The Heme Connection: Linking Erythrocytes and Macrophage Biology

Distinct metabolic responses to heme in inflammatory human and mouse macrophages - Role of nitric oxide

Activation of inflammation is tightly associated with metabolic reprogramming in macrophages. The iron-containing tetrapyrrole heme can induce pro-oxidant and pro-inflammatory effects in murine macrophages, but has been associated with polarization towards an anti-inflammatory phenotype in human macrophages. In the current study, we compared the regulatory responses to heme and the prototypical Toll-like receptor (TLR)4 ligand lipopolysaccharide (LPS) in human and mouse macrophages with a particular focus on alterations of cellular bioenergetics. In human macrophages, bulk RNA-sequencing analysis indicated that heme led to an anti-inflammatory transcriptional profile, whereas LPS induced a classical pro-inflammatory gene response. Co-stimulation of heme with LPS caused opposing regulatory patterns of inflammatory activation and cellular bioenergetics in human and mouse macrophages. Specifically, in LPS-stimulated murine, but not human macrophages, heme led to a marked suppression of oxidative phosphorylation and an up-regulation of glycolysis. The species-specific alterations in cellular bioenergetics and inflammatory responses to heme were critically dependent on the availability of nitric oxide (NO) that is generated in inflammatory mouse, but not human macrophages. Accordingly, studies with an inducible nitric oxide synthase (iNOS) inhibitor in mouse, and a pharmacological NO donor in human macrophages, reveal that NO is responsible for the opposing effects of heme in these cells. Taken together, the current findings indicate that NO is critical for the immunomodulatory role of heme in macrophages.

The clinical relevance of heme detoxification by the macrophage heme oxygenase system

Heme degradation by the heme oxygenase (HMOX) family of enzymes is critical for maintaining homeostasis and limiting heme-induced tissue damage. Macrophages express HMOX1 and 2 and are critical sites of heme degradation in healthy and diseased states. Here we review the functions of the macrophage heme oxygenase system and its clinical relevance in discrete groups of pathologies where heme has been demonstrated to play a driving role. HMOX1 function in macrophages is essential for limiting oxidative tissue damage in both acute and chronic hemolytic disorders. By degrading pro-inflammatory heme and releasing anti-inflammatory molecules such as carbon monoxide, HMOX1 fine-tunes the acute inflammatory response with consequences for disorders of hyperinflammation such as sepsis. We then discuss divergent beneficial and pathological roles for HMOX1 in disorders such as atherosclerosis and metabolic syndrome, where activation of the HMOX system sits at the crossroads of chronic low-grade inflammation and oxidative stress. Finally, we highlight the emerging role for HMOX1 in regulating macrophage cell death via the iron- and oxidation-dependent form of cell death, ferroptosis. In summary, the importance of heme clearance by macrophages is an active area of investigation with relevance for therapeutic intervention in a diverse array of human diseases.

A secretory protein neudesin regulates splenic red pulp macrophages in erythrophagocytosis and iron recycling

Neudesin, originally identified as a neurotrophic factor, has primarily been studied for its neural functions despite its widespread expression. Using 8-week-old neudesin knockout mice, we elucidated the role of neudesin in the spleen. The absence of neudesin caused mild splenomegaly, shortened lifespan of circulating erythrocytes, and abnormal recovery from phenylhydrazine-induced acute anemia. Blood cross-transfusion and splenectomy experiments revealed that the shortened lifespan of erythrocytes was attributable to splenic impairment. Further analysis revealed increased erythrophagocytosis and decreased iron stores in the splenic red pulp, which was linked to the upregulation of Fcγ receptors and iron-recycling genes in neudesin-deficient macrophages. In vitro analysis confirmed that neudesin suppressed erythrophagocytosis and expression of Fcγ receptors through ERK1/2 activation in heme-stimulated macrophages. Finally, we observed that 24-week-old neudesin knockout mice exhibited severe symptoms of anemia. Collectively, our results suggest that neudesin regulates the function of red pulp macrophages and contributes to erythrocyte and iron homeostasis.

Organogermanium, Ge-132, promotes the clearance of senescent red blood cells via macrophage-mediated phagocyte activation

Red blood cells (RBCs) are renewed in a cyclic manner. Aging RBCs are captured and degraded by phagocytic cells, and heme metabolic pigments are subsequently excreted in feces. We evaluated the effect of an organogermanium compound on RBC metabolism and found that the phagocytosis of RAW264.7 macrophage-like cells was increased by treatment with 3-(trihydroxygermyl)propanoic acid (THGP). Additionally, consumption of Ge-132 (a dehydrate polymer of THGP) changed the fecal color to bright yellow and increased the erythrocyte metabolic pigment levels and antioxidant activity in feces. These data suggest that Ge-132 may activate macrophages in the body and promote the degradation of aged RBCs. Furthermore, Ge-132 intake promoted not only increases in RBC degradation but also the induction of erythroblast differentiation in bone marrow cells. The normal hematocrit levels were maintained due to the maintenance of homeostasis, even though Ge-132 ingestion increased erythrocyte degradation. Therefore, Ge-132 enhances the degradation of senescent RBCs by macrophages. In turn, RBC production is increased to compensate for the amount of degradation, and RBC metabolism is increased.

Physiological Conditions Leading to Maternal Subclinical Ketosis in Holstein Dairy Cows Can Impair the Offspring's Postnatal Growth and Gut Microbiome Development

Maternal metabolic disruptions, such as ketosis, can have adverse effects on fetal development and influence postnatal factors. Twelve Holstein calves were randomly enrolled in this study at birth and monitored until 8 weeks of age. The study was conducted from fall 2018 until spring 2019. After completing the data collection period, calves were classified according to their respective dams ketotic condition after parturition. This classification was based on dam blood β-hydroxybutyrate < 1.4 mmol/L nonketotic (NONKET; n = 6 calves) or ≥1.4 mmol/L subclinical-ketotic (SK; n = 6 calves). SK calves had greater birth body weight (p = 0.05) but exhibited a slower growth rate compared to NONKET calves from 1 to 8 weeks (p = 0.02). At birth, SK calves had lower (p < 0.01) levels of non-esterified fatty acids and bilirubin compared to NONKET calves. Analysis of feces alpha diversity indicates that by 3 weeks, NONKET calves had greater diversity, richness, and evenness. Butyricicoccus pullicaecorum and Gallibacterium anatis were more abundant in SK calves (p < 0.05) at 3 weeks. In contrast, NONKET calves had a greater (p < 0.05) abundance of Sharpae azabuensis at 3 weeks. These findings suggest that subclinical ketosis in cows can impact the in-utero development, postnatal growth, and maturing gut microbiome of their offspring.

Macrophage metabolic rewiring improves heme-suppressed efferocytosis and tissue damage in sickle cell disease

Sickle cell disease (SCD) is hallmarked by an underlying chronic inflammatory condition, which is contributed by heme-activated proinflammatory macrophages. Although previous studies addressed heme ability to stimulate macrophage inflammatory skewing through Toll-like receptor4 (TLR4)/reactive oxygen species signaling, how heme alters cell functional properties remains unexplored. Macrophage-mediated immune cell recruitment and apoptotic cell (AC) clearance are relevant in the context of SCD, in which tissue damage, cell apoptosis, and inflammation occur owing to vaso-occlusive episodes, hypoxia, and ischemic injury. Here we show that heme strongly alters macrophage functional response to AC damage by exacerbating immune cell recruitment and impairing cell efferocytic capacity. In SCD, heme-driven excessive leukocyte influx and defective efferocytosis contribute to exacerbated tissue damage and sustained inflammation. Mechanistically, these events depend on heme-mediated activation of TLR4 signaling and suppression of the transcription factor proliferator-activated receptor γ (PPARγ) and its coactivator peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α). These changes reduce efferocytic receptor expression and promote mitochondrial remodeling, resulting in a coordinated functional and metabolic reprogramming of macrophages. Overall, this results in limited AC engulfment, impaired metabolic shift to mitochondrial fatty acid β-oxidation, and, ultimately, reduced secretion of the antiinflammatory cytokines interleukin-4 (IL-4) and IL-10, with consequent inhibition of continual efferocytosis, resolution of inflammation, and tissue repair. We further demonstrate that impaired phagocytic capacity is recapitulated by macrophage exposure to plasma of patients with SCD and improved by hemopexin-mediated heme scavenging, PPARγ agonists, or IL-4 exposure through functional and metabolic macrophage rewiring. Our data indicate that therapeutic improvement of heme-altered macrophage functional properties via heme scavenging or PGC1α/PPARγ modulation significantly ameliorates tissue damage associated with SCD pathophysiology.

Predictive risk markers in alcoholism

The medical disorders of alcoholism rank among the leading public health problems worldwide and the need for predictive and prognostic risk markers for assessing alcohol use disorders (AUD) has been widely acknowledged. Early-phase detection of problem drinking and associated tissue toxicity are important prerequisites for timely initiations of appropriate treatments and improving patient's committing to the objective of reducing drinking. Recent advances in clinical chemistry have provided novel approaches for a specific detection of heavy drinking through assays of unique ethanol metabolites, phosphatidylethanol (PEth) or ethyl glucuronide (EtG). Carbohydrate-deficient transferrin (CDT) measurements can be used to indicate severe alcohol problems. Hazardous drinking frequently manifests as heavy episodic drinking or in combinations with other unfavorable lifestyle factors, such as smoking, physical inactivity, poor diet or adiposity, which aggravate the metabolic consequences of alcohol intake in a supra-additive manner. Such interactions are also reflected in multiple disease outcomes and distinct abnormalities in biomarkers of liver function, inflammation and oxidative stress. Use of predictive biomarkers either alone or as part of specifically designed biological algorithms helps to predict both hepatic and extrahepatic morbidity in individuals with such risk factors. Novel approaches for assessing progression of fibrosis, a major determinant of prognosis in AUD, have also been made available. Predictive algorithms based on the combined use of biomarkers and clinical observations may prove to have a major impact on clinical decisions to detect AUD in early pre-symptomatic stages, stratify patients according to their substantially different disease risks and predict individual responses to treatment.

Heme oxygenase-1 modulates ferroptosis by fine-tuning levels of intracellular iron and reactive oxygen species of macrophages in response to Bacillus Calmette-Guerin infection

Macrophages are the host cells and the frontline defense against Mycobacterium tuberculosis (Mtb) infection, and the form of death of infected macrophages plays a pivotal role in the outcome of Mtb infections. Ferroptosis, a programmed necrotic cell death induced by overwhelming lipid peroxidation, was confirmed as one of the mechanisms of Mtb spread following infection and the pathogenesis of tuberculosis (TB). However, the mechanism underlying the macrophage ferroptosis induced by Mtb infection has not yet been fully understood. In the present study, transcriptome analysis revealed the upregulation of heme oxygenase-1 (HMOX1) and pro-ferroptosis cytokines, but downregulation of glutathione peroxidase 4 (GPX4) and other key anti-lipid peroxidation factors in the peripheral blood of both patients with extra-pulmonary tuberculosis (EPTB) and pulmonary tuberculosis (PTB). This finding was further corroborated in mice and RAW264.7 murine macrophage-like cells infected with Bacillus Calmette-Guerin (BCG). A mechanistic study further demonstrated that heme oxygenase-1 protein (HO-1) regulated the production of reactive oxygen species (ROS) and iron metabolism, and ferroptosis in BCG-infected murine macrophages. The knockdown of Hmox1 by siRNA resulted in a significant increase of intracellular ROS, Fe2+, and iron autophagy-mediated factor Ncoa4, along with the reduction of antioxidant factors Gpx4 and Fsp1 in macrophages infected with BCG. The siRNA-mediated knockdown of Hmox1 also reduced cell survival rate and increased the release of intracellular bacteria in BCG-infected macrophages. By contrast, scavenging ROS by N-acetyl cysteine led to the reduction of intracellular ROS, Fe2+, and Hmox1 concentrations, and subsequently inhibited ferroptosis and the release of intracellular BCG in RAW264.7 cells infected with BCG. These findings suggest that HO-1 is an essential regulator of Mtb-induced ferroptosis, which regulates ROS production and iron accretion to alter macrophage death against Mtb infections.

New Insights on Heme Uptake in Leishmania spp

The protozoan parasite Leishmania, responsible for leishmaniasis, is one of the few aerobic organisms that cannot synthesize the essential molecule heme. Therefore, it has developed specialized pathways to scavenge it from its host. In recent years, some proteins involved in the import of heme, such as LHR1 and LFLVCRB, have been identified, but relevant aspects regarding the process remain unknown. Here, we characterized the kinetics of the uptake of the heme analogue Zn(II) Mesoporphyrin IX (ZnMP) in Leishmania major promastigotes as a model of a parasite causing cutaneous leishmaniasis with special focus on the force that drives the process. We found that ZnMP uptake is an active, inducible, and pH-dependent process that does not require a plasma membrane proton gradient but requires the presence of the monovalent cations Na⁺ and/or K⁺. In addition, we demonstrated that this parasite can efflux this porphyrin against a concentration gradient. We also found that ZnMP uptake differs among different dermotropic or viscerotropic Leishmania species and does not correlate with LHR1 or LFLVCRB expression levels. Finally, we showed that these transporters have only partially overlapping functions. Altogether, these findings contribute to a deeper understanding of an important process in the biology of this parasite.

Erythrophagocytes in hemolytic anemia, wound healing, and cancer

Hemolysis is a ubiquitous pathology defined as premature red blood cell destruction within the circulation or local tissues. One of the most archetypal functions of macrophages is phagocytosis of damaged or extravasated red blood cells, preventing the extracellular release of toxic hemoglobin and heme. Upon erythrophagocytosis, spiking intracellular heme concentrations drive macrophage transformation into erythrophagocytes, leveraging antioxidative and iron recycling capacities to defend against hemolytic stress. This unique phenotype transformation is coordinated by a regulatory network comprising the transcription factors BACH1, SPI-C, NRF2, and ATF1. Erythrophagocytes negatively regulate inflammation and immunity and may modulate disease-specific outcomes in hemolytic anemia, wound healing, atherosclerosis, and cancer. In this opinion article, we outline the known and presumed functions of erythrophagocytes and their implications for therapeutic innovation and research.

New Avenues of Heme Synthesis Regulation

During erythropoiesis, there is an enormous demand for the synthesis of the essential cofactor of hemoglobin, heme. Heme is synthesized de novo via an eight enzyme-catalyzed pathway within each developing erythroid cell. A large body of data exists to explain the transcriptional regulation of the heme biosynthesis enzymes, but until recently much less was known about alternate forms of regulation that would allow the massive production of heme without depleting cellular metabolites. Herein, we review new studies focused on the regulation of heme synthesis via carbon flux for porphyrin synthesis to post-translations modifications (PTMs) that regulate individual enzymes. These PTMs include cofactor regulation, phosphorylation, succinylation, and glutathionylation. Additionally discussed is the role of the immunometabolite itaconate and its connection to heme synthesis and the anemia of chronic disease. These recent studies provide new avenues to regulate heme synthesis for the treatment of diseases including anemias and porphyrias.

Transcriptional Control of Gene Expression and the Heterogeneous Cellular Identity of Erythroblastic Island Macrophages

During definitive erythropoiesis, maturation of erythroid progenitors into enucleated reticulocytes requires the erythroblastic island (EBI) niche comprising a central macrophage attached to differentiating erythroid progenitors. Normally, the macrophage provides a nurturing environment for maturation of erythroid cells. Its critical physiologic importance entails aiding in recovery from anemic insults, such as systemic stress or acquired disease. Considerable interest in characterizing the central macrophage of the island niche led to the identification of putative cell surface markers enriched in island macrophages, enabling isolation and characterization. Recent studies focus on bulk and single cell transcriptomics of the island macrophage during adult steady-state erythropoiesis and embryonic erythropoiesis. They reveal that the island macrophage is a distinct cell type but with widespread cellular heterogeneity, likely suggesting distinct developmental origins and biological function. These studies have also uncovered transcriptional programs that drive gene expression in the island macrophage. Strikingly, the master erythroid regulator EKLF/Klf1 seems to also play a major role in specifying gene expression in island macrophages, including a putative EKLF/Klf1-dependent transcription circuit. Our present review and analysis of mouse single cell genetic patterns suggest novel expression characteristics that will enable a clear enrichment of EBI subtypes and resolution of island macrophage heterogeneity. Specifically, the discovery of markers such as Epor, and specific features for EKLF/Klf1-expressing island macrophages such as Sptb and Add2, or for SpiC-expressing island macrophage such as Timd4, or for Maf/Nr1h3-expressing island macrophage such as Vcam1, opens exciting possibilities for further characterization of these unique macrophage cell types in the context of their critical developmental function.

Lineage-instructive functions of the E26-transformation-specific-family transcription factor Spi-C in immune cell development and disease

Cell fate decisions during hematopoiesis are the consequence of a complex mixture of inputs from cell-intrinsic and cell-extrinsic factors. In rare cases, expression of a single transcription factor, or a few key factors, may be sufficient to dictate lineage differentiation in a precursor cell. The E26-transformation-specific-family transcription factor Spi-C has emerged as an example of a lineage-instructive factor involved in the generation of mature, specialized subsets of both myeloid and lymphoid cells. Spi-C can instruct differentiation of splenic precursors into red pulp macrophages responsible for phagocytosing senescent red blood cells. In the B cell compartment, Spi-C acts as a key regulator of cell fate decisions at the pro-B to pre-B cell stage and for plasma cell differentiation. Spi-C regulates key genes including Nfkb1, Bach2, Syk, and Blnk to regulate cell cycle entry and B cell differentiation. Here, we review the biology of the lineage-instructive transcription factor Spi-C and its contribution to mechanisms of disease in macrophages and B cells. This article is categorized under: Cancer > Molecular and Cellular Physiology Immune System Diseases > Molecular and Cellular Physiology Infectious Diseases > Genetics/Genomics/Epigenetics.

Haemolymph node - an immunomorpholoigcal organ: modeling the haemolymph node by allografting renal tissue in the rat

There is no authoritative identification of the attributes of the haemolymph node since Gibbes first described it in 1884. Early literatures showed that it was found near the kidney in human and animals with the characteristics of numerous erythrocytes in sinuses. Subsequent studies were mainly focused in the field of anatomy and histology, such as the source, distribution and quantity of erythrocytes in sinuses. Recent articles mentioned that the emergence of the haemolymph node was related to immunity, but there was no strong evidence to support the hypothesis. Therefore it is still uncertain whether the haemolymph node is an organ of anatomy, histology or immunology. It has been found that the development of the haemolymph node can be elicited in the parathymic area by stimuli such as Escherichia coli, allogeneic breast cancer cells and renal tissue that were injected/transplanted into the tail of rats in our pilot studies. In this study, the model of the haemolymph node was established by transferring the allogeneic renal tissue in the rat. Intrasinusoidal erythrocytes of the node were the component for producing a red macroscopic appearance, while macrophage-erythrocyte-lymphocyte rosettes were the major immunomorphological changes, which reflects the immune activity against the invasion of the allogeneic tissue within the node. Therefore the haemolymph node is an immunomorphological organ.

An Analysis of the Multifaceted Roles of Heme in the Pathogenesis of Cancer and Related Diseases

Heme is an essential prosthetic group in proteins and enzymes involved in oxygen utilization and metabolism. Heme also plays versatile and fascinating roles in regulating fundamental biological processes, ranging from aerobic respiration to drug metabolism. Increasing experimental and epidemiological data have shown that altered heme homeostasis accelerates the development and progression of common diseases, including various cancers, diabetes, vascular diseases, and Alzheimer's disease. The effects of heme on the pathogenesis of these diseases may be mediated via its action on various cellular signaling and regulatory proteins, as well as its function in cellular bioenergetics, specifically, oxidative phosphorylation (OXPHOS). Elevated heme levels in cancer cells intensify OXPHOS, leading to higher ATP generation and fueling tumorigenic functions. In contrast, lowered heme levels in neurons may reduce OXPHOS, leading to defects in bioenergetics and causing neurological deficits. Further, heme has been shown to modulate the activities of diverse cellular proteins influencing disease pathogenesis. These include BTB and CNC homology 1 (BACH1), tumor suppressor P53 protein, progesterone receptor membrane component 1 protein (PGRMC1), cystathionine-β-synthase (CBS), soluble guanylate cyclase (sGC), and nitric oxide synthases (NOS). This review provides an in-depth analysis of heme function in influencing diverse molecular and cellular processes germane to disease pathogenesis and the modes by which heme modulates the activities of cellular proteins involved in the development of cancer and other common diseases.

Macrophage-fibroblast circuits in the spleen

Macrophages are an integral part of all organs in the body, where they contribute to immune surveillance, protection, and tissue-specific homeostatic functions. This is facilitated by so-called niches composed of macrophages and their surrounding stroma. These niches structurally anchor macrophages and provide them with survival factors and tissue-specific signals that imprint their functional identity. In turn, macrophages ensure appropriate functioning of the niches they reside in. Macrophages thus form reciprocal, mutually beneficial circuits with their cellular niches. In this review, we explore how this concept applies to the spleen, a large secondary lymphoid organ whose primary functions are to filter the blood and regulate immunity. We first outline the splenic micro-anatomy, the different populations of splenic fibroblasts and macrophages and their respective contribution to protection of and key physiological processes occurring in the spleen. We then discuss firmly established and potential cellular circuits formed by splenic macrophages and fibroblasts, with an emphasis on the molecular cues underlying their crosstalk and their relevance to splenic functionality. Lastly, we conclude by considering how these macrophage-fibroblast circuits might be impaired by aging, and how understanding these changes might help identify novel therapeutic avenues with the potential of restoring splenic functions in the elderly.

Counter Regulation of Spic by NF-κB and STAT Signaling Controls Inflammation and Iron Metabolism in Macrophages

Activated macrophages must carefully calibrate their inflammatory responses to balance efficient pathogen control with inflammation-mediated tissue damage, but the molecular underpinnings of this "balancing act" remain unclear. Using genetically engineered mouse models and primary macrophage cultures, we show that Toll-like receptor (TLR) signaling induces the expression of the transcription factor Spic selectively in patrolling monocytes and tissue macrophages by a nuclear factor κB (NF-κB)-dependent mechanism. Functionally, Spic downregulates pro-inflammatory cytokines and promotes iron efflux by regulating ferroportin expression in activated macrophages. Notably, interferon-gamma blocks Spic expression in a STAT1-dependent manner. High levels of interferon-gamma are indicative of ongoing infection, and in its absence, activated macrophages appear to engage a "default" Spic-dependent anti-inflammatory pathway. We also provide evidence for the engagement of this pathway in sterile inflammation. Taken together, our findings uncover a pathway wherein counter-regulation of Spic by NF-κB and STATs attune inflammatory responses and iron metabolism in macrophages.

Macrophage metabolic adaptation to heme detoxification involves CO-dependent activation of the pentose phosphate pathway

Heme is an essential cofactor for numerous cellular functions, but release of free heme during hemolysis results in oxidative tissue damage, vascular dysfunction, and inflammation. Macrophages play a key protective role in heme clearance; however, the mechanisms that regulate metabolic adaptations that are required for effective heme degradation remain unclear. Here we demonstrate that heme loading drives a unique bioenergetic switch in macrophages, which involves a metabolic shift from oxidative phosphorylation toward glucose consumption. Metabolomic and transcriptional analysis of heme-loaded macrophages revealed that glucose is funneled into the pentose phosphate pathway (PPP), which is indispensable for efficient heme detoxification and is required to maintain redox homeostasis. We demonstrate that the metabolic shift to the PPP is controlled by heme oxygenase-dependent generation of carbon monoxide (CO). Finally, we show that PPP upregulation occurs in vivo in organ systems central to heme clearance and that PPP activity correlates with heme levels in mouse sickle cell disease (SCD). Together, our findings demonstrate that metabolic adaptation to heme detoxification in macrophages requires a shift to the PPP that is induced by heme-derived CO, suggesting pharmacologic targeting of macrophage metabolism as a novel therapeutic strategy to improve heme clearance in patients with hemolytic disorders.

Sickle Cell Disease: Role of Oxidative Stress and Antioxidant Therapy

Sickle cell disease (SCD) is the most common hereditary disorder of hemoglobin (Hb), which affects approximately a million people worldwide. It is characterized by a single nucleotide substitution in the β-globin gene, leading to the production of abnormal sickle hemoglobin (HbS) with multi-system consequences. HbS polymerization is the primary event in SCD. Repeated polymerization and depolymerization of Hb causes oxidative stress that plays a key role in the pathophysiology of hemolysis, vessel occlusion and the following organ damage in sickle cell patients. For this reason, reactive oxidizing species and the (end)-products of their oxidative reactions have been proposed as markers of both tissue pro-oxidant status and disease severity. Although more studies are needed to clarify their role, antioxidant agents have been shown to be effective in reducing pathological consequences of the disease by preventing oxidative damage in SCD, i.e., by decreasing the oxidant formation or repairing the induced damage. An improved understanding of oxidative stress will lead to targeted antioxidant therapies that should prevent or delay the development of organ complications in this patient population.

Bone marrow haemophagocytosis indicates severe infection with severe acute respiratory syndrome coronavirus 2

AIMS

Haemophagocytosis in the bone marrow of patients who have succumbed to coronavirus disease 19 (COVID-19) has not been widely studied. The aims of the present study were to perform morphological analyses and morphometry of haemophagocytosis in the bone marrow of patients with severe COVID-19, and to correlate the findings with the clinical course of the disease.

METHODS AND RESULTS

In this single-centre study performed at the University Hospital Jena, bone marrow specimens of 15 deceased patients who had experienced a severe course of COVID-19 were sampled from the vertebral column during autopsy. Slides of the bone marrow were stained with routine stains or immunohistochemically, and further examined for haemophagocytosis by the use of light microscopy. To substantiate the morphological findings, additional slides were stained for CD163 and morphometry was performed. In all bone marrow samples, an increase in cellularity was found. Haemophagocytes with erythrophagocytosis were detected in 67% of the deceased patients. In tissues with low numbers of haemophagocytes or ill-defined haemophagocytes, an increase in iron deposits was frequently seen. Morphological findings were then correlated with several important clinical data, and the HScore (probability of having a reactive hemophagocytic syndrome) was calculated to posthumously confirm the diagnosis of secondary haemophagocytic lymphohistiocytosis. The median duration of disease and the hospitalisation time were lower in patients with haemophagocytosis (n = 10) than in patients without haemophagocytosis (n = 5). In addition, patients with haemophagocytes showed increased inflammatory parameters 2-5 days prior to death, in contrast to patients without haemophagocytes.

CONCLUSIONS

Haemophagocytosis is a common finding in the bone marrow of deceased individuals with severe COVID-19, and may indicate fatal severe acute respiratory syndrome coronavirus 2 infections.

Transcriptome Reprogramming of CD11b+ Bone Marrow Cells by Pancreatic Cancer Extracellular Vesicles

Pancreatic cancers (PC) are highly metastatic with poor prognosis, mainly due to delayed detection. We previously showed that PC-derived extracellular vesicles (EVs) act on macrophages residing in the liver, eliciting extracellular matrix remodeling in this organ and marked hepatic accumulation of CD11b+ bone marrow (BM) cells, which support PC liver metastasis. We here show that PC-EVs also bind to CD11b+ BM cells and induce the expansion of this cell population. Transcriptomic characterization of these cells shows that PC-EVs upregulate IgG and IgA genes, which have been linked to the presence of monocytes/macrophages in tumor microenvironments. We also report here the transcriptional downregulation of genes linked to monocyte/macrophage activation, trafficking, and expression of inflammatory molecules. Together, these results show for the first time the existence of a PC-BM communication axis mediated by EVs with a potential role in PC tumor microenvironments.

Heme Oxygenase-1 Induction by Blood-Feeding Arthropods Controls Skin Inflammation and Promotes Disease Tolerance

Hematophagous vectors lacerate host skin and capillaries to acquire a blood meal, resulting in leakage of red blood cells (RBCs) and inflammation. Here, we show that heme oxygenase-1 (HO-1), a pleiotropic cytoprotective isoenzyme that mitigates heme-mediated tissue damage, is induced after bites of sand flies, mosquitoes, and ticks. Further, we demonstrate that erythrophagocytosis by macrophages, including a skin-residing CD163⁺CD91⁺ professional iron-recycling subpopulation, produces HO-1 after bites. Importantly, we establish that global deletion or transient inhibition of HO-1 in mice increases inflammation and pathology following Leishmania-infected sand fly bites without affecting parasite number, whereas CO, an end product of the HO-1 enzymatic reaction, suppresses skin inflammation. This indicates that HO-1 induction by blood-feeding sand flies promotes tolerance to Leishmania infection. Collectively, our data demonstrate that HO-1 induction through erythrophagocytosis is a universal mechanism that regulates skin inflammation following blood feeding by arthropods, thus promoting early-stage disease tolerance to vector-borne pathogens.

Influence of different erythrocyte storage times on the macrophage response in haemorrhagic shock mice

Objective

To investigate the characteristics of the macrophage response to transfusion of erythrocytes kept at different storage times in the mouse model of haemorrhagic shock.

Methods

Erythrocytes were isolated from mice and stored for 7, 21 or 35 days and samples injected intravenously into haemorrhagic shock mice. Changes in macrophages, inflammatory cytokines and T cell differentiation were assessed using flow cytometry or enzyme-linked immunosorbent assay (ELISA). In a second experiment, haemorrhagic shock mice were injected with 21D-erythrocytes and the expression of nuclear factor erythroid 2 p45-related factor 2 (Nrf2), arginine -1 (Arg-1) and inducible nitrous oxide (iNOS) determined.

Results

The proportion of M1-polarized macrophages was greatest in the 21D group while M2 macrophages tended to increase with the erythrocyte storage time. Levels of inflammatory cytokines and T helper 1 (Th1) cells increased in proportion to erythrocytes storage time. Most regulatory T cells (Treg) were found at 21D. Arg-1 expression was significantly increased in a group that received an heme oxygenase 1 (HO-1) agonist and significantly decreased in a group that received an HO-1 inhibitor but there were no differences in the expression of iNOS or Nrf2.

Conclusion

21D storage time may be an important time point for erythrocyte storage and immunity response and Arg-1 may have a role in the macrophage response to erythrocyte infusion.

A Cytoplasmic Heme Sensor Illuminates the Impacts of Mitochondrial and Vacuolar Functions and Oxidative Stress on Heme-Iron Homeostasis in Cryptococcus neoformans

Invasive fungal diseases are increasing in frequency, and new drug targets and antifungal drugs are needed to bolster therapy. The mechanisms by which pathogens obtain critical nutrients such as iron from heme during host colonization represent a promising target for therapy. In this study, we employed a fluorescent heme sensor to investigate heme homeostasis in Cryptococcus neoformans. We demonstrated that endocytosis is a key aspect of heme acquisition and that vacuolar and mitochondrial functions are important in regulating the pool of available heme in cells. Stress generated by oxidative conditions impacts the heme pool, as do the drugs artemisinin and metformin; these drugs have heme-related activities and are in clinical use for malaria and diabetes, respectively. Overall, our study provides insights into mechanisms of fungal heme acquisition and demonstrates the utility of the heme sensor for drug characterization in support of new therapies for fungal diseases.

Pathogens must compete with hosts to acquire sufficient iron for proliferation during pathogenesis. The pathogenic fungus Cryptococcus neoformans is capable of acquiring iron from heme, the most abundant source in vertebrate hosts, although the mechanisms of heme sensing and acquisition are not entirely understood. In this study, we adopted a chromosomally encoded heme sensor developed for Saccharomyces cerevisiae to examine cytosolic heme levels in C. neoformans using fluorescence microscopy, fluorimetry, and flow cytometry. We validated the responsiveness of the sensor upon treatment with exogenous hemin, during proliferation in macrophages, and in strains defective for endocytosis. We then used the sensor to show that vacuolar and mitochondrial dysregulation and oxidative stress reduced the labile heme pool in the cytosol. Importantly, the sensor provided a tool to further demonstrate that the drugs artemisinin and metformin have heme-related activities and the potential to be repurposed for antifungal therapy. Overall, this study provides insights into heme sensing by C. neoformans and establishes a powerful tool to further investigate mechanisms of heme-iron acquisition in the context of fungal pathogenesis.

Heme protects intestinal mucosal barrier in DSS-induced colitis through regulating macrophage polarization in both HO-1-dependent and HO-1-independent way

Hemoglobin-derived heme was reported to play protective roles in hemorrhagic diseases by modulating the macrophages toward recovery. Mucosal bleeding is one of the pathological features of inflammatory bowel diseases (IBD). However, whether heme provides anti-inflammatory profiles in macrophages, thus contributing to the intestinal mucosal barrier protection, is unclear. In the current study, we investigated the beneficial effects of heme on DSS-induced colitis mice and explored the underlying mechanisms. In vivo, systemic heme supplementation by hemin injection relieved intestinal inflammation and remedied intestinal mucosal barrier damage by correcting abnormal intestinal macrophage polarization. In vitro, we confirmed the reciprocally regulating effects of hemin on M1/M2 macrophage polarization in BMDM. Intriguingly, with knockdown of HO-1, the inhibiting effects of hemin on M1 polarization were maintained, while the promoting effects on M2 polarization were reversed. Further research proved that hemin repressed the inflammatory profiles in macrophages through inhibiting the translocation of NF-κB p65 by disrupting IRF5-NF-κB p65 complex formation in Spi-C-dependent way. In conclusion, these results showed that the modification of colon tissue microenvironment with heme supplementation plays a protective role in DSS-induced colitis mice through regulating the macrophage polarization in both HO-1-dependent and HO-1-independent way, indicating a new choice to therapeutically modulate the macrophage function and prevent IBD.

Role of M1-polarized tumor-associated macrophages in the prognosis of advanced ovarian cancer patients

The identification of prognostic and predictive markers is crucial for choosing the most appropriate management method for ovarian cancer patients. We aimed to assess the prognostic role of tumor-associated macrophage (TAM) polarization in advanced ovarian cancer patients. We carried out a prospective observational study that included 140 consecutive patients with advanced-stage high-grade serous ovarian cancer as well as patients with other histotypes of ovarian cancer and patients with ovarian metastasis from other sites between June 2013 and December 2018. Patients were enrolled at the time of laparoscopic surgery before receiving any antineoplastic treatment. We found that patients with high-grade serous papillary ovarian cancers had a prevalence of M1 TAMs, a higher M1/M2 ratio, and a longer overall survival (OS) and progression-free survival (PFS) than other patients. Regression analysis confirmed that there was a significant positive association between the M1/M2 ratio and an improved OS, PFS and platinum-free interval (PFI), both in the entire population and in patients stratified according to tumor type and initial surgery. Kaplan-Meier analysis was performed after the patients were divided into 2 groups according to the median M1/M2 ratio and revealed that patients with a high M1/M2 ratio had a higher OS, PFS and PFI than those with a low M1/M2 ratio. In conclusion, the prognostic and predictive role of TAM polarization in the tumor microenvironment could be of great clinical relevance and may allow the early identification of patients who are likely to respond to therapy. Further studies in a larger prospective sample are warranted.

Interplay of Heme with Macrophages in Homeostasis and Inflammation

Macrophages are an integral part of the mononuclear phagocyte system that is critical for maintaining immune homeostasis. They play a key role for initiation and modulation of immunological responses in inflammation and infection. Moreover, macrophages exhibit a wide spectrum of tissue-specific phenotypes in steady-state and pathophysiological conditions. Recent clinical and experimental evidence indicates that the ubiquitous compound heme is a crucial regulator of these cells, e.g., in the differentiation of monocytes to tissue-resident macrophages and/ or in activation by inflammatory stimuli. Notably, heme, an iron containing tetrapyrrole, is essential as a prosthetic group of hemoproteins (e.g., hemoglobin and cytochromes), whereas non-protein bound free or labile heme can be harmful via pro-oxidant, pro-inflammatory, and cytotoxic effects. In this review, it will be discussed how the complex interplay of heme with macrophages regulates homeostasis and inflammation via modulating macrophage inflammatory characteristics and/ or hematopoiesis. A particular focus will be the distinct roles of intra- and extracellular labile heme and the regulation of its availability by heme-binding proteins. Finally, it will be addressed how heme modulates macrophage functions via specific transcriptional factors, in particular the nuclear repressor BTB and CNC homologue (BACH)1 and Spi-C.

Mechanism of colorectal carcinogenesis triggered by heme iron from red meat

Colorectal cancer (CRC) is one of the major tumor entities worldwide, with an increasing incidence in younger people. CRC formation is causally linked to various genetic, life-style and dietary risk factors. Among the ladder, the consumption of red meat has emerged as important risk factor contributing to CRC. A large body of evidence shows that heme iron is the critical component of red meat, which promotes colorectal carcinogenesis. In this review, we describe the uptake and cellular fate of both heme and inorganic iron in intestinal epithelial cells. Next, an overview on the DNA damaging properties of heme iron is provided, highlighting the DNA adducts relevant for CRC etiology. Moreover, heme triggered mechanisms leading to colonic hyperproliferation are presented, which are intimately linked to changes in the intestinal microbiota induced by heme. A special focus was set on the impact of heme iron on innate and adaptive immune cells, which could be relevant in the context of CRC. Finally, we recapitulate in vivo studies providing evidence for the tumor-promoting potential of dietary heme iron. Altogether, heme iron affects numerous key pathways involved in the pathogenesis of CRC.

Heme synthesis through the life cycle of the heme auxotrophic parasite Leishmania major

Heme is an essential molecule synthetized through a broadly conserved 8-step route that has been lost in trypanosomatid parasites. Interestingly, Leishmania reacquired by horizontal gene transfer from γ-proteobacteria the genes coding for the last 3 enzymes of the pathway. Here we show that intracellular amastigotes of Leishmania major can scavenge heme precursors from the host cell to fulfill their heme requirements, demonstrating the functionality of this partial pathway. To dissect its role throughout the L. major life cycle, the significance of L. major ferrochelatase (LmFeCH), the terminal enzyme of the route, was evaluated. LmFeCH expression in a heterologous system demonstrated its activity. Knockout promastigotes lacking lmfech were not able to use the ferrochelatase substrate protoporphyrin IX as a source of heme. In vivo infection of Phlebotomus perniciosus with knockout promastigotes shows that LmFeCH is not required for their development in the sandfly. In contrast, the replication of intracellular amastigotes was hampered in vitro by the deletion of lmfech. However, LmFeCH^-/- parasites produced disease in a cutaneous leishmaniasis murine model in a similar way as control parasites. Therefore, although L. major can synthesize de novo heme from macrophage precursors, this activity is dispensable being an unsuited target for leishmaniasis treatment.-Orrego, L. M., Cabello-Donayre, M., Vargas, P., Martínez-García, M., Sánchez, C., Pineda-Molina, E., Jiménez, M., Molina, R., Pérez-Victoria, J. M. Heme synthesis through the life cycle of the heme auxotrophic parasite Leishmania major.

The BACH1-HMOX1 Regulatory Axis Is Indispensable for Proper Macrophage Subtype Specification and Skeletal Muscle Regeneration

The infiltration and subsequent in situ subtype specification of monocytes to effector/inflammatory and repair macrophages is indispensable for tissue repair upon acute sterile injury. However, the chromatin-level mediators and regulatory events controlling this highly dynamic macrophage phenotype switch are not known. In this study, we used a murine acute muscle injury model to assess global chromatin accessibility and gene expression dynamics in infiltrating macrophages during sterile physiological inflammation and tissue regeneration. We identified a heme-binding transcriptional repressor, BACH1, as a novel regulator of this process. Bach1 knockout mice displayed impaired muscle regeneration, altered dynamics of the macrophage phenotype transition, and transcriptional deregulation of key inflammatory and repair-related genes. We also found that BACH1 directly binds to and regulates distal regulatory elements of these genes, suggesting a novel role for BACH1 in controlling a broad spectrum of the repair response genes in macrophages upon injury. Inactivation of heme oxygenase-1 (Hmox1), one of the most stringently deregulated genes in the Bach1 knockout in macrophages, impairs muscle regeneration by changing the dynamics of the macrophage phenotype switch. Collectively, our data suggest the existence of a heme-BACH1--HMOX1 regulatory axis, that controls the phenotype and function of the infiltrating myeloid cells upon tissue damage, shaping the overall tissue repair kinetics.

The commensal bacterium Bacteroides fragilis down-regulates ferroportin expression and alters iron homeostasis in macrophages

The intestinal microbiota has several effects on host physiology. Previous work from our laboratory demonstrated that the microbiota influences systemic iron homeostasis in mouse colitis models by altering inflammation-induced expression of the iron-regulating hormone hepcidin. In the present study, we examined the impact of the gut commensal bacterium Bacteroides fragilis on the expression of the iron exporter ferroportin, the target of hepcidin action, in macrophages, the cell type that plays a pivotal role in iron recycling. Mouse bone marrow-derived macrophages were exposed to B. fragilis and were analyzed by quantitative real-time polymerase chain reaction and Western blotting. We found that B. fragilis down-regulated ferroportin transcription independently of bacterial viability. Medium conditioned by the bacteria also reduced ferroportin expression, indicating the involvement of soluble factors, possibly Toll-like receptor ligands. Consistent with this idea, several of these ligands were able to down-regulate ferroportin. The B. fragilis-induced decrease in ferroportin was functionally important since it produced a significant increase in intracellular iron concentrations that prevented the effects of the iron chelator deferoxamine on Salmonella-induced IL-6 and IL-1β production. Our results thus reveal that B. fragilis can influence macrophage iron handling and inflammatory responses by modulating ferroportin expression.

Alveolar Macrophages in the Resolution of Inflammation, Tissue Repair, and Tolerance to Infection

Pathogen persistence in the respiratory tract is an important preoccupation, and of particular relevance to infectious diseases such as tuberculosis. The equilibrium between elimination of pathogens and the magnitude of the host response is a sword of Damocles for susceptible patients. The alveolar macrophage is the first sentinel of the respiratory tree and constitutes the dominant immune cell in the steady state. This immune cell is a key player in the balance between defense against pathogens and tolerance toward innocuous stimuli. This review focuses on the role of alveolar macrophages in limiting lung tissue damage from potentially innocuous stimuli and from infections, processes that are relevant to appropriate tolerance of potential causes of lung disease. Notably, the different anti-inflammatory strategies employed by alveolar macrophages and lung tissue damage control are explored. These two properties, in addition to macrophage manipulation by pathogens, are discussed to explain how alveolar macrophages may drive pathogen persistence in the airways.

Glutamine via α-ketoglutarate dehydrogenase provides succinyl-CoA for heme synthesis during erythropoiesis

During erythroid differentiation, the erythron must remodel its protein constituents so that the mature red cell contains hemoglobin as the chief cytoplasmic protein component. For this, ∼10⁹ molecules of heme must be synthesized, consuming 10¹⁰ molecules of succinyl-CoA. It has long been assumed that the source of succinyl-coenzyme A (CoA) for heme synthesis in all cell types is the tricarboxylic acid (TCA) cycle. Based upon the observation that 1 subunit of succinyl-CoA synthetase (SCS) physically interacts with the first enzyme of heme synthesis (5-aminolevulinate synthase 2, ALAS2) in erythroid cells, it has been posited that succinyl-CoA for ALA synthesis is provided by the adenosine triphosphate-dependent reverse SCS reaction. We have now demonstrated that this is not the manner by which developing erythroid cells provide succinyl-CoA for ALA synthesis. Instead, during late stages of erythropoiesis, cellular metabolism is remodeled so that glutamine is the precursor for ALA following deamination to α-ketoglutarate and conversion to succinyl-CoA by α-ketoglutarate dehydrogenase (KDH) without equilibration or passage through the TCA cycle. This may be facilitated by a direct interaction between ALAS2 and KDH. Succinate is not an effective precursor for heme, indicating that the SCS reverse reaction does not play a role in providing succinyl-CoA for heme synthesis. Inhibition of succinate dehydrogenase by itaconate, which has been shown in macrophages to dramatically increase the concentration of intracellular succinate, does not stimulate heme synthesis as might be anticipated, but actually inhibits hemoglobinization during late erythropoiesis.

Transfusion-related immunomodulation: a reappraisal

PURPOSE OF REVIEW

This review summarizes current and prior observations regarding transfusion-related immunomodulation (TRIM) and puts these ideas into a modern immunological context, incorporating concepts from innate, adaptive, and nutritional immunity. We propose that TRIM research focus on determining whether there are specific, well-defined immunosuppressive effects from transfusing 'pure' red blood cells (RBCs) themselves, along with the by-products produced by the stored RBCs as a result of the 'storage lesion.' Macrophages are a key cell type involved in physiological and pathological RBC clearance and iron recycling. The plasticity and diversity of macrophages makes these cells potential mediators of immune suppression that could constitute TRIM.

RECENT FINDINGS

Recent reports identified the capacity of macrophages and monocytes to exhibit 'memory.' Exposure to various stimuli, such as engulfment of apoptotic cells and interactions with ß-glucan and lipopolysaccharide, were found to induce epigenetic, metabolic, and functional changes in certain myeloid cells, particularly macrophages and monocytes.

SUMMARY

Macrophages may mediate the immunosuppressive aspects of TRIM that arise as a result of transfused RBCs and their storage lesion induced by-products.

The macrophage heme-heme oxygenase-1 system and its role in inflammation

Heme oxygenase (HO)-1, the inducible isoform of the heme-degrading enzyme HO, plays a critical role in inflammation and iron homeostasis. Regulatory functions of HO-1 are mediated via the catalytic breakdown of heme, which is an iron-containing tetrapyrrole complex with potential pro-oxidant and pro-inflammatory effects. In addition, the HO reaction produces the antioxidant and anti-inflammatory compounds carbon monoxide (CO) and biliverdin, subsequently converted into bilirubin, along with iron, which is reutilized for erythropoiesis. HO-1 is up-regulated by a plethora of stimuli and injuries in most cell types and tissues and provides salutary effects by restoring physiological homeostasis. Notably, HO-1 exhibits critical immuno-modulatory functions in macrophages, which are a major cell population of the mononuclear phagocyte system. Macrophages play key roles as sentinels and regulators of the immune system and HO-1 in these cells appears to be of critical importance for driving resolution of inflammatory responses. In this review, the complex functions and regulatory mechanisms of HO-1 in macrophages will be high-lighted. A particular focus will be the intricate interactions of HO-1 with its substrate heme, which play a contradictory role in distinct physiological and pathophysiological settings. The therapeutic potential of targeted modulation of the macrophage heme-HO-1 system will be discussed in the context of inflammatory disorders.

Interaction between Mitochondrial Reactive Oxygen Species, Heme Oxygenase, and Nitric Oxide Synthase Stimulates Phagocytosis in Macrophages

Background

Macrophages are cells of the innate immune system that populate every organ. They are required not only for defense against invading pathogens and tissue repair but also for maintenance of tissue homeostasis and iron homeostasis.

Aim

The aim of this study is to understand whether heme oxygenase (HO) and nitric oxide synthase (NOS) contribute to the regulation of nicotinamide adenine dinucleotide phosphate oxidase (NOX) activity and phagocytosis, two key components of macrophage function.

Methods

This study was carried out using resting J774A.1 macrophages treated with hemin or vehicle. Activity of NOS, HO, or NOX was inhibited using specific inhibitors. Reactive oxygen species (ROS) formation was determined by Amplex^® red assay, and phagocytosis was measured using fluorescein isothiocyanate-labeled bacteria. In addition, we analyzed the fate of the intracellular heme by using electron spin resonance.

Results

We show that both enzymes NOS and HO are essential for phagocytic activity of macrophages. NOS does not directly affect phagocytosis, but stimulates NOX activity via nitric oxide-triggered ROS production of mitochondria. Treatment of macrophages with hemin results in intracellular accumulation of ferrous heme and an inhibition of phagocytosis. In contrast to NOS, HO products, including carbon monoxide, neither clearly affect NOX activity nor clearly affect phagocytosis, but phagocytosis is accelerated by HO-mediated degradation of heme.

Conclusion

Both enzymes contribute to the bactericidal activity of macrophages independently, by controlling different pathways.

Targeting macrophages for cancer therapy disrupts bone homeostasis and impairs bone marrow erythropoiesis in mice bearing Lewis lung carcinoma tumors

Macrophages are represented in all tissues by phenotypically distinct resident populations that show great functional diversity. Macrophages generally play a protumoral role, and they are attractive targets for cancer therapy. In this study, we found that CD169⁺ macrophages depletion inhibited the growth of established Lewis lung carcinoma tumors in mice. Benefits must be weighed against potential adverse effects in cancer therapy. Here, we investigated the adverse effects of CD169⁺ macrophages depletion on bone and bone marrow in mice bearing Lewis lung carcinoma tumors. Our studies showed that depletion of CD169⁺ macrophages in LLC tumor-bearing mice disrupted bone homeostasis, including bone weight loss and bone mineral density decrease. Further studies revealed that bone marrow erythropoiesis was severely impaired after depletion of CD169⁺ macrophages in LLC tumor-bearing mice. Our findings suggest that depletion of macrophages for cancer therapy may be associated with potential adverse effects that need to be recognized, prevented, and optimally managed.