Immunoregulation of MSC: Creation of an Anti-inflammatory Environment
Let’s take a closer look at how MSCs contribute to and participate in immune regulation. When tissue damage occurs, immune cells begin an inflammatory response to heal the tissue and prevent further infection by secreting cytokines (interferon-γ, IFNγ), tumor necrosis factor-α (TNF-α), IL-1, chemokines, and growth factors. Stimulated by these signals, activated MSCs use various chemokines to travel through the bloodstream to the site of injury (homing). This is because MSCs possess receptors that can recognize chemokines or cytokines secreted from damaged or inflamed tissues. Their ability to “find” the wound site is one of their key features and a great advantage in tissue repair.
The role of MSCs is to help create an environment that suppresses inflammatory responses and allows the body to focus on healing. This role is carried out by directly secreting various anti-inflammatory cytokines, metabolic enzymes, and growth factors, or by packaging mRNA, tRNA, peptides/proteins into vesicles and releasing them outside the cell, thereby transmitting them to nearby cells and inducing the expression of anti-inflammatory genes. This process is called paracrine signaling, which, unlike hormones that travel through the bloodstream to act systemically, refers to a local and transient secretion mode in which a cell releases various factors targeting neighboring cells within the same tissue or microenvironment.
Secretome secreted by MSCs via paracrine or direct secretion
To regulate immune responses, MSCs first need to attract immune cells. Therefore, they release a variety of chemokine signaling molecules (CCL2, CXCL8, CXCL12, CCL5, etc.) into the surrounding environment. Among the major immune-regulating factors secreted by MSCs are indoleamine-2,3-dioxygenase (IDO) and prostaglandin E₂ (PGE₂).
IDO, a key metabolic enzyme secreted by MSCs, can directly suppress T cells by degrading tryptophan—an essential amino acid required for T cell survival and proliferation—into kynurenine (KYN) and other metabolites. When tryptophan is rapidly depleted and its concentration decreases, T cells can no longer proliferate and instead enter cell-cycle arrest, a condition that makes them more susceptible to apoptosis. [1] Furthermore, this metabolic process promotes the differentiation of naïve (undifferentiated) T cells into regulatory T cells (Tregs), which function to suppress immune responses. As a result, T-cell activity is inhibited while Tregs are activated, leading to the establishment of an anti-inflammatory environment.
Prostaglandin E₂ (PGE₂) is a bioactive lipid signaling molecule produced when arachidonic acid, a component of membrane phospholipids, is released into the cytoplasm and converted by the enzyme COX (cyclooxygenase). [2] PGE₂ generated through the COX-1 pathway helps maintain homeostasis in the body by protecting the gastric mucosa and sustaining blood flow, while PGE₂ produced via the COX-2 pathway is induced under inflammatory conditions and causes inflammation symptoms such as pain, fever, and swelling. When you experience a toothache or catch a cold and develop a fever or headache, it is due to the inflammatory response mediated by the COX-2 pathway. Therefore, anti-inflammatory painkillers (NSAIDs) work by inhibiting this pathway. However, because these drugs also inadvertently suppress the COX-1 pathway, which plays an important role in protecting the stomach lining, side effects such as stomach irritation and gastrointestinal discomfort can occur. To address this issue, many drugs have been developed that specifically target COX-2, selectively relieving pain and inflammation while minimizing gastrointestinal side effects.
Returning to MSCs: under inflammatory conditions, MSCs stimulated by inflammatory cytokines secrete PGE₂ through the COX-2 pathway to regulate immunity. MSC-derived PGE₂ induces surrounding immune cells to shift toward anti-inflammatory responses. It reprograms macrophage differentiation, converting pro-inflammatory (M1) macrophages into anti-inflammatory (M2) macrophages. It also inhibits T cell proliferation (activation) and suppresses their differentiation into inflammatory Th1 and Th17 cells while promoting their differentiation into anti-inflammatory regulatory T cells (Tregs). Ultimately, the function of PGE₂ depends on which cell secretes it. Observing this duality reveals how the microenvironment created by cells determines the consequences of immune modulation.
Further aspects of MSC immunoregulation
Beyond T cell suppression via IDO and PGE₂, MSCs also inhibit B cell proliferation, their differentiation into plasma cells, and the release of immunoglobulins (IgE, IgG) from activated B cells. [3] MSC-derived IDO induces the secretion of anti-inflammatory interleukin-10 (IL-10), promoting the generation and differentiation of B cell subtypes that give rise to regulatory T cells (Tregs). In addition to adaptive immunity involving T and B cells, MSCs also modulate innate immunity. Through cytokine signaling (IL-2 or IL-15), they limit the proliferation and activity of natural killer (NK) cells. MSCs suppress monocyte differentiation into dendritic cells, and can even reprogram mature dendritic cells back into immature forms. As mentioned earlier, MSCs also shift macrophages toward an anti-inflammatory phenotype. As part of innate defense against infection, MSCs secrete antimicrobial peptides (AMPs)—such as cathelicidin peptide LL-37, hepcidin, β-defensin 2, and lipocalin 2—which can directly kill bacteria.
MSC secretion of growth factors for tissue repair
MSCs secrete various growth factors that promote tissue repair. These include vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), keratinocyte growth factor (KGF), insulin-like growth factors (IGF), hepatocyte growth factor (HGF), tumor necrosis factor-inducible gene 6 protein (TSG6), chemokine (CCL-2), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and angiopoietin-1 (Ang1). All of these secreted factors suppress inflammatory responses and promote angiogenesis, thereby contributing to faster tissue regeneration.
Secretion via Extracellular Vesicles (EVs)
In addition to the secreted factors mentioned above, MSCs also release vesicles containing miRNA, mRNA, tRNA, peptides, or proteins. Neighboring cells that receive these extracellular vesicles undergo changes in gene expression and function. These vesicles—known as exosomes when generated through membrane budding and enclosed by lipid bilayers—mediate communication with other cells, alleviate oxidative stress, reduce apoptosis, weaken immune cell activation, and help repair damaged tissues. Some vesicles even contain mitochondria, which are transferred to damaged neighboring cells to enhance their energy production and restore metabolic function. Currently, research on cell-free therapies using MSC-derived EVs is being actively pursued. These therapies utilize EVs containing immunomodulatory secretions, mRNA, and miRNA extracted from cultured MSCs. Compared to direct cell transplantation, such cell-free therapies retain the therapeutic functions of parent cells while offering advantages such as lower immunogenicity, higher biocompatibility, ability to cross barriers (e.g., blood–brain barrier), long-term stability at −80 °C, and ease of storage. In addition, therapeutic agents such as antibiotics can be encapsulated within EVs for targeted delivery, making them highly promising for clinical use. [4]
Types of EVs and Exosomes
Extracellular vesicles (EVs) released from cells vary widely in size and origin. Among them, APOs, MVs, and exosomes can be compared. APOs (apoptotic bodies) are large vesicles shed during programmed cell death (apoptosis). MVs (microvesicles) are formed by the outward budding of the plasma membrane. Both of these types of EVs originate directly from the plasma membrane, unlike exosomes, which are produced through the endosomal pathway.
So what is the endosomal pathway? It can be viewed as a system for sorting and distributing materials entering the cell. Early endosomes are the initial vesicles formed when the plasma membrane invaginates during endocytosis. This process not only allows the entry of external materials but also regulates signaling by internalizing ligand-bound receptors from the membrane after signal activation, preventing overactivation. Receptors within endosomes may be recycled back to the membrane or sent to lysosomes for degradation. That is, the early endosome functions like a logistics center that sorts the “packages” coming from outside the cell, and it is located closer to the cell membrane.
Over time, the early endosome becomes more acidic (its internal pH decreases) and matures into a more developed form, gradually moving toward the nucleus at the center of the cell. The endosomal membrane invaginates again to form multiple smaller vesicles inside, creating a multivesicular body (MVB)—a mature form known as the late endosome. MVBs have two fates: they either fuse with lysosomes for degradation of internal vesicles or fuse with the plasma membrane to release their internal vesicles outside the cell. The released vesicles are exosomes, which carry proteins, lipids, mRNA, miRNA, and other cargo that regulate signaling and gene expression in recipient cells.
[References]
[1] Indoleamine 2,3-dioxygenase specific, cytotoxic T cells as immune regulators
doi: 10.1182/blood-2010-06-288498
[2]The Role of COX-2 and PGE2 in the Regulation of Immunomodulation and Other Functions of Mesenchymal Stromal Cells
doi: 10.3390/biomedicines11020445
[3] Role of Mesenchymal Stromal Cells as Therapeutic Agents: Potential Mechanisms of Action and Implications in Their Clinical Use
https://doi.org/10.3390/jcm9020445
[4] Mesenchymal stem cell-derived extracellular vesicles for immunomodulation and regeneration: a next generation therapeutic tool?
https://doi.org/10.1038/s41419-022-05034-x
