Neutrophils can be classified into N1 (Ly6G⁺ CD206^–) and N2 (Ly6G⁺ CD206⁺) phenotypes based on the type of cytokine production, the potential for macrophage activation, and surface molecule expression (16). N1 cells highly expressed pro-inflammatory markers such as IL-1β, IL-12a, TNF-α, classically activated macrophages, and expressed CD49d^high CD11b^low, whereas N2 cells highly expressed anti-inflammatory attributes such as IL-10 or activated macrophages, and expressed CD49d^low CD11b^high (16). The heterogeneity of neutrophils isolated from the left ventricular (LV) after MI was further demonstrated by gene expression profiling and flow cytometry analysis of neutrophils (17). The MI mice model found that although CD206^– N1 neutrophil numbers consistently dominated the first 7 days after MI, CD206⁺ N2 neutrophils gradually increased 5 to 7 days after MI (18, 19). Furthermore, CD206⁺ N2 neutrophils were not found in the peripheral blood of MI mice, suggesting that N2 neutrophils are locally formed in the ischemic cardiac microenvironment. This is consistent with previous reports that the ischemic cardiac microenvironment is sufficient to promote CD206 expression on resident macrophages without IL-4 signaling (19). In vitro studies have shown that interferon-gamma (INF-γ) plus lipopolysaccharide (LPS) induces the N1 phenotype and IL-4 induction induces the N2 phenotype. When exogenous IL-4 is continuously infused 24 h after MI, proinflammatory cytokines are reduced in neutrophils, which implies a conversion of neutrophils to a repair phenotype (20). Nevertheless, the underlying mechanisms of the Ly6G⁺ CD206⁺ subpopulation in the post-MI phase of inflammatory regression remain unclear. To determine the possibility of whether Ly6G⁺ CD206⁺ neutrophils have anti-inflammatory activity, pro-angiogenic, and pro-fibrotic effects, this subpopulation was identified under conditions of induced angiogenesis in transplanted hypoxic tissue. Gene expression analysis demonstrates that this subpopulation of neutrophils has low pro-inflammatory activity and high anti-inflammatory and remodeling activity (21, 22). By correlating N1 and N2 neutrophils with LV functional variables, it was found that LV infarct wall thinning was positively correlated with N1. This may be associated with increased expression of MMP-12 and MMP-25. And LV infarct wall thinning was negatively correlated with N2 neutrophil polarization (17). Moreover, neutrophils can differentiate into other myeloid cell types in an inflammatory environment. Combining neutrophils with discrete cytokines and incubating for long periods can result in antigen-like presentation properties and dendritic cell (DC) characteristics or reprogramming to macrophages (23–26). The neutrophil’s role in injury and repair after MI may be explained in this way.

After MI, the adhesion of integrins to cellular debris increased the activation of calcium and calpain in neutrophils and the size of neutrophil membranes, promoting the function of neutrophils in the phagocytosis of cellular debris (27–29). After completing their phagocytosis, neutrophils must be promptly removed by apoptosis or other means of death. The persistent presence of neutrophils leads to tissue damage and chronic inflammation. Delayed clearance of apoptotic neutrophils is associated with multiple human inflammatory diseases (30). Available studies demonstrate neutrophils contribute to ischemia/reperfusion (I/R) injury. In the ischemic border zone, neutrophils produce and release ROS, leading to acute inflammation and myocardial apoptosis (31). Ultimately, reperfusion damages the originally ischemic myocardium and exacerbates the inflammatory response, causing further impairment of cardiac function and tissue (32). Apoptosis of neutrophils results in corresponding changes on the cell surface, inducing changes in the surface charge of lipids and amino sugars by exposing intracellular molecules such as phosphatidylserine and calreticulin, promoting the exocytosis effect of macrophages (30, 33). In necrotic myocardial tissue, apoptotic neutrophils are predominantly cleared by macrophages, with a small proportion passing through DC to draining lymph nodes (33). In others, a portion of apoptotic neutrophils migrates via the endothelium by reverse migration from the necrotic myocardial tissue into the vessel’s lumen (34).

Clearing apoptotic neutrophils are considered the beginning of inflammation regression and repair after MI (35). First, apoptotic neutrophils can remove cytokines and chemokines to limit their recruitment to the infarcted area. After being phagocytosed by macrophages, it promotes the production of anti-inflammatory factors such as transforming growth factor-β (TGF-β), IL-10, vascular endothelial growth factor (VEGF), and the specialized pro-catabolic mediator SMP to reduce the inflammatory response (36–38). Second, dead or necrotic neutrophils release antimicrobial alpha-defensins that increase and activate macrophage phagocytosis and inhibit inflammatory cytokine release (39). In others, circulating platelets induce neutrophil apoptosis and anti-inflammatory polarization of macrophages through the release of abundant pro-resolution mediators, such as lipotoxin A4, lipolytic E1, and annexin A1 (AnxA1), thereby promoting the regression of inflammation (40–42). Based on a related MI model, inhibition of MMP-12 reduces the expression of CD44, caspase 3, and caspase 8 to inhibit neutrophil apoptosis, leading to a prolonged inflammatory response and further deterioration of LV function (43).

Neutrophil extracellular traps (NETs) are chromatin-based reticular structures, histones, and particles with protein hydrolases that form in the vascular system in infectious and non-infectious diseases. NETs induce monocytes to differentiate into intermediate subpopulations, promoting fibrin deposition and fibrin networks leading to thrombosis (44, 45). Therefore, the magnitude of the intensity of NETs formation directly affects the size of the MI infarct area. In peptide arginine deiminase 4 (PAD4) deficient mice, reduced NET production or increased NET degradation reduced neutrophil infiltration, reduced infarct size, and improved cardiac function (46). Because the NET formation process differs from necrosis or apoptosis, this novel form of neutrophil death in vivo is known as NETosis. Macrophages can also clear NETosis, and the uptake of NETs by macrophages does not induce the secretion of pro-inflammatory cytokines, which may be a potential mechanism by which macrophages can promote the regression of inflammation (47).

Post-MI neutrophils can also promote inflammatory regression and cardiac repair by inducing reparative macrophages, promoting monocyte recruitment, promoting angiogenesis, regulating fibroblast function, and interacting with platelets.

The co-cultivation of neutrophils with activated macrophages inhibits the NF-κB pathway and reduces macrophage-released pro-inflammatory factors (48). The study found that the expression of phagocytic receptor Tyrosine-protein kinase Mer (MerTK), a primary apoptotic cell receptor on macrophages, was reduced in neutrophil-depleted MI mice models, all demonstrating that neutrophils can promote cardiac repair after MI by inducing reparative macrophages (49). In addition, depletion of neutrophils in wild-type mice with anti-Ly6G monoclonal antibody results in reduced Ly6C^high release from the spleen of wild-type mice and failure of timely clearance of necrotic cells and tissue debris, leading to impaired cardiac repair. Neutrophils can also improve cardiac repair after MI by promoting neovascularization through direct or induced macrophage production of VEGF-A, which provides nutrients and oxygen to the tissues surrounding the infarcted area to limit the expansion of the infarcted area. Inhibition of CD49d⁺ VEGFR1^high CXCR4^high neutrophil subpopulation was recently found to impair vascular neogenesis in humans and mice (21, 50).

Neutrophils can interact with fibroblasts to have an impact on cardiac repair. TGF-β promotes the differentiation of cardiac fibroblasts into myofibroblasts after MI by regulating the Smad3 signaling pathway and secretes collagen and other ECM proteins to repair myocardial tissue and temporarily maintain the structure and function of the cardiac (51). Fibroblasts with Smad3 or TGFbR1/2 genes transformed into myofibroblasts upon TGF-β stimulation after infection with adenovirus expressing beta-gal, marked by α-SMA expression. In contrast, fibroblasts infected with Cre’s adenovirus block the expression of TGFbR1/2 or Smad3, leading to impaired transdifferentiation of this cell and thus attenuating cardiac fibrosis and ECM remodeling (52). In a mice model of stress overload, daily injections of Anti-TGF-β neutralizing antibodies starting before surgery inhibited fibroblast activation (53). However, fibroblasts are significant players in the resolution of inflammation and scar formation after MI and are equally important for cardiac repair after MI (54). Whole transcriptome analysis of fibroblasts isolated from infarcted regions of C57BL/6J male mice by RNA sequencing revealed that on day 1 after MI, fibroblasts exhibited a pro-inflammatory phenotype contributing inflammatory cytokines to promote leukocyte recruitment to clear necrotic cardiomyocytes. On day 3 after MI, fibroblasts have anti-inflammatory, pro-fibrotic, and pro-angiogenic effects. On day 7 after MI, fibroblasts produce ECM and inhibit angiogenesis through Thbs1 signaling (54, 55). Co-culture of naïve neutrophils with cardiac fibroblasts in vitro upregulated TGF-β1 expression, further demonstrating that neutrophils can regulate fibroblast function to promote cardiac repair after MI (56).

Platelet-neutrophil complexes (PNCs) are associated with the pathogenesis of myocardial I/R (41, 57, 58). After MI, platelets form PNCs by releasing inflammatory mediators to promote neutrophil recruitment, which further exacerbates the extent of myocardial injury (42). In experiments on male C57BL/6J mice with permanent occlusion of the left coronary artery, a time-dependent accumulation of platelets in the infarcted myocardium was found, leading to a local inflammatory response, ventricular remodeling, and rupture (59). In contrast, antiplatelet therapy inhibits the inflammatory response in the infarcted myocardium and reduces post-infarct ventricular rupture and remodeling (59). Vasodilator-stimulated phosphoprotein (VASP) modulates dynamic changes in the cytoskeleton, thereby regulating the adhesion between neutrophils and platelets (58, 60). Phosphorylation of VASP at Ser153 or Ser235 was observed to reduce the formation of intravascular PNCs and the presence of PNCs in ischemic myocardial tissue in a model of myocardial I/R injury, resulting in a significant reduction in MI size (58). Thus, in addition to mediating ischemic injury after MI, platelets influence inflammatory cell infiltration and negotiate cardiac repair after MI.

Although many trials have shown that antineutrophil therapy can reduce cardiac injury in the acute phase, this has not been the case when applied to the clinical setting (61). The pro-repair effects of neutrophils should be considered when designing anti-inflammatory approaches because of the multifaceted functions of neutrophils. Any therapeutic strategy targeting neutrophils must strike a good balance between reducing the pro-inflammatory effect and maintaining the pro-repair development of neutrophils.

Macrophages exhibit a high degree of plasticity and adaptability in vivo and in vitro. Macrophages evolve into different phenotypes according to the environment in which it lives. The M1 and M2 phenotypes are classified according to surface markers, function and cytokines produced. M1 macrophages are classically activated or pro-inflammatory macrophages that mainly secrete pro-inflammatory factors to promote the development of inflammation and participate in the clearance of pathogens (83). The M2 macrophages are alternatively activated or anti-inflammatory macrophages that stimulate tissue repair and regeneration and have pro-fibrotic and pro-angiogenic effects (84). After MI, the phenotypic profile of macrophages changes significantly. After cardiomyocyte death, myocardial tissue at the infarct site was rapidly infiltrated by Ly6C^high monocytes, peaking at approximately day 3 after MI. Ly6C^high monocytes differentiate into M1 macrophages under the stimulation of IFN-γ, TNF-α and secrete pro-inflammatory and chemokines (IL-12, IL-23, IL-27, TNF, CXCL9, CXCL10, and CXCL11) to exacerbate the inflammatory response while performing the functions of phagocytosis of apoptotic and necrotic cell debris and degradation of ECM (85). It has also been shown that M1-like macrophages can secrete pro-inflammatory miRNAs and pro-inflammatory exosomes to exacerbate myocardial injury after MI and inhibit cardiac healing by exerting anti-angiogenic effects (86). On days 4–7 after MI, Ly6C^low monocytes are preferentially recruited to damaged tissues and differentiate into M2 macrophages with the induction of IL-4 and IL-13. They highly express IL-10, IL-1, CD206, decoy type II receptors, scavenger, and galactose-type receptors. They secrete anti-inflammatory mediators such as CCL17, CCL22, and CCL24, promote the release of Arginase-1 (Arg1), and exert anti-inflammatory effects while initiating ECM remodeling and promoting angiogenesis to participate in tissue repair (86).

However, the origin of M1 and M2 macrophages or whether M2 macrophages are polarized from M1 macrophages remains controversial (87, 88). Some related studies have demonstrated that stimulation of M1 macrophages to the M2 phenotype polarization can reduce poor LV remodeling after MI, thereby improving cardiac function. M2 macrophages secrete cytokines (IL-10 and TGF-β) that initiate tissue remodeling and angiogenesis to inhibit the pro-inflammatory effects of M1 macrophages. The Trib1 gene is associated with the ability to form M2 macrophages. Trib1^–/– mice exhibit impaired fibroblast activation and reduced collagen synthesis in the infarcted region after MI, ultimately leading to frequent cardiac ruptures and a poor prognosis. Inputting exogenous M2 macrophages can rescue this phenomenon and promote myocardial tissue repair after MI (89). IL-4 and IL-10 have been shown to induce the M2 phenotype in infarcted cardiac (90, 91). The administration of long-acting IL-4 complex to mice after coronary artery ligation restored cardiac function, improved cardiac repair, and attenuated adverse ventricular remodeling. However, this effect was not observed in mice unable to form M2 macrophages, which was attributed to the fact that the repairing result of the cardiac was due to M2 macrophage activity rather than IL-4 action (90). The administration of exogenous IL-10 to C57BL/6J mice significantly reduced inflammation in the LV, and a significant increase in gene expression of M2 markers (Arg1, Ym1, and Tgfb5) was observed in the isolated macrophages (91).

As previously mentioned, the level of MHC-II^lowCCR2-macrophages in the YS decreases with age and is replaced by macrophages evolving from circulating monocytes after MI. However, these resident macrophages partially resemble the M2 macrophages phenotype and show cardioprotective properties after MI (89). By reducing resident macrophages, the peri-infarct region undergoes adverse remodeling, and cardiac function is impaired (92). The Lgmn gene is associated with the ability of resident cardiac macrophages to clear apoptotic cardiomyocytes in an MI mice model. Lgmn deficiency exacerbates myocardial injury by promoting the accumulation of apoptotic cardiomyocytes and reducing the index of effluent cells in vivo, and overexpression of Lgmn in cardiac macrophages using an adenoviral vector improves cardiac function in mice after MI (93). Thus, stimulating the generation of resident cardiac macrophages is a cell-mediated approach to cardiac repair after MI.

The anti-inflammatory and tissue repair effects of M2 macrophages are associated with fibroblast activation (91). Activated fibroblasts produce large amounts of prostaglandin E2 (PGE2) to inhibit TNF-α production by macrophages, thereby promoting macrophage phagocytosis of apoptotic neutrophils to mediate the regression of inflammation (94). IL-10-stimulated macrophages exhibited higher proliferation and migration rates. M2 macrophages can increase fibroblast proliferation to reduce the ratio of collagen I and III, thereby significantly decreasing myocardial fibrosis after MI (91, 95). Activated fibroblasts generate contractile force to reorganize ECM to prevent cardiac dilation and rupture after MI (91). Immune-mediated activation of cardiac fibrosis is necessary to avoid cardiac rupture after MI. However, in many cases, the initial beneficial fibroblast activation is uncontrolled, resulting in excessive ECM accumulation that hardens myocardial tissue and deteriorates cardiac function, thereby detrimental to cardiac repair.

M2 anti-inflammatory macrophages can also enhance the ability of mesenchymal stem cells (MSCs) in tissue repair, while MSCs can improve tissue repair by acting on macrophages (96). Animal models have shown this phenomenon. In a mice MI model, intravenous transplantation of human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) stimulated intra- cardiac and extra-cardiac macrophage M2 expression, reduced serum IL-6 and galectin-3 levels to decrease inflammatory responses and improved cardiac function (97). In contrast, the depletion of macrophages with clodronate liposomes attenuated the therapeutic effect of MSCs for MI, with increased infarct size, decreased ejection fraction, accelerated LV remodeling, and increased 30-day mortality in mice (98).

Extracellular vesicle mediates the cardioprotective effects of MSCs through immunomodulatory and anti-inflammatory effects. Targeting MSC-EVs to pro-inflammatory macrophages using engineering techniques can promote local myocardial immune regulation, thereby enhancing inflammation regression and cardiac repair (99, 100). Platelet binding to circulating monocytes is enhanced after MI/R, and the introduction of platelet membrane engineering (P-EVs) also enhances the targeted delivery of EVs to pro-inflammatory macrophages in the local myocardium (59, 101). In a mice model of MI/R injury, P-EVs bind to circulating Ly6C^high monocytes into ischemic myocardial tissue and preferentially differentiate into M1 macrophages after endothelial migration. P-EVs are then endocytosed in situ by M1 macrophages and escape from macrophage lysosomes, releasing miRNAs into the cytoplasm to amplify immunomodulatory effects and promote M1 macrophage polarization into M2 macrophages, ultimately promoting cardiac repair (101).

Adipose-derived stem cell (ADSC) exosomes could also promote macrophage M2 polarization via the S1P/SK1/S1PR1 signaling pathway in an animal model of MI, reducing the inflammatory response to improve cardiac injury after MI (102). Intracoronary infusion of cardiosphere-derived cells (CDC) 48 h after MI in pigs reduces infarct size and decreases microvascular occlusion while promoting the conversion of macrophages to a cardioprotective phenotype (103). In addition to reducing inflammatory myocardial injury, cardiac progenitor cells can promote cardiac repair by regulating monocyte/macrophage phenotypic changes. Cardiac progenitor cell transplantation is a desirable and promising therapeutic measure for MI.

In summary, macrophages play a dual role in the entire pathological process of MI. Among them, the anti-inflammatory and reparative effects of M2 macrophages after myocardial infarction have been recognized. Restoration of cardiac function after MI by M2 macrophages is mainly reflected in three aspects: inhibition of inflammatory response, promotion of angiogenesis and collagen formation, and regulation of macrophage polarization is the key to cardiac repair. Therefore, targeting macrophages offers a potential therapeutic opportunity for cardiac repair after MI.

DNA methylation (DNAm) alters the transcription process by adding methyl groups to specific DNA nucleotides, resulting in inactive gene expression. Myocardial ischemia is protected by aldehyde dehydrogenase 2 (ALDH2). Myocardial ischemic injury caused by the downregulation of ALDH2 after MI was associated with aberrant hypermethylation of the CpG site in the upstream sequence of the ALDH2 promoter, and myocardial injury after MI could be alleviated by enhanced activation of ALDH2 (121).

Histone modifications are mainly post-transcriptional and are the primary mechanism of epigenetic regulation. Phosphorylation, acetylation, methylation, and ubiquitination are the most common modalities (122). Histone deacetylase (HDAC) is a transcriptional regulator, and basic experiments have shown that HDAC inhibitors prevent cardiac remodeling after MI by relying on recovering cardiac fibroblast autophagosomes (123). In addition, HDAC inhibitors may promote cardiac repair and neovascularization in MI. Trazodactin A (TSA) is an effective HDAC inhibitor. Ventricular function was restored in TSA-treated wild-type Kit(+ / +) mice compared with Kit(W)/Kit(W-v) mice. Myocardial function and cardiac repair were restored and promoted by reintroducing TSA-treated wild-type C-kit(+) CSCs into Kit(W)/Kit(W-v) mice with MI. To further validate the beneficial effects of HDAC inhibitors, pretreatment of c-kit + CSC with HDAC by TSA significantly increased c-kit + CSC-derived myocytes and microvessels (124). It is unclear, however, whether specific HDAC4 is involved in the benefits of HDAC inhibition in promoting myocardial repair and maintaining cardiac function. It has even been shown that inhibition of HDAC reduces TNF-α in myocardium and serum, decreases ROS production, avoids cell death, and promotes cell viability, thereby improving recovery of myocardial function after MI (125, 126). TSA can also block α-SMA expression in fibroblasts and reduce AKT activation, thus inhibiting TGF-β1 from promoting fibroblast to myofibroblast transdifferentiation and avoiding adverse cardiac remodeling due to continuous activation of myofibroblasts (127).

In general, non-coding RNAs (ncRNAs) do not encode proteins but function as regulators of protein expression. These RNAs mainly consist of long non-coding RNAs (lncRNAs), microRNAs (miRNAs), small interfering RNAs (siRNAs), and circulating RNAs. A growing body of research shows that ncRNA plays a crucial role in MI and can be used to diagnose and treat MI as a potential biomarker and intervention target (128, 129). MiR-214 is a newly discovered miRNA that can inhibit apoptosis in cardiomyocytes. In the MI rat model, by overexpressing miR-214, infarct size was reduced, cardiac function and hemodynamic status were improved, and LV remodeling was inhibited (130). It has been demonstrated that some miRNAs are essential in reducing the inflammatory response after MI. MiR-155 overexpression increased inflammation and leukocyte infiltration in a mice model of MI. At the same time, this effect was eliminated by knocking down the RNA of SOCS-1, the direct target gene of miR-155 (131). MiR-146a is the miRNA most abundant in CDC exosomes. It reduces inflammation after MI by inhibiting the expression of Irak1 and Traf6. Larger scar masses and thicker infarct wall thickness were observed in MI mice knocked out of miR-146a compared to wild-type mice. Exogenous injection of miR-146a decreased the frequency of cardiomyocyte apoptosis and increased the live tissue of the cardiac (132). In addition, exogenous hydrogen sulfide can attenuate NLRP3 inflammatory vesicle and cysteine-1 formation in a miR-21-dependent manner to reduce the inflammatory response after MI (133). This suggests that cardiac repair after MI may be facilitated by targeting this inflammatory miRNA. NcRNA also regulates angiogenesis and fibrosis in the infarcted region after MI. MiR-126 promotes angiogenesis after MI by acting as a central regulator (134). In addition, plasma levels of miR-126 correlated with platelet activation (135). Overexpression of miR-126 was observed in an MI mice model to enhance functional myocardial angiogenesis in MI by upregulating VEGF, FGF (essential fibroblast growth factor), and DLL4 (notch ligand Δ-4 in mesenchymal stem cells) (136). Studies show that cardiac fibrosis is associated with the miR-34 family after MI and that silencing the entire miR-34 family protects MI mice from pathological cardiac remodeling and improves cardiac function via the TGF-β1/Smad4 pathway (137, 138).

Although the above evidence suggests that modulating epigenetic modifications can promote the regression of the inflammatory response and cardiac repair after MI, the exact mechanisms of how they act on post-MI inflammatory cells have not been elucidated.

Cardiac repair after MI requires neutrophils, and several new studies suggest that HDAC contributes to regulating neutrophil proliferation, differentiation, and apoptosis. Neutrophils in HDAC11-deficient mice exhibited more pro-inflammatory effects, higher migratory and phagocytic capacities, and higher TNF-α and IL-6 expression levels than wild-type mice (139). Therefore, targeting neutrophils by modulating histone modifications may benefit cardiac repair after MI.

Studies have shown that epigenetic modifications increase cardiac repair after MI by regulating monocyte/macrophage phenotypes and inflammation genes (140). Jmjd3, which demethylates H3K27, is essential for M2 polarization in macrophages of mice treated with IL-4. It induces Irf4, Arg1, CD206, and other M2-specific markers (141). Smad3 is an H3K4 methyltransferase that also regulates M2 polarization in macrophages, and its expression level in human monocyte-derived macrophages increases with stimulation of M-CSF, IL-4, and IL-13 (142). In general, inhibition of HDAC exerts anti-inflammatory effects, and monocytes and macrophages using HDAC inhibitors can reduce the production of inflammatory cytokines in an inflammatory environment (143, 144). It was shown that HDAC3 deficiency in macrophages leads to IL-4-induced expression of alternative activating genes and fails to activate most of the inflammatory gene expression program upon LPS stimulation, depending on the activation of the autocrine IFN-β/STAT1 circuit (145, 146). In addition, HDAC4 was also associated with anti-inflammatory effects, and in human monocyte-derived DCs, HDAC4 regulated STAT6 activity and anti-inflammatory gene expression (147). HDAC5 is associated with pro-inflammatory macrophages, and overexpression of HDAC5 in RAW264.7 cells significantly increased the activity of TNF-α, NF-κB, and the secretion of other inflammatory mediators (148). MiR-33 regulates macrophage metabolism, reduces fatty acid oxidation, and promotes glycolysis, thereby maintaining a pro-inflammatory M1 phenotype. Inhibition of miR-33 metabolism repolarizes macrophages to an M2 phenotype with anti-inflammatory and tissue repair effects and induces the accumulation of Foxp3(+) Tregs (149). Although recent studies suggest an interaction between RNA-based mechanisms and histone-modifying enzymes in epigenetic modifications, the exact mechanism in the monocytes/macrophages phenotype is unclear (150).

Transcription factor forkhead box protein 3 (Foxp3) is a master regulator of Treg function, and inhibition of methylation using 5-aza-29-deoxycytidine (DAC) or knockdown of DNA methyltransferase DNMT1 both induce Foxp3 expression and increase Treg number (151). In vitro stimulation of human Tregs results in CpG methylation of conserved regions within the Foxp3 locus in CD4⁺ CD25⁺ CD127^low, leading to Foxp3 expression being lost and pro-inflammatory cytokines being produced (152). In addition, miR-155 is considered critical for maintaining Treg adaptability under adverse conditions. Foxp3-dependent miR-155 enhances the survival of Treg cells by activating STAT5 through the down-regulation of STAT pathway inhibitor SOCS1 protein and enhancing signaling to IL-2 (153).

Therefore, further work is needed to determine which epigenetic mechanisms play a role in neutrophils, monocytes, macrophages, and Treg cells so that these epigenetic modalities can be modulated to promote cardiac tissue repair and inflammation regression in the treatment of post-MI. Although modulating epigenetic modifications can effectively reduce infarct size and promote cardiac repair after MI, applying them to the clinic faces serious challenges. First, we do not fully understand the exact mechanisms of epigenetic modifications. Second, there is some use of vectors in gene therapy, and safety and efficacy still need to be thoroughly evaluated. Thus, the pathway of epigenetic modification has not yet reached the stage of translation to the clinic.