PU.1 enforces quiescence and limits hematopoietic stem cell expansion during inflammatory stress

Hematopoietic stem cells (HSCs) are capable of entering the cell cycle to replenish the blood system in response to inflammatory cues; however, excessive proliferation in response to chronic inflammation can lead to either HSC attrition or expansion. The mechanism(s) that limit HSC proliferation and expansion triggered by inflammatory signals are poorly defined. Here, we show that long-term HSCs (HSCLT) rapidly repress protein synthesis and cell cycle genes following treatment with the proinflammatory cytokine interleukin (IL)-1. This gene program is associated with activation of the transcription factor PU.1 and direct PU.1 binding at repressed target genes. Notably, PU.1 is required to repress cell cycle and protein synthesis genes, and IL-1 exposure triggers aberrant protein synthesis and cell cycle activity in PU.1-deficient HSCs. These features are associated with expansion of phenotypic PU.1-deficient HSCs. Thus, we identify a PU.1-dependent mechanism triggered by innate immune stimulation that limits HSC proliferation and pool size. These findings provide insight into how HSCs maintain homeostasis during inflammatory stress. Overall design: ChIP-seq for PU.1 in LSK Flk2- CD150+ HSPC with whole cell extract controls

造血干细胞（Hematopoietic stem cells, HSCs）可响应炎症信号进入细胞周期以补充血液系统；然而，慢性炎症诱导的过度增殖可引发造血干细胞耗竭或细胞扩增。目前，针对炎症信号触发的造血干细胞增殖与扩增的调控机制仍不甚明确。本研究显示，经促炎细胞因子白细胞介素（IL）-1处理后，长期造血干细胞（long-term HSCs, HSCLT）可快速抑制蛋白质合成与细胞周期相关基因的表达。该基因表达程序与转录因子PU.1的激活，以及PU.1在受抑制靶基因上的直接结合密切相关。值得注意的是，PU.1是抑制细胞周期与蛋白质合成基因所必需的，IL-1暴露会在PU.1缺陷型造血干细胞中引发异常的蛋白质合成与细胞周期活性。上述特征与表型PU.1缺陷型造血干细胞的扩增存在关联。综上，本研究鉴定出一种由先天免疫刺激触发的、依赖PU.1的调控机制，该机制可限制造血干细胞的增殖与细胞群体规模。本研究结果为造血干细胞在炎症应激过程中如何维持稳态提供了新的见解。实验整体设计：以全细胞提取物为对照，对LSK Flk2- CD150+ 造血干祖细胞（hematopoietic stem and progenitor cells, HSPC）开展PU.1的染色质免疫共沉淀测序（ChIP-seq）。