Surface-bound FHR-1 promotes complement activation. (A) Cryostat kidney sections from Cfh^−/−;Cfhr^−/− mice were incubated with 10% NHSΔFHR in presence or absence of 0.5 µM of the different FHR-1 proteins. Bound FHR-1 was detected using the mAb 2C6, and deposits of hC3-activated fragments were detected with the mAb 12.17. (B) Cryostat kidney sections from Cfh^−/−;Cfhr^−/− mice were incubated with 10% NHSΔFHR and 0.25 µM of FHR-1_L290S,A296V in the presence or absence of FH (1 µM) or EDTA (20 mM). (C) Increasing concentrations of FHR-1_WT or FHR-1_L290S,A296V were added to 10% NHSΔFHR and then incubated with cryostat kidney sections from Cfh^−/−;Cfhr^−/− mice. Fluorescence intensity is expressed in arbitrary units. Data are means ± standard deviations of a minimum of 40 glomeruli. All photomicrographs were taken at ×40 magnification.