Adult hematopoietic stem cells activate a normally fetal-restricted program to functionally expand ex vivo.

The ability to robustly expand transplantable hematopoietic stem cells (HSCs) ex vivo enables basic science and clinical therapies otherwise hindered by the scarcity of these self-renewing multipotent cells. Despite recent improvements in long-term ex vivo HSC expansion conditions, the molecular mechanisms required for successful ex vivo expansion of functional HSCs remain unknown. Here we characterized the ex vivo expansion potential of HSCs from mouse fetal liver, young bone marrow, and aged bone marrow at the functional and molecular levels. We find that Lin28b, the in vivo fetal-restricted gene, contributes to the mechanism of both fetal and adult ex vivo HSC expansion. The expression of Lin28b correlates with reconstitution potential, with higher levels observed in HSCs expanded from the fetal liver and young adults. By contrast, expanded aged HSCs fail to robustly express Lin28b and also fail to stably reconstitute the hematopoietic system following transplantation. Consistent with a functional role for this fetal program in ex vivo expanded HSCs, Lin28b-deficient HSCs display aging-associated molecular and functional features following ex vivo expansion cultures. Importantly, Lin28b overexpression during ex vivo expansion was sufficient to enhance reconstitution potential of aged HSCs. In summary, we identify Lin28b as an important regulator of functional HSC expansion ex vivo, suggesting novel opportunities for HSC rejuvenation and clinical applications.