ABSTRACT
OBJECTIVE: CD4+CD25+ regulatory T cells (known as Treg cells) suppress unwanted and autoreactive T cell responses. Treg cells express the costimulatory molecule CTLA-4 intracellularly, but the mechanisms by which Treg cells exploit CTLA-4 signaling remain unclear. The present study was undertaken to investigate the role of CTLA-4 in controlling the homeostasis and suppressive function of Treg cells. METHODS: Murine Treg cells were analyzed by flow cytometry for coexpression of CTLA-4 and typical Treg cell-expressed molecules, and the influence of CTLA-4 on T cell proliferation, suppression, and apoptosis was investigated by in vitro assays. To analyze the importance of CTLA-4 in Treg cell-mediated suppression in vivo, wild-type Treg cells were transferred into CTLA-4-deficient mice displaying lymphoproliferation, and survival was monitored over time. RESULTS: A strong correlation between expression of forkhead box P3 and ex vivo expression of CTLA-4 in Treg cells was observed. Inhibition of CTLA-4 signaling in Treg cells during in vitro stimulation increased cell cycling and led to enhanced activation-induced cell death (AICD), which was mediated by CD95/CD95 ligand-induced activation of caspases. Blockade of CTLA-4 signaling resulted in impairment of the suppressive capacity of Treg cells. Despite these effects, high amounts of Treg cells persisted in CTLA-4-deficient mice. Results of transfer experiments in CTLA-4-deficient mice showed that the mice had a significantly prolonged lifespan when CTLA-4-competent Treg cells were injected. CONCLUSION: Expression of CTLA-4 on Treg cells serves to control T cell proliferation, to confer resistance against AICD, and to maintain the suppressive function of Treg cells.
Subject(s)
Antigens, CD/metabolism , Homeostasis/immunology , Signal Transduction/immunology , T-Lymphocytes, Regulatory/metabolism , Adoptive Transfer , Animals , Antigens, CD/genetics , CD4 Antigens/metabolism , CTLA-4 Antigen , Cell Death/immunology , Cell Division/immunology , Cell Survival/immunology , Cells, Cultured , Flow Cytometry , Forkhead Transcription Factors/metabolism , Interleukin-2 Receptor alpha Subunit/metabolism , Mice , Mice, Inbred C57BL , Mice, Mutant Strains , T-Lymphocytes, Regulatory/cytologyABSTRACT
CD8 T cell expansion and cytokine production is needed to generate an effective defense against viral invasion of the host. These features of CD8 T lymphocytes are regulated, especially during primary responses, by positive and negative costimulation. We show in this study that surface expression of CD152 is highly up-regulated on activated CD8 T lymphocytes during primary immune responses, suggesting a prominent regulatory role. Indeed, production of the proinflammatory cytokine IFN-gamma, but not TNF-alpha, by CD8 T cells was inhibited by CD152 engagement. The inhibition was regulated independent of proliferation and IL-2 production, but dependent on the quality of the TCR signaling. We show that signals induced by CD152 on activated CD8 T lymphocytes reduce the frequency of IFN-gamma(high)-expressing cells. Our data also show that in activated CD8 T cells, the CD152-mediated inhibition of cytokine production is more pronounced than inhibition of their proliferation.
