Glomerular C4d Immunoperoxidase in Chronic Antibody-Mediated Rejection and Transplant Glomerulopathy

INTRODUCTION: The diagnosis of late antibody-mediated rejection (AMR) is compromised by frequent absence of C4d in peritubular capillaries (C4d(ptc)), termed “C4d-negative” AMR. We hypothesized that glomerular capillary C4d (C4d(glom)) reflected endothelial interaction with antibody and could improve immunologic classification of transplant glomerulopathy (TG). METHODS: We evaluated C4d using immunoperoxidase in 3524 consecutive, kidney transplant biopsies from a single center. RESULTS: C4d(glom) was detected in 16.5% and C4d(ptc) in 9.9% of biopsies. C4d(glom) occurred in 60.3% of TG (n = 174) and was absent in normal glomeruli. Epidemiologic risk factors for C4d(glom) were younger, female, living-donor recipients with early AMR, prior treated rejection, and late presentation using multivariable analysis. Semiquantitative C4d(glom) score correlated with donor specific antibody (DSA) level, C4d(ptc), microvascular inflammation (MVI), Banff cg scores, renal dysfunction, and proteinuria. Principal component analysis colocalized C4d(glom) with histologic AMR. Multivariable analysis of TG found DSA, C4d(ptc), and post-transplant time associated with C4d(glom). Addition of C4d(glom) into Banff chronic AMR schema improved its diagnostic sensitivity for TG (verified by electron microscopy [EM]) from 22.2% to 82.4% and accuracy from 59.6% to 93.9%, compared with Banff 2019 using only C4d(ptc). Tissue C4d(glom) and chronic AMR diagnosis incorporating C4d(glom) were associated with death-censored allograft failure in TG (P < 0.001), independent of the severity of glomerulopathy and chronic interstitial fibrosis. CONCLUSION: C4d(glom) is a promising diagnostic biomarker of endothelial interaction with antibody which substantially improved test performance of the Banff schema to correctly classify TG by pathophysiology and prognosticate graft loss. We recommend routine C4d immunoperoxidase to minimize underdiagnosis of late AMR in TG.