Novel malaria vaccines

ABSTRACT

Malaria vaccines hold significant promise for life-saving benefit, especially to children who bear the major burden of malaria mortality. The RTS,S/AS01 malaria vaccine provides moderate efficacy and is being tested in implementation studies. In parallel, multiple strategies are being advanced to test next-generation malaria vaccines, including novel approaches that build on principles learned from RTS,S development, vaccination with radiation-attenuated sporozoites, and development of monoclonal antibodies targeting immunogenic peptides. Novel vaccine delivery approaches are also being advanced, including self-amplifying RNA vaccine delivery, self-assembling protein nanoparticle methods, circumsporozoite protein-based approaches, and whole organism vaccination. Techniques employed for COVID-19 vaccine development should also be considered for malaria vaccination, including sustained release polymer nanoparticle hydrogel vaccination and charge-altering releasable transporters. As vaccine science advances and new approaches optimize knowledge gained, highly effective malaria vaccines that provide sustained protection are within reach.

Introduction

Aside from provision of clean water and sanitation, vaccines represent the most life-saving public health intervention available. The World Health Organization estimates that vaccines prevent 4–5 million deaths per year.¹ Infectious diseases that cause significant morbidity and mortality are perfect targets for vaccines, especially malaria. This is because the burden of malaria disease is high, especially in young children, so the potential for public health benefit of an effective vaccine administered during childhood is large.

To date, no vaccine has been deployed for widespread use against a parasitic pathogen. The RTS,S/AS01 vaccine is the first licensed malaria vaccine to date, and is modestly effective against clinical disease. The European Medicines Agency gave favorable opinion for the vaccine in 2015, and it continues to be evaluated for effectiveness in three sub-Saharan African countries to inform policy makers regarding use.² Numerous methodologies have been employed to develop and deliver candidate vaccines that would improve on this benchmark, and many have advanced to Phase 1 and 2 testing, only to be abandoned when field trials fail to demonstrate efficacy, while others show significant promise.^3,4 In parallel with these efforts to optimize favorable candidates, researchers continue to develop and employ new technologies for both malaria vaccine antigen discovery and vaccination strategies. Many of these build on knowledge gained from studies of RTS,S and irradiated sporozoite vaccines, while others exploit monoclonal antibodies to identify the most promising epitopes for vaccine development. This piece will discuss some of the most cutting-edge technologies currently employed for malaria vaccine development.

The status of malaria elimination efforts

In 2019, malaria caused 229 million clinical illnesses and 409,000 deaths.⁵ The majority of severe malaria disease occurs in children, with over 90% of malaria deaths in children <5 years old.⁵ Since the year 2000, the malaria mortality incidence rate has decreased from 25 to 10 per 100,000 population at risk, yet malaria incidence remained relatively unchanged despite use of new interventions to combat the disease.⁵ To achieve the ambitious targets for 90% reduction in malaria mortality rates and malaria incidence globally, set forth in the Global technical strategy for malaria 2016–2030,⁶ a novel, highly-effective, preventive antimalarial intervention is needed.

Malaria and vaccine development- what new tools are being employed?

Novel tools to advance malaria vaccine development include new antigens discovered through systematic studies and vaccine delivery technologies that successfully generate an effective immune response against malaria. Most efforts target prevention of infection with Plasmodium falciparum, one of five malaria species that infects humans and causes the most malaria morbidity and mortality worldwide, and others aim at either P. vivax or antigens that are conserved across malaria species. Some of these advances include vaccines delivered using self-amplifying RNA, nanoparticles, and novel virus-like particles that display circumsporozoite protein (CSP) epitopes. Other strategies focus on delivery of whole organism vaccines to generate a broad and diverse immune response.

Self-amplifying RNA vaccine delivery

Building on the recent success of vaccines developed for COVID-19 prevention, RNA technology is being employed to deliver a promising antimalarial candidate antigen Plasmodium macrophage migration inhibitory factor (PMIF).⁷ For this vaccination strategy, both the RNA platform and the PMIF antigen are novel for the malaria vaccine field. Bucala and colleagues developed a self-amplifying RNA vaccine that targets PMIF, which is secreted by the parasite and acts to diminish the host inflammatory response against infection, specifically the T-cell response.⁸ A study in mice showed that vaccination with a novel RNA-PMIF construct enhanced the capacity to control liver and blood stage infection, increased differentiation of memory CD4 T-cells and liver-resident CD8 T-cells, and boosted antibody titers directed against the parasite, ultimately leading to protection against reinfection.⁸ These encouraging results may soon lead to human clinical trials to evaluate preliminary safety and immunogenicity. Unlike the current mRNA vaccines manufactured by Pfizer-BioNTech and Moderna that inject a limited amount of mRNA that does not replicate, this self-amplifying RNA vaccine encodes both the antigen and the replication machinery required for intracellular RNA amplification, leading to profuse protein expression. Self-amplifying RNA vaccines thus require a small initial dose of antigen and could potentially lead to a more enhanced immune response that would serve to mitigate the rapid decline of antibody responses after vaccination. Such declines in antibody levels are well-documented after RTS,S/AS01 vaccination.⁹ Self-amplifying RNA vaccines hold promise to reduce the need for multiple booster doses that are more cumbersome to administer, especially outside of routine vaccination schedules in resource-limited areas. Another advantage of self-amplifying RNA vaccines is that unlike mRNA vaccines for COVID-19, they do not require freezer storage, further increasing suitability for delivery to malaria-endemic areas.

Self-assembling protein nanoparticle (SAPN)

After an initial period of development,¹⁰ the SAPN approach to malaria vaccine development has advanced to show increasing promise. The technique involves technology that manipulates the ability of peptides and proteins to self-assemble into particles that are mechanically and chemically stable. These particles include multiple conformational epitopes that generate both cell-mediated and humoral immune responses, driven by engineering a coiled-coil core sequence. These conformation-specific antigens overcome issues with linear peptide antigens that cannot generate protective immune responses and break down rapidly.¹⁰ An additional advantage is the capability to insert multiple epitope targets, including two B cell antigens and antigens from other life cycle stages that can each stimulate a targeted immune response. Correct folding and orientation that is stabilized by the SAPN can lead to increased immune responses that are conformation-appropriate and functionally active.¹¹ A candidate SAPN vaccine was designed to include domains of the P. falciparum CSP, including CD4+ and CD8+ epitopes, part of the NANP repeat motif, and universal T helper cell epitopes, among others. This multivalent vaccine protected mice against infection,¹² and is currently being tested in a clinical trial (NCT04296279). Also for this construct, immune responses were further modulated by adding a TLR5 agonist flagellin, demonstrating the capacity to fine tune immunogenicity of a SAPN-based vaccine.¹³ A separate SAPN construct integrating P. falciparum antigens from different life cycle stages was tested in mice and rabbits, and confirmed that antigen orientation, density, and folding appropriately guided the immune response based on functional and quantitative readouts.¹¹ The ability to extensively manipulate multiple antigen constructs that are conformationally stable, highly immunogenic for both B and T cell epitopes, and enhanced by engineering designs make this platform highly attractive for vaccines against malaria.

Circumsporozoite protein-based approaches

With the success of the RTS,S/AS01 vaccine, which is based on part of the central repeat and carboxyl-terminus regions of P. falciparum CSP, advances have been made to further enhance CSP-induced protection. Approaches include full-length CSP vaccines that include the amino-terminal region that RTS,S/AS01 lacks, together with the central repeat and carboxy-terminal regions. This approach is attractive as monoclonal antibodies directed against the junction between the central repeat region and the amino-terminal region were recently isolated from participants in a clinical trial of an irradiated sporozoite vaccine, and protected individuals against controlled human malaria infection.^14,15 A full-length CSP construct was successfully expressed in a Pseudomonas fluorescens platform,¹⁶ and initial testing in clinical trials demonstrated safety and immunogenicity.¹⁷ Next steps include testing vaccinated participants for protection against controlled human malaria infection (NCT03589794).

Additional advances in CSP-based vaccines include delayed, fractional dosing of the RTS,S/AS01 vaccine, and the novel R21 vaccine adjuvanted with Matrix-M. For RTS,S, a delayed and fractional third dose booster schedule showed 87% efficacy in malaria-naïve adults who underwent malaria challenge,¹⁸ and the strategy is currently being tested in children in Ghana and Kenya (NCT03276962). The R21 vaccine uses the same CSP protein as RTS,S, but with an increased ratio of CSP to hepatitis B surface antigen.¹⁹ First results of a clinical trial of R21 adjuvanted with Matrix-M among children living in a malaria-endemic area showed 77% efficacy over 12 months, the first malaria vaccine to achieve high efficacy in children.³ These promising results will hopefully be followed by durable protection over a second year of follow-up through the end of 2021 and confirmatory studies in children living in different malaria transmission settings.

Whole organism approaches

Several strategies to use whole malaria sporozoites delivered in attenuated form or under chemoprophylaxis continue to advance. A radiation-attenuated, metabolically active, non-replicating sporozoite vaccine manufactured in aseptically reared mosquitoes that demonstrated unprecedented efficacy against controlled human malaria challenge trials when delivered intravenously²⁰ has advanced to testing in malaria-endemic areas and demonstrated encouraging results in adults.^4,21 A related approach administers live, non-attenuated sporozoites while individuals take a blood stage antimalarial. This strategy has proved efficacious in malaria-naïve²² and malaria experienced adults,²³ Dose and regimen optimization studies are underway (NCT03952650). Another method that uses three gene deletions to attenuate whole sporozoites and arrest parasite growth at the liver stage demonstrated safety in initial clinical trials²⁴ and will next be assessed for protective effect against controlled human malaria infection.

Transmission blocking vaccines

One advanced transmission blocking vaccine targets an epitope expressed in the mosquito midgut, the Anopheles alanyl aminopeptidase 1 (AnAPN1).²⁵ As this epitope is common to Anopheles species that spread malaria, the approach is attractive as a method to prevent transmission of all malaria species infecting humans. A study of several monoclonal antibodies directed against active antibody binding sites on AnAPN1 demonstrated transmission blocking in a cooperative manner that maximized effectiveness.²⁵ Subsequent studies showed that adjuvanted vaccination targeting two key AnAPN1 epitopes resulted in highly active transmission blocking activity in mice.²⁶ First-in-human studies of the AnAPN1-based vaccine are planned to advance with support from Global Health Innovative Technology Fund.

What other tools should be considered for malaria vaccines?

A novel approach that can be studied for malaria vaccines include sustained release polymer nanoparticle hydrogel vaccination.²⁷ As malaria vaccines, including RTS,S, are capable of inducing a strong initial antibody response that declines over time, a vaccination strategy that includes sustained antigen release would more likely lead to durable immune responses required to prevent malaria infection over longer periods of time. These hydrogel platforms are made using hydrophically-modified hydroxypropyl methylcellulose derivatives that stabilize the antigen and biodegradable polymeric nanoparticles that contain vaccine.²⁷ This approach is being advanced for COVID-19 vaccination, and could potentially be used for malaria as well.

Another approach that can be considered is the use of charge-altering releasable transporters (CARTs).²⁸ This alternative mRNA vaccination technique, currently being used for a candidate SARS-CoV-2 vaccine, includes antigens together with adjuvants to increase immunostimulatory capacity and generate a robust immune response, including CD4+ and CD8 + T cell memory responses. This adaptable vaccination platform could incorporate multiple vaccine epitopes, enhancing immunity provided by mRNA vaccination, all while potentially reducing the need for multiple booster doses.

Conclusion

As malaria incidence has stalled after several years of decline, new, highly cost-effective tools are needed to combat this deadly disease that disproportionately affects low-income countries. Vaccination is an ideal approach that has the potential to prevent disease transmission and lead to malaria eradication. Many candidate malaria vaccines continue to be advanced and tested in preclinical and clinical settings, and several show significant promise. Although the COVID-19 pandemic has taken a heavy toll on the world’s population, this public health crisis has advanced novel vaccination technology that can be adapted to malaria vaccines. Similar to COVID-19 vaccination development, several approaches that ultimately yield multiple vaccine products are needed to effectively address current needs for malaria elimination. As uptake and optimization of malaria vaccine approaches advance, so does the potential for development of long-lasting, highly effective malaria vaccines that support malaria elimination efforts.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.