The relationship between human thymus-derived regulatory T cells, TGF-β-induced regulatory T cells and conventional CD4+ T cells as revealed by proteomics

The CD4+ regulatory T (Treg) cell lineage, as defined by FOXP3 expression, comprises thymus-derived (t)Treg cells and peripherally induced (p)Treg cells. In human, naïve tTreg cells can be purified from blood, but occur in low abundance, while effector pTreg and tTreg cell populations cannot be purified for lack of discriminating cell surface markers. Therefore, studies often employ TGF-β-induced (i)Treg cells that are generated from CD4+ conventional T (Tconv) cells in vitro . Here, we describe the relationship of iTreg cells to tTreg and Tconv cells, as optimally purified from human blood. Global proteomic analysis revealed that iTreg, tTreg and Tconv cell populations each have a unique protein expression pattern. We next used as a benchmark a previously defined proteomic signature that discerns ex vivo naïve and effector phenotype Treg cells from Tconv cells and reflects unique Treg cell properties. This Treg cell core signature was largely absent from iTreg cells, while clearly present in simultaneously analyzed tTreg cells. In addition, we used a proteomic signature that distinguishes ex vivo effector Treg cells from Tconv cells and naïve Treg cells. This effector Treg cell signature was partially present in iTreg cells. Thus, iTreg cells are distinct from tTreg cells and largely lack the common Treg cell proteomic signature. However, they do have certain protein expression features in common with ex vivo effector Treg cells. These data demonstrate the utility of the core and effector Treg cell signatures as tools to define Treg cell populations and encourage the use of ex vivo Treg cells for functional analyses. ### Competing Interest Statement The authors have declared no competing interest.