Integration of Avidity and Differentiation is enabled by CD8+ T-cell sensing of IFN-γ
The most effective responses to intracellular pathogens have a breadth of T-cell clones with different affinities for their cognate peptide, and a diversity of functional phenotypes, from effector to long-lived memory cells. While high- and low-affinity T-cells are inherently skewed towards becoming effector and memory, respectively, overall, both functional subsets exploit a wide range of affinities. How the breadth of affinities and functionalities are coordinated is therefore unclear. In this study, we provide evidence that direct sensing of the cytokine IFN-{gamma} by CD8+ T-cells is a factor controlling the integration of T-cell affinity and differentiation during infection. IFN-{gamma} increases the expansion of low-affinity T-cells, allowing them to overcome the selective advantage of high-affinity T-cells. Concomitantly, IFN-{gamma} reinforces high-affinity T-cell entry into the memory pool. As a result, direct IFN-{gamma} sensing by CD8+ T-cells increases the avidity of the memory response. This comes at the expense of the primary T-cell response, for which IFN-{gamma} decreases the avidity, leading to sub-optimum immunity to infection. IFN-{gamma} sensing by CD8+ T-cells is paracrine, provided by a distinct subset of CD8+ T-cells called Virtual Memory T-cells, an antigen inexperienced subset that harbors memory features. Overall, we propose that IFN-{gamma} and Virtual Memory T-cells fulfil a critical immunoregulatory role by enabling the coordination of T-cell avidity and fate.