Kitlo hematopoietic stem cells exhibit distinct lymphoid-primed chromatin landscapes that enhance thymic reconstitution

Hematopoietic stem cells (HSC) with multilineage potential are critical for T cell reconstitution after allogeneic hematopoietic cell transplantation (allo-HCT). The Kit(lo) HSC subset is enriched for multipotential precursors, but their T cell potential remains poorly characterized. Using a preclinical allo-HCT mouse model, we demonstrate that Kit(lo) HSCs provide superior thymic recovery and T cell reconstitution, resulting in improved immune responses to post-transplant infection. Kit(lo) HSCs with augmented bone marrow (BM) lymphopoiesis mitigate age-associated thymic alterations and enhance T cell recovery in middle-aged mice. Mechanistically, chromatin profiling reveals Kit(lo) HSCs exhibiting higher activity of lymphoid-specifying transcription factors, such as, ZBTB1. Zbtb1 deletion diminishes HSC engraftment and T cell potential; by contrast, reinstating Zbtb1 in megakaryocytic-biased Kit(hi) HSCs rescues hematopoietic engraftment and T cell potential in vitro and in vivo. Furthermore, age-associated decline in Kit(lo) HSCs is associated with diminished T lymphopoietic potential in aged BM precursors; meanwhile, Kit(lo) HSCs in aged mice maintain enhanced lymphoid potential, but their per-cell capacity is diminished. Lastly, we observe an analogous human BM KIT(lo) HSC subset with enhanced lymphoid potential. Our results thus uncover an age-related epigenetic regulation of lymphoid-competent Kit(lo) HSCs for T cell reconstitution.