*Purpose: Patients with de novo donor specific antibody (dnDSA) after kidney transplantation have a poorer prognosis than patients without dnDSA. Meanwhile, patients with dnDSA do not necessarily elicit antibody mediated rejection (AMR). The underlying mechanism between the development of dnDSA and AMR has not been fully elucidated. In this study, the pathogenecity and immunogenicity of dnDSA were assessed by evaluating epitope and complement binding affinity.

*Methods: We extracted patients who were positive for dnDSA and received graft biopsy to confirm the presence of AMR. The levels of DSA, C1q, and C3d were measured with Luminex Single Antigen Beads. The presence of C4d was evaluated by immunohistochemical staining in biopsy specimen. The specificity of structural epitope such as conformational or linear epitope was examined using HLA Epitope Registry. The binding force between antigen and antibody such as electrostatic interactions and hydrophobic bonding was calculated as electrostatic mismatch score (EMS) and hydrophobic mismatch score (HMS) by Cambridge HLA Immunogenicity Algorithm. The relationship between these data and complement-binding DSA was statistically examined.

*Results: Twenty-six cases with dnDSA positive and graft biopsy were identified. The dnDSAs included 14, 16, and 20 types of DRB1, DRB3/4/5, and DQA1/B1, respectively. The structural epitope specificities for DSAs were 25 linear epitopes and 23 conformational epitopes. Conformational epitope showed significantly stronger binding properties for EMS and HME than linear epitope in DSAs for DQA1/B1 (P <0.01). The positive predictive value for C1q, C3d, and C4d in the diagnosis of AMR was 85.7%, 100% and 64.3%, and the negative predictive value was 83.3%, 100% and 91.7%, respectively. DSAs against the conformational epitope of DQA1/B1 showed more potent binding of C1q and C3d than the linear epitope (P <0.05). Complement binding DSAs showed significantly stronger electrostatic interaction and hydrophobic binding than non-complement binding DSA (P <0.05).

*Conclusions: C3d was the best among clinical complement assays examined for diagnosing AMR in patients with HLA class II dnDSA. The epitope specificity, physicochemical interaction, and complement binding affinity were associated with the development of AMR in patients with dnDSA.

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