Regulation of murine chronic colitis by CD4⁺CD25^- programmed death-1⁺ T cells

Naturally arising CD4⁺CD25⁺ regulatory T (T_R) cells are engaged in the maintenance of self tolerance and prevention of autoimmune diseases. However, accumulating evidence suggests that a fraction of peripheral CD4⁺CD25^- T cells also possesses regulatory activity. Programmed death-1 (PD-1) is a new member of the CD28/CTLA-4 family, which has been implicated in the maintenance of peripheral self tolerance. Here, we identified a subpopulation of CD4⁺CD25^-PD-1⁺ T cells in the spleen of naive mice that constitutively expressed CTLA-4 and FoxP3 and was hypoproliferative in response to anti-CD3 antibody stimulation in vitro. However, the CD4⁺CD25^-PD-1⁺ T cells uniquely produced large amounts of IL-4 and IL-10 in response to anti-CD3 and anti-CD28 mAb stimulation, unlike the CD4⁺CD25⁺ T_R cells. The CD4⁺CD25^-PD-1⁺ T cells exhibited a suppressor activity against the proliferation of anti-CD3 antibody-stimulated CD4⁺CD25^-PD-1^- T cells in vitro, which was partially abrogated by anti-CTLA-4 mAb, but not by anti-IL-10 or anti-PD-1 mAb. Remarkably, the CD4⁺CD25^-PD-1⁺ T cells inhibited the development of colitis induced by adoptive transfer of CD4⁺CD45RB^high T cells into C.B17-scid/scid mice, albeit to a lesser extent than CD4⁺CD25⁺ T_R cells, in a CTLA-4-dependent manner. These results indicate that the CD4⁺CD25^-PD-1⁺ T cells contain substantial amounts of T_R cells that are involved in the maintenance of peripheral tolerance.