Cardiovascular disease (CVD) is a leading cause of global mortality, with over 18.5 million deaths attributable to CVD in 2019, with ischemic heart disease (IHD) accounting for half of all CVD-related deaths worldwide (1). IHD is characterized by reduced blood flow to the heart, leading to an imbalance between myocardial oxygen supply and demand. Ischemia of the myocardium can progress to ischemia-reperfusion arrhythmias, myocardial infarction (MI), and even heart failure (2).

Due to its complex pathophysiological mechanisms, understanding the specific mechanisms involved in IHD occurrence can contribute to the development of more effective treatment methods aimed at improving patient survival rates (3–6).

The underlying mechanisms of most CVDs involve innate and acquired immune responses. Among them, inflammation is one of the important complications following IHD, such as MI or reperfusion injury, and the inflammatory cascade response plays an important role in myocardial tissue injury, repair, and remodeling, and mastery of the cell-specific signaling mechanisms that mediate the inflammatory response is essential for the treatment of MI (7–11).

Immune cells are involved in microenvironmental changes following the development of IHD, and macrophages are among the most abundant immune cells in the heart (12, 13).

In healthy conditions, cardiac-resident macrophages constitute 6–8% of non-myocardial cells in adult mice (14). After myocardial ischemia, cardiac macrophages undergo marked changes in phenotype and function and are capable of massive expansion through their proliferation and recruitment of monocytes, a behavior implicated in both the injurious and reparative responses of the heart (7, 15, 16). This is primarily evidenced by the rapid apoptosis of resident macrophages within 2 hours and their near depletion within 24 hours following MI (17). Meanwhile, disruption of cardiac homeostasis leads to recruitment and differentiation of Ly6C^hi monocytes from the bloodstream into macrophages, replacing resident macrophages and persisting long-term (18–20). Upon ischemic insult, circulating monocytes swiftly transition from a rolling to a flowing state via activation of chemokine ligand 2 (CCL2)/monocyte chemoattractant protein (MCP)-1 signaling (21). This prompts their infiltration into the infarcted area, forming a reservoir of monocytes. Stimulated by factors such as colony-stimulating factor and granulocyte growth factor within macrophage colonies, these monocytes differentiate into mature macrophages, thus constituting the primary source of cardiac macrophages (22).

While resident macrophages are scarce in number, recent research indicates their ability to proliferate following cardiac injury, thereby influencing the subsequent recruitment of monocytes. Following cardiac injury, resident macrophages produce inflammatory and chemotactic factors responsible for clearing and degrading apoptotic cardiomyocytes, impacting cardiac conduction (23–25). These resident macrophages are typically divided into CCR2^- and CCR2⁺ subsets, each with distinct mechanisms and functions (26). Tissue-resident CCR2⁺ macrophages promote monocyte recruitment through a MYD88-dependent mechanism, leading to MCP release and monocyte mobilization, while CCR2^- macrophages inhibit monocyte recruitment (27).

Nevertheless, both in terms of quantity and impact, resident cardiac macrophages are not as influential as circulating monocytes, which play a predominant role in the ischemic heart (28). Circulating monocytes differentiate into M1 type early post-MI and transition to M2 type later (29). Classically activated M1 macrophages primarily engage in phagocytosis, MHC II antigen presentation, and reactive oxygen species production (30–32), whereas M2 macrophages stimulate extracellular matrix production, cell proliferation, and angiogenesis, facilitating tissue remodeling and repair (33–35). In conclusion, the dynamic interplay between resident cardiac macrophages and circulating monocytes plays a pivotal role in the response to MI and subsequent cardiac repair processes. Further investigation into the precise molecular pathways governing macrophage behavior in the injured heart holds promise for the development of targeted strategies to enhance cardiac healing and functional recovery.

Exosomes are extracellular vesicles with a diameter of 40–160 nm that are secreted by cells and serve as important vehicles for paracrine signaling (36). Exosomes play pivotal roles in various macrophage-mediated effects, serving crucial functions in cellular processes. Their biogenesis involves distinct stages, commencing with cellular internalization, where the cell membrane engulfs extracellular material to form vesicles. Subsequent fusion with endosomal compartments generates early endosomes, which mature into intraluminal vesicles (ILVs) within late endosome multivesicular bodies (MVBs). Ultimately, MVBs merge with the cell membrane, releasing ILVs as exosomes (37, 38). Exosomes play a significant role in intracellular and intercellular communication by selectively delivering cargoes such as nucleic acids, proteins, and lipids to target cells and organs. They contribute to important processes including angiogenesis, ventricular remodeling, and immune response regulation following cardiac ischemia (39). The therapeutic benefits of exosomes have been validated in various animal and disease models (40–42). Moreover, the unique physiological properties of exosomes, including their circulatory stability, biocompatibility, low immunogenicity, and low toxicity, make them an excellent carrier for targeted drug delivery, which has been applied in a range of diseases such as cancer and inflammation (43). As typical secretory cells, macrophages are capable of secreting various molecular signaling substances and releasing different types of exosomes in the ischemic microenvironment (44–46). Based on this, this review summarizes the research progress in the regulatory mechanisms and therapeutic effects of different phenotypic macrophages and their exosomes in IHD over the past five years. It evaluates their specific biological functions and provides guidance for the utilization of macrophage-derived exosomes in low-risk, highly targeted therapy for IHD.

Due to the non-regenerative nature of adult cardiac cells, prolonged myocardial ischemia often leads to MI, resulting in irreversible loss of cardiomyocytes and even left ventricular remodeling and progressive heart failure (47). Therefore, protecting cardiomyocytes and preventing ventricular remodeling and heart failure are important strategies for treating IHD.

Studies have shown that ventricular remodeling is closely associated with dysregulated immune responses, and monocytes and macrophages are the key effector cells of the immune system (48). During disease onset, a significant accumulation of blood-derived monocytes occurs at the ischemic site of the heart, which subsequently differentiate into macrophages to participate in the immune response (3). In the early phase of MI, the necrosis of a large number of cardiomyocytes triggers an intense inflammatory response, and the number of M1 macrophages will peak at 3–4 days after MI, participating in the removal of damaged cells and phagocytosis of extracellular matrix debris during this inflammatory phase (49). In the following days, the inflammatory phase gradually transitions into the reparative phase, where macrophages shift towards an M2 phenotype. M2 macrophages become predominant from day 5 post-infarction and secrete anti-inflammatory and pro-fibrotic cytokines to facilitate the repair of injured myocardium. It is noteworthy that this transition depends on the timely suppression of the inflammatory response. Prolonged activation of M1 macrophages can lead to extensive cardiomyocyte death, degradation of extracellular matrix, expansion of the infarct area, and adverse ventricular remodeling, ultimately resulting in heart failure (4, 49–54) ( Figure 1 ).

Exosomes carry a cargo of proteins, RNA, DNA, lipids, and metabolites (such as amino acids, ATP, and acylamide) from the cell surface and interior. The types and levels of exosome cargo are influenced by donor cells, microenvironment, or physiological conditions. Through endocytosis, direct membrane fusion, or binding to cell surface receptors, exosomes can effectively target and deliver these biomolecules carrying important information to recipient cells, playing multifaceted roles in altering cell phenotypes, regulating gene expression, controlling the recruitment of inflammatory cells, etc., and participating in the pathogenesis and development of IHD (36, 131–133). Exosomes mediate signal exchanges between immune cells and cardiomyocytes. Studying the exosome cargo and their functions not only provides insight into the communication between cells in healthy and diseased states but also lays a foundation for the clinical application of exosomes. ( Table 3 ).

Macrophages and their exosomes play important roles in the progression of IHD at different stages. M1 macrophages primarily mediate early inflammatory responses, while M2 macrophages regulate the cardiac repair process. As crucial mediators of intercellular communication, exosomes have significant roles in modulating immune responses and facilitating communication between macrophages, cardiac cells, and the microenvironment. Further exploration of the functions and specific mechanisms of macrophages and their exosomes holds promise for uncovering their potential therapeutic applications.

In recent years, regenerative medicine has been widely applied in the treatment of IHD. The efficacy of stem cell therapy, particularly mesenchymal stem cell (MSC) therapy, has been confirmed by numerous preclinical and clinical studies (143, 144). There is also abundant preclinical evidence supporting the use of exosome therapy in animal models (145). Macrophages, due to their specific functional roles in the pathological process of IHD, often serve as target cells in related research. For instance, the studies conducted by Deng et al. and Xu et al. explored the therapeutic mechanisms of MSC-derived exosomes in the treatment of MI, although they act through different signaling pathways, both studies demonstrated that they improved cardiac injury by modulating macrophage phenotypic polarization (146, 147). There have also been studies on directly transplanting macrophages for the treatment of IHD. As mentioned earlier, transplantation of M2 macrophages has shown beneficial effects in reducing cardiac fibrosis and improving heart function in both I/R rats and heart failure mice models (88, 112). Macrophages have the advantage of an innate ability to migrate and settle into damaged tissue, which contributes to the functional implantation of transplanted cells in the damaged heart (78). However, cell therapy itself faces challenges such as poor recruitment and survival rates after transplantation into ischemic hearts. Additionally, macrophage transplantation presents difficulties in maintaining activated macrophages and has lower clinical feasibility (148). Pretreatment of macrophages may be able to partially address these issues. Chen et al. pre-treated bone marrow-derived macrophages with a sodium-dependent glucose transporter 2 inhibitor (SGLT2i) before transplantation into a mouse model of MI. This resulted in the suppression of inflammation, reduction of myocardial cell apoptosis, and promotion of the transformation of native cardiac macrophages into the M2 phenotype, which contributed to the reduction of adverse ventricular remodeling after MI (149). Similarly, Podaru et al. stimulated bone marrow-derived monocytes with macrophage colony-stimulating factor (M-CSF) and IL-4 to induce their differentiation into M2 macrophages before transplantation, resulting in significant improvement in cardiac function and structure in MI mice. The highlight of this study is that the generated M2 macrophages not only enhance the reparative secretion profile of endogenous reparative macrophages but also possess good stability, maintaining an M2-like phenotype even in the inflammatory environment after MI (150). Overall, macrophage transplantation for the treatment of IHD has accumulated some preclinical research data. However, further optimization of cell delivery routes and studies involving the use of human cells are needed before entering clinical trials. In comparison, a more convenient alternative may be the direct delivery of exosomes that carry the therapeutic functions of parent cells. For macrophages, blocking the transport of M1 macrophage-derived exosomes and promoting targeted delivery of M2 macrophage-derived exosomes, or promoting macrophage transformation from the M1 to M2 phenotype through the uptake of exogenous exosomes can all contribute to the repair of damaged myocardial tissue.

Researchers have further developed and optimized exosomes based on their characteristics. In terms of the therapeutic benefits of exosomes themselves, although exosomes exhibit good targeting ability, they have low persistence, and the use of biologics that can prolong their duration of action would enhance their efficacy. Biomaterials, such as hydrogels, have been widely employed in the delivery of exosomes due to their excellent biocompatibility, stability, and mechanical properties, which effectively extend the duration of action and even enhance therapeutic effects (151). For instance, Zou et al. constructed a composite system called Gel@Exo by combining conductive hydrogel with umbilical cord MSC-derived exosomes to improve their therapeutic effects on MI. This composite system offers advantages such as controllable gel kinetics, injectability, conductivity matching with natural myocardium, adaptability to heartbeats, softness, dynamic stability, and good cellular compatibility. It significantly improves the retention time of exosomes in the heart and optimizes their therapeutic effects (152). Furthermore, engineering modifications of exosomes can enhance their stability, bioactivity, and target binding capability at both cellular and tissue-specific levels, thereby further improving their therapeutic efficacy for diseases (153, 154). Techniques such as pre-treating parent cells and incorporating self-assembling peptides into the exosomes membrane allow the generated exosomes to better cope with the complex physiological environment of ischemic hearts (155, 156).

Clinical trials of macrophages and their exosomes for the treatment of cardiovascular diseases are still in the early stages, but preliminary results have shown promise. In 2013, Perin et al. (NCT00824005) significantly improved the left ventricular ejection fraction (LVEF) of patients with chronic ischemic heart disease by administering non-expanded autologous bone marrow macrophages via transendocardial injections. This trial demonstrated the potential positive outcomes of ex vivo expansion of macrophages for cardiac repair in patients with chronic ischemic heart disease (157). Further investigations revealed a total of 11 completed clinical studies on cell therapy using macrophages, focusing on conditions such as cardiomyopathy, arterial diseases, tumors, and other ailments. Among them, three trials (NCT01670981, NCT01020968, and NCT00765518) involved intramyocardial injection of Ixmyelocel-T, which contained a mixture of macrophages, granulocytes, monocytes, mixed myeloid progenitor cells, lymphocytes, and mesenchymal stem/stromal cells, to treat heart failure due to ischemic dilated cardiomyopathy. These studies showed a reduction in major adverse cardiovascular events and improvement in symptoms, attributing the effectiveness to the M2 macrophages in Ixmyelocel-T. These M2 macrophages were found to be effective in removing apoptotic cells, limiting tissue damage, and promoting wound healing, confirming the efficacy of macrophage therapy (158, 159).Additionally, two studies on autologous M2 macrophage therapy in children with severe cerebral palsy and patients with non-acute stroke confirmed the clinical efficacy of M2 macrophages in promoting neurological recovery without serious adverse events or cellular rejection (160, 161), further demonstrating the safety and efficacy of this approach.

As for exosome-related clinical trials, research has focused on exosomes secreted by plasma, mesenchymal stem cells, and human induced pluripotent stem cells for conditions like cancer, myocardial infarction, and decompensated cirrhosis. However, there have been no studies on exosomes secreted by M1 and M2 macrophages for the treatment of ischemic heart disease (IHD) yet. Nonetheless, a clinical trial investigating the phenotype of macrophages and their exosomes in type 2 diabetic patients who develop myocardial infarction (NCT02768935) suggested that the treatment of IHD with macrophages and their exosomes was progressing toward clinical therapy.

Overall, while there are fewer clinical trials involving macrophages and their exosomes in IHD, the available data have laid the initial groundwork for the development of new therapeutic strategies. As more studies are conducted, we can expect to gather more clinical evidence regarding the potential use of macrophages and their exosomes in the treatment of cardiovascular diseases.

From the perspective of the delivery characteristics of exosomes, as endogenous extracellular vesicles, they possess excellent blood stability, substance transport properties, and high targeting ability, making them promising candidates as nanocarriers for drug delivery (162–165). This has the potential to overcome the limitations associated with traditional treatments for CVDs, such as low bioavailability, poor retention, inadequate targeting, and complex drug resistance. By utilizing methods such as ultrasound, transfection, incubation, transgenesis, freeze-thaw cycles, and heat shock, drugs can be loaded into exosomes for delivery, aiming to achieve therapeutic effects and improve prognosis (162). The study by Gao et al. utilized macrophage-derived exosomes as a nanoplatform for loading the anti-inflammatory drug methylprednisolone acetate, enhancing its anti-inflammatory and antioxidant effects, rescuing the viability of an in vitro inflammatory cardiomyocyte model, and providing preliminary evidence for the effectiveness of macrophage-derived exosomes as a drug carrier for cardiac immunotherapy (166). On the other hand, due to their abundant cell sources and diverse cargo, exosomes exhibit complex functional heterogeneity, which presents tremendous potential in clinical applications, including disease diagnosis. For instance, several studies have indicated that non-coding RNAs, such as miR-183, miR-21, and miR-208a, found in circulating extracellular vesicles, are closely associated with myocardial ischemic injury and hold promise as future biomarkers for IHD (167–169).

In summary, macrophages and their exosomes actively participate in and regulate the progression of IHD, potentially serving as key therapeutic tools for IHD treatment in the future. Additionally, ongoing optimization of cell therapy and advanced engineering techniques for exosomes are driving the development of related therapeutic strategies. Further exploration of the functions and mechanisms of macrophages and their exosomes will contribute to a deeper understanding of the pathophysiological mechanisms underlying IHD and provide new insights for its treatment.

MW: Visualization, Writing – original draft, Writing – review & editing. CL: Writing – original draft, Writing – review & editing. YL: Writing – original draft, Writing – review & editing. YJ: Visualization, Writing – review & editing. YY: Writing – review & editing. XT: Writing – review & editing. CZ: Writing – review & editing.