SEL1L preserves CD8 T-cell survival and homeostasis by fine-tuning PERK signaling and the IL-15 receptor-mediated mTORC1 axis.

SEL1L-mediated endoplasmic reticulum-associated degradation (ERAD) plays critical roles in controlling protein homeostasis by degrading misfolded or terminal unfolded proteins. However, it remains unclear how SEL1L regulates peripheral T-cell survival and homeostasis. Herein, we found that SEL1L deficiency led to a greatly reduced frequency and number of mature T cells, which was further validated by adoptive transfer experiments or bone marrow chimera experiments, accompanied by the induction of multiple forms of cell death. Furthermore, SEL1L deficiency selectively disrupted naïve CD8 T-cell homeostasis, as indicated by the severe loss of the naïve T-cell subset but an increase in the memory T-cell subset. We also found that SEL1L deficiency fueled mTORC1/c-MYC activation and induced a metabolic shift, which was largely attributable to enhanced expression of the IL-15 receptor α and β chains. Mechanistically, single-cell transcriptomic profiling and biochemical analyses further revealed that Sel1l CD8 T cells harbored excessive ER stress, particularly aberrant activation of the PERK-ATF4-CHOP-Bim pathway, which was alleviated by supplementing IL-7 or IL-15. Importantly, PERK inhibition greatly resolved the survival defects of Sel1l CD8 T cells. In addition, IRE1α deficiency decreased mTORC1 signaling in Sel1l naïve CD8 T cells by downregulating the IL-15 receptor α chain. Altogether, these observations suggest that the ERAD adaptor molecule SEL1L acts as an important checkpoint for preserving the survival and homeostasis of peripheral T cells by regulating the PERK signaling cascade and IL-15 receptor-mediated mTORC1 axis.