NF-κB and TNF-α: A Positive Autocrine Loop in Human Lung Mast Cells?
The generation of cytokines, particularly TNF-α, by mast cells is crucial for the initiation of the allergic response. A key transcription factor involved in the synthesis of TNF-α is NF-κB. Using a mAb specific for the activated form of NF-κB, immunocytochemistry, confocal microscopy, and gel shift assays have been used in conjunction to localize this transcription factor to human lung mast cells and to study its activation. Activation of mast cells with stem cell factor (10 ng/ml) and anti-IgE (1 μg/ml) induced maximal activation of NF-κB at 4 and 2 h, respectively. In contrast, with TNF-α (5 ng/ml) maximal activation occurred within 15 min. Parallel falls in IκB were demonstrated. Confocal microscopy demonstrated the localization of the activated form of NF-κB to the nuclei of activated mast cells. NF-κB activation was verified using a gel shift assay. A supershift assay showed mast cell NF-κB to be composed primarily of p50 with smaller amounts of p65. No interaction with Abs for Rel-A, c-Rel, Rel-B, and p52 was seen. Immunocytochemistry and ELISAs showed TNF-α to be stored within mast cells and released into the extracellular environment following activation. The possible participation of TNF-α generated by mast cells in NF-κB activation by anti-IgE was investigated using a blocking Ab for TNF-α. The blocking Ab reduced NF-κB activation by anti-IgE by >50%, suggesting that the release of preformed mast cell-associated TNF-α acts as a positive autocrine feedback signal to augment NF-κB activation and production of further cytokine, including GM-CSF and IL-8.