The p47 GTPase Lrg-47 (Irgm1) links host defense and hematopoietic stem cell proliferation

Type I IFN signaling in the absence of IRGM1 promotes M. tuberculosis replication in immune cells by suppressing T cell responses

Polymorphisms in the IRGM gene are associated with susceptibility to tuberculosis in humans. A murine ortholog of Irgm, Irgm1, is also essential for controlling Mycobacterium tuberculosis (Mtb) infection in mice. Multiple processes have been associated with IRGM1 activity that could impact the host response to Mtb infection, including roles in autophagy-mediated pathogen clearance and expansion of activated T cells. However, what IRGM1-mediated pathway is necessary to control Mtb infection in vivo and the mechanistic basis for this control remains unknown. We dissected the contribution of IRGM1 to immune control of Mtb pathogenesis in vivo and found that Irgm1 deletion leads to higher levels of IRGM3-dependent type I interferon signaling. The increased type I interferon signaling precludes T cell expansion during Mtb infection. The absence of Mtb-specific T cell expansion in Irgm1-/- mice results in uncontrolled Mtb infection in neutrophils and alveolar macrophages, which directly contributes to susceptibility to infection. Together, our studies reveal that IRGM1 is required to promote T cell-mediated control of Mtb infection in neutrophils, which is essential for the survival of Mtb-infected mice. These studies also uncover new ways type I interferon signaling can impact TH1 immune responses.

Clonal dynamics and somatic evolution of haematopoiesis in mouse

Haematopoietic stem cells maintain blood production throughout life. While extensively characterised using the laboratory mouse, little is known about how the population is sustained and evolves with age. We isolated stem cells and progenitors from young and old mice, identifying 221,890 somatic mutations genome-wide in 1845 single cell-derived colonies, and used phylogenetic analysis to infer the ontogeny and population dynamics of the stem cell pool. Mouse stem cells and progenitors accrue ~45 somatic mutations per year, a rate only about 2-fold greater than human progenitors despite the vastly different organismal sizes and lifespans. Phylogenetic patterns reveal that stem and multipotent progenitor cell pools are both established during embryogenesis, after which they independently self-renew in parallel over life. The stem cell pool grows steadily over the mouse lifespan to approximately 70,000 cells, self-renewing about every six weeks. Aged mice did not display the profound loss of stem cell clonal diversity characteristic of human haematopoietic ageing. However, targeted sequencing revealed small, expanded clones in the context of murine ageing, which were larger and more numerous following haematological perturbations and exhibited a selection landscape similar to humans. Our data illustrate both conserved features of population dynamics of blood and distinct patterns of age-associated somatic evolution in the short-lived mouse.

Type I IFN signaling in the absence of IRGM1 promotes M. tuberculosis replication in immune cells by suppressing T cell responses

Polymorphisms in the IRGM gene are associated with susceptibility to tuberculosis in humans. A murine ortholog of Irgm, Irgm1, is also essential for controlling Mycobacterium tuberculosis (Mtb) infection in mice. Multiple processes have been associated with IRGM1 activity that could impact the host response to Mtb infection, including roles in autophagy-mediated pathogen clearance and expansion of activated T cells. However, what IRGM1-mediated pathway is necessary to control Mtb infection in vivo and the mechanistic basis for this control remains unknown. We dissected the contribution of IRGM1 to immune control of Mtb pathogenesis in vivo and found that Irgm1 deletion leads to higher levels of IRGM3-dependent type I interferon signaling. The increased type I interferon signaling precludes T cell expansion during Mtb infection. The absence of Mtb-specific T cell expansion in Irgm1-/- mice results in uncontrolled Mtb infection in neutrophils and alveolar macrophages, which directly contributes to susceptibility to infection. Together, our studies reveal that IRGM1 is required to promote T cell-mediated control of Mtb infection in neutrophils, which is essential for the survival of Mtb-infected mice. These studies also uncover new ways type I interferon signaling can impact TH1 immune responses.

Hematopoietic stem and progenitor cells confer cross-protective trained immunity in mouse models

Recent studies suggest that infection reprograms hematopoietic stem and progenitor cells (HSPCs) to enhance innate immune responses upon secondary infectious challenge, a process called "trained immunity." However, the specificity and cell types responsible for this response remain poorly defined. We established a model of trained immunity in mice in response to Mycobacterium avium infection. scRNA-seq analysis revealed that HSPCs activate interferon gamma-response genes heterogeneously upon primary challenge, while rare cell populations expand. Macrophages derived from trained HSPCs demonstrated enhanced bacterial killing and metabolism, and a single dose of recombinant interferon gamma exposure was sufficient to induce similar training. Mice transplanted with influenza-trained HSPCs displayed enhanced immunity against M. avium challenge and vice versa, demonstrating cross protection against antigenically distinct pathogens. Together, these results indicate that heterogeneous responses to infection by HSPCs can lead to long-term production of bone marrow derived macrophages with enhanced function and confer cross-protection against alternative pathogens.

Infection with hypervirulent Mycobacterium tuberculosis triggers emergency myelopoiesis but not trained immunity

Introduction

During infection, bone marrow (BM) hematopoiesis is reprogrammed toward myeloid cell production, a mechanism named emergency myelopoiesis. In addition to replenishing myeloid cells, emergency myelopoiesis has been linked to trained immunity, a process that allows enhanced innate immune responses to secondary challenges. Although hematopoietic alterations during tuberculosis (TB) have been described and Mycobacterium tuberculosis may colonize the BM, studies using the mouse model of infection and the laboratory reference strain M. tuberculosis H37Rv have demonstrated limited emergency myelopoiesis and trained immunity.

Methods

To further address this issue, we aerosol- infected C57BL/6 mice with high doses of the hypervirulent M. tuberculosis isolate HN878 and monitored alterations to the BM. This experimental model better resembles the human blood immune signature of TB.

Results and discussion

We found increased frequencies of lineage^-Sca-1⁺cKit⁺ (LSK) cells and the granulocyte/macrophage progenitor (GMP) population. At the mature cell level, we observed an increase of monocytes and neutrophils in the blood and lung, likely reflecting the increased BM myeloid output. Monocytes or monocyte-derived macrophages recovered from the BM of M. tuberculosis HN878-infected mice did not show signs of trained immunity, suggesting an uncoupling of emergency myelopoiesis and trained immunity in the BM. Surprisingly, M. tuberculosis HN878-induced emergency myelopoiesis was not fully dependent on IFNγ, as mice lacking this cytokine and infected under the same conditions as wild-type mice still presented BM alterations. These data expand our understanding of the immune response to M. tuberculosis and raise awareness of pathogen strain-imposed differences to host responses.

BATF2 promotes HSC myeloid differentiation by amplifying IFN response mediators during chronic infection

Basic leucine zipper ATF-like transcription factor 2 (BATF2), an interferon-activated immune response regulator, is a key factor responsible for myeloid differentiation and depletion of HSC during chronic infection. To delineate the mechanism of BATF2 function in HSCs, we assessed Batf2 KO mice during chronic infection and found that they produced less pro-inflammatory cytokines, less immune cell recruitment to the spleen, and impaired myeloid differentiation with better preservation of HSC capacity compared to WT. Co-IP analysis revealed that BATF2 forms a complex with JUN to amplify pro-inflammatory signaling pathways including CCL5 during infection. Blockade of CCL5 receptors phenocopied Batf2 KO differentiation defects, whereas treatment with recombinant CCL5 was sufficient to rescue IFNγ-induced myeloid differentiation and recruit more immune cells to the spleen in Batf2 KO mice. By revealing the mechanism of BATF2-induced myeloid differentiation of HSCs, these studies elucidate potential therapeutic strategies to boost immunity while preserving HSC function during chronic infection.

Inflammatory exposure drives long-lived impairment of hematopoietic stem cell self-renewal activity and accelerated aging

Hematopoietic stem cells (HSCs) mediate regeneration of the hematopoietic system following injury, such as following infection or inflammation. These challenges impair HSC function, but whether this functional impairment extends beyond the duration of inflammatory exposure is unknown. Unexpectedly, we observed an irreversible depletion of functional HSCs following challenge with inflammation or bacterial infection, with no evidence of any recovery up to 1 year afterward. HSCs from challenged mice demonstrated multiple cellular and molecular features of accelerated aging and developed clinically relevant blood and bone marrow phenotypes not normally observed in aged laboratory mice but commonly seen in elderly humans. In vivo HSC self-renewal divisions were absent or extremely rare during both challenge and recovery periods. The progressive, irreversible attrition of HSC function demonstrates that temporally discrete inflammatory events elicit a cumulative inhibitory effect on HSCs. This work positions early/mid-life inflammation as a mediator of lifelong defects in tissue maintenance and regeneration.

Schlafen2 is a regulator of quiescence in adult murine hematopoietic stem cells

Even though hematopoietic stem cells (HSC) are characterized by their ability to self-renew and differentiate, they primarily reside in quiescence. Despite the immense importance of this quiescent state, its maintenance and regulation is still incompletely understood. Schlafen2 (Slfn2) is a cytoplasmic protein known to be involved in cell proliferation, differentiation, quiescence, interferon response, and regulation of the immune system. Interestingly, Slfn2 is highly expressed in primitive hematopoietic cells. In order to investigate the role of Slfn2 in the regulation of HSC we have studied HSC function in the elektra mouse model, where the elektra allele of the Slfn2 gene contains a point mutation causing loss of function of the Slfn2 protein. We found that homozygosity for the elektra allele caused a decrease of primitive hematopoietic compartments in murine bone marrow. We further found that transplantation of elektra bone marrow and purified HSC resulted in a significantly reduced regenerative capacity of HSC in competitive transplantation settings. Importantly, we found that a significantly higher fraction of elektra HSC (as compared to wild-type HSC) were actively cycling, suggesting that the mutation in Slfn2 increases HSC proliferation. This additionally caused an increased amount of apoptotic stem and progenitor cells. Taken together, our findings demonstrate that dysregulation of Slfn2 results in a functional deficiency of primitive hematopoietic cells, which is particularly reflected by a drastically impaired ability to reconstitute the hematopoietic system following transplantation and an increase in HSC proliferation. This study thus identifies Slfn2 as a novel and critical regulator of adult HSC and HSC quiescence.

Sex Influences Age-Related Changes in Natural Antibodies and CD5+ B-1 Cells

Natural Abs are primarily produced by B-1 cells and are essential for protection against Streptococcus pneumoniae The incidence and mortality rate for pneumococcal infection increases dramatically after age 65, disproportionately affecting males in both human and murine systems. To date, there is a significant gap in our understanding of the relationship among sex, aging, natural IgM efficacy, and the natural IgM repertoire. Our investigation demonstrates that the protective capacity of serum IgM against pneumococcal infection is maintained in IgM obtained from aged female mice but absent in IgM from aged male mice. To understand this difference in protective capacity, we examined serum Ig, discovering that the protective change was not associated with shifts in levels of phosphorylcholine (PC)- or pneumococcal capsular polysaccharide serotype 3-specific IgM. Interestingly, we observed that aged females have an increase in the total number of CD5⁺ B-1 cells, higher serum IL-5 levels, and a larger percentage of aged female CD5⁺ B-1 cells that express CD86 as compared with aged males. Furthermore, single-cell IgM repertoire analysis from peritoneal PC⁺, splenic PC⁺, and bone marrow CD5⁺ B-1 cell subsets demonstrated greater diversity with age and a higher level of germline status in female mice than previously observed in studies of aged male mice. Aged female CD5⁺ B-1 cells also expressed higher levels of transcripts associated with cell activity and self-renewal, such as Nanog and Hmga2 Taken together, these data indicate that females maintain a more diverse and active CD5⁺ B-1 cell pool and natural IgM repertoire, which has implications for sex-related susceptibility to infection and disease.

Irgm1 regulates metabolism and function in T cell subsets

Immunity Related GTPases (IRG) are a family of proteins produced during infection that regulate membrane remodeling events in cells, particularly autophagy and mitophagy. The human IRGM gene has been strongly associated with Crohn's disease and other inflammatory diseases through Genome-Wide Association studies. Absence of Irgm1 in mice prompts intestinal inflammation, autoimmunity, and impaired immune control of pathogenic bacteria and protozoa. Although prior work has focused on a prominent role for IRGM/Irgm1 in regulating macrophage function, the work described here addresses a potential role of Irgm1 in regulating the function of mature T cells. Irgm1 was found to be highly expressed in T cells in a manner that varied with the particular T cell subset and increased with activation. Mice with a complete lack of Irgm1, or a conditional lack of Irgm1 specifically in T cells, displayed numerous changes in T cell numbers and function in all subsets examined, including CD4+ (Th1 and Treg) and CD8+ T cells. Related to changes in T cell number, apoptosis was found to be increased in Irgm1-deficient CD4+ and CD8+ T cells. Altered T cell metabolism appeared to be a key driver of the phenotypes: Glucose metabolism and glycolysis were increased in Irgm1-deficient CD4+ and CD8+ T cells, and muting these effects with glycolytic inhibitors partially restored T cell function and viability.

Lessons from Toxoplasma: Host responses that mediate parasite control and the microbial effectors that subvert them

The intracellular parasite Toxoplasma gondii has long provided a tractable experimental system to investigate how the immune system deals with intracellular infections. This review highlights the advances in defining how this organism was first detected and the studies with T. gondii that contribute to our understanding of how the cytokine IFN-γ promotes control of vacuolar pathogens. In addition, the genetic tractability of this eukaryote organism has provided the foundation for studies into the diverse strategies that pathogens use to evade antimicrobial responses and now provides the opportunity to study the basis for latency. Thus, T. gondii remains a clinically relevant organism whose evolving interactions with the host immune system continue to teach lessons broadly relevant to host–pathogen interactions.

Chronic infection drives Dnmt3a-loss-of-function clonal hematopoiesis via IFNγ signaling

Age-related clonal hematopoiesis (CH) is a risk factor for malignancy, cardiovascular disease, and all-cause mortality. Somatic mutations in DNMT3A are drivers of CH, but decades may elapse between the acquisition of a mutation and CH, suggesting that environmental factors contribute to clonal expansion. We tested whether infection provides selective pressure favoring the expansion of Dnmt3a mutant hematopoietic stem cells (HSCs) in mouse chimeras. We created Dnmt3a-mosaic mice by transplanting Dnmt3a-/- and WT HSCs into WT mice and observed the substantial expansion of Dnmt3a-/- HSCs during chronic mycobacterial infection. Injection of recombinant IFNγ alone was sufficient to phenocopy CH by Dnmt3a-/- HSCs upon infection. Transcriptional and epigenetic profiling and functional studies indicate reduced differentiation associated with widespread methylation alterations, and reduced secondary stress-induced apoptosis accounts for Dnmt3a-/- clonal expansion during infection. DNMT3A mutant human HSCs similarly exhibit defective IFNγ-induced differentiation. We thus demonstrate that IFNγ signaling induced during chronic infection can drive DNMT3A-loss-of-function CH.

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

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;Rheb1 ^{f l/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.

IRGM1 links mitochondrial quality control to autoimmunity

Mitochondrial abnormalities have been noted in lupus, but the causes and consequences remain obscure. Autophagy-related genes ATG5, ATG7 and IRGM have been previously implicated in autoimmune disease. We reasoned that failure to clear defective mitochondria via mitophagy might be a foundational driver in autoimmunity by licensing mitochondrial DNA-dependent induction of type I interferon. Here, we show that mice lacking the GTPase IRGM1 (IRGM homolog) exhibited a type I interferonopathy with autoimmune features. Irgm1 deletion impaired the execution of mitophagy with cell-specific consequences. In fibroblasts, mitochondrial DNA soiling of the cytosol induced cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING)-dependent type I interferon, whereas in macrophages, lysosomal Toll-like receptor 7 was activated. In vivo, Irgm1-/- tissues exhibited mosaic dependency upon nucleic acid receptors. Whereas salivary and lacrimal gland autoimmune pathology was abolished and lung pathology was attenuated by cGAS and STING deletion, pancreatic pathology remained unchanged. These findings reveal fundamental connections between mitochondrial quality control and tissue-selective autoimmune disease.

Visualizing the Selectivity and Dynamics of Interferon Signaling In Vivo

Interferons (IFN) are pleiotropic cytokines essential for defense against infection, but the identity and tissue distribution of IFN-responsive cells in vivo are poorly defined. In this study, we generate a mouse strain capable of reporting IFN-signaling activated by all three types of IFNs and investigate the spatio-temporal dynamics and identity of IFN-responding cells following IFN injection and influenza virus infection. Despite ubiquitous expression of IFN receptors, cellular responses to IFNs are highly heterogenous in vivo and are determined by anatomical site, cell type, cellular preference to individual IFNs, and activation status. Unexpectedly, type I and II pneumocytes, the primary target of influenza infection, exhibit striking differences in the strength and temporal dynamics of IFN signaling associated with differential susceptibility to the viral infection. Our findings suggest that time- and cell-type-dependent integration of distinct IFN signals govern the specificity and magnitude of IFN responses in vivo.

Guanylate Binding Proteins Restrict Leishmania donovani Growth in Nonphagocytic Cells Independent of Parasitophorous Vacuolar Targeting

Interferon (IFN)-inducible guanylate binding proteins (GBPs) play important roles in host defense against many intracellular pathogens that reside within pathogen-containing vacuoles (PVs). For instance, members of the GBP family translocate to PVs occupied by the protozoan pathogen Toxoplasma and facilitate PV disruption and lytic parasite killing. While the GBP defense program targeting Toxoplasma has been studied in some detail, the role of GBPs in host defense to other protozoan pathogens is poorly characterized. Here, we report a critical role for both mouse and human GBPs in the cell-autonomous immune response against the vector-borne parasite Leishmania donovani Although L. donovani can infect both phagocytic and nonphagocytic cells, it predominantly replicates inside professional phagocytes. The underlying basis for this cell type tropism is unclear. Here, we demonstrate that GBPs restrict growth of L. donovani in both mouse and human nonphagocytic cells. GBP-mediated restriction of L. donovani replication occurs via a noncanonical pathway that operates independent of detectable translocation of GBPs to L. donovan-containing vacuoles (LCVs). Instead of promoting the lytic destruction of PVs, as reported for GBP-mediated killing of Toxoplasma in phagocytic cells, GBPs facilitate the delivery of L. donovani into autolysosomal-marker-positive compartments in mouse embryonic fibroblasts as well as the human epithelial cell line A549. Together our results show that GBPs control a novel cell-autonomous host defense program, which renders nonphagocytic cells nonpermissible for efficient Leishmania replication.IMPORTANCE The obligate intracellular parasite Leishmania causes the disease leishmaniasis, which is transmitted to mammalian hosts, including humans, via the sandfly vector. Following the bite-induced breach of the skin barrier, Leishmania is known to live and replicate predominantly inside professional phagocytes. Although Leishmania is also able to infect nonphagocytic cells, nonphagocytic cells support limited parasitic replication for unknown reasons. In this study, we show that nonphagocytic cells possess an intrinsic property to restrict Leishmania growth. Our study defines a novel role for a family of host defense proteins, the guanylate binding proteins (GBPs), in antileishmanial immunity. Mechanistically, our data indicate that GBPs facilitate the delivery of Leishmania into antimicrobial autolysosomes, thereby enhancing parasite clearance in nonphagocytic cells. We propose that this GBP-dependent host defense program makes nonphagocytic cells an inhospitable host cell type for Leishmania growth.

Emerging single-cell tools are primed to reveal functional and molecular heterogeneity in malignant hematopoietic stem cells

PURPOSE OF REVIEW

The recent emergence of single-cell technologies has permitted unprecedented insight into the molecular drivers of fate choice in blood stem and progenitor cells. This review gives a broad overview of current efforts to understand the molecular regulators of malignant hematopoietic stem cells (HSCs) at the single-cell level.

RECENT FINDINGS

The large-scale adoption of single-cell approaches has allowed extensive description of the transcriptional profiles and functional properties of single HSCs. These techniques are now beginning to be applied to malignant HSCs isolated directly from patients or from mouse models of malignancy. However, these studies have generally struggled to pinpoint the functional regulators of malignant characteristics, since malignant HSCs often differ in more than one property when compared with normal HSCs. Moreover, both normal and malignant populations are complicated by HSC heterogeneity.

SUMMARY

Despite the existence of single-cell gene expression profiling tools, relatively few publications have emerged. Here, we review these studies from recent years with a specific focus on those undertaking single-cell measurements in malignant stem and progenitor cells. We anticipate this to be the tip of the iceberg, expecting the next 2-3 years to produce datasets that will facilitate a much broader understanding of malignant HSCs.

The bone marrow hematopoietic niche and its adaptation to infection

Hematopoiesis is responsible for the formation of all blood cells from hematopoietic stem cells (HSC) in the bone marrow (BM). It is a highly regulated process, in order to adapt its cellular output to changing body requirements. Specific microenvironmental conditions within the BM must exist in order to maintain HSC pluripotency and self-renewal, as well as to ensure appropriate differentiation of progenitor cells towards each hematopoietic lineage. Those conditions were coined "the hematopoietic niche" and their identity in terms of cell types, location and soluble molecular components has been the subject of intense research in the last decades. Infections are one of the environmental challenges to which hematopoiesis must respond, to feed the immune system with functional cell components and compensate for cellular losses. However, how infections impact the bone marrow hematopoietic niche(s) remains elusive and most of the mechanisms involved are still largely unknown. Here, we review the most recent advances on our knowledge on the hematopoietic niche composition and regulation during homeostasis and also on how the niche responds to infectious stress.

Irgm1-deficiency leads to myeloid dysfunction in colon lamina propria and susceptibility to the intestinal pathogen Citrobacter rodentium

IRGM and its mouse orthologue Irgm1 are dynamin-like proteins that regulate vesicular remodeling, intracellular microbial killing, and pathogen immunity. IRGM dysfunction is linked to inflammatory bowel disease (IBD), and while it is thought that defective intracellular killing of microbes underscores IBD susceptibility, studies have yet to address how IRGM/Irgm1 regulates immunity to microbes relevant to intestinal inflammation. Here we find that loss of Irgm1 confers marked susceptibility to Citrobacter rodentium, a noninvasive intestinal pathogen that models inflammatory responses to intestinal bacteria. Irgm1-deficient mice fail to control C. rodentium outgrowth in the intestine, leading to systemic pathogen spread and host mortality. Surprisingly, susceptibility due to loss of Irgm1 function was not linked to defective intracellular killing of C. rodentium or exaggerated inflammation, but was instead linked to failure to remodel specific colon lamina propria (C-LP) myeloid cells that expand in response to C. rodentium infection and are essential for C. rodentium immunity. Defective immune remodeling was most striking in C-LP monocytes, which were successfully recruited to the infected C-LP, but subsequently underwent apoptosis. Apoptotic susceptibility was induced by C. rodentium infection and was specific to this setting of pathogen infection, and was not apparent in other settings of intestinal inflammation. These studies reveal a novel role for Irgm1 in host defense and suggest that deficiencies in survival and remodeling of C-LP myeloid cells that control inflammatory intestinal bacteria may underpin IBD pathogenesis linked to IRGM dysfunction.

Irgm1 knockout indirectly inhibits regeneration after skeletal muscle injury in mice

Immunity-related GTPase family M1 protein (lRGM1) plays an important role in host resistance to infection, immune inflammation, and tumors, and it is expressed in various tissues and cells, including the central nervous system, cardiovascular system, bone marrow-derived cells, glioma, and melanoma. However, the effect of IRGM1 in the muscles has not been reported to date. In this study, Irgm1^-/- mice were used to evaluate the effect of lrgm1 on regeneration after skeletal muscle injury. The tibialis anterior muscle in Irgm1^-/- mice was poorly repaired after BaCl₂-induced injury, whereas lrgm1 knockout itself had no significant effect on the differentiation of myoblasts. However, the microenvironment of Irgm1^-/- mice with a high interferon-gamma level inhibited the differentiation of myoblasts in vivo. These results suggest that lrgm1 knockout indirectly inhibits skeletal muscle regeneration after injury, providing new insights into the biological function of IRGM1.

Ipr1 Regulation by Cyclic GMP-AMP Synthase/Interferon Regulatory Factor 3 and Modulation of Irgm1 Expression via p53

Intracellular pathogen resistance 1 (Ipr1) has been found to be a mediator to integrate cyclic GMP-AMP synthase (cGAS)-interferon regulatory factor 3 (IRF3), activated by intracellular pathogens, with the p53 pathway. Previous studies have shown the process of Ipr1 induction by various immune reactions, including intracellular bacterial and viral infections. The present study demonstrated that Ipr1 is regulated by the cGAS-IRF3 pathway during pathogenic infection. IRF3 was found to regulate Ipr1 expression by directly binding the interferon-stimulated response element motif of the Ipr1 promoter. Knockdown of Ipr1 decreased the expression of immunity-related GTPase family M member 1 (Irgm1), which plays critical roles in autophagy initiation. Irgm1 promoter characterization revealed a p53 motif in front of the transcription start site. P53 was found to participate in regulation of Irgm1 expression and IPR1-related effects on P53 stability by affecting interactions between ribosomal protein L11 (RPL11) and transformed mouse 3T3 cell double minute 2 (MDM2). Our results indicate that Ipr1 integrates cGAS-IRF3 with p53-modulated Irgm1 expression.

Bordetella pertussis Whole Cell Immunization, Unlike Acellular Immunization, Mimics Naïve Infection by Driving Hematopoietic Stem and Progenitor Cell Expansion in Mice

Hematopoietic stem and progenitor cell (HSPC) compartments are altered to direct immune responses to infection. Their roles during immunization are not well-described. To elucidate mechanisms for waning immunity following immunization with acellular vaccines (ACVs) against Bordetella pertussis (Bp), we tested the hypothesis that immunization with Bp ACVs and whole cell vaccines (WCVs) differ in directing the HSPC characteristics and immune cell development patterns that ultimately contribute to the types and quantities of cells produced to fight infection. Our data demonstrate that compared to control and ACV-immunized CD-1 mice, immunization with an efficacious WCV drives expansion of hematopoietic multipotent progenitor cells (MPPs), increases circulating white blood cells (WBCs), and alters the size and composition of lymphoid organs. In addition to MPPs, common lymphoid progenitor (CLP) proportions increase in the bone marrow of WCV-immunized mice, while B220⁺ cell proportions decrease. Upon subsequent infection, increases in maturing B cell populations are striking in WCV-immunized mice. RNAseq analyses of HSPCs revealed that WCV and ACV-immunized mice vastly differ in developing VDJ gene segment diversity. Moreover, gene set enrichment analyses demonstrate WCV-immunized mice exhibit unique gene signatures that suggest roles for interferon (IFN) induced gene expression. Also observed in naïve infection, these IFN stimulated gene (ISG) signatures point toward roles in cell survival, cell cycle, autophagy, and antigen processing and presentation. Taken together, these findings underscore the impact of vaccine antigen and adjuvant content on skewing and/or priming HSPC populations for immune response.

Partners in anti-crime: how interferon-inducible GTPases and autophagy proteins team up in cell-intrinsic host defense

Once pathogens have breached the mechanical barriers to infection, survived extracellular immunity and successfully invaded host cells, cell-intrinsic immunity becomes the last line of defense to protect the mammalian host against viruses, bacteria, fungi and protozoa. Many cell-intrinsic defense programs act as high-precision weapons that specifically target intracellular microbes or cytoplasmic sites of microbial replication while leaving endogenous organelles unharmed. Critical executioners of cell-autonomous immunity include interferon-inducible dynamin-like GTPases and autophagy proteins, which often act cooperatively in locating and antagonizing intracellular pathogens. Here, we discuss possible mechanistic models to account for the functional interactions that occur between these two distinct classes of host defense proteins.

Rheb1 loss leads to increased hematopoietic stem cell proliferation and myeloid-biased differentiation in vivo

Hematopoietic stem cells constitute a unique subpopulation of blood cells that can give rise to all types of mature cells in response to physiological demands. However, the intrinsic molecular machinery that regulates this transformative property remains elusive. In this paper, we demonstrate that small GTPase Rheb1 is a critical regulator of proliferation and differentiation of hematopoietic stem cells in vivo Rheb1 deletion led to increased phenotypic hematopoietic stem cell/hematopoietic progenitor cell proliferation under a steady state condition. Over-proliferating Rheb1-deficient hematopoietic stem cells were severely impaired in functional repopulation assays, and they failed to regenerate the blood system when challenged with hematopoietic ablation by sublethal irradiation. In addition, it was discovered that Rheb1 loss resulted in a lack of maturation of neutrophils / caused neutrophil immaturation by reducing mTORC1 activity, and that activation of the mTORC1 signaling pathway by mTOR activator 3BDO partially restored the maturation of Rheb1-deficient neutrophils. Rheb1 deficiency led to a progressive enlargement of the hematopoietic stem cell population and an eventual excessive myeloproliferation in vivo, including an overproduction of peripheral neutrophils and an excessive expansion of extramedullary hematopoiesis. Moreover, low RHEB expression was correlated with poor survival in acute myeloid leukemia patients with normal karyotype. Our results, therefore, demonstrate a critical and unique role for Rheb1 in maintaining proper hematopoiesis and myeloid differentiation.

Myelopoiesis in the Context of Innate Immunity

An intact and fully functional innate immune system is critical in the defense against pathogens. Indeed, during systemic infection, the ability of the organism to cope with the increased demand for phagocytes depends heavily on sufficient replenishment of mature myeloid cells. This process, designated emergency or demand-adapted myelopoiesis, requires the activation of hematopoietic progenitors in the bone marrow (BM), resulting in their proliferation and differentiation toward the myeloid lineage. Failure of BM progenitors to adapt to the enhanced need for mature cells in the periphery can be life-threatening, as indicated by the detrimental effect of chemotherapy-induced myelosuppression on the outcome of systemic infection. Recent advances demonstrate an important role of not only committed myeloid progenitors but also of hematopoietic stem cells (HSCs) in emergency myelopoiesis. In this regard, pathogen-derived products (e.g., Toll-like receptor ligands) activate HSC differentiation towards the myeloid lineage, either directly or indirectly, by inducing the production of inflammatory mediators (e.g., cytokines and growth factors) by hematopoietic and nonhematopoietic cell populations. The inflammatory mediators driving demand-adapted myelopoiesis target not only HSCs but also HSC-supportive cell populations, collectively known as the HSC niche, the microenvironment where HSCs reside. In this review, we discuss recent findings that have further elucidated the mechanisms that drive emergency myelopoiesis, focusing on the interactions of HSCs with their BM microenvironment.

Regulation of myelopoiesis by proinflammatory cytokines in infectious diseases

Hematopoiesis is hierarchically orchestrated by a very small population of hematopoietic stem cells (HSCs) that reside in the bone-marrow niche and are tightly regulated to maintain homeostatic blood production. HSCs are predominantly quiescent, but they enter the cell cycle in response to inflammatory signals evoked by severe systemic infection or injury. Thus, hematopoietic stem and progenitor cells (HSPCs) can be activated by pathogen recognition receptors and proinflammatory cytokines to induce emergency myelopoiesis during infection. This emergency myelopoiesis counterbalances the loss of cells and generates lineage-restricted hematopoietic progenitors, eventually replenishing mature myeloid cells to control the infection. Controlled generation of such signals effectively augments host defense, but dysregulated stimulation by these signals is harmful to HSPCs. Such hematopoietic failure often results in blood disorders including chronic inflammatory diseases and hematological malignancies. Recently, we found that interleukin (IL)-27, one of the IL-6/IL-12 family cytokines, has a unique ability to directly act on HSCs and promote their expansion and differentiation into myeloid progenitors. This process resulted in enhanced production of neutrophils by emergency myelopoiesis during the blood-stage mouse malaria infection. In this review, we summarize recent advances in the regulation of myelopoiesis by proinflammatory cytokines including type I and II interferons, IL-6, IL-27, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, and IL-1 in infectious diseases.

NR4A1 and NR4A3 restrict HSC proliferation via reciprocal regulation of C/EBPα and inflammatory signaling

Members of the NR4A subfamily of nuclear receptors have complex, overlapping roles during hematopoietic cell development and also function as tumor suppressors of hematologic malignancies. We previously identified NR4A1 and NR4A3 (NR4A1/3) as functionally redundant suppressors of acute myeloid leukemia (AML) development. However, their role in hematopoietic stem cell (HSC) homeostasis remains to be disclosed. Using a conditional Nr4a1/Nr4a3 knockout mouse (CDKO), we show that codepletion of NR4A1/3 promotes acute changes in HSC homeostasis including loss of HSC quiescence, accumulation of oxidative stress, and DNA damage while maintaining stem cell regenerative and differentiation capacity. Molecular profiling of CDKO HSCs revealed widespread upregulation of genetic programs governing cell cycle and inflammation and an aberrant activation of the interferon and NF-κB signaling pathways in the absence of stimuli. Mechanistically, we demonstrate that NR4A1/3 restrict HSC proliferation in part through activation of a C/EBPα-driven antiproliferative network by directly binding to a hematopoietic-specific Cebpa enhancer and activating Cebpa transcription. In addition, NR4A1/3 occupy the regulatory regions of NF-κB-regulated inflammatory cytokines, antagonizing the activation of NF-κB signaling. Taken together, our results reveal a novel coordinate control of HSC quiescence by NR4A1/3 through direct activation of C/EBPα and suppression of activation of NF-κB-driven proliferative inflammatory responses.

Chronic Infection Depletes Hematopoietic Stem Cells through Stress-Induced Terminal Differentiation

Chronic infections affect a third of the world's population and can cause bone marrow suppression, a severe condition that increases mortality from infection. To uncover the basis for infection-associated bone marrow suppression, we conducted repeated infection of WT mice with Mycobacterium avium. After 4-6 months, mice became pancytopenic. Their hematopoietic stem and progenitor cells (HSPCs) were severely depleted and displayed interferon gamma (IFN-γ) signaling-dependent defects in self-renewal. There was no evidence of increased HSPC mobilization or apoptosis. However, consistent with known effects of IFN-γ, transcriptome analysis pointed toward increased myeloid differentiation of HSPCs and revealed the transcription factor Batf2 as a potential mediator of IFN-γ-induced HSPC differentiation. Gain- and loss-of-function studies uncovered a role for Batf2 in myeloid differentiation in both murine and human systems. We thus demonstrate that chronic infection can deplete HSPCs and identify BATF2 as a mediator of infection-induced HSPC terminal differentiation.

Regulation of innate immune functions by guanylate-binding proteins

Guanylate-binding proteins (GBP) are a family of dynamin-related large GTPases which are expressed in response to interferons and other pro-inflammatory cytokines. GBPs mediate a broad spectrum of innate immune functions against intracellular pathogens ranging from viruses to bacteria and protozoa. Several binding partners for individual GBPs have been identified and several different mechanisms of action have been proposed depending on the organisms, the cell type and the pathogen used. Many of these anti-pathogenic functions of GBPs involve the recruitment to and the subsequent destruction of pathogen containing vacuolar compartments, the assembly of large oligomeric innate immune complexes such as the inflammasome, or the induction of autophagy. Furthermore, GBPs often cooperate with immunity-related GTPases (IRGs), another family of dynamin-related GTPases, to exert their anti-pathogenic function, but since most IRGs have been lost in the evolution of higher primates, the anti-pathogenic function of human GBPs seems to be IRG-independent. GBPs and IRGs share biochemical and structural properties with the other members of the dynamin superfamily such as low nucleotide affinity and a high intrinsic GTPase activity which can be further enhanced by oligomerisation. Furthermore, GBPs and IRGs can interact with lipid membranes. In the case of three human and murine GBP isoforms this interaction is mediated by C-terminal isoprenylation. Based on cell biological studies, and in analogy to the function of other dynamins in membrane scission events, it has been postulated that both GBPs and IRGs might actively disrupt the outer membrane of pathogen-containing vacuole leading to the detection and destruction of the pathogen by the cytosolic innate immune system of the host. Recent evidence, however, indicates that GBPs might rather function by mediating membrane tethering events similar to the dynamin-related atlastin and mitofusin proteins, which mediate fusion of the ER and mitochondria, respectively. The aim of this review is to highlight the current knowledge on the function of GBPs in innate immunity and to combine it with the recent progress in the biochemical characterisation of this protein family.

Irgm1 coordinately regulates autoimmunity and host defense at select mucosal surfaces

The pathogenesis of primary Sjogren's syndrome (SS), an autoimmune disease that targets the mucosa of exocrine tissues, is poorly understood. Although several mouse models have been developed that display features of SS, most of these are within the larger context of a lupus-like presentation. Immunity-related GTPase family M protein 1 (Irgm1) is an interferon-inducible cytoplasmic GTPase that is reported to regulate autophagy and mitochondrial homeostasis. Here, we report that naive Irgm1-/- mice display lymphocytic infiltration of multiple mucosal tissues including the lung in a manner reminiscent of SS, together with IgA class-predominant autoantibodies including anti-Ro and anti-La. This phenotype persists in the germ-free state, but is abolished by deletion of Irgm3. Irgm1-/- mice have increased local production in the lung of TECP15-idiotype IgA, a natural antibody with dual reactivity against host and pneumococcal phosphorylcholine. Associated with this, Irgm1-/- mice display enhanced opsonization and clearance of Streptococcus pneumoniae from the lung and increased survival from pneumococcal pneumonia. Taken together, our results identify Irgm1 as a master regulator of mucosal immunity that dually modulates evolutionarily conserved self- and other-directed immune responses at the interface of host with environment.

Toxoplasma Effectors Targeting Host Signaling and Transcription

Early electron microscopy studies revealed the elaborate cellular features that define the unique adaptations of apicomplexan parasites. Among these were bulbous rhoptry (ROP) organelles and small, dense granules (GRAs), both of which are secreted during invasion of host cells. These early morphological studies were followed by the exploration of the cellular contents of these secretory organelles, revealing them to be comprised of highly divergent protein families with few conserved domains or predicted functions. In parallel, studies on host-pathogen interactions identified many host signaling pathways that were mysteriously altered by infection. It was only with the advent of forward and reverse genetic strategies that the connections between individual parasite effectors and the specific host pathways that they targeted finally became clear. The current repertoire of parasite effectors includes ROP kinases and pseudokinases that are secreted during invasion and that block host immune pathways. Similarly, many secretory GRA proteins alter host gene expression by activating host transcription factors, through modification of chromatin, or by inducing small noncoding RNAs. These effectors highlight novel mechanisms by which T. gondii has learned to harness host signaling to favor intracellular survival and will guide future studies designed to uncover the additional complexity of this intricate host-pathogen interaction.

Developmental HSC Microenvironments: Lessons from Zebrafish

Hematopoietic stem cells (HSCs) posses the ability to maintain the blood system of an organism from birth to adulthood. The behavior of HSCs is modulated by its microenvironment. During development, HSCs acquire the instructions to self-renew and differentiate into all blood cell fates by passing through several developmental microenvironments. In this chapter, we discuss the signals and cell types that inform HSC decisions throughout ontogeny with a focus on HSC specification, mobilization, migration, and engraftment.

Stress and Non-Stress Roles of Inflammatory Signals during HSC Emergence and Maintenance

Hematopoietic stem cells (HSCs) are a rare population that gives rise to almost all cells of the hematopoietic system, including immune cells. Until recently, it was thought that immune cells sense inflammatory signaling and HSCs respond only secondarily to these signals. However, it was later shown that adult HSCs could directly sense and respond to inflammatory signals, resulting in a higher output of immune cells. Recent studies demonstrated that inflammatory signaling is also vital for HSC ontogeny. These signals are thought to arise in the absence of pathogens, are active during development, and indispensable for HSC formation. In contrast, during times of stress and disease, inflammatory responses can be activated and can have devastating effects on HSCs. In this review, we summarize the current knowledge about inflammatory signaling in HSC development and maintenance, as well as the endogenous molecular cues that can trigger inflammatory pathway activation. Finally, we comment of the role of inflammatory signaling in hematopoietic diseases.

Hematopoietic Stem Cell Regulation by Type I and II Interferons in the Pathogenesis of Acquired Aplastic Anemia

Aplastic anemia (AA) occurs when the bone marrow fails to support production of all three lineages of blood cells, which are necessary for tissue oxygenation, infection control, and hemostasis. The etiology of acquired AA is elusive in the vast majority of cases but involves exhaustion of hematopoietic stem cells (HSC), which are usually present in the bone marrow in a dormant state, and are responsible for lifelong production of all cells within the hematopoietic system. This destruction is immune mediated and the role of interferons remains incompletely characterized. Interferon gamma (IFNγ) has been associated with AA and type I IFNs (alpha and beta) are well documented to cause bone marrow aplasia during viral infection. In models of infection and inflammation, IFNγ activates HSCs to differentiate and impairs their ability to self-renew, ultimately leading to HSC exhaustion. Recent evidence demonstrating that IFNγ also impacts the HSC microenvironment or niche, raises new questions regarding how IFNγ impairs HSC function in AA. Immune activation can also elicit type I interferons, which may exert effects both distinct from and overlapping with IFNγ on HSCs. IFNα/β increase HSC proliferation in models of sterile inflammation induced by polyinosinic:polycytidylic acid and lead to BM aplasia during viral infection. Moreover, patients being treated with IFNα exhibit cytopenias, in part due to BM suppression. Herein, we review the current understanding of how interferons contribute to the pathogenesis of acquired AA, and we explore additional potential mechanisms by which interferons directly and indirectly impair HSCs. A comprehensive understanding of how interferons impact hematopoiesis is necessary in order to identify novel therapeutic approaches for treating AA patients.

Mycobacterium avium infection induces the resistance of the interferon-γ response in mouse spleen cells at late stages of infection

Background

Bacterial infections cause an increase in the population of hematopoietic stem cells (HSCs). To investigate the downstream factors associated with hematopoietic stem cells, mice are infected with Mycobacterium avium (M. avium).

Results

Mycobacterium avium (M. avium) infection induces the enlargement of the spleen and changes in histopathology, including changes to the lineage populations. A dramatic expansion of Lin⁻c-kit⁺Sca-1⁺ (KSL) cells in mouse bone marrow cells and spleen cells was detected 4 weeks after infection with M. avium; however, there was no difference in the engraft activity between infected and un-infected mouse bone marrow cells. We tested the cytokine and cytokine-related gene expression after M. avium infection and found that IFN-γ expression increased and peaked at 4 weeks in both bone marrow and spleen cells. The expression of Sca-1 gene peaked at 4 weeks in the bone marrow but peaked at 2 weeks in spleen cells, although the Sca-1 surface marker peaked at 4 weeks after infection in both bone marrow and spleen cells. Interferon regulatory factor-2 (IRF-2) expression did not change in the bone marrow cells, whereas it decreased in spleen cells at 4 weeks and IRF-1 expression was up-regulated in both bone marrow and spleen cells after infection. However, the up-regulation of IRF-1 was not correlated with IFN-γ expression in the M. avium-infected mouse spleen cells.

Conclusions

This finding suggests that the IFN-γ production mediated by M. avium infection alters the population of KSL cells during host defense, and the down-regulation of the IFN-γ response in spleen cells occurs at the late stage after M. avium infection.

Concise Review: Hematopoietic Stem Cell Origins: Lessons from Embryogenesis for Improving Regenerative Medicine

Hematopoietic stem cells (HSCs) have extensive regenerative capacity to replace all blood cell types, an ability that is harnessed in the clinic for bone marrow transplantation. Finding appropriate donors remains a major limitation to more extensive usage of HSC‐based therapies. Derivation of patient‐specific HSCs from pluripotent stem cells offers great promise to remedy this problem if scientists could crack the code on how to make robust, transplantable HSCs in a dish. Studies delving into the native origins of HSC production during embryonic development should supply the necessary playbook. This review presents recent discoveries from animal models, with a focus on zebrafish, and discusses the implications of these new advances in the context of prior knowledge. The focus is on the latest research exploring the role of epigenetic regulation, signaling pathways, and niche components needed for proper HSC formation. These studies provide new directions that should be explored for de novo generation and expansion of HSCs for regenerative therapies. Stem Cells Translational Medicine2017;6:60–67

Loss of the interferon-γ-inducible regulatory immunity-related GTPase (IRG), Irgm1, causes activation of effector IRG proteins on lysosomes, damaging lysosomal function and predicting the dramatic susceptibility of Irgm1-deficient mice to infection

Background

The interferon-γ (IFN-γ)-inducible immunity-related GTPase (IRG), Irgm1, plays an essential role in restraining activation of the IRG pathogen resistance system. However, the loss of Irgm1 in mice also causes a dramatic but unexplained susceptibility phenotype upon infection with a variety of pathogens, including many not normally controlled by the IRG system. This phenotype is associated with lymphopenia, hemopoietic collapse, and death of the mouse.

Results

We show that the three regulatory IRG proteins (GMS sub-family), including Irgm1, each of which localizes to distinct sets of endocellular membranes, play an important role during the cellular response to IFN-γ, each protecting specific membranes from off-target activation of effector IRG proteins (GKS sub-family). In the absence of Irgm1, which is localized mainly at lysosomal and Golgi membranes, activated GKS proteins load onto lysosomes, and are associated with reduced lysosomal acidity and failure to process autophagosomes. Another GMS protein, Irgm3, is localized to endoplasmic reticulum (ER) membranes; in the Irgm3-deficient mouse, activated GKS proteins are found at the ER. The Irgm3-deficient mouse does not show the drastic phenotype of the Irgm1 mouse. In the Irgm1/Irgm3 double knock-out mouse, activated GKS proteins associate with lipid droplets, but not with lysosomes, and the Irgm1/Irgm3^−/− does not have the generalized immunodeficiency phenotype expected from its Irgm1 deficiency.

Conclusions

The membrane targeting properties of the three GMS proteins to specific endocellular membranes prevent accumulation of activated GKS protein effectors on the corresponding membranes and thus enable GKS proteins to distinguish organellar cellular membranes from the membranes of pathogen vacuoles. Our data suggest that the generalized lymphomyeloid collapse that occurs in Irgm1^−/− mice upon infection with a variety of pathogens may be due to lysosomal damage caused by off-target activation of GKS proteins on lysosomal membranes and consequent failure of autophagosomal processing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0255-4) contains supplementary material, which is available to authorized users.

Type II interferon promotes differentiation of myeloid-biased hematopoietic stem cells

Interferon gamma (IFNγ) promotes cell division of hematopoietic stem cells (HSCs) without affecting the total HSC number. We postulated that IFNγ stimulates differentiation of HSCs as part of the innate immune response. Here, we report that type II interferon signaling is required, both at baseline and during an animal model of LCMV infection, to maintain normal myeloid development. By separately evaluating myeloid-biased and lymphoid-biased HSC subtypes, we found that myeloid-biased HSCs express higher levels of IFNγ receptor and are specifically activated to divide after recombinant IFNγ exposure in vivo. While both HSC subtypes show increased expression of the transcription factor C/EBPβ after infection, only the myeloid-biased HSCs are transiently depleted from the marrow during the type II interferon-mediated immune response to Mycobacterium avium infection, as measured both functionally and phenotypically. These findings indicate that IFNγ selectively permits differentiation of myeloid-biased HSCs during an innate immune response to infection. This represents the first report of a context and a mechanism for discriminate utilization of the alternate HSC subtypes. Terminal differentiation, at the expense of self-renewal, may compromise HSC populations during states of chronic inflammation.

Pathogenesis of Crohn's disease

Significant progress in our understanding of Crohn's disease (CD), an archetypal common, complex disease, has now been achieved. Our ability to interrogate the deep complexities of the biological processes involved in maintaining gut mucosal homeostasis is a major over-riding factor underpinning this rapid progress. Key studies now offer many novel and expansive insights into the interacting roles of genetic susceptibility, immune function, and the gut microbiota in CD. Here, we provide overviews of these recent advances and new mechanistic themes, and address the challenges and prospects for translation from concept to clinic.

IFNg-induced Irgm1 promotes tumorigenesis of melanoma via dual regulation of apoptosis and Bif-1-dependent autophagy

Interferon gamma (IFNg) has been known as the regulator for both tumor immune surveillance and tumorgenesis. However, mechanisms underlying the resistance of tumor cell to IFNg have yet been fully understood. In the current study, we showed that immunity-related GTPase family member 1 (mouse: Irgm1; human: IRGM) is essential for IFNg-mediated regulation of tumor cell growth in melanoma. IRGM/Irgm1 was highly expressed in human and mouse melanoma. IFNg and starvation synergistically induced Irgm1 expression in melanoma B16 cells. In vivo, injection of Irgm1-siRNA-treated cells significantly reduced the number of tumor nodules and prolonged the mice survival. In vitro, knockdown endogenous or IFNg-induced Irgm1 significantly decreases the proliferation and increases apoptosis of B16 cells. In addition, suppressing Irgm1 decreased the IFNg/starvation-induced autophagy, while overexpressing Irgm1 significantly increased autophagy and rescued starvation-challenged cells. Moreover, IFNg and starvation-induced the co-localization of Irgm1 with Bax-interacting factor 1 (Bif-1). Knockdown of Bif-1 decreased Irgm1-mediated tumor cell autophagy. Taken together, these data reveal an Irgm1-dependent mechanism that promotes the tumorigenesis of melanoma via dual regulation of apoptosis and Bif-1-dependent autophagy.

Striking the Right Balance Determines TB or Not TB

Mycobacterium tuberculosis continues to be one of the most successful pathogens on earth. Upon inhalation of M. tuberculosis by a healthy individual, the host immune system will attempt to eliminate these pathogens using a combination of immune defense strategies. These include the recruitment of macrophages and other phagocytes to the site of infection, production of cytokines that enhance the microbicidal capacity of the macrophages, as well as the activation of distinct subsets of leukocytes that work in concert to fight the infection. However, being as successful as it is, M. tuberculosis has evolved numerous strategies to subvert host immunity at virtual every level. As a consequence, one third of the world inhabitants carry M. tuberculosis, and tuberculosis continuous to cause disease in more than 8 million people with deadly consequences in well over 1 million patients each year. In this review, we discuss several of the strategies that M. tuberculosis employs to circumvent host immunity, as well as describe some of the mechanisms that the host uses to counter such subversive strategies. As for many other infectious diseases, the ultimate outcome is usually defined by the relative strength of the virulence strategies employed by the tubercle bacillus versus the arsenal of immune defense mechanisms of the infected host.

Temporal gene profiling of the 5XFAD transgenic mouse model highlights the importance of microglial activation in Alzheimer's disease

BACKGROUND

The 5XFAD early onset mouse model of Alzheimer's disease (AD) is gaining momentum. Behavioral, electrophysiological and anatomical studies have identified age-dependent alterations that can be reminiscent of human AD. However, transcriptional changes during disease progression have not yet been investigated. To this end, we carried out a transcriptomic analysis on RNAs from the neocortex and the hippocampus of 5XFAD female mice at the ages of one, four, six and nine months (M1, M4, M6, M9).

RESULTS

Our results show a clear shift in gene expression patterns between M1 and M4. At M1, 5XFAD animals exhibit region-specific variations in gene expression patterns whereas M4 to M9 mice share a larger proportion of differentially expressed genes (DEGs) that are common to both regions. Analysis of DEGs from M4 to M9 underlines the predominance of inflammatory and immune processes in this AD mouse model. The rise in inflammation, sustained by the overexpression of genes from the complement and integrin families, is accompanied by an increased expression of transcripts involved in the NADPH oxidase complex, phagocytic processes and IFN-γ related pathways.

CONCLUSIONS

Overall, our data suggest that, from M4 to M9, sustained microglial activation becomes the predominant feature and point out that both detrimental and neuroprotective mechanisms appear to be at play in this model. Furthermore, our study identifies a number of genes already known to be altered in human AD, thus confirming the use of the 5XFAD strain as a valid model for understanding AD pathogenesis and for screening potential therapeutic molecules.

Impact of interferon-γ on hematopoiesis

The proinflammatory cytokine interferon-γ (IFN-γ) is well known for its important role in innate and adaptive immunity against intracellular infections and for tumor control. Yet, it has become clear that IFN-γ also has a strong impact on bone marrow (BM) output during inflammation, as it affects the differentiation of most hematopoietic progenitor cells. Here, we review the impact of IFN-γ on hematopoiesis, including the function of hematopoietic stem cells (HSCs) and more downstream progenitors. We discuss which hematopoietic lineages are functionally modulated by IFN-γ and through which underlying molecular mechanism(s). We propose the novel concept that IFN-γ acts through upregulation of suppressor of cytokine signaling molecules, which impairs signaling of several cytokine receptors. IFN-γ has also gained clinical interest from different angles, and we discuss how chronic IFN-γ production can lead to the development of anemia and BM failure and how it is involved in malignant hematopoiesis. Overall, this review illustrates the wide-ranging effect of IFN-γ on the (patho-)physiological processes in the BM.

Hematopoietic stem cells, infection, and the niche

The immune response to infection is a rapid and multifaceted process. Infection affects homeostasis within the hematopoietic stem cell (HSC) niche, as lost immune cells must be replaced by HSCs. During the immune response, interferon is produced. Surprisingly, HSCs respond directly to interferon, entering the cell cycle from even the most dormant state. The complex response of both the HSCs and the niche to infection is a unique platform on which to consider HSC-niche interactions. Here, we comment on the contribution of the immune system to the niche and on the direct and indirect effect that infection has on HSCs in the niche.

Re-entry into quiescence protects hematopoietic stem cells from the killing effect of chronic exposure to type I interferons

Quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure

Type I interferons (IFN-1s) are antiviral cytokines that suppress blood production while paradoxically inducing hematopoietic stem cell (HSC) proliferation. Here, we clarify the relationship between the proliferative and suppressive effects of IFN-1s on HSC function during acute and chronic IFN-1 exposure. We show that IFN-1–driven HSC proliferation is a transient event resulting from a brief relaxation of quiescence-enforcing mechanisms in response to acute IFN-1 exposure, which occurs exclusively in vivo. We find that this proliferative burst fails to exhaust the HSC pool, which rapidly returns to quiescence in response to chronic IFN-1 exposure. Moreover, we demonstrate that IFN-1–exposed HSCs with reestablished quiescence are largely protected from the killing effects of IFNs unless forced back into the cell cycle due to culture, transplantation, or myeloablative treatment, at which point they activate a p53-dependent proapoptotic gene program. Collectively, our results demonstrate that quiescence acts as a safeguard mechanism to ensure survival of the HSC pool during chronic IFN-1 exposure. We show that IFN-1s can poise HSCs for apoptosis but induce direct cell killing only upon active proliferation, thereby establishing a mechanism for the suppressive effects of IFN-1s on HSC function.

Nfix is a novel regulator of murine hematopoietic stem and progenitor cell survival

Hematopoietic stem cells are both necessary and sufficient to sustain the complete blood system of vertebrates. Here we show that Nfix, a member of the nuclear factor I (Nfi) family of transcription factors, is highly expressed by hematopoietic stem and progenitor cells (HSPCs) of murine adult bone marrow. Although short hairpin RNA-mediated knockdown of Nfix expression in Lineage(-)Sca-1(+)c-Kit(+) HSPCs had no effect on in vitro cell growth or viability, Nfix-depleted HSPCs displayed a significant loss of colony-forming potential, as well as short- and long-term in vivo hematopoietic repopulating activity. Analysis of recipient mice at 4 to 20 days posttransplant revealed that Nfix-depleted HSPCs are established in the bone marrow, but fail to persist due to increased apoptotic cell death. Gene expression profiling of Nfix-depleted HSPCs reveals that loss of Nfix expression in HSPCs is concomitant with a decrease in the expression of multiple genes known to be important for HSPCs survival, such as Erg, Mecom, and Mpl. These data reveal that Nfix is a novel regulator of HSPCs survival posttransplantation and establish a role for Nfi genes in the regulation of this cellular compartment.

Aging of the hematopoietic system

PURPOSE OF REVIEW

Aging of the hematopoietic system is associated with myeloid malignancies, anemia and immune dysfunction. As hematopoietic stem cells (HSCs) generate all cells of the hematopoietic system, age-associated changes in HSCs may underlie many features of the aged hematopoietic system. Recent findings on age-associated changes in HSCs are reviewed here.

RECENT FINDINGS

Aged HSCs are myeloid biased, have acquired DNA damage and are functionally compromised. However, overall function of the HSC compartment is well maintained through age-associated expansion of HSCs. Many age-related changes in the hematopoietic system, in particular the clonal myeloid bias of HSCs and the decrease in B and T-cell development, in fact begin during development. Furthermore, HSCs possess specific protective mechanisms aimed at maintaining their number, even at the expense of accumulating damaged cells.

SUMMARY

We argue that age-related changes in HSCs and in the hematopoietic system may not entirely be due to a degenerative aging process, but are the result of developmental and stem cell-protective mechanisms aimed at maximizing fitness during reproductive life. These mechanisms may be disadvantageous later in life as damaged HSCs accumulate and establishment of responses to neoantigens becomes compromised because of the reduced generation of naive T and B cells.

Irgm1-deficient mice exhibit Paneth cell abnormalities and increased susceptibility to acute intestinal inflammation

Crohn's disease (CD) is a chronic, immune-mediated, inflammatory disorder of the intestine that has been linked to numerous susceptibility genes, including the immunity-related GTPase (IRG) M (IRGM). IRGs comprise a family of proteins known to confer resistance to intracellular infections through various mechanisms, including regulation of phagosome processing, cell motility, and autophagy. However, despite its association with CD, the role of IRGM and other IRGs in regulating intestinal inflammation is unclear. We investigated the involvement of Irgm1, an ortholog of IRGM, in the genesis of murine intestinal inflammation. After dextran sodium sulfate exposure, Irgm1-deficient [Irgm1 knockout (KO)] mice showed increased acute inflammation in the colon and ileum, with worsened clinical responses. Marked alterations of Paneth cell location and granule morphology were present in Irgm1 KO mice, even without dextran sodium sulfate exposure, and were associated with impaired mitophagy and autophagy in Irgm1 KO intestinal cells (including Paneth cells). This was manifested by frequent tubular and swollen mitochondria and increased LC3-positive autophagic structures. Interestingly, these LC3-positive structures often contained Paneth cell granules. These results suggest that Irgm1 modulates acute inflammatory responses in the mouse intestine, putatively through the regulation of gut autophagic processes, that may be pivotal for proper Paneth cell functioning.