The low percentage of Foxp3+ T cells obtained in these

experiments in lymphoreplete mice is in agreement with previous reports by Lathrop et al. [16]. Moreover, identical numbers of recovered T cells were found, arguing against a better engraftment or survival of young T cells (data not shown). Finally, artificially spiking 0.1% of contaminating tTreg in C57BL/6 CD4+ T cells, i.e. >10 times the true contaminating cell-sorting percentage (<0.01%) in purified CD4+eGFP− T cells, led only to 0.1% of eGFP+ cells among recovered donor CD4+ T cells (Fig. 1C). This confirmed that the low conversion of CD4+eGFP− T cells observed here at the steady state could not be attributed to the expansion Vadimezan chemical structure of cotransferred eGFP+ tTreg cells. A straightforward explanation for the defective pTreg-cell production observed in aged mice could be the progressive disappearance from the periphery of recent thymic emigrants (RTE) enriched in pTreg-cell precursors [17, 18]. Precise time-course experiments effectively revealed that pTreg-cell generation was higher in 2-week Tconv cells, which are enriched in Crenolanib RTE, and comparable with that of thymocytes (Fig. 1D). This is consistent with an RTE-dependent conversion process at that very young age. To address the role of RTE in pTreg-cell induction after 5 weeks of age, we isolated CD4+eGFP− Tconv

cells from young donor mice thymectomized 3 or 6 weeks earlier and therefore devoid of RTE. We found that they retained a similar conversion potential as Tconv

cells from nonthymectomized age-matched controls (Fig. 1E). Overall, our results indicated an age-related decline in the steady-state production of pTreg cells in adult mice, independent from a potential loss of conversion-prone RTE. In addition to the progressive disappearance of RTE, aging has been previously associated with accumulation of conversion-resistant CD4+CD44hi T cells secreting proinflammatory cytokines like IL-4 and IFN-γ [19], early defects old in T-cell IL-2 secretion leading to impaired proliferation [14], and narrowing of the T-cell repertoire. To analyze these points in more detail, we switched to a more defined system of in vitro Treg cells induction (iTreg), using plate-bound anti-CD3 stimulation in the presence of exogenous IL-2 and TGF-β [20]. Under these conditions, we found again a reduced induction, as early as day 2, of Foxp3 in CD4+eGFP− Tconv cells isolated from old Foxp3-eGFP mice (Fig. 2A and B). This reduction was also observed in sorted naïve CD44lo Tconv population (Fig. 2C) and held true at all doses of anti-CD3 concentrations tested (data not shown). Saturating amounts of TGF-β were unable to reverse this reduction in old T cells (data not shown). As TGF-β-dependent Th17 induction is enhanced in aged T cells [21, 22], we presumed that TGF-β signaling is intact in aged T cells.