Hemozoin and the human monocyte--a brief review of their interactions

In vitro, human monocytes avidly ingest hemozoin (HZ) that modifies a number of monocyte functions. Inhibitory effects: inhibition of: PMA-elicited respiratory burst, ability to killing and repeat phagocytosis, activity of NADPH-oxidase and PKC, expression of ICAM-1, integrin-CD11c, MHC-class-II (IFN-gamma-mediated), differentiation to functional, antigen-presenting dendritic cells. Stimulatory effects: increase in phagocytosis-related respiratory burst and accumulation of lipoperoxidation products; induction of metalloproteinase-9 and pro-inflammatory cytokines and chemokines. Mechanism of action: HZ generates by nonenzymatic catalysis large amounts of lipoperoxidation products, such as monohydroxy derivatives of arachidonic (HETE) and linoleic (HODE) acid, and 4-hydroxynonenal (HNE). Several HZ effects were reproduced by supplementation with plausible concentrations of HETE or HNE, the first most likely via interaction with PPAR-receptors, the second via adduct or crosslinks formation with critical targets.

Phagocytosis of malarial pigment haemozoin by human monocytes: a confocal microscopy study

Haem from host erythrocyte (RBC) haemoglobin is polymerized in the digestive organelle of Plasmodium falciparum to haemozoin (HZ), a crystaLline, insoluble substance. Human monocytes avidly ingest HZ that persists undigested for long periods of time, and generates potent bioactive lipid peroxide derivatives. Protein kinase C, an effector of signal transduction, phagolysosome formation and acidification, is inhibited in HZ-fed monocytes. Inability to digest HZ might derive from impairment in phagolysosome formation or acidification. Time-course and extent of HZ phagocytosis and acidification of phagolysosomes were studied by quantitative confocal microscopy. From 180 min until 72 h after the start of phagocytosis approximately 75-79% of the monocytes contained massive amounts of HZ. Coincidence between red (HZ) and green (acidic organelles) fluorescent compartments was very high. Confocal images showed that at 30-60 min after the start of phagocytosis, HZ was preferentially present as separated particles, with full co-localization of red and green fluorescence. Later on HZ-laden phagolysosomes tended to fuse together. In conclusion, phagolysosome formation and acidification were normal in HZ-fed monocytes during the 72-h observation time. The presence of HZ in the phagolysosome, the site of antigen processing, may offer a physical link with immunodepression in malaria.

15(S)-hydroxyeicosatetraenoic acid (15-HETE), a product of arachidonic acid peroxidation, is an active component of hemozoin toxicity to monocytes

Several studies have shown that human and murine hemozoin-fed phagocytes are functionally impaired. Unpurified hemozoin contains unspecifically attached unsaturated fatty acids such as arachidonic and linolenic acids. The presence in unpurified hemozoin of large quantities of ferric heme with small amounts of free iron makes hemozoin a generator of oxidative radicals capable of forming lipoperoxides or other breakdown products from polyunsaturated fatty acids. Here we show that delipidized hemozoin had reduced toxicity to monocytes. Phorbol myristate acetate (PMA)-elicited burst was poorly affected by delipidized hemozoin (ca. 17% and 21% burst inhibition by delipidized hemozoin vs ca. 75% and 65% burst inhibition by native hemozoin at 20 min or 17 h post-phagocytosis, respectively). Analysis of the lipid fraction isolated from native hemozoin by HPLC and chiral-phase HPLC showed equimolar amounts of 15(R)- and 15(S)-HETE (HETE, 15-hydroxy-6,8,11,13-eicosatetraenoic acid), most likely by-products of non-enzymatic peroxidation of arachidonic acid. The biologically active isomer, 15(S)-HETE, the product of 15-lipoxygenase, is a powerful mediator of inflammation and the effector of a large number of bioactions. 15(R,S)-HETE was found in native hemozoin (0.24 millimole/mole hemozoin heme), in supernatants of hemozoin-fed monocytes (87 nMol) and in hemozoin-fed monocytes (9.6 microMol). Approximately 84% of 15-HETE attached to hemozoin was in the esterified form. A large preponderance of esterified over free 15-HETE was also noted in supernatants of hemozoin-fed monocytes and in hemozoin-fed monocytes. In the latter cells, remarkable levels of the substance were attained. A dose-dependent curve of inhibition of PMA-elicited oxidative burst was observed. Assuming homogenuous distribution of 15-HETE in hemozoin-fed monocytes, 15(S)-HETE concentrations measured in hemozoin-fed monocytes (8 muMol) would bring about ca. 85% inhibition of PMA-elicited burst. In conclusion, derivatives of lipoperoxidation of unsaturated fatty acids such as 4-hydroxynonenal, 15-HETE and others now under study, appear to be relevant causes of hemozoin toxicity.

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

Human monocytes avidly ingest malarial pigment, hemozoin. Phagocytosed hemozoin persists in the monocyte for a long time and modifies important monocyte functions. Stability of phagocytosed hemozoin may depend on modifications of the hemozoin heme moiety or reduced ability to express heme-inducible heme oxygenase. We show here that the spectral characteristics of alkali-solubilized hemozoin were identical to those of authentic heme, although hemozoin was solubilized by alkali much more slowly than authentic heme. Alkali-solubilized hemozoin was a substrate of microsomal rat heme oxygenase and bilirubin reductase, with bilirubin as the main final product. Hemozoin feeding to human monocytes did not induce heme oxygenase, but authentic heme and alkali-solubilized hemozoin supplemented to hemozoin-fed monocytes induced heme oxygenase and were degraded normally. Lysosomes isolated from hemozoin-fed monocytes released only traces of heme while lysosomes from erythrocyte-fed monocytes liberated considerable quantities of heme. Lack of heme release from hemozoin did not depend on proteolysis-resistant, heme-binding proteins, since lysosomal proteases fully degraded hemozoin-associated proteins but did not solubilize hemozoin. In conclusion, our data indicate that lack of induction of HO1 is due to the intrinsic structural characteristics of hemozoin and not to hemozoin-mediated impairment of the mechanism of HO1 induction.

Phagocytosis of the malarial pigment, hemozoin, impairs expression of major histocompatibility complex class II antigen, CD54, and CD11c in human monocytes

In Plasmodium falciparum malaria, large proportions of resident macrophages and circulating monocytes and leukocytes contain massive amounts of the malarial pigment, hemozoin. Previous studies have shown that important functions (e.g., the generation of the oxidative burst, the ability to repeat phagocytosis, and protein kinase C activity) were severely impaired in hemozoin-loaded monocytes. Expression of membrane antigens directly involved in the immune response and in the phagocytic process, and/or under protein kinase C control, in hemozoin-loaded human monocytes was studied. Expression of major histocompatibility complex (MHC) class II after gamma interferon stimulation was blocked in hemozoin-loaded monocytes at the protein expression and gene transcription levels but was preserved in control monocytes loaded with opsonized latex beads or anti-D(Rho)-immunoglobulin G (IgG)-opsonized human erythrocytes. Expression of CD54 (intracellular adhesion molecule 1) and CD11c (p150,95 integrin) was also decreased in hemozoin-loaded monocytes. Expression of MHC class I, CD16 (low-affinity Fc receptor for aggregated IgG), CD32 (low-affinity Fc receptor for aggregated IgG), CD64 (high-affinity receptor for IgG), CD11b (receptor for complement component iC3b [CR3]), CD35 (receptor for complement components C3b and C4b [CR1]), and CD36 (non-class-A scavenger receptor) was not specifically affected by hemozoin loading. These results suggest that hemozoin loading may contribute to the impairment of the immune response and the derangement of antigen presentation reported in previous studies of P. falciparum malaria.

Malarial pigment (haemozoin): a very active 'inert' substance

Malarial pigment (haemozoin; HZ) is generally considered to be a non-toxic, high-molecular-weight storage form of undigested, toxic, host-haemoglobin haem. The available information on HZ indicates that it is a very heterozygous material. Its exact structure, in terms of constituent proteins (remnants of host globin v. parasite proteins), the type of linkage between the haem moieties (mu-oxo haem dimers further aggregated by non-covalent hydrophobic bonds v. mutually independent haematin monomers), iron status in the haem (penta-co-ordinated, high-spin ferriprotoporphyrin IX v. esa-co-ordinated, low-spin ferriprotoporphyrin IX), and compositions (beta-haematin-like structure without functionally relevant proteins or other constituents v. a ferriprotoporphyrin-IX core with aggregated proteins and phospholipids of host and parasite origin) remains a subject of controversy. When investigated by macrophages, HZ is not inert but affects a number of functional parameters. Crude pigment, as present in infected erythrocytes and shed after schizont rupture, may be considered the 'natural diet' ingested by macrophages in infected blood. It is a powerful source of radicals that may generate lipoperoxides and derived, toxic hydroxyaldehydes such as 4-hydroxynonenal (HNE). High concentrations of HNE, which have been detected in HZ-fed macrophages, inhibit protein kinase C (PKC). Complexes between HNE and PKC have also been detected in immunoprecipitated PKC from HZ-fed macrophages. HNE-mediated inhibition of PKC (and of other, as yet unidentified enzymes and processes) may explain HZ-mediated effects. HZ-mediated inhibition of NADPH-oxidase, the enzyme responsible for oxidative bursts, may only be partially explained by PKC inhibition. As Hz-laden human and murine macrophages produce increased amounts of tumour necrosis factor-alpha, interleukins 1 and 6, and macrophage inflammatory proteins 1 alpha and 1 beta, HZ-macrophage interactions may contribute to the cytokine-mediated manifestations of malaria.

Phagocytosis of malarial pigment hemozoin inhibits NADPH-oxidase activity in human monocyte-derived macrophages

Upon stimulation, inactive subunits of monocyte NADPH oxidase (NOX) are assembled in the membrane to generate the active enzyme responsible for oxidative burst. Phosphorylation of the 47 kDa NOX cytoplasmic subunit (47 kDa band) by protein kinase C (PKC) is important for NOX assembly and activation. Alternatively, NOX is activated in vitro by sodium dodecyl sulfate (SDS) or amphiphiles via a phosphorylation-independent mechanism. Previous data indicate that phagocytosis of malarial pigment hemozoin inhibits oxidative burst and PKC activity (Schwarzer, E., Turrini, F., Giribaldi, G., Cappadoro, M. and Arese, P. (1993) Biochim. Biophys. Acta, 1181, 51-54). We show here that SDS-stimulated NOX activity and phorbol 12-myristate 13-acetate (PMA)-induced oxidative burst dropped by 54% and 46% of control values 2 h after hemozoin phagocytosis, respectively. SDS-stimulated NOX activity remained roughly constant until 12 h, whereas oxidative burst dropped further by approx. 60% and 75% of control values 6 h and 12 h after hemozoin phagocytosis. Reconstitution experiments indicate that damage was localized to cytosolic NOX subunit(s). Membrane assembly of active NOX was defective in PMA-(PKC-dependent stimulation) and FMLP-(PKC-dependent and independent stimulation) stimulated hemozoin-fed monocytes. Labeling experiments with [32P]orthophosphate or [gamma-32P]ATP showed that endogenous PKC-dependent phosphorylation of the 47 kDa band was unaffected 12 h and impaired only 24 h after hemozoin phagocytosis. Thus, only long-term inhibition of NOX may additionally depend on superimposed PKC inhibition.

Increased levels of 4-hydroxynonenal in human monocytes fed with malarial pigment hemozoin. A possible clue for hemozoin toxicity

In human monocytes, lipoperoxides were increased 3-fold at 2 h, 6-fold at 5 h and 7.5-fold at 12 h after hemozoin phagocytosis. 4-Hydroxynonenal (HNE) was also increased, reaching 40 nmol/10(10) cells at 2 h (approximate intracellular concentration [AIE] 8 microM) 230 nmol/10(10) cells at 5 h (AIE 46 microM) and 79 nmol/10(10) cells (AIE 16 microM) at 12 h. A moderate increase in HNE, approx. 20 nmol/10(10) cells (AIE 4 microM) was also observed after phagocytosis of anti-D IgG-opsonized erythrocytes. HNE in unfed controls was approx. 5 nmol/10(10) cells (AIE 1 microM) during the whole incubation period. An increased amount of protein kinase C (PKC)/HNE adduct was demonstrated in hemozoin-fed monocytes. Purified PKC was profoundly inhibited at HNE > 10 microM. The impairment of PKC previously observed in hemozoin-fed monocytes can thus be explained by direct interaction with increased HNE levels.

A luminescence method for the quantitative determination of phagocytosis of erythrocytes, of malaria-parasitized erythrocytes and of malarial pigment

A method is described for the quantitative measurement of phagocytosis of human erythrocytes, malaria-parasitized erythrocytes and isolated malarial pigment by adherent human monocytes. The method utilizes measurement of haem-elicited luminescence both for the assay of ingested haemoglobin or malarial pigment haem and for the quantification of adherent monocytes. The latter is based on assay of luminescence elicited by cytochrome haem. The method utilizes the same low-cost reagents and equipment for assay of ingested haem and for quantification of adherent monocytes. The method is fast and extremely sensitive. The lower sensitivity limit is 500 monocytes and 10 RBC, or RBC equivalents in the case of malarial pigment, per assay. A detailed protocol with full calculations of a typical phagocytosis experiment of oxidatively damaged RBC, malarial pigment and control RBS is presented.

The involvement of hemozoin toxicity in depression of cellular immunity

Human monocyte-derived macrophages are severely impaired after ingestion of isolated malaria pigment (hemozoin) or hemozoin-containing red blood cells'. In this brief review, Franco Turrini, Evelin Schwarzer and Poolo Arese discuss possible pothophysiologicol consequences, outline unresolved problems and provide an outlook for future research.

Phagocytosis of P. falciparum malarial pigment hemozoin by human monocytes inactivates monocyte protein kinase C

Hemozoin (malarial pigment) is a ferriprotoporphyrin IX-rich hemoglobin degradation product present in parasitized RBC. Avidly phagocytosed hemozoin abolishes phagocyte TPA-induced oxidative burst. Membrane-associated PKC increased transiently in hemozoin-fed monocytes by 50% after 30 min and decreased irreversibly to 20% of initial value within 5 h after phagocytosis. Control RBC-fed monocytes showed transient decay of membrane-associated PKC followed by complete recovery 12 h after phagocytosis. Cytosolic PKC was not impaired within 12 h and diminished drastically 24 h after phagocytosis of hemozoin. Results are compatible with increased degradation of membrane-translocated PKC, possibly by iron/H2O2-mediated damage of cysteine-rich regulatory domains of PKC.

Impairment of macrophage functions after ingestion of Plasmodium falciparum-infected erythrocytes or isolated malarial pigment

Human monocyte-derived macrophages ingest diamide-treated red blood cells (RBC), anti-D immunoglobulin (Ig)G-opsonized RBC, or Plasmodium falciparum ring-stage parasitized RBC (RPRBC), degrade ingested hemoglobin rapidly, and can repeat the phagocytic cycle. Monocytes fed with trophozoite-parasitized RBC (TPRBC), which contain malarial pigment, or fed with isolated pigment are virtually unable to degrade the ingested material and to repeat the phagocytic cycle. Monocytes fed with pigment display a long-lasting oxidative burst that does not occur when they phagocytose diamide-treated RBC or RPRBC. The phorbol myristate acetate-elicited oxidative burst is irreversibly suppressed in monocytes fed with TPRBC or pigment, but not in monocytes fed with diamide-treated or IgG-opsonized RBC. This pattern of inhibition of phagocytosis and oxidative burst suggests that malarial pigment is responsible for the toxic effects. Pigment iron released in the monocyte phagolysosome may be the responsible element. 3% of total pigment iron is labile and easily detached under conditions simulating the internal environment of the phagolysosome, i.e., pH 5.5 and 10 microM H2O2. Iron liberated from pigment could account for the lipid peroxidation and increased production of malondialdehyde observed in monocytes fed with pigment or in RBC ghosts and liposomes incubated at pH 6.5 in presence of pigment and low amounts of H2O2. Removal of the labile iron fraction from pigment by repeated treatments with 0.1 mM H2O2 at pH 5.5 reduces pigment toxicity. It is suggested that iron released from ingested pigment is responsible for the intoxication of monocytes. In acute and chronic falciparum infections, circulating and tissue-resident phagocytes are seen filled with TPRBC and pigment particles over long periods of time. Moreover, human monocytes previously fed with TPRBC are unable to neutralize pathogenic bacteria, fungi, and tumor cells, and macrophage responses decline during the course of human and animal malaria. The present results may offer a mechanistic explanation for depression of cellular immunity in malaria.

Connection of electronic medical devices in ICU according to the standard 'MIB'

In the daily routine of an SICU, particularly in cardiac surgery, the nurses are to an increasing extent confronted with problems of connecting and using medical instruments as well as with documentation of the data of these instruments. An increasing number of user errors is to be expected. Use of advanced computer technology can help avoid such errors. In this paper the system 'ALODIN' is presented which is developed in our institutions according to the IEEE standard 'Medical Information Bus' (MIB). ALODIN-I will be part of a CCCS (computerized critical care system) which is installed in the clinic for cardiovascular surgery at Medical School Hannover.