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

Role of 4-hydroxynonenal in the hemozoin-mediated inhibition of differentiation of human monocytes to dendritic cells induced by GM-CSF/IL-4

In falciparum malaria, rupture of parasitized RBC liberates hemozoin (HZ), polymerized heme that contains and generates lipoperoxidation products. In HZ and HZ-loaded monocytes 4-HNE attained approx. 50 and 15 microM, respectively. In malaria, HZ-loaded monocytes are precursors of dendritic cells (DC). Here, the role of 4-HNE as inhibitor of DC differentiation was examined. 4-HNE in HZ was quantified after derivatization by HPLC. DC were differentiated in vitro from human monocytes supplemented with GM-CSF/IL-4 and analyzed for surface antigens and 4-HNE-adducts by FACScan after labelling with specific antibodies. HZ-loading, or treatment with 4-HNE induced large numbers of 4-HNE-protein-adducts on the monocyte membrane. As low as 10 nM 4-HNE inhibited up-regulation of functionally important DC differentiation markers. 1 microM 4-HNE elicited inhibition of up-regulation of DC differentiation markers as follows: MHC-class I and II, -29% and -40%; CD1a, -16%; CD40, -25%; CD54, -27%; and CD83 (the most important DC differentiation marker), -45%, with no signs of apoptosis. The sequence of additions was important, as the inhibitory effect was reduced when 4-HNE was added after GM-CSF/IL-4, indicating that GM-CSF/IL-4 receptors could be modified by 4-HNE. In conclusion, inhibition of DC differentiation by 4-HNE may play a role in malaria immunodepression.

Glycosylphosphatidylinositols in malaria pathogenesis and immunity: potential for therapeutic inhibition and vaccination

Glycosylphosphatidylinositols (GPIs) are found in the outer cell membranes of all eukaryotes. GPIs anchor a diverse range of proteins to the surface of Plasmodium falciparum, but may also exist free of protein attachment. In vitro and in vivo studies have established GPIs as likely candidate toxins in malaria, consistent with the prevailing paradigm that attributes induction of inflammatory cytokines, fever and other pathology to parasite toxins released when schizonts rupture. Although evolutionarily conserved, sufficient structural differences appear to exist that impart upon plasmodial GPIs the ability to activate second messengers in mammalian cells and elicit immune responses. In populations exposed to P. falciparum, the antibody response to purified GPIs is characterised by a predominance of immunoglobulin (Ig)G over IgM and an increase in the prevalence, level and persistence of responses with increasing age. It remains unclear, however, if these antibodies or other cellular responses to GPIs mediate anti-toxic immunity in humans; anti-toxic immunity may comprise either reduction in the severity of disease or maintenance of the malaria-tolerant state (i.e. persistent asymptomatic parasitaemia). P. falciparum GPIs are potentially amenable to specific therapeutic inhibition and vaccination; more needs to be known about their dual roles in malaria pathogenesis and protection for these strategies to succeed.

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.

Sequestration: causes and consequences

Adhesion of Plasmodium falciparum-infected erythrocytes to endothelial cells and to syncytiotrophoblasts lining the placenta is a key feature of malaria pathogenesis. P. falciparum erythrocyte membrane protein 1, a family of variable proteins, mediates adhesion to CD36 and intercellular adhesion molecule 1 in the systemic vasculature, and to chondroitin sulphate A and hyaluronic acid in the placenta. Recent studies of the pathology of fatal cerebral malaria and of placental malaria that follow such sequestration suggest that coagulation disturbances may have a greater role in pathogenesis than previously realized, and that monocyte infiltrates in response to malaria may initiate some of these changes. Chemokines such as macrophage inflammatory protein 1 alpha and beta and monocyte chemoattractant protein 1 may play a key role in attracting monocytes to the placenta and other organs, but the stimulus to chemokine secretion is not presently known.

Ferriprotoporphyrin IX, phospholipids, and the antimalarial actions of quinoline drugs

Two subclasses of quinoline antimalarial drugs are used clinically. Both act on the endolysosomal system of malaria parasites, but in different ways. Treatment with 4-aminoquinoline drugs, such as chloroquine, causes morphologic changes and hemoglobin accumulation in endocytic vesicles. Treatment with quinoline-4-methanol drugs, such as quinine and mefloquine, also causes morphologic changes, but does not cause hemoglobin accumulation. In addition, chloroquine causes undimerized ferriprotoporphyrin IX (ferric heme) to accumulate whereas quinine and mefloquine do not. On the contrary, treatment with quinine or mefloquine prevents and reverses chloroquine-induced accumulation of hemoglobin and undimerized ferriprotoporphyrin IX. This difference is of particular interest since there is convincing evidence that undimerized ferriprotoporphyrin IX in malaria parasites would interact with and serve as a target for chloroquine. According to the ferriprotoporphyrin IX interaction hypothesis, chloroquine would bind to undimerized ferriprotoporphyrin IX, delay its detoxification, cause it to accumulate, and allow it to exert its intrinsic biological toxicities. The ferriprotoporphyrin IX interaction hypothesis appears to explain the antimalarial action of chloroquine, but a drug target in addition to ferriprotoporphyrin IX is suggested by the antimalarial actions of quinine and mefloquine. This article summarizes current knowledge of the role of ferriprotoporphyrin IX in the antimalarial actions of quinoline drugs and evaluates the currently available evidence in support of phospholipids as a second target for quinine, mefloquine and, possibly, the chloroquine-ferriprotoporphyrin IX complex.

Sensing Environmental Lipids by Dendritic Cell Modulates Its Function

Because of its oxidative modification during the acute-phase response to an aggression, low density lipoprotein (LDL) can be regarded as a source of lipid mediators that can act both to promote and inhibit inflammation. This can be exemplified by the production of anti-inflammatory oxidized fatty acids and proinflammatory lysophosphatidylcholine (LPC) during LDL oxidation. We have shown previously that oxidized LDL (oxLDL) plays an active role at the interface between innate and adaptive immunity by delivering instructive molecules such as LPC, which promotes mature dendritic cell (DC) generation from differentiating monocytes. It is shown in this study that LPC affects the signaling pathway of peroxisome proliferator-activated receptors (PPARs). LPC-induced DC maturation is associated with complete inhibition of PPARgamma activity and up-regulation of the activity of an uncharacterized nuclear receptor that bind peroxisome proliferator response element. Oxidized fatty acids generated during LDL oxidation are natural ligands for PPARgamma and inhibit oxLDL- and LPC-induced maturation. Inhibition experiments with synthetic PPARgamma ligands suggested a PPARgamma-dependent and independent effect of LPC on DC maturation. Therefore, the relative amount of oxidized fatty acids and LPC influences the immunological functions of oxLDL on DC, in part by regulating the PPAR pathway. By sensing the biochemical composition of lipoprotein particles, the innate immune system may thus identify various endogenous signals that influence the immune response during the acute-phase reaction. The therapeutic emulsion intralipid also blocks LPC action on PPAR activity and DC maturation. Intralipid may thus be an alternative therapeutic strategy for some chronic inflammatory diseases.

Glutathione--functions and metabolism in the malarial parasite Plasmodium falciparum

When present as a trophozoite in human erythrocytes, the malarial parasite Plasmodium falciparum exhibits an intense glutathione metabolism. Glutathione plays a role not only in antioxidative defense and in maintaining the reducing environment of the cytosol. Many of the known glutathione-dependent processes are directly related to the specific lifestyle of the parasite. Reduced glutathione (GSH) supports rapid cell growth by providing electrons for deoxyribonucleotide synthesis and it takes part in detoxifying heme, a product of hemoglobin digestion. Free radicals generated in the parasite can be scavenged in reaction sequences involving the thiyl radical GS* as well as the thiolate GS-. As a substrate of glutathione S-transferase, glutathione is conjugated to non-degradable compounds including antimalarial drugs. Furthermore, it is the coenzyme of the glyoxalase system which detoxifies methylglyoxal, a byproduct of the intense glycolysis taking place in the trophozoite. Proteins involved in GSH-dependent processes include glutathione reductase, glutaredoxins, glyoxalase I and II, glutathione S-transferases, and thioredoxins. These proteins, as well as the ATP-dependent enzymes of glutathione synthesis, are studied as factors in the pathophysiology of malaria but also as potential drug targets. Methylene blue, an inhibitor of the structurally known P. falciparum glutathione reductase, appears to be a promising antimalarial medication when given in combination with chloroquine.