Designing the RH5.2 malaria vaccine immunogen

Our RH5.2 vaccine immunogen is being tested in clinical trials for its ability to prevent malaria.

The parasite, Plasmodium falciparum, which is responsible for the most-deadly human malaria cases, must get inside our red blood cells to replicate and cause disease. This process of erythrocyte invasion requires an interaction between the PfRH5 protein, found on the parasite surface, and the basigin receptor on the erythrocyte. This makes PfRH5 an excellent candidate for inclusion in a blood stage malaria vaccine. However, while immunisation of people with a full-length PfRH5-based vaccine (also known as RH5.1) reduces the multiplication rate of the parasite, these people are not protected from getting malaria [1]. Can we make it better?

PfRH5 has several shortcomings as a vaccine. First, only some of the antibodies which are induced by immunisation with PfRH5 are protective, while others have no positive effect. Second, it is hard and expensive to produce at the scale required for a global vaccine. We took on these challenges in our structure-guided design of RH5.2.

Our quest to make better PfRH5-based vaccines started when Kate Wright worked out the structure of PfRH5 and showed how it binds to basigin and to inhibitory antibodies [2]. As part of this quest, she removed some flexible parts of PfRH5.1, deleting a long N-terminal extension and a central loop and cutting away the C-terminus using a protease. This allowed Kate to pack PfRH5 molecules into crystals and solve the structure. Her truncations also removed regions of PfRH5 which induce antibodies which are not effective at preventing malaria. These truncations are now included in PfRH5.2.

The second change incorporated into RH5.2 came from our collaboration with Adi Goldenzweig and Sarel Fleishman. They contacted us to ask whether we would be interested in optimising PfRH5 using their Protein Design One-Stop Shop (PROSS) algorithm, which predicts the effect of different mutations on the stability of a protein [3]. Armed with our PfRH5 structure and advice about sequence alignments for PfRH5, they provided three designs. Ivan Campeotto tested these, showing that a design with 18 changes from PfRH5 was best, being produced more readily and showing increased thermal stability, while adopting the same structure [4]. These 18 changes were combined with Kate’s truncations, to generate RH5.2.

So how is RH5.2 doing? It is currently in clinical trials. In both Oxford and Burkino Faso, it is being tested as a fusion with a virus-like particle, with the Matrix-M adjuvant in a phase IIb study (https://classic.clinicaltrials.gov/ct2/show/NCT05790889 and NCT05978037). It is also being tested, combined with the pre-erythrocytic malaria vaccine R21 in the Gambia in phase Ib (NCT05357560).

While we are excited to see how RH5.2 works, it does have its shortcomings. It still contains large regions which induce antibodies which are not protective against malaria [5]. It also cannot be produced in a form directly fused to a virus-like particle but requires an indirect fusion approach. This makes manufacturing challenging. While we wait eagerly to see how protective RH5.2 will prove to be, we are also generating another generation of PfRH5-based vaccines which solve these challenges.

[1] Minassian, A.M., Silk, S.E., Barrett, J.R., Nielsen, C.M., Miura, K., Diouf, A., Loos, C., Fallon, J.K., Michell, A.R., White, M.T. et al (2021) Reduced blood-stage malaria growth and immune correlates in humans following RH5 vaccination. Med 2 701-719

[2] Wright, K.E., Hjerrild, K.A., Bartlett, J., Douglas, A.D., Jin, J., Brown, R.E., Illingworth, J.J., Ashfield R., Clemmensen, S.B., de Jongh, W.A., Draper, S.J. and Higgins, M.K. (2014) Structure of malaria invasion protein RH5 with erythrocyte basigin and blocking antibodies. Nature 515 427-30

[3] Goldenzweig, A., Goldsmith, M., Hill, S.E., Gertman, O., Laurino, P., Ashani, Y., Dym, O., Unger, T., Albeck, S., Prilusky, J., Lieberman, R.L., Aharoni, A., Silman, I., Sussman, J.L., Tawfik, D.S. and Fleishman, S.J. (2016) Automated Structure- and Sequence-Based Design of Proteins for High Bacterial Expression and Stability. Molecular Cell 63 337-346.

[4] Campeotto, I., Goldenzweig, A., Davey, J., Barfod, L., Marshall, J.M., Silk, S.E., Wright, K.E., Draper, S.J., Higgins, M.K.* and Fleishman, S.J.* (2017) One-step design of a stable variant of the malaria invasion protein RH5 for use as a vaccine immunogen. Proc Natl Acad Sci U S A. 114 998-1002 

[5] Alanine, D.G.W, Quinkert, D., Kumarasingha, R., Mehmood, S., Donnellan, F.R., Minkah, N.K., Dadonaite, B., Diouf, A., Galaway, F., Silk, S.E., Jamwal, A., Marshall, J.M., Miura, K., Foquet, L., Elias, S.C., Labbé, G.M., Douglas, A.D^., Jin, J., Payne, R.O., Illingworth, J., Pattinson, D.J., Pulido-Gomez, D., Williams, B.G., de Jongh, W.A., Wright, G.J., Kappe, S.H.I., Robinson, C.V., Long, C.A., Crabb, B.S., Gilson, P.R., Higgins, M.K.* and Draper, S.J.*. (2019) Human antibodies that slow erythrocyte invasion potentiate malaria-neutralizing antibodies. Cell 178 216-228