TRAF6 Mediates Basal Activation of NF-κB Necessary for Hematopoietic Stem Cell Homeostasis.

The nuclear factor-kB (NF-kB) family of transcription factors is important for hematopoietic function, including development, maintenance, and differentiation of different hematopoietic lineages in response to cytokines and infection. Although ligand-independent or basal NF-kB signaling is required for HSC homeostasis in the absence of inflammation, the upstream tonic mediators of NF-kB signaling are not known. Herein we describe TNF receptor associated factor 6 (TRAF6) as an essential regulator of HSC homeostasis by preserving self-renewal and quiescence through basal activation of NF-kB. Hematopoietic-specific deletion of Traf6 resulted in impaired HSC self-renewal and fitness. Gene expression, RNA splicing, and molecular analyses of Traf6-deficient HSPC revealed changes in adaptive immune signaling, innate immune signaling, and NF-kB signaling, indicating that signaling via TRAF6 in the absence of cytokine stimulation and/or infection occurs in HSPC and is required for HSC function. In addition, we established that loss of NF-kB signaling is responsible for the major hematopoietic defects observed in Traf6-deficient HSPC as deletion of IKKb similarly resulted in impaired HSC self-renewal and fitness. Taken together, our observations position TRAF6 as an essential regulator of HSC homeostasis by maintaining a minimal threshold level of IKKb/NF-kB signaling.