Structural characterisation of sporozoite components for a multistage, multi-epitope, anti-malarial vaccine

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.

IMPIPS: the immune protection-inducing protein structure concept in the search for steric-electron and topochemical principles for complete fully-protective chemically synthesised vaccine development

Determining immune protection-inducing protein structures (IMPIPS) involves defining the stereo-electron and topochemical characteristics which are essential in MHC-p-TCR complex formation. Modified high activity binding peptides (mHABP) were thus synthesised to produce a large panel of IMPIPS measuring 26.5 ±3.5Å between the farthest atoms fitting into Pockets 1 to 9 of HLA-DRβ1* structures. They displayed a polyproline II-like (PPII_L) structure with their backbone O and N atoms orientated to establish H-bonds with specific residues from HLA-DRβ1*-peptide binding regions (PBR). Residues having specific charge and gauche⁺ orientation regarding p3χ1, p5χ2, and p7χ1 angles determined appropriate rotamer orientation for perfectly fitting into the TCR to induce an appropriate immune response. Immunological assays in Aotus monkeys involving IMPIPS mixtures led to promising results; taken together with the aforementioned physicochemical principles, non-interfering, long-lasting, protection-inducing, multi-epitope, multistage, minimal subunit-based chemically-synthesised peptides can be designed against diseases scourging humankind.

3D structure and immunogenicity of Plasmodium falciparum sporozoite induced associated protein peptides as components of fully-protective anti-malarial vaccine

SIAP-1 and SIAP-2 are proteins which are implicated in early events involving Plasmodium falciparum infection of the Anopheles mosquito vector and the human host. High affinity HeLa and HepG2 cell binding conserved peptides have been previously identified in these proteins, i.e. SIAP-1 34893 ((421)KVQGLSYLLRRKNGTKHPVY(440)) and SIAP-1 34899 ((541)YVLNSKLLNSRSFDKFKWIQ(560)) and SIAP-2 36879 ((181)LLLYSTNSEDNLDISFGELQ(200)). When amino acid sequences have been properly modified (replacements shown in bold) they have induced high antibody titres against sporozoites in Aotus monkeys (assessed by IFA) and in the corresponding recombinant proteins (determined by ELISA and Western blot). (1)H NMR studies of these conserved native and modified high activity binding peptides (HABPs) revealed that all had α-helical structures in different locations and lengths. Conserved and corresponding modified HABPs displayed different lengths between the residues fitting into MHCII molecule pockets 1-9 and different amino acid orientation based on their different HLA-DRβ1(∗) binding motifs and binding registers, suggesting that such modifications were associated with making them immunogenic. The results suggested that these modified HAPBs could be potential targets for inclusion as components of a fully-effective, minimal sub-unit based, multi-epitope, and multistage anti-malarial vaccine.

The role of amino acid electron-donor/acceptor atoms in host-cell binding peptides is associated with their 3D structure and HLA-binding capacity in sterile malarial immunity induction

Plasmodium falciparum malaria continues being one of the parasitic diseases causing the highest worldwide mortality due to the parasite's multiple evasion mechanisms, such as immunological silence. Membrane and organelle proteins are used during invasion for interactions mediated by high binding ability peptides (HABPs); these have amino acids which establish hydrogen bonds between them in some of their critical binding residues. Immunisation assays in the Aotus model using HABPs whose critical residues had been modified have revealed a conformational change thereby enabling a protection-inducing response. This has improved fitting within HLA-DRβ1(∗) molecules where amino acid electron-donor atoms present in β-turn, random or distorted α-helix structures preferentially bound to HLA-DR53 molecules, whilst HABPs having amino acid electron-acceptor atoms present in regular α-helix structure bound to HLA-DR52. This data has great implications for vaccine development.

Binding activity, structure, and immunogenicity of synthetic peptides derived from Plasmodium falciparum CelTOS and TRSP proteins

Several sporozoite proteins have been associated with Plasmodium falciparum cell traversal and hepatocyte invasion, including the cell-traversal protein for ookinetes and sporozoites (CelTOS), and thrombospondin-related sporozoite protein (TRSP). CelTOS and TRSP amino acid sequences have been finely mapped to identify regions specifically binding to HeLa and HepG2 cells, respectively. Three high-activity binding peptides (HABPs) were found in CelTOS and one HABP was found in TRSP, all of them having high α-helical structure content. These HABPs' specific binding was sensitive to HeLa and HepG2 cells' pre-treatment with heparinase I and chondroitinase ABC. Despite their similarity at three-dimensional (3D) structural level, TRSP and TRAP HABPs located in the TSR domain did not compete for the same binding sites. CelTOS and TRSP HABPs were used as a template for designing modified sequences to then be assessed in the Aotus monkey experimental model. Antibodies directed against these modified HABPs were able to recognize both the native parasite protein by immunofluorescence assay and the recombinant protein (expressed in Escherichia coli) by Western blot and ELISA assays. The results suggested that these modified HABPs could be promising targets in designing a fully effective, antimalarial vaccine.

Synthetic peptides from two Pf sporozoite invasion-associated proteins specifically interact with HeLa and HepG2 cells

Two recently described molecules have been associated with sporozoite traversal ability and hepatocyte entry: sporozoite invasion-associated proteins (SIAP)-1 and -2. The HeLa and HepG2 cell binding ability of synthetic peptides spanning the whole SIAP-1 and -2 sequences has been studied in the search for identifying these proteins' functionally active specific regions. Twelve HepG-2 and seventeen HeLa cell high-activity binding peptides (HABPs) have been identified in SIAP-1, 8 of them having high specific binding affinity for both cell lines. Four HepG2 HABPs and two HeLa HABPs have been identified in SIAP-2, one of them interacting with both HeLa and HepG2 cells. SIAP-1 and SIAP-2 HABPs bound specifically and saturably to heparin sulfate and chondroitin sulfate-type membrane receptors on host cells. Circular dichroism assays have shown high α-helix content in SIAP-1 and SIAP-2 HABP secondary structure. Immunofluorescence analysis has revealed that specific peptides against SIAP proteins are highly immunogenic in mice and that anti-SIAP-1 and -2 antibodies recognize the native protein in Plasmodium falciparum sporozoites. Polymorphism studies have shown that a most SIAP-1 and -2 HABPs are conserved among P. falciparum strains. Our results have suggested that SIAP-1 and -2 participate in host-pathogen interactions during cell-traversal and hepatocyte invasion and highlighted the relevance of the ongoing identification and study of potentially new molecules when designing a fully protective antimalarial vaccine.

Electrostatic potential as a tool to understand interactions between malaria vaccine candidate peptides and MHC II molecules

One of the most important problems in vaccine development consists in understanding receptor-ligand interactions between Class II Major Histocompatibility Complex molecules (MHC II) and antigenic peptides involved in inducing an appropriate immune response. In this study, we used X-ray crystallography structural data provided by the HLA-DRβ1*0301-CLIP peptide interaction to compare native non-immunogenic and specifically-modified immunogenic peptides derived from the malarial SALSA protein, by analyzing molecular electrostatic potential surfaces on the most important regions of the peptide binding groove (Pockets 1, 4, 6 and 9). Important differences were found on the electrostatic potential induced by these peptides, particularly in MHC II conserved residues: Qα9, Sα53, Nα62, Nα69, Yβ30, Yβ60, Wβ61, Qβ70, Kβ71 and Vβ86, the same ones involved in establishing hydrogen bonds between Class II molecule-peptide and the recognition by T cell receptor, it correlating well with the change in their immunological properties. The results clearly suggest that modifications done on the electrostatic potential of these amino acids could favor the induction of different immune responses and therefore, their identification could allow modifying peptides a priori and in silico, so as to render them into immunogenic and protection-inducers and hence suitable components of a chemically-synthesized, multi-antigenic, minimal subunit based vaccine.

Biological and structural characteristics of the binding peptides from the sporozoite proteins essential for cell traversal (SPECT)-1 and -2

The sporozoite microneme proteins essential for cell traversal, SPECT-1 and SPECT-2, are considered attractive pre-erythrocytic immune targets due to the key role they play in crossing of the malaria parasite across the dermis and the liver sinusoidal wall, prior to invasion of hepatocytes. In this study, the sequences of SPECT-1 and SPECT-2 were mapped using 20 mer-long synthetic peptides to identify high-activity binding peptides (HABPs) to HeLa cells. 17 HABPs with enzyme sensitive bindings to HeLa cells were identified: 3 predominantly α-helical in SPECT-1, and 10 α-helical and 4 β-turns/random coils in SPECT-2. Immunofluorescence assays (IFA) with antibodies raised in rabbits against chemically synthesized B-cell epitopes suggests the presence of these two proteins in the micronemes and in sporozoite membrane. (1)H NMR studies showed that HABPs located in the membrane-attack complex/perforin (MACPF) domain of SPECT-2 share high similarity with the 3D structure of C8α. Altogether, the results highlight the potential of including HABPs from SPECT-1 and SPECT-2 as components of a fully effective multistage, multiepitopic, minimal subunit-based synthetic vaccine against Plasmodium falciparum malaria.

The nature and combination of subunits used in epitope-based Schistosoma japonicum vaccine formulations affect their efficacy

Background

Schistosomiasis remains a major public health problem in endemic countries and is caused by infections with any one of three primary schistosome species. Although there are no vaccines available to date, this strategy appears feasible since natural immunity develops in individuals suffering from repeated infection during a lifetime. Since vaccinations resulting in both Th1- and Th2-type responses have been shown to contribute to protective immunity, a vaccine formulation with the capacity for stimulating multiple arms of the immune response will likely be the most effective. Previously we developed partially protective, single Th- and B cell-epitope-based peptide-DNA dual vaccines (PDDV) (T3-PDDV and B3-PDDV, respectively) capable of eliciting immune responses against the Schistosoma japonicum 22.6 kDa tegument antigen (Sj22.6) and a 62 kDa fragment of myosin (Sj62), respectively.

Results

In this study, we developed PDDV cocktails containing multiple epitopes of S. japonicum from Sj22.6, Sj62 and Sj97 antigens by predicting cytotoxic, helper, and B-cell epitopes, and evaluated vaccine potential in vivo. Results showed that mice immunized with a single-epitope PDDV elicited either Tc, Th, or B cell responses, respectively, and mice immunized with either the T3- or B3- single-epitope PDDV formulation were partially protected against infection. However, mice immunized with a multicomponent (3 PDDV components) formulation elicited variable immune responses that were less immunoprotective than single-epitope PDDV formulations.

Conclusions

Our data show that combining these different antigens did not result in a more effective vaccine formulation when compared to each component administered individually, and further suggest that immune interference resulting from immunizations with antigenically distinct vaccine targets may be an important consideration in the development of multicomponent vaccine preparations.

3D structure determination of STARP peptides implicated in P. falciparum invasion of hepatic cells

To block the different stages of Plasmodium falciparum invasion into human hepatocytes and red blood cells, we have focused on those proteins belonging to the pre-erythrocytic stage. One of these proteins is Sporozoite Threonine and Asparagine Rich Protein (STARP), which is a ligand used by P. falciparum parasites to bind Hepatic cells (HepG2). Previous studies on this protein identified two conserved peptides binding with high activity to HepG2 cells (namely 20546 and 20570) with corresponding critical hepatic-cell binding residues and determined an important role for these two peptides in the invasion process. This study shows the results of immunization trials in Aotus monkeys with native STARP peptides and analogues modified in critical hepatic-cell binding residues. The results show that native peptides are not immunogenic but can induce high-antibody titers when their critical residues are replaced by other with similar volume and mass but different polarity. Nuclear Magnetic Resonance ((1)H NMR) studies revealed that native peptides (non-immunogenic) displayed shorter alpha-helical regions compared to their highly immunogenic modified analogues. Binding assays with HLA-DRbeta1* molecules showed that 20546 modified peptides inducing high-antibody titers (24972, 24320 and 24486) bound to HLA-DRbeta1*0301 molecules, while the 20570 modified analogue (24322) bound to HLA-DRbeta1*0101. The results support including these high-immunogenic STARP-derived modified peptides as pre-erythrocytic candidates to be included in the design of a synthetic antimalarial vaccine.

Identification of conserved erythrocyte binding regions in members of the Plasmodium falciparum Cys6 lipid raft-associated protein family

Detergent-resistant lipid raft membrane-associated Pf12, Pf38 and Pf41 proteins belong to the Cys(6) family, whose members are implicated in Plasmodium falciparum invasion to erythrocytes. We have analyzed the interaction between 20-mer-long synthetic peptides spanning the entire Pf12, Pf38 and Pf41 sequences and erythrocytes. Eight high-activity binding peptides (HABPs) were identified in these proteins, which presented saturable bindings susceptible to erythrocytes' enzymatic treatment, and beta-turn, random coil and alpha-helical elements as principal structural features. Some of these HABPs inhibited merozoite invasion in vitro, suggesting a possible role of Pf12, Pf38 and Pf41 during erythrocyte invasion and supporting their inclusion in the design of a fully effective antimalarial vaccine.