Vassal-Stermann Emilie, Lacroix Monique, Gout Evelyne, Laffly Emmanuelle, Pedersen Christian M, Martin Lydie, Amoroso Ana, Schmidt Richard R, Zähringer Ulrich, Gaboriaud Christine, Di Guilmi Anne-Marie, Thielens Nicole M
University of Grenoble Alpes, Institut de Biologie Structurale, F-38044 Grenoble, France; Centre National de la Recherche Scientifique, Institut de Biologie Structurale, F-38044 Grenoble, France; Commissariat à l'Energie Atomique et aux Energies Alternatives, Institut de Biologie Structurale, F-38044 Grenoble, France;
Department of Chemistry, University of Copenhagen, 21000 Copenhagen, Denmark;
J Immunol. 2014 Dec 1;193(11):5699-708. doi: 10.4049/jimmunol.1400127. Epub 2014 Oct 24.
Human L-ficolin is a soluble protein of the innate immune system able to sense pathogens through its fibrinogen (FBG) recognition domains and to trigger activation of the lectin complement pathway through associated serine proteases. L-Ficolin has been previously shown to recognize pneumococcal clinical isolates, but its ligands and especially its molecular specificity remain to be identified. Using solid-phase binding assays, serum and recombinant L-ficolins were shown to interact with serotype 2 pneumococcal strain D39 and its unencapsulated R6 derivative. Incubation of both strains with serum triggered complement activation, as measured by C4b and C3b deposition, which was decreased by using ficolin-depleted serum. Recombinant L-ficolin and its FBG-like recognition domain bound to isolated pneumococcal cell wall extracts, whereas binding to cell walls depleted of teichoic acid (TA) was decreased. Both proteins were also shown to interact with two synthetic TA compounds, each comprising part structures of the complete lipoteichoic acid molecule with two PCho residues. Competition studies and direct interaction measurements by surface plasmon resonance identified PCho as a novel L-ficolin ligand. Structural analysis of complexes of the FBG domain of L-ficolin and PCho revealed that the phosphate moiety interacts with amino acids previously shown to define an acetyl binding site. Consequently, binding of L-ficolin to immobilized acetylated BSA was inhibited by PCho and synthetic TA. Binding of serum L-ficolin to immobilized synthetic TA and PCho-conjugated BSA triggered activation of the lectin complement pathway, thus further supporting the hypothesis of L-ficolin involvement in host antipneumococcal defense.
人L-纤维胶凝蛋白是先天性免疫系统的一种可溶性蛋白，能够通过其纤维蛋白原（FBG）识别结构域感知病原体，并通过相关的丝氨酸蛋白酶触发凝集素补体途径的激活。先前已证明L-纤维胶凝蛋白可识别肺炎球菌临床分离株，但其配体，尤其是其分子特异性仍有待确定。使用固相结合试验，血清和重组L-纤维胶凝蛋白显示与2型肺炎球菌菌株D39及其无荚膜R6衍生物相互作用。用血清孵育这两种菌株会触发补体激活，通过C4b和C3b沉积来测量，而使用缺乏纤维胶凝蛋白的血清时这种激活会减少。重组L-纤维胶凝蛋白及其FBG样识别结构域与分离的肺炎球菌细胞壁提取物结合，而与去除磷壁酸（TA）的细胞壁的结合减少。这两种蛋白还显示与两种合成TA化合物相互作用，每种化合物都包含具有两个磷酸胆碱（PCho）残基的完整脂磷壁酸分子的部分结构。通过表面等离子体共振进行的竞争研究和直接相互作用测量确定PCho为一种新型的L-纤维胶凝蛋白配体。L-纤维胶凝蛋白的FBG结构域与PCho复合物的结构分析表明，磷酸部分与先前已证明可定义乙酰结合位点的氨基酸相互作用。因此，PCho和合成TA可抑制L-纤维胶凝蛋白与固定化乙酰化牛血清白蛋白的结合。血清L-纤维胶凝蛋白与固定化合成TA和PCho偶联牛血清白蛋白的结合触发了凝集素补体途径的激活，从而进一步支持了L-纤维胶凝蛋白参与宿主抗肺炎球菌防御的假说。
