A distinct population of naive CD4⁺/CD25⁺ cells emerged after initiation of IL-2 treatment and remained present for long periods after IL-2 cycles.

Frozen PBMCs were stained before and at different time points (months) after initiation of IL-2. Each arrow represents a 5-day IL-2 cycle (months 2, 4, 6, 8, 10, and 28). Naive CD4⁺ cells were defined as CD45RO⁻/CD27⁺, and the CD25 histograms were gated on these naive CD4⁺ cells.