Explore the vast range of titles available.

M-CSF, a myeloid cytokine released during infection and inflammation, instructs myeloid lineage fate in single haematopoietic stem cells by directly inducing PU.1, a known myeloid lineage master regulator; this shows that specific cytokines can act directly on haematopoietic stem cells to instruct a change of cell identity. Cytokine-directed stem-cell differentiation Lineage-specific cytokines such as macrophage colony-stimulating factor (M-CSF), which is released during infection and inflammation, can potently increase the production of mature cells from lineage-committed progenitors, but whether they influence differentiation decisions of haematopoietic stem cells directly has been a matter of debate. Now Michael Sieweke and colleagues report that M-CSF instructs myeloid lineage fate by directly inducing PU.1, a known myeloid lineage master regulator. Through this mechanism cytokines released during stress may direct stem-cell differentiation towards cells tailored to cope with that specific stress. This might also provide opportunities for the manipulation of stem-cell fate under pathological or transplantation conditions. Under stress conditions such as infection or inflammation the body rapidly needs to generate new blood cells that are adapted to the challenge. Haematopoietic cytokines are known to increase output of specific mature cells by affecting survival, expansion and differentiation of lineage-committed progenitors 1 , 2 , but it has been debated whether long-term haematopoietic stem cells (HSCs) are susceptible to direct lineage-specifying effects of cytokines. Although genetic changes in transcription factor balance can sensitize HSCs to cytokine instruction 3 , the initiation of HSC commitment is generally thought to be triggered by stochastic fluctuation in cell-intrinsic regulators such as lineage-specific transcription factors 4 , 5 , 6 , 7 , leaving cytokines to ensure survival and proliferation of the progeny cells 8 , 9 . Here we show that macrophage colony-stimulating factor (M-CSF, also called CSF1), a myeloid cytokine released during infection and inflammation, can directly induce the myeloid master regulator PU.1 and instruct myeloid cell-fate change in mouse HSCs, independently of selective survival or proliferation. Video imaging and single-cell gene expression analysis revealed that stimulation of highly purified HSCs with M-CSF in culture resulted in activation of the PU.1 promoter and an increased number of PU.1 + cells with myeloid gene signature and differentiation potential. In vivo , high systemic levels of M-CSF directly stimulated M-CSF-receptor-dependent activation of endogenous PU.1 protein in single HSCs and induced a PU.1-dependent myeloid differentiation preference. Our data demonstrate that lineage-specific cytokines can act directly on HSCs in vitro and in vivo to instruct a change of cell identity. This fundamentally changes the current view of how HSCs respond to environmental challenge and implicates stress-induced cytokines as direct instructors of HSC fate.

Therapies reconstituting autologous antiviral immunocompetence may represent an important prophylaxis and treatment for immunosuppressed individuals. Following hematopoietic cell transplantation (HCT), patients are susceptible to Herpesviridae including cytomegalovirus (CMV). We show in a murine model of HCT that macrophage colony‐stimulating factor (M‐CSF) promoted rapid antiviral activity and protection from viremia caused by murine CMV. M‐CSF given at transplantation stimulated sequential myeloid and natural killer (NK) cell differentiation culminating in increased NK cell numbers, production of granzyme B and interferon‐γ. This depended upon M‐CSF‐induced myelopoiesis leading to IL15Rα‐mediated presentation of IL‐15 on monocytes, augmented by type I interferons from plasmacytoid dendritic cells. Demonstrating relevance to human HCT, M‐CSF induced myelomonocytic IL15Rα expression and numbers of functional NK cells in G‐CSF‐mobilized hematopoietic stem and progenitor cells. Together, M‐CSF‐induced myelopoiesis triggered an integrated differentiation of myeloid and NK cells to protect HCT recipients from CMV. Thus, our results identify a rationale for the therapeutic use of M‐CSF to rapidly reconstitute antiviral activity in immunocompromised individuals, which may provide a general paradigm to boost innate antiviral immunocompetence using host‐directed therapies. Synopsis Herpesviridae like CMV are a major cause of morbidity and mortality in patients after HCT. Therapies reconstituting the host's antiviral immunocompetence for prophylaxis and treatment are an unmet medical need since licensed therapies are either insufficiently effective or have severe side effects. M‐CSF protects from lethal murine CMV viremia during leukopenia following hematopoietic cell transplantation, a vulnerable period of immunosuppression, by rapidly reconstituting donor hematopoietic stem and progenitor cells. M‐CSF stimulates a coordinated myeloid and NK cell differentiation program resulting in increased NK cell numbers and activity, which depends on M‐CSF‐induced myelopoiesis generating IL‐15‐producing monocytes and I‐IFN‐producing pDCs. No impairment of long‐term hematopoietic stem cell engraftment or acute graft‐versus‐host‐disease after M‐CSF treatment was observed. In G‐CSF‐mobilized human PBMCs M‐CSF also stimulates monopoiesis, IL15Rα expression in monocytes and functional NK cell differentiation. M‐CSF could provide a general host‐directed antiviral cytokine therapy, to complement pathogen‐directed antiviral therapies in immunosuppressed conditions beyond HCT, such as post‐chemotherapy leukopenia or septicemia. Graphical Abstract Herpesviridae like CMV are a major cause of morbidity and mortality in patients after HCT. Therapies reconstituting the host's antiviral immunocompetence for prophylaxis and treatment are an unmet medical need since licensed therapies are either insufficiently effective or have severe side effects.

Immunosuppressed patients are highly susceptible to viral infections. Therapies reconstituting autologous antiviral immunocompetence could therefore represent an important prophylaxis and treatment. Herpesviridae including cytomegalovirus (CMV) are a major cause of morbidity and mortality in patients after hematopoietic cell transplantation (HCT). Here, we show in a mouse model of HCT that macrophage colony-stimulating factor (M-CSF/CSF-1), a key cytokine for myeloid and monocytic differentiation, promoted rapid antiviral activity and protection from viremia caused by murine CMV. Mechanistically, M-CSF stimulated a coordinated myeloid and natural killer (NK) cell differentiation program culminating in increased NK cell numbers and production of granzyme B and interferon-b. This NK cell response depended upon M-CSF-induced myelopoiesis leading to IL15Ra-mediated presentation of IL-15 on monocytes. Furthermore, M-CSF also induced differentiation of plasmacytoid dendritic cells producing type I interferons, which supported IL-15-mediated protection. In the context of human HCT, M-CSF induced monopoiesis, increased IL15Ra expression on monocytes and elevated numbers of functionally competent NK cells in G-CSF-mobilized human hematopoietic stem and progenitor cells. Together, our data show that M-CSF induces an integrated multistep differentiation program that culminates in increased NK cell numbers and activation, thereby protecting graft recipients from CMV infection. Thus, our results identify a mechanism by which M-CSF-induced myelopoiesis can rapidly reconstitute antiviral activity during episodes of leukopenia.Competing Interest StatementThe authors declare the following potential conflict of interests: Michael Sieweke is a patent holder of WO2014167018A1 (Use of M-CSF for preventing or treating myeloid cytopenia and related complications).

Most cells in adult tissues are nondividing. In skeletal muscle, differentiated myofibers have exited the cell cycle permanently, whereas satellite stem cells withdraw transiently, returning to active proliferation to repair damaged myofibers. We have examined the epigenetic mechanisms operating in conditional quiescence by analyzing the function of a predicted chromatin regulator mixed lineage leukemia 5 (MLL5) in a culture model of reversible arrest. MLL5 is induced in quiescent myoblasts and regulates both the cell cycle and differentiation via a hierarchy of chromatin and transcriptional regulators. Knocking down MLL5 delays entry of quiescent myoblasts into S phase, but hastens S-phase completion. Cyclin A2 (CycA) mRNA is no longer restricted to S phase, but is induced throughout G₀/G₁, with activation of the cell cycle regulated element (CCRE) in the CycA promoter. Overexpressed MLL5 physically associates with the CCRE and impairs its activity. MLL5 also regulates CycA indirectly: Cux, an activator of CycA promoter and S phase is induced in RNAi cells, and Brm/Brg1, CCRE-binding repressors that promote differentiation are repressed. In knockdown cells, H3K4 methylation at the CCRE is reduced, reflecting quantitative global changes in methylation. MLL5 appears to lack intrinsic histone methyl transferase activity, but regulates expression of histone-modifying enzymes LSD1 and SET7/9, suggesting an indirect mechanism. Finally, expression of muscle regulators Pax7, Myf5, and myogenin is impaired in MLL5 knockdown cells, which are profoundly differentiation defective. Collectively, our results suggest that MLL5 plays an integral role in novel chromatin regulatory mechanisms that suppress inappropriate expression of S-phase-promoting genes and maintain expression of determination genes in quiescent cells.

Most cells in adult tissues are nondividing. In skeletal muscle, differentiated myofibers have exited the cell cycle permanently, whereas satellite stem cells withdraw transiently, returning to active proliferation to repair damaged myofibers. We have examined the epigenetic mechanisms operating in conditional quiescence by analyzing the function of a predicted chromatin regulator mixed lineage leukemia 5 (MLL5) in a culture model of reversible arrest. MLL5 is induced in quiescent myoblasts and regulates both the cell cycle and differentiation via a hierarchy of chromatin and transcriptional regulators. Knocking down MLL5 delays entry of quiescent myoblasts into S phase, but hastens S-phase completion. Cyclin A2 (CycA) mRNA is no longer restricted to S phase, but is induced throughout G0/G1, with activation of the cell cycle regulated element (CCRE) in the CycA promoter. Overexpressed MLL5 physically associates with the CCRE and impairs its activity. MLL5 also regulates CycA indirectly: Cux, an activator of CycA promoter and S phase is induced in RNAi cells, and Brm/Brg1, CCRE-binding repressors that promote differentiation are repressed. In knockdown cells, H3K4 methylation at the CCRE is reduced, reflecting quantitative global changes in methylation. MLL5 appears to lack intrinsic histone methyl transferase activity, but regulates expression of histone-modifying enzymes LSD1 and SET7/9, suggesting an indirect mechanism. Finally, expression of muscle regulators Pax7, Myf5, and myogenin is impaired in MLL5 knockdown cells, which are profoundly differentiation defective. Collectively, our results suggest that MLL5 plays an integral role in novel chromatin regulatory mechanisms that suppress inappropriate expression of S-phase-promoting genes and maintain expression of determination genes in quiescent cells.