Immunophenotyping of fetal haemopoietic cells permissive for human parvovirus B19 replication in vitro

Detection and analysis of parvovirus B19 among blood donors in a regional blood center in Eastern China

Background and objective

The B19 virus is mainly transmitted through the respiratory tract; however, studies have shown that it can also be transmitted through blood transfusions or plasma products. This study investigated B19V antibodies, DNA, and gene typing in blood donors at a central blood station in China to evaluate the status of B19V infection.

Materials and methods

A total of 7728 samples from Suzhou Blood Center were collected from July 2022 to April 2023. Samples were detected for the B19V DNA using real-time polymerase chain reaction. Furthermore, 893 selected samples were screened for the seroprevalence of B19V antibodies using enzyme-linked immunosorbent assay. The NS1-VP1u fragment of the B19V DNA-positive samples was amplified using nested PCR, and the sequences were determined. A B19V phylogenetic tree was constructed using neighborhood joint and maximum parsimony methods to discriminate genotypes using the NS1-VP1u sequences.

Results

The percentages of IgG, IgM, and DNA were 19.4 %, 1.9 %, and 0.09 %, respectively. IgG positivity increased with age, and there was a significant difference among the blood groups. The IgG levels of repeat donors were greater than those of first-time donors. There were no apparent differences in the IgM levels in all the participants. Genotyping revealed that the B19 genotype was 1.

Conclusions

The prevalence of B19V antibodies and DNA was lower in these areas than in rest of China, indicating that the risk of B19V transmission via transfusion may be relatively low. However, during transfusion, particular attention should be paid to the B19V-susceptible populations, especially those in high-risk groups.

The N-terminal 5-68 amino acids domain of the minor capsid protein VP1 of human parvovirus B19 enters human erythroid progenitors and inhibits B19 infection

Parvovirus B19 (B19V) infection causes diseases in humans ranging from the mild erythema infectiosum to severe hematological disorders. The unique region of the minor structural protein VP1 (VP1u) of 227 amino acids harbors strong neutralizing epitopes which elicit dominant immune responses in patients. Recent studies have shown that the VP1u selectively binds to and enters B19V permissive cells through an unknown cellular proteinaceous receptor. In the present study, we demonstrated that purified recombinant VP1u effectively inhibits B19V infection of ex vivo expanded primary human erythroid progenitors. Furthermore, we identified the amino acid sequence 5-68 of the VP1 (VP1u^5-68aa) is sufficient to confer the inhibition of B19V infection at a level similar to that of the full-length VP1u. In silico structure prediction suggests that the VP1u^5-68aa contains three α-helices. Importantly, we found that the inhibition capability of the minimal domain VP1u^5-68aa is independent of its dimerization but is likely dependent on the structure of the three predicated α-helices. As VP1u^5-68aa outcompetes the full-length VP1u in entering cells, we believe that VP1u^5-68aa functions as a receptor-binding ligand during virus entry. Finally, we determined the effective inhibition potency of VP1u^5-68aa in B19V infection of human erythroid progenitors, which has a half maximal effective concentration (EC₅₀) of 67 nM, suggesting an anti-viral peptide candidate to combat B19V infection.IMPORTANCEHuman parvovirus B19 infection causes severe hematological disorders, including transient aplastic crisis, pure red cell aplasia, and hydrops fetalis. A productive B19 infection is highly restricted to human erythroid progenitors in human bone marrow and fetal liver. In the current study, we identified that the N-terminal 5-68 amino acids domain of the minor viral capsid protein VP1 enters ex vivo expanded human erythroid progenitors, which is nearly 5 times more efficient than the full-length VP1 unique region (1-227aa). Importantly, purified recombinant 5-68aa of the VP1 has a high efficiency in inhibition of parvovirus B19 infection of human erythroid progenitors, which has a half maximal effective concentration (EC₅₀) of 67 nM and a low cytotoxicity. The N-terminal 5-68 amino acids holds the potential as an effective antiviral of parvovirus B19 caused hematological disorders, as well as a carrier to deliver proteins to human erythroid progenitors.

Endonuclease Activity Inhibition of the NS1 Protein of Parvovirus B19 as a Novel Target for Antiviral Drug Development

Human parvovirus B19 (B19V), a member of the genus Erythroparvovirus of the family Parvoviridae, is a small nonenveloped virus that has a single-stranded DNA (ssDNA) genome of 5.6 kb with two inverted terminal repeats (ITRs). B19V infection often results in severe hematological disorders and fetal death in humans. B19V replication follows a model of rolling hairpin-dependent DNA replication, in which the large nonstructural protein NS1 introduces a site-specific single-strand nick in the viral DNA replication origins, which locate at the ITRs. NS1 executes endonuclease activity through the N-terminal origin-binding domain. Nicking of the viral replication origin is a pivotal step in rolling hairpin-dependent viral DNA replication. Here, we developed a fluorophore-based in vitro nicking assay of the replication origin using the origin-binding domain of NS1 and compared it with the radioactive in vitro nicking assay. We used both assays to screen a set of small-molecule compounds (n = 96) that have potential antinuclease activity. We found that the fluorophore-based in vitro nicking assay demonstrates sensitivity and specificity values as high as those of the radioactive assay. Among the 96 compounds, we identified 8 which have an inhibition of >80% at 10 µM in both the fluorophore-based and radioactive in vitro nicking assays. We further tested 3 compounds that have a flavonoid-like structure and an in vitro 50% inhibitory concentration that fell in the range of 1 to 3 µM. Importantly, they also exhibited inhibition of B19V DNA replication in UT7/Epo-S1 cells and ex vivo-expanded human erythroid progenitor cells.

Recent Advances in Replication and Infection of Human Parvovirus B19

Parvovirus B19 (B19V) is pathogenic to humans and causes bone marrow failure diseases and various other inflammatory disorders. B19V infection exhibits high tropism for human erythroid progenitor cells (EPCs) in the bone marrow and fetal liver. The exclusive restriction of B19V replication to erythroid lineage cells is partly due to the expression of receptor and co-receptor(s) on the cell surface of human EPCs and partly depends on the intracellular factors essential for virus replication. We first summarize the latest developments in the viral entry process and the host cellular factors or pathways critical for B19V replication. We discuss the role of hypoxia, erythropoietin signaling and STAT5 activation in the virus replication. The B19V infection-induced DNA damage response (DDR) and cell cycle arrest at late S-phase are two key events that promote B19V replication. Lately, the virus infection causes G2 arrest, followed by the extensive cell death of EPCs that leads to anemia. We provide the current understanding of how B19V exploits the cellular resources and manipulate pathways for efficient virus replication. B19V encodes a single precursor mRNA (pre-mRNA), which undergoes alternate splicing and alternative polyadenylation to generate at least 12 different species of mRNA transcripts. The post-transcriptional processing of B19V pre-mRNA is tightly regulated through cis-acting elements and trans-acting factors flanking the splice donor or acceptor sites. Overall, in this review, we focus on the recent advances in the molecular virology and pathogenesis of B19V infection.

Human Parvovirus B19 Utilizes Cellular DNA Replication Machinery for Viral DNA Replication

Human parvovirus B19 (B19V) infection of human erythroid progenitor cells (EPCs) induces a DNA damage response and cell cycle arrest at late S phase, which facilitates viral DNA replication. However, it is not clear exactly which cellular factors are employed by this single-stranded DNA virus. Here, we used microarrays to systematically analyze the dynamic transcriptome of EPCs infected with B19V. We found that DNA metabolism, DNA replication, DNA repair, DNA damage response, cell cycle, and cell cycle arrest pathways were significantly regulated after B19V infection. Confocal microscopy analyses revealed that most cellular DNA replication proteins were recruited to the centers of viral DNA replication, but not the DNA repair DNA polymerases. Our results suggest that DNA replication polymerase δ and polymerase α are responsible for B19V DNA replication by knocking down its expression in EPCs. We further showed that although RPA32 is essential for B19V DNA replication and the phosphorylated forms of RPA32 colocalized with the replicating viral genomes, RPA32 phosphorylation was not necessary for B19V DNA replication. Thus, this report provides evidence that B19V uses the cellular DNA replication machinery for viral DNA replication.IMPORTANCE Human parvovirus B19 (B19V) infection can cause transient aplastic crisis, persistent viremia, and pure red cell aplasia. In fetuses, B19V infection can result in nonimmune hydrops fetalis and fetal death. These clinical manifestations of B19V infection are a direct outcome of the death of human erythroid progenitors that host B19V replication. B19V infection induces a DNA damage response that is important for cell cycle arrest at late S phase. Here, we analyzed dynamic changes in cellular gene expression and found that DNA metabolic processes are tightly regulated during B19V infection. Although genes involved in cellular DNA replication were downregulated overall, the cellular DNA replication machinery was tightly associated with the replicating single-stranded DNA viral genome and played a critical role in viral DNA replication. In contrast, the DNA damage response-induced phosphorylated forms of RPA32 were dispensable for viral DNA replication.

Parvovirus B19 NS1 protein induces cell cycle arrest at G2-phase by activating the ATR-CDC25C-CDK1 pathway

Human parvovirus B19 (B19V) infection of primary human erythroid progenitor cells (EPCs) arrests infected cells at both late S-phase and G2-phase, which contain 4N DNA. B19V infection induces a DNA damage response (DDR) that facilitates viral DNA replication but is dispensable for cell cycle arrest at G2-phase; however, a putative C-terminal transactivation domain (TAD2) within NS1 is responsible for G2-phase arrest. To fully understand the mechanism underlying B19V NS1-induced G2-phase arrest, we established two doxycycline-inducible B19V-permissive UT7/Epo-S1 cell lines that express NS1 or NS1mTAD2, and examined the function of the TAD2 domain during G2-phase arrest. The results confirm that the NS1 TAD2 domain plays a pivotal role in NS1-induced G2-phase arrest. Mechanistically, NS1 transactivated cellular gene expression through the TAD2 domain, which was itself responsible for ATR (ataxia-telangiectasia mutated and Rad3-related) activation. Activated ATR phosphorylated CDC25C at serine 216, which in turn inactivated the cyclin B/CDK1 complex without affecting nuclear import of the complex. Importantly, we found that the ATR-CHK1-CDC25C-CDK1 pathway was activated during B19V infection of EPCs, and that ATR activation played an important role in B19V infection-induced G2-phase arrest.

Human Parvoviruses

Parvovirus B19 (B19V) and human bocavirus 1 (HBoV1), members of the large Parvoviridae family, are human pathogens responsible for a variety of diseases. For B19V in particular, host features determine disease manifestations. These viruses are prevalent worldwide and are culturable in vitro, and serological and molecular assays are available but require careful interpretation of results. Additional human parvoviruses, including HBoV2 to -4, human parvovirus 4 (PARV4), and human bufavirus (BuV) are also reviewed. The full spectrum of parvovirus disease in humans has yet to be established. Candidate recombinant B19V vaccines have been developed but may not be commercially feasible. We review relevant features of the molecular and cellular biology of these viruses, and the human immune response that they elicit, which have allowed a deep understanding of pathophysiology.

Frequency and significance of parvovirus B19 infection in patients with rheumatoid arthritis

The present study aims to clarify the possible involvement of parvovirus B19 (B19V) infection in rheumatoid arthritis (RA) pathogenesis by investigating the presence of B19V infection markers (genomic sequences and virus-specific antibodies) in association with the level of cytokines and RA clinical activity and aggressiveness. A total of 118 RA patients and 49 age- and sex-matched healthy volunteers were enrolled in the study. Nested PCR was used to detect B19V sequences in whole blood and cell-free plasma DNA, ELISA to detect virus-specific antibodies and cytokine levels in plasma and recomLine dot blot assay for antibodies to separate B19V antigens. The detection frequency of B19V DNA was higher in patients with RA (25.4 %) in comparison with healthy persons (18.4 %). B19V DNA in cell-free plasma (B19⁺p) was detected significantly often in RA patients in comparison with healthy controls (13.6 vs 2 %; P=0.0002). RA B19⁺p patients had higher disease activity and aggressiveness, decreased haemoglobin and increased erythrocyte sedimentation rates. IL-6 plasma levels were significantly higher in RA patients than in controls. Within the RA patients’ group the IL-6 level was significantly increased in B19⁺p patients with disease activity scores of DAS28>5.2, high C-reactive protein and low haemoglobin. Contrary to the healthy controls, the majority of RA B19⁺p patients did not have antibodies to VP-1S (VP1u) and VP-N (N-terminal half of structural proteins VP1 and VP2), which correspond to the epitopes of neutralizing antibodies. These results indicate that B19V infection at least in some patients is involved in RA pathogenesis.

The VP1u Receptor Restricts Parvovirus B19 Uptake to Permissive Erythroid Cells

Parvovirus B19 (B19V) is a small non-enveloped virus and known as the causative agent for the mild childhood disease erythema infectiosum. B19V has an extraordinary narrow tissue tropism, showing only productive infection in erythroid precursor cells in the bone marrow. We recently found that the viral protein 1 unique region (VP1u) contains an N-terminal receptor-binding domain (RBD), which mediates the uptake of the virus into cells of the erythroid lineage. To further investigate the role of the RBD in connection with a B19V-unrelated capsid, we chemically coupled the VP1u of B19V to the bacteriophage MS2 capsid and tested the internalization capacity of the bioconjugate on permissive cells. In comparison, we studied the cellular uptake and infection of B19V along the erythroid differentiation. The results showed that the MS2-VP1u bioconjugate mimicked the specific internalization of the native B19V into erythroid precursor cells, which further coincides with the restricted infection profile. The successful mimicry of B19V uptake demonstrates that the RBD in the VP1u is sufficient for the endocytosis of the viral capsid. Furthermore, the recombinant VP1u competed with B19V uptake into permissive cells, thus excluding a significant alternative uptake mechanism by other receptors. Strikingly, the VP1u receptor appeared to be expressed only on erythropoietin-dependent erythroid differentiation stages that also provide the necessary intracellular factors for a productive infection. Taken together, these findings suggest that the VP1u binds to a yet-unknown erythroid-specific cellular receptor and thus restricts the virus entry to permissive cells.

Human parvovirus B19 antibodies induce altered membrane protein expression and apoptosis of BeWo trophoblasts

Human parvovirus B19 (B19) is harmful during pregnancy since it can be vertically transmitted to the developing fetus. In addition, the anti‑B19 antibodies induced by B19 infection are believed to have a cytopathic role in B19 transmission; however, knowledge regarding the effects of anti‑B19 antibodies during pregnancy is limited. To investigate the possible roles of anti‑B19 antibodies during pregnancy, the present study examined the effects of anti‑B19‑VP1 unique region (VP1u), anti‑B19‑VP2 and anti‑B19‑nonstructural protein 1 (NS1) immunoglobulin G (IgG) antibodies on BeWo trophoblasts. Briefly, BeWo trophoblasts were incubated with purified IgG against B19‑VP1u, B19‑VP2 and B19‑NS1. Subsequently, the expression of surface proteins and apoptotic molecules were assessed in BeWo trophoblasts using flow cytometry, ELISA and western blotting. The expression levels of human leukocyte antigen (HLA)‑G were significantly increased on BeWo trophoblasts treated with rabbit anti‑B19‑VP1u IgG, and were unchanged in those treated with rabbit anti‑B19‑NS1 and anti‑B19‑VP2 IgG, as compared with the control group. Furthermore, the expression levels of globoside (P blood group antigen) and cluster of differentiation (CD)29 (β1 integrin) were significantly increased in BeWo trophoblasts treated with rabbit anti‑B19‑NS1 and anti‑B19‑VP2 IgG, whereas only CD29 was also significantly increased in cells treated with anti‑B19‑VP1u IgG. In addition, the number of cells at sub‑G1 phase; caspase‑3 activity; and the expression of intrinsic and extrinsic apoptotic molecules, including Fas‑associated death domain protein, activated caspase‑8, activated caspase‑3, B‑cell lymphoma 2‑associated X protein, cytochrome c, apoptotic peptidase activating factor 1 and activated caspase‑9, were significantly increased in BeWo trophoblasts treated with anti‑B19‑VP1u and anti‑B19‑NS1 IgG. In conclusion, the present study demonstrated that antibodies against B19 may have a crucial role in pathological processes during pregnancy. These findings may help to elucidate the mechanisms underlying transmission of the B19 virus during pregnancy.

Human parvovirus B19: a mechanistic overview of infection and DNA replication

Human parvovirus B19 (B19V) is a human pathogen that belongs to genus Erythroparvovirus of the Parvoviridae family, which is composed of a group of small DNA viruses with a linear single-stranded DNA genome. B19V mainly infects human erythroid progenitor cells and causes mild to severe hematological disorders in patients. However, recent clinical studies indicate that B19V also infects nonerythroid lineage cells, such as myocardial endothelial cells, and may be associated with other disease outcomes. Several cell culture systems, including permissive and semipermissive erythroid lineage cells, nonpermissive human embryonic kidney 293 cells and recently reported myocardial endothelial cells, have been used to study the mechanisms underlying B19V infection and B19V DNA replication. This review aims to summarize recent advances in B19V studies with a focus on the mechanisms of B19V tropism specific to different cell types and the cellular pathways involved in B19V DNA replication including cellular signaling transduction and cell cycle arrest.

Human parvovirus B19 infection causes cell cycle arrest of human erythroid progenitors at late S phase that favors viral DNA replication

Human parvovirus B19 (B19V) infection has a unique tropism to human erythroid progenitor cells (EPCs) in human bone marrow and the fetal liver. It has been reported that both B19V infection and expression of the large nonstructural protein NS1 arrested EPCs at a cell cycle status with a 4 N DNA content, which was previously claimed to be "G2/M arrest." However, a B19V mutant infectious DNA (M20(mTAD2)) replicated well in B19V-semipermissive UT7/Epo-S1 cells but did not induce G2/M arrest (S. Lou, Y. Luo, F. Cheng, Q. Huang, W. Shen, S. Kleiboeker, J. F. Tisdale, Z. Liu, and J. Qiu, J. Virol. 86:10748-10758, 2012). To further characterize cell cycle arrest during B19V infection of EPCs, we analyzed the cell cycle change using 5-bromo-2'-deoxyuridine (BrdU) pulse-labeling and DAPI (4',6-diamidino-2-phenylindole) staining, which precisely establishes the cell cycle pattern based on both cellular DNA replication and nuclear DNA content. We found that although both B19V NS1 transduction and infection immediately arrested cells at a status of 4 N DNA content, B19V-infected 4 N cells still incorporated BrdU, indicating active DNA synthesis. Notably, the BrdU incorporation was caused neither by viral DNA replication nor by cellular DNA repair that could be initiated by B19V infection-induced cellular DNA damage. Moreover, several S phase regulators were abundantly expressed and colocalized within the B19V replication centers. More importantly, replication of the B19V wild-type infectious DNA, as well as the M20(mTAD2) mutant, arrested cells at S phase. Taken together, our results confirmed that B19V infection triggers late S phase arrest, which presumably provides cellular S phase factors for viral DNA replication.

Parvovirus infection-induced DNA damage response

Parvoviruses are a group of small DNA viruses with ssDNA genomes flanked by two inverted terminal structures. Due to a limited genetic resource they require host cellular factors and sometimes a helper virus for efficient viral replication. Recent studies have shown that parvoviruses interact with the DNA damage machinery, which has a significant impact on the life cycle of the virus as well as the fate of infected cells. In addition, due to special DNA structures of the viral genomes, parvoviruses are useful tools for the study of the molecular mechanisms underlying viral infection-induced DNA damage response (DDR). This review aims to summarize recent advances in parvovirus-induced DDR, with a focus on the diverse DDR pathways triggered by different parvoviruses and the consequences of DDR on the viral life cycle as well as the fate of infected cells.

Human parvovirus B19 DNA replication induces a DNA damage response that is dispensable for cell cycle arrest at phase G2/M

Human parvovirus B19 (B19V) infection is highly restricted to human erythroid progenitor cells, in which it induces a DNA damage response (DDR). The DDR signaling is mainly mediated by the ATR (ataxia telangiectasia-mutated and Rad3-related) pathway, which promotes replication of the viral genome; however, the exact mechanisms employed by B19V to take advantage of the DDR for virus replication remain unclear. In this study, we focused on the initiators of the DDR and the role of the DDR in cell cycle arrest during B19V infection. We examined the role of individual viral proteins, which were delivered by lentiviruses, in triggering a DDR in ex vivo-expanded primary human erythroid progenitor cells and the role of DNA replication of the B19V double-stranded DNA (dsDNA) genome in a human megakaryoblastoid cell line, UT7/Epo-S1 (S1). All the cells were cultured under hypoxic conditions. The results showed that none of the viral proteins induced phosphorylation of H2AX or replication protein A32 (RPA32), both hallmarks of a DDR. However, replication of the B19V dsDNA genome was capable of inducing the DDR. Moreover, the DDR per se did not arrest the cell cycle at the G(2)/M phase in cells with replicating B19V dsDNA genomes. Instead, the B19V nonstructural 1 (NS1) protein was the key factor in disrupting the cell cycle via a putative transactivation domain operating through a p53-independent pathway. Taken together, the results suggest that the replication of the B19V genome is largely responsible for triggering a DDR, which does not perturb cell cycle progression at G(2)/M significantly, during B19V infection.

Role of erythropoietin receptor signaling in parvovirus B19 replication in human erythroid progenitor cells

Parvovirus B19 (B19V) infection is highly restricted to human erythroid progenitor cells. Although previous studies have led to the theory that the basis of this tropism is receptor expression, this has been questioned by more recent observation. In the study reported here, we have investigated the basis of this tropism, and a potential role of erythropoietin (Epo) signaling, in erythroid progenitor cells (EPCs) expanded ex vivo from CD34(+) hematopoietic cells in the absence of Epo (CD36(+)/Epo(-) EPCs). We show, first, that CD36(+)/Epo(-) EPCs do not support B19V replication, in spite of B19V entry, but Epo exposure either prior to infection or after virus entry enabled active B19V replication. Second, when Janus kinase 2 (Jak2) phosphorylation was inhibited using the inhibitor AG490, phosphorylation of the Epo receptor (EpoR) was also inhibited, and B19V replication in ex vivo-expanded erythroid progenitor cells exposed to Epo (CD36(+)/Epo(+) EPCs) was abolished. Third, expression of constitutively active EpoR in CD36(+)/Epo(-) EPCs led to efficient B19V replication. Finally, B19V replication in CD36(+)/Epo(+) EPCs required Epo, and the replication response was dose dependent. Our findings demonstrate that EpoR signaling is absolutely required for B19V replication in ex vivo-expanded erythroid progenitor cells after initial virus entry and at least partly accounts for the remarkable tropism of B19V infection for human erythroid progenitors.

Parvovirus infection-induced cell death and cell cycle arrest

The cytopathic effects induced during parvovirus infection have been widely documented. Parvovirus infection-induced cell death is often directly associated with disease outcomes (e.g., anemia resulting from loss of erythroid progenitors during parvovirus B19 infection). Apoptosis is the major form of cell death induced by parvovirus infection. However, nonapoptotic cell death, namely necrosis, has also been reported during infection of the minute virus of mice, parvovirus H-1 and bovine parvovirus. Recent studies have revealed multiple mechanisms underlying the cell death during parvovirus infection. These mechanisms vary in different parvoviruses, although the large nonstructural protein (NS)1 and the small NS proteins (e.g., the 11 kDa of parvovirus B19), as well as replication of the viral genome, are responsible for causing infection-induced cell death. Cell cycle arrest is also common, and contributes to the cytopathic effects induced during parvovirus infection. While viral NS proteins have been indicated to induce cell cycle arrest, increasing evidence suggests that a cellular DNA damage response triggered by an invading single-stranded parvoviral genome is the major inducer of cell cycle arrest in parvovirus-infected cells. Apparently, in response to infection, cell death and cell cycle arrest of parvovirus-infected cells are beneficial to the viral cell lifecycle (e.g., viral DNA replication and virus egress). In this article, we will discuss recent advances in the understanding of the mechanisms underlying parvovirus infection-induced cell death and cell cycle arrest.

Ex vivo-generated CD36+ erythroid progenitors are highly permissive to human parvovirus B19 replication

The pathogenic parvovirus B19 (B19V) has an extreme tropism for human erythroid progenitor cells. In vitro, only a few erythroid leukemic cell lines (JK-1 and KU812Ep6) or megakaryoblastoid cell lines (UT7/Epo and UT7/Epo-S1) with erythroid characteristics support B19V replication, but these cells are only semipermissive. By using recent advances in generating large numbers of human erythroid progenitor cells (EPCs) ex vivo from hematopoietic stem cells (HSCs), we produced a pure population of CD36(+) EPCs expanded and differentiated from CD34(+) HSCs and assessed the CD36(+) EPCs for their permissiveness to B19V infection. Over more than 3 weeks, cells grown in serum-free medium expanded more than 800,000-fold, and 87 to 96% of the CD36(+) EPCs were positive for globoside, the cellular receptor for B19V. Immunofluorescence (IF) staining showed that about 77% of the CD36(+) EPCs were positive for B19V infection, while about 9% of UT7/Epo-S1 cells were B19V positive. Viral DNA detected by real-time PCR increased by more than 3 logs in CD36(+) EPCs; the increase was 1 log in UT7/Epo-S1 cells. Due to the extensive permissivity of CD36(+) EPCs, we significantly improved the sensitivity of detection of infectious B19V by real-time reverse transcription-PCR and IF staining 100- and 1,000-fold, respectively, which is greater than the sensitivity of UT7/Epo-S1 cell-based methods. This is the first description of an ex vivo method to produce large numbers of EPCs that are highly permissive to B19V infection and replication, offering a cellular system that mimics in vivo infection with this pathogenic human virus.

Ku80 autoantigen as a cellular coreceptor for human parvovirus B19 infection

Human parvovirus B19 (B19) infects human erythroid cells expressing P antigen. However, some cell lines that were positive for P antigen failed to bind B19, whereas some cell lines had an ability to bind B19 despite undetectable expression of P antigen. We here demonstrate that B19 specifically binds with Ku80 autoantigen on the cell surface. Furthermore, transfection of HeLa cells with the gene of Ku80 enabled the binding of B19 and allowed its entry into cells. Moreover, reduction of cell-surface expression of Ku80 in KU812Ep6 cells, which was a high-sensitive cell line for B19 infection, by short interfering RNA for Ku80 resulted in the marked inhibition of B19 binding in KU812Ep6 cells. Although Ku80 originally has been described as a nuclear protein, human bone marrow erythroid cells with glycophorin A or CD36, B cells with CD20, or T cells with CD3 were all positive for cell-surface expression of Ku80. B19 infection of KU812Ep6 cells and bone marrow cells was inhibited in the presence of anti-Ku80 antibody. Our data suggest that Ku80 functions as a novel coreceptor for B19 infection, and this finding may provide an explanation for the pathologic immunity associated with B19 infection.

Parvovirus B19-associated transient red cell aplasia in children: the role of bone marrow examination in unusual presentations

Human parvovirus B19 (PV B19) infection in children commonly presents as fifth disease. Transient red cell crisis, the other manifestation of PV B19 infection, is usually reported in children with chronic hemolytic anemia, with a worsening of the anemia. However, this condition may pass unrecognized in children without an underlying hemolytic disorder, since the anemia may be of a short duration and self-limiting. The authors report 3 cases of PV B19-induced transient aplastic in different clinical settings--pancytopenia in one child, during induction phase for acute lymphoblastic leukemia in the second, and fever with joint pains in the third. Treatment for PV B19-induced transient aplastic crisis is essentially supportive. There may be a dilemma in patients on immunosuppressive therapy, since initially it is difficult to distinguish between chronic pure red cell aplasia (a condition where intravenous immunoglobulin therapy is beneficial) and transient aplastic crisis, where supportive red cell transfusions suffice. The patient with leukemia also recovered spontaneously despite being on steroids. In all the 3 patients, the pure red cell aplasia recovered spontaneously without administration of intravenous gammaglobulins.

Parvovirus B19 and the pathogenesis of anaemia

Human parvovirus B19 (B19) infection causes human bone marrow failure, by affecting erythroid-lineage cells which are well-known target cells for B19. The anaemia induced by B19 infection is of minor clinical significance in healthy children and adults, however, it becomes critical in those afflicted with haemolytic diseases. This condition is called transient aplastic crisis, and the pathogenesis is explained by the short life-span of red blood cells. Similarly, fetuses are thought to be severely affected by B19-intrauterine infection in the first and second trimester, as the half-life of red blood cells is apparently shorter than RBC at the bone marrow haematopoietic stage. On the other hand, B19 is also the causative agent of persistent anaemia in immunocompromised patients, transplant recipients and infants. The deficiencies of appropriate immune responses to B19 impair viral elimination in vivo, which results in enlargement of B19-infected erythroid-lineage cells. The B19-associated damage of erythroid lineage cells is due to cytotoxicity mediated by viral proteins. B19-infected erythroid-lineage cells show apoptotic features, which are thought to be induced by the non-structural protein, NS1, of B19. In addition, B19 infection induces cell cycle arrests at the G(1) and G(2) phases. The G(1) arrest is induced by NS1 expression prior to apoptosis induction in B19-infected cells, while the G(2) arrest is induced not only by infectious B19 but also by UV-inactivated B19, which lacks the ability to express NS1. In this review, we address the clinical manifestations and molecular mechanisms for B19-induced anaemia in humans and a mouse model, and of B19-induced cell cycle arrests in erythroid cells.

Inactivation of parvovirus B19 during pasteurization of human serum albumin

BACKGROUND

It has been shown that HSA may be contaminated with parvovirus B19 (B19) DNA. However, the presence of B19 DNA does not necessarily indicate infectious virus. HSA is pasteurized at 60 degrees C for 10 hours and it remains unclear whether this procedure inactivates B19. Studies with animal parvoviruses indicate considerable heat resistance at 60 degrees C. However, due to the lack of a suitable cell culture system, the pasteurization process has not been investigated in the past.

STUDY DESIGN AND METHODS

The recently described cell clone KU812Ep6 was used to establish a system for investigation of B19 inactivation during pasteurization. Virus-infected cells were detected by immunofluorescent staining of viral capsid antigen and by RT-PCR assay of virus-specific capsid mRNA.

RESULTS

B19 was inactivated after 10 minutes at 60 degrees C for > or = 4 log. In contrast, porcine parvovirus was widely resistant at 60 degrees C. Inactivation of B19 was independent of the analyzed albumin products (5, 20, and 25% albumin from three manufacturers) and from the specific virus source used for the inactivation experiments. Degradation of B19 DNA by deoxyribonuclease I treatment after pasteurization indicated that the virus capsid is destroyed during heat treatment.

CONCLUSION

Heat resistance of B19 markedly differs from heat resistance of animal parvoviruses. While animal parvoviruses widely withstand pasteurization of albumin, B19 was rapidly inactivated. These results confirm the safety of pasteurized albumin and are in line with its good clinical safety record with respect to B19 infection. However, conclusions regarding the safety of other blood-derived medicinal products should not be derived from B19 inactivation in albumin, because different processes or different composition of product intermediates may significantly influence B19 stability during heat treatment.

Parvovirus B19 and erythroid cells

Parvovirus B19 is a human erythrovirus, i.e. which induces the death of erythroid progenitors. In such cells, until now only ubiquitous transcription factors have been described to regulate promoter driven gene expression. Their possible interactions with erythroid specific transcription factors merit further investigations. Effectively, the high level of replication of B19 in erythroid cells is not well understood. In addition to apoptosis, necrosis or inhibition of cell growth, the death of B19 infected erythroid progenitors has been never clearly reported as the result of immunological attack: this mecanism will merit further investigations. The interactions with other cell types in vitro remain at present not well defined but many obstacles have been mentioned which counteract B19 expression.

Expression of p53 and Ki-67 antigen in bone marrow giant proerythroblasts associated with human parvovirus B19 infection

Giant proerythroblasts are hallmarks of human parvovirus B19 infection. We attempted to characterize these cells in 5 patients with parvovirus B19-induced pure red cell aplasia using immunostaining of paraffin-embedded bone marrow sections with antibodies against erythroid-lineage-specific proteins, viral capsid antigen VP-1, and apoptosis- and cell-cycle-related proteins. Giant proerythroblasts are immunohistochemically consistent with early erythroid precursors of cells in the differentiation stage of CD34-, cytoplasmic spectrin+, glycophorin A-, and band-3-. VP-1 was expressed in the nucleus and cytoplasm of small- to medium-sized spectrin+ erythroid cells but not in giant proerythroblasts. The giant proerythroblasts displayed nuclear staining for p53 (41%+/-16%) and Ki-67 antigen (100%+/-0%) and cytoplasmic staining for Bax (65%+/-11%) and procaspase-3 (78%+/-10%), whereas they were not stained for p21Wafl/Cip1, active form of caspase-3, or terminal deoxynucleotidyltransferase-mediated deoxyuridine nick-end labeling (TUNEL). Antiapoptotic proteins, Bcl-2 and Mcl-1, were not expressed in the giant cells, and Bcl-x was infrequently expressed in these cells (11%+/-4%). These immunohistochemical findings suggest that giant proerythroblasts are proliferating erythroid precursors with accumulation of nonfunctional p53.

Immunohistochemical identification of erythroid precursors in paraffin embedded bone marrow sections: spectrin is a superior marker to glycophorin

AIM

To investigate whether spectrin can be used as an immunohistochemical marker for erythroid precursors in routinely processed paraffin embedded bone marrow sections.

METHODS

Bone marrow biopsies and clot sections were stained with rabbit antihuman erythrocyte spectrin antibodies, specific for erythroid cells as shown by western blotting and bone marrow smears, and compared to sections stained with antiglycophorin monoclonal antibodies (JC159 and Ret49f).

RESULTS

Antispectrin antibodies resulted in diffuse cytoplasmic staining of early erythroblasts and membranous staining of late erythroblasts as well as erythrocytes. In haematopathological samples, immature erythroid cell clusters were clearly identified. In contrast, antiglycophorin monoclonal antibodies resulted in only membranous staining of late erythroblasts, and faint staining of early erythroblasts.

CONCLUSIONS

Spectrin may be a superior marker to glycophorin for the identification of erythroid precursors in paraffin embedded sections.

Novel cytomorphology of the giant proerythroblasts of parvovirus B19 infection

The morphology of the giant proerythroblasts (GPE) in air-dried and Wright-Giemsa-stained smears of bone marrow in 16 patients with pure red cell aplasia (PRCA) caused by parvovirus B19 infection is described. B19 infection was diagnosed by the presence of the virus or viral DNA and/or IgM antibodies. Twelve patients had chronic hemolytic anemia and aplastic crisis and 4 patients had AIDS with chronic PRCA. In patients with chronic hemolytic anemia and aplastic crisis, GPE were not detectable in bone marrow biopsies that showed any degree of recovery of erythropoiesis. The GPE morphology was quite variable. The early (basophilic) GPE measured 25 to 35 microm in diameter, had a narrow rim of intensely blue and often vacuolated cytoplasm with pseudopodia, round nuclei with compact uncondensed chromatin, and an indistinct and inclusion-like purple-colored tinctorial change. The "intermediate" and "late" GPE measured 25 to 45 microm in diameter and showed cytoplasmic swelling, gradual loss of cytoplasmic basophilia, and fraying of the cytoplasm with focal rupture; the nuclei showed an increase in volume, a highly uncondensed and coarse sieve-like chromatin, and 1 to 3 prominent, pale to moderate purple inclusion-like nucleoli or inclusions. Bare nuclei similar in size and chromatin pattern to those of the GPE were present in proximity to the GPE and may have arisen from the GPE by dissolution of the cytoplasm. The glassy intranuclear inclusions with central clearing, the so-called lantern cells described in formalin-fixed tissues of patients with B19 infection, were absent in all cases. These findings suggest that direct toxic cell injury rather than apoptosis may be involved in the pathogenesis of erythroid aplasia in B19 infection.

Parvovirus B19 infection

Human parvovirus B19, discovered in 1974, is a single-stranded DNA virus which causes erythema infectiosum, arthralgia, aplastic crisis in patients with red cell defects, chronic anaemia in immunocompromised patients, and fetal hydrops. Seroprevalence in developed countries is 2-10% in children less than 5 years, 40-60% in adults more than 20 years, and 85% or more in those over 70 years. The virus may be transmitted by the respiratory route and by transfusion of infected blood and blood products. After an incubation period of six to eight days, viraemia occurs, during which reticulocyte numbers fall dramatically resulting in a temporary drop in haemoglobin of 1 g/dl in a normal person. Clearance of viraemia is dependent on development of specific antibody to the B19 structural proteins, VP1 and VP2. The red cell receptor for the virus is blood group P antigen. Diagnosis in immunocompetent persons depends on detection of specific IgM in serum. Diagnosis in immunocompromised persons depends on detection of B19 antigen or DNA in serum. There is no specific treatment for B19 infection; however, human normal immunoglobulin may be used as a source of specific antibody in chronically infected persons. A recombinant parvovirus B19 vaccine is under development.

B19 parvovirus

B19 parvovirus is pathogenic in man and causes a variety of clinical illnesses, among them several haematological diseases. Acute infection of a host with underlying haemolysis produces transient aplastic crisis; of the midtrimester fetus, hydrops fetalis; and of an immunocompromised patient, pure red cell aplasia. The target of B19 parvovirus infection is the human erythroid progenitor cell. Infection is cytotoxic due to expression of the viral nonstructural protein. The virus can be propagated in cultures of human bone marrow, blood, and fetal liver. Humoral immunity normally terminates infection, and commercially available immunoglobulin can be used to treat persistent infection. Recombinant capsids, produced in a baculovirus system, are suitable as a vaccine reagent.

Combined immunocytochemistry and non-isotopic in situ hybridization for the ultrastructural investigation of human parvovirus B19 infection

Parvovirus B19 is a single-stranded DNA virus with a specific tropism for human erythroid precursor cells. The virus codes for two overlapping structural (capsid) proteins and one non-structural protein which is thought to perform essential functions in viral replication, transcription and packaging. The ultrastructural localization of these proteins was achieved in cultured haemopoietic cells derived from fetal liver which had been infected in vitro and subsequently embedded in LR White acrylic resin. Postembedding immunogold detection of B19 structural and non-structural proteins was combined with localization of viral nucleic acid by in situ hybridization using a digoxigenin-labelled probe and different sized gold labels. The majority of the B19 capsid protein and DNA present in cells harvested 48 hours post-infection co-localized within the centri-nuclear region of erythroid cells demonstrating characteristic chromatin margination. Relatively little DNA hybridization signal was present over paracrystalline inclusions strongly labelled with anti-capsid protein monoclonal antibody R92F6. Viral DNA and capsid protein were co-localized in apparent egress from the nucleus through nuclear pores. B19 non-structural protein was detected in association with both nuclear and cytoplasmic arrays of capsids, supporting the view that this protein plays an important role in viral packaging and remains associated with the complete viral particle until its release from the cell. Co-localization of viral nucleic acid and proteins at the ultrastructural level is a flexible, rapid and highly specific tool for examination of viral life-cycles within cells.

A case of pure red cell aplasia: follow-up on different immunosuppressive regimens

A 66-year-old patient was admitted to our hospital in January 1992 for further evaluation of severe normocytic anemia. Hemoglobin (Hb) was 3.5 g/dl, reticulocyte count 1%. Bone marrow showed a nearly complete lack of red cell precursors, thus favoring the diagnosis of acquired pure red cell aplasia (PRCA). Immunosuppressive therapy with prednisolone was started but had to be supplemented with azathioprine because of a further rapid decrease in Hb to 3.7 g/dl after an initial transfusion of 6 U red blood cells. However, with this regimen a renewed decrease in Hb to 6.6 g/dl was noted, and further transfusions were required. Therefore therapy was switched to cyclosporine A (CyA) while tapering off prednisolone. Four months after the initial diagnosis a positive parvovirus B19 IgM antibody was found. After the failure of hematological remission with three immunosuppressive regimens a course of high-dose intravenous immunoglobulins (IVIG) was administered in July 1992. Six weeks after IVIG therapy a peak hemoglobin concentration of 12.3 g/dl was noted, and further transfusion was not required. CyA was tapered off in October 1992. One month later CyA was reinstituted because of a relapse of PRCA but was unsuccessful until January 1993. At this time immunosuppressive CyA therapy was discontinued because of a periodontal abscess. In February 1993 a second IVIG infusion was given, and a second remission of PRCA was noted, showing an increase in hemoglobin up to 14.5 g/dl by November 1993. At the last follow-up visit in February 1994 our patient was still in complete hematological remission.

Parvovirus B19-associated haemophagocytosis in Evans syndrome: aplastic crisis accompanied by severe thrombocytopenia

A 36-year-old man who had been treated for Evans syndrome (ES) developed an aplastic crisis caused by acute human parvovirus B19 (HPV) infection. Profound thrombocytopenia (8.0 x 10(9)/l) followed with a sudden increase in platelet-associated IgG (PAIgG) (1376.9 ng/10(7) plts). Bone marrow examination revealed a considerable number of haemophagocytic histiocytes without any disturbance of megakaryopoiesis. To our knowledge this is the first case of aplastic crisis with virus-associated haemophagocytosis in a patient with ES, which provides an interesting insight into the mechanisms for thrombocytopenia in HPV infection.

Intracellular localization of parvovirus B19 nucleic acid at the ultrastructural level by in situ hybridization with digoxigenin-labelled probes

Conditions suitable for immunogold detection of digoxigenin-labelled DNA probes hybridized to parvovirus B19-infected erythroid cells embedded in Lowicryl K4M and LR White acrylic resins were established at the electron microscope level. The protocol was initially optimized using a positive control probe for whole human DNA which produced signal over the heterochromatin of all nucleated cells. In cultures harvested 2 days postinfection, B19 nucleic acid was detected mainly within the centrinuclear region of erythroid cells exhibiting characteristic margination of the chromatin. The B19 hybridization signal was largely unaffected by denaturation and was resistant to RNase digestion but sensitive to DNase digestion, indicating that it was mainly single-stranded B19 DNA. Relatively few gold particles were found over crystalline arrays of viral capsids, consistent with the observation that they are composed of mainly 'empty' capsids. B19 nucleic acid was detected in apparent transit from nucleus to cytoplasm through pores in the nuclear membrane. While the sensitivity of this system is limited by the fact that hybridization occurs only at the surface of the section, it is a rapid and specific means of localizing viral nucleic acids with a high degree of resolution.

Ultrastructural features of fetal erythroid precursors infected with parvovirus B19 in vitro: evidence of cell death by apoptosis

Human parvovirus B19 cannot be cultured in standard cell lines and relatively little is known about the intracellular life-cycle of the virus. In this study, ultrastructural features of B19 infection were examined using haemopoietic cell suspension cultures derived from human fetal liver. Erythroblasts from infected cultures frequently contained crystalline arrays of both full and empty virus-like particles. The number and size of these arrays increased with the duration of culture, and their location changed from exclusively nuclear at 24 h post-infection to both nuclear and cytoplasmic at 3 days post-infection. Arrays were occasionally found in cytoplasmic protuberances which appeared to be pinching off from the cell. The location of the arrays corresponded to the distribution of viral capsid protein determined by immunolabelling at the light microscope level. Cells containing viral crystalline arrays also exhibited nucleolar degeneration, extreme margination of the nuclear heterochromatin, and cytoplasmic vacuolation. These features are typical of cells undergoing individual programmed cell death or 'apoptosis'. The triggering of apoptosis in erythroid precursors by parvovirus B19 may help to explain the apparent lack of a strong inflammatory response to fetal B19 infection and may have implications for understanding the mechanisms of viral spread throughout the host.