Introduction and Background of the Project

Introduction

Vivax malaria is recognized as a leading neglected tropical disease worldwide. Plasmodium vivax parasite poses a major challenge to malaria elimination due to its ability to cause recurring blood infections. These ‘relapses’ of malaria are caused by hypnozoites, the dormant form of the parasite in the liver. Therefore, hypnozoites is considered as the major challenge towards malaria elimination in vivax endemic countries. The only drugs available to clear hypnozoites are primaquine and tafenoquine. However, these drugs cause severe hemolysis in people with G6PD deficiency and are difficult to deploy in large scale to drive vivax malaria elimination. Vaccination is an alternative and the most cost-effective way to control malaria, however, malaria vaccine which target P. vivax has not been developed.

Project objective

The goal of this project is to develop a novel nucleoside-modified mRNA vaccine targeting Pvs25 protein that induces potent and long-lasting transmission blocking immunity and is able to interrupt transmission of P. vivax from human to mosquito.

Project design

To achieve our objective, we will combine our experiences in malaria vaccine development (Mahidol and Ehime Universities) and mRNA vaccine technology (the University of Pennsylvania) to develop nucleoside-modified mRNA vaccines that block transmission of P. vivax, a major malaria parasite outside Africa. The vaccine target is the protein Pvs25 which is expressed on the surface of the transmission-stage parasite, a well-validated target. In our vaccines, nucleoside-modified mRNA encoding Pvs25 will be delivered by lipid nanoparticles (LNP), an approach which has been shown to be highly effective in other vaccines. Several mRNA-LNP formulations will be tested in animals to identify the best candidate. Several routes of administrations and immunization schedules will also be explored. Vaccine efficacy will be determined by the ability of the immune sera of immunized animals to block mosquito infection using membrane feeding assay with P. vivax parasites isolated in Thailand. In addition to developing a new transmission blocking vaccine, we will also investigate the immune mechanism that results in transmission blocking activity.

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

Malaria continues to place a heavy toll on human health and economic burden in low and middle-income countries. The WHO’s Global Technical Strategy for Malaria (2016–2030) aims to eliminate malaria in a further 35 countries by 2030 compared to 2015. P. vivax can cause relapses which make it difficult to cure or control. A vaccine that interrupts parasite transmission will contribute to the elimination of malaria but no such vaccine has been developed to date.

Short-term Impact: To develop a Pvs25 mRNA-LNP vaccine that elicits robust and long-lasting transmission blocking immunity in mice. The vaccine could be further evaluated in nonhuman primates and humans. A controlled human malaria infection model of P. vivax has recently been established at Mahidol University to test new vivax malaria vaccines.

Long-term Impact: The Pvs25 mRNA-LNP vaccine will be combined with Pre-erythrocytic vaccine (PvCSP mRNA-LNP vaccines have recently been developed by our collaborators). This combined vaccine approach will serve as an essential tool in P. vivax elimination. It is expected to reduce both the number of infected patients and the number of asymptomatic hypnozoite carriers in the endemic communities.

What sort of innovation are you bringing in your project?

To date, two most advanced transmission blocking vaccines for P. vivax utilized Pvs25 protein to immunize human volunteers, but the results were disappointing due to low immune response and adverse reactions from adjuvant in the vaccine formulation. Thus, there is a need to develop/test more effective vaccine platforms and/or immunization schemes. Nucleoside-modified mRNA formulated with lipid nanoparticle (mRNA-LNP) has recently emerged as a promising vaccine platform against other infectious diseases. Therefore, we propose to develop a mRNA-LNP vaccine targeting a prime P. vivax transmission blocking vaccine candidate, Pvs25. Pvs25-encoding nucleoside-modified mRNA-LNP vaccines will be engineered to induce potent and long-lasting transmission blocking immunity with the potential to interrupt transmission from human to mosquito.

Role and Responsibility of Each Partner

This research collaborative partnership comprises three academic institutes: Mahidol University, Thailand, the University of Pennsylvania, USA, and Ehime University, Japan. Mahidol University will be responsible for overall project administration, conducting immunization and transmission blocking experiments, data analyses, and reporting. The University of Pennsylvania will be responsible for designing and producing Pvs25 nucleoside-modified mRNA-LNP vaccines, and conducting in-vitro cell transfection studies to confirm protein production from Pvs25 mRNA. Ehime University will be responsible for the production of Pvs25 protein antigen using their wheat germ cell-free protein synthesis technology for protein/adjuvant immunization, transmission blocking experiments with Mahidol group.

Others (including references if necessary)

WHO Global Technical Strategy for Malaria 2016-2030.

https://www.who.int/malaria/publications/atoz/9789241564991/en/

Final Report

1. Project objective

The goal of this project is to develop a vaccine for Plasmodium vivax malaria, a major mosquito-borne disease common in many parts of the world including Asia, Oceania, East Africa, and Central and South America. This vaccine is designed to invoke human immunity that can block the transmission of the parasite to the mosquito vector. After immunization, patients as well as asymptomatic carriers, will no longer be able to spread the parasite to other people in their community.

2. Project design

The ideal vaccine should be able to induce a strong and long-lasting antibody response that is effective against all strains of P. vivax. To develop such a vaccine, the researchers utilized the nucleoside-modified mRNA-LNP technology to screen many versions of a well-validated vaccine target, Pvs25. The best route of immunization and the optimal dosage were also determined. The potency of test vaccines was evaluated against P. vivax from Thai malaria patients, using the direct membrane-feeding assay in which serum samples from immunized animals were added to P. vivax-infected blood before feeding to the mosquitoes.

3. Results, lessons learned

A highly potent Pvs25-based mRNA-LNP vaccine was developed. This vaccine induced a high level of antibodies in mice that can recognize the parasite developing inside the mosquitoes. With two-dose intramuscular administration of the vaccine, the antibodies reached a level that blocked the parasite transmission 100% and remained fully effective for >6 months. This vaccine performed much better than a recombinant Pvs25 protein vaccine formulated with Montanide ISA-51 as the adjuvant. In addition, the mRNA-LNP vaccine also induced immune cells known to sustain the antibody response. This project has thus yielded a new transmission-blocking vaccine candidate for P. vivax that is ready for non-human primate testing. This study demonstrates that significant improvement can be achieved through optimization of a traditional vaccine candidate using the mRNA-LNP platform.