Solid-Phase C1q/C3d Fixing Readouts Correlate with High Median Fluorescence Intensity (MFI) De Novo Donor-Specific HLA Antibodies and C4d(+) Antibody-Mediated Rejection in Kidney Transplant Recipients

Background: Solid-phase assays to investigate the complement-activating capacity of HLA antibodies have been utilized to optimize organ allocation and improve transplant outcomes. The clinical utility of C1q/C3d-binding characteristics of de novo donor-specific anti-HLA antibodies (dnDSA) associated with C4d-positive antibody-mediated rejection (C4d+ AMR) in kidney transplants (KTx) has not been defined. Material/Methods: Sera from 120 KTx recipients that had dnDSA concurrent with protocol/cause biopsy (median 3.8 years after transplantation) were screened for C1q and C3d-binding dnDSA. The difference in the incidence of C4d+ AMR between recipients with and without C1q/C3d-binding dnDSA was assessed. Results: Over 86% of dnDSAs were class II antibodies. The immunodominant dnDSAs characterized by the highest median fluorescence intensity (MFI) in most recipients were HLA-DQ antibodies (67%). Most recipients (62%, n=74) had either C1q+ (56%), C3d+ (48%), or both C1q+C3d+ (41.2%) dnDSA, while the remaining 38% were negative for both C1q and C3d. Of those with C1q+/C3d+ dnDSA, 87% had high-MFI IgG (MFI=14144 +/- 5363 and 13932 +/- 5278, respectively), while 65% of C1q-C3d- dnDSA had low-MFI IgG (MFI=5970 +/- 3347). The incidence of C4d+ AMR was significantly higher in recipients with C1q+ (66%), C3d+ (74%), and C1q+C3d+ (72%) dnDSA than in those with C1q-C3d- dnDSA (30%) recipients. Recipients with C3d+/C1q+ dnDSA had higher C4d+ scores on biopsy. Conclusions: C1q+/C3d+ dnDSA were associated with C4d+ AMR and high-IgG MFI. Our data call into question the predictive utility of C1q/C3d-binding assays in identifying KTx recipients at risk of allograft failure. In conclusion, IgG MFI is sufficient for clinical management, and the C1q/C3d-assays with added cost do not provide any additional information.