A GBP 130 derived peptide from Plasmodium falciparum binds to human erythrocytes and inhibits merozoite invasion in vitro

Immunization with PfGBP130 generates antibodies that inhibit RBC invasion by P. falciparum parasites

Background

Despite decades of effort, Plasmodium falciparum malaria remains a leading killer of children. The absence of a highly effective vaccine and the emergence of parasites resistant to both diagnosis as well as treatment hamper effective public health interventions.

Methods and results

To discover new vaccine candidates, we used our whole proteome differential screening method and identified PfGBP130 as a parasite protein uniquely recognized by antibodies from children who had developed resistance to P. falciparum infection but not from those who remained susceptible. We formulated PfGBP130 as lipid encapsulated mRNA, DNA plasmid, and recombinant protein-based immunogens and evaluated the efficacy of murine polyclonal anti-PfGBP130 antisera to inhibit parasite growth in vitro. Immunization of mice with PfGBP130-A (aa 111-374), the region identified in our differential screen, formulated as a DNA plasmid or lipid encapsulated mRNA, but not as a recombinant protein, induced antibodies that inhibited RBC invasion in vitro. mRNA encoding the full ectodomain of PfGBP130 (aa 89-824) also generated parasite growth-inhibitory antibodies.

Conclusion

We are currently advancing PfGBP130-A formulated as a lipid-encapsulated mRNA for efficacy evaluation in non-human primates.

Vaccination Strategies against Malaria: novel carrier(s) more than a tour de force

The introduction of vaccine technology has facilitated an unprecedented multi-antigen approach to develop an effective vaccine against complex systemic inflammatory pathogens such as Plasmodium spp. that cause severe malaria. The capacity of multi subunit DNA vaccine encoding different stage Plasmodium antigens to induce CD8(+) cytotoxic T lymphocytes and interferon-γ responses in mice, monkeys and humans has been observed. Moreover, genetic vaccination may be capable of eliciting both cell mediated and humoral immune responses. The cytotoxic T cell responses are categorically needed against intracellular hepatic stage and humoral response with antibodies targeted against antigens from all stages of malaria parasite life cycle. Therefore, the key to success for any DNA based vaccine is to design a vector able to serve as a safe and efficient delivery system. This has encouraged the development of non-viral DNA-mediated gene transfer techniques such as liposome, virosomes, microsphere and nanoparticles. Efficient and relatively safe DNA transfection using lipoplexes makes them an appealing alternative to be explored for gene delivery. Also, liposome-entrapped DNA has been shown to enhance the potency of DNA vaccines, possibly by facilitating uptake of the plasmid by antigen-presenting cells (APC). Another recent technology using cationic lipids has been deployed and has generated substantial interest in this approach to gene transfer. In this review we discussed various aspects that could be decisive in the formulation of efficient and stable carrier system(s) for the development of malaria vaccine.

Functional, structural, and immunological compartmentalisation of malaria invasive proteins

Conserved Plasmodium falciparum merozoite high activity binding peptides (HABPs) involved in red blood cell (RBC) invasion which are present in merozoite surface proteins (MSPs) involved in attachment, rolling over RBC, those derived from soluble proteins loosely bound to the membrane, and those present in microneme and rhoptry organelles have an alpha-helical structure and bind with high affinity to HLA-DR52 molecules. On the contrary, conserved HABPs belonging to molecules anchored to the membrane by a GPI tail, or a transmembranal region, or those molecules presenting PEXEL motifs have a strand, turn or unordered configuration and bind with high affinity to HLA-DR53 molecules. Such functional, cellular, structural, and immunological compartmentalisation has tremendous implications in subunit-based, multi-epitope, synthetic, anti-malarial vaccine development.

Identification, characterization, and inhibition of Plasmodium falciparum beta-hydroxyacyl-acyl carrier protein dehydratase (FabZ)

The emergence of drug-resistant forms of Plasmodium falciparum emphasizes the need to develop new antimalarials. In this context, the fatty acid biosynthesis (FAS) pathway of the malarial parasite has recently received a lot of attention. Due to differences in the fatty acid biosynthesis systems of Plasmodium and man, this pathway is a good target for the development of new and selective therapeutic drugs directed against malaria. In continuation of these efforts we report cloning and overexpression of P. falciparum beta-hydroxyacyl-acyl carrier protein (ACP) dehydratase (PffabZ) gene that codes for a 17-kDa protein. The enzyme catalyzes the dehydration of beta-hydroxyacyl-ACP to trans-2-acyl-ACP, the third step in the elongation phase of the FAS cycle. It has a Km of 199 microM and kcat/Km of 80.4 m-1 s-1 for the substrate analog beta-hydroxybutyryl-CoA but utilizes crotonoyl-CoA, the product of the reaction, more efficiently (Km = 86 microM, kcat/Km = 220 m-1 s-1). More importantly, we also identify inhibitors (NAS-91 and NAS-21) for the enzyme. Both the inhibitors prevented the binding of crotonoyl-CoA to PfFabZ in a competitive fashion. Indeed these inhibitors compromised the growth of P. falciparum in cultures and inhibited the parasite fatty acid synthesis pathway both in cell-free extracts as well as in situ. We modeled the structure of PfFabZ using Escherichia coli beta-hydroxydecanoyl thioester dehydratase (EcFabA) as a template. We also modeled the inhibitor complexes of PfFabZ to elucidate the mode of binding of these compounds to FabZ. The discovery of the inhibitors of FabZ, reported for the first time against any member of this family of enzymes, essential to the type II FAS pathway opens up new avenues for treating a number of infectious diseases including malaria.

MHC allele-specific binding of a malaria peptide makes it become promiscuous on fitting a glycine residue into pocket 6

Peptide 1585 (EVLYLKPLAGVYRSLKKQLE) has a highly conserved amino-acid sequence located in the Plasmodium falciparum main merozoite surface protein (MSP-1) C-terminal region, required for merozoite entry into human erythrocytes and therefore represents a vaccine candidate for P. falciparum malaria. Original sequence-specific binding to five HLA DRB1* alleles (0101, 0102, 0401, 0701, and 1101) revealed this peptide's specific HLA DRB1*0102 allele binding. This peptide's allele-specific binding to HLA DRB1*0102 took on broader specificity for the DRB1*0101, -0401, and -1101 alleles when lysine was replaced by glycine in position 17 (peptide 5198: EVLYLKPLAGVYRSLKG(17)QLE). Binding of the identified G(10)VYRSLKGQLE(20) C-terminal register to these alleles suggests that peptide promiscuous binding relied on fitting Y(12), L(15), and G(17) into P-1, P-4, and P-6, respectively. The implications of the findings and the future of this synthetic vaccine candidate are discussed.

6746 SERA peptide analogues immunogenicity and protective efficacy against malaria is associated with short alpha helix formation: malaria protection associated with peptides alpha helix shortening

Erythrocyte high activity binding peptides (HABPs) have been identified for the Plasmodium falciparum serine repeat antigen (SERA). HABP 6746, located in this protein's 50 kDa fragment had its critical binding residues replaced by amino acids having similar mass but different charge to change their immunologic properties. This peptide analogues were used to immunize Aotus monkeys that were challenged later on with a virulent P. falciparum strain to determine their protective efficacy. A shortening in alpha helix structure was found in the immunogenic and protective ones when their secondary structure was analyzed by NMR, to correlate their structure with their immunologic properties. These data, together with results from previous studies, suggest that this shortening in HABP helical configuration may lead to better fitting with immune system molecules, rendering them immunogenic and protective and therefore making them excellent candidates for consideration as components of a subunit based multicomponent synthetic vaccine against malaria.

P. falciparum: merozoite surface protein-8 peptides bind specifically to human erythrocytes

This work determined Plasmodium falciparum merozoite surface protein-8 (MSP-8) regions specifically binding to membrane surface receptors on human erythrocytes. Five high activity binding peptides (HABPs), whose binding to erythrocytes became saturable and sensitive on being treated with neuraminidase and chymotrypsin were identified from the MSP-8 protein. Those amino acids directly involved in interaction with erythrocytes were also determined for each one of the HABPs. Some of them specifically recognized 28, 46, and 73 kDa erythrocyte membrane proteins. Some HABPs inhibited in vitro P. falciparum merozoite invasion of erythrocytes by up to 98%, suggesting the MSP-8 protein's possible role in the invasion process.

Alpha helix shortening in 1522 MSP-1 conserved peptide analogs is associated with immunogenicity and protection against P. falciparum malaria

1522 is a nonimmunogenic conserved high-activity binding peptide (HABP) belonging to Plasmodium falciparum MSP-1 protein N-terminal fragment. The key amino acids in binding to red blood cells (RBC) were identified and replaced by others having similar mass but different charge. Because conserved HABPs are not antigenic nor immunogenic, immunogenicity and protectivity studies were then conducted on them in the Aotus monkey. 1H-NMR studies included the lead peptide 1522 as well as the analogs 9782, 13446, 13448, and 13442 to relate their structure to biological function. All the peptides presented alpha-helical structure, with differences observed in helix location and extension. The nonprotective 1522 peptide was totally helical from the N- to the C-terminus, very similar to nonprotective 13442 and 13448 peptides whose extension was almost totally helical. The 9782 and 13446 protective peptides, however, possessed a shorter helical region where modified critical binding residues were not included. A more flexible region was generated at the C-terminus in those peptides with a shorter helical region, leading to a greater number of conformers. These data suggest that peptide flexibility results in increased interaction with immune system molecules, generating protective immunity.

Constructing a useful tool for characterizing amino acid conformers by means of quantum chemical and graph theory indices

The aim of this work is to construct a tool to assist in the prediction of peptidic properties resulting from the exchange of two amino acids in a proteic chain. In the past others have used experimental properties for this purpose. However, the nature of these data sets severely limits their access to important properties pertaining to secondary structure, and hence the indices used cannot characterize different backbone conformers like alpha helix and beta strands, or side-chain conformations like gauche +, gauche - and trans. In this study we explore the importance of backbone and side-chain angles with regard to conformer similarity measured with theoretical properties calculated in an ab initio manner. For each of the 20 genetically encoded amino acids, we studied five conformers that correspond to alpha helical and beta strand structures, with three different side chain conformations for each, defined solely by their angles phi, psi and chi1. This methodology allowed each of the 108 conformers to be represented by a mathematical object without ambiguity. The peptidic chain was emulated using two capping models to simulate the effect of nearest neighbors. These are OHC-Xaa-NH2 and Ala-Xaa-Ala, where Xaa is the conformer of interest. We then calculated 40 ab initio quantum chemical and graph theory indices for each backbone-side-chain conformer to obtain a characterization and classification scheme. We found that: (1) while backbone structure is very important to conformer similarity, side-chain conformations do not cluster together in a top-level manner; (2) amino acids with pi electrons group together independent of backbone conformation.

Plasmodium falciparum acid basic repeat antigen (ABRA) peptides: erythrocyte binding and biological activity

Non overlapping 20-mer peptides, covering the complete sequence of acid basic repeat antigen (ABRA) of Plasmodium falciparum, were synthesised and tested in binding assays to erythrocytes. Five peptides localised in the N-terminal region coded 2148 (121LQSHKKLIKALKKNIESYQN(140)), 2149 (141KKHLIYKNKSYNPLLLSCVK(160)), 2150 (161KMNMLKENVDYIQKNQNLFK(180)), 2152 (201YKSQGHKKETSQNQNENNDN(220)) and 2153 (221QKYQEVNDEDDVNDEEDTND(240)) specifically bind to erythrocytes. These peptides bind independently of the peptide and erythrocyte charge, with high affinity (Kd between 70 and 180 nM) and the hydrophobic interaction is important for this binding ( approximately 30% hydrophobic critical residues). These results allow us define a specific erythrocyte binding region (residues 121-240), which may bound to at least three different binding sites on erythrocytes. Peptide 2153 shares the underlined sequence 221QKYQEVNDEDDVNDEEDTND(240) with an earlier 18-mer peptide recognised by human exposed sera. Peptides number 2148 and 2149 in vitro inhibit erythrocyte invasion by merozoites. We found that 2149 peptide and some of its glycine analogues show specific haemolytic and/or antimicrobial activity. We discuss a possible role of ABRA or its regions in the merozoite invasion of erythrocyte.