CD8+ T cell differentiation into effector cells is initiated early after antigen encounter by signals from the T cell antigen receptor and costimulatory molecules. The molecular mechanisms that establish the timing and rate of differentiation however are not defined. Here we show that the RNA binding proteins (RBP) ZFP36 and ZFP36L1 limit the rate of differentiation of activated naïve CD8+ T cells and the potency of the resulting cytotoxic lymphocytes. The RBP function in an early and short temporal window to enforce dependency on costimulation via CD28 for full T cell activation and effector differentiation by directly binding mRNA of NF-κB, Irf8 and Notch1 transcription factors and cytokines, including Il2. Their absence in T cells, or the adoptive transfer of small numbers of CD8+ T cells lacking the RBP, promotes resilience to influenza A virus infection without immunopathology. These findings highlight ZFP36 and ZFP36L1 as nodes for the integration of the early T cell activation signals controlling the speed and quality of the CD8+ T cell response.

How individual T cells compete for and respond to IL2 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 non-redundant role in suppressing negative regulators of cytokine signalling and mediating a selection mechanism based on competition for IL2. We suggest that ZFP36L1 acts as a sensor of antigen affinity and establishes dominance of high affinity T cells by installing a hierarchical response to IL2.