mRNA vaccination overcomes haemozoin-mediated impairment of whole-parasite malaria vaccines in mice.

Immunization with radiation-attenuated sporozoites (RAS) drives effective sterilizing immunity against liver-stage Plasmodium infection. However, protection is compromised in individuals living in malaria endemic regions and the mechanisms of vaccine failure are unclear. Here we show that previous blood-stage exposure in a mouse model of Plasmodium yoelii infection compromises Plasmodium berghei RAS-induced essential CD8+ T cell responses and subsequent protection. The persisting malarial pigment haemozoin mediates impaired CD8+ T cell responses owing to impaired antigen uptake by dendritic cells, leading to reduced T cell activation. We designed a lipid nanoparticle-encapsulated mRNA vaccine that encodes a string of Plasmodium CD8+ T cell epitopes, which overcomes the defective T cell response and restores protection in Plasmodium-exposed mice. A combined RAS-plus-mRNA vaccine regimen enhances liver-resident memory T cells and protection in murine malaria-experienced hosts. The identification of haemozoin as a potential obstacle to vaccine efficacy in malaria endemic areas can inform the design of more effective malaria vaccines.