Age-Associated Myeloid Biased Hematopoiesis Depends on Relative Decrease of Short-Term Hematopoietic Stem Cell

Aging in the human hematopoietic system is known to be associated with reduced self-renewal capacity, myeloid biased hematopoiesis, and increased incidence of hematological disorders. Amongst these changes, myeloid-biased hematopoiesis leads to decreased adaptive immune system function and increased propensity for myeloid malignancies in aged individuals. Recent reports have supported the concept that myeloid-biased alteration is most likely due to cell intrinsic alterations in the hematopoietic stem cell (HSC) compartment and a clonal shift toward myeloid-biased HSCs. Recently, we demonstrated that Hoxb5 specifically marks HSCs with long-term self-renewal capacity (LT-HSC) within the bone marrow of young mice. Additionally, we further demonstrated that around 80% of the cells included within the immunophenotypic HSC fraction (defined by the cell surface markers: Lineage-c-Kit+Sca-1+Flk2-CD150+CD34-/lo) (hereafter referred to as pHSCs) are not LT-HSCs (Chen JY et al., Nature 2016). In this study, using our LT-HSC-specific reporter mouse model, we sought to understand the mechanisms underlying the myeloid-biased alteration with age. First, to verify that Hoxb5 can be used as a LT-HSC-specific marker in aged mice, we utilized a transplantation assay in which 10 cells of Hoxb5+ or Hoxb5- pHSCs isolated from 2-year-aged mice were transplanted with supporting bone marrow cells into lethally irradiated mice. Any mice showing long-term (≧16week) granulocyte reconstitution (≧1% generation in peripheral blood) were considered as a positive for long-term hematopoiesis. Refer to this criteria, continuous hematopoiesis was observed only within the Hoxb5+pHSC recipients, indicating that Hoxb5 marks LT-HSCs within the pHSC compartment throughout the mouse lifespan. Interestingly, the lineage output derived from transplanted aged-Hoxb5+-HSCs was not skewed towards the myeloid lineage when compared to their young counterparts (the frequency of myeloid lineage contribution in donor cells: Aged vs Young = 42.6% vs 50.5%, n.s.). Our findings suggest that the balance between myeloid and lymphoid lineage output of the LT-HSC (Hoxb5+ pHSC) persists throughout life. To further confirm this hypothesis, we co-transplanted 10 cells of 2-year-aged Hoxb5+ pHSC with 10 cells of EGFP-overexpressed young Hoxb5+ pHSC along with supporting bone marrow cells into the same recipients. At 12 weeks posttransplant, peripheral blood (PB) analysis demonstrated that the frequency of myeloid outpu