Institute of Infection & Immunity, School of Medicine, Cardiff University, Cardiff CF14 4XN, United Kingdom;
J Immunol. 2014 May 15;192(10):4844-51. doi: 10.4049/jimmunol.1303131. Epub 2014 Apr 11.
Dysregulation of the complement alternative pathway can cause disease in various organs that may be life-threatening. Severe alternative pathway dysregulation can be triggered by autoantibodies to the C3 convertase, termed nephritic factors, which cause pathological stabilization of the convertase enzyme and confer resistance to innate control mechanisms; unregulated complement consumption followed by deposition of C3 fragments in tissues ensues. The mAb, 3E7, and its humanized derivative, H17, have been shown previously to specifically bind activated C3 and prevent binding of both the activating protein, factor B, and the inhibitor, factor H, which are opposite effects that complicate its potential for therapy. Using ligand binding assays, functional assays, and electron microscopy, we show that these Abs bind C3b via a site that overlaps the binding site on C3 for the Ba domain within factor B, thereby blocking an interaction essential for convertase formation. Both Abs also bind the preformed convertase, C3bBb, and provide powerful inhibition of complement activation by preventing cleavage of C3. Critically, the Abs also bound and inhibited C3 cleavage by the nephritic factor-stabilized convertase. We suggest that by preventing enzyme formation and/or cleavage of C3 to its active downstream fragments, H17 may be an effective therapy for conditions caused by severe dysregulation of the C3 convertase and, in particular, those that involve nephritic factors, such as dense deposit disease.
补体替代途径的失调可能导致各种危及生命的器官疾病。严重的替代途径失调可能由针对 C3 转化酶的自身抗体（称为肾炎因子）触发，这些抗体导致转化酶酶的病理性稳定，并对先天控制机制产生抗性；随后不受调节的补体消耗以及 C3 片段在组织中的沉积。先前已经表明，mAb 3E7 及其人源化衍生物 H17 特异性结合活化的 C3，并阻止激活蛋白因子 B 和抑制剂因子 H 的结合，这是相反的作用，这使其治疗潜力变得复杂。通过配体结合测定、功能测定和电子显微镜，我们表明这些抗体通过与因子 B 内 Ba 结构域结合 C3 的位点结合 C3b，从而阻断转化酶形成所必需的相互作用。两种抗体也结合预形成的转化酶 C3bBb，并通过防止 C3 的裂解来提供对补体激活的有力抑制。至关重要的是，抗体还结合并抑制肾炎因子稳定的转化酶切割 C3。我们认为，通过防止酶形成和/或 C3 裂解为其活性下游片段，H17 可能是治疗 C3 转化酶严重失调引起的疾病的有效疗法，特别是涉及肾炎因子的疾病，例如致密沉积物病。
