Translational repression of pre-formed cytokine mRNA prevents chronic activation of memory T cells

Memory T cells are critical to protect us from recurring infections. Their instantaneous reactivity to pathogens is empowered by persistent expression of cytokine mRNA. How aberrant protein production of this pre-formed mRNA is prevented in the absence of infection, however, remains unresolved. We show that protein production in memory T cells is blocked through a 3’untranslated region (3’UTR)-mediated process, and that AU-rich elements (AREs) are key herein. Germ-line deletion of AREs leads to chronic IFN- production in bona fide memory T cells. Strikingly, the aberrant protein production does not result from increased mRNA levels and/or half-life. Rather, AREs block the recruitment of cytokine mRNA to ribosomes, a process that is mediated by the ARE-binding protein ZFP36L2. Thus, AREs are crucial elements for translational repression that allow murine and human memory T cells to contain pre-formed cytokine mRNAs, while preventing undesirable protein production in the absence of infection.