The Synergistic Effect Of IL-6 And TGF-β Signal Promotes The Degradation Of FOXP3 Protein

In human immune system, naive CD4+T cells can be differentiated into various effector cell types, including T helper1(Thl), Th2,Thl7and induced regulatory T (iTreg) cells, which polarize according to the local cytokine environment in which they are stimulated. Th17cells are proinflammatory, characterized by the production of inflammatory cytokines such as interleukin (IL)-17, IL-6and tumor necrosis factor (TNF), which are not only involved in mediating host defense mechanisms but can also promote autoimmune disease; In contrast, iTreg cells, marked by expression of the forkhead/winged helix transcription factor Forkhead BoxP3(FOXP3) are highly capable of suppressing autoimmunity and can limit excessive tissue damage. Transforming growth factor-β (TGF-β), along with T cell receptor stimulation, is required for the induction of FOXP3expression in conventional T cells to be fully differentiated to iTreg cells. In contrast, TGF-β, in conjunction with IL-6, can stimulate the differentiation of Th17cells where IL-6signaling completely inhibits the generation of FOXP3+Treg cells induced by TGF-β, but the exact molecular mechanisms underlying IL-6and TGF-P mediated regulation of FOXP3expression remain unclear.FOXP3mRNA expression level was down-regulated with IL-6/TGF-β stimulation in human Treg.To study the effect of IL-6on FOXP3expression, the Jurkat-HA-FOXP3stable cell line was produced in our lab via lentiviral transduction.We found that FOXP3expression level had no change by IL-6treatment alone, but IL-6/TGF-P costimulation dramatically downregulated FOXP3protein level after12h of stimulation. we hypothesized that the decrease in FOXP3expression level is due to FOXP3protein degradation by the proteasome. We treated Jurkat-HA-FOXP3T cells with IL-6/TGF-β/MG132.We found that the proteasome inhibitor MG132could antagonize IL-6/TGF-β-mediated reduction of FOXP3expression. Our data suggests that FOXP3protein may be degraded by the ubiquitin-proteasome-dependent pathway upon IL-6/TGF-β co-stimulation. So, FOXP3post-translational regulation is crucial for FOXP3stabilization and function and T cells differentiation.