The liver centralizes the systemic metabolism and thus controls and modulates the functions of the central and peripheral nervous systems, the immune system, and the endocrine system. In addition, the liver intervenes between the splanchnic and systemic venous circulation, determining an abdominal portal circulatory system. The liver displays a powerful regenerative potential that rebuilds the parenchyma after an injury. This regenerative mission is mainly carried out by resident liver cells. However, in many cases this regenerative capacity is insufficient and organ failure occurs. In normal livers, if the size of the liver is at least 30% of the original volume, hepatectomy can be performed safely. In cirrhotic livers, the threshold is 50% based on current practice and available data. Typically, portal vein embolization of the part of the liver that is going to be resected is employed to allow liver regeneration in two-stage liver resection after portal vein occlusion (PVO). However, hepatic resection often cannot be performed due to advanced disease progression or because it is not indicated in patients with cirrhosis. In such cases, liver transplantation is the only treatment possibility, and the need for transplantation is the common outcome of progressive liver disease. It is the only effective treatment and has high survival rates of 83% after the first year. However, donated organs are becoming less available, and mortality and the waiting lists have increased, leading to the initiation of living donor liver transplantations. This type of transplant has overall complications of 38%. In order to improve the treatment of hepatic injury, much research has been devoted to stem cells, in particular mesenchymal stem cells (MSCs), to promote liver regeneration. In this review, we will focus on the advances made using MSCs in animal models, human patients, ongoing clinical trials, and new strategies using 3D organoids.
The liver has two functional characteristics that are fundamental to the maintenance of the organism’s homeostasis. First, it centralizes the systemic metabolism and thus controls and modulates the functions of the central and peripheral nervous systems, the immune system, and the endocrine system. Hence, liver failure can cause encephalopathy, immunosuppression, and diabetes, respectively. Second, it intervenes between the splanchnic and systemic venous circulation, determining an abdominal portal circulatory system. For this reason, hepatic pathology can be the cause of portal vein flow obstruction with hypertension in the splanchnic venous circulation and development of portosystemic collateral circulation [
When the liver suffers an injury, either by viruses (hepatitis A, B, or C), toxic substances (alcohol), or immune (primary biliary cholangitis), metabolic (nonalcoholic fatty liver disease (NAFLD)), or tumoral (hepatocarcinoma) diseases, it displays a great capacity for regeneration [
Liver failure is the consequence of a pathological progression that begins with hepatic parenchymal dysfunction and continues with progressive degrees of insufficiency until organ failure. At present, three types of liver failure are fully characterized:
Also, to evaluate short-term mortality, a Model for End-Stage Liver Disease (MELD) has been instituted, based on the determination of creatinine and bilirubin, and it is an international normalized ratio. MELD is mainly used to prioritize treatment by liver transplant to patients with poorer prognoses [
All of the abovementioned types of hepatic insufficiency would benefit from treatment by mesenchymal stem cell transplantation or by stimulating the intrinsic regenerative capacity of the hepatic parenchyma. In this sense, in chronic liver failure it appears more appropriate to test “in situ” regenerative therapies as there is a hepatic functional reserve susceptible to be activated. Thus, in chronic liver failure, a dedifferentiating stimulus of the remaining hepatocytes could constitute the establishment of regenerative niches of the parenchyma. In turn, in acute liver failure, it is predictable that the associated inflammatory response would hamper the effectiveness of intrinsic stem cell activation therapy. Conversely, the administration of mesenchymal stem cells or other cell therapy would be capable of counteracting this harmful stimulus by oxidative and enzymatic stresses, due to their anti-inflammatory and immunosuppressive properties, providing the necessary hepatocyte cellular support that substitutes the functional capacity which has been suppressed. Finally, in cases of acute-on-chronic liver failure, as in the case of acute liver failure, patients present a severe short-term prognosis, which limits their survival as well as the period of time necessary for cell replacement to take place effectively, so extrinsic MSC therapy and exquisite timing to be administered must be taken into account.
The liver is a clearance organ and thus is subject to harmful substances, and it requires a powerful regenerative potential that rebuilds the injured parenchyma. This regenerative mission is mainly carried out by resident liver cells, either mature (hepatocytes and cholangiocytes) or with certain embryonic characteristics (hepatic stem/progenitor cells and biliary stem/progenitor cells) [
Hepatocytes and cholangiocytes have a great proliferative ability, and they stand out in terms of physiological hepatic turnover. In the liver lobule, the hepatocytes have various functional abilities depending on their location. While
At the same time, the hepatocytes have various pathways to reconstitute the liver mass depending on the type of injury. This characteristic has been demonstrated by performing various types of hepatectomies. Depending on the amount of hepatic parenchyma removed, such as 30%, 60%, and 80-90%, regeneration is mainly by hypertrophy, hyperplasia, or dedifferentiation in progenitor cells, respectively [
Cells with certain embryonic or immature characteristics involved in hepatobiliary regeneration are called stem/progenitor cells and are of two types: the hepatic stem/progenitor cells, with intrahepatic location, both in the canals of Hering and in the bile ductules, and the biliary stem/progenitor cells, which are located in the peribiliary glands of the large bile ducts and therefore are intra- and extrahepatic [
The hepatic stem/progenitor population exhibits bipotential differentiation capacity in both hepatocytes and cholangiocytes and expresses stem cell markers such as Sox 9, CD44, CD133, epithelial cell adhesion molecules (EpCAM), neural cell adhesion molecules (NCAM), and cholangiocyte (CK7, CK19) and hepatocyte (CK18) cytokeratins [
The activation of hepatic stem/progenitor cells depends on the cause of the injury and displays various phenotypes. In situations of hepatocyte injury (NAFLD, nonalcoholic steatohepatitis, cirrhosis, acute hepatitis, or cholangiopathies), an intermediate phenotype between stem and mature hepatocytes, so-called intermediate hepatocytes, is induced [
One of the consequences of the ductular reaction of hepatic stem/progenitor cells is the production of cirrhotic regeneration nodules, which do not possess the functional capacity of the hepatic lobule. These nodules are surrounded by fibrous tracts and cause portal hypertension with the development of collateral portosystemic circulation, both extra- (esophageal varices) and intrahepatic [
Histological images of a normal rat (a) and long-term cholestatic (b) liver parenchyma. Note the severe epithelial bile cell proliferation associated with fibrosis and hepatocyte death by necrosis and apoptosis in (b). V: portal vein, A: hepatic artery, B: biliary duct, and H: hepatocytes.
The biliary stem/progenitor cells can differentiate into cholangiocytes, hepatocytes, and pancreatic islets [
Peribiliary gland vascularization originates from branches of the hepatic artery, and for this reason, in the case of hepatic arterial ischemia, they suffer from hypoxia with subsequent oxidative stress which, in turn, activates NF-
Inflammatory liver pathologies such as cholestatic diseases and benign and malignant tumors induce a dedifferentiation process in which structures that are common in its embryonic development are created and are histologically characterized by a massively increased number of bile duct structures [
In the liver, the ductular reactions (bottom) could adopt ductal plate configurations (superior). In addition, the normal hepatic structure, represented by a functional hepatic unit (middle), is also based on the ductal plate configuration.
Three types of ductular reactions are recognized:
In essence, ductular reactions are characterized by the proliferation of reactive bile ducts and are secondary to liver injuries [
Posthepatectomy hepatic failure remains at 10% of cases; one of the most frequently used criteria to predict prognosis in clinical practice is the 50-50 criterion that combines with PT
In order to treat hepatic lesions, much research has been performed on stem cells, especially mesenchymal stem cells (MSCs), to promote liver regeneration after hepatic injury. MSCs have the ability to differentiate into hepatocytes and also to induce immunomodulatory and anti-inflammatory responses [
Mesenchymal stem cells in culture under phase-contrast microcopy. (a) Bone marrow-derived MSC. (b) Adipose tissue-derived MSC. Original magnification 200x.
Chemokines and cytokines secreted by MSCs might be effective in reducing inflammation and hepatocyte apoptosis in both acute and chronic liver injuries. MSCs have been shown to secrete epidermal growth factor (EGF), which promotes hepatocyte proliferation and function during liver regeneration [
Several animal models for both acute and chronic cirrhosis treatment with MSCs have shown benefits. Fang et al. [
Differences in cell membrane CD expression and differentiation capacity between BM-MSC and AD-MSC. Data from [
Surface markers | Differentiation capacity | ||||
---|---|---|---|---|---|
AD-MSC | BM-MSC | AD-MSC | BM-MSC | ||
CD9 | + | + | |||
CD10 | + | + | PPAR |
High | High |
CD11b | + | + | LPL | High | High |
CD13 | + | + | |||
CD29 | + | + | Osterix | Low | High |
CD34 | Unstable | − | Alk phosphatase | High | High |
CD44 | + | + | Osteocalcin | Low | High |
CD45 | − | − | |||
CD49d | + | − | Type II collagen | High | Low |
CD54 | + | Unstable | Aggrecan | Low | High |
CD55 | + | + | Type X collagen | High | Low |
CD58 | + | + | |||
CD71 | + | + | Insulin | Positive | ND |
CD73 | + | + | |||
CD90 | + | + | Sarcomeric actin | Positive | ND |
CD91 | + | + | GATA4 | Positive | ND |
CD105 | + | + | |||
CD106 | + | + | Albumin | Positive | ND |
CD140 | − | + | |||
CD146 | + | + | |||
CD166 | − | + |
Our studies on extrahepatic cholestasis-induced acute-on-chronic liver failure in rats demonstrated that isogenic hepatocyte-predifferentiated AD-MSCs intraparenchymally injected 2 weeks after the cholestasis were able to improve hepatic and extrahepatic complications [
In our model, isogenic transplantation of hepatocyte-predifferentiated AD-MSCs after microsurgical extrahepatic cholestasis reduced the hepatic and extrahepatic pathology secondary to long-term evolution, suggesting that AD-MSC-derived hepatocyte-like cells might be useful for the treatment of end-stage cholestatic liver disease. The direct incorporation of these cells into the fibrotic cholestatic liver could effectively improve the specialized hepatic metabolism and revert changes in the spleen and gonads that are a result of the inflammatory response [
In rats with obstructive cholestasis, portal fibroblasts are the first responders to liver injury [
MSCs might also exert their antifibrotic effects through the secretion of matrix metalloproteinases (MMP-9, MMP-13). These enzymes are normally upregulated during liver fibrosis in response to collagen accumulation, and an increase in their activity could allow a more efficient degradation of the extracellular matrix [
Clinical application of hepatocyte transplantation is prevented by the scarcity of donors, who are logically prioritized for whole organ transplant. Therefore, the use of pluripotent or multipotent cells differentiated toward hepatocytes has been the subject of intense research in patients (see [
There are currently 46 listed clinical trials involving MSC therapy for liver diseases, most focusing on cirrhosis (70%) but also on other acute liver diseases, such as liver failure and hepatitis [
Summary of clinical trials with MSC for liver failure.
Trial PI | Number of patients | Cell type | Cell number | Administration route | Disease |
---|---|---|---|---|---|
Kharaziha et al. [ |
8 | BM-MSCs | Portal vein | Chronic liver failure | |
Amer et al. [ |
40 | BM-MSCs | Intrasplenic vs. intrahepatic | End-stage liver failure | |
Kantarcıoğlu et al. [ |
12 | BM-MSCs | Peripheral vein | Liver cirrhosis | |
Suk et al. [ |
55 | BM-MSCs | Hepatic artery | Liver cirrhosis | |
El-Ansary et al. [ |
12 | BM-MSCs | Intrasplenic vs. peripheral vein | Chronic liver failure | |
Peng et al. [ |
23 | BM-MSCs | Hepatic artery | Liver failure | |
Mohamadnejad et al. [ |
25 | BM-MSCs | Peripheral vein | Decompensated liver cirrhosis | |
Zhang et al. [ |
46 | UC-MSCs | Peripheral vein | Decompensated liver cirrhosis | |
Yu et al. [ |
35 | BM-MSCs | Peripheral vein | End-stage liver failure | |
Zhang et al. [ |
30 | UC-MSCs | Hepatic artery | Decompensated liver cirrhosis | |
Liu et al. [ |
35 | UC-MSCs | Peripheral vein vs. hepatic artery | Acute-on-chronic liver failure | |
Sakai et al. [ |
4 | AD-MSCs | Hepatic artery | Liver cirrhosis |
Most of these studies have not yet reported data. Three studies are not yet recruiting; one will attempt to use Stemchymal (commercial adipose-derived mesenchymal stem cells), and is estimated to be completed in 2020, and the other two will perform a classical MSC infusion via the peripheral vein. Eight of the studies are recruiting: five in China, two in Japan, and one in Spain. There is a long-term follow-up being performed of a completed clinical trial involving Livercellgram (autologous bone marrow-derived MSCs), enrolling by invitation. One of the trials using umbilical cord MSC transfusion in patients with severe liver cirrhosis has been suspended. Twenty-three of these trials have passed their completion date; however, their status has not been verified in more than 2 years. Ten studies have been completed; among them, we highlight those that are outstanding for the breadth of the research (phase 2 studies of end-stage liver failure) and the data provided.
In Kharaziha et al.’s group study [
In another study [
The study by Suk et al. [
Amer et al. [
Similar results were reported by Peng et al. [
New strategies for liver regeneration will take advantage of the progress in tissue engineering and the use of 3D scaffolds. Efforts have focused on
With respect to treatment with organoids, remarkably Takebe et al. [
In conclusion, despite the huge regenerative capacity of the liver after an injury, many diseases involving inflammation or advanced pathology require new strategies to promote liver regeneration
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be considered as a potential conflict of interest.
The authors are most grateful for financing from Roche Farma SA and Foundation Domingo Martínez and Jesús Antolín Garciarena Grants. The authors thank Juliette Siegfried and her team at