased vascular permeability and leukocyte infiltration. MSC-derived factors such as Ang-1, VEGF, HGF, EGF, PDGF, FGF, KGF, and TGF-β act directly on endothelial cells to maintain vascular integrity. These trophic factors regulate endothelial proliferation and extracellular matrix production, reduce permeability, and inhibit leukocyte-endothelial interactions. Moreover, these same signals stimulate fibroblasts, which contribute to wound healing by secreting extracellular matrix proteins and matrix metalloproteinases. In vivo studies have demonstrated that MSC-secreted growth factors significantly enhance wound healing and recovery from injuries such as myocardial infarction, emphasizing their clinical potential in regenerative therapies.
In addition to promoting tissue repair, MSCs have robust immunomodulatory properties. MSCs affect cells of both innate immunity and adaptive immunity, but their
immunosuppressive ability is not constitutive; instead, it is induced by inflammatory cytokines,
such as those in the inflammatory microenvironment. They inhibit the overactivation of inflammatory T cells while promoting the expansion of regulatory T cells (Tregs), and modulate dendritic cell activity, which is particularly relevant in autoimmune conditions such as inflammatory bowel disease and arthritis. Mesenchymal stem cells (MSCs) modulate immune responses through the secretion of various cytokines and soluble factors that act on multiple immune cell types. Interleukin-6 (IL-6) inhibits the maturation of dendritic cell progenitors, preventing their development into fully functional antigen-presenting cells. Human leukocyte antigen-G (HLA-G) suppresses neutrophil migration to sites of injury and reduces reactive oxygen species (ROS) generation, limiting tissue damage. Prostaglandin E2 (PGE2) impairs B cell proliferation, disrupts chemotaxis, and inhibits their terminal differentiation into plasma cells. Indoleamine 2,3-dioxygenase (IDO) suppresses both cellular and mitogen-induced T cell proliferation and alters the cytokine secretion profile of naive and effector T cells, skewing them toward a more anti-inflammatory phenotype. Nitric oxide (NO) affects natural killer (NK) cells by reducing their proliferation, cytokine secretion, and cytotoxicity—especially against HLA class I-expressing targets. Additionally, interleukin-10 (IL-10) enhances the expansion and function of regulatory T cells (Tregs), contributing to immune tolerance. Collectively, these mechanisms highlight the multifaceted role of MSCs in dampening inflammation and promoting immune homeostasis.
MSCs also secrete IGF-2, which reprograms macrophages toward an anti-inflammatory M2 phenotype through metabolic shifts involving oxidative phosphorylation and the IGF2R-GSK3α/β-Dnmt3a signaling axis. Moreover, chemokines such as MCP-1, CCL2, CXCL12, and CCL5 secreted by MSCs guide immune cell migration and polarization, contributing to immune homeostasis and in some contexts, influencing tumor metastasis.
A significant mechanism of MSC function lies in their secretion of extracellular vesicles (EVs), including exosomes and microvesicles. These EVs deliver proteins, RNAs, and organelles that modulate immune responses and promote tissue repair. For instance, MSC-derived exosomes can suppress T-cell proliferation and shift macrophages toward the M2 phenotype via STAT3 activation. In models of periodontitis and neuroinflammation, EVs from MSCs and dental pulp stem cells (DPSCs) deliver anti-inflammatory molecules such as IL-10 and TGF-β, reducing osteoclastogenesis and bone loss. Additionally, MSCs under oxidative stress release mitochondria via vesicles, which are then taken up by macrophages to enhance their bioenergetic capacity and reduce inflammatory signaling. These multifactorial capabilities position MSCs as key players in regenerative medicine and immunotherapy.
multipotent capacity
One of the typical properties of MSCs is their multipotency capacity in which these stem cells are able to differentiate into a number of tissues in vitro [59]. Chondrogenic differentiation of MSCs in vitro occurs commonly via culturing them in the existence of TGF-β1 or TGF-β3, IGF-1, FGF-2, or BMP-2 [60,61,62,63]. MSC differentiation into chondroblasts is characterized by the increasing of various genes such as collagen type II, IX, aggrecan, and proliferation of chondroblast cell morphology. During the process of chondrogenesis, FGF-2 promotes the MSCs induced with TGF-β1 or TGF-β3 and/ or IGF-1 [64]. According to the literature works, several molecular pathways such as hedgehog, Wnt/β-catenin, TGF-βs, BMPs, and FGFs can regulate chondrogenesis [65]. In addition, MSCs can exert the osteogenesis function by inducing MSCs with ascorbic acid, β-glycerophosphate, vitamin D3, and/or BMP-2, BMP-4, BMP-6, and BMP-7 [66].
regenerative medicine
Certainly, various investigations have revealed anti-inflammatory, anti-aging, reconstruction, and wound healing potentials of MSCs in many in vitro and in vivo models. their therapeutic potential arises not only from differentiation but also from their immunomodulatory and paracrine properties. This essay explores the key mechanisms by which MSCs exert therapeutic effects and highlights the evidence supporting their clinical use.
subheading: mechanisms of action
a) Differentiation and Engraftment
Many studies have attempted to exploit the potential of MSCs to differentiate and thus
replace damaged resident cells, such as endothelial cells, smooth muscle cells,
cardiomyocytes or hepatocytes, and thereby promote tissue regeneration in various organs
such as the heart, kidneys and liver. In most cases, however, the rate of MSC engraftment is
poor, and engrafted MSCs tend to be short-lived, which indicates that there must be other
mechanisms by which MSCs exert their therapeutic effects.
Also, in experimental animal models, such as liver cirrhosis, myocardial infarction, renal
failure and neural degeneration, the success of MSC therapy does not correlate with the
efficiency of cell engraftment and replacement.
Furthermore, inflammatory diseases have been effectively treated with only the culture
supernatants of MSCs (the ‘MSC secretome’) containing growth factors, which indicates
that extended engraftment of MSCs is unnecessary for such therapies.
Many preclinical and clinical studies have provided growing evidence of the efficacy of
MSC-based treatments.
Therefore, the therapeutic effects of MSCs may depend largely on the capacity of MSCs
to regulate inflammation and tissue homeostasis via an array of immunosuppressive
factors, cytokines, growth factors and differentiation factors
They can release bioactive molecules, which is a key benefit in tissue regeneration [4, 5]. These properties result in progression of treatments for a wide range of diseases, such as diseases affecting the bone, neuron, lung, liver, heart, kidney, etc. [4]. Due to these features, it is obvious that MSCs will hold a major therapeutic role in clinical trials.
A key mechanism underlying the therapeutic potential of MSCs is their paracrine signaling and immunomodulatory capacity. Rather than relying solely on engraftment and differentiation, MSCs exert their effects primarily through the secretion of a wide array of bioactive molecules, collectively known as the MSC secretome. This includes cytokines, growth factors, and extracellular vesicles (EVs)/exosomes. Key components such as interleukin-10 (IL-10), transforming growth factor-beta (TGF-β), prostaglandin E2 (PGE2), and hepatocyte growth factor (HGF) contribute to creating a reparative microenvironment. These molecules enable MSCs to suppress the activation of T cells, B cells, and natural killer (NK) cells, while promoting the induction of regulatory T cells (Tregs). This immune modulation makes MSCs highly promising for treating autoimmune and inflammatory conditions.
One prominent application of this immunomodulatory effect is in wound healing, a complex process that involves hemostasis, inflammation, proliferation, and tissue remodeling. MSCs have demonstrated the ability to enhance all phases of this process by migrating to the injury site, dampening local inflammation, and stimulating fibroblasts, keratinocytes, and endothelial cells. In preclinical models, MSC treatment accelerated wound closure and reduced scar formation. Clinically, Falanga et al. showed that autologous bone marrow-derived MSCs (BMSCs) significantly improved healing in patients with chronic wounds, leading to substantial ulcer size reduction or full closure within 4–5 months and no reported adverse effects.
Beyond the skin, MSCs have also shown therapeutic promise in other inflammatory conditions, such as renal injury, where inflammation plays a central role in both damage and repair. In a 2021 study, Swaminathan et al. demonstrated that allogeneic BMSCs treated with SBI-101 promoted immune modulation and supported recovery in patients with acute kidney injury, highlighting another avenue for MSC-based therapy. These examples underscore how MSCs’ secretory and immunosuppressive capabilities contribute significantly to their regenerative potential across various tissues.
More recent investigation has been directed to the secretion of paracrine immunomodulatory factors, which are packaged into extracellular vesicles (EVs) to form the bioactive fraction of the MSC secretome [83]. This has elucidated the mechanisms by which the MSC secretome mediates its effector functions and has provided multiple examples of the potential therapeutic properties of the EVs [84]. EVs are heterogeneous structures that can be subtyped to exosomes, microvesicles, and apoptotic bodies. Exosomes are created by an endosomal route and are typically 30 to 150 nm in diameter. They are derived when MSCs exchange genetic material between cells, particularly microRNA and mRNA
At the cellular level, the immunosuppressive function of MSCs is not only mediated by soluble factors but also through direct cell–cell interactions. An inflammatory environment induces MSCs to secrete multiple chemokines and upregulate the expression of ICAM1 and VCAM1, which attract and engage T-cells to MSCs [79]. The clinical relevance of these interactions is highlighted by showing that the blockade or deletion of ICAM1 and VCAM1 could significantly reverse MSC-mediated immunosuppression in vitro and in vivo [103]. MSCs inhibit the proliferation of T-cells, specifically pro-inflammatory populations of T-helper cells (Th17 and Th1), decrease the ratio of Th1/Th2 T-helper cell populations, and promote an anti-inflammatory profile by activation of Treg cells [77]. These findings could be translated into therapies for autoimmune and autoinflammatory diseases such as rheumatoid arthritis (RA), which are characterised by a predominance of pro-inflammatory CD4+ T cells with the hyper-proliferative capacity to differentiate into Th1 and Th17 pathogenic T cells [104].
In rheumatoid arthritis (RA), MSCs have shown promise due to their ability to suppress pro-inflammatory T-helper cells (Th1 and Th17) and promote regulatory T cells (Tregs). In a clinical report by Ghoryani et al. (2019) [45], autologous bone marrow-derived MSCs (BM-MSCs) were administered intravenously to nine patients with refractory RA at a dose of 1 × 10⁶ cells/kg. Post-treatment, there was a significant increase in Treg cells and a decrease in Th17 cells, indicating a shift toward immune tolerance. These findings underscore how MSC-based therapies can effectively alleviate inflammatory symptoms in autoimmune disorders by rebalancing dysregulated T cell responses.
The induction of Tregs by MSCs has been considered to be caused by direct cell–cell contact as well as the secretion of PGE2, TGFβ1, IL10, and soluble human leukocyte antigen-G (sHLA-G) [106,107]. The balance between Treg cells and Th17 cells determines the efficacy of immune therapy and thus underscores the importance of MSCs as tools for moderating autoimmune and autoinflammatory diseases
In addition to the secretion of individual paracrine factors, MSCs can secrete EV, mostly exosomes (Basalova et al., 2020), whose composition also varies depending on external signals (Lopatina et al., 2014). The significance of this cell communication mechanism is crucial because MSCs can transmit molecules of various nature, including proteins, lipids, and nucleic acids such as mRNA and regulatory non-coding RNAs, within EV to target cells (Kalinina et al., 2015b; Yáñez-Mó et al., 2015; Efimenko et al., 2016; Basalova et al., 2020). Due to the possible targeted effects of EV, their transfer to nearby cells can help fine-tune the effects of the MSC secretome (Hoshino et al., 2015). The EV secreted by MSCs contain a large number of micrornas that are capable of inhibiting the translation of mRNA in target cells both in vitro and in vivo (Friedman et al., 2009; Wahid et al., 2010). Among the most represented in the MSC-produced EV, microRNAs were found to regulate the maintenance of the stem cell pool by changing the expression of the components of the Wnt, PDGF, and TGF-beta signal transmission pathways. The EV secreted by MSCs can also contribute to tissue regeneration due to their effect on the microenvironment. In particular, studies have demonstrated that they contain micrornas that suppress the formation of myofibroblasts and, accordingly, the development of fibrosis by suppressing the TGF-beta2/SMAD2 pathway and the production of ECM proteins (Fang et al., 2016; Basalova et al., 2020).
MSCs have shown significant promise in restoring function following myocardial injury. Cardiomyocyte death due to ischemic events, such as myocardial infarction, leads to replacement by fibrotic tissue and progressive heart failure. However, MSCs can counteract this deterioration. For instance, the intracoronary administration of Wharton’s jelly-derived MSCs (WJ-MSCs) has been demonstrated to enhance myocardial perfusion, increase left ventricular ejection fraction (LVEF), and reduce end-systolic and end-diastolic volumes, indicating improved cardiac contractility and remodeling. Similarly, autologous bone marrow MSCs (BMSCs) have been employed during surgical interventions such as coronary artery bypass grafting, where they contribute to regional improvements in myocardial contractility and significant reductions in angina symptoms. Notably, MSC-based interventions are associated with favorable safety profiles and improvements in patient-reported outcomes like quality of life. Moreover, allogeneic BMSC treatments in patients with non-ischemic dilated cardiomyopathy have been linked to reductions in inflammatory markers such as TNF-α, further supporting their immunomodulatory role in cardiac repair. Altogether, MSCs facilitate cardiac regeneration not only through direct tissue integration but also by orchestrating a reparative microenvironment, making them a cornerstone of next-generation cell therapies for heart disease.
Induced pluripotent stem cell-derived mesenchymal stromal cells (iMSCs) have emerged as a promising and more standardized alternative to primary MSCs, offering higher proliferative capacity and scalability for therapeutic applications such as bone repair, nerve regeneration, and cancer therapy. Unlike primary MSCs, iMSCs can be generated in large quantities with consistent quality, enabling off-the-shelf, patient-specific treatments and reducing donor variability. Their exosomes—nano-sized vesicles loaded with proteins, RNAs, and signaling molecules—are increasingly being explored as cell-free therapeutics due to their ability to replicate the anti-inflammatory and regenerative effects of MSCs in vivo, while avoiding challenges associated with live cell therapy, such as immune rejection and tumor formation. Early clinical trials have already demonstrated the translational viability of these approaches: for example, CYP-001, an iPSC-derived MSC product, showed an 87% overall response rate in a Phase I trial for steroid-resistant acute graft-versus-host disease (aGvHD), with a 60% two-year survival rate. Similarly, MSC-derived exosomes are being studied in clinical settings for Crohn’s disease, stroke, and type 1 diabetes, with preliminary data indicating favourable safety profiles and regenerative outcomes. These innovations reflect a shift toward next-generation MSC-based therapies that may overcome current manufacturing and safety hurdles, positioning iMSCs and their exosomes at the forefront of future regenerative medicine.
