Browsing by Subject "Hematopoietic stem cell"

Hematopoietic stem cell (HSC) self-renewal, proliferation and differentiation are regulated by signaling through protein tyrosine kinases (PTK) such as c-kit, Flt-3 and Tie2. PTKs work in concert with receptor protein tyrosine phosphatases (PTPs) to maintain cellular equilibrium. The functions of PTPs in counterbalancing PTK signaling in HSCs however remain incompletely understood. Our laboratory has demonstrated that a heparin binding growth factor, Pleiotrophin (PTN), promotes the expansion of murine long-term (LT)-HSCs via binding to a PTP, protein tyrosine phosphatase receptor type Z (PTPRZ). The addition of PTN to murine PTPRZ-/- c-Kit+Sca-1+Lineage- (KSL) cells caused no expansion of HSCs in culture, suggesting that PTPRZ mediates PTN effects on HSC growth. We subsequently screened for the expression of other receptor PTPs in murine HSCs. Among 21 different receptor PTPs, we found that protein tyrosine phosphatase receptor type S (PTPRS) was significantly overexpressed in mouse and human HSCs compared to more mature hematopoietic cells. Ptprs-/- mice displayed no difference in mature blood counts or phenotypic HSC frequency compared to Ptprs+/+ mice. However, competitive transplantation of bone marrow (BM) cells from Ptprs-/- mice resulted in more than 8-fold increased multilineage hematopoietic repopulation in primary and secondary recipient mice compared to mice transplanted with BM cells from Ptprs+/+ mice. While Ptprs-/- mice displayed no differences in cell cycle status, HSC survival or homing capability compared to Ptprs+/+ mice, PTPRS-/- BM cells expressed significantly increased levels of activated Rac1, a RhoGTPase which regulates HSC engraftment capacity, compared to PTPRS+/+ BM cells. PTPRS-/- BM cells displayed significantly increased transendothelial migration capacity and cobblestone area forming cells (CAFC), consistent with increased Rac1 activation. Furthermore, inhibition of Rac1 abrogated the increased transendothelial migration capacity of PTPRS-/- BM cells, suggesting that the augmented engraftment capacity of PTPRS-/- BM cells was mediated via Rac1. Translationally, we demonstrated that negative selection of human cord blood Lin-CD34+CD38-CD45RA- cells for PTPRS expression yielded a 15-fold enrichment for human long term HSCs compared to Lin-CD34+CD38-CD45RA- cells or Lin-CD34+CD38-CD45RA- PTPRS+ cells. These data suggest that PTPRS regulates HSC repopulating capacity via inhibition of Rac1 and selection of human PTPRS - negative HSCs is a translatable strategy to significantly enrich human cord blood HSCs for transplantation.

Normally, neutrophil pools are maintained by "steady-state" granulopoiesis. Infections and inflammation, however, trigger neutrophilias that are supported by a hematopoietic program of accelerated granulopoiesis known as "emergency" granulopoiesis. Steady-state and emergency granulopoiesis are thought to depend on distinct members of the CCAAT enhancer binding protein (C/EBP) family of transcription factors, yet the extracellular cues that determine these developmental pathways are unclear. I hypothesize that inflammation elicits IL-1 which acts directly on hematopoietic progenitor cells for the induction of emergency granulopoiesis. Indeed, IL-1RI-/- mice fail to mount reactive neutrophilias in response to adjuvant-induced inflammation. Analysis of this specific impairment revealed an unanticipated role for IL-1RI in supporting increased proliferation by granulocyte/macrophage progenitors (GMP) and, surprisingly, more primitive multipotent progenitors (MPP) and hematopoietic stem cells (HSC). Whereas IL-1 drives HSC proliferation directly in vitro, inflammation induces comparable rates of proliferation in IL-1RI deficient and -sufficient HSC, MPP, and GMP in mixed chimeric mice. Thus, IL-1RI signals play a necessary, but indirect role in the support of alum-induced neutrophilias by expanding both pluripotent and myeloid progenitor compartments to accelerate granulopoiesis.

The lack of alum-induced neutrophilia in IL-1RI-/- mice is due to defective mobilization of bone marrow (BM) neutrophils and impaired proliferation of hematopoietic stem and progenitor cells (HSPC). Coincident defects in neutrophil mobilization and HSPC proliferation suggest that the trigger for emergency granulopoiesis might be the exhaustion of neutrophil compartments rather than inflammatory inductions of growth factors. Consistent with this hypothesis, non-inflammatory reductions in BM neutrophil numbers elicit granulopoietic responses similar to those induced by adjuvant. Alum mobilizes BM neutrophils via G-CSF, but increased HSPC proliferation results from a density-dependent mechanism that is only partially dependent on G-CSF. Notably, C/EBPβ, thought to be necessary for enhanced generative capacity of BM, is dispensable for increased proliferation of HSPC, but plays a role in the terminal differentiation of neutrophils. These observations indicate that the draining of BM neutrophil pools is sufficient to activate a latent, homeostatic mechanism of accelerated granulopoiesis. I propose a common model for the regulation of neutrophil production that explains both steady-state and emergency granulopoiesis through negative feedback.