CD8 T cell independent immunity after single dose infection-treatment-vaccination (ITV) against Plasmodium yoelii

Designing a multi-epitope chimeric protein from different potential targets: A potential vaccine candidate against Plasmodium

Malaria is an infectious disease that has been a continuous threat to mankind since the time immemorial. Owing to the complex multi-staged life cycle of the plasmodium parasite, an effective malaria vaccine which is fully protective against the parasite infection is urgently needed to deal with the challenges. In the present study, essential parasite proteins were identified and a chimeric protein with multivalent epitopes was generated. The designed chimeric protein consists of best potential B and T cell epitopes from five different essential parasite proteins. Physiochemical studies of the chimeric protein showed that the modeled vaccine construct was thermo-stable, hydrophilic and antigenic in nature. And the binding of the vaccine construct with Toll-like receptor-4 (TLR-4) as revealed by the molecular docking suggests the possible interaction and role of the vaccine construct in activating the innate immune response. The constructed vaccine being a chimeric protein containing epitopes from different potential candidates could target different stages or pathways of the parasite. Moreover, the approach used in this study is time and cost effective, and can be applied in the discoveries of new potential vaccine targets for other pathogens.

Antibody-dependent immune responses elicited by blood stage-malaria infection contribute to protective immunity to the pre-erythrocytic stages

Advances in transcriptomics and proteomics have revealed that different life-cycle stages of the malaria parasite, Plasmodium, share antigens, thus allowing for the possibility of eliciting immunity to a parasite life-cycle stage that has not been experienced before. Using the Plasmodium chabaudi (AS strain) model of malaria in mice, we investigated how isolated exposure to blood-stage infection, bypassing a liver-stage infection, yields significant protection to sporozoite challenge resulting in lower liver parasite burdens. Antibodies are the main immune driver of this protection. Antibodies induced by blood-stage infection recognise proteins on the surface of sporozoites and can impair sporozoite gliding motility in vitro, suggesting a possible function in vivo. Furthermore, mice lacking B cells and/or secreted antibodies are not protected against a sporozoite challenge in mice that had a previous blood-stage infection. Conversely, effector CD4⁺ and CD8⁺ T cells do not seem to play a role in protection from sporozoite challenge of mice previously exposed only to the blood stages of P. chabaudi. The protective response against pre-erythrocytic stages can be induced by infections initiated by serially passaged blood-stage parasites as well as recently mosquito transmitted parasites and is effective against a different strain of P. chabaudi (CB strain), but not against another rodent malaria species, P. yoelii. The possibility to induce protective cross-stage antibodies advocates the need to consider both stage-specific and cross-stage immune responses to malaria, as natural infection elicits exposure to all life-cycle stages. Future investigation into these cross-stage antibodies allows the opportunity for candidate antigens to contribute to malaria vaccine development.

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.

Plasmodium infection and drug cure for malaria vaccine development

Introduction: Despite decades of research into the development of a vaccine to combat the malaria parasite, a highly efficacious malaria vaccine is not yet available. Different whole parasite-based vaccine approaches, including deliberate Plasmodium infection and drug cure (IDC), have been evaluated in pre-clinical and early phase clinical trials. The advantage of whole parasite vaccines is that they induce immune responses against multiple parasite antigens, thus lowering the impact of antigenic diversity. Deliberate Plasmodium IDC, as a vaccine approach, involves administering infectious, live parasites in combination with an anti-malarial drug, which controls the infection and enables induction of protective immune responses.

Immune responses to malaria pre-erythrocytic stages: Implications for vaccine development

Radiation-attenuated sporozoites induce sterilizing immunity and remain the 'gold standard' for malaria vaccine development. Despite practical challenges in translating these whole sporozoite vaccines to large-scale intervention programmes, they have provided an excellent platform to dissect the immune responses to malaria pre-erythrocytic (PE) stages, comprising both sporozoites and exoerythrocytic forms. Investigations in rodent models have provided insights that led to the clinical translation of various vaccine candidates-including RTS,S/AS01, the most advanced candidate currently in a trial implementation programme in three African countries. With advances in immunology, transcriptomics and proteomics, and application of lessons from past failures, an effective, long-lasting and wide-scale malaria PE vaccine remains feasible. This review underscores the progress in PE vaccine development, focusing on our understanding of host-parasite immunological crosstalk in the tissue environments of the skin and the liver. We highlight possible gaps in the current knowledge of PE immunity that can impact future malaria vaccine development efforts.

Synthetic DNA Vaccines Adjuvanted with pIL-33 Drive Liver-Localized T Cells and Provide Protection from Plasmodium Challenge in a Mouse Model

The need for a malaria vaccine is indisputable. A single vaccine for Plasmodium pre-erythrocytic stages targeting the major sporozoite antigen circumsporozoite protein (CSP) has had partial success. Additionally, CD8+ T cells targeting liver-stage (LS) antigens induced by live attenuated sporozoite vaccines were associated with protection in human challenge experiments. To further evaluate protection mediated by LS antigens, we focused on exported pre-erythrocytic proteins (exported protein 1 (EXP1), profilin (PFN), exported protein 2 (EXP2), inhibitor of cysteine proteases (ICP), transmembrane protein 21 (TMP21), and upregulated in infective sporozoites-3 (UIS3)) expressed in all Plasmodium species and designed optimized, synthetic DNA (synDNA) immunogens. SynDNA antigen cocktails were tested with and without the molecular adjuvant plasmid IL-33. Immunized animals developed robust T cell responses including induction of antigen-specific liver-localized CD8+ T cells, which were enhanced by the co-delivery of plasmid IL-33. In total, 100% of mice in adjuvanted groups and 71%–88% in non-adjuvanted groups were protected from blood-stage disease following Plasmodium yoelii sporozoite challenge. This study supports the potential of synDNA LS antigens as vaccine components for malaria parasite infection.

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

Malaria parasite infection continues to inflict extensive morbidity and mortality in resource-poor countries. The insufficiently understood parasite biology, continuously evolving drug resistance and the lack of an effective vaccine necessitate intensive research on human malaria parasites that can inform the development of new intervention tools. Humanized mouse models have been greatly improved over the last decade and enable the direct study of human malaria parasites in vivo in the laboratory. Nevertheless, no small animal model developed so far is capable of maintaining the complete life cycle of Plasmodium parasites that infect humans. The ultimate goal is to develop humanized mouse systems in which a Plasmodium infection closely reproduces all stages of a parasite infection in humans, including pre-erythrocytic infection, blood stage infection and its associated pathology, transmission as well as the human immune response to infection. Here, we discuss current humanized mouse models and the future directions that should be taken to develop next-generation models for human malaria parasite research.

Humanized DRAGA mice immunized with Plasmodium falciparum sporozoites and chloroquine elicit protective pre-erythrocytic immunity

Background

Human-immune-system humanized mouse models can bridge the gap between humans and conventional mice for testing human vaccines. The HLA-expressing humanized DRAGA (HLA-A2.HLA-DR4.Rag1KO.IL2RγcKO.NOD) mice reconstitute a functional human-immune-system and sustain the complete life cycle of Plasmodium falciparum. Herein, the DRAGA mice were investigated for immune responses following immunization with live P. falciparum sporozoites under chloroquine chemoprophylaxis (CPS-CQ), an immunization approach that showed in human trials to confer pre-erythrocytic immunity.

Results

The CPS-CQ immunized DRAGA mice (i) elicited human CD4 and CD8 T cell responses to antigens expressed by P. falciparum sporozoites (Pfspz) and by the infected-red blood cells (iRBC). The Pfspz-specific human T cell responses were found to be systemic (spleen and liver), whereas the iRBCs-specific human T cell responses were more localized to the liver, (ii) elicited stronger antibody responses to the Pfspz than to the iRBCs, and (iii) they were protected against challenge with infectious Pfspz but not against challenge with iRBCs.

Conclusions

The DRAGA mice represent a new pre-clinical model to investigate the immunogenicity and protective efficacy of P. falciparum malaria vaccine candidates.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2264-y) contains supplementary material, which is available to authorized users.

Tissue-specific cellular immune responses to malaria pre-erythrocytic stages

Complete and long-lasting protective immunity against malaria can be achieved through vaccination with invasive live attenuated Plasmodium sporozoites, the motile stage inoculated in the host skin during a mosquito bite. Protective immunity relies primarily on effector CD8⁺ T cells targeting the parasite in the liver. Understanding the tissue-specific features of the immune response is emerging as a vital requirement for understanding protective immunity. The small parasite inoculum, the scarcity of infected cells and the tolerogenic properties of the liver represent hurdles for the establishment of protective immunity in endemic areas. In this review, we discuss recent advances on liver-specific features of immunity including innate recognition of malaria pre-erythrocytic stages, CD8⁺ T cell interactions with infected hepatocytes, antigen presentation for effective CD8⁺ T cell responses and generation of liver-resident memory CD8⁺ T cells. A better understanding of the factors involved in the induction and maintenance of effector CD8⁺ T cell immunity against malaria pre-erythrocytic stages is crucial for the development of an effective vaccine targeting the initial phase of malaria infection.

Profiling the Targets of Protective CD8+ T Cell Responses to Infection

T cells are critical effectors of host immunity that target intracellular pathogens, such as the causative agents of HIV, tuberculosis, and malaria. The development of vaccines that induce effective cell-mediated immunity against such pathogens has proved challenging; for tuberculosis and malaria, many of the antigens targeted by protective T cells are not known. Here, we report a novel approach for screening large numbers of antigens as potential targets of T cells. Malaria provides an excellent model to test this antigen discovery platform because T cells are critical mediators of protection following immunization with live sporozoite vaccines and the specific antigen targets are unknown. We generated an adenovirus array by cloning 312 highly expressed pre-erythrocytic Plasmodium yoelii antigens into adenovirus vectors using high-throughput methodologies. The array was screened to identify antigen-specific CD8⁺ T cells induced by a live sporozoite vaccine regimen known to provide high levels of sterile protection mediated by CD8⁺ T cells. We identified 69 antigens that were targeted by CD8⁺ T cells induced by this vaccine regimen. The antigen that recalled the highest frequency of CD8⁺ T cells, PY02605, induced protective responses in mice, demonstrating proof of principle for this approach in identifying antigens for vaccine development.

Trimethoprim-Sulfamethoxazole Prophylaxis During Live Malaria Sporozoite Immunization Induces Long-Lived, Homologous, and Heterologous Protective Immunity Against Sporozoite Challenge

Trimethoprim-sulfamethoxazole (TMP-SMX) is widely used in malaria-endemic areas in human immunodeficiency virus (HIV)-infected children and HIV-uninfected, HIV-exposed children as opportunistic infection prophylaxis. Despite the known effects that TMP-SMX has in reducing clinical malaria, its impact on development of malaria-specific immunity in these children remains poorly understood. Using rodent malaria models, we previously showed that TMP-SMX, at prophylactic doses, can arrest liver stage development of malaria parasites and speculated that TMP-SMX prophylaxis during repeated malaria exposures would induce protective long-lived sterile immunity targeting pre-erythrocytic stage parasites in mice. Using the same models, we now demonstrate that repeated exposures to malaria parasites during TMP-SMX administration induces stage-specific and long-lived pre-erythrocytic protective anti-malarial immunity, mediated primarily by CD8⁺ T-cells. Given the HIV infection and malaria coepidemic in sub-Saharan Africa, clinical studies aimed at determining the optimum duration of TMP-SMX prophylaxis in HIV-infected or HIV-exposed children must account for the potential anti-infection immunity effect of TMP-SMX prophylaxis.

Repetitive live sporozoites inoculation under arteether chemoprophylaxis confers protection against subsequent sporozoite challenge in rodent malaria model

Inoculation with live sporozoites under prophylactic antimalarial cover (CPS-immunization) represents an alternate approach to develop sterile, reproducible, and long-term protection against malaria. Here, we have employed arteether (ART), a semi synthetic derivative of artemisinin to explore its potential as a chemoprophylaxis candidate in CPS approach and systematically compared the protective potential of arteether with mefloquine, azithromycin and primaquine. Blood stage patency and quantitative RT-PCR of liver stage parasite load were monitored as primary key end-points for protection against malaria challenge infection. For this purpose, sequential exposures of Plasmodium yoelii sporozoites under prophylactic treatment with arteether (ART), mefloquine (MFQ), azithromycin (AZ) or primaquine (PQ) was conducted in experimental Swiss mice. Our results show that during the first three sequential exposures (1st, 2nd and 3rd challenge) no marked difference in the blood stage patency was observed between control and CPS-ART group. However, delayed patency was recorded following 4th sporozoite challenge and mice enjoyed sterile protection after 5th sporozoite challenge. A similar response was observed in CPS-MFQ group, whereas earlier protection was recorded in CPS-AZ group i.e., after 4th sprozoite challenge. However, mice under PQ cover did not show any protection/delay in patency even after five sequential sporozoite inoculations, possibly due to inhibition of liver stage development. Furthermore, protection acquired by CPS-immunization is stage-specific as the protected mice remained susceptible to challenge with blood stage parasites. In short, the present study demonstrates that sporozoite administration under ART, MFQ or AZ treatment confers strong protection against subsequent sporozoite infection and the acquired response is dependent on the presence of liver stage parasites.

Discriminating Protective from Nonprotective Plasmodium-Specific CD8+ T Cell Responses

Despite decades of research, malaria remains a global health crisis. Current subunit vaccine approaches do not provide efficient long-term, sterilizing immunity against Plasmodium infections in humans. Conversely, whole parasite vaccinations with their larger array of target Ags have conferred long-lasting sterilizing protection to humans. Similar studies in rodent models of malaria reveal that CD8(+) T cells play a critical role in liver-stage immunity after whole parasite vaccination. However, it is unknown whether all CD8(+) T cell specificities elicited by whole parasite vaccination contribute to protection, an issue of great relevance for enhanced subunit vaccination. In this article, we show that robust CD8(+) T cell responses of similar phenotype are mounted after prime-boost immunization against Plasmodium berghei glideosome-associated protein 5041-48-, sporozoite-specific protein 20318-325-, thrombospondin-related adhesion protein (TRAP) 130-138-, or circumsporozoite protein (CSP) 252-260-derived epitopes in mice, but only CSP252-260- and TRAP130-138-specific CD8(+) T cells provide sterilizing immunity and reduce liver parasite burden after sporozoite challenge. Further, CD8(+) T cells specific to sporozoite surface-expressed CSP and TRAP proteins, but not intracellular glideosome-associated protein 50 and sporozoite-specific protein 20, efficiently recognize sporozoite-infected hepatocytes in vitro. These results suggest that: 1) protection-relevant antigenic targets, regardless of their immunogenic potential, must be efficiently presented by infected hepatocytes for CD8(+) T cells to eliminate liver-stage Plasmodium infection; and 2) proteins expressed on the surface of sporozoites may be good target Ags for protective CD8(+) T cells.

A Plasmodium Promiscuous T Cell Epitope Delivered within the Ad5 Hexon Protein Enhances the Protective Efficacy of a Protein Based Malaria Vaccine

A malaria vaccine is a public health priority. In order to produce an effective vaccine, a multistage approach targeting both the blood and the liver stage infection is desirable. The vaccine candidates also need to induce balanced immune responses including antibodies, CD4+ and CD8+ T cells. Protein-based subunit vaccines like RTS,S are able to induce strong antibody response but poor cellular reactivity. Adenoviral vectors have been effective inducing protective CD8+ T cell responses in several models including malaria; nonetheless this vaccine platform exhibits a limited induction of humoral immune responses. Two approaches have been used to improve the humoral immunogenicity of recombinant adenovirus vectors, the use of heterologous prime-boost regimens with recombinant proteins or the genetic modification of the hypervariable regions (HVR) of the capsid protein hexon to express B cell epitopes of interest. In this study, we describe the development of capsid modified Ad5 vectors that express a promiscuous Plasmodium yoelii T helper epitope denominated PyT53 within the hexon HVR2 region. Several regimens were tested in mice to determine the relevance of the hexon modification in enhancing protective immune responses induced by the previously described protein-based multi-stage experimental vaccine PyCMP. A heterologous prime-boost immunization regime that combines a hexon modified vector with transgenic expression of PyCMP followed by protein immunizations resulted in the induction of robust antibody and cellular immune responses in comparison to a similar regimen that includes a vector with unmodified hexon. These differences in immunogenicity translated into a better protective efficacy against both the hepatic and red blood cell stages of P. yoelii. To our knowledge, this is the first time that a hexon modification is used to deliver a promiscuous T cell epitope. Our data support the use of such modification to enhance the immunogenicity and protective efficacy of adenoviral based malaria vaccines.

Novel approaches to whole sporozoite vaccination against malaria

The parasitic disease malaria threatens more than 3 billion people worldwide, resulting in more than 200 million clinical cases and almost 600,000 deaths annually. Vaccines remain crucial for prevention and ultimately eradication of infectious diseases and, for malaria, whole sporozoite based immunization has been shown to be the most effective in experimental settings. In addition to immunization with radiation-attenuated sporozoites, chemoprophylaxis and sporozoites (CPS) is a highly efficient strategy to induce sterile protection in humans. Genetically attenuated parasites (GAP) have demonstrated significant protection in rodent studies, and are now being advanced into clinical testing. This review describes the existing pre-clinical and clinical data on CPS and GAP, discusses recent developments and examines how to transform these immunization approaches into vaccine candidates for clinical development.

Live attenuated pre-erythrocytic malaria vaccines

Although recent control measures have significantly reduced malaria cases and deaths in many endemic areas, an effective vaccine will be essential to eradicate this parasitic disease. Malaria vaccine strategies developed to date focus on different phases of the parasite's complex life cycle in the human host and mosquito vector, and include both subunit-based and whole-parasite vaccines. This review focuses on the 3 live-attenuated malaria vaccination strategies that have been tested in humans to date, and discusses their progress, challenges and the immune correlates of protection that have been identified.

Mechanisms of stage-transcending protection following immunization of mice with late liver stage-arresting genetically attenuated malaria parasites

Malaria, caused by Plasmodium parasite infection, continues to be one of the leading causes of worldwide morbidity and mortality. Development of an effective vaccine has been encumbered by the complex life cycle of the parasite that has distinct pre-erythrocytic and erythrocytic stages of infection in the mammalian host. Historically, malaria vaccine development efforts have targeted each stage in isolation. An ideal vaccine, however, would target multiple life cycle stages with multiple arms of the immune system and be capable of eliminating initial infection in the liver, the subsequent blood stage infection, and would prevent further parasite transmission. We have previously shown that immunization of mice with Plasmodium yoelii genetically attenuated parasites (GAP) that arrest late in liver stage development elicits stage-transcending protection against both a sporozoite challenge and a direct blood stage challenge. Here, we show that this immunization strategy engenders both T- and B-cell responses that are essential for stage-transcending protection, but the relative importance of each is determined by the host genetic background. Furthermore, potent anti-blood stage antibodies elicited after GAP immunization rely heavily on FC-mediated functions including complement fixation and FC receptor binding. These protective antibodies recognize the merozoite surface but do not appear to recognize the immunodominant merozoite surface protein-1. The antigen(s) targeted by stage-transcending immunity are present in both the late liver stages and blood stage parasites. The data clearly show that GAP-engendered protective immune responses can target shared antigens of pre-erythrocytic and erythrocytic parasite life cycle stages. As such, this model constitutes a powerful tool to identify novel, protective and stage-transcending T and B cell targets for incorporation into a multi-stage subunit vaccine.

Correlates of protective immunity following whole sporozoite vaccination against malaria

Human infection with Plasmodium parasites remains a serious global health crisis, leading to more than 600,000 deaths annually. Currently, no licensed vaccine is available to alleviate this malaria disease burden and vaccination with the most advanced antimalarial vaccine candidate, RTS,S, provides limited protection that wanes over time. To date, the only vaccination strategy capable of inducing complete, long-lasting protection in human subjects is administration of attenuated whole sporozoites. Several approaches for vaccination with attenuated whole sporozoites have been clinically tested in humans and include vaccination with radiation or genetically attenuated sporozoites or with virulent sporozoites concurrent with administration of antimalarial drug cover. Rodent studies with these three attenuated whole sporozoite vaccination (WSV) approaches provide insights into the immune correlates of vaccine-induced protection. The majority of these studies have identified a critical role for liver-stage parasite-directed CD8 T cells in providing protection with possible contributions from Plasmodium-specific CD4 T cells or antibodies. Together, rodent and human vaccination studies with attenuated WSV may lead to an understanding of the correlates of protective immunity against malarial disease, and the development of new, highly efficacious vaccines.

Blood-stage immunity to Plasmodium chabaudi malaria following chemoprophylaxis and sporozoite immunization

Protection against malaria in humans can be achieved by repeated exposure to infected mosquito bites during prophylactic chloroquine treatment (chemoprophylaxis and sporozoites (CPS)). We established a new mouse model of CPS immunization to investigate the stage and strain-specificity of malaria immunity. Immunization with Plasmodium chabaudi by mosquito bite under chloroquine cover does not generate pre-erythrocytic immunity, which is acquired only after immunization with high sporozoite doses. Instead, CPS immunization by bite elicits long-lived protection against blood-stage parasites. Blood-stage immunity is effective against a virulent, genetically distinct strain of P. chabaudi. Importantly, if exposure to blood-stage parasitemia is extended, blood-stage parasites induce cross-stage immunity targeting pre-erythrocytic stages. We therefore show that CPS immunization can induce robust, long-lived heterologous blood-stage immunity, in addition to protection against pre-erythrocytic parasites following high dose sporozoite immunization. Cross-stage immunity elicited by blood-stage parasites may further enhance efficacy of this immunization regimen.

DOI:http://dx.doi.org/10.7554/eLife.05165.001

Malaria is a life-threatening infectious disease in humans that is caused by a single-celled parasite called Plasmodium. The parasite is carried between people by mosquitos; when an infected mosquito bites a human, the parasite is injected into the bloodstream with the mosquito's saliva. Plasmodium first infects liver cells but then re-enters the bloodstream, where it infects red blood cells leading to symptoms of disease. If another mosquito bites the infected individual at this so-called ‘blood-stage’, the parasite can be passed to this mosquito and the cycle of transmission continues.

Currently there are no vaccines available that can effectively protect against malaria. Although an experimental vaccine containing a weakened form of the parasite can protect against the liver-stage parasites, it fails to prevent the parasite from multiplying in the red blood cells. Therefore, the individuals remain susceptible to severe malaria.

Recently, researchers have developed a new strategy for immunization that provides exposure to both liver-stage and blood-stage parasites. Human volunteers taking an anti-malarial drug were deliberately exposed to mosquitos carrying the parasite on three separate occasions. Although the volunteers were infected with the parasite, the anti-malarial drug killed the parasites inside the red blood cells. After the end of the drug treatment, the volunteers were exposed to mosquitos carrying the parasite and they were still protected from infection. These results are promising, but it is not clear if the volunteers have acquired immunity to liver-stage or blood-stage parasites, or even both.

To answer this important question, Nahrendorf et al. developed a similar immunization strategy in mice. Just like the human volunteers, the mice were treated with an anti-malarial drug and exposed to mosquitos carrying Plasmodium on three separate occasions. Although the immunizations did not protect the mice against early infection in the liver, they did provide long-term protection against parasites multiplying in the red-blood cells.

The immunity generated by this immunization strategy also protected the mice against another strain of Plasmodium, different to the one used in the immunizations. The experiments also show that prolonged exposure to the blood-stage parasites can even lead to immunity against the liver-stage parasites.

Nahrendorf et al.'s findings show that this immunization strategy can protect individuals against both the liver-stage and blood-stage parasites. The next challenges are to find out how the immunity generated by one stage of infection can protect against the other stages, and to discover which molecules on the parasite the immune system targets.

DOI:http://dx.doi.org/10.7554/eLife.05165.002

Artesunate versus chloroquine infection-treatment-vaccination defines stage-specific immune responses associated with prolonged sterile protection against both pre-erythrocytic and erythrocytic Plasmodium yoelii infection

Sterile protection against malaria infection can be achieved through vaccination of mice and humans with whole Plasmodium spp. parasites. One such method, known as infection-treatment-vaccination (ITV), involves immunization with wild type sporozoites (spz) under drug coverage. In this work, we used the different effects of antimalarial drugs chloroquine (CQ) and artesunate (AS) on blood stage (BS) parasites to dissect the stage-specific immune responses in mice immunized with Plasmodium yoelii spz under either drug, as well as their ability to protect mice against challenge with spz or infected RBCs (iRBCs). Whereas CQ-ITV induced sterile protection against challenge with both spz and iRBCs, AS-ITV only induced sterile protection against spz challenge. Importantly, AS-ITV delayed the onset of BS infection, indicating that both regimens induced cross-stage immunity. Moreover, both CQ- and AS-ITV induced CD8(+) T cells in the liver that eliminated malaria-infected hepatocytes in vitro, as well as Abs that recognized pre-erythrocytic parasites. Sera from both groups of mice inhibited spz invasion of hepatocytes in vitro, but only CQ-ITV induced high levels of anti-BS Abs. Finally, passive transfer of sera from CQ-ITV-treated mice delayed the onset of erythrocytic infection in the majority of mice challenged with P. yoelii iRBCs. Besides constituting the first characterization, to our knowledge, of AS-ITV as a vaccination strategy, our data show that ITV strategies that lead to subtle differences in the persistence of parasites in the blood enable the characterization of the resulting immune responses, which will contribute to future research in vaccine design and malaria interventions.