Aim Antibody-mediated rejection (AMR) is a major cause of kidney graft loss, yet assessment of individual risk at diagnosis is impeded by the lack of a reliable prognosis assay. Here, we tested whether the capacity of HLA antibodies to bind complement component C3d allows accurate risk stratification at the time of AMR diagnosis. Methods Sera from kidney transplant recipients, who underwent a protocol or for-cause kidney biopsy and had detectable de novo DSA (by One Lambda) at the time of biopsy (median 3.8 yrs post-tx), were included in this study. These serum samples were re-tested using the Immucor single antigen beads with and without C3d detection system. Results This study included samples from 123 kidney recipients (70 males; 14 re-Tx; 46 LD) transplanted in our center between 1989 and 2011. Fifty-seven patients (46%) had C3d-binding DSA. Most C3d-binding DSAs were high MFI DSAs (11700 + 5188), and only 4/57 (7%) C3d-binding DSAs had 5000 MFI. Seventy percent of the patients with C3d-binding DSA (40/57) had AMR (18 aAMR and 22 cAMR) and C4d-positive biopsies; twenty-six percent (15/57) had ACR, and four percent (2/57) had negative biopsies. Fifty-two present of the patients (34/66) in the C3d-negative DSA group had DSA with MFI 5000. Some of the weak and moderate DSA detected by One Lambda single antigen bead reagents were not detected with the Immucor SAB. Among the patients with C3d-negative DSA, thirty-five percent (23/66) had AMR (7 aAMR and 16 cAMR); twelve percent (8/66) had ACR and thirty-three percent (22/66) had C4d-positive biopsies. In most cases 15/22 (68%), the C4d-positive biopsies were observed in patients with strong DSA (MFI > 5000). Conclusions Our data indicate a strong correlation between the presence of C3d-binding DSAs and AMR. C3d-binding antibodies seem to be prevalent to stronger antibodies. Further studies are needed to evaluate whether the presence of C3d-binding donor-specific antibodies can predict AMR and identify patients who are at increased risk of allograft failure. Download : Download full-size image