The different effects of indirubin on effector and CD4⁺CD25 ⁺ regulatory T cells in mice: Potential implication for the treatment of autoimmune diseases

CD4⁺CD25⁺ regulatory T (Treg) cells play an essential role in the induction and maintenance of peripheral self-tolerance. Indirubin, a traditional Chinese medicine, was clinically used in the treatment of chronic myelocytic leukemia as well as some autoimmune diseases, including Alzheimer's disease, diabetes, and so on. The effects of indirubin on CD4 ⁺CD25⁺Treg cells, which play a critical role in controlling autoimmunity, have not been addressed. In the present study, we observed the cell levels, phenotypes, and immunoregulatory function of CD4 ⁺CD25⁺Treg cells in indirubin-treated mice. Treatment with indirubin significantly enhanced the ratios of CD4⁺CD25 ⁺Treg cells or CD4⁺CD25⁺Foxp3⁺Treg cells to CD4⁺T cells in peripheral blood, lymph nodes, and spleens (P<0.01 compared with control mice). CD4⁺CD25 ⁺Foxp3⁺Treg cells to CD4 single positive cells in the thymi of indirubin-treated mice were significantly higher than those in control mice. Furthermore, splenic CD4⁺CD25⁺Treg cells in indirubin-treated mice showed immunosuppressive ability on the immune response of T effector cells to alloantigens or mitogen as efficiently as the control CD4⁺CD25⁺Treg cells in vitro. The present studies indicate that CD4⁺CD25⁺Treg cells are more resistant to indirubin than effector T cells in vivo. The selectively enhanced CD4⁺CD25 ⁺Treg cell levels by indirubin made host to be more favorable for immune tolerance induction, which opened one possibility for indirubin to treat autoimmune diseases.