Li Huiqing, Mo Kai-For, Wang Qun, Stover C Kendall, DiGiandomenico Antonio, Boons Geert-Jan
Complex Carbohydrate Research Center, The University of Georgia, Athens, GA 30602 (USA).
Chemistry. 2013 Dec 16;19(51):17425-31. doi: 10.1002/chem.201302916. Epub 2013 Nov 18.
Pseudomonas aeruginosa is an opportunistic Gram-negative bacterium that can cause life-threatening infections in critically ill and cystic fibrosis patients. The Psl exopolysaccharide of P. aeruginosa offers an attractive serotype-independent antigen for the development of immunotherapies. Here, the first chemical synthesis of a panel of oligosaccharides derived from the exopolysaccharide of P. aeruginosa by a synthetic strategy that efficiently deals with the stereoselective installation of several β-mannosides and the formation of a mannoside that is extended by saccharide moieties at C-1, C-2, and C-3 in a crowded 1,2,3-cis configuration is described. The approach was employed to prepare tetra-, penta-, and hexa- and decasaccharide part structures. The compounds were employed to define the epitope requirements of several functionally active monoclonal antibodies (mAbs) that can bind three distinct epitopes of Psl (class I, II, and III). The class II mAb reacted potently with each oligosaccharide indicating its epitope resides within the tetrasaccharide and does not require the branched mannoside of Psl. The class III antibody did not bind the tetra- or pentasaccharide; however, it did react potently with the hexasaccharide and weakly with the decasaccharide, suggesting a terminal glucoside is required for optimal binding. Unexpectedly, the class I mAb did not bind any of the oligosaccharides indicating that Psl contains a yet to be elucidated sub-stoichiometric isoform. This study demonstrates that functional activity of a mAb does not only depend on the avidity of binding but also on the location of an epitope within a bacterial polysaccharide. The results also provide a strong impetus to analyze further the structure of Psl to identify the class I epitope, that is expected to provide an attractive target for the development of a synthetic vaccine for P. aeruginosa.
铜绿假单胞菌是一种机会性革兰氏阴性菌，可在重症患者和囊性纤维化患者中引起危及生命的感染。铜绿假单胞菌的Psl胞外多糖为免疫疗法的开发提供了一种有吸引力的血清型非依赖性抗原。在此，描述了通过一种合成策略首次化学合成一组源自铜绿假单胞菌胞外多糖的寡糖，该策略有效地处理了几个β-甘露糖苷的立体选择性安装以及在拥挤的1,2,3-顺式构型中在C-1、C-2和C-3处由糖部分延伸的甘露糖苷的形成。该方法用于制备四糖、五糖、六糖和十糖部分结构。这些化合物用于确定几种功能性活性单克隆抗体（mAb）的表位要求，这些单克隆抗体可以结合Psl的三个不同表位（I类、II类和III类）。II类mAb与每种寡糖都有强烈反应，表明其表位位于四糖内，不需要Psl的分支甘露糖苷。III类抗体不结合四糖或五糖；然而，它与六糖有强烈反应，与十糖反应较弱，表明最佳结合需要末端葡萄糖苷。出乎意料的是，I类mAb不结合任何寡糖，表明Psl含有一种尚未阐明的亚化学计量异构体。这项研究表明，单克隆抗体的功能活性不仅取决于结合亲和力，还取决于表位在细菌多糖中的位置。这些结果也为进一步分析Psl的结构以鉴定I类表位提供了强大动力，预计该表位将为开发铜绿假单胞菌合成疫苗提供有吸引力的靶点。
