Stromal-derived IL-6 alters the balance of myeloerythroid progenitors during Toxoplasma gondii infection

Young minds, deeper insights: a recap of the BMAS Summer School 2023, ranging from basic research to clinical implications of bone marrow adipose tissue

Bone marrow adiposity (BMA) is a rapidly growing yet very young research field that is receiving worldwide attention based on its intimate relationship with skeletal and metabolic diseases, as well as hematology and cancer. Moreover, increasing numbers of young scientists and students are currently and actively working on BMA within their research projects. These developments led to the foundation of the International Bone Marrow Adiposity Society (BMAS), with the goal to promote BMA knowledge worldwide, and to train new generations of researchers interested in studying this field. Among the many initiatives supported by BMAS, there is the BMAS Summer School, inaugurated in 2021 and now at its second edition. The aim of the BMAS Summer School 2023 was to educate and train students by disseminating the latest advancement on BMA. Moreover, Summer School 2023 provided suggestions on how to write grants, deal with negative results in science, and start a laboratory, along with illustrations of alternative paths to academia. The event was animated by constructive and interactive discussions between early-career researchers and more senior scientists. In this report, we highlight key moments and lessons learned from the event.

Posttraumatic pneumonia exacerbates bone marrow erythropoietic dysfunction

INTRODUCTION

Pneumonia is a common complication after severe trauma that is associated with worse outcomes with increased mortality. Critically ill trauma patients also have persistent inflammation and bone marrow dysfunction that manifests as persistent anemia. Terminal erythropoiesis, which occurs in bone marrow structures called erythroblastic islands (EBIs), has been shown to be impacted by trauma. Using a preclinical model of polytrauma (PT) and pneumonia, we sought to determine the effect of infection on bone marrow dysfunction and terminal erythropoiesis.

METHODS

Male and female Sprague-Dawley rats aged 9 to 11 weeks were subjected to either PT (lung contusion, hemorrhagic shock, cecectomy, and bifemoral pseudofracture) or PT with postinjury day 1 Pseudomonas pneumonia (PT-PNA) and compared with a naive cohort. Erythroblastic islands were isolated from bone marrow samples and imaged via confocal microscopy. Hemoglobin, early bone marrow erythroid progenitors, erythroid cells/EBI, and % reticulocytes/EBI were measured on day 7. Significance was defined as p < 0.05.

RESULTS

Day 7 hemoglobin was significantly lower in both PT and PT-PNA groups compared with naive (10.8 ± 0.6 and 10.9 ± 0.7 vs. 12.1 ± 0.7 g/dL [ p < 0.05]). Growth of bone marrow early erythroid progenitors (colony-forming units-granulocyte, erythrocyte, monocyte, megakaryocyte; erythroid burst-forming unit; and erythroid colony-forming unit) on day 7 was significantly reduced in PT-PNA compared with both PT and naive. Despite a peripheral reticulocytosis following PT and PT-PNA, the percentage of reticulocytes/EBI was not different between naive, PT, and PT-PNA. However, the number of erythroblasts/EBI was significantly lower in PT-PNA compared with naive (2.9 ± 1.5 [ p < 0.05] vs. 8.9 ± 1.1 cells/EBI macrophage). In addition to changes in EBI composition, EBIs were also found to have significant structural changes following PT and PT-PNA.

CONCLUSION

Multicompartmental PT altered late-stage erythropoiesis, and these changes were augmented with the addition of pneumonia. To improve outcomes following trauma and pneumonia, we need to better understand how alterations in EBI structure and function impact persistent bone marrow dysfunction and anemia.

Modulation of haematopoiesis by protozoal and helminth parasites

During inflammation, haematopoietic stem cells (HSCs) in the bone marrow (BM) and periphery rapidly expand and preferentially differentiate into myeloid cells that mediate innate immune responses. HSCs can be directed into quiescence or differentiation by sensing alterations to the haematopoietic niche, including cytokines, chemokines, and pathogen-derived products. Most studies attempting to identify the mechanisms of haematopoiesis have focused on bacterial and viral infections. From intracellular protozoan infections to large multicellular worms, parasites are a global health burden and represent major immunological challenges that remain poorly defined in the context of haematopoiesis. Immune responses to parasites vary drastically, and parasites have developed sophisticated immunomodulatory mechanisms that allow development of chronic infections. Recent advances in imaging, genomic sequencing, and mouse models have shed new light on how parasites induce unique forms of emergency haematopoiesis. In addition, parasites can modify the haematopoiesis in the BM and periphery to improve their survival in the host. Parasites can also induce long-lasting modifications to HSCs, altering future immune responses to infection, inflammation or transplantation, a term sometimes referred to as central trained immunity. In this review, we highlight the current understanding of parasite-induced haematopoiesis and how parasites target this process to promote chronic infections.

Monocytes as primary defenders against Toxoplasma gondii infection

Monocytes are recruited from the bone marrow to sites of infection where they release cytokines and chemokines, function in antimicrobial immunity, and differentiate into macrophages and dendritic cells to control infection. Although many studies have focused on monocyte-derived macrophages and dendritic cells, recent work has examined the unique roles of monocytes during infection to promote immune defense. We focus on the effector functions of monocytes during infection with the parasite Toxoplasma gondii, and discuss the signals that mobilize monocytes to sites of infection, their production of inflammatory cytokines and antimicrobial mediators, their ability to shape the adaptive immune response, and their immunoregulatory functions. Insights from other infections, including Plasmodium and Listeria are also included for comparison and context.

Comparison of Interleukin-6 with Other Markers in Diagnosis of Ovarian Cancer

The lack of specific symptoms in ovarian cancer delays onset of the diagnostic process. Hence, most cases are recognized in late stages of the disease. The aim of this study was to confirm the role of Il-6 compared to other markers in diagnosis and survival in ovarian cancer. The database was collected from 13 January 2021 to 15 February 2023. In total, 101 patients with pelvic tumors with a mean age of 57.86 ± 16.39 participated in the study. In every case, CA125, HE4, CEA, CA19-9, Il-6, C-reactive protein and procalcitonin measurements were taken. Patients with ovarian borderline tumor and metastatic ovarian tumors were excluded from further analysis. Statistically significant correlations were found between diagnosis of ovarian cancer and levels of CA125, HE4, CRP, PCT and Il-6. Comparison of Il-6 with other markers revealed that longer overall survival correlated with lower values of Il-6. In the case of a higher concentration of Il-6, OS and PFS were shorter. Sensitivity and specificity of Il-6 in diagnosis of ovarian cancer were 46.8% and 77.8%, respectively, while for CA125, CRP and PCT were 76.6% and 63%; 68% and 57.5%; 36% and 77%, respectively. More investigations are needed to identify the most specific and sensitive marker for ovarian cancer.

HSC-derived fatty acid oxidation in steady-state and stressed hematopoiesis

Metabolism impacts all cellular functions and plays a fundamental role in physiology. Metabolic regulation of hematopoiesis is dynamically regulated under steady-state and stress conditions. It is clear that hematopoietic stem cells (HSCs) impose different energy demands and flexibility during maintenance compared with stressed conditions. However, the cellular and molecular mechanisms underlying metabolic regulation in HSCs remain poorly understood. In this review, we focus on defining the role of fatty acid oxidation (FAO) in HSCs. We first review the existing literature describing FAO in HSCs under steady-state hematopoiesis. Next, we describe the models used to examine HSCs under stress conditions, and, finally, we describe how infection causes a shift toward FAO in HSCs and the impact of using this pathway on emergency hematopoiesis.

The role of IL-6/JAK2/STAT3 signaling pathway in cancers

Interleukin-6 (IL-6) is a pleiotropic cytokine involved in immune regulation. It can activate janus kinase 2 (JAK2)-signal transducer and activator of transcription 3 (STAT3) signaling pathway. As one of the important signal transduction pathways in cells, JAK2/STAT3 signaling pathway plays a critical role in cell proliferation and differentiation by affecting the activation state of downstream effector molecules. The activation of JAK2/STAT3 signaling pathway is involved in tumorigenesis and development. It contributes to the formation of tumor inflammatory microenvironment and is closely related to the occurrence and development of many human tumors. This article focuses on the relationship between IL-6/JAK2/STAT3 signaling pathway and liver cancer, breast cancer, colorectal cancer, gastric cancer, lung cancer, pancreatic cancer and ovarian cancer, hoping to provide references for the research of cancer treatment targeting key molecules in IL-6/JAK2/STAT3 signaling pathway.

You are what you eat: How to best fuel your immune system

Normal bone marrow (BM) homeostasis ensures consistent production of progenitor cells and mature blood cells. This requires a reliable supply of nutrients in particular free fatty acids, carbohydrates and protein. Furthermore, rapid changes can occur in response to stress such as infection which can alter the demand for each of these metabolites. In response to infection the haematopoietic stem cells (HSCs) must respond and expand rapidly to facilitate the process of emergency granulopoiesis required for the immediate immune response. This involves a shift from the use of glycolysis to oxidative phosphorylation for energy production and therefore an increased demand for metabolites. Thus, the right balance of each dietary component helps to maintain not only normal homeostasis but also the ability to quickly respond to systemic stress. In addition, some dietary components can drive chronic inflammatory changes in the absence of infection or immune stress, which in turn can impact on overall immune function. The optimal nutrition for the best immunological outcomes would therefore be a diet that supports the functions of immune cells allowing them to initiate effective responses against pathogens but also to resolve the response rapidly when necessary and to avoid any underlying chronic inflammation. In this review we discuss how these key dietary components can alter immune function, what is their impact on bone marrow metabolism and how changes in dietary intake of each of these can improve the outcomes of infections.

Biomarkers of Inflammation among Patients with COVID-19: A Single-Centre Prospective Study from Prishtina, Kosovo

SARS-CoV-2 infection involves the phase of viral replication and inflammatory response predicting the severity of COVID-19. The aim of the study was to analyze the association between IL-6 and hematological and inflammatory parameters and outcomes of patients with COVID-19. Plasma interleukin 6 (IL-6) levels and other inflammatory and hematological parameters were analyzed in 86 adult patients diagnosed with SARS-CoV-2 infection in Kosovo. The median age of patients was 61.50 (49.75–67.25) years. Over half of patients were categorised as severe (58%) and had comorbidities (69%) with hypertension being the most common. The overall mortality rate was 4.7%. The distribution of biochemical parameters across disease severity groups was significantly different for C-reactive protein (CRP), lactate dehydrogenase (LDH), erythrocyte sedimentation rate (ESR), white blood cells (WBC), and granulocytes with higher median values in more severe and critically ill patients whereas lower percentage of lymphocytes, monocytes, and platelet count in severe and critically ill patients. IL-6 levels were increased in 63% of patients with significant differences in the distribution across the following groups; age, disease severity, hospitalisation status, pulmonary infiltrates, oxygen therapy, and hypertension status. IL-6 significantly correlated with CRP, LDH, CK, ESR, and percentages of granulocytes. IL-6 and other inflammatory and hematological parameters were strongly associated with disease severity and may predict the outcome of the SARS-CoV-2 infection.

Preclinical advances and the immunophysiology of a new therapeutic chagas disease vaccine

INTRODUCTION

Chronic infection with the protozoal parasite Trypanosoma cruzi leads to a progressive cardiac disease, known as chronic Chagasic cardiomyopathy (CCC). A new therapeutic Chagas disease vaccine is in development to augment existing antiparasitic chemotherapy drugs.

AREAS COVERED

We report on our current understanding of the underlying immunologic and physiologic mechanisms that lead to CCC, including parasite immune escape mechanisms that allow persistence and the subsequent inflammatory and fibrotic processes that lead to clinical disease. We report on vaccine design and the observed immunotherapeutic effects including induction of a balanced T_H1/T_H2/T_H17 immune response that leads to reduced parasite burdens and tissue pathology. Further, we report vaccine-linked chemotherapy, a dose sparing strategy to further reduce parasite burdens and tissue pathology.

EXPERT OPINION

Our vaccine-linked chemotherapeutic approach is a multimodal treatment strategy, addressing both the parasite persistence and the underlying deleterious host inflammatory and fibrotic responses that lead to cardiac dysfunction. In targeting treatment towards patients with chronic indeterminate or early determinate Chagas disease, this vaccine-linked chemotherapeutic approach will be highly economical and will reduce the global disease burden and deaths due to CCC.

Hematopoietic stem and progenitor cells improve survival from sepsis by boosting immunomodulatory cells

New therapeutic strategies to reduce sepsis-related mortality are urgently needed, as sepsis accounts for one in five deaths worldwide. Since hematopoietic stem and progenitor cells (HSPCs) are responsible for producing blood and immune cells, including in response to immunological stress, we explored their potential for treating sepsis. In a mouse model of Group A Streptococcus (GAS)-induced sepsis, severe immunological stress was associated with significant depletion of bone marrow HSPCs and mortality within approximately 5–7 days. We hypothesized that the inflammatory environment of GAS infection drives rapid HSPC differentiation and depletion that can be rescued by infusion of donor HSPCs. Indeed, infusion of 10,000 naïve HSPCs into GAS-infected mice resulted in rapid myelopoiesis and a 50–60% increase in overall survival. Surprisingly, mice receiving donor HSPCs displayed a similar pathogen load compared to untreated mice. Flow cytometric analysis revealed a significantly increased number of myeloid-derived suppressor cells in HSPC-infused mice, which correlated with reduced inflammatory cytokine levels and restored HSPC levels. These findings suggest that HSPCs play an essential immunomodulatory role that may translate into new therapeutic strategies for sepsis.

Bone Marrow Mesenchymal Stromal Cells: Identification, Classification, and Differentiation

Bone marrow mesenchymal stromal cells (BMSCs), identified as pericytes comprising the hematopoietic niche, are a group of heterogeneous cells composed of multipotent stem cells, including osteochondral and adipocyte progenitors. Nevertheless, the identification and classification are still controversial, which limits their application. In recent years, by lineage tracing and single-cell sequencing, several new subgroups of BMSCs and their roles in normal physiological and pathological conditions have been clarified. Key regulators and mechanisms controlling the fate of BMSCs are being revealed. Cross-talk among subgroups of bone marrow mesenchymal cells has been demonstrated. In this review, we focus on recent advances in the identification and classification of BMSCs, which provides important implications for clinical applications.

Regulation of emergency granulopoiesis during infection

During acute infectious and inflammatory conditions, a large number of neutrophils are in high demand as they are consumed in peripheral organs. The hematopoietic system rapidly responds to the demand by turning from steady state to emergency granulopoiesis to expedite neutrophil generation in the bone marrow (BM). How the hematopoietic system integrates pathogenic and inflammatory stress signals into the molecular cues of emergency granulopoiesis has been the subject of investigations. Recent studies in the field have highlighted emerging concepts, including the direct sensing of pathogens by BM resident or sentinel hematopoietic stem and progenitor cells (HSPCs), the crosstalk of HSPCs, endothelial cells, and stromal cells to convert signals to granulopoiesis, and the identification of novel inflammatory molecules, such as C/EBP-β, ROS, IL-27, IFN-γ, CXCL1 with direct effects on HSPCs. In this review, we will provide a detailed account of emerging concepts while reassessing well-established cellular and molecular players of emergency granulopoiesis. While providing our views on the discrepant results and theories, we will postulate an updated model of granulopoiesis in the context of health and disease.

Small animal models of thermal injury

Burns are a severe form of trauma that account for 1.1 million cases necessitating medical attention and 4500 mortalities annually in the United States alone. Importantly, the initial trauma is succeeded by extensive, prolonged physiological alterations that detrimentally impact multiple organ systems. Given the complexity of post-burn pathophysiology, in vitro experiments are insufficient to model thermal injuries. Therefore, compatible animal burn models are essential for studying burn-related phenomena. In this chapter, we discuss commonly employed small animal burn models and their comparability and applicability to human studies. In particular, we compare post-burn wound healing between the species as well as relevant hypermetabolic and inflammatory characteristics, providing a better understanding of the pros and cons of utilizing a small animal surrogate for human burns. We further provide an overview of the rodent scald burn model methodology as well as a comparison between elderly, aged and young animals, providing a guide for tailoring animal model choice based on the relevant research question.

Remodeling of the Bone Marrow Stromal Microenvironment During Pathogenic Infections

The bone marrow (BM) is the primary hematopoietic organ and a hub in which organismal demands for blood cellular output are systematically monitored. BM tissues are additionally home to a plethora of mature immune cell types, providing functional environments for the activation of immune responses and acting as preferred anatomical reservoirs for cells involved in immunological memory. Stromal cells of the BM microenvironment crucially govern different aspects of organ function, by structuring tissue microanatomy and by directly providing essential regulatory cues to hematopoietic and immune components in distinct niches. Emerging evidence demonstrates that stromal networks are endowed with remarkable functional and structural plasticity. Stress-induced adaptations of stromal cells translate into demand-driven hematopoiesis. Furthermore, aberrations of stromal integrity arising from pathological conditions critically contribute to the dysregulation of BM function. Here, we summarize our current understanding of the alterations that pathogenic infections and ensuing inflammatory conditions elicit on the global topography of the BM microenvironment, the integrity of anatomical niches and cellular interactions, and ultimately, on the regulatory function of diverse stromal subsets.

Skeletal stem cell fate defects caused by Pdgfrb activating mutation

Autosomal dominant PDGFRβ gain-of-function mutations in mice and humans cause a spectrum of wasting and overgrowth disorders afflicting the skeleton and other connective tissues, but the cellular origin of these disorders remains unknown. We demonstrate that skeletal stem cells (SSCs) isolated from mice with a gain-of-function D849V point mutation in PDGFRβ exhibit colony formation defects that parallel the wasting or overgrowth phenotypes of the mice. Single-cell RNA transcriptomics with SSC-derived polyclonal colonies demonstrates alterations in osteogenic and chondrogenic precursors caused by PDGFRβD849V. Mutant cells undergo poor osteogenesis in vitro with increased expression of Sox9 and other chondrogenic markers. Mice with PDGFRβD849V exhibit osteopenia. Increased STAT5 phosphorylation and overexpression of Igf1 and Socs2 in PDGFRβD849V cells suggests that overgrowth in mice involves PDGFRβD849V activating the STAT5-IGF1 axis locally in the skeleton. Our study establishes that PDGFRβD849V causes osteopenic skeletal phenotypes that are associated with intrinsic changes in SSCs, promoting chondrogenesis over osteogenesis.

Bone marrow CD73+ mesenchymal stem cells display increased stemness in vitro and promote fracture healing in vivo

Mesenchymal stem cells (MSCs) are multipotent and considered to be of great potential for regenerative medicine. We could show recently (Breitbach, Kimura et al. 2018) that a subpopulation of MSCs as well as sinusoidal endothelial cells (sECs) in the bone marrow (BM) of CD73-EGFP reporter mice could be labeled in vivo.

We took advantage of this model to explore the plasticity and osteogenic potential of CD73-EGFP⁺ MSCs in vitro and their role in the regenerative response upon bone lesion in vivo. Herein we show that isolated CD73-EGFP⁺ MSCs displayed more pronounced stemness and stronger in vitro differentiation capacity into the osteogenic lineage compared to CD73-EGFP⁻ MSCs. In a bone fracture model, endogenous BM-resident CD73-EGFP⁺ MSCs were found to migrate to the fracture site and differentiate into cartilage and bone cells. Our analysis also showed that CD73-EGFP⁺ sECs contributed to the neovascularization of the fracture site. In addition, grafting of CD73-EGFP⁺ MSCs into acute bone lesions revealed their capacity to differentiate into chondrocytes and osteocytes in vivo and their contribution to callus formation in the regeneration process of fracture healing. Thus, CD73⁺ MSCs display enhanced stemness and osteogenic differentiation potential in vitro and in vivo illustrating a prominent role of the CD73⁺ MSC subpopulation to promote fracture repair.

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CD73+ mesenchymal stem cells (MSCs) display pronounced proliferation potential.

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CD73+ MSCs show high in vitro osteogenic differentiation capacity.

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Endogenous bone marrow-resident CD73+ MSCs contribute to fracture healing.

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Grafted CD73+ MSCs promote bone formation upon fracture repair.

FBW7 couples structural integrity with functional output of primary cilia

Structural defects in primary cilia have robust effects in diverse tissues and systems. However, how disorders of ciliary length lead to functional outcomes are unknown. We examined the functional role of a ciliary length control mechanism of FBW7-mediated destruction of NDE1, in mesenchymal stem cell (MSC) differentiation. We show that FBW7 functions as a master regulator of both negative (NDE1) and positive (TALPID3) regulators of ciliogenesis, with an overall positive net effect on primary cilia formation, MSC differentiation to osteoblasts, and bone architecture. Deletion of Fbxw7 suppresses ciliation, Hedgehog activity, and differentiation, which are partially rescued in Fbxw7/Nde1-null cells. We also show that NDE1, despite suppressing ciliogenesis, promotes MSC differentiation by increasing the activity of the Hedgehog pathway by direct binding and enhancing GLI2 activity in a cilia-independent manner. We propose that FBW7 controls a protein-protein interaction network coupling ciliary structure and function, which is essential for stem cell differentiation.

Petsouki et al. dissect the importance of FBW7-mediated regulation of NDE1 and TALPID3 in mesenchymal stem cells (MSCs). They find that by modulating the abundance of negative (NDE1) and positive (TALPID3) cilia regulators, FBW7 contributes to both the assembly and signaling functions of primary cilia that are necessary for osteoblast differentiation.

A spatiotemporal microenvironment model to improve design of a 3D bioreactor for red cell production

Cellular microenvironments provide stimuli including paracrine and autocrine growth factors and physico-chemical cues, which support efficient in vivo cell production unmatched by current in vitro biomanufacturing platforms. While three-dimensional (3D) culture systems aim to recapitulate niche architecture and function of the target tissue/organ, they are limited in accessing spatiotemporal information to evaluate and optimize in situ cell/tissue process development. Herein, a mathematical modelling framework is parameterized by single-cell phenotypic imaging and multiplexed biochemical assays to simulate the non-uniform tissue distribution of nutrients/metabolites and growth factors in cell niche environments. This model is applied to a bone marrow mimicry 3D perfusion bioreactor containing dense stromal and hematopoietic tissue with limited red blood cell (RBC) egress. The model characterized an imbalance between endogenous cytokine production and nutrient starvation within the microenvironmental niches, and recommended increased cell inoculum density and enhanced medium exchange, guiding the development of a miniaturized prototype bioreactor. The second-generation prototype improved the distribution of nutrients and growth factors and supported a 50-fold increase in RBC production efficiency. This image-informed bioprocess modelling framework leverages spatiotemporal niche information to enhance biochemical factor utilization and improve cell manufacturing in 3D systems.

Tumor Immune Evasion Induced by Dysregulation of Erythroid Progenitor Cells Development

Cancer cells harness normal cells to facilitate tumor growth and metastasis. Within this complex network of interactions, the establishment and maintenance of immune evasion mechanisms are crucial for cancer progression. The escape from the immune surveillance results from multiple independent mechanisms. Recent studies revealed that besides well-described myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) or regulatory T-cells (Tregs), erythroid progenitor cells (EPCs) play an important role in the regulation of immune response and tumor progression. EPCs are immature erythroid cells that differentiate into oxygen-transporting red blood cells. They expand in the extramedullary sites, including the spleen, as well as infiltrate tumors. EPCs in cancer produce reactive oxygen species (ROS), transforming growth factor β (TGF-β), interleukin-10 (IL-10) and express programmed death-ligand 1 (PD-L1) and potently suppress T-cells. Thus, EPCs regulate antitumor, antiviral, and antimicrobial immunity, leading to immune suppression. Moreover, EPCs promote tumor growth by the secretion of growth factors, including artemin. The expansion of EPCs in cancer is an effect of the dysregulation of erythropoiesis, leading to the differentiation arrest and enrichment of early-stage EPCs. Therefore, anemia treatment, targeting ineffective erythropoiesis, and the promotion of EPC differentiation are promising strategies to reduce cancer-induced immunosuppression and the tumor-promoting effects of EPCs.

Obesity-induced inflammation: The impact of the hematopoietic stem cell niche

Obesity and obesity-related diseases like type 2 diabetes (T2D) are prominent global health issues; therefore, there is a need to better understand the mechanisms underlying these conditions. The onset of obesity is characterized by accumulation of proinflammatory cells, including Ly6c^hi monocytes (which differentiate into proinflammatory macrophages) and neutrophils, in metabolic tissues. This shift toward chronic, low-grade inflammation is an obese-state hallmark and highly linked to metabolic disorders and other obesity comorbidities. The mechanisms that induce and maintain increased inflammatory myelopoiesis are of great interest, with a recent focus on how obesity affects more primitive hematopoietic cells. The hematopoietic system is constantly replenished by proper regulation of hematopoietic stem and progenitor (HSPC) pools in the BM. While early research suggests that chronic obesity promotes expansion of myeloid-skewed HSPCs, the involvement of the hematopoietic stem cell (HSC) niche in regulating obesity-induced myelopoiesis remains undefined. In this review, we explore the role of the multicellular HSC niche in hematopoiesis and inflammation, and the potential contribution of this niche to the hematopoietic response to obesity. This review further aims to summarize the potential HSC niche involvement as a target of obesity-induced inflammation and a driver of obesity-induced myelopoiesis.

Structural organization of the bone marrow and its role in hematopoiesis

PURPOSE OF REVIEW

The bone marrow is the main site for hematopoiesis. It contains a unique microenvironment that provides niches that support self-renewal and differentiation of hematopoietic stem cells (HSC), multipotent progenitors (MPP), and lineage committed progenitors to produce the large number of blood cells required to sustain life. The bone marrow is notoriously difficult to image; because of this the anatomy of blood cell production -- and how local signals spatially organize hematopoiesis -- are not well defined. Here we review our current understanding of the spatial organization of the mouse bone marrow with a special focus in recent advances that are transforming our understanding of this tissue.

RECENT FINDINGS

Imaging studies of HSC and their interaction with candidate niches have relied on ex-vivo imaging of fixed tissue. Two recent manuscripts demonstrating live imaging of subsets of HSC in unperturbed bone marrow have revealed unexpected HSC behavior and open the door to examine HSC regulation, in situ, over time. We also discuss recent findings showing that the bone marrow contains distinct microenvironments, spatially organized, that regulate unique aspects of hematopoiesis.

SUMMARY

Defining the spatial architecture of hematopoiesis in the bone marrow is indispensable to understand how this tissue ensures stepwise, balanced, differentiation to meet organism demand; for deciphering alterations to hematopoiesis during disease; and for designing organ systems for blood cell production ex vivo.

Bone Marrow Stromal Cell Assays: In Vitro and In Vivo

Populations of bone marrow stromal cells (BMSCs, also known as bone marrow-derived "mesenchymal stem cells") contain a subset of cells that are able to recapitulate the formation of a bone/marrow organ (skeletal stem cells, SSCs). It is now apparent that cells with similar but not identical properties can be isolated from other skeletal compartments (growth plate, periosteum). The biological properties of BMSCs, and these related stem/progenitor cells, are assessed by a variety of assays, both in vitro and in vivo. Application of these assays in an appropriate fashion provide a great deal of information on the role of BMSCs, and the subset of SSCs, in health and in disease.

The bone marrow niche components are adversely affected in sepsis

Multiple organ dysfunction is an important cause of death in patients with sepsis. Currently, few studies have focused on the impact of sepsis on bone marrow (BM), especially on the cell components of BM niche. In this study, we performed mouse sepsis models by intraperitoneal injection of LPS and cecal ligation and puncture (CLP). The changes of niche major components in the mouse BM among vascular structures, mesenchymal stem cells and Treg cells were observed and analyzed. The results showed that pathological changes in BM was earlier and more prominent than in other organs, and various cell components of the BM niche changed significantly, of which vascular endothelial cells increased transiently with vascular remodeling and the regulatory T cells decreased over a long period of time. These results indicated that the components of the BM niche underwent series of adaptive changes in sepsis.

The Role of the Bone Marrow Microenvironment in the Response to Infection

Hematopoiesis in the bone marrow (BM) is the primary source of immune cells. Hematopoiesis is regulated by a diverse cellular microenvironment that supports stepwise differentiation of multipotent stem cells and progenitors into mature blood cells. Blood cell production is not static and the bone marrow has evolved to sense and respond to infection by rapidly generating immune cells that are quickly released into the circulation to replenish those that are consumed in the periphery. Unfortunately, infection also has deleterious effects injuring hematopoietic stem cells (HSC), inefficient hematopoiesis, and remodeling and destruction of the microenvironment. Despite its central role in immunity, the role of the microenvironment in the response to infection has not been systematically investigated. Here we summarize the key experimental evidence demonstrating a critical role of the bone marrow microenvironment in orchestrating the bone marrow response to infection and discuss areas of future research.

G-CSF shifts erythropoiesis from bone marrow into spleen in the setting of systemic inflammation

The anemia of inflammation is related in part to abnormal erythropoiesis in bone marrow. G-CSF regulates granulopoiesis and is increased during systemic inflammation. Here, we have showed that high levels of G-CSF are associated with repression of bone marrow erythropoiesis and expansion of splenic erythropoiesis in Escherichia coli-infected mice and lipopolysaccharide-treated mice. Under lipopolysaccharide-induced systemic inflammatory conditions in mice, G-CSF neutralization with antibody alleviated the blockage of bone marrow erythropoiesis, prevented the enhancement of splenic erythropoiesis, ameliorated splenomegaly, and reduced the brittleness of spleen. We further demonstrated that after lipopolysaccharide treatment, TLR4-knockout mice display low levels of G-CSF, healthy bone marrow erythropoiesis, almost no stress erythropoiesis in the spleen, and normal size and toughness of spleen. In addition, we found HIF-mediated erythropoietin production is essential for splenic erythropoiesis in the setting of G-CSF-induced suppression of bone marrow erythropoiesis. Our findings identify G-CSF as a critical mediator of inflammation-associated erythropoiesis dysfunction in bone marrow and offer insight into the mechanism of G-CSF-induced splenic erythropoiesis. We provide experimentally significant dimension to the biology of G-CSF.

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.

The role of macrophages in protective and pathological responses to Toxoplasma gondii

The ability of Toxoplasma gondii to cause clinical disease in immune-competent and immune-deficient hosts coupled with its ease of use in vitro and availability of murine models has led to its use as a model organism to study how the immune system controls an intracellular infection. This article reviews the studies that established the role of the cytokine IFN-γ in the activation of macrophages to control T gondii and the events that lead to the mobilization and expansion of macrophage populations and their ability to limit parasite replication. Macrophages also have pro-inflammatory functions that promote protective NK and T-cell activities as well as regulatory properties that facilitate the resolution of inflammation. Nevertheless, while macrophages are important in determining the outcome of infection, T gondii has evolved mechanisms to subvert macrophage activation and can utilize their migratory activities to promote dissemination and these two properties underlie the ability of this parasite to persist and cause disease.

Dynamic responses of the haematopoietic stem cell niche to diverse stresses

Adult haematopoietic stem cells (HSCs) mainly reside in the bone marrow, where stromal and haematopoietic cells regulate their function. The steady state HSC niche has been extensively studied. In this Review, we focus on how bone marrow microenvironment components respond to different insults including inflammation, malignant haematopoiesis and chemotherapy. We highlight common and unique patterns among multiple cell types and their environment and discuss current limitations in our understanding of this complex and dynamic tissue.

Coordination of Immune-Stroma Crosstalk by IL-6 Family Cytokines

Stromal cells are a subject of rapidly growing immunological interest based on their ability to influence virtually all aspects of innate and adaptive immunity. Present in every bodily tissue, stromal cells complement the functions of classical immune cells by sensing pathogens and tissue damage, coordinating leukocyte recruitment and function, and promoting immune response resolution and tissue repair. These diverse roles come with a price: like classical immune cells, inappropriate stromal cell behavior can lead to various forms of pathology, including inflammatory disease, tissue fibrosis, and cancer. An important immunological function of stromal cells is to act as information relays, responding to leukocyte-derived signals and instructing leukocyte behavior in kind. In this regard, several members of the interleukin-6 (IL-6) cytokine family, including IL-6, IL-11, oncostatin M (OSM), and leukemia inhibitory factor (LIF), have gained recognition as factors that mediate crosstalk between stromal and immune cells, with diverse roles in numerous inflammatory and homeostatic processes. This review summarizes our current understanding of how IL-6 family cytokines control stromal-immune crosstalk in health and disease, and how these interactions can be leveraged for clinical benefit.

The Impact of Integrin β2 on Granulocyte/Macrophage Progenitor Proliferation

Previously, we reported that although the HSPC frequency in bone marrow cells (BMC) was comparable between β2-/- and β2+/+ mice, transplantation of β2-/- BMC into lethally irradiated CD45.1 recipient resulted in more myeloid cell production than β2+/+ BMC. The objective of this study is to address if integrin β2 deficiency skews granulocyte/macrophage progenitor (GMP) proliferation. FACS analysis demonstrated that GMP frequency and cell number were higher and megakaryocyte/erythrocyte progenitor frequency and cell number were lower in β2-/- mice than β2+/+ mice. However, the common myeloid progenitors (CMP) frequency and cell number were similar between the two groups. The increased GMP number was due to GMP proliferation as evidenced by the percentage of BrdU-incorporating GMP. Whole genome transcriptome analysis identified increased FcεRIα expression in β2-/- CMP compared to β2+/+ CMP. FcεRIα expression on β2-/- GMP was detected increased in β2-/- mice by qRT-PCR and FACS. Although transplantation of FcεRIα^hi GMP or FcεRIα^lo GMP into lethally irradiated CD45.1 recipient resulted in comparable myeloid cell production, transplantation of β2 deficient FcεRIα^hi GMP generated more myeloid cells than β2+/+ FcεRIα^hi GMP. GATA2 expression was increased in β2-/- GMP. Using a luciferase reporter assay, we demonstrated that mutation of the GATA2 binding site in the FcεRIα promoter region diminished FcεRIα transcription. In vitro, the addition of IgE, the ligand of FcεRIα, promoted GMP expansion, which was abrogated by inhibition of JNK phosphorylation. Integrin β2 deficiency promoted GMP proliferation and myeloid cell production, which was mediated via FcεRIα/IgE-induced JNK phosphorylation in GMP. Stem Cells 2019;37:430-440.

Modulation of Iron Metabolism in Response to Infection: Twists for All Tastes

Iron is an essential nutrient for almost all living organisms, but is not easily made available. Hosts and pathogens engage in a fight for the metal during an infection, leading to major alterations in the host’s iron metabolism. Important pathological consequences can emerge from the mentioned interaction, including anemia. Several recent reports have highlighted the alterations in iron metabolism caused by different types of infection, and several possible therapeutic strategies emerge, based on the targeting of the host’s iron metabolism. Here, we review the most recent literature on iron metabolism alterations that are induced by infection, the consequent development of anemia, and the potential therapeutic approaches to modulate iron metabolism in order to correct iron-related pathologies and control the ongoing infection.

Local exchange of metabolites shapes immunity

Immune cell differentiation and function depend on metabolic changes for the provision of energy and metabolites. Consequently, cellular metabolism relies on the availability of micronutrients such as vitamins and energy‐rich sources including amino acids and fatty acids. The bone marrow controls the continuous production of blood cells and is thereby reliant on the sophisticated interplay of progenitor and mature immune cells with its stromal microenvironment. The significance of stromal subsets in immunopoiesis is undisputed; however, our current knowledge is limited to their role in the production and secretion of a variety of soluble proteins such as cytokines. In contrast, the role of the haematopoietic niche in controlling and providing nutrients such as fatty acids, amino acids and vitamins, which are required for immune cell differentiation and function, remains largely elusive. In this review, we summarize the current understanding of local nutritional exchange and control between immune and stromal cells in peripheral tissue and, when it is known, in the bone marrow. The parallels found between peripheral tissues and bone marrow stroma raises the question of how local metabolism is capable of influencing haematopoiesis and immunopoiesis. A better understanding of the local exchange of nutrients in the bone marrow can be used to improve immune cell formation during ageing, after haematopoietic stem cell transplantation and during immune challenge.

The Role of Interferon-Gamma in Hematopoietic Stem Cell Development, Homeostasis, and Disease

Purpose of Review

Interferon-gamma (IFN-γ) is a pro-inflammatory cytokine that participates in the regulation of hematopoietic stem cells (HSC) during development and under homeostatic conditions. IFN-γ also plays a key pathogenic role in several diseases that affect hematopoiesis including aplastic anemia, hemophagocytic lymphohistiocytosis, and cirrhosis of the liver.

Recent Findings

Studies have shown that increased IFN-γ negatively affects HSC homeostasis, skewing HSC towards differentiation over self-renewal and eventually causing exhaustion of the HSC compartment.

Summary

Here, we explore the mechanisms by which IFN-γ regulates HSC in both normal and pathological conditions. We focus on the role of IFN-γ signaling in HSC fate decisions, and the transcriptional changes it elicits. Elucidating the mechanisms through which IFN-γ regulates HSCs may lead to new therapeutic options to prevent or treat adverse hematologic effects of the many diseases to which IFN-γ contributes.

Interleukin-6 in Schizophrenia-Is There a Therapeutic Relevance?

Renewing interest in immune aspects of schizophrenia and new findings about the brain-fat axis encourage us to discuss the possible role of interleukin-6 (IL-6) in schizophrenia. Previously, it was suggested that a primary alteration of the innate immune system may be relevant in schizophrenia. Functional dichotomy of IL-6 suggests that this chemical messenger may be responsible for regulating the balance between pro- and anti-inflammatory responses, with tissue-specific properties at the periphery and in the central nervous system. Specific phase of this chronic and deteriorating disorder must be considered, which can involve IL-6 in acute or possible chronic inflammation and/or autoimmunity. We give an overview of IL-6 role in the onset and progression of this disorder, also considering cognitive impairment and metabolic changes in patients with schizophrenia. Data suggest that decreased serum level of IL-6 following antipsychotic therapy could be predisposing factor for the development of obesity and obesity-related metabolic disorders in schizophrenia. As we reviewed, the IL-6 plays significant role in disease genesis and progression, so the use of specific inhibitors may not only be beneficial for exacerbation and alleviation of positive symptoms, but may attenuate cognitive impairment in patients with schizophrenia.

Mesenchymal Stem Cells in Myeloid Malignancies: A Focus on Immune Escaping and Therapeutic Implications

The importance of the bone marrow microenvironment forming the so-called niche in physiologic hemopoiesis is largely known, and recent evidences support the presence of stromal alterations from the molecular to the cytoarchitectural level in hematologic malignancies. Various alterations in cell adhesion, metabolism, cytokine signaling, autophagy, and methylation patterns of tumor-derived mesenchymal stem cells have been demonstrated, contributing to the genesis of a leukemic permissive niche. This niche allows both the ineffective haematopoiesis typical of myelodysplastic syndromes and the differentiation arrest, proliferation advantage, and clone selection which is the hallmark of acute myeloid leukemia. Furthermore, the immune system, both adaptive and innate, encompassing mesenchymal-derived cells, has been shown to take part to the leukemic niche. Here, we critically review the state of art about mesenchymal stem cell role in myelodysplastic syndromes and acute myeloid leukemia, focusing on immune escaping mechanisms as a target for available and future anticancer therapies.

"Mesenchymal stem cells": fact or fiction, and implications in their therapeutic use

The concept of a post-natal “mesenchymal stem cell” (“MSC”) originated from studies focused on bone marrow stromal cells (BMSCs), which are non-hematopoietic adherent cells, a subset of which are skeletal stem cells (SSCs), able to form cartilage, bone, hematopoiesis-supportive stroma, and marrow adipocytes based on rigorous clonal and differentiation assays. Subsequently, it was speculated that BMSCs could form other mesodermal derivatives and even cell types from other germ layers. Based on BMSC surface markers, representative of fibroblastic cells, and imprecise differentiation assays, it was further imagined that “MSCs” are ubiquitous and equipotent. However, “MSCs” do not have a common embryonic origin and are not a lineage, but recent studies indicate that they are tissue-specific stem/progenitor cells. These cells share cell surface features owing to their fibroblastic nature, but they are not identical. They display different differentiation capacities based on their tissue origin but do not “trans-differentiate” outside of their lineage, based on rigorous assays. For these reasons, the “MSC” term should be abandoned. Tissue-specific stem/progenitor cells provide the opportunity to devise methods for tissue regeneration by the cells themselves (tissue engineering). Their use in other forms of regenerative medicine based on paracrine, immunosuppressive, and immunomodulatory effects is far less clear.

The Role of the Bone Marrow Stromal Compartment in the Hematopoietic Response to Microbial Infections

Continuous production of blood cells unfolds within a complex three-dimensional tissue scaffold established by highly organized stromal cell networks of mesenchymal, neural, and vascular origin inside bone marrow (BM) cavities. Collectively, stromal cells have been shown to serve two principal roles; first as primary participants of bone remodeling and metabolism and second as master regulators of different stages of blood cell development and production. Indeed, ample evidence demonstrates that stromal cells can sense and integrate systemic signals to shape hematopoietic responses and that these regulatory mechanisms are subverted in multiple pathologic conditions. Microbial infections are stressors that elicit potent inflammatory reactions and induce substantial alterations of hematopoietic output. Whether the cellular components of the BM stromal microenvironment are targeted by infections and participate in infection-induced hematopoiesis has not been investigated in sufficient detail to date. In this manuscript, we provide a succinct updated overview of the different cell populations that are currently known to form BM stroma. We discuss experimental evidence demonstrating that different stromal components are actively damaged or functionally altered by pathogens and/or ensuing inflammatory signals and review how these effects are known to contribute to the hematologic manifestations observed during infections.

Mesenchymal Stem and Progenitor Cells in Normal and Dysplastic Hematopoiesis-Masters of Survival and Clonality?

Myelodysplastic syndromes (MDS) are malignant hematopoietic stem cell disorders that have the capacity to progress to acute myeloid leukemia (AML). Accumulating evidence suggests that the altered bone marrow (BM) microenvironment in general, and in particular the components of the stem cell niche, including mesenchymal stem cells (MSCs) and their progeny, play a pivotal role in the evolution and propagation of MDS. We here present an overview of the role of MSCs in the pathogenesis of MDS, with emphasis on cellular interactions in the BM microenvironment and related stem cell niche concepts. MSCs have potent immunomodulatory capacities and communicate with diverse immune cells, but also interact with various other cellular components of the microenvironment as well as with normal and leukemic stem and progenitor cells. Moreover, compared to normal MSCs, MSCs in MDS and AML often exhibit altered gene expression profiles, an aberrant phenotype, and abnormal functional properties. These alterations supposedly contribute to the “reprogramming” of the stem cell niche into a disease-permissive microenvironment where an altered immune system, abnormal stem cell niche interactions, and an impaired growth control lead to disease progression. The current article also reviews molecular targets that play a role in such cellular interactions and possibilities to interfere with abnormal stem cell niche interactions by using specific targeted drugs.

A Hitchhiker's Guide to Myeloid Cell Subsets: Practical Implementation of a Novel Mononuclear Phagocyte Classification System

The classification of mononuclear phagocytes as either dendritic cells or macrophages has been mainly based on morphology, the expression of surface markers, and assumed functional specialization. We have recently proposed a novel classification system of mononuclear phagocytes based on their ontogeny. Here, we discuss the practical application of such a classification system through a number of prototypical examples we have encountered while hitchhiking from one subset to another, across species and between steady-state and inflammatory settings. Finally, we discuss the advantages and drawbacks of such a classification system and propose a number of improvements to move from theoretical concepts to concrete guidelines.

IL-6 as a keystone cytokine in health and disease

Interleukin 6 (IL-6) has a broad effect on cells of the immune system and those not of the immune system and often displays hormone-like characteristics that affect homeostatic processes. IL-6 has context-dependent pro- and anti-inflammatory properties and is now regarded as a prominent target for clinical intervention. However, the signaling cassette that controls the activity of IL-6 is complicated, and distinct intervention strategies can inhibit this pathway. Clinical experience with antagonists of IL-6 has raised new questions about how and when to block this cytokine to improve disease outcome and patient wellbeing. Here we discuss the effect of IL-6 on innate and adaptive immunity and the possible advantages of various antagonists of IL-6 and consider how the immunobiology of IL-6 may inform clinical decisions.

Disruption of bone and skeletal muscle in severe burns

Severe burn injury triggers the body's nonspecific adaptive responses to acute insult, including the systemic inflammatory and stress responses, as well as the sympathetic response to immobilization. These responses trigger inflammatory bone resorption followed by glucocorticoid-induced apoptosis of osteoblasts and probably osteocytes. Because these patients are catabolic, they suffer concomitant muscle wasting and negative nitrogen balance. The use of anabolic agents such as recombinant human growth hormone and oxandrolone results in improved bone mineral content and muscle strength after approximately 1 year. Use of bisphosphonates within the first 10 days of a severe burn completely blocks the resorptive bone loss and has the added advantage of appearing to preserve muscle protein from excessive breakdown. The mechanism for the protective effect on muscle is not currently known. However, if the effect of bisphosphonates on muscle can be confirmed, it raises the possibility that bone communicates with muscle.

Host immune response to infection and cancer: unexpected commonalities

Both microbes and tumors activate innate resistance, tissue repair, and adaptive immunity. Unlike acute infection, tumor growth is initially unapparent; however, inflammation and immunity affect all phases of tumor growth from initiation to progression and dissemination. Here, we discuss the shared features involved in the immune response to infection and cancer including modulation by commensal microbiota, reactive hematopoiesis, chronic immune responses and regulatory mechanisms to prevent collateral tissue damage. This comparative analysis of immunity to infection and cancer furthers our understanding of the basic mechanisms underlying innate resistance and adaptive immunity and their translational application to the design of new therapeutic approaches.

"Mesenchymal" stem cells

Two opposing descriptions of so-called mesenchymal stem cells (MSCs) exist at this time. One sees MSCs as the postnatal, self-renewing, and multipotent stem cells for the skeleton. This cell coincides with a specific type of bone marrow perivascular cell. In skeletal physiology, this skeletal stem cell is pivotal to the growth and lifelong turnover of bone and to its native regeneration capacity. In hematopoietic physiology, its role as a key player in maintaining hematopoietic stem cells in their niche and in regulating the hematopoietic microenvironment is emerging. In the alternative description, MSCs are ubiquitous in connective tissues and are defined by in vitro characteristics and by their use in therapy, which rests on their ability to modulate the function of host tissues rather than on stem cell properties. Here, I discuss how the two views developed, conceptually and experimentally, and attempt to clarify the confusion arising from their collision.