Sporozoite immunization: innovative translational science to support the fight against malaria

Meeting report: Expert consultation on late arresting replication competent (LARC) malaria sporozoite vaccine research & development

In June 2024, the National Institute of Allergy and Infectious Diseases (NIAID) convened an expert consultation to guide strategic priorities for the research and development of next-generation whole Plasmodium falciparum sporozoite vaccine candidates. The focus was on genetically attenuated, late (liver stage) arresting replication competent (LARC) sporozoite vaccines. The meeting reviewed the World Health Organization (WHO) Preferred Product Characteristics (PPCs) for future malaria vaccines and drew lessons from previous malaria vaccine R&D and implementation efforts. Key discussions centered on critical features related to the Target Product Profiles (TPP) of LARC vaccines for various specified indications and relevant development strategies. The consultation identified knowledge gaps and suggested priorities and opportunities for future research and product development for LARC malaria sporozoite vaccines.

Current Developments in Malaria Vaccination: A Concise Review on Implementation, Challenges, and Future Directions

Introduction

Malaria remains a persistent challenge in global health, disproportionately affecting populations in endemic regions (eg, sub-Saharan Africa). Despite decades of international collaborative efforts, malaria continues to claim hundreds of thousands of lives each year, with young children and pregnant women enduring the heaviest burden. This concise review aimed to provide an up-to-date assessment of malaria vaccines progress, challenges, and future directions.

Methods

A PubMed/MEDLINE search (2015-2024) was conducted to identify studies on malaria vaccine development, implementation barriers, efficacy, and vaccination hesitancy. Clinical trials, reviews, and global health reports were included based on relevance to the review aims. No strict inclusion criteria were applied, and selection was guided by key review themes and policy relevance.

Results

The introduction of pre-erythrocytic malaria vaccines (RTS,S/AS01 and R21/Matrix-M), represents an important milestone in malaria control efforts with promising results from the erythrocytic vaccine RH5.1/Matrix-M in recent clinical trials. However, the approval of these vaccines is accompanied by significant challenges such as the limited efficacy, the complexity of multi-dose regimens, and numerous barriers to widespread implementation in resource-limited settings. The review identified the complex challenges to broad malaria vaccination coverage, including logistical barriers, healthcare infrastructure effect, financial limitations, malaria vaccine hesitancy, among other obstacles in malaria-endemic regions. Promising developments in malaria vaccination, such as next-generation candidates (eg, mRNA-based vaccines), hold the potential to offer improved efficacy, longer-lasting protection, and greater scalability. There is a critical need to integrate malaria vaccination efforts with established malaria control interventions (eg, insecticide-treated bed nets, vector control strategies, and anti-malarial drugs).

Conclusion

Achieving sustained control of malaria morbidity and mortality will require strong global collaboration, sufficient funding, and continuous efforts to address inequities in access and delivery of malaria control measures including the malaria vaccines.

Whole-genome sieve analysis: Identification of protective malaria antigens by leveraging allele-specific vaccine efficacy

Discovery of new protective malaria antigens will enable the development of novel vaccine formulations with potentially higher efficacy. While several high-throughput experimental approaches enable the identification of novel immunogens, none so far has been designed to selectively identify protective antigens. Here, we propose that sieve analysis conducted on the whole genome (SAWG) can be used specifically for this purpose. We review available medium- to high-throughput methods for antigen identification and contextualize the need for the identification of protective antigens. We then provide the rationale for why SAWG is ideally suited for the identification of protective antigens in recombining pathogens with large genome size, describe conditions for optimal use, and discuss potential pitfalls. Most importantly, this approach can be applied to the discovery of new protective targets in any recombining organism for which there is a whole organism-based vaccine that can be safely deployed in a disease-endemic region.

Safety, tolerability and immunogenicity of a condensed, multi-dose prime regimen of PfSPZ Vaccine for the prevention of Plasmodium falciparum malaria infection

BACKGROUND

The World Health Organization (WHO) has called for new malaria vaccines with > 90% efficacy against Plasmodium falciparum infection to expand the anti-disease benefit provided by the RTS,S/AS01 and R21/Matrix M subunit vaccines currently administered to infants and young children in sub-Saharan Africa. Attenuated P. falciparum sporozoites (PfSPZ) are being developed as a traveller's vaccine and to fulfill WHO's call for high-level efficacy in endemic countries to support malaria elimination.

METHODS

PfSPZ Vaccine, comprised of radiation-attenuated PfSPZ, was compared with normal saline placebo in a randomized, double-blind trial targeting 60 malaria-naive US adults to assess safety, tolerability, immunogenicity, and efficacy against heterologous controlled human malaria infection three and twelve weeks after immunization. Pharmacists provided syringes to blinded clinicians using 3:1 (vaccine:placebo) blocked randomization, for administration by direct venous inoculation on days 1 and 8 (multidose prime) and day 29 (boost), a condensed regimen with superior efficacy. Primary outcomes included adverse events and antibody responses to the P. falciparum circumsporozoite protein (PfCSP).

RESULTS

31 participants were screened, randomized and immunized twice (V1, V2) 5-7 days apart, with one withdrawal after an intercurrent adverse event. A vial issue, later traced to the vial manufacturer, halted further immunizations. Solicited local and systemic adverse events recorded for 2 and 7 days after immunizations, respectively, occurred with equal frequency and severity in the 23 vaccinees and 7 controls receiving two immunizations, as did unsolicited adverse events recorded for 28 days and laboratory abnormalities 1 and 5 weeks after V2. Four of 23 vaccinees and one of 7 controls (p = 1.00) developed grade 2 adverse events including subjective fever, headache, malaise, fatigue, rigors, arthralgia and myalgia after V2 but not V1, these symptoms generally resolving within 24 h. Twenty-two of 23 (96%) vaccinees developed IgG (median 99-fold increase over baseline) and IgM (median 1,110-fold increase) antibodies to PfCSP one week after V2. Antibody responses were not associated with reactogenicity.

CONCLUSIONS

The two-dose priming immunization regimen was safe, well tolerated and highly immunogenic. Larger studies may better define the adverse event profile of condensed regimens of PfSPZ Vaccine in malaria-naive adults.

TRIAL REGISTRATION NUMBER

clinicaltrial.gov NCT05604521.

Protective targets of PfSPZ vaccines identified from whole-genome sieve analysis of isolates from malaria vaccine efficacy trials in West Africa

Identification of antigens targeted by a protective response is a central quest in malaria vaccinology. Whole-genome sieve analysis (SAWG) in samples collected from placebo-controlled field trials of Plasmodium falciparum (Pf) sporozoite (SPZ) vaccines may enable identification of Pf pre-erythrocytic antigens. We applied SAWG to genomic data generated from Pf isolates collected during two field trials measuring the efficacy, in malaria-exposed African adults, of two PfSPZ vaccines. These randomized, double-blind, placebo-controlled trials were conducted in regions of Mali and Burkina Faso characterized by high seasonal transmission, where parasite genetic diversity is high. Genomic sites in which the vaccine allelic state was significantly underrepresented among breakthrough infections in vaccinees relative to placebo recipients were termed "target sites". Protein-coding loci containing target sites that changed amino acids were termed "target loci". The SAWG conducted on clinical trial samples from the Burkina Faso and Mali trials identified 138 and 80 single-copy protein-coding target loci in the Burkinabe and Malian data sets, respectively, with twelve common to both, a number significantly higher than expected (E = 3.9; 99%CI = [0, 9]). Among these was the thrombospondin-related anonymous protein locus, which encodes PfSSP2|TRAP, one of the most abundant and well-characterized pre-erythrocytic stage antigen as well as other genes encoding membrane-associated proteins of unknown function. These results identify SAWG as a potentially powerful tool for identifying protective vaccine antigens in recombining pathogens with large genome size and reveals potential new protective Pf antigens.

Liver stage P. falciparum antigens highly targeted by CD4+ T cells in malaria-exposed Ugandan children

T cell responses against liver stage Plasmodium help protect against reinfection, but the antigens and epitopes targeted by these T cells are largely unknown. This knowledge gap has impeded mechanistic studies to identify the effector functions most critical for protection. We performed a bioinformatic analysis of gene expression datasets to identify plasmodial genes that are highly and selectively expressed during liver stage infection and predict epitopes within them likely to bind MHC-II molecules prevalent in Uganda. We then tested their recognition by malaria-exposed Ugandan children. In over two-thirds of the children, we detected a peripheral blood CD4+ T cell response to one or more antigens. The most highly targeted antigen, LISP1, contained several epitopes, including one that was promiscuously presented and recognized by most participants. These novel liver stage P. falciparum epitopes should be explored as potential vaccine targets and will facilitate the development of tools to interrogate antimalarial immunity at the single-cell level and inform future vaccine development efforts.

Single immunization with genetically attenuated Pf∆mei2 (GA2) parasites by mosquito bite in controlled human malaria infection: a placebo-controlled randomized trial

Malaria vaccines consisting of metabolically active Plasmodium falciparum (Pf) sporozoites can offer improved protection compared with currently deployed subunit vaccines. In a previous study, we demonstrated the superior protective efficacy of a three-dose regimen of late-arresting genetically attenuated parasites administered by mosquito bite (GA2-MB) compared with early-arresting counterparts (GA1-MB) against a homologous controlled human malaria infection. Encouraged by these results, we explored the potency of a single GA2-MB immunization in a placebo-controlled randomized trial. Primary outcomes were safety and tolerability, time-to-parasitemia and protective efficacy. Humoral and cellular immunological results were considered secondary outcomes. Here we report the safe administration of GA2-MB with no breakthrough malaria and sterile protection in nine of ten participants at 6 weeks after a single immunization with 50 GA2-infected mosquitoes, compared with none of five mock-immunized participants, against a homologous controlled human malaria infection. Immunization increased circulating Pf-specific polyfunctional effector memory CD4+ T cells coexpressing tumor necrosis factor and interleukin-2. This unprecedented 90% protective efficacy after a single low-dose immunization holds great promise for the potency of GA2 immunization. Future studies should demonstrate whether GA2 is similarly efficacious in pre-exposed populations and whether the favorable safety profile reported here holds up in larger groups. ClinicalTrials.gov registration: NCT05468606 .

Identifying Plasmodium P36 and P52 antigens for coadministration with circumsporozoite protein to enhance vaccine efficacy

Vaccines targeting the complex pre-erythrocytic stage of Plasmodium parasites may benefit from the inclusion of multiple antigens. However, discerning protective effects can be difficult because newer candidates may not be as protective as leading antigens like the circumsporozoite protein (CSP) in the conventional pre-clinical mouse model. We developed a modified mouse model challenge strategy that maximizes the contribution of T cells induced by novel candidate antigens at the sporozoite challenge time point and used this approach to test Plasmodium P36 and P52 vaccine candidates alone and in concert with non-protective doses of CSP. Co-administration of P36 and/or P52 with CSP achieved 80-100% sterile protection in mice, compared to only 7-30% protection for each individual antigen. P36 and P52 vaccination induced murine CD4+ and CD8+ T cell responses, but not antibody responses. This study adds P36 and P52 as promising vaccine antigens that may enhance the protection achieved by CSP vaccination.

Malaria vaccines: a new era of prevention and control

Malaria killed over 600,000 people in 2022, a death toll that has not improved since 2015. Additionally, parasites and mosquitoes resistant to existing interventions are spreading across Africa and other regions. Vaccines offer hope to reduce the mortality burden: the first licensed malaria vaccines, RTS,S and R21, will be widely deployed in 2024 and should substantially reduce childhood deaths. In this Review, we provide an overview of the malaria problem and the Plasmodium parasite, then describe the RTS,S and R21 vaccines (the first vaccines for any human parasitic disease), summarizing their benefits and limitations. We explore next-generation vaccines designed using new knowledge of malaria pathogenesis and protective immunity, which incorporate antigens and platforms to elicit effective immune responses against different parasite stages in human or mosquito hosts. We describe a decision-making process that prioritizes malaria vaccine candidates for development in a resource-constrained environment. Future vaccines might improve upon the protective efficacy of RTS,S or R21 for children, or address the wider malaria scourge by preventing pregnancy malaria, reducing the burden of Plasmodium vivax or accelerating malaria elimination.

Safety and efficacy of PfSPZ Vaccine against malaria in healthy adults and women anticipating pregnancy in Mali: two randomised, double-blind, placebo-controlled, phase 1 and 2 trials

BACKGROUND

Plasmodium falciparum parasitaemia during pregnancy causes maternal, fetal, and infant mortality. Poor pregnancy outcomes are related to blood-stage parasite sequestration and the ensuing inflammatory response in the placenta, which decreases over successive pregnancies. A radiation-attenuated, non-replicating, whole-organism vaccine based on P falciparum sporozoites (PfSPZ Vaccine) has shown efficacy at preventing infection in African adults. Here, we aimed to examine vaccine safety and efficacy of the PfSPZ Vaccine in adults and women who anticipated conception.

METHODS

Two randomised, double-blind, placebo-controlled trials (phase 1 MLSPZV3 and phase 2 MLSPZV4) were conducted at a clinical research centre in Mali. MLSPZV3 included adults aged 18-35 years and MLSPZV4 included non-pregnant women aged 18-38 years who anticipated conception within a year of enrolment. In MLSPZV3, participants were stratified by village and randomly assigned (2:1) using block randomisation to receive three doses of 9 × 105 PfSPZ Vaccine or saline placebo at weeks 0, 1, and 4 (4-week schedule) or at weeks 0, 8, and 16 (16-week schedule) and a booster dose around 1 year later. In MLSPZV4, women received presumptive artemether-lumefantrine twice per day for 3 days 2 weeks before dose one and were randomly assigned (1:1:1) using block randomisation to receive three doses of 9 × 105 or 1·8 × 106 PfSPZ Vaccine or saline placebo all administered at weeks 0, 1, and 4 (4-week schedule). Participants in both studies received artemether-lumefantrine 2 weeks before dose three and additionally 2 weeks before dose four (booster dose) in MLSPZV3. Investigators and participants were masked to group assignment. The primary outcome, assessed in the as-treated population, was PfSPZ Vaccine safety and tolerability within 7 days after each dose. The secondary outcome, assessed in the modified intention-to-treat population, was vaccine efficacy against P falciparum parasitaemia (defined as the time-to-first positive blood smear) from dose three until the end of transmission season. In exploratory analyses, MLSPZV4 evaluated incidence of maternal obstetric and neonatal outcomes as safety outcomes, and vaccine efficacy against P falciparum parasitaemia during pregnancy (defined as time-to-first positive blood smear post-conception). In MLSPZV4, women were followed at least once a month with human chorionic gonadotropin testing, and those who became pregnant received standard of care (including intermittent presumptive sulfadoxine-pyrimethamine antimalarial drugs after the first trimester) during routine antenatal visits. These studies are registered with ClinicalTrials.gov, NCT03510481 and NCT03989102.

FINDINGS

Participants were enrolled for vaccination during the onset of malaria seasons for two sequential studies conducted from 2018 to 2019 for MLSPZV3 and from 2019 to 2021 for MLSPZV4, with follow-up during malaria seasons across 2 years. In MLSPZV3, 478 adults were assessed for eligibility, of whom 220 were enrolled between May 30 and June 12, 2018, and then between Aug 13 and Aug 18, 2018, and 210 received dose one. 66 (96%) of 69 participants who received the 16-week schedule and 68 (97%) of 70 who received the 4-week schedule of the 9 × 105 PfSPZ Vaccine and 70 (99%) of 71 who received saline completed all three doses in year 1. In MLSPZV4, 407 women were assessed for eligibility, of whom 324 were enrolled from July 3 to July 27, 2019, and 320 received dose one of presumptive artemether-lumefantrine. 300 women were randomly assigned with 100 per group (PfSPZ Vaccine 9 × 105, 1·8 × 106, or saline) receiving dose one. First trimester miscarriages were the most commonly reported serious adverse event but occurred at a similar rate across study groups (eight [15%] of 54 with 9 × 105 PfSPZ Vaccine, 12 [21%] of 58 with 1·8 × 106 PfSPZ Vaccine, and five [12%] of 43 with saline). One unrelated maternal death occurred 425 days after the last vaccine dose in the 1·8 × 106 PfSPZ Vaccine group due to peritonitis shortly after childbirth. Most related adverse events reported in MLSPZV3 and MLSPZV4 were mild (grade 1) and frequency of adverse events in the PfSPZ Vaccine groups did not differ from that in the saline group. Two unrelated serious adverse events occurred in MLSPZV3 (one participant had appendicitis in the 9 × 105 PfSPZ Vaccine group and the other in the saline group died due to a road traffic accident). In MLSPZV3, the 9 × 105 PfSPZ Vaccine did not show vaccine efficacy against parasitaemia with the 4-week (27% [95% CI -18 to 55] in year 1 and 42% [-5 to 68] in year 2) and 16-week schedules (16% [-34 to 48] in year 1 and -14% [-95 to 33] in year 2); efficacies were similar or worse against clinical malaria compared with saline. In MLSPZV4, the PfSPZ Vaccine showed significant efficacy against parasitaemia at doses 9 × 105 (41% [15 to 59]; p=0·0069 in year 1 and 61% [36 to 77]; p=0·0011 in year 2) and 1·8 × 106 (54% [34 to 69]; p<0·0001 in year 1 and 45% [13 to 65]; p=0·029 in year 2); and against clinical malaria at doses 9 × 105 (47% [20 to 65]; p=0·0045 in year 1 and 56% [22 to 75]; p=0·0081 in year 2) and 1·8 × 106 (48% [22 to 65]; p=0·0013 in year 1 and 40% [2 to 64]; p=0·069 in year 2). Vaccine efficacy against post-conception P falciparum parasitaemia during first pregnancies that arose in the 2-year follow-up was 57% (14 to 78; p=0·017) in the 9 × 105 PfSPZ Vaccine group versus 49% (3 to 73; p=0·042) in the 1·8 × 106 PfSPZ Vaccine group. Among 55 women who became pregnant within 24 weeks after dose three, vaccine efficacy against parasitaemia was 65% (23 to 84; p=0·0088) with the 9 × 105 PfSPZ Vaccine and 86% (64 to 94; p<0·0001) with the 1·8 × 106 PfSPZ Vaccine. When combined in a post-hoc analysis, women in the PfSPZ Vaccine groups had a non-significantly reduced time-to-first pregnancy after dose one compared with those in the saline group (log-rank test p=0·056). Exploratory maternal obstetric and neonatal outcomes did not differ significantly between vaccine groups and saline.

INTERPRETATION

PfSPZ Vaccine was safe and well tolerated in adults in Mali. The 9 × 105 and 1·8 × 106 doses of PfSPZ Vaccine administered as per the 4-week schedule, which incorporated presumptive antimalarial treatment before the first vaccine dose, showed significant efficacy against P falciparum parasitaemia and clinical malaria for two malaria transmission seasons in women of childbearing age and against pregnancy malaria. PfSPZ Vaccine without presumptive antimalarial treatment before the first vaccine dose did not show efficacy.

FUNDING

National Institute of Allergy and Infectious Diseases, National Institutes of Health, and Sanaria.

Safety and Efficacy of Immunization with a Late-Liver-Stage Attenuated Malaria Parasite

BACKGROUND

Currently licensed and approved malaria subunit vaccines provide modest, short-lived protection against malaria. Immunization with live-attenuated Plasmodium falciparum malaria parasites is an alternative vaccination strategy that has potential to improve protection.

METHODS

We conducted a double-blind, controlled clinical trial to evaluate the safety, side-effect profile, and efficacy of immunization, by means of mosquito bites, with a second-generation genetically attenuated parasite (GA2) - a mei2 single knockout P. falciparum NF54 parasite (sporozoite form) with extended development into the liver stage. After an open-label dose-escalation safety phase in which participants were exposed to the bites of 15 or 50 infected mosquitoes (stage A), healthy adults who had not had malaria were randomly assigned to be exposed to 50 mosquito bites per immunization of GA2, an early-arresting parasite (GA1), or placebo (bites from uninfected mosquitoes) (stage B). After the completion of three immunization sessions with 50 mosquito bites per session, we compared the protective efficacy of GA2 against homologous P. falciparum controlled human malaria infection with that of GA1 and placebo. The primary end points were the number and severity of adverse events (in stages A and B) and blood-stage parasitemia greater than 100 P. falciparum parasites per milliliter after bites from GA2-infected mosquitoes (in stage A) and after controlled human malaria infection (in stage B).

RESULTS

Adverse events were similar across the trial groups. Protective efficacy against subsequent controlled human malaria infection was observed in 8 of 9 participants (89%) in the GA2 group, in 1 of 8 participants (13%) in the GA1 group, and in 0 of 3 participants in the placebo group. A significantly higher frequency of P. falciparum-specific polyfunctional CD4+ and Vδ2+ γδ T cells were observed among participants who received GA2 than among those who received GA1, whereas GA2 and GA1 induced similar antibody titers targeting the P. falciparum circumsporozoite protein.

CONCLUSIONS

In this small trial, GA2 was associated with a favorable immune induction profile and protective efficacy, findings that warrant further evaluation. (Funded by the Bontius Foundation; ClinicalTrials.gov number, NCT04577066.).

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.

PfSPZ Vaccine induces focused humoral immune response in HIV positive and negative Tanzanian adults

BACKGROUND

PfSPZ Vaccine, a promising pre-erythrocytic stage malaria vaccine candidate based on whole, radiation-attenuated Plasmodium falciparum (Pf) sporozoites (SPZ), has proven safe and effective in mediating sterile protection from malaria in malaria-naïve and exposed healthy adults. Vaccine-induced protection presumably depends on cellular responses to early parasite liver stages, but humoral immunity contributes.

METHODS

On custom-made Pf protein microarrays, we profiled IgG and IgM responses to PfSPZ Vaccine and subsequent homologous controlled human malaria infection (CHMI) in 21 Tanzanian adults with (n = 12) or without (n = 9) HIV infection. Expression of the main identified immunogens in the pre-erythrocytic parasite stage was verified by immunofluorescence detection using freshly purified PfSPZ and an in vitro model of primary human hepatocytes.

FINDINGS

Independent of HIV infection status, immunisation induced focused IgG and IgM responses to circumsporozoite surface protein (PfCSP) and merozoite surface protein 5 (PfMSP5). We show that PfMSP5 is detectable on the surface and in the apical complex of PfSPZ.

INTERPRETATION

Our data demonstrate that HIV infection does not affect the quantity of the total IgG and IgM antibody responses to PfCSP and PfMSP5 after immunization with PfSPZ Vaccine. PfMSP5 represents a highly immunogenic, so far underexplored, target for vaccine-induced antibodies in malaria pre-exposed volunteers.

FUNDING

This work was supported by the Equatorial Guinea Malaria Vaccine Initiative (EGMVI), the Clinical Trial Platform of the German Center for Infection Research (TTU 03.702), the Swiss Government Excellence Scholarships for Foreign Scholars and Artists (grant 2016.0056) and the Interdisciplinary Center for Clinical Research doctoral program of the Tübingen University Hospital. The funders had no role in design, analysis, or reporting of this study.

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.

Identifying Plasmodium P36 and P52 antigens for co-administration with circumsporozoite protein to enhance vaccine efficacy

Vaccines targeting the complex pre-erythrocytic stage of Plasmodium parasites may benefit from inclusion of multiple antigens. However, discerning protective effects can be difficult because newer candidates may not be as protective as leading antigens like the circumsporozoite protein (CSP) in the conventional pre-clinical mouse model. We developed a modified mouse model challenge strategy that maximizes the contribution of T cells induced by novel candidate antigens at the sporozoite challenge time point and used this approach to test Plasmodium P36 and P52 vaccine candidates alone and in concert with non-protective doses of CSP. Co-administration of P36 and/or P52 with CSP achieved 80-100% sterile protection in mice, compared to only 7-30% protection for each individual antigen. P36 and P52 vaccination induced murine CD4+ and CD8+ T cell responses, but not antibody responses. This study adds P36 and P52 as promising vaccine antigens that may enhance protection achieved by CSP vaccination.

20 years of BioMalPar: Building a collaborative malaria research network

In 2004 the first annual BioMalPar meeting was held at EMBL Heidelberg, bringing together researchers from around the world with the goal of building connections between malaria research groups in Europe. Twenty years on it is time to reflect on what was achieved and to look ahead to the future.

Vaccines and monoclonal antibodies: new tools for malaria control

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

Hepatocytes and the art of killing Plasmodium softly

The Plasmodium parasites that cause malaria undergo asymptomatic development in the parenchymal cells of the liver, the hepatocytes, prior to infecting erythrocytes and causing clinical disease. Traditionally, hepatocytes have been perceived as passive bystanders that allow hepatotropic pathogens such as Plasmodium to develop relatively unchallenged. However, now there is emerging evidence suggesting that hepatocytes can mount robust cell-autonomous immune responses that target Plasmodium, limiting its progression to the blood and reducing the incidence and severity of clinical malaria. Here we discuss our current understanding of hepatocyte cell-intrinsic immune responses that target Plasmodium and how these pathways impact malaria.

Implementation of a Randomized, Placebo-Controlled Trial of Live Attenuated Malaria Sporozoite Vaccines in an Indonesian Military Study Population

Malaria eradication efforts prioritize safe and efficient vaccination strategies, although none with high-level efficacy against malaria infection are yet available. Among several vaccine candidates, Sanaria® PfSPZ Vaccine and Sanaria PfSPZ-CVac are, respectively, live radiation- and chemo-attenuated sporozoite vaccines designed to prevent infection with Plasmodium falciparum, the leading cause of malaria-related morbidity and mortality. We are conducting a randomized normal saline placebo-controlled trial called IDSPZV1 that will analyze the safety, tolerability, immunogenicity, and efficacy of PfSPZ Vaccine and PfSPZ-CVac administered pre-deployment to malaria-naive Indonesian soldiers assigned to temporary duties in a high malaria transmission area. We describe the manifold challenges of enrolling and immunizing 345 soldier participants at their home base in western Indonesia before their nearly 6,000-km voyage to eastern Indonesia, where they are being monitored for incident P. falciparum and Plasmodium vivax malaria cases during 9 months of exposure. The unique regulatory, ethical, and operational complexities of this trial demonstrate the importance of thorough planning, frequent communication, and close follow-up with stakeholders. Effective engagement with the military community and the ability to adapt to unanticipated events have proven key to the success of this trial.

A paradigm for Africa-centric vaccine development in Equatorial Guinea

The Equatorial Guinea Malaria Vaccine Initiative (EGMVI) highlights how long-term African government and international energy industry investment, plus novel partnerships, can enable clinical development of vaccines in Africa, for Africa. We review achievements and challenges of this pioneering, award-winning, public-private partnership which offers a model for future Africa-centric clinical research and development (R&D).

Plasmodium falciparum merozoite surface protein 1 as asexual blood stage malaria vaccine candidate

INTRODUCTION

Malaria represents a public health challenge in tropical and subtropical regions, and currently deployed control strategies are likely insufficient to drive elimination of malaria. Development and improvement of malaria vaccines might be key to reduce disease burden. Vaccines targeting asexual blood stages of the parasite have shown limited efficacy when studied in human trials conducted over the past decades.

AREAS COVERED

Vaccine candidates based on the merozoite surface protein 1 (MSP1) were initially envisioned as one of the most promising approaches to provide immune protection against asexual blood-stage malaria. Successful immunization studies in monkey involved the use of the full-length MSP1 (MSP1FL) as vaccine construct. Vaccines using MSP1FL for immunization have the potential benefit of including numerous conserved B-cell and T-cell epitopes. This could result in improved parasite strain-transcending, protective immunity in the field. We review outcomes of clinical trials that utilized a variety of MSP1 constructs and formulations, including MSP1FL, either alone or in combination with other antigens, in both animal models and humans.

EXPERT OPINION

Novel approaches to analyze breadth and magnitude of effector functions of MSP1-targeting antibodies in volunteers undergoing experimental vaccination and controlled human malaria infection will help to define correlates of protective immunity.

Determine the enzymatic kinetic characteristics of CYP3A4 variants utilizing artemether-lumefantrine

Artemether-lumefantrine is an artemisinin-based combination therapy for the treatment of malaria, which are primarily metabolized by cytochrome P450 3A4. Therapeutic difference caused by gene polymorphisms of CYP3A4 may lead to uncertain adverse side effects or treatment failure. The aim of this study was to evaluate the effect of CYP3A4 gene polymorphism on artemether-lumefantrine metabolism in vitro. Enzyme kinetics assay was performed using recombinant human CYP3A4 cell microsomes. The analytes, dihydroartimisinin and desbutyl-lumefantrine, were detected by ultra-performance liquid chromatography tandem mass spectrometry. The results demonstrated that compared to CYP3A4.1, the intrinsic clearance of CYP3A4.4, 5, 9, 16, 18, 23, 24, 28, 31-34 significantly reduced for artemether (58.5%-93.3%), and CYP3A4.17 almost loss catalytic activity. Simultaneously, CYP3A4.5, 14, 17, 24 for lumefantrine were decreased by 56.1%-99.6%, and CYP3A4.11, 15, 18, 19, 23, 28, 29, 31-34 for lumefantrine was increased by 51.7%-296%. The variation in clearance rate indicated by molecular docking could be attributed to the disparity in the binding affinity of artemether and lumefantrine with CYP3A4. The data presented here have enriched our understanding of the effect of CYP3A4 gene polymorphism on artemether-lumefantrine metabolizing. These findings serve as a valuable reference and provide insights for guiding the treatment strategy involving artemether-lumefantrine.

Models of Protective Immunity against Schistosomes: Implications for Vaccine Development

After many decades of research, a schistosome vaccine still looks to be a distant prospect. These helminths can live in the human bloodstream for years, even decades, surrounded by and feeding on the components of the immune response they provoke. The original idea of a vaccine based on the killing of invading cercariae in the skin has proven to be illusory. There has also been a realisation that even if humans develop some protection against infection over a protracted period, it very likely involves IgE-mediated responses that cannot provide the basis for a vaccine. However, it has also become clear that both invasive migrating larvae and adult worms must expose proteins and release secretions into the host environment as part of their normal biological activities. The application of modern 'omics approaches means that we now have a much better idea of the identity of these potential immune targets. This review looks at three animal models in which acquired immunity has been demonstrated and asks whether the mechanisms might inform our vaccine strategies to achieve protection in model hosts and humans. Eliciting responses, either humoral or cellular, that can persist for many months is a challenge. Arming of the lungs with effector T cells, as occurs in mice exposed to the radiation-attenuated cercarial vaccine, is one avenue. Generating IgG antibody titres that reach levels at which they can exert sustained immune pressure to cause worm elimination, as occurs in rhesus macaques, is another. The induction of memory cell populations that can detect trickle invasions of larval stages remains to be explored. One promising approach is the analysis of protective antibodies using high-density peptide arrays of target proteins to identify reactive regions. These can be combined in multi-epitope constructs to immunise a host against many targets simultaneously and cheaply.