Three non-repeated transmission blocking epitopes recognized in the 21 kD surface antigen of zygotes-ookinetes of Plasmodium berghei

Effects of transmission-blocking vaccines simultaneously targeting pre- and post-fertilization antigens in the rodent malaria parasite Plasmodium yoelii

Background

Transmission-blocking vaccine (TBV) is a promising strategy for interrupting the malaria transmission cycle. Current TBV candidates include both pre- and post-fertilization antigens expressed during sexual development of the malaria parasites.

Methods

We tested whether a TBV design combining two sexual-stage antigens has better transmission-blocking activity. Using the rodent malaria model Plasmodium yoelii, we pursued a DNA vaccination strategy with genes encoding the gametocyte antigen Pys48/45 and the major ookinete surface protein Pys25.

Results

Immunization of mice with DNA constructs expression either Pys48/45 or Pys25 elicited strong antibody responses, which specifically recognized a ~45 and ~25 kDa protein from gametocyte and ookinete lysates, respectively. Immune sera from mice immunized with DNA constructs expressing Pys48/45 and Pys25 individually and in combination displayed evident transmission-blocking activity in in vitro ookinete culture and direct mosquito feeding experiments. With both assays, the Pys25 sera had higher transmission-blocking activity than the Pys48/45 sera. Intriguingly, compared with the immunization with the individual DNA vaccines, immunization with both DNA constructs produced lower antibody responses against individual antigens. The resultant immune sera from the composite vaccination had significantly lower transmission-blocking activity than those from Pys25 DNA immunization group, albeit the activity was substantially higher than that from the Pys48 DNA vaccination group.

Conclusions

This result suggested that vaccination with the two DNA constructs did not achieve a synergistic effect, but rather caused interference in inducing antigen-specific antibody responses. This result has important implications for future design of composite vaccines targeting different sexual antigens.

Identification of three ookinete-specific genes and evaluation of their transmission-blocking potentials in Plasmodium berghei

With a renewed hope for malaria elimination, interventions that prevent transmission of parasites from humans to mosquitoes have received elevated attention. Transmission-blocking vaccines (TBVs) targeting the sexual stages are well suited for this task. Here, through bioinformatic analysis, we selected two putative Plasmodium berghei ookinete-stage proteins (PBANKA_111920, and PBANKA_145770) and a previously characterized ookinete protein PBANKA_135340 (PSOP7) for evaluation of their transmission-blocking potentials. Fragments of these predicted proteins were expressed in bacteria and purified recombinant proteins were used to immunize mice. Antisera against these recombinant proteins recognized proteins of predicted sizes from ookinete lysates and localized their expression on the surface of ookinetes. Inclusion of these antisera in in vitro ookinete culture significantly inhibited ookinete formation. Mosquitoes fed on mice immunized with the recombinant proteins also showed significantly reduced oocyst densities (60.0-70.7%) and modest reductions of oocyst prevalence (10.7-37.4%). These data, together with the conservation of these genes in Plasmodium, suggest that these three ookinete proteins could be new promising targets for TBVs and are worth of future investigations in the human malaria parasites.

The glutathione biosynthetic pathway of Plasmodium is essential for mosquito transmission

Infection of red blood cells (RBC) subjects the malaria parasite to oxidative stress. Therefore, efficient antioxidant and redox systems are required to prevent damage by reactive oxygen species. Plasmodium spp. have thioredoxin and glutathione (GSH) systems that are thought to play a major role as antioxidants during blood stage infection. In this report, we analyzed a critical component of the GSH biosynthesis pathway using reverse genetics. Plasmodium berghei parasites lacking expression of gamma-glutamylcysteine synthetase (γ-GCS), the rate limiting enzyme in de novo synthesis of GSH, were generated through targeted gene disruption thus demonstrating, quite unexpectedly, that γ-GCS is not essential for blood stage development. Despite a significant reduction in GSH levels, blood stage forms of pbggcs⁻ parasites showed only a defect in growth as compared to wild type. In contrast, a dramatic effect on development of the parasites in the mosquito was observed. Infection of mosquitoes with pbggcs⁻ parasites resulted in reduced numbers of stunted oocysts that did not produce sporozoites. These results have important implications for the design of drugs aiming at interfering with the GSH redox-system in blood stages and demonstrate that de novo synthesis of GSH is pivotal for development of Plasmodium in the mosquito.

The antioxidant systems of malaria parasites (Plasmodium spp.) are potential targets for the development of antimalarials. The glutathione (GSH) redox system constitutes one of the Plasmodium primary lines of defense against damage caused by reactive oxygen species and other forms of chemical stress. GSH is synthesized de novo by the sequential action of gamma-glutamylcysteine synthase (γ-GCS) and GSH synthase (GS). Biochemical studies have suggested that parasite survival depends on functional de novo GSH synthesis. Using reverse genetics we interrupted the GSH biosynthetic pathway in the rodent malaria Plasmodium berghei by disrupting the pbggcs gene. The mutation caused minor changes in parasite growth in the mammalian host but development in the mosquito was completely arrested at the oocyst stage. These results suggest that the GSH biosynthetic pathway, while essential for mosquito stage development, is not an appropriate target for antimalarials against blood stages of the parasite.

Murine model for assessment of Plasmodium falciparum transmission-blocking vaccine using transgenic Plasmodium berghei parasites expressing the target antigen Pfs25

Currently, there is no animal model for Plasmodium falciparum challenge to evaluate malaria transmission-blocking vaccines based on the well-established Pfs25 target antigen. The biological activity of transmission-blocking antibodies is typically assessed using an assay known as the membrane feeding assay (MFA). It is an in vitro method that involves mixing antibodies with cultured P. falciparum gametocytes and feeding them to mosquitoes through an artificial membrane followed by assessment of infection in the mosquitoes. We genetically modified Plasmodium berghei to express Pfs25 and demonstrated that the transgenic parasites (TrPfs25Pb) are susceptible to anti-Pfs25 antibodies during mosquito-stage development. The asexual growth kinetics and mosquito infectivity of TrPfs25Pb were comparable to those of wild-type parasites, and TrPfs25Pb displayed Pfs25 on the surface of ookinetes. Immune sera from nonhuman primates immunized with a Pfs25-based vaccine when passively transferred to mice blocked transmission of TrPfs25Pb to Anopheles stephensi. Furthermore, mice immunized with Pfs25 DNA vaccine and challenged with TrPfs25Pb displayed reduced malaria transmission compared to mice immunized with wild-type plasmid. These studies describe development of an animal malaria model alternative to the in vitro MFA and show that the model can facilitate P. falciparum transmission-blocking vaccine evaluation based on the target antigen Pfs25. We believe that an animal model to test transmission-blocking vaccines would be superior to the MFA, since there may be additional immune factors that synergize the transmission-blocking activity of antibodies in vivo.

Gene expression in Plasmodium berghei ookinetes and early oocysts in a co-culture system with mosquito cells

Using an in vitro development system for Plasmodium berghei sporogonic stages and microarray technology we examined parasite gene expression during ookinete invasion of Aedes cells and the ensuing oocyst development. A number of genes were found to be differentially expressed. The most prominent class of up-regulated elements corresponded to products involved in protein synthesis and metabolism. Furthermore, several previously studied genes with a known in vivo developmental profile matched published data. A large number of genes with a hitherto unknown function during the life cycle stages studied also show a differential pattern of expression, indicating the involvement of their products in control and execution of active developmental processes.

Plasmodium berghei ookinetes bind to Anopheles gambiae and Drosophila melanogaster annexins

Using a proteomic approach we identified polypeptides from Anopheles gambiae and Drosophila melanogaster protein extracts that selectively bind purified Plasmodium berghei ookinetes in vitro; these were two and three distinct polypeptides, respectively, with an apparent molecular weight of about 36 kDa. Combining two-dimensional electrophoresis and MALDI-TOF (matrix-associated laser desorption ionization time of flight) mass spectrometry we determined that the polypeptides correspond to isomorphs of the annexin B11 protein of the fruit fly. When protein extracts derived from A. gambiae and D. melanogaster tissue culture cells were further fractionated, the binding activity matching the annexin protein could be localized in the fraction derived from cell membranes in both diptera. Antibody staining showed that annexin also binds to ookinetes during the invasion of the mosquito midgut. Finally, inclusion of antiannexin antisera in a mosquito blood meal impaired parasite development, suggesting a facilitating role for annexins in the infection of the mosquito by Plasmodium.

Distinct roles for pbs21 and pbs25 in the in vitro ookinete to oocyst transformation of Plasmodium berghei

We have developed an in vitro culture system for early sporogonic stages of Plasmodium berghei, which can be used to study developmental events normally taking place in the midgut of an infected mosquito. These include penetration of insect cells by the mature ookinete, transformation into oocysts and the early development of the latter, sustained through several rounds of nuclear division. The system, based upon co-culture of enriched ookinetes with several established insect cell lines, was used to study the development of mutant ookinetes lacking both the Pbs21 and Pbs25 surface proteins. Motility and entry of double knockout and Pbs21 single knockout ookinetes into the insect cells are normal, but the number of ookinetes successfully transforming into oocysts expressing the CSP protein are substantially reduced. Finally, using the yeast two-hybrid system we also show that Pbs25 has the capacity to homodimerise as well as to form heterodimers with Pbs21.

The roles of the glycosylphosphatidylinositol anchor on the production and immunogenicity of recombinant ookinete surface antigen Pbs21 of Plasmodium berghei when prepared in a baculovirus expression system

Malarial ookinetes express an immunodominant surface protein (P28) that is a priority candidate for the development of transmission-blocking vaccines. The full length P28 gene from Plasmodium berghei [Pbs21(1-213)] and a deletion construct [Pbs21(1-188)] encoding a protein that lacks the 25 C-terminal amino acids, including the glycosylphosphatidylinositol (GPI) anchor signal, were expressed in insect cells using baculovirus vectors. Pbs21(1-213) protein is strongly hydrophobic, found in the cytoplasm and on the surface of Spodoptera Sf21 cells, and in the culture medium. Pbs21(1-188) protein was largely found in the aqueous phase of the medium and in the cytoplasm of Sf21 cells, but was not detected on the cell surface. The presence of 25 C-terminal amino acids is therefore critical to the attachment of recombinant Pbs21 to the parasite plasma membrane. Mice were immunized subcutaneously or intramuscularly with affinity purified recombinant Pbs21(1-213), Pbs21(1-188) or native Pbs21 proteins. Following two immunizations, native Pbs21 induces higher titres when administered by either route, than the recombinant protein bearing an insect GPI anchor, which in turn is markedly more immunogenic than the recombinant polypeptide lacking a GPI anchor. When specific anti Pbs21 antibody titres exceeded 1 mg/ml all three antigens were capable of inducing transmission blockade > or = 90%, below 1 mg/ml blockade did not correlate with antibody concentration.

A single-chain antibody fragment specific for the Plasmodium berghei ookinete protein Pbs21 confers transmission blockade in the mosquito midgut

Mouse monoclonal antibody 13.1 (mAb 13.1) directed against Pbs21, a 21-kDa sexual-stage surface protein of Plasmodium berghei, is known to inhibit oocyst development from gametocytes and ookinetes in the mosquito midgut. To examine the properties and potential uses of a single-chain antibody fragment (scFv) for blocking transmission of malaria parasites to mosquitoes, we have cloned and sequenced the genes encoding variable regions of the immunoglobulin heavy and light chains (V(H) and V(L)) of mAb 13.1. The V(H) and V(L) genes were assembled as an scFv gene, and expressed in a baculovirus expression system. Following purification of 13.1 scFv, Western blotting and inhibition ELISA assays confirmed that 13.1 scFv retained the binding specificity of the parent mAb 13.1 for Pbs21. Furthermore, 13.1 scFv bound to the surface of P. berghei ookinetes, and blocked oocyst development in the mosquito midgut by at least 93%, as assessed by oocyst counts in mosquitoes. We suggest that the 13.1 scFv gene could be useful not only in studying the mechanism of transmission blockade, but also in generating, by mosquito germline transformation, a model system to evaluate the production of mosquitoes refractory to malaria.

Studies on the immunogenicity of a recombinant ookinete surface antigen Pbs21 from Plasmodium berghei expressed in Escherichia coli

Plasmodium berghei ookinete surface antigen (Pbs21), was produced as a fusion product with maltose binding protein (MBP) in Escherichia coli and used to induce transmission-blocking immunity in mice. Specificity of induced antibody was confirmed by Western blotting with native ookinete Pbs21, and by the indirect immunofluorescent antibody test on ookinete bloodfilms. Immunized mice were infected with P. berghei and transmission to Anopheles stephensi mosquitoes determined by both the intensity and prevalence of oocyst infections. Compared with a control group immunized with MBP alone the maximum blockade of oocyst intensity was 66% in the mice immunized with recombinant MBP-Pbs21. Over nine experiments blockade averaged only 33%. By comparison with native Pbs21 protein, which usually induces > or = 90% blockade, our data suggests the recombinant protein produced in this bacterial system is a less effective immunogen despite expressing epitopes recognized by known transmission-blocking monoclonal antibodies.

Structure and expression of a post-transcriptionally regulated malaria gene encoding a surface protein from the sexual stages of Plasmodium berghei

The sexual stage-specific protein Pbs21 of the rodent malaria parasite Plasmodium berghei, expressed on the surface of zygotes and ookinetes, has been shown to induce an effective and long-lasting transmission blocking immunity. The gene encoding Pbs21 was cloned by screening a cDNA library prepared from enriched zygotes and ookinetes using the monoclonal antibody 13.1.15, which is capable of blocking subsequent parasite sexual development in the mosquito vector. The Pbs21 gene encoded a protein of 213 amino acids which contained a putative amino-terminal signal sequence and a putative carboxy-terminal hydrophobic membrane anchor. The amino-acid sequence was characterised by a large number of cysteine residues which were organized into 4 epidermal growth factor-like domains. The spacing of the cysteine residues was highly conserved when compared to the 25-kDa ookinete proteins of Plasmodium falciparum (Pfs25), Plasmodium reichenowi (Prs25) and Plasmodium gallinaceum (Pgs25) which were approximately 45%, 45% and 40% homologous to Pbs21 respectively. The gene is located on chromosome 5 and cross-hybridizes to a similarly defined gene unit in the other rodent malaria species Plasmodium chabaudi, Plasmodium vinckei and Plasmodium yoelii. The gene is internally disposed and not in the subtelomeric region of chromosome 5. The gene is transcribed in a stage-specific manner giving rise to an abundant 1.5-kb transcript. This mRNA is synthesised in the precursor cells to female gametes (gametocytes) however the protein is observed only after activation of the gametes, suggesting that translation of the mRNA is controlled by a post-transcriptional process. The Pbs21 gene and the P. berghei parasite system provide an excellent vehicle for the study of stage-specific transcriptional and post-transcriptional control in malaria.