Functionally relevant proteins in Plasmodium falciparum host cell invasion

Spatiotemporal analysis of Plasmodium falciparum erythrocyte binding antigen-175 gene dimorphism in Ghana

BACKGROUND

Malaria remains a leading cause of death worldwide, claiming over 600,000 lives each year. Over 90% of these deaths, mostly among children under 5 years, occur in sub-Saharan Africa and are caused by Plasmodium falciparum. The merozoites stage of the parasite, crucial for asexual development invade erythrocytes through ligand-receptor interactions. Erythrocyte binding antigen (EBA)-175 is one of the key ligands facilitating invasion via interaction with glycoprotein A (GpA) receptors on the erythrocytes. EBA-175 is known to exist in two dimorphic allelic (F and C) forms with each found to infer different virulence. There is paucity of data on the prevalence of these alleles and their epidemiology in the Ghanaian malaria landscape and hence this study.

METHODS

Parasite gDNA was extracted from archived Dried Blood Spots (DBS) prepared from 700 confirmed malaria-infected individuals and analysed for P. falciparum EBA-175 dimorphism. Selective eba-175 gene amplification via nested PCR and allele scoring using agarose gel electrophoresis for F, C and F/C alleles.

RESULTS

Of the total 632 successfully genotyped samples, prevalence of F, C, and F/C allelic forms were 61.2% (n = 387), 20.7% (n = 131), and 18.0% (n = 114), respectively. Seasonality analysis did not reveal a statistically significant difference in the prevalence of dimorphic forms between the wet (n = 475) and dry (n = 157) seasons (p = 0.051). The prevalence ratio (wet/dry) for C, F and F/C were determined to be 1.0, 1.1 and 1.4, respectively. Between 2019 and 2022, the prevalence of the alleles changed significantly (χ2 = 6.5427, p = 0.03). Geometric mean parasite density for the C, F, and F/C alleles were 21,477.1 [95%CI 15,749.2 - 29,288.1], 18,308.0 [95%CI 15,149.9-22,124.5] and 22,690.4[95% CI 16,891.9-30,479.2], respectively.

CONCLUSION

The F-allele was the most prevalent form across all age groups, followed by the C allele and mixed F/C alleles. No significant difference in allele prevalence was observed between the high malaria season (wet) and low malaria season (dry). However, a statistically significant difference in the temporal prevalence of pure alleles (F & C) between two time points was observed. The current study adds to the existing body of knowledge on eba-175 allelic dimorphism and highlights the co-circulation of alleles in high malaria endemic areas in Ghana.

Identification of Novel, Potent, and Selective Compounds against Malaria Using Glideosomal-Associated Protein 50 as a Drug Target

Phylum apicomplexan consists of parasites, such as Plasmodium and Toxoplasma. These obligate intracellular parasites enter host cells via an energy-dependent process using specialized machinery, called the glideosome. In the present study, we used Plasmodium falciparum GAP50, a glideosome-associated protein, as a target to screen 951 different compounds from diverse chemical libraries. Using different screening methods, eight compounds (Hayatinine, Curine, MMV689758 (Bedaquiline), MMV1634402 (Brilacidin), and MMV688271, MMV782353, MMV642550, and USINB4-124-8) were identified, which showed promising binding affinity (KD < 75 μM), along with submicromolar range antiparasitic efficacy and selectivity index > 100 fold for malaria parasite. These eight compounds were effective against Chloroquine-resistant PfINDO and Artemisinin-resistant PfCam3.1R359T strains. Studies on the effect of these compounds at asexual blood stages showed that these eight compounds act differently at different developmental stages, indicating the binding of these compounds to other Plasmodium proteins, in addition to PfGAP50. We further studied the effects of compounds (Bedaquiline and USINB4-124-8) in an in vivoPlasmodium berghei mouse model of malaria. Importantly, the oral delivery of Bedaquiline (50 mg/kg b. wt.) showed substantial suppression of parasitemia, and three out of seven mice were cured of the infection. Thus, our study provides new scaffolds for the development of antimalarials that can act at multiple Plasmodium lifecycle stages.

Escaping the enemy's bullets: an update on how malaria parasites evade host immune response

Malaria continues to cause untold hardship to inhabitants of malaria-endemic regions, causing significant morbidity and mortality that severely impact global health and the economy. Considering the complex life cycle of malaria parasites (MPs) and malaria biology, continued research efforts are ongoing to improve our understanding of the pathogenesis of the diseases. Female Anopheles mosquito injects MPs into its hosts during a blood meal, and MPs invade the host skin and the hepatocytes without causing any serious symptoms. Symptomatic infections occur only during the erythrocytic stage. In most cases, the host's innate immunity (for malaria-naïve individuals) and adaptive immunity (for pre-exposed individuals) mount severe attacks and destroy most MPs. It is increasingly understood that MPs have developed several mechanisms to escape from the host's immune destruction. This review presents recent knowledge on how the host's immune system destroys invading MPs as well as MPs survival or host immune evasion mechanisms. On the invasion of host cells, MPs release molecules that bind to cell surface receptors to reprogram the host in a way to lose the capacity to destroy them. MPs also hide from the host immune cells by inducing the clustering of both infected and uninfected erythrocytes (rosettes), as well as inducing endothelial activation. We hope this review will inspire more research to provide a complete understanding of malaria biology and promote interventions to eradicate the notorious disease.

Mosquirix™ RTS, S/AS01 Vaccine Development, Immunogenicity, and Efficacy

Malaria is a parasitic infection caused by bites from Plasmodium falciparum (P. falciparum)-infected mosquitoes with a present scale of symptoms ranging from moderate fever to neurological disorders. P. falciparum is the most lethal of the five strains of malaria, and is a major case of morbidity and mortality in endemic regions. Recent advancements in malaria diagnostic tools and prevention strategies have improved conjugation antimalarial therapies using fumigation and long-lasting insecticidal sprays, thus lowering malarial infections. Declines in the total number of infected individuals have been correlated with antimalarial drugs. Despite this, malaria remains a major health threat, affecting more than 30 million men, women, and children around the globe, and 20 percent of all children around the globe have malaria parasites in their blood. To overcome this life-threatening condition, novel therapeutic strategies, including immunization, are urgently needed to tackle this infection around the world. In line with this, the development of the RTS, S vaccine was a significant step forward in the fight against malaria. RTS, S is a vaccine for P. falciparum in which R specifies central repeat units, T the T-cell epitopes, and S indicates surface antigen. The RTS, S/AS01 malarial vaccine was synthesized and screened in several clinical trials between 2009 and 2014, involving thousands of young children in seven African countries, showing that children who received the vaccine did not suffer from severe malaria. Mosquirix™ was approved by the World Health Organization in 2021, indicating it to be safe and advocating its integration into routine immunization programs and existing malaria control measures. This paper examines the various stages of the vaccine's development, including the evaluation of its immunogenicity and efficacy on the basis of a total of 2.3 million administered doses through a routine immunization program. The protection and effectiveness provided by the vaccine are strong, and evidence shows that it can be effectively delivered through the routine child immunization platform. The economic cost of the vaccine remains to be considered.

Malaria: Paving the way to developing peptide-based vaccines against invasion in infectious diseases

Malaria remains a large-scale public health problem, killing more than 400,000 people and infecting up to 230 million worldwide, every year. Unfortunately, despite numerous efforts and research concerning vaccine development, results to date have been low and/or strain-specific. This work describes a strategy involving Plasmodium falciparum Duffy binding-like (DBL) and reticulocyte-binding protein homologue (RH) family-derived minimum functional peptides, netMHCIIpan3.2 parental and modified peptides' in silico binding prediction and modeling some Aotus major histocompatibility class II (MHCII) molecules based on known human molecules' structure to understand their differences. These are used to explain peptides' immunological behaviour when used as vaccine components in the Aotus model. Despite the great similarity between human and Aotus immune system molecules, around 50% of Aotus allele molecules lack a counterpart in the human immune system which could lead to an Aotus-specific vaccine. It was also confirmed that functional Plasmodium falciparum' conserved proteins are immunologically silent (in both the animal model and in-silico prediction); they must therefore be modified to elicit an appropriate immune response. Some peptides studied here had the desired behaviour and can thus be considered components of a fully-protective antimalarial vaccine.

From a basic to a functional approach for developing a blood stage vaccine against Plasmodium vivax

Introduction: Numerous challenges have hampered developing an anti-malarial vaccine against the most widespread malarial parasite worldwide: Plasmodium vivax. Despite the progress achieved in studying proteins in short-term in vitro culture or in experimental models, there is still no clear method for defining which antigens or their regions should be prioritized for including them in a vaccine.Areas covered: The methods used by research groups so far which have focused on the functional analysis of P. vivax blood stage antigens have been reviewed here. A logical strategy orientated toward resolving two of the most commonly occurring problems in designing vaccines against this species has thus been proposed (i.e. the search for candidates and evaluating/ascertaining their functional role in the invasion of such molecules).Expert commentary: Advances in knowledge regarding P. vivax biology have been extremely slow. Only two key receptor-ligand interactions concerning merozoite entry to reticulocytes have been reported during the last 20 years: PvDBP1-DARC and PvRBP2b-CD71. Despite increasing knowledge about the parasite's intimate preference for its host cells, it has yet to be determined which regions of the merozoite molecules characterized to date meet the requirement of inducing protective immune responses effectively blocking heterologous parasite entry to human cells.

Preliminary Evaluation of the Safety and Immunogenicity of an Antimalarial Vaccine Candidate Modified Peptide (IMPIPS) Mixture in a Murine Model

Malaria continues being a high-impact disease regarding public health worldwide; the WHO report for malaria in 2018 estimated that ~219 million cases occurred in 2017, mostly caused by the parasite Plasmodium falciparum. The disease cost the lives of more than 400,000 people, mainly in Africa. In spite of great efforts aimed at developing better prevention (i.e., a highly effective vaccine), diagnosis, and treatment methods for malaria, no efficient solution to this disease has been advanced to date. The Fundación Instituto de Inmunología de Colombia (FIDIC) has been developing studies aimed at furthering the search for vaccine candidates for controlling P. falciparum malaria. However, vaccine development involves safety and immunogenicity studies regarding their formulation in animal models before proceeding to clinical studies. The present work has thus been aimed at evaluating the safety and immunogenicity of a mixture of 23 chemically synthesised, modified peptides (immune protection-inducing protein structure (IMPIPS)) derived from different P. falciparum proteins. Single and repeat dose assays were thus used with male and female BALB/c mice which were immunised with the IMPIPS mixture. It was found that single and repeat dose immunisation with the IMPIPS mixture was safe, both locally and systemically. It was observed that the antibodies so stimulated recognised the parasite's native proteins and inhibited merozoite invasion of red blood cells in vitro when evaluating the humoral immune response induced by the IMPIPS mixture. Such results suggested that the IMPIPS peptide mixture could be a safe candidate to be tested during the next stage involved in developing an antimalarial vaccine, evaluating local safety, immunogenicity, and protection in a nonhuman primate model.

Plasmodium falciparum Blood Stage Antimalarial Vaccines: An Analysis of Ongoing Clinical Trials and New Perspectives Related to Synthetic Vaccines

Plasmodium falciparum malaria is a disease causing high morbidity and mortality rates worldwide, mainly in sub-Saharan Africa. Candidates have been identified for vaccines targeting the parasite's blood stage; this stage is important in the development of symptoms and clinical complications. However, no vaccine that can directly affect morbidity and mortality rates is currently available. This review analyzes the formulation, methodological design, and results of active clinical trials for merozoite-stage vaccines, regarding their safety profile, immunological response (phase Ia/Ib), and protective efficacy levels (phase II). Most vaccine candidates are in phase I trials and have had an acceptable safety profile. GMZ2 has made the greatest progress in clinical trials; its efficacy has been 14% in children aged less than 5 years in a phase IIb trial. Most of the available candidates that have shown strong immunogenicity and that have been tested for their protective efficacy have provided good results when challenged with a homologous parasite strain; however, their efficacy has dropped when they have been exposed to a heterologous strain. In view of these vaccines' unpromising results, an alternative approach for selecting new candidates is needed; such line of work should be focused on how to increase an immune response induced against the highly conserved (i.e., common to all strains), functionally relevant, protein regions that the parasite uses to invade target cells. Despite binding regions tending to be conserved, they are usually poorly antigenic and/or immunogenic, being frequently discarded as vaccine candidates when the conventional immunological approach is followed. The Fundación Instituto de Inmunología de Colombia (FIDIC) has developed a logical and rational methodology based on including conserved high-activity binding peptides (cHABPs) from the main P. falciparum biologically functional proteins involved in red blood cell (RBC) invasion. Once appropriately modified (mHABPs), these minimal, subunit-based, chemically synthesized peptides can be used in a system covering the human immune system's main genetic variables (the human leukocyte antigen HLA-DR isotype) inducing a suitable, immunogenic, and protective immune response in most of the world's populations.

Specific β-Turns Precede PPIIL Structures Binding to Allele-Specific HLA-DRβ1* PBRs in Fully-Protective Malaria Vaccine Components

The 3D structural analysis of 62 peptides derived from highly pathogenic Plasmodium falciparum malaria parasite proteins involved in host cell invasion led to finding a striking association between particular β-turn types located in the N-terminal peripheral flanking residue region (preceding the polyproline II left-handed structures fitting into the HLA-DRβ^* allele family) and modified immune protection-inducing protein structure induced long-lasting protective immunity. This is the first time association between two different secondary structures associated with a specific immunological function has been described: full, long-lasting protective immunity.

Conserved Binding Regions Provide the Clue for Peptide-Based Vaccine Development: A Chemical Perspective

Synthetic peptides have become invaluable biomedical research and medicinal chemistry tools for studying functional roles, i.e., binding or proteolytic activity, naturally-occurring regions’ immunogenicity in proteins and developing therapeutic agents and vaccines. Synthetic peptides can mimic protein sites; their structure and function can be easily modulated by specific amino acid replacement. They have major advantages, i.e., they are cheap, easily-produced and chemically stable, lack infectious and secondary adverse reactions and can induce immune responses via T- and B-cell epitopes. Our group has previously shown that using synthetic peptides and adopting a functional approach has led to identifying Plasmodium falciparumconserved regions binding to host cells. Conserved high activity binding peptides’ (cHABPs) physicochemical, structural and immunological characteristics have been taken into account for properly modifying and converting them into highly immunogenic, protection-inducing peptides (mHABPs) in the experimental Aotus monkey model. This article describes stereo–electron and topochemical characteristics regarding major histocompatibility complex (MHC)-mHABP-T-cell receptor (TCR) complex formation. Some mHABPs in this complex inducing long-lasting, protective immunity have been named immune protection-inducing protein structures (IMPIPS), forming the subunit components in chemically synthesized vaccines. This manuscript summarizes this particular field and adds our recent findings concerning intramolecular interactions (H-bonds or π-interactions) enabling proper IMPIPS structure as well as the peripheral flanking residues (PFR) to stabilize the MHCII-IMPIPS-TCR interaction, aimed at inducing long-lasting, protective immunological memory.