Distinct causes of three phenotypic hallmarks of hematopoietic aging.

Hematopoietic aging is characterized by chronic inflammation associated with myeloid bias, HSC accumulation, and functional HSC impairment. Yet it remains unclear how inflammation promotes these aging phenotypes. NFkappaB both responds to and directs inflammation, and we present an experimental model of elevated NFkappaB activity (IkappaBminus) to dissect its role in hematopoietic aging phenotypes. We found that while elevated NFkappaB activity is not sufficient for HSC accumulation, HSC-autonomous NFkappaB activity impairs their functionality, leading to reduced bone marrow reconstitution. In contrast, myeloid bias is driven by the IkappaBminus proinflammatory bone marrow milieu as observed functionally, epigenomically, and transcriptomically. A new scRNA-seq HSPC labeling framework enabled comparisons with aged murine and human HSC datasets, documenting an association between HSC-intrinsic NFkappaB 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.