Based on HLA-DRβ1* Allele Binding Specificities, Striking Differences in Distance and TCR Contacting Residue Orientation can be Observed in Modified Protection-Inducing Malarial Synthetic Peptides

Abstract

An anti-malarial vaccine is urgently needed, especially against P. falciparum which causes 2 to 3 million deaths each year, mostly in Sub-Saharan African children. This vaccine should contain molecules from the parasite's different developmental stages due to the parasite's remarkable complexity and genetic variability. The first approach using synthetic peptides from different parasite stage molecules (the SPf66 malaria vaccine) conferred limited protective efficacy in Aotus monkeys and in large field-trials carried out in different parts of the world SPf66 contains red blood cell (RBC) binding merozoite peptides for which immune responses against them are genetically controlled by HLA-DR region. Therefore, a systematic search of conserved high activity binding peptides (HABP) was undertaken aimed at using them as immunogens. However, these peptides were poorly immunogenic and had poor protection-inducing capacity against experimental challenge with a P. falciparum strain highly infective for Aotus monkeys an experimental model with an immune system quite similar to humans. Modifications were thus made to key residues to render them immunogenic and protection-inducing. These native and modified HABPs' three-dimensional structure was determined by (1)H-NMR studies and their ability in forming stable Major Histocompatibility Class II - peptide (MHCII-peptide) complexes was correlated with their ability to bind in vitro to purified HLA-DR beta1* molecules. Our experimental data suggests a correlation between modified HABPs' three-dimensional structure, HLA-DR beta1* binding preferences and their protection-inducing capacity in monkeys. Furthermore, the data presented here indicates that a synthetic peptide vaccine's three-dimensional structural features dictate both HLA-DR beta1* allele binding preference (imposing genetic restriction on the immune response) and on these vaccines' protection-inducing value. Basic knowledge of a parasite's functionally active peptides, their 3D structure and their interaction for forming the MHC II- peptide-TCR complex will thus contribute towards designing fully effective multi-component, multi-stage subunit-based malarial vaccines.

Functional, Immunological and Three-Dimensional Analysis of Chemically Synthesised Sporozoite Peptides as Components of a Fully-Effective Antimalarial Vaccine

“…One of the most important observations to emerge from such study has been that conserved HABPs have poor antigenicity, poor immunogenicity and poor or no protection-inducing activity, thereby making them silent to a host's immune system. Our results have also shown that each specifically identified HABP's immunological properties have been directly related to its 3D structure [5,14,16,71,106]. This has suggested a strong correlation between binding (functional approach), 3D structure and immune response (biological activity).…”

“…Our results with merozoite-modified HABPs has shown that such modifications have resulted in better presentation and binding to MHC molecules and the formation of a more stable TCRpeptide-MHC (TCR-p-MHC) complex, highlighting these specifically-modified HABPs potential as components of a fully-effective antimalarial vaccine [5,14,16,84,106].…”

“…Such modifications have specifically affected their critical binding residues' polarity but have conserved the same volume and mass [5,10,14,16,71,78,84,85,87,145,158]. This feature has been closely related to conserved HABPs' 3D structure which may be specifically modified to enable modified HABPs' enhanced interaction with immune system molecules and facilitate stable Class II molecule-peptide-T-cell receptor (MHCIIpeptide-TCR) formation [106]. Establishing this trimer complex has been vital for inducing an effective immune response, as we have very recently shown at atom level [159].…”

“…This has also been based on in-depth molecular analysis of fifty of the most relevant proteins involved in RBC invasion by recognising their conserved HABPs [10], where their critical binding residues should be modified, as well as the 1 H-NMR 3D structure of 90 native and modified HABPs to unravel their atomic characteristics [16] in MHC-peptide-TCR complex formation to render them highly immunogenic, protection-inducers [106]. This work has led to a large number of modified HABPs having been identified which have been able to induce sterilising immunity against this stage of P. falciparum malaria when infection has been due to the most virulent Aotus monkey-adapted P. falciparum strain [5,10,14,16].…”

Emerging Rules for Subunit-Based, Multiantigenic, Multistage Chemically Synthesized Vaccines

“…41,44,[50][51][52][53][54][55][56][57][58] This distance was 6.5 ( 0.5 Å and 4.5 ( 1.5 Å shorter in short-lived and long-lasting antibody-inducing but non-protection-inducing modified HABPs, respectively, than in immunogenic, protection-inducing ones; residue orientation was also different. 42,43 In essence, immunogenic protection-inducing modified conserved HABPs have been modified so that they can fit perfectly into the MHC II-peptide-TCR complex for triggering an appropriate immune response, providing tremendous support for using chemically synthesized, specifically modified conserved HABPs in vaccine development.…”