ABSTRACT
Modulation of cytokine responsiveness following T cell activation represents an important mechanism that shapes the fate of T cells after encounters with antigens. We activated T cells in mice with superantigen and assessed their ability to phosphorylate Stat1 in response to interferon-gamma (IFN-gamma) and IFN-alpha. After 4 h of activation in vivo, T cells became deficient in their ability to phosphorylate Stat1 in response to either cytokine. The loss of IFN sensitivity was accompanied by increased mRNA transcription for multiple suppressors of cytokine signaling (SOCS) genes (SOCS1, SOCS3, and SOCS7). The transcript levels of these SOCS were elevated only during the early hours after activation and were at or below normal levels by 60 h. Likewise, the activation-induced inhibition of IFN-alpha signaling was transient, and sensitivity was restored by 3 days postactivation. The loss of sensitivity to IFN-gamma persisted, however, and was still evident at 3 days. These data suggest that SOCS-independent mechanisms specific for inhibition of IFN-gamma signaling may be present at later stages of the T cell response. The loss of Stat1 signaling may be a factor in differentiation of T cells during and after activation, and it could also represent a protective mechanism against the toxic effects of IFN-gamma during immune responses.
Subject(s)
DNA-Binding Proteins/metabolism , Interferons/antagonists & inhibitors , Lymphocyte Activation , T-Lymphocytes/immunology , Trans-Activators/metabolism , Animals , Antibodies/pharmacology , CD28 Antigens/metabolism , CD3 Complex/metabolism , Carrier Proteins/genetics , Carrier Proteins/metabolism , DNA-Binding Proteins/genetics , Enterotoxins/immunology , Female , Interferon-alpha/antagonists & inhibitors , Interferon-alpha/genetics , Interferon-gamma/antagonists & inhibitors , Interferon-gamma/genetics , Interferons/genetics , Janus Kinase 1 , Lymphocyte Activation/immunology , Mice , Mice, Inbred C57BL , Phosphorylation , Protein-Tyrosine Kinases/metabolism , RNA, Messenger/analysis , Receptors, Interferon/metabolism , Repressor Proteins/genetics , Repressor Proteins/metabolism , STAT1 Transcription Factor , Signal Transduction , Suppressor of Cytokine Signaling 1 Protein , Suppressor of Cytokine Signaling 3 Protein , Suppressor of Cytokine Signaling Proteins , Trans-Activators/genetics , Transcription Factors/genetics , Transcription Factors/metabolism , Up-Regulation/genetics , Interferon gamma ReceptorABSTRACT
Interleukin-7 is widely accepted as a major homeostatic factor involved in T cell development. To assess the IL-7 responsiveness of thymocytes involved in selection processes, we used a new sensitive flow cytometry-based assay to detect intracellular phosphorylation of STAT-5 induced by IL-7 in defined mouse thymocyte subsets. Using this method, we found the earliest thymocyte subset (CD4(-)CD8(-)CD25(-)CD44(+)) to contain both IL-7-responsive and nonresponsive cells. Transition through the next stages of development (CD4(-)CD8(-)CD25(+)CD44(+ and -)) was associated with responsiveness of all thymocytes within these populations. Passage of thymocytes through beta-selection resulted in a significant reduction in IL-7 sensitivity. In the next phases of development (TCR(-) and TCR(low)CD69(-)), thymocytes were completely insensitive to the effects of IL-7. STAT-5 phosphorylation in response to IL-7 was again observed, however, in thymocytes involved in the positive selection process (TCR(low)CD69(+) and TCR(intermediate)). As expected, CD4 and CD8 single-positive thymocytes were responsive to IL-7. These findings delineate an IL-7-insensitive population between the beta-selection and positive selection checkpoints encompassing thymocytes predicted to die by neglect due to failure of positive selection. This pattern of sensitivity suggests a two-signal mechanism by which survival of thymocytes at these checkpoints is governed.
