Strain-Dependent Inhibition of Erythrocyte Invasion by Monoclonal Antibodies Against Plasmodium falciparum CyRPA

The highly conserved Plasmodium falciparum cysteine-rich protective antigen (PfCyRPA) is a key target for next-generation vaccines against blood-stage malaria. PfCyRPA constitute the core of a ternary complex, including the reticulocyte binding-like homologous protein 5 (PfRh5) and the Rh5-interacting protein (PfRipr), and is fundamental for merozoite invasion of erythrocytes. In this study, we show that monoclonal antibodies (mAbs) specific to PfCyRPA neutralize the in vitro growth of Ghanaian field isolates as well as numerous laboratory-adapted parasite lines. We identified subsets of mAbs with neutralizing activity that bind to distinct sites on PfCyRPA and that in combination potentiate the neutralizing effect. As antibody responses against multiple merozoite invasion proteins are thought to improve the efficacy of blood-stage vaccines, we also demonstrated that combinations of PfCyRPA- and PfRh5 specific mAbs act synergistically to neutralize parasite growth. Yet, we identified prominent strain-dependent neutralization potencies, which our results suggest is independent of PfCyRPA expression level and polymorphism, demonstrating the importance of addressing functional converseness when evaluating blood-stage vaccine candidates. Finally, our results suggest that blood-stage vaccine efficacy can be improved by directing the antibody response towards defined protective epitopes on multiple parasite antigens.

Identification and characterization of a Babesia bigemina thrombospondin-related superfamily member, TRAP-1: a novel antigen containing neutralizing epitopes involved in merozoite invasion

Background Thrombospondin-related anonymous protein (TRAP) has been described as a potential vaccine candidate for several diseases caused by apicomplexan parasites. However, this protein and members of this family have not yet been characterized in Babesia bigemina, one of the most prevalent species causing bovine babesiosis. Methods The 3186-bp Babesia bigemina TRAP-1 (BbiTRAP-1) gene was identified by a bioinformatics search using the B. bovis TRAP-1 sequence. Members of the TRAP and TRAP-related protein families (TRP) were identified in Babesia and Theileria through a search of the TSP-1 adhesive domain, which is the hallmark motif in both proteins. Structural modeling and phylogenetic analysis were performed with the identified TRAP proteins. A truncated recombinant BbiTRAP-1 that migrates at approximately 107 kDa and specific antisera were produced and used in Western blot analysis and indirect fluorescent antibody tests (IFAT). B-cell epitopes with neutralizing activity in BbiTRAP-1 were defined by enzyme-linked immunosorbent assays (ELISA) and invasion assays. Results Three members of the TRAP family of proteins were identified in B. bigemina (BbiTRAP-1 to -3). All are type 1 transmembrane proteins containing the von Willebrand factor A (vWFA), thrombospondin type 1 (TSP-1), and cytoplasmic C-terminus domains, as well as transmembrane regions. The BbiTRAP-1 predicted structure also contains a metal ion-dependent adhesion site for interaction with the host cell. The TRP family in Babesia and Theileria species contains the canonical TSP-1 domain but lacks the vWFA domain and together with TRAP define a novel gene superfamily. A variable number of tandem repeat units are present in BbiTRAP-1 and could be used for strain genotyping. Western blot and IFAT analysis confirmed the expression of BbiTRAP-1 by blood-stage parasites. Partial recognition by a panel of sera from B. bigemina-infected cattle in ELISAs using truncated BbiTRAP-1 suggests that this protein is not an immunodominant antigen. Additionally, bovine anti-recombinant BbiTRAP-1 antibodies were found to be capable of neutralizing merozoite invasion in vitro. Conclusions We have identified the TRAP and TRP gene families in several Babesia and Theileria species and characterized BbiTRAP-1 as a novel antigen of B. bigemina. The functional relevance and presence of neutralization-sensitive B-cell epitopes suggest that BbiTRAP-1 could be included in tests for future vaccine candidates against B. bigemina.

Pfcerli2, a duplicated gene in the malaria parasite Plasmodium falciparum essential for invasion of erythrocytes as revealed by phylogenetic and cell biological analysis

ABSTRACTMerozoite invasion of host red blood cells (RBCs) is essential for survival of the human malaria parasite Plasmodium falciparum. Proteins involved with RBC binding and invasion are secreted from dual-club shaped organelles at the apical tip of the merozoite called the rhoptries. Here we characterise P. falciparum Cytosolically Exposed Rhoptry Leaflet Interacting protein 2 (PfCERLI2), as a rhoptry bulb protein that is essential for merozoite invasion. Phylogenetic analyses show that cerli2 arose through an ancestral gene duplication of cerli1, a related cytosolically exposed rhoptry bulb protein. We show that PfCERLI2 is essential for blood-stage growth and localises to the cytosolic face of the rhoptry bulb. Inducible knockdown of PfCERLI2 led to an inhibition of merozoite invasion after tight junction formation. PfCERLI2 knockdown was associated with inhibition of rhoptry antigen processing and a significant elongation of the rhoptries, suggesting that the inability of merozoites to invade is caused by aberrant rhoptry function due to PfCERLI2 deficiency. These findings identify PfCERLI2 as a protein that has key roles in rhoptry biology during merozoite invasion.

Identification and characterization of a Babesia bigemina thrombospondin-related superfamily member, TRAP-1: a novel antigen containing neutralizing epitopes involved in merozoite invasion

Background The thrombospondin-related anonymous protein (TRAP) has been described as a potential vaccine candidate in several apicomplexan parasites. However, this protein and members of this family have not been characterized yet in Babesia bigemina, one of the most prevalent species causing bovine babesiosis.Methods The Babesia bigemina TRAP-1 (BbiTRAP-1) gene of 3186 bp was identified by a bioinformatics search using the B. bovis TRAP-1 sequence. Members of the TRAP and TRAP-related protein families (TRP) were identified in Babesia and Theileria through the search of the TSP-1 adhesive domain, the hallmark motif in both proteins. Structural modelling and phylogenetic analysis were performed with the identified TRAP proteins. A truncated recombinant BbiTRAP-1 that migrates at ~107 kDa and specific antisera were produced and used in Western blot analysis and indirect fluorescent antibody test (IFAT). B-cell epitopes with neutralizing activity in BbiTRAP-1 were defined by ELISA and invasion assays.Results TRAP family has 3 members in B. bigemina (BbiTRAP-1-3). All are type 1 transmembrane proteins containing the von Willebrand factor A (vWFA), thrombospondin type 1 (TSP-1) and cytoplasmic C-terminus domains along with transmembrane regions. The BbiTRAP-1 predicted structure also contains a metal ion-dependent adhesion site (MIDAS) for interaction with the host cell. The TRP family in Babesia and Theileria species contains the canonical TSP-1 domain but lacks the vWFA domain and together with TRAP define a novel gene superfamily. A variable number of tandem repeat units is present in BbiTRAP-1 and could be used for strain genotyping. Western blot and IFAT analysis confirmed expression of BbiTRAP-1 by blood stage parasites. Partial recognition by a panel of sera from B. bigemina infected cattle in ELISA using truncated BbiTRAP-1 suggests that this protein is not an immunodominant antigen. Additionally, bovine anti-recombinant BbiTRAP-1 antibodies were capable of neutralizing merozoite invasion in vitro. Conclusions We identified the TRAP and TRP gene families in several Babesia and Theileria species, and characterized BbiTRAP-1 as a novel antigen of B. bigemina. The functional relevance and presence of neutralization-sensitive B-cell epitopes suggest that BbiTRAP-1 could be included in future vaccine candidates against B. bigemina.

Identification and characterization of a Babesia bigemina thrombospondin-related superfamily member, TRAP-1: a novel antigen containing neutralizing epitopes involved in merozoite invasion

BackgroundThe thrombospondin-related anonymous protein (TRAP) has been described as a potential vaccine candidate in several apicomplexan parasites. However, this protein and members of this family have not been characterized yet in Babesia bigemina, one of the most prevalent species causing bovine babesiosis.MethodsThe Babesia bigemina TRAP-1 (BbiTRAP-1) gene of 3186 bp was identified by a bioinformatics search using the B. bovis TRAP-1 sequence. Members of the TRAP and TRAP-related protein families (TRP) were identified in Babesia and Theileria through the search of the TSP-1 adhesive domain, the hallmark motif in both proteins. Structural modelling and phylogenetic analysis were performed with the identified TRAP proteins. A truncated recombinant BbiTRAP-1 that migrates at ~107 kDa and specific antisera were produced and used in Western blot analysis and indirect fluorescent antibody test (IFAT). B-cell epitopes with neutralizing activity in BbiTRAP-1 were defined by ELISA and invasion assays.ResultsTRAP family has 3 members in B. bigemina (BbiTRAP-1-3). All are type 1 transmembrane proteins containing the von Willebrand factor A (vWFA), thrombospondin type 1 (TSP-1) and cytoplasmic C-terminus domains along with transmembrane regions. The BbiTRAP-1 predicted structure also contains a metal ion-dependent adhesion site (MIDAS) for interaction with the host cell. The TRP family in Babesia and Theileria species contains the canonical TSP-1 domain but lacks the vWFA domain and together with TRAP define a novel gene superfamily. A variable number of tandem repeat units is present in BbiTRAP-1 and could be used for strain genotyping. Western blot and IFAT analysis confirmed expression of BbiTRAP-1 by blood stage parasites. Partial recognition by a panel of sera from B. bigemina infected cattle in ELISA using truncated BbiTRAP-1 suggests that this protein is not an immunodominant antigen. Additionally, bovine anti-recombinant BbiTRAP-1 antibodies were capable of neutralizing merozoite invasion in vitro. ConclusionsWe identified the TRAP and TRP gene families in several Babesia and Theileria species, and characterized BbiTRAP-1 as a novel antigen of B. bigemina. The functional relevance and presence of neutralization-sensitive B-cell epitopes suggest that BbiTRAP-1 could be included in future vaccine candidates against B. bigemina.

Identification and characterization of a Babesia bigemina thrombospondin-related superfamily member, TRAP-1: a novel antigen containing neutralizing epitopes involved in merozoite invasion

Background The Thrombospondin-Related Anonymous Protein (TRAP) has been described as a potential vaccine candidate in several apicomplexan parasites. However, this protein and members of this family have not been characterized yet in Babesia bigemina, one of the most prevalent species causing bovine babesiosis.Methods The Babesia bigemina TRAP-1 (BbiTRAP-1) gene was identified by a bioinformatics search using the B. bovis TRAP-1 sequence. Members of the TRAP and TRAP-related protein family (TRP) families were identified in Babesia and Theileria through the search of the TSP-1 adhesive domain, the hallmark motif of TRAP proteins. Structural modelling and phylogenetic analysis were performed with the identified TRAP proteins. A truncated recombinant BbiTRAP-1 and specific antisera were produced, and used in Western blot analysis and indirect fluorescent antibody test (IFAT). B-cell epitopes with neutralizing activity in BbiTRAP-1 were defined by ELISA and invasion assays.Results TRAP family has 3 members in B. bigemina (BbiTRAP-1-3). All are type 1 transmembrane proteins containing the von Willebrand factor A (vWFA), thrombospondin type 1 (TSP-1) and cytoplasmic C-terminus domains along with transmembrane regions. The BbiTRAP-1 predicted structure also contains a metal ion-dependent adhesion site (MIDAS) for interaction with the host cell. The TRP family in Babesia and Theileria species contains the canonical TSP-1 domain but lack the vWFA domain,and together with TRAP define a novel gene superfamily. A variable number of tandem repeat units is present in BbiTRAP-1 and could be used for strain genotyping. Western blot and IFAT analysis confirmed expression of BbiTRAP-1 by blood stage parasites. Partial recognition by a panel of sera from B. bigemina infected cattle in ELISA using truncated BbiTRAP-1 suggests that this protein is not an immunodominant antigen. Additionally, bovine anti- recombinant BbiTRAP-1 antibodies were capable of neutralize merozoite invasion in vitro. Conclusions We identified the TRAP and TRP gene families in several Babesia and Theileria species, and characterized BbiTRAP-1 as a novel antigen of B. bigemina. The functional relevance and presence of neutralization-sensitive B-cell epitopes suggest that BbiTRAP-1 could be included in future vaccine candidates against B. bigemina.

Phosphorylation-Dependent Assembly of a 14-3-3 Mediated Signaling Complex During Red Blood Cell Invasion by Plasmodium falciparum Merozoites

Red blood cell (RBC) invasion by Plasmodium merozoites requires multiple steps that are regulated by signaling pathways. Exposure of P. falciparum merozoites to the physiological signal of low K+, as found in blood plasma, leads to a rise in cytosolic Ca2+, which mediates microneme secretion, motility, and invasion. We have used global phosphoproteomic analysis of merozoites to identify signaling pathways that are activated during invasion. Using quantitative phosphoproteomics we found 394 protein phosphorylation site changes in merozoites subjected to different ionic environments (high K+/ low K+) out of which 143 were Ca2+-dependent. These included a number of signaling proteins such as catalytic and regulatory subunits of protein kinase A (PfPKAc and PfPKAr) and calcium-dependent protein kinase 1 (PfCDPK1). Proteins of the 14-3-3 family interact with phosphorylated target proteins to assemble signaling complexes. Here, using co-immunoprecipitation and gel filtration chromatography, we demonstrate that Pf14-3-3I binds phosphorylated PfPKAr and PfCDPK1 to mediate the assembly of a multi-protein complex in P. falciparum merozoites. A phospho-peptide, P1, based on the Ca2+ dependent phosphosites of PKAr, binds Pf14-3-3I and disrupts assembly of the Pf14-3-3I-mediated multi-protein complex. Disruption of the multi-protein complex with P1 inhibits microneme secretion and RBC invasion. This study thus identifies a novel signaling complex that plays a key role in merozoite invasion of RBCs. Disruption of this signaling complex could serve as a novel approach to inhibit blood stage growth of malaria parasites.ImportanceInvasion of red blood cells (RBCs) by Plasmodium falciparum merozoites is a complex process that is regulated by intricate signaling pathways. Here, we have used phosphoproteomic profiling to identify the key proteins involved in signaling events during invasion. We found changes in the phosphorylation of various merozoite proteins including multiple kinases previously implicated in the process of invasion. We also found that a phosphorylation dependent multi-protein complex including signaling kinases assembles during the process of invasion. Disruption of this multi-protein complex impairs merozoite invasion of RBCs providing a novel approach for the development of inhibitors to block the growth of blood stage malaria parasites.

Overexpression of the HECT ubiquitin ligase PfUT prolongs the intraerythrocytic cycle and reduces invasion efficiency of Plasmodium falciparum

The glms ribozyme system has been used as an amenable tool to conditionally control expression of genes of interest. It is generally assumed that insertion of the ribozyme sequence does not affect expression of the targeted gene in the absence of the inducer glucosamine-6-phosphate, although experimental support for this assumption is scarce. Here, we report the unexpected finding that integration of the glms ribozyme sequence in the 3′ untranslated region of a gene encoding a HECT E3 ubiquitin ligase, termed Plasmodium falciparum ubiquitin transferase (PfUT), increased steady state RNA and protein levels 2.5-fold in the human malaria parasite P. falciparum. Overexpression of pfut resulted in an S/M phase-associated lengthening of the parasite’s intraerythrocytic developmental cycle and a reduced merozoite invasion efficiency. The addition of glucosamine partially restored the wild type phenotype. Our study suggests a role of PfUT in controlling cell cycle progression and merozoite invasion. Our study further raises awareness regarding unexpected effects on gene expression when inserting the glms ribozyme sequence into a gene locus.