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

Evaluation of transmission-blocking potential of PvPSOP25 using transgenic murine malaria parasite and clinical isolates

BACKGROUND

Malaria transmission-blocking vaccines (TBVs) aim to inhibit malaria parasite development in mosquitoes and prevent further transmission to the human host. The putative-secreted ookinete protein 25 (PSOP25), highly conserved in Plasmodium spp., is a promising TBV target. Here, we investigated PvPSOP25 from P. vivax as a TBV candidate using transgenic murine parasite P. berghei and clinical P. vivax isolates.

METHODS AND FINDINGS

A transgenic P. berghei line expressing PvPSOP25 (TrPvPSOP25Pb) was generated. Full-length PvPSOP25 was expressed in the yeast Pichia pastoris and used to immunize mice to obtain anti-rPvPSOP25 sera. The transmission-blocking activity of the anti-rPvPSOP25 sera was evaluated through in vitro assays and mosquito-feeding experiments. The antisera generated by immunization with rPvPSOP25 specifically recognized the native PvPSOP25 antigen expressed in TrPvPSOP25Pb ookinetes. In vitro assays showed that the immune sera significantly inhibited exflagellation and ookinete formation of the TrPvPSOP25Pb parasite. Mosquitoes feeding on mice infected with the transgenic parasite and passively transferred with the anti-rPvPSOP25 sera showed a 70.7% reduction in oocyst density compared to the control group. In a direct membrane feeding assay conducted with five clinical P. vivax isolates, the mouse anti-rPvPSOP25 antibodies significantly reduced the oocyst density while showing a negligible influence on mosquito infection prevalence.

CONCLUSIONS

This study supported the feasibility of transgenic murine malaria parasites expressing P. vivax antigens as a useful tool for evaluating P. vivax TBV candidates. Meanwhile, the moderate transmission-reducing activity of the generated anti-rPvPSOP25 sera necessitates further research to optimize its efficacy.

Evaluation of different types of adjuvants in a malaria transmission-blocking vaccine

Adjuvants are critical components for vaccines, which enhance the strength and longevity of the antibody response and influence the types of immune response. Limited research has been conducted on the immunogenicity and protective efficacy of various adjuvants in malaria transmission-blocking vaccines (TBVs). In this study, we formulated a promising TBV candidate antigen, the P. berghei ookinete surface antigen PSOP25, with different types of adjuvants, including the TLR4 agonist monophosphoryl lipid A (MPLA), the TLR9 agonist cytosine phosphoguanosine oligodeoxynucleotides (CpG ODN 1826) (CpG), a saponin adjuvant QS-21, aluminum hydroxide (Alum), and two combination adjuvants MPLA + QS-21 and QS-21 + CpG. We demonstrated that adjuvanted vaccines results in elevated elicited antibody levels, increased proliferation of plasma cells, and efficient formation of germinal centers (GCs), leading to enhanced long-term protective immune responses. Furthermore, CpG group exhibited the most potent inhibition of ookinete formation and transmission-blocking activity. We found that the rPSOP25 with CpG adjuvant was more effective than MPLA, QS-21, MPLA + QS-21, QS-21 + CpG adjuvants in dendritic cells (DCs) activation and differentiation. Additionally, the CpG adjuvant elicited more rubust immune memory response than Alum adjuvant. CpG and QS-21 adjuvants could activate the Th1 response and promote the secretion of IFN-γ and TNF-α. PSOP25 induced a higher number of Tfh cells in splenocytes when combined with MPLA, CpG, and QS-21 + CpG; and there was no increase in these cell populations when PSOP25 was administered with Alum. In conclusion, CpG may confer enhanced efficacy for the rPSOP25 vaccine, as evidenced by the ability of the elicited antisera to induce protective immune responses and improved transmission-blocking activity.

Functional characterization of a conserved membrane protein, Pbs54, involved in gamete fertilization in Plasmodium berghei

The successful completion of gamete fertilization is essential for malaria parasite transmission, and this process can be targeted by intervention strategies. In this study, we identified a conserved gene (PBANKA_0813300) in the rodent malaria parasite Plasmodium berghei, which encodes a protein of 54 kDa (designated as Pbs54). Localization studies indicated that Pbs54 is associated with the plasma membranes of gametes and ookinetes. Functional studies by gene disruption showed that the Δpbs54 parasites had no defect in asexual proliferation, gametocyte development, or gametogenesis. However, the interactions between male and female gametes were significantly decreased compared with wild-type parasites. The Δpbs54 lines did not show a further reduction in zygote and ookinete numbers during in vitro culture, indicating that the defects were probably restricted to gamete fertilization. Consistent with this finding, mosquitoes fed on Δpbs54-infected mice showed a 30.1% reduction in infection prevalence and a 74.7% reduction in oocyst intensity. Cross-fertilization assay indicated that both male and female gametes were impaired in the Δpbs54 parasites. To evaluate its transmission-blocking potential, we obtained polyclonal antibodies from mice immunized with the recombinant Pbs54 (rPbs54) protein. In vitro assays showed that anti-rPbs54 sera inhibited ookinete formation by 42.7%. Our experiments identified Pbs54 as a fertility factor required for mosquito transmission and a novel candidate for a malaria transmission-blocking vaccine.

Structural definition of babesial RAP-1 proteins identifies a novel protein superfamily across Apicomplexa

Apicomplexan protozoa are intracellular parasites of medical and economic importance. These parasites contain specialized apical complex organelles, including rhoptries, that participate in the process of host cell invasion. Conserved antigens expressed in the rhoptries are rational vaccine targets, but whether conservation of protein structure is a functional requirement for invasion remains unknown. Novel protein structural modeling enables identification of structurally conserved protein families that are not evident by sequence analysis alone. Here we show by AlphaFold2 structural modeling that the rhoptry-associated protein 1 superfamily of the Piroplasmida hemoparasites Babesia and Theileria (pRAP-1) is structurally conserved, with the core conserved region being composed of a globin-like and a 4-helix bundle subdomain. Search for structurally related members of this protein family in other apicomplexan parasites revealed structural homologues of pRAP-1 in several species of Plasmodium, Toxoplasma gondii and other members of the Sarcocystidae family. Based on these structural findings, pRAP-1 is a conserved apical complex protein, but whether these proteins share functional features in different species remains unknown. Identification of widely conserved elements involved in infection in these parasites will enhance our knowledge of invasion mechanisms, and facilitate the design of methods for controlling diseases that affect humans and animals globally.

A nanoparticle vaccine displaying the ookinete PSOP25 antigen elicits transmission-blocking antibody response against Plasmodium berghei

BACKGROUND

Safe and effective vaccines are crucial for the control and eventual elimination of malaria. Novel approaches to optimize and improve vaccine efficacy are urgently required. Nanoparticle-based delivery platforms are considered potent and powerful tools for vaccine development.

METHODS

In this study, we developed a transmission-blocking vaccine against malaria by conjugating the ookinete surface antigen PSOP25 to the Acinetobacter phage coat protein AP205, forming virus-like particles (VLPs) using the SpyTag/SpyCatcher adaptor system. The combination of AP205-2*SpyTag with PSOP25-SpyCatcher resulted in the formation of AP205-PSOP25 complexes (VLP-PSOP25). The antibody titers and avidity of serum from each immunization group were assessed by ELISA. Western blot and IFA were performed to confirm the specific reactivity of the elicit antisera to the native PSOP25 in Plasmodium berghei ookinetes. Both in vitro and in vivo assays were conducted to evaluate the transmission-blocking activity of VLP-PSOP25 vaccine.

RESULTS

Immunization of mice with VLP-PSOP25 could induced higher levels of high-affinity antibodies than the recombinant PSOP25 (rPSOP25) alone or mixtures of untagged AP205 and rPSOP25 but was comparable to rPSOP25 formulated with alum. Additionally, the VLP-PSOP25 vaccine enhanced Th1-type immune response with remarkably increased levels of IgG2a subclass. The antiserum generated by VLP-PSOP25 specifically recognizes the native PSOP25 antigen in P. berghei ookinetes. Importantly, antisera generated by inoculation with the VLP-PSOP25 could inhibit ookinete development in vitro and reduce the prevalence of infected mosquitoes or oocyst intensity in direct mosquito feeding assays.

CONCLUSIONS

Antisera elicited by immunization with the VLP-PSOP25 vaccine confer moderate transmission-reducing activity and transmission-blocking activity. Our results support the utilization of the AP205-SpyTag/SpyCatcher platform for next-generation TBVs development.

Evaluation of transmission-blocking potential of Pv22 using clinical Plasmodium vivax infections and transgenic Plasmodium berghei

Antigens expressed during the sexual development of malaria parasites are transmission-blocking vaccine (TBV) targets. Pb22, a protein expressed and localized to the plasma membrane of gametes and ookinetes in Plasmodium berghei, is an excellent TBV candidate. Here, we evaluated the TB potential of the Plasmodium vivax ortholog Pv22 using a transgenic P. berghei parasite line and P. vivax clinical isolates. The full-length recombinant Pv22 (rPv22) protein was produced and used to immunize mice and rabbits to obtain antibodies. We generated a transgenic P. berghei line (TrPv22Pb) by inserting the pv22 gene into the pb22 locus and showed that Pv22 expression completely rescued the defects in male gametogenesis of the pb22 deletion parasite. Since Pv22 in the transgenic parasite showed similar expression and localization patterns to Pb22, we used the TrPv22Pb parasite as a surrogate to evaluate the TB potential of Pv22. In mosquito feeding assays, mosquitoes feeding on rPv22-immunized mice infected with TrPv22Pb parasites showed a 49.3-53.3 % reduction in the oocyst density compared to the control group. In vitro assays showed that the rPv22 immune sera significantly inhibited exflagellation and ookinete formation of the TrPv22Pb parasites. In a direct membrane feeding assay using three clinical P. vivax isolates, the rabbit anti-rPv22 antibodies also significantly decreased the oocyst density by 53.7, 30.2, and 26.2 %, respectively. This study demonstrated the feasibility of using transgenic P. berghei parasites expressing P. vivax antigens as a potential tool to evaluate TBV candidates. However, the much weaker TB activity of Pv22 obtained from two complementary assays suggest that Pv22 may not be a promising TBV candidate for P. vivax.

Host-parasite interactions during Plasmodium infection: Implications for immunotherapies

Malaria is a global infectious disease that remains a leading cause of morbidity and mortality in the developing world. Multiple environmental and host and parasite factors govern the clinical outcomes of malaria. The host immune response against the Plasmodium parasite is heterogenous and stage-specific both in the human host and mosquito vector. The Plasmodium parasite virulence is predominantly associated with its ability to evade the host's immune response. Despite the availability of drug-based therapies, Plasmodium parasites can acquire drug resistance due to high antigenic variations and allelic polymorphisms. The lack of licensed vaccines against Plasmodium infection necessitates the development of effective, safe and successful therapeutics. To design an effective vaccine, it is important to study the immune evasion strategies and stage-specific Plasmodium proteins, which are targets of the host immune response. This review provides an overview of the host immune defense mechanisms and parasite immune evasion strategies during Plasmodium infection. Furthermore, we also summarize and discuss the current progress in various anti-malarial vaccine approaches, along with antibody-based therapy involving monoclonal antibodies, and research advancements in host-directed therapy, which can together open new avenues for developing novel immunotherapies against malaria infection and transmission.

Characterization of PSOP26 as an ookinete surface antigen with improved transmission-blocking activity when fused with PSOP25

Background

The Plasmodium zygote-to-ookinete developmental transition is an essential step for establishing an infection in the mosquito vector, and antigens expressed during this stage are potential targets for transmission-blocking vaccines (TBVs). The secreted ookinete protein 26 (PSOP26) is a newly identified ookinete surface protein. The anti-PSOP26 serum has moderate transmission-blocking activity, indicating the benefit of further investigating this protein as a target for TBVs.

Methods

The function of psop26 was analyzed by targeted gene disruption. A chimeric PSOP25-PSOP26 protein was expressed in the Escherichia coli system. The PSOP25-PSOP26 fusion protein, along with mixed (PSOP25 + PSOP26) or single proteins (PSOP26 or PSOP25), were used for the immunization of mice. The antibody titers and immunogenicity of individual sera were analyzed by enzyme-linked immunoassay (ELISA), indirect immunofluorescence assay (IFA), and Western blot. The transmission-blocking activity of sera from different immunization schemes was assessed using in vitro and in vivo assays.

Results

PSOP26 is a surface protein expressed in Plasmodium gametes and ookinetes. The protein is dispensable for asexual blood-stage development, gametogenesis, and zygote formation, but is essential for the zygote-to-ookinete developmental transition. Specifically, both the prevalence of infections and oocyst densities were decreased in mosquitoes fed on psop26-null mutants. Mixtures of individual PSOP25 and PSOP26 fragments (PSOP25 + PSOP26), as well as chimeras (PSOP25-PSOP26), elicited high antibody levels in mice, with no immunological interference. Antisera against the mixed and fusion proteins elicited higher transmission-reducing activity (TRA) than antisera against the single PSOP26 antigen, but comparable to antisera against PSOP25 antigen alone.

Conclusions

PSOP26 plays a critical role in the zygote-to-ookinete developmental transition. PSOP25 is a promising TBV candidate that could be used alone to target the ookinete stage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05294-8.

A G-Protein-Coupled Receptor Modulates Gametogenesis via PKG-Mediated Signaling Cascade in Plasmodium berghei

Gametogenesis is essential for malaria parasite transmission, but the molecular mechanism of this process remains to be refined. Here, we identified a G-protein-coupled receptor 180 (GPR180) that plays a critical role in signal transduction during gametogenesis in Plasmodium. The P. berghei GPR180 was predominantly expressed in gametocytes and ookinetes and associated with the plasma membrane in female gametes and ookinetes. Knockout of pbgpr180 (Δpbgpr180) had no noticeable effect on blood-stage development but impaired gamete formation and reduced transmission of the parasites to mosquitoes. Transcriptome analysis revealed that a large proportion of the dysregulated genes in the Δpbgpr180 gametocytes had assigned functions in cyclic nucleotide signal transduction. In the Δpbgpr180 gametocytes, the intracellular cGMP level was significantly reduced, and the cytosolic Ca2+ mobilization showed a delay and a reduction in the magnitude during gametocyte activation. These results suggest that PbGPR180 functions upstream of the cGMP-protein kinase G-Ca2+ signaling pathway. In line with this functional prediction, the PbGPR180 protein was found to interact with several transmembrane transporter proteins and the small GTPase Rab6 in activated gametocytes. Allele replacement of pbgpr180 with the P. vivax ortholog pvgpr180 showed equal competence of the transgenic parasite in sexual development, suggesting functional conservation of this gene in Plasmodium spp. Furthermore, an anti-PbGPR180 monoclonal antibody and the anti-PvGPR180 serum possessed robust transmission-blocking activities. These results indicate that GPR180 is involved in signal transduction during gametogenesis in malaria parasites and is a promising target for blocking parasite transmission. IMPORTANCE Environmental changes from humans to mosquitoes activate gametogenesis of the malaria parasite, an obligative process for parasite transmission, but how the signals are relayed remains poorly understood. Here, we show the identification of a Plasmodium G-protein-coupled receptor, GPR180, and the characterization of its function in gametogenesis. In P. berghei, GPR180 is dispensable for asexual development and gametocytogenesis, but its deletion impairs gametogenesis and reduces transmission to mosquitoes. GPR180 appears to function upstream of the cGMP-protein kinase G-Ca2+ signaling pathway and is required for the maximum activity of this pathway. Genetic complementation shows that the GPR180 ortholog from the human malaria parasite P. vivax was fully functional in P. berghei, indicating functional conservation of GPR180 in Plasmodium spp. With predominant expression and membrane association of GPR180 in sexual stages, GPR180 is a promising target for blocking transmission, and antibodies against GPR180 possess robust transmission-blocking activities.

Identification of Novel Malaria Transmission-Blocking Vaccine Candidates

Control measures have significantly reduced malaria morbidity and mortality in the last two decades; however, the downward trends have stalled and have become complicated by the emergence of COVID-19. Significant efforts have been made to develop malaria vaccines, but currently only the RTS,S/AS01 vaccine against Plasmodium falciparum has been recommended by the WHO, for widespread use among children in sub-Saharan Africa. The efficacy of RTS,S/AS01 is modest, and therefore the development of more efficacious vaccines is still needed. In addition, the development of transmission-blocking vaccines (TBVs) to reduce the parasite transmission from humans to mosquitoes is required toward the goal of malaria elimination. Few TBVs have reached clinical development, and challenges include low immunogenicity or high reactogenicity in humans. Therefore, novel approaches to accelerate TBV research and development are urgently needed, especially novel TBV candidate discovery. In this mini review we summarize the progress in TBV research and development, novel TBV candidate discovery, and discuss how to accelerate novel TBV candidate discovery.

Evaluation of two Plasmodium vivax sexual stage antigens as transmission-blocking vaccine candidates

BACKGROUND

Plasmodium vivax transmission-blocking vaccines (TBVs) are receiving increasing attention. Based on excellent transmission-blocking activities of the PbPH (PBANKA_0417200) and PbSOP26 (PBANKA_1457700) antigens in Plasmodium berghei, their orthologs in P. vivax, PVX_098655 (PvPH) and PVX_101120 (PvSOP26), were selected for the evaluation of their potential as TBVs.

METHODS

Fragments of PvPH (amino acids 22-304) and PvSOP26 (amino acids 30-272) were expressed in the yeast expression system. The recombinant proteins were used to immunize mice to obtain antisera. The transmission-reducing activities of these antisera were evaluated using the direct membrane feeding assay (DMFA) using Anopheles dirus mosquitoes and P. vivax clinical isolates.

RESULTS

The recombinant proteins PvPH and PvSOP26 induced robust antibody responses in mice. The DMFA showed that the anti-PvSOP26 sera significantly reduced oocyst densities by 92.0 and 84.1% in two parasite isolates, respectively, whereas the anti-PvPH sera did not show evident transmission-reducing activity. The variation in the DMFA results was unlikely due to the genetic polymorphisms of the two genes since their respective sequences were identical in the clinical P. vivax isolates.

CONCLUSION

PvSOP26 could be a promising TBV candidate for P. vivax, which warrants further evaluation.

PSOP1, putative secreted ookinete protein 1, is localized to the micronemes of Plasmodium yoelii and P. berghei ookinetes

Plasmodium parasites cause malaria in mammalian hosts and are transmitted by Anopheles mosquitoes. Activated gametocytes in the mosquito midgut egress from erythrocytes followed by fertilization and zygote formation. Zygotes differentiate into motile invasive ookinetes, which penetrate the midgut epithelium before forming oocysts beneath the basal lamina. Ookinete development and traversal across the mosquito midgut wall are major bottlenecks in the parasite life cycle. In ookinetes, surface proteins and proteins stored in apical organelles have been shown to be involved in parasite-host interactions. A group of ookinete proteins that are predicted to have such functions are named PSOPs (putative secreted ookinete protein). PSOP1 is possibly involved in migration through the midgut wall, and here its subcellular localization was examined in ookinetes by immunoelectron microscopy. PSOP1 localizes to the micronemes of Plasmodium yoelii and Plasmodium berghei ookinetes, indicating that it is stored and possibly apically secreted during ookinete penetration through the mosquito midgut wall.

Evaluation of two sexual-stage antigens as bivalent transmission-blocking vaccines in rodent malaria

BACKGROUND

Transmission-blocking vaccine (TBV) is a promising strategy for malaria elimination. It is hypothesized that mixing or fusing two antigens targeting different stages of sexual development may provide higher transmission-blocking activity than these antigens used individually.

METHODS

A chimeric protein composed of fragments of Pbg37 and PSOP25 was designed and expressed the recombinant protein in Escherichia coli Rosetta-gami B (DE3). After immunizing mice with individual recombinant proteins Pbg37 and PSOP25, mixed proteins (Pbg37+PSOP25), or the fusion protein (Pbg37-PSOP25), the antibody titers of individual sera were analyzed by ELISA. IFA and Western blot were performed to test the reactivity of the antisera with the native proteins in the parasite. The transmission-blocking activity of the different immunization schemes was assessed using in vitro and in vivo assays.

RESULTS

When Pbg37 and PSOP25 were co-administered in a mixture or as a fusion protein, they elicited similar antibody responses in mice as single antigens without causing immunological interference with each other. Antibodies against the mixed or fused antigens recognized the target proteins in the gametocyte, gamete, zygote, and ookinete stages. The mixed proteins or the fusion protein induced antibodies with significantly stronger transmission-reducing activities in vitro and in vivo than individual antigens.

CONCLUSIONS

There was no immunological interference between Pbg37 and PSOP25. The bivalent vaccines, which expand the portion of the sexual development during which the transmission-blocking antibodies act, produced significantly stronger transmission-reducing activities than single antigens. Altogether, these data provide the theoretical basis for the development of combination TBVs targeting different sexual stages.

Identification of Three Novel Plasmodium Factors Involved in Ookinete to Oocyst Developmental Transition

Plasmodium falciparum malaria remains a major cause of global morbidity and mortality, mainly in sub-Saharan Africa. The numbers of new malaria cases and deaths have been stable in the last years despite intense efforts for disease elimination, highlighting the need for new approaches to stop disease transmission. Further understanding of the parasite transmission biology could provide a framework for the development of such approaches. We phenotypically and functionally characterized three novel genes, PIMMS01, PIMMS57, and PIMMS22, using targeted disruption of their orthologs in the rodent parasite Plasmodium berghei. PIMMS01 and PIMMS57 are specifically and highly expressed in ookinetes, while PIMMS22 transcription starts already in gametocytes and peaks in sporozoites. All three genes show strong phenotypes associated with the ookinete to oocyst transition, as their disruption leads to very low numbers of oocysts and complete abolishment of transmission. PIMMS22 has a secondary essential function in the oocyst. Our results enrich the molecular understanding of the parasite-vector interactions and identify PIMMS01, PIMMS57, and PIMMS22 as new targets of transmission blocking interventions.

Identification of Plasmodium falciparum-specific protein PIESP2 as a novel virulence factor related to cerebral malaria

Cerebral malaria (CM) is the most severe complication caused by Plasmodium falciparum infection. The pathophysiological changes caused by parasite virulence factors and the human immune response to parasites contribute to CM. To date, very few parasite virulence proteins have been found to participate in CM. Here, we employed comparative genomics analysis and identified parasite-infected erythrocyte specific protein 2 (PIESP2) to be a CM-related protein. We conducted further experimental investigations and found that PIESP2 is an immunogenic protein. PIESP2 expression begins at the early trophozoite stage and progressively increases with parasite development. Although PIESP2 proteins mainly reside within infected red blood cells (IRBCs), some of them are present on the IRBC surface at the pigmented stage. Moreover, blockage of PIESP2 by antiserum apparently inhibited the adhesion of IRBCs to brain microvascular endothelial cells (BMECs). Western blot analysis detected the binding of PIESP2 to BMECs. Transcriptional analysis revealed that the binding of PIESP2 to BMECs can increase the expression of genes involved in the inflammatory response but decrease the expression of genes related to the anchoring junction. Overall, PIESP2 might be associated with CM by mediating the sequestration of IRBCs, inducing the inflammation response, and impairing the integrity of blood-brain barrier.

A conserved malaria parasite antigen Pb22 plays a critical role in male gametogenesis in Plasmodium berghei

Gametogenesis, the formation of gametes from gametocytes, an essential step for malaria parasite transmission, is targeted by transmission-blocking drugs and vaccines. We identified a conserved protein (PBANKA_0305900) in Plasmodium berghei, which encodes a protein of 22 kDa (thus named Pb22) and is expressed in both asexual stages and gametocytes. Its homologues are present in all Plasmodium species and its closely related, Hepatocystis, but not in other apicomplexans. Pb22 protein was localised in the cytosols of schizonts, as well as male and female gametocytes. During gamete-to-ookinete development, Pb22 became localised on the plasma membranes of gametes and ookinetes. Compared to the wild-type (WT) parasites, P. berghei with pb22 knockout (KO) showed a significant reduction in exflagellation (~89%) of male gametocytes and ookinete number (~97%) during in vitro ookinete culture. Mosquito feeding assays showed that ookinete and oocyst formation of the pb22-KO line in mosquito midguts was almost completely abolished. These defects were rescued in parasites where pb22 was restored. Cross-fertilisation experiments with parasite lines defective in either male or female gametes confirmed that the defects in the pb22-KO line were restricted to the male gametes, whereas female gametes in the pb22-KO line were fertile at the WT level. Detailed analysis of male gametogenesis showed that 30% of the male gametocytes in the pb22-KO line failed to assemble the axonemes, whereas ~48.9% of the male gametocytes formed flagella but failed to egress from the host erythrocyte. To explore its transmission-blocking potential, recombinant Pb22 (rPb22) was expressed and used to immunise mice. in vitro assays showed that the rPb22-antisera significantly inhibited exflagellation by ~64.8% and ookinete formation by ~93.4%. Mosquitoes after feeding on rPb22-immunised mice also showed significant decreases in infection prevalence (83.3-93.3%) and oocyst density (93.5-99.6%). Further studies of the Pb22 orthologues in human malaria parasites are warranted.

PIMMS43 is required for malaria parasite immune evasion and sporogonic development in the mosquito vector

Malaria is transmitted among humans through mosquito bites. Here, we characterize a protein found on the surface of mosquito stages of malaria parasites and reveal that it serves to evade the mosquito immune system and ensure disease transmission. Neutralization of PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43), either by eliminating it from the parasite genome or by preincubating parasites with antibodies that bind to the PIMMS43 protein, inhibits mosquito infection with malaria parasites. Differences in PIMMS43 detected between African malaria parasite populations suggest that these populations have adapted for transmission by different mosquito vectors that are also differentially distributed across the continent. We conclude that targeting PIMMS43 can block malaria parasites inside mosquitoes before they can infect humans.

After being ingested by a female Anopheles mosquito during a bloodmeal on an infected host, and before they can reach the mosquito salivary glands to be transmitted to a new host, Plasmodium parasites must establish an infection of the mosquito midgut in the form of oocysts. To achieve this, they must first survive a series of robust innate immune responses that take place prior to, during, and immediately after ookinete traversal of the midgut epithelium. Understanding how parasites may evade these responses could highlight new ways to block malaria transmission. We show that an ookinete and sporozoite surface protein designated as PIMMS43 (Plasmodium Infection of the Mosquito Midgut Screen 43) is required for parasite evasion of the Anopheles coluzzii complement-like response. Disruption of PIMMS43 in the rodent malaria parasite Plasmodium berghei triggers robust complement activation and ookinete elimination upon mosquito midgut traversal. Silencing components of the complement-like system through RNAi largely restores ookinete-to-oocyst transition but oocysts remain small in size and produce a very small number of sporozoites that additionally are not infectious, indicating that PIMMS43 is also essential for sporogonic development in the oocyst. Antibodies that bind PIMMS43 interfere with parasite immune evasion when ingested with the infectious blood meal and significantly reduce the prevalence and intensity of infection. PIMMS43 genetic structure across African Plasmodium falciparum populations indicates allelic adaptation to sympatric vector populations. These data add to our understanding of mosquito–parasite interactions and identify PIMMS43 as a target of malaria transmission blocking.

Malaria transmission-blocking vaccines: wheat germ cell-free technology can accelerate vaccine development

Introduction: Highly effective malaria vaccines are essential component toward malaria elimination. Although the leading malaria vaccine, RTS,S/AS01, with modest efficacy is being evaluated in a pilot feasibility trial, development of a malaria transmission-blocking vaccine (TBV) could make a major contribution toward malaria elimination. Only a few TBV antigens have reached pre-clinical or clinical development but with several challenges including difficulties in the expression of malaria recombinant proteins and low immunogenicity in humans. Therefore, novel approaches to accelerate TBV research to preclinical development are critical to generate an efficacious TBV.Areas covered: PubMed was searched to review the progress and future prospects of malaria TBV research and development. We also reviewed registered trials at ClinicalTrials.gov as well as post-genome TBV candidate discovery research including our efforts.Expert opinion: Wheat germ cell-free protein synthesis technology can accelerate TBV development by overcoming some current challenges of TBV research.

An MFS-Domain Protein Pb115 Plays a Critical Role in Gamete Fertilization of the Malaria Parasite Plasmodium berghei

Sexual reproduction is an essential process in the Plasmodium life cycle and a vulnerable step for blocking transmission from the human host to mosquitoes. In this study, we characterized the functions of a conserved cell membrane protein P115 in the rodent malaria parasite Plasmodium berghei ANKA. Pb115 was expressed in both asexual stages (schizonts) and sexual stages (gametocytes, gametes, and ookinetes), and was localized on the plasma membrane of gametes and ookinetes. In P. berghei, genetic deletion of Pb115 (Δpb115) did not affect asexual multiplication, nor did it affect gametocyte development or exflagellation of the male gametocytes. However, mosquitoes fed on Δpb115-infected mice showed 74% reduction in the prevalence of infection and 96.5% reduction in oocyst density compared to those fed on wild-type P. berghei-infected mice. The Δpb115 parasites showed significant defects in the interactions between the male and female gametes, and as a result, very few zygotes were formed in ookinete cultures. Cross fertilization with the male-defective Δpbs48/45 line and the female-defective Δpfs47 line further indicated that the fertilization defects of the Δpb115 lines were present in both male and female gametes. We evaluated the transmission-blocking potential of Pb115 by immunization of mice with a recombinant Pb115 fragment. In vivo mosquito feeding assay showed Pb115 immunization conferred modest, but significant transmission reducing activity with 44% reduction in infection prevalence and 39% reduction in oocyst density. Our results described functional characterization of a conserved membrane protein as a fertility factor in Plasmodium and demonstrated transmission-blocking potential of this antigen.

Antimalarial Transmission-Blocking Interventions: Past, Present, and Future

Malaria remains a major global health challenge. Appropriate use of current antimalarial tools has reduced the disease burden, but morbidity and mortality remain unacceptably high. It is widely accepted that, to achieve long-term control/eradication, it will be necessary to use interventions that inhibit the transmission of parasites to mosquitoes - these tools are termed transmission-blocking interventions (TBIs). This article aims to outline the rationale for the development of TBIs, with a focus on transmission-blocking drugs and (parasite-derived) transmission-blocking vaccines. We describe and summarise the current status of each of these intervention classes and attempt to identify future requirements in development, with a focus on the challenges of establishing each method within an integrated malarial control programme in the future.

Characterization of Plasmodium berghei Pbg37 as Both a Pre- and Postfertilization Antigen with Transmission-Blocking Potential

Transmission-blocking vaccines (TBVs) interrupting malaria transmission are an integrated tool for malaria eradication. We characterized a sexual-stage-specific gene (PBANKA_060330) from Plasmodium berghei and studied its potential for use as a TBV. This gene, referred to as pbg37, encodes a protein of 37 kDa with a signal peptide and multiple transmembrane domains and is preferentially expressed in gametocytes. A recombinant Pbg37 (rPbg37) protein targeting the N-terminal 63 amino acids (amino acids 26 to 88) expressed in bacteria elicited strong antibody responses in mice. Western blotting demonstrated Pbg37 expression in gametocytes, zygotes, and, to a lesser extent, ookinetes and its predominant association with the membranes of gametocytes. Indirect immunofluorescence assay showed an abundant surface localization of Pbg37 on gametes and zygotes but reduced amounts on retorts and ookinetes. Knockout of pbg37 (Δpbg37) led to a considerable reduction in gametocytemia, which translated into a ~92.1% decrease in the oocyst number in mosquitoes. Deletion of pbg37 had a more substantial influence on the development and maturation of microgametocytes. As a result, the Δpbg37 lines exhibited a higher female/male gametocyte ratio, fewer mature male gametocytes, and defects in the exflagellation of mature microgametocytes. To test the transmission-blocking potential of Pbg37, an in vitro ookinete assay showed that the major inhibitory effects of anti-Pbg37 antiserum were on the exflagellation and fertilization processes. Direct feeding of mosquitoes on mice immunized with rPbg37 or a control protein showed that rPbg37-immunized and P. berghei-infected mice had a significant reduction (49.1%) in oocyst density compared to the controls. The conservation of this gene in Plasmodium warrants further investigations in human malaria parasites.

Characterization of Pb51 in Plasmodium berghei as a malaria vaccine candidate targeting both asexual erythrocytic proliferation and transmission

Background

A vaccine that targets multiple developmental stages of malaria parasites would be an effective tool for malaria control and elimination.

Methods

A conserved gene in Plasmodium, the Plasmodium berghei gene (PBANKA_020570) encoding a 51 kDa protein (pb51 gene), was identified through search of the PlasmoDB database using a combination of expression and protein localization criteria. A partial domain of the Pb51 protein was expressed in a prokaryotic expression system (rPb51) and used for immunization in mice. The protein expression profile and localization were studied by Western blot and indirect immunofluorescence assay (IFA), respectively. The inhibitory effect of the anti-rPb51 antibodies on parasite proliferation was evaluated in erythrocytes in vivo. The transmission-blocking activity of the immune sera was determined by in vitro ookinete conversion assay and by direct mosquito feeding assay (DFA).

Results

The rPb51 elicited specific antibodies in mice. Western blot confirmed Pb51 expression in schizonts, gametocytes and ookinetes. IFA showed localization of Pb51 on the outer membranes of schizonts, gametocytes, zygotes, retorts, ookinetes and sporozoites of P. berghei. Mice immunized with the rPb51 protein significantly reduced parasite proliferation and gametocyte conversion in vivo. Moreover, the rPb51 antisera also significantly reduced the in vitro ookinete conversion when added into the ookinete culture medium. In DFA, mice immunized with the rPb51 reduced the prevalence of mosquito infection by 21.3% and oocyst density by 54.8%.

Conclusions

In P. berghei, P51 was expressed in both asexual erythrocytic and sexual stages and localized on the surface of these stages with the exception of the ring stage. The anti-rPb51 antibodies inhibited both P. berghei proliferation in mice and transmission of the parasite to mosquitoes.

Electronic supplementary material

The online version of this article (10.1186/s12936-017-2107-2) contains supplementary material, which is available to authorized users.

Functional characterization of Plasmodium berghei PSOP25 during ookinete development and as a malaria transmission-blocking vaccine candidate

Background

Plasmodium ookinete surface proteins as post-fertilization target antigens are potential malaria transmission-blocking vaccine (TBV) candidates. Putative secreted ookinete protein 25 (PSOP25) is a highly conserved ookinete surface protein, and has been shown to be a promising novel TBV target. Here, we further investigated the TBV activities of the full-length recombinant PSOP25 (rPSOP25) protein in Plasmodium berghei, and characterized the potential functions of PSOP25 during the P. berghei life-cycle.

Methods

We expressed the full-length P. berghei PSOP25 protein in a prokaryotic expression system, and developed polyclonal mouse antisera and a monoclonal antibody (mAb) against the recombinant protein. Indirect immunofluorescence assay (IFA) and Western blot were used to test the specificity of antibodies. The transmission-blocking (TB) activities of antibodies were evaluated by the in vitro ookinete conversion assay and by direct mosquito feeding assay (DFA). Finally, the function of PSOP25 during Plasmodium development was studied by deleting the psop25 gene.

Results

Both polyclonal mouse antisera and anti-rPSOP25 mAb recognized the PSOP25 proteins in the parasites, and IFA showed the preferential expression of PSOP25 on the surface of zygotes, retorts and mature ookinetes. In vitro, these antibodies significantly inhibited ookinetes formation in an antibody concentration-dependent manner. In DFA, mice immunized with the rPSOP25 and those receiving passive transfer of the anti-rPSOP25 mAb reduced the prevalence of mosquito infection by 31.2 and 26.1%, and oocyst density by 66.3 and 63.3%, respectively. Genetic knockout of the psop25 gene did not have a detectable impact on the asexual growth of P. berghei, but significantly affected the maturation of ookinetes and the formation of midgut oocysts.

Conclusions

The full-length rPSOP25 could elicit strong antibody response in mice. Polyclonal and monoclonal antibodies against PSOP25 could effectively block the formation of ookinetes in vitro and transmission of the parasites to mosquitoes. Genetic manipulation study indicated that PSOP25 is required for ookinete maturation in P. berghei. These results support further testing of the PSOP25 orthologs in human malaria parasites as promising TBV candidates.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1932-4) contains supplementary material, which is available to authorized users.