Transiently Reduced PI3K/Akt Activity Drives the Development of Regulatory Function in Antigen-Stimulated Naïve T-Cells

Regulatory T-cells (T(regs)) are central for immune homeostasis and divided in thymus-derived natural T(regs) and peripherally induced iT(reg). However, while phenotype and function of iT(regs) are well known, a remarkable lack exists in knowledge about signaling mechanisms leading to their generation from naïve precursors in peripheral tissues. Using antigen specific naïve T-cells from mice, we investigated CD4+ CD25+ FoxP3- iT(reg) induction during antigen-specific T-cell receptor (TCR) stimulation with weak antigen presenting cells (APC). We show that early signaling pathways such as ADAM-17-activation appeared similar in developing iT(reg) and effector cells (T(eff)) and both initially shedded CD62-L. But iT(reg) started reexpressing CD62-L after 24 h while T(eff) permanently downmodulated it. Furthermore, between 24 and 72 hours iT(reg) presented with significantly lower phosphorylation levels of Akt-S473 suggesting lower activity of the PI3K/Akt-axis. This was associated with a higher expression of the Akt hydrophobic motif-specific phosphatase PHLPP1 in iT(reg). Importantly, the lack of costimulatory signals via CD28 from weak APC was central for the development of regulatory function in iT(reg) but not for the reappearance of CD62-L. Thus, T-cells display a window of sensitivity after onset of TCR triggering within which the intensity of the PI3K/Akt signal controls entry into either effector or regulatory pathways.