Transforming Growth Factor-β1 Inhibits the Maturation of Dendritic Cells Via Toll-Like Receptor 4 Signaling Pathway.

We have demonstrated that transforming growth factor-β1(TGF-β1) inhibits the maturation of mouse bone marrow derived dendritic cells (DCs). TGF-β1 treated DCs (TGFβ-DCs) are resistant to maturation stimulus -lipopolysaccharide (LPS) and might have some correlation with the down-modulation of Toll-like receptor 4 (TLR4) expression. It was known TLR4 binds LPS from Gram-negative bacteria, triggering signaling pathways that lead to the activation of NF-κB, ERK1/2 and p38 MAPK and ensuing gene expression of proinflammatory factors. In the current study, we further estimated the activities of NF-κB, ERK1/2 and p38 proteins involved in TLR4 signaling pathway. Using EMSA method, we found the NF-κB DNA binding activity in immature DCs (imDCs) was significantly increased in response to LPS, but addition of TGF-β1 to DCs inhibited NF-κB binding. Moreover, TGF-β1 was effective in suppressing LPS-induced activation of ERK1/2 and p38 kinase, the level of phosphorylation of ERK1/2 and p38 kinase were lower than imDCs measured by Western Blot. After treatment of imDCs and TGFβ-DCs with LPS for 24 h, the production of IL-12p70 of TGFβ-DCs was significantly less than that of imDCs(115.4±15.2 pg/ml vs 517.0±29.7 pg/ml, P<0.01), but the level of Th2 cytokine-IL-10 was elevated(132.1±17.5 pg/ml vs 75.1±16.6 pg/ml, P<0.05), indicating that exposure to TGF-β1 impaired the capability of DCs to produce high amounts of bioactive IL-12p70. According to the semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), the expressions of chemokines MIP-1α mRNA on TGFβ-DCs after LPS stimulation were lower than imDCs at each time point. The lower expressions of MCP-1 and IP-10 on TGFβ-DCs at irregular pattern after LPS treatment, whereas the expressions RANTES were no different. Hence, the results suggested TGF-β1 maybe directly inhibit TLR4 expression on DCs, and then interfere with the activity of downstream key proteins, such as NF-κB, ERK1/2 and p38. Ultimately, TGF-β1 treated DCs were resistance to LPS, down-regulated the expression of costimulatory molecular on DCs and decreased the secretion of inflammatory cytokines.