Plasmodium falciparum associated with severe childhood malaria preferentially expresses PfEMP1 encoded by group A var genes

Cytoadherence and genotype of Plasmodium falciparum strains from symptomatic children in Franceville, southeastern Gabon

BACKGROUND

Plasmodium falciparum causes severe clinical manifestations by sequestering parasitized red blood cells (PRBC) in the microvasculature of major organs such as the brain. This sequestration results from PRBC adherence to vascular endothelial cells via erythrocyte membrane protein 1, a variant parasite surface antigen.

OBJECTIVE

To determine whether P. falciparum multiple genotype infection (MGI) is associated with stronger PRBC cytoadherence and greater clinical severity.

METHODS

Nested polymerase chain reaction was used to genotype P. falciparum isolates from symptomatic children and to distinguish between single genotype infection (SGI) and MGI. PRBC cytoadhesion was studied with cultured human lung endothelial cells.

RESULTS

Analysis of two highly polymorphic regions of the merozoite surface antigen (MSP)-1 and MSP-2 genes and a dimorphic region of the erythrocyte binding antigen-175 gene showed that 21.4% and 78.6% of the 42 children had SGI and MGI, respectively. It also showed that 37 (89%) of the 42 PRBC samples expressed MSP-1 allelic family K1. Cytoadherence values ranged from 58 to 1811 PRBC/mm(2) of human lung endothelial cells monolayer in SGI and from 5 to 5744 PRBC/mm(2) in MGI. MGI was not associated with higher cytoadherence values or with more severe malaria.

CONCLUSIONS

These results suggested that infection of the same individual by multiple clones of P. falciparum does not significantly influence PRBC cytoadherence or disease severity and confirmed the predominance of the MSP-1 K1 genotype in southeastern Gabon.

Molecular epidemiology of malaria

Malaria persists as an undiminished global problem, but the resources available to address it have increased. Many tools for understanding its biology and epidemiology are well developed, with a particular richness of comparative genome sequences. Targeted genetic manipulation is now effectively combined with in vitro culture assays on the most important human parasite, Plasmodium falciparum, and with in vivo analysis of rodent and monkey malaria parasites in their laboratory hosts. Studies of the epidemiology, prevention, and treatment of human malaria have already been influenced by the availability of molecular methods, and analyses of parasite polymorphisms have long had useful and highly informative applications. However, the molecular epidemiology of malaria is currently undergoing its most substantial revolution as a result of the genomic information and technologies that are available in well-resourced centers. It is a challenge for research agendas to face the real needs presented by a disease that largely exists in extremely resource-poor settings, but it is one that there appears to be an increased willingness to undertake. To this end, developments in the molecular epidemiology of malaria are reviewed here, emphasizing aspects that may be current and future priorities.

Comparison of cellular and humoral responses to recombinant protein and synthetic peptides of exon2 region of Plasmodium falciparum erythrocyte membrane protein1 (PfEMP1) among malaria patients from an endemic region

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) expressed on the surface of parasitized red blood cells (PRBCs) mediate adhesion of PRBCs to host vascular endothelial receptors and is considered responsible for pathogenesis of severe P. falciparum malaria. The present study was undertaken to measure cellular immune responses and serum antibody responses against recombinant exon2 protein, the most conserved region of PfEMP1, and its synthetic peptides. T cell recognizing this domain could provide universal help to B cells in recognizing variant epitopes located in the extracellular region of PfEMP1. Human peripheral blood mononuclear cells from malaria-exposed immune adults (IA), malaria patients with varying severity, and malaria unexposed healthy donors were stimulated with recombinant exon2 protein and six synthetic peptides from its sequence to estimate the proliferative, IFN-gamma, and IL-4 responses. Antibody responses against these synthetic peptides and exon2 protein were also studied. Positive proliferative, IFN-gamma, and IL-4 responses in IA group each were 60% with recombinant exon2 protein and 27-47% with different synthetic peptides. Antibody recognition was observed in 67% with exon2 and between 40 and 53% with different peptides. In malaria patients, frequency and magnitude of proliferative response, IL-4 concentration, and antibody recognition were far less than immune adults but IFN-gamma response was almost similar. Proportion of positive responders and the magnitude of response to synthetic peptides were low. Also, there was no consistency in response of different peptides towards proliferative, cytokine, and antibody responses in IA and malaria patient groups except for peptide 1. We presume peptide 1 is a potential vaccine candidate and different cocktails containing peptide 1 are being evaluated for their T cell immunogenicity.

Programmed transcription of the var gene family, but not of stevor, in Plasmodium falciparum gametocytes

The var genes encode Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) proteins, a set of highly diverse surface-expressed proteins that mediate adhesion of erythrocytes infected with asexual blood-stage parasites to host endothelium. Switching among expressed PfEMP1 variants in the course of a blood-stage infection is a key component of antigenic variation, and thus immune evasion, by the parasite. The majority of var loci are found in the subtelomeric regions of P. falciparum chromosomes associated with members of other multigene families, including stevor. Both PfEMP1 and STEVOR are expressed in gametocytes, the transmissible parasite stage, but the role of these proteins in the biology of sexual-stage parasites remains unknown. PfEMP1 may continue to mediate antigenic variation in gametocytes, which need to persist in the host for many days before reaching maturity. Using quantitative reverse transcription-PCR and Northern hybridization, we demonstrate that transcription of a defined subset of type C var loci occurs during gametocyte development in vitro. This transcriptional program occurs in gametocytes regardless of the var expression phenotype of their asexual progenitors and therefore is subject to regulatory processes distinct from those that manage antigenic variation in the asexual parasite. In contrast, the same stevor variants are transcribed in both gametocytes and their asexual progenitors. We also provide evidence that for both asexual parasites and gametocytes, var and stevor transcription patterns are not linked to each other.

Patterns of gene recombination shape var gene repertoires in Plasmodium falciparum: comparisons of geographically diverse isolates

Background

Var genes encode a family of virulence factors known as PfEMP1 (Plasmodium falciparum erythrocyte membrane protein 1) which are responsible for both antigenic variation and cytoadherence of infected erythrocytes. Although these molecules play a central role in malaria pathogenesis, the mechanisms generating variant antigen diversification are poorly understood. To investigate var gene evolution, we compared the variant antigen repertoires from three geographically diverse parasite isolates: the 3D7 genome reference isolate; the recently sequenced HB3 isolate; and the IT4/25/5 (IT4) parasite isolate which retains the capacity to cytoadhere in vitro and in vivo.

Results

These comparisons revealed that only two var genes (var1csa and var2csa) are conserved in all three isolates and one var gene (Type 3 var) has homologs in IT4 and 3D7. While the remaining 50 plus genes in each isolate are highly divergent most can be classified into the three previously defined major groups (A, B, and C) on the basis of 5' flanking sequence and chromosome location. Repertoire-wide sequence comparisons suggest that the conserved homologs are evolving separately from other var genes and that genes in group A have diverged from other groups.

Conclusion

These findings support the existence of a var gene recombination hierarchy that restricts recombination possibilities and has a central role in the functional and immunological adaptation of var genes.

Immunoglobulin G antibody reactivity to a group A Plasmodium falciparum erythrocyte membrane protein 1 and protection from P. falciparum malaria

Variant surface antigens (VSA) on the surface of Plasmodium falciparum-infected red blood cells play a major role in the pathogenesis of malaria and are key targets for acquired immunity. The best-characterized VSA belong to the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family. In areas where P. falciparum is endemic, parasites causing severe malaria and malaria in young children with limited immunity tend to express semiconserved PfEMP1 molecules encoded by group A var genes. Here we investigated antibody responses of Tanzanians who were 0 to 19 years old to PF11_0008, a group A PfEMP1. PF11_0008 has previously been found to be highly transcribed in a nonimmune Dutch volunteer experimentally infected with NF54 parasites. A high proportion of the Tanzanian donors had antibodies against recombinant PF11_0008 domains, and in children who were 4 to 9 years old the presence of antibodies to the PF11_0008 CIDR2beta domain was associated with reduced numbers of malaria episodes. These results indicate that homologues of PF11_0008 are present in P. falciparum field isolates and suggest that PF11_0008 CIDR2beta-reactive antibodies might be involved in protection against malaria episodes.

Heterochromatin-mediated control of virulence gene expression

In recent years, the sequencing and annotation of complete genomes, together with the development of genetic and proteomic techniques to study previously intractable eukaryotic microbes, has revealed interesting new themes in the control of virulence gene expression. Families of variantly expressed genes are found adjacent to telomeres in the genomes of both pathogenic and non-pathogenic organisms. This subtelomeric DNA is normally heterochromatic and higher-order chromatin structure has now come to be recognized as an important factor controlling both the evolution and expression dynamics of these multigene families. In eukaryotic cells, higher-order chromatin structure plays a central role in many DNA processes including the control of chromosome integrity and recombination, DNA partitioning during cell division, and transcriptional control. DNA can be packaged in two distinct forms: euchromatin is relatively accessible to DNA binding proteins and generally contains active genes, while heterochromatin is densely packaged, relatively inaccessible and usually transcriptionally silent. These features of chromatin are epigenetically inherited from cell cycle to cell cycle. This review will focus on the epigenetic mechanisms used to control expression of virulence genes in medically important microbial pathogens. Examples of such control have now been reported in several evolutionarily distant species, revealing what may be a common strategy used to regulate many very different families of genes.

3D7-DerivedPlasmodium falciparumErythrocyte Membrane Protein 1 Is a Frequent Target of Naturally Acquired Antibodies Recognizing Protein Domains in a Particular Pattern Independent of Malaria Transmission Intensity

Protection against Plasmodium falciparum malaria is largely mediated by IgG against surface Ags such as the erythrocyte membrane protein 1 family (PfEMP1) responsible for antigenic variation and sequestration of infected erythrocytes. PfEMP1 molecules can be divided into groups A, B/A, B, C, and B/C. We have previously suggested that expression of groups A and B/A PfEMP1 is associated with severe disease and that Abs to these molecules are acquired earlier in life than Abs to PfEMP1 belonging to groups B, B/C, and C PfEMP1. In this study, we compared the acquisition of IgG to 20 rPfEMP1 domains derived from 3D7 in individuals living under markedly different malaria transmission intensity and were unable to find differences in the Ab acquisition rate to PfEMP1 of different groupings (A, B, or C) or domain type (alpha, beta, gamma, delta, epsilon, or x). Abs were acquired early in life in individuals living in the high transmission village and by the age of 2-4 years most individuals had Abs against most constructs. This level of reactivity was found at the age of 10-20 years in the medium transmission village and was never reached by individuals living under low transmission. Nevertheless, the sequence by which individuals acquired Abs to particular constructs was largely the same in the three villages. This indicates that the pattern of PfEMP1 expression by parasites transmitted at the different sites was similar, suggesting that PfEMP1 expression is nonrandom and shaped by host-parasite relationship factors operating at all transmission intensities.

Epitope mapping and topographic analysis of VAR2CSA DBL3X involved in P. falciparum placental sequestration

Pregnancy-associated malaria is a major health problem, which mainly affects primigravidae living in malaria endemic areas. The syndrome is precipitated by accumulation of infected erythrocytes in placental tissue through an interaction between chondroitin sulphate A on syncytiotrophoblasts and a parasite-encoded protein on the surface of infected erythrocytes, believed to be VAR2CSA. VAR2CSA is a polymorphic protein of approximately 3,000 amino acids forming six Duffy-binding-like (DBL) domains. For vaccine development it is important to define the antigenic targets for protective antibodies and to characterize the consequences of sequence variation. In this study, we used a combination of in silico tools, peptide arrays, and structural modeling to show that sequence variation mainly occurs in regions under strong diversifying selection, predicted to form flexible loops. These regions are the main targets of naturally acquired immunoglobulin gamma and accessible for antibodies reacting with native VAR2CSA on infected erythrocytes. Interestingly, surface reactive anti-VAR2CSA antibodies also target a conserved DBL3X region predicted to form an alpha-helix. Finally, we could identify DBL3X sequence motifs that were more likely to occur in parasites isolated from primi- and multigravidae, respectively. These findings strengthen the vaccine candidacy of VAR2CSA and will be important for choosing epitopes and variants of DBL3X to be included in a vaccine protecting women against pregnancy-associated malaria.

Human pregnancy-associated malaria-specific B cells target polymorphic, conformational epitopes in VAR2CSA

Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes (IEs) that bind to chondroitin sulphate A (CSA) in the placenta by PAM-associated clonally variant surface antigens (VSA). Pregnancy-specific VSA (VSA_PAM), which include the PfEMP1 variant VAR2CSA, are targets of IgG-mediated protective immunity to PAM. Here, we report an investigation of the specificity of naturally acquired immunity to PAM, using eight human monoclonal IgG1 antibodies that react exclusively with intact CSA-adhering IEs expressing VSA_PAM. Four reacted in Western blotting with high-molecular-weight (> 200 kDa) proteins, while seven reacted with either the DBL3-X or the DBL5-ε domains of VAR2CSA expressed either as Baculovirus constructs or on the surface of transfected Jurkat cells. We used a panel of recombinant antigens representing DBL3-X domains from P. falciparum field isolates to evaluate B-cell epitope diversity among parasite isolates, and identified the binding site of one monoclonal antibody using a chimeric DBL3-X construct. Our findings show that there is a high-frequency memory response to VSA_PAM, indicating that VAR2CSA is a primary target of naturally acquired PAM-specific protective immunity, and demonstrate the value of human monoclonal antibodies and conformationally intact recombinant antigens in VSA characterization.

Limited cross-reactivity among domains of the Plasmodium falciparum clone 3D7 erythrocyte membrane protein 1 family

The var gene-encoded Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family is responsible for antigenic variation and sequestration of infected erythrocytes during malaria. We have previously grouped the 60 PfEMP1 variants of P. falciparum clone 3D7 into groups A and B/A (category A) and groups B, B/C, and C (category non-A). Expression of category A molecules is associated with severe malaria, and that of category non-A molecules is associated with uncomplicated malaria and asymptomatic infection. Here we assessed cross-reactivity among 60 different recombinant PfEMP1 domains derived from clone 3D7 by using a competition enzyme-linked immunosorbent assay and a pool of plasma from 63 malaria-exposed Tanzanian individuals. We conclude that naturally acquired antibodies are largely directed toward epitopes varying between different domains with a few, mainly category A, domains sharing cross-reactive antibody epitopes. Identification of groups of serological cross-reacting molecules is pivotal for the development of vaccines based on PfEMP1.

The frequency of BDCA3-positive dendritic cells is increased in the peripheral circulation of Kenyan children with severe malaria

The ability of Plasmodium falciparum-infected erythrocytes to adhere to host endothelial cells via receptor molecules such as ICAM-1 and CD36 is considered a hallmark for the development of severe malaria syndromes. These molecules are also expressed on leukocytes such as dendritic cells. Dendritic cells are antigen-presenting cells that are crucial for the initiation of adaptive immune responses. In many human diseases, their frequency and function is perturbed. We analyzed the frequency of peripheral blood dendritic cell subsets and the plasma concentrations of interleukin-10 (IL-10) and IL-12 in Kenyan children with severe malaria and during convalescence and related these parameters to the adhesion phenotype of the acute parasite isolates. The frequency of CD1c(+) dendritic cells in children with acute malaria was comparable to that in healthy controls, but the frequency of BDCA3(+) dendritic cells was significantly increased. Analysis of the adhesion phenotypes of parasite isolates revealed that adhesion to ICAM-1 was associated with the frequency of peripheral blood CD1c(+) dendritic cells, whereas the adhesion of infected erythrocytes to CD36 correlated with high concentrations of IL-10 and low concentrations of IL-12 in plasma.

Differential var gene transcription in Plasmodium falciparum isolates from patients with cerebral malaria compared to hyperparasitaemia

The Plasmodium falciparum variant erythrocyte surface antigens known as PfEMP1, encoded by the var gene family, are thought to play a crucial role in malaria pathogenesis because they mediate adhesion to host cells and immuno-modulation. Var genes have been divided into three major groups (A, B and C) and two intermediate groups (B/A and B/C) on the basis of their genomic location and upstream sequence. We analysed expressed sequence tags of the var gene DBLα domain to investigate var gene transcription in relation to disease severity in Malian children. We found that P. falciparum isolates from children with cerebral malaria (unrousable coma) predominantly transcribe var genes with DBLα1-like domains that are characteristic of Group A or B/A var genes. In contrast, isolates from children with equally high parasite burdens but no symptoms or signs of severe malaria (hyperparasitaemia patients) predominantly transcribe var genes with DBLα0-like domains that are characteristic of the B and C-related var gene groups. These results suggest that var genes with DBLα1-like domains (Group A or B/A) may be implicated in the pathogenesis of cerebral malaria, while var genes with DBLα0-like domains promote less virulent malaria infections.

Baculovirus-expressed constructs induce immunoglobulin G that recognizes VAR2CSA on Plasmodium falciparum-infected erythrocytes

We raised specific antisera against recombinant VAR2CSA domains produced in Escherichia coli and in insect cells. All were reactive in enzyme-linked immunosorbent assay, but only insect cell-derived constructs induced immunoglobulin G (IgG) that was reactive with native VAR2CSA on the surface of infected erythrocytes. Our data show that five of the six VAR2CSA Duffy-binding-like domains are surface exposed and that induction of surface-reactive VAR2CSA-specific IgG depends critically upon antigen conformation. These findings have implications for the development of vaccines against pregnancy-associated Plasmodium falciparum malaria.

Differential expression of var gene groups is associated with morbidity caused by Plasmodium falciparum infection in Tanzanian children

The var gene family of Plasmodium falciparum encodes the variant surface antigen Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 is considered an important pathogenicity factor in P. falciparum infection because it mediates cytoadherence to host cell endothelial receptors. var genes can be grouped into three major groups, A, B, and C, and the conserved var genes, var1-4, according to sequence similarities in coding and noncoding upstream regions. Using real-time quantitative PCR in a study conducted in Tanzania, the var transcript abundances of the different var gene groups were compared among patients with severe, uncomplicated, and asymptomatic malaria. Transcripts of var group A and B genes were more abundant in patients with severe malaria than in patients with uncomplicated malaria. In general, the transcript abundances of var group A and B genes were higher for children with clinical malaria than for children with asymptomatic infections. The var group C and var1-like transcript abundances were similar between the three sample groups. A transcript abundance pattern similar to that for var group A was observed for var2csa and var3-like genes. These results suggest that substantial and systematic differences in var gene expression exist between different clinical presentations.

Detection sensitivity and quantitation of Plasmodium falciparum var gene transcripts by real-time RT-PCR in comparison with conventional RT-PCR

Antigenic variation in Plasmodium falciparum erythrocyte membrane protein 1, caused by a switch in transcription of the encoding var gene, is an important feature of malaria. In this study, we quantified the relative abundance of var gene transcripts present in P. falciparum parasite clones using real-time reverse transcription-polymerase chain reaction (RT-PCR) and conventional RT-PCR combined with cloning and sequencing, with the aim of directly comparing the results obtained. When there was sufficient abundance of RNA for the real-time RT-PCR assay to be operating within the region of good reproducibility, RT-PCR and real-time RT-PCR tended to identify the same dominant transcript, although some transcript-specific issues were identified. When there were differences in the estimated relative amounts of minor transcripts, the RT-PCR assay tended to produce higher estimates than real-time RT-PCR. These results provide valuable information comparing RT-PCR and real-time RT-PCR analysis of samples with small quantities of RNA as might be expected in the analysis of field or clinical samples.

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.

Variant antigen gene expression in malaria

Pathogens of the genus Plasmodium are unicellular parasites that infect a variety of animals, including reptiles, birds and mammals. All Plasmodium species target host erythrocytes and replicate asexually within this niche. In humans, proliferation within erythrocytes causes disease symptoms ranging from asymtomatic infection to severe disease, including mild to severe febrile and respiratory symptoms, profound anaemia and obstruction of blood flow. The most serious form of human malaria is caused by Plasmodium falciparum, a pathogen that is responsible for several million deaths annually throughout the developing world. Malaria parasites succeed in evading the host immune response to establish long-term, persistent infections, thus increasing the efficiency by which they are transmitted to the mosquito vector. The ability to evade the host immune system, in particular the avoidance of antibody-mediated immunity against parasite-encoded surface proteins, is the result of amplification of extensive repertoires of multicopy, hypervariable gene families that encode infected erythrocyte or merozoite surface proteins. Via switching between antigenically diverse genes within these large families, populations of parasites have the capacity for rapid variation in antigenicity and virulence over the course of an infection. Here we review the amplification and generation of antigenic diversity within the Plasmodium variant gene families, as well as discuss the mechanisms underlying their tightly controlled gene expression and antigenic switching.

Virulence of malaria is associated with differential expression of Plasmodium falciparum var gene subgroups in a case-control study

Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is a major pathogenicity factor in falciparum malaria that mediates cytoadherence. PfEMP1 is encoded by approximately 60 var genes per haploid genome. Most var genes are grouped into 3 subgroups: A, B, and C. Evidence is emerging that the specific expression of these subgroups has clinical significance. Using field samples from children from Papua New Guinea with severe, mild, and asymptomatic malaria, we compared proportions of transcripts of var groups, as determined by quantitative polymerase chain reaction. We found a significantly higher proportion of var group B transcripts in children with clinical malaria (mild and severe), whereas a large proportion of var group C transcripts was found in asymptomatic children. These data from naturally infected children clearly show that major differences exist in var gene expression between parasites causing clinical disease and those causing asymptomatic infections. Furthermore, parasites forming rosettes showed a significant up-regulation of var group A transcripts.

Levels of plasma immunoglobulin G with specificity against the cysteine-rich interdomain regions of a semiconserved Plasmodium falciparum erythrocyte membrane protein 1, VAR4, predict protection against malarial anemia and febrile episodes

Antibodies to variant surface antigen have been implicated as mediators of malaria immunity in studies measuring immunoglobulin G (IgG) binding to infected erythrocytes. Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) is an important target for these antibodies, but no study has directly linked the presence of PfEMP1 antibodies in children to protection. We measured plasma IgG levels to the cysteine-rich interdomain region 1alpha (CIDR1alpha) of VAR4 (VAR4-CIDR1alpha), a member of a semiconserved PfEMP1 subfamily, by enzyme-linked immunosorbent assay in 561 Tanzanian individuals, who were monitored clinically for 7 months. The participants resided in Mkokola (a high-transmission village where malaria is holoendemic) or Kwamasimba (a moderate-transmission village). For comparison, plasma IgG levels to two merozoite surface protein 1 (MSP1) constructs, MSP1-19 and MSP1 block 2, and a control CIDR1 domain were measured. VAR4-CIDR1alpha antibodies were acquired at an earlier age in Mkokola than in Kwamasimba, but after the age of 10 years the levels were comparable in the two villages. After controlling for age and other covariates, the risk of having anemia at enrollment was reduced in VAR4-CIDR1alpha responders for Mkokola (adjusted odds ratio [AOR], 0.49; 95% confidence interval [CI], 0.29 to 0.88; P = 0.016) and Kwamasimba (AOR, 0.33; 95% CI, 0.16 to 0.68; P = 0.003) villages. The risk of developing malaria fever was reduced among individuals with a measurable VAR4-CIDR1alpha response from Mkokola village (AOR, 0.51; 95% CI, 0.29 to 0.89; P = 0.018) but not in Kwamasimba. Antibody levels to the MSP1 constructs and the control CIDR1alpha domain were not associated with morbidity protection. These data strengthen the concept of developing vaccines based on PfEMP1.

Harbouring in the brain: A focus on immune evasion mechanisms and their deleterious effects in malaria and human African trypanosomiasis

Malaria and human African trypanosomiasis represent the two major tropical vector-transmitted protozoan infections, displaying different prevalence and epidemiological patterns. Death occurs mainly due to neurological complications which are initiated at the blood-brain barrier level. Adapted host-immune responses present differences but also similarities in blood-brain barrier/parasite interactions for these diseases: these are the focus of this review. We describe and compare parasite evasion mechanisms, the initiating mechanisms of central nervous system pathology and major clinical and neuropathological features. Finally, we highlight the common immune mediated mechanisms leading to brain involvement. In both diseases neurological damage is caused mainly by cytokines (interferon-gamma, tumour necrosis factor-alpha and IL-10), nitric oxide and endothelial cell apoptosis. Such a comparative analysis is expected to be useful in the comprehension of disease mechanisms, which may in turn have implications for treatment strategies.

Determinants of Variant Surface Antigen Antibody Response in Severe Plasmodium falciparum Malaria in an Area of Low and Unstable Malaria Transmission

The variant surface antigens (VSA) of infected erythrocytes are important pathogenic markers, a set of variants (VSA(SM)), were assumed to be associated with severe malaria (SM), while SM constitutes clinically diverse forms, such as, severe malarial anemia (SMA) and cerebral malaria (CM). This study was conducted in Eastern Sudan, an area of seasonal and unstable malaria transmission. Parasites and plasma were obtained from patients with different clinical grades of malaria, and flow cytometry was used for analysis of VSA antibody (Ab) response. We found that individuals recognized a broader range of isolates had a higher level of VSA Ab against the recognized isolates (correlation coefficient, 0.727, P<0.001). Unexpectedly, at the time of malaria diagnosis, plasma from patients with CM recognized a significantly larger number of isolates than did the plasma from patients with SMA (P<0.001). Parasites obtained from patients with SMA or from children were better recognized than isolates obtained from patients with uncomplicated malaria or from adults, P<0.001, P=0.021, respectively. Taken together, the above findings suggest that the limitations in the VSA immunoglobulin G repertoire were most probably contributing to the pathogenesis of SMA but not to that of CM.

Strict pairing of var promoters and introns is required for var gene silencing in the malaria parasite Plasmodium falciparum

The human malaria parasite, Plasmodium falciparum, maintains a persistent infection altering the proteins expressed on the surface of the infected red blood cells, thus avoiding the host immune response. The primary surface antigen, a protein called PfEMP1, is encoded by a multicopy gene family called var. Each individual parasite only expresses a single var gene at a time, maintaining all other members of the family in a transcriptionally silent state. Previous work using reporter genes in transiently transfected plasmid constructs implicated a conserved intron found in all var genes in the silencing process. Here we have utilized episomal recombination within stably transformed parasites to generate different var promoter and intron arrangements and show that loss of the intron results in var promoter activation. Further, in multicopy plasmid concatamers, each intron could only silence a single promoter, suggesting a one-to-one pairing requirement for silencing. Transcriptionally active, "unpaired" promoters remained active after integration into a chromosome; however, they were not recognized by the pathway that maintains mutually exclusive var gene expression. The data indicate that intron/promoter pairing is responsible for silencing each individual var gene and that disruption of silencing of one gene does not affect the transcriptional activity of neighboring var promoters. This suggests that silencing is regulated at the level of individual genes rather than by assembly of silent chromatin throughout a chromosomal region, thus providing a possible explanation of how a var gene can be maintained in a silent state while the immediately adjacent var gene is transcriptionally active.

Immune effector mechanisms in malaria

That humans in endemic areas become immune to malaria offers encouragement to the idea of developing protective vaccines. However natural immunity is relatively inefficient, being bought at the cost of substantial childhood mortality, and current vaccines are only partially protective. Understanding potential targets and mechanisms of protective immunity is important in the development and evaluation of future vaccines. Some of the problems in identifying such targets and mechanisms in humans naturally exposed to malaria may stem from conceptual and methodological issues related to defining who in a population is susceptible, problems in defining immune responsiveness at single time points and issues related to antigenic polymorphism, as well as the failure of many current approaches to examine functional aspects of the immune response. Protective immune responses may be directed to the pre erythrocytic parasite, to the free merozoite of the blood stage parasite or to new antigens induced on the infected red cell surface. Tackling the methodological issues of defining protection and immune response, together with studies that combine functional assays with new approaches such as allelic exchange and gene knock out offer opportunities for better defining key targets and mechanisms.

Early interactions between blood-stage plasmodium parasites and the immune system

Accumulating evidence provides strong support for the importance of innate immunity in shaping the subsequent adaptive immune response to blood-stage Plasmodium parasites, the causative agents of malaria. Early interactions between blood-stage parasites and cells of the innate immune system, including dendritic cells, monocytes/macrophages, natural killer (NK) cells, NKT cells, and gamma6 T cells, are important in the timely control of parasite replication and in the subsequent elimination and resolution of the infection. The major role of innate immunity appears to be the production of immunoregulatory cytokines, such as interleukin (IL)-12 and interferon (IFN)-gamma, which are critical for the development of type 1 immune responses involving CD4+ Thl cells, B cells, and effector cells which mediate cell-mediated and antibody-dependent adaptive immune responses. In addition, it is likely that cells of the innate immune system, especially dendritic cells, serve as antigen-presenting cells. Here, we review recent data from rodent models of blood-stage malaria and from human studies, and outline the early interactions of infected red blood cells with the innate immune system. We compare and contrast the results derived from studies in infected laboratory mice and humans. These host species are sufficiently different with respect to the identity of the infecting Plasmodium species, the resulting pathologies, and immune responses, particularly where the innate immune response is concerned. The implications of these findings for the development of an effective and safe malaria vaccine are also discussed.

A var gene promoter controls allelic exclusion of virulence genes in Plasmodium falciparum malaria

Mono-allelic expression of gene families is used by many organisms to mediate phenotypic variation of surface proteins. In the apicomplexan parasite Plasmodium falciparum, responsible for the severe form of malaria in humans, this is exemplified by antigenic variation of the highly polymorphic P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1, encoded by the 60-member var gene family, represents a major virulence factor due to its central role in immune evasion and intravascular parasite sequestration. Mutually exclusive expression of PfEMP1 is controlled by epigenetic mechanisms involving chromatin modification and perinuclear var locus repositioning. Here we show that a var promoter mediates the nucleation and spreading of stably inherited silenced chromatin. Transcriptional activation of this promoter occurs at the nuclear periphery in association with chromosome-end clusters. Additionally, the var promoter sequence is sufficient to infiltrate a transgene into the allelic exclusion programme of var gene expression, as transcriptional activation of this transgene results in silencing of endogenous var gene transcription. These results show that a var promoter is sufficient for epigenetic silencing and mono-allelic transcription of this virulence gene family, and are fundamental for our understanding of antigenic variation in P. falciparum. Furthermore, the PfEMP1 knockdown parasites obtained in this study will be important tools to increase our understanding of P. falciparum-mediated virulence and immune evasion.

Plasmodium falciparum variant surface antigen expression patterns during malaria

The variant surface antigens expressed on Plasmodium falciparum–infected erythrocytes are potentially important targets of immunity to malaria and are encoded, at least in part, by a family of var genes, about 60 of which are present within every parasite genome. Here we use semi-conserved regions within short var gene sequence “tags” to make direct comparisons of var gene expression in 12 clinical parasite isolates from Kenyan children. A total of 1,746 var clones were sequenced from genomic and cDNA and assigned to one of six sequence groups using specific sequence features. The results show the following. (1) The relative numbers of genomic clones falling in each of the sequence groups was similar between parasite isolates and corresponded well with the numbers of genes found in the genome of a single, fully sequenced parasite isolate. In contrast, the relative numbers of cDNA clones falling in each group varied considerably between isolates. (2) Expression of sequences belonging to a relatively conserved group was negatively associated with the repertoire of variant surface antigen antibodies carried by the infected child at the time of disease, whereas expression of sequences belonging to another group was associated with the parasite “rosetting” phenotype, a well established virulence determinant. Our results suggest that information on the state of the host–parasite relationship in vivo can be provided by measurements of the differential expression of different var groups, and need only be defined by short stretches of sequence data.

Hope that it will be possible to develop a malaria vaccine is supported by the fact that individuals who have grown up in malaria endemic regions learn to carry malarial infections without suffering disease. Surprisingly little is still known about how this immunity develops. Much current research focuses on how the host develops immune responses to parasite antigens that are exposed to the host immune system. A major family of such antigens are inserted into the surface of parasite-infected erythrocytes, where they undergo antigenic switching to evade a developing antibody response. These proteins are encoded by a family of approximately 60 var genes, variants of which are present in every parasite genome.

The extreme diversity of the var genes has prevented meaningful comparison of their expression in clinical isolates. However, the authors of this paper show that var genes can be placed in groups that have a similar representation in the genomes of all parasites that the authors collected from Kenyan children. Having demonstrated an underlying similarity at the genomic level, the authors show that the var expression patterns vary markedly between different patients. The expression levels of specific groups of var genes was associated with poorly developed antibody responses in the children and a well-established parasite virulence phenotype. The study provides tools for exploring how host and parasite adapt to one another as immunity develops.

Naturally acquired immunity to Plasmodium falciparum malaria in Africa

Infection by Plasmodium falciparum parasites can lead to substantial protective immunity to malaria, and available evidence suggest that acquisition of protection against some severe malaria syndromes can be fairly rapid. Although these facts have raised hopes that the development of effective vaccines against this major cause of human misery is a realistic goal, the uncertainty regarding the antigenic targets of naturally acquired protective immunity and the immunological mechanisms involved remain major vaccine development obstacles. Nevertheless, a coherent theoretical framework of how protective immunity to P. falciparum malaria is acquired following natural exposure to the parasites is beginning to emerge, not least thanks to studies that have combined clinical and epidemiological data with basic immunological research. This framework involves IgG with specificity for clonally variant antigens on the surface of the infected erythrocytes, can explain some of the difficulties in relating particular immune responses with specificity for well-defined antigenic targets to clinical protection, and suggests a radically new approach to controlling malaria-related morbidity and mortality by immunological means.

Clinical and molecular aspects of severe malaria

The erythrocytic cycle of Plasmodium falciparum presents a particularity in relation to other Plasmodium species that infect man. Mature trophozoites and schizonts are sequestered from the peripheral circulation due to adhesion of infected erythrocytes to host endothelial cells. Modifications in the surface of infected erythrocytes, termed knobs, seem to facilitate adhesion to endothelium and other erythrocytes. Adhesion provides better maturation in the microaerophilic venous atmosphere and allows the parasite to escape clearance by the spleen which recognizes the erythrocytes loss of deformability. Adhesion to the endothelium, or cytoadherence, has an important role in the pathogenicity of the disease, causing occlusion of small vessels and contributing to failure of many organs. Cytoadherence can also describe adhesion of infected erythrocytes to uninfected erythrocytes, a phenomenon widely known as rosetting. Clinical aspects of severe malaria, as well as the host receptors and parasite ligands involved in cytoadherence and rosetting, are reviewed here. The erythrocyte membrane protein 1 of P. falciparum (PfEMP1) appears to be the principal adhesive ligand of infected erythrocytes and will be discussed in more detail. Understanding the role of host receptors and parasite ligands in the development of different clinical syndromes is urgently needed to identify vaccination targets in order to decrease the mortality rates of this disease.

Malaria virulence genes controlling expression through chromatin modification

Mutually exclusive expression within the var gene family of the malaria parasite Plasmodium falciparum is important for parasite survival and virulence. In this issue of Cell, Duraisingh et al. and Freitas-Junior et al. provide evidence for the role of Sir2-dependent alterations in chromatin structure and changes in subnuclear chromatin localization in regulating var gene expression (Duraisingh et al., 2005; Freitas-Junior et al., 2005).

Expression of Plasmodium falciparum erythrocyte membrane protein 1 in experimentally infected humans

Background

Parasites causing severe malaria in non-immune patients express a restricted subset of variant surface antigens (VSA), which are better recognized by immune sera than VSA expressed during non-severe disease in semi-immune individuals. The most prominent VSA are the var gene-encoded Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family, which is expressed on the surface of infected erythrocytes where it mediates binding to endothelial receptors. Thus, severe malaria may be caused by parasites expressing PfEMP1 variants that afford parasites optimal sequestration in immunologically naïve individuals and high effective multiplication rates.

Methods

var gene transcription was analysed using real time PCR and PfEMP1 expression by western blots as well as immune plasma recognition of parasite cultures established from non-immune volunteers shortly after infection with NF54 sporozoites.

Results

In cultures representing the first generation of parasites after hepatic release, all var genes were transcribed, but GroupA var genes were transcribed at the lowest levels. In cultures established from second or third generation blood stage parasites of volunteers with high in vivo parasite multiplication rates, the var gene transcription pattern differed markedly from the transcription pattern of the cultures representing first generation parasites. This indicated that parasites expressing specific var genes, mainly belonging to group A and B, had expanded more effectively in vivo compared to parasites expressing other var genes. The differential expression of PfEMP1 was confirmed at the protein level by immunoblot analysis. In addition, serological typing showed that immune sera more often recognized second and third generation parasites than first generation parasites.

Conclusion

In conclusion, the results presented here support the hypothesis that parasites causing severe malaria express a subset of PfEMP1, which bestows high parasite growth rates in individuals with limited pre-existing immunity.

Malaria

Malaria is the most important parasitic infection in people, accounting for more than 1 million deaths a year. Malaria has become a priority for the international health community and is now the focus of several new initiatives. Prevention and treatment of malaria could be greatly improved with existing methods if increased financial and labour resources were available. However, new approaches for prevention and treatment are needed. Several new drugs are under development, which are likely to be used in combinations to slow the spread of resistance, but the high cost of treatments would make sustainability difficult. Insecticide-treated bed-nets provide a simple but effective means of preventing malaria, especially with the development of longlasting nets in which insecticide is incorporated into the net fibres. One malaria vaccine, RTS,S/AS02, has shown promise in endemic areas and will shortly enter further trials. Other vaccines are being studied in clinical trials, but it will probably be at least 10 years before a malaria vaccine is ready for widespread use.

Immunoglobulin G isotype responses to erythrocyte surface-expressed variant antigens of Plasmodium falciparum predict protection from malaria in African children

We assessed immunoglobulin G (IgG) isotype responses to variant surface antigens (VSA) expressed on parasite-infected erythrocytes of a panel of heterologous isolates during and after acute episodes in groups of Gabonese children presenting with either mild or severe Plasmodium falciparum malaria. In the acute and convalescent phases IgG3 and IgG1 anti-VSA antibodies, respectively, predominated. In the absence of infection, the levels of both cytophilic isotypes waned, while those of IgG4 increased, particularly in those admitted with severe malaria. Prospective analyses showed significantly longer delays between malaria attacks associated both (i) with increasing IgG1 responses with specificity for VSA of isolates from children with mild malaria and (ii) with increasing IgG4 responses with specificity for VSA of isolates from children with severe malaria. These findings imply that the predictive value of prospectively measured cross-reactive VSA-specific IgG antibodies with respect to protection against malaria in African children depends both on their isotype and on their fine specificity.

DEVELOPMENT AND REGULATION OF CELL-MEDIATED IMMUNE RESPONSES TO THE BLOOD STAGES OF MALARIA: Implications for Vaccine Research

The immune response to the malaria parasite is complex and poorly understood. Although antibodies and T cells can control parasite growth in model systems, natural immunity to malaria in regions of high endemicity takes several years to develop. Variation and polymorphism of antibody target antigens are known to impede immune responses, but these factors alone cannot account for the slow acquisition of immunity. In human and animal model systems, cell-mediated responses can control parasite growth effectively, but such responses are regulated by parasite load via direct effects on dendritic cells and possibly on T and B cells as well. Furthermore, high parasite load is associated with pathology, and cell-mediated responses may also harm the host. Inflammatory cytokines have been implicated in the pathogenesis of cerebral malaria, anemia, weight loss, and respiratory distress in malaria. Immunity without pathology requires rapid parasite clearance, effective regulation of the inflammatory anti-parasite effects of cellular responses, and the eventual development of a repertoire of antibodies effective against multiple strains. Data suggest that this may be hastened by exposure to malaria antigens in low dose, leading to augmented cellular immunity and rapid parasite clearance.

Proteomic studies of Plasmodium knowlesi SICA variant antigens demonstrate their relationship with P. falciparum EMP1

Malaria variant antigens are encoded by large multigene families and expressed on the surface of infected erythrocytes. The Plasmodium knowlesi Schizont-infected cell agglutination (SICA) antigens are encoded by the SICAvar multigene family, and the P. falciparum erythrocyte membrane protein-1 (PfEMP1) antigens are encoded by the var gene family. Although these variant antigens share many fundamental features, P. knowlesi and P. falciparum are phylogenetically distantly related, and so far a significant level of sequence identity has not been observed in alignments of either the SICAvar and var gene families or their encoded proteins. In support of their orthologous relationship, however, here we demonstrate through proteomic analysis that the P. knowlesi SICA variant antigens share significant common sequences with P. falciparum EMP1 molecules. As many as forty P. knowlesi SICA peptides show identity with a particular P. falciparum EMP1, mapping throughout all characterized domains, including the externally exposed cysteine-rich domains that are characteristic of both proteins. These findings provide further validation of the classical in vivo P. knowlesi-rhesus monkey model system for advancing our understanding of the immunobiology of antigenic variation and variant antigen gene expression in Plasmodium.

Clinical features and pathogenesis of severe malaria

A major change in recent years has been the recognition that severe malaria, predominantly caused by Plasmodium falciparum, is a complex multi-system disorder presenting with a range of clinical features. It is becoming apparent that syndromes such as cerebral malaria, which were previously considered relatively clear cut, are not homogenous conditions with a single pathological correlate or pathogenic process. This creates challenges both for elucidating key mechanisms of disease and for identifying suitable targets for adjunctive therapy. The development of severe malaria probably results from a combination of parasite-specific factors, such as adhesion and sequestration in the vasculature and the release of bioactive molecules, together with host inflammatory responses. These include cytokine and chemokine production and cellular infiltrates. This review summarizes progress in several areas presented at a recent meeting.

Evidence for the involvement of VAR2CSA in pregnancy-associated malaria

In Plasmodium falciparum–endemic areas, pregnancy-associated malaria (PAM) is an important health problem. The condition is precipitated by accumulation of parasite-infected erythrocytes (IEs) in the placenta, and this process is mediated by parasite-encoded variant surface antigens (VSA) binding to chondroitin sulfate A (CSA). Parasites causing PAM express unique VSA types, VSA_PAM, which can be serologically classified as sex specific and parity dependent. It is sex specific because men from malaria-endemic areas do not develop VSA_PAM antibodies; it is parity dependent because women acquire anti-VSA_PAM immunoglobulin (Ig) G as a function of parity. Previously, it was shown that transcription of var2csa is up-regulated in placental parasites and parasites selected for CSA binding. Here, we show the following: (a) that VAR2CSA is expressed on the surface of CSA-selected IEs; (b) that VAR2CSA is recognized by endemic plasma in a sex-specific and parity-dependent manner; (c) that high anti-VAR2CSA IgG levels can be found in pregnant women from both West and East Africa; and (d) that women with high plasma levels of anti-VAR2CSA IgG give birth to markedly heavier babies and have a much lower risk of delivering low birth weight children than women with low levels.

Pregnancy-associated malaria and the prospects for syndrome-specific antimalaria vaccines

Aided by the Plasmodium falciparum genome project, recent discoveries regarding the molecular basis of malaria pathogenesis have led to a better understanding of the interactions between host and parasite. Although vaccines that prevent infection by malaria parasites remain only hopes for the future, there are now more immediate prospects for vaccines that protect against specific disease syndromes. Here, we discuss the latest advances in the development of a vaccine that specifically targets pregnancy-associated malaria (PAM).

Variant surface antigens, virulence genes and the pathogenesis of malaria

The first Molecular Approaches to Malaria meeting was held 2-5 February 2000 in Lorne, Australia. Following the meeting, Brian Cooke, Mats Wahlgren and Ross Coppel predicted that research into the molecular details of the mechanisms behind sequestration of parasitized erythrocytes would "become increasingly more complicated, with further interactions, receptors, ligands and functional domains". Furthermore, they cautioned that "the challenge will be not to lose ourselves in the molecular detail, but remain focused on the role of [the var genes and other multigene families] in pathogenesis of malaria". We contemplate on these statements, following the recent second Molecular Approaches to Malaria meeting, which was held at the same venue on 2-5 February 2004.