Five decades of clinical assessment of whole-sporozoite malaria vaccines

A Plasmodium late liver stage arresting GAP provides superior protection in mice

Liver-stage genetically attenuated malaria parasites (GAPs) are powerful immunogens that provide protection against sporozoite challenge. We previously generated two late liver-stage-arresting GAPs by deleting the stearoyl-CoA desaturase (Scd) or sporozoite conserved orthologous transcript 1 (Scot1) genes in Plasmodium berghei. Immunization with Scd or Scot1 GAP conferred complete protection against a sporozoite challenge. In a safety study, we observed rare breakthrough blood-stage infections in mice inoculated with high doses of sporozoites, indicating that both GAPs were incompletely attenuated. In this study, we generated a Scd/Scot1 GAP by dual gene deletion. This resulted in complete attenuation of the parasites in the liver and did not transition to blood-stage infection despite a high-dose sporozoite challenge. The Scd/Scot1 KO and WT GFP parasites were indistinguishable during blood, mosquito and early liver stage development. Moreover, Scd/Scot1 KO liver-stage schizonts exhibited an abnormal apicoplast biogenesis and nuclear division phenotype, failed to form hepatic merozoites, and exhibited late liver-stage arrest. Compared with early-arresting Speld KO immunization, late-stage liver-arresting Scd/Scot1 KO induces greater and broader CD8+ T-cell responses and elicits stage-transcending immunity that provides superior protection in C57BL/6 mice. These data prove that multiple gene deletions lead to complete attenuation of the parasite and support the development of late liver stage-arresting P. falciparum GAP.

Whole-sporozoite malaria vaccines: where we are, where we are going

The malaria vaccination landscape has seen significant advancements with the recent endorsement of RTS,S/AS01 and R21/Matrix-M vaccines, which target the pre-erythrocytic stages of Plasmodium falciparum (Pf) infection. However, several challenges remain to be addressed, including the incomplete protection afforded by these vaccines, their dependence on a single Pf antigen, and the fact that they were not designed to protect against P. vivax (Pv) malaria. Injectable formulations of whole-sporozoite (WSpz) malaria vaccines offer a promising alternative to existing subunit vaccines, with recent developments including genetically engineered parasites and optimized administration regimens. Clinical evaluations demonstrate varying efficacy, influenced by factors, such as immune status, prior exposure to malaria, and age. Despite significant progress, a few hurdles persist in vaccine production, deployment, and efficacy in malaria-endemic regions, particularly in children. Concurrently, transgenic parasites expressing Pv antigens emerge as potential solutions for PvWSpz vaccine development. Ongoing clinical studies and advancements in vaccine technology, including the recently described PfSPZ-LARC2 candidate, signify a hopeful future for WSpz malaria vaccines, which hold great promise in the global fight against malaria.

Circadian rhythms mediate malaria transmission potential

Malaria transmission begins when infected female Anopheles mosquitos deposit Plasmodium parasites into the mammalian host's skin during a bloodmeal. The salivary gland-resident sporozoite parasites migrate to the bloodstream, subsequently invading and replicating within hepatocytes. As Anopheles mosquitos are more active at night, with a 24-hour rhythm, we investigated whether their salivary glands are under circadian control, anticipating bloodmeals and modulating sporozoite biology for host encounters. Here we show that approximately half of the mosquito salivary gland transcriptome, particularly genes essential for efficient bloodmeals such as anti-blood clotting factors, exhibits circadian rhythmic expression. Furthermore, we demonstrate that mosquitoes prefer to feed during nighttime, with the amount of blood ingested varying cyclically throughout the day. Notably, we show a substantial subset of the sporozoite transcriptome cycling throughout the day. These include genes involved in parasite motility, potentially modulating the ability to initiate infection at different times of day. Thus, although sporozoites are typically considered quiescent, our results demonstrate their transcriptional activity, revealing robust daily rhythms of gene expression. Our findings suggest a circadian evolutionary relationship between the vector, parasite and mammalian host that together modulate malaria transmission.

A new malaria vaccination tool based on replication-competent Plasmodium falciparum parasites

D. Moita and M. Prudêncio discuss the study by Goswami et al, in this issue of EMBO Mol. Med ., on the generation and pre-clinical characterization of late liver stage-arresting genetically attenuated Plasmodium parasites as a potential vaccine against malaria.

Immune tolerance caused by repeated P. falciparum infection against SE36 malaria vaccine candidate antigen and the resulting limited polymorphism

The call for second generation malaria vaccines needs not only the identification of novel candidate antigens or adjuvants but also a better understanding of immune responses and the underlying protective processes. Plasmodium parasites have evolved a range of strategies to manipulate the host immune system to guarantee survival and establish parasitism. These immune evasion strategies hamper efforts to develop effective malaria vaccines. In the case of a malaria vaccine targeting the N-terminal domain of P. falciparum serine repeat antigen 5 (SE36), now in clinical trials, we observed reduced responsiveness (lowered immunogenicity) which may be attributed to immune tolerance/immune suppression. Here, immunogenicity data and insights into the immune responses to SE36 antigen from epidemiological studies and clinical trials are summarized. Documenting these observations is important to help identify gaps for SE36 continued development and engender hope that highly effective blood-stage/multi-stage vaccines can be achieved.

A Diversity Covering (DiCo) Plasmodium vivax apical membrane antigen-1 vaccine adjuvanted with RFASE/RSL10 yields high levels of growth-inhibitory antibodies

Plasmodium vivax malaria is increasingly recognized as a major global health problem and the socio-economic impact of P.vivax-induced burden is huge. Vaccine development against P. vivax malaria has been hampered by the lack of an in vitro culture system and poor access to P. vivax sporozoites. The recent generation of Plasmodium falciparum parasites that express a functional P. vivax AMA1 molecule has provided a platform for in vitro evaluation of PvAMA1 as a potential blood stage vaccine. Three so-called PvAMA1 Diversity Covering (DiCo) proteins were designed to assess their potential to induce a functional and broad humoral immune response to the polymorphic PvAMA1 molecule. Rabbits were immunized with the mixture of three, Pichia-produced, PvAMA1 DiCo proteins, as well as with 2 naturally occurring PvAMA1 alleles. For these three groups, the experimental adjuvant raffinose fatty acid sulfate ester (RFASE) was used, while in a fourth group the purified main mono-esterified constituent (RSL10) of this adjuvant was used. Animals immunized with the mixture of the three PvAMA1 DiCo proteins in RFASE showed high anti-PvAMA1 antibody titers against three naturally occurring PvAMA1variants while also high growth-inhibitory capacity was observed against P. falciparum parasites expressing PvAMA1. This supports further clinical development of the PvAMA1 DiCo mixture as a potential malaria vaccine. However, as the single allele PvAMA1 SalI-group showed similar characteristics in antibody titer and inhibition levels as the PvAMA1 DiCo mixture-group, this raises the question whether a mixture is really necessary to overcome the polymorphism in the vaccine candidate. RFASE induced strong humoral responses, as did the animals immunized with the purified component, RSL10. This suggests that RSL10 is the active ingredient. However, one of the RSL10-immunized animal showed a delayed response, necessitating further research into the clinical development of RSL10.

Systemic inflammatory response syndrome triggered by blood-borne pathogens induces prolonged dendritic cell paralysis and immunosuppression

Blood-borne pathogens can cause systemic inflammatory response syndrome (SIRS) followed by protracted, potentially lethal immunosuppression. The mechanisms responsible for impaired immunity post-SIRS remain unclear. We show that SIRS triggered by pathogen mimics or malaria infection leads to functional paralysis of conventional dendritic cells (cDCs). Paralysis affects several generations of cDCs and impairs immunity for 3-4 weeks. Paralyzed cDCs display distinct transcriptomic and phenotypic signatures and show impaired capacity to capture and present antigens in vivo. They also display altered cytokine production patterns upon stimulation. The paralysis program is not initiated in the bone marrow but during final cDC differentiation in peripheral tissues under the influence of local secondary signals that persist after resolution of SIRS. Vaccination with monoclonal antibodies that target cDC receptors or blockade of transforming growth factor β partially overcomes paralysis and immunosuppression. This work provides insights into the mechanisms of paralysis and describes strategies to restore immunocompetence post-SIRS.

Profiling the antibody response of humans protected by immunization with Plasmodium vivax radiation-attenuated sporozoites

Malaria sterile immunity has been reproducibly induced by immunization with Plasmodium radiation-attenuated sporozoites (RAS). Analyses of sera from RAS-immunized individuals allowed the identification of P. falciparum antigens, such as the circumsporozoite protein (CSP), the basis for the RTS, S and R21Matrix-M vaccines. Similar advances in P. vivax (Pv) vaccination have been elusive. We previously reported 42% (5/12) of sterile protection in malaria-unexposed, Duffy-positive (Fy +) volunteers immunized with PvRAS followed by a controlled human malaria infection (CHMI). Using a custom protein microarray displaying 515 Pv antigens, we found a significantly higher reactivity to PvCSP and one hypothetical protein (PVX_089630) in volunteers protected against P. vivax infection. In mock-vaccinated Fy + volunteers, a strong antibody response to CHMI was also observed. Although the Fy- volunteers immunized with non-irradiated Pv-infected mosquitoes (live sporozoites) did not develop malaria after CHMI, they recognized a high number of antigens, indicating the temporary presence of asexual parasites in peripheral blood. Together, our findings contribute to the understanding of the antibody response to P. vivax infection and allow the identification of novel parasite antigens as vaccine candidates.Trial registration: ClinicalTrials.gov number: NCT01082341.

Variable long-term protection by radiation-, chemo-, and genetically-attenuated Plasmodium berghei sporozoite vaccines

Long-term protection against malaria remains one of the greatest challenges of vaccination against this deadly parasitic disease. Whole-sporozoite (WSp) malaria vaccine formulations, which target the Plasmodium parasite's pre-erythrocytic stages, include radiation-attenuated sporozoites (RAS), early- and late-arresting genetically-attenuated parasites (EA-GAP and LA-GAP, respectively), and chemoprophylaxis with sporozoites (CPS). Although all these four vaccine formulations induce protective immune responses in the clinic, data on the longevity of the antimalarial protection they afford remain scarce. We employed a mouse model of malaria to assess protection conferred by immunization with P. berghei (Pb)-based surrogates of these four WSp formulations over a 36-week period. We show that EA-GAP WSp provide the lowest overall protection against an infectious Pb challenge, and that while immunization with RAS and LA-GAP WSp elicits the most durable protection, the protective efficacy of CPS WSp wanes rapidly over the 36-week period, most notably at higher immunization dosages. Analyses of liver immune cells show that CD44hi CD8+ T cells in CPS WSp-immunized mice express increased levels of the co-inhibitory PD-1 and LAG-3 markers compared to mice immunized with the other WSp formulations. This indicates that memory CD8+ T cells elicited by CPS WSp immunization display a more exhausted phenotype, which may explain the rapid waning of protection conferred by the former. These results emphasize the need for a detailed comparison of the duration of protection of different WSp formulations in humans and suggest a more beneficial effect of RAS and LA-GAP WSp compared to EA-GAP or CSP WSp.

The effect of dosage on the protective efficacy of whole-sporozoite formulations for immunization against malaria

Immunization with Plasmodium sporozoites, either attenuated or administered under the cover of an antimalarial drug, can induce strong protection against malaria in pre-clinical murine models, as well as in human trials. Previous studies have suggested that whole-sporozoite (WSpz) formulations based on parasites with longer liver stage development induce higher protection, but a comparative analysis of four different WSpz formulations has not been reported. We employed a rodent model of malaria to analyze the effect of immunization dosage on the protective efficacy of WSpz formulations consisting of (i) early liver arresting genetically attenuated parasites (EA-GAP) or (ii) radiation-attenuated sporozoites (RAS), (iii) late arresting GAP (LA-GAP), and (iv) sporozoites administered under chemoprophylaxis, that are eliminated upon release into the bloodstream (CPS). Our results show that, unlike all other WSpz formulations, EA-GAP fails to confer complete protection against an infectious challenge at any immunization dosage employed, suggesting that a minimum threshold of liver development is required to elicit fully effective immune responses. Moreover, while immunization with RAS, LA-GAP and CPS WSpz yields comparable, dosage-dependent protection, protection by EA-GAP WSpz peaks at an intermediate dosage and markedly decreases thereafter. In-depth immunological analyses suggest that effector CD8+ T cells elicited by EA-GAP WSpz immunization have limited developmental plasticity, with a potential negative impact on the functional versatility of memory cells and, thus, on protective immunity. Our findings point towards dismissing EA-GAP from prioritization for WSpz malaria vaccination and enhance our understanding of the complexity of the protection elicited by these WSpz vaccine candidates, guiding their future optimization.

Antimalarial Agents Derived from Metal-Amodiaquine Complexes with Activity in Multiple Stages of the Plasmodium Life Cycle

Malaria is the one of the deadliest infectious diseases worldwide. Chemically, quinolines are excellent ligands for metal coordination and are deployed as drugs for malaria treatment. There is a growing body of evidence indicating that metal complexes can be conjugated with antimalarial quinolines to be used as chemical tools to overcome the disadvantages of quinolines, improving their bioactive speciation, cellular distribution, and subsequently broadening the spectrum of activity to multiple stages of the complex Plasmodium life cycle. In this study, four novel complexes of ruthenium(II)- and gold(I)-containing amodiaquine (AQ) were synthesized, and a careful chemical characterization revealed the precise coordination site of AQ to the metals. Their speciation in solution was investigated, demonstrating the stability of the quinoline-metal bond. RuII - and AuI -AQ complexes were demonstrated to be potent and efficacious in inhibiting parasite growth in multiple stages of the Plasmodium life cycle as assayed in vitro and in vivo. These properties could be attributed to the ability of the metal-AQ complexes to reproduce the suppression of heme detoxification induced by AQ, while also inhibiting other processes in the parasite life cycle; this can be attributed to the action of the metallic species. Altogether, these findings indicate that metal coordination with antimalarial quinolines is a potential chemical tool for drug design and discovery in malaria and other infectious diseases susceptible to quinoline treatment.

mRNA vaccine against malaria tailored for liver-resident memory T cells

Malaria is caused by Plasmodium species transmitted by Anopheles mosquitoes. Following a mosquito bite, Plasmodium sporozoites migrate from skin to liver, where extensive replication occurs, emerging later as merozoites that can infect red blood cells and cause symptoms of disease. As liver tissue-resident memory T cells (Trm cells) have recently been shown to control liver-stage infections, we embarked on a messenger RNA (mRNA)-based vaccine strategy to induce liver Trm cells to prevent malaria. Although a standard mRNA vaccine was unable to generate liver Trm or protect against challenge with Plasmodium berghei sporozoites in mice, addition of an agonist that recruits T cell help from type I natural killer T cells under mRNA-vaccination conditions resulted in significant generation of liver Trm cells and effective protection. Moreover, whereas previous exposure of mice to blood-stage infection impaired traditional vaccines based on attenuated sporozoites, mRNA vaccination was unaffected, underlining the potential for such a rational mRNA-based strategy in malaria-endemic regions.

Malaria Genomics, Vaccine Development, and Microbiome

Recent advances in malaria genetics and genomics have transformed many aspects of malaria research in areas of molecular evolution, epidemiology, transmission, host-parasite interaction, drug resistance, pathogenicity, and vaccine development. Here, in addition to introducing some background information on malaria parasite biology, parasite genetics/genomics, and genotyping methods, we discuss some applications of genetic and genomic approaches in vaccine development and in studying interactions with microbiota. Genetic and genomic data can be used to search for novel vaccine targets, design an effective vaccine strategy, identify protective antigens in a whole-organism vaccine, and evaluate the efficacy of a vaccine. Microbiota has been shown to influence disease outcomes and vaccine efficacy; studying the effects of microbiota in pathogenicity and immunity may provide information for disease control. Malaria genetics and genomics will continue to contribute greatly to many fields of malaria research.

Babesia microti alleviates disease manifestations caused by Plasmodium berghei ANKA in murine co-infection model of complicated malaria

Malaria remains one of the most significant health issues worldwide, accounting for 2.6% of the total global disease burden, and efforts to eliminate this threat continue. The key focus is to develop an efficient and long-term immunity to this disease via vaccination or therapeutic approach, and innovative strategies would enable us to achieve this target. Previously, using a mouse co-infection disease model, cross-protection was illustrated between Babesia microti and Plasmodium chabaudi. Hence, this study was planned to elucidate the impact of acute B. microti Peabody mjr and Plasmodium berghei ANKA co-infection on the consequence of complicated malaria in the C57BL/6J mouse model of malaria. Furthermore, immune response and pathological features were analyzed, and the course of the disease was compared among experimental groups. Our study established that acute B. microti infection activated immunity which was otherwise suppressed by P. berghei. The immunosuppressive tissue microenvironment was counteracted as evidenced by the enhanced immune cell population in co-infected mice, in contrast to P. berghei-infected control mice. Parasite sequestration in the brain, liver, lung, and spleen of co-infected mice was significantly decreased and tissue injury was ameliorated. Meanwhile, the serum levels of IFN-γ, TNF-α, and IL-12p70 were reduced while the secretion of IL-10 was promoted in co-infected mice. Eventually, co-infected mice showed an extended rate of survival. Hereby, the principal cytokines associated with the severity of malaria by P. berghei infection were TNF-α, IFN-γ, and IL-12p70. Moreover, it was evident from our flow cytometry results that innate immunity is crucial and macrophages are at the frontline of immunity against P. berghei infection. Our study recommended further investigations to shed light on the effects of babesiosis in suppressing malaria with the goal of developing Babesia-based therapy against malaria.

A Critical Review on Human Malaria and Schistosomiasis Vaccines: Current State, Recent Advancements, and Developments

Malaria and schistosomiasis are two major parasitic diseases that remain leading causes of morbidity and mortality worldwide. Co-infections of these two parasites are common in the tropics, where both diseases are endemic. The clinical consequences of schistosomiasis and malaria are determined by a variety of host, parasitic, and environmental variables. Chronic schistosomiasis causes malnutrition and cognitive impairments in children, while malaria can cause fatal acute infections. There are effective drugs available to treat malaria and schistosomiasis. However, the occurrence of allelic polymorphisms and the rapid selection of parasites with genetic mutations can confer reduced susceptibility and lead to the emergence of drug resistance. Moreover, the successful elimination and complete management of these parasites are difficult due to the lack of effective vaccines against Plasmodium and Schistosoma infections. Therefore, it is important to highlight all current vaccine candidates undergoing clinical trials, such as pre-erythrocytic and erythrocytic stage malaria, as well as a next-generation RTS,S-like vaccine, the R21/Matrix-M vaccine, that conferred 77% protection against clinical malaria in a Phase 2b trial. Moreover, this review also discusses the progress and development of schistosomiasis vaccines. Furthermore, significant information is provided through this review on the effectiveness and progress of schistosomiasis vaccines currently under clinical trials, such as Sh28GST, Sm-14, and Sm-p80. Overall, this review provides insights into recent progress in malarial and schistosomiasis vaccines and their developmental approaches.

The mRNA Vaccine Technology Era and the Future Control of Parasitic Infections

Despite intensive long-term efforts, with very few exceptions, the development of effective vaccines against parasitic infections has presented considerable challenges, given the complexity of parasite life cycles, the interplay between parasites and their hosts, and their capacity to escape the host immune system and to regulate host immune responses. For many parasitic diseases, conventional vaccine platforms have generally proven ill suited, considering the complex manufacturing processes involved and the costs they incur, the inability to posttranslationally modify cloned target antigens, and the absence of long-lasting protective immunity induced by these antigens. An effective antiparasite vaccine platform is required to assess the effectiveness of novel vaccine candidates at high throughput. By exploiting the approach that has recently been used successfully to produce highly protective COVID mRNA vaccines, we anticipate a new wave of research to advance the use of mRNA vaccines to prevent parasitic infections in the near future. This article considers the characteristics that are required to develop a potent antiparasite vaccine and provides a conceptual foundation to promote the development of parasite mRNA-based vaccines. We review the recent advances and challenges encountered in developing antiparasite vaccines and evaluate the potential of developing mRNA vaccines against parasites, including those causing diseases such as malaria and schistosomiasis, against which vaccines are currently suboptimal or not yet available.

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.