Chemically Attenuated Blood-Stage Plasmodium yoelii Parasites Induce Long-Lived and Strain-Transcending Protection

Sterile protection against P. vivax malaria by repeated blood stage infection in the Aotus monkey model

The malaria parasite Plasmodium vivax remains a major global public health challenge, and no vaccine is approved for use in humans. Here, we assessed whether P. vivax strain-transcendent immunity can be achieved by repeated infection in Aotus monkeys. Sterile immunity was achieved after two homologous infections, whereas subsequent heterologous challenge provided only partial protection. IgG levels based on P. vivax lysate ELISA and protein microarray increased with repeated infections and correlated with the level of homologous protection. Parasite transcriptional profiles provided no evidence of major antigenic switching upon homologous or heterologous challenge. However, we observed significant sequence diversity and transcriptional differences in the P. vivax core gene repertoire between the two strains used in the study, suggesting that partial protection upon heterologous challenge is due to molecular differences between strains rather than immune evasion by antigenic switching. Our study demonstrates that sterile immunity against P. vivax can be achieved by repeated homologous blood stage infection in Aotus monkeys, thus providing a benchmark to test the efficacy of candidate blood stage P. vivax malaria vaccines.

Evaluating the stability of host-reference gene expression and simultaneously quantifying parasite burden and host immune responses in murine malaria

The efficacy of pre-erythrocytic stage malaria antigens or vaccine platforms is routinely assessed in murine models challenged with Plasmodium sporozoites. Relative liver-stage parasite burden is quantified using reverse transcription quantitative PCR (RTqPCR), which relies on constitutively expressed endogenous control reference genes. However, the stability of host-reference gene expression for RTqPCR analysis following Plasmodium challenge and immunization has not been systematically evaluated. Herein, we evaluated the stability of expression of twelve common RTqPCR reference genes in a murine model of Plasmodium yoelii sporozoite challenge and DNA-adenovirus IV 'Prime-Target' immunization. Significant changes in expression for six of twelve reference genes were shown by one-way ANOVA, when comparing gene expression levels among challenge, immunized, and naïve mice groups. These changes were attributed to parasite challenge or immunization when comparing group means using post-hoc Bonferroni corrected multiple comparison testing. Succinate dehydrogenase (SDHA) and TATA-binding protein (TBP) were identified as stable host-reference genes suitable for relative RTqPCR data normalisation, using the RefFinder package. We defined a robust threshold of 'partial-protection' with these genes and developed a strategy to simultaneously quantify matched host parasite burden and cytokine responses following immunisation or challenge. This is the first report systematically identifying reliable host reference genes for RTqPCR analysis following Plasmodium sporozoite challenge. A robust RTqPCR protocol incorporating reliable reference genes which enables simultaneous analysis of host whole-liver cytokine responses and parasite burden will significantly standardise and enhance results between international malaria vaccine efficacy studies.

A CAF01-adjuvanted whole asexual blood-stage liposomal malaria vaccine induces a CD4+ T-cell-dependent strain-transcending protective immunity in rodent models

IMPORTANCE

Malaria is a devastating disease that has claimed many lives, especially children <5 years of age in Sub-Saharan Africa, as documented in World Malaria Reports by WHO. Even though vector control and chemoprevention tools have helped with elimination efforts in some, if not all, endemic areas, these efforts have been hampered by serious issues (including drug and insecticide resistance and disruption to social cohesion caused by the COVID-19 pandemic). Development of an effective malaria vaccine is the alternative preventative tool in the fight against malaria. Vaccines save millions of lives each year and have helped in elimination and/or eradication of global diseases. Development of a highly efficacious malaria vaccine that will ensure long-lasting protective immunity will be a "game-changing" prevention strategy to finally eradicate the disease. Such a vaccine will need to counteract the significant obstacles that have been hampering subunit vaccine development to date, including antigenic polymorphism, sub-optimal immunogenicity, and waning vaccine efficacy.

Malaria vaccines: the 60-year journey of hope and final success-lessons learned and future prospects

BACKGROUND

The world has made great strides towards beating malaria, although about half of the world population is still exposed to the risk of contracting malaria. Developing an effective malaria vaccine was a huge challenge for medical science. In 2021 the World Health Organization (WHO) approved the first malaria vaccine, RTS,S/AS01 vaccine (Mosquirix™), for widespread use. This review highlights the history of development, and the different approaches and types of malaria vaccines, and the literature to date. It covers the developmental stages of RTS,S/AS01 and recommends steps for its deployment. The review explores other potential vaccine candidates and their status, and suggests options for their further development. It also recommends future roles for vaccines in eradicating malaria. Questions remain on how RTS,S vaccine will work in widespread use and how it can best be utilized to benefit vulnerable communities.

CONCLUSION

Malaria vaccines have been in development for almost 60 years. The RTS,S/AS01 vaccine has now been approved, but cannot be a stand-alone solution. Development should continue on promising candidates such as R21, PfSPZ and P. vivax vaccines. Multi-component vaccines may be a useful addition to other malaria control techniques in achieving eradication of malaria.

Boosting Live Malaria Vaccine with Cytomegalovirus Vector Can Prolong Immunity through Innate and Adaptive Mechanisms

Vaccines to persistent parasite infections have been challenging, and current iterations lack long-term protection. Cytomegalovirus (CMV) chronic vaccine vectors drive protection against SIV, tuberculosis and liver-stage malaria correlated with antigen-specific CD8 T cells with a Tem phenotype. This phenotype is likely driven by a combination of antigen-specific and innate adjuvanting effects of the vector, though these mechanisms are less well understood. Sterilizing immunity from live Plasmodium chabaudi vaccination lasts less than 200 days. While P. chabaudi-specific antibody levels remain stable after vaccination, the decay of parasite-specific T cells correlates with loss of challenge protection. Therefore, we enlisted murine CMV as a booster strategy to prolong T cell responses against malaria. To study induced T cell responses, we included P. chabaudi MSP-1 epitope B5 (MCMV-B5). We found that MCMV vector alone significantly protected against a challenge P. chabaudi infection 40-60 days later, and that MCMV-B5 was able to make B5-specific Teff, in addition to previously-reported Tem, that survive to the challenge timepoint. Used as a booster, MCMV-B5 prolonged protection from heterologous infection beyond day 200, and increased B5 TCR Tg T cell numbers, including both a highly-differentiated Tem phenotype and Teff, both previously reported to protect. B5 epitope expression was responsible for maintenance of Th1 and Tfh B5 T cells. In addition, the MCMV vector had adjuvant properties, contributing non-specifically through prolonged stimulation of IFN-γ. In vivo neutralization of IFN-γ, but not IL-12 and IL-18, late in the course of MCMV, led to loss of the adjuvant effect. Mechanistically, sustained IFN-γ from MCMV increased CD8α+ dendritic cell numbers, and led to increased IL-12 production upon Plasmodium challenge. In addition, neutralization of IFN-γ before challenge reduced the polyclonal Teff response to challenge. Our findings suggest that, as protective epitopes are defined, an MCMV vectored booster can prolong protection through the innate effects of IFN-γ.

Live attenuated vaccines, a favorable strategy to provide long-term immunity against protozoan diseases

The control of diseases caused by protozoan parasites is one of the United Nations' Sustainable Development Goals. In recent years much research effort has gone into developing a new generation of live attenuated vaccines (LAVs) against malaria, Chagas disease and leishmaniasis. However, there is a bottleneck related to their biosafety, production, and distribution that slows downs further development. The success of irradiated or genetically attenuated sporozoites against malaria, added to the first LAV against leishmaniasis to be evaluated in clinical trials, is indicative that the drawbacks of LAVs are gradually being overcome. However, whether persistence of LAVs is a prerequisite for sustained long-term immunity remains to be clarified, and the procedures necessary for clinical evaluation of vaccine candidates need to be standardized.

Development and Evaluation of a Cryopreserved Whole-Parasite Vaccine in a Rodent Model of Blood-Stage Malaria

Infection with malaria parasites continues to be a major global public health issue. While current control measures have enabled a significant decrease in morbidity and mortality over the last 20 years, additional tools will be required if we are to progress toward malaria parasite eradication. Malaria vaccine research has focused on the development of subunit vaccines; however, more recently, interest in whole-parasite vaccines has reignited. Whole-parasite vaccines enable the presentation of a broad repertoire of antigens to the immune system, which limits the impact of antigenic polymorphism and genetic restriction of the immune response. We previously reported that whole-parasite vaccines can be prepared using chemically attenuated parasites within intact red blood cells or using killed parasites in liposomes, although liposomes were less immunogenic than attenuated parasites. If they could be frozen or freeze-dried and be made more immunogenic, liposomal vaccines would be ideal for vaccine deployment in areas where malaria is endemic. Here, we develop and evaluate a Plasmodium yoelii liposomal vaccine with enhanced immunogenicity and efficacy due to incorporation of TLR4 agonist, 3D(6-acyl) PHAD, and mannose to target the liposome to antigen-presenting cells. Following vaccination, mice were protected, and strong cellular immune responses were induced, characterized by parasite-specific splenocyte proliferation and a mixed Th1/Th2/Th17 cytokine response. Parasite-specific antibodies were induced, predominantly of the IgG1 subclass. CD4⁺ T cells and gamma interferon were critical components of the protective immune response. This study represents an important development toward evaluation of this whole-parasite blood-stage vaccine in a phase I clinical trial. IMPORTANCE Malaria is a mosquito-borne infectious disease that is caused by parasites of the genus, Plasmodium. There are seven different Plasmodium spp. that can cause malaria in humans, with P. falciparum causing the majority of the morbidity and mortality. Malaria parasites are endemic in 87 countries and continue to result in >200 million cases of malaria and >400,000 deaths/year, mostly children <5 years of age. Malaria infection initially presents as a flu-like illness but can rapidly progress to severe disease in nonimmune individuals if treatment is not initiated promptly. Existing control strategies for the mosquito vector (insecticides) and parasite (antimalarial drugs) are becoming increasingly less effective due to the development of resistance. While artemisinin combination therapies are frontline treatment for P. falciparum malaria, resistance has been documented in numerous countries. A highly effective malaria vaccine is urgently required to reduce malaria-attributable clinical disease and death and enable progression toward the ultimate goal of eradication.

A non-reactive natural product precursor of the duocarmycin family has potent and selective antimalarial activity

We identify a selective nanomolar inhibitor of blood-stage malarial proliferation from a screen of microbial natural product extracts. The responsible compound, PDE-I₂, is a precursor of the anticancer duocarmycin family that preserves the class's sequence-specific DNA binding but lacks its signature DNA alkylating cyclopropyl warhead. While less active than duocarmycin, PDE-I₂ retains comparable antimalarial potency to chloroquine. Importantly, PDE-I₂ is >1,000-fold less toxic to human cell lines than duocarmycin, with mitigated impacts on eukaryotic chromosome stability. PDE-I₂ treatment induces severe defects in parasite nuclear segregation leading to impaired daughter cell formation during schizogony. Time-of-addition studies implicate parasite DNA metabolism as the target of PDE-I₂, with defects observed in DNA replication and chromosome integrity. We find the effect of duocarmycin and PDE-I₂ on parasites is phenotypically indistinguishable, indicating that the DNA binding specificity of duocarmycins is sufficient and the genotoxic cyclopropyl warhead is dispensable for the parasite-specific selectivity of this compound class.

Pre-clinical evaluation of a whole-parasite vaccine to control human babesiosis

Babesia spp. are tick-transmitted intra-erythrocytic protozoan parasites that infect humans and animals, causing a flu-like illness and hemolytic anemia. There is currently no human vaccine available. People most at risk of severe disease are the elderly, immunosuppressed, and asplenic individuals. B. microti and B. divergens are the predominant species affecting humans. Here, we present a whole-parasite Babesia vaccine. To establish proof-of-principle, we employed chemically attenuated B. microti parasitized red blood cells from infected mice. To aid clinical translation, we produced liposomes containing killed parasite material. Vaccination significantly reduces peak parasitemia following challenge. B cells and anti-parasite antibodies do not significantly contribute to vaccine efficacy. Protection is abrogated by the removal of CD4⁺ T cells or macrophages prior to challenge. Importantly, splenectomized mice are protected by vaccination. To further facilitate translation, we prepared a culture-based liposomal vaccine and demonstrate that this performs as a universal vaccine inducing immunity against different human Babesia species.

Whole-Killed Blood-Stage Vaccine: Is It Worthwhile to Further Develop It to Control Malaria?

Major challenges have been encountered regarding the development of highly efficient subunit malaria vaccines, and so whole-parasite vaccines have regained attention in recent years. The whole-killed blood-stage vaccine (WKV) is advantageous as it can be easily manufactured and efficiently induced protective immunity against a blood-stage challenge, as well as inducing cross-stage protection against both the liver and sexual-stages. However, it necessitates a high dose of parasitized red blood cell (pRBC) lysate for immunization, and this raises concerns regarding its safety and low immunogenicity. Knowledge of the major components of WKV that can induce or evade the host immune response, and the development of appropriate human-compatible adjuvants will greatly help to optimize the WKV. Therefore, we argue that the further development of the WKV is worthwhile to control and potentially eradicate malaria worldwide.

Malaria vaccines since 2000: progress, priorities, products

Malaria vaccine development entered a new era in 2015 when the pre-erythrocytic Plasmodium falciparum candidate RTS,S was favorably reviewed by the European Medicines Agency and subsequently introduced into national pilot implementation programs, marking the first human anti-parasite vaccine to pass regulatory scrutiny. Since the first trials published in 1997, RTS,S has been evaluated in a series of clinical trials culminating in Phase 3 testing, while testing of other pre-erythrocytic candidates (that target sporozoite- or liver-stage parasites), particularly whole sporozoite vaccines, has also increased. Interest in blood-stage candidates (that limit blood-stage parasite growth) subsided after disappointing human efficacy results, although new blood-stage targets and concepts may revive activity in this area. Over the past decade, testing of transmission-blocking vaccines (that kill mosquito/sexual-stage parasites) advanced to field trials and the first generation of placental malaria vaccines (that clear placenta-sequestering parasites) entered the clinic. Novel antigen discovery, human monoclonal antibodies, structural vaccinology, and improved platforms promise to expand on RTS,S and improve existing vaccine candidates.

A genetically hmgb2 attenuated blood stage P. berghei induces crossed-long live protection

Due to the lack of efficiency to control malaria elicited by sub-unit vaccine preparations, vaccination with live-attenuated Plasmodium parasite as reported 70 years ago with irradiated sporozoites regained recently a significant interest. The complex life cycle of the parasite and the different stages of development between mammal host and anopheles do not help to propose an easy vaccine strategy. In order to achieve a complete long-lasting protection against Plasmodium infection and disease, we considered a genetically attenuated blood stage parasite in the hmgb2 gene coding for the high-mobility-group-box 2 (HMGB2). This Plasmodium protein belongs to the HMGB family and hold as the mammal proteins, a double life since it acts first as a nuclear factor involved in chromatin remodelling and transcription regulation and second, when secreted as an active pro-inflammatory alarmin protein. Even though the number of reports on whole living attenuated blood stage parasites is limited when compared to attenuated sporozoites, the results reported with Plasmodium KO parasites are very encouraging. In this report, we present a novel strategy based on pre-immunization with Δhmgb2PbNK65 parasitized red blood cells that confer long-lasting protection in a murine experimental cerebral malaria model against two highly pathogenic homologous and heterologous parasites.

Malaria vaccines in the eradication era: current status and future perspectives

INTRODUCTION

The challenge to eradicate malaria is an enormous task that will not be achieved by current control measures, thus an efficacious and long-lasting malaria vaccine is required. The licensing of RTS, S/AS01 is a step forward in providing some protection, but a malaria vaccine that protects across multiple transmission seasons is still needed. To achieve this, inducing beneficial immune responses while minimising deleterious non-targeted effects will be essential.

AREAS COVERED

This article discusses the current challenges and advances in malaria vaccine development and reviews recent human clinical trials for each stage of infection. Pubmed and ScienceDirect were searched, focusing on cell mediated immunity and how T cell subsets might be targeted in future vaccines using novel adjuvants and emerging vaccine technologies.

EXPERT COMMENTARY

Despite decades of research there is no highly effective licensed malaria vaccine. However, there is cause for optimism as new adjuvants and vaccine systems emerge, and our understanding of correlates of protection increases, especially regarding cellular immunity. The new field of heterologous (non-specific) effects of vaccines also highlights the broader consequences of immunization. Importantly, the WHO led Malaria Vaccine Technology Roadmap illustrates that there is a political will among the global health community to make it happen.

Controlled Infection Immunization Using Delayed Death Drug Treatment Elicits Protective Immune Responses to Blood-Stage Malaria Parasites

Naturally acquired immunity to malaria is robust and protective against all strains of the same species of Plasmodium. This develops as a result of repeated natural infection, taking several years to develop.

Naturally acquired immunity to malaria is robust and protective against all strains of the same species of Plasmodium. This develops as a result of repeated natural infection, taking several years to develop. Evidence suggests that apoptosis of immune lymphocytes due to uncontrolled parasite growth contributes to the slow acquisition of immunity. To hasten and augment the development of natural immunity, we studied controlled infection immunization (CII) using low-dose exposure to different parasite species (Plasmodium chabaudi, P. yoelii, or P. falciparum) in two rodent systems (BALB/c and C57BL/6 mice) and in human volunteers, with drug therapy commencing at the time of initiation of infection. CIIs with infected erythrocytes and in conjunction with doxycycline or azithromycin, which are delayed death drugs targeting the parasite’s apicoplast, allowed extended exposure to parasites at low levels. In turn, this induced strong protection against homologous challenge in all immunized mice. We show that P. chabaudi/P. yoelii infection initiated at the commencement of doxycycline therapy leads to cellular or antibody-mediated protective immune responses in mice, with a broad Th1 cytokine response providing the best correlate of protection against homologous and heterologous species of Plasmodium. P. falciparum CII with doxycycline was additionally tested in a pilot clinical study (n = 4) and was found to be well tolerated and immunogenic, with immunological studies primarily detecting increased cell-associated immune responses. Furthermore, we report that a single dose of the longer-acting drug, azithromycin, given to mice (n = 5) as a single subcutaneous treatment at the initiation of infection controlled P. yoelii infection and protected all mice against subsequent challenge.

Metformin Promotes the Protection of Mice Infected With Plasmodium yoelii Independently of γδ T Cell Expansion

Adaptive immune responses are critical for protection against infection with Plasmodium parasites. The metabolic state dramatically changes in T cells during activation and the memory phase. Recent findings suggest that metformin, a medication for treating type-II diabetes, enhances T-cell immune responses by modulating lymphocyte metabolism. In this study, we investigated whether metformin could enhance anti-malaria immunity. Mice were infected with Plasmodium yoelii and administered metformin. Levels of parasitemia were reduced in treated mice compared with those in untreated mice, starting at ~2 weeks post-infection. The number of γδ T cells dramatically increased in the spleens of treated mice compared with that in untreated mice during the later phase of infection, while that of αβ T cells did not. The proportions of Vγ1⁺ and Vγ2⁺ γδ T cells increased, suggesting that activated cells were selectively expanded. However, these γδ T cells expressed inhibitory receptors and had severe defects in cytokine production, suggesting that they were in a state of exhaustion. Metformin was unable to rescue the cells from exhaustion at this stage. Depletion of γδ T cells with antibody treatment did not affect the reduction of parasitemia in metformin-treated mice, suggesting that the effect of metformin on the reduction of parasitemia was independent of γδ T cells.

A step forward for an attenuated blood-stage malaria vaccine

Efforts to develop an effective malaria vaccine have encountered multiple challenges, and have had limited success to date. As the need remains urgent, novel approaches must be explored. One concept that has gained attention uses whole malaria parasites. Building on preclinical studies in animal models, Stanisic et al. describe the development of a vaccine based on chemically attenuated Plasmodium falciparum blood-stage parasites, with an evaluation of safety and immunogenicity in malaria-naïve human subjects. The vaccine was shown to be safe, well tolerated, and capable of priming antigen-specific T cells. This work, and the completion of an initial clinical trial in human subjects, represents a significant advance. While the path forward for this attenuated vaccine remains challenging, these initial findings are encouraging. Importantly, the results provide the foundation and framework for testing modified immunization protocols, and designing subsequent clinical trials to further evaluate safety, test for enhanced immunogenicity, and ultimately measure protective efficacy.Please see related article: https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-018-1173-9.

Vaccination with chemically attenuated Plasmodium falciparum asexual blood-stage parasites induces parasite-specific cellular immune responses in malaria-naïve volunteers: a pilot study

BACKGROUND

The continuing morbidity and mortality associated with infection with malaria parasites highlights the urgent need for a vaccine. The efficacy of sub-unit vaccines tested in clinical trials in malaria-endemic areas has thus far been disappointing, sparking renewed interest in the whole parasite vaccine approach. We previously showed that a chemically attenuated whole parasite asexual blood-stage vaccine induced CD4⁺ T cell-dependent protection against challenge with homologous and heterologous parasites in rodent models of malaria.

METHODS

In this current study, we evaluated the immunogenicity and safety of chemically attenuated asexual blood-stage Plasmodium falciparum (Pf) parasites in eight malaria-naïve human volunteers. Study participants received a single dose of 3 × 10⁷ Pf pRBC that had been treated in vitro with the cyclopropylpyrolloindole analogue, tafuramycin-A.

RESULTS

We demonstrate that Pf asexual blood-stage parasites that are completely attenuated are immunogenic, safe and well tolerated in malaria-naïve volunteers. Following vaccination with a single dose, species and strain transcending Plasmodium-specific T cell responses were induced in recipients. This included induction of Plasmodium-specific lymphoproliferative responses, T cells secreting the parasiticidal cytokines, IFN-γ and TNF, and CD3⁺CD45RO⁺ memory T cells. Pf-specific IgG was not detected.

CONCLUSIONS

This is the first clinical study evaluating a whole parasite blood-stage malaria vaccine. Following administration of a single dose of completely attenuated Pf asexual blood-stage parasites, Plasmodium-specific T cell responses were induced while Pf-specific antibodies were not detected. These results support further evaluation of this chemically attenuated vaccine in humans.

TRIAL REGISTRATION

Trial registration: ACTRN12614000228684 . Registered 4 March 2014.

Induction of immunity following vaccination with a chemically attenuated malaria vaccine correlates with persistent antigenic stimulation

Objectives

Blood stage malaria parasites attenuated with seco-cyclopropyl pyrrolo indole (CPI) analogues induce robust immunity in mice to homologous and heterologous malaria parasites and are being considered for the development of a human vaccine. However, it is not understood how attenuated parasites induce immunity. We showed that following vaccination, parasite DNA persisted in blood for several months, raising the possibility that ongoing immune stimulation may be critical. However, parasites were not seen microscopically beyond 24 h postvaccination. We aimed to provide a mechanistic understanding of immune induction.

Methods

Mice were vaccinated with chemically attenuated Plasmodium chabaudi parasites. PCR and adoptive transfer studies were used to determine the presence of parasites and antigen in vivo. In other experiments, Plasmodium falciparum parasitised red blood cells were attenuated in vitro and RNA and antigen expression studied.

Results

We show that blood transferred from vaccinated mice into naïve mice activates T cells and induces complete protective immunity in the recipient mice strongly suggesting that there is persistence of parasite antigen postvaccination. This is supported by the presence of parasite RNA in vaccinated mice and both RNA and antigen expression in P. falciparum cultures treated with CPI drugs in vitro. In addition, drugs that block parasite growth also prevent the induction of immunity in vaccinated mice, indicating that some growth of attenuated parasites is required for immune induction.

Conclusions

Attenuated parasites persist at submicroscopic levels in the blood of mice postvaccination with the ability to activate T cells and induce ongoing protective immune responses.

Interpreting challenge data from early phase malaria blood stage vaccine trials

INTRODUCTION

As the quest for an effective blood stage malaria vaccine continues, there is increasing reliance on the use of controlled human malaria infections (CHMI) in non-endemic settings to test vaccine efficacy at the earliest possible time. This is seen as a way to accelerate vaccine research and quickly eliminate candidates with poor efficacy. Areas covered: The data from these studies need to be carefully examined and interpreted in light of the very different roles that antibody and cellular immunity play in protection and within the context of the distinct clinical sensitivities of volunteers living in malaria-non-endemic countries compared to those living in endemic countries. With current strategies, it is likely that vaccines with protective immunological 'signatures' will be missed and potentially good candidates discarded. Expert commentary: Efficacy data from early phase vaccine trials in non-endemic countries should not be used to decide whether or not to proceed to vaccine trials in endemic countries.

Gold(i) phosphine compounds as parasite attenuating agents for malaria vaccine and drug development

The asexual blood-stagePlasmodiumparasite attenuating properties of gold(i) phosphine compounds are exploited in a novel strategy for malaria vaccine development.

Exploiting the apicoplast: apicoplast-targeting drugs and malaria vaccine development

The apicoplast, a relic plastid found in most Apicomplexan parasites, is a notable drug target. Certain antibiotics elicit a delayed death phenotype by targeting this organelle. Here, we review apicoplast-targeting drugs and their targets, particularly those that cause delayed death, and highlight its potential uses in malaria vaccine development.

Advances in malaria vaccine development: report from the 2017 malaria vaccine symposium

The Malaria Vaccine Symposium occurred at Johns Hopkins University in Baltimore, MD, USA on April 25th, 2017, coinciding with World Malaria Day and the WHO announcement that the RTS,S malaria vaccine would begin pilot implementation programs in Ghana, Kenya, and Malawi in 2018. Scientists from several disciplines reported progress on an array of malaria vaccine concepts and product candidates, including pre-erythrocytic vaccines that prevent infection, blood-stage vaccines that limit infection and disease, and transmission-blocking vaccines that interrupt the spread of infection. Other speakers highlighted the immunological and genetic considerations that must be addressed by vaccinologists to yield the most efficacious vaccines. Here, we highlight the advances in malaria vaccinology that were reported at the symposium.

Immunological memory to blood-stage malaria infection is controlled by the histamine releasing factor (HRF) of the parasite

While most subunit malaria vaccines provide only limited efficacy, pre-erythrocytic and erythrocytic genetically attenuated parasites (GAP) have been shown to confer complete sterilizing immunity. We recently generated a Plasmodium berghei (PbNK65) parasite that lacks a secreted factor, the histamine releasing factor (HRF) (PbNK65 hrfΔ), and induces in infected mice a self-resolving blood stage infection accompanied by a long lasting immunity. Here, we explore the immunological mechanisms underlying the anti-parasite protective properties of the mutant PbNK65 hrfΔ and demonstrate that in addition to an up-regulation of IL-6 production, CD4⁺ but not CD8⁺ T effector lymphocytes are indispensable for the clearance of malaria infection. Maintenance of T cell-associated protection is associated with the reduction in CD4⁺PD-1⁺ and CD8⁺PD-1⁺ T cell numbers. A higher number of central and effector memory B cells in mutant-infected mice also plays a pivotal role in protection. Importantly, we also demonstrate that prior infection with WT parasites followed by a drug cure does not prevent the induction of PbNK65 hrfΔ-induced protection, suggesting that such protection in humans may be efficient even in individuals that have been infected and who repeatedly received antimalarial drugs.

Chemical Attenuation in the Development of a Whole-Organism Malaria Vaccine

Malaria vaccine development has been dominated by the subunit approach; however, many subunit vaccine candidates have had limited efficacy in settings of malaria endemicity. As our search for an efficacious malaria vaccine continues, the development of a whole-organism vaccine is now receiving much scrutiny. One strategy currently being explored in the development of a whole-organism vaccine involves chemical attenuation of the malaria parasite. In vivo and in vitro chemical attenuation of both liver-stage and blood-stage Plasmodium parasites has been investigated. Here, we discuss both approaches of chemical attenuation in the development of a whole-organism vaccine against malaria.