van den Dobbelsteen M E, van der Woude F J, Schroeijers W E, Klar-Mohamad N, van Es L A, Daha M R
University Hospital Leiden, Dept. of Nephrology, The Netherlands.
J Immunol. 1993 Oct 15;151(8):4315-24.
Previous reports have shown the presence of C1Q-R on monocytes, macrophages, polymorphonuclear cells, fibroblasts, platelets, lymphocytes, and endothelial cells. The present study demonstrates a functional C1Q-R on rat renal mesangial cells (MC). Incubation of MC with increasing concentrations of [125I]C1Q resulted in a dose-dependent binding of [125I]C1Q to MC; the binding of [125I]C1Q was inhibitable by excess unlabeled C1Q or C1Q stalks whereas BSA and C1Q globular heads had no effect. Scatchard analysis of the data revealed the presence of 6.2 x 10(7) binding sites/cell with an affinity of 4.9 x 10(6) M-1 for C1Q. Immunoprecipitation of 125I-labeled MC membrane proteins with C1Q or mAb directed against human C1Q-R revealed a single 66- to 68-kDa band under reducing conditions. We have shown previously that soluble stable aggregates of IgG bind to rat MC in a dose-dependent fashion. In addition the presence of a receptor for IgG has been described on rat MC. In order to find out whether there is a cooperative effect between C1Q and AlgG in binding of [125I]AlgG to MC, we incubated [125I]AlgG in the presence of increasing concentrations of C1Q, and showed a 5- to 15-fold enhancement of binding of [125I]AlgG to MC. Neither heat-inactivated C1Q nor C1Q stalks were able to enhance the binding of [125I]AlgG to MC. Enhanced binding by C1Q was only observed when aggregated IgG was used; the binding of monomeric IgG to MC was not affected by C1Q. These studies indicate that there is a cooperative effect between Fc gamma R and C1Q-R on MC in the recognition of immune complexes.
先前的报告显示，单核细胞、巨噬细胞、多形核细胞、成纤维细胞、血小板、淋巴细胞和内皮细胞上存在C1Q-R。本研究证明大鼠肾系膜细胞（MC）上存在功能性C1Q-R。用浓度递增的[125I]C1Q孵育MC，导致[125I]C1Q与MC呈剂量依赖性结合；[125I]C1Q的结合可被过量未标记的C1Q或C1Q柄抑制，而牛血清白蛋白和C1Q球状头部则无影响。对数据进行Scatchard分析显示，每个细胞存在6.2×10⁷个结合位点，对C1Q的亲和力为4.9×10⁶ M⁻¹。用C1Q或针对人C1Q-R的单克隆抗体对125I标记的MC膜蛋白进行免疫沉淀，在还原条件下显示出一条单一的66至68 kDa条带。我们先前已表明，IgG的可溶性稳定聚集体以剂量依赖性方式与大鼠MC结合。此外，已报道大鼠MC上存在IgG受体。为了弄清楚C1Q和IgG在[125I]IgG与MC结合中是否存在协同作用,我们在浓度递增的C1Q存在下孵育[125I]IgG，结果显示[125I]IgG与MC的结合增强了5至15倍。热灭活的C1Q和C1Q柄均不能增强[125I]IgG与MC的结合。仅当使用聚集的IgG时才观察到C1Q增强结合；单体IgG与MC的结合不受C1Q影响。这些研究表明，在免疫复合物的识别中，MC上的FcγR和C1Q-R之间存在协同作用。
