Blood stem cells are like spare keys to a house: they open the door to any room, but over time they wear down and start to stick. In an elderly person, such keys handle the locks worse, and the blood renews itself more slowly. Scientists have long been searching for a way to make a duplicate that stays new even after heavy use.
In the Retro Biosciences lab, researchers took ordinary cells from six adult donors aged 18 to 60 and returned them to an embryo-like state. This trick is called reprogramming: the cell forgets its profession but keeps an age passport that can be read from chemical marks on DNA. After reprogramming, the marks reset, as if the cell had just been born. Then, from these "rejuvenated" cells, they grew blood stem cells and transplanted them into immunocompromised mice.
The cells not only stayed young but also learned to work like adults. They colonized the mice's bone marrow and produced all types of blood cells: red blood cells, white blood cells, platelets. At the same time, their protective caps on the ends of chromosomes, the telomeres, turned out to be longer than those of ordinary adult human stem cells. This means the cells can divide longer before they age.
It was once thought that maturation and aging go hand in hand: a cell cannot become fully functional without accumulating age-related changes. This work shows that the two processes can be separated. The cell undergoes training for an adult profession but keeps a child's body.
So far this is a mouse study, but it changes the way we look at stem cell transplantation. If the method can be transferred to humans, it will be possible to create universal stocks of young blood cells. Then an older patient won't have to wait for a suitable donor and put up with the burden of that donor's lived years. The spare key will be new, no matter how many times it is used.
The Horvath epigenetic clocks used in the work were calibrated on thousands of human tissue samples and can predict age to within a few years. Here they showed that cells that went through reprogramming and two transplants remain younger than seven years, even though their "real" age, counting the donor and the time in culture, could exceed 60 years.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.