Flow cytometric analysis is superior to the Ham test and permits concomitant diagnosis of PNH in about 20% of patients with myelodysplasia (a rate similar to that seen in patients with aplastic anemia). The presence of GPI-anchored protein-deficient cells in myelodysplasia predicts responsiveness to immunosuppressive therapy. Early emergence of GPI-anchored protein-deficient hematopoiesis in a patient with marrow failure may point to an underlying immune pathogenesis.

N PATIENTS WITH APLASTIC ANEMIA, bone marrow failure leads to pancytopenia; death occurs secondary to infection, bleeding, or complications of severe anemia. 1 Clinical observations and laboratory experiments have implicated an immune pathophysiological effect, in which T cells destroy hematopoietic stem and progenitor cells. 2 Allogeneic bone marrow transplantation replaces stem cells. Current transplant methods produce high cure rates, but only a minority of patients have histocompatible siblings, and graft vs host disease is a frequent complication in older recipients. 3 Observation of recovery despite failed bone marrow transplant led to the treatment of aplastic anemia with immunosuppressive therapy alone. Antithymocyte globulin improved blood cell counts in about half of treated patients. 4,5 Cyclosporine improved outcomes of many who had not responded to antithymocyte globulin. 6,7 The combination of antithymocyte globulin, which lyses lymphocytes, and cyclosporine, which blocks T-cell function, has led to survival rates comparable with those observed with transplant recipients. [8][9][10] However, immunosuppression is an imperfect treatment. About a third of patients fail to respond, and even responders often have chronically low blood cell counts. Late complications include relapse of pancytopenia and development of secondary clonal hematologic diseases like myelodysplasia. 11 These problems have resulted in pessimistic commentary suggesting that immunosuppression only "postpones the inevitable" 12 use of more toxic thera-pies such as high-dose cyclophosphamide 13 and uncertainty in the timing of risky alternative donor transplants in

B19 parvovirus is pathogenic in humans, causing fifth disease, transient aplastic crisis, some cases of hydrops fetalis, and acquired pure red cell aplasia. Efforts to develop serologic assays and vaccine development have been hampered by the virus's extreme tropism for human bone marrow and the absence of a convenient culture system. We constructed recombinants containing either the major (VP2) or minor (VP1) structural proteins of B19 in a baculovirus-based plasmid, from which the polyhedrin gene had been deleted; these recombinant plasmids were used to generate recombinant infectious baculovirus. Subsequent infection of insect cells in vitro resulted in high-level expression ofeither B19 VP1 or VP2. Parvovirus capsids were obtained by self-assembly in cell cultures coinfected with either VP1-and VP2-containing baculoviruses or, surprisingly, VP2-containing baculoviruses alone. Empty B19 capsids composed of VP1 and VP2 could replace serum virus as a source of antigen in a conventional inmmunoassay for detection ofeither IgG or IgM antiparvovirus antibodies in human serum. Immunization of rabbits with capsids composed of VP1 and VP2 resulted in production of antisera that recognized serum parvovirus on immunoblot and neutralized parvovirus infectivity for human erythroid progenitor cells. Baculovirus-derived parvovirus antigen can substitute for scarce viral antigen in immunoassays and should be suitable as a human vaccine.B19 parvovirus is the only member ofthe family Parvoviridae pathogenic in humans (1, 2). Acute infection results in fifth disease, a common childhood exanthem that in adults more usually manifests as an arthralgia/arthritis syndrome. In persons with underlying hemolysis, acute infection produces transient aplastic crisis, precipitous anemia due to hypoproliferative erythropoiesis. B19 infection can be persistent (3). In the setting of immunodeficiency, due to congenital, acquired, or iatrogenic immunodeficiency, persistent parvovirus results in chronic pure red cell aplasia. In utero infection causes hydrops fetalis, with fetal death due to severe anemia and congestive heart failure (4).Clinical studies of B19 parvovirus infection have been hampered by limited supplies of viral antigen. The virus has extraordinary tropism for human erythroid progenitor cells and has only been propagated in explanted human bone marrow (5-8), fetal liver (9), and (to a lesser degree) erythroleukemia cells (10). We describe here expression of the virus's structural proteins in a baculovirus system. Large quantities of empty viral capsids were produced, which can substitute for serum virus in clinical assays and stimulate the production of neutralizing antibodies in animals. Only the major protein was required for capsid assembly; the minor capsid protein serves an important role in virus function independent of virion assembly. MATERIALS AND METHODSCell Culture and Virus Stocks. Autographa californica nuclear polyhedrosis virus (AcMNPV) and recombinant viruses were grown in monolayers of Sf9 cells. Sf9 c...

Immunosuppressive therapy can produce hematologic improvement in a large proportion of patients with severe aplastic anemia. Antithymocyte globulin (ATG) is the current treatment of choice for patients who do not have histocompatible sibling donors or who are otherwise inegligible for allogeneic bone marrow transplantation. About 50% of patients respond to an initial course of ATG, and many nonresponders can be salvaged by subsequent treatment with cyclosporine (CsA). To determine whether simultaneous administration of these agents could further improve response rates, we enrolled 55 patients in a therapeutic trial of 4 days of ATG and 6 months of CsA. Among the 51 patients who had not received previous courses of ATG or CsA, 67% had responded by 3 months, and 78% had responded by 1 year (response was defined as an increase in peripheral blood counts sufficient that a patient no longer met the criteria for severe disease). There was a high incidence of relapse (36% actuarial risk at 2 years), but most relapsed patients responded to additional courses of immunosuppression, and relapse was not associated with a significant survival disadvantage. Evolution to myelodysplastic syndromes and acute leukemia was rare (1 of 51 patients), but the later appearance of paroxysmal nocturnal hemoglobinuria was more common (5 of 51 patients). Actuarial survival was 86% at 1 year and 72% at 2 years. These data support the use of a combination immunosuppressive regimen containing both ATG and CsA as first-line therapy for severe aplastic anemia.