Increased binding of polymeric A-IgA to cultured human mesangial cells in IgA nephropathy

Abstract

IgA nephropathy (IgAN) is characterized by raised plasma λ-IgA1 and mesangial polymeric λ-IgA1 deposits. It remains uncertain whether the predominant glomerular λ-IgA1 deposits represent a selective uptake of polymeric IgA or a non-specific uptake due to elevated circulating λ-IgA1 levels in response to an unidentified antigen. In this study, we explored whether there is an increased binding of monomeric IgA1 (mIgA1) or polymeric IgA1 (pIgA1) from patients with IgAN to cultured human mesangial cells (HMC). Total IgA1 in plasma from patients or healthy controls was isolated by jacalin-agarose column as jacalin-bound proteins (JBP). Monomeric IgA1 and pIgA1 were distinctly separated by FPLC. HMC were incubated with IgA preparations and IgA bound to HMC was determined by flow cytometry analysis using standard curves constructed by known concentrations of κ-IgA1 or λ-IgA1. In order to avoid any increased binding of IgA to HMC due to elevated κ- or λ-IgA concentrations in JBP samples from patients, JBP samples from patients or controls were appropriately diluted to achieve comparable levels of total IgA1. No differences in the total mIgA1 or pIgA1 concentration, percentage of mIgA1 or pIgA1, or the κ/λ ratio of mIgA1 or pIgA1 were found between adjusted JBP samples from patients or healthy controls. We found a sharp rise in percentage of pIgA1 among IgA1 bound to HMC (70%), despite the fact that only 3% of the IgA1 in the adjusted JBP samples were polymeric, suggesting that pIgA1 had a higher affinity to HMC than mIgA1. Furthermore, the κ/λ ratios of pIgA1 bound to HMC were significantly lower than the κ/λ ratios of pIgA1 in adjusted JBP only with IgAN patients but not healthy controls (P = 0.0026). Our data suggest a preferential mesangial binding of polymeric λ-IgA1 from patients with IgAN. These polymeric λ-IgA immune complexes are likely to be "pathogenic" and are important in the pathogenesis of IgAN.