Otte Jan-Michel, Cario Elke, Podolsky Daniel K
Gastrointestinal Unit, Center for the Study of Inflammatory Bowel Disease, Massachusetts General Hospital, Boston, USA.
Gastroenterology. 2004 Apr;126(4):1054-70. doi: 10.1053/j.gastro.2004.01.007.
BACKGROUND & AIMS: Despite the ability to participate in immune responses and the continuous presence of bacteria and bacterial products, functional responses of intestinal epithelial cells (IEC) seem to be muted. Previously, tolerance to Toll-like receptors (TLRs) ligands has been described in monocytic cells. However, mechanisms in the intestine are unknown.
The effect of purified lipopolysaccharide (LPS) and lipoteichoic acid (LTA) on expression and function of TLRs in intestinal epithelial cells (Colo205, SW480, T84) was assessed by Northern and Western blot and FACS analysis, kinase activity assays, immunohistochemistry, and ELISA.
Expression of TLRs except 10 was detected in primary IEC and TLR1-10 in the cultured cells. Short-term stimulation with LPS or LTA activated proinflammatory signaling cascades in IEC, including phosphorylation of IRAK and MAP kinases and increased IL-8 secretion, whereas prolonged incubation resulted in a state of hyporesponsiveness with no reactivation of the cells by a second challenge with either substance detected. The cells remained responsive to tumor necrosis factor (TNF). Hyporesponsive cells showed no alteration in expression of TLR or signaling molecules but revealed a decrease in TLR surface expression and IRAK activity. Toll-interacting protein (Tollip) mRNA and protein expression were increased in hyporesponsive cells, and overexpression of Tollip in IEC resulted in a significantly decreased proinflammatory response.
Continuous presence of specific bacterial components results in a status of hyporesponsiveness in otherwise reactive IEC. Down-regulation of TLR surface expression and up-regulation of inhibitory Tollip with decreased phosphorylation of IRAK might all contribute to this hyporesponsiveness.
尽管肠道上皮细胞（IEC）能够参与免疫反应，且细菌及细菌产物持续存在，但IEC的功能反应似乎受到抑制。此前，单核细胞中已发现对Toll样受体（TLR）配体存在耐受性。然而，肠道中的机制尚不清楚。
通过Northern印迹、Western印迹、FACS分析、激酶活性测定、免疫组织化学和ELISA，评估纯化的脂多糖（LPS）和脂磷壁酸（LTA）对肠道上皮细胞（Colo205、SW480、T84）中TLR表达及功能的影响。
在原代IEC中检测到除TLR10外的其他TLR表达，在培养细胞中检测到TLR1 - 10的表达。LPS或LTA短期刺激可激活IEC中的促炎信号级联反应，包括IRAK和MAP激酶的磷酸化以及IL - 8分泌增加，而长时间孵育则导致低反应状态，再次用任何一种物质刺激细胞均未检测到再激活现象。细胞对肿瘤坏死因子（TNF）仍有反应。低反应性细胞的TLR或信号分子表达未发生改变，但TLR表面表达和IRAK活性降低。低反应性细胞中Toll相互作用蛋白（Tollip）的mRNA和蛋白表达增加，在IEC中过表达Tollip会导致促炎反应显著降低。
特定细菌成分的持续存在导致原本具有反应性的IEC处于低反应状态。TLR表面表达下调、抑制性Tollip上调以及IRAK磷酸化降低可能共同导致了这种低反应性。
