Thymic selection of innate self-specific T cells: Both Timing and Signal Strength matter

Abstract

Abstract Naïve CD4 and CD8 T cells are selected in the thymus in response to low-avidity TCR interactions. Such weak TCR signals are crucial for installing a quiescent program that guides them to the lymph nodes yet allows for immune activation upon encounter of high-affinity ligands in the periphery. In contrast, agonist-selected T cells receive strong self-based TCR signals in the thymus that imprint them to guard the mucosal borders and maintain quiescence at steady state. Although the TCR signal strength is a decisive factor in instructing these different outcomes, the mechanism that generates protective self-specific T cells remains largely enigmatic. Previous work suggests that the self-specific double negative (DN) T cell lineage already diverges early during T cell development. We have explored the role of developmental timing by Cre-mediated deletion of components of the TCR signaling pathway. The Lck proximal promoter drives Cre expression before b-selection and the CD4 promoter drives Cre expression after b-selection but before positive selection. As TCR targets we investigated the lack of Themis expression, essential for the generation of conventional naïve T cells, but dispensable for agonist selection. Vice versa, we analyzed the requirement for Stim1 and Stim2, driving agonist selection, but not conventional selection. Both Lck prox Cre- and CD4 Cre-driven deletion of Themis lead to a drastic decrease in naïve T cells. However, DN T cells are particularly dependent on early strong TCR signals at the Pre-TCR stage, as only Lck prox Cre- but not CD4 Cre-driven deletion of Stim1/2 eliminates this subset. Our results suggest that strong signals at the Pre-TCR checkpoint are crucial for the generation of innate self-specific T cells.