Immunization with the MAEBL M2 Domain Protects against Lethal Plasmodium yoelii Infection

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.

Preliminary characterization of Plasmodium vivax sporozoite antigens as pre-erythrocytic vaccine candidates

Plasmodium vivax pre-erythrocytic (PE) vaccine research has lagged far behind efforts to develop Plasmodium falciparum vaccines. There is a critical gap in our knowledge of PE antigen targets that can induce functionally inhibitory neutralizing antibody responses. To overcome this gap and guide the selection of potential PE vaccine candidates, we considered key characteristics such as surface exposure, essentiality to infectivity and liver stage development, expression as recombinant proteins, and functional immunogenicity. Selected P. vivax sporozoite antigens were surface sporozoite protein 3 (SSP3), sporozoite microneme protein essential for cell traversal (SPECT1), sporozoite surface protein essential for liver-stage development (SPELD), and M2 domain of MAEBL. Sequence analysis revealed little variation occurred in putative B-cell and T-cell epitopes of the PE candidates. Each antigen was tested for expression as refolded recombinant proteins using an established bacterial expression platform and only SPELD failed. The successfully expressed antigens were immunogenic in vaccinated laboratory mice and were positively reactive with serum antibodies of P. vivax-exposed residents living in an endemic region in Thailand. Vaccine immune antisera were tested for reactivity to native sporozoite proteins and for their potential vaccine efficacy using an in vitro inhibition of liver stage development assay in primary human hepatocytes quantified on day 6 post-infection by high content imaging analysis. The anti-PE sera produced significant inhibition of P. vivax sporozoite invasion and liver stage development. This report provides an initial characterization of potential new PE candidates for a future P. vivax vaccine.

Plasmodium vivax vaccine: What is the best way to go?

Malaria is one of the most devastating human infectious diseases caused by Plasmodium spp. parasites. A search for an effective and safe vaccine is the main challenge for its eradication. Plasmodium vivax is the second most prevalent Plasmodium species and the most geographically distributed parasite and has been neglected for decades. This has a massive gap in knowledge and consequently in the development of vaccines. The most significant difficulties in obtaining a vaccine against P. vivax are the high genetic diversity and the extremely complex life cycle. Due to its complexity, studies have evaluated P. vivax antigens from different stages as potential targets for an effective vaccine. Therefore, the main vaccine candidates are grouped into preerythrocytic stage vaccines, blood-stage vaccines, and transmission-blocking vaccines. This review aims to support future investigations by presenting the main findings of vivax malaria vaccines to date. There are only a few P. vivax vaccines in clinical trials, and thus far, the best protective efficacy was a vaccine formulated with synthetic peptide from a circumsporozoite protein and Montanide ISA-51 as an adjuvant with 54.5% efficacy in a phase IIa study. In addition, the majority of P. vivax antigen candidates are polymorphic, induce strain-specific and heterogeneous immunity and provide only partial protection. Nevertheless, immunization with recombinant proteins and multiantigen vaccines have shown promising results and have emerged as excellent strategies. However, more studies are necessary to assess the ideal vaccine combination and test it in clinical trials. Developing a safe and effective vaccine against vivax malaria is essential for controlling and eliminating the disease. Therefore, it is necessary to determine what is already known to propose and identify new candidates.

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.

MAEBL Contributes to Plasmodium Sporozoite Adhesiveness

The sole currently approved malaria vaccine targets the circumsporozoite protein-the protein that densely coats the surface of sporozoites, the parasite stage deposited in the skin of the mammalian host by infected mosquitoes. However, this vaccine only confers moderate protection against clinical diseases in children, impelling a continuous search for novel candidates. In this work, we studied the importance of the membrane-associated erythrocyte binding-like protein (MAEBL) for infection by Plasmodium sporozoites. Using transgenic parasites and live imaging in mice, we show that the absence of MAEBL reduces Plasmodium berghei hemolymph sporozoite infectivity to mice. Moreover, we found that maebl knockout (maebl-) sporozoites display reduced adhesion, including to cultured hepatocytes, which could contribute to the defects in multiple biological processes, such as in gliding motility, hepatocyte wounding, and invasion. The maebl- defective phenotypes in mosquito salivary gland and liver infection were reverted by genetic complementation. Using a parasite line expressing a C-terminal myc-tagged MAEBL, we found that MAEBL levels peak in midgut and hemolymph parasites but drop after sporozoite entry into the salivary glands, where the labeling was found to be heterogeneous among sporozoites. MAEBL was found associated, not only with micronemes, but also with the surface of mature sporozoites. Overall, our data provide further insight into the role of MAEBL in sporozoite infectivity and may contribute to the design of future immune interventions.

A Synthetic Nanoparticle Based Vaccine Approach Targeting MSP4/5 Is Immunogenic and Induces Moderate Protection Against Murine Blood-Stage Malaria

Malaria remains a significant health problem in many tropical and sub-tropical regions. The development of vaccines against the clinically active blood-stage of infection needs to consider variability and polymorphism in target antigens, and an adjuvant system able to induce broad spectrum immunity comprising both antibodies and helper T cells. Moreover, recent studies have shown some conventional pro-inflammatory adjuvants can also promote expansion of immunosuppressive regulatory T cells (Treg) and myeloid derived suppressor cells (MDSC), both of which could negatively impact malaria disease progression. Herein, we explore the ability of a model nanoparticle delivery system (polystyrene nanoparticles; PSNPs), previously proven to not induce conventional inflammation, Treg or MDSC, to induce immunity to MSP4/5 from Plasmodium yoelii, a member of the MSP4 and MSP5 family of proteins which are highly conserved across diverse malaria species including P. falciparum. The results show PSNPs-MSP4/5 conjugates are highly immunogenic, inducing immune responses comprising both T helper 1 (Th1) and Th2 cellular immunity, and a spectrum of antibody subclasses including IgG1, IgG2a, and IgG2b. Benchmarked against Alum and Complete Freund's Adjuvant (CFA), the immune responses that were induced were of comparable or higher magnitude, for both T cell frequencies by ELISpot and antibody responses in terms of ELISA end titer. Importantly, immunization with PSNPs-MSP4/5 induced partial protection against malaria blood-stage infection (50–80%) shown to be mechanistically dependent on interferon gamma (IFN-γ) production. These results expand the scope of adjuvants considered for malaria blood-stage vaccine development to those that do not use conventional adjuvant pathways and emphasizes the critical role of cellular immunity and specifically IFN-γ producing cells in providing moderate protection against blood-stage malaria comparable to Freunds adjuvant.

Identifying Potential Plasmodium vivax Sporozoite Stage Vaccine Candidates: An Analysis of Genetic Diversity and Natural Selection

Parasite antigen genetic diversity represents a great obstacle when designing a vaccine against malaria caused by Plasmodium vivax. Selecting vaccine candidate antigens has been focused on those fulfilling a role in invasion and which are conserved, thus avoiding specific-allele immune responses. Most antigens described to date belong to the blood stage, thereby blocking parasite development within red blood cells, whilst studying antigens from other stages has been quite restricted. Antigens from different parasite stages are required for developing a completely effective vaccine; thus, pre-erythrocyte stage antigens able to block the first line of infection becoming established should also be taken into account. However, few antigens from this stage have been studied to date. Several P. falciparum sporozoite antigens are involved in invasion. Since 77% of genes are orthologous amongst Plasmodium parasites, P. vivax sporozoite antigen orthologs to those of P. falciparum might be present in its genome. Although these genes might have high genetic diversity, conserved functionally-relevant regions (ideal for vaccine development) could be predicted by comparing genetic diversity patterns and evolutionary rates. This study was thus aimed at searching for putative P. vivax sporozoite genes so as to analyse their genetic diversity for determining their potential as vaccine candidates. Several DNA sequence polymorphism estimators were computed at each locus. The evolutionary force (drift, selection and recombination) drawing the genetic diversity pattern observed was also determined by using tests based on polymorphism frequency spectrum as well as the type of intra- and inter-species substitutions. Likewise, recombination was assessed both indirectly and directly. The results showed that sporozoite genes were more conserved than merozoite genes evaluated to date. Putative domains implied in cell traversal, gliding motility and hepatocyte interaction had a negative selection signal, being conserved amongst different species in the genus. PvP52, PvP36, PvSPATR, PvPLP1, PvMCP1, PvTLP, PvCelTOS, and PvMB2 antigens or functionally restricted regions within them would thus seem promising vaccine candidates and could be used when designing a pre-erythrocyte and/or multi-stage vaccine against P. vivax to avoid allele-specific immune responses that could reduce vaccine efficacy.

In silico epitope mapping and experimental evaluation of the Merozoite Adhesive Erythrocytic Binding Protein (MAEBL) as a malaria vaccine candidate

BACKGROUND

Technical limitations for culturing the human malaria parasite Plasmodium vivax have impaired the discovery of vaccine candidates, challenging the malaria eradication agenda. The immunogenicity of the M2 domain of the Merozoite Adhesive Erythrocytic Binding Protein (MAEBL) antigen cloned from the Plasmodium yoelii murine parasite, has been previously demonstrated.

RESULTS

Detailed epitope mapping of MAEBL through immunoinformatics identified several MHCI, MHCII and B cell epitopes throughout the peptide, with several of these lying in the M2 domain and being conserved between P. vivax, P. yoelii and Plasmodium falciparum, hinting that the M2-MAEBL is pan-reactive. This hypothesis was tested through functional assays, showing that P. yoelii M2-MAEBL antisera are able to recognize and inhibit erythrocyte invasion from both P. falciparum and P. vivax parasites isolated from Thai patients, in ex vivo assays. Moreover, the sequence of the M2-MAEBL is shown to be highly conserved between P. vivax isolates from the Amazon and Thailand, indicating that the MAEBL antigen may constitute a vaccine candidate outwitting strain-specific immunity.

CONCLUSIONS

The MAEBL antigen is promising candidate towards the development of a malaria vaccine.

Functionally relevant proteins in Plasmodium falciparum host cell invasion

A totally effective, antimalarial vaccine must involve sporozoite and merozoite proteins (or their fragments) to ensure complete parasite blocking during critical invasion stages. This Special Report examines proteins involved in critical biological functions for parasite survival and highlights the conserved amino acid sequences of the most important proteins involved in sporozoite invasion of hepatocytes and merozoite invasion of red blood cells. Conserved high activity binding peptides are located in such proteins’ functionally strategic sites, whose functions are related to receptor binding, nutrient and protein transport, enzyme activity and molecule–molecule interactions. They are thus excellent targets for vaccine development as they block proteins binding function involved in invasion and also their biological function.

Species-specific escape of Plasmodium sporozoites from oocysts of avian, rodent, and human malarial parasites

Background

Malaria is transmitted when an infected mosquito delivers Plasmodium sporozoites into a vertebrate host. There are many species of Plasmodium and, in general, the infection is host-specific. For example, Plasmodium gallinaceum is an avian parasite, while Plasmodium berghei infects mice. These two parasites have been extensively used as experimental models of malaria transmission. Plasmodium falciparum and Plasmodium vivax are the most important agents of human malaria, a life-threatening disease of global importance. To complete their life cycle, Plasmodium parasites must traverse the mosquito midgut and form an oocyst that will divide continuously. Mature oocysts release thousands of sporozoites into the mosquito haemolymph that must reach the salivary gland to infect a new vertebrate host. The current understanding of the biology of oocyst formation and sporozoite release is mostly based on experimental infections with P.berghei, and the conclusions are generalized to other Plasmodium species that infect humans without further morphological analyses.

Results

Here, it is described the microanatomy of sporozoite escape from oocysts of four Plasmodium species: the two laboratory models, P. gallinaceum and P. berghei, and the two main species that cause malaria in humans, P.vivax and P. falciparum. It was found that sporozoites have species-specific mechanisms of escape from the oocyst. The two model species of Plasmodium had a common mechanism, in which the oocyst wall breaks down before sporozoites emerge. In contrast, P. vivax and P. falciparum sporozoites show a dynamic escape mechanism from the oocyst via polarized propulsion.

Conclusions

This study demonstrated that Plasmodium species do not share a common mechanism of sporozoite escape, as previously thought, but show complex and species-specific mechanisms. In addition, the knowledge of this phenomenon in human Plasmodium can facilitate transmission-blocking studies and not those ones only based on the murine and avian models.

Dissecting the interface between apicomplexan parasite and host cell: Insights from a divergent AMA-RON2 pair

Plasmodium falciparum and Toxoplasma gondii are widely studied parasites in phylum Apicomplexa and the etiological agents of severe human malaria and toxoplasmosis, respectively. These intracellular pathogens have evolved a sophisticated invasion strategy that relies on delivery of proteins into the host cell, where parasite-derived rhoptry neck protein 2 (RON2) family members localize to the host outer membrane and serve as ligands for apical membrane antigen (AMA) family surface proteins displayed on the parasite. Recently, we showed that T. gondii harbors a novel AMA designated as TgAMA4 that shows extreme sequence divergence from all characterized AMA family members. Here we show that sporozoite-expressed TgAMA4 clusters in a distinct phylogenetic clade with Plasmodium merozoite apical erythrocyte-binding ligand (MAEBL) proteins and forms a high-affinity, functional complex with its coevolved partner, TgRON2L1. High-resolution crystal structures of TgAMA4 in the apo and TgRON2L1-bound forms complemented with alanine scanning mutagenesis data reveal an unexpected architecture and assembly mechanism relative to previously characterized AMA-RON2 complexes. Principally, TgAMA4 lacks both a deep surface groove and a key surface loop that have been established to govern RON2 ligand binding selectivity in other AMAs. Our study reveals a previously underappreciated level of molecular diversity at the parasite-host-cell interface and offers intriguing insight into the adaptation strategies underlying sporozoite invasion. Moreover, our data offer the potential for improved design of neutralizing therapeutics targeting a broad range of AMA-RON2 pairs and apicomplexan invasive stages.