抗C3b抗体在狼疮性肾炎发病机制中的研究

The pathogenesis of lupus nephritis (LN) has not been fully understood. The complement deficiency and persistence of apoptotic cells had been demonstrated to play important roles in the process of LN. In our preliminary study, we found that the complement alternative pathway activation might be crucial for the disease development. Since C3b is one of the key proteins in alternative pathway activation and clearance of immunocomplex, we speculate that anti-C3b antibodies may be involved in the pathogenesis of lupus nephritis through suppression of C3b functions. We had found that patients with anti-C3b antibodies had lower serum C3 levels and higher anti-ds-DNA antibody titers, which suggested the pathogenic effect of anti-C3b antibody. However, the actual roles of anti-C3b antibody were still unknown. In this study, we aimed to elucidate whether the anti-C3b antibody was involved in the pathogenesis of lupus nephritis through affecting the functions of C3b, disturbing the complement activation and apoptosis clearance both in vivo and in vitro, and we also intended to observe the associations between anti-C3b antibody and renal injury parameters in MRL/lpr mice models. This study would illuminate one of the etiologies of lupus nephritis and be meaningful in providing new directions for the diagnosis and treatment of this disease.

狼疮性肾炎发病机制目前未完全阐明，补体缺陷与凋亡物质清除障碍在其中起重要作用。申请者前期研究发现补体旁路异常活化是本病发病机制中的重要环节。鉴于C3b是补体旁路活化的中心环节，也是免疫复合物清除的关键因子，我们推测抗C3b抗体引起的C3b功能异常可能参与了本病的发病。我们的前期研究发现抗C3b抗体阳性的狼疮性肾炎患者血清C3水平更低、抗ds-DNA抗体滴度更高，提示抗C3b抗体可能参与了致病，但其确切机制仍不明确。本研究拟通过体外及动物实验，探讨抗C3b抗体对C3b功能的影响，以阐明抗C3b抗体在狼疮性肾炎补体活化及清除凋亡过程中的致病作用，为临床防治该病提供新的理论和实验依据。

本课题纳入134例本中心狼疮性肾炎患者，并匹配相应的对照，发现抗C3b抗体在狼疮性肾炎患者中阳性率为47.8%。抗C3b抗体阳性患者SLEDAI评分更高 (19±6 vs. 16±6, P=0.003)；C3 (0.37 (0.15-0.78) vs. 0.50 (0.15-1.38) g/L, P<0.001) 和 C4 (0.07 (0-0.34) vs. 0.11(0-1.01) g/L, P<0.001) 水平更低。抗C3b抗体水平与肾组织上C3c和C1q的沉积呈正相关 (分别为 r=0.266，P=0.003 和 r=0.185, P=0.039)，且存在活动性肾脏病理损伤指标 (系膜细胞增生、中性粒细胞浸润/核碎裂及白金耳) 的患者抗C3b抗体水平也明显升高 (均 P<0.05)。随着临床病情缓解，抗C3b抗体水平较活动期明显下降 (372.00 (213.17-938.35) AU/mL vs. 0.00(0.00-82.55) AU/mL，P<0.001)。多因素Cox 回归发现抗C3b抗体与肾脏预后不良独立相关 (HR 3.100 (95% CI:1.543-6.227, P=0.001))。体外功能学实验显示，向正常人血清中加入抗 C3b 抗体后，其血清C3a的水平显著升高 (20.36 (16.92-22.90) g/ml vs. 11.93(8.77-14.18) g/ml，P=0.034)。抗C3b抗体可分别抑制C3b与H因子及CR1的结合 (抑制率分别是 20.7%32.4%和 21.3%45.9%)，抗C3b抗体可抑制H因子辅助I因子裂解C3b；此外，抗C3b抗体导致H因子不能有效地抑制绵羊红细胞溶血 (18.9 (13.2-18.95) vs. 0.45 (0.05-3.60)%,P=0.024) 。多数MRL/lpr狼疮鼠存在血浆抗C3b抗体(6/8, 75%；野生型对照鼠抗C3b抗体为阴性)。狼疮鼠血浆抗体水平与蛋白尿平行。本课题研究结果表明，血浆抗C3b抗体与狼疮性肾炎患者肾脏损伤及预后密切相关。抗C3b抗体可以促进补体旁路途径在血循环中以及细胞表面上的过度活化，从而参与狼疮性肾炎的发病。