TGase2–TfR1 interaction amplifies the deposition of the IgA1–sCD89 complexes and its pathological consequences. (A and B) Binding of IgA1 on TfR1 after the addition of TGase2. (A) ELISA plates coated with sTfR1 were incubated with the indicated proteins in the order shown at 37°C and revealed with an AP-coupled polyclonal anti–human IgA antibody. n = 3 experiments. (B) HMCs were incubated with the different proteins in the order shown at 4°C, and binding was detected using anti–IgA-FITC. The graph shows the mean of IgA1 binding (±SEM) as a ratio of IgA1 binding mean fluorescence intensity/background mean fluorescence intensity of six independent experiments. (C and D) Immunostaining for the detection of human IgA1 deposits (C) and C3, MBL deposits, CD11b⁺ and TfR1⁺ cells (D) in frozen kidney sections from 12-wk-old α1KI-CD89Tg-TGase2^−/− mice compared with α1KI-CD89Tg mice. Similar results were obtained with F4/80 mAb staining (not depicted). Arrows indicate CD11b⁺ cells. The graphs show the corresponding number of positive cells per glomerulus counted in 20 randomly chosen fields for each mouse at 200 magnification. n = 3 mice per group. Bars: (C) 10 µm; (D) 5 µm. (A, B, and D) Error bars indicate SEM. (E) Hematuria of 12-wk-old α1KI-CD89Tg and α1KI-CD89Tg-TGase2^−/− mice. 12–23 mice per group. Bars represent the mean. *, P < 0.05; **, P < 0.01; and ***, P < 0.001 (using the Mann-Whitney U test).