Dissecting the CD8+ T cell responses to pre-erythrocytic malaria antigens

The pre-erythrocytic stages of malaria are the preliminary steps to a disease of massive global health importance. Following transmission of sporozoites by mosquito bite to a human host, a clinically silent period of parasite development in the liver preludes the harmful blood stages that characterise malarial infection. Creating a vaccine that targets these stages is a very attractive notion as it would prevent this burden of disease. Protection against the pre-erythrocytic stages has been shown to be dependent on CD8+ T cells. However, very few antigens that induce anti-Plasmodium CD8+ T cell responses have been identified, especially those expressed by the parasite when developing inside hepatocytes. Using mouse models, experimental genetics and bioinformatics tools, I present work that has progressed our understanding of CD8+ T cells induced in the preerythrocytic stages of malaria and assessed the ability of antigen-specific CD8+ T cells to protect against subsequent challenge following vaccination. I have investigated and compared the differences in CD8+ T cell responses to a sporozoite (Circumsporozoite Protein) and a vacuolar membrane liver stage protein (Upregulated in Infectious Sporozoites gene 4) and showed that despite a divergence in immunogenicity when immunising with radiation attenuated sporozoites, both types of antigen are equally protective when mice are vaccinated with viral vectors to induce large antigen-specific CD8+ T cell populations. The natural immunogenicity of the liver stage antigen does not improve when liver stage development is extended by using drug prophylaxis. Additionally, I have compared the protection induced by liver stage antigens expressed constitutively after hepatocyte invasion and those expressed only after at least 12 hours post invasion. I have shown that some protection can be induced by the mid-late expressed Liver Specific Proteins 1 and 2, suggesting that these antigens are effectively presented and recognised by CD8+ T cells. This highlights the potential for the incorporation of liver stage antigens into next-generation malaria vaccines. 4 Additionally, I have investigated the role of the immunodominant CD8+ T cell epitope of Circumsporozoite Protein and showed that a significant level of protection is mediated by CD8+ T cells specific for this epitope. Nonetheless, following multiple immunisations with a parasite lacking this epitope, sterile protection can still be achieved, suggesting other antigens are important for parasite-induced protection. Thus, finally I go on to identify a number of novel CD8+ T cell epitopes from antigens expressed in the sporozoite and liver stage parasite, to further broaden our view of the CD8+ T cell responses induced during the pre-erythrocytic stages of malaria.