Characterization of a differential immunoscreen epitope of Plasmodium falciparum using combinatorial agents

Strain-transcending neutralization of malaria parasite by antibodies against Plasmodium falciparum enolase

Background

Plasmodium enolase is a target for the growth neutralizing antibodies. Interestingly, the three invasive stages i.e. sporozoites, merozoites, and ookinetes express this protein on their cell surface. Polyclonal anti-Plasmodium falciparum enolase (Pfeno) antibodies disrupt traversal of ookinete through mosquito mid-gut wall as well as have inhibitory effect on parasite growth at erythrocytic stage. In a recent study, it was observed that immunization with a unique epitope of parasite enolase (EWGWS) could confer partial protection against mouse malaria. Further validation is needed for the protective potential of this unique epitope in otherwise highly conserved enolase.

Methods

In order to investigate the efficacy of growth inhibitory potential of the epitope of P falciparum enolase, a monoclonal antibody specific to EWGWS is generated. In vitro parasite growth inhibition assays and passive immunization of Plasmodium yoelii (or Plasmodium berghei) infected mice were used to assess the parasite growth neutralizing activity of the antibody.

Results

Screening a panel of monoclonal antibodies raised against recombinant Pfeno that were specific to EWGWS resulted in isolation of H12E1. This antibody recognized only EWGWS epitope containing enolases. H12E1 strongly inhibited parasite growth in culture. This inhibition was strain transcending. Passive infusion of this antibody in P. yoelii or P. berghei infected mice showed significant reduction in parasitemia as compared to controls (p < 0.001). Surface Plasmon Resonance measurements indicated high affinity binding of H12E1 to P. falciparum enolase (K_D ~ 7.6 × 10⁻⁹M).

Conclusions

A monoclonal antibody directed against EWGWS epitope of Pfeno was shown to inhibit the growth of blood stage malarial parasites. This inhibition was species/strain transcending and is likely to arise due to blockade of enolase on the surface of merozoites, functionally implicating Pfeno in invasion related events. Presence of enolase on the cell surface of merozoites and ookinetes could potentially result in inhibition of host cell invasions at erythrocytic and transmission stages in the parasite life cycle. It is suggested that antibodies against EWGWS epitope have the potential to confer dual stage, species and strain transcending protection against malaria.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2455-6) contains supplementary material, which is available to authorized users.

Protective immunity induced by phage displayed mitochondrial related peptides of Schistosoma japonicum

New antigens and strategies are necessary for vaccine development against schistosomiasis japonica. Using a pool of 43 high titred anti-SWA sera from individuals residing in an Schistosoma japonicum endemic area of China, we have cloned a S. japonicum gene by cDNA library screening. The recombinant Sj338 protein has 44-46% identity to a mitochondrial precursor receptor protein of humans and rats. Immunization of mice with the recombinant Sj338 conferred 27-32% (p<0.01) reduction in worm burdens following cercarial challenge. In an effort to identify protective epitopes in Sj338 and increase the level of protection, we screened a random 12-mer peptide library constructed in M13 using a polyspecific anti-Sj338 rabbit serum. After five rounds of biopanning, we identified 30 reactive clones consisting of 11 distinct peptide sequences. These clones shared limited primary sequence homology with the recombinant Sj338 protein. Anti-sera raised against these phage clones recognized recombinant Sj338 and SWAP by Western blot. In murine vaccination experiments using whole recombinant phage without adjuvant, four of these clones demonstrated worm reductions of 11.6-25.1% (p=ns - 0.05) compared to M13 vaccinated animals. Animals vaccinated with all four of these phage demonstrated 34.2% (p<0.01) worm reduction compared to controls vaccinated with M13 clone. These data suggest that mimotope peptides are potential vaccine candidates for S. japonicum.

A protective merozoite protein of Plasmodium falciparum shares an epitope with surface antigens of Paramecium

A Plasmodium falciparum cDNA expression clone, lambdaPf9, had been identified earlier as a protective epitope, using anti-lambdaPf9 antibodies and combinatorial phagotopes. A segment of the Pf9 gene showed homology with Paramecium immobilization surface antigens such as 51B, 51A and 156G. A synthetic Pf9-peptide was designed from this region, and specific antibodies were raised. Each of these anti-Pf9 antibodies and combinatorial reagents, as well as anti-Paramecium 51B antibodies, recognized the Pf9-peptide on ELISA, and the same protein band in parasite immunoblots. The P. falciparum protein was released from the merozoite membrane fraction on treatment with PI-PLC, indicating the presence of a GPI anchor. Anti-Pf9-peptide antibodies specifically inhibited the growth of P. falciparum in culture. Immunofluorescence assays showed the reactivity of anti-Pf9-peptide sera with P. falciparum merozoites and gametocytes, as well as on the surface of Paramecium tetraurelia. The Pf9-peptide was able to induce proliferation of splenic lymphocytes obtained from mice infected with the rodent malarial parasites Plasmodium berghei and Plasmodium yoelii. These results point towards Plasmodium Pf9 as a conserved novel protective protein, sharing an epitope with Paramecium surface antigens.

The P domain of the P0 protein of Plasmodium falciparum protects against challenge with malaria parasites

Monoclonal antibodies (MAbs) specific for the P domain of the Plasmodium falciparum P0 phosphoriboprotein (PfP0) blocked the invasion of RBCs by P. falciparum. Vaccination with this P-domain peptide protected mice upon malaria parasite challenge. The absolute specificity of the MAbs and the PfP0 P peptide makes them potential protective malaria reagents.

High throughput immuno-screening of cDNA expression libraries produced by in vitro recombination; exploring the Plasmodium falciparum proteome

Improved Plasmodium falciparum cDNA expression libraries were constructed by combining mRNA oligo-capping with in vitro recombination and directional cloning of cDNA inserts into a plasmid vector that expresses sequences as thioredoxin fusion proteins. A novel procedure has also been developed for the rapid identification of seropositive clones on high-density filters, using direct labelling of P. falciparum immune immunoglobulin with fluorescein isothiocynate (FITC). This approach combines the advantages of recombination-assisted cDNA cloning with high throughput, non-radioactive serological screening of expression libraries. Production of replicate colony matrices allows the identification of antigens recognised by different pools with different specificities from residents of a malaria endemic region. Analyses of DNA sequences derived from sero-reactive colonies indicate that this is an effective method for producing recombinant proteins that react with antibodies from malaria-exposed individuals. This approach permits the systematic construction of a database of antigenic proteins recognised by sera from malaria-exposed individuals.