Table 5. Characteristics of subsets of Treg cell.

Subset	Specific markers	Secretory products	Actions	Location
nTreg	CD4, CD25, Foxp3	IL-10, TGF-β	Block T cell proliferation, suppression of DCs, inhibition of effector Th1, Th2 and Th17 cells; eliminate production of allergen-specific IgE, induce IgG4 secretion; suppress mast cells, basophils and eosinophils; interact with resident tissue cells and participate tissue remodelling	Thymus [188]
ICOS(+) Treg	CD4, CD25, Foxp3, ICOS	IL-10, IL-17, IFN-γ	Suppress hapten-reactive CD8(+) T cells	Generated from nTregs
iTreg	CD4, Foxp3	IL-10, TGF-β	Similar to nTreg	Periphery
Tr1	CD4, CD25	IL-10	Suppress effector Th cell migration and functions [186]; suppress mast cells, basophils, and eosinophils [187]	Generated from non-Treg cell precursors and home lungs, and draining lymph nodes
CD8(+)Treg	CD8, Foxp3, CD25 (not for tonsil origin), CD28	IL-10, TNF-α, IFN-γ, GB	Block activation of naïve or effector T cells; suppress IgG/IgE antibody responses [188], IL-4 expression, and the proliferation of CD4(+) T cells.	Generated from OT-1 CD8 cells [188] and tonsils
IL-17-producing Foxp3 (+) Treg	CD4, Foxp3, CCR6, ROR- GTF	IL-17	Inhibit the proliferation of CD4(+) effector T cells [189].	Differentiated from CD4(+)Foxp3(+) CCR6(−) Tregs in peripheral blood and lymphoid tissue [189]

nTreg, natural regulatory T cell; ICOS, inducible costimulator; iTreg, inducible/adaptive regulatory T cell; Tr1 cell, IL-10-producing type 1 regulatory T cell; GB, granzyme B; RORGTF, ROR gammat transcription factor (adapted from Zhang and colleagues [183]).