TCR-contacting residues orientation and HLA-DRß* binding preference determine long-lasting protective immunity against malaria.

Fully-protective, long-lasting, immunological (FPLLI) memory against Plasmodium falciparum malaria regarding immune protection-inducing protein structures (IMPIPS) vaccinated into monkeys previously challenged and re-challenged 60 days later with a lethal Aotus monkey-adapted P. falciparum strain was found to be associated with preferential high binding capacity to HLA-DRß1* allelic molecules of the major histocompatibility class II (MHC-II), rather than HLA-DRß3*, ß4*, ß5* alleles. Complete PPIIL 3D structure, a longer distance (26.5 Å ± 1.5 Å) between residues perfectly fitting into HLA-DRß1*PBR pockets 1 and 9, a gauche(-) rotamer orientation in p8 TCR-contacting polar residue and a larger volume of polar p2 residues was also found. This data, in association with previously-described p3 and p7 apolar residues having gauche(+) orientation to form a perfect MHC-II-peptide-TCR complex, determines the stereo-electronic and topochemical characteristics associated with FPLLI immunological memory.

Protecting capacity against malaria of chemically defined tetramer forms based on the Plasmodium falciparum apical sushi protein as potential vaccine components.

Developing novel generations of subunit-based antimalarial vaccines in the form of chemically-defined macromolecule systems for multiple antigen presentation represents a classical problem in the field of vaccine development. Many efforts involving synthesis strategies leading to macromolecule constructs have been based on dendrimer-like systems, the condensation of large building blocks and conventional asymmetric double dimer constructs, all based on lysine cores. This work describes novel symmetric double dimer and condensed linear constructs for presenting selected peptide multi-copies from the apical sushi protein expressed in Plasmodium falciparum. These molecules have been proved to be safe and innocuous, highly antigenic and have shown strong protective efficacy in rodents challenged with two Plasmodium species. Insights into systematic design, synthesis and characterisation have led to such novel antigen systems being used as potential platforms for developing new anti-malarial vaccine candidates.

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.

Shedding Light on the Role of the Skin in Vaccine-Induced Protection against the Malaria Sporozoite.

The most advanced vaccine against Plasmodium falciparum malaria, RTS,S/AS01, provides partial protection in infants and children living in areas of malaria endemicity. Further understanding its mechanisms of protection may allow the development of improved second-generation vaccines. The RTS,S/AS01 vaccine targets the sporozoites injected by mosquito vectors into the dermis which then travel into the blood stream to establish infection in the liver. Flores-Garcia et al. (Y. Flores-Garcia, G. Nasir, C. S. Hopp, C. Munoz, et al., mBio 9:e02194-18, 2018, https://doi.org/10.1128/mBio.02194-18) shed light on early protective responses occurring in the dermis in immunized animals. They demonstrated that immunization impairs sporozoite motility and entry into blood vessels. Furthermore, they established that challenge experiments performed using a dermal route conferred greater protection than intravenous challenge in immunized mice. Thus, the dermal challenge approach captures the additional protective mechanisms occurring in the dermis that reflect the natural physiology of infection. Those studies highlighted the fascinating biology of skin-stage sporozoites and provided additional insights into vaccine-induced protection.

On the Evolution and Function of Plasmodium vivax Reticulocyte Binding Surface Antigen (pvrbsa).

The RBSA protein is encoded by a gene described in Plasmodium species having tropism for reticulocytes. Since this protein is antigenic in natural infections and can bind to target cells, it has been proposed as a potential candidate for an anti-Plasmodium vivax vaccine. However, genetic diversity (a challenge which must be overcome for ensuring fully effective vaccine design) has not been described at this locus. Likewise, the minimum regions mediating specific parasite-host interaction have not been determined. This is why the rbsa gene's evolutionary history is being here described, as well as the P. vivax rbsa (pvrbsa) genetic diversity and the specific regions mediating parasite adhesion to reticulocytes. Unlike what has previously been reported, rbsa was also present in several parasite species belonging to the monkey-malaria clade; paralogs were also found in Plasmodium parasites invading reticulocytes. The pvrbsa locus had less diversity than other merozoite surface proteins where natural selection and recombination were the main evolutionary forces involved in causing the observed polymorphism. The N-terminal end (PvRBSA-A) was conserved and under functional constraint; consequently, it was expressed as recombinant protein for binding assays. This protein fragment bound to reticulocytes whilst the C-terminus, included in recombinant PvRBSA-B (which was not under functional constraint), did not. Interestingly, two PvRBSA-A-derived peptides were able to inhibit protein binding to reticulocytes. Specific conserved and functionally important peptides within PvRBSA-A could thus be considered when designing a fully-effective vaccine against P. vivax.

Gauche(+) side-chain orientation as a key factor in the search for an immunogenic peptide mixture leading to a complete fully protective vaccine.

Topological and stereo-electron characteristics are essential in major histocompability class II-peptide-T-cell receptor (MHC-p-TCR) complex formation for inducing an appropriate immune response. Modified high activity binding peptides (mHABPs) were synthesised for complete full protection antimalarial vaccine development producing a large panel of individually fully protection-inducing protein structures (FPIPS) and very high long-lasting antibody-inducing (VHLLAI) mHABPs. Most of those which did not interfere, compete, inhibit or suppress their individual VHLLAI or FPIPS activity contained or displayed a polyproline II-like (PPIIL) structure when mixed. Here we show that amino acid side-chains located in peptide binding region (PBR) positions p3 and p7 displayed specific electron charges and side-chain gauche(+) orientation for interacting with the TCR. Based on the above, and previously described physicochemical principles, non-interfering, long-lasting, full protection-inducing, multi-epitope, multistage, minimal subunit-based chemically synthesised mHABP mixtures can be designed for developing vaccines against diseases scourging humankind, malaria being one of them.