A screen for Plasmodium falciparum sporozoite surface protein binding to human hepatocyte surface receptors identifies novel host-pathogen interactions.

BACKGROUND: Sporozoite invasion of hepatocytes is an essential step in the Plasmodium life-cycle and has similarities, at the cellular level, to merozoite invasion of erythrocytes. In the case of the Plasmodium blood-stage, efforts to identify host-pathogen protein-protein interactions have yielded important insights including vaccine candidates. In the case of sporozoite-hepatocyte invasion, the host-pathogen protein-protein interactions involved are poorly understood. METHODS: To gain a better understanding of the protein-protein interaction between the sporozoite ligands and host receptors, a systematic screen was performed. The previous Plasmodium falciparum and human surface protein ectodomain libraries were substantially extended, resulting in the creation of new libraries comprising 88 P. falciparum sporozoite protein coding sequences and 182 sequences encoding human hepatocyte surface proteins. Having expressed recombinant proteins from these sequences, a plate-based assay was used, capable of detecting low affinity interactions between recombinant proteins, modified for enhanced throughput, to screen the proteins for interactions. The novel interactions identified in the screen were characterized biochemically, and their essential role in parasite invasion was further elucidated using antibodies and genetically manipulated Plasmodium parasites. RESULTS: A total of 7540 sporozoite-hepatocyte protein pairs were tested under conditions capable of detecting interactions of at least 1.2 µM KD. An interaction between the human fibroblast growth factor receptor 4 (FGFR4) and the P. falciparum protein Pf34 is identified and reported here, characterizing its affinity and demonstrating the blockade of the interaction by reagents, including a monoclonal antibody. Furthermore, further interactions between Pf34 and a second P. falciparum rhoptry neck protein, PfRON6, and between human low-density lipoprotein receptor (LDLR) and the P. falciparum protein PIESP15 are identified. Conditional genetic deletion confirmed the essentiality of PfRON6 in the blood-stage, consistent with the important role of this protein in parasite lifecycle. Pf34 was refractory to attempted genetic modification. Antibodies to Pf34 abrogated the interaction and had a modest effect upon sporozoite invasion into primary human hepatocytes. CONCLUSION: Pf34 and PfRON6 may be members of a functionally important invasion complex which could be a target for future interventions. The modified interaction screening assay, protein expression libraries and P. falciparum mutant parasites reported here may be a useful tool for protein interaction discovery and antigen candidate screening which could be of wider value to the scientific community.

Integrating 3Rs approaches in WHO guidelines for the batch release testing of biologicals: Responses from a survey of vaccines and biological therapeutics manufacturers.

The UK National Centre for the Replacement, Refinement, and Reduction of Animals in Research (NC3Rs) has been tasked by the World Health Organization (WHO) to review the extent to which animal-based testing methods are described in their manuals, guidelines and recommendations for vaccines and biotherapeutics. The aim is to identify and recommend where updates to these documents can lead to an increased and more harmonised adoption of 3Rs principles (i.e. Replacement, Reduction and Refinement of animal tests) in the quality control and batch release testing requirements for vaccines and biotherapeutics. Developing recommendations that are widely applicable by both the manufacturers and national regulatory authorities for vaccines and biologicals globally requires a detailed understanding of how different organisations view the opportunities and barriers to better integration of the 3Rs. To facilitate this, we developed and distributed a survey aimed at vaccine and biotherapeutics manufacturers in July 2021. In this paper, we present the key findings from this survey and how these will help inform the recommendations for wider integration of 3Rs approaches by WHO in their guidance documents applicable to the quality control and batch testing of vaccines and biotherapeutics.

A physical wiring diagram for the human immune system.

The human immune system is composed of a distributed network of cells circulating throughout the body, which must dynamically form physical associations and communicate using interactions between their cell-surface proteomes1. Despite their therapeutic potential2, our map of these surface interactions remains incomplete3,4. Here, using a high-throughput surface receptor screening method, we systematically mapped the direct protein interactions across a recombinant library that encompasses most of the surface proteins that are detectable on human leukocytes. We independently validated and determined the biophysical parameters of each novel interaction, resulting in a high-confidence and quantitative view of the receptor wiring that connects human immune cells. By integrating our interactome with expression data, we identified trends in the dynamics of immune interactions and constructed a reductionist mathematical model that predicts cellular connectivity from basic principles. We also developed an interactive multi-tissue single-cell atlas that infers immune interactions throughout the body, revealing potential functional contexts for new interactions and hubs in multicellular networks. Finally, we combined targeted protein stimulation of human leukocytes with multiplex high-content microscopy to link our receptor interactions to functional roles, in terms of both modulating immune responses and maintaining normal patterns of intercellular associations. Together, our work provides a systematic perspective on the intercellular wiring of the human immune system that extends from systems-level principles of immune cell connectivity down to mechanistic characterization of individual receptors, which could offer opportunities for therapeutic intervention.

An invariant Trypanosoma vivax vaccine antigen induces protective immunity.

Trypanosomes are protozoan parasites that cause infectious diseases, including African trypanosomiasis (sleeping sickness) in humans and nagana in economically important livestock1,2. An effective vaccine against trypanosomes would be an important control tool, but the parasite has evolved sophisticated immunoprotective mechanisms-including antigenic variation3-that present an apparently insurmountable barrier to vaccination. Here we show, using a systematic genome-led vaccinology approach and a mouse model of Trypanosoma vivax infection4, that protective invariant subunit vaccine antigens can be identified. Vaccination with a single recombinant protein comprising the extracellular region of a conserved cell-surface protein that is localized to the flagellum membrane (which we term 'invariant flagellum antigen from T. vivax') induced long-lasting protection. Immunity was passively transferred with immune serum, and recombinant monoclonal antibodies to this protein could induce sterile protection and revealed several mechanisms of antibody-mediated immunity, including a major role for complement. Our discovery identifies a vaccine candidate for an important parasitic disease that has constrained socioeconomic development in countries in sub-Saharan Africa5, and provides evidence that highly protective vaccines against trypanosome infections can be achieved.

Systematic Identification of Plasmodium Falciparum Sporozoite Membrane Protein Interactions Reveals an Essential Role for the p24 Complex in Host Infection.

Sporozoites are a motile form of malaria-causing Plasmodium falciparum parasites that migrate from the site of transmission in the dermis through the bloodstream to invade hepatocytes. Sporozoites interact with many cells within the host, but the molecular identity of these interactions and their role in the pathology of malaria is poorly understood. Parasite proteins that are secreted and embedded within membranes are known to be important for these interactions, but our understanding of how they interact with each other to form functional complexes is largely unknown. Here, we compile a library of recombinant proteins representing the repertoire of cell surface and secreted proteins from the P. falciparum sporozoite and use an assay designed to detect extracellular interactions to systematically identify complexes. We identify three protein complexes including an interaction between two components of the p24 complex that is involved in the trafficking of glycosylphosphatidylinositol-anchored proteins through the secretory pathway. Plasmodium parasites lacking either gene are strongly inhibited in the establishment of liver-stage infections. These findings reveal an important role for the p24 complex in malaria pathogenesis and show that the library of recombinant proteins represents a valuable resource to investigate P. falciparum sporozoite biology.

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

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.IMPORTANCE 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.

Rapid and sensitive large-scale screening of low affinity extracellular receptor protein interactions by using reaction induced inhibition of Gaussia luciferase.

Extracellular protein interactions mediated by cell surface receptors are essential for intercellular communication in multicellular organisms. Assays to detect extracellular interactions must account for their often weak binding affinities and also the biochemical challenges in solubilising membrane-embedded receptors in an active form. Methods based on detecting direct binding of soluble recombinant receptor ectodomains have been successful, but genome-scale screening is limited by the usual requirement of producing sufficient amounts of each protein in two different forms, usually a "bait" and "prey". Here, we show that oligomeric receptor ectodomains coupled to concatenated units of the light-generating Gaussia luciferase enzyme robustly detected low affinity interactions and reduced the amount of protein required by several orders of magnitude compared to other reporter enzymes. Importantly, we discovered that this flash-type luciferase exhibited a reaction-induced inhibition that permitted the use of a single protein preparation as both bait and prey thereby halving the number of expression plasmids and recombinant proteins required for screening. This approach was tested against a benchmarked set of quantified extracellular interactions and shown to detect extremely weak interactions (KDs ≥ µM). This method will facilitate large-scale receptor interaction screening and contribute to the goal of mapping networks of cellular communication.

Resurrection of the ancestral RH5 invasion ligand provides a molecular explanation for the origin of P. falciparum malaria in humans.

Many important infectious diseases are the result of zoonoses, in which pathogens that normally infect animals acquire mutations that enable the breaching of species barriers to permit the infection of humans. Our understanding of the molecular events that enable host switching are often limited, and yet this is a fundamentally important question. Plasmodium falciparum, the etiological agent of severe human malaria, evolved following a zoonotic transfer of parasites from gorillas. One gene-rh5-which encodes an essential ligand for the invasion of host erythrocytes, is suspected to have played a critical role in this host switch. Genome comparisons revealed an introgressed sequence in the ancestor of P. falciparum containing rh5, which likely allowed the ancestral parasites to infect both gorilla and human erythrocytes. To test this hypothesis, we resurrected the ancestral introgressed reticulocyte-binding protein homologue 5 (RH5) sequence and used quantitative protein interaction assays to demonstrate that this ancestral protein could bind the basigin receptor from both humans and gorillas. We also showed that this promiscuous receptor binding phenotype of RH5 was shared with the parasite clade that transferred its genome segment to the ancestor of P. falciparum, while the other lineages exhibit host-specific receptor binding, confirming the central importance of this introgression event for Plasmodium host switching. Finally, since its transfer to humans, P. falciparum, and also the RH5 ligand, have evolved a strong human specificity. We show that this subsequent restriction to humans can be attributed to a single amino acid mutation in the RH5 sequence. Our findings reveal a molecular pathway for the origin and evolution of human P. falciparum malaria and may inform molecular surveillance to predict future zoonoses.

Human Antibodies that Slow Erythrocyte Invasion Potentiate Malaria-Neutralizing Antibodies.

The Plasmodium falciparum reticulocyte-binding protein homolog 5 (PfRH5) is the leading target for next-generation vaccines against the disease-causing blood-stage of malaria. However, little is known about how human antibodies confer functional immunity against this antigen. We isolated a panel of human monoclonal antibodies (mAbs) against PfRH5 from peripheral blood B cells from vaccinees in the first clinical trial of a PfRH5-based vaccine. We identified a subset of mAbs with neutralizing activity that bind to three distinct sites and another subset of mAbs that are non-functional, or even antagonistic to neutralizing antibodies. We also identify the epitope of a novel group of non-neutralizing antibodies that significantly reduce the speed of red blood cell invasion by the merozoite, thereby potentiating the effect of all neutralizing PfRH5 antibodies as well as synergizing with antibodies targeting other malaria invasion proteins. Our results provide a roadmap for structure-guided vaccine development to maximize antibody efficacy against blood-stage malaria.

Human vaccination against RH5 induces neutralizing antimalarial antibodies that inhibit RH5 invasion complex interactions.

The development of a highly effective vaccine remains a key strategic goal to aid the control and eventual eradication of Plasmodium falciparum malaria. In recent years, the reticulocyte-binding protein homolog 5 (RH5) has emerged as the most promising blood-stage P. falciparum candidate antigen to date, capable of conferring protection against stringent challenge in Aotus monkeys. We report on the first clinical trial to our knowledge to assess the RH5 antigen - a dose-escalation phase Ia study in 24 healthy, malaria-naive adult volunteers. We utilized established viral vectors, the replication-deficient chimpanzee adenovirus serotype 63 (ChAd63), and the attenuated orthopoxvirus modified vaccinia virus Ankara (MVA), encoding RH5 from the 3D7 clone of P. falciparum. Vaccines were administered i.m. in a heterologous prime-boost regimen using an 8-week interval and were well tolerated. Vaccine-induced anti-RH5 serum antibodies exhibited cross-strain functional growth inhibition activity (GIA) in vitro, targeted linear and conformational epitopes within RH5, and inhibited key interactions within the RH5 invasion complex. This is the first time to our knowledge that substantial RH5-specific responses have been induced by immunization in humans, with levels greatly exceeding the serum antibody responses observed in African adults following years of natural malaria exposure. These data support the progression of RH5-based vaccines to human efficacy testing.

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.

Comparative Antibody Responses Against three Antimalarial Vaccine Candidate Antigens from Urban and Rural Exposed Individuals in Gabon.

The analysis of immune responses in diverse malaria endemic regions provides more information to understand the host's immune response to Plasmodium falciparum. Several plasmodial antigens have been reported as targets of human immunity. PfAMA1 is one of most studied vaccine candidates; PfRH5 and Pf113 are new promising vaccine candidates. The aim of this study was to evaluate humoral response against these three antigens among children of Lastourville (rural area) and Franceville (urban area). Malaria was diagnosed using rapid diagnosis tests. Plasma samples were tested against these antigens by enzyme-linked immunosorbent assay (ELISA). We found that malaria prevalence was five times higher in the rural area than in the urban area (p < 0.0001). The anti-PfAMA1 and PfRh5 response levels were significantly higher in Lastourville than in Franceville (p < 0.0001; p = 0.005). The anti-AMA1 response was higher than the anti-Pf113 response, which in turn was higher than the anti-PfRh5 response in both sites. Anti-PfAMA1 levels were significantly higher in infected children than those in uninfected children (p = 0.001) in Franceville. Anti-Pf113 and anti-PfRh5 antibody levels were lowest in children presenting severe malarial anemia. These three antigens are targets of immunity in Gabon. Further studies on the role of Pf113 in antimalarial protection against severe anemia are needed.

MS2 viruslike particles: a robust, semisynthetic targeted drug delivery platform.

We show that viruslike particles (VLPs) reassembled in vitro with the RNA bacteriophage MS2 coat protein and an RNA conjugate encompassing a siRNA and a known capsid assembly signal can be targeted to HeLa cells by covalent attachment of human transferrin. The siRNA VLPs protect their cargoes from nuclease, have a double-stranded conformation in the capsid and carry multiple drug and targeting ligands. The relative efficiency of VLP reassembly has been assessed, and conditions have been determined for larger scale production. Targeted VLPs have been purified away from unmodified VLPs for the first time allowing improved analysis of the effects of this synthetic virion system. The particles enter cells via receptor-mediated endocytosis and produce siRNA effects at low nanomolar concentrations. Although less effective than a commercial cationic lipid vector at siRNA delivery, the smaller amounts of internalized RNA with VLP delivery had an effect as good as if not better than the lipid transfection route. This implies that the siRNAs delivered by this route are more accessible to the siRNA pathway than identical RNAs delivered in complex lipid aggregates. The data suggest that the MS2 system continues to show many of the features that will be required to create an effective targeted drug delivery system. The fluorescence assays of siRNA effects described here will facilitate the combinatorial analysis of both future formulations and dosing regimes.