Newly Generated CD4⁺ T Cells Acquire Metabolic Quiescence after Thymic Egress

Mature naive T cells circulate through the secondary lymphoid organs in an actively enforced quiescent state. Impaired cell survival and cell functions could be found when T cells have defects in quiescence. One of the key features of T cell quiescence is low basal metabolic activity. It remains unclear at which developmental stage T cells acquire this metabolic quiescence. We compared mitochondria among CD4 single-positive (SP) T cells in the thymus, CD4⁺ recent thymic emigrants (RTEs), and mature naive T cells in the periphery. The results demonstrate that RTEs and naive T cells had reduced mitochondrial content and mitochondrial reactive oxygen species when compared with SP thymocytes. This downregulation of mitochondria requires T cell egress from the thymus and occurs early after young T cells enter the circulation. Autophagic clearance of mitochondria, but not mitochondria biogenesis or fission/fusion, contributes to mitochondrial downregulation in RTEs. The enhanced apoptosis signal-regulating kinase 1/MAPKs and reduced mechanistic target of rapamycin activities in RTEs relative to SP thymocytes may be involved in this mitochondrial reduction. These results indicate that the gain of metabolic quiescence is one of the important maturation processes during SP-RTE transition. Together with functional maturation, it promotes the survival and full responsiveness to activating stimuli in young T cells.