Ohnishi Takahiro, Muroi Masashi, Tanamoto Ken-ichi
Division of Microbiology, National Institute of Health Sciences, Tokyo, Japan.
FEMS Immunol Med Microbiol. 2007 Oct;51(1):84-91. doi: 10.1111/j.1574-695X.2007.00281.x. Epub 2007 Jul 5.
We analysed the lipopolysaccharide (LPS)-recognition mechanism in cells expressing TLR4 and CD14 but lacking MD-2. When TLR4 and CD14 were transiently expressed in HEK293 cells, cell-surface expression of TLR4 was observed, although the expression level was lower than that in cells coexpressing MD-2. We found that membrane CD14-TLR4 complexes were formed in these cells in response to LPS stimulation even in the absence of MD-2 expression, although NF-kappaB-dependent reporter activity was not induced. A strong activation of NF-kappaB was observed when these cells were stimulated with LPS followed by soluble MD-2 in this order, even when excess LPS was removed after formation of the CD14-TLR4 complex by washing cells prior to sMD-2 addition. From these results, we propose an additional LPS-recognition mechanism. In cells expressing TLR4 and CD14 but lacking MD-2, LPS is first transferred to membrane CD14 with the aid of LPS binding protein, which leads to the formation of the TLR4-CD14 complex. Then, the binding of soluble MD-2 to this complex triggers the transmembrane signal transduction. Cells expressing TLR4 and CD14 but lacking MD-2, such as airway epithelial cells, may be activated in response to LPS by this mechanism.
我们分析了在表达TLR4和CD14但缺乏MD-2的细胞中的脂多糖（LPS）识别机制。当TLR4和CD14在HEK293细胞中瞬时表达时，虽然表达水平低于共表达MD-2的细胞，但仍可观察到TLR4的细胞表面表达。我们发现，即使在没有MD-2表达的情况下，这些细胞在LPS刺激下也会形成膜CD14-TLR4复合物，尽管未诱导NF-κB依赖性报告基因活性。当用LPS刺激这些细胞，然后依次加入可溶性MD-2时，即使在加入sMD-2之前通过洗涤细胞在形成CD14-TLR4复合物后去除过量LPS，也观察到NF-κB的强烈激活。基于这些结果，我们提出了一种额外的LPS识别机制。在表达TLR4和CD14但缺乏MD-2的细胞中，LPS首先在LPS结合蛋白的帮助下转移到膜CD14上，这导致TLR4-CD14复合物的形成。然后，可溶性MD-2与该复合物的结合触发跨膜信号转导。表达TLR4和CD14但缺乏MD-2的细胞，如气道上皮细胞，可能通过这种机制对LPS作出反应而被激活。
