Notch-induced endoplasmic reticulum-associated degradation governs mouse thymocyte ß-selection.

Signals from the pre-T cell receptor and Notch coordinately instruct ß-selection of CD4-CD8-double negative (DN) thymocytes to generate αß T cells in the thymus. However, how these signals ensure a high-fidelity proteome and safeguard the clonal diversification of the pre-selection TCR repertoire given the considerable translational activity imposed by ß-selection is largely unknown. Here, we identify the endoplasmic reticulum (ER)-associated degradation (ERAD) machinery as a critical proteostasis checkpoint during ß-selection. Expression of the SEL1L-HRD1 complex, the most conserved branch of ERAD, is directly regulated by the transcriptional activity of the Notch intracellular domain. Deletion of Sel1l impaired DN3 to DN4 thymocyte transition and severely impaired mouse αß T cell development. Mechanistically, Sel1l deficiency induced unresolved ER stress that triggered thymocyte apoptosis through the PERK pathway. Accordingly, genetically inactivating PERK rescued T cell development from Sel1l-deficient thymocytes. In contrast, IRE1α/XBP1 pathway was induced as a compensatory adaptation to alleviate Sel1l-deficiency-induced ER stress. Dual loss of Sel1l and Xbp1 markedly exacerbated the thymic defect. Our study reveals a critical developmental signal controlled proteostasis mechanism that enforces T cell development to ensure a healthy adaptive immunity.

Pregnancy promotes tolerance to future offspring by programming selective dysfunction in long-lived maternal T cells.

Fetal antigen available during pregnancy induces the proliferation of maternal T cells. It is unknown, however, whether these antigen-activated T cells differentiate into long-lived memory T cells that are capable of mediating rapid-recall responses to tissue antigens. To test the hypothesis that pregnancy induces an alternative fate in fetal-specific maternal T cells, we used a murine model to track longitudinally fetal-specific T cells in pregnant and postpartum animals and test the response of these cells when challenged with the same antigen during sequential pregnancy or skin transplantation. Fetal-specific CD8+ T cells were robustly primed during pregnancy but failed to acquire robust effector functions. These primed cells persisted long term in postpartum animals, frequently maintained a programmed death 1 (PD-1)+ phenotype, and failed to expand or produce cytokines robustly in response to second pregnancy or skin transplantation. However, whereas there was no impact on second pregnancy as a result of the persistence of fetal-primed memory CD8+ T cells in the mother, skin grafts bearing the same antigen were rejected more rapidly. Altogether, our data suggest that fetal antigen exposure during pregnancy induces the differentiation of long-lived maternal CD8+ T cells with context-dependent, selective effector dysfunction. This programmed effector dysfunction provides temporal and systemic restraint of maternal anti-fetal alloreactivity to promote reproductive fitness efficiently, while preserving potentially protective effector T cell responses.