Plasmodium falciparum merozoite surface protein 8 is a ring-stage membrane protein that localizes to the parasitophorous vacuole of infected erythrocytes

A Chimeric Plasmodium vivax Merozoite Surface Protein Antibody Recognizes and Blocks Erythrocytic P. cynomolgi Berok Merozoites In Vitro

Research on erythrocytic Plasmodium vivax merozoite antigens is critical for identifying potential vaccine candidates in reducing P. vivax disease. However, many P. vivax studies are constrained by its inability to undergo long-term culture in vitro Conserved across all Plasmodium spp., merozoite surface proteins are essential for invasion into erythrocytes and highly expressed on erythrocytic merozoites, thus making it an ideal vaccine candidate. In clinical trials, the P. vivax merozoite surface protein 1 (PvMSP1-19) vaccine candidate alone has shown to have limited immunogenicity in patients; hence, we incorporate the highly conserved and immunogenic C terminus of both P. vivax merozoite surface protein 8 (PvMSP8) and PvMSP1-19 to develop a multicomponent chimeric protein rPvMSP8+1 for immunization of mice. The resulted chimeric rPvMSP8+1 antibody was shown to recognize native protein MSP8 and MSP1-19 of mature P. vivax schizonts. In the immunized mice, an elevated antibody response was observed in the rPvMSP8+1-immunized group compared to that immunized with single-antigen components. In addition, we examined the growth inhibition of these antibodies against Plasmodium cynomolgi (Berok strain) parasites, which is phylogenetically close to P. vivax and sustains long-term culture in vitro Similarly, the chimeric anti-rPvMSP8+1 antibodies recognize P. cynomolgi MSP8 and MSP1-19 on mature schizonts and showed strong inhibition in vitro via growth inhibition assay. This study provides support for a new multiantigen-based paradigm rPvMSP8+1 to explore potential chimeric vaccine candidates against P. vivax malaria using sister species P. cynomolgi.

The Malaria Cell Atlas: Single parasite transcriptomes across the complete Plasmodium life cycle

Malaria parasites adopt a remarkable variety of morphological life stages as they transition through multiple mammalian host and mosquito vector environments. We profiled the single-cell transcriptomes of thousands of individual parasites, deriving the first high-resolution transcriptional atlas of the entire Plasmodium berghei life cycle. We then used our atlas to precisely define developmental stages of single cells from three different human malaria parasite species, including parasites isolated directly from infected individuals. The Malaria Cell Atlas provides both a comprehensive view of gene usage in a eukaryotic parasite and an open-access reference dataset for the study of malaria parasites.

The acquisition of long-lived memory B cell responses to merozoite surface protein-8 in individuals with Plasmodium vivax infection

BACKGROUND

The ability of a malaria antigen to induce effective, long-lasting immune responses is important for the development of a protective malaria vaccine. Plasmodium vivax merozoite surface protein-8 (PvMSP8) has been shown to be immunogenic in natural P. vivax infections and produces both cell-mediated and antibody-mediated immunity. Thus, PvMSP8 has been proposed as a vaccine candidate following fusion with other merozoite antigens in blood stage vaccine design. Here, the long-term responses of antibodies and memory B cells (MBCs) specific to PvMSP8 in individuals were monitored in a longitudinal cohort study.

METHODS

Both cross-sectional surveys and cohort studies were utilized to explore the persistence of antibody and MBC responses to PvMSP8. Antibody titers were detected in individuals with acute disease and those who recovered from an infection for 4 years. The dominant peptide epitope of PvMSP8 recognized by naturally acquired antibodies was examined to observe the durability of the post-infection antibody response. PvMSP8-specific MBCs were also in subjects 4 years post-infection using an enzyme-linked immunospot assay.

RESULTS

The prevalence of antibodies to PvMSP8 was high during and after infection. The antibody levels in individual responders were monitored for up to 12 months post-infection and showed that most patients maintained their seropositive response. Interestingly, the anti-PvMSP8 antibody responses stably persisted in some patients who had recovered from an infection for 4 years. Positive PvMSP8-specific MBCs were also detected at 4 years post-infection. However, analysis in these individuals showed no correlation with the presence or titer of circulating antibody.

CONCLUSION

PvMSP8 had the ability to induce a long-term humoral immune response. The antibodies and MBCs specific for this antigen developed and persisted in subjects who acquired a natural P. vivax infection. Inclusion of the PvMSP8 antigen in blood stage vaccine design should be considered.

An in silico down-scaling approach uncovers novel constituents of the Plasmodium-containing vacuole

During blood stage development the malaria parasite resides in a membrane-bound compartment, termed the parasitophorous vacuole (PV). The reasons for this intravacuolar life style and the molecular functions of this parasite-specific compartment remain poorly defined, which is mainly due to our limited knowledge about the molecular make-up of this unique niche. We used an in silico down-scaling approach to select for Plasmodium-specific candidates that harbour signatures of PV residency. Live co-localisation of five endogenously tagged proteins confirmed expression in the PV of Plasmodium berghei blood and liver stages. ER retention was ruled out by addition of the respective carboxyterminal tetrapeptides to a secreted reporter protein. Although all five PV proteins are highly expressed, four proved to be dispensable for parasite development in the mammalian and mosquito host, as revealed by targeted gene deletion. In good agreement with their redundant roles, the knockout parasites displayed no detectable deficiencies in protein export, sequestration, or PV morphology. Together, our approach improved the catalogue of the Plasmodium PV proteome and provides experimental genetics evidence for functional redundancy of several PV proteins.

Evaluation of a Plasmodium-Specific Carrier Protein To Enhance Production of Recombinant Pfs25, a Leading Transmission-Blocking Vaccine Candidate

Challenges with the production and suboptimal immunogenicity of malaria vaccine candidates have slowed the development of a Plasmodium falciparum multiantigen vaccine. Attempting to resolve these issues, we focused on the use of highly immunogenic merozoite surface protein 8 (MSP8) as a vaccine carrier protein. Previously, we showed that a genetic fusion of the C-terminal 19-kDa fragment of merozoite surface protein 1 (MSP119) to P. falciparum MSP8 (PfMSP8) facilitated antigen production and folding and the induction of neutralizing antibodies to conformational B cell epitopes of MSP119 Here, using the PfMSP1/8 construct, we further optimized the recombinant PfMSP8 (rPfMSP8) carrier by the introduction of two cysteine-to-serine substitutions (CΔS) to improve the yield of the monomeric product. We then sought to test the broad applicability of this approach using the transmission-blocking vaccine candidate Pfs25. The production of rPfs25-based vaccines has presented challenges. Antibodies directed against the four highly constrained epidermal growth factor (EGF)-like domains of Pfs25 block sexual-stage development in mosquitoes. The sequence encoding mature Pfs25 was codon harmonized for expression in Escherichia coli We produced a rPfs25-PfMSP8 fusion protein [rPfs25/8(CΔS)] as well as unfused, mature rPfs25. rPfs25 was purified with a modest yield but required the incorporation of refolding protocols to obtain a proper conformation. In comparison, chimeric rPfs25/8(CΔS) was expressed and easily purified, with the Pfs25 domain bearing the proper conformation without renaturation. Both antigens were immunogenic in rabbits, inducing IgG that bound native Pfs25 and exhibited potent transmission-reducing activity. These data further demonstrate the utility of PfMSP8 as a parasite-specific carrier protein to enhance the production of complex malaria vaccine targets.

Identification of novel parasitophorous vacuole proteins in P. falciparum parasites using BioID

Malaria blood stage parasites develop within red blood cells where they are contained in a vacuolar compartment known as the parasitophorous vacuole (PV). This compartment holds a key role in the interaction of the parasite with its host cell. However, the proteome of this compartment has so far not been comprehensively analysed. Here we used BioID in asexual blood stages of the most virulent human malaria parasite Plasmodium falciparum to identify new proteins of the PV. The resulting proteome contained many of the already known PV proteins and validation by GFP-knock-in of 10 previously in P. falciparum uncharacterised hits revealed 5 new PV proteins and two with a partial PV localisation. This included proteins peripherally attached to the inner face of the PV membrane as well as proteins anchored in the parasite plasma membrane that protrude into the PV. Using selectable targeted gene disruption we generated mutants for 2 of the 10 candidates. In contrast we could not select parasites with disruptions for another 3 candidates, strongly suggesting that they are important for parasite growth. Interestingly, one of these included the orthologue of UIS2, a protein previously proposed to regulate protein translation in the parasite cytoplasm but here shown to be an essential PV protein. This work extends the number of known PV proteins and provides a starting point for further functional analyses of this compartment.

Naturally acquired humoral and cellular immune responses to Plasmodium vivax merozoite surface protein 8 in patients with P. vivax infection

Background

Thirty-one glycosylphosphatidylinositol (GPI)-anchored proteins of Plasmodium vivax, merozoite surface protein 1 (MSP1), MSP1 paralogue, MSP4, MSP5, MSP8, and MSP10 have been reported from homologs of Plasmodium falciparum by gene annotation with bioinformatics tools. These GPI-anchored proteins contain two epidermal growth factor (EGF)-like domains at its C-terminus. Here, P. vivax merozoite surface protein 8 (PvMSP8) are considered as potential targets of protective immunity.

Methods

Recombinant PvMSP8 (rPvMSP8) was expressed, purified, and used for the assessment of humoral and cellular immune responses in P. vivax-infected patients and immune mice. Moreover, the target epitope of ant-PvMSP8 antibodies and subcellular localization of PvMSP8 was also determined.

Results

The rPvMSP8 was successfully expressed and purified as soluble form as ~55 kDa. PvMSP8 was localized to the outer circle of pigments associated with the food vacuole. The rPvMSP8 protein had a high antigenicity (73.2% in sensitivity and 96.2% in specificity) in patients infected with P. vivax. IgG2 antibody subtype was the predominantly responses to this antigen. Antibody response to PvMSP8 increased up to day 7 and after that slightly decreased within a month. The longevity of anti-PvMSP8 antibody was stably sustained up to 12-year recovery patient samples. Most anti-PvMSP8 antibodies recognized two epitopes that were located outside the C-terminal EGF-like domain. The cellular immune response in P. vivax-exposed individuals produced high levels of IFN-γ and IL-10 upon PvMSP8 antigen stimulation in vitro.

Conclusions

All data in this study suggest that PvMSP8 antigen has a potential to induce both humoral and cellular immune responses in patients with P. vivax infection. The subcellular localization of PvMSP8 confirmed that it was associated with the parasite food vacuole in blood-stage parasites. A further characterization of this protein will be useful for blood stage P. vivax vaccine development.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1837-5) contains supplementary material, which is available to authorized users.

Immunogenicity of a chimeric Plasmodium falciparum merozoite surface protein vaccine in Aotus monkeys

Background

The production of properly folded, recombinant sub-unit Plasmodium falciparum malaria vaccine candidates in sufficient quantities is often a challenge. Success in vaccine immunogenicity studies in small animal models does not always predict immunogenicity in non-human primates and/or human subjects. The aim of this study was to assess the immunogenicity of a chimeric blood-stage malaria vaccine in Aotus monkeys. This vaccine candidate includes the neutralizing B cell epitopes of P. falciparum merozoite surface protein 1 (rPfMSP1₁₉) genetically linked to a highly immunogenic, well-conserved P. falciparum merozoite surface protein 8 (rPfMSP8 (ΔAsn/Asp)) partner.

Methods

Aotus nancymaae monkeys were immunized with purified rPfMSP1/8 or rPfMSP8 (ΔAsn/Asp) formulated with Montanide ISA 720 as adjuvant, or with adjuvant alone. Antibody responses to MSP1₁₉ and MSP8 domains were measured by ELISA following primary, secondary and tertiary immunizations. The functionality of vaccine-induced antibodies was assessed in a standard P. falciparum blood-stage in vitro growth inhibition assay. Non-parametric tests with corrections for multiple comparisons when appropriate were used to determine the significance of differences in antigen-specific IgG titres and in parasite growth inhibition.

Results

The chimeric rPfMSP1/8 vaccine was shown to be well tolerated and highly immunogenic with boost-able antibody responses elicited to both PfMSP8 and PfMSP1₁₉ domains. Elicited antibodies were highly cross-reactive between FVO and 3D7 alleles of PfMSP1₁₉ and potently inhibited the in vitro growth of P. falciparum blood-stage parasites.

Conclusions

Similar to previous results with inbred and outbred mice and with rabbits, the PfMSP1/8 vaccine was shown to be highly effective in eliciting P. falciparum growth inhibitory antibodies upon immunization of non-human primates. The data support the further assessment of PfMSP1/8 as a component of a multivalent vaccine for use in human subjects. As important, the data indicate that rPfMSP8 (ΔAsn/Asp) can be used as a malaria specific carrier protein to: (1) drive production of antibody responses to neutralizing B cell epitopes of heterologous vaccine candidates and (2) facilitate production of properly folded, recombinant P. falciparum subunit vaccines in high yield.

Analysis of a Multi-component Multi-stage Malaria Vaccine Candidate--Tackling the Cocktail Challenge

Combining key antigens from the different stages of the P. falciparum life cycle in the context of a multi-stage-specific cocktail offers a promising approach towards the development of a malaria vaccine ideally capable of preventing initial infection, the clinical manifestation as well as the transmission of the disease. To investigate the potential of such an approach we combined proteins and domains (11 in total) from the pre-erythrocytic, blood and sexual stages of P. falciparum into a cocktail of four different components recombinantly produced in plants. After immunization of rabbits we determined the domain-specific antibody titers as well as component-specific antibody concentrations and correlated them with stage specific in vitro efficacy. Using purified rabbit immune IgG we observed strong inhibition in functional in vitro assays addressing the pre-erythrocytic (up to 80%), blood (up to 90%) and sexual parasite stages (100%). Based on the component-specific antibody concentrations we calculated the IC₅₀ values for the pre-erythrocytic stage (17–25 μg/ml), the blood stage (40–60 μg/ml) and the sexual stage (1.75 μg/ml). While the results underline the feasibility of a multi-stage vaccine cocktail, the analysis of component-specific efficacy indicates significant differences in IC₅₀ requirements for stage-specific antibody concentrations providing valuable insights into this complex scenario and will thereby improve future approaches towards malaria vaccine cocktail development regarding the selection of suitable antigens and the ratios of components, to fine tune overall and stage-specific efficacy.

Genome-level determination of Plasmodium falciparum blood-stage targets of malarial clinical immunity in the Peruvian Amazon

BACKGROUND

Persons with blood-stage Plasmodium falciparum parasitemia in the absence of symptoms are considered to be clinically immune. We hypothesized that asymptomatic subjects with P. falciparum parasitemia would differentially recognize a subset of P. falciparum proteins on a genomic scale.

METHODS AND FINDINGS

Compared with symptomatic subjects, sera from clinically immune, asymptomatically infected individuals differentially recognized 51 P. falciparum proteins, including the established vaccine candidate PfMSP1. Novel, hitherto unstudied hypothetical proteins and other proteins not previously recognized as potential vaccine candidates were also differentially recognized. Genes encoding the proteins differentially recognized by the Peruvian clinically immune individuals exhibited a significant enrichment of nonsynonymous nucleotide variation, an observation consistent with these genes undergoing immune selection.

CONCLUSIONS

A limited set of P. falciparum protein antigens was associated with the development of naturally acquired clinical immunity in the low-transmission setting of the Peruvian Amazon. These results imply that, even in a low-transmission setting, an asexual blood-stage vaccine designed to reduce clinical malaria symptoms will likely need to contain large numbers of often-polymorphic proteins, a finding at odds with many current efforts in the design of vaccines against asexual blood-stage P. falciparum.

Antimalarial activity of granzyme B and its targeted delivery by a granzyme B-single-chain Fv fusion protein

We present here the first evidence that granzyme B acts against Plasmodium falciparum (50% inhibitory concentration [IC50], 1,590 nM; 95% confidence interval [95% CI], 1,197 to 2,112 nM). We created a novel antimalarial fusion protein consisting of granzyme B fused to a merozoite surface protein 4 (MSP4)-specific single-chain Fv protein (scFv), which targets the enzyme to infected erythrocytes, with up to an 8-fold reduction in the IC50 (176 nM; 95% CI, 154 to 202 nM). This study confirms the therapeutic efficacies of recombinant antibody-mediated antimalarial immunotherapeutics based on granzyme B.

Ycf93 (Orf105), a small apicoplast-encoded membrane protein in the relict plastid of the malaria parasite Plasmodium falciparum that is conserved in Apicomplexa

Malaria parasites retain a relict plastid (apicoplast) from a photosynthetic ancestor shared with dinoflagellate algae. The apicoplast is a useful drug target; blocking housekeeping pathways such as genome replication and translation in the organelle kills parasites and protects against malaria. The apicoplast of Plasmodium falciparum encodes 30 proteins and a suite of rRNAs and tRNAs that facilitate their expression. orf105 is a hypothetical apicoplast gene that would encode a small protein (PfOrf105) with a predicted C-terminal transmembrane domain. We produced antisera to a predicted peptide within PfOrf105. Western blot analysis confirmed expression of orf105 and immunofluorescence localised the gene product to the apicoplast. Pforf105 encodes a membrane protein that has an apparent mass of 17.5 kDa and undergoes substantial turnover during the 48-hour asexual life cycle of the parasite in blood stages. The effect of actinonin, an antimalarial with a putative impact on post-translational modification of apicoplast proteins like PfOrf105, was examined. Unlike other drugs perturbing apicoplast housekeeping that induce delayed death, actinonin kills parasites immediately and has an identical drug exposure phenotype to the isopentenyl diphosphate synthesis blocker fosmidomycin. Open reading frames of similar size to PfOrf105, which also have predicted C-terminal trans membrane domains, occur in syntenic positions in all sequenced apicoplast genomes from Phylum Apicomplexa. We therefore propose to name these genes ycf93 (hypothetical chloroplast reading frame 93) according to plastid gene nomenclature convention for conserved proteins of unknown function.

A chimeric Plasmodium falciparum merozoite surface protein vaccine induces high titers of parasite growth inhibitory antibodies

The C-terminal 19-kDa domain of Plasmodium falciparum merozoite surface protein 1 (PfMSP119) is an established target of protective antibodies. However, clinical trials of PfMSP142, a leading blood-stage vaccine candidate which contains the protective epitopes of PfMSP119, revealed suboptimal immunogenicity and efficacy. Based on proof-of-concept studies in the Plasmodium yoelii murine model, we produced a chimeric vaccine antigen containing recombinant PfMSP119 (rPfMSP119) fused to the N terminus of P. falciparum merozoite surface protein 8 that lacked its low-complexity Asn/Asp-rich domain, rPfMSP8 (ΔAsn/Asp). Immunization of mice with the chimeric rPfMSP1/8 vaccine elicited strong T cell responses to conserved epitopes associated with the rPfMSP8 (ΔAsn/Asp) fusion partner. While specific for PfMSP8, this T cell response was adequate to provide help for the production of high titers of antibodies to both PfMSP119 and rPfMSP8 (ΔAsn/Asp) components. This occurred with formulations adjuvanted with either Quil A or with Montanide ISA 720 plus CpG oligodeoxynucleotide (ODN) and was observed in both inbred and outbred strains of mice. PfMSP1/8-induced antibodies were highly reactive with two major alleles of PfMSP119 (FVO and 3D7). Of particular interest, immunization with PfMSP1/8 elicited higher titers of PfMSP119-specific antibodies than a combined formulation of rPfMSP142 and rPfMSP8 (ΔAsn/Asp). As a measure of functionality, PfMSP1/8-specific rabbit IgG was shown to potently inhibit the in vitro growth of blood-stage parasites of the FVO and 3D7 strains of P. falciparum. These data support the further testing and evaluation of this chimeric PfMSP1/8 antigen as a component of a multivalent vaccine for P. falciparum malaria.

Evaluation of the immunogenicity and vaccine potential of recombinant Plasmodium falciparum merozoite surface protein 8

The C-terminal 19-kDa domain of merozoite surface protein 1 (MSP1₁₉) is the target of protective antibodies but alone is poorly immunogenic. Previously, using the Plasmodium yoelii murine model, we fused P. yoelii MSP1₁₉ (PyMSP1₁₉) with full-length P. yoelii merozoite surface protein 8 (MSP8). Upon immunization, the MSP8-restricted T cell response provided help for the production of high and sustained levels of protective PyMSP1₁₉- and PyMSP8-specific antibodies. Here, we assessed the vaccine potential of MSP8 of the human malaria parasite, Plasmodium falciparum. Distinct from PyMSP8, P. falciparum MSP8 (PfMSP8) contains an N-terminal asparagine and aspartic acid (Asn/Asp)-rich domain whose function is unknown. Comparative analysis of recombinant full-length PfMSP8 and a truncated version devoid of the Asn/Asp-rich domain, PfMSP8(ΔAsn/Asp), showed that both proteins were immunogenic for T cells and B cells. All T cell epitopes utilized mapped within rPfMSP8(ΔAsn/Asp). The dominant B cell epitopes were conformational and common to both rPfMSP8 and rPfMSP8(ΔAsn/Asp). Analysis of native PfMSP8 expression revealed that PfMSP8 is present intracellularly in late schizonts and merozoites. Following invasion, PfMSP8 is found distributed on the surface of ring- and trophozoite-stage parasites. Consistent with a low and/or transient expression of PfMSP8 on the surface of merozoites, PfMSP8-specific rabbit IgG did not inhibit the in vitro growth of P. falciparum blood-stage parasites. These studies suggest that the further development of PfMSP8 as a malaria vaccine component should focus on the use of PfMSP8(ΔAsn/Asp) and its conserved, immunogenic T cell epitopes as a fusion partner for protective domains of poor immunogens, including PfMSP1₁₉.

Evidence of purifying selection on merozoite surface protein 8 (MSP8) and 10 (MSP10) in Plasmodium spp

Evidence for natural selection, positive or negative, on gene encoding antigens may indicate variation or functional constraints that are immunologically relevant. Most malaria surface antigens with high genetic diversity have been reported to be under positive-diversifying selection. However, antigens with limited genetic variation are usually ignored in terms of the role that natural selection may have in generating such patterns. We investigated orthologous genes encoding two merozoite proteins, MSP8 and MSP10, among several mammalian Plasmodium spp. These antigens, together with MSP1, are among the few MSPs that have two epidermal growth factor-like domains (EGF) at the C-terminal. Those EGF are relatively conserved (low levels of genetic polymorphism) and have been proposed to act as ligands during the invasion of RBCs. We use several evolutionary genetic methods to detect patterns consistent with natural selection acting on MSP8 and MSP10 orthologs in the human parasites Plasmodium falciparum and P. vivax, as well as closely related malarial species found in non-human primates (NHPs). Overall, these antigens have low polymorphism in the human parasites in comparison with the orthologs from other Plasmodium spp. We found that the MSP10 gene polymorphism in P. falciparum only harbor non-synonymous substitutions, a pattern consistent with a gene under positive selection. Evidence of purifying selection was found on the polymorphism observed in both orthologs from P. cynomolgi, a non-human primate parasite closely related to P. vivax, but it was not conclusive in the human parasite. Yet, using phylogenetic base approaches, we found evidence for purifying selection on both MSP8 and MSP10 in the lineage leading to P. vivax. Such antigens evolving under strong functional constraints could become valuable vaccine candidates. We discuss how comparative approaches could allow detecting patterns consistent with negative selection even when there is low polymorphism in the extant populations.

Structural and functional characterization of Bc28.1, major erythrocyte-binding protein from Babesia canis merozoite surface

Babesiosis (formerly known as piroplasmosis) is a tick-borne disease caused by the intraerythrocytic development of protozoa parasites from the genus Babesia. Like Plasmodium falciparum, the agent of malaria, or Toxoplasma gondii, responsible for human toxoplasmosis, Babesia belongs to the Apicomplexa family. Babesia canis is the agent of the canine babesiosis in Europe. Clinical manifestations of this disease range from mild to severe and possibly lead to death by multiple organ failure. The identification and characterization of parasite surface proteins represent major goals, both for the understanding of the Apicomplexa invasion process and for the vaccine potential of such antigens. Indeed, we have already shown that Bd37, the major antigenic adhesion protein from Babesia divergens, the agent of bovine babesiosis, was able to induce complete protection against various parasite strains. The major merozoite surface antigens of Babesia canis have been described as a 28-kDa membrane protein family, anchored at the surface of the merozoite. Here, we demonstrate that Bc28.1, a major member of this multigenic family, is expressed at high levels at the surface of the merozoite. This protein is also found in the parasite in vitro culture supernatants, which are the basis of effective vaccines against canine babesiosis. We defined the erythrocyte binding function of Bc28.1 and determined its high resolution solution structure using NMR spectroscopy. Surprisingly, although these proteins are thought to play a similar role in the adhesion process, the structure of Bc28.1 from B. canis appears unrelated to the previously published structure of Bd37 from B. divergens. Site-directed mutagenesis experiments also suggest that the mechanism of the interaction with the erythrocyte membrane could be different for the two proteins. The resolution of the structure of Bc28 represents a milestone for the characterization of the parasite erythrocyte binding and its interaction with the host immune system.

Cerebral malaria: insights from host-parasite protein-protein interactions

Background

Cerebral malaria is a form of human malaria wherein Plasmodium falciparum-infected red blood cells adhere to the blood capillaries in the brain, potentially leading to coma and death. Interactions between parasite and host proteins are important in understanding the pathogenesis of this deadly form of malaria. It is, therefore, necessary to study available protein-protein interactions to identify lesser known interactions that could throw light on key events of cerebral malaria.

Methods

Sequestration, haemostasis dysfunction, systemic inflammation and neuronal damage are key processes of cerebral malaria. Key events were identified from literature as being crucial to these processes. An integrated interactome was created using available experimental and predicted datasets as well as from literature. Interactions from this interactome were filtered based on Gene Ontology and tissue-specific annotations, and further analysed for relevance to the key events.

Results

PfEMP1 presentation, platelet activation and astrocyte dysfunction were identified as the key events influencing the disease. 48896 host-parasite along with other host-parasite, host-host and parasite-parasite protein-protein interactions obtained from a disease-specific corpus were combined to form an integrated interactome. Filtering of the interactome resulted in five host-parasite PPI, six parasite-parasite and two host-host PPI. The analysis of these interactions revealed the potential significance of apolipoproteins and temperature/Hsp expression on efficient PfEMP1 presentation; role of MSP-1 in platelet activation; effect of parasite proteins in TGF-β regulation and the role of albumin in astrocyte dysfunction.

Conclusions

This work links key host-parasite, parasite-parasite and host-host protein-protein interactions to key processes of cerebral malaria and generates hypotheses for disease pathogenesis based on a filtered interaction dataset. These hypotheses provide novel and significant insights to cerebral malaria.

Systematic genetic analysis of the Plasmodium falciparum MSP7-like family reveals differences in protein expression, location, and importance in asexual growth of the blood-stage parasite

Proteins located on Plasmodium falciparum merozoites, the invasive form of the parasite's asexual blood stage, are of considerable interest in vaccine research. Merozoite surface protein 7 (MSP7) forms a complex with MSP1 and is encoded by a member of a multigene family located on chromosome 13. The family codes for MSP7 and five MSP7-related proteins (MSRPs). In the present study, we have investigated the expression and the effect of msrp gene deletion at the asexual blood stage. In addition to msp7, msrp2, msrp3, and msrp5 are transcribed, and mRNA was easily detected by hybridization analysis, whereas mRNA for msrp1 and msrp4 could be detected only by reverse transcription (RT)-PCR. Notwithstanding evidence of transcription, antibodies to recombinant MSRPs failed to detect specific proteins, except for antibodies to MSRP2. Sequential proteolytic cleavages of MSRP2 resulted in 28- and 25-kDa forms. However, MSRP2 was absent from merozoites; the 25-kDa MSRP2 protein (MSRP2(25)) was soluble and secreted upon merozoite egress. The msrp genes were deleted by targeted disruption in the 3D7 line, leading to ablation of full-length transcripts. MSRP deletion mutants had no detectable phenotype, with growth and invasion characteristics comparable to those of the parental parasite; only the deletion of MSP7 led to a detectable growth phenotype. Thus, within this family some of the genes are transcribed at a significant level in asexual blood stages, but the corresponding proteins may or may not be detectable. Interactions of the expressed proteins with the merozoite also differ. These results highlight the potential for unexpected differences of protein expression levels within gene families.

The transcriptome of Plasmodium vivax reveals divergence and diversity of transcriptional regulation in malaria parasites

Plasmodium vivax causes over 100 million clinical infections each year. Primarily because of the lack of a suitable culture system, our understanding of the biology of this parasite lags significantly behind that of the more deadly species P. falciparum. Here, we present the complete transcriptional profile throughout the 48-h intraerythrocytic cycle of three distinct P. vivax isolates. This approach identifies strain specific patterns of expression for subsets of genes predicted to encode proteins associated with virulence and host pathogen interactions. Comparison to P. falciparum revealed significant differences in the expression of genes involved in crucial cellular functions that underpin the biological differences between the two parasite species. These data provide insights into the biology of P. vivax and constitute an important resource for the development of therapeutic approaches.

Deletion of the Plasmodium falciparum merozoite surface protein 7 gene impairs parasite invasion of erythrocytes

Merozoite surface proteins have been implicated in the initial attachment to the host red blood cell membrane that begins the process of invasion, an important step in the life cycle of the malaria parasite. In Plasmodium falciparum, merozoite surface proteins include several glycosylphosphatidyl inositol-anchored proteins and peripheral proteins attached to the membrane through protein-protein interactions. The most abundant of these proteins is the merozoite surface protein 1 (MSP1) complex, encoded by at least three genes: msp1, msp6, and msp7. The msp7 gene is part of a six-member multigene family in Plasmodium falciparum. We have disrupted msp7 in the Plasmodium falciparum D10 parasite, as confirmed by Southern hybridization. Immunoblot and indirect immunofluorescence analyses confirmed the MSP7 null phenotype of D10DeltaMSP7 parasites. The synthesis, distribution, and processing of MSP1 were not affected in this parasite line. The level of expression and cellular distribution of the proteins MSP1, MSP3, MSP6, MSP9, and SERA5 remained comparable to those for the parental line. Furthermore, no significant change in the expression of MSP7-related proteins, except for that of MSRP5, was detected at the transcriptional level. The lack of MSP7 was not lethal at the asexual blood stage, but it did impair invasion of erythrocytes by merozoites to a significant degree. Despite this reduction in efficiency, D10DeltaMSP7 parasites did not show any obvious preference for alternate pathways of invasion.

Formation of the food vacuole in Plasmodium falciparum: a potential role for the 19 kDa fragment of merozoite surface protein 1 (MSP1(19))

Plasmodium falciparum Merozoite Surface Protein 1 (MSP1) is synthesized during schizogony as a 195-kDa precursor that is processed into four fragments on the parasite surface. Following a second proteolytic cleavage during merozoite invasion of the red blood cell, most of the protein is shed from the surface except for the C-terminal 19-kDa fragment (MSP1₁₉), which is still attached to the merozoite via its GPI-anchor. We have examined the fate of MSP1₁₉ during the parasite's subsequent intracellular development using immunochemical analysis of metabolically labeled MSP1₁₉, fluorescence imaging, and immuno-electronmicroscopy. Our data show that MSP1₁₉ remains intact and persists to the end of the intracellular cycle. This protein is the first marker for the biogenesis of the food vacuole; it is rapidly endocytosed into small vacuoles in the ring stage, which coalesce to form the single food vacuole containing hemozoin, and persists into the discarded residual body. The food vacuole is marked by the presence of both MSP1₁₉ and the chloroquine resistance transporter (CRT) as components of the vacuolar membrane. Newly synthesized MSP1 is excluded from the vacuole. This behavior indicates that MSP1₁₉ does not simply follow a classical lysosome-like clearance pathway, instead, it may play a significant role in the biogenesis and function of the food vacuole throughout the intra-erythrocytic phase.

Identification of protein complexes in detergent-resistant membranes of Plasmodium falciparum schizonts

Merozoite surface proteins of the human malaria parasite Plasmodium falciparum are involved in initial contact with target erythrocytes, a process that begins a cascade of events required for successful invasion of these cells. In order to identify complexes that may play a role in invasion we purified detergent-resistant membranes (DRMs), known to be enriched in merozoite surface proteins, and used blue native-polyacrylamide gel electrophoresis (BN-PAGE) to isolate high molecular weight complexes for identification by mass spectrometry. Sixty-two proteins were detected and these mostly belonged to expected DRM proteins classes including GPI-anchored, multi-membrane spanning and rhoptry proteins. Proteins from seven known complexes were identified including MSP-1/7, the low (RAP1/2 and RAP1/3), and high (RhopH1/H2/H3) molecular weight rhoptry complexes, and the invasion motor complex (GAP45/GAP50/myosinA). Remarkably, a large proportion of identified spectra were derived from only 4 proteins: the GPI-anchored proteins MSP-1 and Pf92, the putative GPI-anchored protein Pf113 and RAP-1, the core component of the two RAP complexes. Each of these proteins predominated in high molecular weight species suggesting their aggregation in much larger complexes than anticipated. To demonstrate that the procedure had isolated novel complexes we focussed on MSP-1, which predominated as a distinct species at approximately 500 kDa by BN-PAGE, approximately twice its expected size. Chemical cross-linking supports the existence of a stable MSP-1 oligomer of approximately 500 kDa, probably comprising a highly stable homodimeric species. Our observations also suggests that oligomerization of MSP-1 is likely to occur outside the C-terminal epidermal growth factor (EGF)-like domains. Confirmation of MSP-1 oligomerization, together with the isolation of a number of known complexes by BN-PAGE, makes it highly likely that novel interactions occur amongst members of this proteome.

Enhanced protection against malaria by a chimeric merozoite surface protein vaccine

The 42-kDa processed fragment of Plasmodium falciparum merozoite surface protein 1 (MSP-1(42)) is a prime candidate for a blood-stage malaria vaccine. Merozoite surface protein 8 contains two C-terminal epidermal growth factor (EGF)-like domains that may function similarly to those of MSP-1(42). Immunization with either MSP-1 or MSP-8 induces protection that is mediated primarily by antibodies against conformation-dependent epitopes. In a series of comparative immunogenicity and efficacy studies using the Plasmodium yoelii rodent model, we tested the ability of recombinant P. yoelii MSP-8 (rPyMSP-8) to complement rPyMSP-1-based vaccines. Unlike MSP-1, PyMSP-8-dependent protection required immunization with the full-length protein and was not induced with recombinant antigens that contained only the C-terminal EGF-like domains. Unlike PyMSP-8, the immunogenicity of the PyMSP-1 EGF-like domains was low when present as part of the rPyMSP-1(42) antigen. Immunization with a mixture of rPyMSP-1(42) and rPyMSP-8 further inhibited the antibody response to protective epitopes of rPyMSP-1(42) and did not improve vaccine efficacy. To improve PyMSP-1 immunogenicity, we produced a chimeric antigen containing the EGF-like domains of PyMSP-1 fused to the N terminus of PyMSP-8. Immunization with the chimeric rPyMSP-1/8 antigen induced high and comparable antibody responses against the EGF-like domains of both PyMSP-1 and PyMSP-8. This enhanced MSP-1-specific antibody response and the concurrent targeting of MSP-1 and MSP-8 resulted in improved, nearly complete protection against lethal P. yoelii 17XL malaria. Unexpectedly, immunization with rPyMSP-1/8 failed to protect against challenge infection with reticulocyte-restricted P. yoelii 17X parasites. Overall, these data establish an effective strategy to improve the efficacy of P. falciparum MSP-based vaccines.

A set of glycosylphosphatidyl inositol-anchored membrane proteins of Plasmodium falciparum is refractory to genetic deletion

Targeted gene disruption has proved to be a powerful approach for studying the function of important ligands involved in erythrocyte invasion by the extracellular merozoite form of the human malaria parasite, Plasmodium falciparum. Merozoite invasion proceeds via a number of seemingly independent alternate pathways, such that entry can proceed with parasites lacking particular ligand-receptor interactions. To date, most focus in this regard has been on single-pass (type 1) membrane proteins that reside in the secretory organelles. Another class of merozoite proteins likely to include ligands for erythrocyte receptors are the glycosylphosphatidyl inositol (GPI)-anchored membrane proteins that coat the parasite surface and/or reside in the apical organelles. Several of these are prominent vaccine candidates, although their functions remain unknown. Here, we systematically attempted to disrupt the genes encoding seven of the known GPI-anchored merozoite proteins of P. falciparum by using a double-crossover gene-targeting approach. Surprisingly, and in apparent contrast to other merozoite antigen classes, most of the genes (six of seven) encoding GPI-anchored merozoite proteins are refractory to genetic deletion, with the exception being the gene encoding merozoite surface protein 5 (MSP-5). No distinguishable growth rate or invasion pathway phenotype was detected for the msp-5 knockout line, although its presence as a surface-localized protein was confirmed.

Expression, localization, and erythrocyte binding activity of Plasmodium yoelii merozoite surface protein-8

PyMSP-8 is a member of a family of merozoite surface proteins that have been described in Plasmodium that are characterized by the presence of a glycolipid membrane anchor and 1-2 epidermal growth factor-like domains. Immunization with recombinant PyMSP-8 has also been shown to protect mice against lethal Plasmodium yoelii malaria. In this report, we demonstrate that PyMSP-8 expression is detectable throughout the entire erythrocytic life cycle of P. yoelii 17XL, reaching peak level during trophozoite development. As determined by immunofluorescence, PyMSP-8 co-localizes with PyMSP-1 on the surface of merozoites in segmented schizonts and on the surface of ring stages in newly invaded erythrocytes. PyMSP-8 binds to the surface of uninfected mouse RBCs in a species-dependent manner, suggesting a potential role in merozoite attachment to and/or invasion of erythrocytes. The receptor for PyMSP-8 on RBCs is sensitive to trypsin digestion but is resistant to treatment with chymotrypsin or neuraminidase and is putatively identified as a approximately 105kDa membrane protein. Since PyMSP-8 binds to both mature RBCs as well as reticulocytes, it appears unlikely that the function of PyMSP-8 is restricted to the invasion of normocytes. While proper folding and conformation of PyMSP-8 are important, linear determinants of PyMSP-8 also contribute to erythrocyte binding. Unexpectedly, however, PyMSP-8 specific antibodies that are protective in vivo, do not disrupt the binding of rPyMSP-8 to its receptor on erythrocytes. The data indicate that protective anti-PyMSP-8 antibodies mediate their effect in vivo by an alternate mechanism(s).

Identification and stoichiometry of glycosylphosphatidylinositol-anchored membrane proteins of the human malaria parasite Plasmodium falciparum

Most proteins that coat the surface of the extracellular forms of the human malaria parasite Plasmodium falciparum are attached to the plasma membrane via glycosylphosphatidylinositol (GPI) anchors. These proteins are exposed to neutralizing antibodies, and several are advanced vaccine candidates. To identify the GPI-anchored proteome of P. falciparum we used a combination of proteomic and computational approaches. Focusing on the clinically relevant blood stage of the life cycle, proteomic analysis of proteins labeled with radioactive glucosamine identified GPI anchoring on 11 proteins (merozoite surface protein (MSP)-1, -2, -4, -5, -10, rhoptry-associated membrane antigen, apical sushi protein, Pf92, Pf38, Pf12, and Pf34). These proteins represent approximately 94% of the GPI-anchored schizont/merozoite proteome and constitute by far the largest validated set of GPI-anchored proteins in this organism. Moreover MSP-1 and MSP-2 were present in similar copy number, and we estimated that together these proteins comprise approximately two-thirds of the total membrane-associated surface coat. This is the first time the stoichiometry of MSPs has been examined. We observed that available software performed poorly in predicting GPI anchoring on P. falciparum proteins where such modification had been validated by proteomics. Therefore, we developed a hidden Markov model (GPI-HMM) trained on P. falciparum sequences and used this to rank all proteins encoded in the completed P. falciparum genome according to their likelihood of being GPI-anchored. GPI-HMM predicted GPI modification on all validated proteins, on several known membrane proteins, and on a number of novel, presumably surface, proteins expressed in the blood, insect, and/or pre-erythrocytic stages of the life cycle. Together this work identified 11 and predicted a further 19 GPI-anchored proteins in P. falciparum.

Distinct protein classes including novel merozoite surface antigens in Raft-like membranes of Plasmodium falciparum

Glycosylphosphatidylinositol (GPI)-anchored proteins coat the surface of extracellular Plasmodium falciparum merozoites, of which several are highly validated candidates for inclusion in a blood-stage malaria vaccine. Here we determined the proteome of gradient-purified detergent-resistant membranes of mature blood-stage parasites and found that these membranes are greatly enriched in GPI-anchored proteins and their putative interacting partners. Also prominent in detergent-resistant membranes are apical organelle (rhoptry), multimembrane-spanning, and proteins destined for export into the host erythrocyte cytosol. Four new GPI-anchored proteins were identified, and a number of other novel proteins that are predicted to localize to the merozoite surface and/or apical organelles were detected. Three of the putative surface proteins possessed six-cysteine (Cys6) motifs, a distinct fold found in adhesive surface proteins expressed in other life stages. All three Cys6 proteins, termed Pf12, Pf38, and Pf41, were validated as merozoite surface antigens recognized strongly by antibodies present in naturally infected individuals. In addition to the merozoite surface, Pf38 was particularly prominent in the secretory apical organelles. A different cysteine-rich putative GPI-anchored protein, Pf92, was also localized to the merozoite surface. This insight into merozoite surfaces provides new opportunities for understanding both erythrocyte invasion and anti-parasite immunity.