Complementary Signaling through flt3 and Interleukin-7 Receptor α Is Indispensable for Fetal and Adult B Cell Genesis

Extensive studies of mice deficient in one or several cytokine receptors have failed to support an indispensable role of cytokines in development of multiple blood cell lineages. Whereas B1 B cells and Igs are sustained at normal levels throughout life of mice deficient in IL-7, IL-7Rα, common cytokine receptor gamma chain, or flt3 ligand (FL), we report here that adult mice double deficient in IL-7Rα and FL completely lack visible LNs, conventional IgM+ B cells, IgA+ plasma cells, and B1 cells, and consequently produce no Igs. All stages of committed B cell progenitors are undetectable in FL−/− × IL-7Rα−/− BM that also lacks expression of the B cell commitment factor Pax5 and its direct target genes. Furthermore, in contrast to IL-7Rα−/− mice, FL−/− × IL-7Rα−/− mice also lack mature B cells and detectable committed B cell progenitors during fetal development. Thus, signaling through the cytokine tyrosine kinase receptor flt3 and IL-7Rα are indispensable for fetal and adult B cell development

The earliest thymic T cell progenitors sustain B cell and myeloid lineage potential

The stepwise commitment from hematopoietic stem cells in the bone marrow to T lymphocyte-restricted progenitors in the thymus represents a paradigm for understanding the requirement for distinct extrinsic cues during different stages of lineage restriction from multipotent to lineage-restricted progenitors. However, the commitment stage at which progenitors migrate from the bone marrow to the thymus remains unclear. Here we provide functional and molecular evidence at the single-cell level that the earliest progenitors in the neonatal thymus had combined granulocyte-monocyte, T lymphocyte and B lymphocyte lineage potential but not megakaryocyte-erythroid lineage potential. These potentials were identical to those of candidate thymus-seeding progenitors in the bone marrow, which were closely related at the molecular level. Our findings establish the distinct lineage-restriction stage at which the T cell lineage-commitment process transits from the bone marrow to the remote thymus. © 2012 Nature America, Inc. All rights reserved

Acute Multiple Organ Failure in Adult Mice Deleted for the Developmental Regulator Wt1

There is much interest in the mechanisms that regulate adult tissue homeostasis and their relationship to processes governing foetal development. Mice deleted for the Wilms' tumour gene, Wt1, lack kidneys, gonads, and spleen and die at mid-gestation due to defective coronary vasculature. Wt1 is vital for maintaining the mesenchymal–epithelial balance in these tissues and is required for the epithelial-to-mesenchyme transition (EMT) that generates coronary vascular progenitors. Although Wt1 is only expressed in rare cell populations in adults including glomerular podocytes, 1% of bone marrow cells, and mesothelium, we hypothesised that this might be important for homeostasis of adult tissues; hence, we deleted the gene ubiquitously in young and adult mice. Within just a few days, the mice suffered glomerulosclerosis, atrophy of the exocrine pancreas and spleen, severe reduction in bone and fat, and failure of erythropoiesis. FACS and culture experiments showed that Wt1 has an intrinsic role in both haematopoietic and mesenchymal stem cell lineages and suggest that defects within these contribute to the phenotypes we observe. We propose that glomerulosclerosis arises in part through down regulation of nephrin, a known Wt1 target gene. Protein profiling in mutant serum showed that there was no systemic inflammatory or nutritional response in the mutant mice. However, there was a dramatic reduction in circulating IGF-1 levels, which is likely to contribute to the bone and fat phenotypes. The reduction of IGF-1 did not result from a decrease in circulating GH, and there is no apparent pathology of the pituitary and adrenal glands. These findings 1) suggest that Wt1 is a major regulator of the homeostasis of some adult tissues, through both local and systemic actions; 2) highlight the differences between foetal and adult tissue regulation; 3) point to the importance of adult mesenchyme in tissue turnover

Cytokine Regulation of Hematopoietic Stem Cells and Lymphopoiesis

Large numbers of blood cells need to be continuously replaced in order to sustain the crucial functions of the immune system, oxygen transport and blood clotting. The large diversity of cell types required to maintain the integrity of the blood system are all produced from blood forming or hematopoietic stem cells (HSCs). HSCs have the unique property of self renewal, ensuring life-long replenishment of all the blood cell types. Hematopoietic growth factors, so called cytokines, have previously been demonstrated to be important for the survival and proliferation of committed progenitor cells and development of different blood cell lineages. However, their potential role in regulating HSCs remains to a large degree unresolved. Herein, we provide data that establish the adaptor protein LNK as an important negative regulator of postnatal HSC expansion, acting as an inhibitor of the cytokine Thrombopoietin, known to be important for HSC maintenance and/or expansion. Furthermore, whereas previous studies demonstrated that another cytokine, fms-like tyrosine kinase receptor 3 (FLT3) ligand (FL), is an important regulator of B cell progenitors but has a redundant role in steady state maintenance of mature B cells, we here demonstrate that FL plays a crucial role in the regeneration of not only B cell progenitors but also mature B cells after bone marrow (BM) transplantation and chemotherapy. In addition, through studies of FL-deficient mice we show that FL plays an important role in maintaining conventional B cells with age, but is redundant in maintaining a normal compartment of fetally/postnatally derived B1 and marginal zone B cells. It has been disputed whether FL also is important in HSC regulation. Our present findings establish that FL is not important for regulating HSC maintenance or expansion neither during fetal development or in adult steady state hematopoiesis, nor following BM transplantation or chemotherapy-induced BM ablation. Finally, through identification of a multipotent progenitor in adult BM, with sustained granulocyte-monocyte and lymphocyte potential, but little or no megakaryocyte and erythroid potential, we provide evidence for a strict separation of myelopoiesis and lymphopoiesis, not being the first lineage commitment step of HSCs

Delineating the cellular pathways of hematopoietic lineage commitment.

The prevailing model for adult hematopoiesis postulates that the first lineage commitment step results in a strict separation of common myeloid and common lymphoid pathways. However, the recent identification of granulocyte/monocyte (GM)-lymphoid restricted lymphoid-primed multipotent progenitors (LMPPs) and primitive common myeloid progenitors (CMPs) within the "HSC" compartment provide compelling support for establishment of independent GM-megakaryocyte/erythroid (GM-MkE) and GM-lymphoid commitment pathways as decisive early lineage fate decisions. These changes in lineage potentials are corroborated by corresponding changes in multilineage transcriptional priming, as LMPPs down-regulate MkE priming but become GM-lymphoid transcriptionally primed, whereas CMPs are GM-MkE primed. These distinct biological and molecular relationships are established already in the fetal liver

Crucial role of FLT3 ligand in immune reconstitution following bone marrow transplantation and high dose chemotherapy.

Almost 5 decades after the first clinical transplantations, delayed immune reconstitution remains a considerable hurdle in bone marrow transplantation, and the mechanisms regulating immune reconstitution after transplantation remain to be established. Whereas adult fms-like tyrosine kinase 3 ligand-deficient (FL-/-) mice have reduced numbers of early Band T-cell progenitors, they sustain close to normal levels of mature B and T cells. Herein, we demonstrate that adult bone marrow cells fail to reconstitute B-cell progenitors and conventional B cells in lethally irradiated FL-/- recipients, which also display delayed kinetics of T-cell reconstitution. Similarly, FL is essential for B-cell regeneration after chemotherapy-induced myeloablation. In contrast, fetal progenitors reconstitute B lymphopoiesis in FL-/- mice, albeit at reduced levels. A critical role of FL in adult B lymphopoiesis is further substantiated by an age-progressive decline in peripheral conventional B cells in FL-/- mice, whereas fetally and early postnatally derived B1 and marginal zone B cells are sustained in a FL-independent manner. Thus, FL plays a crucial role in sustaining conventional B lymphopoiesis in adult mice and, as a consequence, our findings implicate a critical role of FL in promoting immune reconstitution after myeloablation and bone marrow transplantation

Key Role of flt3 Ligand in Regulation of the Common Lymphoid Progenitor but Not in Maintenance of the Hematopoietic Stem Cell Pool

AbstractThe first lineage commitment step of hematopoietic stem cells (HSC) results in separation into distinct lymphoid and myeloid differentiation pathways, reflected in the generation of common lymphoid and myeloid progenitors (CLP and CMP, respectively). In this report we present the first evidence for a nonredundant regulator of this process, in that adult mice deficient in expression of the flt3 ligand (FL) have severely (10-fold) reduced levels of the CLP, accompanied by reductions in the earliest identifiable B and T cell progenitors. In contrast, CMP and HSC are unaffected in FL-deficient mice. Noteworthy, CLP express high levels of both the flt3 receptor and ligand, indicating a potential autocrine role of FL in regulation of the earliest lymphoid commitment step from HSC

PRDM11 is dispensable for the maintenance and function of hematopoietic stem and progenitor cells

AbstractHematopoietic stem cells (HSC)11Hematopoietic stem cell (HSC), hematopoietic stem and progenitor cell (HSPC), bone marrow (BM), bone marrow transplantation (BMT), acute myeloid leukemia (AML), peripheral blood (PB), multipotent progenitor (MPP), pre-megakaryocyte/erythroid (preMegE), megakaryocytic progenitor (MkP), pre-granulocyte/macrophage (preGM), granulocyte/macrophage progenitors (GMP), common lymphoid progenitors (CLP), colony forming unit erythroid (CFU-E), proErythroid (proE), colony forming unit megakaryocyte (CFU-Mk), colony forming unit granulocyte macrophage (CFU-GM), megakaryocyte (Mk), LSK (Lineage−, Sca1+, c-Kithi). supply organisms with life-long output of mature blood cells. To do so, the HSC pool size has to be maintained by HSC self-renewing divisions. PRDM3 and PRDM16 have been documented to regulate HSC self-renewal, maintenance and function. We found Prdm11 to have similar expression patterns in the hematopoietic stem and progenitor cell (HSPC) compartments as Prdm3 and Prdm16. Therefore, we undertook experiments to test if PRDM11 regulates HSC self-renewal, maintenance and function by investigating the Prdm11−/− mice. Our data shows that phenotypic HSPCs are intact in bone marrow (BM) of one-year-old Prdm11−/− mice. In addition, Prdm11−/− mice were able to fully regenerate the hematopoietic system upon BM transplantation (BMT) into lethally irradiated mice with a mild drop in lymphoid output only. Taken together, this suggests that PRDM11, in contrast to PRDM3 and PRDM16, is not directly involved in regulation of HSPCs in mice