Characterization of a modified red cell membrane protein expressed on erythrocytes infected with the human malaria parasite Plasmodium falciparum: possible role as a cytoadherent mediating protein

A large scale Plasmodium vivax- Saimiri boliviensis trophozoite-schizont transition proteome

Plasmodium vivax is a complex protozoan parasite with over 6,500 genes and stage-specific differential expression. Much of the unique biology of this pathogen remains unknown, including how it modifies and restructures the host reticulocyte. Using a recently published P. vivax reference genome, we report the proteome from two biological replicates of infected Saimiri boliviensis host reticulocytes undergoing transition from the late trophozoite to early schizont stages. Using five database search engines, we identified a total of 2000 P. vivax and 3487 S. boliviensis proteins, making this the most comprehensive P. vivax proteome to date. PlasmoDB GO-term enrichment analysis of proteins identified at least twice by a search engine highlighted core metabolic processes and molecular functions such as glycolysis, translation and protein folding, cell components such as ribosomes, proteasomes and the Golgi apparatus, and a number of vesicle and trafficking related clusters. Database for Annotation, Visualization and Integrated Discovery (DAVID) v6.8 enriched functional annotation clusters of S. boliviensis proteins highlighted vesicle and trafficking-related clusters, elements of the cytoskeleton, oxidative processes and response to oxidative stress, macromolecular complexes such as the proteasome and ribosome, metabolism, translation, and cell death. Host and parasite proteins potentially involved in cell adhesion were also identified. Over 25% of the P. vivax proteins have no functional annotation; this group includes 45 VIR members of the large PIR family. A number of host and pathogen proteins contained highly oxidized or nitrated residues, extending prior trophozoite-enriched stage observations from S. boliviensis infections, and supporting the possibility of oxidative stress in relation to the disease. This proteome significantly expands the size and complexity of the known P. vivax and Saimiri host iRBC proteomes, and provides in-depth data that will be valuable for ongoing research on this parasite’s biology and pathogenesis.

Localization of anionic constituents in mast cell granules of brachymorphic (bm/bm) mice by using avidin-conjugated colloidal gold

We used the egg avidin gold complex as a polycationic probe for the localization of negatively charged sites in the secretory granules of mouse mast cells. We compared the binding of this reagent to mast cell granules in wild-type mice and in congenic brachymorphic mice in which mast cell secretory granules contained undersulfated proteoglycans. We localized anionic sites by post-embedding labeling of thin sections of mouse skin and tongue tissues fixed in Karnovsky's fixative and OsO(4) and embedded in Araldite. Transmission electron microscopy revealed that the mast cell granules of bm/bm mice had a lower optical density than those of wild-type mice (P<0.001) and a lower avidin gold binding density (by approximately 50%, P<0.001). The latter result provided additional evidence that the contents of mast cell granules in bm/bm mice were less highly sulfated than in those of wild-type mice. In both wild-type and bm/bm mast cells, the distribution of granule equivalent volumes was multimodal, but the unit granule volume was approximately 19% lower in bm/bm cells than in wild-type cells (P<0.05). Thus, bm/bm mast cells develop secretory granules that differ from those of wild-type mice in exhibiting a lower optical density and slightly smaller unit granules, however the processes that contribute to granule maturation and granule-granule fusion in mast cells are operative in bm/bm cells.

Current knowledge about the functional roles of phosphorylative changes of membrane proteins in normal and diseased red cells

With the advent of proteomic techniques the number of known post-translational modifications (PTMs) affecting red cell membrane proteins is rapidly growing but the understanding of their role under physiological and pathological conditions is incompletely established. The wide range of hereditary diseases affecting different red cell membrane functions and the membrane modifications induced by malaria parasite intracellular growth represent a unique opportunity to study PTMs in response to variable cellular stresses. In the present review, some of the major areas of interest in red cell membrane research have been considered as modifications of erythrocyte deformability and maintenance of the surface area, membrane transport alterations, and removal of diseased and senescent red cells. In all mentioned research areas the functional roles of PTMs are prevalently restricted to the phosphorylative changes of the more abundant membrane proteins. The insufficient information about the PTMs occurring in a large majority of the red membrane proteins and the general lack of mass spectrometry data evidence the need of new comprehensive, proteomic approaches to improve the understanding of the red cell membrane physiology.

Band 3/complement-mediated recognition and removal of normally senescent and pathological human erythrocytes

Band 3 modifications that normally occur during physiological red blood cell (RBC) senescence in humans, and occasionally in pathological conditions are described in the context of their role in enhancing RBC recognition and phagocytic removal. Band 3 modifications are mostly due to oxidative insults that gradually accumulate during the RBC lifespan or impact massively in a shorter time period in pathological conditions. The oxidative insults that impact on the RBC, the protective mechanisms that counteract those damages and the phenotypic modifications that accumulate during the RBC lifespan are described. It is shown how specific oxidative as well as non-oxidative band 3 modifications enhance RBC membrane affinity for normally circulating anti-band 3 antibodies, and how membrane-bound anti-band 3 antibodies bring about a limited complement activation and membrane deposition of complement C3 fragments. The partially covalent complexes between anti-band 3 antibodies and complement C3 fragments are very powerful opsonins readily recognized by the CR1 complement receptor on the phagocyte. Band 3 modifications typically encountered in old RBCs have crystallized to a number of band 3-centered models of RBC senescence. One of those band 3-centered models, the so-called 'band 3/complement RBC removal model' first put up by Lutz et al. is discussed in more detail. Finally, it is shown how the genetic deficiency of glucose-6-phosphate dehydrogenase (G6PD) plus fava bean consumption, and a widespread RBC parasitic disease, P. falciparum malaria, may lead to massive and rapid destruction of RBCs by a mechanism comparable to a dramatic, time-compressed enhancement of normal RBC senescence.

Band 3 clustering promotes the exposure of neoantigens in Plasmodium falciparum-infected erythrocytes

Erythrocytes infected with the human malaria parasite Plasmodium falciparum become structurally and antigenically modified as a consequence of intracellular parasite development. The new antigens that appear on the surface of the infected erythrocyte originate from parasite-encoded proteins and by modification of the erythrocyte membrane protein band 3. Here, we show that anti-peptide antibodies generated against an amino acid sequence (YETFSKLIKIFQDH) of human band 3, and previously identified as mediating adhesion of infected erythrocytes to CD36, recognized P. falciparum-infected erythrocytes. In addition, sera from individuals living in a malaria endemic area (and who are presumably immune) contained immunoglobulins specific for this region of band 3. The anti-peptide antibodies reacted with the surface excrescences (knobs) on falciparum-infected erythrocytes. In uninfected erythrocytes, the band 3 region was cryptic and its exposure on the falciparum-infected erythrocyte surface required clustering of band 3 protein. Thus, a parasite-induced modification of band 3 promotes adhesion and induces antigenic changes in the P. falciparum-infected erythrocyte.

The sticky secrets of sequestration

Sequestration, the adherence of infected erythrocytes containing the more mature stages of parasite development (trophozoites and schizonts) to the endothelial cells lining the capillaries and post-capillary venules, is characteristic of Plasmodium falciparum infections. In this review, Irwin Sherman and his colleagues discuss recent advances in the characterization of the adhesive molecules on the surface of malaria-infected erythrocytes and the receptors on the endothelium to which they bind.

Antigenic variation and cytoadhesion in Babesia bovis and Plasmodium falciparum: different logics achieve the same goal

Babesia bovis is a protozoal hemoparasite of cattle which behaves in certain crucial respects like Plasmodium falciparum, despite being phylogenetically distant and having many differences in its life cycle. The shared behavioral attributes of rapid antigenic variation and cytoadhesion/sequestration are thought to contribute significantly to immune evasion, establishment of persistent infections, and disease pathology. Although differing in their genetic and biochemical strategies for achieving these behaviors, information from studies of each parasite may further our understanding of the overall host-parasite interaction. In this review we contrast the molecular basis and 'genetic logic' for these critical behaviors in the two parasites, with emphasis on the biology of B. bovis.

Malaria pathogenesis: a jigsaw with an increasing number of pieces

Plasmodium falciparum malaria remains as one of the most devastating global health problems of today. It is estimated that around 150 million individuals get the disease every year and of these 2-3 million die from it. Our knowledge of the mechanisms underlying the pathology has expanded greatly over the last decades, but many aspects of the molecular biology, immunology and epidemiology that govern the pathogenesis and spread of this parasite are still unclear. As new insights are gained we are also revealing a challenging biological complexity. Piecing this information together is the key to vaccine development and production of new antimalarial drugs.

Plasmodium chabaudi chabaudi AS: modification of acute infection in CBA/Ca mice as a result of pre-treatment with erythrocyte band 3 in adjuvant

In this paper, in vivo data are presented that suggest a role for host recognition of erythrocyte band 3 in the control of malaria parasitaemia. The course of Plasmodium chabaudi chabaudi AS acute infection in CBA/Ca mice was suppressed or enhanced as a result of treatment on two occasions with enriched preparations of normal erythrocyte band 3 in adjuvant. Co-treatment with band 3 and a recombinant polypeptide encoding the C-terminal region of the P. c. chabaudi AS merozoite surface protein 1, which on its own had no clear effect on parasitaemia, appeared to modulate band 3-induced inhibition. Despite several-fold reductions in ascending parasitaemias in some band 3-immunized groups, there was a lack of obvious or unexpected anaemia prior to, or during infection, indicating a degree of specificity in the parasitaemia modifying response for infected rather than uninfected erythrocytes. These findings support a role for modified host recognition of erythrocyte band 3 in the partial immunity that transcends phenotypic and genotypic antigenic variation by malaria parasites.

Amino terminus of Plasmodium falciparum acidic basic repeat antigen interacts with the erythrocyte membrane through band 3 protein

The acidic basic repeat antigen (ABRA) of Plasmodium falciparum is localised in the parasitophorous vacuole, and associates with the merozoite surface at the time of schizont rupture. By virtue of its protease-like activity, it is implicated in the process of merozoite invasion and schizont rupture, and therefore, possibly interacts with erythrocyte membrane proteins to execute its function during these events. In this study, using Escherichia coli expressed recombinant fragments of ABRA, we have demonstrated that ABRA interacts with red blood cells through its N-terminus. Out of the four human erythrocyte proteins tested, namely, band 3, glycophorin A and B and spectrin, ABRA showed dose-dependent and saturable binding with the band 3 protein. This binding was lost on chymotrypsin treatment of erythrocytes or their membrane extract. Studies with the deletion constructs of the N-terminus revealed that the binding domain lies in the cysteine-rich N-proximal region of ABRA. In addition to the recombinant fragments, native ABRA derived from the P. falciparum-infected erythrocytes also showed binding to band 3 protein. Sequencing of the cysteine-rich 528 bp region, amplified from fifteen field isolates of P. falciparum, showed that not only the five cysteines of mature ABRA but also the whole sequence is fully conserved, even at the nucleotide level. This sequence conservation of the N-terminus and its role in RBC binding suggests that this region may be crucial for any putative function of ABRA, therefore emphasising its importance as a vaccine/drug target.

Growth of Plasmodium falciparum induces stage-dependent haemichrome formation, oxidative aggregation of band 3, membrane deposition of complement and antibodies, and phagocytosis of parasitized erythrocytes

Plasmodium falciparum-parasitized erythrocytes (RBCs) are progressively transformed into non-self cells, phagocytosed by human monocytes. Haemichromes, aggregated band 3 (Bd3) and membrane-bound complement fragment C3c and IgG were assayed in serum-opsonized stage-separated parasitized RBCs. All parameters progressed from control to rings to trophozoites to schizonts: haemichromes, nil; 0.64 +/- 0.12; 5.6 +/- 1.91; 8.4 +/- 2.8 (nmol/ml membrane); Bd3, 1 +/- 0.1; 4.3 +/- 1.5; 23 +/- 5; 25 +/- 6 (percentage aggregated); C3c, 31 +/- 11; 223 +/- 86; 446 +/- 157; 620 +/- 120 (mOD405/min/ml membrane); IgG, 35 +/- 12; 65 +/- 23; 436 +/- 127; 590 +/- 196 (mOD405/min/ml membrane). All increments in rings versus controls and in trophozoites versus rings were highly significant. Parasite development in the presence of 100 micromol/l beta-mercaptoethanol largely reverted haemichrome formation, Bd3 aggregation, C3c and IgG deposition and phagocytosis. Membrane proteins extracted by detergent C12E8 were separated on Sepharose CL-6B. Haemichromes, C3c and IgG were present exclusively in the high-molecular-weight fractions together with approximately 30% of Bd3, indicating the oxidative formation of immunogenic Bd3 aggregates. Immunoblots of separated membrane proteins with anti-Bd3 antibodies confirmed Bd3 aggregates that, in part, did not enter the gel. Immunoprecipitated antibodies eluted from trophozoites reacted preferentially with aggregated Bd3. Changes in parasitized RBC membranes and induction of phagocytosis were similar to oxidatively damaged, senescent or thalassaemic RBC, indicating that parasite-induced oxidative modifications of Bd3 were per se sufficient to induce and enhance phagocytosis of malaria-parasitized RBC.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Plasmodium falciparum histidine-rich protein 1 associates with the band 3 binding domain of ankyrin in the infected red cell membrane

Infection of erythrocytes by the malaria parasite Plasmodium falciparum results in the export of several parasite proteins into the erythrocyte cytoplasm. Changes occur in the infected erythrocyte due to altered phosphorylation of proteins and to novel interactions between host and parasite proteins, particularly at the membrane skeleton. In erythrocytes, the spectrin based red cell membrane skeleton is linked to the erythrocyte plasma membrane through interactions of ankyrin with spectrin and band 3. Here we report an association between the P. falciparum histidine-rich protein (PfHRP1) and phosphorylated proteolytic fragments of red cell ankyrin. Immunochemical, biochemical and biophysical studies indicate that the 89 kDa band 3 binding domain and the 62 kDa spectrin-binding domain of ankyrin are co-precipitated by mAb 89 against PfHRP1, and that native and recombinant ankyrin fragments bind to the 5' repeat region of PfHRP1. PfHRP1 is responsible for anchoring the parasite cytoadherence ligand to the erythrocyte membrane skeleton, and this additional interaction with ankyrin would strengthen the ability of PfEMP1 to resist shear stress.

Sulfated glycoconjugates enhance CD36-dependent adhesion ofPlasmodium falciparum–infected erythrocytes to human microvascular endothelial cells

A novel adhesive pathway that enhances the adhesion ofPlasmodium falciparum-infected erythrocytes (IEs) to endothelial cells has been identified. The sulfated glycoconjugates heparin, fucoidan, dextran sulfate 5000, and dextran sulfate 500 000 caused a dramatic increase in adhesion of IEs to human dermal microvascular endothelial cells. The same sulfated glycoconjugates had little effect on IE adhesion to human umbilical vein endothelial cells, a CD36-negative cell line. The effect was abolished by a monoclonal antibody directed against CD36, suggesting that enhanced adhesion to endothelium is dependent on CD36. No effect was observed on adhesion to purified platelet CD36 cells immobilized on plastic. The same sulfated glycoconjugates enhanced adhesion of infected erythrocytes to COS cells transfected with CD36, and this was inhibited by the CD36 monoclonal antibody. These findings demonstrate a role for sulfated glycoconjugates in endothelial adherence that may be important in determining the location and magnitude of sequestration through endogenous carbohydrates. In addition, they highlight possible difficulties that may be encountered from the proposed use of sulfated glycoconjugates as antiadhesive agents in patients with severe malaria.

Sulfated glycoconjugates enhance CD36-dependent adhesion ofPlasmodium falciparum–infected erythrocytes to human microvascular endothelial cells

A novel adhesive pathway that enhances the adhesion ofPlasmodium falciparum-infected erythrocytes (IEs) to endothelial cells has been identified. The sulfated glycoconjugates heparin, fucoidan, dextran sulfate 5000, and dextran sulfate 500 000 caused a dramatic increase in adhesion of IEs to human dermal microvascular endothelial cells. The same sulfated glycoconjugates had little effect on IE adhesion to human umbilical vein endothelial cells, a CD36-negative cell line. The effect was abolished by a monoclonal antibody directed against CD36, suggesting that enhanced adhesion to endothelium is dependent on CD36. No effect was observed on adhesion to purified platelet CD36 cells immobilized on plastic. The same sulfated glycoconjugates enhanced adhesion of infected erythrocytes to COS cells transfected with CD36, and this was inhibited by the CD36 monoclonal antibody. These findings demonstrate a role for sulfated glycoconjugates in endothelial adherence that may be important in determining the location and magnitude of sequestration through endogenous carbohydrates. In addition, they highlight possible difficulties that may be encountered from the proposed use of sulfated glycoconjugates as antiadhesive agents in patients with severe malaria.

clag9: A cytoadherence gene in Plasmodium falciparum essential for binding of parasitized erythrocytes to CD36

The propensity of isolates of the malaria parasite Plasmodium falciparum to delete a segment of chromosome 9 has provided positional information that has allowed us to identify a gene necessary for cytoadherence. It has been termed the cytoadherence-linked asexual gene (clag9). clag9 encodes at least nine exons and is expressed in blood stages. The hydrophobicity profile of the predicted CLAG9 protein identifies up to four transmembrane domains. We show here that targeted gene disruption of clag9 ablated cytoadherence to C32 melanoma cells and purified CD36. DNA-induced antibodies to the clag9 gene product reacted with a polypeptide of 220 kDa in the parental malaria clone but not in clones with a disrupted clag9 gene.

Plasmodium falciparum-infected erythrocyte adhesion to the type 3 repeat domain of thrombospondin-1 is mediated by a modified band 3 protein

Previously, the binding site for the Plasmodium falciparum-infected erythrocyte (PE) was determined to be the C-terminal 120 or 140 kDa region but not the N-terminal 25 kDa domain of thrombospondin (TSP). In this work, we have localized the TSP binding site for PE more precisely. PE adhered to glutathione-S-transferase-fusion proteins containing the type 3 repeat (T3) of TSP, but not to other functional domains of TSP (i.e. N-terminal domain, procollagen domain, type 1 and 2 repeat, and C-terminal domain). Soluble T3 inhibited PE binding to immobilized TSP. PE binding to immobilized T3 was inhibited by soluble TSP, a monoclonal antibody directed against the T3, glycine-arginine-glycine-aspartic acid-serine-proline (GRGDSP) peptide, and *cysteine-GRGDSP-cysteine*, where *cysteine and cysteine* form a disulfide linkage, suggesting involvement of an RGD-containing motif in the T3. In support of this, a fusion protein which excluded the RGD motif showed no PE binding activity. Earlier it was shown that the amino acid sequence of the band 3 protein, histidine-proline-leucine-glutamine-lysine-threonine-tyrosine (HPLQKTY), was exposed on PE and mediated PE binding to TSP. Monoclonal antibodies, which recognize HPLQKTY and inhibit PE binding to TSP, also inhibited PE binding to the T3. The involvement of the sequence was confirmed by the fact that an octamer of HPLQKTY-containing peptide bound to the T3 but not to the RGD motif-excluded fusion protein and the binding to T3 was inhibited by GRGDSP peptide. Thus, PE binding to the T3 domain of TSP is mediated by the peptidic sequence HPLQKTY of band 3 which is exposed on PE.

Role of red blood cells in thrombosis

Most biomedical textbooks teach that coagulation and thrombosis are primarily a function of endothelial cells, platelets, and soluble coagulation factors. Red blood cells, in contrast, are generally regarded as innocent bystanders, passively entrapped in a developing thrombus as they flow through the vasculature. This review summarizes evidence that demonstrates an active role for red cells in normal and pathologic hemostasis. We then evaluate the possible molecular mechanisms whereby a usually inert erythrocyte can actively contribute to the processes of clot formation.

Increase of a calcium independent transglutaminase activity in the erythrocyte during the infection with Plasmodium falciparum

We have studied the activity of a calcium dependent transglutaminase (EC 2.3.2.13) during the growth of the parasite Plasmodium falciparum inside the infected human erythrocyte. There is only one detectable transglutaminase in the two-cell-system, and its origin is erythrocytic. No activity was detected in preparations of the parasite devoid of erythrocyte cytoplasm. The Michaelis Menten constants (Km) of the enzyme for the substrates N'N' dimethylcaseine and putrescine were undistinguishable whether the cell extracts used in their determination were obtained from normal or from infected red cells. The total activity of transglutaminase in stringently synchronized cultures, measured at 0.5 mM Ca2+, decreased with the maturation of the parasite. However, a fraction which became irreversibly activated and independent of calcium concentration was detected. The proportion of this fraction grew with maturation; it represented only 20% of the activity in 20 hr-old-trophozoites while in 48-hr-schizonts it was more than 85% of the total activity. The activation of this fraction of transglutaminase did not depend on an increase in the erythrocyte cytoplasmic calcium, since most of the calcium was shown to be located in the parasite.

Analysis of adhesive domains from the A4VAR Plasmodium falciparum erythrocyte membrane protein-1 identifies a CD36 binding domain

The A4VAR is a variant antigen expressed by a clonal line that binds CD36 and intercellular adhesion molecule-1, ICAM-1. We have cloned and sequenced the extracellular domain coded by the A4var gene. To probe the relationship between A4var expression and parasite adhesion to ICAM-1, var mRNA and protein expression were analyzed in an enriched population of A4 parasites that displayed higher ICAM-1 binding. By Northern analyses, A4var was the predominant var message and antisera raised against a recombinant A4VAR protein reacted with the majority of infected erythrocytes, reinforcing previous conclusions that A4VAR binds ICAM-1. A4VAR contains five Duffy-binding like (DBL) domains, and two cysteine-rich interdomain regions (CIDR) domains. DBL and CIDR domains from A4VAR were expressed in mammalian cells to determine which regions mediate binding to CD36 and ICAM-1. Using several different binding assays, the A4VAR CIDR1 was the only domain found to bind CD36. In contrast, the same assays were unable to identify the ICAM-1 binding domain in A4VAR. This is the first time that each of the DBL and CIDR domains from a Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) have been systematically expressed and tested for binding. These results confirm that CIDR1 is sufficient to bind CD36 without any apparent contribution from other domains.

Identification of peripheral membrane proteins associated with the tubo-vesicular network of Plasmodium falciparum infected erythrocytes

During intracellular development of the malarial parasite numerous membranous vesicles appear in the infected erythrocyte cytoplasm between the parasitophorous vacuolar membrane (PVM) and the erythrocyte plasma membrane. In this study we describe the characterization of a monoclonal antibody which recognizes two major parasite-encoded proteins of 50 and 41 kDa. Immunofluorescence and immunoelectron microscopy demonstrated that the monoclonal antibody reacts with cytoplasmic vesicles of Plasmodium falciparum infected erythrocyte referred to as Maurer's clefts. The antigens recognized by the monoclonal antibody were expressed very early during the erythrocytic life cycle of the parasite, and remained tightly associated within membrane vesicles even after merozoites are released from infected erythrocytes. The antigens were partially soluble in non-ionic detergents, and were released from the membrane by alkali treatment, indicating that the proteins recognized by the monoclonal antibody are peripheral membrane proteins. It is proposed that the 50 and 41 kDa antigens might be part of an underlying membrane skeletal network that provides structural support to vesicles and tubules present in the infected erythrocyte cytoplasm.

Molecular basis for evasion of host immunity and pathogenesis in malaria

The article relates the ability of the malaria parasite Plasmodium falciparum to avoid a protective immune response, and to induce pathological changes, to the properties of specific parasite molecules. Cytoadherence and rosetting are important features of cerebral malaria and involve proteins located on the surface of the infected red blood cell. Proinflammatory cytokines, particularly tumour necrosis factor (TNF), play a role in protective immunity and in inducing pathology. Glycophosphatidyl inositol membrane anchors of parasite proteins possess insulin like activity and induce TNF synthesis. People subject to repeated infections in malaria endemic areas rarely develop complete or sterile immunity to malaria. They frequently carry small numbers of parasites in the blood, with little symptoms of the disease, illustrating a phenomenon termed semi-immunity. The basis for semi-immunity is incompletely understood. Malaria parasites are susceptible to several immunological effector mechanisms. The presence of extensive repetitive regions is a feature of many P. falciparum proteins. Available evidence suggests that the structural characteristics of the repeats and their location on the surface of parasite proteins promote immunogenicity. The repeats may help the parasite evade host immunity by (i) exhibiting sequence polymorphism, (ii) preventing the normal affinity and isotype maturation of an immune response, (iii) functioning possibly as B cell superantigens, (iv) generating predominantly thymus independent antibody responses, and (v) acting as a sink for binding protective antibodies. Sequence diversity in non-repetitive regions and antigenic variation in parasite molecules located on the surface of infected red blood cells also play a role in immune evasion. Some sequence homologies between parasite and human proteins may be due to molecular mimicry. Homologies in other instances can cause autoimmune responses. The immune evasion mechanisms of the parasite need to be considered in developing vaccines. Protective immunity and pathology may be delicately balanced in malaria.

Identification of novel membrane structures in Plasmodium falciparum infected erythrocytes

Little is known about the molecular mechanisms underlying the release of merozoites from malaria infected erythrocytes. In this study membranous structures present in the culture medium at the time of merozoite release have been characterized. Biochemical and ultrastructural evidence indicate that membranous structures consist of the infected erythrocyte membrane, the parasitophorous vacuolar membrane and a residual body containing electron dense material. These are subcellular compartments expected in a structure that arises as a consequence of merozoite release from the infected cell. Ultrastructural studies show that a novel structure extends from the former parasite compartment to the surface membrane. Since these membrane modifications are detected only after merozoites have been released from the infected erythrocyte, it is proposed that they might play a role in the release of merozoites from the host cell.

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.

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.

Plasmodium falciparum: the adherence of erythrocytes infected with human malaria can be mimicked using pfalhesin-coated microspheres

Infection of human erythrocytes with the malaria parasite, Plasmodium falciparum, results in the exposure of amino acid residues 542-555 of the anion-exchange protein, band 3, in a conformation that enables the cell to adhere to C32 amelanotic melanoma cells. Attempts to isolate this adhesive form from infected cells by immunoaffinity were unsuccessful, and so other approaches were utilized. Chinese hamster ovary (CHO) cells transfected with cDNA encoding the first 578 amino acid residues of human band 3 protein transiently expressed the protein efficiently. A murine monoclonal antibody (MAb) that specifically recognizes the adhesin exposed on the surface of erythrocytes bearing mature stages of P. falciparum immunostained some transfected cells, confirming that the first 578 amino residues are sufficient for the adhesive conformation. As a more efficient alternative to transgenic expression of the adhesin, microspheres with covalently bound peptides fashioned on band 3 sequences previously found to be adherent (residues 546-553 and 820-829 and called pfalhesin) were produced. The pfalhesin-coated microspheres specifically bound to C32 amelanotic melanoma cells, whereas microspheres coupled with a scrambled version of residues 546-553 had little binding capacity for melanoma cells. These results demonstrate that the previously identified band 3-related peptides that inhibit cytoadherence interact directly with target cells and suggest that microspheres with covalently coupled peptides might constitute novel 'artificial' P. falciparum-infected erythrocytes for use in in vitro and in vivo studies.

Detection of a LFA-1-like epitope on the surface of erythrocytes infected with a strain of Plasmodium falciparum

The adhesion of erythrocytes infected with Plasmodium falciparum (P. falciparum) is one of the major pathological features of severe malaria. Several potential receptors to endothelium for falciparum-infected erythrocyte on endothelium have been described. Recently, the malaria binding site on ICAM-1(CD54) has been mapped to a site distinct but overlapping with the LFA-1 (CD11a/CD18) site. We detected by flow cytometry, confocal laser microscopy and immunoprecipitation, a molecule expressed at the surface of erythrocytes infected with mature stages of the M96 strain of P. falciparum that was recognized by a monoclonal antibody (mAb) (TS1/22) directed against an LFA-1 epitope. However, this molecule was not recognized by mAbs directed against other epitopes of LFA-1 or against other integrins. Furthermore, the mAb TS1/22 partially inhibited cytoadherence of parasitized red blood cells to human-brain microvascular endothelial cells. The expression of a molecule sharing an epitope with human LFA-1 integrin on the parasitized erythrocyte surface could be involved in the sequestration of these cells and thus in the pathogenesis of severe disease.

Cytoadherence-related homologous motifs in Plasmodium falciparum antigen Pf155/RESA and erythrocyte band 3 protein

Cytoadherence of Plasmodium falciparum-infected erythrocytes plays an important role in the pathogenesis of cerebral malaria. The identity of cell surface molecules on parasitized erythrocytes involved in cytoadherence is of great interest to understand the molecular basis of this mechanism. Peptide sequences derived from exofacial loops of the erythrocyte antigen band 3 from parasitized erythrocytes have previously been shown to inhibit cytoadherence. We now report that a non-repeated region of Pf155/RESA (residues 213-218) contains a hexapeptide motif being highly homologous to cytoadherence inhibitory sequences from band 3. Synthetic peptides containing the hexapeptide motif of Pf155/RESA inhibited the binding of P. falciparum-infected erythrocytes to melanoma cells in vitro. Furthermore, individuals residing in malaria-endemic areas have antibodies reactive with epitopes involving these motifs in band 3 and in Pf155/RESA.

Monoclonal antibodies that react with human band 3 residues 542-555 recognize different conformations of this protein in uninfected and Plasmodium falciparum infected erythrocytes

A monoclonal antibody generated against synthetic peptides patterned on amino acids 542-555 of human band 3, designated 1F4, specifically immunostained Plasmodium falciparum-infected erythrocytes and inhibited the cytoadherence of P. falciparum-infected erythrocytes to C32 amelanotic melanoma cells. 1F4 did not recognize intact band 3 protein on immunoblots, however it was reactive towards proteolytic fragments of band 3. The binding region of another murine monoclonal antibody previously reported to recognize the membrane spanning domain of human band 3, designated B6, was found to also recognize residues 542-555, however its properties differed from 1F4. Mab B6 recognized both infected and uninfected red cells, and reacted only with intact band 3 on immunoblots. Mab B6 was without effect on cytoadherence. These results demonstrate that monoclonal antibodies reactive against a common peptide sequence may bind to different conformations of the peptide sequence and suggest that the adherent competency of P. falciparum-infected erythrocytes may result from a change in the surface topography of human band 3 protein.

Immune Evasion by Babesia bovis and Plasmodium falciparum: Cliff-dwellers of the Parasite World

Erythrocyte-dwelling parasites, such as Babesia bovis and Plasmodium falciparum, are not accessible to the host immune system during most of their asexual reproductive cycle because they are intracellular. While intracellular, the host immune response must be directed toward the surface of the infected erythrocyte. Immune individuals mount protective antibody and cell-mediated responses which eliminate most of the parasites, yet some survive to establish chronic infections. In this review, David Allred discusses some of the mechanisms used by these parasites to evade individual immune mechanisms targeting the infected erythrocyte to survive in the hostile environment of an effective immune response.

Immune responses to band 3 neoantigens on Plasmodium falciparum-infected erythrocytes in subjects living in an area of intense malaria transmission are associated with low parasite density and high hematocrit value

During the intracellular development of the human malarial parasite, Plasmodium falciparum, cryptic regions of the erythrocyte band 3 protein are exposed. Antibodies against these band 3-related neoantigens block cytoadherence, and peptides based on amino acid sequences of putative exofacial loops of band 3 protein block the in vitro and in vivo adherence of P. falciparum-infected erythrocytes. At present, it is not known whether reactivity to these antigens is related to exposure to the malaria parasite or is correlated with protective immunity. The reactivities of plasma to peptides containing amino acid sequences of putative exofacial loops 3 and 7 of human band 3 protein were determined for children and adults living in an area of perennial malaria transmission (Liberia) and for donors who had never been exposed to malaria (Denmark). Plasma samples from children and adults living in an area of intense malaria transmission showed a much higher reactivity with the band 3 peptides than did those from nonimmune individuals. High reactivity to the loop 3 peptide (amino acids 546 to 555) was correlated with lower mean parasite density in children in the 5- to 9-year-old age group. The presence of antibodies against loop 3 and 7 peptides was not associated with a low packed erythrocyte volume (hematocrit); in fact, higher-than-average reactivities to both peptides were positively correlated with high hematocrit values, indicating that antibodies which specifically recognize the band 3-related neoantigens are not involved in hemolysis (autoimmunity).

Antibodies to synthetic peptides based on band 3 motifs react specifically with Plasmodium falciparum (human malaria)-infected erythrocytes and block cytoadherence

Rabbit polyclonal and mouse monoclonal antibodies (Mabs) prepared against synthetic peptides patterned on exofacial loops 3 (amino acids 546-555) and 7 (821-834) of the human anion transport protein band 3 inhibited the cytoadherence of Plasmodium falciparum-infected erythrocytes to C32 amelanotic melanoma cells. Mabs directed against exofacial loop 4 (amino acids 628-642) did not inhibit adherence to a significant degree. The murine Mabs recognized only P. falciparum-infected cryptic in uninfected erythrocytes.

Cytoadherence-related neoantigens on Plasmodium falciparum (human malaria)-infected human erythrocytes result from the exposure of normally cryptic regions of the band 3 protein

Murine monoclonal antibodies (Mabs) were produced by vaccination of Balb/c mice with live Plasmodium falciparum-infected red cells (iRBC). The iRBC Mabs recognized altered forms of the human erythrocyte membrane protein band 3; however, these Mabs did not recognize the band 3 molecule in uninfected or ring-infected red cells. The location of epitopes was determined by studying the binding of the iRBC Mabs after selective proteolysis of band 3 as well as by the reactivity of these Mabs to synthetic peptides that corresponded to putative exofacial regions of band 3. Treatment of uninfected red cell membranes with trypsin under low ionic strength conditions resulted in exposure of cryptic epitopes of band 3 which were recognized by the iRBC Mabs. Several of the anti-iRBC Mabs (two of which were described previously) inhibited the in vitro adherence of infected erythrocytes to C32 amelanotic melanoma cells. A mouse polyclonal serum against a synthetic peptide based on an amino acid sequence motif of band 3 reacted (by immunostaining) only with the surface of iRBC and blocked adhesion. Thus, it appears that cryptic residues of the band 3 protein become exposed upon parasitization, and their presence contributes to the increased adhesiveness of the P. falciparum-infected red cell.

Proliferative responses of lymphocytes from malaria patients and healthy controls to isolated, Plasmodium falciparum schizont antigens

The specificity of lymphocyte proliferative responses of malaria patients and healthy control subjects was analysed using antigen fractions from soluble extracts of purified Plasmodium falciparum schizonts. Fractions of 14-250 kDa were separated by SDS-PAGE, blotted to nitrocellulose membranes and eluted for use in lymphocyte stimulation studies. Lymphocyte proliferation following stimulation with the separated protein fractions demonstrated that the fractions were recognized only by patients' T cells. Moreover, only the fractions including proteins of 36-250 kDa were immunogenic to the T cells. The pattern of response against each fraction differed between patients, indicating an HLA-dependent genetic restriction in the T cell activation.

Rosetting

Why do some individuals get severe falciparum malaria while others don't? Rosetting (the binding of uninfected erythrocytes to Plasmodium falciparum-infected erythrocytes), together with endothelial cytoadherence, has been shown to play a crucial role in the obstruction of the microvosculoture in P. falciparum malaria. Here, Mats Wahlgren, Victor Fernandez, Carin Scholonder and Johan Carlson review the literature surrounding rosetting.

Synthetic peptides based on motifs present in human band 3 protein inhibit cytoadherence/sequestration of the malaria parasite Plasmodium falciparum

Synthetic peptides patterned on the amino acid sequences found in two exofacial regions of band 3 protein (residues 824-829 of loop 7 and residues 547-553 of loop 3) blocked, in a dose-dependent fashion, the in vitro adherence of Plasmodium falciparum-infected erythrocytes to C32 amelanotic melanoma cells. Intravenous infusion of these synthetic peptides into Aotus and Saimiri monkeys infected with sequestering isolates of P. falciparum resulted in the appearance of mature forms of the parasite in the peripheral circulation. The finding that the peptides were effective as adhesion blockers in the micromolar range suggests that cerebral malaria could be managed through antiadhesion therapy.

In vitro phagocytosis inhibition assay for the screening of potential candidate antigens for sub-unit vaccines against the asexual blood stage of Plasmodium falciparum

We have previously established a direct correlation between immune protection against the asexual blood stage Plasmodium falciparum infection and the presence of opsonizing antibodies promoting phagocytosis of parasitized red blood cells. In the present communication we describe an in vitro assay for measuring phagocytosis inhibition (PIA) specific for P. falciparum-infected erythrocytes. The phagocytosis inhibition assay is a simple procedure for screening potential candidates for sub-unit vaccines against P. falciparum based on the correlation between opsonizing antibodies and immunoprotection. The assay was used to analyse 18 recombinant molecules, corresponding to 11 distinct antigens of P. falciparum. Pre-incubation and selective antibody depletion experiments demonstrate the antigen-antibody specificity of the PIA. The presence of epitopes participating as targets of opsonic antibodies were demonstrated in six distinct polypeptide antigens.

An 88-kDa protein of Plasmodium falciparum is related to the band-3-binding domain of human erythrocyte ankyrin

Three tryptic-peptide sequences of an 88-kDa pair of phosphoproteins of the malaria parasite Plasmodium falciparum were determined. They exhibit a striking similarity to corresponding sequences of the 89-kDa domain of human erythrocyte ankyrin. [35S]Methionine labeling of the two proteins demonstrated their parasitic origin. Using an appropriate oligonucleotide probe, Southern-blot analysis of genomic malaria DNA and Northern-blot analysis of malaria RNA suggest the existence of ankyrin-related sequences in the parasite genome and the presence of an ankyrin-related transcript of about 3.2 kb. Our studies provide further evidence of malaria-specific analogues of host-cell proteins, implying an unusual kind of parasite/host interaction.

Disruption of Plasmodium falciparum-infected erythrocyte cytoadherence to human melanoma cells with inhibitors of glycoprotein processing

Adherence of Plasmodium falciparum-infected erythrocytes (IE) to the venular endothelium in brain and other organs is characteristic of cerebral malaria, an often fatal complication in infected individuals. It has been shown that cytoadherence may be mediated through interaction of IE with glycoproteins on host target cell surfaces, including CD36 (GPIV), intercellular adhesion molecule-1 (ICAM-1), and thrombospondin. Inhibitors of glycoprotein synthesis and processing were tested for their abilities to decrease IE adherence to C32 human melanoma cells. The alpha-glucosidase inhibitor, castanospermine, was effective in disrupting cytoadherence in vitro when incubated with C32 cells (IC50 = 600-700 microM). Castanospermine-6-butyrate was even more effective than the parent compound (IC50 = 9 microM) in disrupting cytoadherence. The mannosidase inhibitors, swainsonine and deoxymannojirimycin, had no effect on cytoadherence at concentrations up to 2 mM. No effect on cytoadherence was observed when the glucosidase and mannosidase inhibitors were incubated with IE rather than the C32 cell cultures. The level of CD36 on the C32 cell surface was decreased as measured by fluorescence-activated cell sorting (FACS) analysis with the same inhibitors which inhibited cytoadherence. Cells labeled with fluorescein isothiocyanate (FITC) OKM5 monoclonal antibody, which recognizes CD36 and disrupts cytoadherence, showed decreased fluorescence when treated with tunicamycin and castanospermine-6-butyrate but not when treated with swainsonine or deoxymannojirimycin. ICAM-1 levels, as measured by surface labeling of C32 cells with FITC CD54 monoclonal antibody, were decreased in cells treated with tunicamycin. However, incubation of cells with castanospermine-6-butyrate or deoxymannojirimycin decreased cell surface ICAM-1 levels only slightly. These findings suggest that (1) in C32 cells, levels of cell surface CD36, and not ICAM-1, change proportionally to the level of cytoadherence; (2) drugs which can affect the carbohydrate moiety of cellular glycoproteins decrease cytoadherence of IE to C32 cells; and (3) protection against the development of cerebral malaria may be possible with inhibitors of glycoprotein biosynthesis.

Plasmodium falciparum (human malaria)-induced modifications in human erythrocyte band 3 protein

A monoclonal antibody, 1C4, was produced which recognizes a 65 kDa protein that is localized to the plasma membrane of human erythrocytes infected with Plasmodium falciparum. By immunofluorescence the antigen was visualized as dots on the surface of the infected cell. The 65 kDa protein was present in 4 strains of diverse geographical origin, and in erythrocytes infected with a knobless strain. The 65 kDa protein was insoluble in non-ionic detergents, but was partly soluble in SDS and some high (1 M) salt solutions. The 65 kDa protein is recognized by antibodies specific for the cytoplasmic domain and the N-terminal side of the membrane-spanning region of human band 3, but was not recognized by an antibody specific to the C-terminal side of the membrane-spanning region. The results of treatment of the 65 kDa protein with trypsin and chymotrypsin are consistent with the 65 kDa protein being a truncated and covalently modified band 3 molecule which consists of the first 540 amino acids of human band 3.

Malaria and ovalocytosis--molecular mimicry?

Two recently published reports have described findings which will have a profound impact on the understanding of molecular mechanisms of human resistance to malaria infection. In Melanesian ovalocytosis, a genetic polymorphism found in Papua New Guinea and parts of South East Asia, the red cells are highly resistant to invasion by various species of malaria parasite. The molecular nature of the defect in ovalocytic erythrocytes was not known. Recent reports by Liu et al. (Liu, S.-C., Zhai, S., Palek, J., Golan, D., Amato, D., Hassan, K., Nurse, G., Babona, D., Coetzer, T., Jarolim, P. Zaik, M. and Borwein, S. (1990) N. Engl. J. Med. 323, 1530-1538.) and Jones et al. (Jones, G.L., Edmundson, H.M., Wesche, D. and Saul, A. (1991) Biochim. Biophys. Acta 1096, 33-40.) have now identified the abnormality in the band 3 protein of ovalocytic red cell membranes. A major discovery in the Jones et al. study is the presence of an extended peptide at the N-terminus of ovalocyte band 3 protein. This novel 13 amino acid extended sequence is not found in the primary structure of normal band 3 protein and was suggested to be the cause of band 3 defect in ovalocytes. We have analyzed this extended sequence through Genbank using SWISS-PROT database and found that an almost identical sequence exists in a malaria parasite protein called RESA.