An epigenetically distinct HSC subset supports thymic reconstitution

Hematopoietic stem cells (HSCs) with multilineage potential are critical for effective T cell reconstitution and restoration of the adaptive immune system after allogeneic Hematopoietic Cell Transplantation (allo-HCT). The Kitlo subset of HSCs is enriched for multipotential precursors,1, 2 but their T-cell lineage potential has not been well-characterized. We therefore studied the thymic reconstituting and T-cell potential of Kitlo HSCs. Using a preclinical allo-HCT model, we demonstrate that Kitlo HSCs support better thymic recovery, and T-cell reconstitution resulting in improved T cell responses to infection post-HCT. Furthermore, Kitlo HSCs with augmented BM lymphopoiesis mitigate age-associated thymic alterations, thus enhancing T-cell recovery in middle-aged hosts. We find the frequency of the Kitlo subset declines with age, providing one explanation for the reduced frequency of T-competent HSCs and reduced T-lymphopoietic potential in BM precursors of aged mice.3, 4, 5 Chromatin profiling revealed that Kitlo HSCs exhibit higher activity of lymphoid-specifying transcription factors (TFs), including Zbtb1. Deletion of Zbtb1 in Kitlo HSCs diminished their T-cell potential, while reinstating Zbtb1 in megakaryocytic-biased Kithi HSCs rescued T-cell potential, in vitro and in vivo. Finally, we discover an analogous Kitlo HSC subset with enhanced lymphoid potential in human bone marrow. Our results demonstrate that Kitlo HSCs with enhanced lymphoid potential have a distinct underlying epigenetic program.