Characterization of cis-acting and NS1 protein-responsive elements in the p6 promoter of parvovirus B19

For better or worse: crosstalk of parvovirus and host DNA damage response

Parvoviruses are a group of non-enveloped DNA viruses that have a broad spectrum of natural infections, making them important in public health. NS1 is the largest and most complex non-structural protein in the parvovirus genome, which is indispensable in the life cycle of parvovirus and is closely related to viral replication, induction of host cell apoptosis, cycle arrest, DNA damage response (DDR), and other processes. Parvovirus activates and utilizes the DDR pathway to promote viral replication through NS1, thereby increasing pathogenicity to the host cells. Here, we review the latest progress of parvovirus in regulating host cell DDR during the parvovirus lifecycle and discuss the potential of cellular consequences of regulating the DDR pathway, targeting to provide the theoretical basis for further elucidation of the pathogenesis of parvovirus and development of new antiviral drugs.

Celastrol attenuates human parvovirus B19 NS1‑induced NLRP3 inflammasome activation in macrophages

Human parvovirus B19 (B19V) has been strongly associated with a variety of inflammatory disorders, such as rheumatoid arthritis (RA), inflammatory bowel disease and systemic lupus erythematosus. Non‑structural protein 1 (NS1) of B19V has been demonstrated to play essential roles in the pathological processes of B19V infection due to its regulatory properties on inflammatory cytokines. Celastrol, a quinone methide isolated from Tripterygium wilfordii, has displayed substantial potential in treating inflammatory diseases, such as psoriasis and RA. However, little is known about the effects of celastrol on B19V NS1‑induced inflammation. Therefore, cell viability assay, migration assay, phagocytosis analysis, zymography assay, ELISA and immunoblotting were conducted to verify the influences of celastrol on macrophages. The present study reported the attenuating effects of celastrol on B19V NS1‑induced inflammatory responses in macrophages derived from human acute monocytic leukemia cell lines, U937 and THP‑1. Although the migration was not significantly decreased by celastrol in both U937 and THP‑1 macrophages, significantly decreased viability, migration and phagocytosis were detected in both B19V NS1‑activated U937 and THP‑1 macrophages in the presence of celastrol. Additionally, celastrol significantly decreased MMP‑9 activity and the levels of inflammatory cytokines, including IL‑6, TNF‑α and IL‑1β, in B19V NS1‑activated U937 and THP‑1 cells. Notably, significantly decreased levels of NLR family pyrin domain‑containing 3, apoptosis‑associated speck‑like, caspase‑1 and IL‑18 proteins were observed in both B19V NS1‑activated U937 and THP‑1 cells in the presence of celastrol, indicating the involvement of the inflammasome pathway. To the best of our knowledge, the present study is the first to report on the attenuating effects of celastrol on B19V NS1‑induced inflammatory responses in macrophages, suggesting a therapeutic role for celastrol in B19V NS1‑related inflammatory diseases.

Tracking of Human Parvovirus B19 Virus-Like Particles Using Short Peptide Tags Reveals a Membrane-Associated Extracellular Release of These Particles

B19 virus (B19V) is a pathogenic human parvovirus that infects erythroid progenitor cells. Because there are limited in vitro culture systems to propagate this virus, little is known about the molecular mechanisms by which it propagates in cells. In this study, we introduced a HiBiT peptide tag into various loops of VP2 located on the surface of B19V particles and evaluated their ability to form virus-like particles (VLPs). Three independent sites were identified as permissive sites for peptide tag insertion without affecting VLP formation. When the HiBiT tag was introduced into B19V clones (pB19-M20) and transfected into a semipermissive erythroleukemia cell line (UT7/Epo-S1), HiBiT-dependent luciferase activities (HiBiT activities) increased depending on helicase activity of viral NS1. Furthermore, we used a GFP11 tag-split system to visualize VLPs in the GFP1-10-expressing live cells. Time-lapse imaging of green fluorescent protein (GFP)-labeled VLPs revealed that nuclear VLPs were translocated into the cytoplasm only after cell division, suggesting that the breakdown of the nuclear envelope during mitosis contributes to VLP nuclear export. Moreover, HiBiT activities of culture supernatants were dependent on the presence of a detergent, and the released VLPs were associated with extracellular vesicles, as observed under electron microscopy. Treatment with an antimitotic agent (nocodazole) enhanced the release of VLPs. These results suggest that the virions accumulated in the cytoplasm are constitutively released from the cell as membrane-coated vesicles. These properties are likely responsible for viral escape from host immune responses and enhance membrane fusion-mediated transmission. IMPORTANCE Parvovirus particles are expected to be applied as nanoparticles in drug delivery systems. However, little is known about how nuclear-assembled B19 virus (B19V) virions are released from host cells. This study provides evidence of mitosis-dependent nuclear export of B19V and extracellular vesicle-mediated virion release. Moreover, this study provides methods for modifying particle surfaces with various exogenous factors and contributes to the development of fine nanoparticles with novel valuable functions. The pB19-M20 plasmid expressing HiBiT-tagged VP2 is a novel tool to easily quantify VP2 expression. Furthermore, this system can be applied in high-throughput screening of reagents that affect VP2 expression, which might be associated with viral propagation.

A Functional Minigenome of Parvovirus B19

Parvovirus B19 (B19V) is a human pathogenic virus of clinical relevance, characterized by a selective tropism for erythroid progenitor cells in bone marrow. Relevant information on viral characteristics and lifecycle can be obtained from experiments involving engineered genetic systems in appropriate in vitro cellular models. Previously, a B19V genome of defined consensus sequence was designed, synthesized and cloned in a complete and functional form, able to replicate and produce infectious viral particles in a producer/amplifier cell system. Based on such a system, we have now designed and produced a derived B19V minigenome, reduced to a replicon unit. The genome terminal regions were maintained in a form able to sustain viral replication, while the internal region was clipped to include only the left-side genetic set, containing the coding sequence for the functional NS1 protein. Following transfection in UT7/EpoS1 cells, this minigenome still proved competent for replication, transcription and production of NS1 protein. Further, the B19V minigenome was able to complement B19-derived, NS1-defective genomes, restoring their ability to express viral capsid proteins. The B19V genome was thus engineered to yield a two-component system, with complementing functions, providing a valuable tool for studying viral expression and genetics, suitable to further engineering for purposes of translational research.

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.

The 11-Kilodalton Nonstructural Protein of Human Parvovirus B19 Facilitates Viral DNA Replication by Interacting with Grb2 through Its Proline-Rich Motifs

Lytic infection of human parvovirus B19 (B19V) takes place exclusively in human erythroid progenitor cells of bone marrow and fetal liver, which disrupts erythropoiesis. During infection, B19V expresses three nonstructural proteins (NS1, 11-kDa, and 7.5-kDa) and two structural proteins (VP1 and VP2). While NS1 is essential for B19V DNA replication, 11-kDa enhances viral DNA replication significantly. In this study, we confirmed the enhancement role of 11-kDa in viral DNA replication and elucidated the underlying mechanism. We found that 11-kDa specially interacts with cellular growth factor receptor-bound protein 2 (Grb2) during virus infection and in vitro We determined a high affinity interaction between 11-kDa and Grb2 that has an equilibrium dissociation constant (KD ) value of 18.13 nM. In vitro, one proline-rich motif was sufficient for 11-kDa to sustain a strong interaction with Grb2. In consistence, in vivo during infection, one proline-rich motif was enough for 11-kDa to significantly reduce phosphorylation of extracellular signal-regulated kinase (ERK). Mutations of all three proline-rich motifs of 11-kDa abolished its capability to reduce ERK activity and, accordingly, decreased viral DNA replication. Transduction of a lentiviral vector encoding a short hairpin RNA (shRNA) targeting Grb2 decreased the expression of Grb2 as well as the level of ERK phosphorylation, which resulted in an increase of B19V replication. These results, in concert, indicate that the B19V 11-kDa protein interacts with cellular Grb2 to downregulate ERK activity, which upregulates viral DNA replication.IMPORTANCE Human parvovirus B19 (B19V) infection causes hematological disorders and is the leading cause of nonimmunological fetal hydrops during pregnancy. During infection, B19V expresses two structural proteins, VP1 and VP2, and three nonstructural proteins, NS1, 11-kDa, and 7.5-kDa. While NS1 is essential, 11-kDa plays an enhancing role in viral DNA replication. Here, we elucidated a mechanism underlying 11-kDa protein-regulated B19V DNA replication. 11-kDa is tightly associated with cellular growth factor receptor-bound protein 2 (Grb2) during infection. In vitro, 11-kDa interacts with Grb2 with high affinity through three proline-rich motifs, of which at least one is indispensable for the regulation of viral DNA replication. 11-kDa and Grb2 interaction disrupts extracellular signal-regulated kinase (ERK) signaling, which mediates upregulation of B19V replication. Thus, our study reveals a novel mechanism of how a parvoviral small nonstructural protein regulates viral DNA replication by interacting with a host protein that is predominately expressed in the cytoplasm.

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.

Parvovirus B19 integration into human CD36+ erythroid progenitor cells

The pathogenic autonomous human parvovirus B19 (B19V) productively infects erythroid progenitor cells (EPCs). Functional similarities between B19V nonstructural protein (NS1), a DNA binding endonuclease, and the Rep proteins of Adeno-Associated Virus (AAV) led us to hypothesize that NS1 may facilitate targeted nicking of the human genome and B19 vDNA integration. We adapted an integration capture sequencing protocol (IC-Seq) to screen B19V infected human CD36+ EPCs for viral integrants, and discovered 40,000 unique B19V integration events distributed throughout the human genome. Computational analysis of integration patterns revealed strong correlations with gene intronic regions, H3K9me3 sites, and the identification of 41 base pair consensus sequence with an octanucleotide core motif. The octanucleotide core has homology to a single region of B19V, adjacent to the P6 promoter TATA box. We present the first direct evidence that B19V infection of erythroid progenitor cells disrupts the human genome and facilitates viral DNA integration.

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.

The formation and modification of chromatin-like structure of human parvovirus B19 regulate viral genome replication and RNA processing

B19 virus (B19V) is a single stranded virus in the genus of Erythroparvovirus in the family of Parvoviridae. One of the limiting steps of B19V infection is the replication of viral genome which affected the alternative processing of its RNA. Minute virus of mice (MVM) and adeno-associated virus (AAV) has been reported to form chromatin-like structure within hours after infection of cells. However, the role of chromatin-like structure is unclear. In the present study, we found that B19V formed chromatin-like structure after 12h when B19V infectious clone was co-transfected with pHelper plasmid to HEK293T cells. Interestingly, the inhibitor of DNA methyl-transferase (5-Aza-2'-deoxycytidine, DAC) inhibited not only the formation of chromatin-like structure, but also the replication of the viral genomic DNA. More importantly, the splicing of the second intron at splice acceptor sites (A2-1, and A2-2) were reduced and polyadenylation at (pA)p increased when transfected HEK293T cells were treated with DAC. Our results showed that the formation and modification of chromatin-like structure are a new layer to regulate B19V gene expression and RNA processing.

Structural proteins of Helicoverpa armigera densovirus 2 enhance transcription of viral genes through transactivation

Herein, we report the identification of putative promoters for the non-structural proteins (NS) and capsid structural proteins (VP) of Helicoverpa armigera densovirus (HaDV2) as well as a potential mechanism for how these promoters might be regulated. For the first time, we report that VP is able to transactivate the VP promoter and, to a lesser degree, the NS promoter in densoviruses. In addition to this, another promoter-like sequence designated P2, when co-transfected with the VP gene, enhanced luciferase activity by approximately 35 times compared to a control. This suggests that there are two promoters for VP in HaDV2 and that the VP of parvoviruses might play a more important role in viral transcription than previously appreciated.

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.

DNA Binding and Cleavage by the Human Parvovirus B19 NS1 Nuclease Domain

Infection with human parvovirus B19 (B19V) has been associated with a myriad of illnesses, including erythema infectiosum (Fifth disease), hydrops fetalis, arthropathy, hepatitis, and cardiomyopathy, and also possibly the triggering of any number of different autoimmune diseases. B19V NS1 is a multidomain protein that plays a critical role in viral replication, with predicted nuclease, helicase, and gene transactivation activities. Herein, we investigate the biochemical activities of the nuclease domain (residues 2-176) of B19V NS1 (NS1-nuc) in sequence-specific DNA binding of the viral origin of replication sequences, as well as those of promoter sequences, including the viral p6 and the human p21, TNFα, and IL-6 promoters previously identified in NS1-dependent transcriptional transactivation. NS1-nuc was found to bind with high cooperativity and with multiple (five to seven) copies to the NS1 binding elements (NSBE) found in the viral origin of replication and the overlapping viral p6 promoter DNA sequence. NS1-nuc was also found to bind cooperatively with at least three copies to the GC-rich Sp1 binding sites of the human p21 gene promoter. Only weak or nonspecific binding of NS1-nuc to the segments of the TNFα and IL-6 promoters was found. Cleavage of DNA by NS1-nuc occurred at the expected viral sequence (the terminal resolution site), but only in single-stranded DNA, and NS1-nuc was found to covalently attach to the 5' end of the DNA at the cleavage site. Off-target cleavage by NS1-nuc was also identified.

The human parvovirus B19 non-structural protein 1 N-terminal domain specifically binds to the origin of replication in the viral DNA

The non-structural protein 1 (NS1) of human parvovirus B19 plays a critical role in viral DNA replication. Previous studies identified the origin of replication in the viral DNA, which contains four DNA elements, namely NSBE1 to NSBE4, that are required for optimal viral replication (Guan et al., 2009). Here we have demonstrated in vitro that the NS1 N-terminal domain (NS1N) binds to the origin of replication in a sequence-specific, length-dependent manner that requires NSBE1 and NSBE2, while NSBE3 and NSBE4 are dispensable. Mutagenesis analysis has identified nucleotides in NSBE1 and NSBE2 that are critical for NS1N binding. These results suggest that NS1 binds to the NSBE1-NSBE2 region in the origin of replication, while NSBE3 and NSBE4 may provide binding sites for potential cellular factors. Such a specialized nucleoprotein complex may enable NS1 to nick the terminal resolution site and separate DNA strands during replication.

Key elements of the human bocavirus type 1 (HBoV1) promoter and its trans-activation by NS1 protein

Background

Human bocavirus (HBoV), a parvovirus, is suspected to be an etiologic agent of respiratory disease and gastrointestinal disease in humans. All mRNAs of HBoV1 are transcribed from a single promoter.

Methods

In this study, we constructed EGFP and luciferase reporter gene vectors under the control of the HBoV1 full promoter (nt 1–252) and its mutated variants, respectively. Fluorescence microscopy was used to observe expression activities of the EGFP. Dual-luciferase reporter vectors were employed in order to evaluate critical promoter elements and the effect of NS1 protein on promoter activity.

Results

The HBoV1 promoter activity was about 2.2-fold and 1.9-fold higher than that of the CMV promoter in 293 T and HeLa cells, respectively. The putative transcription factor binding region of the promoter was identified to be located between nt 96 and nt 145. Mutations introduced in the CAAT box of the HBoV1 promoter reduced promoter activity by 34%, whereas nucleotide substitutions in the TATA box had no effect on promoter activity. The HBoV1 promoter activities in 293 T and HeLa cells, in the presence of NS1 protein, were 2- to 2.5-fold higher than those in the absence of NS1 protein.

Conclusion

The HBoV1 promoter was highly active in 293 T and HeLa cell lines, and the sequence from nt 96 to nt 145 was critical for the activity of HBoV1 promoter. The CAAT box, in contrast to the TATA-box, was important for optimum promoter activity. In addition, the transcriptional activity of this promoter could be trans-activated by the viral nonstructural protein NS1 in these cells.

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.

Structure of the NS1 protein N-terminal origin recognition/nickase domain from the emerging human bocavirus

Human bocavirus is a newly identified, globally prevalent, parvovirus that is associated with respiratory infection in infants and young children. Parvoviruses encode a large nonstructural protein 1 (NS1) that is essential for replication of the viral single-stranded DNA genome and DNA packaging and may play versatile roles in virus-host interactions. Here, we report the structure of the human bocavirus NS1 N-terminal domain, the first for any autonomous parvovirus. The structure shows an overall fold that is canonical to the histidine-hydrophobic-histidine superfamily of nucleases, which integrates two distinct DNA-binding sites: (i) a positively charged region mediated by a surface hairpin (residues 190 to 198) that is responsible for recognition of the viral origin of replication of the double-stranded DNA nature and (ii) the nickase active site that binds to the single-stranded DNA substrate for site-specific cleavage. The structure reveals an acidic-residue-rich subdomain that is present in bocavirus NS1 proteins but not in the NS1 orthologs in erythrovirus or dependovirus, which may mediate bocavirus-specific interaction with DNA or potential host factors. These results provide insights into recognition of the origin of replication and nicking of DNA during bocavirus genome replication. Mapping of variable amino acid residues of NS1s from four human bocavirus species onto the structure shows a scattered pattern, but the origin recognition site and the nuclease active site are invariable, suggesting potential targets for antivirals against this clade of highly diverse human viruses.

Parvovirus B19 Achievements and Challenges

Parvovirus B19 is a widespread human pathogenic virus, member of the Erythrovirus genus in the Parvoviridae family. Infection can be associated with an ample range of pathologies and clinical manifestations, whose characteristics and outcomes depend on the interplay between the pathogenetic potential of the virus, its adaptation to different cellular environments, and the physiological and immune status of the infected individuals. The scope of this review is the advances in knowledge on the biological characteristics of the virus and of virus-host relationships; in particular, the interactions of the virus with different cellular environments in terms of tropism and ability to achieve a productive replicative cycle, or, on the contrary, to establish persistence; the consequences of infection in terms of interference with the cell physiology; the process of recognition of the virus by the innate or adaptive immune system, hence the role of the immune system in controlling the infection or in the development of clinical manifestations. Linked to these issues is the continuous effort to develop better diagnostic algorithms and methods and the need for development of prophylactic and therapeutic options for B19V infections.

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.

Parvovirus b19 DNA CpG dinucleotide methylation and epigenetic regulation of viral expression

CpG DNA methylation is one of the main epigenetic modifications playing a role in the control of gene expression. For DNA viruses whose genome has the ability to integrate in the host genome or to maintain as a latent episome, a correlation has been found between the extent of DNA methylation and viral quiescence. No information is available for Parvovirus B19, a human pathogenic virus, which is capable of both lytic and persistent infections. Within Parvovirus B19 genome, the inverted terminal regions display all the characteristic signatures of a genomic CpG island; therefore we hypothesised a role of CpG dinucleotide methylation in the regulation of viral genome expression.

The analysis of CpG dinucleotide methylation of Parvovirus B19 DNA was carried out by an aptly designed quantitative real-time PCR assay on bisulfite-modified DNA. The effects of CpG methylation on the regulation of viral genome expression were first investigated by transfection of either unmethylated or in vitro methylated viral DNA in a model cell line, showing that methylation of viral DNA was correlated to lower expression levels of the viral genome. Then, in the course of in vitro infections in different cellular environments, it was observed that absence of viral expression and genome replication were both correlated to increasing levels of CpG methylation of viral DNA. Finally, the presence of CpG methylation was documented in viral DNA present in bioptic samples, indicating the occurrence and a possible role of this epigenetic modification in the course of natural infections.

The presence of an epigenetic level of regulation of viral genome expression, possibly correlated to the silencing of the viral genome and contributing to the maintenance of the virus in tissues, can be relevant to the balance and outcome of the different types of infection associated to Parvovirus B19.

Roles of E4orf6 and VA I RNA in adenovirus-mediated stimulation of human parvovirus B19 DNA replication and structural gene expression

Despite its very narrow tropism for erythroid progenitor cells, human parvovirus B19 (B19V) has recently been shown to replicate and form infectious progeny virus in 293 cells in the presence of early adenoviral functions provided either by infection with adenovirus type 5 or by addition of the pHelper plasmid encoding the E2a, E4orf6, and VA RNA functions. In the present study we dissected the individual influence of these functions on B19V genome replication and expression of structural proteins VP1 and VP2. We show that, in the presence of the constitutively expressed E1A and E1B, E4orf6 alone is able to promote B19V DNA replication, resulting in a concomitant increase in VP expression levels. The stimulatory effects of E4orf6 require the integrity of the BC box motifs, which target cellular proteins such as p53 and the Mre11 DNA repair complex for proteosomal degradation through formation of an E3 ubiquitin ligase complex with E1B. VA RNA also strongly induces VP expression but, in contrast to E4orf6, in a replication-independent manner. This stimulation could be attributed exclusively to the VA I RNA transcript and does not involve major activating effects at the level of the B19V p6 promoter, but the nucleotide residues required for the well-defined pathway of VA I RNA mediated stimulation of translation through functional inactivation of protein kinase R. These data show that the cellular pathways regulating B19V replication may be very similar to those governing the productive cycle of the helper-dependent parvoviruses, the adeno-associated viruses.

Diagnosis, management and possibilities to prevent parvovirus B19 infection in pregnancy

Human parvovirus B19 (B19V) infection in pregnancy can cause severe fetal anemia and nonimmune hydrops fetalis, which may be associated with spontaneous abortion and fetal death. Approximately 30–40% of women of child-bearing age are not immune to B19V infection. The risk to fetal life is particularly high if maternal infection occurs during the first 20 weeks of gestation. In this article we intend to give an overview on the molecular biology, epidemiology and management of B19V infection during pregnancy. These data will be combined with an assessment of the clinical situation of the infected fetus and the possibilities for avoiding and/or preventing B19V infection in pregnant women. Currently B19V infection is the causative agent of one of the most frequently occurring infectious complications in pregnancy that endangers fetal life, and so the necessity to develop a preventive vaccine is discussed.

Transactivation of human parvovirus B19 gene expression in endothelial cells by adenoviral helper functions

Human parvovirus B19 (B19V) DNA is highly prevalent in endothelial cells lining up intramyocardial arterioles and postcapillary venules of patients with chronic myocarditis and cardiomyopathies. We addressed the question of a possible stimulation of B19V gene expression in endothelial cells by infection with adenoviruses. Adenovirus infection led to a strong augmentation of B19V structural and nonstructural proteins in individual endothelial cells infected with B19V or transfected with an infectious B19V genome. Transactivation was mostly mediated at the level of transcription and not due to adenovirus-mediated induction of second-strand synthesis from the single-stranded parvoviral genome. The main adenoviral functions required were E1A and E4orf6, which displayed synergistic effects. Furthermore, a limited B19V genome replication could be demonstrated in endothelial cells and adenovirus infection induced the appearance of putative dimeric replication intermediates. Thus the almost complete block in B19V gene expression seen in endothelial cells can be abrogated by infection with other viruses.

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.

Human parvovirus B19 causes cell cycle arrest of human erythroid progenitors via deregulation of the E2F family of transcription factors

Human parvovirus B19 (B19V) is the only human pathogenic parvovirus. It causes a wide spectrum of human diseases, including fifth disease (erythema infectiosum) in children and pure red cell aplasia in immunocompromised patients. B19V is highly erythrotropic and preferentially replicates in erythroid progenitor cells (EPCs). Current understanding of how B19V interacts with cellular factors to regulate disease progression is limited, due to a lack of permissive cell lines and animal models. Here, we employed a recently developed primary human CD36(+) EPC culture system that is highly permissive for B19V infection to identify cellular factors that lead to cell cycle arrest after B19V infection. We found that B19V exploited the E2F family of transcription factors by downregulating activating E2Fs (E2F1 to E2F3a) and upregulating repressive E2Fs (E2F4 to E2F8) in the primary CD36(+) EPCs. B19V nonstructural protein 1 (NS1) was a key viral factor responsible for altering E2F1-E2F5 expression, but not E2F6-E2F8 expression. Interaction between NS1 and E2F4 or E2F5 enhanced the nuclear import of these repressive E2Fs and induced stable G₂ arrest. NS1-induced G₂ arrest was independent of p53 activation and increased viral replication. Downstream E2F4/E2F5 targets, which are potentially involved in the progression from G₂ into M phase and erythroid differentiation, were identified by microarray analysis. These findings provide new insight into the molecular pathogenesis of B19V in highly permissive erythroid progenitors.

Antibody to parvovirus B19 nonstructural protein is associated with chronic arthralgia in patients with chronic fatigue syndrome/myalgic encephalomyelitis

Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a neuro-immune disease of uncertain pathogenesis. Human parvovirus B19 infection has been shown to occur just prior to development of the onset of CFS/ME in several cases, although B19 seroprevalence studies do not show any significant differences between CFS/ME and controls. In this study, we analysed parvovirus B19 markers in CFS/ME patients (n=200), diagnosed according to Fukuda CDC criteria, and normal blood donors (n=200). Serum from each subject was tested for anti-B19 VP2 IgM and IgG (by Biotrin ELISA), anti-B19 NS1 IgM and IgG (by immunofluorescence), and B19 DNA (by real-time PCR). CFS/ME patients and normal blood donors had a similar B19 seroprevalence (75 % versus 78 %, respectively). Eighty-three CFS patients (41.5 %) as compared with fourteen (7 %) normal blood donors tested positive for anti-B19 NS1 IgG (chi(2)=64.8; P<0.0001; odds ratio=9.42, CI 5.11-17.38). Of these 83 patients, 61 complained of chronic joint pain, while 22 did not. Parvovirus B19 DNA was detected in serum of 11 CFS patients and none of the controls by Taqman real-time PCR (chi(2)=9.35, P<0.002). Positivity for anti-B19 NS1 IgG was associated with higher expression levels of the human CFS-associated genes NHLH1 and GABPA. As NS1 antibodies are thought to indicate chronic or severe courses of B19 infection, these findings suggest that although the seroprevalence of B19 in CFS patients is similar to controls, the immune control of the virus in these patients may not be efficient.

Analysis of nucleotide sequences of human parvovirus B19 genome reveals two different modes of evolution, a gradual alteration and a sudden replacement: a retrospective study in Sapporo, Japan, from 1980 to 2008

There have been no long-term systematic analyses of the molecular epidemiology of human parvovirus B19 (B19V). We investigated the variations of nucleotide sequences of B19V strains collected in Sapporo, Japan, from 1980 to 2008. In that period, six outbreaks of erythema infectiosum occurred regularly at 5-year intervals. The B19V strains collected successively, regardless of the outbreak, were analyzed for nucleotide variation in the subgenomic NS1-VP1u junction. The isolated strains can be classified into 10 subgroups. Two patterns of change of endemic strains were observed. One was a dynamic replacement of strains that occurred almost every 10 years, and the other was a gradual change consisting of an accumulation of point mutations.

Parvovirus B19: its role in chronic arthritis

B19 infection-associated joint symptoms occur most frequently in adults, usually presenting as a self-limited, acute symmetric polyarthritis affecting the small joints of the hands, wrists, and knees. A small percentage of patients persist with chronic polyarthritis that mimics rheumatoid arthritis raising the question of whether B19 virus may have a role as a concomitant or precipitating factor in the pathogenesis of autoimmune conditions. Comprehensive and updated reviews address different aspects of human parvovirus infection. This article focuses on the evidence supporting the arthritogenic potential of the B19 virus and the proposed mechanisms that underlie it.

Human bocavirus--a novel parvovirus to infect humans

For almost three decades parvovirus B19 has been described as the only member of the Parvoviridae to infect and cause illness in humans. This statement was correct until 2005 when a group of Swedish scientists identified a previously uncharacterized virus in pools of human nasopharyngeal aspirates obtained from individuals suffering from diseases of the respiratory tract. Comprehensive sequence and phylogenetic analysis allowed the identification of the new virus as a member of the Parvoviridae. Based on its close relation to the minute virus of canines and the bovine parvovirus, it was named human bocavirus (HBoV). Since the identification of HBoV, viral genomes have been frequently detected worldwide in nasopharyngeal swabs, serum and fecal samples almost exclusively derived from young children with various symptoms of the respiratory or the gastrointestinal tract. The detection of HBoV genomes tends to be associated with elevated rates of coinfections with further respiratory viruses, e.g. respiratory syncytial virus or metapneumovirus. First studies on virus-specific immune responses have described the presence of ubiquitous humoral and cellular immune reactions against HBoV in adults and adolescents, indicating a high seroprevalence of this new virus in humans.

Characterization of the promoter elements and transcription profile of Periplaneta fuliginosa densovirus nonstructural genes

Periplaneta fuliginosa Densovirus (PfDNV), an autonomous invertebrate parvovirus that infects the cockroach, is unusual in that alternative splicing is involved in the structural gene expression. The expression strategy for nonstructural (NS) genes has yet not been reported. Northern blot analysis of cockroach larvae infected with PfDNV revealed two transcripts for the NS genes, one of 2.6 kb, and the other of 1.9 kb. The two transcripts were shown to begin at a common initiator consensus sequence, CAGT, located in the terminus of ITR. The 1.9 kb transcript was produced by splicing out the ns3 gene from the 2.6 kb transcript. To understand the mechanism of transcriptional regulation of NS genes, the 5'-flanking sequence of ns3 gene (325 bp), which encompasses the region from the 5'-terminus of the viral genome to the initiator ATG codon of the ns3 gene, was cloned and fused to a luciferase reporter gene. The luciferase reporter assay showed that this sequence possessed promoter activity in Sf9, Ld652, Tn368, and S2 cell lines. Subsequent promoter deletion analysis showed that the promoter exhibited TATA-dependent and TATA-independent transcriptional activities. Moreover, we found that the promoter activity of the 325-bp fragment in S2 cells could be enhanced significantly by co-transfection of the nonstructural protein NS1 and that the NS1 binding element, (CAC)(4) repeat, mediated the promoter activity activated by NS1 protein.

Tissue persistence and prevalence of B19 virus types 1–3

Human parvovirus B19 is a minute ssDNA virus that causes a wide variety of diseases, including erythema infectiosum, arthropathy, anemias and fetal death. In addition to the B19 prototype, two new variants (B19 types 2 and 3) have been identified. After primary infection, B19 genomic DNA has been shown to persist in solid tissues of not only symptomatic but also of constitutionally healthy, immunocompetent individuals. The viral DNA persists as an intact molecule without persistence-specific mutations, and via a storage mechanism with life-long capacity. Thus, the mere presence of B19 DNA in tissue cannot be used as a diagnostic criterion, although a possible role in the pathology of diseases, for example through mRNA or protein production, cannot be excluded. The molecular mechanism, host-cell type and possible clinical significance of tissue persistence are yet to be elucidated.

Biological and immunological relations among human parvovirus B19 genotypes 1 to 3

The human parvovirus B19 is now divided into three genotypes: type 1 (prototype), type 2 (A6- and LaLi-like), and type 3 (V9-like). In overall DNA sequence, the three virus types differ by approximately 10%. The most striking DNA dissimilarity, of >20%, is observed within the p6 promoter region. Because of the scarcity of data on the biological activities and pathogenetic potentials of virus types 2 and 3, we examined the functional characteristics of these virus types. We found the activities of the three p6 promoters to be of equal strength and to be most active in B19-permissive cells. Virus type 2 capsid protein VP2, alone or together with VP1, was expressed with the baculovirus system and was shown to assemble into icosahedral parvovirus-like particles, which were reactive in the hemagglutination assay. Furthermore, sera containing DNA of any of the three B19 types were shown to hemagglutinate. The infectivities of these sera were examined in two B19-permissive cell lines. Reverse transcription-PCR revealed synthesis of spliced B19 mRNAs, and immunofluorescence verified the production of NS and VP proteins in the infected cells. All three genotypes showed similar functional characteristics in all experiments performed, showing that the three virus types indeed belong to the same species, i.e., human parvovirus B19. Additionally, the antibody activity in sera from B19 type 1- or type 2-infected subjects (long-term immunity) was examined with homo- and heterologous virus-like particles. Cross-reactivity of 100% was observed, indicating that the two B19 genotypes comprise a single serotype.

Phospholipase A2 activity-dependent stimulation of Ca2+ entry by human parvovirus B19 capsid protein VP1

Recent reports demonstrated an association of human parvovirus B19 with inflammatory cardiomyopathy (iCMP), which is accompanied by endothelial dysfunction. As intracellular Ca(2+) activity is a key regulator of cell function and participates in mechanisms leading to endothelial dysfunction, the present experiments explored the effects of the B19 capsid proteins VP1 and VP2. A secreted phospholipase A2 (PLA2)-like activity has been located in the VP1 unique region of the B19 minor capsid protein. As PLA2 has recently been shown to activate the store-operated or capacitative Ca(2+) channel I(CRAC), we analyzed the impact of the viral PLA2 motif on Ca(2+) entry. We cloned the VP1 and VP2 genes isolated from a patient suffering from fatal B19 iCMP into eukaryotic expression vectors. We also generated a B19 replication-competent plasmid to demonstrate PLA2 activity under the control of the complete B19 genome. After the transfection of human endothelial cells (HMEC-1), cytosolic Ca(2+) activity was determined by utilizing Fura-2 fluorescence. VP1 and VP2 expression did not significantly modify basal cytosolic Ca(2+) activity or the decline of cytosolic Ca(2+) activity following the removal of extracellular Ca(2+). However, expression of VP1 and of the full-length B19 clone, but not of VP2, significantly accelerated the increase of cytosolic Ca(2+) activity following the readdition of extracellular Ca(2+) in the presence of thapsigargin, indicating an activation of I(CRAC.) The effect of VP1 was mimicked by the PLA2 product lysophosphatidylcholine and abolished by an inactivating mutation of the PLA2-encoding region of the VP1 gene. Our observations point to the activation of Ca(2+) entry by VP1 PLA2 activity, an effect likely participating in the pathophysiology of B19 infection.

Parvovirus B19 genome as a single, two-state replicative and transcriptional unit

The variation in the amount of parvovirus B19 DNA and different classes of RNA in permissive and non-permissive infected cells was analysed by means of quantitative real-time PCR and RT-PCR assays. In the permissive bone marrow mononuclear cells, UT7/Epo and KU812Ep6 cells, viral DNA usually increased within 48 hpi, rarely exceeding 2 Logs with respect to input DNA. Viral RNA was always present within 2-6 hpi, its increase paralleled that of viral DNA up to 36-48 hpi, and all the different classes of viral RNA were constantly represented in stable relative amounts throughout the infection cycle. In the non-permissive TF-1 cells, viral DNA did not increase and only one most represented single class of viral RNA was detected. Our data do not support the current model for B19 virus replication and transcription, consisting in different early and late expression patterns, but suggest an alternative model, indicating that the B19 virus genome should be considered a single, two-state replicative and transcriptional unit.

Parvovirus B19: the causative agent of dilated cardiomyopathy or a harmless passenger of the human myocard?

Parvovirus B19 infections may cause a widespread benign and self-limiting disease in children and adults known as erythema infectiosum (fifth disease). Several further manifestations are associated with B19 infections, such as arthralgias, arthritis, leucopenia and thrombocytopenia, anaemia and vasculitis and spontaneous abortion and hydrops fetalis in pregnant women. Persistent infections with continuous virus production may occur in immunocompetent as well as in immunosuppressed individuals. Parvovirus B19 infections have been frequently implicated as a cause or trigger of various forms of autoimmune diseases affecting joints, connective tissue and large and small vessels. Autoimmune neutropenia, thrombocytopenia and haemolytic anaemia are known as sequelae of B19 infections. The molecular basis of the autoimmune phenomena is unclear. Many patients with these long-lasting symptoms are not capable of eliminating the virus or controlling its propagation. Furthermore, latent viral genomes have been detected in cells of various organs and tissues by PCR. At present, it is not clear if these cells produce viral proteins and/or infectious B19 particles, if the virus genome can be reactivated to productive replication and if the presence of viral DNA indicates a causative role of parvovirus B19 with distinct diseases.

Characterization of Parvovirus B19 genotype 2 in KU812Ep6 cells

An infectious parvovirus B19 (B19V) genotype 2 variant was identified as a high-titer contaminant in a human plasma donation. Genome analysis revealed a 138-bp insertion within the p6 promoter. The inserted sequence was represented by an additional 30 bp from the end of the inverted terminal repeat adjacent to a 108-bp element found also, in inverted orientation, at the extreme right end of the unique sequence of the genome. However, despite the profound variations in the promoter region, the pattern of gene expression and DNA replication did not differ between genotype 1 and genotype 2 in permissive erythroid KU812Ep6 cells. Capsid proteins of both genotypes differ in their amino acid sequences. However, equivalent kinetics of virus inactivation at 56 degrees C or pH 4 indicated a comparable physicochemical stability of virus capsids. Sera from six individuals infected by B19V genotype 1 were investigated on cross-neutralization of B19V genotype 2 in vitro. Similar neutralization of both B19V genotypes was observed in sera from three individuals, while the sera from three other individuals showed weaker cross-neutralization for genotype 2. In conclusion, the in vitro replication characteristics and physical stability of B19V capsids are very similar between human parvovirus B19 genotypes 1 and 2, and cross-neutralization indicates a close antigenic relation of genotypes 1 and 2.

Human Parvovirus B19 nonstructural protein transactivates the p21/WAF1 through Sp1

The expression of human Parvovirus B19 nonstructural protein 1 (NS1) induces cell cycle arrest at the G1 phase and is accompanied by increased expression of the cyclin-dependent kinase inhibitor, p21/WAF1. Here, we provide direct evidence that NS1 mediates the transactivation of p21/WAF1. Up-regulation of p21/WAF1 by wild-type NS1 but not an NS1 mutant deleted of its NTP binding motif was observed. We also demonstrated that the wild-type NS1 is unable to induce G1 arrest in p21-deficient cells. Using reporter plasmids containing various mutants of the p21/WAF1 promoter, luciferase assay further revealed that the binding sites of the promoter to the transcription factor Sp1 are critical for NS1-mediated transactivation. Indeed Sp1 interacts only with the wild-type NS1 but not the NS1 mutant. These results indicate a cooperative contribution of NS1 and Sp1 to the transactivation of p21/WAF1, which leads to G1 arrest.

Cardiotropic viruses in the myocardium of children with end-stage heart disease

BACKGROUND

Transplantation has become a lifesaving procedure for children with end-stage heart failure. The long-term outcome for children who undergo transplantation has been of considerable interest, but the causes of graft failure and death are largely unknown, and the role of pre-transplant viral infection is unclear.

METHODS

Myocardial samples from 80 explanted hearts from children with end-stage heart disease caused by congenital heart disease (CHD), cardiomyopathy, or chronic rejection were analyzed using polymerase chain reaction and reverse-transcriptase polymerase chain reaction for cardiotropic viruses using virus-specific primers. We used immunohistochemical analysis of cytoskeletal proteins to evaluate myocyte architecture.

RESULTS

We identified parvoviral genomes in 6 patients (3 with CHD and 3 with cardiomyopathy). We detected no other viruses. Immunohistochemistry showed normal staining for key components of the cytoskeleton/sarcolemma, sarcomere, and nuclear membrane in the 6 virus-positive samples. The clinical outcome of these children was worse (4 long-term survivors, but 2 deaths) than for individuals without the genome.

CONCLUSIONS

Detecting viruses within the myocardium at the point of end-stage heart failure is not common, regardless of the primary pathology. However, the presence of viruses may result in poor outcome for the patient.

Hypoxia enhances human B19 erythrovirus gene expression in primary erythroid cells

Human B19 erythrovirus replicates in erythroid progenitors present in bone marrow and fetal tissues where partial oxygen tension is low. Here we show that infected human primary erythroid progenitor cells exposed to hypoxia (1% O2) in vitro increase viral capsid protein synthesis, virus replication, and virus production. Hypoxia-inducible factor-1 (HIF-1), the main transcription factor involved in the cellular response to reduced oxygenation, is shown to bind an HIF binding site (HBS) located in the distal part of the B19 promoter region, but the precise mechanism involved in the oxygen-sensitive upregulation of viral gene expression remains to be elucidated.

B19 virus genome diversity: epidemiological and clinical correlations

Genetic analysis of parvovirus B19 has been carried out mainly to establish a framework to track molecular epidemiology of the virus and to correlate sequence variability with different pathological and clinical manifestations of the virus. A good amount of information regarding B19 virus sequence variability is available, and presently there are about 400 sequence records deposited in the nucleotide database of NCBI. A few are almost complete genomic sequences, and these allow the construction of a global alignment framework. Many others are partial genomic sequences, limited to selected regions, and these allow comparison of a higher number of isolates from well-defined epidemiological settings and/or pathological conditions. Most studies showed that the genetic variability of B19 virus is low, that molecular epidemiology is possible only on a limited geographical and temporal setting, and that no clear correlations are present between genome sequence and distinctive pathological and clinical manifestations. More recently, several viral isolates have been identified that show remarkable sequence diversity with respect to reference sequences. The identification of variant isolates added to the knowledge of genetic diversity in this virus group and allowed the identification of three divergent genetic clusters, about 10% divergent from each other and still quite distinct from other parvoviruses, that can be thought of as different genotypes within the human erythrovirus group and that show clearly resolved phylogenetic relationship. These variant isolates pose interesting questions regarding the real extent of genetic variability in the human erythroviruses, the relevance of these viruses in terms of epidemiology and their possible implication in the pathogenesis of erythrovirus-related diseases.

Antiphospholipid antibodies in pediatric and adult patients with rheumatic disease are associated with parvovirus B19 infection

OBJECTIVE

To show a possible association between parvovirus B19 infection and the presence of antiphospholipid antibodies (aPL) in patients with rheumatic diseases.

METHODS

Serum samples obtained from 88 children with various forms of juvenile rheumatic disease and from 40 adults with systemic lupus erythematosus, the antiphospholipid syndrome, or other rheumatic disease, who had previously been tested and shown to be positive for IgG aPL, were analyzed for the presence of B19 DNA, for antibodies against the B19 viral proteins VP1, VP2, and NS1, and for IgG aPL (anticardiolipin, anti-beta(2)-glycoprotein I, and antiphosphatidylserine). As controls, serum samples obtained from 135 children with noninflammatory bone diseases or growth retardation were also analyzed.

RESULTS

Twenty-four (27%) of the 88 children with rheumatic diseases had detectable amounts of IgG aPL. Fourteen (58%) of these 24 IgG aPL-positive patients showed IgG against VP1/VP2 and viral genomes, indicating the presence of acute (2 patients) or persistent (12 patients) infection. Past parvovirus B19 infection was identified in 7 (29%) of 24 IgG aPL-positive children, as indicated by VP1/VP2-specific IgG in the absence of viral DNA. Three (12%) of 24 IgG aPL-positive children had not been infected with B19. Sixty-nine (51%) of 135 control children displayed VP1/VP2-specific IgG. Three (2%) of these 135 children were IgG aPL positive (2 children had past parvovirus B19 infection, and 1 was negative for parvovirus B19). Analysis of the parvovirus B19 status of 40 adult IgG aPL-positive patients showed that 33 (83%) were anti-IgG VP1/VP2-positive, and viral DNA was detected in 11 patients (28%). Ten of these 11 viremic patients were in the subgroup of 28 IgG aPL-positive SLE patients.

CONCLUSION

Antiphospholipid antibodies are preferentially found in serum of children with juvenile idiopathic arthritis who have been previously infected with parvovirus B19 and have established, persistent infection. Adult patients with IgG aPL positivity have a high incidence of persistent parvovirus B19 infection. We conclude that parvovirus B19 might be directly involved in the elicitation of autoimmune reactions partly mediated by aPL.

Frequent infection with a viral pathogen, parvovirus B19, in rheumatic diseases of childhood

OBJECTIVE

To find further evidence of the association of parvovirus B19 infection with juvenile rheumatic diseases, and to get new insights into the immunopathogenesis of these diseases.

METHODS

Paired serum and synovial fluid samples from 74 children with rheumatic disease were analyzed with respect to their content of viral DNA and antibodies directed against the B19 viral proteins VP1, VP2, and NS1. Control sera from 124 children with noninflammatory bone diseases or growth retardation were also analyzed. The sequence of the viral DNA, amplified by polymerase chain reaction (PCR), was determined. IgG-complexed virus was isolated from sera and synovial fluid by adsorption to protein A beads. The amount of free virus versus immunocomplexed virus particles was determined by quantification of the viral genomes by quantitative PCR.

RESULTS

Twenty-six of the 74 patients (35%) had detectable amounts of parvovirus B19 DNA in the serum (n = 22 [30%]) and/or the synovial fluid (n = 16 [22%]), whereas only 9 of the 124 control sera (7%) were positive for the viral DNA (P < 0.0001). Forty-six patients (62%) had serum IgG against the structural proteins, indicating past infection with B19. NS1-specific antibodies were detected in sera from 29 patients (39%) and 27 controls (22%) (P < 0.001). In addition, 3 patients (4%) showed VP2-specific IgM. In 15 patients, viral DNA could be repeatedly detected in followup samples of serum and synovial fluid. Sequencing revealed low-degree nucleotide variations that are in the range of genotype 1 of parvovirus B19. Immunocomplexed virus was present in varying amounts, both in the sera and in the synovial fluid samples.

CONCLUSION

Parvovirus B19 is frequently found in serum or synovial fluid of children with rheumatism. The rate of persistent B19 infection in these patients is significantly higher than in age-matched controls.

A new parvovirus genotype persistent in human skin

Parvovirus B19 is the exclusive human pathogen of the Erythrovirus genus. In classical view, the B19 DNA sequence shows little variability, with no disease-specific or tissue type specific associations. We examined skin biopsies from patients with B19-unrelated skin disease or from constitutionally healthy adults by polymerase chain reaction assays for four different genomic regions of the B19 virus. Sequencing showed that the skin-derived viral DNA differed within the protein-coding region from the B19 reference sequences by 10.8% and from the V9 variant by 8.6% and within the noncoding region (covering nucleotides 189-435 of the promoter region) by 26.5 and 17.2%, respectively. Despite this sequence difference, the promoter region was shown by a luciferase gene expression assay to be biologically active. We have detected a new B19 virus genotype, K71, which differs extensively from the known B19-virus genotypes and is persistently carried in human skin.

Genetic diversity within human erythroviruses: identification of three genotypes

B19 virus is a human virus belonging to the genus Erythrovirus: The genetic diversity among B19 virus isolates has been reported to be very low, with less than 2% nucleotide divergence in the whole genome sequence. We have previously reported the isolation of a human erythrovirus isolate, termed V9, whose sequence was markedly distinct (>11% nucleotide divergence) from that of B19 virus. To date, the V9 isolate remains the unique representative of a new variant in the genus Erythrovirus, and its taxonomic position is unclear. We report here the isolation of 11 V9-related viruses. A prospective study conducted in France between 1999 and 2001 indicates that V9-related viruses actually circulate at a significant frequency (11.4%) along with B19 viruses. Analysis of the nearly full-length genome sequence of one V9-related isolate (D91.1) indicates that the D91.1 sequence clusters together with but is notably distant from the V9 sequence (5.3% divergence) and is distantly related to B19 virus sequences (13.8 to 14.2% divergence). Additional phylogenetic analysis of partial sequences from the V9-related isolates combined with erythrovirus sequences available in GenBank indicates that the erythrovirus group is more diverse than thought previously and can be divided into three well-individualized genotypes, with B19 viruses corresponding to genotype 1 and V9-related viruses being distributed into genotypes 2 and 3.

Chronic human parvovirus B19 infection in rheumatic disease of childhood and adolescence

Parvovirus B19 causes erythema infectiosum in children, but the virus is associated with an increasing range of different diseases. About 20% of infections are associated with delayed virus elimination and viremia persisting over several months or years. These persistent B19-infections are characterised by the presence of IgG against the non-structural protein NS1. This study aimed to find further evidence for an association of parvovirus B19 persistence with VP1/2- and NS1-specific IgG-antibodies in children suffering from rheumatic diseases of childhood. Forty-eight children and adolescents with joint complaints lasting longer than 1 year including patients with juvenile systemic sclerosis and juvenile dermatomyositis showed antibodies against the viral NS1-protein. Laboratory markers of inflammation, humoral immune response against parvovirus B19 proteins and the presence of viral genomes in patients' sera as well as in 124 healthy children were investigated. Almost 50% of the patients showed laboratory signs of chronic inflammation. B19-DNA was amplified in 31% of patients' sera and 7% of the controls (P<0.0001). VP2-specific IgM was detectable in 50% of the patients' and 6% of control sera. NS1-specific immune reactions were linked to persistent B19-infection as indicated by the presence of viral genomes in the peripheral blood and of VP2-specific IgM years after disease onset. To estimate the severity of the disease and the clinical course, the number of affected and functionally impaired joints were noted and compared with the records from patients' initial visit in the hospital. Disease related complications were registered. Impairment of activities of daily living was assessed by Childhood Health Assessment Questionnaire (CHAQ)- and Munich Quality of Life Questionnaire (KINDL)-tests. During observation the clinical state of four patients worsened, 27 improved, the others remained stable. Twenty-four children were restricted in their daily activities.

Current molecular epidemiology and human parvovirus B19 infection

Viruses evolve gradually through replication. Therefore, isolates of a virus species can have different genome sequences, albeit slightly, if isolates are epidemiologically unrelated. The difference in virus genome involves difference in virus functions and clinical manifestations of virus infection. Molecular epidemiology of virus infection is a relatively new field directed at infection in humans but not other animals. Analyses are based on genomic differences between virus strains with advances in methodology related to DNA analyses, progress is being made. Classification of virus strains, tracing of transmission of a strain, analyses of outbreaks (including nosocomial infection), and analyses of pathogenesis of virus infection in humans (a natural host) are given attention in molecular epidemiological studies. Human parvovirus B19 is a common human pathogen associated with a wide variety of diseases, including erythema infectiosum, aplastic crisis, hydrops fetalis, and arthritis. B19 is not propagatable in conventional cell lines, hence, molecular cloning of B19 DNA directly from clinical materials has to be done. Events concerning B19 infection were analyzed based on the concept of molecular epidemiology and studies proved to be productive to better understand the pathogenesis of B19 infection.