C4d is produced from the direct interaction between antibodies and tissue injury at an antibody binding site in a graft. C4d deposition along peritubular capillaries (PTCs) in a renal allograft is a characteristic finding of antibody-mediated rejection (AMR), and is a useful diagnostic tool of AMR. The C4d along PTCs is associated with poor graft survival. Therefore C4d is regarded as a biomarker of AMR and was included in the diagnosis criteria of AMR at 2007 Banff conference. However, although C4d assay is widely used, it has several limitations. ABO-incompatible transplantations develop C4d along the PTCs in the majority of grafts but this seems to be graft accommodation rather than AMR. Recent studies reported that more than half of renal allograft biopsies with chronic AMR were C4d-negative. Without treatment, the C4d-negative AMR can cause scarring within the graft, transplant glomerulopathy (TG) or even graft loss. C4d is not a certain indicator of antibody-mediated rejection and C4d staining is not always highly sensitive for detecting AMR. Measuring endothelial gene expression in kidney graft biopsies with alloantibody can be another sensitive and specific method to diagnose AMR and predict graft outcomes. Because of these complexities, at the 2011 Banff meeting, criteria for diagnosis of chronic AMR in the kidney were refined, and the need for inclusion of C4d-negative AMR in the Banff classification was investigated.

Keywords: C4d, Antibody, Rejection, Limitation