ciated viruses are dependoviruses. The autonomous parvoviruses, which replicate in the absence of other viruses, are common animal pathogens; feline panleucopenia virus, Aleutian mink disease virus, minute virus of mice, and Kilham rat virus are examples of members of this genus. Despite differences in their biological behavior, both types of parvovirus have similar genetic organizations. The coding potential of the short genome is increased by utilization of overlapping transcripts and multiple reading frames. Structurally, their DNA is characterized by the presence of terminal hairpin structures that serve as initiation sites for replication through double-stranded intermediate forms. Two or three structurally similar capsid proteins are encoded by the right side of the genome by separate but overlapping RNA species. One to three noncapsid proteins that are thought to provide replicative functions are encoded by the left side. In the parvoviruses studied so far, separate promoters drive transcription from the left side and the middle of the genome. The pathogenic B19 human virus is, for a member of the Parvoviridae family, remarkable in its highly restricted tissue range. The molecular basis of this unusual biologic behavior remains uncertain. In erythroid bone marrow cells cultured in vitro, the pattern of B19 DNA replication resembles that of other parvoviruses (26). The protein species that compose the capsid and the noncapsid proteins also are analogous in number and size to those of other parvoviruses (13, 27). As we report here, however, the B19 transcription map differs in several fundamental aspects from that of other Parvoviridae. MATERIALS AND METHODS Cell culture. B19 parvovirus was propagated in human erythroid bone marrow cells that were obtained from patients with sickle cell disease after informed consent and 2395

The B19 parvovirus is a cause of bone marrow failure in humans. B19 is toxic to erythroid progenitor cells in vitro. Viral products possibly responsible for toxicity were explored by transfection of cloned B19 genome into HeLa cells. The nonstructural (NS) protein was detected in cells 30 h after transfection. Plasmids containing the B19 genome were transfected with selectable marker genes in stable transformation assays. Plasmids that contained the left side of the B19 genome, which encodes the NS protein of the virus, inhibited antibiotic-resistant colony formation. Transformation occurred when NS protein expression was blocked by mutation. Suppression of transformation by NS protein was not tissue specific, suggesting a role for NS protein in toxicity for nonpermissive cells without parvovirus replication or virion accumulation.

B19 parvovirus is a small single-stranded DNA virus with a genome that encodes only two structural proteins, designated VP1 and VP2. 60 copies of the structural proteins assemble into the viral capsid, with approximately 95% VP2 and 5% VP1. Recombinant empty capsids composed of VP2 alone or of VP2 and VP1 self-assemble into particles that are morphologically indistinguishable from full virions. Empty capsids containing both VP2 and VP1 elicit a strong neutralizing antibody response when used to immunize rabbits. Capsids containing only VP2 are similarly antigenic but elicit only weak neutralizing activity. We performed fine structure epitope mapping by measuring the reactivity of antisera raised against capsids composed of VP2 and VP1 or VP2 alone against 85 overlapping peptides spanning the sequence of the two structural proteins. A profile of the antigenic difference between empty capsids with and without VP1 was produced from the resulting data. This profile divided the sequence of the structural proteins into four regions that correlated well with expected viral structures. Thus, the addition of a small number of VP1 residues altered the antigenicity of the entire capsid. The major area of enhanced antigenicity is homologous to the spike of canine parvovirus, an area known to contain both neutralizing and host-range determinants. Our data are consistent with a model in which the unique region of VP1 is necessary for the virus to assume its mature capsid conformation.

In contrast to reports of documented nosocomial transmission of B19 parvovirus from patients in transient aplastic crisis, nosocomial transmission did not occur--even in the absence of isolation precautions--presumably from the lower level of B19 viremia in our chronically infected (rather than acutely infected) patient.

We have analyzed the coding capacity of B19 parvovirus transcripts by in vitro translation using the negative hybrid selection technique. Five different antisense oligonucleotides (18-mers) corresponding to different portions of the B19 genome were hybridized to RNA samples extracted from human erythroid bone marrow cells infected with B19 parvovirus in vitro, and RNase H was added to cleave specific B19 RNA molecules at selected sites. B19-specific translation products of these RNA samples were determined by immunoprecipitation. We localized the B19 nonstructural protein to the left-side transcript and the two capsid proteins to overlapping transcripts from the right side of the genome.

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OBJECTIVE: To assess the potential for nosocomial spread of parvovirus B19 from a chronically infected patient. DESIGN: Employees exposed to the index case and control (unexposed) employees were evaluated by baseline and follow up parvovirus B19 serologies and hematologic assessments, and completed baseline and follow up epidemiologic questionnaires. SETTING: A chronically infected patient was hospitalized on a hematology ward in a research referral hospital for 3.5 weeks prior to a diagnosis of parvovirus B19 infection and the institution of isolation precautions. METHODS: Sera were screened for parvovirus B19 DNA (dot blot analysis), and IgG and IgM anti-B19 antibodies (capture immunoassay). Hematologic assessment included CBC, differential, and reticulocyte count.

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