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B19 parvovirus has been shown to persist in some immunocompromised patients, and treatment with specific antibodies can lead to decreased quantities of circulating virus and hematologic improvement. A defective immune response to B19 parvovirus in these patients was shown by comparison of results using a capture RIA and immunoblotting. In normal individuals, examination of paired sera showed that the dominant humoral immune response during early convalescence was to the virus major capsid protein (58 kD) and during late convalescence to the minor capsid species (83 kD). In patients with persistent parvovirus infection, variable titers against intact particles were detected by RIA, but the sera from these patients had minimal or no IgG to capsid proteins determined by Western analysis. Competition experiments suggested that this discrepancy was not explicable on the basis of immune complex formation alone and that these patients may have a qualitative abnormality in antibody binding to virus. In neutralization experiments, in which erythroid colony formation in vitro was used as an assay of parvovirus activity, sera from patients with poor reactivity on immunoblotting were also inadequate in inhibiting viral infectivity. A cellular response to purified B19 parvovirus could not be demonstrated using proliferation assays and PBMC from individuals with serologic evidence of exposure to virus. These results suggest that production of neutralizing antibody to capsid protein plays a major role in limiting parvovirus infection in man.

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...