Cross-Species Immune Recognition Between Plasmodium vivax Duffy Binding Protein Antibodies and the Plasmodium falciparum Surface Antigen VAR2CSA

Background

In pregnancy, Plasmodium falciparum parasites express the surface antigen VAR2CSA, which mediates adherence of red blood cells to chondroitin sulfate A (CSA) in the placenta. VAR2CSA antibodies are generally acquired during infection in pregnancy and are associated with protection from placental malaria. We observed previously that men and children in Colombia also had antibodies to VAR2CSA, but the origin of these antibodies was unknown. Here, we tested whether infection with Plasmodium vivax is an alternative mechanism of acquisition of VAR2CSA antibodies.

Methods

We analyzed sera from nonpregnant Colombians and Brazilians exposed to P. vivax and monoclonal antibodies raised against P. vivax Duffy binding protein (PvDBP). Cross-reactivity to VAR2CSA was characterized by enzyme-linked immunosorbent assay, immunofluorescence assay, and flow cytometry, and antibodies were tested for inhibition of parasite binding to CSA.

Results

Over 50% of individuals had antibodies that recognized VAR2CSA. Affinity-purified PvDBP human antibodies and a PvDBP monoclonal antibody recognized VAR2CSA, showing that PvDBP can give rise to cross-reactive antibodies. Importantly, the monoclonal antibody inhibited parasite binding to CSA, which is the primary in vitro correlate of protection from placental malaria.

Conclusions

These data suggest that PvDBP induces antibodies that functionally recognize VAR2CSA, revealing a novel mechanism of cross-species immune recognition to falciparum malaria.

Differential adhesion-inhibitory patterns of antibodies raised against two major variants of the NTS-DBL2X region of VAR2CSA

BACKGROUND

VAR2CSA is a large polymorphic Plasmodium falciparum protein expressed on infected erythrocytes (IE) that allows their binding in the placenta, thus precipitating placental malaria (PM). The N-terminal part of VAR2CSA that contains the binding site to placental chondroitin sulfate A (CSA) is currently recognized as the most attractive region for vaccine development. An ultimate challenge is to define epitopes in this region that induce a broad cross-reactive adhesion inhibitory antibody response.

METHODS

Based on phylogenetic data that identified a dimorphic sequence motif in the VAR2CSA DBL2X, we raised antibodies against the NTS-DBL2X constructs containing one sequence or the other (3D7 and FCR3) and tested their functional properties on P. falciparum isolates from pregnant women and on laboratory-adapted strains.

RESULTS

The CSA binding inhibitory capacity of the antibodies induced varied from one parasite isolate to another (range, 10%–100%), but the combined analysis of individual activity highlighted a broader functionality that increased the total number of isolates inhibited. Interestingly, the differential inhibitory effect of the antibodies observed on field isolates resulted in significant inhibition of all field isolates tested, suggesting that optimal inhibitory spectrum on field isolates from pregnant women might be achieved with antibodies targeting limited variants of the N-terminal VAR2CSA.

CONCLUSIONS

Our findings indicate that the NTS-DBL2X region of VAR2CSA can elicit strain-transcending anti-adhesion antibodies and suggest that the combination of the two major variants used here could represent the basis for an effective bivalent VAR2CSA-based vaccine.

Electroporation-mediated genetic vaccination for antigen mapping: application to Plasmodium falciparum VAR2CSA protein

Genetic vaccination, consisting in delivering a genetically engineered plasmid DNA by a non-viral vector or technique into a tissue, is currently of great interest. New delivery technique including DNA transfer by electroporation recently greatly improved the potency of this concept. Because it avoids the step of producing a recombinant protein, it is particularly of use in studying the immunogenic properties of large proteins. Here we describe the use of electroporation mediated DNA immunization to identify important protective epitopes from the large VAR2CSA protein from Plasmodium falciparum implicated in the pathology of placental malaria. Immunizing mice and rabbit with DNA plasmids encoding different fragments of VAR2CSA leads to high titer antisera. Moreover an N-terminal region of the protein was found to induce protective functional antibodies.

The NTS-DBL2X region of VAR2CSA induces cross-reactive antibodies that inhibit adhesion of several Plasmodium falciparum isolates to chondroitin sulfate A

BACKGROUND

Binding to chondroitin sulfate A by VAR2CSA, a parasite protein expressed on infected erythrocytes, allows placental sequestration of Plasmodium falciparum-infected erythrocytes. This leads to severe consequences such as maternal anemia, stillbirths, and intrauterine growth retardation. The latter has been clearly associated to increased morbidity and mortality of the infants. Acquired anti-VAR2CSA antibodies have been associated with improved pregnancy outcomes, suggesting a vaccine could prevent the syndrome. However, identifying functionally important regions in the large VAR2CSA protein is difficult.

METHODS

Using genetic immunization, we raised polyclonal antisera against overlapping segments of VAR2CSA in mice and rabbits. The adhesion-inhibition capacities of induced antisera and of specific antibodies purified from plasma of malaria-exposed pregnant women were assessed on laboratory-adapted parasite lines and field isolates expressing VAR2CSA. Competition enzyme-linked immunosorbent assay (ELISA) was employed to analyze functional resemblance between antibodies induced in animals and those naturally acquired by immune multigravidae.

RESULTS

Antibodies targeting the N-terminal sequence (NTS) up to DBL2X (NTS-DBL2X) efficiently blocked parasite adhesion to chondroitin sulfate A in a manner similar to that of antibodies raised against the entire VAR2CSA extracellular domain. Interestingly, naturally acquired antibodies and those induced by vaccination against NTS-DBL2X target overlapping strain-transcendent anti-adhesion epitopes.

CONCLUSIONS

This study highlights an important step achieved toward development of a protective vaccine against placental malaria.

Insight into antigenic diversity of VAR2CSA-DBL5ε domain from multiple Plasmodium falciparum placental isolates

Background

Protection against pregnancy associated malaria (PAM) is associated with high levels of anti-VAR2CSA antibodies. This protection is obtained by the parity dependent acquisition of anti-VAR2CSA antibodies. Distinct parity-associated molecular signatures have been identified in VAR2CSA domains. These two observations combined point to the importance of identifying VAR2CSA sequence variation, which facilitate parasitic evasion or subversion of host immune response. Highly conserved domains of VAR2CSA such as DBL5ε are likely to contain conserved epitopes, and therefore do constitute attractive targets for vaccine development.

Methodology/Principal Findings

VAR2CSA DBL5ε-domain sequences obtained from cDNA of 40 placental isolates were analysed by a combination of experimental and in silico methods. Competition ELISA assays on two DBL5ε variants, using plasma samples from women from two different areas and specific mice hyperimmune plasma, indicated that DBL5ε possess conserved and cross-reactive B cell epitopes. Peptide ELISA identified conserved areas that are recognised by naturally acquired antibodies. Specific antibodies against these peptides labelled the native proteins on the surface of placental parasites. Despite high DBL5ε sequence homology among parasite isolates, sequence analyses identified motifs in DBL5ε that discriminate parasites according to donor's parity. Moreover, recombinant proteins of two VAR2CSA DBL5ε variants displayed diverse recognition patterns by plasma from malaria-exposed women, and diverse proteoglycan binding abilities.

Conclusions/Significance

This study provides insights into conserved and exposed B cell epitopes in DBL5ε that might be a focus for cross reactivity. The importance of sequence variation in VAR2CSA as a critical challenge for vaccine development is highlighted. VAR2CSA conformation seems to be essential to its functionality. Therefore, identification of sequence variation sites in distinct locations within VAR2CSA, affecting antigenicity and/or binding properties, is critical to the effort of developing an efficient VAR2CSA-based vaccine. Motifs associated with parasite segregation according to parity constitute one such site.

Plasmodium falciparum population dynamics in a cohort of pregnant women in Senegal

BACKGROUND

Pregnant women acquire protective antibodies that cross-react with geographically diverse placental Plasmodium falciparum isolates, suggesting that surface molecules expressed on infected erythrocytes by pregnancy-associated malaria (PAM) parasites have conserved epitopes and, that designing a PAM vaccine may be envisaged. VAR2CSA is the main candidate for a pregnancy malaria vaccine, but vaccine development may be complicated by its sequence polymorphism.

METHODS

The dynamics of P. falciparum genotypes during pregnancy in 32 women in relation to VAR2CSA polymorphism and immunity was determined. The polymorphism of the msp2 gene and five microsatellites was analysed in consecutive parasite isolates, and the DBL5epsilon + Interdomain 5 (Id5) part of the var2csa gene of the corresponding samples was cloned and sequenced to measure variation.

RESULTS

In primigravidae, the multiplicity of infection in the placenta was associated with occurrence of low birth weight babies. Some parasite genotypes were able to persist over several weeks and, still be present in the placenta at delivery particularly when the host anti-VAR2CSA antibody level was low. Comparison of diversity among genotyping markers confirmed that some PAM parasites may harbour more than one var2csa gene copy in their genome.

CONCLUSIONS

Host immunity to VAR2CSA influences the parasite dynamics during pregnancy, suggesting that the acquisition of protective immunity requires pre-exposure to a limited number of parasite variants. Presence of highly conserved residues in surface-exposed areas of the VAR2CSA immunodominant DBL5epsilon domain, suggest its potential in inducing antibodies with broad reactivity.

ZO-1 is involved in trophoblastic cell differentiation in human placenta

Trophoblastic cell-cell fusion is an essential event required during human placental development. Several membrane proteins have been described to be directly involved in this process, including connexin 43 (Cx43), syncytin 1 (Herv-W env), and syncytin 2 (Herv-FRD env glycoprotein). Recently, zona occludens (ZO) proteins (peripheral membrane proteins associated with tight junctions, adherens junctions, and gap junctions) were shown to be involved in mouse placental development. Moreover, zona occludens 1 (ZO-1) was localized mainly at the intercellular boundaries between human trophoblastic cells. Therefore the role of ZO-1 in the dynamic process of human trophoblastic cell-cell fusion was investigated using primary trophoblastic cells in culture. In vitro as in situ, ZO-1 was localized mainly at the intercellular boundaries between trophoblastic cells where its expression substantially decreased during differentiation and during fusion. At the same time, Cx43 was localized at the interface of trophoblastic cells and its expression increased during differentiation. To determine a functional role for ZO-1 during trophoblast differentiation, small interfering RNA (siRNA) was used to knock down ZO-1 expression. Cytotrophoblasts treated with ZO-1 siRNA fused poorly, but interestingly, decreased Cx43 expression without altering the functionality of trophoblastic cell-cell communication as measured by relative permeability time constant determined using gap-FRAP experiments. Because kinetics of Cx43 and ZO-1 proteins show a mirror image, a potential association of these two proteins was investigated. By using coimmunoprecipitation experiments, a physical interaction between ZO-1 and Cx43 was demonstrated. These results demonstrate that a decrease in ZO-1 expression reduces human trophoblast cell-cell fusion and differentiation.

Cloning and characterization of a novel isoform of iodotyrosine dehalogenase 1 (DEHAL1) DEHAL1C from human thyroid: comparisons with DEHAL1 and DEHAL1B

The human iodotyrosine dehalogenase 1 (DEHAL1) gene is composed of six exons. Two isoforms (DEHAL1 and DEHAL1B) have been published in GenBank, both of which have a nitroreductase domain and arise from differential splicing in exon 5. We recently showed that the DEHAL1 isoform is a transmembrane protein that efficiently catalyzes the NADPH-dependent deiodination of mono (L-MIT) and diiodotyrosine (L-DIT) in human embryonic kidney-293 (HEK293) cells. In the present study, we establish the existence of a new transcript, DEHAL1C, in the human thyroid with a terminal exon that lacks in the DEHAL1 transcript. This exon is the complete exon 5, which is spliced in the DEHAL1B mRNA variant. These two variants encode proteins with differing C-terminal domains. Using quantitative reverse transcription polymerase chain reaction, we found that the expression of the mRNA of DEHAL1C and DEHAL1B was lower than that of DEHAL1 mRNA in the thyroid. We also observed that human DEHAL1B and DEHAL1C proteins are rapidly degraded in stably transfected HEK293 cells, unlike the DEHAL1 protein, and that exposure to the proteasome inhibitor MG132 resulted in accumulation of these proteins that was markedly time- and concentration-dependent. These findings show that the cytoplasmic tail could play a role in the stability of the protein.

Dual oxidase-2 has an intrinsic Ca2+-dependent H2O2-generating activity

Duox2 (and probably Duox1) is a glycoflavoprotein involved in thyroid hormone biosynthesis, as the thyroid H2O2 generator functionally associated with Tpo (thyroperoxidase). So far, because of the impairment of maturation and of the targeting process, transfecting DUOX into nonthyroid cell lines has not led to the expression of a functional H2O2-generating system at the plasma membrane. For the first time, we investigated the H2O2-generating activity in the particulate fractions from DUOX2- and DUOX1-transfected HEK293 and Chinese hamster ovary cells. The particulate fractions of these cells stably or transiently transfected with human or porcine DUOX cDNA demonstrate a functional NADPH/Ca2+-dependent H2O2-generating activity. The immature Duox proteins had less activity than pig thyrocyte particulate fractions, and their activity depended on their primary structures. Human Duox2 seemed to be more active than human Duox1 but only half as active as its porcine counterpart. TPO co-transfection produced a slight increase in the enzymatic activity, whereas p22(phox), the 22-kDa subunit of the leukocyte NADPH oxidase, had no effect. In previous studies on the mechanism of H2O2 formation, it was shown that mature thyroid NADPH oxidase does not release O2*- but H2O2. Using a spin-trapping technique combined with electron paramagnetic resonance spectroscopy, we confirmed this result but also demonstrated that the partially glycosylated form of Duox2, located in the endoplasmic reticulum, generates superoxide in a calcium-dependent manner. These results suggest that post-translational modifications during the maturation process of Duox2 could be implicated in the mechanism of H2O2 formation by favoring intramolecular superoxide dismutation.

Dual oxidase2 is expressed all along the digestive tract

The dual oxidase (Duox)2 flavoprotein is strongly expressed in the thyroid gland, where it plays a critical role in the synthesis of thyroid hormones by providing thyroperoxidase with H2O2. DUOX2 mRNA was recently detected by RT-PCR and in-situ hybridization experiments in other tissues, such as rat colon and rat and human epithelial cells from the salivary excretory ducts and rectal glands. We examined Duox2 expression at the protein level throughout the porcine digestive tract and in human colon. Western blot analysis identified Duox2 as the same two molecular species (M(r) 165 and 175 kDa) as detected in the thyroid. It was expressed in all the tissues tested, but the highest levels were found in the cecum and sigmoidal colon. Immunohistochemical studies showed that Duox2 protein is mainly present in these parts of the gut and located at the apical membrane of the enterocytes in the brush border, indicating that it is expressed only in highly differentiated cells. A Ca2+/NADPH-dependent H2O2-generating system was associated with Duox2 protein expression, which had the same biochemical characteristics as the NADPH oxidase in the thyroid. Indeed, treatment of the thyroid and cecum particulate fractions with phenylarsine oxide resulted in complete calcium desensitization of both enzymes. A marked increase in DUOX2 expression was also found during spontaneous differentiation of postconfluent Caco-2 cells. The discovery of Duox2 as a novel source of H2O2 in the digestive tract, particularly in the cecum and colon, makes it a new candidate mediator of physiopathological processes.

Iodotyrosine dehalogenase 1 (DEHAL1) is a transmembrane protein involved in the recycling of iodide close to the thyroglobulin iodination site

In the thyroid, iodotyrosine dehalogenase acts on the mono and diiodotyrosines released during the hydrolysis of thyroglobulin to liberate iodide, which can then reenter the hormone-producing pathways. It has been reported that the deiodination of iodotyrosines occurs predominantly in the microsomes and is mediated by NADPH. Recently, two cDNAs, 7401- and 7513-base pairs long that encode proteins with a conserved nitroreductase domain were published in GenBank as iodotyrosine dehalogenase 1 (DEHAL1) and iodotyrosine dehalogenase 1B (DEHAL1B), respectively. We report here our investigation of the localization and activity of one of these isoforms, DEHAL1. DEHAL1 mRNA is highly expressed in the thyroid, is up-regulated by cAMP, and encodes a transmembrane protein that efficiently catalyzes the NADPH-dependent deiodination of mono (L-MIT) and diiodotyrosine (L-DIT), with greater activity vs. L-MIT. Iodotyrosine deiodinase was active in HEK293 cells transfected by DEHAL1 cDNA, but not in CHO cells. A fraction of DEHAL1 protein is exposed to the cell surface, as indicated by biotinylation experiments. Immunohistochemistry studies showed that DEHAL1 proteins accumulate at the apical pole of thyrocytes. Taken together, these findings indicate that the deiodination reaction occurs at the apical pole of the thyrocyte and is involved in a rapid iodide recycling process at and/or close to the organification site.

Targeting of the dual oxidase 2 N-terminal region to the plasma membrane

Dual oxidase 2 (Duox2) is a cell surface glycoprotein that probably provides thyroperoxidase with the H2O2 required to catalyze thyroid hormone synthesis. No functional H2O2-generating system has yet been obtained after transfecting Duox2 into non-thyroid cell lines, because it is retained in the endoplasmic reticulum (ER). We investigated the level of maturation of various Duox2 truncated proteins in an attempt to identify the region of Duox2 responsible for its remaining in the ER. Duox2-Q686X mutant, corresponding to the N-terminal ectodomain including the first putative transmembrane domain, was expressed in different cell lines. Carbohydrate content analysis revealed that complex type-specific Golgi apparatus (GA) oligosaccharides were present on pig Duox2-Q686X, whereas human truncated Duox2 carried only high mannose-type sugar chains characteristic of the ER. Further characterization using surface biotinylation and flow cytometry assays indicated that pig Duox2-Q686X was present at the plasma membrane, whereas human Duox2-Q686X remained inside the cell. The replacement of the last 90 residues of the human Duox2-Q686X with the pig equivalent region allowed the chimerical peptide to reach the Golgi apparatus. Pig mutants containing the complete first intracellular loop with or without the second transmembrane domain accumulated in the ER. These findings show that 1) the human Duox2-Q686X region encompassing residues 596-685 prevents mutant exportation from the ER and 2) there is a pig Duox2 retention domain in the first intracellular loop. In addition, missense mutations of four cysteines (Cys-351, -370, -568, or -582) completely inhibited the emergence of pig Duox2-Q686X from the ER compartment, indicating their importance in Duox2 maturation.