C4d binding correlated with strong HLA antibodies involved in graft failures

Capillary C4d was an established marker of antibody-mediated rejection in kidney transplantation. Recently, C4d fixation to antibodies against HLA-coated beads were found to be a novel way to test for complement-fixing antibodies in the serum. To find differences between complement-fixing and nonfixing antibody assays, we tested HLA antibodies and C4d in 102 sera of graft-failure patients using the LABScreen and FlowPRA bead methods. HLA antibodies were observed in the sera of patients who subsequently failed and who continued to function. C4d fixing antibodies were present in the sera of patients who subsequently experienced graft failures and those with high fluorescence intensity of HLA antibodies by FlowPRA. In antibody dilution studies with LABScreen, we noted that C4d fixation occurred only at high antibody titers, and fluorescence was lost as sera were diluted. In conclusion, the fluorescence levels of antibodies were more important that the C4d content. Quantitative aspects of antibody testing using the fluorescence value appears to be a valuable factor when examining anti-HLA antibodies posttransplantation.