The Molecular Phenotype of Kidney Transplants

Microarray studies of kidney transplant biopsies provide an opportunity to define the molecular phenotype. To facilitate this process, we used experimental systems to annotate transcripts as members of pathogenesis‐based transcript sets (PBTs) representing biological processes in injured or diseased tissue. Applying this annotation to microarray results revealed that changes in single molecules and PBTs reflected a large‐scale coordinate disturbance, stereotyped across various diseases and injuries, without absolute specificity of individual molecules or PBTs for rejection. Nevertheless, expression of molecules and PBTs was quantitatively specific: IFNG effects for rejection; T cell and macrophage transcripts for T cell‐mediated rejection; endothelial and NK transcripts for antibody‐mediated rejection. Various diseases and injuries induced the same injury–repair response, undetectable by histopathology, involving epithelium, stroma and endothelium, with increased expression of developmental, cell cycle and apoptosis genes and decreased expression of differentiated epithelial features. Transcripts reflecting this injury–repair response were the best correlates of functional disturbance and risk of future graft loss. Late biopsies with atrophy‐fibrosis, reflecting their cumulative burden of injury, displayed more transcripts for B cells, plasma cells and mast cells. Thus the molecular phenotype is best described in terms of three elements: specific diseases, including rejection; the injury–repair response and the cumulative burden of injury. The molecular features of transplant biopsies are best summarized in terms of three elements: specific diseases; the active injury‐repair response; and the cumulative burden of injury.