近期，Cell Research在线报道了上海生科院生化与细胞所丁建平组关于单克隆抗体药物daclizumab抑制人白细胞介素2(IL-2)信号通路的研究成果。近年来丁建平组对一些重要治疗性抗体和抗原复合物结构和功能展开了系统性的研究，已发表一系列文章，特别是于今年初已揭示了治疗性抗体basiliximab抑制IL-2信号通路的分子机制。该项工作是这些研究工作的延续。
IL-2是免疫应答过程中发挥重要作用的细胞因子，其受体由α、β和γ三个亚基组成，其中α亚基(IL-2Rα)是IL-2的特异性受体。研究表明，IL-2Rα在参与器官移植排斥反应、某些自身免疫性疾病、以及T细胞白血病等病理过程的T细胞表面上高表达，因此是很好的药物靶标。抗IL-2Rα抗体可以抑制其与IL-2的结合，阻断IL-2信号通路，从而抑制T细胞的活化和增殖，减少器官移植后的免疫排斥反应。单克隆抗体药物daclizumab是第一株通过美国FDA认证的治疗性抗体，主要应用于器官移植，尤其是肾脏移植的免疫抑制。丁建平组的博士生杨荟和王建船等人解析了daclizumab的Fab片段与IL-2Rα胞外区的复合物的晶体结构，分析了抗原-抗体之间的相互作用。进一步通过对复合物的结构分析和与已报道的生化数据的比较，鉴定了daclizumab的抗原决定表位。结构分析表明，Fab片段的CDR环区主要通过一个带正电的表面及其两侧两个相对疏水的区域与IL-2Rα两个“shushi-like”结构域D1和D2发生较强的亲水和疏水相互作用，因此daclizumab对IL-2Rα的识别具有高亲和力和高特异性。进一步与已报道的IL-2/IL-2Rα及IL-2/IL-2Rαβγc复合物的结构比较，发现daclizumab的抗原表位与IL-2识别IL-2Raα区域有很大程度上的重叠，而且和IL-2相比daclizumab与IL-2Rα具有更强的相互作用，因此可以竞争性地结合IL-2Rα，从而阻断IL-2与IL-2Rα的结合而抑制IL-2信号通路的激活。这些研究结果在分子水平上揭示了daclizumab抑制IL-2信号通路的分子机制，并合理地解释了已有的生物化学和免疫学数据。基于研究结果，研究者们提出了通过对daclizumab进行定点突变以研发具有更高特异性和更强亲和力的抗体药物的策略。这项研究成果对于抗IL-2Rα的抗体药物的改造和新药物的研发具有重要的指导意义。
该项工作得到国家科技部、国家自然科学基金委、中科院和上海市科委的经费支持。
生物谷推荐英文摘要：
Cell Research , (7 September 2010) | doi:10.1038/cr.2010.130
Structural basis of immunosuppression by the therapeutic antibody daclizumab
Hui Yang, Jianchuan Wang, Jiamu Du, Chen Zhong, Dapeng Zhang, Huaizu Guo, Yajun Guo and Jianping Ding
Interleukin-2 (IL)-2 signaling plays a pivotal role in the activation of immune responses, and drugs that block this pathway have been shown to be effective for the immunosuppression in patients with organ transplantation to alleviate/eliminate allograft rejection. The first humanized monoclonal antibody (mAb) daclizumab falls into this category and shows high specificity and affinity against a key component of the IL-2 receptor complex, namely IL-2Rα. To reveal the molecular mechanism of the inhibition of the IL-2 signaling pathway by daclizumab, we determined the crystal structures of the daclizumab Fab in free form and in complex with the IL-2Rα ectodomain at 2.6 and 2.8 ? resolution, respectively. The daclizumab Fab adopts a similar conformation in the presence or absence of the IL-2Rα ectodomain. The antigen-binding site of daclizumab is mainly composed of five complementarity determining regions (CDRs) that form a large positively charged surface depression and two flanking patches that are generally hydrophobic. The conformational epitope consists of several discontinuous segments of the IL-2Rα ectodomain, a large portion of which overlaps with the regions that interact with IL-2, suggesting that the binding of daclizumab to IL-2Rα would prevent the IL-2 binding to IL-2Rα and the subsequent formation of the IL-2/IL-2Rαβγc complex, and therefore block the IL-2 signaling pathway. These results also have implications for the design and development of improved mAb drugs targeting IL-2Rα.
