Division of Innate Immunity, The Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Tokyo 108-8639, Japan.
Int Immunol. 2013 Jan;25(1):45-52. doi: 10.1093/intimm/dxs084. Epub 2012 Sep 7.
A cell surface heterodimer Toll-like receptor 4 (TLR4)/MD-2 senses lipopolysaccharide (LPS), a principal membrane component of Gram-negative bacteria. LPS binds to MD-2 and induces dimerization of TLR4/MD-2. Dimerized TLR4 activates downstream signaling. TLR4 polymorphism replacing Asp299 with Gly and Thr399 with Ile (D299G/T399I) causes LPS hyporesponsiveness, and is associated with a variety of infectious and noninfectious diseases. However, a molecular mechanism underlying the LPS hyporesponsiveness remains controversial. We here asked whether the TLR4 polymorphism influenced cell surface expression of TLR4/MD-2, ligand-dependent TLR4/MD-2 dimerization or TLR4/MD-2 responses to a weak agonist monophosphoryl lipid A (MPL). A newly established anti-TLR4 mAb detected D299G/T399I TLR4/MD-2 on Ba/F3 cells, whereas a previous anti-TLR4 mAb did will this fit on the line above?, suggesting that the D299G/T399I polymorphism caused a conformational change in TLR4. Hyporesponsiveness of D299G/T399I TLR4/MD-2 was much more apparent when cells were stimulated with MPL than with lipid A. MPL-dependent TLR4/MD-2 dimerization was impaired by the D299G/T399I polymorphism. The D299G/T399I polymorphism did not alter LPS-binding to soluble TLR4/MD-2, but impaired its dimerization. These results suggest that the D299G/T399I TLR4 polymorphism impairs TLR4/MD-2 responses by altering ligand-dependent dimerization.
细胞表面异二聚体 Toll 样受体 4（TLR4）/MD-2 可识别脂多糖（LPS），LPS 是革兰氏阴性菌主要的膜成分。LPS 与 MD-2 结合并诱导 TLR4/MD-2 二聚化。二聚化的 TLR4 激活下游信号。TLR4 多态性将天冬氨酸 299 替换为甘氨酸和苏氨酸 399 替换为异亮氨酸（D299G/T399I）导致 LPS 低反应性，并与多种感染和非感染性疾病相关。然而，导致 LPS 低反应性的分子机制仍存在争议。我们在这里询问 TLR4 多态性是否影响 TLR4/MD-2 的细胞表面表达、配体依赖性 TLR4/MD-2 二聚化或 TLR4/MD-2 对弱激动剂单磷酰脂质 A（MPL）的反应。新建立的抗 TLR4 mAb 可检测到 Ba/F3 细胞上的 D299G/T399I TLR4/MD-2，而之前的抗 TLR4 mAb 则不能，这表明 D299G/T399I 多态性导致 TLR4 构象发生变化。与脂质 A 相比，MPL 刺激时 D299G/T399I TLR4/MD-2 的低反应性更为明显。D299G/T399I 多态性损害了 MPL 依赖性 TLR4/MD-2 二聚化。D299G/T399I 多态性不改变可溶性 TLR4/MD-2 与 LPS 的结合，但损害其二聚化。这些结果表明，D299G/T399I TLR4 多态性通过改变配体依赖性二聚化来损害 TLR4/MD-2 反应。
