CD4+ CD25bright+ regulatory T cells can mediate donor nonreactivity in long-term immunosuppressed kidney allograft patients

CD4(+) CD25(bright+) FoxP3(+) T cells are potent regulators of T-cell reactivity, but their possible involvement in donor-specific nonresponsiveness after clinical kidney transplantation remains to be elucidated. We assessed the proliferative donor-reactivity in 33 kidney allograft recipients who were maintained on a combination of proliferation inhibitors (mycophenolate mofetil (MMF) or Azathioprine (Aza)) and prednisone, long (> 5 years) after transplantation. Of the 33 patients, 8 still exhibited donor-reactivity, whereas 25 were classified as donor nonreactive patients. Within these 25 donor nonreactive patients, we assessed the involvement of CD4(+) CD25(bright+) regulatory T cells both by depleting them from the responder population as well as by reconstituting them to the CD25(-/dim) effector population. The absence of proliferation in these 25 patients, was abolished in 7 (28%) recipients upon depletion of the CD4(+) CD25(bright+) T cells. Reconstitution of these cells suppressed the donor-reactivity in a dose-dependent manner. Adding-back CD4(+) CD25(bright+) T cells inhibited the anti-third party response in all recipients, indicating that functional CD4(+) CD25(bright+) T cells circulate despite more then 5 years of immunosuppressive treatment. Altogether, we conclude that in long-term immunosuppressed kidney allograft patients functional regulatory CD4(+) CD25(bright+) T cells circulate but that these cells mediate donor non reactivity only in a subset of patients.