An improved microassay for Plasmodium falciparum cytoadherence using stable transformants of Chinese hamster ovary cells expressing CD36 or intercellular adhesion molecule-1

Plasmodium falciparum Acetyl-CoA Synthetase Is Essential for Parasite Intraerythrocytic Development and Chromatin Modification

The malaria parasite Plasmodium falciparum possesses a unique Acetyl-CoA Synthetase (PfACS), which provides acetyl moieties for different metabolic and regulatory cellular pathways. We characterized PfACS and studied its role focusing on epigenetic modifications using the var gene family as reporter genes. For this, mutant lines to modulate plasmodial ACS expression by degron-mediated protein degradation and ribozyme-induced transcript decay were created. Additionally, an inhibitor of the human Acetyl-CoA Synthetase 2 was tested for its effectiveness in interfering with PfACS. The knockdown of PfACS or its inhibition resulted in impaired parasite growth. Decreased levels of PfACS also led to differential histone acetylation patterns, altered variant gene expression, and concomitantly decreased cytoadherence of infected red blood cells containing knocked-down parasites. Further, ChIP analysis revealed the presence of PfACS in many loci in ring stage parasites, underscoring its involvement in the regulation of chromatin. Due to its central function in the plasmodial metabolism and significant differences to human ACS, PfACS is an interesting target for drug development.

Blood outgrowth endothelial cells (BOECs) as a novel tool for studying adhesion of Plasmodium falciparum-infected erythrocytes

The lack of suitable animal models for the study of cytoadhesion of P. falciparum-infected erythrocytes (IEs) has necessitated in vitro studies employing a range of cell lines of either human tumour origin (e.g., BeWo and C32 cells) or non-human origin (e.g., CHO cells). Of the human cells available, many were isolated from adults, or derived from a pool of donors (e.g., HBEC-5i). Here we demonstrate, for the first time, the successful isolation of blood outgrowth endothelial cells (BOECs) from frozen stabilates of peripheral blood mononuclear cells obtained from small-volume peripheral blood samples from paediatric malaria patients. BOECs are a sub-population of human endothelial cells, found within the peripheral blood. We demonstrate that these cells express receptors such as Intercellular Adhesion Molecule 1 (ICAM-1/CD54), Endothelial Protein C Receptor (EPCR/CD201), platelet/endothelial cell adhesion molecule 1 (PECAM-1/CD31), Thrombomodulin (CD141), and support adhesion of P. falciparum IEs.

Antibody recognition of Plasmodium falciparum infected red blood cells by symptomatic and asymptomatic individuals in the Brazilian Amazon

In the Amazon Region, there is a virtual absence of severe malaria and few fatal cases of naturally occurring Plasmodium falciparum infections; this presents an intriguing and underexplored area of research. In addition to the rapid access of infected persons to effective treatment, one cause of this phenomenon might be the recognition of cytoadherent variant proteins on the infected red blood cell (IRBC) surface, including the var gene encoded P. falciparum erythrocyte membrane protein 1. In order to establish a link between cytoadherence, IRBC surface antibody recognition and the presence or absence of malaria symptoms, we phenotype-selected four Amazonian P. falciparum isolates and the laboratory strain 3D7 for their cytoadherence to CD36 and ICAM1 expressed on CHO cells. We then mapped the dominantly expressed var transcripts and tested whether antibodies from symptomatic or asymptomatic infections showed a differential recognition of the IRBC surface. As controls, the 3D7 lineages expressing severe disease-associated phenotypes were used. We showed that there was no profound difference between the frequency and intensity of antibody recognition of the IRBC-exposed P. falciparum proteins in symptomatic vs. asymptomatic infections. The 3D7 lineages, which expressed severe malaria-associated phenotypes, were strongly recognised by most, but not all plasmas, meaning that the recognition of these phenotypes is frequent in asymptomatic carriers, but is not necessarily a prerequisite to staying free of symptoms.

Fucosylated chondroitin sulfate inhibits Plasmodium falciparum cytoadhesion and merozoite invasion

Sequestration of Plasmodium falciparum-infected erythrocytes (Pf-iEs) in the microvasculature of vital organs plays a key role in the pathogenesis of life-threatening malaria complications, such as cerebral malaria and malaria in pregnancy. This phenomenon is marked by the cytoadhesion of Pf-iEs to host receptors on the surfaces of endothelial cells, on noninfected erythrocytes, and in the placental trophoblast; therefore, these sites are potential targets for antiadhesion therapies. In this context, glycosaminoglycans (GAGs), including heparin, have shown the ability to inhibit Pf-iE cytoadherence and growth. Nevertheless, the use of heparin was discontinued due to serious side effects, such as bleeding. Other GAG-based therapies were hampered due to the potential risk of contamination with prions and viruses, as some GAGs are isolated from mammals. In this context, we investigated the effects and mechanism of action of fucosylated chondroitin sulfate (FucCS), a unique and highly sulfated GAG isolated from the sea cucumber, with respect to P. falciparum cytoadhesion and development. FucCS was effective in inhibiting the cytoadherence of Pf-iEs to human lung endothelial cells and placenta cryosections under static and flow conditions. Removal of the sulfated fucose branches of the FucCS structure virtually abolished the inhibitory effects of FucCS. Importantly, FucCS rapidly disrupted rosettes at high levels, and it was also able to block parasite development by interfering with merozoite invasion. Collectively, these findings highlight the potential of FucCS as a candidate for adjunct therapy against severe malaria.

On cytoadhesion of Plasmodium vivax: raison d'être?

It is generally accepted that Plasmodium vivax, the most widely distributed human malaria parasite, causes mild disease and that this species does not sequester in the deep capillaries of internal organs. Recent evidence, however, has demonstrated that there is severe disease, sometimes resulting in death, exclusively associated with P. vivax and that P. vivax-infected reticulocytes are able to cytoadhere in vitro to different endothelial cells and placental cryosections. Here, we review the scarce and preliminary data on cytoadherence in P. vivax, reinforcing the importance of this phenomenon in this species and highlighting the avenues that it opens for our understanding of the pathology of this neglected human malaria parasite.

Spleen modulation of cytoadherence properties of Plasmodium falciparum

Cytoadherence to CD36 and ICAM-1 and var gene expression of P. falciparum parasites from a splenectomized patient were studied. These parasites lacked cytoadherence and showed expression of a non-coding cDNA sequence. Hence the spleen's modulation of parasite cytoadherence by mechanisms that affect the parasite's surface antigen expression is suggested.

Microfluidic modeling of cell-cell interactions in malaria pathogenesis

The clinical outcomes of human infections by Plasmodium falciparum remain highly unpredictable. A complete understanding of the complex interactions between host cells and the parasite will require in vitro experimental models that simultaneously capture diverse host-parasite interactions relevant to pathogenesis. Here we show that advanced microfluidic devices concurrently model (a) adhesion of infected red blood cells to host cell ligands, (b) rheological responses to changing dimensions of capillaries with shapes and sizes similar to small blood vessels, and (c) phagocytosis of infected erythrocytes by macrophages. All of this is accomplished under physiologically relevant flow conditions for up to 20 h. Using select examples, we demonstrate how this enabling technology can be applied in novel, integrated ways to dissect interactions between host cell ligands and parasitized erythrocytes in synthetic capillaries. The devices are cheap and portable and require small sample volumes; thus, they have the potential to be widely used in research laboratories and at field sites with access to fresh patient samples.

Differences in human antibody reactivity to Plasmodium falciparum variant surface antigens are dependent on age and malaria transmission intensity in northeastern Tanzania

Plasmodium falciparum variant surface antigens (VSA) are involved in the pathogenesis of malaria. Immunoglobulin G (IgG) with specificity for VSA (anti-VSA IgG) is therefore considered important for acquired immunity. To better understand the nature and dynamics of variant-specific IgG responses at population level, we conducted an immunoepidemiological study in nearby communities in northeastern Tanzania, situated at different altitudes and therefore exposed to different levels of P. falciparum transmission intensity. Samples of plasma and infected red blood cells (IRBC) were collected from 759 individuals aged 0 to 19 years. Plasma levels of IgG with specificity for VSA expressed by a panel of different parasite isolates were measured by flow cytometry, while the ability of plasma to inhibit IRBC adhesion to CD36 was examined in cellular assays. The level and repertoire of the heterologous anti-VSA IgG response developed dramatically in individuals at 1 to 2 years of age in the high-transmission area, reaching a maximum level at around 10 years of age; only a modest further increase was observed among older children and adults. In contrast, at lower levels of malaria transmission, anti-VSA IgG levels were lower and the repertoire was more narrow, and similar age- and transmission-dependent differences were observed with regard to the ability of the plasma samples to inhibit adhesion of IRBC to CD36. These differences indicate a strong and dynamic relationship between malaria exposure and functional characteristics of the variant-specific antibody response, which is likely to be important for protection against malaria.

Var transcription profiling of Plasmodium falciparum 3D7: assignment of cytoadherent phenotypes to dominant transcripts

BACKGROUND

Cytoadherence of Plasmodium falciparum-infected red blood cells is mediated by var gene-encoded P. falciparum erythrocyte membrane protein-1 and host receptor preference depends in most cases on which of the 50-60 var genes per genome is expressed. Enrichment of phenotypically homogenous parasites by panning on receptor expressing cells is fundamental for the identification of the corresponding var transcript.

METHODS

P. falciparum 3D7 parasites were panned on several transfected CHO-cell lines and their var transcripts analysed by i) reverse transcription/PCR/cloning/sequencing using a universal DBLalpha specific oligonucleotide pair and ii) by reverse transcription followed by quantitative PCR using 57 different oligonucleotide pairs.

RESULTS

Each cytoadherence selected parasite line also adhered to untransfected CHO-745 cells and upregulation of the var gene PFD995/PFD1000c was consistently associated with cytoadherence to all but one CHO cell line. In addition, parasites panned on different CHO cell lines revealed candidate var genes which reproducibly associated to the respective cytoadherent phenotype. The transcription profile obtained by RT-PCR/cloning/sequencing differed significantly from that of RT-quantitative PCR.

CONCLUSION

Transfected CHO cell lines are of limited use for the creation of monophenotypic cytoadherent parasite lines. Nevertheless, 3D7 parasites can be reproducibly selected for the transcription of different determined var genes without genetic manipulation. Most importantly, var transcription analysis by RT-PCR/cloning/sequencing may lead to erroneous interpretation of var transcription profiles.

Sialic acid is a cellular receptor for coxsackievirus A24 variant, an emerging virus with pandemic potential

Binding to target cell receptors is a critical step in the virus life cycle. Coxsackievirus A24 variant (CVA24v) has pandemic potential and is a major cause of acute hemorrhagic conjunctivitis, but its cellular receptor has hitherto been unknown. Here we show that CVA24v fails to bind to and infect CHO cells defective in sialic acid expression. Binding of CVA24v to and infection of corneal epithelial cells are efficiently inhibited by treating cells with a sialic acid-cleaving enzyme or sialic acid-binding lectins and by treatment of the virus with soluble, multivalent sialic acid. Protease treatment of cells efficiently inhibited virus binding, suggesting that the receptor is a sialylated glycoprotein. Like enterovirus type 70 and influenza A virus, CVA24v can cause pandemics. Remarkably, all three viruses use the same receptor. Since several unrelated viruses with tropism for the eye use this receptor, sialic acid-based antiviral drugs that prevent virus entry may be useful for topical treatment of such infections.

The impact of IgG antibodies to recombinant Plasmodium falciparum 732var CIDR-1alpha domain in mothers and their newborn babies

Different domains of a novel full-length var gene (termed 732var) isolated from a placenta of a malaria-infected woman were expressed in Escherichia coli as recombinant proteins and analysed biochemically and immunologically. Two of these, the Duffy binding-like (DBL)-3gamma domain and the cysteine-rich interdomain region (CIDR)-1alpha were able to bind chondroitin sulfate A and CD36, respectively. The DBL-3gamma domain was investigated in a previous study and confirmed here to exhibit anti-disease characteristics related to pregnancy-associated malaria. Mothers with high anti-DBL-3gamma antibody levels were protected from placental infection. The novel finding in this study is that babies born to mothers carrying anti-CIDR-1alpha antibodies had a delayed time to the first infection.

Adherence of Plasmodium falciparum infected erythrocytes to CHO-745 cells and inhibition of binding by protein A in the presence of human serum

Adhesion of erythrocytes infected with the malaria parasite Plasmodium falciparum to human host receptors is a process associated with severe malarial pathology. A number of in vitro cell lines are available as models for these adhesive processes, including Chinese hamster ovary (CHO) cells which express the placental adhesion receptor chondroitin-4-sulphate (CSA) on their surface. CHO-745 cells, a glycosaminoglycan-negative mutant CHO cell line lacking CSA and other reported P. falciparum adhesion receptors, are often used for recombinant expression of host receptors and for receptor binding studies. In this study we show that P. falciparum-infected erythrocytes can be easily selected for adhesion to an endogenous receptor on the surface of CHO-745 cells, bringing into question the validity of using these cells as a tool for P. falciparum adhesin expression studies. The adhesive interaction between CHO-745 cells and parasitized erythrocytes described here is not mediated by the known P. falciparum adhesion receptors CSA, CD36, or ICAM-1. However, we found that CHO-745-selected parasitized erythrocytes bind normal human IgM and that adhesion to CHO-745 cells is inhibited by protein A in the presence of serum, but not in its absence, indicating a non-specific inhibitory effect. Thus, protein A, which has been used as an inhibitor for a recently described interaction between infected erythrocytes and the placenta, may not be an appropriate in vitro inhibitor for understanding in vivo adhesive interactions.

Dimeric sesquiterpenoids isolated from Chloranthus japonicus inhibited the expression of cell adhesion molecules

In the search for cell adhesion inhibitors from natural sources, three active compounds were isolated from Chloranthus japonicus Sieb. (Chloranthaceae) roots. The compounds were identified as dimeric sesquiterpenoids of shizukaol B (1), cycloshizukaol A (2) and shizukaol F (3). These compounds inhibited PMA-induced homotypic aggregation of HL-60 cells without cytotoxicity with MIC values of 34.1 nM (1), 0.9 microM (2) and 27.3 nM (3), respectively. Although 1-3 did not affect the direct binding of LFA-1 to ICAM-1, these compounds markedly inhibited ICAM-1 expression in HL-60 cells in a dose-dependent fashion. On the other hand, when HUVEC were pretreated with 1-3 and stimulated with TNF-alpha, adhesion of THP-1 cells to HUVEC decreased in dose-dependent manner with IC(50) values of 54.6 nM, 1.2 microM and 34.1 nM, respectively. In fact, 1 inhibited TNF-alpha-induced surface expression of the ICAM-1, VCAM-1 and E-selectin in HUVEC with IC(50) values of 5.4 nM, 13.6 microM and 95.6 nM, respectively. The present findings suggest that 1-3 prevent monocyte adhesion to HUVEC through the inhibition of cell adhesion molecules expression stimulated by TNF-alpha.

Chondroitin sulphate A as an adherence receptor for Plasmodium falciparum-infected erythrocytes

Until recently, the sequestration of erythrocytes infected with Plasmodium falciparum has been thought to be due to one of a number of protein-protein interactions. In this article, Stephen Rogerson and Graham Brown summarize the emerging evidence that, in vitro, infected erythrocytes can also adhere to the glycosaminoglycan chondroitin sulphate A (CSA) expressed on the surface of cells and immobilized on plastic. In vivo, binding of infected erythrocytes to CSA could be crucial to the development of malarial infection of the placenta, and possibly to sequestration in the lung and brain. The consequences of this may include maternal morbidity and mortality, low birth weight in the infant, pulmonary oedema and cerebral malaria. They discuss the need to characterize the molecular basis of this interaction, and to investigate the possible therapeutic role of CSA in malaria. Chondroitin sulphates are nontoxic compounds already in use for other diseases in humans. Vaccines based on inhibiting this receptor-ligand interaction could also be appropriate.

Entamoeba histolytica: identification of a distinct beta2 integrin-like molecule with a potential role in cellular adherence

Entamoeba histolytica infection causes dysentery, intestinal colitis, and hepatic abscess in an estimated 50 million people worldwide. Attachment of E. histolytica trophozoites to intestinal epithelium and vascular endothelium during liver metastasis results in an inflammatory process. We report the identification of a distinct amebic beta2 integrin (CD18)-like molecule which affords adherence to TNF-alpha-activated endothelial cells. Data from flow cytometry and indirect immunofluorescence assays suggest the amebic beta2 integrin was localized to focal adhesion plates and was present in both E. histolytica and Entamoeba dispar. The amebic beta2 integrin appeared to be distinct from the amebic Gal/GalNAc lectin based on recombinant expression, amebic colocalization, and ELISA studies. Trophozoite adherence to endothelial cells expressing ICAM-1 (CD54) following activation with TNF-alpha or ICAM-1-transfected CHO cells was specifically inhibited with anti-CD18 or anti-CD54 MAbs. In summary, evidence in support of a distinct beta2 integrin-like molecule participating in amebic adherence to TNF-alpha-activated endothelial cells expressing ICAM-1 is presented. The presence of integrin-dependent binding may allow trophozoites to opportunistically adhere to activated intestinal epithelium or vascular endothelium expressing ICAM-1 during amebic colitis or hepatic abscess.

In vitro selection of Plasmodium falciparum 3D7 for expression of variant surface antigens associated with severe malaria in African children

P. falciparum-infected red blood cells (IRBC) can adhere to endothelial host receptors through parasite-encoded, clonally variant surface antigens (VSA). The VSA-mediated IRBC adhesion and the acquired VSA-specific antibody response have both been linked to IRBC organ tropism and disease severity. Parasites isolated from young children with severe malaria (SM) tend to express a limited and conserved set of VSA (VSASM) that are both stronger and more commonly recognized by IgG in the plasma of malaria-exposed individuals than VSA (VSAUM) expressed by parasites causing uncomplicated malaria (UM) in older semi-immune children. Establishment of the genetic mechanism underlying changes in VSA expression in response to in vitro selective pressure is now possible because of the availability of the entire genomic sequence of the P. falciparum clone 3D7. As a first step towards direct molecular identification of VSASM-encoding genes in 3D7, we report here a method of enforcing expression of VSASM-like antigens in this parasite clone by a novel selection method using plasma from semi-immune children with low VSAUM-specific, but high VSASM-specific, IgG reactivity. In addition to the resulting increase in VSA-specific IgG recognition, VSASM-expressing 3D7(3D7-Dodowa1) showed reduced adhesion to CD36. Finally, levels of IgG specific for the VSA expressed by 3D7-Dodowa1 were uniformly higher than those of IgG with specificity for VSA expressed by the unselected 3D7 in plasma samples from geographically and epidemiologically diverse areas of endemic parasite transmission. The described selection method appears a useful tool in the identification of genes encoding VSA involved in severe and life-threatening P. falciparum malaria.

Cytotoxic and antioxidant activities of alkylated benzoquinones from Maesa lanceolata

The natural and semi-synthetic analogs of substituted 1,4-benzoquinones were evaluated for in vitro cytotoxic and antioxidant activities. Maesanin, dihydromaesanin, maesanin dimethyl ether and isomeric mixtures of 3-[(Z)-10'-pentadecenyl]-benzoquinone derivatives exhibited cytotoxic activity against HL-60 cell line (IC50 values 4.5, 2.2, 0.43 and 2.8 microg/mL, respectively), while it was found to be inactive against ROS (Reactive Oxygen Species) generation in HL-60. In contrast, the isomeric acylated benzoquinones with shorter alkyl substituents, namely, 2-acetoxy-5-hydoxy-6-methyl-3-tridecyl-1,4-benzoquinone and 2-hydoxy-5-acetoxy-6-methyl-3-tridecyl-1,4-benzoquinone showed most prominent antioxidant and antiproliferative effect on HL-60 (IC50 values 6.2 and 2.2 microg/mL, respectively), as well as cytotoxicities against SK-MEL, KB, BT-549 and SK-OV-3 carcinomas (IC50 values <1.1-4.2 microg/mL). All benzoquinones were found to be inactive against cell aggregation and cell adhesion assays, thus showing no effect on immune responses and inflammation.

Pathogenesis of cerebral malaria: recent experimental data and possible applications for humans

Malaria still is a major public health problem, partly because the pathogenesis of its major complication, cerebral malaria, remains incompletely understood. Experimental models represent useful tools to better understand the mechanisms of this syndrome. Here, data generated by several models are reviewed both in vivo and in vitro; we propose that some pathogenic mechanisms, drawn from data obtained from experiments in a mouse model, may be instrumental in humans. In particular, tumor necrosis factor (TNF) receptor 2 is involved in this syndrome, implying that the transmembrane form of TNF may be more important than the soluble form of the cytokine. It has also been shown that in addition to differences in immune responsiveness between genetically resistant and susceptible mice, there are marked differences at the level of the target cell of the lesion, namely, the brain endothelial cell. In murine cerebral malaria, a paradoxical role of platelets has been proposed. Indeed, platelets appear to be pathogenic rather than protective in inflammatory conditions because they can potentiate the deleterious effects of TNF. More recently, it has been shown that interactions among platelets, leukocytes, and endothelial cells have phenotypic and functional consequences for the endothelial cells. A better understanding of these complex interactions leading to vascular injury will help improve the outcome of cerebral malaria.

Characterization of peroxynitrite-oxidized low density lipoprotein binding to human CD36

Peroxynitrite-mediated oxidation may be an important physiological mechanism for oxidation of low density lipoprotein (LDL), however, the molecular basis for the interaction of peroxynitrite oxidized LDL (OxLDL) with scavenger receptors such as CD36, has not been characterized. In this study, we compared the biochemical characteristics and receptor binding of LDL that was oxidized using: (1) Cu2+, a standard method of oxidizing LDL in vitro; and (2) 3-morpholinosydnonimine (SIN-1), a source of peroxynitrite. Both methods of oxidation caused an increase in electrophoretic migration of LDL, but greater mobility was observed with Cu2+-OxLDL. In addition, greater fragmentation of apolipoprotein B was observed following Cu2+ oxidation than after SIN-1 oxidation. The levels of lipid peroxides and thiobarbituric acid reactive substances were similar after 20 h of oxidation by both methods, although the time-course was distinct. Cu2+ and SIN-1-OxLDL bound specifically to the macrophage scavenger receptor CD36 with high affinity. Binding of the 20 h SIN-1 treated LDL to CD36 was comparable to a 4 h Cu2+ modified LDL. The binding of Cu2+ and SIN-1-OxLDL to CD36 was similar under different biochemical conditions and modifications of the receptor, suggesting that OxLDL particles, generated by either method, bind to the same domain of CD36. The results demonstrate that SIN-1 produced an oxidized LDL particle that binds specifically to CD36 and suggests that peroxynitrite OxLDL may represent a more physiologically relevant model than Cu2+-OxLDL for studying the interactions of OxLDL with cells and lipoprotein receptors in vitro.

Plasma antibodies from malaria-exposed pregnant women recognize variant surface antigens on Plasmodium falciparum-infected erythrocytes in a parity-dependent manner and block parasite adhesion to chondroitin sulfate A

In areas of intense Plasmodium falciparum transmission, clinical immunity is acquired during childhood, and adults enjoy substantial protection against malaria. An exception to this rule is pregnant women, in whom malaria is both more prevalent and severe than in nonpregnant women. Pregnancy-associated malaria (PAM) in endemic areas is concentrated in the first few pregnancies, indicating that protective immunity to PAM is a function of parity. The placenta is often heavily infected in PAM, and placental parasites show a striking preference for chondroitin sulfate A (CSA) as an adhesion receptor. Plasma Abs from malaria-exposed multiparous women are able to interfere with binding of P. falciparum parasites to CSA in vitro, and acquisition of Abs interfering with CSA-specific parasite sequestration thus appears to be a critical element in acquired protection against PAM. Here we show that adults from an area of hyperendemic P. falciparum transmission generally possessed low levels of Abs specifically recognizing surface Ags expressed by a CSA-adhering parasite isolate, while unselected isolates were well recognized. In marked contrast, most third-trimester pregnant women from that area had very high plasma levels of such Abs. Plasma levels of Abs specifically recognizing the CSA-adhering isolate strongly depended on parity, whereas recognition of CSA-nonadhering isolates did not. Finally, we demonstrate a clear correlation between plasma levels of Abs recognizing the CSA-specific isolate and the ability to interfere with its sequestration to CSA in vitro. Our study supports the hypothesis that Abs inhibiting CSA-specific parasite sequestration are important in acquisition of protection against PAM.

Plasmodium falciparum: cloned and expressed CIDR domains of PfEMP1 bind to chondroitin sulfate A

Adherence of erythrocytes infected with mature asexual Plasmodium falciparum parasites (iRBC) to microvascular endothelial cells contributes to the pathology of P. falciparum malaria. It has been shown that the variant P. falciparum erythrocyte membrane protein 1 (PfEMP1) confers adhesion to a wide range of cell surface receptors. Previously, the cysteine-rich interdomain region (CIDR) of PfEMP1 has been identified as binding site to CD36. We provide evidence that the same region can also mediate binding to chondroitin sulfate A (CSA). CIDR domains of two different parasite strains were expressed in Escherichia coli as a 6xHis-tagged protein. Purified recombinant protein bound to Chinese hamster ovary (CHO) cells which naturally express chondroitin sulfate A. Treatment of wild-type CHO cells with chondroitinase ABC reduced binding up to 94.4%. Competitive binding using soluble CSA inhibited binding to CHO cells by up to 100% at 2 mg/ml and by 62.4% at 0.5 mg/ml, whereas 1 mg/ml heparan sulfate had only a little effect (18.1%). In contrast, a recombinant 6xHis-tagged DBL1 domain showed no binding to wild-type CHO cells. Such an approach of analyzing various domains of PfEMP1 as recombinant proteins may elucidate their functions and may lead to novel anti-adherence therapeutics, especially for maternal malaria infections.

Ex vivo desequestration of Plasmodium falciparum-infected erythrocytes from human placenta by chondroitin sulfate A

We performed ex vivo experiments with Plasmodium falciparum-infected human placentas from primi- and multigravida women from Cameroon. All women, independent of their gravida status, had anti-chondroitin sulfate A (CSA) adhesion antibodies which cross-reacted with heterologous strains, such as FCR3 and Palo-Alto(FUP)1, which were selected for CSA binding. These antibodies, directed against the surface of infected erythrocytes obtained by flushing with CSA (IRBC(CSA)), were restricted to the immunoglobulin G3 isotypes. Massive desequestration of parasites was achieved with soluble CSA but not with anti-ICAM-1 and anti-CD36 monoclonal antibodies. All of the CSA-flushed parasites were analyzed immediately by using in vitro assays of binding to Saimiri brain endothelial cells (SBEC) expressing various adhesion receptors. Parasites derived from all six placentas displayed the CSA adhesion phenotype. However, only partial inhibition of adhesion was observed in the presence of soluble CSA or when Sc1D SBEC were treated with chondroitinase ABC. These results suggest that an additional adhesive molecule of IRBC(CSA) which binds to an unidentified receptor is present in the placenta. This new phenotype was lost once the parasites adapted to in vitro culture. We observed additional differences in the CSA adhesion phenotype between placental parasites and in vitro-cultured parasites panned on endothelial cells carrying CSA. The minimum size of fractionated CSA required for a significant inhibition of placental IRBC(CSA) adhesion to Sc1D cells was 1 to 2 kDa, which contrasts with the 4-kDa size necessary to reach equivalent levels of inhibition with panned IRBC(CSA) of this phenotype. All placental IRBC(CSA) cytoadhered to Sc17 SBEC, which express only the CSA receptor. Panning of IRBC(CSA) on these cells resulted in a significant quantitative increase of IRBC cytoadhering to the CSA of Sc1D cells but did not change their capacity for adhesion to CSA on normal placenta cryosections. Our results indicate that the CSA binding phenotype is heterogeneous and that several distinct genes may encode P. falciparum-CSA ligands with distinct binding properties.

A high capacity in vitro assay for measuring the cytoadherence of Plasmodium falciparum-infected erythrocytes

A simple, efficient, sensitive, reproducible and high throughput assay for measuring the cytoadhesion of Plasmodium falciparum-infected red blood cells (human malaria) is described. The assay format uses 96-well microplates, with the number of P. falciparum parasitized erythrocytes bound determined by measuring Plasmodium specific lactic dehydrogenase activity colorimetrically (absorbance at 655 nm) using the 3-acetylpyridine analog of nicotinamide adenine dinucleotide, nitro blue tetrazolium and diaphorase. The results of the described microplate assay were found to be comparable to those using microscopic analysis but much less time consuming.

CD36 Peptides That Block Cytoadherence Define the CD36 Binding Region for Plasmodium falciparum-Infected Erythrocytes

Mature Plasmodium falciparum parasitized erythrocytes (PE) sequester from the circulation by adhering to microvascular endothelial cells. PE sequestration contributes directly to the virulence and severe pathology of falciparum malaria. The scavenger receptor, CD36, is a major host receptor for PE adherence. PE adhesion to CD36 is mediated by the malarial variant antigen, P. falciparumerythrocyte membrane protein 1 (PfEMP1), and particularly by its cysteine-rich interdomain region 1 (CIDR-1). Several peptides from the extended immunodominant domain of CD36 (residues 139-184), including CD36 139-155, CD36 145-171, CD36 146-164, and CD36 156-184 interfered with the CD36-PfEMP1 interaction. Each of these peptides affected binding at the low micromolar range in 2 independent assays. Two peptides, CD36 145-171 and CD36 156-184, specifically blocked PE adhesion to CD36 without affecting binding to the host receptor intercellular adhesion molecule-1 (ICAM-1). Moreover, an adhesion blocking peptide from the ICAM-1 sequence inhibits the PfEMP1–ICAM-1 interaction without affecting adhesion to CD36. These results confirm earlier observations that PfEMP1 is also a receptor for ICAM-1. Thus, the region 139-184 and particularly the 146-164 or the 145-171 regions of CD36 form the adhesion region for P. falciparum PE. Adherence blocking peptides from this region may be useful for modeling the PE/PfEMP1 interaction with CD36 and for development of potential anti-adhesion therapeutics.

CD36 Peptides That Block Cytoadherence Define the CD36 Binding Region for Plasmodium falciparum-Infected Erythrocytes

Mature Plasmodium falciparum parasitized erythrocytes (PE) sequester from the circulation by adhering to microvascular endothelial cells. PE sequestration contributes directly to the virulence and severe pathology of falciparum malaria. The scavenger receptor, CD36, is a major host receptor for PE adherence. PE adhesion to CD36 is mediated by the malarial variant antigen, P. falciparumerythrocyte membrane protein 1 (PfEMP1), and particularly by its cysteine-rich interdomain region 1 (CIDR-1). Several peptides from the extended immunodominant domain of CD36 (residues 139-184), including CD36 139-155, CD36 145-171, CD36 146-164, and CD36 156-184 interfered with the CD36-PfEMP1 interaction. Each of these peptides affected binding at the low micromolar range in 2 independent assays. Two peptides, CD36 145-171 and CD36 156-184, specifically blocked PE adhesion to CD36 without affecting binding to the host receptor intercellular adhesion molecule-1 (ICAM-1). Moreover, an adhesion blocking peptide from the ICAM-1 sequence inhibits the PfEMP1–ICAM-1 interaction without affecting adhesion to CD36. These results confirm earlier observations that PfEMP1 is also a receptor for ICAM-1. Thus, the region 139-184 and particularly the 146-164 or the 145-171 regions of CD36 form the adhesion region for P. falciparum PE. Adherence blocking peptides from this region may be useful for modeling the PE/PfEMP1 interaction with CD36 and for development of potential anti-adhesion therapeutics.

Inhibitory Activity of Human Lactoferrin and Its Peptide on Chondroitin Sulfate A-, CD36-, and Thrombospondin-Mediated Cytoadherence ofPlasmodium falciparum–Infected Erythrocytes

Lactoferrin (LF), a human serum protein, strongly inhibited the adherence of Plasmodium falciparum–infected erythrocytes (PE) to immobilized chondroitin sulfate A (CSA)–conjugated albumin at a concentration of 100 μg/mL and blocked the PE binding to CD36-expressing Chinese hamster ovary (CHO) cells, as well as immobilized CD36 at concentrations of 5 μg/mL and 100 μg/mL, respectively. Biotinylated LF bound to CD36 in a saturable manner, and such binding was inhibited by unlabeled LF and the anti-CD36 monoclonal antibody, 8A6, suggesting specificity of binding. Additionally, LF inhibited PE binding to immobilized thrombospondin (TSP) at a concentration of 100 μg/mL, and specific binding of LF to TSP was confirmed using biotinylated LF. LF inhibited PE binding to C32 amelanotic melanoma cells in a dose-dependent manner. A peptide of LF, Arg-Asn-Met Arg-Lys-Val Arg-Gly-Pro-Pro-Val-Ser-Cys (amino acid residues 25-37 of LF), which has been suggested to contribute to LF binding to various materials, including CSA, inhibited PE binding to immobilized CSA-conjugated albumin, immobilized CD36, CD36-expressing CHO cells, immobilized TSP, and C32 amelanotic melanoma cells, as well as LF itself. These results suggest that LF peptide may provide the basis for developing agents that are able to inhibit CSA-, CD36-, and TSP-mediated cytoadherence of PE.

An in vitro microassay to assess the ability of Plasmodium falciparum-infected erythrocytes to bind to the human syncytiotrophoblast

PROBLEM

Erythrocytes parasitized by matures stages of Plasmodium falciparum are frequently sequestered in human placenta. The consequences of this sequestration have been well described, but little is known about the mechanisms used by the parasite to concentrate in the placenta.

METHOD OF STUDY

We developed an in vitro assay to study their binding capacity to the human syncytiotrophoblast. Our cytoadherence test was scaled down, and each step of the assay was optimized to enhance the sensitivity of our model.

RESULTS

Cytoadherence assays between P. falciparum-infected erythrocytes and human trophoblasts are easily performed using low numbers of trophoblast cells. The process can also be used to carry out immunofluorescence and immunostaining techniques.

CONCLUSIONS

The test may be adapted to any kind of cell, is inexpensive, and allows the culture of virtually any kind of cell for several weeks.

Biological and biochemical characteristics of cytoadhesion of Plasmodium falciparum-infected erythrocytes to chondroitin-4-sulfate

The cytoadhesion of Plasmodium falciparum laboratory strains and clones to Saimiri brain microvascular endothelial cells (SBEC 17), with chondroitin-4-sulfate (CSA) as the only adhesion receptor, was tested. Only one strain had significant cytoadhesion. However, CSA-specific infected erythrocytes (IRBCs) were detected in all strains after selection of a CSA-specific subpopulation by culturing the few adherent IRBCs. This demonstrates the lack of sensitivity of cytoadhesion microassays for detecting small quantities of CSA-specific IRBCs in cultures or field isolates. Cytoadhesion to CSA is maximal at 24 h of the cycle and decreases with the onset of schizogony, reaching a minimum just before reinvasion. This fluctuation must be taken into account in comparisons of the cytoadhesion of different strains or isolates. The minimum size of CSA for active inhibition was 4 kDa, and a mass of 9 kDa was required for inhibition similar to that obtained with the 50-kDa CSA. In contrast to cytoadhesion to CSA, which is pH independent or maximal at physiological pH (depending on the target endothelial cells), adhesion to CD36 and intercellular adhesion molecule 1 was pH dependent, requiring acidic conditions to be maximal in all cases. Cytoadhesion to CSA may trigger the occlusion of microvessels and cause the acidosis necessary for the other receptors to be fully efficient. If this key role in the mechanisms of sequestration were to be confirmed in vivo, prevalence studies of the CSA cytoadhesion phenotype would have to be reevaluated, because simple cytoadhesion assays do not detect CSA-specific parasites present in very low numbers, and these parasites might then be undetected in the peripheral blood but present in organs in which sequestration occurs, such as the placenta (M. Fried and P. E. Duffy, Science 272:1502-1504, 1996).

Antigenic variation in malaria: in situ switching, relaxed and mutually exclusive transcription of var genes during intra-erythrocytic development in Plasmodium falciparum

Members of the Plasmodium falciparum var gene family encode clonally variant adhesins, which play an important role in the pathogenicity of tropical malaria. Here we employ a selective panning protocol to generate isogenic P.falciparum populations with defined adhesive phenotypes for CD36, ICAM-1 and CSA, expressing single and distinct var gene variants. This technique has established the framework for examining var gene expression, its regulation and switching. It was found that var gene switching occurs in situ. Ubiquitous transcription of all var gene variants appears to occur in early ring stages. However, var gene expression is tightly regulated in trophozoites and is exerted through a silencing mechanism. Transcriptional control is mutually exclusive in parasites that express defined adhesive phenotypes. In situ var gene switching is apparently mediated at the level of transcriptional initiation, as demonstrated by nuclear run-on analyses. Our results suggest that an epigenetic mechanism(s) is involved in var gene regulation.

Inhibition of binding of malaria-infected erythrocytes by a tetradecasaccharide fraction from chondroitin sulfate A

Adherence of parasite-infected erythrocytes (IEs) to the microvascular endothelium of various organs, a process known as sequestration, is a feature of Plasmodium falciparum malaria. This event is mediated by specific adhesive interactions between parasite proteins, expressed on the surface of IEs, and host molecules. P. falciparum IEs can bind to purified chondroitin sulfate A (CS-A), to the proteoglycan thrombomodulin through CS-A side chains, and to CS-A present on the surface of brain and lung endothelial cells and placental syncytiotrophoblasts. In order to identify structural characteristics of CS-A important for binding, oligosaccharide fragments ranging in size from 2 to 20 monosaccharide units were isolated from CS-A and CS-C, following controlled chondroitin lyase digestion, and used as competitive inhibitors of IE binding to immobilized ligands. Inhibition of binding to CS-A was highly dependent on molecular size: a CS-A tetradecasaccharide fraction was the minimum length able to almost completely inhibit binding. The effect was dose dependent and similar to that of the parent polysaccharide, and the same degree of inhibition was not found with the CS-C oligosaccharides. There was no effect on binding of IEs to other ligands, e.g., CD36 and intercellular adhesion molecule 1. Hexadeca- and octadecasaccharide fractions of CS-A were required for maximum inhibition of binding to thrombomodulin. Analyses of oligosaccharide fractions and polysaccharides by electrospray mass spectrometry and high-performance liquid chromatography suggest that the differences between the activities of CS-A and CS-C oligosaccharides can be attributed to differences in sulfate content and sulfation pattern and that iduronic acid is not involved in IE binding.

Multiple adhesive phenotypes linked to rosetting binding of erythrocytes in Plasmodium falciparum malaria

The cerebral form of severe malaria is associated with excessive intravascular sequestration of Plasmodium falciparum-infected erythrocytes (PRBC). Retention and accumulation of PRBC may lead to occlusion of brain microvessels and direct the triggering of acute pathologic changes. Here we report that by selection, cloning, and subcloning, we have identified rare P. falciparum parasites expressing a pan-adhesive phenotype linked to erythrocyte rosetting, a previously identified correlate of cerebral malaria. Rosetting PRBC not only bound uninfected erythrocytes but also formed autoagglutinates, adhered to endothelial cells, and bound to CD36, immunoglobulins, and the blood group A antigen. The linkage of rosetting, autoagglutination, and cytoadherence involved the coexpression on a single PRBC of ligands with multiple specificities and the binding to two or more receptors on erythrocytes and to at least two other cell adhesion molecules, including a new endothelial cell receptor for P. falciparum-infected erythrocytes. Limited proteolysis that differentially cleaved the rosetting ligand PfEMP1 from the PRBC surface abrogated all the binding phenotypes of these parasites, implicating the variant antigen PfEMP1 as a carrier of multiple ligand specificities. The results encourage the further study of pan-adhesion as a potentially important parasite phenotype in the pathogenesis of severe P. falciparum malaria.

Identification of a Region of PfEMP1 That Mediates Adherence of Plasmodium falciparum Infected Erythrocytes to CD36: Conserved Function With Variant Sequence

Adherence of mature parasitized erythrocytes (PE) of Plasmodium falciparum to microvascular endothelial cells contributes directly to the virulence and pathology of this human malaria. The malarial variant antigen, P falciparum erythrocyte membrane protein 1 (PfEMP1), has been implicated as the PE receptor for CD36 on endothelial cells. We identified the region of PfEMP1 that mediates adherence of PE to CD36 and showed that a recombinant protein fragment from this region blocked and reversed adherence of antigenically different parasites. Sequence variation was evident in the CD36 binding domain of different PfEMP1 genes, yet many highly conserved residues, particularly cysteine residues, are evident. This suggests a highly conserved shape that mediates adherence to CD36. Immunization with the CD36-binding domain elicited sera that are cross-reactive with the different recombinant proteins but are strain-specific for the PE surface. Novel anti-adherence therapeutics and a malaria vaccine may derived from exploitation of the structure of the CD36 binding domain of PfEMP1.

Cytoadherence of Plasmodium falciparum to intercellular adhesion molecule 1 and chondroitin-4-sulfate expressed by the syncytiotrophoblast in the human placenta

Late stages of Plasmodium falciparum-infected erythrocytes (IRBCs) frequently sequester in the placentas of pregnant women, a phenomenon associated with low birth weight of the offspring. To investigate the physiological mechanism of this sequestration, we developed an in vitro assay for studying the cytoadherence of IRBCs to cultured term human trophoblasts. The capacity for binding to the syncytiotrophoblast varied greatly among P. falciparum isolates and was mediated by intercellular adhesion molecule 1 (ICAM-1), as binding was totally inhibited by 84H10, a monoclonal antibody specific for ICAM-1. Binding of the P. falciparum line RP5 to the syncytiotrophoblast involves chondroitin-4-sulfate (CSA), as this binding was dramatically impaired by addition of free CSA to the binding medium or by preincubation of the syncytiotrophoblast with chondroitinase ABC. ICAM-1 and CSA were visualized on the syncytiotrophoblast by immunofluorescence, while CD36, E-selectin, and vascular cell adhesion molecule 1 were not expressed even on tumor necrosis factor alpha (TNF-alpha)-stimulated syncytiotrophoblast tissue, and monoclonal antibodies against these cell adhesion molecules did not inhibit cytoadherence. ICAM-1 expression and cytoadherence of wild isolates was upregulated by TNF-alpha, a cytokine that can be secreted by the numerous mononuclear phagocytes present in malaria-infected placentas. These results suggest that cytoadherence may be involved in the placental sequestration and broaden the understanding of the physiopathology of the malaria-infected placenta.

Functional expression of CD11a on CD8+ cells is suppressed in regional lymph nodes with cancer involvement in patients with gastrointestinal carcinoma

BACKGROUND

To evaluate the immunologic activity of regional lymph nodes, the phenotype of lymphocytes and the functional expression of cell adhesion molecules (CAMs) on lymph node lymphocytes (LNL-: uninvolved, LNL+: involved) were investigated in patients with gastrointestinal carcinoma.

METHODS

The lymphocyte subpopulation and the expression of CD11a, CD44, and CD29 on CD4+ and CD8+ cells in peripheral blood lymphocytes (PBL), LNL- and LNL+ derived from 37 patients with gastrointestinal carcinoma were studied. In addition, the adherence of CD8+ cells to ICAM-1 which reflects the adhesive function of CD11a, was examined, and changes in this adherence were studied by experimental coculture with cancer cells (DLD-1).

RESULTS

Although there were no differences in the overall proportion of T cells between the groups, CD8+ cells and CD16+ cells were considerably diminished in LNL+. The expression of CD11a and CD29 on CD4+ and CD8+ cells was significantly lower in LNL than in PBL, whereas the expression of CD44 showed no significant differences. The expression levels of these CAMs were almost the same in LNL- and LNL+. Only CD11a expression on CD8+ cells in LNL+ was significantly lower than that in LNL- (P < 0.005). The adherence of CD8+ cells in LNL+ to ICAM-1 was lower than that in PBL and LNL-, and was extremely enhanced by experimental coculture with cancer cells (DLD-1).

CONCLUSIONS

These data indicate that the functional expression of CD11a (LFA-1) on CD8+ T cells is suppressed in cancer-involved regional lymph nodes in patients with gastrointestinal carcinoma.

Plasmodium falciparum gametocyte adhesion to C32 cells via CD36 is inhibited by antibodies to modified band 3

Plasmodium falciparum gametocyte-infected erythrocytes are characterized by their ability to sequester in the microvasculature of various organs, primarily the spleen and bone marrow. This phenomenon is thought to play a critical role in the development and survival of the sexual stages. Little is known, however, about ligands on the gametocyte-infected erythrocyte. Infection of erythrocytes with mature asexual stages of P. falciparum (trophozoites and schizonts) has been shown to induce modification of the erythrocyte anion transporter, band 3, and this has been linked to the acquisition of an adherent phenotype. Here, we demonstrate for the first time that immature gametocyte-infected erythrocytes also express modified band 3. In vitro binding assays demonstrate that gametocyte-infected erythrocytes of the 3D7 strain utilize this surface receptor for adhesion to C32 amelanotic melanoma cells via the host cell receptor CD36 (platelet glycoprotein IIIb). Adhesion of gametocyte-infected erythrocytes to CD36-transfected CHO cells is also dependent on modified band 3. However, modified band 3 does not mediate adhesion of gametocyte-infected erythrocytes to intercellular adhesion molecule 1, a second host receptor for gametocytes expressed on C32 cells.

Studies of Plasmodium falciparum cytoadherence using immortalized human brain capillary endothelial cells

The cytoadherence of Plasmodium falciparum-infected erythrocytes was studied using immortalized human brain capillary endothelial cells. The immortalized cells, denoted as BB19, derived from the human brain endothelium, were transformed with the E6E7 genes of human papilloma virus and retained their endothelial nature, i.e. tubule formation occurred with Matrigel as a substratum and the cells stained positive for Factor VIII-related antigen, or vonWillebrand's factor. Surface expression of ICAM-1, VCAM, E-selectin, and CD36 was demonstrated by immunofluorescence staining with monoclonal antibodies to these ligands. Exposure to cytokines (TNF, IFN gamma, IL-1 alpha, and IL-6) and lipopolysaccharide resulted in an increase in expression of ICAM-1, VCAM, E-selectin, and CD36. The BB19 cells bound P. falciparum-infected red blood cells with both the FCR-3 and the ITO4 strains. Antibodies to CD36 and ICAM-1 partially inhibited the binding of the FCR-3 and the ITO4 lines, respectively. These findings suggest that BB19 cells may be useful in the analysis of receptor-based cytoadherence and sequestration, as well as in the cell biology of microvessel formation.

Plasmodium falciparum erythrocyte membrane protein 1 is a parasitized erythrocyte receptor for adherence to CD36, thrombospondin, and intercellular adhesion molecule 1

Adherence of mature Plasmodium falciparum parasitized erythrocytes (PRBCs) to microvascular endothelium contributes directly to acute malaria pathology. We affinity purified molecules from detergent extracts of surface-radioiodinated PRBCs using several endothelial cell receptors known to support PRBC adherence, including CD36, thrombospondin (TSP), and intercellular adhesion molecule 1 (ICAM-1). All three host receptors affinity purified P. falciparum erythrocyte membrane protein 1 (PfEMP1), a very large malarial protein expressed on the surface of adherent PRBCs. Binding of PfEMP1 to particular host cell receptors correlated with the binding phenotype of the PRBCs from which PfEMP1 was extracted. Preadsorption of PRBC extracts with anti-PfEMP1 antibodies, CD36, or TSP markedly reduced PfEMP1 binding to CD36 or TSP. Mild trypsinization of intact PRBCs of P. falciparum strains shown to express antigenically different PfEMP1 released different (125)I-labeled tryptic fragments of PfEMP1 that bound specifically to CD36 and TSP. In clone C5 and strain MC, these activities resided on different tryptic fragments, but a single tryptic fragment from clone ItG-ICAM bound to both CD36 and TSP. Hence, the CD36- and TSP-binding domains are distinct entities located on a single PfEMP1 molecule. PfEMP1, the malarial variant antigen on infected erythrocytes, is therefore a receptor for CD36, TSP, and ICAM-1. A therapeutic approach to block or reverse adherence of PRBCs to host cell receptors can now be pursued with the identification of PfEMP1 as a malarial receptor for PRBC adherence to host proteins.

CD36 and intercellular adhesion molecule 1 mediate adhesion of developing Plasmodium falciparum gametocytes

Plasmodium falciparum trophozoite-infected erythrocytes adhere to the amelanotic melanoma C32 cell line in vitro. Here we demonstrate for the first time that immature gametocyte-infected erythrocytes also adhere to C32 cells, albeit at lower levels than trophozoites. However, anti-CD36 and anti-intercellular adhesion molecule 1 antibodies inhibit asexual and gametocyte adhesion by comparable percentages, suggesting a common dependency for binding to these cellular receptors.

Primary culture of human lung microvessel endothelial cells: a useful in vitro model for studying Plasmodium falciparum-infected erythrocyte cytoadherence

In the past, several cell lines have been used as in vitro models for studying cytoadherence, which refers to the specific binding of Plasmodium falciparum-parasitized red blood cells (PRBC) to host endothelium of microvessels. These models include: (a) human cells, including human umbilical vein endothelial cells (HUVEC), C32 amelanotic melanoma cells and monocytes; (b) non-human cells transfected with human genes, including COS and CHO cells; and (c) purified candidate receptor molecules. However, endothelial cells from malaria target organs are rarely investigated. In this study, we describe the efficient isolation and characterization of human lung endothelial cells (HLEC). This is the first in vitro study of P. falciparum PRBC cytoadherence to human lung endothelium, one of the target organs during severe malaria. The endothelial nature of the HLEC lines was confirmed by the presence of the von Willebrand factor, anti-human platelet endothelial adhesion molecule-1 and E-selectin antigens as specific endothelial markers. After exposure of HLEC to human cytokines, FACScan analysis indicated the coexpression of PRBC receptors CD36, intercellular adhesion molecule-1 (ICAM-1), E-selectin and vascular cell adhesion molecule-1 (VCAM-1). The laboratory-adapted P. falciparum strains adhered specifically in vitro to these HLEC. The binding of PRBC could be inhibited with variable efficiency by various monoclonal antibodies (anti-CD36 > anti-ICAM-1 > anti-VCAM-1 > anti-E-selectin). Target organ specific cell lines such as HLEC expressing a variety of potential P. falciparum PRBC cytoadherence receptors may provide in vitro systems for studying the pathophysiology of severe malaria and identifying new therapeutic agents designed to directly block adhesive events involved in severe malaria.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells and secretion of cytokines (IL-1-beta, IL-1RA, IL-6, IL-8, IL-10, TGF beta, TNF alpha, G-CSF, GM-CSF

The scientific interest in the physical interaction of Plasmodium falciparum-infected erythrocytes with host cells stems from the suggestion that excessive binding in the microvasculature leads to severe malaria. The authors studied, therefore, two parasites for their ability to adhere to normal human cells and to induce cytokine production, one parasite lacking a binding capacity (DD2) and one which adhered to CD36+ transfected CHO cells (MCAMP). The MCAMP parasites readily bound to platelets and erythrocytes and to monocytes, polymorphonuclear granulocytes and EBV-transformed B cells as seen by light and electron microscopy. Platelets were frequently attached in large numbers to the infected erythrocyte surface and groups of infected erythrocytes were sometimes held together by several platelets. Nine out of 17 cytokines tested were found to be secreted into the culture supernatants after 35 h of co-cultures containing monocytes or unfractionated peripheral blood mononuclear cells (PBMC) and parasites (IL-1RA, IL-6, IL-8, IL-10, TGF beta, TNF alpha, G-CSF, IL-1-beta, and GM-CSF). Three additional cytokines were also present in low levels (< 200 pg/ml, IL-2, IL-4, IFN gamma) in the culture supernatants after incubation of the cells for 4 days. TNF alpha, IL-RA, and IL-8 were secreted from polymorphonuclear granulocytes, LGLs and T cells. Platelets and, to a lesser degree, monocytes and T cells secreted large amounts of TGF beta (10-30 ng/ml). Cytokines may participate in the pathogenesis but also the suppression of immune responses seen during acute malarial infections.

Cytoadhesion and Falciparum Malaria: Going with the flow

Sequestration of parasitized red blood cells in the cerebral vasculature is the predisposing event to the development of cerebral malaria during infection with Plasmodium falciparum. The adhesive interaction between these cells and receptors on the endothelial cell (cytoadhesion) occurs in the dynamic environment of the microcirculation, but most studies have neglected this factor and have concentrated on measuring adhesion in static (no flow) assays. Such studies ignore the markedly different rheological properties of parasitized red blood cells that become apparent when adhesion is examined under dynamic, flow conditions that resemble those of the circulation in vivo. Here, Brian Cooke and Ross Coppel review a number of novel aspects of cytoadhesion that have been identified using flow-based assays, and discuss their relevance to the pathophysiology, investigation and clinical management of falciparum malaria.

Isolation and characterization of brain microvascular endothelial cells from Saimiri monkeys. An in vitro model for sequestration of Plasmodium falciparum-infected erythrocytes

The adhesion of parasitized red blood cells (PRBC) to the endothelium (sequestration) may contribute to the pathogenic events in severe human malaria caused by P. falciparum. However, the factors involved in the pathophysiology, especially cerebral malaria are poorly understood. Previously, we have shown that the squirrel monkey Saimiri sciureus is a potential model for human cerebral malaria. In this paper we describe five stable clones of endothelial cell lines isolated immediately postmortem from different regions of the brain of Saimiri monkeys. The endothelial cell characteristics of these clones were confirmed by analyzing their ultrastructural aspects by transmission electron microscopy and by immunodetection of various endothelial cell markers. The Saimiri brain endothelial cell clones (SBEC) varied in their expression of different surface molecules. For example, various combinations of receptors involved in P. falciparum PRBC adherence such as CD36, ICAM-1 and E-selectin, were expressed at baseline values and could be up-regulated by human srTNF-alpha and human srIFN-gamma. One of the SBEC clones showed a strong cytoadherence for various laboratory strains of P. falciparum despite the absence of surface expression of any of the known endothelial receptors implicated in PRBC adherence. This finding suggests the existence of a new and uncharacterized PRBC binding receptor. The use of target organ specific endothelial cell lines expressing a number of different potential P. falciparum PRBC cytoadherence receptors, will be a useful in vitro system for the evaluation of strategies for the development of vaccine and antimalarial drugs to prevent human cerebral malaria.

Cloning the P. falciparum gene encoding PfEMP1, a malarial variant antigen and adherence receptor on the surface of parasitized human erythrocytes

Plasmodium falciparum-infected human erythrocytes evade host immunity by expression of a cell-surface variant antigen and receptors for adherence to endothelial cells. These properties have been ascribed to P. falciparum erythrocyte membrane protein 1 (PfEMP1), an antigenically diverse malarial protein of 200-350 kDa on the surface of parasitized erythrocytes (PEs). We describe the cloning of two related PfEMP1 genes from the Malayan Camp (MC) parasite strain. Antibodies generated against recombinant protein fragments of the genes were specific for MC strain PfEMP1 protein. These antibodies reacted only with the surface of MC strain PEs and blocked adherence of these cells to CD36 but without effect on adherence to thrombospondin. Multiple forms of the PfEMP1 gene are apparent in MC parasites. The molecular basis for antigenic variation in malaria and adherence of infected erythrocytes to host cells can now be pursued.

Chondroitin-4-sulphate (proteoglycan), a receptor for Plasmodium falciparum-infected erythrocyte adherence on brain microvascular endothelial cells

Adherence of Plasmodium falciparum parasitized erythrocytes to the microvascular endothelium is mediated by different receptors expressed by endothelial cells. The study of the adherence of P. falciparum-infected erythrocytes to Saimiri monkey brain microvascular endothelial cells revealed the presence of an additional receptor, which was identified and further characterized. This receptor was also found on the surface of primary human lung endothelial cells (HLEC). We developed two mAbs to this receptor which very efficiently blocked the adherence of parasite strains to Saimiri brain endothelial cells (SBEC). The ability of these mAb to bind to SBEC was partially blocked by chondroitin-4-sulphate (CSA). Competitive inhibition assays on adherence of parasitized red blood cells (PRBC) showed that CSA, but not hyaluronic acid, chondroitin-6-sulphate, dermatan sulphate, keratane sulphate, heparan sulphate or chondroitin-4S-disaccharide, was able to almost completely inhibit PRBC adherence. The same effect was obtained with chondroitinase ABC and AC, but not B, hyaluronidase or heparinase. These results strongly suggest that a member of the chondroitin-glycosaminoglycan family, CSA, represents an additional receptor used by P. falciparum PRBC to cytoadhere to microvascular endothelial cells.

Chondroitin sulfate A is a cell surface receptor for Plasmodium falciparum-infected erythrocytes

Adherence of Plasmodium falciparum-infected erythrocytes to cerebral postcapillary venular endothelium is believed to be a critical step in the development of cerebral malaria. Some of the possible receptors mediating adherence have been identified, but the process of adherence in vivo is poorly understood. We investigated the role of carbohydrate ligands in adherence, and we identified chondroitin sulfate (CS) as a specific receptor for P. falciparum-infected erythrocytes. Parasitized cells bound to Chinese hamster ovary (CHO) cells and C32 melanoma cells in a chondroitin sulfate-dependent manner, whereas glycosylation mutants lacking chondroitin sulfate A (CSA) supported little or no binding. Chondroitinase treatment of wild-type CHO cells reduced binding by up to 90%. Soluble CSA inhibited binding to CHO cells by 99.2 +/- 0.2% at 10 mg/ml and by 72.5 +/- 3.8% at 1 mg/ml, whereas a range of other glycosaminoglycans such as heparan sulfate had no effect. Parasite lines selected for increased binding to CHO cells and most patient isolates bound specifically to immobilized CSA. We conclude that P. falciparum can express or expose proteins at the surface of the infected erythrocyte that mediate specific binding to CSA. This mechanism of adherence may contribute to the pathogenesis of P. falciparum malaria, but has wider implications as an example of an infectious agent with the capacity to bind specifically to cell-associated or immobilized CS.

PfEMP3 and HRP1: co-expressed genes localized to chromosome 2 of Plasmodium falciparum

A malarial protein, Plasmodium falciparum erythrocyte membrane protein 3 (PfEMP3), has been recently characterized as a high-molecular-mass component (approx. 315 kDa) localized to the erythrocyte membrane of knob-bearing (K+), cytoadherent (C+) mature stages of P. falciparum-parasitized erythrocytes (PE) [Pasloske et al., Mol. Biochem. Parasitol. 59 (1993) 59-72]. Knobless (K-), non-cytoadherent (C-) parasites of the same strain were shown to lack the PfEMP3 gene. In view of the biological importance of the knobby and cytoadherent phenotypes with regard to parasite virulence, we extended the analysis of PfEMP3 and its gene product to other K+/K- and C+/C- parasites. Previously, other studies have shown that the malarial protein, knob-associated histidine-rich protein 1 (HRP1), is also strongly correlated with knob expression. Here, we show that PfEMP3 and HRP1 were absent from all the K- parasites tested, including the Palo Alto (PA) K-C+ strain. This result demonstrates that PfEMP3 and HRP1 are not essential for cytoadherence. PfEMP3 was localized to chromosome 2 of the K+ parasites, within no more than 130 kb of HRP1, between the telomere and HRP1. Stage-specific analysis of the mRNA for HRP1 and PfEMP3 indicated maximal transcription of the genes in ring-stage parasites, with little or no mRNA present during the mature parasite stages. Analysis of PfEMP3 and HRP1 by immunofluorescence assay (IFA) revealed identical staining patterns of fixed PE at all stages of the asexual life cycle. Hence, PfEMP3 and HRP1 are adjacent to each other in chromosome 2, co-expressed temporally and their gene products co-localized to the PE membrane.