Binding of FHR-1 to glomeruli from Cfh^−/− mice. (A) Illustration of the rationale for the experiments using cryostat sections of Cfh^−/− mouse kidneys. Photomicrograph shows staining with a biotinylated anti–mouse C3 antibody. (B) Three different concentrations of FHR-1 proteins were added to cryostat kidney sections of Cfh^−/− mice. Bound FHR-1 was detected with an in-house biotinylated anti–FHR-1 mAb (2C6). All FHR-1 proteins bound exclusively to the glomerulus. Fluorescence intensity (FI) is expressed in arbitrary units. Data are means ± standard deviations of a minimum of 20 glomeruli. FI values obtained from FHR-1_WT were used to make a reference curve. FI values of all FHR-1 mutants were interpolated in this curve to calculate the relative binding between the FHR-1 proteins. SIA, sialic acid. All photomicrographs were taken at ×40 magnification. Average diameter of mouse glomeruli is 70 μm.