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 inhibited proliferation, 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 G, phase. High levels of the IL-2-dependent cyclin-dependent kinase (cdk) inhibitor p27(Kip1) 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 G, 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 p27(Kip1) and a complete inhibition of the antigen-specific regulatory function as demonstrated by high proliferation and enhanced IFN-gamma production of co-cultured T cells. Further experiments demonstrated that p27(Kip)-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 G, phase of the cell cycle mediated by increased levels of the IL-2- and CTLA-4-dependent cdk inhibitor P27(Kip1).