Generation of liver tissue-resident memory T cells with novel NKT cell agonists and mRNA vaccines

mRNA vaccines have become a central part of vaccine research over the last decade due to their application in cancer immunotherapy, and more recently, the rapid response to the SARS-CoV-2 pandemic leading to two clinically approved mRNA vaccines that generate cellular and humoral immunity. These vaccines have saved countless lives and have paved the way for the prevention and treatment of infectious diseases with new mRNA vaccines. One area of ongoing investigation is the longevity and specificity of the adaptive immune response which incorporates both humoral (i.e. antibody) and cellular (T cell) memory. Whilst the focus of the SARS-CoV-2 vaccines was humoral-mediated immunity another emerging field of immunology and vaccine development is the function and generation of tissue-resident memory T(rm) cells. These specialised memory cells reside in tissues awaiting the presence of a pathogen and “raise the alarm” when they encounter the pathogen. This involves the production of inflammatory cytokines that attract other circulating immune cells and have a rapid cytotoxic function that eliminates infected cells. Previous work has shown that the activation of “innate-like” natural killer T (NKT) cells can improve adaptive immune responses, including the production of Trm cells in the liver that can provide sterile protection from malaria sporozoites. This work explored the use of and effect on the adaptive cellular immune system of vaccines comprising NKT cell agonists and mRNA encoding pathogen antigens co-delivered in lipid-delivery vehicles. The inclusion of an NKT cell agonist improved the generation of circulating effector memory CD8+ T(em) cells and the type of ligand affected the magnitude of the priming effector response. Surprisingly, whilst the magnitude of Tem responses generally predicted other memory responses, liver Trm responses were detached from priming responses with regard to specific agonist compounds. In particular, several novel NKT cell agonists, including CI058 generated substantially more liver Trm than the prototypical NKT cell agonist, aGalCer, even though aGalCer induced stronger priming Tem responses. The liver Trm inducing capacity of the novel NKT cell agonists was dependent on modification at the 6’ position of the galactose of a-linked glycolipids. The vaccines developed during this work were able to generate protective liver Trm that could be boosted and protect from Plasmodium sporozoite challenge to model and disease-relevant Plasmodium antigens. Additionally, global research efforts have demonstrated vaccine or infection-acquired immunity (antibody or liver Trm) is impaired by previous red blood cell (RBC) infection by Plasmodium. In contrast, the liver CD8+ Trm cells generated by these NKT cell-adjuvanted mRNA vaccines were unaffected by previous RBC infection. The specific signals from NKT cells that contribute to this improvement in liver Trm formation are unclear, but the response is dependent on both the mRNA vaccine and NKT cell signalling. The response was abrogated in the absence of NKT cells and its magnitude was dependent on a NKT cell phenotype. Modulation of the mRNA component could either impair or improve the overall response and there is synergy between NKT cell and TLR signalling in DC licencing and IL-12 production that positively correlates with liver Trm.