Genetic diversity in the Plasmodium falciparum next-generation blood stage vaccine candidate antigen PfCyRPA in Senegal

The Plasmodium falciparum cysteine-rich protective antigen (PfCyRPA) is a promising target as a next-generation blood-stage malaria vaccine and together with PCRCR complex members, the reticulocyte binding-like homologous protein 5 (PfRh5) and the Rh5-interacting protein (PfRipr), are currently being evaluated in clinical trials. PfCyRPA is essential for merozoite invasion and appears to be highly conserved within the P. falciparum parasite populations. Here, we used a targeted deep amplicon next-generation sequencing approach to assess the breadth of PfCyRPA genetic diversity in 95 P. falciparum clinical isolates from Kédougou, an area with a high seasonal malaria transmission in Senegal. Our data show the dominant prevalence of PfCyRPA wild type reference allele, while we also identify a total of 15 single nucleotide polymorphisms (SNPs). Of these, only five have previously been reported, while the majority of the SNPs were present as singletons within our sampled population. Moreover, the variant read frequency of the identified SNPs varied from 2.6 to 100%, while the majority of the SNPs were present at frequencies greater than 25% in polygenomic samples. We also applied a structure-based modelling approach to thread these SNPs onto PfCyRPA crystal structures and showed that these polymorphisms have different predicted functional impacts on the interactions with binding partner PfRH5 or neutralizing antibodies. Our prediction revealed that the majority of these SNPs have minor effects on PfCyRPA antibodies, while others alter its structure, stability, or interaction with PfRH5. Altogether, our present findings reveal conserved PfCyRPA epitopes which will inform downstream investigations on next-generation structure-guided malaria vaccine design.

Leveraging Immunofocusing and Virus-like Particle Display to Enhance Antibody Responses to the Malaria Blood-Stage Invasion Complex Antigen PfCyRPA

A vaccine protecting against malaria caused by Plasmodium falciparum is urgently needed. The blood-stage invasion complex PCRCR consists of the five malarial proteins PfPTRAMP, PfCSS, PfRipr, PfCyRPA, and PfRH5. As each subcomponent represents an essential and highly conserved antigen, PCRCR is considered a promising vaccine target. Furthermore, antibodies targeting the complex can block red blood cell invasion by the malaria parasite. However, extremely high titers of neutralizing antibodies are needed for this invasion-blocking effect, and a vaccine based on soluble PfRH5 protein has proven insufficient in inducing a protective response in a clinical trial. Here, we present the results of two approaches to increase the neutralizing antibody titers: (A) immunofocusing and (B) increasing the immunogenicity of the antigen via multivalent display on capsid virus-like particles (cVLPs). The immunofocusing strategies included vaccinating with peptides capable of binding the invasion-blocking anti-PfCyRPA monoclonal antibody CyP1.9, as well as removing non-neutralizing epitopes of PfCyRPA through truncation. Vaccination with PfCyRPA coupled to the AP205 cVLP induced nearly two-fold higher IgG responses compared to vaccinating with soluble PfCyRPA protein. Immunofocusing using a linear peptide greatly increased the neutralizing capacity of the anti-PfCyRPA antibodies. However, significantly lower total anti-PfCyRPA titers were achieved using this strategy. Our results underline the potential of a cVLP-based malaria vaccine including full-length PfCyRPA, which could be combined with other leading malaria vaccine antigens presented on cVLPs.

Immunogenicity of PvCyRPA, PvCelTOS and Pvs25 chimeric recombinant protein of Plasmodium vivax in murine model

In the Americas, P. vivax is the predominant causative species of malaria, a debilitating and economically significant disease. Due to the complexity of the malaria parasite life cycle, a vaccine formulation with multiple antigens expressed in various parasite stages may represent an effective approach. Based on this, we previously designed and constructed a chimeric recombinant protein, PvRMC-1, composed by PvCyRPA, PvCelTOS, and Pvs25 epitopes. This chimeric protein was strongly recognized by naturally acquired antibodies from exposed population in the Brazilian Amazon. However, there was no investigation about the induced immune response of PvRMC-1. Therefore, in this work, we evaluated the immunogenicity of this chimeric antigen formulated in three distinct adjuvants: Stimune, AddaVax or Aluminum hydroxide (Al(OH)3) in BALB/c mice. Our results suggested that the chimeric protein PvRMC-1 were capable to generate humoral and cellular responses across all three formulations. Antibodies recognized full-length PvRMC-1 and linear B-cell epitopes from PvCyRPA, PvCelTOS, and Pvs25 individually. Moreover, mice's splenocytes were activated, producing IFN-γ in response to PvCelTOS and PvCyRPA peptide epitopes, affirming T-cell epitopes in the antigen. While aluminum hydroxide showed notable cellular response, Stimune and Addavax induced a more comprehensive immune response, encompassing both cellular and humoral components. Thus, our findings indicate that PvRMC-1 would be a promising multistage vaccine candidate that could advance to further preclinical studies.

Development of an improved blood-stage malaria vaccine targeting the essential RH5-CyRPA-RIPR invasion complex

Reticulocyte-binding protein homologue 5 (RH5), a leading blood-stage Plasmodium falciparum malaria vaccine target, interacts with cysteine-rich protective antigen (CyRPA) and RH5-interacting protein (RIPR) to form an essential heterotrimeric "RCR-complex". We investigate whether RCR-complex vaccination can improve upon RH5 alone. Using monoclonal antibodies (mAbs) we show that parasite growth-inhibitory epitopes on each antigen are surface-exposed on the RCR-complex and that mAb pairs targeting different antigens can function additively or synergistically. However, immunisation of female rats with the RCR-complex fails to outperform RH5 alone due to immuno-dominance of RIPR coupled with inferior potency of anti-RIPR polyclonal IgG. We identify that all growth-inhibitory antibody epitopes of RIPR cluster within the C-terminal EGF-like domains and that a fusion of these domains to CyRPA, called "R78C", combined with RH5, improves the level of in vitro parasite growth inhibition compared to RH5 alone. These preclinical data justify the advancement of the RH5.1 + R78C/Matrix-M™ vaccine candidate to Phase 1 clinical trial.

A targetable receptor-binding site on PfCyRPA to aid in the fight against malaria

Recently, Day et al. identified a receptor-binding site on the malaria parasite protein PfCyRPA that binds the host sugar Neu5Ac, and they found that disrupting this interaction impedes parasite growth. A map of the receptor-binding site identifies an attractive target for antimalarial vaccines and therapeutics.

The essential malaria protein PfCyRPA targets glycans to invade erythrocytes

Plasmodium falciparum is a human-adapted apicomplexan parasite that causes the most dangerous form of malaria. P. falciparum cysteine-rich protective antigen (PfCyRPA) is an invasion complex protein essential for erythrocyte invasion. The precise role of PfCyRPA in this process has not been resolved. Here, we show that PfCyRPA is a lectin targeting glycans terminating with α2-6-linked N-acetylneuraminic acid (Neu5Ac). PfCyRPA has a >50-fold binding preference for human, α2-6-linked Neu5Ac over non-human, α2-6-linked N-glycolylneuraminic acid. PfCyRPA lectin sites were predicted by molecular modeling and validated by mutagenesis studies. Transgenic parasite lines expressing endogenous PfCyRPA with single amino acid exchange mutants indicated that the lectin activity of PfCyRPA has an important role in parasite invasion. Blocking PfCyRPA lectin activity with small molecules or with lectin-site-specific monoclonal antibodies can inhibit blood-stage parasite multiplication. Therefore, targeting PfCyRPA lectin activity with drugs, immunotherapy, or a vaccine-primed immune response is a promising strategy to prevent and treat malaria.

The dual action of human antibodies specific to Plasmodium falciparum PfRH5 and PfCyRPA: Blocking invasion and inactivating extracellular merozoites

The Plasmodium falciparum reticulocyte-binding protein homolog 5 (PfRH5) is the current leading blood-stage malaria vaccine candidate. PfRH5 functions as part of the pentameric PCRCR complex containing PTRAMP, CSS, PfCyRPA and PfRIPR, all of which are essential for infection of human red blood cells (RBCs). To trigger RBC invasion, PfRH5 engages with RBC protein basigin in a step termed the RH5-basigin binding stage. Although we know increasingly more about how antibodies specific for PfRH5 can block invasion, much less is known about how antibodies recognizing other members of the PCRCR complex can inhibit invasion. To address this, we performed live cell imaging using monoclonal antibodies (mAbs) which bind PfRH5 and PfCyRPA. We measured the degree and timing of the invasion inhibition, the stage at which it occurred, as well as subsequent events. We show that parasite invasion is blocked by individual mAbs, and the degree of inhibition is enhanced when combining a mAb specific for PfRH5 with one binding PfCyRPA. In addition to directly establishing the invasion-blocking capacity of the mAbs, we identified a secondary action of certain mAbs on extracellular parasites that had not yet invaded where the mAbs appeared to inactivate the parasites by triggering a developmental pathway normally only seen after successful invasion. These findings suggest that epitopes within the PfCyRPA-PfRH5 sub-complex that elicit these dual responses may be more effective immunogens than neighboring epitopes by both blocking parasites from invading and rapidly inactivating extracellular parasites. These two protective mechanisms, prevention of invasion and inactivation of uninvaded parasites, resulting from antibody to a single epitope indicate a possible route to the development of more effective vaccines.

Construction, Expression, and Evaluation of the Naturally Acquired Humoral Immune Response against Plasmodium vivax RMC-1, a Multistage Chimeric Protein

The PvCelTOS, PvCyRPA, and Pvs25 proteins play important roles during the three stages of the P. vivax lifecycle. In this study, we designed and expressed a P. vivax recombinant modular chimeric protein (PvRMC-1) composed of the main antigenic regions of these vaccine candidates. After structure modelling by prediction, the chimeric protein was expressed, and the antigenicity was assessed by IgM and IgG (total and subclass) ELISA in 301 naturally exposed individuals from the Brazilian Amazon. The recombinant protein was recognized by IgG (54%) and IgM (40%) antibodies in the studied individuals, confirming the natural immunogenicity of the epitopes that composed PvRMC-1 as its maintenance in the chimeric structure. Among responders, a predominant cytophilic response mediated by IgG1 (70%) and IgG3 (69%) was observed. IgM levels were inversely correlated with age and time of residence in endemic areas (p < 0.01). By contrast, the IgG and IgM reactivity indexes were positively correlated with each other, and both were inversely correlated with the time of the last malaria episode. Conclusions: The study demonstrates that PvRMC-1 was successfully expressed and targeted by natural antibodies, providing important insights into the construction of a multistage chimeric recombinant protein and the use of naturally acquired antibodies to validate the construction.

The malaria blood stage antigen PfCyRPA formulated with the TLR-4 agonist adjuvant GLA-SE elicits parasite growth inhibitory antibodies in experimental animals

BACKGROUND

Plasmodium falciparum cysteine-rich protective antigen (PfCyRPA) is an invasion complex protein essential for erythrocyte invasion. In contrast to several previously clinically tested merozoite vaccine candidate antigens, PfCyRPA is not polymorphic, making it a promising candidate antigen for blood stage vaccine development.

METHODS

Mice and rabbits were immunized with vaccine formulations of recombinantly expressed PfCyRPA adjuvanted either with the glucopyranosyl lipid A (GLA) containing adjuvants GLA-LSQ, GLA-SE, GLA-Alum or with Nanoalum. ELISA and indirect immunofluorescence assays (IFA) were used to analyse elicited IgG titers and the P. falciparum growth inhibitory activity was determined with a standardized in vitro [³H]-hypoxanthine incorporation assay.

RESULTS

In the mouse experiments, the GLA adjuvanted formulations were superior to the Nanoalum formulation with respect to antibody titer development, IFA sero-conversion rates and in vitro parasite growth-inhibitory activity. In rabbits, the highest titers of parasite growth inhibitory antibodies were obtained with the GLA-SE formulation. Comparable mean ELISA IgG endpoint titers were reached in rabbits after three immunizations with GLA-SE adjuvanted PfCyRPA doses of 5, 25 and 100 µg, but with 100 µg of antigen, only two immunizations were required to reach this titer.

CONCLUSION

PfCyRPA formulated with the human-compatible adjuvant GLA-SE represents an attractive vaccine candidate for early clinical testing in a controlled P. falciparum blood stage challenge trial.

RH5.1-CyRPA-Ripr antigen combination vaccine shows little improvement over RH5.1 in a preclinical setting

Background

RH5 is the leading vaccine candidate for the Plasmodium falciparum blood stage and has shown impact on parasite growth in the blood in a human clinical trial. RH5 binds to Ripr and CyRPA at the apical end of the invasive merozoite form, and this complex, designated RCR, is essential for entry into human erythrocytes. RH5 has advanced to human clinical trials, and the impact on parasite growth in the blood was encouraging but modest. This study assessed the potential of a protein-in-adjuvant blood stage malaria vaccine based on a combination of RH5, Ripr and CyRPA to provide improved neutralizing activity against P. falciparum in vitro.

Methods

Mice were immunized with the individual RCR antigens to down select the best performing adjuvant formulation and rats were immunized with the individual RCR antigens to select the correct antigen dose. A second cohort of rats were immunized with single, double and triple antigen combinations to assess immunogenicity and parasite neutralizing activity in growth inhibition assays.

Results

The DPX® platform was identified as the best performing formulation in potentiating P. falciparum inhibitory antibody responses to these antigens. The three antigens derived from RH5, Ripr and CyRPA proteins formulated with DPX induced highly inhibitory parasite neutralising antibodies. Notably, RH5 either as a single antigen or in combination with Ripr and/or CyRPA, induced inhibitory antibodies that outperformed CyRPA, Ripr.

Conclusion

An RCR combination vaccine may not induce substantially improved protective immunity as compared with RH5 as a single immunogen in a clinical setting and leaves the development pathway open for other antigens to be combined with RH5 as a next generation malaria vaccine.

Process development and preclinical evaluation of a major Plasmodium falciparum blood stage vaccine candidate, Cysteine-Rich Protective Antigen (CyRPA)

Plasmodium falciparum Cysteine-Rich Protective Antigen (CyRPA) is an essential, highly conserved merozoite antigen that forms an important multi-protein complex (RH5/Ripr/CyRPA) necessary for erythrocyte invasion. CyRPA is a promising blood-stage vaccine target that has been shown to elicit potent strain-transcending parasite neutralizing antibodies. Recently, we demonstrated that naturally acquired immune anti-CyRPA antibodies are invasion-inhibitory and therefore a correlate of protection against malaria. Here, we describe a process for the large-scale production of tag-free CyRPA vaccine in E. coli and demonstrate its parasite neutralizing efficacy with commonly used adjuvants. CyRPA was purified from inclusion bodies using a one-step purification method with high purity (>90%). Biochemical and biophysical characterization showed that the purified tag-free CyRPA interacted with RH5, readily detected by a conformation-specific CyRPA monoclonal antibody and recognized by sera from malaria infected individuals thus indicating that the recombinant antigen was correctly folded and retained its native conformation. Tag-free CyRPA formulated with Freund’s adjuvant elicited highly potent parasite neutralizing antibodies achieving inhibition of >90% across diverse parasite strains. Importantly, we identified tag-free CyRPA/Alhydrogel formulation as most effective in inducing a highly immunogenic antibody response that exhibited efficacious, cross-strain in vitro parasite neutralization achieving ~80% at 10 mg/ml. Further, CyRPA/Alhydrogel vaccine induced anti-parasite cytokine response in mice. In summary, our study provides a simple, scalable, cost-effective process for the production of tag-free CyRPA that in combination with human-compatible adjuvant induces efficacious humoral and cell-mediated immune response.

Expanding the Malaria Antibody Toolkit: Development and Characterisation of Plasmodium falciparum RH5, CyRPA, and CSP Recombinant Human Monoclonal Antibodies

Malaria, an infection caused by apicomplexan parasites of the genus Plasmodium, continues to exact a significant toll on public health with over 200 million cases world-wide, and annual deaths in excess of 600,000. Considerable progress has been made to reduce malaria burden in endemic countries in the last two decades. However, parasite and mosquito resistance to frontline chemotherapies and insecticides, respectively, highlights the continuing need for the development of safe and effective vaccines. Here we describe the development of recombinant human antibodies to three target proteins from Plasmodium falciparum: reticulocyte binding protein homologue 5 (PfRH5), cysteine-rich protective antigen (PfCyRPA), and circumsporozoite protein (PfCSP). All three proteins are key targets in the development of vaccines for blood-stage or pre-erythrocytic stage infections. We have developed potent anti-PfRH5, PfCyRPA and PfCSP monoclonal antibodies that will prove useful tools for the standardisation of assays in preclinical research and the assessment of these antigens in clinical trials. We have generated some very potent anti-PfRH5 and anti-PfCyRPA antibodies with some clones >200 times more potent than the polyclonal anti-AMA-1 antibodies used for the evaluation of blood stage antigens. While the monoclonal and polyclonal antibodies are not directly comparable, the data provide evidence that these new antibodies are very good at blocking invasion. These antibodies will therefore provide a valuable resource and have potential as biological standards to help harmonise pre-clinical malaria research.

Scalable Process for High-Yield Production of PfCyRPA Using Insect Cells for Inclusion in a Malaria Virosome-Based Vaccine Candidate

Plasmodium falciparum cysteine-rich protective antigen (PfCyRPA) has been identified as a promising blood-stage candidate antigen to include in a broadly cross-reactive malaria vaccine. In the last couple of decades, substantial effort has been committed to the development of scalable cost-effective, robust, and high-yield PfCyRPA production processes. Despite insect cells being a suitable expression system due to their track record for protein production (including vaccine antigens), these are yet to be explored to produce this antigen. In this study, different insect cell lines, culture conditions (baculovirus infection strategy, supplementation schemes, culture temperature modulation), and purification strategies (affinity tags) were explored aiming to develop a scalable, high-yield, and high-quality PfCyRPA for inclusion in a virosome-based malaria vaccine candidate. Supplements with antioxidants improved PfCyRPA volumetric titers by 50% when added at the time of infection. In addition, from three different affinity tags (6x-His, 4x-His, and C-tag) evaluated, the 4x-His affinity tag was the one leading to the highest PfCyRPA purification recovery yields (61%) and production yield (26 mg/L vs. 21 mg/L and 13 mg/L for 6x-His and C-tag, respectively). Noteworthy, PfCyRPA expressed using High Five cells did not show differences in protein quality or stability when compared to its human HEK293 cell counterpart. When formulated in a lipid-based virosome nanoparticle, immunized rabbits developed functional anti-PfCyRPA antibodies that impeded the multiplication of P. falciparum in vitro. This work demonstrates the potential of using IC-BEVS as a qualified platform to produce functional recombinant PfCyRPA protein with the added benefit of being a non-human expression system with short bioprocessing times and high expression levels.

Enhancing neutralization of Plasmodium falciparum using a novel monoclonal antibody against the rhoptry-associated membrane antigen

The pathogenesis of malaria is associated with blood-stage infection and there is strong evidence that antibodies specific to parasite blood-stage antigens can control parasitemia. This provides a strong rational for applying blood-stage antigen components in a multivalent vaccine, as the induced antibodies in combination can enhance protection. The Plasmodium falciparum rhoptry-associated membrane antigen (PfRAMA) is a promising vaccine target, due to its fundamental role in merozoite invasion and low level of polymorphism. Polyclonal antibodies against PfRAMA are able to inhibit P. falciparum growth and interact synergistically when combined with antibodies against P. falciparum reticulocyte-binding protein 5 (PfRh5) or cysteine-rich protective antigen (PfCyRPA). In this study, we identified a novel PfRAMA-specific mAb with neutralizing activity, which in combination with PfRh5- or PfCyRPA-specific mAbs potentiated the neutralizing effect. By applying phage display technology, we mapped the protective epitope to be in the C-terminal region of PfRAMA. Our results confirmed previous finding of synergy between PfRAMA-, PfRh5- and PfCyRPA-specific antibodies, thereby paving the way of testing these antigens (or fragments of these antigens) in combination to improve the efficacy of blood-stage malaria vaccines. The results emphasize the importance of directing antibody responses towards protective epitopes, as the majority of anti-PfRAMA mAbs were unable to inhibit merozoite invasion of erythrocytes.

Heterotypic interactions drive antibody synergy against a malaria vaccine candidate

Understanding mechanisms of antibody synergy is important for vaccine design and antibody cocktail development. Examples of synergy between antibodies are well-documented, but the mechanisms underlying these relationships often remain poorly understood. The leading blood-stage malaria vaccine candidate, CyRPA, is essential for invasion of Plasmodium falciparum into human erythrocytes. Here we present a panel of anti-CyRPA monoclonal antibodies that strongly inhibit parasite growth in in vitro assays. Structural studies show that growth-inhibitory antibodies bind epitopes on a single face of CyRPA. We also show that pairs of non-competing inhibitory antibodies have strongly synergistic growth-inhibitory activity. These antibodies bind to neighbouring epitopes on CyRPA and form lateral, heterotypic interactions which slow antibody dissociation. We predict that such heterotypic interactions will be a feature of many immune responses. Immunogens which elicit such synergistic antibody mixtures could increase the potency of vaccine-elicited responses to provide robust and long-lived immunity against challenging disease targets.

Antibodies can have synergistic effects, but mechanisms are not well understood. Here, Ragotte et al. identify three antibodies that bind neighbouring epitopes on CyRPA, a malaria vaccine candidate, and show that lateral interactions between the antibodies slow dissociation and inhibit parasite growth synergistically.

PCRCR complex is essential for invasion of human erythrocytes by Plasmodium falciparum

The most severe form of malaria is caused by Plasmodium falciparum. These parasites invade human erythrocytes, and an essential step in this process involves the ligand PfRh5, which forms a complex with cysteine-rich protective antigen (CyRPA) and PfRh5-interacting protein (PfRipr) (RCR complex) and binds basigin on the host cell. We identified a heteromeric disulfide-linked complex consisting of P. falciparum Plasmodium thrombospondin-related apical merozoite protein (PfPTRAMP) and P. falciparum cysteine-rich small secreted protein (PfCSS) and have shown that it binds RCR to form a pentameric complex, PCRCR. Using P. falciparum lines with conditional knockouts, invasion inhibitory nanobodies to both PfPTRAMP and PfCSS, and lattice light-sheet microscopy, we show that they are essential for merozoite invasion. The PCRCR complex functions to anchor the contact between merozoite and erythrocyte membranes brought together by strong parasite deformations. We solved the structure of nanobody-PfCSS complexes to identify an inhibitory epitope. Our results define the function of the PCRCR complex and identify invasion neutralizing epitopes providing a roadmap for structure-guided development of these proteins for a blood stage malaria vaccine.

Plasmodium falciparum Cysteine-Rich Protective Antigen (CyRPA) Elicits Detectable Levels of Invasion-Inhibitory Antibodies during Natural Infection in Humans

Plasmodium falciparum Cysteine-Rich Protective Antigen (CyRPA) is a conserved component of an essential erythrocyte invasion complex (RH5/Ripr/CyRPA) and a target of potent cross-strain parasite-neutralizing antibodies. While, naturally acquired human RH5 antibodies have been functionally characterized, there are no similar reports on CyRPA. Thus, we analyzed the parasite neutralizing activity of naturally acquired human CyRPA antibodies. In this regard, CyRPA human antibodies were measured and purified from malaria infected sera obtained from central India and analyzed for their parasite neutralizing activity in in vitro growth inhibition assays (GIA). We report that despite being susceptible to antibody, CyRPA being a highly conserved antigen does not appear to be under substantial immune selection pressure as a very low acquisition of anti-CyRPA antibodies was reported in malaria-exposed Indians. We demonstrate for the first time that the low amounts of natural CyRPA antibodies exhibited functional parasite-neutralizing activity and that a CyRPA based vaccine formulation induces highly potent antibodies in rabbits. Importantly, the vaccine induced CyRPA antibodies exhibited a robust IC₅₀ of 21.96 μg/ml that is comparable to IC₅₀ of antibodies against the leading blood stage vaccine candidate, RH5. Our data support CyRPA as a unique vaccine target that is highly susceptible to immune attack but highly conserved compared to other leading candidates such as MSP-1, AMA-1, further substantiating its promise as a leading blood-stage vaccine candidate.

Genetic Diversity of Plasmodium vivax Cysteine-Rich Protective Antigen (PvCyRPA) in Field Isolates from Five Different Areas of the Brazilian Amazon

The Plasmodium vivax Cysteine-Rich Protective Antigen (PvCyRPA) has an important role in erythrocyte invasion and has been considered a target for vivax malaria vaccine development. Nonetheless, its genetic diversity remains uncharted in Brazilian malaria-endemic areas. Therefore, we investigated the pvcyrpa genetic polymorphism in 98 field isolates from the Brazilian Amazon and its impact on the antigenicity of predicted B-cell epitopes. Genetic diversity parameters, population genetic analysis, neutrality test and the median-joining network were analyzed, and the potential amino acid polymorphism participation in B-cell epitopes was investigated. One synonymous and 26 non-synonymous substitutions defined fifty haplotypes. The nucleotide diversity and Tajima’s D values varied across the coding gene. The exon-1 sequence had greater diversity than those of exon-2. Concerning the prediction analysis, seven sequences were predicted as linear B cell epitopes, the majority contained in conformational epitopes. Moreover, important amino acid polymorphism was detected in regions predicted to contain residues participating in B-cell epitopes. Our data suggest that the pvcyrpa gene presents a moderate polymorphism in the studied isolates and such polymorphisms alter amino acid sequences contained in potential B cell epitopes, an important observation considering the antigen potentiality as a vaccine candidate to cover distinct P. vivax endemic areas worldwide.

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.

Using Plasmodium knowlesi as a model for screening Plasmodium vivax blood-stage malaria vaccine targets reveals new candidates

Plasmodium vivax is responsible for the majority of malaria cases outside Africa. Unlike P. falciparum, the P. vivax life-cycle includes a dormant liver stage, the hypnozoite, which can cause infection in the absence of mosquito transmission. An effective vaccine against P. vivax blood stages would limit symptoms and pathology from such recurrent infections, and therefore could play a critical role in the control of this species. Vaccine development in P. vivax, however, lags considerably behind P. falciparum, which has many identified targets with several having transitioned to Phase II testing. By contrast only one P. vivax blood-stage vaccine candidate based on the Duffy Binding Protein (PvDBP), has reached Phase Ia, in large part because the lack of a continuous in vitro culture system for P. vivax limits systematic screening of new candidates. We used the close phylogenetic relationship between P. vivax and P. knowlesi, for which an in vitro culture system in human erythrocytes exists, to test the scalability of systematic reverse vaccinology to identify and prioritise P. vivax blood-stage targets. A panel of P. vivax proteins predicted to function in erythrocyte invasion were expressed as full-length recombinant ectodomains in a mammalian expression system. Eight of these antigens were used to generate polyclonal antibodies, which were screened for their ability to recognize orthologous proteins in P. knowlesi. These antibodies were then tested for inhibition of growth and invasion of both wild type P. knowlesi and chimeric P. knowlesi lines modified using CRISPR/Cas9 to exchange P. knowlesi genes with their P. vivax orthologues. Candidates that induced antibodies that inhibited invasion to a similar level as PvDBP were identified, confirming the utility of P. knowlesi as a model for P. vivax vaccine development and prioritizing antigens for further follow up.

The Plasmodium falciparum Rh5 invasion protein complex reveals an excess of rare variant mutations

BACKGROUND

The invasion of the red blood cells by Plasmodium falciparum merozoites involves the interplay of several proteins that are also targets for vaccine development. The proteins PfRh5-PfRipr-PfCyRPA-Pfp113 assemble into a complex at the apical end of the merozoite and are together essential for erythrocyte invasion. They have also been shown to induce neutralizing antibodies and appear to be less polymorphic than other invasion-associated proteins, making them high priority blood-stage vaccine candidates. Using available whole genome sequencing data (WGS) and new capillary sequencing data (CS), this study describes the genetic polymorphism in the Rh5 complex in P. falciparum isolates obtained from Kilifi, Kenya.

METHODS

162 samples collected in 2013 and 2014 were genotyped by capillary sequencing (CS) and re-analysed WGS from 68 culture-adapted P. falciparum samples obtained from a drug trial conducted from 2005 to 2007. The frequency of polymorphisms in the merozoite invasion proteins, PfRh5, PfRipr, PfCyRPA and PfP113 were examined and where possible polymorphisms co-occurring in the same isolates.

RESULTS

From a total 70 variants, including 2 indels, 19 SNPs [27.1%] were identified by both CS and WGS, while an additional 15 [21.4%] and 36 [51.4%] SNPs were identified only by either CS or WGS, respectively. All the SNPs identified by CS were non-synonymous, whereas WGS identified 8 synonymous and 47 non-synonymous SNPs. CS identified indels in repeat regions in the p113 gene in codons 275 and 859 that were not identified in the WGS data. The minor allele frequencies of the SNPs ranged between 0.7 and 34.9% for WGS and 1.1-29.6% for CS. Collectively, 12 high frequency SNPs (> 5%) were identified: four in Rh5 codon 147, 148, 203 and 429, two in p113 at codons 7 and 267 and six in Ripr codons 190, 259, 524, 985, 1003 and 1039.

CONCLUSION

This study reveals that the majority of the polymorphisms are rare variants and confirms a low level of genetic polymorphisms in all proteins within the Rh5 complex.

Antibodies from malaria-exposed Malians generally interact additively or synergistically with human vaccine-induced RH5 antibodies

Reticulocyte-binding protein homolog 5 (RH5) is a leading Plasmodium falciparum blood-stage vaccine candidate. Another possible candidate, apical membrane antigen 1 (AMA1), was not efficacious in malaria-endemic populations, likely due to pre-existing antimalarial antibodies that interfered with the activity of vaccine-induced AMA1 antibodies, as judged by in vitro growth inhibition assay (GIA). To determine how pre-existing antibodies interact with vaccine-induced RH5 antibodies, we purify total and RH5-specific immunoglobulin Gs (IgGs) from malaria-exposed Malians and malaria-naive RH5 vaccinees. Infection-induced RH5 antibody titers are much lower than those induced by vaccination, and RH5-specific IgGs show differences in the binding site between the two populations. In GIA, Malian polyclonal IgGs show additive or synergistic interactions with RH5 human monoclonal antibodies and overall additive interactions with vaccine-induced polyclonal RH5 IgGs. These results suggest that pre-existing antibodies will interact favorably with vaccine-induced RH5 antibodies, in contrast to AMA1 antibodies. This study supports RH5 vaccine trials in malaria-endemic regions.

RH5 IgG titers induced by infection are lower than those induced by RH5 vaccination

Infection- and vaccine-induced RH5 IgGs have different specificity and avidity

Infection- and vaccine-induced RH5 IgGs interact differently with RH5 mAbs

Infection-induced IgGs generally do not reduce the activity of vaccine-induced IgGs

Malaria in the 'Omics Era'

Genomics has revolutionised the study of the biology of parasitic diseases. The first Eukaryotic parasite to have its genome sequenced was the malaria parasite Plasmodium falciparum. Since then, Plasmodium genomics has continued to lead the way in the study of the genome biology of parasites, both in breadth-the number of Plasmodium species' genomes sequenced-and in depth-massive-scale genome re-sequencing of several key species. Here, we review some of the insights into the biology, evolution and population genetics of Plasmodium gained from genome sequencing, and look at potential new avenues in the future genome-scale study of its biology.

Erythrocyte CD55 mediates the internalization of Plasmodium falciparum parasites

Invasion of human erythrocytes by the malaria parasite Plasmodium falciparum is a multi-step process. Previously, a forward genetic screen for P. falciparum host factors identified erythrocyte CD55 as essential for invasion, but its specific role and how it interfaces with the other factors that mediate this complex process are unknown. Using CRISPR-Cas9 editing, antibody-based inhibition, and live cell imaging, here we show that CD55 is specifically required for parasite internalization. Pre-invasion kinetics, erythrocyte deformability, and echinocytosis were not influenced by CD55, but entry was inhibited when CD55 was blocked or absent. Visualization of parasites attached to CD55-null erythrocytes points to a role for CD55 in stability and/or progression of the moving junction. Our findings demonstrate that CD55 acts after discharge of the parasite’s rhoptry organelles, and plays a unique role relative to all other invasion receptors. As the requirement for CD55 is strain-transcendent, these results suggest that CD55 or its interacting partners may hold potential as therapeutic targets for malaria.

Plasmodium vivax Cysteine-Rich Protective Antigen Polymorphism at Exon-1 Shows Recombination and Signatures of Balancing Selection

Plasmodium vivax Cysteine-Rich Protective Antigen (CyRPA) is a merozoite protein participating in the parasite invasion of human reticulocytes. During natural P. vivax infection, antibody responses against PvCyRPA have been detected. In children, low anti-CyRPA antibody titers correlated with clinical protection, which suggests this protein as a potential vaccine candidate. This work analyzed the genetic and amino acid diversity of pvcyrpa in Mexican and global parasites. Consensus coding sequences of pvcyrpa were obtained from seven isolates. Other sequences were extracted from a repository. Maximum likelihood phylogenetic trees, genetic diversity parameters, linkage disequilibrium (LD), and neutrality tests were analyzed, and the potential amino acid polymorphism participation in B-cell epitopes was investigated. In 22 sequences from Southern Mexico, two synonymous and 21 nonsynonymous mutations defined nine private haplotypes. These parasites had the highest LD-R² index and the lowest nucleotide diversity compared to isolates from South America or Asia. The nucleotide diversity and Tajima's D values varied across the coding gene. The exon-1 sequence had greater diversity and Rm values than those of exon-2. Exon-1 had significant positive values for Tajima's D, β-α values, and for the Z (HA: dN > dS) and MK tests. These patterns were similar for parasites of different origin. The polymorphic amino acid residues at PvCyRPA resembled the conformational B-cell peptides reported in PfCyRPA. Diversity at pvcyrpa exon-1 is caused by mutation and recombination. This seems to be maintained by balancing selection, likely due to selective immune pressure, all of which merit further study.

Antibody Combinations Targeting the Essential Antigens CyRPA, RH5, and MSP-119 Potently Neutralize Plasmodium falciparum Clinical Isolates From India and Africa

BACKGROUND

Targeting multiple key antigens that mediate distinct Plasmodium falciparum erythrocyte invasion pathways is an attractive approach for the development of blood-stage malaria vaccines. However, the challenge is to identify antigen cocktails that elicit potent strain-transcending parasite-neutralizing antibodies efficacious at low immunoglobulin G concentrations feasible to achieve through vaccination. Previous reports have screened inhibitory antibodies primarily against well adapted laboratory parasite clones. However, validation of the parasite-neutralizing efficacy against clinical isolates with minimal in vitro cultivation is equally significant to better ascertain their prospective in vivo potency.

METHODS

We evaluated the parasite-neutralizing activity of different antibodies individually and in combinations against laboratory adapted clones and clinical isolates. Clinical isolates were collected from Central India and Mozambique, Africa, and characterized for their invasion properties and genetic diversity of invasion ligands.

RESULTS

In our portfolio, we evaluated 25 triple antibody combinations and identified the MSP-Fu+CyRPA+RH5 antibody combination to elicit maximal parasite neutralization against P. falciparum clinical isolates with variable properties that underwent minimal in vitro cultivation.

CONCLUSIONS

The MSP-Fu+CyRPA+RH5 combination exhibited highly robust parasite neutralization against P. falciparum clones and clinical isolates, thus substantiating them as promising candidate antigens and establishing a proof of principle for the development of a combinatorial P. falciparum blood-stage malaria vaccine.

The Structure of the Cysteine-Rich Domain of Plasmodium falciparum P113 Identifies the Location of the RH5 Binding Site

Malaria is a deadly infectious disease primarily caused by the parasite Plasmodium falciparum. It remains a major global health problem, and there is no highly effective vaccine. A parasite protein called RH5 is centrally involved in the invasion of host red blood cells, making it—and the other parasite proteins it interacts with—promising vaccine targets. We recently identified a protein called P113 that binds RH5, suggesting that it anchors RH5 to the parasite surface. In this paper, we use structural biology to locate and characterize the RH5 binding region on P113. These findings will be important to guide the development of new antimalarial vaccines to ultimately prevent this disease, which affects some of the poorest people on the planet.

Plasmodium falciparum RH5 is a secreted parasite ligand that is essential for erythrocyte invasion through direct interaction with the host erythrocyte receptor basigin. RH5 forms a tripartite complex with two other secreted parasite proteins, CyRPA and RIPR, and is tethered to the surface of the parasite through membrane-anchored P113. Antibodies against RH5, CyRPA, and RIPR can inhibit parasite invasion, suggesting that vaccines containing these three components have the potential to prevent blood-stage malaria. To further explore the role of the P113-RH5 interaction, we selected monoclonal antibodies against P113 that were either inhibitory or noninhibitory for RH5 binding. Using a Fab fragment as a crystallization chaperone, we determined the crystal structure of the RH5 binding region of P113 and showed that it is composed of two domains with structural similarities to rhamnose-binding lectins. We identified the RH5 binding site on P113 by using a combination of hydrogen-deuterium exchange mass spectrometry and site-directed mutagenesis. We found that a monoclonal antibody to P113 that bound to this interface and inhibited the RH5-P113 interaction did not inhibit parasite blood-stage growth. These findings provide further structural information on the protein interactions of RH5 and will be helpful in guiding the development of blood-stage malaria vaccines that target RH5.

The RH5-CyRPA-Ripr Complex as a Malaria Vaccine Target

Despite ongoing efforts, a highly effective vaccine against Plasmodium falciparum remains elusive. Vaccines targeting the pre-erythrocytic stages of the P. falciparum life cycle are the most advanced to date, affording moderate levels of efficacy in field trials. However, the discovery that the members of the merozoite PfRH5-PfCyRPA-PfRipr (RCR) complex are capable of inducing strain-transcendent neutralizing antibodies has renewed enthusiasm for the possibility of preventing disease by targeting the parasite during the blood stage of infection. With Phase I/II clinical trials now underway using first-generation vaccines against PfRH5, and more on the horizon for PfCyRPA and PfRipr, this review explores the rationale and future potential of the RCR complex as a P. falciparum vaccine target.

Designing a multi-epitope vaccine against blood-stage of Plasmodium falciparum by in silico approaches

Plasmodium falciparum causes the most severe form of malaria disease and is the major cause of infection-related mortalities in the world. Due to increasing in P. falciparum resistance to the first-line antimalarial drugs, an effective vaccine for the control and elimination of malaria infection is urgent. Because the pathogenesis of malaria disease results from blood-stage infection, and all of the symptoms and clinical illness of malaria occur during this stage, there is a strong rationale to develop vaccine against this stage. In the present study, different structural-vaccinology and immuno informatics tools were applied to design an effective antibody-inducing multi-epitope vaccine against the blood-stage of P. falciparum. The designed multi-epitope vaccine was composed of three main parts including B cell epitopes, T helper (Th) cell epitopes, and two adjuvant motives (HP91 and RS09), which were linked to each other via proper linkers. B cell and T cell epitopes were derived from four protective antigens expressed on the surface of merozoites, which are critical to invade the erythrocytes. HP91 and RS09 adjuvants and Th cell epitopes were used to induce, enhance and direct the best form of humoral immune-response against P. falciparum surface merozoite antigens. The vaccine construct was modeled, and after model quality evaluation and refinement by different software, the high-quality 3D-structure model of the vaccine was achieved. Analysis of immunological and physicochemical features of the vaccine showed acceptable results. We believe that this multi-epitope vaccine can be effective for preventing malaria disease caused by P. falciparum.

Antibodies against a short region of PfRipr inhibit Plasmodium falciparum merozoite invasion and PfRipr interaction with Rh5 and SEMA7A

Plasmodium falciparum merozoite invasion into erythrocytes is an essential step of the blood-stage cycle, survival of parasites, and malaria pathogenesis. P. falciparum merozoite Rh5 interacting protein (PfRipr) forms a complex with Rh5 and CyRPA in sequential molecular events leading to erythrocyte invasion. Recently we described PfRipr as a conserved protein that induces strain-transcending growth inhibitory antibodies in in vitro assays. However, being a large and complex protein of 1086 amino acids (aa) with 87 cysteine residues, PfRipr is difficult to express in conventional expression systems towards vaccine development. In this study we sought to identify the most potent region of PfRipr that could be developed to overcome difficulties related to protein expression, as well as to elucidate the invasion inhibitory mechanism of anti-PfRipr antibodies. Using the wheat germ cell-free system, Ecto- PfRipr and truncates of approximately 200 aa were expressed as soluble proteins. We demonstrate that antibodies against PfRipr truncate 5 (PfRipr_5: C₇₂₀-D₉₃₄), a region within the PfRipr C-terminal EGF-like domains, potently inhibit merozoite invasion. Furthermore, the antibodies strongly block PfRipr/Rh5 interaction, as well as that between PfRipr and its erythrocyte-surface receptor, SEMA7A. Taken together, PfRipr_5 is a potential candidate for further development as a blood-stage malaria vaccine.

Molecular Characterization and Immuno-Reactivity Patterns of a Novel Plasmodium falciparum Armadillo-Type Repeat Protein, PfATRP

Nearly half of the genes in the Plasmodium falciparum genome have not yet been functionally investigated. We used homology-based structural modeling to identify multiple copies of Armadillo repeats within one uncharacterized gene expressed during the intraerythrocytic stages, PF3D7_0410600, subsequently referred to as P. falciparum Armadillo-Type Repeat Protein (PfATRP). Soluble recombinant PfATRP was expressed in a bacterial expression system, purified to apparent homogeneity and the identity of the recombinant PfATRP was confirmed by mass spectrometry. Affinity-purified α-PfATRP rabbit antibodies specifically recognized the recombinant protein. Immunofluorescence assays revealed that α-PfATRP rabbit antibodies reacted with P. falciparum schizonts. Anti-PfATRP antibody exhibited peripheral staining patterns around the merozoites. Given the localization of PfATRP in merozoites, we tested for an egress phenotype during schizont arrest assays and demonstrated that native PfATRP is inaccessible on the surface of merozoites in intact schizonts. Dual immunofluorescence assays with markers for the inner membrane complex (IMC) and microtubules suggest partial colocalization in both asexual and sexual stage parasites. Using the soluble recombinant PfATRP in a screen of plasma samples revealed that malaria-infected children have naturally acquired PfATRP-specific antibodies.

Cell invasion by intracellular parasites - the many roads to infection

Intracellular parasites from the genera Toxoplasma, Plasmodium, Trypanosoma, Leishmania and from the phylum Microsporidia are, respectively, the causative agents of toxoplasmosis, malaria, Chagas disease, leishmaniasis and microsporidiosis, illnesses that kill millions of people around the globe. Crossing the host cell plasma membrane (PM) is an obstacle these parasites must overcome to establish themselves intracellularly and so cause diseases. The mechanisms of cell invasion are quite diverse and include (1) formation of moving junctions that drive parasites into host cells, as for the protozoans Toxoplasma gondii and Plasmodium spp., (2) subversion of endocytic pathways used by the host cell to repair PM, as for Trypanosoma cruzi and Leishmania, (3) induction of phagocytosis as for Leishmania or (4) endocytosis of parasites induced by specialized structures, such as the polar tubes present in microsporidian species. Understanding the early steps of cell entry is essential for the development of vaccines and drugs for the prevention or treatment of these diseases, and thus enormous research efforts have been made to unveil their underlying biological mechanisms. This Review will focus on these mechanisms and the factors involved, with an emphasis on the recent insights into the cell biology of invasion by these pathogens.

Vaccination with virosomally formulated recombinant CyRPA elicits protective antibodies against Plasmodium falciparum parasites in preclinical in vitro and in vivo models

The Plasmodium falciparum (Pf) cysteine-rich protective antigen (PfCyRPA) has emerged as a promising blood-stage candidate antigen for inclusion into a broadly cross-reactive malaria vaccine. This highly conserved protein among various geographical strains plays a key role in the red blood cell invasion process by P. falciparum merozoites, and antibodies against PfCyRPA can efficiently prevent the entry of the malaria parasites into red blood cells. The aim of the present study was to develop a human-compatible formulation of the PfCyRPA vaccine candidate and confirming its activity in preclinical studies. Recombinant PfCyRPA expressed in HEK 293 cells was chemically coupled to phosphoethanolamine and then incorporated into the membrane of unadjuvanted influenza virosomes approved as antigen delivery system for humans. Laboratory animals were immunised with the virosome-based PfCyRPA vaccine to determine its immunogenic properties and in particular, its capacity to elicit parasite binding and growth-inhibitory antibodies. The vaccine elicited in mice and rabbits high titers of PfCyRPA-specific antibodies that bound to the blood-stage parasites. At a concentration of 10 mg/mL, purified total serum IgG from immunised rabbits inhibited parasite growth in vitro by about 80%. Furthermore, in a P. falciparum infection mouse model, passive transfer of 10 mg of purified total IgG from PfCyRPA vaccinated rabbits reduced the in vivo parasite load by 77%. Influenza virosomes thus represent a suitable antigen delivery system for the induction of protective antibodies against the recombinant PfCyRPA, designating it as a highly suitable component for inclusion into a multivalent and multi-stage virosomal malaria vaccine.

Plasmodium vivax Reticulocyte Binding Proteins for invasion into reticulocytes

Plasmodium vivax is responsible for most of the malaria infections outside Africa and is currently the predominant malaria parasite in countries under elimination programs. P. vivax preferentially enters young red cells called reticulocytes. Advances in understanding the molecular and cellular mechanisms of entry are hampered by the inability to grow large numbers of P. vivax parasites in a long-term in vitro culture. Recent progress in understanding the biology of the P. vivax Reticulocyte Binding Protein (PvRBPs) family of invasion ligands has led to the identification of a new invasion pathway into reticulocytes, an understanding of their structural architecture and PvRBPs as targets of the protective immune response to P. vivax infection. This review summarises current knowledge on the role of reticulocytes in P. vivax infection, the function of the PvRBP family of proteins in generating an immune response in human populations, and the characterization of anti-PvRBP antibodies in blocking parasite invasion.

Neutralising antibodies block the function of Rh5/Ripr/CyRPA complex during invasion of Plasmodium falciparum into human erythrocytes

An effective vaccine is a priority for malaria control and elimination. The leading candidate in the Plasmodium falciparum blood stage is PfRh5. PfRh5 assembles into trimeric complex with PfRipr and PfCyRPA in the parasite, and this complex is essential for erythrocyte invasion. In this study, we show that antibodies specific for PfRh5 and PfCyRPA prevent trimeric complex formation. We identify the EGF-7 domain on PfRipr as a neutralising epitope and demonstrate that antibodies against this region act downstream of complex formation to prevent merozoite invasion. Antibodies against the C-terminal region of PfRipr were more inhibitory than those against either PfRh5 or PfCyRPA alone, and a combination of antibodies against PfCyRPA and PfRipr acted synergistically to reduce invasion. This study supports prioritisation of PfRipr for development as part of a next-generation antimalarial vaccine.

Functional Comparison of Blood-Stage Plasmodium falciparum Malaria Vaccine Candidate Antigens

The malaria genome encodes over 5,000 proteins and many of these have also been proposed to be potential vaccine candidates, although few of these have been tested clinically. RH5 is one of the leading blood-stage Plasmodium falciparum malaria vaccine antigens and Phase I/II clinical trials of vaccines containing this antigen are currently underway. Its likely mechanism of action is to elicit antibodies that can neutralize merozoites by blocking their invasion of red blood cells (RBC). However, many other antigens could also elicit neutralizing antibodies against the merozoite, and most of these have never been compared directly to RH5. The objective of this study was to compare a range of blood-stage antigens to RH5, to identify any antigens that outperform or synergize with anti-RH5 antibodies. We selected 55 gene products, covering 15 candidate antigens that have been described in the literature and 40 genes selected on the basis of bioinformatics functional prediction. We were able to make 20 protein-in-adjuvant vaccines from the original selection. Of these, S-antigen and CyRPA robustly elicited antibodies with neutralizing properties. Anti-CyRPA IgG generally showed additive GIA with anti-RH5 IgG, although high levels of anti-CyRPA-specific rabbit polyclonal IgG were required to achieve 50% GIA. Our data suggest that further vaccine antigen screening efforts are required to identify a second merozoite target with similar antibody-susceptibility to RH5.

Spy&Go purification of SpyTag-proteins using pseudo-SpyCatcher to access an oligomerization toolbox

Peptide tags are a key resource, introducing minimal change while enabling a consistent process to purify diverse proteins. However, peptide tags often provide minimal benefit post-purification. We previously designed SpyTag, forming an irreversible bond with its protein partner SpyCatcher. SpyTag provides an easy route to anchor, bridge or multimerize proteins. Here we establish Spy&Go, enabling protein purification using SpyTag. Through rational engineering we generated SpyDock, which captures SpyTag-fusions and allows efficient elution. Spy&Go enabled sensitive purification of SpyTag-fusions from Escherichia coli, giving superior purity than His-tag/nickel-nitrilotriacetic acid. Spy&Go allowed purification of mammalian-expressed, N-terminal, C-terminal or internal SpyTag. As an oligomerization toolbox, we established a panel of SpyCatcher-linked coiled coils, so SpyTag-fusions can be dimerized, trimerized, tetramerized, pentamerized, hexamerized or heptamerized. Assembling oligomers for Death Receptor 5 stimulation, we probed multivalency effects on cancer cell death. Spy&Go, combined with simple oligomerization, should have broad application for exploring multivalency in signaling.

PfMSA180 is a novel Plasmodium falciparum vaccine antigen that interacts with human erythrocyte integrin associated protein (CD47)

Malaria symptoms and pathology are initiated by invasion of host erythrocytes by Plasmodium merozoites in a complex process that involves interactions between parasite and host erythrocyte proteins. Erythrocyte invasion presents attractive targets for malaria vaccine and drug development. Recently it was observed that antibodies against PfMSA180 (PF3D7_1014100) are associated with protection from symptomatic malaria, suggesting that this protein is a target of naturally acquired protective antibodies. Here we characterize PfMSA180, a ~170 kDa merozoite surface antigen that is potentially involved in erythrocyte invasion. PfMSA180 synthesized by the wheat germ cell-free system was used to raise antibodies in rabbits. Growth inhibition assays revealed that parasite invasion is inhibited by antibodies to the PfMSA180 C-terminal region, which contains an erythrocyte-binding domain. Surface plasmon resonance analysis showed that PfMSA180 specifically interacts with human erythrocyte integrin associated protein (CD47), suggesting that PfMSA180 plays a role during merozoite invasion of erythrocytes. Polymorphism analysis revealed that pfmsa180 is highly conserved among field isolates. We show that naturally acquired PfMSA180-specific antibodies responses are associated with protective immunity in a malaria-exposed Thai population. In sum, the data presented here supports further evaluation of the conserved erythrocyte-binding C-terminal region of PfMSA180 as an asexual blood-stage malaria vaccine candidate.

Engineering a Rugged Nanoscaffold To Enhance Plug-and-Display Vaccination

Nanoscale organization is crucial to stimulating an immune response. Using self-assembling proteins as multimerization platforms provides a safe and immunogenic system to vaccinate against otherwise weakly immunogenic antigens. Such multimerization platforms are generally based on icosahedral viruses and have led to vaccines given to millions of people. It is unclear whether synthetic protein nanoassemblies would show similar potency. Here we take the computationally designed porous dodecahedral i301 60-mer and rationally engineer this particle, giving a mutated i301 (mi3) with improved particle uniformity and stability. To simplify the conjugation of this nanoparticle, we employ a SpyCatcher fusion of mi3, such that an antigen of interest linked to the SpyTag peptide can spontaneously couple through isopeptide bond formation (Plug-and-Display). SpyCatcher-mi3 expressed solubly to high yields in Escherichia coli, giving more than 10-fold greater yield than a comparable phage-derived icosahedral nanoparticle, SpyCatcher-AP205. SpyCatcher-mi3 nanoparticles showed high stability to temperature, freeze-thaw, lyophilization, and storage over time. We demonstrate approximately 95% efficiency coupling to different transmission-blocking and blood-stage malaria antigens. Plasmodium falciparum CyRPA was conjugated to SpyCatcher-mi3 nanoparticles and elicited a high avidity antibody response, comparable to phage-derived virus-like particles despite their higher valency and RNA cargo. The simple production, precise derivatization, and exceptional ruggedness of this nanoscaffold should facilitate broad application for nanobiotechnology and vaccine development.

Kinetics of antibody responses to PfRH5-complex antigens in Ghanaian children with Plasmodium falciparum malaria

Plasmodium falciparum PfRH5 protein binds Ripr, CyRPA and Pf113 to form a complex that is essential for merozoite invasion of erythrocytes. The inter-genomic conservation of the PfRH5 complex proteins makes them attractive blood stage vaccine candidates. However, little is known about how antibodies to PfRH5, CyRPA and Pf113 are acquired and maintained in naturally exposed populations, and the role of PfRH5 complex proteins in naturally acquired immunity. To provide such data, we studied 206 Ghanaian children between the ages of 1–12 years, who were symptomatic, asymptomatic or aparasitemic and healthy. Plasma levels of antigen-specific IgG and IgG subclasses were measured by ELISA at several time points during acute disease and convalescence. On the day of admission with acute P. falciparum malaria, the prevalence of antibodies to PfRH5-complex proteins was low compared to other merozoite antigens (EBA175, GLURP-R0 and GLURP-R2). At convalescence, the levels of RH5-complex-specific IgG were reduced, with the decay of PfRH5-specific IgG being slower than the decay of IgG specific for CyRPA and Pf113. No correlation between IgG levels and protection against P. falciparum malaria was observed for any of the PfRH5 complex proteins. From this we conclude that specific IgG was induced against proteins from the PfRH5-complex during acute P. falciparum malaria, but the prevalence was low and the IgG levels decayed rapidly after treatment. These data indicate that the levels of IgG specific for PfRH5-complex proteins in natural infections in Ghanaian children were markers of recent exposure only.

The Molecular Basis of Erythrocyte Invasion by Malaria Parasites

Plasmodium species cause malaria by proliferating in human erythrocytes. Invasion of immunologically privileged erythrocytes provides a relatively protective niche as well as access to a rich source of nutrients. Plasmodium spp. target erythrocytes of different ages, but share a common mechanism of invasion. Specific engagement of erythrocyte receptors defines target cell tropism, activating downstream events and resulting in the physical penetration of the erythrocyte, powered by the parasite's actinomyosin-based motor. Here we review the latest in our understanding of the molecular composition of this highly complex and fascinating biological process.

Synergistic malaria vaccine combinations identified by systematic antigen screening

Malaria still kills hundreds of thousands of children each year. Malaria vaccine development is complicated by high levels of parasite genetic diversity, which makes single target vaccines vulnerable to the development of variant-specific immunity. To overcome this hurdle, we systematically screened a panel of 29 blood-stage antigens from the most deadly human malaria parasite, Plasmodium falciparum. We identified several targets that were able to inhibit erythrocyte invasion in two genetically diverse strains. Testing these targets in combination identified several pairs that blocked invasion more effectively in combination than in isolation. Video microscopy and studies of natural immune responses to malaria in patients suggest that targeting multiple steps in invasion is more likely to produce a synergistic vaccine response.

A highly effective vaccine would be a valuable weapon in the drive toward malaria elimination. No such vaccine currently exists, and only a handful of the hundreds of potential candidates in the parasite genome have been evaluated. In this study, we systematically evaluated 29 antigens likely to be involved in erythrocyte invasion, an essential developmental stage during which the malaria parasite is vulnerable to antibody-mediated inhibition. Testing antigens alone and in combination identified several strain-transcending targets that had synergistic combinatorial effects in vitro, while studies in an endemic population revealed that combinations of the same antigens were associated with protection from febrile malaria. Video microscopy established that the most effective combinations targeted multiple discrete stages of invasion, suggesting a mechanistic explanation for synergy. Overall, this study both identifies specific antigen combinations for high-priority clinical testing and establishes a generalizable approach that is more likely to produce effective vaccines.

Structural basis for inhibition of erythrocyte invasion by antibodies to Plasmodium falciparum protein CyRPA

Plasmodium falciparum causes malaria in humans with over 450,000 deaths annually. The asexual blood stage involves invasion of erythrocytes by merozoites, in which they grow and divide to release daughter merozoites, which in turn invade new erythrocytes perpetuating the cycle responsible for malaria. A key step in merozoite invasion is the essential binding of PfRh5/CyRPA/PfRipr complex to basigin, a step linked to the formation of a pore between merozoites and erythrocytes. We show CyRPA interacts directly with PfRh5. An invasion inhibitory monoclonal antibody to CyRPA blocks binding of CyRPA to PfRh5 and complex formation thus illuminating the molecular mechanism for inhibition of parasite growth. We determined the crystal structures of CyRPA alone and in complex with an antibody Fab fragment. CyRPA has a six-bladed β-propeller fold, and we identify the region that interacts with PfRh5. This functionally conserved epitope is a potential target for vaccines against P. falciparum.

Structure of the malaria vaccine candidate antigen CyRPA and its complex with a parasite invasion inhibitory antibody

Invasion of erythrocytes by Plasmodial merozoites is a composite process involving the interplay of several proteins. Among them, the Plasmodium falciparum Cysteine-Rich Protective Antigen (PfCyRPA) is a crucial component of a ternary complex, including Reticulocyte binding-like Homologous protein 5 (PfRH5) and the RH5-interacting protein (PfRipr), essential for erythrocyte invasion. Here, we present the crystal structures of PfCyRPA and its complex with the antigen-binding fragment of a parasite growth inhibitory antibody. PfCyRPA adopts a 6-bladed β-propeller structure with similarity to the classic sialidase fold, but it has no sialidase activity and fulfills a purely non-enzymatic function. Characterization of the epitope recognized by protective antibodies may facilitate design of peptidomimetics to focus vaccine responses on protective epitopes. Both in vitro and in vivo anti-PfCyRPA and anti-PfRH5 antibodies showed more potent parasite growth inhibitory activity in combination than on their own, supporting a combined delivery of PfCyRPA and PfRH5 in vaccines.

DOI:http://dx.doi.org/10.7554/eLife.20383.001

Malaria is one of the deadliest infectious diseases worldwide, killing over 400,000 people a year. About 200 million people are infected every year, placing a huge social and medical burden especially on developing countries. Microscopic parasites known as Plasmodium are responsible for causing this disease. Plasmodium parasites have a complex life cycle involving both mosquito and mammal hosts. This includes a stage where the parasites infect the mammal’s red blood cells, which causes the symptoms of the disease. In 2012, a team of researchers discovered that a protein called CyRPA forms a group (or ‘complex’) with several other proteins to allow the parasites to enter red blood cells.

Developing a vaccine is one of the most promising approaches to prevent malaria. Vaccines help the body to recognise and fight an invading microbe by triggering an immune response that results in the production of proteins called antibodies, which can bind to specific molecules on the surface of the microbe. If the microbe later enters the body, these antibodies can be produced quickly to eliminate the microbe before it causes disease. However, efforts to develop a highly effective vaccine against malaria have so far been unsuccessful.

Favuzza et al. – including some of the researchers involved in the 2012 work – used a technique called X-ray crystallography to investigate the three-dimensional structure of the CyRPA protein. The experiments show that an antibody is able to bind to a region of CyRPA – a designated ‘protective epitope’ – that is similar in the CyRPA proteins of all Plasmodium falciparum strains. These antibodies can prevent the parasite from entering the red blood cells, and vaccines containing CyRPA may therefore be effective at protecting individuals from malaria. The findings of Favuzza et al. also suggest that using CyRPA in combination with another protein in the complex called RH5 could make the vaccine more powerful as it would make it harder for the parasite to become resistant.

The next step following on from this work is to design a vaccine containing protective CyRPA epitopes that triggers an immune response in mammals that is strong enough to reduce the numbers of parasites in the blood. A future challenge will be to develop a vaccine that combines several proteins involved in different stages of the parasite’s life cycle to provide full protection against malaria.

DOI:http://dx.doi.org/10.7554/eLife.20383.002

P113 is a merozoite surface protein that binds the N terminus of Plasmodium falciparum RH5

Invasion of erythrocytes by Plasmodium falciparum merozoites is necessary for malaria pathogenesis and is therefore a primary target for vaccine development. RH5 is a leading subunit vaccine candidate because anti-RH5 antibodies inhibit parasite growth and the interaction with its erythrocyte receptor basigin is essential for invasion. RH5 is secreted, complexes with other parasite proteins including CyRPA and RIPR, and contains a conserved N-terminal region (RH5Nt) of unknown function that is cleaved from the native protein. Here, we identify P113 as a merozoite surface protein that directly interacts with RH5Nt. Using recombinant proteins and a sensitive protein interaction assay, we establish the binding interdependencies of all the other known RH5 complex components and conclude that the RH5Nt-P113 interaction provides a releasable mechanism for anchoring RH5 to the merozoite surface. We exploit these findings to design a chemically synthesized peptide corresponding to RH5Nt, which could contribute to a cost-effective malaria vaccine.

The secreted Plasmodium falciparum protein RH5 is essential for invasion of erythrocytes and is a promising vaccine candidate. Here, Galaway et al. show that the N-terminal region of RH5 binds the GPI-anchored merozoite surface protein P113 and can elicit invasion-blocking antibodies.

The association between naturally acquired IgG subclass specific antibodies to the PfRH5 invasion complex and protection from Plasmodium falciparum malaria

Understanding the targets and mechanisms of human immunity to malaria is important for advancing the development of highly efficacious vaccines and serological tools for malaria surveillance. The PfRH5 and PfRipr proteins form a complex on the surface of P. falciparum merozoites that is essential for invasion of erythrocytes and are vaccine candidates. We determined IgG subclass responses to these proteins among malaria-exposed individuals in Papua New Guinea and their association with protection from malaria in a longitudinal cohort of children. Cytophilic subclasses, IgG1 and IgG3, were predominant with limited IgG2 and IgG4, and IgG subclass-specific responses were higher in older children and those with active infection. High IgG3 to PfRH5 and PfRipr were significantly and strongly associated with reduced risk of malaria after adjusting for potential confounding factors, whereas associations for IgG1 responses were generally weaker and not statistically significant. Results further indicated that malaria exposure leads to the co-acquisition of IgG1 and IgG3 to PfRH5 and PfRipr, as well as to other PfRH invasion ligands, PfRH2 and PfRH4. These findings suggest that IgG3 responses to PfRH5 and PfRipr may play a significant role in mediating naturally-acquired immunity and support their potential as vaccine candidates and their use as antibody biomarkers of immunity.

Generation of Plasmodium falciparum parasite-inhibitory antibodies by immunization with recombinantly-expressed CyRPA

BACKGROUND

The pathogenesis of malaria is primarily associated with blood-stage infection and there is strong evidence that antibodies specific for parasite blood-stage antigens can control parasitaemia. This provides a strong rationale for incorporation of asexual blood-stage antigen components into an effective multivalent malaria subunit vaccine. On the basis of available genome-wide transcriptomic and proteomic data, previously uncharacterized Plasmodium falciparum open reading frames were screened for new blood stage vaccine candidates. This has led to the identification of the cysteine-rich protective antigen (PfCyRPA), which forms together with PfRH5 and PfRipr a multiprotein complex that is crucial for erythrocyte invasion.

METHODS

Glycosylated and non-glycosylated variants of recombinant PfCyRPA were expressed and produced as secreted protein in mammalian cells. Adjuvanted formulations of purified PfCyRPA were tested to assess whether they can effectively elicit parasite inhibitory antibodies, and to investigate whether or not the glycosylation status affects antibody binding. For this purpose, two sets of PfCyRPA-specific mouse monoclonal antibodies (mAbs) have been raised and evaluated for functional activity.

RESULTS

Generated PfCyRPA-specific mAbs, irrespective of the immunogen's glycosylation status, showed substantial parasite in vitro growth-inhibitory activity due to inhibition of erythrocyte invasion by merozoites. Furthermore, passive immunization experiments in P. falciparum infected NOD-scid IL2Rγ (null) mice engrafted with human erythrocytes demonstrated potent in vivo growth-inhibitory activity of generated mAbs.

CONCLUSIONS

Recombinantly expressed PfCyRPA tested as adjuvanted vaccine formulations in mice elicited antibodies that significantly inhibit P. falciparum asexual blood stage parasite growth both in vitro and in vivo. These findings render PfCyRPA a promising blood-stage candidate antigen for inclusion into a multicomponent malaria subunit vaccine.

Merozoite surface proteins in red blood cell invasion, immunity and vaccines against malaria

Malaria accounts for an enormous burden of disease globally, with Plasmodium falciparum accounting for the majority of malaria, and P. vivax being a second important cause, especially in Asia, the Americas and the Pacific. During infection with Plasmodium spp., the merozoite form of the parasite invades red blood cells and replicates inside them. It is during the blood-stage of infection that malaria disease occurs and, therefore, understanding merozoite invasion, host immune responses to merozoite surface antigens, and targeting merozoite surface proteins and invasion ligands by novel vaccines and therapeutics have been important areas of research. Merozoite invasion involves multiple interactions and events, and substantial processing of merozoite surface proteins occurs before, during and after invasion. The merozoite surface is highly complex, presenting a multitude of antigens to the immune system. This complexity has proved challenging to our efforts to understand merozoite invasion and malaria immunity, and to developing merozoite antigens as malaria vaccines. In recent years, there has been major progress in this field, and several merozoite surface proteins show strong potential as malaria vaccines. Our current knowledge on this topic is reviewed, highlighting recent advances and research priorities.

The authors summarize current knowledge of merozoite surface proteins of malaria parasites; their function in invasion, processing of surface proteins before, during and after invasion, their importance as targets of immunity, and the current status of malaria vaccines that target merozoite surface proteins.