Since Orlic et al. reported the potential of bone marrow cells to replace damaged adult myocardium in 2001, a finding that was later challenged, MSCs-based therapies for myocardial regeneration have been extensively studied. These studies have utilized differentiated or undifferentiated MSCs from allogeneic, autologous, and even xenogeneic sources, employing various delivery approaches (Huang et al., 2010; Jansen Of Lorkeers et al., 2015; Luger et al., 2017; Tomita et al., 1999). Meta-analyses have reported an overall 12% increase in left ventricular ejection fraction (LVEF) in rodent studies following MSCs administration compared to untreated groups. Additionally, a 7% increase in LVEF was observed in large species such as pigs, with only 7 out of 16 studies showing favorable results for MSCs. Correspondingly, an 8% reduction in infarct size was observed in half of the preclinical studies in rodents, and a 6.4% reduction in pig hearts was noted in the seven studies that reported favorable results for MSCs administration (Kanelidis et al., 2017).

Most clinical trials of cell therapy for IHD have concentrated on BM-, with AD- and UC-MSCs also being studied in recent years (Ward et al., 2018). These trials have demonstrated favorable safety and tolerability. The treatment of acute myocardial infarction (AMI) typically involves the intracoronary injection of MSCs following percutaneous coronary intervention (PCI), with intravenous injection being used in some cases. A meta-analysis of 13 clinical trials in AMI reported a highly significant 3.78% increase in LVEF for the MSCs-treated group compared to the control group (Attar et al., 2021). MSCs have also been investigated for the treatment of chronic ischemia and ischemic cardiomyopathy. The primary route of treatment for chronic myocardial infarction (MI) is intramyocardial, rather than intracoronary or intravenous (Wang et al., 2015a). A meta-analysis demonstrated the efficacy of MSCs, highlighting the overall safety and efficacy of autologous BM-MSCs in chronic MI treatment (Kanelidis et al., 2017). A randomized controlled trial by Zhang et al. assessed the efficacy of MSCs transplantation in patients with chronic MI, reporting significant improvements in LVEF and reduced infarct size (Wang et al., 2015b). These findings supported the long-term outcomes of MSCs therapy, observing improved survival rates, reduced rehospitalization, and enhanced quality of life among treated patients (Afzal et al., 2015). Nowadays, there are 2 related MSC products approved for the treatment of peripheral vascular disease. One is Stempeucel for atherosclerotic and non-atherosclerotic critical limb ischemia, developed by Stempeutics; and the other is Vescell (ACP-01), under development by Hemostemix, for the treatment of IHD. Several clinical trials are currently underway for the treatment of IHD using UC-MSCs (Table 1).

The dramatic pathological changes following myocardial ischemia include the massive loss of cardiomyocytes, destruction of microvessels, and recruitment and activation of immune cells (Figure 1). Current knowledge indicates that the regenerative capacity of cardiomyocytes is extremely weak, leading to fibrotic repair to maintain the heart’s integrity. Various fibrotic mediators and cytokines released by macrophages, lymphocytes, and other cells create a fibrotic microenvironment in the ischemic area, stimulating the transformation of fibroblasts into myofibroblasts (Xiao et al., 2023). These myofibroblasts produce large amounts of extracellular matrix proteins, leading to collagen deposition and cardiac fibrosis, which induces adverse remodeling that can gradually progress to heart failure (Prabhu and Frangogiannis, 2016).

Appropriate vasculature is crucial for the survival and normal function of cardiac cells, as it provides essential oxygen, nutrients, growth factors, and cytokines (Brutsaert, 2003). Ischemic injury, arising from the obstruction of blood vessels, can lead to further structural damage in these vessels (Xiao et al., 2021a). Endothelial cells, in particular, are highly susceptible to ischemic insults and may experience impaired integrity and increased permeability. This disruption of the endothelial barrier facilitates the leakage of fluid and macromolecules into the surrounding tissue, exacerbating the ischemic injury (Chu et al., 2023). Additionally, ischemic insults can damage the basement membrane, resulting in the loss of its structural support and regulatory functions. Furthermore, smooth muscle cells in the vessel wall may undergo apoptosis or dysfunction, compromising the structural integrity of blood vessels and contributing to the progression of vascular remodeling and dysfunction (Heitzer et al., 2001). In response to ischemia-induced hypoxia, angiogenesis is initiated, sprouting from existing endothelial cells and expanding towards the injured site to recover blood supply (Nofi et al., 2018). However, angiogenesis is often not sustained, and vascular remodeling further impairs the ability of blood vessels to withstand hemodynamic stress (Xiao et al., 2021a).

The ischemic injury leading to vascular changes directly affect the survival and migration of MSCs. As blood vessels are the primary route for MSCs to obtain oxygen, nutrients, and growth factors, vascular obstruction or damage disrupts the microenvironment, compromising their survival and normal functions (Kang and Zheng, 2013). On the other hand, endothelial cells losing their integrity and increasing vascular permeability might facilitate the migration of MSCs from blood vessels to the injured site (Chu et al., 2023). Additionally, in response to ischemia-induced hypoxia, MSCs might activate their paracrine effects on immunomodulation and suppress inflammatory responses (Xiao et al., 2021b). Some studies showed that hypoxia might shift the differentiation preference of MSCs towards vascular repair-related cell types, including endothelial cells and smooth muscle cells (Tian et al., 2022).

Following ischemic injury, the heart undergoes a series of intricate immune responses involving the activation and infiltration of various immune cells. These include both innate immune cells, such as neutrophils and macrophages, and adaptive immune cells, including T and B cells (Nahrendorf et al., 2007; Frangogiannis, 2014). These immune cells accumulate at the site of cardiac injury and release a multitude of inflammatory mediators and cytokines, such as interleukins (IL-1, IL-6, IL-17), tumor necrosis factor-alpha (TNF-α), chemokines (CXCL1, CCL2), and reactive oxygen species (ROS), which further promote the inflammatory response and tissue repair (Swirski and Nahrendorf, 2013). The immune response typically peaks within 4–7 days following ischemic injury and subsequently reduces to a stable state over time (Xiao et al., 2021a).

MSCs respond to cytokines and chemokines released from the injured myocardium, rapidly mobilizing and migrating towards the site of injury under the guidance of these factors (Kang and Zheng, 2013). Once in the injured myocardium, MSCs interact with various immune cells. Macrophages play a pivotal role in the inflammatory response following ischemic heart injury, with the ability to polarize into M1 (pro-inflammatory) or M2 (anti-inflammatory) phenotypes, each exerting distinct effects on MSCs (Ben-Mordechai et al., 2013). M1 macrophages release pro-inflammatory cytokines that can compromise MSCs survival and function, while M2 macrophages foster a more conducive environment for MSCs-mediated tissue repair. Reciprocally, MSCs can influence macrophage polarization, favoring the M2 phenotype, which aids in reducing inflammation and promoting healing (Neupane et al., 2023). Additionally, MSCs can mitigate the inflammatory response by suppressing T cell proliferation and activation (Behm et al., 2024), as well as inhibiting NK cell activation and cytotoxicity, thereby protecting heart tissue from immune-mediated damage. The interplay between MSCs and immune cells is dynamic, with MSCs promoting a more balanced immune response that supports tissue repair (Najar et al., 2010).

On the other hand, the ECM forms the native cellular support network and has a strong interplay with its residing cells. Following myocardial ischemic injury, the ECM undergoes significant remodeling, particularly in its composition (Chu et al., 2021). Activated cardiac fibroblasts proliferate and increase the synthesis of collagenous proteins, primarily types I and III collagen, which are then deposited in the ECM. This excessive deposition of collagenous proteins leads to fibrosis, altering the structural integrity of the myocardium (Xiao et al., 2023). In addition to collagen, non-collagenous components of the ECM, such as glycosaminoglycans, proteoglycans, elastin, and laminin, may also undergo changes in their content and distribution, further contributing to ECM remodeling (Bonnans et al., 2014).

This remodeling process creates a new microenvironment that supports MSCs migration and affects their biological behavior. Firstly, the remodeled ECM provides a structural framework and essential cues for MSCs migration. Specific signals and adhesion molecules, including growth factors, cytokines, and chemokines embedded within the remodeled ECM, could guide MSCs to the injured area, acting as chemoattractants for efficient homing and tissue repair processes (Zhu et al., 2006). Additionally, the mechanical stress altered by the remodeled ECM might change MSCs biological functions, including their paracrine effects to attenuate myofibroblast transition-induced fibrosis (Galie and Stegemann, 2014) and promote angiogenesis (Piao et al., 2005). Furthermore, MSCs could acquire contraction ability when exposed to mechanical stretch, contributing to functional recovery after ischemic injury (Choi et al., 2017; Girao-Silva et al., 2014; Izadpanah et al., 2022). However, excessive collagen deposition leads to fibrotic scar formation, resulting in a deteriorated microenvironment for MSCs characterized by stiff ECM, intensive mechanical stress, and reduced blood vessels (Li et al., 2021). Therefore, this situation should be avoided when utilizing MSCs treatment for IHDs.

The HIF-1 signaling pathway is a central regulator of angiogenesis, orchestrating the expression of multiple pro-angiogenic factors and metabolic adaptations essential for vessel formation (Semenza, 2014). HIF-1 is a heterodimeric transcription factor with two subunits: HIF-1α and HIF-1β (ARNT). HIF-1α, the oxygen-sensitive subunit, is tightly regulated by cellular oxygen levels. Under hypoxic conditions, HIF-1α escapes hydroxylation and proteasomal degradation, translocates to the nucleus, and dimerizes with HIF-1β to form an active transcription complex (Wang et al., 1995). This complex binds to hypoxia-responsive elements (HREs) in target gene promoters, inducing their expression. One crucial target of HIF-1 in angiogenesis is VEGF, which promotes endothelial cell proliferation, migration, and tube formation (Liu et al., 2018; Cheng et al., 2016). HIF-1 directly upregulates VEGF expression, initiating and enhancing the angiogenic response and revascularization to improve tissue perfusion (Liu et al., 2018; Li et al., 2012).

However, the angiogenic response following myocardial ischemic injury is often impaired (Zhang et al., 2016b). While angiogenesis is rapidly activated in response to ischemic injury, peaking at day 4 and declining after 1 week, HIF-1α consistently accumulates in the ischemic myocardium (Xiao et al., 2020; Xiao et al., 2021a). The primary issue may be impaired activation of HIF-1 regulation of angiogenic factors, leading to an imbalance between angiogenic factors and their inhibitors. This imbalance hinders new blood vessel formation and limits tissue revascularization, ultimately affecting MSCs-mediated tissue repair. Additionally, the inflammatory response following myocardial ischemia releases pro-inflammatory cytokines that inhibit angiogenesis and promote fibrosis (Gomez-Ferrer et al., 2021).

Several mechanisms underlie the beneficial effects of angiogenesis on the ischemic myocardial microenvironment and MSCs function (Shi et al., 2021). Firstly, new blood vessel formation increases oxygen and nutrient delivery to the injured tissue and removes waste products, creating a more conducive environment for MSCs survival and proliferation (Sun et al., 2020b). Secondly, angiogenesis stimulates the release of growth factors and cytokines beneficial for MSCs function. For instance, VEGF and FGF, potent angiogenic factors, also promote MSCs proliferation, migration, and differentiation (King et al., 2021). The restoration of functional vasculature in the ischemic myocardium provides a route for MSCs to migrate to the injury site and plays a key role in ECM reconstruction and inflammatory response modulation (Gomez-Ferrer et al., 2021; Sun et al., 2020b). Vasculature-derived ECM supports MSCs adhesion and migration, essential for efficient recruitment to damaged tissue (King et al., 2021). Angiogenesis also modulates the inflammatory response by promoting anti-inflammatory cell infiltration and reducing pro-inflammatory cytokine levels, creating a less hostile microenvironment for MSCs, allowing for better cell survival and function (Chin et al., 2021).

Promoting angiogenesis in ischemic heart disease can be achieved through activating angiogenic factors (Lupu et al., 2020). Direct intravenous injection, intramyocardial injection, or gene therapy with VEGF and FGF can significantly increase capillary density (Henning, 2016). Overexpression of HIF-1α via adenoviral or lentiviral vectors stimulates angiogenic factor expression (Huang et al., 2014). A recent study showed that while HIF-1α proteins consistently accumulate in the infarct zone, angiogenic factor expression is impaired, possibly due to dysfunction in selective HIF-1 signaling regulation. Supplementing the trace element copper can retune this dysregulation and reactivate HIF-1 target angiogenic factor expression without excessively increasing HIF-1α accumulation (Xiao et al., 2023; Zhang et al., 2016b).

Other techniques, including physical stimuli like low-intensity pulsed ultrasound (LIPUS) and electrical stimulation, have also been explored as non-invasive means to enhance VEGF expression and promote capillary formation (Amaral et al., 2001; Li et al., 2022). Electrical stimulation stimulates the release of angiogenic growth factors from endogenous cells and promotes neovascularization (Zhao et al., 2021). Both techniques have shown positive effects on angiogenesis and cardiac function in preclinical studies.

Another approach involves cell-based therapies, particularly using endothelial progenitor cells (EPCs). EPCs exhibit inherent angiogenic properties and secrete a diverse range of growth factors promoting blood vessel formation. When injected into the ischemic myocardium, EPCs can differentiate into vascular endothelial cells, directly participating in new blood vessel formation (Ghem et al., 2017; Zhang et al., 2008). Combining MSCs and EPCs for ischemic heart disease treatment represents an innovative and promising therapeutic approach. MSCs, known for their immunomodulatory effects, reduce inflammation and promote tissue repair, while EPCs specifically target endothelial cells regeneration. This combination harnesses the regenerative and angiogenic potential of these stem cells to promote cardiac tissue repair and restore blood flow to ischemic regions of the heart.