Molecular analysis of the cytoadherence phenotype of a Plasmodium falciparum field isolate that binds intercellular adhesion molecule—1

A C2 domain containing plasma membrane protein of Plasmodium falciparum merozoites mediates calcium-dependent binding and invasion to host erythrocytes

BACKGROUND

Invasion of red blood cells by Plasmodium falciparum merozoites is governed by multiple receptor-ligand interactions which are critical for bridging the two cells together. The critical function of these ligands for invasion and their direct exposure to the host immune system makes them lucrative vaccine candidates. This necessitates the discovery of new adhesins with less redundancy that mediates the binding of merozoite to the red cell, and furthermore invasion into it. Here we have identified a novel membrane associated antigen (PfC2DMA) that is conserved throughout the Plasmodium species and has a membrane targeting C2 domain at its extreme N-terminal region.

METHODS

Recombinant C2_dom was expressed heterologously in bacteria and purified to homogeneity. Mice antisera against C2_dom was raised and used to check the expression and intraparasitic localization of the protein. RBC and Ca²⁺ ion binding activity of C2_dom was also checked.

RESULTS

C2_dom exhibited specific binding to Ca²⁺ ions and not to Mg²⁺ ions. PfC2DMA localized to the surface of merozoite and recombinant C2_dom bound to the surface of human RBCs. RBC receptor modification by treatment with different enzymes showed that binding of C2_dom to RBC surface is neuraminidase sensitive. Mice antisera raised against C2_dom of Pf C2DMA showed invasion inhibitory effects.

CONCLUSION

Our findings suggest that C2_dom of PfC2DMA binds to surface of red cell in a Ca²⁺-dependent manner, advocating a plausible role in invasion and can serve as a potential novel blood stage vaccine candidate.

Accelerator or Brake: Immune Regulators in Malaria

Malaria is a life-threatening infectious disease, affecting over 250 million individuals worldwide each year, eradicating malaria has been one of the greatest challenges to public health for a century. Growing resistance to anti-parasitic therapies and lack of effective vaccines are major contributing factors in controlling this disease. However, the incomplete understanding of parasite interactions with host anti-malaria immunity hinders vaccine development efforts to date. Recent studies have been unveiling the complexity of immune responses and regulators against Plasmodium infection. Here, we summarize our current understanding of host immune responses against Plasmodium-derived components infection and mainly focus on the various regulatory mechanisms mediated by recent identified immune regulators orchestrating anti-malaria immunity.

Structural insights into diverse modes of ICAM-1 binding by Plasmodium falciparum-infected erythrocytes

A major determinant of pathogenicity in malaria caused by Plasmodium falciparum is the adhesion of parasite-infected erythrocytes to the vasculature or tissues of infected individuals. This occludes blood flow, leads to inflammation, and increases parasitemia by reducing spleen-mediated clearance of the parasite. This adhesion is mediated by PfEMP1, a multivariant family of around 60 proteins per parasite genome which interact with specific host receptors. One of the most common of these receptors is intracellular adhesion molecule-1 (ICAM-1), which is bound by 2 distinct groups of PfEMP1, A-type and B or C (BC)-type. Here, we present the structure of a domain from a B-type PfEMP1 bound to ICAM-1, revealing a complex binding site. Comparison with the existing structure of an A-type PfEMP1 bound to ICAM-1 shows that the 2 complexes share a globally similar architecture. However, while the A-type PfEMP1 bind ICAM-1 through a highly conserved binding surface, the BC-type PfEMP1 use a binding site that is more diverse in sequence, similar to how PfEMP1 interact with other human receptors. We also show that A- and BC-type PfEMP1 present ICAM-1 at different angles, perhaps influencing the ability of neighboring PfEMP1 domains to bind additional receptors. This illustrates the deep diversity of the PfEMP1 and demonstrates how variations in a single domain architecture can modulate binding to a specific ligand to control function and facilitate immune evasion.

Mapping the Binding Site of a Cross-Reactive Plasmodium falciparum PfEMP1 Monoclonal Antibody Inhibitory of ICAM-1 Binding

The virulence of Plasmodium falciparum is linked to the ability of infected erythrocytes (IE) to adhere to the vascular endothelium, mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1). In this article, we report the functional characterization of an mAb that recognizes a panel of PfEMP1s and inhibits ICAM-1 binding. The 24E9 mouse mAb was raised against PFD1235w DBLβ3_D4, a domain from the group A PfEMP1s associated with severe malaria. 24E9 recognizes native PfEMP1 expressed on the IE surface and shows cross-reactivity with and cross-inhibition of the ICAM-1 binding capacity of domain cassette 4 PfEMP1s. 24E9 Fab fragments bind DBLβ3_D4 with nanomolar affinity and inhibit ICAM-1 binding of domain cassette 4–expressing IE. The antigenic regions targeted by 24E9 Fab were identified by hydrogen/deuterium exchange mass spectrometry and revealed three discrete peptides that are solvent protected in the complex. When mapped onto a homology model of DBLβ3_D4, these cluster to a defined, surface-exposed region on the convex surface of DBLβ3_D4. Mutagenesis confirmed that the site most strongly protected is necessary for 24E9 binding, which is consistent with a low-resolution structure of the DBLβ3_D4::24E9 Fab complex derived from small-angle x-ray scattering. The convex surface of DBLβ3_D4 has previously been shown to contain the ICAM-1 binding site of DBLβ domains, suggesting that the mAb acts by occluding the ICAM-1 binding surface. Conserved epitopes, such as those targeted by 24E9, are promising candidates for the inclusion in a vaccine interfering with ICAM-1–specific adhesion of group A PfEMP1 expressed by P. falciparum IE during severe malaria.

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.

Patterns of nucleotide and haplotype diversity at ICAM-1 across global human populations with varying levels of malaria exposure

Malaria is one of the strongest selective pressures in recent human evolution. African populations have been and continue to be at risk for malarial infections. However, few studies have re-sequenced malaria susceptibility loci across geographically and genetically diverse groups in Africa. We examined nucleotide diversity at Intercellular adhesion molecule-1 (ICAM-1), a malaria susceptibility candidate locus, in a number of human populations with a specific focus on diverse African ethnic groups. We used tests of neutrality to assess whether natural selection has impacted this locus and tested whether SNP variation at ICAM-1 is correlated with malaria endemicity. We observe differing patterns of nucleotide and haplotype variation in global populations and higher levels of diversity in Africa. Although we do not observe a deviation from neutrality based on the allele frequency distribution, we do observe several alleles at ICAM-1, including the ICAM-1 (Kilifi) allele, that are correlated with malaria endemicity. We show that the ICAM-1 (Kilifi) allele, which is common in Africa and Asia, exists on distinct haplotype backgrounds and is likely to have arisen more recently in Asia. Our results suggest that correlation analyses of allele frequencies and malaria endemicity may be useful for identifying candidate functional variants that play a role in malaria resistance and susceptibility.

Diversity of the var gene family of Indonesian Plasmodium falciparum isolates

Background

The large polymorphic protein PfEMP1 is encoded by the var gene family. PfEMP1 has been shown to play an important role as cytoadherence ligand on the surface of infected erythrocytes and thereby contributes to the distinct pathogenesis of malaria. The study explored the diversity of the DBL1α and DBL2β-C2 domains of the protein from Indonesian Plasmodium falciparum field isolates.

Methods

Samples of patients with severe and uncomplicated malaria from two different malaria-endemic areas in Indonesia were collected and DNA directly extracted. Dried blood on filter paper was prepared for RNA extraction. PCR amplicons were either cloned and subsequently sequenced or directly sequenced for analysis on nucleotide and amino acid level. Recently published as well as self-designed primers were used for amplification.

Results

Blood from eight patients was finally used for analysis. Seventy-one different sequences out of over 500 DBL1α sequenced clones were observed, resulting in an average of 8.9 different DBL1α sequences per isolate. The average DBL1α sequence similarity within isolates was similar to between isolates. Phylogenetic analysis demonstrated no clustering of sequences regarding strain or geographical origin. The DBL1α sequences were analysed by distribution of semi-conserved features (cysteine/PoLV1-4 grouping) and classified into six sequence groups. The DBL1α cys2 type was observed in all expressed sequences in vivo. Expression of certain DBL sequences implied potential involvement in the pathogenesis. As expected, the DBL2β-C2 domains showed high to moderate homology among each other.

Conclusion

The DBL1α domains of PfEMP1 from clinical Indonesian isolates showed high divergence among same isolates and some similarities with other Asia-Pacific strains. Further investigations of important var gene domains with a larger sample size are required to confirm with statistical significance observed associations with severe malaria in Indonesian samples.

Molecular architecture of a complex between an adhesion protein from the malaria parasite and intracellular adhesion molecule 1

The adhesion of Plasmodium falciparum-infected erythrocytes to human tissues or endothelium is central to the pathology caused by the parasite during malaria. It contributes to the avoidance of parasite clearance by the spleen and to the specific pathologies of cerebral and placental malaria. The PfEMP1 family of adhesive proteins is responsible for this sequestration by mediating interactions with diverse human ligands. In addition, as the primary targets of acquired, protective immunity, the PfEMP1s are potential vaccine candidates. PfEMP1s contain large extracellular ectodomains made from CIDR (cysteine-rich interdomain regions) and DBL (Duffy-binding-like) domains and show extensive variation in sequence, size, and domain organization. Here we use biophysical methods to characterize the entire ∼300-kDa ectodomain from IT4VAR13, a protein that interacts with the host receptor, intercellular adhesion molecule-1 (ICAM-1). We show through small angle x-ray scattering that IT4VAR13 is rigid, elongated, and monomeric. We also show that it interacts with ICAM-1 through the DBLβ domain alone, forming a 1:1 complex. These studies provide a first low resolution structural view of a PfEMP1 ectodomain in complex with its ligand. They show that it combines a modular domain arrangement consisting of individual ligand binding domains, with a defined higher order architecture that exposes the ICAM-1 binding surface to allow adhesion.

A novel domain cassette identifies Plasmodium falciparum PfEMP1 proteins binding ICAM-1 and is a target of cross-reactive, adhesion-inhibitory antibodies

Cerebral Plasmodium falciparum malaria is characterized by adhesion of infected erythrocytes (IEs) to the cerebral microvasculature. This has been linked to parasites expressing the structurally related group A subset of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of IE adhesion ligands and to IEs with affinity for ICAM-1. However, recent evidence has cast doubt on both these associations, tempering hopes of the feasibility of developing a vaccine based on ICAM-1-binding PfEMP1. In this study, we report the identification of a domain cassette (DC) present in group A var genes from six genetically distinct P. falciparum parasites. The three domains in the cassette, which we call DC4, had a high level of sequence identity and cluster together phylogenetically. Erythrocytes infected by these parasites and selected in vitro for expression of DC4 adhered specifically to ICAM-1. The ICAM-1-binding capacity of DC4 was mapped to the C-terminal third of its Duffy-binding-like β3 domain. DC4 was the target of broadly cross-reactive and adhesion-inhibitory IgG Abs, and levels of DC4-specific and adhesion-inhibitory IgG increased with age among P. falciparum-exposed children. Our study challenges earlier conclusions that group A PfEMP1 proteins are not central to ICAM-1-specific IE adhesion and support the feasibility of developing a vaccine preventing cerebral malaria by inhibiting cerebral IE sequestration.

Investigating the host binding signature on the Plasmodium falciparum PfEMP1 protein family

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family plays a central role in antigenic variation and cytoadhesion of P. falciparum infected erythrocytes. PfEMP1 proteins/var genes are classified into three main subfamilies (UpsA, UpsB, and UpsC) that are hypothesized to have different roles in binding and disease. To investigate whether these subfamilies have diverged in binding specificity and test if binding could be predicted by adhesion domain classification, we generated a panel of 19 parasite lines that primarily expressed a single dominant var transcript and assayed binding against 12 known host receptors. By limited dilution cloning, only UpsB and UpsC var genes were isolated, indicating that UpsA var gene expression is rare under in vitro culture conditions. Consequently, three UpsA variants were obtained by rosette purification and selection with specific monoclonal antibodies to create a more representative panel. Binding assays showed that CD36 was the most common adhesion partner of the parasite panel, followed by ICAM-1 and TSP-1, and that CD36 and ICAM-1 binding variants were highly predicted by adhesion domain sequence classification. Binding to other host receptors, including CSA, VCAM-1, HABP1, CD31/PECAM, E-selectin, Endoglin, CHO receptor “X”, and Fractalkine, was rare or absent. Our findings identify a category of larger PfEMP1 proteins that are under dual selection for ICAM-1 and CD36 binding. They also support that the UpsA group, in contrast to UpsB and UpsC var genes, has diverged from binding to the major microvasculature receptor CD36 and likely uses other mechanisms to sequester in the microvasculature. These results demonstrate that CD36 and ICAM-1 have left strong signatures of selection on the PfEMP1 family that can be detected by adhesion domain sequence classification and have implications for how this family of proteins is specializing to exploit hosts with varying levels of anti-malaria immunity.

The malaria parasite Plasmodium falciparum persists in the human host partly by avoiding elimination in the spleen during blood stage infection. This strategy depends principally upon members of the large and diverse PfEMP1 family of proteins that are exported to the surface of infected erythrocytes. PfEMP1 proteins are important targets for host protective antibody responses and encode binding to several different host receptor proteins. Switches in PfEMP1 expression allow parasites to evade host antibodies and may precipitate severe disease when infected erythrocytes accumulate in brain or placenta. Consequently, the severity of malaria infection may depend on the type of PfEMP1 protein expressed. In this study, we employ a representative panel of distinct PfEMP1 types and host receptor proteins to demonstrate that CD36 and ICAM-1 binding properties of full-length PfEMP1 are highly predicted by their domain composition. We also find that CD36 binding is under strong selection in many PfEMP1 proteins, but that a group of PfEMP1s associated with more severe infections does not bind CD36 and may utilize alternative means to sequester infected erythrocytes. These findings have implications for understanding the molecular basis for severe malaria.

Surface co-expression of two different PfEMP1 antigens on single plasmodium falciparum-infected erythrocytes facilitates binding to ICAM1 and PECAM1

The Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) antigens play a major role in cytoadhesion of infected erythrocytes (IE), antigenic variation, and immunity to malaria. The current consensus on control of variant surface antigen expression is that only one PfEMP1 encoded by one var gene is expressed per cell at a time. We measured var mRNA transcript levels by real-time Q-PCR, analysed var gene transcripts by single-cell FISH and directly compared these with PfEMP1 antigen surface expression and cytoadhesion in three different antibody-selected P. falciparum 3D7 sub-lines using live confocal microscopy, flow cytometry and in vitro adhesion assays. We found that one selected parasite sub-line simultaneously expressed two different var genes as surface antigens, on single IE. Importantly, and of physiological relevance to adhesion and malaria pathogenesis, this parasite sub-line was found to bind both CD31/PECAM1 and CD54/ICAM1 and to adhere twice as efficiently to human endothelial cells, compared to infected cells having only one PfEMP1 variant on the surface. These new results on PfEMP1 antigen expression indicate that a re-evaluation of the molecular mechanisms involved in P. falciparum adhesion and of the accepted paradigm of absolutely mutually exclusive var gene transcription is required.

Plasmodium falciparum is the most pathogenic human malaria parasite and its virulence has been linked to its capacity to express different adhesion proteins that enable the developing parasitized erythrocyte to bind to capillaries of the host, thereby avoiding removal by the spleen. Each parasite has approximately 60 genes encoding different versions of this adhesion protein, and a switch in surface display of these proteins enables the parasite to evade the immune system. Here we show that different variants of these binding proteins can be found expressed simultaneously on single infected red blood cells mediating binding to different endothelial receptors.

Competitive endothelial adhesion between Plasmodium falciparum isolates under physiological flow conditions

BACKGROUND

Sequestration of parasitized red blood cells in the microvasculature of major organs involves a sequence of events that is believed to contribute to the pathogenesis of severe falciparum malaria. Plasmodium falciparum infections are commonly composed of multiple subpopulations of parasites with varied adhesive properties. A key question is: do these subpopulations compete for adhesion to endothelium? This study investigated whether, in a laboratory model of cytoadherence, there is competition in binding to endothelium between pRBC infected with P. falciparum of variant adhesive phenotypes, particularly under flow conditions.

METHODS

Four different P. falciparum isolates, of known adherence phenotypes, were matched in pairs, mixed in different proportions and allowed to bind to cultured human endothelium. Using in vitro competitive static and flow-based adhesion assays, that allow simultaneous testing of the adhesive properties of two different parasite lines, adherence levels of paired P. falciparum isolates were quantified and analysed using either non-parametric Wilcoxon's paired signed rank test or Student paired test.

RESULTS

Study findings show that P. falciparum parasite lines show marked differences in the efficiency of adhesion to endothelium.

CONCLUSION

Plasmodium falciparum variants will compete for adhesion to endothelia and variants can be ranked by their efficiency of binding. These findings suggest that variants from a mixed infection will not show uniform cytoadherence and so may vary in their ability to cause disease.

Sequence variation of PfEMP1-DBLalpha in association with rosette formation in Plasmodium falciparum isolates causing severe and uncomplicated malaria

BACKGROUND

Rosetting and cytoadherence of Plasmodium falciparum-infected red blood cells have been associated with severity of malaria. ICAM-1 and CD36 are the main host cell receptors, while PfEMP1-DBLalpha is a major parasite ligand, which can contribute to rosette formation. This study is aimed at demonstrating whether the highly polymorphic PfEMP1-DBLalpha sequences occurring among Thai isolates causing severe and uncomplicated malaria are associated with their ability to form rosettes and reflected the clinical outcome of the patients.

METHODS

Two hundred and ninety five PfEMP1-DBLalpha sequences from Thai clinical isolates causing severe and uncomplicated malaria were evaluated by sequencing and direct comparison using the specific text string analysis functions in Microsoft Excel and Perl. The relationships between the PfEMP1-DBLalpha sequences were also analysed by network analysis. The binding abilities of parasitized red blood cells (PRBCs) to CD36, wild type ICAM-1, ICAM-1Kilifi and ICAM-1S22/A under static condition were included.

RESULTS

Two hundred and eighty one non-identical amino acid sequences were identified (< 95% sequence identity). When the distributions of semi-conserved features (PoLV1-4 and sequence group) within the rosetting domain PfEMP1-DBLalpha were observed, close similarity was found between isolates from the two disease groups. The sequence group 1 representing uncomplicated malaria was significantly different from the sequence group 3 representing the majority of severe malaria (p = 0.027). By using a simple non-phylogenetic approach to visualize the sharing of polymorphic blocks (position specific polymorphic block, PSPB) and cys/PoLV among DBLalpha sequences, the sequence group 1 was split from the other five sequence groups. The isolates belonging to sequence group 5 gave the highest mean rosetting rate (21.31%). However, within sequence group 2 and group 6, the isolates causing severe malaria had significantly higher rosetting rate than those causing uncomplicated malaria (p = 0.014, p = 0.007, respectively).

CONCLUSION

This is the first report of PfEMP1-DBLalpha analysis in clinical Thai isolates using semi-conserved features (cys/PoLV and PSPBs). The cys/PoLV group 5 gave the highest rosetting rate. PfEMP1-DBLalpha domains in Thai isolates are highly diverse, however, clinical isolates from severe and uncomplicated malaria shared common sequences.

A model of the complex between the PfEMP1 malaria protein and the human ICAM-1 receptor

Malaria is caused by protozoan parasites of the genus Plasmodium. Four species of Plasmodium can infect humans: P. falciparum, P. malariae, P. vivax, and P. ovale. P. falciparum is the only able to cytoadhere to the surface of postcapillary endothelial cells. A key role in cytoadherence is played by the interaction between the PfEMP1 P. falciparum protein and the human intracellular adhesion molecule (ICAM-1) although very little is known about the molecular details of this complex. Here we propose a model for this interaction on the basis of a homology model of the functional domain of PfEMP1 and of the ICAM-1 three dimensional structures. Our model is consistent with the results of many experimental observations, provides a rational explanation for the different binding abilities of different strains of P. falciparum and explains the reduced binding affinity of the A4 strain of P. falciparum for the ICAM-1(Kilifi) polymorphism. On the basis of our model, we can also explain why the murine ICAM-1, although sharing 70% sequence similarity with its human homologue, does not bind PfEMP1, and why the binding of fibrinogen and PfEMP1 to ICAM-1 is mutually exclusive. The model of the complex proposed here can serve as a useful tool for the design and interpretation of biochemical and immunological experimental results.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

Malaria and bacterial sepsis: similar mechanisms of endothelial apoptosis and its prevention in vitro

OBJECTIVE AND DESIGN

Apoptotic endothelial damage contributes to multiorgan failure in Plasmodium falciparum malaria and in sepsis. In malaria, endothelial apoptosis is amplified by neutrophils and their secretory products, and reduced by inhibitors of neutrophil-derived substances in vitro. We compared the mechanisms of endothelial apoptosis in malaria and in sepsis, using the human umbilical vein endothelial cell as a model.

INTERVENTIONS

Endothelial cells were incubated with patient sera (P. falciparum malaria, Escherichia coli sepsis, Staphylococcus aureus sepsis) or culture supernatants of the respective organisms, with or without neutrophils. Ascorbic acid or ulinastatin was used to neutralize reactive oxygen species or elastase secreted by neutrophils. Transwell sieve inserts or antibodies against leukocyte function antigen 1 or intercellular adhesion molecule 1 was used to study the effect of direct interaction between neutrophils and endothelial cells. The rate of apoptotic endothelial cells was determined by TUNEL and annexin staining.

MEASUREMENTS AND MAIN RESULTS

Incubation of endothelial cells with patient sera or culture supernatants (P. falciparum, E. coli, S. aureus) lead to higher apoptosis rates, compared with incubation with control sera or control supernatants. Addition of neutrophils augmented the apoptosis rate further. Addition of ascorbic acid or ulinastatin reduced endothelial apoptosis in the presence of neutrophils. Separation of neutrophils from endothelial cells with Transwell sieve inserts, or addition of anti-leukocyte function antigen-1 antibodies also reduced endothelial cell apoptosis. However, addition of anti-intercellular adhesion molecule-1 antibodies restored high apoptosis rates that had been reduced by Transwell inserts.

CONCLUSIONS

These in vitro results show how neutrophils can contribute to endothelial damage in malaria and in sepsis, both by their secretory products and by binding to intercellular adhesion molecule-1 on endothelial cells. The presence of similar pathomechanisms suggests that similar antiapoptotic strategies may offer potential benefit in malaria and in sepsis.

The cysteine-rich interdomain region from the highly variable plasmodium falciparum erythrocyte membrane protein-1 exhibits a conserved structure

Plasmodium falciparum malaria parasites, living in red blood cells, express proteins of the erythrocyte membrane protein-1 (PfEMP1) family on the red blood cell surface. The binding of PfEMP1 molecules to human cell surface receptors mediates the adherence of infected red blood cells to human tissues. The sequences of the 60 PfEMP1 genes in each parasite genome vary greatly from parasite to parasite, yet the variant PfEMP1 proteins maintain receptor binding. Almost all parasites isolated directly from patients bind the human CD36 receptor. Of the several kinds of highly polymorphic cysteine-rich interdomain region (CIDR) domains classified by sequence, only the CIDR1α domains bind CD36. Here we describe the CD36-binding portion of a CIDR1α domain, MC179, as a bundle of three α-helices that are connected by a loop and three additional helices. The MC179 structure, containing seven conserved cysteines and 10 conserved hydrophobic residues, predicts similar structures for the hundreds of CIDR sequences from the many genome sequences now known. Comparison of MC179 with the CIDR domains in the genome of the P. falciparum 3D7 strain provides insights into CIDR domain structure. The CIDR1α three-helix bundle exhibits less than 20% sequence identity with the three-helix bundles of Duffy-binding like (DBL) domains, but the two kinds of bundles are almost identical. Despite the enormous diversity of PfEMP1 sequences, the CIDR1α and DBL protein structures, taken together, predict that a PfEMP1 molecule is a polymer of three-helix bundles elaborated by a variety of connecting helices and loops. From the structures also comes the insight that DBL1α domains are approximately 100 residues larger and that CIDR1α domains are approximately 100 residues smaller than sequence alignments predict. This new understanding of PfEMP1 structure will allow the use of better-defined PfEMP1 domains for functional studies, for the design of candidate vaccines, and for understanding the molecular basis of cytoadherence.

Malaria parasites express proteins of the erythrocyte membrane protein-1 family (PfEMP1) on the surfaces of the human red blood cells that they infect. These large proteins vary in sequence extensively, yet bind to host receptors to allow infected cells to adhere to host tissues. PfEMP1 proteins help parasites evade the immune system, as the 60 PfEMP1 genes are expressed one at a time. Sequence comparisons predict that PfEMP1 molecules are modular, made up of Duffy binding-like (DBL) and cysteine-rich interdomain region (CIDR) domains. Many CIDR domains bind to the human receptor CD36. We have analyzed the structure of the CD36-binding portion, known as MC179, of a CIDR domain. The MC179 protein is composed of a bundle of three helices connected by a loop and three additional helices. Based on the structure and sequence similarities, MC179 is a good model for the hundreds of known CIDR sequences. In addition, the MC179 three-helix bundle is remarkably similar to subdomain 3 of the known DBL structures. MC179 provides insight into the relatedness of both kinds of PfEMP1 domains and predicts that the large PfEMP1 molecules are polymers of three-helix bundles and their connecting polypeptides.

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.

Mapping a common interaction site used by Plasmodium falciparum Duffy binding-like domains to bind diverse host receptors

The Duffy binding-like (DBL) domain is a key adhesive module in Plasmodium falciparum, present in both erythrocyte invasion ligands (EBLs) and the large and diverse P. falciparum erythrocyte membrane protein 1 (PfEMP1) family of cytoadherence receptors. DBL domains bind a variety of different host receptors, including intercellular adhesion molecule 1 (ICAM-1), a receptor interaction that may have a role in infected erythrocyte binding to cerebral blood vessels and cerebral malaria. In this study, we expressed the nearly full complement of DBLbeta-C2 domains from the IT4/25/5 (IT4) parasite isolate and showed that ICAM-1-binding domains (DBLbeta-C2(ICAM-1)) were confined to group B and group C PfEMP1 proteins and were not present in group A, suggesting that ICAM-1 selection pressure differs between PfEMP1 groups. To further dissect the molecular determinants of binding, we modelled a DBLbeta-C2(ICAM-1) domain on a solved DBL structure and created alanine substitution mutants in two DBLbeta-C2(ICAM-1) domains. This analysis indicates that the DBLbeta-C2::ICAM-1 interaction maps to the equivalent glycan binding region of EBLs, and suggests a general model for how DBL domains evolve under dual selection for host receptor binding and immune evasion.

A family affair: var genes, PfEMP1 binding, and malaria disease

An immunovariant adhesion protein family in Plasmodium falciparum named erythrocyte membrane protein 1 (PfEMP1), encoded by var genes, is responsible for both antigenic variation and cytoadhesion of infected erythrocytes at blood microvasculature sites throughout the body. Elucidation of the genome sequence of P. falciparum has revealed that var genes can be classified into different groups, each with distinct 5' flanking sequences, chromosomal locations and gene orientations. Recent binding and serological comparisons suggest that this genomic organization might cause var genes to diversify into separately recombining adhesion groups that have different roles in infection and disease. Detailed understanding of PfEMP1 expression and receptor binding mechanisms during infection and of the antigenic relatedness of disease variants might lead to new approaches in prevention of malaria disease.

The role of ICAM-1 in Plasmodium falciparum cytoadherence

Parasite sequestration at microvascular sites is a fundamental phenomenon in the manifestation of the symptoms of malaria and the progression to severe disease. Here, we review the endothelial cell-expressed intercellular adhesion molecule-1 (ICAM-1) and its role in mediating the interaction between the parasitised red blood cell (PRBC) and the vascular endothelium. We highlight the nature of the interaction between ICAM-1 and the parasite-expressed PfEMP-1 molecule at the molecular level. The review also discusses the complexity of the PRBC-endothelial cell interaction and the mechanisms that underlie parasite cytoadherence.

Functional interdependence of the DBLbeta domain and c2 region for binding of the Plasmodium falciparum variant antigen to ICAM-1

Cytoadherence of Plasmodium falciparum-infected erythrocytes is associated with severe malaria and is primarily mediated through binding of the variant surface antigen P. falciparum erythrocyte membrane protein 1 (PfEMP1) to specific host ligands. Infected erythrocyte binding to Intercellular Adhesion Molecule 1 (ICAM-1) has been implicated as having a role in cerebral malaria, a major cause of death from P. falciparum infection. We have examined ICAM-1-binding PfEMP1 proteins in the cytoadhesive P. falciparum strain IT4/25/5 in order to extend our understanding of binding. For A4tres, the ICAM-1 binding region was previously shown to reside within contiguous DBL2beta and c2 domains. We determined the gene sequence encoding IT-ICAM var, and showed that ICAM-1 binding in this protein also maps to DBL2betac2 domains that have 48% amino acid identity to A4tres. By truncation and chimera analysis, most of the DBL2beta and the first half of the c2 region were required for A4tres binding to ICAM-1, suggesting this tandem should be considered a structural-functional combination for ICAM-1 binding. Of interest, a chimera formed between two different ICAM-1 binding domains did not bind ICAM-1, suggesting a functional interdependence between DBL2beta and c2 from the same protein. As gene recombination and gene conversion are important mechanisms for generating diversity in the PfEMP1 protein family, this finding implies an extra level of constraint on the functional evolution of binding traits. Knowledge about the PfEMP1::ICAM-1 interaction may allow the development of interventions to prevent binding and disease.

Lymphatic metastasis and nm23H1 genetic instability in Chinese colon cancer patients

AIM: To investigate the pathogenic mechanism of colon cancer at the molecular level and to elucidate the relationship between intercellular adhesion molecule-1 (ICAM-1) and nm23H₁ genes and Chinese patients with colon cancer.

METHODS: DNA was extracted from paraffin-embedded materials. Polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) was used to analyze MSI and LOH. Expression of ICAM-1 was detected by Envision immuno-histochemistry. Experimental results were analyzed with Leica-Qwin computer imaging techniques and SPSS software of statistics.

RESULTS: ICAM-1 expression of lymphatic endothelium was negative in normal colon and positive in colon cancer respectively. The number of lymphatics positive for ICAM-1 was gradually increased with degree of cancer invasion (P < 0.01). In the group with metastasis of colon cancer, the number of lymphatics positive for ICAM-1 in lymph nodes was more than that in the group with no metastasis (P < 0.01). The frequency of MSI, LOH and nm23H₁ protein was 26.67%, 20.00% and 53.33% in colon cancer, respectively. In TNM staging, MSI (43.75%) and nm23H₁ protein (81.25%) in stages I + II were detected more easily than the corresponding indexes (MSI: 7.14%, P < 0.05 and nm23H₁: 21.43%, P < 0.01) in stages III + IV. By comparison, the frequency of LOH (35.71%) in stages III + IV was more than that of LOH (6.25%, P < 0.05) in stages I + II. LOH exhibited a rising trend along with the Duke’s staging. nm23H₁ protein in the group of tubular adenocarcinoma (60.00%) was higher expressed than that in the group of mucoid adenocarcinoma (20.00%) (P < 0.01), and exhibited a rising trend with the differentiation degrees of tubular adenocarcinoma. nm23H₁ protein in MSI positive group was higher expressed (75%) than that in MSI negative group (45.45%, P < 0.05).

CONCLUSION: The expression of ICAM-1 in lymphatic vessels is beneficial to the judgement of the invasion and metastasis ability of colon cancer and the anti-tumor immunity function, and shows an important clinical significance in predicting lymphatic metastasis of colon cancer. MSI and LOH may separately control the development of sporadic colon cancer with different pathways. LOH mostly arises in the late period of sporadic colon cancer and endows a high aggressive and poor prognostic phenotype. By compassion, MSI may be an early period molecule marker for sporadic colon cancer, enhanced expression of nm23H₁ protein can effectively inhibit colon cancer metastasis and improve prognosis of sporadic colon cancer patients.

Receptor-binding residues lie in central regions of Duffy-binding-like domains involved in red cell invasion and cytoadherence by malaria parasites

Erythrocyte invasion by malaria parasites and cytoadherence of Plasmodium falciparum-infected erythrocytes to host capillaries are 2 key pathogenic mechanisms in malaria. The receptor-binding domains of erythrocyte-binding proteins (EBPs) such as Plasmodium falciparum EBA-175, which mediate invasion, and P falciparum erythrocyte membrane protein 1 (PfEMP-1) family members, which are encoded by var genes and mediate cytoadherence, have been mapped to conserved cysteine-rich domains referred to as Duffy-binding-like (DBL) domains. Here, we have mapped regions within DBL domains from EBPs and PfEMP-1 that contain receptor-binding residues. Using biochemical and molecular methods we demonstrate that the receptor-binding residues of parasite ligands that bind sialic acid on glycophorin A for invasion as well as complement receptor-1 and chondroitin sulfate A for cytoadherence map to central regions of DBL domains. In contrast, binding to intercellular adhesion molecule 1 (ICAM-1) requires both the central and terminal regions of DBLbetaC2 domains. Determination of functional regions within DBL domains is the first step toward understanding the structure-function bases for their interaction with diverse host receptors.

Divergent binding sites on intercellular adhesion molecule-1 (ICAM-1) for variant Plasmodium falciparum isolates

Adhesion of human erythrocytes infected with the malaria parasite Plasmodium falciparum to host endothelium has been associated with severe forms of this disease. A number of endothelial receptors have been identified, and there is evidence that one of these, intercellular adhesion molecule-1 (ICAM-1), may play an important role in the pathology of cerebral malaria. Mutagenesis of domain 1 of ICAM-1, which is involved in parasite adhesion, shows that the binding sites for different parasite variants overlap to a large extent, but that there are subtle differences between them that correlate with their adhesive phenotypes. This suggests that the ability to bind to ICAM-1 has arisen from a common variant, but that subsequent changes have led to differences in binding avidity, which may affect pathogenesis. The definition of common binding determinants and the elucidation of links between ICAM-1 binding phenotype and disease will provide new leads in the design of therapeutic interventions.