Dissecting the role of gd T cells in T cell priming for liver stage immunity

Liver resident memory T cells (TRM) are poised for protection against repeat infection and rapidly form a robust defence against tissue-specific insults such as liver stage malaria. A direct correlation between liver stage immunity and gd T cells has been observed both in mice (Zaidi et al. 2017) and in humans (Seder et al. 2013; Ishizuka et al. 2016), but the precise molecular mechanisms by which these gd T cells exert their protective effect are yet to be defined. In mice, intravenous injection with radiation-attenuated sporozoites (RAS) confers sterile protection against challenge with live sporozoites. This protection is mediated by responding antigen-specific CD8+ and CD4+ T cells that migrate to the liver and form resident-memory T cells (TRM). In the absence of gd T cells, protective CD8+ liver TRM are not generated, leaving mice susceptible to reinfection. Using Plasmodium-specific T cells as a readout for effective immunity, we determined that IL-4 is important for the accumulation of CD8+ and CD4+ T cells. By utilising complex in vivo systems including mixed-bone marrow chimeras and adoptive transfer of gd T cells, we revealed that gd T cell-derived IL-4 is crucial for the expansion of antigen-specific CD8+ T cells. In addition, in vivo neutralisation of IL-12 or IFN-g confirmed a partial dependency for these cytokines, despite their traditionally opposing function to IL-4. Given IL-4, IFN-g and IL-12 all have a clear role in CD8+ T cell priming following RAS vaccination, we hypothesised that IL-4 and IFN-g synergise to enhance cDC1 activity. These findings led to our development of a novel model to reconstitute cDC1-deficient mice using CRISPR-edited primary dendritic cells. This model enabled the investigation of the mechanism by which gd T cell derived IL-4 leads to DC activation and therefore effective CD8+ T cell expansion for memory development. Collectively, this project has shown a significant role for IL-4 in the priming of malaria-specific CD8+ T