Acute liver failure (ALF) refers to a potentially reversible disorder that was the result of severe liver injury, with an onset of encephalopathy within 8 weeks of symptom appearance and in the absence of pre-existing liver disease. Acute-on-chronic liver failure refers to a liver failure syndrome in which some patients with chronic liver disease with relatively stable liver function suffer from acute liver decompensation and liver failure due to the effects of various acute injury factors. Liver transplantation is the only curative treatment for this type of end-stage liver disease. The potential of MSCs to repair or regenerate damaged tissue and suppress immune responses makes them promising in the treatment of liver diseases, especially in the field of liver transplantation. Many studies have shown that MSC-based therapies can reduce the symptoms of liver disease due to their paracrine effects. Therefore, compared to the cells they derive from, mesenchymal stem cells-derived extracellular vesicles (MSC-EV) are gradually gaining attention for their enhanced safety, as they do not replicate or cause microvascular embolism, and can be easily stored without losing their properties. It represents a novel and effective cell-free therapeutic agent as alternative to cell-based therapies for liver diseases, and liver failure was also concerned. This study was designed to evaluate the safety and tolerability of MSC-EV in acute-on-chronic liver failure after liver transplantation.

In the MSC-EV group (experimental group), 15 patients will receive a single injection of MSC-EV after their first liver transplantation. In the non-MSC-EV group (control group), 15 patients will not receive MSC-EV therapy after their first liver transplantation.

The outcome of the experimental group will be compared with that of similar control patients undergoing liver transplantation but who will not receive MSC-EV. Both of the two groups will receive standard immunosuppressive therapy( a regimen based on tacrolimus (TAC), mycophenolate mofetyl (MMF) and steroids). Patients participated in the experimental cohort will be infused with a single dose of 10 E10 MSC-EV particles per 100ml, at an appropriate time during the first 1-5 days after transplantation.

Inclusion Criteria:

• aged 18-70 years;
• Acute on chronic liver failure-which is characterized by acute hepatic insult manifesting as jaundice (serum total bilirubin [TBil] ≥ 10×ULN umol/L) and coagulopathy (international normalized ratio [INR] ≥ 1.5 or prothrombin activity < 40%), complicated within 4 weeks by ascites and/or encephalopathy as determined by physical examination, in patients with previously diagnosed or undiagnosed chronic liver disease; Requiring liver transplantation due to acute on chronic liver failure;
• Obtain the patients' consent after informing patients of the purpose and method of the clinical trial;

Exclusion Criteria:

• Past history of malignant disease
• Active uncontrolled infection;
• Combined transplantation
• EBV-negative;
• HIV or HCV positive;
• Retransplantation;