Zfp36l1 establishes the high-affinity CD8 T-cell response by directly linking TCR affinity to cytokine sensing.

How individual T cells compete for and respond to IL-2 at the molecular level, and, as a consequence, how this shapes population dynamics and the selection of high-affinity clones is still poorly understood. Here we describe how the RNA binding protein ZFP36L1, acts as a sensor of TCR affinity to promote clonal expansion of high-affinity CD8 T cells. As part of an incoherent feed-forward loop, ZFP36L1 has a nonredundant role in suppressing multiple negative regulators of cytokine signaling and mediating a selection mechanism based on competition for IL-2. We suggest that ZFP36L1 acts as a sensor of antigen affinity and establishes the dominance of high-affinity T cells by installing a hierarchical response to IL-2.