Down-Regulation of Toll-Like Receptor 4 Expression in Transforming Growth Factor β1 Treated Murine Dendritic Cells: Implications of Maturation Resistant to Lipopolysaccharide.

Evidences accumulated that immature dendritic cell (iDC) could inhibit alloantigen-specific T cell responses and prolong the survival time of allografts. However, the tolerogenic properties of these iDCs were often unstable or inconsistent because of in vivo maturation, such as lipopolysaccharide (LPS) stimulating. Toll-like receptor 4(TLR4) has been reported to act as a receptor for LPS and LPS can stimulate iDC to mature DC (mDC) via TLR4 signal transduction pathway. In this study, we investigated the effects of transforming growth factor β1 on murine bone marrow derived DCs. Murine bone marrow cells were cultured with GM-CSF and TGF-β1 to generate TGF-β1 treated DCs (TGFβ-DCs). Compared to iDCs cultured by GM-CSF alone, the TGFβ-DCs had no significant alterations in ultrastructure after LPS stimulation. Surface expression of CD80, CD86, CD40, MHC-II were inhibited by addition of TGF-β1, especially in CD80, CD86 (p<0.05). Furthermore, the iDCs were sensitive to further maturation in response to LPS by showing increased levels of MHC class II, CD80, CD86 and CD40. In marked contrast, TGF-β1 prevented this LPS-mediated maturation and maintained the cells in the immature state, with low levels of surface costimulatory molecules expression. Using BrdU incorporation method, after 96 h mix lymphocyte reaction, TGFβ-DCs had weaker allogeneic stimulating capacity than iDCs. Importantly, LPS stimulating strongly promoted the allostimulatory capacity of iDCs, whereas only slightly affected TGFβ-DCs. TGFβ-DCs also showed decreased IL-12p70 production and impaired NF-κB activation after LPS stimulation. We also found the expression of TLR4 mRNA on TGFβ-DCs was weaker than that on iDCs by RT-PCR. Moreover, the results of flow cytometry revealed the positive expression percentages of TLR4/MD2 complex on iDCs and TGFβ-DCs were (51.8±3.89% vs. 15.7±4.13%, p<0.01) and the mean fluorescence intensities (MFIs) were (2.37±0.26 vs. 1.36±0.17, p<0.05). These results agreed with previous findings that TGFβ-DCs responded weakly to LPS. In summary, TGFβ-DC is resistant to maturation stimulus (LPS) and might have some correlation with the down-modulation of TLR4 expression.