Delineation of the Common Developmental Pathway for Granulocyte/Monocyte and Lymphoid Lineages by Using an Expression Reporter for PU.1.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 1656-1656 ◽  
Author(s):  
Yojiro Arinobu ◽  
Shin-ichi Mizuno ◽  
Hirokazu Shigematsu ◽  
Hidetoshi Ozawa ◽  
Yong Chong ◽  
...  
Keyword(s):  
Cell Biology ◽  
Lymphoid Cells ◽  
Early Event ◽  
Hematopoietic Stem ◽  
Multipotent Cells ◽  
Gfp Reporter ◽  
Rag 1 ◽  
High Level

Abstract Understanding how multipotent cells commit to each of their terminal fate potentials is an important aspect of stem cell biology. Hematopoietic stem cells (HSCs) of Lin −Sca-1+c-Kit+ (LSK) phenotype have been purified, which were further divided into CD34-long-term and CD34+ short-term (ST)-HSCs. The existence of phenotypically isolatable common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs) downstream of ST-HSCs suggests that the first commitment step after the HSC stage is the bifurcation of lymphoid vs. myeloid pathway. Recent studies, however, suggest that the loss of MegE potential could be an early event in HSC stage. For example, LSK cells activating RAG-1 or Flt-3 expression retained granulocyte/monocyte (GM) but not megakaryocyte/erythrocyte (MegE) potential together with lymphoid potential, suggesting the existence of common progenitor for GM and lymphoid lineages. Here we report that a fraction of ST-LSK cells expressing high levels of PU.1, a transcription factor necessary for GM and lymphoid development, represents GM/lymphoid bipotent progenitors. In mice harboring knock-in GFP reporter for PU.1, LSK cells were divided into GFP high and GFP low subpopulations. Although PU.1low LSK cells were multipotent, PU.1high LSK cells differentiated only into GM and lymphoid cells in vitro and in vivo. We also found that single PU.1high LSK cells differentiated into GM, T and B cells in vivo. These data formally prove the existence of the third major early progenitor population activating PU.1 at a high level, the granulocyte/monocyte/lymphoid progenitor (GMLP). The existence of prospectively isolatable GMLPs strongly suggests that HSCs sequentially lose MegE then GM potential during their lymphoid commitment.

The Role of Nucleophosmin In Hematopoietic Stem Cells and the Pathogenesis of Myelodysplastic Syndrome

Blood ◽  
2010 ◽  
Vol 116 (21) ◽  
pp. 95-95 ◽  
Author(s):  
Keisuke Ito ◽  
Paolo Sportoletti ◽  
John G Clohessy ◽  
Grisendi Silvia ◽  
Pier Paolo Pandolfi
Keyword(s):  
Stem Cells ◽  
Stem Cell ◽  
Myelodysplastic Syndrome ◽  
Cell Biology ◽  
De Novo ◽  
Stem Cell Biology ◽  
Hematopoietic Stem

Abstract Abstract 95 Myelodysplastic syndrome (MDS) is an incurable stem cell disorder characterized by ineffective hematopoiesis and an increased risk of leukemia transformation. Nucleophosmin (NPM) is directly implicated in primitive hematopoiesis, the pathogenesis of hematopoietic malignancies and more recently of MDS. However, little is known regarding the molecular role and function of NPM in MDS pathogenesis and in stem cell biology. Here we present data demonstrating that NPM plays a critical role in the maintenance of hematopoietic stem cells (HSCs) and the transformation of MDS into leukemia. NPM is located on chromosome 5q and is frequently lost in therapy-related and de novo MDS. We have previously shown that Npm1 acts as a haploinsufficient tumor suppressor in the hematopoietic compartment and Npm1+/− mice develop a hematologic syndrome with features of human MDS, including increased susceptibility to leukemogenesis. As HSCs have been demonstrated to be the target of the primary neoplastic event in MDS, a functional analysis of the HSC compartment is essential to understand the molecular mechanisms in MDS pathogenesis. However, the role of NPM in adult hematopoiesis remains largely unknown as Npm1-deficiency leads to embryonic lethality. To investigate NPM function in adult hematopoiesis, we have generated conditional knockout mice of Npm1, using the Cre-loxP system. Analysis of Npm1 conditional mutants crossed with Mx1-Cre transgenic mice reveals that Npm1 plays a crucial role in adult hematopoiesis and ablation of Npm1 in adult HSCs leads to aberrant cycling and followed by apoptosis. Analysis of cell cycle status revealed that HSCs are impaired in their ability to maintain quiescence after Npm1-deletion and are rapidly depleted in vivo as well as in vitro. Competitive reconstitution assay revealed that Npm1 acts cell-autonomously to maintain HSCs. Conditional inactivation of Npm1 leads to an MDS phenotype including a profoundly impaired ability to differentiate into cells of the erythroid lineage, megakaryocyte dyspoiesis and centrosome amplification. Furthermore, Npm1 loss evokes a p53-dependent response and Npm1-deleted HSCs undergo apoptosis in vivo and in vitro. Strikingly, transfer of the Npm1 mutation into a p53-null background rescued the apoptosis of Npm1-ablated HSCs and resulted in accelerated transformation to an aggressive and lethal form of acute myeloid leukemia. Our findings highlight the crucial role of NPM in stem cell biology and identify a new mechanism by which MDS can progress to leukemia. This has important therapeutic implications for de novo MDS as well as therapy-related MDS, which is known to rapidly evolve to leukemia with frequent loss or mutation of TRP53. Disclosures: No relevant conflicts of interest to declare.

scholarly journals Rheb1-Deficient Neutrophils Promote Hematopoietic Stem/Progenitor Cell Proliferation via Mesenchymal Stem Cells

2021 ◽  
Vol 9 ◽  
Author(s):  
Juan Gao ◽  
Shuaibing Hou ◽  
Shengnan Yuan ◽  
Yuxia Wang ◽  
Yanan Gao ◽  
...  
Keyword(s):  
Stem Cells ◽  
Mesenchymal Stem Cells ◽  
Myeloid Cells ◽  
Human Mscs ◽  
Hematopoietic Stem ◽  
Progenitor Cell Proliferation ◽  
High Level ◽  
Lower Expression

Myeloid cells have been identified as hematopoietic stem cell (HSC)-regulating cells. However, the mechanisms by which myeloid cells regulate the function of HSCs are not fully defined. Our previous study indicated that the HSCs are over-expanded in Vav1-Cre;Rheb1fl/fl mice. Here, using in vivo and in vitro models, we found that Rheb1-deficient neutrophils remodeled the bone marrow environment and induced expansion of HSCs in vivo. Further studies showed that loss of Rheb1 impaired neutrophils’ ability to secrete IL-6, led mesenchymal stem cells (MSCs) to produce more SCF, and promote HSC proliferation. We further found that IL-6 suppressed SCF mRNA expression in human MSCs. Interesting, the high level of IL-6 was also related with poor survival of chronic myeloid leukemia (CML) patients, and higher expression of IL-6 in CML cells is associated with the lower expression of SCF in MSCs in patients. Our studies suggested that blocking IL-6 signaling pathway might stimulate MSCs to secrete more SCF, and to support hematopoietic stem/progenitor cells proliferation.

scholarly journals Innate lymphoid cells lacking surface expression of CD90 are functional

2021 ◽  
Author(s):  
J-H Schroeder ◽  
T Zabinski ◽  
J F Neves ◽  
GM Lord
Keyword(s):  
Steady State ◽  
Cell Biology ◽  
Surface Expression ◽  
Innate Lymphoid Cells ◽  
Lymphoid Cells ◽  
T Cell Biology ◽  
First Time ◽  
Made In

ABSTRACTHuge progress has been made in understanding the biology of innate lymphoid cells (ILC) by adopting several well-known concepts of T cell biology. As such flow cytometry gating strategies and markers, such as CD90, to identify ILC were discovered. Here we report that most non-NK intestinal ILC have a high expression of CD90 as expected, but surprisingly some have only a low or even no expression of this marker. CD90-negative CD127+ ILC were identified among all ILC subsets in the gut. CD90-negative cLP ILC2 were frequent at steady state. The frequency of CD90-negative CD127+ ILC was dependent on stimulatory cues in vitro and in vivo, and CD90-negative CD127+ ILC played a functional role as a source of IL-13, IFNγ and IL-17A at steady state and upon dextran sulphate sodium-elicited colitis. Hence, this study highlights for the first time that CD90 is not constitutively expressed by functional ILC in the gut.

scholarly journals Trajectory of chemical cocktail-induced neutrophil reprogramming

2020 ◽  
Vol 13 (1) ◽  
Author(s):  
Yi Zhou ◽  
Chuijin Wei ◽  
Shumin Xiong ◽  
Liaoliao Dong ◽  
Zhu Chen ◽  
...  
Keyword(s):  
Cell Types ◽  
Lymphoid Cells ◽  
Great Promise ◽  
Target Cells ◽  
Differentiation Potential ◽  
Hematopoietic Stem ◽  
Multipotent Progenitors ◽  
First Time

AbstractHematopoietic reprogramming holds great promise for generating functional target cells and provides new angle for understanding hematopoiesis. We reported before for the first time that diverse differentiated hematopoietic cell lineages could be reprogrammed back into hematopoietic stem/progenitor cell-like cells by chemical cocktail. However, the exact cell types of induced cells and reprogramming trajectory remain elusive. Here, based on genetic tracing method CellTagging and single-cell RNA sequencing, it is found that neutrophils could be reprogrammed into multipotent progenitors, which acquire multi-differentiation potential both in vitro and in vivo, including into lymphoid cells. Construction of trajectory map of the reprogramming procession shows that mature neutrophils follow their canonical developmental route reversely into immature ones, premature ones, granulocyte/monocyte progenitors, common myeloid progenitors, and then the terminal cells, which is stage by stage or skips intermediate stages. Collectively, this study provides a precise dissection of hematopoietic reprogramming procession and sheds light on chemical cocktail-induction of hematopoietic stem cells.

scholarly journals EGFR confers exquisite specificity of Wnt9a-Fzd9b signaling in hematopoietic stem cell development

2018 ◽  
Author(s):  
Stephanie Grainger ◽  
Nicole Nguyen ◽  
Jenna Richter ◽  
Jordan Setayesh ◽  
Brianna Lonquich ◽  
...  
Keyword(s):  
Stem Cell ◽  
Wnt Signaling ◽  
Hematopoietic Stem Cell ◽  
Cell Biology ◽  
Embryonic Stem ◽  
Cell Development ◽  
List Type ◽  
Hematopoietic Stem

SummaryThe mechanisms of Wnt-Frizzled (Fzd) signaling selectivity and their biological implications remain unclear. We demonstrate for the first time that the epidermal growth factor receptor (EGFR) is required as a co-factor for Wnt signaling. Using genetic studies in zebrafish, paired within vitrocell biology and biochemistry, we have determined that Fzd9b signals specifically with Wnt9ain vivoandin vitroto elicit β-catenin dependent Wnt signals that regulate hematopoietic stem and progenitor cell (HSPC) development in the dorsal aorta. This requirement is conserved in the derivation of HSPCs from human embryonic stem cells. Wnt9a-Fzd9b specificity requires two intracellular domains in Fzd9b, which interact with EGFR as a required co-factor to promote signal transduction. EGFR phosphorylates one tyrosine residue on Fzd9b, a requirement for the Wnt signal. These findings indicate that Wnt signaling interactions can be exquisitely specific and inform protocols for derivation of HSPCsin vitro.HighlightsAnin vitrosignaling screen identifies Fzd9b as a Wnt9a-specific receptor.Fzd9b and Wnt9a regulate hematopoietic stem cell development as a cognate pair.WNT9A and FZD9 are required for HSPC derivation from human pluripotent cellsin vitro.EGFR confers specificity to Wnt9a-Fzd9b signaling in zebrafish and human cells.

scholarly journals IL-1R1-Dependent Signals Improve Control of Cytosolic Virulent Mycobacteria In Vivo

mSphere ◽  
2021 ◽  
Vol 6 (3) ◽  
Author(s):  
Sanne van der Niet ◽  
Maaike van Zon ◽  
Karin de Punder ◽  
Anita Grootemaat ◽  
Sofie Rutten ◽  
...  
Keyword(s):  
Mycobacterium Tuberculosis ◽  
Cell Biology ◽  
Interleukin 1 ◽  
Severe Combined Immunodeficiency ◽  
Content Type ◽  
Adaptive Immune ◽  
Pathogenicity Factor ◽  
High Level

ABSTRACT Mycobacterium tuberculosis infections claim more than a million lives each year, and better treatments or vaccines are required. A crucial pathogenicity factor is translocation from phagolysosomes to the cytosol upon phagocytosis by macrophages. Translocation from the phagolysosome to the cytosol is an ESX-1-dependent process, as previously shown in vitro. Here, we show that in vivo, mycobacteria also translocate to the cytosol but mainly when host immunity is compromised. We observed only low numbers of cytosolic bacilli in mice, armadillos, zebrafish, and patient material infected with M. tuberculosis, M. marinum, or M. leprae. In contrast, when innate or adaptive immunity was compromised, as in severe combined immunodeficiency (SCID) or interleukin-1 receptor 1 (IL-1R1)-deficient mice, significant numbers of cytosolic M. tuberculosis bacilli were detected in the lungs of infected mice. Taken together, in vivo, translocation to the cytosol of M. tuberculosis is controlled by adaptive immune responses as well as IL-1R1-mediated signals. IMPORTANCE For decades, Mycobacterium tuberculosis has been one of the deadliest pathogens known. Despite infecting approximately one-third of the human population, no effective treatment or vaccine is available. A crucial pathogenicity factor is subcellular localization, as M. tuberculosis can translocate from phagolysosome to the cytosol in macrophages. The situation in vivo is more complicated. In this study, we establish that high-level cytosolic escape of mycobacteria can indeed occur in vivo but mainly when host resistance is compromised. The IL-1 pathway is crucial for the control of the number of cytosolic mycobacteria. The establishment that immune signals result in the clearance of cells containing cytosolic mycobacteria connects two important fields, cell biology and immunology, which is vital for the understanding of the pathology of M. tuberculosis.

Overexpression of the TGF-Beta Modulator .Bambi Promotes Hematopoetic Stem Cell Proliferation.

Blood ◽  
2009 ◽  
Vol 114 (22) ◽  
pp. 1446-1446
Author(s):  
Rui Mao ◽  
Olga Sirin ◽  
Margaret Goodell
Keyword(s):  
Cell Cycle ◽  
Bone Marrow ◽  
Conflicts Of Interest ◽  
Receptor Activation ◽  
Lymphoid Cells ◽  
Quiescent State ◽  
Tgf Beta ◽  
Hematopoietic Stem

Abstract Abstract 1446 Poster Board I-469 Hematopoietic stem cells (HSC) normally reside in a quiescent state in the bone marrow. During times of stress, HSCs are activated to begin differentiation and self-renewal, replenishing the supply of myeloid and lymphoid cells present in the blood. The mechanisms regulating this rapid activation have not been fully elucidated. We previously identified the TGF-beta modulator Bambi (BMP and activin membrane-bound inhibitor) to be upregulated four-fold in HSCs compared to differentiated cells. Bambi codes for a transmembrane pseudoreceptor that inhibits TGF-beta receptor activation. Since TGF-beta signaling has been established to be important for induction of HSC quiescence as well as cell-cycle inhibition in long-term progenitors, we hypothesize that Bambi may play an important role in the regulation of HSCs. Using a retroviral vector, we overexpressed Bambi in bone marrow cells. Overexpression of Bambi resulted in increased colony-formation in vitro when compared to control cells. Furthermore, transduced cells expressed higher levels of the cell-cycle marker Ki-67, indicating a greater proportion of cells in active stages of the cell cycle. To verify the results of these assays in vivo, bone marrow overexpressing Bambi was transplanted into lethally irradiated recipient mice. Bambi-overexpressing cells demonstrated a higher level of engraftment in all lineages than control cells at several time points, which confirms the previous in vitro data suggesting greater cell cycle activity. Moreover, we identified the pathway through which Bambi acts by monitoring the levels of phosphorylated Smad2 (pSmad2), a downstream target of TGF-beta. Overexpression of Bambi resulted in a distinctly lower level of pSmad2, which explains the cell-cycle effects seen in vivo and in vitro. These studies show that Bambi functions to promote HSC proliferation and a probable mode of action in HSCs is through decreased pSmad2 levels from inhibition of the TGF-beta pathway. Bambi has been shown to be upregulated in certain leukemias, and a more complete understanding of the mechanism through which Bambi acts will provide better opportunities for therapeutic innovation. This research was graciously funded by an NIH grant and the ASH Trainee Research Award. Disclosures No relevant conflicts of interest to declare.

Comprehensive Clonal Mapping of Hematopoiesis in Vivo in Humans By Retroviral Vector Insertional Barcoding

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 5-5
Author(s):  
Luca Biasco ◽  
Serena Scala ◽  
Francesca Dionisio ◽  
Andrea Calabria ◽  
Luca Basso Ricci ◽  
...  
Keyword(s):  
Detailed Analysis ◽  
Graphical Models ◽  
Integration Site ◽  
Cell Types ◽  
Retroviral Vector ◽  
Lymphoid Cells ◽  
Hematopoietic Stem

Abstract Hematopoietic stem cells (HSC) are endowed with the unique role of generating an adequate and efficient pool of blood cells throughout human life. Data derived from clonal tracking of HSC activity and hematopoietic dynamics directly in vivo in humans would be of paramount importance for the design of therapies for hematological disorders and cancers. Our gene therapy (GT) clinical trials for adenosine deaminase (ADA) deficient-SCID and Wiskott-Aldrich Syndrome (WAS) based on the infusion of genetically engineered HSC, constitute unique clinical settings where each vector-marked progenitors and its blood cell progeny is traceable being univocally barcoded by a vector integration site (IS). To study early dynamics of hematopoietic reconstitution in humans, we collected by LAM-PCR + Illumina-Miseq sequencing 14.807.407 sequence reads corresponding to 71.981 IS tagging clones belonging to 13 different cell types purified from the bone marrow and the peripheral blood of 4 WAS patients up to 36 months after GT. We firstly identified and quantified identical IS shared among CD34+ progenitors, and mature Myeloid/Lymphoid cells as marker of the real-time clonal output of individual vector-marked HSC clones in vivo. We unraveled the timing of short, intermediate and long term HSC output showing that CD34+ clones active at 3-6 months after GT are not detectable at later follow up. By unsupervised clustering of IS similarities among lineages we unveiled diverse input of HSPC clonal differentiation towards lymphoid, myeloid and megakaryo-erythroid cells and found that NK cells have a distinct relationship with HSPC as compared to T and B cells. We also profiled the level of HSPC output overtime showing that early reconstitution is markedly skewed towards myeloid production. Importantly, clonogenic progenitors generated in vitro from ex vivo purified CD34+ patients’ cells, showed a IS profile coherent with that of freshly purified BM and PB cell types from the same time-point. We also studied population clonal entropy through 7 different diversity indexes and uncovered that progenitor output occurs in distinct waves during the first 6-9 months after transplantation reaching a “homeostatic equilibrium” only by 12 months after GT. At steady state we estimated by mark-recapture mathematical approaches that 1900-7000 transduced HSC clones were stably contributing to the progenitors repertoire for up to 3 years after infusion of gene corrected CD34+ cells. To evaluate the long-term preservation of activity by transplanted HSC we exploited data derived from the IS-based tracking of 4.845 clones in ADA-SCID patients performed for up to 6 years after GT. We showed that identical IS are consistently detected at multiple lineages level even several years after GT. Strikingly, by semi-quantitative PCRs on specific vector-genome junctions we tracked a fluctuating but consistent output of marked HSC over a period of 5 years without the manifestation of clonal quiescence phases. Additionally, since the gamma-retroviral vector used in ADA-SCID HSC-GT trial is able to transduce only actively replicating cells, we provided the first evidence that in vitro activated HSC, “awaken” from dormancy, can still, once infused, retain in vivo long-term activity in humans. We exploited IS similarities among the lineages for both WAS and ADA-SCID datasets to reconstruct the hematopoietic hierarchy by combining conditional probability distributions and static/dynamic graphical models of dependencies. Notably, preliminary data unveiled a link between myeloid progenitors and mature lymphoid cells that supports the recently suggested model of hematopoiesis based on a delayed branching of myeloid and lymphoid lineages. Further mathematical models are being applied to specifically study population dynamics and single HSPC contribution to hematopoiesis including stochastic models of neutral clonal drift. More detailed analysis are also being performed on IS collected from 7 distinct CD34+ subtypes isolated from GT patients and FACS sorted according to the most recent markers of HSPC differentiation. Overall our work constitute the first molecular tracking of individual hematopoietic clones in humans providing an unprecedented detailed analysis of HSC activity and dynamics in vivo. The information gathered will be crucial for the design of therapeutic approaches for a broad spectrum of hematological diseases and tumors. Disclosures Neduva: GSK: Employment. Dow:GSK: Employment.

scholarly journals Ontogeny stage-independent and high-level clonal expansion in vitro of mouse hematopoietic stem cells stimulated by an engineered NUP98-HOX fusion transcription factor

Blood ◽  
2011 ◽  
Vol 118 (16) ◽  
pp. 4366-4376 ◽  
Author(s):  
Sanja Sekulovic ◽  
Maura Gasparetto ◽  
Véronique Lecault ◽  
Corinne A. Hoesli ◽  
David G. Kent ◽  
...  
Keyword(s):  
Stem Cells ◽  
Hematopoietic Stem Cells ◽  
Single Cells ◽  
Hematopoietic Stem ◽  
Image Tracking ◽  
Large Numbers ◽  
High Level

Abstract Achieving high-level expansion of hematopoietic stem cells (HSCs) in vitro will have an important clinical impact in addition to enabling elucidation of their regulation. Here, we couple the ability of engineered NUP98-HOXA10hd expression to stimulate > 1000-fold net expansions of murine HSCs in 10-day cultures initiated with bulk lin−Sca-1+c-kit+ cells, with strategies to purify fetal and adult HSCs and analyze their expansion clonally. We find that NUP98-HOXA10hd stimulates comparable expansions of HSCs from both sources at ∼ 60% to 90% unit efficiency in cultures initiated with single cells. Clonally expanded HSCs consistently show balanced long-term contributions to the lymphoid and myeloid lineages without evidence of leukemogenic activity. Although effects on fetal and adult HSCs were indistinguishable, NUP98-HOXA10hd–transduced adult HSCs did not thereby gain a competitive advantage in vivo over freshly isolated fetal HSCs. Live-cell image tracking of single transduced HSCs cultured in a microfluidic device indicates that NUP98-HOXA10hd does not affect their proliferation kinetics, and flow cytometry confirmed the phenotype of normal proliferating HSCs and allowed reisolation of large numbers of expanded HSCs at a purity of 25%. These findings point to the effects of NUP98-HOXA10hd on HSCs in vitro being mediated by promoting self-renewal and set the stage for further dissection of this process.

scholarly journals IL-1R1 dependent signals improve clearance of cytosolic virulent mycobacteria in vivo

2020 ◽  
Author(s):  
Sanne van der Niet ◽  
Maaike van Zon ◽  
Karin de Punder ◽  
Anita Grootemaat ◽  
Sofie Rutten ◽  
...  
Keyword(s):  
Mycobacterium Tuberculosis ◽  
Immune Responses ◽  
Cell Biology ◽  
Host Immunity ◽  
Adaptive Immune ◽  
Pathogenicity Factor ◽  
High Level ◽  
Deficient Mice

AbstractMycobacterium tuberculosis infections claim more than a million lives each year and better treatments or vaccines are required. A crucial pathogenicity factor is translocation from the phago-lysosomes to the cytosol upon phagocytosis by macrophages. The translocation from the phago-lysosome into the cytosol is an ESX-1 dependent process as previously shown in vitro. Here we show that in vivo, mycobacteria also translocate to the cytosol but mainly when host immunity is compromised. We observed only low numbers of cytosolic bacilli in mice, armadillo, zebrafish and patient material infected with M. tuberculosis, M. marinum or M. leprae. In contrast, when innate or adaptive immunity was compromised, as in SCID or IL-1R1 deficient mice, a significant number of cytosolic M. tuberculosis bacilli were detected in lungs of infected mice. Taken together, M. tuberculosis infection is controlled by adaptive immune responses as well as IL-1R1-mediated signals that result in clearance of cells containing cytosolic mycobacteria in vivo.ImportanceFor decades, Mycobacterium tuberculosis is one of the deathliest pathogens known. Despite infecting approximately one third of the human population, no effective treatment or vaccine is available. A crucial pathogenicity factor is the subcellular localization, as M. tuberculosis can translocate from the phago-lysosome to the cytosol in macrophages. The situation in vivo is more complicated. In this study we establish that high level cytosolic escape of mycobacteria can indeed occur in vivo, but mainly when host resistance is compromised. The IL-1 pathway is crucial for the control of the number of cytosolic mycobacteria. The establishment that immune signals result in clearance of cells containing cytosolic mycobacteria, connects two important fields: cell-biology and immunology which is vital for the understanding of the pathology of M. tuberculosis.

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