Antibody-Mediated Rejection in ABO-Incompatible Kidney Transplantation

Background and hypothesis: In ABO-incompatible (ABOi) kidney transplantation, C4d deposition is associated with accommodation rather than rejection. Isoagglutinins targeting blood group antigens A/B are also classified as donor-specific antibodies (DSA). Therefore, the diagnosis of antibody-mediated rejection (AMR) relies primarily on microvascular inflammation (MVI). Methods: We analyzed 66 ABOi and 251 ABO-compatible (ABOc) KTRs concerning anti-HLA DSA development. 46 protocol biopsies from ABOi KTRs were classified according to Banff 2022. In addition, 25 ABOi protocol biopsies were assessed by the Molecular Microscope Diagnostics System (MMDx) and compared to ABOc biopsies: (1) 35 DSA-negative, C4d-negative cases with MVI<2, (2) 16 C4d-positive cases with MVI<2, (3) 35 DSA-positive, C4d-negative cases with MVI=1 (probable AMR), and (4) 87 C4d-negative/positive cases with MVI≥2. Results: ABOi KTRs showed lower rates of de novo anti-HLA DSA (p=0.001) and clinical AMR (p=0.018) than ABOc KTRs. Among 25 ABOi protocol biopsies analyzed with MMDx, 56% met AMR criteria due to anti-A/B DSA: 20% active AMR, 20% probable AMR, 16% chronic AMR. However, molecular AMR was confirmed in only 14% by MMDx (p<0.001). ABOi and DSA-negative, C4d-negative ABOc biopsies with MVI below threshold did not differ in molecular rejection, archetype, and lesion scores (p>0.05) and showed stable graft function. Molecular AMR classifier scores were significantly lower in ABOi and DSA-negative, C4d-negative ABOc cases with MVI below threshold compared to C4d-positive ABOc and ABOc cases with probable AMR (p=0.007). Notably, C4d drives molecular AMR activity in ABOc biopsies already at C4d1 levels by immunofluorescence (p=0.011) and even in the absence of a histological Banff AMR diagnosis (p=0.003). Conclusion: ABOi transplantation reduces the risk of developing de novo anti-HLA DSA. Banff 2022 criteria may over-diagnose AMR. Biopsy-based transcript diagnostics differentiate anti-HLA- and anti-A/B-mediated alloimmune injury from C4d deposition due to accommodation. Interestingly, C4d deposition drives molecular AMR activity in ABOc biopsies.