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Abstract: Background: Antibody-mediated rejection (AMR) of cardiac allografts is associated with reduced long-term graft survival, but not every patient with AMR develops premature graft failure. The tissue level mechanisms leading to graft failure in some patients with antibody-mediated rejection are poorly characterized. Methods: We assessed changes in myocardial microvessel density (number of capillaries per unit area) in endomyocardial biopsies over time using whole-slide microscopic imaging of CD34-stained slides and computer-assisted image analysis. Changes were compared among eight heart transplant recipients with multiple episodes of pathologic AMR who died from cardiovascular causes, eight age- and gender-matched patients with pathologic AMR who were still alive at a similar follow-up interval, and six matched controls without AMR or cellular rejection. Results: Microvessel density decreased in the last biopsies (mean 6.52 years post-transplant) from patients with pathologic AMR and cardiovascular mortality compared to their biopsies at 6 and 12 months post-transplant [respectively, −22% (P=.02) and −25% (P=.02)]. A similar decrease was not seen for the other groups. Conclusions: Significantly reduced myocardial microvessel density does occur in a subset of patients with pathologic AMR who have a worse outcome. These data provide insights into the interplay between AMR, microvascular injury, and clinical outcomes. [Copyright &y& Elsevier]