Purpose of Review This review summarizes the role of hypoxia and hypoxia-inducible factors (HIFs) in the regulation of stem cell biology, specifically focusing on maintenance, differentiation, and stress responses in the context of several stem cell systems. Stem cells for different lineages/tissues reside in distinct niches, and are exposed to diverse oxygen concentrations. Recent studies have revealed the importance of the hypoxia signaling pathway for stem cell functions. Recent Findings Hypoxia and HIFs contribute to maintenance of embryonic stem cells, generation of induced pluripotent stem cells, functionality of hematopoietic stem cells, and survival of leukemia stem cells. Harvest and collection of mouse bone marrow and human cord blood cells in ambient air results in fewer hematopoietic stem cells recovered due to the phenomenon of Extra PHysiologic Oxygen Shock/Stress (EPHOSS). Summary Oxygen is an important factor in the stem cell microenvironment. Hypoxia signaling and HIFs play important roles in modeling cellular metabolism in both stem cells and niches to regulate stem cell biology, and represent an additional dimension that allows stem cells to maintain an undifferentiated status and multilineage differentiation potential.

There is an ongoing shift in demographics such that older persons will outnumber young persons in the coming years, and with it age-associated tissue attrition and increased diseases and disorders. There has been increased information on the association of the aging process with dysregulation of hematopoietic stem (HSC) and progenitor (HPC) cells, and hematopoiesis. This review provides an extensive up-to date summary on the literature of aged hematopoiesis and HSCs placed in context of potential artifacts of the collection and processing procedure, that may not be totally representative of the status of HSCs in their in vivo bone marrow microenvironment, and what the implications of this are for understanding aged hematopoiesis. This review covers a number of interactive areas, many of which have not been adequately explored. There are still many unknowns and mechanistic insights to be elucidated to better understand effects of aging on the hematopoietic system, efforts that will take multidisciplinary approaches, and that could lead to means to ameliorate at least some of the dysregulation of HSCs and HPCs associated with the aging process. Graphical Abstract

Mobilized peripheral blood (mPB) hematopoietic stem (HSCs) and progenitor (HPCs) cells are primary sources for hematopoietic cell transplantation (HCT). Successful HCT requires threshold numbers of high-quality HSCs to reconstitute hematopoiesis long-term. Nevertheless, considerable percentages of patients and healthy donors fail to achieve required thresholds of HSCs with current mobilization regimens. In this present study we demonstrate that similar to mouse bone marrow (BM) and human cord blood, collection and processing of mouse Granulocyte Colony Stimulating Factor (G-CSF)-, AMD3100/Plerixafor-or G-CSF plus AMD3100/Plerixaformobilized HSCs in 3% O 2 results in enhanced numbers of rigorously-defined phenotypic and for G-CSF -and G-CSF plus AMD3100/Plerixafor -mPB enhanced functionally-engrafting HSCs. These results may be of potential clinical utility.

LIST OF ABBREVIATIONS AGM Aorta-genital ridge-mesonephros ALL Acute lymphoblastic leukemia AML Acute myeloid leukemia ANGPT1 Angiopoietin 1 BCSC Breast cancer stem cell BFU-E Burst-forming unit-erythroid BM Bone marrow CB Cord blood CFU Colony-forming unit CFU-GEMM CFU-granulocyte, erythrocyte, macrophage, megakaryocyte CFU-GM CFU-granulocyte,macrophage CH Clonal hematopoiesis CHIP Clonal hematopoiesis of indeterminate potential CLP Common lymphoid progenitor CMP Common myeloid progenitor CPT1 Carnitinepalmitoyl transferase 1 CRU Competitive repopulating unit CSC Cancer stem cell DPP4 Dipeptidyl peptidase 4 E Embryonic EC Endothelial cell EIF Eukaryotic translation initiation factor in type 2 diabetes • Techniques: qRT-PCR.

Understanding physiologic T‐cell development from hematopoietic stem (HSCs) and progenitor cells (HPCs) is essential for development of improved hematopoietic cell transplantation (HCT) and emerging T‐cell therapies. Factors in the thymic niche, including Notch 1 receptor ligand, guide HSCs and HPCs through T‐cell development in vitro. We report that physiologically relevant oxygen concentration (5% O2, physioxia), an important environmental thymic factor, promotes differentiation of cord blood CD34+ cells into progenitor T (proT) cells in serum‐free and feeder‐free culture system. This effect is enhanced by a potent reducing and antioxidant agent, ascorbic acid. Human CD34+ cell‐derived proT cells in suspension cultures maturate into CD3+ T cells in an artificial thymic organoid (ATO) culture system more efficiently when maintained under physioxia, compared to ambient air. Low oxygen tension acts as a positive regulator of HSC commitment and HPC differentiation toward proT cells in the feeder‐free culture system and for further maturation into T cells in the ATO. Culturing HSCs/HPCs in physioxia is an enhanced method of effective progenitor T and mature T‐cell production ex vivo and may be of future use for HCT and T‐cell immunotherapies.