Infections by pathogens targeting the liver, major health burdens globally. While a robust and sustained CD8 T cell response is a positive predictor of resolution of infection, CD4 T cells play a critical role in protecting against these pathogens. It is well known that one of the many roles of the CD4 T cell response is to promote an effective CD8 T cell response through a process termed CD4 help. However, due to several experimental limitations, the role CD4 help plays in the immune response against hepatotropic pathogens has been difficult to investigate.

To determine the role of CD4 help in influencing the CD8 T cell response to hepatocyte-expressed antigen, we have established a novel murine model utilising recombinant adeno-associated viral (rAAV) vector encoding for a fusion antigen containing both CD8 and CD4 epitopes that are recognised by OT-1 and P25 TCR transgenic T cells respectively. Furthermore, altered peptide ligands of the CD8 epitope allow us to vary the affinity at which the OT-1 T cells recognise their cognation peptide antigen. The rAAV vector facilitates hepatocyte-restricted expression of this fusion antigen, allowing us to investigate the role of CD4 help in shaping CD8 T cell responses to hepatocyte-expressed antigens.

We show that CD4 help promoted a 3-fold increase in the number of functional CD8 T cells following high affinity rAAV treatment, where function was assessed by their ability to produce IFN-g and release cytotoxic granules upon ex vivo restimulation. When the intermediate affinity CD8 epitope assessed, CD4 help promoted a 15-fold increase in the total number of CD8 T cells responding to hepatocyte expressed Ag.

These results suggest a model in which CD4 help boosts the number and function of effector virus-specific CD8 T cells that recognise their cognate peptide-antigen at high and intermediate affinities during the primary immune response.