New Insights into the Pathophysiology of Acquired Cytopenias.

This review addresses three related bone marrow failure diseases, the study of which has generated important insights in hematopoiesis, red cell biology, and immune-mediated blood cell injury. In Section I, Dr. Young summarizes the current knowledge of acquired aplastic anemia. In most patients, an autoimmune mechanism has been inferred from positive responses to nontransplant therapies and laboratory data. Cytotoxic T cell attack, with production of type I cytokines, leads to hematopoietic stem cell destruction and ultimately pancytopenia; this underlying mechanism is similar to other human disorders of lymphocyte-mediated, tissue-specific organ destruction (diabetes, multiple sclerosis, uveitis, colitis, etc.). The antigen that incites disease is unknown in aplastic anemia as in other autoimmune diseases; post-hepatitis aplasia is an obvious target for virus discovery. Aplastic anemia can be effectively treated by either stem cell transplantation or immunosuppression. Results of recent trials with antilymphocyte globulins and high dose cyclophosphamide are reviewed. Dr. Abkowitz discusses the diagnosis and clinical approach to patients with acquired pure red cell aplasia, both secondary and idiopathic, in Section II. The pathophysiology of various PRCA syndromes including immunologic inhibition of red cell differentiation, viral infection (especially human parvovirus B19), and myelodysplasia are discussed. An animal model of PRCA (secondary to infection with feline leukemia virus [FeLV], subgroup C) is presented. Understanding the mechanisms by which erythropoiesis is impaired provides for insights into the process of normal red cell differentiation, as well as a rational strategy for patient management. Among the acquired cytopenias paroxysmal nocturnal hemoglobinuria (PNH) is relatively rare; however, it can pose formidable management problems. Since its first recognition as a disease, PNH has been correctly classified as a hemolytic anemia; however, the frequent co-existence of other cytopenias has hinted strongly at a more complex pathogenesis. In Section III, Dr. Luzzatto examines recent progress in this area, with special emphasis on the somatic mutations in the PIG-A gene and resulting phenotypes. Animal models of PNH and the association of PNH with bone marrow failure are also reviewed. Expansion of PNH clones must reflect somatic cell selection, probably as part of an autoimmune process. Outstanding issues in treatment are illustrated through clinical cases of PNH. Biologic inferences from PNH may be relevant to our understanding of more common marrow failure syndromes like myelodysplasia.