FOXP3 and lymphopenia: In peripheral lymphoid tissues, naïve CD4+Foxp3? T cells can acquire a Foxp3+ Treg cell phenotype in a variety of experimental settings, such as lymphopenia-driven homeostatic expansion [13], [14], [15] and subimmunogenic administration of either free Ag [16], [17], [18], [19], [20] or DEC-205+ dendritic cell-targeted foreign [21], [22], [23], [24] and self-Ag [25], [26].