Inflammation is an immunological condition that has an important role in controlling microbial infections and is the basis of many natural physiological events of the body in healthy conditions (1). This physiological process plays a vital role in embryo implantation, tissue repair, fetal growth, etc. (2). These inflammations must be carefully regulated to maintain the homeostasis of various tissues and organs. Failure to control inflammation can lead to multiple injuries to the tissues involved and lead to the loss of part or all of that tissue (3). Hemostatic inflammation is an integral part of liver health, and treatment strategies should focus on reversing pathological inflammation to hemostatic inflammation (4).

The liver is one of the vital organs that has a great ability to regenerate itself due to liver cells’ characteristics (5, 6). But when the amount of damage is so much and exceeds liver cells’ ability to regenerate, the function of this tissue is impaired (7). Acute liver failure (ALF) is more common in adolescents and is associated with high mortality (8). In ALF, the metabolic and immunological function of the liver is impaired. It presents with manifestations such as blood coagulation defects, cardiovascular instability, liver encephalopathy, susceptibility to infection, and progressive multiple organ failure (9, 10). The interval between the onset of symptoms and the onset of hepatic encephalopathy distinguishes different forms of the disease (11). In this way, if the time between them is a few hours, it is called hyperacute liver failure (12), but if this interval occurs more slowly and lasts from a few days to a few weeks, it is called acute or sub-acute damage (13). Fibrosis is an intrinsic response to injury that maintains the integrity of the various organs involved in tissue damage due to extensive necrosis or apoptosis (14). Chronic liver disease occurs during persistent inflammation and is associated with the destruction and regeneration of the liver parenchyma, which eventually leads to fibrosis and cirrhosis. There are many different causes for chronic liver disease, autoimmune diseases, including long-term alcohol abuse, toxins, infections, genetic and metabolic disorders (15). For example, during myocardial infarction, fibrosis and collagen fibres become apparent by the end of the first week, preventing heart rupture by maintaining the heart’s structure (16). Tissue fibrosis is also seen in the kidney after ischemia (17) and in the lung after H5N1 influenza and COVID-19 infection (18–20). Figure 1 summarizes some acute and chronic liver diseases and the main mechanisms involved in these diseases development ( Figure 1 ). It is worth noting that various treatments can prevent the progression of liver fibrosis (21) but do not lead to the reversion of fibrous tissue, and there is no definitive treatment for fibrosis.

There is currently no specific drug for treating liver disease, and many of the current treatments are used only in very acute cases due to their side effects (22). Therefore, there is an urgent need to find appropriate treatment methods to prevent liver disease progression and repair liver tissue. Today cell therapy is one of the fields that has raised many hopes and interests in the field of regenerative medicine. Cells such as mesenchymal stromal/stem cells, hepatocytes, hematopoietic cells, immune system cells and endothelial progenitor cells have been used in various studies to treat liver disease (23).

There are advantages and disadvantages to using each of these cells, which are summarized in Table 1 . Among these cells, mesenchymal stem cells have been used extensively in studies due to their desirable properties and have shown promising results. The desirable characteristics of these cells are various isolation sources, easy isolation methods, easy culture methods, expansion and storage, low immunogenicity, self-renewal ability, ability to differentiate into different cell types, immunomodulatory properties, The ability to produce soluble factors such as exosomes and growth factors and the ability to migrate to the site of tissue damage were noted (39, 40). Due to the favourable properties of MSCs-derived extracellular vesicles (EVs), the therapeutic ability of these vesicles has been investigated. This review discusses the effect of inflammation on the progression of liver disease, the role of liver cells and immune cells in the pathogenesis of the liver disease, and studies of MSCs and their EVs (mechanisms).

Under physiological conditions, the liver constantly contacts food-derived foreign proteins, drugs, chemicals, toxins, and intestinal microbiota (41). Kupffer cells (KCs) and dendritic cells (DCs), as well as circulating immune cells such as bone marrow-derived macrophages, natural killer (NK) cells, and neutrophils (42), play an important role in the formation of the liver immune microenvironment and hepatic immune responses (43).

Kupffer cells become highly active in the liver during this disease and produce inflammatory cytokines, including IL-1, IL-6, and TNF-α. In addition, due to the activation of other immune system cells that are recruited from the bloodstream to the liver tissue, systemic inflammation develops that can stimulate necrosis and apoptosis in hepatocytes (44, 45). Overall, inflammation, immune cells and liver cells inflammatory responses play an essential role in developing liver-related diseases (46).

Hepatic steatosis, or fatty liver disease (FLD), is the most common liver disease in the United States (47), occurs in response to alcohol, chemotherapy, toxins such as vanilla chloride, and insulin-related metabolic syndrome and can lead to liver tissue damage (48). This damage is associated with inflammation and fibrosis, alters hepatocyte gene expression, and leads to increased TLRs ligands, TGF-β, CXCL10, and IL-1A receptors (49). In hepatitis, activation of the NF-κB signaling pathway in activated hepatocytes has also been observed. In this pathway, the activated cells release several pro-inflammatory cytokines and chemokines such as TNF-α, IL-6, and CCL2, which mediate liver inflammation (50). Inflammation can lead to necrosis and necroptosis of liver cells (especially in hepatocytes) and result in releasing of a group of molecules called alarmin from them. Alarmins such as the high-mobility group B1 (HMGB1), IL-33, ATP, and formyl peptide is released from necrotic cells (51). In fact, ATP and formyl peptide act as absorbers for neutrophils to the site of tissue damage in the liver. Activation of inflammasomes by reactive oxygen species (ROS) from damaged cells and other damage-associated molecular patterns (DAMPs) can also play a role in liver inflammation (52, 53). As a result of liver inflammation, extensive necrosis and apoptosis in hepatocytes eventually lead to loss of liver function (54). Therefore, controlling inflammatory responses is of particular importance and, if left untreated, can lead to fibrosis (55).

Fibrosis that has occurred in chronic liver disease and inflammation has been extensively studied, but its underlying mechanism in acute liver failure is still unclear (56). The progression of fibrosis leads to cirrhosis, hepatocellular carcinoma, liver failure, and portal hypertension (57). Liver fibrosis is caused by the activation of hepatic stellate cells (HSCs) and the extracellular matrix (ECM) deposition (58). When these cells are activated, the amount of vitamin A and adipogenic-related transcription factors decreases and leads to their differentiation into myofibroblasts, which are the main source of ECM production in liver fibrosis (59). In addition, HSCs contribute to the progression of liver fibrosis by producing inflammatory cytokines and chemokines (60). Immune system cells, especially KCs and circulating macrophages, play a significant role in the TGF-β1-mediated activation of the liver HSCs and increase their survival by the NF-κB-dependent manner (61). reactive oxygen species (ROS) production by KCs and macrophages is another fibrosis stimulant in hepatitis that stimulates the production of collagen 1 in HSCs/myofibroblasts (62).

In response to lipopolysaccharide (LPS), HSCs express chemokines such as IP-10, MCP-1, MIP-1α, MIP-1β, MIP-2, RANTES, and adhesive molecules E-selectin, ICAM-1, VCAM-1, and leading to migration of immune system cells into the liver (63). If liver fibrosis is left untreated, Cirrhosis develops. Cirrhosis is the end stage of progressive fibrosis that lacks effective and comprehensive medical treatment (64). Liver cancer is another consequence of fibrosis and liver inflammation.

Studies have identified the importance of inflammation-related signaling pathways and transcription factors, including STAT3 (55) and NF-κB (65), in liver cancer progression by four mechanisms. (1) Increases the expression of epithelial-to-mesenchymal transition (EMT) associated genes, including matrix metalloproteinases (MMPs) 2 and 9 (in breast cancer), E-cadherin, TWIST (in nasopharyngeal cancer), and cathepsins B. (2) Stimulates angiogenesis and increases tumor growth by regulating the expression of vascular endothelial growth factor (VEGF) (66). (3) Increases the expression of Bcl-2 members and cFLIP families and helps neoplastic cells proliferation and inhibit their apoptosis (67). (4) Increase production of inflammatory cytokines such as IL-1α, TNF-α, IL-1β, EGF-R, IL-6, and CCL2 (50, 68).

There are some major types of cells in the liver that regulate liver different functions, including hepatocytes, hepatic sinusoidal endothelial cells (LSECs), bile epithelial cells (cholangiocytes), HSCs, and KCs (69). Each of these cells performs their specific function, and a defect in their function leads to a defect in the function of the liver (70). In addition, some of these cells play vital roles in regulating the liver microenvironment and participate in inflammation and immune responses ( Figure 2 ).

In vitro studies and co-culture of MSCs with liver tissue-derived cells can increase our understanding of their therapeutic properties and their effectiveness in the treatment of liver disease (106) ( Figure 3 ). In vivo injections of MSCs into animals to treat various liver diseases have shown very promising results (107). These cells exert their therapeutic functions in laboratory models through various mechanisms (108), and in many studies, only some of the therapeutic aspects of these cells have been addressed. Overall these studies show that MSCs exert their therapeutic effect by modulating the immune environment and increasing the regeneration of liver tissue.

EVs are a significant component of intercellular communication that affect the actions of the target cell by transporting various substances, including proteins, lipids, and nucleic acids, from the producing cell to the target cell (150). These vesicles are divided into microvesicles (MVs), apoptotic bodies and exosomes according to their size, internal contents, biogenesis and different surface markers (151). However, the fourth group of these vesicles, smaller than the rest, has been identified as named exomer (152). Each of these vesicles has different contents, typically containing various proteins (including enzymes, cytokines, growth factors and heat shock proteins), different types of nucleic acids (including DNA, microRNA and lncRNAs) and different types of lipids (153). These molecules change target cell function after transferring by EVs bilipid membrane. As a consequence, and depending on the cell type and its physiological conditions, exosomes can lead to differentiation, increase proliferation and survival, or lead to increased apoptosis and decreased cell activity (154).

Many studies have examined the therapeutic effects of MSC derived EVs (MSC-EVs) in different diseases such as pulmonary fibrosis, osteoporosis, skin diseases, cardiovascular diseases, and various liver-related diseases. The results of new studies show that the application of MSCs-EVs has similar therapeutic effects to MSCs (155–157). In fact, the use of EVs is cell-free cell therapy because it preserves the benefits associated with cell therapy and bypasses its disadvantages. These benefits include: (1) lack of immunogenicity, (2) the ability to store them easily, (3) drug loading and increase their efficiency as a drug delivery system and (4) application as a ready-to-use drug (cryopreservation of EVs) (158).

Given that the results of many in vitro and animal model studies have shown that MSCs can migrate to TME, it is thought to be they can be used as carriers for tumor-targeted therapies (159). However, there is little clinical evidence of MSCs recruitment in hepatocellular carcinoma (160). Also, due to the proliferative and differentiating characteristics of MSCs, considering the possibility of malignant transformation and promotion of tumor progression by these cells, most studies are still in the preclinical stage (159). Therefore, to use the therapeutic properties of these cells, researchers used MSC-EVs that do not have these deficiencies (161). Recent studies show that adipose tissue MSCs derived exosomes (MSC-Ex) can increase the chemical sensitivity of liver cancer cells by affecting the mTOR related signaling pathway and leading to increased expression of chemo-sensitive related genes in cancer cells (162). Further studies have shown that these exosomes perform this function by transferring miR-199a to cancer cells (162). MiR-222-3p in BM-MSC-Ex has also been shown to inhibit cancer cell proliferation and increase their apoptosis (163). Another study showed that miR-302a, carried by UC-MSC-Ex, suppressed cyclin D1 as well as the AKT signaling pathway and thereby suppress tumor progression (164). Also, various anti-tumour drugs can be loaded into exosomes and used as targeted therapy (165). In addition, it has been shown that exosomes can be engineered to migrate to the target site, integrate with cancer cells, and deliver anti-tumor drugs to them (166, 167). However, the application of MSC-Ex is still in its initiation, and further studies are needed for their optimal use.

As mentioned, due to the size of MSCs, when injected intravenously, they accumulate in the lungs while EVs are much smaller and can migrate to the site of injury (168). MSC-EVs exert their therapeutic functions through 3 mechanisms that have been studied in vitro, pre-clinical, and clinical trials. These mechanisms include (1) proliferation induction/apoptosis suppression, (2) modulation of immune system responses, (3) reduction of fibrosis. Table 4 summarizes several new studies on the application of MSC-EVs ( Figure 4 ).

Due to the urgent need to treat tissue degenerative diseases, methods that can accelerate tissue repair are of great value. Using MSCs as multipotent stem cells is one of these widely used methods due to its properties. As mentioned, these cells uses different mechanisms such as differentiation into hepatocyte-like cells, reducing apoptosis and increasing hepatocyte proliferation, reducing inflammation, suppressing tissue-damaging immune cells, producing various growth factors, suppressing the function of HSC, and improving the function of LSECs to improve liver diseases. The use of these cells not only prevents further damage to the liver tissue but also accelerates and increases the repair of liver tissue. MSCs migrate to the site of liver injury after intravenous injection due to their chemokine receptors and perform their therapeutic actions there. As mentioned earlier, due to the limitations of cell therapy, MSC-EVs are used today. Many studies have been conducted on the application of MSC-EVs in preclinical studies. The results of these studies indicate the potential ability of these vesicles to improve liver cell function and modulate the immune system (182). Some studies comparing the therapeutic effect of MSCs and MSC-EVs have shown that MSC-EVs have a higher therapeutic potential and better outcomes. Although the therapeutic effects of EVs on liver repair in this study are divided into three distinct mechanisms, these processes are interrelated and complement each other’s functions. In fact, the therapeutic potential of MSC-EVs likely lies in their ability to act simultaneously through multiple signaling pathways. Therefore, using these vesicles does not have cell therapy disadvantages. Due to its benefits, it can perform better therapeutic performance in liver injury animals models.

Although there are numerous clinical trials on the use of MSCs in the treatment of liver disease, to date, no clinical trials have been performed on the use of MSC-EVs despite their benefits. However, it has been used to treat many disorders, including orthopaedic disease (151), neurodegenerative disease (183), myocardial disease (184), renal disease, graft versus host disease (GvHD), pancreatic cancer and type 1 diabetes and has shown promising results (185). Despite the therapeutic potential of MSC-EVs and their beneficial effects in preclinical studies, several issues related to the clinical use of these vesicles remain unresolved. Exosomes as an EV circulate in the blood and transfer their cargos to the target cell through phagocytosis, receptor-mediated endocytosis, fusion, micropinocytosis (186). In the body, the half-life of EVs is estimated at a few minutes and are removed from the bloodstream within a few hours. Therefore, we need multiple injections to treat diseases. Due to this issue, producing EVs in an industrial scale and repeatable method is significant. It is also important to note that despite effective and proven results in animal models, the improvement of these models is usually incomplete. Therefore, more research is needed to investigate the long-term effects of MSC-EVs.

AH: Conceptualization, Investigation, Writing - original draft, Writing - review and editing, Validation. KM: Writing - review and editing, Validation. SS: Edited final version of the manuscript. SH: Supervision and final approval of the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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