Chronic liver disease: how could gene and cell therapy help?

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Chronic liver disease is a significant global health and economic problem, and is the eleventh leading cause of death worldwide. When the liver undergoes enough serious damage, it loses the ability both to function adequately and to repair itself. This is a life-threatening problem. The only treatment currently available is a liver transplant. Could regenerative medicine help?

What do we know?

The liver is the only internal organ in the human body capable of regenerating itself after being damaged.

In chronic liver disease, damage to the liver over long periods of time leads to the accumulation of scar tissue, that limits the ability of the liver to function and repair itself (cirrhosis). This disease is the fifth largest killer in the EU and presently can only be treated with liver transplants.

Cell-based therapy for liver disease is an attractive alternative to liver transplant; it addresses both the challenges of donor tissue availability, and of finding a compatible donor from a suitable tissue match. Researchers have successfully used pluripotent stem cells and adult stem cells to make new liver cells in laboratories. Researchers are also investigating whether cells derived from a patient’s own blood could be used to repair damaged tissue and promote regeneration. This approach is currently being tested in patients to assess its viability for future clinical use.

What are researchers working on?

Researchers are studying hepatocytes (the most common cell type in the liver) and hepatic progenitor cells (stem-like cells located in the liver), to learn how they can regenerate liver tissue. It may be possible to develop treatments that harness the natural ability of liver cells to regenerate the liver.

Researchers are also investigating treatments using other types of stem cells; for example, pluripotent stem cells could be used to develop transplantable cells, capable of replenishing a damaged liver.

Researchers are also developing bio-artificial liver devices. These are devices external to the body which filter the blood plasma of patients with severe liver damage. However, unlike traditional medical devices, bio-artificial devices house healthy human cells to act as biological filters. Researchers are also investigating whether it would be possible to grow liver tissue for transplant using bioengineering techniques.

Studies are also currently exploring the use of cells that suppress inflammation and break down scar tissue in the damaged liver, facilitating regeneration.

What are the challenges?

The number of patients in need of a liver transplant for liver cirrhosis far exceeds the number of eligible donors. Successful transplants also have long-term health implications, as patients are required to take immunosuppressants so that their immune system does not reject the transplanted tissue.

Many stem cell therapies could potentially avoid the issue of immune rejection, as the patient would provide their own cells for transplant. However, further work must still be carried out to make sure that stem cell treatments, particularly pluripotent stem cell treatments, create mature and functional liver cells that are safe for transplantation in the large numbers needed to integrate successfully into the existing tissue. As these technologies progress, researchers will also need to assess the long-term safety and efficacy of these therapies.

Introduction to chronic liver disease

The liver is the largest internal organ of the human body. It performs many vital functions, including removing toxins from the blood, helping to digest food, and fighting infections. It is the only internal organ in the body that can regenerate itself after damage.

The cells carrying out this work in the liver are called hepatocytes. On average, each hepatocyte lives for around 200 to 300 days. In a healthy liver, hepatocytes can divide to make copies of themselves. This means they can replace the cells that die, and can even repair some kinds of damage.

If the liver is severely injured, another type of liver cell is involved in the healing response. These cells are called hepatic progenitor cells (HPCs). HPCs are the liver’s resident stem cells and have been shown in mice to produce new hepatocytes during severe injury. Scientists have developed techniques to extract HPCs from the liver, grow them in large numbers in the lab, and then transplant these cells into injured mice, where they produce new hepatocytes. Scientists have shown that cells with similar properties to mouse HPCs are present in human livers and are investigating their potential to be a useful type of cell for therapy, including how to grow HPCs using clinically appropriate methods, and at a scale where they could be used in patients in the future.

In chronic liver disease, a lot of liver damage occurs over a long period of time – for example, due to a long-term infection. This damage reduces the amount of functional tissue in the liver. This severe damage also impairs the ability of hepatocytes to divide and repair the liver by inducing a process called ‘senescence' (causing cells to stop dividing, and so preventing damaged tissue from regenerating). Furthermore, long-term liver injury causes scar formation in the liver (cirrhosis).

Cirrhosis can lead to a wide range of health complications. These include swelling or bleeding of the veins in the oesophagus and stomach, reduced ability to form blood clots (carrying a risk of severe bleeding), and cognitive issue such as confusion. As cirrhosis progresses, it often causes a build-up of fluid in the lags or ankles (peripheral oedema) or in the spaces within the abdomen (ascites). This fluid build-up can be painful and restrict movement. In severe cases ascites fluid may becoming infected, and it may need to be drained with a tube.

Current treatments

The methods for managing the effects of cirrhosis vary depending on the cause of the damage. Patients may be advised to make lifestyle changes, such as changing their diet or avoiding alcohol. Medicines may also be used to reduce swelling or prevent infection.

The only currently available treatment for patients with advanced cirrhosis is liver transplantation. However, the number of patients in need of organ donation greatly exceeds the number of eligible donors. The process of matching donors with patients is made more complex by the need to make sure that they have matching ‘tissue types’, so that the tissue is not rejected by the body. When a successful transplant can occur, the process is costly, and, and the patient must undergo lifelong immunosuppression. Alternative therapies must therefore be found for patients with liver cirrhosis.

How might gene and cell therapy help?

Researchers are investigating whether gene and cell-based therapies might be used to treat and repair chronic cirrhosis. A major barrier to treating patients in need of a liver transplant is the availability of donor tissue. Being able to grow suitable numbers of donor hepatocytes in the lab could overcome this issue.

Another issue intrinsic to organ transplantation is the need for matching the donor tissue to the recipient, and the risk of rejection by the immune system. Technologies which produce hepatocytes from a patient’s own cells – such as iPS cells – could overcome this risk of rejection. Researchers are also investigating whether immune cells could be used to repair damaged tissue and reduce the patient’s immune response, reducing damage.

Tissue engineers are investigating whether it is possible to grow liver tissue suitable for transplant in lab setting. This would be a highly complex process, as the organ would need to be composed of multiple tissue types, correctly arranged and organised, to ensure its long-term survival and functioning in the body.

Current research

Transplanting donated hepatocytes

Preliminary clinical trials have seen limited success in transplanting donor hepatocytes into a patient’s liver to treat chronic liver disease. However, there are several significant obstacles to this approach.

The attachment and integration (or engraftment) of these new cells into the liver tissue is often very low, due to the harsh environment of the injured liver. Researchers are investigating means of overcoming this issue - for example, by encapsulating donor cells in a protective substance. Another approach being explored is to simultaneously transplant cells which suppress the inflammatory environment, which could potentially reduce the risk of transplant rejection.

Using stem cells to create a supply of cells for transplant

A major barrier to the viability of cell transplants as a therapy for cirrhosis is that it requires large numbers of new cells, which are not readily available. Stem cells, such as induced pluripotent stem cells (iPSCs) or HPCs, combined with new cell culture techniques for growing cells in the lab, could offer significant help in creating enough new hepatocytes for transplants. Using iPSCs to create new hepatocytes may also help avoid attack by the immune system, since iPSCs can be made from cells taken from a patient, such as their skin cells. Researchers are also investigating whether tissue engineering techniques can be used to modify cells for transplant, reducing the risk of immune rejection.

There are still several fundamental questions that must be answered before hepatocyte transplants using stem cells can be used clinically. It will be important to confirm that stem-cell derived hepatocytes are safe for use, and that they function as normal hepatocytes. If these cells are been approved for clinical applications, longer-term studies will need to be carried out to assess the long-term safety and efficacy of their use.

Liver progenitor cells

Researchers have identified liver progenitor cells – precursor cells which can produce new hepatocytes. They are currently undertaking studies to understand how they might be used to both as an investigative tool and to treat patients with chronic liver disease.

Researchers have established techniques to identify and isolate liver stem cells from donor tissue, and grow them in the lab as ‘organoids’ (three-dimensional, cellular ‘models’ of an organ, which can be grown and maintained in a lab). As a three-dimensional construct, organoids more closely resembles the natural environment in the liver than traditional cell culture. These cells grow rapidly and stably. These organoids can be used to investigate new therapies, as well as to try to recreate and understand the mechanisms of disease. In the longer term, liver progenitor cell organoids may provide a tissue source for clinical transplantation; studies in mice, using human cells, suggests that this is a promising avenue of research.

Using immune cells to change the injured liver environment

Another route to new treatments might be to use cell therapies that target the damaged liver environment to reduce inflammation and scarring, encouraging regeneration of remaining liver tissue. Several types of cells are currently being investigated in clinical trials for this purpose, based on their natural role in the body.

Macrophages are a type of immune cell that ‘clean up’ the dead cells and debris left after an infection or injury. They also promote regeneration. Macrophage therapy is potentially promising to help repair damaged liver tissue. Cells in the blood called ‘monocytes’ (macrophage precursors) can be isolated and treated in the laboratory to produce large quantities of macrophages, which have been shown to have a role in resolving liver scarring. Recent work has shown that macrophages secrete enzymes that break down scar tissue directly, and help remove cells that produce the scar tissue in the first place.

When tested in mice with damaged livers, macrophage therapy showed beneficial roles in both reversing scar formation and promoting liver regeneration. When the amount of scar tissue is reduced, the liver functions better. Human macrophages have been assessed in clinical trials and shown to be safe for use in patients with cirrhosis. They are currently being assessed to determine whether they are effective for improving liver function in patients with cirrhosis, compared to the standard care regime.

Macrophages are an attractive cell therapy because they can be derived from a patient’s own blood cells and therefore would not be rejected by the immune system, as sometimes happens with organ transplants.

Next steps

Bio-artificial livers

Bio-artificial livers are a growing area of research that combine technology with biology. Similar to a dialysis machine, these are devices external to the body, through which patient’s blood would be passed and returned to the body. Unlike traditional devices, which use synthetic membranes to filter out toxins and waste, these devices would contain a chamber of healthy, living, hepatocytes. In theory, the cells in this device could remove toxins, produce essential proteins for the body (such as albumin for blood serum), and provide other vital roles of a healthy liver. As with hepatocyte transplant treatments, obtaining large enough numbers of human hepatocytes to live in these bio-artificial livers is a major obstacle. Researchers are attempting to address this issue by using hepatocytes from animal donors (such as pigs) or human stem cells (such as iPSCs) to grow large numbers of fully functional hepatocytes. Clinical results in studies examining bio-artificial livers have been mixed, with some showing no benefits over current treatment methods. However, next generation technologies for growing cells in these bio-artificial livers may improve the quality of the cells, cell metabolic activity and overall effectiveness of these devices for treatment.

Tissue engineering the liver

Tissue engineering research is advancing technologies to bioengineer whole organs, including the liver. Bioengineering a liver is complex, requiring the ability to assemble extremely large numbers of functional hepatocytes into specific three-dimensional structures with other types of cells. Researchers have determined how to create scaffolds for hepatocytes to grow on and direct their general structure, but there are still substantial challenges.

Once again, the need for large numbers of human hepatocytes is a problem. Stem cells, particularly iPSCs, offer the potential for creating the large numbers of hepatocytes needed, but researchers must make sure that these cells show the correct metabolic activity, and will not continue to multiply once in the body. Unregulated growth of stem cell-made hepatocytes has potential to lead to tumours.

As well as the issue of cell numbers, researchers are still investigating how to create bioengineered tissue which incorporates vascular tissue and other cells types, which would be essential for the organs integration, functioning, and survival in the body.

Bioengineering the liver with stem cells is an ambitious goal but has the potential to avoid or solve many problems associated with other treatment methods. Such a technology would completely replace the damaged liver with a new liver, and potentially avoid attack by the immune system if made with iPSCs.