Microbiology & Immunology - Theses
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ItemGenerating CD8+ liver-resident memory T cell immunity against malaria( 2022)Liver resident memory CD8+ T (Trm) cells are attractive vaccine targets for malaria (Plasmodium) liver-stage immunity and can be effectively generated by glycolipid-peptide (GLP) vaccines. To gain insight into underlying mechanisms, we examined the requirements for priming, differentiation, long-term maintenance, and secondary boosting of liver Trm cells. We found that type I conventional dendritic cells (cDC1) were essential for priming CD8+ T cell responses, during which exposure to IL-4, most likely provided by activated type I natural killer T (NKT) cells, enhanced liver Trm cell formation. In addition, optimal generation of liver Trm cells required exposure to a combination of vaccine-derived inflammatory and antigenic signals post-priming, with antigen recognition being associated with enhanced Trm cell longevity. After primary immunisation with GLP vaccines, boosting of liver Trm cells could be achieved with the same GLP vaccine but a substantial delay was required for optimal boosting. This appeared to be due to NKT cell anergy post-priming as NKT cell-independent heterologous boosting could be achieved much earlier. Overall, our study revealed that the generation of liver Trm cells by GLP vaccination is IL-4 and cDC1 dependent, with longevity increased by post-priming antigenic signals and homologous boosting influenced by NKT cell recovery. Like many other malaria subunit vaccines, however, the utility of GLP vaccines is somewhat limited by the scarcity of protective CD8+ T cell epitopes. This issue is particularly prominent in the context of rodent P. berghei ANKA (PbA) infection of B6 mice, an extensively studied model of malaria. Using a combination of mass-spectrometry and in-silico approaches, we generated a library of 400 PbA-derived MHC I-restricted epitopes, from which we identified 4 immunogenic candidates that each reproducibly stimulated CD8+ T cells after pre-erythrocytic and blood-stage infections of B6 mice. Further characterisation of one of these peptide candidates, Db163, revealed cross-reactivity with a known immunogenic, but non-protective peptide PbA GAP5040-48. Targeting two additional epitopes, Db100 and Db177, by GLP vaccines induced substantial CD8+ liver Trm cells but these responses lacked protective efficacy against sporozoite challenge. The fourth epitope is derived from the PbA X, a predominantly late liver-stage antigen. Promisingly, this epitope could be targeted by a GLP vaccine to evoke liver Trm cell-mediated immunity against malaria in B6 mice. This protective immunity was remarkably long-lived with liver Trm cells persisting for at least 210 days. Furthermore, we demonstrated that X-specific liver Trm cells could execute a protective immune response cooperatively with those specific for PbA TRAP130-138, leading to improved sterile immunity even against high-dose sporozoite challenges. Lastly, the discovery of two novel HLA-A 02:01-restricted epitopes within the P. falciparum X proteins provides a future opportunity to dissect their usefulness as human vaccine candidates. Overall, this thesis provides novel mechanistic insights to maximise liver Trm cell formation and longevity after vaccination. Additionally, this thesis identifies novel antigenic targets of liver Trm cells that could be exploited for vaccination to induce immunity against malaria.