Cells with Treg-specific FOXP3 demethylation but low CD25 are prevalent in autoimmunity

Identification of alterations in the cellular composition of the human immune system is key to understanding the autoimmune process. Recently, a subset of FOXP3⁺ cells with low CD25 expression was found to be increased in peripheral blood from systemic lupus erythematosus (SLE) patients, although its functional significance remains controversial. Here we find in comparisons with healthy donors that the frequency of FOXP3⁺ cells within CD127^lowCD25^low CD4⁺ T cells (here defined as CD25^lowFOXP3⁺ T cells) is increased in patients affected by autoimmune disease of varying severity, from combined immunodeficiency with active autoimmunity, SLE to type 1 diabetes. We show that CD25^lowFOXP3⁺ T cells share phenotypic features resembling conventional CD127^lowCD25^highFOXP3⁺ Tregs, including demethylation of the Treg-specific epigenetic control region in FOXP3, HELIOS expression, and lack of IL-2 production. As compared to conventional Tregs, more CD25^lowFOXP3⁺HELIOS⁺ T cells are in cell cycle (33.0% vs 20.7% Ki-67⁺; P = 1.3 × 10^-9) and express the late-stage inhibitory receptor PD-1 (67.2% vs 35.5%; P = 4.0 × 10^-18), while having reduced expression of the early-stage inhibitory receptor CTLA-4, as well as other Treg markers, such as FOXP3 and CD15s. The number of CD25^lowFOXP3⁺ T cells is correlated (P = 3.1 × 10^-7) with the proportion of CD25^highFOXP3⁺ T cells in cell cycle (Ki-67⁺). These findings suggest that CD25^lowFOXP3⁺ T cells represent a subset of Tregs that are derived from CD25^highFOXP3⁺ T cells, and are a peripheral marker of recent Treg expansion in response to an autoimmune reaction in tissues.