Decreased binding of aHUS-associated FHR-1 mutants to desialylated Cfh^−/− mouse glomeruli. Optimal concentrations of FHR-1_WT (1 μM), FHR-1_L290V (0.3 μM), FHR-1_296A (1 μM), FHR-1_L290S (0.3 μM), and FHR-1_L290S,A296V (0.3 μM) in PBS/BSA were added to normal and neuraminidase (NA)-treated cryostat sections of Cfh^−/− mouse kidneys and incubated at 37°C for 30 minutes. Bound FHR-1 was detected with an anti–FHR-1 mAb (2C6). Images of representative glomeruli are shown to illustrate the decreased binding to desialylated glomeruli of the aHUS-associated FHR-1 mutants compared with the binding to untreated glomeruli. Loss of binding for the different FHR-1 proteins was calculated basically as described in Figure 3. All FHR-1 proteins bound similarly to desialylated sections of Cfh^−/− mouse kidneys when they were used at the same concentration (data not shown). All photomicrographs were taken at ×40 magnification.