‘Supercooling’ keeps livers longer

Scientists say they have tripled the amount of time human livers can be safely kept in storage, giving greater flexibility for doctors and transplant recipients.

Currently livers are kept at four degrees Celsius to avoid the irreparable damage that can occur when cells freeze, so they only remain viable for around nine hours.

The new technique allows them to be “supercooled” without ice storage to minus four degrees, maintaining tissue for up to 27 hours.

The breakthrough, by an international team led by Massachusetts General Hospital (MGH) and Harvard Medical School, US, is described in the journal Nature Biotechnology.

In previous studies, researchers at MGH succeeded in cooling rat livers to minus six degrees by augmenting the protective solution used to cool them with both a modified glucose compound, which lowers the temperature at which the cells freeze, and an ingredient used in anti-freeze, to protect against cold.

However, the technique was unsuccessful when applied to human livers, which are 200 times larger. The size difference significantly increased the risk that ice crystals would start to spontaneously form, making the organ unusable for transplantation. {%recommended 6925%}

The latest work, which was led by Reinier de Vries, Shannon Tessier and Korkut Uygun, added three new steps.

The first was to limit the contact of the storage liquid with air. The researchers found this greatly increased the risk of ice crystals forming. 

Next they included two additional ingredients in the protective solution: trehalose, a glucose derivative, to help protect the cell as well as stabilise the cell membranes, and glycerol to support the protective properties of the glucose compound.

Both additives have been used in the cryogenic preservation of cells in the laboratory, the researchers say, but had not been used in the preservation of organs for transplantation.

Finally, they developed a new method of delivering the preservation solution to the liver. 

Previous studies have manually flushed it through the tissue, but the new solution is thicker, and can damage the cell lining. It also does not spread uniformly, increasing the chance of ice nucleation spreading and freezing the liver. 

The new approach uses machine perfusion – a way of delivering oxygen and nutrients to capillaries in biological tissues while outside the body – at four degrees with the traditional protective solution, then slowly lowers the temperature while increasing the concentration of the new protective additives. 

The staggered approach, the researchers say, allows the tissue time to adjust and ensures the solution spreads throughout the organ more uniformly.

“With supercooling, as the volume increases it becomes exponentially more difficult to prevent ice formation at sub-zero temperatures,” says de Vries. 

“Before, there were a lot of experts who said, ‘well this is amazing in small rats, but it will not work in human organs,’ and now we have successfully scaled it up 200 times from rat to human livers using a combination of technologies.”

The researchers have not yet implanted a liver preserved using this new method into a human subject, but they say traditional standards of assessing liver viability indicate that this process will not negatively affect the organ.