Cross‐priming of TCD8+ specific for cell‐associated antigens

Abstract

TCD8+ of the adaptive immune system play critical roles in the host defence against viruses and other pathogens through elimination of infected host cells. After infection, TCD8+ proliferate and adopt effector functions following recognition of specific antigenic‐peptides in the groove of a MHC class I molecule expressed by antigen presenting cells (APC). Professional APCs such as dendritic cells (DCs) are specialized for the priming and activation of naïve TCD8+. DCs are the key cell type responsible for T cell priming. They can either be directly infected and present viral antigens (direct‐priming) or phagocytose infected cell debris and process and present phagocytosed antigens to the specific TCD8+ (cross‐priming). Little is known about the actual contribution of direct versus cross‐priming during an immune response against viral infections. Understanding how DCs regulate TCD8+ responses is central for our ability to favourably manipulate the immune system as well as develop effective targeting strategies for optimal anti‐viral and antitumoral vaccination. To determine the contributions of direct versus cross‐priming to the clearance of an in vivo viral infection, we generated a Cre‐indicator transgenic mouse model utilising cutaneous HSV‐Cre infection to conditionally trigger model antigen expression in infected cells. This was expected to enable us to discriminate between virally infected and hence direct‐presenting DCs from uninfected and cross‐presenting ones in vivo for the first time. Unexpectedly, all generated transgenic mouse lines showed various levels of tolerance towards our model antigen due to undesired protein leakiness at steady state. However, TCD8+ in these neo‐transgene expresser transgenic mice possessed antigen ignorant properties and were non‐specifically activated following acute viral infection and the introduction of other innate immune cell ligands. Therefore, although these generated transgenic mice could not be used for their initial study purpose, their varying expression levels of model antigen make them an excellent model for studying peripheral tolerance induction and its maintenance.

Cross‐priming is especially important for anti‐tumour immunity as tumour cells, although carrying tumour associated antigens, do not activate naïve TCD8+ efficiently due to an absence of co‐stimulatory molecules. Remarkably, our group has recently shown that influenza A virus (IAV) infection of allogeneic cells led to tumour protection due to enhanced cross‐priming of TCD8+ specific to cellular antigen. We have previously demonstrated that this enhancement was partially mediated through TLR7 sensing and entirely dependent on MyD88 and IFN signalling pathways, yet independent of the IL‐1β‐inflammasome. To further increase our understanding of cross‐priming enhancement mechanisms found in our system, we have additionally investigated the involvement of other immunological mechanisms in this thesis. Here, we show that IAV enhanced crosspriming is independent from the IL‐18‐inflammasome signalling pathway but that TCD4+ helper play a surprisingly important role for optimal enhancement. Also, through investigations using Batf3‐/‐ mice, we not only confirm the specificity of CD8α+ and CD103+ DC subsets for cross‐presenting, but also demonstrated that there are two types of cross‐priming outcomes: a baseline cross‐priming that is innate signalling independent and an innate immune signal enhanced crosspriming pathway. Interestingly, both types of cross‐priming events were abolished in Batf3‐/‐ mice. This knowledge will be useful to aid future efforts to develop more robust cancer vaccines.

Finally, efficient antigen processing and presentation of the TCD8+ epitope is a central factor that determines the extent of specific T‐cell responses. Our group has identified the most immunodominant T‐cell response in the Balb/c mice after IAV infection. Interestingly, this novel epitope is encoded by the intronic region of the non‐structural protein mRNA. To dissect the mechanism of such epitope translation, as being either through spliced mRNA translation or generation using an alternative open reading frame (AltORF), we disabled splicing mechanisms and discovered that generation of the novel epitope occurred through AltORF translation. We will focus on the identification of the exact mechanism(s) underlying the efficient generation of this novel epitope as such mechanisms may provide a great opportunity for developing more efficient IAV T‐cell based vaccine strategies.