Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive lung disease that causes irreversible damage to the alveoli and leads to pulmonary interstitial fibrosis. Patients with IPF often experience severe difficulty breathing, which can result in respiratory failure and death. The disease is challenging to diagnose, has a high mortality rate, and a median survival of only three to five years after diagnosis, which is worse than many forms of cancer.
Current treatments primarily focus on supportive care, such as lung transplantation, mechanical ventilation, and oxygen therapy. Drugs like pirfenidone and nintedanib can slow disease progression but do not repair damaged lung tissue. For this reason, researchers are exploring the use of mesenchymal stem cells (MSCs) as a potential new therapy for IPF. MSCs are multipotent stem cells capable of self-renewal, differentiation, and secreting a variety of factors that may reduce inflammation, promote tissue repair, and regulate immune responses.
As part of this review, Li et al. summarize recent studies on MSCs in reducing lung inflammation and fibrosis, highlighting their potential mechanisms, such as migration and differentiation, secretion of soluble factors and extracellular vesicles, and regulation of endogenous repair processes.
Pathological Changes in IPF
The main pathological features of IPF include widespread alveolar damage, excessive proliferation of fibroblasts, and deposition of extracellular matrix (ECM) proteins. Fibroblastic foci, areas of active fibroblast and myofibroblast accumulation, are a hallmark of the disease and strongly correlate with patient outcomes. Fibroblasts in these foci arise from three primary mechanisms: proliferation of resident fibroblasts, epithelial-mesenchymal transition (EMT), and bone marrow-derived fibrocytes.
Resident fibroblasts proliferate and differentiate into myofibroblasts under the influence of factors like transforming growth factor-β (TGF-β). Myofibroblasts produce collagen and other ECM proteins, which contribute to tissue stiffness and fibrosis. EMT occurs when alveolar epithelial cells lose epithelial markers and acquire mesenchymal traits, becoming fibroblast-like cells that contribute to ECM deposition. TGF-β is a key driver of EMT, acting through pathways such as Ras/ERK/MAPK signaling. Endothelial cells can also undergo a similar transition, producing collagen and contributing to fibrosis. Bone marrow-derived fibrocytes, circulating in the blood, migrate to damaged lung tissue and differentiate into fibroblasts. Their accumulation is linked to poor prognosis and is guided by chemokine signaling pathways like CXCL12/CXCR4 and CCL3/CCR5.
Properties of Mesenchymal Stem Cells
MSCs, first discovered in 1968, are multipotent cells that can differentiate into bone, cartilage, and fat. They can be sourced from bone marrow, adipose tissue, and umbilical cord blood, and are identified by fibroblast-like shape, plastic adherence, and surface markers (CD44, CD29, CD90) while lacking hematopoietic markers (CD45).
MSCs have low immunogenicity, can modulate the immune system, and support tissue repair. Transplantation in animal models of lung injury shows promise with minimal side effects, but human safety and efficacy remain uncertain due to species differences and small clinical trials. Potential risks include tumor formation and unwanted angiogenesis, especially in immunocompromised patients.
Mobilizing endogenous MSCs is also being studied, as these cells can migrate to injured tissue, secrete reparative factors, and aid repair, with agents like G-CSF enhancing mobilization, though outcomes vary.
Mechanisms of MSC Therapy in Pulmonary Fibrosis
Mesenchymal stem cells (MSCs) help repair lung injury through multiple, interconnected mechanisms: migration to injury sites, differentiation, secretion of bioactive factors, immune modulation, and regulation of lung defenses.
MSCs are guided to damaged lung areas by chemokines such as stromal cell-derived factor-1 (SDF-1) and interleukin-8 (CXCL8). Once at the injury site, they can differentiate into type II alveolar epithelial cells, supporting tissue repair. This differentiation is influenced by Wnt signaling pathways, though in some cases, MSCs may become fibroblast-like cells, which could worsen fibrosis.
A key part of MSC therapy is the secretome, a collection of soluble factors. Growth factors like KGF, HGF, EGF, Ang-1, and VEGF restore alveolar and endothelial function, maintain lung barrier integrity, and reduce fluid buildup. Anti-inflammatory molecules such as IL-1ra, IL-10, PGE2, and TSG-6 help control inflammation and promote repair. MSCs also encourage macrophages to shift from a pro-inflammatory (M1) to an anti-inflammatory (M2) state, aiding recovery. Early administration during acute inflammation provides the most benefit.
MSCs exert immunomodulatory effects by secreting chemokines, adhesion molecules, and regulatory factors like nitric oxide (NO) and indoleamine-2,3-dioxygenase (IDO), which suppress T-cell activity. They influence B cells and support regulatory T cells to maintain immune balance. MSCs can also secrete TGF-β, which can either aid healing or promote fibrosis depending on context and timing.
Extracellular vesicles (EVs), including exosomes and microvesicles, are another way MSCs deliver therapeutic benefits. They carry proteins, RNAs, and other molecules that reduce inflammation and promote tissue repair. EV-based therapy may offer many of the benefits of MSCs while minimizing risks associated with cell transplantation.
Finally, MSCs regulate molecules involved in oxidative stress, inflammation, and tissue repair. They decrease pro-fibrotic and inflammatory signals like matrix metalloproteinases and TGF-β1 while increasing antioxidant enzymes and repair-promoting proteins such as FoxM1, stanniocalcin, and Miro1, all of which protect lung tissue and combat fibrosis.
Advancing MSC Therapy for Pulmonary Fibrosis
Mesenchymal stem cell therapy represents a promising approach for treating idiopathic pulmonary fibrosis. Its benefits involve multiple mechanisms, including homing to injured tissue, differentiation, secretion of growth factors and cytokines, immunomodulation, and enhancement of endogenous lung defenses. MSCs are most effective when administered early in the inflammatory phase of lung injury, highlighting the importance of timing. Despite encouraging preclinical and early clinical results, safety and efficacy in humans remain under investigation, and some contradictory findings underscore the complexity of MSC therapy.
Li et al. conclude that future research should focus on optimizing MSC mobilization, improving therapeutic efficacy, exploring the role of microRNAs, and advancing clinical trials to establish MSC-based therapies as viable treatments for IPF.
Source: Li X, Yue S, Luo Z. Mesenchymal stem cells in idiopathic pulmonary fibrosis. Oncotarget. 2017 May 23;8(60):102600-102616. doi: 10.18632/oncotarget.18126. PMID: 29254275; PMCID: PMC5731985.
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