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  • Title: Human intestinal intraepithelial lymphocytes keep TNF alpha levels low by cell uptake and feedback inhibition of transcription.
    Author: Ebert EC, Mehta V.
    Journal: Cell Immunol; 2006 May; 241(1):7-13. PubMed ID: 16914126.
    Abstract:
    BACKGROUND: Human intestinal intraepithelial lymphocytes (IELs) are mainly TCRalphabeta(+)CD8(+) T cells, juxtaposed between epithelial cells. Their release of TNFalpha, a cytokine that is toxic to the epithelium, is likely to be tightly regulated. This study documents a novel mechanism to keep TNFalpha levels low with stimuli that cause large fluctuations in IFNgamma release. METHODS: IELs were stimulated with combinations of IL-2, IL-12, and IL-15, without TCR ligation. TNFalpha and IFNgamma production was assessed at both the mRNA and protein levels. Uptake of both cytokines was analyzed by radioligand binding. RESULTS: By 72h with IL-15, IFNgamma concentrations increased markedly, while TNFalpha remained low. With IL-12 supplementation, IFNgamma increased 10-fold, while TNFalpha remained low. Initially, IELs had similar amounts of transcripts for IFNgamma and TNFalpha, indicating equivalent synthetic rates. Two events explain the low extracellular accumulation of TNFalpha. First, IELs took up 25-30% of bound (125)I-TNFalpha without undergoing apoptosis. In contrast, they incorporated little IFNgamma. Second, TNFalpha was under strict feedback control as shown by the reduction in transcription with the addition of TNFalpha. No feedback control was seen with IFNgamma. CONCLUSIONS: Extracellular TNFalpha, but not IFNgamma, levels are kept low by uptake of free cytokine followed by a decline in transcription, thereby avoiding accumulation of this potentially toxic cytokine.
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