Human intestinal intraepithelial lymphocytes keep TNFα levels low by cell uptake and feedback inhibition of transcription

Background. Human intestinal intraepithelial lymphocytes (IELs) are mainly TCRαβ+CD8+ T cells, juxtaposed between epithelial cells. Their release of TNFα, a cytokine that is toxic to the epithelium, is likely to be tightly regulated. This study documents a novel mechanism to keep TNFα levels low with stimuli that cause large fluctuations in IFNγ release.Methods. IELs were stimulated with combinations of IL-2, IL-12, and IL-15, without TCR ligation. TNFα and IFNγ production was assessed at both the mRNA and protein levels. Uptake of both cytokines was analyzed by radioligand binding.Results. By 72 h with IL-15, IFNγ concentrations increased markedly, while TNFα remained low. With IL-12 supplementation, IFNγ increased 10-fold, while TNFα remained low. Initially, IELs had similar amounts of transcripts for IFNγ and TNFα, indicating equivalent synthetic rates. Two events explain the low extracellular accumulation of TNFα. First, IELs took up 25–30% of bound 125I-TNFα without undergoing apoptosis. In contrast, they incorporated little IFNγ. Second, TNFα was under strict feedback control as shown by the reduction in transcription with the addition of TNFα. No feedback control was seen with IFNγ.Conclusions. Extracellular TNFα, but not IFNγ, levels are kept low by uptake of free cytokine followed by a decline in transcription, thereby avoiding accumulation of this potentially toxic cytokine.