Department of Pediatric Nephrology, Radboud Institute for Molecular Life Sciences, Amalia Children's Hospital, Radboud University Medical Center, Nijmegen, Netherlands.
Department of Laboratory Medicine, Radboud University Medical Center, Nijmegen, Netherlands.
Front Immunol. 2021 Aug 11;12:715704. doi: 10.3389/fimmu.2021.715704. eCollection 2021.
The rare and heterogeneous kidney disorder C3 glomerulopathy (C3G) is characterized by dysregulation of the alternative pathway (AP) of the complement system. C3G is often associated with autoantibodies stabilizing the AP C3 convertase named C3 nephritic factors (C3NeF). The role of classical pathway (CP) convertase stabilization in C3G and related diseases such as immune complex-mediated membranoproliferative glomerulonephritis (IC-MPGN) remains largely unknown. Here, we investigated the CP convertase activity in patients with C3G and IC-MPGN. Using a refined two-step hemolytic assay, we measured the stability of CP convertases directly in the serum of 52 patients and 17 healthy controls. In four patients, CP convertase activity was prolonged compared to healthy controls, i.e. the enzymatic complex was stabilized. In three patients (2 C3G, 1 IC-MPGN) the convertase stabilization was caused by immunoglobulins, indicating the presence of autoantibodies named C4 nephritic factors (C4NeFs). Importantly, the assay also enabled detection of non-immunoglobulin-mediated stabilization of the CP convertase in one patient with C3G. Prolonged CP convertase activity coincided with C3NeF activity in all patients and for up to 70 months of observation. Crucially, experiments with C3-depleted serum showed that C4NeFs stabilized the CP C3 convertase (C4bC2a), that does not contain C3NeF epitopes. All patients with prolonged CP convertase activity showed clear signs of complement activation, i.e. lowered C3 and C5 levels and elevated levels of C3d, C3bc, C3bBbP, and C5b-9. In conclusion, this work provides new insights into the diverse aspects and (non-)immunoglobulin nature of factors causing CP convertase overactivity in C3G/IC-MPGN.
补体系统替代途径（AP）失调是导致罕见且异质性的肾脏疾病 C3 肾小球病（C3G）的特征。C3G 常与稳定 AP C3 转化酶的自身抗体有关，这种转化酶被称为 C3 肾炎因子（C3NeF）。经典途径（CP）转化酶稳定在 C3G 和相关疾病（如免疫复合物介导的膜增生性肾小球肾炎（IC-MPGN））中的作用在很大程度上仍然未知。在这里，我们研究了 C3G 和 IC-MPGN 患者的 CP 转化酶活性。使用改良的两步溶血测定法，我们直接测量了 52 名患者和 17 名健康对照者血清中的 CP 转化酶稳定性。在 4 名患者中，与健康对照组相比，CP 转化酶活性延长，即酶复合物被稳定。在 3 名患者（2 名 C3G，1 名 IC-MPGN）中，转化酶的稳定是由免疫球蛋白引起的，这表明存在被称为 C4 肾炎因子（C4NeFs）的自身抗体。重要的是，该测定法还能够在 1 名 C3G 患者中检测到非免疫球蛋白介导的 CP 转化酶稳定。在所有患者中，CP 转化酶活性延长与 C3NeF 活性一致，最长观察时间为 70 个月。至关重要的是，用 C3 耗尽的血清进行的实验表明，C4NeFs 稳定了不含 C3NeF 表位的 CP C3 转化酶（C4bC2a）。所有 CP 转化酶活性延长的患者均显示出明显的补体激活迹象，即 C3 和 C5 水平降低，C3d、C3bc、C3bBbP 和 C5b-9 水平升高。总之，这项工作为 C3G/IC-MPGN 中导致 CP 转化酶过度活跃的因素的多样性和（非）免疫球蛋白性质提供了新的见解。
