Charge-dependent binding of polymeric IgA 1 to human mesangial cells in IgA nephropathy

Abstract

Background. IgA nephropathy (IgAN) is characterized by raised serum IgA 1 and predominant mesangial IgA 1 deposits of polymeric nature. The mechanism of polymeric IgA 1 (pIgA 1) deposition in the kidney mesangium is poorly understood in IgAN. It has been suggested that increased sialic acid content and anionic charge of the pIgA 1 molecules may be operational in the IgA 1 deposition in human mesangial cells (HMCs). The present study examined the binding of pIgA 1 with different surface charges to HMCs. The binding characteristics of IgA 1 to HMCs in the presence of polycation (poly-n-lysine) or polyanion (heparin) were also investigated. Methods. IgA 1 was purified in sera from patients with IgAN and from healthy controls by jacalin affinity chromatography. IgA 1 was further separated into pIgA 1 and monomeric IgA 1 (mIgA 1 by fast protein liquid chromatography (FPLC). pIgA 1 or mIgA 1 with different net charges on their surface were resolved by ion exchange chromatography (IEC) with a Mono Q column. The binding characteristics of pIgA 1 and mIgA 1 to HMCs in the presence or absence of polycation or polyanion were examined by flow cytometry. Results. In patients with IgAN, the absolute amount of mIgA 1 and pIgA 1 is significantly higher than that of healthy controls (P < 0.001). There was significant increase in binding of pIgA 1 from patients with IgAN to HMC and cell lysate. pIgA 1 that interacted strongly with the ion exchanger also bound more to HMCs when compared with IgA 1 interacted weakly with the ion exchanger (P < 0.001). The anionic charged pIgA 1 from patients was significantly higher than that of healthy controls (P < 0.001). Preincubation with poly-L-lysine increased the binding of pIgA 1 to HMCs. The binding of pIgA 1 to HMCs was decreased by preincubation with heparin. Conclusions. The binding of IgA to HMCs is charge dependent. Polymeric IgA with the highest net negative charge binds more to HMCs. Preincubation with polyanion decreased the binding of polymeric IgA to HMCs. These results suggest an important role for anionic charge in IgA 1 deposition onto the kidney mesangial cells.