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Summary: The transcription factor hepatic leukemia factor (HLF) is strongly expressed in hematopoietic stem cells (HSCs) and is thought to influence both HSC self-renewal and leukemogenesis. However, the physiological role of HLF in hematopoiesis and HSC function is unclear. Here, we report that mice lacking Hlf are viable with essentially normal hematopoietic parameters, including an intact HSC pool during steady-state hematopoiesis. In contrast, when challenged through transplantation, Hlf-deficient HSCs showed an impaired ability to reconstitute hematopoiesis and became gradually exhausted upon serial transplantation. Transcriptional profiling of Hlf-deficient HSCs revealed changes associated with enhanced cellular activation, and cell-cycle analysis demonstrated a significant reduction of quiescent HSCs. Accordingly, toxic insults targeting dividing cells completely eradicated the HSC pool in Hlf-deficient mice. In summary, our findings point to HLF as a critical regulator of HSC quiescence and as an essential factor for maintaining the HSC pool during regeneration. : Komorowska et al. report that the transcription factor HLF is required to maintain hematopoietic stem cell (HSC) function during regeneration. Moreover, Hlf-deficient HSCs are less quiescent. In accordance with this, toxic insults targeting dividing cells completely eradicate the HSC pool in Hlf-deficient mice. Keywords: HSC, transcription factor, HLF, quiescence, self-renewal, cell cycle, reconstitution, transplantation, 5FU, stress

Summary A gradual restriction in lineage potential of multipotent stem/progenitor cells is a hallmark of adult hematopoiesis, but the underlying molecular events governing these processes remain incompletely understood. Here, we identified robust expression of the leukemia-associated transcription factor hepatic leukemia factor (Hlf) in normal multipotent hematopoietic progenitors, which was rapidly downregulated upon differentiation. Interference with its normal downregulation revealed Hlf as a strong negative regulator of lymphoid development, while remaining compatible with myeloid fates. Reciprocally, we observed rapid lymphoid commitment upon reduced Hlf activity. The arising phenotypes resulted from Hlf binding to active enhancers of myeloid-competent cells, transcriptional induction of myeloid, and ablation of lymphoid gene programs, with Hlf induction of nuclear factor I C (Nfic) as a functionally relevant target gene. Thereby, our studies establish Hlf as a key regulator of the earliest lineage-commitment events at the transition from multipotency to lineage-restricted progeny, with implications for both normal and malignant hematopoiesis., Graphical Abstract, Highlights • During hematopoiesis, Hlf is sharply downregulated upon exit from multipotency • Prolonged Hlf expression instructs GMLPs to adopt myeloid fates • Failure to downregulate Hlf is incompatible with appropriate B and T lymphopoiesis • Hlf governs molecular programs driving myelopoiesis and inhibiting lymphopoiesis, Regulators of early blood cell formation are important in both health and disease. Wahlestedt et al. identify abrupt downregulation of the transcription factor Hlf during hematopoietic differentiation. Failure to downregulate Hlf leads to a drastically skewed output of mature blood cells, positioning Hlf as a critical regulator of hematopoiesis.