Designing a PfRIPR-based blood stage malaria vaccine: from understanding human antibody responses to structure-guided design

Malaria remains one of the deadliest diseases of humanity, causing over 600,000 deaths and two hundred million cases each year. Malaria treatment and prevention relies on various measures but has been hampered by lack of a vaccine. In recent major advances, the first two malaria vaccines, RTS,S and R21, have been licenced. However, these require multiple doses, provide only 30-50% protection in phase III trials and wane over a few years. It is imperative that new vaccines are developed.

The parasites that cause malaria first invade liver cells before moving into the blood. While current vaccines prevent liver invasion, targeting the blood stage is extremely promising. Both symptoms and transmission of malaria occur as parasites replicate in blood and these could be prevented by a vaccine which blocks red blood cell (RBC) invasion.

The most-deadly malaria parasite, Plasmodium falciparum, requires a five-component protein complex, PfPCRCR, to invade RBCs. PfRH5 is the best studied component. It makes an essential interaction with human basigin and antibodies targeting PfRH5 can prevent RBC invasion.

In phase I clinical trials, current PfRH5-based vaccines slow parasite growth in vaccinated volunteers but do not induce enough protective antibodies to prevent malaria. Therefore, we must also assess other PfPCRCR components as blood stage malaria vaccines.

This proposal focuses on PfRIPR, the central component of PfPCRCR. This large, complex protein is essential for RBC invasion and is a target of highly neutralising antibodies. However, until recently, little has been known about its structure and function.

In 2023 we revealed the PfRIPR structure, showing its complex molecular architecture, consisting of a compact 'core' and an elongated 'tail'. We also demonstrated how PfRIPR bridges the PfRH5 protein, which interacts with the RBC membrane, to the PfCSS:PfPTRAMP complex on the parasite surface. These new insights now make it possible for us to apply the tools of structure-guided vaccine design to PfRIPR.

Structure-guided reverse vaccinology uses rational approaches to design vaccine immunogens which induce only neutralising antibody responses. Vaccination with a complex immunogen induces antibodies which bind to various parts of the immunogen. In the case of PfRIPR, only a fraction of these antibodies prevent RBC invasion. Structure-guided vaccinology will allow us to discover which regions of PfRIPR induce the most effective antibodies and to test these PfRIPR fragments as vaccine immunogens.

To achieve this, we will use the latest methods to produce the first ever panel of human PfRIPR-targeting monoclonal antibodies, isolated from volunteers who have suffered from malaria. We will determine which of these prevent RBC invasion by parasites and will use structural studies to discover where on PfRIPR these neutralising antibodies bind and how they work.

Finally, we will test PfRIPR-based vaccine immunogens using a proven pre-clinical model involving rodent immunisation and assess how effectively the antibodies induced prevent RBC invasion. We will determine which human-compatible vaccine adjuvant is most effective in combination with PfRIPR to induce a strong immune response. Then, we will use structure-guided rational design to generate the 15 individual modules which make up PfRIPR as individual proteins.

Those which bind neutralising antibodies will be tested in the preclinical model, side-by-side with the best PfRH5-based vaccine immunogen.

The outcomes of this study will be the first human PfRIPR-targeting neutralising monoclonal antibodies and the first fully tested PfRIPR-based vaccine immunogens ready for clinical testing in future malaria vaccines.