Kidney transplantation is life-saving, but many kidney grafts fail years after transplantation from chronic antibody mediated rejection (CAMR). Antibodies can cause harm to grafts via variety of mechanisms: deposition of complement, recruitment of inflammatory cells to the graft, or direct endothelial injury. New therapeutic modalities targeting antibodies are under investigation, and accurate diagnosis of CAMR would enable patients to be appropriately triaged into clinical trials or for therapeutic intervention. Te current diagnostic criteria for CAMR relies on detection of the complement component C4d in a kidney biopsy. Recently, the C4d assay has been shown to be insensitive, missing between 30% and 70% of CAMR cases. The proposed research aims to develop a novel algorithm that will improve the diagnosis of CAMR based on a retrospective cohort of late transplant kidney biopsies. A combination of computer-aided digital morphometry and immunohistochemical stains will be used to highlight the recruitment of inflammatory natural killer (NK) cells and endothelial injury and activation markers at the primary location of antibody mediated injury, the microcirculation. Regression analysis will be used to determine whether NK cell infiltration, endothelial injury, or a variety of additional histologic and laboratory characteristics are associated with graft failure from antibodies and poor outcome. Statistically significant features will be used to assemble a new diagnostic algorithm for CAMR, which will be subsequently validated in both a prospective late biopsy cohort and a protocol early biopsy cohort. The ultimate goal of the proposed research is to enable diagnosis of antibody mediated injury during its subclinical stage, before irreversible injury and progression to CAMR. Targeting these patients for therapeutic intervention would prevent the morbidity and mortality associated with return to dialysis, and reduce the number of second kidney transplants, therefore shortening the kidney transplant waiting list. The proposed training plan will provide 5 years of protected research time for Dr. Evan Farkash, as he transitions from his current role as a clinical fellow in pathology at Massachusetts General Hospital/Harvard Medical School and post-doctoral research fellow in the Transplantation Biology Research Center into a tenured junior faculty position and primary investigator in the Department of Pathology. Dr. Farkash will receive regular structured advice and guidance from 3 senior faculty mentors, Dr. Robert Colvin, Dr. David Sachs, and Dr. Susan Saidman, all of whom have extensive expertise in the biology and pathophysiology of renal transplant rejection. The primary mentor, Dr. Colvin, is NIH funded, a world-renowned renal pathologist, and has an extensive track record of training research and clinical fellows. Dr. Farkash has access to laboratory facilities, equipment, and supplies to enable him to address the experimental questions within the confines of the proposed budget, and will be performing the research in an environment with abundant opportunities for collaboration. The ultimate goal of Dr. Farkash's training is to apply for a R01 or equivalent research support with the aim of becoming a fully independent physician-scientist.