Serum Proteomic Profiles of Microvascular Inflammation in Donor-specific Antibody-negative and C4d-negative in Kidney Transplantation

Background. Microvascular inflammation (MVI) has long been considered a hallmark of antibody-mediated rejection (AMR). However, the 2022 Banff classification introduced isolated MVI (iMVI; MVI + /donor-specific antibody DSA − /C4d − ) as a distinct phenotype. The biological characteristics and clinical significance of iMVI remain incompletely understood. Methods. Kidney transplant recipients who underwent allograft biopsy between 2013 and 2024 were classified into iMVI, AMR (MVI + /DSA + /C4d + ), and acute tubular injury (ATI; MVI − /DSA − /C4d − ). After propensity score matching, 80 patients (30 iMVI, 30 AMR, and 20 ATI) compromised the main cohort. An independent validation cohort (n = 88; 26 iMVI, 26 AMR, 26 T-cell–mediated rejection, 10 ATI) was additionally assembled. Serum proteomic profiling was performed using a proximity extension assay. Results. iMVI exhibited a distinct proteomic profile enriched for innate and cytotoxic immune activation proteins, including tumor necrosis factor receptor superfamily member 9, programmed death-ligand 1, interleukin-15 receptor subunit alpha, fibroblast growth factor 19, C-X3-C motif chemokine ligand 1, C-C motif chemokine ligand 23, signaling lymphocytic activation molecule family member 1, and interleukin-10 receptor subunit beta with enrichment of natural killer cell differentiation and leukocyte migration pathway. In contrast, AMR showed higher expression of adaptive and humoral immune mediators, including interleukin-33, interleukin-2, TNF-related apoptosis-inducing ligand, and interleukin-2 receptor subunit beta with enrichment of immunoglobulin production and interferon-γ–related adaptive immune pathways. Protein risk scores showed strong discriminative performance in the main cohort (area under the curve, 0.856 for iMVI; 0.879 for AMR) and retained significant ability in the validation cohort (area under the curve, 0.708 and 0.689, respectively). Several iMVI-enriched proteins were significantly associated with increased risk of death-censored graft loss. Conclusions. iMVI is characterized by dominant innate and cytotoxic immune activation distinct from AMR and is associated with adverse graft outcomes, supporting its recognition as a clinically meaningful phenotype beyond conventional AMR.