Figure 1. The subpopulation of human Treg cells. Three distinct subpopulations of human Treg cells are defined based on the expression level of CD45RA and Foxp3 as follows: CD45RA⁺Foxp3^low resting Treg cells (subpopulation I), CD45RA⁻Foxp3^high activated Treg cells (subpopulation II), and CD45RA⁻Foxp3^low cytokine-secreting non-suppressive cells (subpopulation III) (32).