Thiol-based Drugs Effectively Reduce the Circulating Poly-IgA Immune Complex but Reduce the Soluble CD89-IgA Complex with Different Rates in the Peripheral Blood of IgA Nephropathy

Author(s): Aibing Rao, Yuanmei Qiu, Yayun Wu, Ronghua Li, Ruifan Li, Ying Tang, Yulei Chen, Junzhe Chen, Min Chen, Xinfang Xie, Jicheng Lv, Hong Zhang, Jing Jin

Background and objectives: Immunoglobulin A nephropathy (IgAN) is the most common type of glomerulonephritis and is due to the deposit of circulating IgA immune complex in kidney. There is evidence showing that reduction of disulfides may be able to break down IgA immune complexes. The goal of this research is to measure and compare the reduction rates of a representative thiol-based reducer on two types of IgA immune complex in-vitro by using plasma or serum samples from IgAN patients, other kidney diseases and healthy controls. Methods: The thiol-based drug, n-acetylcysteine (NAC), was used as an representative after being compared with cysteine (CYS), and cysteamine (CA) for reaction conditions. Using plasma or serum samples from different patient groups, two types of IgA immune complex were quantified with house-developed ELISA kits prior and post NAC reduction. One is the poly-IgA immune complex which can be captured by CD89, the IgA Fc receptor, hence it is not combined with CD89 in the circulation, denoted as Poly-IgA-IC; the other is the IgA immune complex which has already been combined with CD89 in the circulation, called soluble CD89-IgA immune complex, denoted as CD89-IgA-IC. Both types were quantified with ELISA kits in parallel. The reduction rates were summarized and compared within each sample group. At last ROC analysis were performed to assess the clinical utility as a novel biomarker for IgAN using the CD89-IgA-IC residuals after reduction. Results: N-acetylcysteine and cysteamine achieved reduction saturation of Poly-IgA-IC using 5 mM under room temperature or 37^oC for one hour, while Cysteine didn’t achieve saturation even at 10 mM with a relatively smaller reduction rate. Using 10 mM NAC as a reducer, plasma or serum samples of three patient groups, IgA nephropathy (IgAN), presumably none-IgAN kidney disease (KD) and healthy control group (HC) were tested for Poly-IgA-IC and CD89- IgA-IC reduction in parallel. For Poly-IgA-IC, the reduction rates showed no difference among the groups, with an average of 82% for IgAN and KD and 78% for HC. On the other hand, for CD89- IgA-IC, a large number of IgAN and some KD samples had greater portions of none-reduced residuals and had much smaller reduction rates than HC, suggesting that CD89-IgA-IC might consist of the reducible and the irreducible subtypes. The reducible type is related to the normal immunity and the irreducible type might be clinically related to IgAN. This was indicated by the ROC analysis using the irreducible CD89-IgA-IC residues to predict the biopsy results, whether IgAN was diagnosed , with an AUC of 0.69, an specificity of 66%, and an sensitivity of 73%. Conclusions: IgA immune complex consists of two types, Poly-IgA-IC and CD89-IgA-IC. This in-vitro study showed that in the plasma or sera, thiol-based compounds can effectively reduce the Poly-IgA-IC for all tested samples but can only effectively reduce CD89-IgA-IC in less than half of IgAN samples. Re-purposing of these drugs such as NAC may be a novel approach to IgA nephropathy treatment and management. For CD89-IgA-IC type, the irreducible residuals in a large number of IgAN samples and in some presumably none-IgAN KD samples might be clinically related to IgAN and need for further investigation.