Retro generated a graft from cells donated by six adults that restored human blood formation through two rounds of transplantation in mice and retained a DNA methylation age of about five years
Retro generated a graft from cells donated by six adults that restored human blood formation through two rounds of transplantation in mice and retained a DNA methylation age of about five years
On July 17, the Retro team posted a non-peer-reviewed preprint describing cells from donors aged 18–60 years. After the second transplant, 18 of 23 immunodeficient mice had more than 1% human cells in their bone marrow. The Horvath clock estimated the age of the isolated human CD45-positive hematopoietic cells at 5.28 ± 1.45 years.
Blood stem cells continuously replenish red blood cells, platelets, and immune cells. After transplantation, they must engraft in the bone marrow and continue producing all of these lineages over the long term. This capacity declines with age.
Reprogramming an adult cell into an induced pluripotent stem cell, or iPSC, resets many age-related DNA marks and lengthens telomeres. For transplantation, the cell must then be converted into a blood stem cell and tested in a living organism. Resetting these marks does not remove all damage. In serially cloned mice, cloning success fell sharply by the 58th generation because genomic errors had accumulated.
In 2024, Elizabeth Ng’s group generated cells from iPSCs that engrafted in mouse bone marrow over the long term and produced several blood lineages. In April, Retro described the path from iPSCs to hematopoietic cell transplantation. The new preprint tests two questions: whether these cells can repopulate the bone marrow after a second transplant and whether they retain a youthful pattern of age-related DNA marks.
The team generated hematopoietic stem cells, called iHSCs, from iPSCs in 15 days and injected them into immunodeficient NBSGW mice. This mouse strain permits human grafts to colonize the bone marrow. After 20 weeks, the researchers transplanted bone marrow from recipients representing three donor lines into other mice. In the second cycle, 18 of 23 animals had more than 1% human cells in their bone marrow. These cells again produced red blood cells and several types of immune cells.
In the first transplant, each mouse received five million iHSCs, while control animals received between 50 thousand umbilical cord blood cells and one million adult hematopoietic stem cells. This design shows that the cultured cells can restore blood production at this dose, but it does not compare the potency of a single iHSC with that of a conventional graft.
During 15 days of differentiation, the iHSCs switched off the pluripotency program and activated the hematopoietic program, although their methylation pattern still differed from that of adult blood stem cells. After the cells had lived in mouse bone marrow, their pattern became more similar to the adult profile. Methylation was measured in a mixed population of isolated human CD45-positive cells. The adult profile could therefore reflect either cell maturation in the bone marrow or expansion of a cell subset that was already better able to engraft.
The methylation pattern addressed two separate questions: whether the cells had become similar to adult blood stem cells and whether they retained a young age according to DNA marks. The Horvath clock converts a methylation pattern into an age estimate. After the second transplant, it gave an estimate of 5.28 ± 1.45 years. In this mouse model, cells derived from adult donors acquired features of an adult hematopoietic graft while retaining a young age according to DNA marks.