The developmental fate of hematopoietic stem and progenitor cells is influenced by their physiological context. Although most hematopoietic stem and progenitor cells are found in the bone marrow of ...the adult, some are found in other tissues, including the spleen. The extent to which the fate of stem cells is determined by the tissue in which they reside is not clear. In this study, we identify a new progenitor population, which is enriched in the mouse spleen, defined by cKit
CD71
CD24
expression. This previously uncharacterized population generates exclusively myeloid lineage cells, including erythrocytes, platelets, monocytes, and neutrophils. These multipotent progenitors of the spleen (MPPS) develop from MPP2, a myeloid-biased subset of hematopoietic progenitors. We find that NR4A1, a transcription factor expressed by myeloid-biased long term-hematopoietic stem cells, guides the lineage specification of MPPS. In vitro, NR4A1 expression regulates the potential of MPPS to differentiate into erythroid cells. MPPS that express NR4A1 differentiate into a variety of myeloid lineages, whereas those that do not express NR4A1 primarily develop into erythroid cells. Similarly, in vivo, after adoptive transfer,
deficient MPPS contribute more to erythrocyte and platelet populations than do wild-type MPPS. Finally, unmanipulated
mice harbor significantly higher numbers of erythroid progenitors in the spleen compared with wild-type mice. Together, our data show that NR4A1 expression by MPPS limits erythropoiesis and megakaryopoeisis, permitting development to other myeloid lineages. This effect is specific to the spleen, revealing a unique molecular pathway that regulates myeloid bias in an extramedullary niche.
Abstract
Adult hematopoiesis occurs primarily in the bone marrow (BM), generating mature blood cells as well as maintaining a heterogeneous pool of self-renewing hematopoietic stem cells (HSCs). ...However, studies suggest that the BM may not be the only site of adult hematopoiesis: during times of BM hematopoietic distress stemming from radiation or disease, the spleen can independently reconstitute the blood. Given that the spleen does indeed contain a small, resident population of lineage-sca1+ckit+CD48-CD150+ long-term (LT)-HSCs with potent reconstitution ability, we investigated the peripheral blood cell contribution of these extra-medullary HSCs by transplanting UBC-GFP Tg reporter splenic fragments into a splenectomized syngenic wildtype mouse and tracking the blood cell production over time. While the spleen gives rise to both mature myeloid and lymphoid cells, we find that the proportion is skewed in favor of T cells. We see spleen-derived donor HSCs generating CD4+CD8+ double positive thymocytes for over five months post-transplant, suggesting that this peripheral T cell population derives from a long-lived stem or progenitor cell resident in the spleen. RNA sequencing demonstrates that splenic LT-HSCs are more responsive to Notch signaling than BM LT-HSCS, as indicated by increased levels of Hes1 transcription and decreased levels of the granulocyte/monocyte (GM) lineage priming genes Mpo, Gfi1, and Fcgr2b. Taken together with our finding that splenic HSCs have higher levels of cytoplasmic Notch2, we suggest that the Notch2-Hes1 axis is simultaneously repressing GM priming and enforcing T cell programming even among the most immature HSCs resident in the spleen.
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