TGF-β-induced myelin peptide-specific regulatory T cells mediate antigen-specific suppression of induction of experimental autoimmune encephalomyelitis

The low number of natural regulatory T cells (nTregs) in the circulation specific for a particular Ag and concerns about the bystander suppressive capacity of expanded nTregs presents a major clinical challenge for nTreg-based therapeutic treatment of autoimmune diseases. In the current study, we demonstrate that naive CD4⁺CD25^-Foxp3^- T cells specific for the myelin proteolipid protein (PLP)_139-151 peptide can be converted into CD25⁺Foxp3⁺ induced Treg cells (iTregs) when stimulated in the presence of TGF-β, retinoic acid, and IL-2. These PLP_139-151-specific iTregs (139-iTregs) have a phenotype similar to nTregs, but additionally express an intermediate level of CD62L and a high level of CD103. Upon transfer into SJL/J mice, 139-iTregs undergo Ag-driven proliferation and are effective at suppressing induction of experimental autoimmune encephalomyelitis induced by the cognate PLP_139-151 peptide, but not PLP_178-191 or a mixture of the two peptides. Furthermore, 139-iTregs inhibit delayed-type hypersensitivity responses to PLP_139-151, but not PLP_178-191, myelin oligodendrocyte glycoprotein (MOG)_35-55, or OVA_323-339 in mice primed with a mixture of PLP_139-151 and the other respective peptides. Additionally, 139-iTregs suppress the proliferation and activation of PLP _139-151-, but not MOG_35-55-specific CD4⁺ T cells in SJL/B6 F1 mice primed with a combination of PLP_139-151 and MOG_35-55. These findings suggest that Ag-specific iTregs are amplified in vivo when exposed to cognate Ag under inflammatory conditions, and these activated iTregs suppress CD4⁺ responder T cells in an Ag-specific manner.