Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibody’s constant (Fc) domain recruits complement component C1 to initiate a proteolytic cascade that generates lytic pores and stimulates phagocytosis. The C1 complex (C1qr₂s₂) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r₂s₂). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various 2,4-dinitrophenol [DNP]-coated surfaces and pathogenic Staphylococcus aureus). The extent to which C1r₂s₂ contributes to C1q–IgG stability strongly differs between human IgG subclasses. Using antibody engineering of monoclonal IgG, we reveal that hexamer-enhancing mutations improve C1q–IgG stability, both in the absence and presence of C1r₂s₂. In addition, hexamer-enhanced IgGs targeting S. aureus mediate improved complement-dependent phagocytosis by human neutrophils. Altogether, these molecular insights into complement binding to surface-bound IgGs could be important for optimal design of antibody therapies.

补体是抗体介导的感染和肿瘤细胞清除的重要效应机制。与靶细胞结合后，抗体的恒定 (Fc) 结构域会募集补体成分 C1 以启动蛋白水解级联反应，从而产生裂解孔并刺激吞噬作用。C1 复合物 (C1qr ₂ s ₂ ) 由大识别蛋白 C1q 和蛋白酶 C1r 和 C1s (C1r ₂ s ₂ )的异源四聚体组成。虽然 C1 和 IgG-Fc 之间的相互作用被认为是由 C1q 的球状头介导的，但我们在这里发现 C1r ₂ s ₂蛋白酶影响 C1q 与表面结合的 IgG 分子（在各种 2,4-二硝基苯酚 [DNP] 涂层表面和致病性金黄色葡萄球菌上）形成强烈复合物的能力。C1r ₂ s ₂对 C1q–IgG 稳定性的贡献程度在人类 IgG 亚类之间存在很大差异。使用单克隆 IgG 的抗体工程，我们揭示了在 C1r ₂ s ₂不存在和存在的情况下，六聚体增强突变提高了 C1q–IgG 稳定性。此外，针对金黄色葡萄球菌的六聚体增强型 IgG介导人中性粒细胞改善的补体依赖性吞噬作用。总而言之，这些关于补体与表面结合 IgG 结合的分子见解对于抗体疗法的优化设计可能很重要。