Different regulatory pathways employed in cytokine-enhanced expression of secretory component and epithelial HLA class I genes

Abstract

The transmembrane secretory component (SC, or plg receptor) plays a crucial role in mucosal immunity by translocating dimeric IgA and pentameric IgM through exocrine epithelia. This receptor is up-regulated by cytokines in parallel with increased epithelial HLA expression. By use of the human epithelial cell line HT-29m3, we show that IFN-γ, TNF-α and IL-4 activate transcription of the SC gene. This activation was slow, suggesting mediation via newly synthesized protein factors. IFN-γ and TNF-α, but not IL-4, also up-regulated expression of HLA class I genes. However, this gene induction was rapid and did not depend on new protein synthesis. Nuclear run-on experiments showed that the transcription rate of HLA class I genes nearly peaked after only 30 min of IFN-γ or TNF-α stimulation, whereas the SC transcription rate did not peak until after 20-36 h of IFN-γ, TNF-α or IL-4 stimulation. Gel electrophoresis mobility shift assays demonstrated binding of nuclear proteins from cytokine-stimulated HT-29 cells to consensus elements in the promoter of the SC gene, involving the binding site for the nuclear factor-xB p50 subunit after TNF-α stimulation, and IFN-stimulated response element after IFN-γ stimulation (and weakly after TNF-α). Our observations in vitro likely parallel events in vivo by which activated mucosal T cells and macrophages enhance plg receptor-mediated external transport of secretory IgA and IgM and up-regulate epithelial HLA expression.