Both IgG- and C1q-Receptors Play a Role in the Enhanced Binding of IgG Complexes to Human Mesangial Cells

Abstract

The presence of immunoglobulin G (IgG) in the mesangial area in kidneys of patients with different forms of glomerulonephritis suggests a role for IgG in the inflammatory process. This study investigates whether IgG is able to bind to cultured human mesangial cells (MC) in vitro. Incubation of MC with 125I-aggregated IgG (125I-AIgG), as a model for immune complexes (IC), at 4°C resulted in a time- and dose-dependent binding of 125I-AIgG to MC. The binding of 125I-AIgG to MC was inhibited by excess AIgG or Fc-fragments and not by F(ab′)2-fragments or human serum albumin (HSA). Scatchard analysis revealed the presence of 2.8·106 receptors/cell with an affinity of 9.7-107 M-1. Incubation of MC with 125I-C1q resulted in a time- and dose-dependent binding of 125I-C1q to MC. The binding of 125I-C1q was inhibited by excess C1q or C1q tails and not by HSA. Scatchard analysis revealed the presence of 3.2-107 binding sites/cell with an affinity of 1.4·107 M-1. Immunoprecipitation of 125I-labeled MC membrane proteins with C1q or monoclonal antibodies directed against human C1q-R revealed a single 66 to 68 kd band under reducing conditions. Fluorescence-activated cell-sorter analysis revealed an average of 60.1% ± 5.4% of the cells positive with a mean channel of fluorescence of 592. A cooperative effect between C1q-R and Fcγ-R in the binding of 125I-AIgG to MC, was assessed by incubation of 125I-AIgG in the presence of increasing concentrations of C1q, C1q tails, or δC1q. Only intact C1q showed a 6-to 11-fold enhancement in binding of 125I-AIgG to MC. These studies demonstrate the occurrence of C1q-R and Fcγ-R on MC and indicate that binding of IC is enhanced after interaction of IC with C1q.