*Purpose: The anti-human leukocyte antigen (HLA) antibodies are well known for risk factor of rejection or allograft loss. Additionally, de novo complement component 1q-binding donor-specific anti-HLA antibodies (C1q-binding DSAs) are already reported to be associated with an increased risk of acute allograft rejection in kidney transplantation (KT). Recently, the clinical impact of preformed C1q-binding DSAs has been discussed. This study investigated the clinical usefulness of preformed C1q-binding DSAs for predicting graft outcomes in KT.

*Methods: From December 2016 to December 2018, 323 recipients underwent kideny transplantation at Seoul St. Mary’s Hospital. If the results of panel reactive antibodies (PRA) were positive in the pre-transplant examination, DSAs and C1q-binding DSAs were performed using Luminex Single Antigen Bead Assay (SAB) at the same time. Graft outcomes in term as Chronic Kidney Disease-Epidemiology Collaboration estimated Glomerular Filtration Rate (CKD-EPI eGFR), acute rejection and graft survival were compared between recipients with preformed C1q-binding DSAs and recipients without preformed C1q-binding DSAs.

*Results: Eighty-two of 323 recipients (25.4%) were evaluated DSAs and C1q-binding DSAs before transplantation. Among them, 40 recipients (48.8%) had preformed DSAs and 8 recipients (9.9%) had preformed C1q-binding DSAs. There were no significant difference in basic demographics except for administration of bortezomib for desensitization between C1q-binding DSAs(-) group and C1q-binding DSAs(+) group. The higher MFI values of DSAs had the higher rate of prevalence of C1q-binding DSAs (9263.9 ± 3670.3 vs. 5955.3 ± 5245.5; p = 0.050). Especially, there was a strong correlation between the presence of DSAs against Class II and C1q-binding DSAs (p = 0.007; CI 95%, OR 9.333). Five of 21 patients (23.8%) with positive at least one of complement-dependent cytotoxicity (CDC) or flow cytometry crossmatch (FCXM) had preformed C1q-binding DSAs. There was a correlation between positivity of crossmatch and preformed C1q-binding DSAs (p = 0.024;CI 95%, OR 6.042). Four of 8 recipients (50%) in C1q-binding DSAs(+) group were confirmed acute antibody-mediated rejection by allograft biopsy. This result showed that C1q-binding DSAs(+) group had significantly higher incidence of acute antibody mediated rejection than C1q-binding DSAs(-) group (p=0.044; CI 95%, OR 4.286).

*Conclusions: The identification of preformed C1q-binding DSAs may be important in predicting acute antibody-mediated rejection in KT. Therefore, the surveillance such as protocol allograft biopsy is required for early detection of acute antibody-mediated rejection after transplantation for patients with preformed C1q-binding DSAs.

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