Microbiota-derived compounds drive steady-state granulopoiesis via MyD88/TICAM signaling.

Neutropenia is probably the strongest known predisposition to infection with otherwise harmless environmental or microbiota-derived species. Because initial swarming of neutrophils at the site of infection occurs within minutes, rather than the hours required to induce "emergency granulopoiesis," the relevance of having high numbers of these cells available at any one time is obvious. We observed that germ-free (GF) animals show delayed clearance of an apathogenic bacterium after systemic challenge. In this article, we show that the size of the bone marrow myeloid cell pool correlates strongly with the complexity of the intestinal microbiota. The effect of colonization can be recapitulated by transferring sterile heat-treated serum from colonized mice into GF wild-type mice. TLR signaling was essential for microbiota-driven myelopoiesis, as microbiota colonization or transferring serum from colonized animals had no effect in GF MyD88(-/-)TICAM1(-/-) mice. Amplification of myelopoiesis occurred in the absence of microbiota-specific IgG production. Thus, very low concentrations of microbial Ags and TLR ligands, well below the threshold required for induction of adaptive immunity, sets the bone marrow myeloid cell pool size. Coevolution of mammals with their microbiota has probably led to a reliance on microbiota-derived signals to provide tonic stimulation to the systemic innate immune system and to maintain vigilance to infection. This suggests that microbiota changes observed in dysbiosis, obesity, or antibiotic therapy may affect the cross talk between hematopoiesis and the microbiota, potentially exacerbating inflammatory or infectious states in the host.

Cytotoxic CD8+ T cells stimulate hematopoietic progenitors by promoting cytokine release from bone marrow mesenchymal stromal cells.

Cytotoxic CD8(+) T cells (CTLs) play a major role in host defense against intracellular pathogens, but a complete clearance of pathogens and return to homeostasis requires the regulated interplay of the innate and acquired immune systems. Here, we show that interferon γ (IFNγ) secreted by effector CTLs stimulates hematopoiesis at the level of early multipotent hematopoietic progenitor cells and induces myeloid differentiation. IFNγ did not primarily affect hematopoietic stem or progenitor cells directly. Instead, it promoted the release of hematopoietic cytokines, including interleukin 6 from bone marrow mesenchymal stromal cells (MSCs) in the hematopoietic stem cell niche, which in turn reduced the expression of the transcription factors Runx-1 and Cebpα in early hematopoietic progenitor cells and increased myeloid differentiation. Therefore, our study indicates that, during an acute viral infection, CTLs indirectly modulate early multipotent hematopoietic progenitors via MSCs in order to trigger the temporary activation of emergency myelopoiesis and promote clearance of the infection.

WIPI-dependent autophagy during neutrophil differentiation of NB4 acute promyelocytic leukemia cells.

Members of the WD-repeat protein interacting with phosphoinositides (WIPI) family are phosphatidylinositol 3-phosphate (PI3P) effectors that are essential for the formation of autophagosomes. Autophagosomes, unique double-membraned organelles, are characteristic for autophagy, a bulk degradation mechanism with cytoprotective and homeostatic function. Both, WIPI-1 and WIPI-2 are aberrantly expressed in several solid tumors, linking these genes to carcinogenesis. We now found that the expression of WIPI-1 was significantly reduced in a large cohort of 98 primary acute myeloid leukemia (AML) patient samples (complex karyotypes; t(8;21); t(15,17); inv(16)). In contrast, the expression of WIPI-2 was only reduced in acute promyelocytic leukemia (APL), a distinct subtype of AML (t(15,17)). As AML cells are blocked in their differentiation, we tested if the expression levels of WIPI-1 and WIPI-2 increase during all-trans retinoic acid (ATRA)-induced neutrophil differentiation of APL. According to the higher WIPI-1 expression in granulocytes compared with immature blast cells, WIPI-1 but not WIPI-2 expression was significantly induced during neutrophil differentiation of NB4 APL cells. Interestingly, the induction of WIPI-1 expression was dependent on the transcription factor PU.1, a master regulator of myelopoiesis, supporting our notion that WIPI-1 expression is reduced in AML patients lacking proper PU-1 activity. Further, knocking down WIPI-1 in NB4 cells markedly attenuated the autophagic flux and significantly reduced neutrophil differentiation. This result was also achieved by knocking down WIPI-2, suggesting that both WIPI-1 and WIPI-2 are functionally required and not redundant in mediating the PI3P signal at the onset of autophagy in NB4 cells. In line with these data, downregulation of PI3KC3 (hVPS34), which generates PI3P upstream of WIPIs, also inhibited neutrophil differentiation. In conclusion, we demonstrate that both WIPI-1 and WIPI-2 are required for the PI3P-dependent autophagic activity during neutrophil differentiation, and that PU.1-dependent WIPI-1 expression is significantly repressed in primary AML patient samples and that the induction of autophagic flux is associated with neutrophil differentiation of APL cells.

IL-33 signaling contributes to the pathogenesis of myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are characterized by the clonal expansion of one or more myeloid cell lineage. In most cases, proliferation of the malignant clone is ascribed to defined genetic alterations. MPNs are also associated with aberrant expression and activity of multiple cytokines; however, the mechanisms by which these cytokines contribute to disease pathogenesis are poorly understood. Here, we reveal a non-redundant role for steady-state IL-33 in supporting dysregulated myelopoiesis in a murine model of MPN. Genetic ablation of the IL-33 signaling pathway was sufficient and necessary to restore normal hematopoiesis and abrogate MPN-like disease in animals lacking the inositol phosphatase SHIP. Stromal cell-derived IL-33 stimulated the secretion of cytokines and growth factors by myeloid and non-hematopoietic cells of the BM, resulting in myeloproliferation in SHIP-deficient animals. Additionally, in the transgenic JAK2V617F model, the onset of MPN was delayed in animals lacking IL-33 in radio-resistant cells. In human BM, we detected increased numbers of IL-33-expressing cells, specifically in biopsies from MPN patients. Exogenous IL-33 promoted cytokine production and colony formation by primary CD34+ MPN stem/progenitor cells from patients. Moreover, IL-33 improved the survival of JAK2V617F-positive cell lines. Together, these data indicate a central role for IL-33 signaling in the pathogenesis of MPNs.