Malaria, a mosquito-borne disease caused by Plasmodium species, remains a major global health threat responsible for approximately 600,000 deaths annually. Current vaccine efficacy is compromised by their reliance on antibody responses that occur within the limited timeframe before hepatocyte invasion, and furthermore show limited durability due to waning of humoral responses. These drawbacks underscore the need for next-generation vaccine strategies. The liver is the replication site of malaria parasites. Our team discovered that sterile protection hinges on effective liver immunity, particularly CD8+ liver-resident memory T (Trm) cells. To address this, we developed a novel mRNA-lipoplex (LPX) vaccine that generates durable immunity through liver Trm cells in a murine model. This vaccine targets a highly conserved Plasmodium antigen, ribosomal protein L6 (RPL6). We aimed to improve our prototype RPL6-LPX vaccine by identifying cytokine responses that are detrimental to liver Trm formation, and incorporate manufacturing modifications to address this limitation.
We demonstrated that liver Trm generation is hampered by the type I interferon (IFN) response. Type I IFNs interfered with mRNA vaccine-induced antigen expression, reducing antigen levels during priming. As Trm differentiation generally requires high antigen exposure, priming conditions were likely unfavourable for Trm development. Importantly, product-like contaminants within the LPX vaccine activated the innate immune system, inducing type I IFN production particularly by dendritic cells. To reduce contaminant levels, we incorporated purification, base modification and co-transcriptional capping/tailing processes during production of the mRNA component of the vaccine. The resulting formulation induced lower levels of type I IFNs and a significantly larger Trm response than our original prototype vaccine. Furthermore, sterile protection was observed in all mice upon low level malaria challenge and half of the mice challenged with a high level of malaria, highlighting enhanced vaccine efficacy in the presence of lower type I IFN levels. Altogether, despite existing studies reporting conflicting evidence regarding the role of type I IFNs in generating long-lasting CD8+ immunity, our findings suggest that type I IFNs are detrimental in the context of mRNA vaccine-induced liver Trm immunity. This knowledge can aid in informing future vaccine designs that aim to promote liver Trm cell generation to combat not only malaria, but also other liver diseases.