Combined deletion of ZFP36L1 and ZFP36L2 drives superior cytokine production in T cells at the cost of cell fitness

A key feature of cytotoxic CD8 ⁺ T cells for eliminating pathogens and malignant cells is their capacity to produce pro-inflammatory cytokines, which includes TNF and IFNγ. Provided that these cytokines are highly toxic, a tight control of their production is imperative. RNA-binding proteins (RBPs) are essential for the fine-tuning of cytokine production. The role of the RBP ZFP36L1 and its sister protein ZFP36L2 herein has been established, however, their relative contribution to cytokine production is not well known. We here compared the effect of ZFP36L1 and ZFP36L2 single and double deficiency in murine effector CD8 ⁺ T cells. Whereas single deficient T cells significantly increased cytokine production, double deficiency completely unleashed the cytokine production. Not only the TNF production was substantially prolonged in double-deficient T cells. Also, the production of IFNγ reached unprecedented levels with >90% IFNγ–producing T cells compared to 3% in WT T cells, even after 3 days of continuous activation. This continuous cytokine production by double-deficient T cells was also observed in tumor-infiltrating lymphocytes in vivo, however, with no effect on tumor growth. Rather, ZFP36L1 and ZFP36L2 double deficiency resulted in decreased cell viability, impaired STAT5 signaling, and dysregulated cell cycle progression. In conclusion, while combined deletion in ZFP36L1 and ZFP36L2 can drive continuous cytokine production even under chronic activation, safeguards are in place to counteract such super-cytokine producers.