In Vitro Detection of C4d-Fixing HLA Alloantibodies: Associations With Capillary C4d Deposition in Kidney Allografts

Capillary C4d deposition is a valuable marker of antibody-mediated rejection (AMR). In this analysis, flow cytometric detection of alloantibody-triggered C4d deposition to HLA antigen-coated microparticles (C4d]FlowPRA) was evaluated for its value as a marker for C4d deposition in renal allografts. For comparative analysis, 105 first renal biopsies performed for graft dysfunction and an equal number of concurrent sera were subjected to immunohistochemistry and C4d] plus standard IgG]FlowPRA, respectively. C4d deposition/fixation was detected in 17 biopsies and, applying C4d]FlowPRA HLA class I and II screening, also in a small number of corresponding sera (N = 20). IgG reactivity detected by standard IgG]FlowPRA was more frequent (49% of sera). Comparing C4d]FlowPRA screening with capillary C4d staining, we found a high level of specificity (0.92 95% confidence interval: 0.86–0.98]), which far exceeded that calculated for IgG]FlowPRA (0.60 0.50–0.70]). IgG]FlowPRA screening, however, turned out to be superior in terms of sensitivity (0.94 0.83–1.05] vs. 0.76 0.56–0.97] calculated for C4d-fixing panel reactivity). Remarkably, posttransplant single antigen testing for identification of complement-fixing donor-specific alloreactivities failed to improve the predictive value of FlowPRA-based serology. In conclusion, our results suggest that detection of complement-fixing HLA panel reactivity could provide a specific tool for monitoring of C4d-positive AMR.