Thioredoxin expression is reduced at the plasma membrane of CD4+ T-cells in mid-life adults compared to young adults

Thioredoxin is a 2-cys oxidoreductase enzyme involved in maintaining cellular redox state. A number of human thioredoxin isoforms have been described; Trx1 is located in the cytoplasm, on the exofacial surface and as a secreted protein whereas Trx2 is located in the mitochondria. Secreted Trx1 has previously been implicated in T-cell signaling. As T-cell regulation declines with age we have investigated the age effects on T-cell Trx1. In the present study, we demonstrate an increase in Trx1 expression (p=0.0317; n=9) on total lymphocytes from healthy mid-life adults (age, 50-78) when compared to young adults (age, 25-30), however, a decrease in the level of Trx1 (p=0.0024) on the cell surface of CD4+ T-cells was observed. To further investigate the mechanisms underpinning the age-related decrease in cell surface Trx1, Jurkat T-cell line was treated with or without the addition of buthionine sulfoximine (BSO) to modulate intracellular GSH levels. BSO reduced cellular GSH content in a dose dependent manner with a 50% decrease after 25μ M BSO for 24 hours. BSO treatment is also associated with an increase in Trx1 expression (1.5-fold increase) as measured by western blot analysis. However, with a selective pull-down of cell surface proteins we observed a decrease in thioredoxin expression (3-fold decrease) at the plasma membrane. Furthermore, BSO treatment decreased mitogenic activation of Jurkats as measured by PHA-induced interleukin-2 secretion. These data suggest that in response to GSH depletion Jurkat T-cells increase cellular levels of Trx1 and alter subcellular localization resulting in decreased surface expression of Trx1. This may reflect adaptation by retention of thioredoxin in the cytoplasm to maintain intracellular protein integrity. Redox homeostasis within the immune synapse is vital for efficient antigen mediated T-cell activation; the impact of decreased expression of thioredoxin with age is currently unclear but it could impact on T-cell function.