The infusion of autologous BMCs into the hepatic artery or the peripheral veins of patients with cirrhosis has been reported to be safe and feasible. Moreover, the infusion of autologous BMCs increases the serum albumin level and improves liver function[16-23]. Terai et al[24] reported that the infusion of BMCs repopulated the damaged liver and differentiated into albumin-producing hepatocytes in a mouse model of chronic liver injury induced by continuous administration of carbon tetrachloride (CCl₄). These authors also showed that the infusion of BMCs elevated the levels of matrix metalloproteinase (MMP)-2, MMP-9 and MMP-14, reduced liver fibrosis and improved the survival rate[25-28]. Currently, MMPs are considered beneficial factors associated with a reduction in liver fibrosis, and several reports have indicated that adenoviral delivery of MMPs into the liver ameliorates experimental cirrhosis[29-31]. However, the therapeutic mechanism of BMC infusion remains controversial. Several studies have suggested that hematopoietic cells among BMCs may generate hepatocyte-like cells, whereas others have hypothesized that they act primarily by fusion with hepatocytes or through a paracrine effect[32-35]. To generate a greater beneficial effect of BMC infusion, granulocyte-colony-stimulating factor (G-CSF) has been used to mobilize BMCs (monitored by the number of CD34⁺ cells) into the peripheral blood[36].

HSCs can be isolated from the bone marrow or peripheral blood after the administration of G-CSF, which induces the mobilization of CD34⁺ cells into the peripheral blood. HSCs express hematopoietic markers[37-39], such as c-kit, Sca-1, Thy-1, CD34, CD45 and CD133, and can differentiate into hepatocytes. Gordon and colleagues reported that the infusion of CD34⁺ cells isolated from the peripheral blood after the administration of G-CSF via the hepatic artery or portal vein improves the level of serum albumin[40]. Moreover, Pai et al[41] reported that the autologous infusion of expanded mobilized CD34⁺ cells improves the level of serum albumin and the Child-Pugh score. However, the therapeutic mechanism by which HSC infusion ameliorates liver damage remains unclear, although some authors have proposed that infused HSCs can differentiate into hepatocytes through cell fusion or through a paracrine effect[32-35].

More recently, clinical studies using bone marrow-derived MSCs have been conducted. MSCs have several advantages over other cell types, such as their relatively simple acquisition and strong proliferative capacity. Moreover, a sufficient number of MSCs required for clinical trials may be expanded ex vivo, without a loss of differentiation or proliferation potential, and then cryo-preserved until needed. Therefore, MSCs can be injected repeatedly without a concomitant loss of their viability and function. In one study, autologous bone marrow-derived MSCs were infused through the peripheral veins of 4 patients with decompensated cirrhosis. No side effects were observed, and the Mayo End-Stage Liver Disease (MELD) score was improved in half of the patients. Furthermore, the quality of life for all four patients improved by the end of the follow-up period[42]. Kharaziha et al[43] also reported improved liver function in patients with cirrhosis who were injected with autologous MSCs via the peripheral or portal veins. In another study, Jang et al[44] demonstrated the beneficial effects of transplanting autologous bone marrow MSCs for the treatment of alcoholic cirrhosis. MSCs (5 × 10⁷ cells) were injected into the hepatic artery twice at weeks 4 and 8. According to the Laennec fibrosis system, histological improvement was observed in 6 of the 11 patients (54.5%).

We conducted a systematic review to evaluate the safety, feasibility and effects of MSC therapy in patients with liver disease and to explore possible future directions (Table 1). We searched the OVID-Medline, EMBASE and Cochrane library databases for studies published through November 2014 to identify studies in which MSC therapy was administered to patients with liver disease. The main search strategy combined the terms that indicated MSC and liver disease. The methodological quality of the studies was assessed with the SIGN (Scottish Intercollegiate Guidelines Network) checklist. Two authors independently extracted the studies with predefined data fields and included indicators of study quality. Of the 568 studies identified, 14 were eligible for inclusion. These studies evaluated a mean sample size of 32 patients and a mean follow-up of 11.6 mo. The publication year of the studies ranged from 2007 to 2014. The majority of the study designs were small single-cohort studies, clinical trials, or case control studies. Overall, the study quality was moderate or poor. Most of the studies used bone marrow-derived MSCs, and 3 used umbilical cord-derived cells. The majority of the studies used the peripheral route, two used the hepatic artery, one used the portal vein, and one used the intrasplenic route for cell delivery. One study compared the administration of cells by intrasplenic injection and by peripheral administration, whereas another investigation compared the intrasplenic and intrahepatic administration of cells. Although marked heterogeneity was observed among studies with respect to the injection dose, cell source, delivery route and study design, MSC therapy was shown to be safe and feasible. The majority of analyzed studies demonstrated improved liver function, which was measured by biochemical outcome, changes in liver function or associated prognostic indicators. The bilirubin, albumin, aspartate aminotransferase [AST, serum glutamic oxaloacetic transaminase (SGOT)), alanine aminotransferase [ALT, serum glutamic pyruvic transaminase (SGPT)], MELD, Child-Pugh and histological scores (Laennec system) demonstrated a statistically significant improvement in 9/10, 11/11, 7/8, 9/9, 8/8, 4/4 and 1/1 studies, respectively. Additionally, critical adverse events or complications were not observed in any of the studies. Hence, although MSC therapy is a much-needed possibility for treating liver disease, further robust clinical trials and evidence regarding the preferred source of cells, dose and route of delivery are required.

Terai et al[46] demonstrated that BMCs could differentiate into albumin-producing hepatocytes in a mouse model of chronic liver injury. These authors also showed that the infusion of BMCs caused an elevation in the levels of MMP-2, MMP-9 and MMP-14, which was associated with reduced liver fibrosis[25,26]. Furthermore, MSCs present among cultured BMCs may represent candidates for treating cirrhosis[17]. However, Thomas et al[48] reported that macrophages among BMCs were beneficial for repairing cirrhosis. Indeed, macrophages, cells of hematopoietic origin, are known to play a critical role in the regulation of liver fibrosis in murine models[48,49]. Furthermore, the intravenous administration of autologous BMCs caused hepatic homing of the injected cells, which suggests that this easy, safe route may represent an adequate option for BMC infusion in patients with cirrhosis[25,50].

Although HSCs can differentiate into hepatocyte-like cells under specified culture conditions in vitro or in animals with liver injury[24,51-54], controversy still exists concerning the therapeutic mechanisms by which HSCs contribute to hepatocyte regeneration or liver repair. Some authors have proposed that conversion to hepatocytes may occur via cell fusion in vivo[32,33,55]. Other possible explanations for target organ regeneration and improvements in function include the activation of endogenous hepatic progenitor cells and the release of vascular endothelial growth factor (VEGF), which would increase the blood supply to the cells and aid in the repair of the damaged tissue[33,56]. It has also been suggested that HSCs may act in a regenerative capacity simply through upregulating the expression of the B-cell leukemia/lymphoma-2 gene (Bcl-2), which would suppress apoptosis[57,58], and downregulating immune responses in the diseased organ via the interleukin-6 (IL-6) pathway[59].

Recently, several studies have emphasized the critical importance of cell types other than hepatocytes, such as macrophages, hepatic stellate cells and lymphocytes, in the regulation of liver regeneration[60-62]. Moreover, unbalanced immune cell populations or infiltration of the liver by immune cells can disrupt the immune-privileged state of the liver, which may cause liver injury or fibrosis. Accumulating evidence indicates that the therapeutic mechanisms of MSC treatments are better defined than those of BMCs or HSCs in terms of liver regeneration (Figure 1). For instance, MSCs can differentiate into hepatocyte-like cells both in vitro and in vivo[63-65] and can secrete trophic factors, including growth factors, cytokines and chemokines, which promote the regeneration of the impaired liver. Trophic factors expressed by MSCs are known not only to reduce the inflammation, apoptosis and fibrosis of damaged tissues but also to stimulate angiogenesis and tissue regeneration[66-68]. Moreover, trophic factors [i.e., IL-10, hepatocyte growth factor (HGF), transforming growth factor-beta 3 (TGF-β3) and tumor necrosis factor alpha (TNF-α)] secreted by MSCs inhibit the proliferation of hepatic stellate cells and decrease collagen synthesis[69,70]. In addition, MSCs have immune-modulatory properties and are able to migrate to damaged tissues. MSCs can also express various soluble factors, such as nitric oxide (NO), prostaglandin E2 (PGE2), indoleamine 2,3-dioxygenase (IDO), IL-6, IL-10 and human leukocyte antigen G (HLA-G). These soluble factors regulate the proliferation and functions of a variety of immune cells, and they have also been shown to induce regulatory T (Treg) cells[71]. The beneficial effects of autologous bone marrow MSC transplantation in patients with cirrhosis are listed in Table 1.

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Figure 1 Potential role of mesenchymal stem cells in cirrhosis. Potential protective mechanisms of mesenchymal stem cells (MSCs) include the following: (1) trans-differentiation into hepatocyte-like cells; (2) suppression of immune reactions; (3) secretion of trophic factors to suppress activated hepatic stellate cells and to increase the proliferation of both resident hepatocytes and hepatic progenitor cells; and (4) anti-fibrotic action that results from the regulation of activated hepatic stellate cells and immune cells. Solid lines and dashed lines indicate stimulatory and inhibitory modifications, respectively. The + sign represents tentative stimulatory effects. The shadows represent extracellular matrix (ECM) that is secreted from hepatic stellate cells.