Department of Immunology, Institute for Cell Biology, University of Tübingen, Tübingen, Germany.
Eur J Immunol. 2013 Aug;43(8):2101-13. doi: 10.1002/eji.201142143.
Polymorphonuclear leukocytes (PMNs) represent one of the first lines of defense against pathogens. TLR9 is normally expressed in endosomes/lysosomes where it is activated by pathogen-derived DNA. Here we show that freshly isolated human and mouse primary PMNs express TLR9 at the cell surface ex vivo. Moreover, surface TLR9 expression is upregulated upon activation of PMNs with different stimuli and not only TLR9 agonists. Importantly, surface TLR9 is processed, active, and functional. TLR9 ligands, oligo-nucleotides containing unmethylated CpG motifs, indeed bind to surface TLR9 and binding was strongly observed at the cell surface of human cells expressing surface TLR9 and at the surface of WT but not TLR9-deficient mouse PMNs. Finally, CpG oligonucleotides cross-linked onto a solid phase and having no access to intracellular TLR9 are able to trigger cell surface TLR9 and induce neutrophil activation, even when endosomal acidification is inhibited. This is the first demonstration of a functional TLR9 expressed at the cell surface of human primary cells. This pathway may be triggered when pathogen-derived TLR9 ligands cannot reach the endosome, offering a rescue mechanism for neutrophil activation.
多形核白细胞（PMN）是抵御病原体的第一道防线之一。TLR9 通常在内涵体/溶酶体中表达，在那里它被病原体衍生的 DNA 激活。在这里，我们表明，新鲜分离的人和小鼠原代 PMN 在体外表达 TLR9 于细胞表面。此外，PMN 被不同的刺激激活后，表面 TLR9 表达上调，而不仅仅是 TLR9 激动剂。重要的是，表面 TLR9 被加工、激活和功能化。TLR9 配体，即含有未甲基化 CpG 基序的寡核苷酸，确实与表面 TLR9 结合，并且在表达表面 TLR9 的人细胞表面和 WT 细胞表面上强烈观察到结合，但在 TLR9 缺陷型小鼠 PMN 表面上则没有。最后，CpG 寡核苷酸交联到固相上，并且没有进入细胞内 TLR9 的途径，能够触发细胞表面 TLR9 并诱导中性粒细胞活化，即使抑制内涵体酸化也是如此。这是在人原代细胞表面表达功能性 TLR9 的第一个证明。当病原体衍生的 TLR9 配体无法到达内涵体时，可能会触发该途径，为中性粒细胞活化提供一种挽救机制。
