Introduction and Background of the Project

Introduction

As a valuable addition to sustain realistic and affordable malaria control, a candidate anti-malarial vaccine must perform safely and efficiently during neonatal and early life vaccination in resource-poor settings. Accordingly, such a malaria childhood vaccine would ideally be tailored for integration into the current Expanded Programme on Immunization (EPI) vaccines. At the same time, we should consider host factors which may severely impair vaccine efficacy. Because RTS,S vaccine can induce higher protective immune response for volunteers in developed countries, compared with those in developing countries and infants in Africa. We (and others) hypothesize that helminths and maternal antibodies are critical host factors to be considered when developing a malaria vaccine. Significant numbers of individuals living in tropical areas are demonstrably co-infected with helminths, which are known to adversely affect immune responses to a number of different existing vaccines. In addition to the host factors, there is an inevitable risk involved in monovalent vaccines such as RTS,S. Gene polymorphisms and mutations may cause drastic reduction of protective efficacy, resulting in vaccine failure. A recent study has reported that vaccines capable of inducing both antibodies against pre-erythrocytic stage (protection) and sexual-stage parasites (transmission blocking [TB]) possess strong synergistic effects on parasite reduction in prevalence.

Project objective

Development of a highly effective and durable next-generation multistage malaria vaccine effective against both pre-erythrocytic stage and sexual-stage parasites based on two viral vectors for African infants with pre-existing helminths and maternal antibodies. It is proven that successful implementation of this strategy would lead to enhanced parasitic elimination.

Project design

Two viral-vectored vaccines expressing both pre-erythrocytic-stage and sexual-stage antigens will be generated. Protective and transmission blocking (TB) efficacies of the heterologous prime-boost immunization regimen will be assessed by sporozoite challenge and Direct Membrane Feeding Assay (DMFA) in a robust and proven mouse model, and then the regime will be further optimized (e.g., dose, route, interval and outbred mice). Desired protection rate >90%. Surrogate markers responsible for protection will be identified. This will be key to allow efficient and robust measurements of efficacy. Humoral and cellular immune responses induced by the heterologous prime-boost immunization regimen will be assessed.

How can your partnership (project) address global health challenges?

The disappointing results of RTS,S Phase III trials in Africa has made the WHO change the strategy of vaccine development. A new malaria roadmap led by the WHO states two following objectives.

(1.)   Developing malaria vaccines with protective efficacy of at least 75 percent against clinical malaria.

(2.)   Developing malaria vaccines that reduce transmission of the parasites in order to substantially reduce the incidence of human malaria infection.

To meet with the roadmap, a new effective malaria vaccine is required to induce potent CTL and antibodies against both pre-erythrocytic stage and sexual-stage parasites to African infants with EPI vaccines, without any interference with current EPI vaccines, pre-existing helminths and maternal antibodies. Our multi-stage malaria vaccine effective both protection and TB is logically designed to develop a next-generation malaria vaccine for African infants who have pre-existing helminths and maternal antibodies, with real world application.

What sort of innovation are you bringing in your project?

A multi-stage vaccine, a vaccine targeting different stages of parasite life cycle in one construct, may provide a more cost-effective solution than a vaccination approach that uses mixtures of multiple single-stage vaccines. This approach may also be more convenient for the vaccine recipients than the co-administration of multiple vaccines. Most importantly, syngeneic effects of our bivalent multi-stage vaccine effective both for protection and transmission-blocking has great advantages to overcome the potential risks of reducing vaccine efficacy due to gene mutation and gene polymorphism. Our bivalent multi-stage vaccine platform consisting two viral-vectored vaccines is powerful technology for delivering heterologous antigens with minimal disadvantages. These vectors provide a key advantage over subunit vaccines, since CD8⁺ T cells are critical for the elimination of intracellular liver-stage parasites. Regarding its originality, two viral-vectored vaccines have never been utilized previously before for malaria vaccine development. We propose that the multi-stage vaccine regimen has the potential to fulfill the landmark goals of the malaria vaccine technology roadmap by achieving sterile protection and long-term TB efficacy.

Role and Responsibility of Each Partner

(1.)   Kanazawa university, Japan: Overall project management and proposal design, production of viral-vectored vaccine platform, mouse immunization studies, immunological assays, challenge tests against sporozoites. Kanazawa University has a sophisticated facility capable of evaluating pre-erythrocytic-stage vaccines using sporozoite-infected mosquitoes.

(2.)   Jichi Medical University, Japan: Proposal design/consultation and production of viral-vectored vaccine platform.

(3.)   Hokkaido University, Japan: Proposal design/consultation and production of viral-vectored vaccine platform.

(4.)   University of Toyama, Japan: Proposal design, immunological assays

(5.)   University of Cambridge, UK: Proposal design, mouse immunization studies, DMFA conducted in Burkina Faso (in collaboration with IRSS), whom routinely perform these experiments. These results, obtained in a field, “human-only” assay, will facilitate future translation to further human trials. University of Cambridge has a sophisticated facility capable of evaluating TB vaccines using P. falciparum gametocyte culture and field blood samples from malaria patients.

All partners will be co-responsible for data interpretation.

Others (including references if necessary)

(1.)   World Health Organization (WHO). World Malaria Report 2019. Geneva: WHO; 2019 Licence: CC BY-NC-SA 3.0 IGO.

(2.)   Sherrard-Smith E et al.:  Synergy in anti-malarial pre-erythrocytic and transmission-blocking antibodies is achieved by reducing parasite density. eLife 7,2018.

(3.)   Moorthy VS et al.: Malaria vaccine technology roadmap. Lancet. 2013 Nov 23;382(9906):1700-1. doi: 10.1016/S0140-6736(13)62238-2.

(4.)   WHO. Malaria Policy Advisory Committee. 2020. http://www.who.int/malaria/mpac/en/ (accessed March 4, 2020).

Final Report

1. Project objective

Our aim is to develop not only a highly effective and durable multistage vaccine both against pre-erythrocytic and sexual stages of Plasmodium falciparum for infants. In animal models, protective and transmission blocking (TB) efficacies of the heterologous LC16m8 (m8)-prime and AAV-boost immunization regimen will be assessed by sporozoite challenge and direct membrane feeding assay in a robust and proven mouse model, and then the regime will be further optimized (e.g., dose, route, interval and outbred mice).

2. Project design

Our malaria vaccine is based on a viral-vectored vaccine platform, consisting of a highly attenuated vaccinia strain; m8, and adeno-associated virus (AAV) expressing P. falciparum Pfs25-PfCSP fusion protein. m8 and AAV viral vectors are powerful technologies for delivering heterologous antigens with minimal disadvantages. These vectors provide a key advantage over subunit vaccines, since CD8⁺ T cells are critical for the elimination of intracellular liver-stage parasites. Regarding its originality, m8/AAV viral vectors have never been utilized previously before for malaria vaccine development.

3. Results, lessons learned

The heterologous m8-prime/AAV-boost immunization regimen in mice provided both 100% protection against PfCSP-transgenic P. berghei sporozoites and up to 100% transmission blocking efficacy, as determined by a direct membrane feeding assay using parasites from P. falciparum-positive, naturally-infected donors from endemic settings. Remarkably, the persistence of vaccine-induced immune responses were over 7 months and additionally provided complete protection against repeated parasite challenge in a murine model.

Malaria Vaccine Initiative (MVI) and other relevant bodies suggested that it would be hugely advantageous to the future development of our vaccine platform to demonstrate its effectiveness within a non-human primate (NHP) model. Therefore, we expanded on these initial findings in the murine model to the NHP model. Four rhesus monkeys were immunized with heterologous m8-prime and AAV-boost regimen. Before and after priming/boosting, blood samples were collected, and hematologic and biochemical analyses was performed. We have confirmed that our vaccine is safe, with acceptable reactogenicity and no detectable systemic toxicity. Our vaccine persists high level of anti-PfCSP and Pfs25 antibody titers over a minimum of 6 months. Regarding vaccine efficacy, the monkey immune sera possessed not only high levels of in vitro sporozoite neutralizing activity but also robust levels of transmission blocking efficacy.

Recently, more than 80,000 cases of Mpox have been reported in 100 non-African nations, with most cases occurring in younger age groups. Historically, vaccination against smallpox was protective against Mpox. We speculate that our m8∆-based malaria vaccine can prevent not only endemic malaria but also Mpox outbreaks. In fact, our recent work has confirmed that our vaccine can also provide strong neutralizing effect against Mpox.

Thus, we demonstrate the safety profile, immunogenicity, and crucially, efficacy of this vaccine platform in the non-human primate (NHP) model. These findings are of key importance for the development of the m8∆/AAV platform, and the transition from rodent and NHP models to clinical study.