There is an urgent need for progress in developing prophylactic and therapeutic vaccination strategies that induce polyfunctional, strongly protective cytotoxic CD8 T cell responses. These could shield us from pathogens against which the presently available, neutralizing antibody-inducing vaccine approaches confer limited or no protection and they could be used to eliminate tumors or chronic infections. In contrast to this need, we currently fail to induce effector and memory CD8 T cells in numbers high enough to effectively impact an infection or the growth of tumors. As protective CD8 T cell responses are readily generated during several viral infections, we need to improve our insight into how pathogen protection is naturally achieved, identify why immune protection sometimes fails, and use this knowledge to develop novel vaccine strategies. We will use a well balanced approach of hypothesis stimulated and unbiased multisystem observations to exploit novel mechanisms and to find ways to augment the CD8 T cell response to a vaccine. We have established model systems that are uniquely suited to extract and test molecules that determine T cell differentiation and expansion magnitude. Along with that we aim to enhance our insight of immune responses in vaccinated individuals to prevent creating situation in which vaccines fail to confer protection or may cause adverse effects. We recently made very unexpected observations that challenge our current concept of T cell differentiation in chronic infections, which proposes that T cells terminally differentiate and become senescent. We therefore aim to redefine our understanding of T cell responses in such infections. This will also be pursued to unravel novel strategies to reactivate T cells in persisting infections. Overall, the project will strongly further our insight into CD8 T cell responses during infections and will support the development of more effective vaccine strategies to induce antigen-specific CD8 T cells