Division of Infectious Genetics, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, 4-6-1 Shirokanedai, Minatoku, Tokyo 108-8639, Japan.
Int Immunol. 2011 Aug;23(8):503-10. doi: 10.1093/intimm/dxr044. Epub 2011 Jun 28.
Toll-like receptor (TLR)4/MD-2, a sensor for LPS, delivers the MyD88-dependent signal from the cell surface, then traffics to endolysosomes and delivers the TRIF/TICAM-1-dependent signal. Both signals are thought to be dependent on cell surface TLR4/MD-2. Although TLR4/MD-2 is located also in recycling endosomes, the Golgi apparatus or the endoplasmic reticulum, little is known about a role for intracellular TLR4/MD-2 in LPS responses. We here studied intracellular LPS sensing in macrophages. PRAT4A (protein associated with TLR4 A) is a cochaperone for a general chaperone gp96 and required for cell surface expression of TLR4/MD-2. Cell surface TLR4/MD-2 was undetectable on PRAT4A(-/-) thioglycollate-elicited peritoneal macrophages (P-Macs) and bone marrow-derived macrophages (BM-Macs). LPS responses were all abolished in PRAT4A(-/-) P-Macs, whereas a part of LPS responses remained detectable in PRAT4A(-/-) BM-Macs. Of note, LPS responses in PRAT4A(-/-) BM-Macs were not necessarily dependent on TRIF/TICAM-1 signaling. PRAT4A(-/-) BM-Macs showed unimpaired production of both TRIF/TICAM-1-dependent chemokine RANTES (CCL5) and MyD88-dependent chemokine MCP-1 (CCL2). Moreover, up-regulation of co-stimulatory molecules, CD40 and CD86 was not altered. In contrast, TRIF/TICAM-1-dependent production of type I IFN was profoundly impaired. In response to heat-killed bacteria Escherichia coli, BM-Macs also required PRAT4A-independent TLR4/MD-2 for production of MCP-1 (CCL2) and RANTES (CCL5) and for up-regulation of CD40 and CD86, indicating that intracellular TLR4/MD-2 is able to sense phagocytosed bacteria and activate immune responses. These results demonstrate that intracellular TLR4/MD-2 is responsible for unique set of LPS responses.
Toll 样受体 (TLR)4/MD-2 是 LPS 的传感器，从细胞表面传递 MyD88 依赖性信号，然后转运到内溶酶体并传递 TRIF/TICAM-1 依赖性信号。这两种信号都被认为依赖于细胞表面的 TLR4/MD-2。尽管 TLR4/MD-2 也位于再循环内体、高尔基体或内质网中，但关于细胞内 TLR4/MD-2 在 LPS 反应中的作用知之甚少。我们在这里研究了巨噬细胞中的细胞内 LPS 感应。PRAT4A（与 TLR4A 相关的蛋白）是一种通用伴侣蛋白 gp96 的共伴侣，并且是 TLR4/MD-2 细胞表面表达所必需的。PRAT4A(-/-) 巯基乙醇酸盐诱导的腹腔巨噬细胞 (P-Mac) 和骨髓来源的巨噬细胞 (BM-Mac) 中无法检测到细胞表面 TLR4/MD-2。PRAT4A(-/-) P-Mac 中的 LPS 反应均被完全消除，而 PRAT4A(-/-) BM-Mac 中仍可检测到部分 LPS 反应。值得注意的是，PRAT4A(-/-) BM-Mac 中的 LPS 反应不一定依赖于 TRIF/TICAM-1 信号。PRAT4A(-/-) BM-Mac 显示 TRIF/TICAM-1 依赖性趋化因子 RANTES（CCL5）和 MyD88 依赖性趋化因子 MCP-1（CCL2）的产生均未受影响。此外，共刺激分子 CD40 和 CD86 的上调没有改变。相比之下，TRIF/TICAM-1 依赖性 I 型 IFN 的产生受到严重损害。对热灭活细菌大肠杆菌的反应中，BM-Mac 还需要 PRAT4A 非依赖性 TLR4/MD-2 来产生 MCP-1（CCL2）和 RANTES（CCL5）以及上调 CD40 和 CD86，表明细胞内 TLR4/MD-2 能够感应吞噬的细菌并激活免疫反应。这些结果表明，细胞内 TLR4/MD-2 负责一组独特的 LPS 反应。
