Heme-Mediated Induction of CXCL10 and Depletion of CD34+ Progenitor Cells Is Toll-Like Receptor 4 Dependent

Insights into the molecular basis and mechanism of heme-triggered TLR4 signalling: The role of heme-binding motifs in TLR4 and MD2

Haemolytic disorders, such as sickle cell disease, are accompanied by the release of high amounts of labile heme into the intravascular compartment resulting in the induction of proinflammatory and prothrombotic complications in affected patients. In addition to the relevance of heme-regulated proteins from the complement and blood coagulation systems, activation of the TLR4 signalling pathway by heme was ascribed a crucial role in the progression of these pathological processes. Heme binding to the TLR4-MD2 complex has been proposed recently, however, essential mechanistic information of the processes at the molecular level, such as heme-binding kinetics, the heme-binding capacity and the respective heme-binding sites (HBMs) is still missing. We report the interaction of TLR4, MD2 and the TLR4-MD2 complex with heme and the consequences thereof by employing biochemical, spectroscopic, bioinformatic and physiologically relevant approaches. Heme binding occurs transiently through interaction with up to four HBMs in TLR4, two HBMs in MD2 and at least four HBMs in their complex. Functional studies highlight that mutations of individual HBMs in TLR4 preserve full receptor activation by heme, suggesting that heme interacts with TLR4 through different binding sites independently of MD2. Furthermore, we confirm and extend the major role of TLR4 for heme-mediated cytokine responses in human immune cells.

Modulation of Heme-Induced Inflammation Using MicroRNA-Loaded Liposomes: Implications for Hemolytic Disorders Such as Malaria and Sickle Cell Disease

Hemolytic disorders, like malaria and sickle cell disease (SCD), are responsible for significant mortality and morbidity rates globally, specifically in the Americas and Africa. In both malaria and SCD, red blood cell hemolysis leads to the release of a cytotoxic heme that triggers the expression of unique inflammatory profiles, which mediate the tissue damage and pathogenesis of both diseases. MicroRNAs (miRNAs), such as miR-451a and let-7i-5p, contribute to a reduction in the pro-inflammatory responses induced by circulating free hemes. MiR-451a targets both IL-6R (pro-inflammatory) and 14-3-3ζ (anti-inflammatory), and when this miRNA is present, IL-6R is reduced and 14-3-3ζ is increased. Let-7i-5p targets and reduces TLR4, which results in anti-inflammatory signaling. These gene targets regulate inflammation via NFκB regulation and increase anti-inflammatory signaling. Additionally, they indirectly regulate the expression of key heme scavengers, such as heme-oxygenase 1 (HO-1) (coded by the HMOX1 gene) and hemopexin, to decrease circulating cytotoxic heme concentration. MiRNAs can be transported within extracellular vesicles (EVs), such as exosomes, offering insights into the mechanisms of mitigating heme-induced inflammation. We tested the hypothesis that miR-451a- or let-7i-5p-loaded artificial EVs (liposomes) will reduce heme-induced inflammation in brain vascular endothelial cells (HBEC-5i, ATCC: CRL-3245) and macrophages (THP-1, ATCC: TIB-202) in vitro. We completed arginase and nitric oxide assays to determine anti- and pro-inflammatory macrophage presence, respectively. We also assessed the gene expression of IL-6R, TLR4, 14-3-3ζ, and NFκB by RT-qPCR for both cell lines. Our findings revealed that the exposure of HBEC-5i and THP-1 to liposomes loaded with miR-451a or let-7i-5p led to a reduced mRNA expression of IL-6R, TLR4, 14-3-3ζ, and NFκB when treated with a heme. It also resulted in the increased expression of HMOX1 and hemopexin. Finally, macrophages exhibited a tendency toward adopting an anti-inflammatory differentiation phenotype. These findings suggest that miRNA-loaded liposomes can modulate heme-induced inflammation and can be used to target specific cellular pathways, mediating inflammation common to hematological conditions, like malaria and SCD.

Identifying effective diagnostic biomarkers for childhood cerebral malaria in Africa integrating coexpression analysis with machine learning algorithm

BACKGROUND

Cerebral malaria (CM) is a manifestation of malaria caused by plasmodium infection. It has a high mortality rate and severe neurological sequelae, existing a significant research gap and requiring further study at the molecular level.

METHODS

We downloaded the GSE117613 dataset from the Gene Expression Omnibus (GEO) database to determine the differentially expressed genes (DEGs) between the CM group and the control group. Weighted gene coexpression network analysis (WGCNA) was applied to select the module and hub genes most relevant to CM. The common genes of the key module and DEGs were selected to perform further analysis. The least absolute shrinkage and selection operator (LASSO) logistic regression and support vector machine recursive feature elimination (SVM-RFE) were applied to screen and verify the diagnostic markers of CM. Eventually, the hub genes were validated in the external dataset. Gene set enrichment analysis (GSEA) was applied to investigate the possible roles of the hub genes.

RESULTS

The GO and KEGG results showed that DEGs were enriched in some neutrophil-mediated pathways and associated with some lumen structures. Combining LASSO and the SVM-RFE algorithms, LEF1 and IRAK3 were identified as potential hub genes in CM. Through the GSEA enrichment results, we found that LEF1 and IRAK3 participated in maintaining the integrity of the blood-brain barrier (BBB), which contributed to improving the prognosis of CM.

CONCLUSIONS

This study may help illustrate the pathophysiology of CM at the molecular level. LEF1 and IRAK3 can be used as diagnostic biomarkers, providing new insight into the diagnosis and prognosis prediction in pediatric CM.

MiR-451a and let-7i-5p loaded extracellular vesicles attenuate heme-induced inflammation in hiPSC-derived endothelial cells

Hemolysis is associated with many pathologies, including trauma, sepsis, hemorrhagic stroke, malaria, and genetic disorders such as sickle cell disease (SCD). When hemolysis occurs, free-heme drives vascular inflammation, resulting in oxidative tissue damage and cardiometabolic complications. A better understanding of heme clearance and detoxification is essential to preventing sustained tissue damage. Human induced pluripotent stem cell (hiPSC)-derived endothelial cells (hiPSC-ECs) provide a novel source of patient-specific cells and tissues for disease modeling, drug discovery, and regenerative therapeutics. Here we report the use of hiPSC-ECs to elucidate the role of miR-451a and let-7i-5p-loaded extracellular vesicles (EVs, such as exosomes) in the inflammatory response to free-heme as a model for heme-induced inflammation. We provide evidence of a significant correlation between miR-451a and let-7i-5p-loaded circulating exosomes in plasmodium-infected patients with reported clinical benchmarks of malaria-severity (e.g., Hemoglobin (Hb) levels, white blood cell counts). Additionally, we determined that exposure of Plasmodium falciparum (Pf) parasites to EVs, loaded with either miRNA, significantly reduces their counts in vitro. Using hiPSCs derived from individuals with wild-type Hb (HbAA) or homozygous sickle cell mutated Hb (HbSS) genotypes, we demonstrate that heme-treated hiPSC-ECs secreted inflammatory products (cytokines, chemokines and growth factors) into supporting media at concentrations that were similar to that reported in HbAA and HbSS serum. This inflammatory response was attenuated by exposure with miR-451a or let-7i-5p-loaded EVs. We also found a decrease in transcription of ICAM1 and P-Selectin, as well as the secretion of key inflammatory cytokines (e.g., CXCL10, TNF-α, and IFN-γ). Based on these findings, we propose a model in which increased levels of exosomal miR-451a and let-7i-5p in Plasmodium-infected individuals will attenuate inflammatory responses to free-heme and parasite-derived products. As a result, infected erythrocytes will less likely adhere to the endothelium, sequester in brain micro vessels, and reduce vaso-occlusive crises that exacerbate cerebral malaria.

Brain endothelial STING1 activation by Plasmodium-sequestered heme promotes cerebral malaria via type I IFN response

CM results from loss of blood–brain endothelial barrier function caused by unrestrained inflammatory response in the natural course of infection by Plasmodium parasites. However, the role of brain endothelium in triggering inflammatory mechanisms is still undetermined. We found that the innate immune sensor STING1 is crucial for production of IFNβ by brain endothelial cells in Plasmodium-infected mice. This in turn stimulates CXCL10-mediated recruitment of leukocytes and subsequent brain inflammation and tissue damage. We identified within extracellular particles released from Plasmodium-infected erythrocytes, a fraction containing products of hemoglobin degradation, namely, heme, which we show can bind STING1. Our results unravel a mechanism of CM immunopathogenesis: Heme contained in extracellular particles triggers the STING/IFNβ/CXCL10 axis in brain endothelial cells.

Cerebral malaria (CM) is a life-threatening form of Plasmodium falciparum infection caused by brain inflammation. Brain endothelium dysfunction is a hallmark of CM pathology, which is also associated with the activation of the type I interferon (IFN) inflammatory pathway. The molecular triggers and sensors eliciting brain type I IFN cellular responses during CM remain largely unknown. We herein identified the stimulator of interferon response cGAMP interactor 1 (STING1) as the key innate immune sensor that induces Ifnβ1 transcription in the brain of mice infected with Plasmodium berghei ANKA (Pba). This STING1/IFNβ-mediated response increases brain CXCL10 governing the extent of brain leukocyte infiltration and blood–brain barrier (BBB) breakdown, and determining CM lethality. The critical role of brain endothelial cells (BECs) in fueling type I IFN–driven brain inflammation was demonstrated in brain endothelial–specific IFNβ-reporter and STING1-deficient Pba-infected mice, which were significantly protected from CM lethality. Moreover, extracellular particles (EPs) released from Pba-infected erythrocytes activated the STING1-dependent type I IFN response in BECs, a response requiring intracellular acidification. Fractionation of the EPs enabled us to identify a defined fraction carrying hemoglobin degradation remnants that activates STING1/IFNβ in the brain endothelium, a process correlated with heme content. Notably, stimulation of STING1-deficient BECs with heme, docking experiments, and in vitro binding assays unveiled that heme is a putative STING1 ligand. This work shows that heme resultant from the parasite heterotrophic activity operates as an alarmin, triggering brain endothelial inflammatory responses via the STING1/IFNβ/CXCL10 axis crucial to CM pathogenesis and lethality.

Dendritic cell responses to Plasmodium falciparum in a malaria-endemic setting

Background

Plasmodium falciparum causes the majority of malaria cases worldwide and children in sub-Saharan Africa are the most vulnerable group affected. Non-sterile clinical immunity that protects from symptoms develops slowly and is relatively short-lived. Moreover, current malaria vaccine candidates fail to induce durable high-level protection in endemic settings, possibly due to the immunomodulatory effects of the malaria parasite itself. Because dendritic cells play a crucial role in initiating immune responses, the aim of this study was to better understand the impact of cumulative malaria exposure as well as concurrent P. falciparum infection on dendritic cell phenotype and function.

Methods

In this cross-sectional study, the phenotype and function of dendritic cells freshly isolated from peripheral blood samples of Malian adults with a lifelong history of malaria exposure who were either uninfected (n = 27) or asymptomatically infected with P. falciparum (n = 8) was assessed. Additionally, plasma cytokine and chemokine levels were measured in these adults and in Malian children (n = 19) with acute symptomatic malaria.

Results

With the exception of lower plasmacytoid dendritic cell frequencies in asymptomatically infected Malian adults, peripheral blood dendritic cell subset frequencies and HLA-DR surface expression did not differ by infection status. Peripheral blood myeloid dendritic cells of uninfected Malian adults responded to in vitro stimulation with P. falciparum blood-stage parasites by up-regulating the costimulatory molecules HLA-DR, CD80, CD86 and CD40 and secreting IL-10, CXCL9 and CXCL10. In contrast, myeloid dendritic cells of asymptomatically infected Malian adults exhibited no significant responses above the uninfected red blood cell control. IL-10 and CXCL9 plasma levels were elevated in both asymptomatic adults and children with acute malaria.

Conclusions

The findings of this study indicate that myeloid dendritic cells of uninfected adults with a lifelong history of malaria exposure are able to up-regulate co-stimulatory molecules and produce cytokines. Whether mDCs of malaria-exposed individuals are efficient antigen-presenting cells capable of mounting an appropriate immune response remains to be determined. The data also highlights IL-10 and CXCL9 as important factors in both asymptomatic and acute malaria and add to the understanding of asymptomatic P. falciparum infections in malaria-endemic areas.

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.

Modelling heme-mediated brain injury associated with cerebral malaria in human brain cortical organoids

Human cerebral malaria (HCM), a severe encephalopathy associated with Plasmodium falciparum infection, has a 20-30% mortality rate and predominantly affects African children. The mechanisms mediating HCM-associated brain injury are difficult to study in human subjects, highlighting the urgent need for non-invasive ex vivo human models. HCM elevates the systemic levels of free heme, which damages the blood-brain barrier and neurons in distinct regions of the brain. We determined the effects of heme on induced pluripotent stem cells (iPSCs) and a three-dimensional cortical organoid system and assessed apoptosis and differentiation. We evaluated biomarkers associated with heme-induced brain injury, including a pro-inflammatory chemokine, CXCL-10, and its receptor, CXCR3, brain-derived neurotrophic factor (BDNF) and a receptor tyrosine-protein kinase, ERBB4, in the organoids. We then tested the neuroprotective effect of neuregulin-1 (NRG-1) against heme treatment in organoids. Neural stem and mature cells differentially expressed CXCL-10, CXCR3, BDNF and ERBB4 in the developing organoids and in response to heme-induced neuronal injury. The organoids underwent apoptosis and structural changes that were attenuated by NRG-1. Thus, cortical organoids can be used to model heme-induced cortical brain injury associated with HCM pathogenesis as well as for testing agents that reduce brain injury and neurological sequelae.

CCL2 and CXCL10 are associated with poor outcome after intracerebral hemorrhage

OBJECTIVE

Intracerebral hemorrhage carries a high mortality and survivors are frequently left with significant disability. Immunological mechanisms may play an important role in hemorrhage-induced brain injury, however, research linking these mechanisms with clinical outcome remains limited. We aim to identify serum inflammatory mediators that are associated with outcome after intracerebral hemorrhage in order to translate data from experimental models to a patient cohort and identify potential targets worthy of reverse translation.

METHODS

A prospective cohort study at two comprehensive stroke centers enrolled patients with spontaneous intracerebral hemorrhage. Peripheral blood was collected at 6, 24, and 72 h from onset. Functional outcome was assessed at 90 days using the modified Rankin Scale (mRS). Serum inflammatory mediators were measured using multiplex ELISA. Multivariable modeling identified serum biomarkers independently associated with functional outcome at 90 days.

RESULTS

115 patients completed the study. At 6 h after onset, patients with elevated CCL2 had worse mRS score at day 90 (OR 4.07, 95% CI 1.27-13.10, P = 0.02) after adjusting for age, gender, ICH volume, IVH, infratentorial location and NIHSS score. At 24 and 72 h after onset, elevation in CXCL10 was independently associated with worse 90 days mRS score (24 h: OR 8.08, 95% CI 2.69-24.30, P < 0.001; 72 h: OR 3.89, 95% CI 1.12-13.49, P = 0.03).

INTERPRETATION

Acute and subacute elevations in specific immune factors are associated with poor outcome, highlighting potential pathways that may contribute to ongoing brain injury in patients with intracerebral hemorrhage.

The Glycosylphosphatidylinositol Transamidase Complex Subunit PbGPI16 of Plasmodium berghei Is Important for Inducing Experimental Cerebral Malaria

In animal models of experimental cerebral malaria (ECM), the glycosylphosphatidylinositols (GPIs) and GPI anchors are the major factors that induce nuclear factor kappa B (NF-κB) activation and proinflammatory responses, which contribute to malaria pathogenesis. GPIs and GPI anchors are transported to the cell surface via a process called GPI transamidation, which involves the GPI transamidase (GPI-T) complex. In this study, we showed that GPI16, one of the GPI-T subunits, is highly conserved among Plasmodium species. Genetic knockout of pbgpi16 (Δpbgpi16) in the rodent malaria parasite Plasmodium berghei strain ANKA led to a significant reduction of the amounts of GPIs in the membranes of merozoites, as well as surface display of several GPI-anchored merozoite surface proteins. Compared with the wild-type parasites, Δpbgpi16 parasites in C57BL/6 mice caused much less NF-κB activation and elicited a substantially attenuated T helper type 1 response. As a result, Δpbgpi16 mutant-infected mice displayed much less severe brain pathology, and considerably fewer Δpbgpi16 mutant-infected mice died from ECM. This study corroborated the GPI toxin as a significant inducer of ECM and further suggested that vaccines against parasite GPIs may be a promising strategy to limit the severity of malaria.

Monocyte dysregulation and systemic inflammation during pediatric falciparum malaria

BACKGROUND

Inflammation and monocytes are thought to be important to human malaria pathogenesis. However, the relationship of inflammation and various monocyte functions to acute malaria, recovery from acute malaria, and asymptomatic parasitemia in endemic populations is poorly understood.

METHODS

We evaluated plasma cytokine levels, monocyte subsets, monocyte functional responses, and monocyte inflammatory transcriptional profiles of 1- to 10-year-old Kenyan children at the time of presentation with acute uncomplicated malaria and at recovery 6 weeks later; these results were compared with analogous data from asymptomatic children and adults in the same community.

RESULTS

Acute malaria was marked by elevated levels of proinflammatory and regulatory cytokines and expansion of the inflammatory "intermediate" monocyte subset that returned to levels of healthy asymptomatic children 6 weeks later. Monocytes displayed activated phenotypes during acute malaria, with changes in surface expression of markers important to innate and adaptive immunity. Functionally, acute malaria monocytes and monocytes from asymptomatic infected children had impaired phagocytosis of P. falciparum-infected erythrocytes relative to asymptomatic children with no blood-stage infection. Monocytes from both acute malaria and recovery time points displayed strong and equivalent cytokine responsiveness to innate immune agonists that were independent of infection status. Monocyte transcriptional profiles revealed regulated and balanced proinflammatory and antiinflammatory and altered phagocytosis gene expression patterns distinct from malaria-naive monocytes.

CONCLUSION

These observations provide insights into monocyte functions and the innate immune response during uncomplicated malaria and suggest that asymptomatic parasitemia in children is not clinically benign.

FUNDING

Support for this work was provided by NIH/National Institute of Allergy and Infectious Diseases (R01AI095192-05), the Burroughs Wellcome Fund/American Society of Tropical Medicine and Hygiene, and the Rainbow Babies & Children's Foundation.

Impact of heme on specific antibody production in mice: promotive, inhibitive or null outcome is determined by its concentration

Free heme is an endogenous danger signal that provokes innate immunity. Active innate immunity provides a precondition of an effective adaptive immune response. However, heme catabolites, CO, biliverdin and bilirubin trigger immunosuppression. Furthermore, free heme induces expression of heme oxygenase-1 to increase production of CO, biliverdin and bilirubin. As such, free heme can play a paradoxical role in adaptive immunity. What is the outcome of the animal immune response to an antigen in the presence of free heme? This question remains to be explored. Here, we report the immunization results of rats and mice after intraperitoneal injection of formulations containing BSA and heme. When the heme concentrations were below 1 μM, between 1 μM and 5 μM and above 5 μM, production of anti-BSA IgG and IgM was unaffected, enhanced and suppressed, respectively. The results suggest that heme can influence adaptive immunity by double concentration-thresholds. If the heme concentrations are less than the first threshold, there is no effect on adaptive immunity; if the concentrations are more than the first but less than the second threshold, there is promotion effect; and if the concentrations are more than the second threshold, there is an inhibitory effect. A hypothesis is also presented here to explain the mechanism.

Host Biomarkers Are Associated With Response to Therapy and Long-Term Mortality in Pediatric Severe Malaria

Background.  Host responses to infection are critical determinants of disease severity and clinical outcome. The development of tools to risk stratify children with malaria is needed to identify children most likely to benefit from targeted interventions. Methods.  This study investigated the kinetics of candidate biomarkers of mortality associated with endothelial activation and dysfunction (angiopoietin-2 [Ang-2], soluble FMS-like tyrosine kinase-1 [sFlt-1], and soluble intercellular adhesion molecule-1 [sICAM-1]) and inflammation (10 kDa interferon γ-induced protein [CXCL10/IP-10] and soluble triggering receptor expressed on myeloid cells-1 [sTREM-1]) in the context of a randomized, double-blind, placebo-controlled, parallel-arm trial evaluating inhaled nitric oxide versus placebo as adjunctive therapy to parenteral artesunate for severe malaria. One hundred eighty children aged 1-10 years were enrolled at Jinja Regional Referral Hospital in Uganda and followed for up to 6 months. Results.  There were no differences between the 2 study arms in the rate of biomarker recovery. Median levels of Ang-2, CXCL10, and sFlt-1 were higher at admission in children who died in-hospital (n = 15 of 180; P < .001, P = .027, and P = .004, respectively). Elevated levels of Ang-2, sTREM-1, CXCL10, and sICAM-1 were associated with prolonged clinical recovery times in survivors. The Ang-2 levels were also associated with postdischarge mortality (P < .0001). No biomarkers were associated with neurodisability. Conclusions.  Persistent endothelial activation and dysfunction predict survival in children admitted with severe malaria.

Red cell DAMPs and inflammation

Intravascular hemolysis, or the destruction of red blood cells in the circulation, can occur in numerous diseases, including the acquired hemolytic anemias, sickle cell disease and β-thalassemia, as well as during some transfusion reactions, preeclampsia and infections, such as those caused by malaria or Clostridium perfringens. Hemolysis results in the release of large quantities of red cell damage-associated molecular patterns (DAMPs) into the circulation, which, if not neutralized by innate protective mechanisms, have the potential to activate multiple inflammatory pathways. One of the major red cell DAMPs, heme, is able to activate converging inflammatory pathways, such as toll-like receptor signaling, neutrophil extracellular trap formation and inflammasome formation, suggesting that this DAMP both activates and amplifies inflammation. Other potent DAMPs that may be released by the erythrocytes upon their rupture include heat shock proteins (Hsp), such as Hsp70, interleukin-33 and Adenosine 5' triphosphate. As such, hemolysis represents a major inflammatory mechanism that potentially contributes to the clinical manifestations that have been associated with the hemolytic diseases, such as pulmonary hypertension and leg ulcers, and likely plays a role in specific complications of sickle cell disease such as endothelial activation, vaso-occlusive processes and tissue injury.