BOSTON End-stage organ failure is the number one cause of death in the United States with a mortality exceeding that of cancer. The ability to replace failing organs through transplantation-on-demand has the potential to save or improve millions of lives each year across the globe.

Although part of the problem is a limited supply of donor organs, the key issue is the limited time that donor organs can survive outside the body. Organ and tissue preservation has been identified as one of the key challenges facing biomedicine today.

Casie Pendexter and McLean Taggart discussing the partial freezing protocol.

The clinical standard for organ preservation is hypothermic preservation at +4°C. However, this limits the time that vascular and metabolically active tissues such as the liver can be stored outside the body to hours.

For the first time, scientists from Massachusetts General Hospital (MGH) and Harvard Medical School (HMS) succeeded in successfully freezing rodent livers to extend preservation duration five-fold by adapting strategies used by animals to endure freezing temperatures in winter.

Freeze tolerance is an effective strategy utilized by multiple organisms in nature. For example, Wood frogs (Rana Sylvatica) can survive in a frozen state at -6°C to -16°C for weeks.

By studying these processes and adopting their learnings, researchers from Center for Engineering in Medicine & Surgery (CEMS) at MGH developed new methods for freezing whole rat livers for five times longer than current methods without losing key indicators of transplant viability.

The central strategy of this protocol is to promote a thermodynamically stable frozen state, while maintaining a sufficient unfrozen fraction to limit ice damage and excessive dehydration; therefore, the approach is coined “partial freezing.”

Importantly, this new technique leverages machine perfusion technology to evenly precondition the organs before subzero storage and to bring the organs back from their state of suspended animation.

Photo of a rat liver being loaded with cryoprotective agents. Photo Credits: Katherine A. Flock, Casie Pendexter, Korkut Uygun.

Shannon Tessier, PhD, the lead author of a Nature Communications study detailing the new protocol, says the extra preservation time can make a major difference when it comes to matching transplanted organs with waiting donors. Tessier is an investigator in the CEMS and Assistant Professor of Surgery at HMS.

“When you have more time, you can search a wider area, and can find an excellent match,” Tessier says. “That means that you have less organ discard, more organs for recipients, and organs that will last longer for the recipient.”

With a focus on conferring a non-injurious partially frozen state, the researchers challenge a central paradigm in cryopreservation that ice should be completely avoided.

Most subzero preservation efforts to date have focused on low cryogenic temperature ranges (<-80°C). At these temperatures, freezing or vitrification approaches can suffer from either lethal intracellular ice formation, mechanical and thermal stresses, cryoprotectant toxicity and/or limited scalability from cell to human organ sized systems.

Instead, there is a potentially advantageous – yet relatively unexplored – “high subzero” temperature range from -4°C to -20C which may enable deeper metabolic stasis than clinical hypothermic storage at +4°C, while avoiding many of the above-mentioned challenges of deep cryogenic storage.

“This approach brings together the cutting edge of thermodynamics, cryobiology and transplantation to realize what the wood frog does routinely, which is get to a thermodynamically stable temperature around -15°C that can keep them alive for weeks and months,” says Mehmet Toner, PhD, the co-corresponding author of the study.

“There is more work to be done, but this success opens the door for banking human organs for durations well beyond what any current technique can achieve.”

Toner is the co-founder of the CEMS and a Professor of Surgery and Health Sciences Technology at HMS.

“With supercooling, we repeated what these animals can do in a way that works for humans,” says co-corresponding author Korkut Uygun, PhD, an investigator at CEMS and an Associate Professor of Surgery at Harvard Medical School.

“Partial freezing takes it a step further, to the best nature can achieve. We are very excited to make a rat liver do what it has never done before – be frozen, thawed and show liver function.”

The work has been supported by funding from the DoD, NIH, NSF, MGH ECOR and public benefit corporation Sylvatica Biotech. The team would like to acknowledge The National Science Foundation Engineering Research Center for Advanced Technologies for the Preservation of Biological Systems (ATP-Bio).

About the Massachusetts General Hospital

Massachusetts General Hospital, founded in 1811, is the original and largest teaching hospital of Harvard Medical School. The Mass General Research Institute conducts the largest hospital-based research program in the nation, with annual research operations of more than $1 billion and comprises more than 9,500 researchers working across more than 30 institutes, centers and departments. In August 2021, Mass General was named #5 in the U.S. News & World Report list of "America’s Best Hospitals." MGH is a founding member of the Mass General Brigham healthcare system.