| Hematopoietic cell transplantation (HCT) is frequently complicated by graft-versus-host disease (GVHD). During the past three decades, experimental studies and clinical observations have elucidated the pathophysiology of acute GVHD, but the biology of chronic GVHD is much less well understood. Recommendations of the NIH Consensus Development Project on Criteria for Clinical Trials in Chronic GVHD have begun to standardize the diagnosis and clinical assessment of the disease. These criteria have emphasized the importance of qualitative differences, as opposed to time of onset after HCT, in making the distinction between acute and chronic GVHD. Experimental studies have generated at least four theories to explain the pathophysiology of chronic GVHD. These theories include 1) thymic damage and defective negative selection of T cells generated from marrow progenitors after HCT, 2) aberrant production of transforming growth factor-β, 3) auto-antibody production, and 4) deficiency of T-regulatory cells. Recent studies in humans have corroborated a possible role for each of these mechanisms in humans. No animal model fully replicates all of the features of chronic GVHD in humans, and it appears likely that multiple biological mechanisms account for the diverse features the disease. Chronic GVHD may represent a "syndrome" with diverse causes among individual patients. In the future, it might become possible to tailor specific therapeutic interventions for patients as individually needed for each distinct pathophysiologic mechanism involved in development of the disease.
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