GM-CSF Controls Proliferation and Survival of the Granulocyte Lineage through the Transcription Factors STAT5A/B.

Neutrophils, one kind of granulocytes, are the most abundant type of white blood cells in human peripheral blood and form an integral part of the immune system. In addition, the majority of acute myelogeneous leukemia (AML) cells are from the granulocyte lineage. Granulocyte-colony stimulating factor (G-CSF) and granulocyte macrophage colony-stimulating factor (GM-CSF) control migration, proliferation and survival of granulocytes. G-CSF and GM-CSF activate the transcription factors STAT5A/B (STAT5), which are essential for the development of T and B cells and the erythroid lineage. However, it is not clear to what extent G-CSF or GM-CSF signaling through STAT5 controls the differentiation, proliferation, survival in granulocyte lineage. STAT5 is not only essential for normal development and its constitutive activation has been linked to AML patients with Flt3 mutations. The objective of this study was to explore the contribution of STAT5 in G-CSF- and GM-CSF-induced granulopoiesis and to elucidate the underlying molecular mechanisms. Towards this goal, the Stat5a/b genes were deleted in mouse hematopoietic stem cells in vivo using Cre-loxP-mediated recombination (mutant mice). Injection of 5-FU resulted in a cytokine storm, which in controls, but not in mutant mice, led to a 10-fold elevation of neutrophils. Strikingly, the distribution of myeloid progenitor populations in bone marrow was not altered in STAT5-null animals in homeostasis. Colony assays were performed to address which cytokine controls granulopoiesis from these progenitors. While common multipotent progenitor cells (CMPs) and granulocyte macrophage progenitor cells (GMPs) from control mice formed large colonies in the presence of GM-CSF, mutant cells responded poorly. No difference between control and mutant colonies was observed in the presence of G-CSF. To investigate GM-CSF-mediated survival, apoptosis-assays were performed with peritoneal neutrophils. Greatly elevated apoptosis was observed with STAT5-null neutrophils. To further dissect the contribution of apoptosis and/or proliferation in the observed defects, long-term time-lapse imaging and single cell tracking was applied. Control and STAT5-null GMPs were cultured with GM-CSF and individual cells and all their progeny were continuously observed for 5 generations. Despite an equal number of initial GMPs responding to GM-CSF, the generation time of STAT5-null GMP-derived progeny was significantly prolonged in each generation and the number of cell death events increased dramatically from generation to generation. Therefore, GM-CSF-mediated STAT5 signaling is necessary to generate high numbers of granulocytic cells from GMPs by providing pro-survival and pro-proliferation signals. To identify GM-CSF-mediated and STAT5-dependent genetic cascades that control proliferation and survival of the granulocyte lineage, we performed gene expression profiling and ChIP-seq of control and STAT5-null CMPs, GMPs and neutrophils. STAT5 target genes specific to CMPs, GMPs and neutrophils were identified and their contribution to normal granulopoiesis is currently being investigated.