Hematopoietic stem cells (HSC) can be harmed by disease, chemotherapy, radiation and normal aging. We now show that damage also occurs in mice repeatedly treated with very low doses of lipopolysaccharide (LPS). Overall health of the animals was good, and there were relatively minor changes in marrow hematopoietic progenitors. However, HSC were unable to maintain quiescence, and transplantation revealed them to be myeloid skewed. Moreover, HSC from treated mice were not sustained in serial transplants and produced lymphoid progenitors with low levels of the E47 transcription factor. This phenomenon was previously seen in normal aging. Screening identified monoclonal antibodies that resolve HSC subsets, and relative proportions of these HSC changed with age and/or chronic LPS treatment. For example, minor CD150Hi CD48− populations lacking CD86 or CD18 expanded. Simultaneous loss of CD150Lo/− CD48− HSC and gain of the normally rare subsets, in parallel with diminished transplantation potential would be consistent with age or Tolllike receptor (TLR) related injury. On the other hand, HSC in old mice differed from those in LPS treated animals with respect to VCAM-1 or CD41 expression, and lacked proliferation abnormalities. HSC can be exposed to endogenous and pathogen derived TLR ligands during persistent low-grade infections. This stimulation might contribute in part to HSC senescence and ultimately compromise immunity.

Key Points• Human hematopoietic cells develop within human iPSCderived teratomas in immunodeficient mice.• Co-transplantation of OP9 stromal cells along with human iPSCs increases hematopoietic specification within teratomas.

SUMMARY To provide a lifelong supply of blood cells, hematopoietic stem cells (HSCs) need to carefully balance both self-renewing cell divisions and quiescence. Although several regulators that control this mechanism have been identified, we demonstrate that the transcription factor PU.1 acts upstream of these regulators. So far, attempts to uncover PU.1’s role in HSC biology have failed because of the technical limitations of complete loss-of-function models. With the use of hypomorphic mice with decreased PU.1 levels specifically in phenotypic HSCs, we found reduced HSC long-term repopulation potential that could be rescued completely by restoring PU.1 levels. PU.1 prevented excessive HSC division and exhaustion by controlling the transcription of multiple cell-cycle regulators. Levels of PU.1 were sustained through autoregulatory PU.1 binding to an upstream enhancer that formed an active looped chromosome architecture in HSCs. These results establish that PU.1 mediates chromosome looping and functions as a master regulator of HSC proliferation.