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The deposition of the complement split fragment C4d and its association with acute antibody-mediated rejection (AMR) in pancreas transplant (PTx) is not well defined. To characterize the deposition of C4d in PTx, we analyzed 27 PTx biopsies from 18 patients transplanted between 2004 and 2007 at the University of Wisconsin.The presence of C4d was graded in interacinar capillaries (IAC), islets, interstitium, and small- and medium-size vessels. Sera obtained at the time or within 5 days of the biopsy were tested for antidonor-specific antibodies (DSA).16 biopsies (59.26%) showed at least 5% C4d+ IAC (range 5%–90%). Of those, five biopsies (18.5%) revealed diffuse labeling (>50% C4d+ IAC) and 11 (40.74%) showed focal staining (5%–50% C4d+ IAC). C4d+ IAC (>5%) was significantly associated with the presence of strong DSA for class I or class II (P<0.018). C4d staining of the media or endothelium of small and medium-size vessels was a common finding in all biopsies without any association with DSA. Similarly, staining of islets and parenchymal interstitium was not statistically associated with AMR. The majority of patients received intravenous corticosteroid bolus and taper, with specific cases requiring thymoglobulin, IVIg, rituximab, or plasmapheresis. Forty-six percent of patients who demonstrated AMR returned to insulin therapy because of chronic graft damage and loss of C-peptide.Our findings support the potential role of C4d labeling of PTx biopsies in the diagnosis of AMR and emphasize the staining of IAC as a valuable histologic tool for the diagnosis.