Hemozoin stability and dormant induction of heme oxygenase in hemozoin-fed human monocytes

Hemozoin-induced IFN-γ production mediates innate immune protection against sporozoite infection

Hemozoin is a crystal synthesized by Plasmodium parasites during hemoglobin digestion in the erythrocytic stage. The hemozoin released when the parasites egress from the red blood cell, which is complexed with parasite DNA, is cleared from the circulation by circulating and tissue-resident monocytes and macrophages, respectively. Recently, we reported that intravenous administration of purified hemozoin complexed with Plasmodium berghei DNA (HzPbDNA) resulted in an innate immune response that blocked liver stage development of sporozoites that was dose-dependent and time-limited. Here, we further characterize the organismal, cellular, and molecular events associated with this protective innate response in the liver and report that a large proportion of the IV administered HzPbDNA localized to F4/80+ cells in the liver and that the rapid and strong protection against liver-stage development waned quickly such that by 1 week post-HzPbDNA treatment animals were fully susceptible to infection. RNAseq of the liver after IV administration of HzPbDNA demonstrated that the rapid and robust induction of genes associated with the acute phase response, innate immune activation, cellular recruitment, and IFN-γ signaling observed at day 1 was largely absent at day 7. RNAseq analysis implicated NK cells as the major cellular source of IFN-γ. In vivo cell depletion and IFN-γ neutralization experiments supported the hypothesis that tissue-resident macrophages and NK cells are major contributors to the protective response and the NK cell-derived IFN-γ is key to induction of the mechanisms that block sporozoite development in the liver. These findings advance our understanding of the innate immune responses that prevent liver stage malaria infection.

Innate immunity to malaria-The role of monocytes

Monocytes are innate immune cells essential for host protection against malaria. Upon activation, monocytes function to help reduce parasite burden through phagocytosis, cytokine production, and antigen presentation. However, monocytes have also been implicated in the pathogenesis of severe disease through production of damaging inflammatory cytokines, resulting in systemic inflammation and vascular dysfunction. Understanding the molecular pathways influencing the balance between protection and pathology is critical. In this review, we discuss recent data regarding the role of monocytes in human malaria, including studies of innate sensing of the parasite, immunometabolism, and innate immune training. Knowledge gained from these studies may guide rational development of novel antimalarial therapies and inform vaccine development.

Plasma Proteins and Platelets Modulate Neutrophil Clearance of Malaria-Related Hemozoin Crystals

Hemozoin is an insoluble crystalline pigment produced by the malaria parasite Plasmodia upon digesting host hemoglobin inside red blood cells. Red blood cell rupture releases hemozoin crystals into the circulation from where they are cleared by phagocytes such as neutrophils. We speculated that plasma proteins would affect the ability of neutrophils to clear hemozoin crystals. To test this, we cultured human blood neutrophils with hemozoin ex vivo and found that neutrophils ingested hemozoin (0.1-1 µm crystal size) in a dose-dependent manner into phagosomes and vesicles/vacuoles, resulting in morphological changes including nuclear enlargement, and vesicle formation, but not cell membrane rupture or release of neutrophil extracellular traps. The presence of human plasma significantly inhibited the ability of neutrophils to ingest hemozoin crystals. Platelet-poor plasma further inhibited the uptake of hemozoin by neutrophils. Selective exposure to fibrinogen completely replicated the plasma effect. Taken together, neutrophils cleared hemozoin crystals from the extracellular space via endocytosis into phagosomes and vesicles without inducing the release of neutrophil extracellular traps. However, human plasma components such as fibrinogen limited hemozoin clearance, whereas the presence of platelets augmented this process. These factors may influence the pro-inflammatory potential of hemozoin crystals in malaria.

Phagocytosis of hemozoin by RAW 264.7 cells, but not THP-1 cells, promotes infection by Leishmania donovani with a nitric oxide-independent mechanism

During its intra-erythrocytic development, the malaria parasite Plasmodium falciparum synthesizes insoluble hemozoin (HZ) crystals that are released into the circulation upon rupture of parasitized red blood cells, and rapidly phagocytized by host mononuclear cells. Here, HZ persists undigested, causing functional impairment and possibly leading to increased host susceptibility to secondary infections. In patients with malaria and visceral leishmaniasis (VL) co-infections, HZ-loaded macrophages are likely to co-harbor Leishmania donovani parasites, but whether this might influence the course of the Leishmania infection is unknown. In this study, L. donovani amastigote growth was monitored in mouse RAW 264.7 macrophages and PMA-differentiated THP-1 cells previously exposed to increasing amounts of HZ or its synthetic analogue β-hematin (BH). Latex beads were used as a phagocytic control. Data demonstrate that phagocytosis of HZ and BH by RAW 264.7 cells promoted infection therein by L. donovani parasites in a dose-dependent fashion. Similar results were not observed when using THP-1 cells, despite a clear persistence of undigested heme up to 48h after phagocytosis. Conditioning with lipopolysaccharide (LPS)/interferon (IFN)-γ prior to Leishmania infection triggered the release in RAW 264.7 cells of nitric oxide (NO), a highly leishmanicidal metabolite. However, neither HZ nor BH pre-ingestion were able to inhibit NO production following stimulation with LPS/IFN-γ, suggesting that the HZ- and BH-promoting effect on L. donovani infection occurred with an NO-independent mechanism. In conclusion, these preliminary findings highlight a possible detrimental effect of HZ on the course of VL, warranting further investigation into the clinical relevance of the current models.

Expression of ABCG2 (BCRP) in mouse models with enhanced erythropoiesis

Haem is a structural component of numerous cellular proteins which contributes significantly to iron metabolic processes in mammals but its toxicity demands that cellular levels must be tightly regulated. Breast Cancer Resistance Protein (BCRP/ABCG2), an ATP Binding Cassette G-member protein has been shown to possess porphyrin/haem efflux function. The current study evaluated the expression and regulation of Abcg2 mRNA and protein levels in mouse tissues involved in erythropoiesis. Abcg2 mRNA expression was enhanced in bone marrow hemopoietic progenitor cells from mice that were treated with phenylhydrazine (PHZ). Abcg2 mRNA expression was increased particularly in the extramedullary haematopoietic tissues from all the mice models with enhanced erythropoiesis. Haem oxygenase (ho1) levels tended to increase in the liver of mice with enhanced erythropoiesis and gene expression patterns differed from those observed in the spleen. Efflux of haem biosynthetic metabolites might be dependent on the relative abundance of Abcg2 or ho1 during erythropoiesis. Abcg2 appears to act principally as a safety valve regulating porphyrin levels during the early stages of erythropoiesis and its role in systemic haem metabolism and erythrophagocytosis, in particular, awaits further clarification.

Immunopathological effects of malaria pigment or hemozoin and other crystals

Blood-stage malaria parasites produce insoluble hemozoin (Hz) crystals that are released in the blood circulation upon schizont rupture. In general, endogenous crystal formation or inhalation of crystalline materials is often associated with pathology. As the immune system responds differently to crystalline particles than to soluble molecules, in this review, the properties, immunological recognition, and pathogenic responses of Hz are discussed, and compared with two other major pathogenic crystals, monosodium urate (MSU) and asbestos. Because of the size and shape of MSU crystals and asbestos fibers, phagolysosomal formation is inefficient and often results in leakage of lysosomal content in the cell cytoplasm and/or in the extracellular environment with subsequent cell damage and cell death. Phagolysosomal formation after Hz ingestion is normal, but Hz remains stored inside these cells for months or even longer without any detectable degradation. Nonetheless, the different types of crystals are recognized by similar immune receptors, involving Toll-like receptors, the inflammasome, antibodies, and/or complement factors, and through similar signaling cascades, they activate both proinflammatory and anti-inflammatory immune responses that contribute to inflammation-associated pathology.

Prolonged neutrophil dysfunction after Plasmodium falciparum malaria is related to hemolysis and heme oxygenase-1 induction

It is not known why people are more susceptible to bacterial infections such as nontyphoid Salmonella during and after a malaria infection, but in mice, malarial hemolysis impairs resistance to nontyphoid Salmonella by impairing the neutrophil oxidative burst. This acquired neutrophil dysfunction is a consequence of induction of the cytoprotective, heme-degrading enzyme heme oxygenase-1 (HO-1) in neutrophil progenitors in bone marrow. In this study, we assessed whether neutrophil dysfunction occurs in humans with malaria and how this relates to hemolysis. We evaluated neutrophil function in 58 Gambian children with Plasmodium falciparum malaria [55 (95%) with uncomplicated disease] and examined associations with erythrocyte count, haptoglobin, hemopexin, plasma heme, expression of receptors for heme uptake, and HO-1 induction. Malaria caused the appearance of a dominant population of neutrophils with reduced oxidative burst activity, which gradually normalized over 8 wk of follow-up. The degree of neutrophil impairment correlated significantly with markers of hemolysis and HO-1 induction. HO-1 expression was increased in blood during acute malaria, but at a cellular level HO-1 expression was modulated by changes in surface expression of the haptoglobin receptor (CD163). These findings demonstrate that neutrophil dysfunction occurs in P. falciparum malaria and support the relevance of the mechanistic studies in mice. Furthermore, they suggest the presence of a regulatory pathway to limit HO-1 induction by hemolysis in the context of infection and indicate new targets for therapeutic intervention to abrogate the susceptibility to bacterial infection in the context of hemolysis in humans.

Mechanisms of toxicity induced by SiO2 nanoparticles of in vitro human alveolar barrier: effects on cytokine production, oxidative stress induction, surfactant proteins A mRNA expression and nanoparticles uptake

An in vitro human alveolar barrier established by the coculture of epithelial human cell line NCI-H441 with endothelial human cell line ISO-HAS1 was used to evaluate the effects of amorphous silicon dioxide nanoparticles (SiNPs), in the presence or absence of THP-1 cells (monocytes). SiNPs exposure induced production of proinflammatory cytokine and oxidative stress. A high release of TNF-α and IL-8 by epithelial/endothelial cells, potentiated in the presence of THP-1 cells could contribute to the observed downregulation of surfactant proteins A mRNA expression resulting in the damage of the alveolar barrier. The obtained results suggested that in vitro approach can be used to study pulmonary toxicity as long as the applied in vitro model mimics closely the complexity of in vivo situation.

Inducing healing-like human primary macrophage phenotypes by 3D hydrogel coated nanofibres

Immune cells are present in the blood and in resident tissues, and the nature of their reaction towards biomaterials is decisive for materials success or failure. Macrophages may for example be classically activated to trigger inflammation (M1), or alternatively activated which supports healing and vascularisation (M2). Here, we have generated 3D nanofibrous meshes in different porosities and precisely controlled surface chemistries comprising PLGA, hydrogel-coated protein repellant and protein repellant endowed with the bioactive peptide sequences GRGDS or GLF. We also prepared 2D substrates with corresponding surface chemistry for a systematic evaluation of primary human macrophage adhesion, migration, transcriptome expression, cytokine release and surface marker expression. Our data show that material morphology is a powerful means in biomaterial design to influence immune cell response. Flat substrates lead to an increased number of M2 classified CD163(+) macrophages. However, these M2 cells released large amounts of pro-inflammatory cytokines. In contrast, 3D nanofibres with corresponding surface chemistry yielded M1 classified 27E10(+) macrophages with a significantly increased release of pro-angiogenic chemokines and angiogenesis related molecules and a strong decrease of pro-inflammatory cytokines. We thus suggest that, for macrophages in contact with biomaterials, cytokine release is taken as main criterion instead of surface-markers for macrophage classifications.

Host fibrinogen stably bound to hemozoin rapidly activates monocytes via TLR-4 and CD11b/CD18-integrin: a new paradigm of hemozoin action

Natural hemozoin (nHZ), prepared after schizogony, consists of crystalline ferriprotoporphyrin-IX dimers from undigested heme bound to host and parasite proteins and lipids. Phagocytosed nHZ alters important functions of host phagocytes. Most alterations are long-term effects. We show that host fibrinogen (FG) was constantly present (at ~ 1 FG per 25 000 HZ-heme molecules) and stably bound to nHZ from plasma-cultured parasites. FG was responsible for the rapid 100-fold stimulation of reactive oxygen species production and 50-fold increase of TNF and monocyte chemotactic protein 1 by human monocytes. Those effects, starting within minutes after nHZ cell contact, were because of interaction of FG with FG-receptors TLR4 and integrin CD11b/CD18. Receptor blockage by specific mAbs or removal of FG from nHZ abrogated the effects. nHZ-opsonizing IgGs contribute to the stimulatory response but are not essential for FG effects. Immediate increase in reactive oxygen species and TNF may switch on previously described long-term effects of nHZ, largely because of HZ-generated lipo-peroxidation products 15(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid and 4-hydroxynonenal. The FG/HZ effects mediated by TLR4/integrins represent a novel paradigm of nHZ activity and allow expansion of nHZ effects to nonphagocytic cells, such as endothelia and airway epithelia, and lead to a better understanding of organ pathology in malaria.

Role of the NF-κB transcription pathway in the haemozoin- and 15-HETE-mediated activation of matrix metalloproteinase-9 in human adherent monocytes

Haemozoin (HZ, malarial pigment) is a crystalline ferriprotoporphyrin IX polymer derived from undigested host haemoglobin haem, present in late stages of Plasmodium falciparum-parasitized RBCs and in residual bodies shed after schizogony. It was shown previously that phagocytosed HZ or HZ-containing trophozoites increased monocyte matrix metalloproteinase-9 (MMP-9) activity and enhanced production of MMP-9-related cytokines TNF and IL-1beta. Here we show that in human monocytes the HZ/trophozoite phagocytosis effects and their recapitulation by 15(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid (15-HETE), a potent lipoperoxidation derivative generated by HZ from arachidonic acid via haem catalysis, were mediated via activation of NF-κB transcription pathway. After phagocytosis of HZ/trophozoites or treatment with 15-HETE, the NF-κB complex migrated to the nuclear fraction while the inhibitory cytosolic IκBalpha protein was phosphorylated and degraded. All HZ/trophozoite/15-HETE effects on MMP-9 activity and TNF/IL-1beta production were abrogated by quercetin, artemisinin and parthenolide, inhibitors of IκBalpha phosphorylation and subsequent degradation, NF-κB nuclear translocation, and NF-κB-p65 binding to DNA respectively. In conclusion, enhanced activation of MMP-9, and release of pro-inflammatory cytokines TNF and IL-1beta, a triad of effects involved in malaria pathogenesis, elicited in human monocytes by trophozoite and HZ phagocytosis and recapitulated by 15-HETE, appear to be causally connected to persisting activation of the NF-κB system.

Robust erythrophagocytosis leads to macrophage apoptosis via a hemin-mediated redox imbalance: role in hemolytic disorders

MP from the RES are responsible for the clearance of senescent RBC. Although the frequency of senescent RBC is low under steady-state conditions, it increases dramatically during hemolytic disorders, resulting in enhanced erythrophagocytosis. As erythrophagocytosis has been involved in MP dysfunction and as certain hemolytic disorders associate to MP apoptosis, a possible link between erythrophagocytosis and the viability of phagocytes was investigated herein. To mimic hemolytic disorders, two distinct in vitro models, artificially oxidized RBC and DSRBC, were chosen to study the erythrophagocytosis impact on the viability of J774A.1 MP. Although CRBC were weakly phagocytosed and did not affect MP viability significantly, erythrophagocytosis of oxidized RBC and DSRBC was robust and resulted in a sharp decrease of MP viability via apoptosis. Under these conditions, Hb-derived HE was shown to be involved in the induction of apoptosis. Moreover, oxidized RBC, DSRBC, and HE generated ROS species, which were responsible for the apoptosis of MP. Furthermore, HO-1, strongly induced in response to treatment with oxidized RBC, DSRBC, or HE, was shown to protect MP partially against apoptosis, suggesting that robust erythro-phagocytosis may exceed the detoxification capabilities of MP. Taken together, these results suggest that enhanced erythrophagocytosis associated to hemolytic disorders leads to MP apoptosis in vitro and may have critical implications for the control of malaria infection and for the exacerbated susceptibility to bacterial infections during hemolytic disorders.

Innate inflammatory response to the malarial pigment hemozoin

Malaria is an infectious disease caused by parasites of the genus Plasmodium. This intraerythrocytic protozoan produces hemozoin (HZ), an insoluble crystalline metabolite resulting from the heme detoxification mechanism. This review will focus on HZ biosynthesis and synthetic preparation, but in particular on its effect on host's innate inflammatory responses.

Rapid uptake of gold nanorods by primary human blood phagocytes and immunomodulatory effects of surface chemistry

Nanoparticle-based in vivo applications should consider the omnipresence of the phagocytes in the bloodstream and tissue. We have studied the nanoparticle uptake capacities of the most important human primary leukocyte populations using a nanoparticle library encompassing both rod-shaped and spherical gold nanoparticles with diameters between 15 and 50 nm and a variety of surface chemistries. Cetyltrimethylammoniumbromide (CTAB)-stabilized nanoparticles were internalized rapidly within 15 min and in large amounts by macrophages and to a lower extent also by monocytes. Interestingly, we found that the uptake of nanorods by macrophages was more efficient than that of nanospheres. Blocking experiments and electron microscopic studies revealed macropinocytosis as the major uptake mechanism. Grafting of poly(ethylene oxide) (PEO) onto the nanorods was found to significantly delay their internalization for several hours. The long-term uptake of PEO-coated nanoparticles with positively or negatively charged end groups was almost identical. Particle surface chemistry strongly influenced the expression of inflammation-related genes within 1 day. Furthermore, the macrophage phenotype was significantly affected after 7 days of culture with nanorods depending on the surface chemistry. Thus, in vivo application of nanoparticles with certain surface functionalities may lead to inflammation upon particle accumulation. However, our data also suggest that chemical modifications of nanoparticles may be useful for immunomodulation.

Efficient phagosomal maturation and degradation of Plasmodium-infected erythrocytes by dendritic cells and macrophages

Dendritic cells (DC) and macrophages phagocytose pathogens and degrade them in their phagosomes to allow for proper presentation of foreign antigens to other cells of the immune system. The Plasmodium parasite, causative agent of malaria, infects RBC that are phagocytosed by DC and macrophages during the course of infection. Under specific conditions, the functionality of these cells can be affected by phagocytosis of Plasmodium-infected RBC. We investigated whether phagosomal maturation and degradation of Plasmodium yoelii-infected RBC in phagosomes is affected in DC and macrophages. We show that recruitment of the phagolysosomal marker Lamp-1 and of MHC-II, as well as acidification of phagosomes, was achieved in a timely manner. Using P. yoelii-infected RBC labelled with a fluorescent dye or transgenic green fluorescent protein (GFP)-expressing parasites, we found a gradual, rapid decrease in the phagosome fluorescence signal, indicating that P. yoelii-infected RBC are efficiently degraded in macrophages and DC. We also observed that pre-incubation of DC with infected RBC did not affect phagosomal maturation of newly internalized P. yoelii-infected RBC. In conclusion, after phagocytosis, Plasmodium-infected RBC are degraded by DC and macrophages, suggesting that the process of phagosomal maturation is effectively completed in malaria.

Phagocytosis independent extracellular nanoparticle clearance by human immune cells

It has recently been discovered that human immune cells, especially neutrophil granulocytes, form neutrophil extracellular traps (NETs) that abolish pathogens. Our study provides evidence that extracellular traps formed by neutrophils, monocytes and macrophages act as physical barriers for nanoparticles, thus presenting a new nanomaterial clearance mechanism of the human immune system. While particle shape is of minor importance, positive charges significantly enhance particle trapping.

Minireview: Invasive fungal infection complicating acute Plasmodium falciparum malaria

Malaria is the most important parasitic infection in people, affecting 5-10% of the world's population with more than two million deaths a year. Whereas invasive bacterial infections are not uncommon during severe Plasmodium falciparum malaria, only a few cases of opportunistic fungal infections have been reported. Here, we present a fatal case of disseminated hyalohyphomycosis associated with acute P. falciparum malaria in a non-immune traveller, review the cases reported in the literature and discuss the theoretical foundations for the increased susceptibility of non-immune individuals with severe P. falciparum malaria to opportunistic fungal infections. Apart from the availability of free iron as sequelae of massive haemolysis, tissue damage, acidosis and measures of advanced life support, patients with complicated P. falciparum malaria also are profoundly immunosuppressed by the organism's interaction with innate and adaptive host immune mechanisms.

Phagocytosis of haemozoin (malarial pigment) enhances metalloproteinase-9 activity in human adherent monocytes: role of IL-1beta and 15-HETE

BACKGROUND

It has been shown previously that human monocytes fed with haemozoin (HZ) or trophozoite-parasitized RBCs displayed increased matrix metalloproteinase-9 (MMP-9) enzyme activity and protein/mRNA expression and increased TNF production, and showed higher matrix invasion ability. The present study utilized the same experimental model to analyse the effect of phagocytosis of: HZ, delipidized HZ, beta-haematin (lipid-free synthetic HZ) and trophozoites on production of IL-1beta and MMP-9 activity and expression. The second aim was to find out which component of HZ was responsible for the effects.

METHODS

Native HZ freshly isolated from Plasmodium falciparum (Palo Alto strain, Mycoplasma-free), delipidized HZ, beta-haematin (lipid-free synthetic HZ), trophozoites and control meals such as opsonized non-parasitized RBCs and inert latex particles, were fed to human monocytes. The production of IL-1beta by differently fed monocytes, in presence or absence of specific MMP-9 inhibitor or anti-hIL-1beta antibodies, was quantified in supernatants by ELISA. Expression of IL-1beta was analysed by quantitative real-time RT-PCR. MMP-9 activity and protein expression were quantified by gelatin zymography and Western blotting.

RESULTS

Monocytes fed with HZ or trophozoite-parasitized RBCs generated increased amounts of IL-1beta and enhanced enzyme activity (in cell supernatants) and protein/mRNA expression (in cell lysates) of monocyte MMP-9. The latter appears to be causally related to enhanced IL-1beta production, as enhancement of both expression and enzyme activity were abrogated by anti-hIL-1beta Abs. Upregulation of IL-1beta and MMP-9 were absent in monocytes fed with beta-haematin or delipidized HZ, indicating a role for HZ-attached or HZ-generated lipid components. 15-HETE (15(S,R)-hydroxy-6,8,11,13-eicosatetraenoic acid) a potent lipoperoxidation derivative generated by HZ from arachidonic acid via haem-catalysis was identified as one mediator possibly responsible for increase of both IL-1beta production and MMP-9 activity.

CONCLUSION

Results indicate that specific lipoperoxide derivatives generated by HZ may play a role in modulating production of IL-1beta and MMP-9 expression and activity in HZ/trophozoite-fed human monocytes. Results may clarify aspects of cerebral malaria pathogenesis, since MMP-9, a metalloproteinase able to disrupt the basal lamina is possibly involved in generation of hallmarks of cerebral malaria, such as blood-brain barrier endothelium dysfunction, localized haemorrhages and extravasation of phagocytic cells and parasitized RBCs into brain tissues.

Cytotoxicity of zinc-containing bioactive glasses in contact with human osteoblasts

Bioactive glasses such as Hench's 45S5 have applications to tissue engineering and bone repair: the insertion of zinc has been proposed to improve their bone-bonding ability and to slacken their dissolution in extracellular body fluids. In view of a potential clinical application, we have investigated whether zinc-containing 45S5 (HZ) glasses might be cytotoxic for human MG-63 osteoblasts. In our experimental conditions, after 24h of incubation HZ glasses released significant amounts of Zn(2+) and induced in MG-63 cells release of lactate dehydrogenase (index of cytotoxicity) and the following indexes of oxidative stress: (i) accumulation of intracellular malonyldialdehyde, (ii) increased activity of pentose phosphate pathway, (iii) increased expression of heme oxygenase-1, (iv) increased activity of Cu,Zn-superoxide dismutase, (v) decreased level of intracellular thiols. These effects were inversely related to the zinc content of glass powders, were mimicked by ZnCl(2) solutions and were prevented by either metal chelators (EDTA, NTA) or the antioxidant ascorbate, suggesting that Zn(2+) released fastly from HZ glasses can cause MG-63 cell damage via an oxidative stress. This work highlights the importance of designing Zn-containing bioactive glasses without cytotoxic effects and gives supplementary information about the prooxidant role of zinc in living systems.

The basis of the immunomodulatory activity of malaria pigment (hemozoin)

The most common and deadly form of the malaria parasite, Plasmodium falciparum, is responsible for 1.5-2.7 million deaths and 300-500 million acute illnesses annually [Bremen in J. Trop. Med. Hyg. 64:1-11 (2001); World Health Organization (2002)]. Hemozoin, the biomineral formed to detoxify the free heme produced during parasitic hemoglobin catabolism, has long been suspected of contributing to the pathological immunodeficiencies that occur during malarial infection. While there is a growing consensus in the literature that native hemozoin maintains immunosuppressive activity, there is considerable controversy over the reactivity of the synthetic form, beta-hematin (BH). Given the emerging importance of hemozoin in modulating a host immune response to malarial infection, a careful examination of the effects of the constitutive components of the malaria pigment on macrophage response has been made in order to clarify the understanding of this process. Herein, we present evidence that BH alone is unable to inhibit stimulation of NADPH oxidase and inducible nitric oxide synthase, the key enzymes involved in oxidative burst, and is sensitive to the microbicidal agents of these enzymes both in vitro and in vivo. Further, by systematically examining each of the malaria pigment's components, we were able to dissect their impact on the immune reactivity of a macrophage model cell line. Reactions between BH and red blood cell (RBC) ghosts effectively reconstituted the observed immunomodulatory reactivity of native hemozoin. Together, these results suggest that the interaction between hemozoin and the RBC lipids results in the generation of toxic products and that these products are responsible for disrupting macrophage function in vivo.

Bioavailable iron and heme metabolism in Plasmodium falciparum

Iron metabolism is essential for cell function and potentially toxic because iron can catalyze oxygen radical production. Malaria-attributable anemia and iron deficiency anemia coincide as being treatable diseases in the developing world. In absolute amounts, more than 95% of Plasmodium metal biochemistry occurs in the acidic digestive vacuole where heme released from hemoglobin catabolism forms heme crystals. The antimalarial quinolines interfere with crystallization. Despite the completion of the Plasmodium genome, many 'gene gaps' exist in components of the metal pathways described in mammalian or yeast cells. Present evidence suggests that parasite bioavailable iron originates from a labile erythrocyte cytosolic pool rather than from abundant heme iron. Indeed the parasite has to make its own heme within two separate organelles, the mitochondrion and the apicomplast. Paradoxically, despite the abundance of iron within the erythrocyte, iron chelators are cytocidal to the Plasmodium parasite. Hemozoin has become a sensitive biomarker for laser desorption mass spectrometry detection of Plasmodium infection in both mice and humans.

Phagocytosis of hemozoin enhances matrix metalloproteinase-9 activity and TNF-alpha production in human monocytes: role of matrix metalloproteinases in the pathogenesis of falciparum malaria

Matrix metalloproteinase-9 (MMP-9), secreted by activated monocytes, degrades matrix proteins, disrupts basal lamina, and activates TNF-alpha from its precursors. In turn, TNF-alpha enhances synthesis of MMP-9 in monocytes. We show here that trophozoite-parasitized RBCs/hemozoin-fed adherent human monocytes displayed increased MMP-9 activity and protein/mRNA expression, produced TNF-alpha time-dependently, and showed higher matrix invasion ability. MMP-9 activation was specific for trophozoite/hemozoin-fed monocytes, was dependent on TNF-alpha production, and abrogated by anti-TNF-alpha Ab and by a specific inhibitor of MMP-9/MMP-13 activity. Hemozoin-induced enhancement of MMP-9 and TNF-alpha production would have a 2-fold effect: to start and feed a cyclic reinforcement loop in which hemozoin enhances production of TNF-alpha, which in turn induces both activation of MMP-9 and shedding of TNF-alpha into the extracellular compartment; and, second, to disrupt the basal lamina of endothelia. Excess production of TNF-alpha and disruption of the basal lamina with extravasation of blood cells into perivascular tissues are hallmarks of severe malaria. Pharmacological inhibition of MMP-9 may offer a new chance to control pathogenic mechanisms in malaria.

Modulation of cytokine profiles by malaria pigment--hemozoin: role of IL-10 in suppression of proliferative responses of mitogen stimulated human PBMC

The malaria parasite pigment hemozoin (Hz) is internalized by circulating and resident phagocytes and modulates their functions. We report here that Hz from Plasmodium falciparum inhibits proliferative responses of PHA stimulated human peripheral blood mononuclear cells (PBMC) in a dose dependent manner. Hz phagocytosed monocyte/macrophages (MO/MQ) secreted high levels of IL-10, IL-1beta and TNF-alpha, but inhibition of proliferation was mediated by IL-10 alone which was reversed by neutralization of the cytokine. Drastic decrease in the levels of IL-2, IL-12 and IFN-gamma were observed in supernatants from PBMC stimulated in the presence of Hz loaded MO/MQ cells. Exogenous addition of these cytokines did not abrogate immunosuppression indicating the inability of these cytokines to enhance proliferation in the presence of IL-10. We provide additional data that the IL-10 levels correlated positively with the load of Hz in the MO/MQ. Kinetics of IL-10 secretion analyzed up to day 6 in MO/MQ cultures fed with Hz revealed that high levels of IL-10 were secreted during the first 48 h after ingestion and decreased drastically at later time points.

Hemozoin (malarial pigment) inhibits differentiation and maturation of human monocyte-derived dendritic cells: a peroxisome proliferator-activated receptor-gamma-mediated effect

Acute and chronic Plasmodium falciparum malaria are accompanied by severe immunodepression possibly related to subversion of dendritic cells (DC) functionality. Phagocytosed hemozoin (malarial pigment) was shown to inhibit monocyte functions related to immunity. Hemozoin-loaded monocytes, frequently found in circulation and adherent to endothelia in malaria, may interfere with DC development and play a role in immunodepression. Hemozoin-loaded and unloaded human monocytes were differentiated in vitro to immature DC (iDC) by treatment with GM-CSF and IL-4, and to mature DC (mDC) by LPS challenge. In a second setting, hemozoin was fed to iDC further cultured to give mDC. In both settings, cells ingested large amounts of hemozoin undegraded during DC maturation. Hemozoin-fed monocytes did not apoptose but their differentiation and maturation to DC was severely impaired as shown by blunted expression of MHC class II and costimulatory molecules CD83, CD80, CD54, CD40, CD1a, and lower levels of CD83-specific mRNA in hemozoin-loaded iDC and mDC compared with unfed or latex-loaded DC. Further studies indicated activation of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) in hemozoin-loaded iDC and mDC, associated with increased expression of PPAR-gamma mRNA, without apparent involvement of NF-kappaB. Moreover, expression of PPAR-gamma was induced and up-regulation of CD83 was inhibited by supplementing iDC and mDC with plausible concentrations of 15(S)-hydroxyeicosatetraenoic acid, a PPAR-gamma ligand abundantly produced by hemozoin via heme-catalyzed lipoperoxidation.

Malaria-parasitized erythrocytes and hemozoin nonenzymatically generate large amounts of hydroxy fatty acids that inhibit monocyte functions

Plasmodium falciparum digests up to 75% of erythrocyte (red blood cell [RBC]) hemoglobin and forms hemozoin. Phagocytosed hemozoin and trophozoites inhibit important monocyte functions. Delipidized trophozoites and hemozoin were remarkably less toxic to monocytes. Parasitized RBCs and hemozoin contained large amounts of mostly esterified monohydroxy derivatives (OH-PUFAs), the stable end products of peroxidation of polyenoic fatty acids. The concentrations of OH-PUFA were 1.8 micromoles per liter RBCs in nonparasitized RBCs, 11.1 micromoles per liter RBCs in rings, 35 micromoles per liter RBCs in trophozoites; and approximately 90 micromoles per liter RBC equivalents in hemozoin. In parasitized RBCs and hemozoin a complex mixture of monohydroxy derivatives of arachidonic (HETEs) and linoleic (HODEs) acid was determined. Respectively, 13- and 9-HODE and 9- and 12-HETE were predominant in hemozoin and parasitized RBCs. The estimated concentrations of all HETE isomers were 33 and 39 micromoles per liter RBCs or RBC equivalents in trophozoites and hemozoin, respectively. No evidence of lipoxygenase activity was found, whereas the large number of positional and optical isomers, the racemic structure, and their generation by incubation of arachidonic acid with hemozoin indicated nonenzymatic origin via heme-catalysis. Sub/low micromolar concentrations of 12- and 15-HETE were toxic to monocytes, whereas HODE isomers were ineffective. Low micromolar concentrations of HETE isomers were estimated to be similarly present in monocytes after phagocytosis of trophozoites or hemozoin. Thus, specific products of heme-catalyzed lipid peroxidation appear to contribute to hemozoin toxicity to phagocytes and may thus play a role in increased cytoadherence, vascular permeability, and chemotaxis, as well as in immunodepression in malaria.

Sorting of proteins into multivesicular bodies: ubiquitin-dependent and -independent targeting

Yeast endosomes, like those in animal cells, invaginate their membranes to form internal vesicles. The resulting multivesicular bodies fuse with the vacuole, the lysosome equivalent, delivering the internal vesicles for degradation. We have partially purified internal vesicles and analysed their content. Besides the known component carboxypeptidase S (Cps1p), we identified a polyphosphatase (Phm5p), a presumptive haem oxygenase (Ylr205p/Hmx1p) and a protein of unknown function (Yjl151p/Sna3p). All are membrane proteins, and appear to be cargo molecules rather than part of the vesicle-forming machinery. We show that both Phm5p and Cps1p are ubiquitylated, and that in a doa4 mutant, which has reduced levels of free ubiquitin, Cps1p, Phm5p and Hmx1p are mis-sorted to the vacuolar membrane. Mutation of Lys 6 in the cytoplasmic tail of Phm5p disrupts its sorting, but sorting is restored, even in doa4 cells, by the biosynthetic addition of a single ubiquitin chain. In contrast, Sna3p enters internal vesicles in a ubiquitin-independent manner. Thus, ubiquitin acts as a signal for the partitioning of some, but not all, membrane proteins into invaginating endosomal vesicles.

Macrophage preconditioning with synthetic malaria pigment reduces cytokine production via heme iron-dependent oxidative stress

Hemozoin (malaria pigment), a polymer of hematin (ferri-protoporphyrin IX) derived from hemoglobin ingested by intraerythrocytic plasmodia, modulates cytokine production by phagocytes. Mouse peritoneal macrophages (PM) fed with synthetic beta-hematin (BH), structurally identical to native hemozoin, no longer produce tumor necrosis factor alpha (TNFalpha) and nitric oxide (NO) in response to lipopolysaccharide (LPS). Impairment of NO synthesis is due to inhibition of inducible nitric oxide synthase (iNOS) production. BH-mediated inhibition of PM functions cannot be ascribed to iron release from BH because neither prevention by iron chelators nor down-regulation of iron-regulatory protein activity was detected. Inhibition appears to be related to pigment-induced oxidative stress because (a) thiol compounds partially restored PM functions, (b) heme oxygenase (HO-1) and catalase mRNA levels were up-regulated, and (c) free radicals production increased in BH-treated cells. The antioxidant defenses of the cells determine the response to BH: microglia cells, which show a lower extent of induction of HO-1 and catalase mRNAs and lower accumulation of oxygen radicals, are less sensitive to the inhibitory effect of BH on cytokine production. Results indicate that BH is resistant to degradation by HO-1 and that heme-iron mediated oxidative stress may contribute to malaria-induced immunosuppression. This study may help correlate the different clinical manifestations of malaria, ranging from uncomplicated to severe disease, with dysregulation of phagocyte functions and promote better therapeutic strategies to counteract the effects of hemozoin accumulation.