Distinct roles for NF-?B in hematopoietic stem cells and the bone marrow milieu in promoting hematopoietic aging

Hematopoietic aging is characterized by chronic inflammation associated with myeloid bias, hematopoietic stem cell (HSC) accumulation, and functional HSC impairment. Yet it remains unclear how inflammation promotes aging phenotypes. Nuclear factor ?B (NF-?B) both responds to and directs inflammation, and we present an experimental model of elevated NF-?B activity ("inhibitor of ?B deficient" [I?B-]) to dissect its role in hematopoietic aging phenotypes. We find that while elevated NF-?B activity is not sufficient for HSC accumulation, HSC-autonomous NF-?B activity impairs their functionality, leading to reduced bone marrow reconstitution. In contrast, myeloid bias is driven by the I?B- proinflammatory bone marrow milieu, as observed functionally, epigenomically, and transcriptomically. A single-cell RNA sequencing (scRNA-seq) HSPC labeling framework enables comparisons with aged murine and human HSC datasets, documenting an association between HSC-intrinsic NF-?B activity and quiescence but not myeloid bias. These findings delineate separate regulatory mechanisms that underlie the three hallmarks of hematopoietic aging, suggesting that they are specifically and independently therapeutically targetable. Overall design: CD45.1+ bone marrow was transplanted into young CD45.2 WT and I?B– recipient mice. After 13-15 weeks, donor LT-HSC, MPP2, MPP3 and MPP4 HSPC subsets were isolated from the bone marrow of the bilateral femur and tibia by flow sorting.