荷兰科学家最近成功“冻结”进行中的免疫反应，用于免疫学研究。

　　补体系统是最古老的机体抵御异物的免疫反应之一。补体系统是天然免疫反应，没有特异性，利用一连串的蛋白质反应消灭病原体和细胞。自身免疫系统变态反应也通常与补体系统有关。

　　在进行一个连续反应体系研究时，我们需要查明每步反应的详细情况才能把握整个反应体系。科学家最新发明了一种金黄葡萄球菌补体抑制蛋白(SCIN)的方法可以使反应体系停滞在一步特定反应。通过冻结免疫反应，研究人员可以研究反应体系中蛋白质复合体的结构，蛋白质间如何结合，每一步反应在反应体系中的作用等。

　　通过利用SCIN，研究人员发现了正常细胞不会受到补体系统攻击的机制。H因子是保证补体系统不攻击正常细胞的重要因素。通过分析研究人员发现，H因子可与正常细胞结合使补体系统不被激活，从而使正常细胞不受补体系统攻击。

　　生物谷推荐原始出处：

　　Nature Immunology 10, 721 - 727 (2009)  7 June 2009 | doi:10.1038/ni.1756

　　Structural and functional implications of the alternative complement pathway C3 convertase stabilized by a staphylococcal inhibitor

　　Suzan H M Rooijakkers1,5, Jin Wu2,5, Maartje Ruyken1, Robert van Domselaar1, Karel L Planken3, Apostolia Tzekou4, Daniel Ricklin4, John D Lambris4, Bert J C Janssen2, Jos A G van Strijp1 & Piet Gros2

　　Abstract

　　Activation of the complement system generates potent chemoattractants and leads to the opsonization of cells for immune clearance. Short-lived protease complexes cleave complement component C3 into anaphylatoxin C3a and opsonin C3b. Here we report the crystal structure of the C3 convertase formed by C3b and the protease fragment Bb, which was stabilized by the bacterial immune-evasion protein SCIN. The data suggest that the proteolytic specificity and activity depend on the formation of dimers of C3 with C3b of the convertase. SCIN blocked the formation of a productive enzyme-substrate complex. Irreversible dissociation of the complex of C3b and Bb is crucial to complement regulation and was determined by slow binding kinetics of the Mg2+-adhesion site in Bb. Understanding the mechanistic basis of the central complement-activation step and microbial immune evasion strategies targeting this step will aid in the development of complement therapeutics.

　　1 Medical Microbiology, University Medical Center Utrecht, Utrecht, The Netherlands.

　　2 Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Department of Chemistry, Faculty of Science, Utrecht University, Utrecht, The Netherlands.

　　3 Van 't Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, Utrecht, The Netherlands.

　　4 Department of Pathology & Laboratory Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.

　　5 These authors contributed equally to this work.