Characterization of plasma labile heme in hemolytic conditions

Quantitative analysis of heme and hemoglobin for the detection of intravascular hemolysis

BACKGROUND

Intravascular hemolysis is associated with massive release of hemoglobin and consequently labile heme into the blood, resulting in prothrombotic and proinflammatory events in patients. Though heme is well-known to participate in these adverse effects, it is not monitored. Instead, haptoglobin and hemoglobin serve as clinical biomarkers. The quantification of labile heme together with hemoglobin, however, should be considered in clinical diagnosis as well, to obtain a complete picture of the hemolytic state in patients. So far, quantification techniques for labile heme were not yet systematically analyzed and compared for their clinical application potential, especially in the presence of hemoglobin.

RESULTS

Two commercial assays (Heme Assay Kit®, Hemin Assay Kit®) and five common approaches (pyridine hemochromogen assay, apo-horseradish peroxidase-based assay, UV/Vis spectroscopy, HPLC, mass spectrometry) were analyzed concerning their linearity, accuracy, and precision, as well as their ability to distinguish between hemoglobin-bound heme and labile heme. Further, techniques for the quantification of hemoglobin (Harboe method, SLS method, Hemastix®) were included to study their selectivity for hemoglobin and potential interference by the presence of labile heme. Both, indirect and direct approaches were suitable for the determination of a wide concentration of heme (∼0.02-45 μM) and hemoglobin (∼0.002-17 μM). A clear distinction between hemoglobin-bound heme and labile heme with one method was not possible. Thus, a novel combined approach is presented and applied to human and porcine plasma samples for the determination of hemoglobin and labile heme.

SIGNIFICANCE

Our results demonstrate the need to develop improved techniques to differentiate labile and protein-bound heme for early detection of intravascular hemolysis. Here, we present a novel strategy by combining two spectroscopic methods, which is most reliable as an easy-to-use tool for the determination of hemoglobin and heme levels in plasma samples for the diagnosis of intravascular hemolysis and in basic biomedical research.

Export of heme by the feline leukemia virus C receptor regulates mitochondrial biogenesis and redox balance in the hematophagous insect Rhodnius prolixus

Heme is a prosthetic group of proteins involved in vital physiological processes. It participates, for example, in redox reactions crucial for cell metabolism due to the variable oxidation state of its central iron atom. However, excessive heme can be cytotoxic due to its prooxidant properties. Therefore, the control of intracellular heme levels ensures the survival of organisms, especially those that deal with high concentrations of heme during their lives, such as hematophagous insects. The export of heme initially attributed to the feline leukemia virus C receptor (FLVCR) has recently been called into question, following the discovery of choline uptake by the same receptor in mammals. Here, we found that RpFLVCR is a heme exporter in the midgut of the hematophagous insect Rhodnius prolixus, a vector for Chagas disease. Silencing RpFLVCR decreased hemolymphatic heme levels and increased the levels of intracellular dicysteinyl-biliverdin, indicating heme retention inside midgut cells. FLVCR silencing led to increased expression of heme oxygenase (HO), ferritin, and mitoferrin mRNAs while downregulating the iron importers Malvolio 1 and 2. In contrast, HO gene silencing increased FLVCR and Malvolio expression and downregulated ferritin, revealing crosstalk between heme degradation/export and iron transport/storage pathways. Furthermore, RpFLVCR silencing strongly increased oxidant production and lipid peroxidation, reduced cytochrome c oxidase activity, and activated mitochondrial biogenesis, effects not observed in RpHO-silenced insects. These data support FLVCR function as a heme exporter, playing a pivotal role in heme/iron metabolism and maintenance of redox balance, especially in an organism adapted to face extremely high concentrations of heme.

Targeting circulating labile heme as a defense strategy against malaria

Severe presentations of malaria emerge as Plasmodium (P.) spp. parasites invade and lyse red blood cells (RBC), producing extracellular hemoglobin (HB), from which labile heme is released. Here, we tested whether scavenging of extracellular HB and/or labile heme, by haptoglobin (HP) and/or hemopexin (HPX), respectively, counter the pathogenesis of severe presentations of malaria. We found that circulating labile heme is an independent risk factor for cerebral and non-cerebral presentations of severe P. falciparum malaria in children. Labile heme was negatively correlated with circulating HP and HPX, which were, however, not risk factors for severe P. falciparum malaria. Genetic Hp and/or Hpx deletion in mice led to labile heme accumulation in plasma and kidneys, upon Plasmodium infection This was associated with higher incidence of mortality and acute kidney injury (AKI) in ageing but not adult Plasmodium-infected mice, and was corroborated by an inverse correlation between heme and HPX with serological markers of AKI in P. falciparum malaria. In conclusion, HP and HPX act in an age-dependent manner to prevent the pathogenesis of severe presentation of malaria in mice and presumably in humans.

Extracellular G-quadruplexes and Z-DNA protect biofilms from DNase I, and G-quadruplexes form a DNAzyme with peroxidase activity

Many bacteria form biofilms to protect themselves from predators or stressful environmental conditions. In the biofilm, bacteria are embedded in a protective extracellular matrix composed of polysaccharides, proteins and extracellular DNA (eDNA). eDNA most often is released from lysed bacteria or host mammalian cells, and it is the only matrix component most biofilms appear to have in common. However, little is known about the form DNA takes in the extracellular space, and how different non-canonical DNA structures such as Z-DNA or G-quadruplexes might contribute to its function in the biofilm. The aim of this study was to determine if non-canonical DNA structures form in eDNA-rich staphylococcal biofilms, and if these structures protect the biofilm from degradation by nucleases. We grew Staphylococcus epidermidis biofilms in laboratory media supplemented with hemin and NaCl to stabilize secondary DNA structures and visualized their location by immunolabelling and fluorescence microscopy. We furthermore visualized the macroscopic biofilm structure by optical coherence tomography. We developed assays to quantify degradation of Z-DNA and G-quadruplex DNA oligos by different nucleases, and subsequently investigated how these enzymes affected eDNA in the biofilms. Z-DNA and G-quadruplex DNA were abundant in the biofilm matrix, and were often present in a web-like structures. In vitro, the structures did not form in the absence of NaCl or mechanical shaking during biofilm growth, or in bacterial strains deficient in eDNA or exopolysaccharide production. We thus infer that eDNA and polysaccharides interact, leading to non-canonical DNA structures under mechanical stress when stabilized by salt. We also confirmed that G-quadruplex DNA and Z-DNA was present in biofilms from infected implants in a murine implant-associated osteomyelitis model. Mammalian DNase I lacked activity against Z-DNA and G-quadruplex DNA, while Micrococcal nuclease could degrade G-quadruplex DNA and S1 Aspergillus nuclease could degrade Z-DNA. Micrococcal nuclease, which originates from Staphylococcus aureus, may thus be key for dispersal of biofilm in staphylococci. In addition to its structural role, we show for the first time that the eDNA in biofilms forms a DNAzyme with peroxidase-like activity in the presence of hemin. While peroxidases are part of host defenses against pathogens, we now show that biofilms can possess intrinsic peroxidase activity in the extracellular matrix.

Free heme and hemopexin in acute kidney injury after cardiopulmonary bypass and transient renal ischemia

Free heme is released from hemoproteins during hemolysis or ischemia reperfusion injury and can be pro-inflammatory. Most studies on nephrotoxicity of hemolysis-derived proteins focus on free hemoglobin (fHb) with heme as a prosthetic group. Measurement of heme in its free, non-protein bound, form is challenging and not commonly used in clinical routine diagnostics. In contrast to fHb, the role of free heme in acute kidney injury (AKI) after cardiopulmonary bypass (CPB) surgery is unknown. Using an apo-horseradish peroxidase-based assay, we identified free heme during CPB surgery as predictor of AKI in patients undergoing cardiac valve replacement (n = 37). Free heme levels during CPB surgery correlated with depletion of hemopexin (Hx), a heme scavenger-protein. In mice, the impact of high levels of circulating free heme on the development of AKI following transient renal ischemia and the therapeutic potential of Hx were investigated. C57BL/6 mice were subjected to bilateral renal ischemia/reperfusion injury for 15 min which did not cause AKI. However, additional administration of free heme in this model promoted overt AKI with reduced renal function, increased renal inflammation, and reduced renal perfusion on functional magnetic resonance imaging. Hx treatment attenuated AKI. Free heme administration to sham operated control mice did not cause AKI. In conclusion, free heme is a predictor of AKI in CPB surgery patients and promotes AKI in transient renal ischemia. Depletion of Hx in CPB surgery patients and attenuation of AKI by Hx in the in vivo model encourage further research on Hx therapy in patients with increased free heme levels during CPB surgery.

The Key Role of GSH in Keeping the Redox Balance in Mammalian Cells: Mechanisms and Significance of GSH in Detoxification via Formation of Conjugates

Glutathione (GSH) is a ubiquitous tripeptide that is biosynthesized in situ at high concentrations (1-5 mM) and involved in the regulation of cellular homeostasis via multiple mechanisms. The main known action of GSH is its antioxidant capacity, which aids in maintaining the redox cycle of cells. To this end, GSH peroxidases contribute to the scavenging of various forms of ROS and RNS. A generally underestimated mechanism of action of GSH is its direct nucleophilic interaction with electrophilic compounds yielding thioether GSH S-conjugates. Many compounds, including xenobiotics (such as NAPQI, simvastatin, cisplatin, and barbital) and intrinsic compounds (such as menadione, leukotrienes, prostaglandins, and dopamine), form covalent adducts with GSH leading mainly to their detoxification. In the present article, we wish to present the key role and significance of GSH in cellular redox biology. This includes an update on the formation of GSH-S conjugates or GSH adducts with emphasis given to the mechanism of reaction, the dependence on GST (GSH S-transferase), where this conjugation occurs in tissues, and its significance. The uncovering of the GSH adducts' formation enhances our knowledge of the human metabolome. GSH-hematin adducts were recently shown to have been formed spontaneously in multiples isomers at hemolysates, leading to structural destabilization of the endogenous toxin, hematin (free heme), which is derived from the released hemoglobin. Moreover, hemin (the form of oxidized heme) has been found to act through the Kelch-like ECH associated protein 1 (Keap1)-nuclear factor erythroid 2-related factor-2 (Nrf2) signaling pathway as an epigenetic modulator of GSH metabolism. Last but not least, the implications of the genetic defects in GSH metabolism, recorded in hemolytic syndromes, cancer and other pathologies, are presented and discussed under the framework of conceptualizing that GSH S-conjugates could be regarded as signatures of the cellular metabolism in the diseased state.

Assessment of total and unbound cell-free heme in plasma of patients with sickle cell disease

Intravascular hemolysis results in the release of cell-free hemoglobin and heme in plasma. In sickle cell disease, the fragility of the sickle red blood cell leads to chronic hemolysis, which can contribute to oxidative damage and activation of inflammatory pathways. The scavenger proteins haptoglobin and hemopexin provide pathways to remove hemoglobin and heme, respectively, from the circulation. Heme also intercalates in membranes of blood cells and endothelial cells in the vasculature and associates with other plasma components such as albumin and lipoproteins. Hemopexin has a much higher affinity and can strip heme from the other pools and detoxify plasma from cell-free circulatory heme. However, due to chronic hemolysis, hemopexin is depleted in individuals with sickle cell disease. Thus, cell-free unbound heme is expected to accumulate in plasma. We developed a methodology for the accurate quantification of the fraction of heme, which is pathologically relevant in sickle cell disease, that does not appear to be sequestered to a plasma compartment. Our data show significant variation in the concentration of unbound heme, and rather unexpectedly, the size of the unbound fraction does not correlate to the degree of hemolysis, as measured by the concentration of bound heme. Very high heme concentrations (>150 µM) were obtained in some plasma with unbound concentrations that were several fold lower than in plasma with much lower hemolysis (<50 µM). These findings underscore the long-term effects of chronic hemolysis on the blood components and of the disruption of the essential equilibrium between release of hemoproteins/heme in the circulation and adaptative response of the scavenging/removal mechanisms. Understanding the clinical implications of this loss of response may provide insights into diagnostic and therapeutic targets in patients with sickle cell disease.

Rabbit derived VL single-domains as promising scaffolds to generate antibody-drug conjugates

Antibody-drug conjugates (ADCs) are among the fastest-growing classes of therapeutics in oncology. Although ADCs are in the spotlight, they still present significant engineering challenges. Therefore, there is an urgent need to develop more stable and effective ADCs. Most rabbit light chains have an extra disulfide bridge, that links the variable and constant domains, between Cys80 and Cys171, which is not found in the human or mouse. Thus, to develop a new generation of ADCs, we explored the potential of rabbit-derived VL-single-domain antibody scaffolds (sdAbs) to selectively conjugate a payload to Cys80. Hence, a rabbit sdAb library directed towards canine non-Hodgkin lymphoma (cNHL) was subjected to in vitro and in vivo phage display. This allowed the identification of several highly specific VL-sdAbs, including C5, which specifically target cNHL cells in vitro and present promising in vivo tumor uptake. C5 was selected for SN-38 site-selective payload conjugation through its exposed free Cys80 to generate a stable and homogenous C5-DAB-SN-38. C5-DAB-SN-38 exhibited potent cytotoxicity activity against cNHL cells while inhibiting DNA-TopoI activity. Overall, our strategy validates a platform to develop a novel class of ADCs that combines the benefits of rabbit VL-sdAb scaffolds and the canine lymphoma model as a powerful framework for clinically translation of novel therapeutics for cancer.

Renal control of life-threatening malarial anemia

Iron recycling prevents the development of anemia under homeostatic conditions. Whether iron recycling was co-opted as a defense strategy to prevent the development of anemia in response to infection is unclear. We find that in severe Plasmodium falciparum malaria, the onset of life-threatening anemia is associated with acute kidney injury (AKI), irrespective of parasite load. Using a well-established experimental rodent model of malaria anemia, we identify a transcriptional response that endows renal proximal tubule epithelial cells (RPTECs) with the capacity to store and recycle iron during P. chabaudi chabaudi (Pcc) infection. This response encompasses the induction of ferroportin 1/SLC40A1, which exports iron from RPTECs and counteracts AKI while supporting compensatory erythropoiesis and preventing the onset of life-threatening malarial anemia. Iron recycling by myeloid cells is dispensable to this protective response, suggesting that RPTECs provide an iron-recycling salvage pathway that prevents the pathogenesis of life-threatening malarial anemia.

Changes in Heme Levels During Acute Vaso-occlusive Crisis in Sickle Cell Anemia

OBJECTIVE/BACKGROUND

Sickle cell anemia (SCA) is associated with increased levels of extracellular heme, which is a key mediator of inflammation in this condition. Despite abundant evidence supporting this concept in cell and animal models, few studies addressed the association between heme levels and the development and severity of acute vasoocclusive crises (VOC) in humans.

METHODS

A cross-sectional study was conducted in patients with acute VOC. Total extracellular heme levels were measured in both plasma and serum at admission and after convalescence, and correlated with other clinical and laboratory markers of SCA severity.

RESULTS

A total of 28 episodes of VOC in 25 patients were included. Heme levels were similar between admission and convalescence, and correlated with the difference between pre and post hemoglobin, and SCA severity estimated by a composite score of clinical and laboratory markers. Heme levels were neither associated with VOC severity nor with markers of hemostasis activation, and were similar to those reported in an independent population of SCA patients at steady state.

DISCUSSION

Acute VOC are not characterized by significant increases in total extracellular heme levels. Studies measuring the fraction of free extracellular heme unbound to proteins are warranted to further refine our understanding of the role of heme in acute VOC.

Glutathione-Hemin/Hematin Adduct Formation to Disintegrate Cytotoxic Oxidant Hemin/Hematin in Human K562 Cells and Red Blood Cells' Hemolysates: Impact of Glutathione on the Hemolytic Disorders and Homeostasis

Hemin, an oxidized form of heme, acts as potent oxidant to regulate glutathione (GSH) content in pro-erythroid K562 nucleated cells, via activation of the KEAP1/NRF2 defensive signaling pathway. Moreover, GSH, as an essential metabolite, is involved in the regulation of cell-redox homeostasis and proposed to scavenge cytotoxic free heme, which is released from hemoglobin of damaged red blood cells (RBCs) during different hemolytic disorders. In the present study, we aimed to uncover the molecular mechanism by which GSH inhibits hemin-induced cytotoxicity (HIC) by affecting hemin’s structural integrity in K562 cells and in RBC hemolysates. GSH, along with other thiols (cysteine, thioglycolic acid, and mercaptoethanol) altered the spectrum of hemin, while each of them co-added with hemin in cultures of K562 cells prevented HIC and growth arrest and markedly reduced the intracellular level of hemin. In addition, GSH endogenous levels served as a barrier to HIC in K562 cells, as shown by the depletion in GSH. LC-MS/MS analysis of the in vitro reaction between hemin and GSH revealed at least five different isomers of GSH–hemin adducts, as well as hydroxy derivatives as reaction products, which are characterized by unique mass spectra (MS). The latter allowed the detection of adducts in human RBC hemolysates. Based on these findings, we proposed a molecular mechanism via which GSH prevents HIC and structurally disintegrates heme. An analogous reaction was observed in RBC hemolysates via direct inter-reaction between hematin (ferric and hydroxide heme) released from hemoglobin and GSH. Overall, GSH–hematin adducts could be considered as novel entities of the human metabolome of RBCs in hemolytic disorders.

A haem-sequestering plant peptide promotes iron uptake in symbiotic bacteria

Symbiotic partnerships with rhizobial bacteria enable legumes to grow without nitrogen fertilizer because rhizobia convert atmospheric nitrogen gas into ammonia via nitrogenase. After Sinorhizobium meliloti penetrate the root nodules that they have elicited in Medicago truncatula, the plant produces a family of about 700 nodule cysteine-rich (NCR) peptides that guide the differentiation of endocytosed bacteria into nitrogen-fixing bacteroids. The sequences of the NCR peptides are related to the defensin class of antimicrobial peptides, but have been adapted to play symbiotic roles. Using a variety of spectroscopic, biophysical and biochemical techniques, we show here that the most extensively characterized NCR peptide, 24 amino acid NCR247, binds haem with nanomolar affinity. Bound haem molecules and their iron are initially made biologically inaccessible through the formation of hexamers (6 haem/6 NCR247) and then higher-order complexes. We present evidence that NCR247 is crucial for effective nitrogen-fixing symbiosis. We propose that by sequestering haem and its bound iron, NCR247 creates a physiological state of haem deprivation. This in turn induces an iron-starvation response in rhizobia that results in iron import, which itself is required for nitrogenase activity. Using the same methods as for L-NCR247, we show that the D-enantiomer of NCR247 can bind and sequester haem in an equivalent manner. The special abilities of NCR247 and its D-enantiomer to sequester haem suggest a broad range of potential applications related to human health.

A hypometabolic defense strategy against malaria

Hypoglycemia is a clinical hallmark of severe malaria, the often-lethal outcome of Plasmodium falciparum infection. Here, we report that malaria-associated hypoglycemia emerges from a non-canonical resistance mechanism, whereby the infected host reduces glycemia to starve Plasmodium. This hypometabolic response is elicited by labile heme, a byproduct of hemolysis that induces illness-induced anorexia and represses hepatic glucose production. While transient repression of hepatic glucose production prevents unfettered immune-mediated inflammation, organ damage, and anemia, when sustained over time it leads to hypoglycemia, compromising host energy expenditure and adaptive thermoregulation. The latter arrests the development of asexual stages of Plasmodium via a mechanism associated with parasite mitochondrial dysfunction. In response, Plasmodium activates a transcriptional program associated with the reduction of virulence and sexual differentiation toward the generation of transmissible gametocytes. In conclusion, malaria-associated hypoglycemia represents a trade-off of a hypometabolic-based defense strategy that balances parasite virulence versus transmission.

DAMPs and Innate Immune Training

The ability to remember a previous encounter with pathogens was long thought to be a key feature of the adaptive immune system enabling the host to mount a faster, more specific and more effective immune response upon the reencounter, reducing the severity of infectious diseases. Over the last 15 years, an increasing amount of evidence has accumulated showing that the innate immune system also has features of a memory. In contrast to the memory of adaptive immunity, innate immune memory is mediated by restructuration of the active chromatin landscape and imprinted by persisting adaptations of myelopoiesis. While originally described to occur in response to pathogen-associated molecular patterns, recent data indicate that host-derived damage-associated molecular patterns, i.e. alarmins, can also induce an innate immune memory. Potentially this is mediated by the same pattern recognition receptors and downstream signaling transduction pathways responsible for pathogen-associated innate immune training. Here, we summarize the available experimental data underlying innate immune memory in response to damage-associated molecular patterns. Further, we expound that trained immunity is a general component of innate immunity and outline several open questions for the rising field of pathogen-independent trained immunity.

Trained innate immunity, long-lasting epigenetic modulation, and skewed myelopoiesis by heme

During infection, extracellular “labile” heme, released from damaged red blood or parenchymal cells, acts as prototypical alarmin stimulating myeloid cells. A characteristic hallmark of myeloid cell activation is the development of trained immunity, specified as long-lasting adaptations based on transcriptional and epigenetic modifications. In vivo, this is maintained by the rerouting of hematopoiesis. We found that heme is a previously unrecognized trained immunity inducer promoting resistance to bacterial infection in mice. This goes along with extensive long-lasting epigenetic memory in hematopoietic stem cells provoking drastic changes in the transcription factor–binding landscape of myeloid progenitor cells. Given the critical role of heme during infections, we propose that trained immunity is a more general component of innate immunity than previously suggested.

Trained immunity defines long-lasting adaptations of innate immunity based on transcriptional and epigenetic modifications of myeloid cells and their bone marrow progenitors [M. Divangahi et al., Nat. Immunol. 22, 2–6 (2021)]. Innate immune cells, however, do not exclusively differentiate between foreign and self but also react to host-derived molecules referred to as alarmins. Extracellular “labile” heme, released during infections, is a bona fide alarmin promoting myeloid cell activation [M. P. Soares, M. T. Bozza, Curr. Opin. Immunol. 38, 94–100 (2016)]. Here, we report that labile heme is a previously unrecognized inducer of trained immunity that confers long-term regulation of lineage specification of hematopoietic stem cells and progenitor cells. In contrast to previous reports on trained immunity, essentially mediated by pathogen-associated molecular patterns, heme training depends on spleen tyrosine kinase signal transduction pathway acting upstream of c-Jun N-terminal kinases. Heme training promotes resistance to sepsis, is associated with the expansion of self-renewing hematopoetic stem cells primed toward myelopoiesis and to the occurrence of a specific myeloid cell population. This is potentially evoked by sustained activity of Nfix, Runx1, and Nfe2l2 and dissociation of the transcriptional repressor Bach2. Previously reported trained immunity inducers are, however, infrequently present in the host, whereas heme abundantly occurs during noninfectious and infectious disease. This difference might explain the vanishing protection exerted by heme training in sepsis over time with sustained long-term myeloid adaptations. Hence, we propose that trained immunity is an integral component of innate immunity with distinct functional differences on infectious disease outcome depending on its induction by pathogenic or endogenous molecules.

Alpha1-antitrypsin counteracts heme-induced endothelial cell inflammatory activation, autophagy dysfunction and death

Free heme toxicity in the vascular endothelium is critical for the pathogenesis of hemolytic disorders including sickle cell disease. In the current study, it is demonstrated that human alpha1-antitrypsin (A1AT), a serine protease inhibitor with high binding-affinity for heme, rescues endothelial cell (EC) injury caused by free heme. A1AT provided endothelial protection against free heme toxicity via a pathway that differs from human serum albumin and hemopexin, two prototypical heme-binding proteins. A1AT inhibited heme-mediated pro-inflammatory activation and death of ECs, but did not affect the increase in intracellular heme levels and up-regulation of the heme-inducible enzyme heme oxygenase-1. Moreover, A1AT reduced heme-mediated generation of mitochondrial reactive oxygen species. Extracellular free heme led to an increased up-take of A1AT by ECs, which was detected in lysosomes and was found to reduce heme-dependent alkalization of these organelles. Finally, A1AT was able to restore heme-dependent dysfunctional autophagy in ECs. Taken together, our findings show that A1AT rescues ECs from free heme-mediated pro-inflammatory activation, cell death and dysfunctional autophagy. Hence, A1AT therapy may be useful in the treatment of hemolytic disorders such as sickle cell disease.

Automated Measurement of Plasma Cell-Free Hemoglobin Using the Hemolysis Index Check Function

BACKGROUND

The Roche Cobas chemistry analyzer's hemolysis index (HI) check function can directly report hemoglobin (Hb) concentrations. We aimed to validate the HI check function for the measurement of plasma cell-free Hb.

METHODS

Plasma samples (6 μl) were taken by the analyzer and diluted in normal saline to measure the absorbance for Hb at 570 and 600 nm. Hb concentrations were calculated based on the molar extinction coefficient. Imprecision, lower limit of quantification (LLOQ), and analytical measurement range (AMR) of the assay were evaluated. The accuracy was determined by comparing the results between the new method and an existing spectrophotometric method. We further studied interference of icterus and lipemia and carryover. The performance of the assay in proficiency testing was also evaluated. The reference range was transferred from the existing method.

RESULTS

Within-run and total CVs were 1.7%-4.2% and 2.1%-7.0%, respectively (n = 20). The LLOQ was 11 mg/dL (CV = 8.1%) with the upper limit of AMR of 506 mg/dL. The results of the new method correlated well with the existing reference assay: Y (new method) = 0.974 x (reference method) + 4.9, r = 0.9990, n = 52. Bilirubin with a concentration up to 60 mg/dL and lipemic index up to 389 did not show significant interference. No significant carryover was detected. The average standard deviation index in proficiency testing was 0.03 ± 0.29. The reference range was <22 mg/dL.

CONCLUSIONS

Plasma cell-free Hb measurement using the HI check function meets the analytical requirements of the plasma cell-free Hb assays. It is simple and cost-effective.

Heme cytotoxicity is the consequence of endoplasmic reticulum stress in atherosclerotic plaque progression

Hemorrhage and hemolysis with subsequent heme release are implicated in many pathologies. Endothelial cells (ECs) encounter large amount of free heme after hemolysis and are at risk of damage from exogenous heme. Here we show that hemorrhage aggravates endoplasmic reticulum (ER) stress in human carotid artery plaques compared to healthy controls or atheromas without hemorrhage as demonstrated by RNA sequencing and immunohistochemistry. In EC cultures, heme also induces ER stress. In contrast, if cultured ECs are pulsed with heme arginate, cells become resistant to heme-induced ER (HIER) stress that is associated with heme oxygenase-1 (HO-1) and ferritin induction. Knocking down HO-1, HO-2, biliverdin reductase, and ferritin show that HO-1 is the ultimate cytoprotectant in acute HIER stress. Carbon monoxide-releasing molecules (CORMs) but not bilirubin protects cultured ECs from HIER stress via HO-1 induction, at least in part. Knocking down HO-1 aggravates heme-induced cell death that cannot be counterbalanced with any known cell death inhibitors. We conclude that endothelium and perhaps other cell types can be protected from HIER stress by induction of HO-1, and heme-induced cell death occurs via HIER stress that is potentially involved in the pathogenesis of diverse pathologies with hemolysis and hemorrhage including atherosclerosis.

Linking COVID-19 and Heme-Driven Pathophysiologies: A Combined Computational-Experimental Approach

The SARS-CoV-2 outbreak was declared a worldwide pandemic in 2020. Infection triggers the respiratory tract disease COVID-19, which is accompanied by serious changes in clinical biomarkers such as hemoglobin and interleukins. The same parameters are altered during hemolysis, which is characterized by an increase in labile heme. We present two computational–experimental approaches aimed at analyzing a potential link between heme-related and COVID-19 pathophysiologies. Herein, we performed a detailed analysis of the common pathways induced by heme and SARS-CoV-2 by superimposition of knowledge graphs covering heme biology and COVID-19 pathophysiology. Focus was laid on inflammatory pathways and distinct biomarkers as the linking elements. In a second approach, four COVID-19-related proteins, the host cell proteins ACE2 and TMPRSS2 as well as the viral proteins 7a and S protein were computationally analyzed as potential heme-binding proteins with an experimental validation. The results contribute to the understanding of the progression of COVID-19 infections in patients with different clinical backgrounds and may allow for a more individual diagnosis and therapy in the future.

Measurement of labile and protein-bound heme in fixed prostate cancer cells and in cellular fractions

Labile heme is present in the cells at very low concentrations, either unbound or loosely bound to molecules, and accessible for signaling as alarmin. Our recent work suggests that extracellular heme can be taken up and detected in the nuclei of cancer cells. Here, we describe the detailed protocol for detection of labile and total heme in prostate cancer cells and its measurement in subcellular compartments in vitro. The protocol can be adapted to be used for other cell types.

For complete details on the use and execution of this protocol, please refer to Canesin et al. (2020).

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Preparation of heme and treatment of cells with heme

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Detection of labile and total heme in prostate cancer cells

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Measurement of heme in subcellular compartments in prostate cancer cells

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Detection of heme in fixed cells

The Worst Things in Life are Free: The Role of Free Heme in Sickle Cell Disease

Hemolysis is a pathological feature of several diseases of diverse etiology such as hereditary anemias, malaria, and sepsis. A major complication of hemolysis involves the release of large quantities of hemoglobin into the blood circulation and the subsequent generation of harmful metabolites like labile heme. Protective mechanisms like haptoglobin-hemoglobin and hemopexin-heme binding, and heme oxygenase-1 enzymatic degradation of heme limit the toxicity of the hemolysis-related molecules. The capacity of these protective systems is exceeded in hemolytic diseases, resulting in high residual levels of hemolysis products in the circulation, which pose a great oxidative and proinflammatory risk. Sickle cell disease (SCD) features a prominent hemolytic anemia which impacts the phenotypic variability and disease severity. Not only is circulating heme a potent oxidative molecule, but it can act as an erythrocytic danger-associated molecular pattern (eDAMP) molecule which contributes to a proinflammatory state, promoting sickle complications such as vaso-occlusion and acute lung injury. Exposure to extracellular heme in SCD can also augment the expression of placental growth factor (PlGF) and interleukin-6 (IL-6), with important consequences to enthothelin-1 (ET-1) secretion and pulmonary hypertension, and potentially the development of renal and cardiac dysfunction. This review focuses on heme-induced mechanisms that are implicated in disease pathways, mainly in SCD. A special emphasis is given to heme-induced PlGF and IL-6 related mechanisms and their role in SCD disease progression.

Linking Labile Heme with Thrombosis

Thrombosis is one of the leading causes of death worldwide. As such, it also occurs as one of the major complications in hemolytic diseases, like hemolytic uremic syndrome, hemorrhage and sickle cell disease. Under these conditions, red blood cell lysis finally leads to the release of large amounts of labile heme into the vascular compartment. This, in turn, can trigger oxidative stress and proinflammatory reactions. Moreover, the heme-induced activation of the blood coagulation system was suggested as a mechanism for the initiation of thrombotic events under hemolytic conditions. Studies of heme infusion and subsequent thrombotic reactions support this assumption. Furthermore, several direct effects of heme on different cellular and protein components of the blood coagulation system were reported. However, these effects are controversially discussed or not yet fully understood. This review summarizes the existing reports on heme and its interference in coagulation processes, emphasizing the relevance of considering heme in the context of the treatment of thrombosis in patients with hemolytic disorders.

Endothelial Barrier Integrity Is Disrupted In Vitro by Heme and by Serum From Sickle Cell Disease Patients

Free extracellular heme has been shown to activate several compartments of innate immunity, acting as a danger-associated molecular pattern (DAMP) in hemolytic diseases. Although localized endothelial barrier (EB) disruption is an important part of inflammation that allows circulating leukocytes to reach inflamed tissues, non-localized/deregulated disruption of the EB can lead to widespread microvascular hyperpermeability and secondary tissue damage. In mouse models of sickle cell disease (SCD), EB disruption has been associated with the development of a form of acute lung injury that closely resembles acute chest syndrome (ACS), and that can be elicited by acute heme infusion. Here we explored the effect of heme on EB integrity using human endothelial cell monolayers, in experimental conditions that include elements that more closely resemble in vivo conditions. EB integrity was assessed by electric cell-substrate impedance sensing in the presence of varying concentrations of heme and sera from SCD patients or healthy volunteers. Heme caused a dose-dependent decrease of the electrical resistance of cell monolayers, consistent with EB disruption, which was confirmed by staining of junction protein VE-cadherin. In addition, sera from SCD patients, but not from healthy volunteers, were also capable to induce EB disruption. Interestingly, these effects were not associated with total heme levels in serum. However, when heme was added to sera from SCD patients, but not from healthy volunteers, EB disruption could be elicited, and this effect was associated with hemopexin serum levels. Together our in vitro studies provide additional support to the concept of heme as a DAMP in hemolytic conditions.

SLC46A1 contributes to hepatic iron metabolism by importing heme in hepatocytes

BACKGROUND

Iron is finely regulated due to its vital roles in organisms and the peroxidase reactivity if excess. Solute Carrier Family 46 Member 1 (SLC46A1), also named PCFT or HCP1, is the main importer of heme‑iron in the intestine, but has a high abundance in the liver. Since the liver has a central role in iron homeostasis, whether SLC46A1 regulates hepatic iron metabolism is of interest to be identified.

METHODS

The recombinant adeno-associated virus vectors were used to hepatic-specifically inhibit SLC46A1 expression to observe its effects on hepatic iron metabolism. Then the abilities of SLC46A1 in importing heme and folate, and consequent alterations of iron content in hepatocytes were determined. Furthermore, effects of iron on SLC46A1 expression were investigated both in vitro and in vivo.

RESULTS

The hepatocyte-specific inhibition of SLC46A1 decreases iron content in the liver and increases iron content in serum. Expressions of iron-related molecules, transferrin receptor 1, hepcidin and ferroportin, are correspondingly altered. Interestingly, free heme concentration in serum is increased, indicating a decreased import of heme by the liver. In hepatocytes, SLC46A1 is capable of importing hemin, increasing intracellular iron content. The import of hemin by SLC46A1 is unaffected by its other substrate, folate. Instead, hemin treatment decreases SLC46A1 expression, reducing the import of folate. In addition, SLC46A1 itself shows to be iron-responsive both in vivo and in vitro, making it available for regulating iron metabolism.

CONCLUSION

The results elucidate that SLC46A1 regulates iron metabolism in the liver through a folate-independent manner of importing heme. The iron-responsive characters of SLC46A1 give us a new clue to link heme or iron overload with folate deficiency diseases.

Heme is involved in the systemic inflammatory response following radiofrequency ablation of hepatic hemangiomas

OBJECTIVE

Radiofrequency ablation (RFA) is an effective and minimally invasive treatment for managing hepatic hemangiomas. Systemic inflammatory response syndrome (SIRS) often occurs with hemoglobinuria, and its underlying pathophysiological mechanism is unknown. Heme can trigger inflammation by inducing the generation of reactive oxygen species (ROS) and the production of inflammatory mediators. We therefore investigated whether circulating heme is involved in SIRS following RFA of hepatic hemangiomas.

METHODS

We enrolled 65 patients with hepatic hemangioma who underwent RFA. Serum concentrations of free heme, ROS, and tumor necrosis factor α (TNF-α) were measured after RFA. Univariate analysis and a multivariate binary logistic regression model were used to evaluate the contribution of 17 risk factors for SIRS after RFA.

RESULTS

Fifty-nine (59/65, 90.8%) patients developed hemoglobinuria, among which 25 (25/59, 42.4%) experienced SIRS shortly after RFA. In the SIRS group, the serum concentrations of heme, ROS, and TNF-α were immediately elevated after RFA compared with baseline and slowly regained their normal levels 3 days after RFA. Moreover, the concentrations of circulating heme significantly correlated with those of ROS (r = 0.805, P < 0.001) and TNF-α (r = 0.797, P < 0.001). Multivariate analysis showed that the volume of hemangioma [odds ratio (OR) = 1.293, P = 0.031], time of ablation (OR = 1.194, P = 0.029) as well as the concentrations of heme (OR = 1.430, P = 0.017), ROS (OR = 1.251, P = 0.031), and TNF-α (OR = 1.309, P = 0.032) were significantly associated with SIRS.

CONCLUSION

Circulating heme was associated with the induction of ROS and the production of TNF-α, which may contribute to the induction of SIRS following RFA of hepatic hemangiomas.

Heme Determination and Quantification Methods and Their Suitability for Practical Applications and Everyday Use

Many research institutions, clinical diagnostic laboratories, and blood banks are desperately searching for a possibility to identify and quantify heme in different physiological and pathological settings as well as various research applications. The reasons for this are the toxicity of the heme and the fact that it acts as a hemolytic and pro-inflammatory molecule. Heme only exerts these severe and undesired effects when it is not incorporated in hemoproteins. Upon release from the hemoproteins, it enters a biologically available state (labile heme), in which it is loosely associated with proteins, lipids, nucleic acids, or other molecules. While the current methods and procedures for quantitative determination of heme have been used for many years in different settings, their value is limited by the challenging chemical properties of heme. A major cause of inadequate quantification is the separation of labile and permanently bound heme and its high aggregation potential. Thus, none of the current methods are utilized as a generally applicable, standardized approach. The aim of this Feature is to describe and summarize the most common and frequently used chemical, analytical, and biochemical methods for the quantitative determination of heme. Based on this overview, the most promising approaches for future solutions to heme quantification are highlighted.

Formation and Detection of Highly Oxidized Hemoglobin Forms in Biological Fluids during Hemolytic Conditions

Hemolytic diseases are characterized by an accelerated breakdown of red blood cells (RBCs) and the release of hemoglobin (Hb). Following, RBC lysis Hb oxidation occurs with the formation of different redox states of Hb (metHb and ferrylHb) and the release of heme. ferrylHb is unstable and decomposes to metHb with the concomitant formation of globin radicals and eventually covalently crosslinked Hb multimers. The goal of the present study was to determine the concentrations of the different redox states of Hb in biological samples during hemolytic conditions. We used plasma and urine samples of mice with intravascular hemolysis and human cerebrospinal fluid (CSF) samples following intraventricular hemorrhage. Because ferrylHb is highly unstable, we also addressed the fate of this species. metHb and free heme time-dependently accumulate in plasma and CSF samples following intravascular hemolysis and intraventricular hemorrhage, respectively. ferrylHb is hardly detectable in the biological samples during hemolytic conditions. Under in vitro conditions, ferrylHb decomposes quickly to metHb, which process is associated with the formation of covalently crosslinked Hb multimers. We detected these covalently crosslinked Hb multimers in plasma, urine, and CSF samples during hemolytic conditions. Because globin modification is specific for these Hb forms, we propose to call this heterogeneous form of Hb produced during ferrylHb decomposition as globin-modified oxidized Hb (gmoxHb). Understanding the formation and the contribution of gmoxHb species to the pathogenesis of hemolytic conditions could have therapeutic implications in the treatment of hemolytic diseases.

Formation of novel N-acetylcysteine-hemin adducts abrogates hemin-induced cytotoxicity and suppresses the NRF2-driven stress response in human pro-erythroid K562 cells

Heme (iron protoporphyrin IX), as the prosthetic group in hemoproteins, regulates vital cellular functions in human tissues. However, free heme released during hemolysis events promotes severe complications to millions of people worldwide. Over the years, thiols like glutathione (GSH) were known to antagonize heme toxicity. In this study, we have uncovered the underlying molecular mechanism by which N-acetylcysteine (NAC), a well-known thiol prevents hemin-induced cytotoxicity (HIC). Hemin-responsive human pro-erythroid K562 cells were employed to assess hemin intracellular accumulation and cytotoxicity at concentrations ≥50 μΜ, in cultures exposed only to hemin and/or both hemin and NAC. NAC inhibited the intracellular accumulation of hemin and prevented hemin-induced cell growth inhibition, cell death, oxidative stress, and accumulation of ubiquitinated proteins. Meanwhile, the activation of the NF-E2-related factor-2 (NRF2)-driven stress gene activation, a key element involved in HIC, was suppressed by NAC. A refined mechanism of the chemical reaction between NAC and hemin leading to adduct formation via a nucleophilic attack on hemin was uncovered for the first time by tandem mass spectrometry analysis (LC-MS/MS). Such thiol-hemin adducts acted as intermediates to mitigate HIC and to suppress hemin-induced NRF2-driven gene activation. Our findings support the concept that NAC-hemin adduct formation is the major novel molecular mechanism rather than the reactive oxygen species-scavenging capacity of thiols to protect cells from HIC. Our results imply that thiols and their derivatives can be of potential therapeutic value in hemolytic disorders.

Activation of KEAP1/NRF2 stress signaling involved in the molecular basis of hemin-induced cytotoxicity in human pro-erythroid K562 cells

During hemolysis, free heme released from damaged RBCs impairs adjacent cells. As a response, heme induces its metabolic degradation via heme oxygenase-1 (HO-1), activated by NF-E2-related factor 2 (NRF2), the master stress response transcription factor. Heme is well considered a signaling molecule, but how heme does activate NRF2 is not well understood. K562, human pro-erythroid cells responding to hemin (ferric chloride heme), were employed to uncover the major role of Kelch-like ECH-associated protein 1 (KEAP1)/NRF2 stress response signaling, embedded in hemin-induced cytotoxicity (HIC), at ≥50 μM. The intracellular pools of hemin were found to determine the progression from the reversible cell growth inhibition to non-apoptotic cell death. Hemin-induced accumulation of both reactive oxygen species (ROS) and ubiquitinated proteins provoked disturbed cellular proteostasis. Immediate accumulation and nuclear translocation of NRF2 were recorded as defensive adaptation. The NRF2-driven genes encoding glutamate-cysteine ligase (GCLC) and cystine/glutamate antiporter (xCT) were substantially activated. Hemin orchestrated a defensive pathway involving the management of cellular non-protein thiols, via an increase in GSH levels and secretion of cysteine. Mechanistically, hemin stabilized NRF2 protein levels selectively by inhibiting the KEAP1-driven ubiquitination of NRF2, while allowing KEAP1 ubiquitination. High-molecular-weight ubiquitinated KEAP1 variants formed in hemin-treated cells degraded in proteasomes, while a portion of them translocated into the nucleus. The KEAP1/NRF2 system can be revealed as a basic homeostatic mechanism, activated in cells encountering free heme, both in healthy and diseased state. Its activation provides a multi-target cytoprotective platform to develop agents preventing heme toxicity.

A Computational Approach for Mapping Heme Biology in the Context of Hemolytic Disorders

Heme is an iron ion-containing molecule found within hemoproteins such as hemoglobin and cytochromes that participates in diverse biological processes. Although excessive heme has been implicated in several diseases including malaria, sepsis, ischemia-reperfusion, and disseminated intravascular coagulation, little is known about its regulatory and signaling functions. Furthermore, the limited understanding of heme's role in regulatory and signaling functions is in part due to the lack of curated pathway resources for heme cell biology. Here, we present two resources aimed to exploit this unexplored information to model heme biology. The first resource is a terminology covering heme-specific terms not yet included in standard controlled vocabularies. Using this terminology, we curated and modeled the second resource, a mechanistic knowledge graph representing the heme's interactome based on a corpus of 46 scientific articles. Finally, we demonstrated the utility of these resources by investigating the role of heme in the Toll-like receptor signaling pathway. Our analysis proposed a series of crosstalk events that could explain the role of heme in activating the TLR4 signaling pathway. In summary, the presented work opens the door to the scientific community for exploring the published knowledge on heme biology.

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.

Learning from Artemisinin: Bioinspired Design of a Reaction-Based Fluorescent Probe for the Selective Sensing of Labile Heme in Complex Biosystems

Labile heme (LH) is an important signaling molecule in virtually all organisms. However, specifically detecting LH remains an outstanding challenge. Herein, by learning from the bioactivation mechanism of artemisinin, we have developed the first LH-responsive small-molecule fluorescent probe, HNG, based on a 4-amino-1,8-naphthalimide (NG) fluorophore. HNG showed high selectivity for LH without interference from hemin, protein-interacting heme, and zinc protoporphyrin. Using HNG, the changes of LH levels in live cells were imaged, and a positive correlation of LH level with the degree of hemolysis was uncovered in hemolytic mice. Our study not only presents the first molecular probe for specific LH detection but also provides a strategy to construct probes with high specificity through a bioinspired approach.

Hemolysis Derived Products Toxicity and Endothelium: Model of the Second Hit

Vascular diseases are multifactorial, often requiring multiple challenges, or ‘hits’, for their initiation. Intra-vascular hemolysis illustrates well the multiple-hit theory where a first event lyses red blood cells, releasing hemolysis-derived products, in particular cell-free heme which is highly toxic for the endothelium. Physiologically, hemolysis derived-products are rapidly neutralized by numerous defense systems, including haptoglobin and hemopexin which scavenge hemoglobin and heme, respectively. Likewise, cellular defense mechanisms are involved, including heme-oxygenase 1 upregulation which metabolizes heme. However, in cases of intra-vascular hemolysis, those systems are overwhelmed. Heme exerts toxic effects by acting as a damage-associated molecular pattern and promoting, together with hemoglobin, nitric oxide scavenging and ROS production. In addition, it activates the complement and the coagulation systems. Together, these processes lead to endothelial cell injury which triggers pro-thrombotic and pro-inflammatory phenotypes. Moreover, among endothelial cells, glomerular ones display a particular susceptibility explained by a weaker capacity to counteract hemolysis injury. In this review, we illustrate the ‘multiple-hit’ theory through the example of intra-vascular hemolysis, with a particular focus on cell-free heme, and we advance hypotheses explaining the glomerular susceptibility observed in hemolytic diseases. Finally, we describe therapeutic options for reducing endothelial injury in hemolytic diseases.

Impaired antioxidant capacity causes a disruption of metabolic homeostasis in sickle erythrocytes

This study examined particularly relevant redox pathways such as glycolysis, pentose phosphate pathway (PPP), metHb reductase and nucleotide metabolism, in order to better address how sickle cells deal with redox metabolism disruption. We also investigated the generation of specific oxidative lesions, and the levels of an unexplored antioxidant that could act as a candidate biomarker for oxidative status in sickle cell anemia (SCA). We adopted rigorous exclusion criteria to obtain the studied groups, which were composed by 10 subjects without hemoglobinopathies and 10 SCA patients. We confirmed that sickle cells overwhelm the antioxidant defense system, leading to an impaired antioxidant capacity that significantly contributed to the increase in cholesterol oxidation (ChAld) and hemolysis. Among the antioxidants evaluated, ergothioneine levels decreased in SCA (two-fold). We found strong correlations of ergothioneine levels with other erythrocyte metabolism markers, suggesting its use as an antioxidant therapy alternative for SCA treatment. Moreover, we found higher activities of MetHb reductase, AChE, G6PDH, HXK, and LDH, as well as levels of NADPH, ATP and hypoxanthine in sickle cells. On this basis, we conclude that impaired antioxidant capacity leaves to a loss of glycolysis and PPP shifting mechanism control and further homeostasis rupture, contributing to a decreased lifespan of sickle cells.

Innate Nutritional Immunity

Iron (Fe) is an essential micronutrient for both microbes and their hosts. The biologic importance of Fe derives from its inherent ability to act as a universal redox catalyst, co-opted in a variety of biochemical processes critical to maintain life. Animals evolved several mechanisms to retain and limit Fe availability to pathogenic microbes, a resistance mechanism termed "nutritional immunity." Likewise, pathogenic microbes coevolved to deploy diverse and efficient mechanisms to acquire Fe from their hosts and in doing so overcome nutritional immunity. In this review, we discuss how the innate immune system regulates Fe metabolism to withhold Fe from pathogenic microbes and how strategies used by pathogens to acquire Fe circumvent these resistance mechanisms.

Renal control of disease tolerance to malaria

Malaria, the disease caused by Plasmodium spp. infection, remains a major global cause of morbidity and mortality. Host protection from malaria relies on immune-driven resistance mechanisms that kill Plasmodium However, these mechanisms are not sufficient per se to avoid the development of severe forms of disease. This is accomplished instead via the establishment of disease tolerance to malaria, a defense strategy that does not target Plasmodium directly. Here we demonstrate that the establishment of disease tolerance to malaria relies on a tissue damage-control mechanism that operates specifically in renal proximal tubule epithelial cells (RPTEC). This protective response relies on the induction of heme oxygenase-1 (HMOX1; HO-1) and ferritin H chain (FTH) via a mechanism that involves the transcription-factor nuclear-factor E2-related factor-2 (NRF2). As it accumulates in plasma and urine during the blood stage of Plasmodium infection, labile heme is detoxified in RPTEC by HO-1 and FTH, preventing the development of acute kidney injury, a clinical hallmark of severe malaria.

Cross-Talk Between Iron and Glucose Metabolism in the Establishment of Disease Tolerance

Infectious diseases are associated with disruption of host homeostasis. This can be triggered directly by pathogens or indirectly by host immune-driven resistance mechanisms. Disease tolerance is a defense strategy against infection that sustains host homeostasis, without exerting a direct negative impact on pathogens. The mechanisms governing disease tolerance encompass host metabolic responses that maintain vital homeostatic parameters within a range compatible with survival. Central to this defense strategy is the host's ability to sense and adapt to variations in nutrients, such as iron and glucose. Here we address how host responses regulating iron and glucose metabolism interact to establish disease tolerance and possibly modulate resistance to infection.

Oxidized hemoglobin forms contribute to NLRP3 inflammasome-driven IL-1β production upon intravascular hemolysis

Damage associated molecular patterns (DAMPs) are released form red blood cells (RBCs) during intravascular hemolysis (IVH). Extracellular heme, with its pro-oxidant, pro-inflammatory and cytotoxic effects, is sensed by innate immune cells through pattern recognition receptors such as toll-like receptor 4 and nucleotide-binding domain and leucine rich repeat containing family, pyrin domain containing 3 (NLRP3), while free availability of heme is strictly controlled. Here we investigated the involvement of different hemoglobin (Hb) forms in hemolysis-associated inflammatory responses. We found that after IVH most of the extracellular heme molecules are localized in oxidized Hb forms. IVH was associated with caspase-1 activation and formation of mature IL-1β in plasma and in the liver of C57BL/6 mice. We showed that ferrylHb (FHb) induces active IL-1β production in LPS-primed macrophages in vitro and triggered intraperitoneal recruitment of neutrophils and monocytes, caspase-1 activation and active IL-1β formation in the liver of C57BL/6 mice. NLRP3 deficiency provided a survival advantage upon IVH, without influencing the extent of RBC lysis or the accumulation of oxidized Hb forms. However, both hemolysis-induced and FHb-induced pro-inflammatory responses were largely attenuated in Nlrp3^-/- mice. Taken together, FHb is a potent trigger of NLRP3 activation and production of IL-1β in vitro and in vivo, suggesting that FHb may contribute to hemolysis-induced inflammation. Identification of RBC-derived DAMPs might allow us to develop new therapeutic approaches for hemolytic diseases.

Antigenic response to CT-P13 and infliximab originator in inflammatory bowel disease patients shows similar epitope recognition

AIM

To test the cross-immunogenicity of anti-CT-P13 IBD patients' sera to CT-P13/infliximab originator and the comparative antigenicity evoked by CT-P13/infliximab originator sera.

METHODS

Sera of patients with IBD with measurable anti-CT-P13 antibodies were tested for their cross-reactivity to 5 batches of infliximab originator and CT-P13. Anti-drug antibody positive sera from treated patients were used to compare antigenic epitopes.

RESULTS

All 42 anti-CT-P13 and 37 anti-infliximab originator IBD sera were cross-reactive with infliximab originator and CT-P13 respectively. Concentration of anti-drug antibodies against infliximab originator or CT-P13 were strongly correlated both for IgG1 and IgG4 (P < 0.001). Anti-CT-P13 sera of patients with IBD (n = 32) exerted similar functional inhibition on CT-P13 or infliximab originator TNF binding capacity and showed reduced binding to CT-P13 in the presence of five different batches of CT-P13 and infliximab originator. Anti-CT-P13 and anti-infliximab originator IBD sera selectively enriched phage-peptides from the VH (CDR1 and CDR3) and VL domains (CDR2 and CDR3) of infliximab. Sera reactivity detected major infliximab epitopes in these regions of infliximab in 60%-79% of patients, and no significant differences were identified between CT-P13 and infliximab originator immunogenic sera. Minor epitopes were localised in framework regions of infliximab with reduced antibody reactivity shown, in 30%-50% of patients. Monoclonal antibodies derived from naïve individuals and ADA-positive IBD patients treated with CT-P13 provided comparable epitope specificity to five different batches of CT-P13 and infliximab originator.

CONCLUSIONS

These results strongly support a similar antigenic profile for infliximab originator and CT-P13, and point toward a safe switching between the two drugs in anti-drug antibody negative patients.

Intravascular hemolysis activates complement via cell-free heme and heme-loaded microvesicles

In hemolytic diseases, such as sickle cell disease (SCD), intravascular hemolysis results in the release of hemoglobin, heme, and heme-loaded membrane microvesicles in the bloodstream. Intravascular hemolysis is thus associated with inflammation and organ injury. Complement system can be activated by heme in vitro. We investigated the mechanisms by which hemolysis and red blood cell (RBC) degradation products trigger complement activation in vivo. In kidney biopsies of SCD nephropathy patients and a mouse model with SCD, we detected tissue deposits of complement C3 and C5b-9. Moreover, drug-induced intravascular hemolysis or injection of heme or hemoglobin in mice triggered C3 deposition, primarily in kidneys. Renal injury markers (Kim-1, NGAL) were attenuated in C3-/- hemolytic mice. RBC degradation products, such as heme-loaded microvesicles and heme, induced alternative and terminal complement pathway activation in sera and on endothelial surfaces, in contrast to hemoglobin. Heme triggered rapid P selectin, C3aR, and C5aR expression and downregulated CD46 on endothelial cells. Importantly, complement deposition was attenuated in vivo and in vitro by heme scavenger hemopexin. In conclusion, we demonstrate that intravascular hemolysis triggers complement activation in vivo, encouraging further studies on its role in SCD nephropathy. Conversely, heme inhibition using hemopexin may provide a novel therapeutic opportunity to limit complement activation in hemolytic diseases.

Heme and hemoglobin suppress amyloid β-mediated inflammatory activation of mouse astrocytes

Glial immune activity is a key feature of Alzheimer's disease (AD). Given that the blood factors heme and hemoglobin (Hb) are both elevated in AD tissues and have immunomodulatory roles, here we sought to interrogate their roles in modulating β-amyloid (Aβ)-mediated inflammatory activation of astrocytes. We discovered that heme and Hb suppress immune activity of primary mouse astrocytes by reducing expression of several proinflammatory cytokines (e.g. RANTES (regulated on activation normal T cell expressed and secreted)) and the scavenger receptor CD36 and reducing internalization of Aβ(1-42) by astrocytes. Moreover, we found that certain soluble (>75-kDa) Aβ(1-42) oligomers are primarily responsible for astrocyte activation and that heme or Hb association with these oligomers reverses inflammation. We further found that heme up-regulates phosphoprotein signaling in the phosphoinositide 3-kinase (PI3K)/Akt pathway, which regulates a number of immune functions, including cytokine expression and phagocytosis. The findings in this work suggest that dysregulation of Hb and heme levels in AD brains may contribute to impaired amyloid clearance and that targeting heme homeostasis may reduce amyloid pathogenesis. Altogether, we propose heme as a critical molecular link between amyloid pathology and AD risk factors, such as aging, brain injury, and stroke, which increase Hb and heme levels in the brain.

Danger signals from mitochondrial DAMPS in trauma and post-injury sepsis

In all multicellular organisms, immediate host responses to both sterile and infective threat are initiated by very primitive systems now grouped together under the general term 'danger responses'. Danger signals are generated when primitive 'pattern recognition receptors' (PRR) encounter activating 'alarmins'. These molecular species may be of pathogenic infective origin (pathogen-associated molecular patterns) or of sterile endogenous origin (danger-associated molecular patterns). There are many sterile and infective alarmins and there is considerable overlap in their ability to activate PRR, but in all cases the end result is inflammation. It is the overlap between sterile and infective signals acting via a relatively limited number of PRR that generally underlies the great clinical similarity we see between sterile and infective systemic inflammatory responses. Mitochondria (MT) are evolutionarily derived from bacteria, and thus they sit at the crossroads between sterile and infective danger signal pathways. Many of the molecular species in mitochondria are alarmins, and so the release of MT from injured cells results in a wide variety of inflammatory events. This paper discusses the known participation of MT in inflammation and reviews what is known about how the major.