Suppressor activity of anergic T cells induced by IL‐10‐treated human dendritic cells: association with IL‐2‐ and CTLA‐4‐dependent G1 arrest of the cell cycle regulated by p27Kip1

We have previously shown that human IL‐10‐treated dendritic cells (DC) induce an antigen‐specific anergy in CD4+ T lymphocytes. These anergic T cells are characterized by an inhibitedproliferation, a reduced production of IL‐2, and additionally display antigen‐specific suppressor activity. In this study we investigated the mechanisms underlying the anergic state and regulatory function of these T cells. We did not observe enhanced rates of programmed cell death of anergic CD4+ suppressor T cells compared to T cells stimulated with mature DC. Cell cycle analysis by DNA staining and Western blot experiments revealed an arrest of anergic CD4+ T suppressor cells in the G1 phase. High levels of the IL‐2‐dependent cyclin‐dependent kinase (cdk) inhibitor p27Kip1 were found in anergic CD4+ suppressor T cells resulting in an inhibited activation of retinoblastoma protein and an arrest of cell cycle progression in the G1 phase. Addition of IL‐2, but not blocking of the CTLA‐4 pathway restored the proliferation of the suppressor T cells. In contrast, both treatments induced a down‐regulation of p27Kip1 and acomplete inhibition of the antigen‐specific regulatory function as demonstrated by high proliferation and enhanced IFN‐γ production of co‐cultured T cells. Further experiments demonstrated thatp27Kip‐expressing regulatory CD4+CD25+ T cells did not contribute to induction of T cell anergy in this model. Our data show that regulatory function of anergic CD4+ suppressor T cells is associated with an arrest in the G1 phase of the cell cycle mediated by increased levels of the IL‐2‐ and CTLA‐4‐dependent cdk inhibitor p27Kip1.