Muñoz E, Vidarte L, Casado M T, Pastor C, Vivanco F
Department of Immunology, Fundación Jiménez Díaz, Madrid, Spain.
Int Immunol. 1998 Feb;10(2):97-106. doi: 10.1093/intimm/10.2.97.
The covalent binding of C3 to antigen-antibody complexes [immune complexes (IC)] plays a pivotal role in the elimination of antigens. C3 prevents the formation of large IC lattices promoting their solubilization. Subsequently, bound C3 fragments determine the efficacy of antigen presentation, and the generation of antibody responses and immunological memory. C3 binding to IgG-IC generates IgG-C3b-C3b complexes which are detected by SDS-PAGE as two major bands: C3alpha65-heavy chain and C3alpha65-C3alpha43 covalent complexes. Using human heat-aggregated IgG1 as a model of IC, a C3b binding site was localized only in the Cgamma1 domain. However, with true IC of ovalbumin and rabbit IgG anti-ovalbumin, C3b binds to both the Fab and Fc regions of IgG. To study the binding of C3b to the different domains of IgG and particularly to evaluate the involvement of the Cgamma1 domain, we have constructed recombinant single-chain antibodies without Cgamma1, which have the structure: V(H)-linker-V(L)-hinge-Cgamma2-Cgamma3 (scAb). The variable domains were from a mouse mAb anti-HSA and the constant region (hinge-C(H)2-C(H)3) from human IgG1 or rabbit IgG. C3 binds very efficiently to IC formed with human (h-scAb) or rabbit (r-scAb) recombinant antibodies (scAb-HSA) and generates also two bands on SDS-PAGE (C3alpha65-scAb and C3alpha65-C3alpha43), which are the counterparts of those of the complete antibody. In addition, IC formed with scAb activate the alternative pathway to a similar extent as IC of the entire IgG. These data indicate that the Cgamma1 domain is a dispensable region for C3b binding and that the remaining constant domains are as efficient as Cgamma1 in C3b binding. Overall these results support the view that C3 does not specifically recognize a unique site in the Cgamma1 domain. Rather it seems to be able to attach along the antibody molecule. Probably this implies an advantage for effective processing of C3b-IC and elimination of antigens in vivo.
补体3（C3）与抗原 - 抗体复合物[免疫复合物（IC）]的共价结合在抗原清除过程中起关键作用。C3可防止大的IC晶格形成，促进其溶解。随后，结合的C3片段决定了抗原呈递、抗体反应产生及免疫记忆的效果。C3与IgG - IC结合产生IgG - C3b - C3b复合物，通过SDS - PAGE检测为两条主要条带：C3α65 - 重链和C3α65 - C3α43共价复合物。以人热聚集IgG1作为IC模型时，C3b结合位点仅定位于Cγ1结构域。然而，对于卵清蛋白与兔抗卵清蛋白IgG形成的真实IC，C3b可结合至IgG的Fab和Fc区域。为研究C3b与IgG不同结构域的结合，特别是评估Cγ1结构域的作用，我们构建了不含Cγ1的重组单链抗体，其结构为：V(H)-连接子-V(L)-铰链区-Cγ2-Cγ3（scAb）。可变结构域来自小鼠抗人血清白蛋白单克隆抗体，恒定区（铰链区-C(H)2-C(H)3）来自人IgG1或兔IgG。C3能非常有效地与由人（h - scAb）或兔（r - scAb）重组抗体（scAb - HSA）形成的IC结合，并且在SDS - PAGE上也产生两条条带（C3α65 - scAb和C3α65 - C3α43），它们与完整抗体产生的条带相对应。此外，由scAb形成的IC激活替代途径的程度与整个IgG的IC相似。这些数据表明，Cγ1结构域对于C3b结合而言是一个可有可无的区域，并且其余恒定结构域在C3b结合方面与Cγ1一样有效。总体而言，这些结果支持这样的观点，即C3并非特异性识别Cγ1结构域中的独特位点。相反，它似乎能够沿着抗体分子附着。这可能意味着在体内有效处理C3b - IC和清除抗原方面具有优势。
