The Plasmodium falciparum Cell-Traversal Protein for Ookinetes and Sporozoites as a Candidate for Preerythrocytic and Transmission-Blocking Vaccines

Vaccines and monoclonal antibodies: new tools for malaria control

SUMMARYMalaria remains one of the biggest health problems in the world. While significant reductions in malaria morbidity and mortality had been achieved from 2000 to 2015, the favorable trend has stalled, rather significant increases in malaria cases are seen in multiple areas. In 2022, there were 249 million estimated cases, and 608,000 malaria-related deaths, mostly in infants and children aged under 5 years, globally. Therefore, in addition to the expansion of existing anti-malarial control measures, it is critical to develop new tools, such as vaccines and monoclonal antibodies (mAbs), to fight malaria. In the last 2 years, the first and second malaria vaccines, both targeting Plasmodium falciparum circumsporozoite proteins (PfCSP), have been recommended by the World Health Organization to prevent P. falciparum malaria in children living in moderate to high transmission areas. While the approval of the two malaria vaccines is a considerable milestone in vaccine development, they have much room for improvement in efficacy and durability. In addition to the two approved vaccines, recent clinical trials with mAbs against PfCSP, blood-stage vaccines against P. falciparum or P. vivax, and transmission-blocking vaccine or mAb against P. falciparum have shown promising results. This review summarizes the development of the anti-PfCSP vaccines and mAbs, and recent topics in the blood- and transmission-blocking-stage vaccine candidates and mAbs. We further discuss issues of the current vaccines and the directions for the development of next-generation vaccines.

Naturally acquired antibodies against Plasmodium vivax pre-erythrocytic stage vaccine antigens inhibit sporozoite invasion of human hepatocytes in vitro

In Plasmodium vivax, the most studied vaccine antigens are aimed at blocking merozoite invasion of erythrocytes and disease development. Very few studies have evaluated pre-erythrocytic (PE) stage antigens. The P. vivax circumsporozoite protein (CSP), is considered the leading PE vaccine candidate, but immunity to CSP is short-lived and variant specific. Thus, there is a need to identify other potential candidates to partner with CSP in a multivalent vaccine to protect against infection and disease. We hypothesize that sporozoite antigens important for host cell infection are considered potential targets. In this study, we evaluated the magnitude and quality of naturally acquired antibody responses to four P. vivax PE antigens: sporozoite surface protein 3 (SSP3), sporozoite protein essential for traversal 1 (SPECT1), cell traversal protein of ookinetes and sporozoites (CelTOS) and CSP in plasma of P. vivax infected patients from Thailand. Naturally acquired antibodies to these antigens were prevalent in the study subjects, but with significant differences in magnitude of IgG antibody responses. About 80% of study participants had antibodies to all four antigens and only 2% did not have antibodies to any of the antigens. Most importantly, these antibodies inhibited sporozoite infection of hepatocytes in vitro. Significant variations in magnitude of antigen-specific inhibitory antibody responses were observed with individual samples. The highest inhibitory responses were observed with anti-CelTOS antibodies, followed by anti-SPECT1, SSP3 and CSP antibodies respectively. These data highlight the vaccine potential of these antigens in protecting against hepatocyte infection and the need for a multi-valent pre-erythrocytic vaccine to prevent liver stage development of P. vivax sporozoites.

Barnes S, H. Adams J, Chootong P. Longitudinal analysis of antibody responses to Plasmodium vivax sporozoite antigens following natural infection

BACKGROUND

P. vivax malaria is a major global health burden hindering social and economic development throughout many tropical and sub-tropical countries. Pre-erythrocytic (PE) vaccines emerge as an attractive approach for the control and elimination of malaria infection. Therefore, evaluating the magnitude, longevity and prevalence of naturally acquired IgG antibody responses against PE candidate antigens is useful for vaccine design.

METHODOLOGY/PRINCIPAL FINDINGS

The antigenicity of five recombinant PE antigens (PvCSP-VK210, PvSSP3, PvM2-MAEBL, PvCelTOS and PvSPECT1) was evaluated in plasma samples from individuals residing in low transmission areas in Thailand (Ranong and Chumphon Provinces). The samples were collected at the time of acute vivax malaria and 90, 270 and 360 days later. The prevalence, magnitude and longevity of total IgG and IgG subclasses were determined for each antigen using the longitudinal data. Our results showed that seropositivity of all tested PE antigens was detected during infection in at least some subjects; anti-PvCSP-VK210 and anti-PvCelTOS antibodies were the most frequent. Titers of these antibodies declined during the year of follow up, but notably seropositivity persisted. Among seropositive subjects at post-infection, high number of subjects possessed antibodies against PvCSP-VK210. Anti-PvSSP3 antibody responses had the longest half-life. IgG subclass profiling showed that the predominant subclasses were IgG1 and IgG3 (cytophilic antibodies), tending to remain detectable for at least 360 days after infection.

CONCLUSIONS/SIGNIFICANCE

The present study demonstrated the magnitude and longevity of serological responses to multiple PE antigens of P. vivax after natural infection. This knowledge could contribute to the design of an effective P. vivax vaccine.

Malaria Vaccines: From the Past towards the mRNA Vaccine Era

Plasmodium spp. is the etiological agent of malaria, a life-threatening parasitic disease transmitted by infected mosquitoes. Malaria remains a major global health challenge, particularly in endemic regions. Over the years, various vaccine candidates targeting different stages of Plasmodium parasite life-cycle have been explored, including subunit vaccines, vectored vaccines, and whole organism vaccines with Mosquirix, a vaccine based on a recombinant protein, as the only currently approved vaccine for Plasmodium falciparum malaria. Despite the aforementioned notable progress, challenges such as antigenic diversity, limited efficacy, resistant parasites escaping protective immunity and the need for multiple doses have hindered the development of a highly efficacious malaria vaccine. The recent success of mRNA-based vaccines against SARS-CoV-2 has sparked renewed interest in mRNA vaccine platforms. The unique mRNA vaccine features, including their potential for rapid development, scalability, and flexibility in antigen design, make them a promising avenue for malaria vaccine development. This review provides an overview of the malaria vaccines' evolution from the past towards the mRNA vaccine era and highlights their advantages in overcoming the limitations of previous malaria vaccine candidates.

In Silico Analysis Reveals High Levels of Genetic Diversity of Plasmodium knowlesi Cell Traversal Protein for Ookinetes and Sporozoites (PkCelTOS) in Clinical Samples

The cell-traversal protein for ookinetes and sporozoites (CelTOS), expressed on the surface of ookinetes and sporozoitesin Plasmodium species, is a promising malaria vaccine candidate. CelTOS is essential for parasite invasion into mosquito midgut and human hepatocytes, thereby contributing to malaria transmission and disease pathogenesis. This study explores the genetic diversity, polymorphisms, haplotypes, natural selection, phylogenetic analysis, and epitope prediction in the full-length Plasmodium knowlesi CelTOS gene in clinical samples from Sarawak, Malaysian Borneo, and long-term laboratory strains from Peninsular Malaysia and the Philippines. Our analysis revealed a high level of genetic variation in the PkCelTOS gene, with a nucleotide diversity of π ~ 0.021, which was skewed towards the 3' end of the gene. This level of diversity is double that observed in PfCelTOS and 20 times that observed in PvCelTOS from worldwide clinical samples. Tests of natural selection revealed evidence for positive selection within clinical samples. Phylogenetic analysis of the amino acid sequence of PkCelTOS revealed the presence of two distinct groups, although no geographical clustering was observed. Epitope prediction analysis identified two potential epitopes (96AQLKATA102 and 124TIKPPRIKED133) using the IEDB server and one epitope (125IKPPRIKED133) by Bcepred server on the C' terminal region of PkCelTOS protein. Both the servers predicted a common epitope region of nine amino acid length (IKPPRIKED) peptide, which can be studied in the future as a potential candidate for vaccine development. These findings shed light on the genetic diversity, polymorphism, haplotypes, and natural selection within PkCelTOS in clinical samples and provide insights about its future prospects as a potential candidate for P. knowlesi malaria vaccine development.

Construction, Expression, and Evaluation of the Naturally Acquired Humoral Immune Response against Plasmodium vivax RMC-1, a Multistage Chimeric Protein

The PvCelTOS, PvCyRPA, and Pvs25 proteins play important roles during the three stages of the P. vivax lifecycle. In this study, we designed and expressed a P. vivax recombinant modular chimeric protein (PvRMC-1) composed of the main antigenic regions of these vaccine candidates. After structure modelling by prediction, the chimeric protein was expressed, and the antigenicity was assessed by IgM and IgG (total and subclass) ELISA in 301 naturally exposed individuals from the Brazilian Amazon. The recombinant protein was recognized by IgG (54%) and IgM (40%) antibodies in the studied individuals, confirming the natural immunogenicity of the epitopes that composed PvRMC-1 as its maintenance in the chimeric structure. Among responders, a predominant cytophilic response mediated by IgG1 (70%) and IgG3 (69%) was observed. IgM levels were inversely correlated with age and time of residence in endemic areas (p < 0.01). By contrast, the IgG and IgM reactivity indexes were positively correlated with each other, and both were inversely correlated with the time of the last malaria episode. Conclusions: The study demonstrates that PvRMC-1 was successfully expressed and targeted by natural antibodies, providing important insights into the construction of a multistage chimeric recombinant protein and the use of naturally acquired antibodies to validate the construction.

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.

Exploring in vitro expression and immune potency in mice using mRNA encoding the Plasmodium falciparum malaria antigen, CelTOS

The secreted malarial protein, Cell-Traversal protein for Ookinetes and Sporozoites (CelTOS), is highly conserved among Plasmodium species, and plays a role in the invasion of mosquito midgut cells and hepatocytes in the vertebrate host. CelTOS was identified as a potential protective antigen based on a proteomic analysis, which showed that CelTOS stimulated significant effector T cells producing IFN-γ in peripheral blood mononuclear cells (PBMCs) from radiation attenuated sporozoite-immunized, malaria-naïve human subjects. In a rodent malaria model, recombinant full-length CelTOS protein/adjuvant combinations induced sterile protection, and in several studies, functional antibodies were produced that had hepatocyte invasion inhibition and transmission-blocking activities. Despite some encouraging results, vaccine approaches using CelTOS will require improvement before it can be considered as an effective vaccine candidate. Here, we report on the use of mRNA vaccine technology to induce humoral and cell-mediated immune responses using this antigen. Several pfceltos encoding mRNA transcripts were assessed for the impact on protein translation levels in vitro. Protein coding sequences included those to evaluate the effects of signal sequence, N-glycosylation on translation, and of nucleoside substitutions. Using in vitro transfection experiments as a pre-screen, we assessed the quality of the expressed CelTOS target relative to the homogeneity, cellular localization, and durability of expression levels. Optimized mRNA transcripts, which demonstrated highest protein expression levels in vitro were selected for encapsulation in lipid nanoparticles (LNP) and used to immunize mice to assess for both humoral and cellular cytokine responses. Our findings indicate that mRNA transcripts encoding pfceltos while potent for inducing antigen-specific cellular cytokine responses in mice, were less able to mount PfCelTOS-specific antibody responses using a two-dose regimen. An additional booster dose was needed to overcome low seroconversion rates in mice. With respect to antibody fine specificities, N-glycosylation site mutated immunogens yielded lower immune responses, particularly to the N-terminus of the molecule. While it remains unclear the impact on CelTOS antigen as immunogen, this study highlights the need to optimize antigen design for vaccine development.

Five decades of clinical assessment of whole-sporozoite malaria vaccines

In 1967, pioneering work by Ruth Nussenzweig demonstrated for the first time that irradiated sporozoites of the rodent malaria parasite Plasmodium berghei protected mice against a challenge with infectious parasites of the same species. This remarkable finding opened up entirely new prospects of effective vaccination against malaria using attenuated sporozoites as immunization agents. The potential for whole-sporozoite-based immunization in humans was established in a clinical study in 1973, when a volunteer exposed to X-irradiated P. falciparum sporozoites was found to be protected against malaria following challenge with a homologous strain of this parasite. Nearly five decades later, much has been achieved in the field of whole-sporozoite malaria vaccination, and multiple reports on the clinical evaluation of such candidates have emerged. However, this process has known different paces before and after the turn of the century. While only a few clinical studies were published in the 1970’s, 1980’s and 1990’s, remarkable progress was made in the 2000’s and beyond. This article reviews the history of the clinical assessment of whole-sporozoite malaria vaccines over the last forty-nine years, highlighting the impressive achievements made over the last few years, and discussing some of the challenges ahead.

Development of New Strategies for Malaria Chemoprophylaxis: From Monoclonal Antibodies to Long-Acting Injectable Drugs

Drug discovery for malaria has traditionally focused on orally available drugs that kill the abundant, parasitic blood stage. Recently, there has also been an interest in injectable medicines, in the form of monoclonal antibodies (mAbs) with long-lasting plasma half-lives or long-lasting depot formulations of small molecules. These could act as prophylactic drugs, targeting the sporozoites and other earlier parasitic stages in the liver, when the parasites are less numerous, or as another intervention strategy targeting the formation of infectious gametocytes. Generally speaking, the development of mAbs is less risky (costly) than small-molecule drugs, and they have an excellent safety profile with few or no off-target effects. Therefore, populations who are the most vulnerable to malaria, i.e., pregnant women and young children would have access to such new treatments much faster than is presently the case for new antimalarials. An analysis of mAbs that were successfully developed for oncology illustrates some of the feasibility aspects, and their potential as affordable drugs in low- and middle-income countries.

Monoclonal antibodies for malaria prevention

Monoclonal antibodies are highly specific proteins that are cloned from a single B cell and bind to a single epitope on a pathogen. These laboratory-made molecules can serve as prophylactics or therapeutics for infectious diseases and have an impressive capacity to modulate the progression of disease, as demonstrated for the first time on a large scale during the COVID-19 pandemic. The high specificity and natural starting point of monoclonal antibodies afford an encouraging safety profile, yet the high cost of production remains a major limitation to their widespread use. While a monoclonal antibody approach to abrogating malaria infection is not yet available, the unique life cycle of the malaria parasite affords many opportunities for such proteins to act, and preliminary research into the efficacy of monoclonal antibodies in preventing malaria infection, disease, and transmission is encouraging. This review examines the current status and future outlook for monoclonal antibodies against malaria in the context of the complex life cycle and varied antigenic targets expressed in the human and mosquito hosts, and provides insight into the strengths and limitations of this approach to curtailing one of humanity’s oldest and deadliest diseases.

Human antibodies against non-circumsporozoite proteins block Plasmodium falciparum parasite development in hepatocytes

Sporozoite-based approaches currently represent the most effective vaccine strategies for induction of sterile protection against Plasmodium falciparum (Pf) malaria. Clinical development of subunit vaccines is almost exclusively centered on the circum-sporozoite protein (CSP), an abundantly expressed protein on the sporozoite membrane. Anti-CSP antibodies are able to block sporozoite invasion and development in human hepatocytes and subsequently prevent clinical malaria. Here, we have investigated whether sporozoite-induced human antibodies with specificities different from CSP can reduce Pf-liver stage development. IgG preparations were obtained from 12 volunteers inoculated with a protective immunization regime of whole sporozoites under chloroquine prophylaxis. These IgGs were depleted for CSP specificity by affinity chromatography. Recovered non-CSP antibodies were tested for sporozoite membrane binding and for functional inhibition of sporozoite invasion of a human hepatoma cell line and hepatocytes both in vitro and in vivo. Postimmunization IgGs depleted for CS specificity of 9 of 12 donors recognized sporozoite surface antigens. Samples from 5 of 12 donors functionally reduced parasite-liver cell invasion or development using the hepatoma cell line HC-04 and FRG-huHep mice containing human liver cells. The combined data provide clear evidence that non-CSP proteins, as yet undefined, do represent antibody targets for functional immunity against Pf parasites responsible for malaria.

Towards large-scale identification of HLA-restricted T cell epitopes from four vaccine candidate antigens in a malaria endemic community in Ghana

Sterile protection against clinical malaria has been achieved in animal models and experimental human challenge studies involving immunization with radiation attenuated Plasmodium falciparum sporozoite vaccines as well as by live sporozoites under chloroquine prophylaxis. Parasite-specific IFN-γ and granzyme B-secreting CD8 + T cells have been identified as key mediators of protection. Although the exact parasite targets of protective CD8 + T cell responses are not fully defined, responses against a handful of vaccine candidate antigens have been associated with protection. Identifying the T cell targets in these antigens will facilitate the development of simpler, cost-effective, and efficacious next generation multi-epitope vaccines. The aim of this study was to identify immunodominant portions of four malaria vaccine candidate antigens using peripheral blood mononuclear cells (PBMCs) from adults with life-long exposure to malaria parasites. Cryopreserved PBMCs from 291 HLA-typed subjects were stimulated with pools of overlapping 15mer peptides spanning the entire sequences of P. falciparum circumsporozoite protein (CSP, 9 pools), apical membrane antigen 1 (AMA1, 12 pools), thrombospondin related anonymous protein (TRAP, 6 pools) and cell traversal for ookinetes and sporozoites (CelTOS, 4 pools) in FluoroSpot assays. 125 of 291 subjects made IFN-γ responses to 30 of the 31 peptide pools tested and 22 of 291 made granzyme B responses, with 20 making dual responses. The most frequent responses were to the CSP C-terminal region and the least frequent responses were to TRAP and CelTOS. There was no association between FluoroSpot responses and active malaria infection, detected by either microscopy, RDT, or PCR. In conclusion, CSP and AMA1 have relatively higher numbers of epitopes that trigger IFN-γ and granzyme B-secreting T cells in adults with life-long malaria parasite exposure compared to the other two antigens tested, and highlights the continued relevance of these two antigens as vaccine candidates.

Molecular interactions between parasite and mosquito during midgut invasion as targets to block malaria transmission

Despite considerable effort, malaria remains a major public health burden. Malaria is caused by five Plasmodium species and is transmitted to humans via the female Anopheles mosquito. The development of malaria vaccines against the liver and blood stages has been challenging. Therefore, malaria elimination strategies advocate integrated measures, including transmission-blocking approaches. Designing an effective transmission-blocking strategy relies on a sophisticated understanding of the molecular mechanisms governing the interactions between the mosquito midgut molecules and the malaria parasite. Here we review recent advances in the biology of malaria transmission, focusing on molecular interactions between Plasmodium and Anopheles mosquito midgut proteins. We provide an overview of parasite and mosquito proteins that are either targets for drugs currently in clinical trials or candidates of promising transmission-blocking vaccines.

Inferior T cell immunogenicity of a Plasmodium berghei model liver stage antigen expressed throughout pre-erythrocytic maturation

Sporozoite antigens are the basis of a number of malaria vaccines being tested, but the contribution of antigens expressed during subsequent liver stage development to pre-erythrocytic stage immunity is poorly understood. We previously showed that, following immunisation with radiation attenuated sporozoites (RAS), a model epitope embedded in a sporozoite surface protein elicited robust CD8⁺ T cell responses, whilst the same epitope in a liver stage antigen induced inferior responses. Since RAS arrest early in their development in host hepatocytes, we hypothesised that extending parasite maturation in the liver could considerably improve the epitope-specific CD8⁺ T cell response. Here, we employed a late liver stage arrested parasite model, azithromycin prophylaxis alongside live sporozoites, to increase expression of the model epitope until full liver stage maturation. Strikingly, this alternative immunisation strategy, which has been shown to elicit superior protection, failed to improve the resulting epitope-specific CD8⁺ T cell responses. Our findings support the notion that liver stage antigens are poorly immunogenic and provide additional caution about prioritising antigens for vaccine development based solely on immunogenicity.

A universal vaccine candidate against Plasmodium vivax malaria confers protective immunity against the three PvCSP alleles

Malaria is a highly prevalent parasitic disease in regions with tropical and subtropical climates worldwide. Among the species of Plasmodium causing human malaria, P. vivax is the second most prevalent and the most geographically widespread species. A major target of a pre-erythrocytic vaccine is the P. vivax circumsporozoite protein (PvCSP). In previous studies, we fused two recombinant proteins representing three allelic variants of PvCSP (VK210, VK247 and P. vivax-like) to the mumps virus nucleocapsid protein to enhance immune responses against PvCSP. The objective of the present study was to evaluate the protective efficacy of these recombinants in mice challenged with transgenic P. berghei parasites expressing PvCSP allelic variants. Formulations containing Poly (I:C) or Montanide ISA720 as adjuvants elicited high and long-lasting IgG antibody titers specific to each PvCSP allelic variant. Immunized mice were challenged with two existing chimeric P. berghei parasite lines expressing PvCSP-VK210 and PvCSP-VK247. We also developed a novel chimeric line expressing the third allelic variant, PvCSP-P. vivax-like, as a new murine immunization-challenge model. Our formulations conferred partial protection (significant delay in the time to reach 1% parasitemia) against challenge with the three chimeric parasites. Our results provide insights into the development of a vaccine targeting multiple strains of P. vivax.

Screening of viral-vectored P. falciparum pre-erythrocytic candidate vaccine antigens using chimeric rodent parasites

To screen for additional vaccine candidate antigens of Plasmodium pre-erythrocytic stages, fourteen P. falciparum proteins were selected based on expression in sporozoites or their role in establishment of hepatocyte infection. For preclinical evaluation of immunogenicity of these proteins in mice, chimeric P. berghei sporozoites were created that express the P. falciparum proteins in sporozoites as an additional copy gene under control of the uis4 gene promoter. All fourteen chimeric parasites produced sporozoites but sporozoites of eight lines failed to establish a liver infection, indicating a negative impact of these P. falciparum proteins on sporozoite infectivity. Immunogenicity of the other six proteins (SPELD, ETRAMP10.3, SIAP2, SPATR, HT, RPL3) was analyzed by immunization of inbred BALB/c and outbred CD-1 mice with viral-vectored (ChAd63 or ChAdOx1, MVA) vaccines, followed by challenge with chimeric sporozoites. Protective immunogenicity was determined by analyzing parasite liver load and prepatent period of blood stage infection after challenge. Of the six proteins only SPELD immunized mice showed partial protection. We discuss both the low protective immunogenicity of these proteins in the chimeric rodent malaria challenge model and the negative effect on P. berghei sporozoite infectivity of several P. falciparum proteins expressed in the chimeric sporozoites.

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.

The PfAP2-G2 transcription factor is a critical regulator of gametocyte maturation

Differentiation from asexual blood stages to mature sexual gametocytes is required for the transmission of malaria parasites. Here, we report that the ApiAP2 transcription factor, PfAP2-G2 (PF3D7_1408200) plays a critical role in the maturation of Plasmodium falciparum gametocytes. PfAP2-G2 binds to the promoters of a wide array of genes that are expressed at many stages of the parasite life cycle. Interestingly, we also find binding of PfAP2-G2 within the gene body of almost 3,000 genes, which strongly correlates with the location of H3K36me3 and several other histone modifications as well as Heterochromatin Protein 1 (HP1), suggesting that occupancy of PfAP2-G2 in gene bodies may serve as an alternative regulatory mechanism. Disruption of pfap2-g2 does not impact asexual development, but the majority of sexual parasites are unable to mature beyond stage III gametocytes. The absence of pfap2-g2 leads to overexpression of 28% of the genes bound by PfAP2-G2 and none of the PfAP2-G2 bound genes are downregulated, suggesting that it is a repressor. We also find that PfAP2-G2 interacts with chromatin remodeling proteins, a microrchidia (MORC) protein, and another ApiAP2 protein (PF3D7_1139300). Overall our data demonstrate that PfAP2-G2 establishes an essential gametocyte maturation program in association with other chromatin-related proteins.

A review of combination adjuvants for malaria vaccines: a promising approach for vaccine development

It is obvious that there is a critical need for an efficient malaria vaccine to accelerate malaria eradication. Currently, recombinant subunit vaccination against malaria using proteins and peptides is gaining attention. However, one of the major drawbacks of this approach is the lack of an efficient and durable immune response. Therefore, subunit vaccines require adjuvants to make the vaccine sufficiently immunogenic. Considering the history of the RTS,S vaccine, it seems likely that no single adjuvant is capable of eliciting all the protective immune responses required in many malarial subunit vaccines and the use of combination adjuvants will be increasingly important as the science of malaria vaccines advances. In light of this, it appears that identifying the most effective mixture of adjuvants with minimal adverse effects offers tremendous opportunities in improving the efficacy of vaccines against malaria. Owing to the importance of a multi-adjuvanted approach in subunit malaria vaccine development, this review paper outlines some of the best known combination adjuvants used in malaria subunit vaccines, focusing on their proposed mechanisms of action, their immunological properties, and their notable results. The aim of the present review is to consolidate these findings to aid the application of these combination adjuvants in experimental malaria vaccines.

The Stem Cell Revolution Revealing Protozoan Parasites' Secrets and Paving the Way towards Vaccine Development

Protozoan infections are leading causes of morbidity and mortality in humans and some of the most important neglected diseases in the world. Despite relentless efforts devoted to vaccine and drug development, adequate tools to treat and prevent most of these diseases are still lacking. One of the greatest hurdles is the lack of understanding of host-parasite interactions. This gap in our knowledge comes from the fact that these parasites have complex life cycles, during which they infect a variety of specific cell types that are difficult to access or model in vitro. Even in those cases when host cells are readily available, these are generally terminally differentiated and difficult or impossible to manipulate genetically, which prevents assessing the role of human factors in these diseases. The advent of stem cell technology has opened exciting new possibilities to advance our knowledge in this field. The capacity to culture Embryonic Stem Cells, derive Induced Pluripotent Stem Cells from people and the development of protocols for differentiation into an ever-increasing variety of cell types and organoids, together with advances in genome editing, represent a huge resource to finally crack the mysteries protozoan parasites hold and unveil novel targets for prevention and treatment.

Orientation of Antigen Display on Self-Assembling Protein Nanoparticles Influences Immunogenicity

Self-assembling protein nanoparticles (SAPN) serve as a repetitive antigen delivery platform with high-density epitope display; however, antigen characteristics such as size and epitope presentation can influence the immunogenicity of the assembled particle and are aspects to consider for a rationally designed effective vaccine. Here, we characterize the folding and immunogenicity of heterogeneous antigen display by integrating (a) dual-stage antigen SAPN presenting the P. falciparum (Pf) merozoite surface protein 1 subunit, PfMSP1₁₉, and Pf cell-traversal protein for ookinetes and sporozoites, PfCelTOS, in addition to (b) a homogenous antigen SAPN displaying two copies of PfCelTOS. Mice and rabbits were utilized to evaluate antigen-specific humoral and cellular induction as well as functional antibodies via growth inhibition of the blood-stage parasite. We demonstrate that antigen orientation and folding influence the elicited immune response, and when appropriately designed, SAPN can serve as an adaptable platform for an effective multi-antigen display.

A Hetero-Multimeric Chitinase-Containing Plasmodium falciparum and Plasmodium gallinaceum Ookinete-Secreted Protein Complex Involved in Mosquito Midgut Invasion

Malaria parasites are transmitted by Anopheles mosquitoes. During its life cycle in the mosquito vector the Plasmodium ookinete escapes the proteolytic milieu of the post-blood meal midgut by traversing the midgut wall. This process requires penetration of the chitin-containing peritrophic matrix lining the midgut epithelium, which depends in part on ookinete-secreted chitinases. Plasmodium falciparum ookinetes have one chitinase (PfCHT1), whereas ookinetes of the avian-infecting parasite, P. gallinaceum, have two, a long and a short form, PgCHT1 and PgCHT2, respectively. Published data indicates that PgCHT2 forms a high molecular weight (HMW) reduction-sensitive complex; and one binding partner is the ookinete-produced von Willebrand A-domain-containing protein, WARP. Size exclusion chromatography data reported here show that P. gallinaceum PgCHT2 and its ortholog, P. falciparum PfCHT1 are covalently-linked components of a HMW chitinase-containing complex (> 1,300 kDa). Mass spectrometry of ookinete-secreted proteins isolated using a new chitin bead pull-down method identified chitinase-associated proteins in P. falciparum and P. gallinaceum ookinete-conditioned culture media. Mass spectrometry of this complex showed the presence of several micronemal proteins including von Willebrand factor A domain-related protein (WARP), ookinete surface enolase, and secreted ookinete adhesive protein (SOAP). To test the hypothesis that ookinete-produced PfCHT1 can form a high molecular homo-multimer or, alternatively, interacts with P. berghei ookinete-produced proteins to produce an HMW hetero-multimer, we created chimeric P. berghei parasites expressing PfCHT1 to replace PbCHT1, enabling the production of large numbers of PfCHT1-expressing ookinetes. We show that chimeric P. berghei ookinetes express monomeric PfCHT1, but a HMW complex containing PfCHT1 is not present. A better understanding of the chitinase-containing HMW complex may enhance development of next-generation vaccines or drugs that target malaria transmission stages.

RTS,S/AS01E malaria vaccine induces IgA responses against CSP and vaccine-unrelated antigens in African children in the phase 3 trial

BACKGROUND

The evaluation of immune responses to RTS,S/AS01 has traditionally focused on immunoglobulin (Ig) G antibodies that are only moderately associated with protection. The role of other antibody isotypes that could also contribute to vaccine efficacy remains unclear. Here we investigated whether RTS,S/AS01_E elicits antigen-specific serum IgA antibodies to the vaccine and other malaria antigens, and we explored their association with protection.

METHODS

Ninety-five children (age 5-17 months old at first vaccination) from the RTS,S/AS01_E phase 3 clinical trial who received 3 doses of RTS,S/AS01_E or a comparator vaccine were selected for IgA quantification 1 month post primary immunization. Two sites with different malaria transmission intensities (MTI) and clinical malaria cases and controls, were included. Measurements of IgA against different constructs of the circumsporozoite protein (CSP) vaccine antigen and 16 vaccine-unrelated Plasmodium falciparum antigens were performed using a quantitative suspension array assay.

RESULTS

RTS,S vaccination induced a 1.2 to 2-fold increase in levels of serum/plasma IgA antibodies to all CSP constructs, which was not observed upon immunization with a comparator vaccine. The IgA response against 13 out of 16 vaccine-unrelated P. falciparum antigens also increased after vaccination, and levels were higher in recipients of RTS,S than in comparators. IgA levels to malaria antigens before vaccination were more elevated in the high MTI than the low MTI site. No statistically significant association of IgA with protection was found in exploratory analyses.

CONCLUSIONS

RTS,S/AS01_E induces IgA responses in peripheral blood against CSP vaccine antigens and other P. falciparum vaccine-unrelated antigens, similar to what we previously showed for IgG responses. Collectively, data warrant further investigation of the potential contribution of vaccine-induced IgA responses to efficacy and any possible interplay, either synergistic or antagonistic, with protective IgG, as identifying mediators of protection by RTS,S/AS01_E immunization is necessary for the design of improved second-generation vaccines.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov: NCT008666191.

Diversify and Conquer: The Vaccine Escapism of Plasmodium falciparum

Over the last century, a great deal of effort and resources have been poured into the development of vaccines to protect against malaria, particularly targeting the most widely spread and deadly species of the human-infecting parasites: Plasmodium falciparum. Many of the known proteins the parasite uses to invade human cells have been tested as vaccine candidates. However, precisely because of the importance and immune visibility of these proteins, they tend to be very diverse, and in many cases redundant, which limits their efficacy in vaccine development. With the advent of genomics and constantly improving sequencing technologies, an increasingly clear picture is emerging of the vast genomic diversity of parasites from different geographic areas. This diversity is distributed throughout the genome and includes most of the vaccine candidates tested so far, playing an important role in the low efficacy achieved. Genomics is a powerful tool to search for genes that comply with the most desirable attributes of vaccine targets, allowing us to evaluate function, immunogenicity and also diversity in the worldwide parasite populations. Even predicting how this diversity might evolve and spread in the future becomes possible, and can inform novel vaccine efforts.

Preclinical optimization of an enterotoxigenic Escherichia coli adjuvanted subunit vaccine using response surface design of experiments

Enterotoxigenic E. coli (ETEC) is a leading cause of moderate-to-severe diarrhoea. ETEC colonizes the intestine through fimbrial tip adhesin colonization factors and produces heat-stable and/or heat-labile (LT) toxins, stimulating fluid and electrolyte release leading to watery diarrhoea. We reported that a vaccine containing recombinant colonization factor antigen (CfaEB) targeting fimbrial tip adhesin of the colonization factor antigen I (CFA/I) and an attenuated LT toxoid (dmLT) elicited mucosal and systemic immune responses against both targets. Additionally, the toll-like receptor 4 ligand second-generation lipid adjuvant (TLR4-SLA) induced a potent mucosal response, dependent on adjuvant formulation. However, a combination of vaccine components at their respective individual optimal doses may not achieve the optimal immune profile. We studied a subunit ETEC vaccine prototype in mice using a response surface design of experiments (DoE), consisting of 64 vaccine dose-combinations of CfaEB, dmLT and SLA in four formulations (aqueous, aluminium oxyhydroxide, squalene-in-water stable nanoemulsion [SE] or liposomes containing the saponin Quillaja saponaria-21 [LSQ]). Nine readouts focusing on antibody functionality and plasma cell response were selected to profile the immune response of parenterally administered ETEC vaccine prototype. The data were integrated in a model to identify the optimal dosage of each vaccine component and best formulation. Compared to maximal doses used in mouse models (10 µg CfaEB, 1 µg dmLT and 5 µg SLA), a reduction in the vaccine components up to 37%, 60% and 88% for CfaEB, dmLT and SLA, respectively, maintained or even maximized immune responses, with SE and LSQ the best formulations. The DoE approach can help determine the best vaccine composition with a limited number of experiments and may accelerate development of multi-antigen/component ETEC vaccines.

Plasmodium vivax Cell Traversal Protein for Ookinetes and Sporozoites (CelTOS) Functionally Restricted Regions Are Involved in Specific Host-Pathogen Interactions

Following the injection of Plasmodium sporozoites by a female Anopheles mosquito into the dermis, they become engaged on a long journey to hepatic tissue where they must migrate through different types of cell to become established in parasitophorous vacuoles in hepatocytes. Studies have shown that proteins such as cell traversal protein for Plasmodium ookinetes and sporozoites (CelTOS) play a crucial role in cell-traversal ability. Although CelTOS has been extensively studied in various species and included in pre-clinical assays it remains unknown which P. vivax CelTOS (PvCelTOS) regions are key in its interaction with traversed or target cells (Kupffer or hepatocytes) and what type of pressure, association and polymorphism these important regions could have to improve their candidacy as important vaccine antigens. This work has described producing a recombinant PvCelTOS which was recognized by ~30% P. vivax-infected individuals, thereby confirming its ability for inducing a natural immune response. PvCelTOS' genetic diversity in Colombia and its ability to interact with HeLa (traversal cell) and/or HepG2 cell (target cell) external membrane have been assessed. One region in the PvCelTOS amino-terminal region and another in its C-terminus were seen to be participating in host-pathogen interactions. These regions had important functional constraint signals (ω < 0.3 and several sites under negative selection) and were able to inhibit specific rPvCelTOS binding to HeLa cells. This led to suggesting that sequences between aa 41-60 (40833) and 141-160 (40838) represent promising candidates for an anti-P. vivax subunit-based vaccine.

Codon harmonization reduces amino acid misincorporation in bacterially expressed P. falciparum proteins and improves their immunogenicity

Codon usage frequency influences protein structure and function. The frequency with which codons are used potentially impacts primary, secondary and tertiary protein structure. Poor expression, loss of function, insolubility, or truncation can result from species-specific differences in codon usage. “Codon harmonization” more closely aligns native codon usage frequencies with those of the expression host particularly within putative inter-domain segments where slower rates of translation may play a role in protein folding. Heterologous expression of Plasmodium falciparum genes in Escherichia coli has been a challenge due to their AT-rich codon bias and the highly repetitive DNA sequences. Here, codon harmonization was applied to the malarial antigen, CelTOS (Cell-traversal protein for ookinetes and sporozoites). CelTOS is a highly conserved P. falciparum protein involved in cellular traversal through mosquito and vertebrate host cells. It reversibly refolds after thermal denaturation making it a desirable malarial vaccine candidate. Protein expressed in E. coli from a codon harmonized sequence of P. falciparum CelTOS (CH-PfCelTOS) was compared with protein expressed from the native codon sequence (N-PfCelTOS) to assess the impact of codon usage on protein expression levels, solubility, yield, stability, structural integrity, recognition with CelTOS-specific mAbs and immunogenicity in mice. While the translated proteins were expected to be identical, the translated products produced from the codon-harmonized sequence differed in helical content and showed a smaller distribution of polypeptides in mass spectra indicating lower heterogeneity of the codon harmonized version and fewer amino acid misincorporations. Substitutions of hydrophobic-to-hydrophobic amino acid were observed more commonly than any other. CH-PfCelTOS induced significantly higher antibody levels compared with N-PfCelTOS; however, no significant differences in either IFN-γ or IL-4 cellular responses were detected between the two antigens.

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.

Combining Monophosphoryl Lipid A (MPL), CpG Oligodeoxynucleotide (ODN), and QS-21 Adjuvants Induces Strong and Persistent Functional Antibodies and T Cell Responses against Cell-Traversal Protein for Ookinetes and Sporozoites (CelTOS) of Plasmodium falciparum in BALB/c Mice

Plasmodium falciparum cell-traversal protein for ookinetes and sporozoites (PfCelTOS) is an advanced vaccine candidate that has a crucial role in the traversal of the malaria parasite in both mosquito and mammalian hosts. As recombinant purified proteins are normally poor immunogens, they require to be admixed with an adjuvant(s); therefore, the objective of the present study was to evaluate the capacity of different vaccine adjuvants, monophosphoryl lipid A (MPL), CpG, and Quillaja saponaria Molina fraction 21 (QS-21), alone or in combination (MCQ [MPL/CpG/QS-21]), to enhance the immunogenicity of Escherichia coli-expressed PfCelTOS in BALB/c mice. This goal was achieved by the assessment of anti-PfCelTOS IgG antibodies (level, titer, IgG isotype profile, avidity, and persistence) and extracellular Th1 cytokines using an enzyme-linked immunosorbent assay (ELISA) on postimmunized BALB/c mouse sera and PfCelTOS-stimulated splenocytes, respectively. Also, an assessment of the transmission-reducing activity (TRA) of anti-PfCelTOS obtained from different vaccine groups was carried out in female Anopheles stephensi mosquitoes by using a standard membrane feeding assay (SMFA). In comparison to PfCelTOS alone, administration of PfCelTOS with three distinct potent Th1 adjuvants in vaccine mouse groups showed enhancement and improvement of PfCelTOS immunogenicity that generated more bias toward a Th1 response with significantly enhanced titers and avidity of the anti-PfCelTOS responses that could impair ookinete development in A. stephensi However, immunization of mice with PfCelTOS with MCQ mixture adjuvants resulted in the highest levels of induction of antibody titers, avidity, and inhibitory antibodies in oocyst development (88%/26.7% reductions in intensity/prevalence) in A. stephensi It could be suggested that adjuvant combinations with different mechanisms stimulate better functional antibody responses than adjuvants individually against challenging diseases such as malaria.

Cell-traversal protein for ookinetes and sporozoites (CelTOS) formulated with potent TLR adjuvants induces high-affinity antibodies that inhibit Plasmodium falciparum infection in Anopheles stephensi

Background

Plasmodium falciparum parasite is the most deadly species of human malaria, and the development of an effective vaccine that prevents P. falciparum infection and transmission is a key target for malarial elimination and eradication programmes. P. falciparum cell-traversal protein for ookinetes and sporozoites (PfCelTOS) is an advanced vaccine candidate. A comparative study was performed to characterize the immune responses in BALB/c mouse immunized with Escherichia coli-expressed recombinant PfCelTOS (rPfCelTOS) in toll-like receptor (TLR)-based adjuvants, CpG and Poly I:C alone or in combination (CpG + Poly I:C), followed by the assessment of transmission-reducing activity (TRA) of anti-rPfCelTOS antibodies obtained from different vaccine groups in Anopheles stephensi.

Methods

The aim of the current work was achieved by head-to-head comparison of the vaccine groups using conventional and avidity enzyme-linked immunosorbent assay (ELISA), immunofluorescence test (IFAT), and standard membrane feeding assay (SMFA).

Results

Comparing to rPfCelTOS alone, administration of rPfCelTOS with two distinct TLR-based adjuvants in vaccine mouse groups showed a significant increase in responses (antibody level, IgG subclass analysis, avidity, and Th1 cytokines) and was able to induce reasonable transmission-reducing activity. Also, comparable functional activity of anti-rPfCelTOS antibodies was found in group that received antigen in either CpG or Poly I:C (69.9%/20% and 73.5%/24.4%, respectively, reductions in intensity/prevalence). However, the vaccine group receiving rPfCelTOS in combination with CpG + Poly I:C showed a significant induction in antibody titers and inhibitory antibodies in oocysts development (78.3%/19.6% reductions in intensity/prevalence) in An. stephensi.

Conclusions

A key finding in this investigation is that rPfCelTOS administered alone in BALB/c mouse is poorly immunogenic, with relatively low IgG level, avidity, inhibitory antibodies, and mixed Th1/Th2 responses. However, immunological characteristic (IgG level, cytophilic IgG2a and IgG2b, avidity, and Th1 cytokines) and TRA of anti-rPfCelTOS significantly enhanced in the presence of co-administration of TLR-based adjuvants, confirming that targeting TLRs would be an effective means for the enhancement of inducing TRA against rPfCelTOS.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2773-3) contains supplementary material, which is available to authorized users.

Expression of Recombinant PfCelTOS Antigen in the Chloroplast of Chlamydomonas reinhardtii and its Potential Use in Detection of Malaria

Malaria is a serious but preventable and treatable infectious disease that is found in over 100 countries around the world. Correct and rapid diagnosis of malaria infection can rescue the patient of getting sicker and reduces the risk of disease spreading among humans. Chlamydomonas reinhardtii chloroplast is an attractive platform for expressing malaria antigens because it is capable of folding complex proteins, including those requiring disulfide bond formation, while lack the ability to glycosylate proteins; a valuable quality of any malaria protein expression system, since the Plasmodium parasite lacks N-linked glycosylation machinery. In this study, Cell-traversal protein for ookinetes and sporozoites (CelTOS) antigen from Plasmodium falciparum was expressed in the chloroplast of C. reinhardtii and a highly sensitive and specific indirect ELISA test was developed using C. reinhardtii expressed PfCelTOS to detect malaria. Results obtained demonstrated that expressed recombinant PfCelTOS accumulates as a soluble, properly folded and functional protein within C. reinhardtii chloroplast and indirect ELISA using sera from malaria-positive donors suggested the potential use of expressed PfCelTOS as a malaria antigen for diagnosis tests.

Immune Responses to the Sexual Stages of Plasmodium falciparum Parasites

Malaria infections remain a serious global health problem in the world, particularly among children and pregnant women in Sub-Saharan Africa. Moreover, malaria control and elimination is hampered by rapid development of resistance by the parasite and the vector to commonly used antimalarial drugs and insecticides, respectively. Therefore, vaccine-based strategies are sorely needed, including those designed to interrupt disease transmission. However, a prerequisite for such a vaccine strategy is the understanding of both the human and vector immune responses to parasite developmental stages involved in parasite transmission in both man and mosquito. Here, we review the naturally acquired humoral and cellular responses to sexual stages of the parasite while in the human host and the Anopheles vector. In addition, updates on current anti-gametocyte, anti-gamete, and anti-mosquito transmission blocking vaccines are given. We conclude with our views on some important future directions of research into P. falciparum sexual stage immunity relevant to the search for the most appropriate transmission-blocking vaccine.

Heterogeneity in the acquisition of naturally acquired antibodies to cell-traversal protein for ookinetes and sporozoites (CelTOS) and thrombospondin-related adhesion protein (TRAP) of Plasmodium falciparum in naturally infected patients from unstable malaria areas in Iran

Currently, there is no subunit malaria vaccine capable of providing long-lasting protection, and a vaccine based on a single-antigen has shown moderate to unsatisfactory efficacies in clinical trials. As in malaria elimination and eradication strategies, the primary objective is reduction in disease and death due to P. falciparum, in the present investigation, for the first time, we attempted to determine and compare the naturally acquired immune responses to two well-recognized sporozoite antigens, cell-traversal protein for ookinetes and sporozoites (CelTOS) and thrombospondin-related adhesion protein (TRAP), in P. falciparum-infected individuals (n = 204) in low malaria transmission settings of Iran using ELISA. Besides, the profile of IgG isotype responses, the avidity of IgG, IgG1, and IgG3, and the association of anti-PfCelTOS and -PfTRAP antibodies with host age were evaluated. Positive antibody responses to PfCelTOS and PfTRAP antigens were detected in 16.2% and 31.9% of Iranian P. falciparum-infected individuals, respectively, indicating significantly lower immune response to PfCelTOS than PfTRAP (P <0.0001, McNemar's test). Also, among the positive samples for anti-PfCelTOS (n = 33) and -PfTRAP (n = 65) total IgG, the cytophilic IgG1 and IgG3 antibodies were predominant. A significant proportion of the examined positive responders had high- and intermediate-avidity for IgG (93.9%, 87.7%), IgG1 (96.3%, 87.7%), and IgG3 (76%, 78.7%) antibodies to both PfCelTOS and PfTRAP antigens, respectively, with no correlation with age (P >0.05; Spearman's correlation test). In conclusion, the present data suggests the acquisition of heterogenic immune responses to both antigens in the same patients naturally infected with P. falciparum from settings of low malaria transmission intensity in Iran in which their role in protection to malaria needs further study.

The Antibody Response to Plasmodium falciparum: Cues for Vaccine Design and the Discovery of Receptor-Based Antibodies

Plasmodium falciparum remains a serious public health problem and a continuous challenge for the immune system due to the complexity and diversity of the pathogen. Recent advances from several laboratories in the characterization of the antibody response to the parasite have led to the identification of critical targets for protection and revealed a new mechanism of diversification based on the insertion of host receptors into immunoglobulin genes, leading to the production of receptor-based antibodies. These advances have opened new possibilities for vaccine design and passive antibody therapies to provide sterilizing immunity and control blood-stage parasites.

Plasmodium genomics: an approach for learning about and ending human malaria

Malaria causes high levels of morbidity and mortality in human beings worldwide. According to the World Health Organization (WHO), about half a million people die of this disease each year. Malaria is caused by six species of parasites belonging to the Plasmodium genus: P. falciparum, P. knowlesi, P. vivax, P. malariae, P. ovale curtisi, and P. ovale wallikeri. Currently, malaria is being kept under control with varying levels of elimination success in different countries. The development of new molecular tools as well as the use of next-generation sequencing (NGS) technologies and novel bioinformatic approaches has improved our knowledge of malarial epidemiology, diagnosis, treatment, vaccine development, and surveillance strategies. In this work, the genetics and genomics of human malarias have been analyzed. Since the first P. falciparum genome was sequenced in 2002, various population-level genetic and genomic surveys, together with transcriptomic and proteomic studies, have shown the importance of molecular approaches in supporting malaria elimination.

Synthetic Toll-like receptor agonists for the development of powerful malaria vaccines: a patent review

INTRODUCTION

Currently, there is no efficient vaccine available against clinical malaria. However, continuous efforts have been committed to develop powerful antimalarial vaccine by discovery of novel antigens with in-depth understanding of its nature, immunogenicity, and presentation (delivery adjuvants). Moreover, another important part of vaccine development includes discovery of better immunostimulatory formulation components (immunostimulants). A protective vaccine against malaria requires antigen-specific B and T helper cell responses as well as cytotoxic T lymphocyte (CTL) responses. A long-lasting B and T memory cell production is also required for effective malaria vaccine. Since activation of Toll-like receptors (TLRs) promotes both innate inflammatory responses as well as the induction of adaptive immunity, several initiatives have been mounted during the last few years for the use of TLR agonists as malaria vaccine adjuvants.

AREAS COVERED

The review summarizes reports related to the use and development of TLR agonists as malaria vaccine adjuvants and describes various strategies involved for the selection of specific antigens and TLR agonists.

EXPERT OPINION

TLR agonists are promising adjuvants for the development of effective malaria vaccine, allowing for both innate inflammatory responses as well as the induction of adaptive immunity.

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.

Analysis of genetic diversity and population structure of gene encoding cell-traversal protein for ookinetes and sporozoites (CelTOS) vaccine candidate antigen in global Plasmodium falciparum populations

Plasmodium falciparum cell-traversal protein for ookinetes and sporozoites (PfCelTOS) has been reported as one of the most attractive malaria vaccine candidate antigens. To design a broadly effective malaria vaccine based on this antigen, it is crucial to have adequate information on genetic diversity in global PfCelTOS. Therefore, the extent of sequence diversity at the full-length of the pfceltos was assessed among both natural P. falciparum isolates collected from Iran (n = 93) and from available global pfceltos sequence data retrieved from PlasmoDB database (n = 159). Also, recombination, natural selection, the degree of genetic differentiation as well as the predicted immunodominant regions in PfCelTOS were analyzed. In total, 40 SNPs (including 1 synonymous and 39 non-synonymous) were detected in 34 positions, as compared to 3D7 sequence, which led to 66 distinct haplotypes with different frequencies. Among those haplotypes, 34 (51.5%, excluded from further analysis) were singleton haplotype and mostly detected among Senegalese parasite isolates. PfCelt-1 was found as predominant haplotype (32.6% total frequency) that was only detected in Iranian P. falciparum isolates. Nucleotide diversity was low in French Guiana (0.00236 ± 0.00203) and Iranian (0.00259 ± 0.00048) P. falciparum isolates in comparison with African populations. Evidence for positive selection by host immunity and intragenic recombination were detected that are two key factors responsible for gene evolution and genetic diversity of pfceltos gene. The results of Fst analysis and haplotype network revealed that PfCelTOS antigen displayed evident genetic structure between geographical parasite populations. In conclusion, the present analysis demonstrates that there is a limited antigenic diversity and geographic variation in global PfCelTOS, and this finding may be associated with the critical function of this antigen in cell traversal of the parasite in sporozoite and ookinete. Besides, most of the predicted B- and T-cell epitopes were located in the conserved region of the gene, but most of the amino acid replacements were located at the C-terminal region of PfCelTOS. The obtained results in this investigation could provide knowledge for better design of PfCelTOS-based malaria vaccine.

Plasmodium yoelii S4/CelTOS is important for sporozoite gliding motility and cell traversal

Gliding motility and cell traversal by the Plasmodium ookinete and sporozoite invasive stages allow penetration of cellular barriers to establish infection of the mosquito vector and mammalian host, respectively. Motility and traversal are not observed in red cell infectious merozoites, and we have previously classified genes that are expressed in sporozoites but not merozoites (S genes) in order to identify proteins involved in these processes. The S4 gene has been described as criticaly involved in Cell Traversal for Ookinetes and Sporozoites (CelTOS), yet knockout parasites (s4/celtos¯) do not generate robust salivary gland sporozoite numbers, precluding a thorough analysis of S4/CelTOS function during host infection. We show here that a failure of oocysts to develop or survive in the midgut contributes to the poor mosquito infection by Plasmodium yoelii (Py) s4/celtos¯ rodent malaria parasites. We rescued this phenotype by expressing S4/CelTOS under the ookinete-specific circumsporozoite protein and thrombospondin-related anonymous protein-related protein (CTRP) promoter (S4/CelTOS^CTRP ), generating robust numbers of salivary gland sporozoites lacking S4/CelTOS that were suitable for phenotypic analysis. Py S4/CelTOS^CTRP sporozoites showed reduced infectivity in BALB/c mice when compared to wild-type sporozoites, although they appeared more infectious than sporozoites deficient in the related traversal protein PLP1/SPECT2 (Py plp1/spect2¯). Using in vitro assays, we substantiate the role of S4/CelTOS in sporozoite cell traversal, but also uncover a previously unappreciated role for this protein for sporozoite gliding motility.

Assessment of the Combined Effect of Epstein-Barr Virus and Plasmodium falciparum Infections on Endemic Burkitt Lymphoma Using a Multiplex Serological Approach

Epstein-Barr virus (EBV) is a necessary cause of endemic Burkitt lymphoma (eBL), while the role of Plasmodium falciparum in eBL remains uncertain. This study aimed to generate new hypotheses on the interplay between both infections in the development of eBL by investigating the IgG and IgM profiles against several EBV and P. falciparum antigens. Serum samples collected in a childhood study in Malawi (2005-2006) from 442 HIV-seronegative children (271 eBL cases and 171 controls) between 1.4 and 15 years old were tested by quantitative suspension array technology against a newly developed multiplex panel combining 4 EBV antigens [Z Epstein-Barr replication activator protein (ZEBRA), early antigen-diffuse component (EA-D), EBV nuclear antigen 1, and viral capsid antigen p18 subunit (VCA-p18)] and 15 P. falciparum antigens selected for their immunogenicity, role in malaria pathogenesis, and presence in different parasite stages. Principal component analyses, multivariate logistic models, and elastic-net regressions were used. As expected, elevated levels of EBV IgG (especially against the lytic antigens ZEBRA, EA-D, and VCA-p18) were strongly associated with eBL [high vs low tertile odds ratio (OR) = 8.67, 95% confidence interval (CI) = 4.81-15.64]. Higher IgG responses to the merozoite surface protein 3 were observed in children with eBL compared with controls (OR = 1.29, 95% CI = 1.02-1.64), showing an additive interaction with EBV IgGs (OR = 10.6, 95% CI = 5.1-22.2, P = 0.05). Using elastic-net regression models, eBL serological profile was further characterized by lower IgM levels against P. falciparum preerythrocytic-stage antigen CelTOS and EBV lytic antigen VCA-p18 compared with controls. In a secondary analysis, abdominal Burkitt lymphoma had lower IgM to EBV and higher IgG to EA-D levels than cases with head involvement. Overall, this exploratory study confirmed the strong role of EBV in eBL and identified differential IgG and IgM patterns to erythrocytic vs preerythrocytic P. falciparum antigens that suggest a more persistent/chronic malaria exposure and a weaker IgM immune response in children with eBL compared with controls. Future studies should continue exploring how the malaria infection status and the immune response to P. falciparum interact with EBV infection in the development of eBL.

Structural Features of Apicomplexan Pore-Forming Proteins and Their Roles in Parasite Cell Traversal and Egress

Apicomplexan parasites cause diseases, including malaria and toxoplasmosis, in a range of hosts, including humans. These intracellular parasites utilize pore-forming proteins that disrupt host cell membranes to either traverse host cells while migrating through tissues or egress from the parasite-containing vacuole after replication. This review highlights recent insight gained from the newly available three-dimensional structures of several known or putative apicomplexan pore-forming proteins that contribute to cell traversal or egress. These new structural advances suggest that parasite pore-forming proteins use distinct mechanisms to disrupt host cell membranes at multiple steps in parasite life cycles. How proteolytic processing, secretion, environment, and the accessibility of lipid receptors regulate the membranolytic activities of such proteins is also discussed.

Limited genetic diversity in the global Plasmodium vivax Cell traversal protein of Ookinetes and Sporozoites (CelTOS) sequences; implications for PvCelTOS-based vaccine development

Cell traversal protein of Ookinetes and Sporozoites (CelTOS) is a new malaria vaccine candidate antigen. Since one of the main challenges in malaria vaccine development is the extensive antigenic diversity of this parasite, local and global gene diversity analysis is of particular importance. Therefore, in this study, the genetic diversity of pvceltos gene was investigated among Iranian P. vivax isolates (n=46) and compared with available worldwide pvceltos sequences. One synonymous (C109A) and three amino acid replacements (V118L, K178T, and G179R) were observed in Iranian pvceltos sequences in compare with Sal-1 sequence leading to five haplotypes including PvCelt-A (GSVKGL, 13%), PvCelt-B (GSLKGL, 50%), PvCelt-C (GSLTGL, 17.4%), PvCelt-D (GSVTGL, 13%) and PvCelt-E (GSLTRL, 6.5%). However, amino acid replacements were observed in six positions (G10S, S40N, V118L/M, K178T, G179R/D and L181R) in PvCelTOS antigen of global isolates leading to 11 distinct haplotypes. PvCelt-A and PvCelt-B haplotypes were the most common haplotypes in the world. The overall nucleotide diversity for Iranian isolates was 0.00169, while, the level of nucleotide diversity was ranged from 0.00252 for Thailand to 0.00022 for Peru populations in the world. The analysis of SNPs in relation with the predicted immunodominant regions revealed that only K178T and G179R SNPs are located in putative B-cell epitopes. All replacements were located in CD4+ and/or CD8+ T-cell epitopes. However, the majority of epitopes are located in conserved regions. Knowing whether these changes may alter the affinity of the epitopes for antibodies and/or MHC molecules remains to be investigated in experimental studies. In conclusion, the present study showed a very limited genetic diversity in pvceltos gene among the global clinical isolates that can be regarded as a potential candidate antigen to apply for vivax-based malaria vaccine development.

Malaria parasite LIMP protein regulates sporozoite gliding motility and infectivity in mosquito and mammalian hosts

Gliding motility allows malaria parasites to migrate and invade tissues and cells in different hosts. It requires parasite surface proteins to provide attachment to host cells and extracellular matrices. Here, we identify the Plasmodium protein LIMP (the name refers to a gliding phenotype in the sporozoite arising from epitope tagging of the endogenous protein) as a key regulator for adhesion during gliding motility in the rodent malaria model P. berghei. Transcribed in gametocytes, LIMP is translated in the ookinete from maternal mRNA, and later in the sporozoite. The absence of LIMP reduces initial mosquito infection by 50%, impedes salivary gland invasion 10-fold, and causes a complete absence of liver invasion as mutants fail to attach to host cells. GFP tagging of LIMP caused a limping defect during movement with reduced speed and transient curvature changes of the parasite. LIMP is an essential motility and invasion factor necessary for malaria transmission.

Evaluation of Plasmodium vivax Cell-Traversal Protein for Ookinetes and Sporozoites as a Preerythrocytic P. vivax Vaccine

Four different vaccine platforms, each targeting the human malaria parasite Plasmodium vivax cell-traversal protein for ookinetes and sporozoites (PvCelTOS), were generated and assessed for protective efficacy. These platforms consisted of a recombinant chimpanzee adenoviral vector 63 (ChAd63) expressing PvCelTOS (Ad), a recombinant modified vaccinia virus Ankara expressing PvCelTOS (MVA), PvCelTOS conjugated to bacteriophage Qβ virus-like particles (VLPs), and a recombinant PvCelTOS protein expressed in eukaryotic HEK293T cells (protein). Inbred BALB/c mice and outbred CD-1 mice were immunized using the following prime-boost regimens: Ad-MVA, Ad-VLPs, and Ad-protein. Protective efficacy against sporozoite challenge was assessed after immunization using a novel chimeric rodent Plasmodium berghei parasite (Pb-PvCelTOS). This chimeric parasite expresses P. vivax CelTOS in place of the endogenous P. berghei CelTOS and produces fully infectious sporozoites. A single Ad immunization in BALB/c and CD-1 mice induced anti-PvCelTOS antibodies which were boosted efficiently using MVA, VLP, or protein immunization. PvCelTOS-specific gamma interferon- and tumor necrosis factor alpha-producing CD8⁺ T cells were induced at high frequencies by all prime-boost regimens in BALB/c mice but not in CD-1 mice; in CD-1 mice, they were only marginally increased after boosting with MVA. Despite the induction of anti-PvCelTOS antibodies and PvCelTOS-specific CD8⁺ T-cell responses, only low levels of protective efficacy against challenge with Pb-PvCelTOS sporozoites were obtained using any immunization strategy. In BALB/c mice, no immunization regimens provided significant protection against a Pb-PvCelTOS chimeric sporozoite challenge. In CD-1 mice, modest protective efficacy against challenge with chimeric P. berghei sporozoites expressing either PvCelTOS or P. falciparum CelTOS was observed using the Ad-protein vaccination regimen.