#725 Incorporating molecular rejection patterns to differentiate histologic activity and chronicity features in antibody-mediated rejection

Abstract Background and Aims Biopsy-based transcripts associated with antibody-mediated rejection (AMR) hold promise as substitutes for C4d positivity, according to the Banff Meeting Report of 2022. However, incorporating histologic and molecular features of disease activity and chronicity is crucial to potentially guide patient management. Method We analyzed 365 kidney allograft biopsies by histology and the Molecular Microscope Diagnostic System (MMDx) at the University Hospital Zurich from July 2021 to November 2023. Histologic findings were classified according to Banff 2022 into (1) active AMR (n = 24), (2) chronic active AMR (MVI, n = 47), and (3) chronic AMR (n = 30). The corresponding histologic subgroups with DSA negativity were used for comparison. Results Fourteen out of 24 cases (58%) with active AMR, 28 of 47 cases (60%) with chronic active AMR, but only 1 of 30 cases (3%) with chronic AMR showed molecular AMR. Among cases with molecular AMR the sum of the fully-developed AMR-related (R5) and late-stage AMR-related (R6) phenotype scores was significantly higher in chronic active AMR compared to active AMR cases (p = 0.0017). This finding was even more pronounced in chronic active AMR cases with double contours (cg)1 (p = 0.00062). Similarly, in the cases without molecular AMR the sum of the fully-developed AMR-related (R5) and late-stage AMR-related (R6) phenotype scores was higher in chronic active AMR compared to active AMR cases (p = 0.039) and more pronounced in cases with cg1 (p = 0.008). However, rejection phenotype scores (R1-R6) among chronic AMR cases did not differ from DSA-positive or DSA-negative cases without microvascular inflammation (p = 0.705 for R5+R6). Conclusion Incorporating molecular rejection phenotype patterns may further differentiate disease activity and chronicity in cases with active and chronic active AMR by histology. However, molecular subthreshold findings do not appear relevant in chronic AMR cases.