The 3' untranslated region of the B19 parvovirus capsid protein mRNAs inhibits its own mRNA translation in nonpermissive cells

Human parvovirus B19 interacts with globoside under acidic conditions as an essential step in endocytic trafficking

The glycosphingolipid (GSL) globoside (Gb4) is essential for parvovirus B19 (B19V) infection. Historically considered the cellular receptor of B19V, the role of Gb4 and its interaction with B19V are controversial. In this study, we applied artificial viral particles, genetically modified cells, and specific competitors to address the interplay between the virus and the GSL. Our findings demonstrate that Gb4 is not involved in the binding or internalization process of the virus into permissive erythroid cells, a function that corresponds to the VP1u cognate receptor. However, Gb4 is essential at a post-internalization step before the delivery of the single-stranded viral DNA into the nucleus. In susceptible erythroid Gb4 knockout cells, incoming viruses were arrested in the endosomal compartment, showing no cytoplasmic spreading of capsids as observed in Gb4-expressing cells. Hemagglutination and binding assays revealed that pH acts as a switch to modulate the affinity between the virus and the GSL. Capsids interact with Gb4 exclusively under acidic conditions and dissociate at neutral pH. Inducing a specific Gb4-mediated attachment to permissive erythroid cells by acidification of the extracellular environment led to a non-infectious uptake of the virus, indicating that low pH-mediated binding to the GSL initiates active membrane processes resulting in vesicle formation. In summary, this study provides mechanistic insight into the interaction of B19V with Gb4. The strict pH-dependent binding to the ubiquitously expressed GSL prevents the redirection of the virus to nonpermissive tissues while promoting the interaction in acidic intracellular compartments as an essential step in infectious endocytic trafficking.

The neutral glycosphingolipid globoside (Gb4) has been historically considered the cellular receptor of B19V, however, its wide expression profile does not correlate well with the restricted tropism of the virus. Here, we show that Gb4 is essential for the infection at a step following virus uptake and before the delivery of the viral ssDNA into the nucleus. B19V interacts with Gb4 exclusively under acidic conditions, prohibiting the interaction on the plasma membrane and promoting it inside the acidic endosomal compartments, which are engaged by the virus and the GSL after internalization. In the absence of Gb4, incoming viruses are retained in the endocytic compartment and the infection is aborted. This study reveals the mechanism of the interaction between the virus and the glycosphingolipid and redefines the role of Gb4 as an essential intracellular partner required for infectious entry.

Parvovirus B19 DNA detection in treatment-naïve HIV anemic patients in Lagos, Nigeria: a case control study

BACKGROUND

Parvovirus B19 (B19) has tropism for cells of the erythroid lineage, which may lead to transient inhibition of erythropoiesis. Several studies and case reports suggested that B19 infection may contribute significantly to severe chronic anemia in HIV infected persons.

OBJECTIVE

To detect parvovirus B19 DNA in treatment-naïve HIV patients.

METHODS

This was a case control retrospective study. One hundred nineteen anemic and 81 non-anemic treatment-naïve HIV infected patients participated in the study at the Lagos University Teaching Hospital, Lagos, Nigeria. Polymerase chain reaction was used to detect B19 DNA.

RESULTS

Out of 200 patients analysed, 13(6.5%) had parvovirus B19 DNA. Eight HIV patients with anemia had B19 DNA while five non-anemic HIV patients had B19 DNA. This suggests that the presence of B19 DNA in the blood of HIV positive individuals may contribute to anemia because the majority (61.5%) who were positive for B19 DNA had anemia as compared to the non-anemic control group (38.5%).

CONCLUSION

This study shows that the presence of B19 DNA in anemic HIV infected patients is not associated with chronic anaemia in HIV infection because no significant association exist.

Human parvovirus B19 in solid organ transplantation: Guidelines from the American society of transplantation infectious diseases community of practice

Clinical manifestations of human parvovirus B19 infection can vary widely and may be atypical in solid organ transplant (SOT) recipients. However, disease is apparent when there is destruction of erythrocyte progenitor cells leading to severe acute or chronic anemia with lack of an appropriate reticulocyte response in the setting of active parvovirus B19 infection. Serology may not reliably establish the diagnosis. High-level viremia is more likely to be associated with symptomatic disease. Conversely, ongoing DNAemia after infection may not be clinically significant, if detected at low levels. Despite lack of robust data, intravenous immunoglobulin (IVIG) is frequently used for the treatment of SOT recipients with symptomatic parvovirus B19 infection. Although the optimal dosage and duration of IVIG is not known, most patients receive a total of 2 g/kg over a period of 2-5 days. A daily dose of 1 g/kg or more seems to be associated with higher incidence of toxicity. Application of standard and droplet isolation precautions remains the cornerstone for preventing human parvovirus B19 transmission. Additional research is needed to assess the efficacy of current and novel therapies and to develop a safe and effective parvovirus B19 vaccine.

Possible involvement of miRNAs in tropism of Parvovirus B19

Human Parvovirus B19 (PVB19) is one of the most important pathogens that targets erythroid lineage. Many factors were mentioned for restriction to erythroid progenitor cells (EPCs). Previous studies showed that in non-permissive cells VP1 and VP2 (structural proteins) mRNAs were detected but could not translate to proteins. A bioinformatics study showed that this inhibition might be due to specific microRNAs (miRNAs) present in non-permissive cells but not in permissive EPCs. To confirm the hypothesis, we evaluated the effect of miRNAs on VP expression. CD34(+) HSCs were separated from cord blood. Then, CD34(+) cells were treated with differentiation medium to obtain CD36(+) EPCs. To evaluate the effect of miRNAs on VP expression in MCF7 and HEK-293 cell lines (non-permissive cells) and CD36(+) EPCs, dual luciferase assay was performed in presence of shRNAs against Dicer and Drosha to disrupt miRNA biogenesis. QRT-PCR was performed to check down-regulation of Dicer and Drosha after transfection. All measurements were done in triplicate. Data means were compared using one-way ANOVAs. MicroRNA prediction was done by the online microRNA prediction tools. No significant difference was shown in luciferase activity of CD36(+) EPCs after co-transfection with shRNAs, while it was significant in non-permissive cells. Our study revealed that miRNAs may be involved in inhibition of VP expression in non-permissive cells, although further studies are required to demonstrate which miRNAs exactly are involved in regulation of PVB19 replication.

Antibody-mediated enhancement of parvovirus B19 uptake into endothelial cells mediated by a receptor for complement factor C1q

Despite its strong host tropism for erythroid progenitor cells, human parvovirus B19 (B19V) can also infect a variety of additional cell types. Acute and chronic inflammatory cardiomyopathies have been associated with a high prevalence of B19V DNA in endothelial cells of the myocardium. To elucidate the mechanisms of B19V uptake into endothelium, we first analyzed the surface expression of the well-characterized primary B19V receptor P antigen and the putative coreceptors α5β1 integrins and Ku80 antigen on primary and permanent endothelial cells. The receptor expression pattern and also the primary attachment levels were similar to those in the UT7/Epo-S1 cell line regarded as functional for B19V entry, but internalization of the virus was strongly reduced. As an alternative B19V uptake mechanism in endothelial cells, we demonstrated antibody-dependent enhancement (ADE), with up to a 4,000-fold increase in B19V uptake in the presence of B19V-specific human antibodies. ADE was mediated almost exclusively at the level of virus internalization, with efficient B19V translocation to the nucleus. In contrast to monocytes, where ADE of B19V has been described previously, enhancement does not rely on interaction of the virus-antibody complexes with Fc receptors (FcRs), but rather, involves an alternative mechanism mediated by the heat-sensitive complement factor C1q and its receptor, CD93. Our results suggest that ADE represents the predominant mechanism of endothelial B19V infection, and it is tempting to speculate that it may play a role in the pathogenicity of cardiac B19V infection. Importance: Both efficient entry and productive infection of human parvovirus B19 (B19V) seem to be limited to erythroid progenitor cells. However, in vivo, the viral DNA can also be detected in additional cell types, such as endothelial cells of the myocardium, where its presence has been associated with acute and chronic inflammatory cardiomyopathies. In this study, we demonstrated that uptake of B19V into endothelial cells most probably does not rely on the classical receptor-mediated route via the primary B19V receptor P antigen and coreceptors, such as α5β1 integrins, but rather on antibody-dependent mechanisms. Since the strong antibody-dependent enhancement (ADE) of B19V entry requires the CD93 surface protein, it very likely involves bridging of the B19V-antibody complexes to this receptor by the complement factor C1q, leading to enhanced endocytosis of the virus.

Parvovirus B19 uptake is a highly selective process controlled by VP1u, a novel determinant of viral tropism

The VP1 unique region (VP1u) of human parvovirus B19 (B19V) is the immunodominant part of the viral capsid. Originally inaccessible, the VP1u becomes exposed upon primary attachment to the globoside receptor. To study the function of the exposed VP1u in B19V uptake, we expressed this region as a recombinant protein. Here, we report that purified recombinant VP1u binds and is internalized in UT7/Epo cells. By means of truncations and specific antibodies, we identified the most N-terminal amino acid residues of VP1u as the essential region for binding and internalization. Furthermore, the recombinant VP1u was able to block B19V uptake, suggesting that the protein and the virus undertake the same internalization pathway. Assays with different erythroid and nonerythroid cell lines showed that the N-terminal VP1u binding was restricted to a few cell lines of the erythroid lineage, which were also the only cells that allowed B19V internalization and infection. These results together indicate that the N-terminal region of VP1u is responsible for the internalization of the virus and that the interacting receptor is restricted to B19V-susceptible cells. The highly selective uptake mechanism represents a novel determinant of the tropism and pathogenesis of B19V.

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.

Codon optimization of human parvovirus B19 capsid genes greatly increases their expression in nonpermissive cells

Parvovirus B19 (B19V) is pathogenic for humans and has an extreme tropism for human erythroid progenitors. We report cell type-specific expression of the B19V capsid genes (VP1 and VP2) and greatly increased B19V capsid protein production in nonpermissive cells by codon optimization. Codon usage limitation, rather than promoter type and the 3' untranslated region of the capsid genes, appears to be a key factor in capsid protein production in nonpermissive cells. Moreover, B19 virus-like particles were successfully generated in nonpermissive cells by transient transfection of a plasmid carrying both codon-optimized VP1 and VP2 genes.

Advances in human B19 erythrovirus biology

Since its discovery, human parvovirus B19 (B19V), now termed erythrovirus, has been associated with many clinical situations (neurological and myocardium infections, persistent B19V DNAemia) in addition to the prototype clinical manifestations, i.e., erythema infectiosum and erythroblastopenia crisis. In 2002, the use of new molecular tools led to the characterization of three different genotypes of human B19 erythrovirus. Although the genomic organization is conserved, the geographic distribution of the different genotypes varies worldwide, and the nucleotidic divergences can impact the molecular diagnosis of B19 virus infection. The cell cycle of the virus remains partially unresolved; however, recent studies have shed light on the mechanism of cell entry and the interactions of B19V proteins with apoptosis pathways.

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.

Clinical aspects of parvovirus B19 infection

Parvovirus B19 is a significant human pathogen that causes a wide spectrum of clinical complications ranging from mild, self-limiting erythema infectiosum in immunocompetent children to lethal cytopenias in immunocompromised patients and intrauterine foetal death in primary infected pregnant women. The infection may also be persistent and can mimic or trigger autoimmune inflammatory disorders. Another important clinical aspect to consider is the risk of infection through B19-contaminated blood products. Recent advances in diagnosis and pathogenesis, new insights in the cellular immune response and newly discovered genotypes of human parvoviruses form a platform for the development of modern therapeutic and prophylactic alternatives.

Apoptosis of liver-derived cells induced by parvovirus B19 nonstructural protein

Parvovirus B19 has been implicated in some cases of acute fulminant non-A, non-B, non-C, non-G liver failure. Our laboratory previously demonstrated that B19 infection of hepatocytes induces apoptosis and that the B19 viral nonstructural protein, NS1, may play a critical role. To study the involvement of NS1 in apoptosis of liver cells, we generated a fusion protein of NS1 with enhanced green fluorescent protein (eGFP) in a system allowing for inducible gene expression. Transfection of the liver-derived cell line HepG2 with the eGFP/NS1 vector allowed expression of the fusion protein, which was visualized by fluorescence microscopy and demonstrated by immunoblotting. The fusion protein localized to discrete domains in the nucleus. Transfection of HepG2 cells with the eGFP/NS1 vector led to apoptosis of 35% of transfected cells, a sevenfold increase over cells transfected with the parent eGFP expression vector. Mutation of the eGFP/NS1 vector to eliminate the nucleoside triphosphate-binding site of NS1 significantly decreased apoptosis, as did treatment of transfected cells with inhibitors of caspase 3 or 9. Neutralization of tumor necrosis factor alpha or Fas ligand had no effect on apoptosis. These results demonstrate that NS1 is sufficient to induce apoptosis in liver-derived cells and that it does so through the initiation of an intrinsic caspase pathway.

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.

Pathogenesis of Parvovirus B19 Infection: Host Gene Variability, and Possible Means and Effects of Virus Persistence

Since conducting follow-up studies of patients with acute symptomatic parvovirus B19 infection which showed that a significant proportion of patients develop prolonged arthritis and chronic fatigue syndrome (CFS), we have become interested in the mechanisms of this phenomenon. We showed that these cases have high levels of pro-inflammatory cytokines in their circulation and that this correlates with the symptoms. However, the underlying mechanisms were not apparent, and we have used various approaches to begin studying this phenomenon. DNA polymorphisms were looked for and several were shown to be more common in these subjects compared with controls; these occur within genes of both the immune response [human leucocyte antigen (HLA)-DRB1, HLA-B, transforming growth factor (TGF)-beta1] and those involved in several other cellular functions (predominantly the cytoskeleton and cell adhesion). Interestingly, one particular single-nucleotide polymorphism (SNP) which is associated with symptomatic B19 infection occurs in the Ku80 gene which has recently been shown to be a B19 co-receptor. B19 persistence is probably the key to this phenomenon, and some new data are presented on short regions of sequence homology (17-26 bp) between human, mouse and rat parvoviruses and their respective hosts which occur in many host genes. This homology may provide a foothold for virus persistence and may also play a role in the genesis of disease through gene disruption. Finally, we used microarrays and TaqMan real-time polymerase chain reaction in 108 normal persons to study human gene expression in persons who are B19-seropositive versus B19-seronegative (age- and sex-matched) to examine the hypothesis that gene regulation may be altered in subjects harbouring the B19 virus DNA. Six genes were found to be differentially expressed with roles in the cytoskeleton (SKIP, MACF1, SPAG7, FLOT1), integrin signalling (FLOT1, RASSF5), HLA class III (c6orf48), and tumour suppression (RASSF5). These results have implications not only for B19 but also for other persistent viruses as well and confirmation is required. In conclusion, these disparate findings contribute to our understanding of the pathogenesis of B19 disease. We are using these studies as a starting point to study the phenomenon of chronic immune activation following B19 infection.

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.

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.

Alpha5beta1 integrin as a cellular coreceptor for human parvovirus B19: requirement of functional activation of beta1 integrin for viral entry

Replication of the pathogenic human parvovirus B19 is restricted to erythroid progenitor cells. Although blood group P antigen has been reported to be the cell surface receptor for parvovirus B19, a number of nonerythroid cells, which express P antigen, are not permissive for parvovirus B19 infection. We have documented that P antigen is necessary for parvovirus B19 binding but not sufficient for virus entry into cells. To test whether parvovirus B19 utilizes a cell surface coreceptor for entry, we used human erythroleukemia cells (K562), which allow parvovirus B19 binding but not entry. We report here that upon treatment with phorbol esters, K562 cells become adherent and permissive for parvovirus B19 entry, which is mediated by alpha 5 beta 1 integrins, but only in their high-affinity conformation. Mature human red blood cells (RBCs), which express high levels of P antigen, but not alpha 5 beta 1 integrins, bind parvovirus B19 but do not allow viral entry. In contrast, primary human erythroid progenitor cells express high levels of both P antigen and alpha 5 beta 1 integrins and allow beta1 integrin-mediated entry of parvovirus B19. Thus, in a natural course of infection, RBCs are likely exploited for a highly efficient systemic dissemination of parvovirus B19.

Identification of a nonconventional motif necessary for the nuclear import of the human parvovirus B19 major capsid protein (VP2)

Human parvovirus B19 replicates and encapsidates its genome in the nucleus of erythroid progenitors in vivo and in vitro. We wanted to understand the determinants necessary for the nuclear transport of the major coat protein, VP2, which makes up about 96% of the viral capsid proteins. A nonconsensus basic motif, KLGPRKATGRW, necessary for the nuclear localization of VP2 was identified and shown to be able to import reporter proteins into the nucleus. The sequence is conserved among the VP2 C-terminal region of erythroviruses. This newly identified sequence will facilitate the understanding of the replication of these viruses.

Evaluation of anti-parvovirus B19 activity in sera by assay using quantitative polymerase chain reaction

Human parvovirus B19 (B19) infects cells of erythroid lineage. Production of neutralizing antibodies (Abs) is indispensable for recovery from B19-related disease state. In this study, we used a convenient method to measure neutralizing activities in human sera by using a real-time quantitative PCR based assay. Erythroid cell line KU812Ep6 was incubated with test sera before infection with B19 virus. The copy number of B19-DNA in cultures was decreased in the presence of the sera from patients who recovered from acute B19 infection, whereas no decrease in B19-DNA was in cultures incubated with sera from healthy volunteers who had no B19 infection. The decrease in B19-DNA copy number was calculated and the inhibition percentage was expressed as neutralizing activity to B19. A clinical study showed that the levels of neutralizing ability were high in patients who recovered soon after acute B19 infection, but were low in some patients with a prolonged clinical course for recovery from B19 infection. This method is simple and convenient compared with methods described previously, showing its usefulness to evaluate the neutralizing activity to B19.

No association between human parvovirus B19 and testicular germ cell cancer

The incidence of testicular germ cell cancer, which is the most common cancer among young male adults, is increasing. The aetiology remains unknown, although a virus has been proposed. A previous study has shown a high prevalence of human parvovirus B19 (B19) DNA in the testes of patients with testicular germ cell tumours (85%) and suggested that B19 may play a role in tumour development. To address this question of causality, seroreactivity to B19 was studied among cases (n=80) and controls (n=241) using serum samples drawn before the onset of disease, in addition to an elucidation of the frequency of virus DNA in a retrospectively collected 2-year testicular carcinoma series. No association was found between B19 seropositivity and the risk of testicular cancer (odds ratio=1.03; 95% confidence interval=0.60-1.77) nor was there any dose-response relation (P for trend=0.53). This study did, however, confirm the observation that B19 DNA can be detected in testicular carcinoma tissue, as 4 of 24 cases were found to be positive, while no B19 DNA could be detected in the control cases. It is speculated that this finding may be due to susceptibility of the carcinoma cells to B19 virus owing to high-level expression of the viral receptor glycosphingolipid (Gb4) and possible other putative cellular factors resulting in a localized persistence initiated after the development of cancer.

Human parvovirus B19

Parvovirus B19 (B19) was discovered in 1974 and is the only member of the family Parvoviridae known to be pathogenic in humans. Despite the inability to propagate the virus in cell cultures, much has been learned about the pathophysiology of this virus, including the identification of the cellular receptor (P antigen), and the control of the virus by the immune system. B19 is widespread, and manifestations of infection vary with the immunologic and hematologic status of the host. In healthy immunocompetent individuals B19 is the cause of erythema infectiosum and, particularly in adults, acute symmetric polyarthropathy. Due to the tropism of B19 to erythroid progenitor cells, infection in individuals with an underlying hemolytic disorder causes transient aplastic crisis. In the immunocompromised host persistent B19 infection is manifested as pure red cell aplasia and chronic anemia. Likewise, the immature immune response of the fetus may render it susceptible to infection, leading to fetal death in utero, hydrops fetalis, or development of congenital anemia. B19 has also been suggested as the causative agent in a variety of clinical syndromes, but given the common nature, causality is often difficult to infer. Diagnosis is primarily based on detection of specific antibodies by enzyme-linked immunosorbent assay or detection of viral DNA by dot blot hybridization or PCR. Treatment of persistent infection with immunoglobulin reduces the viral load and results in a marked resolution of anemia. Vaccine phase I trials show promising results.

Detection of parvovirus B19 NS1-specific antibodies by ELISA and western blotting employing recombinant NS1 protein as antigen

Human parvovirus B19 (B19) encodes a number of nonstructural proteins, including the major protein, NS1, and two structural proteins, VP1 and VP2. The use of denatured NS1 in enzyme-linked immunosorbent assay (ELISA) and Western blot (WB) assay has provided an opportunity to study some of the immunologic properties of NS1, but the results have been equivocal and the diagnostic sensitivity poor, probably because of the absence of conformational epitopes. Various viral isolates and baculovirus vectors were employed to produce recombinant B19 NS1 under nondenaturing conditions for the first time. To assess the antigenicity of purified B19 NS1, the reaction patterns of 252 samples were compared by B19 NS1 and VP2 ELISA. In sera from individuals with past infection (VP2 IgG-positive), the use of this new antigen increased significantly the sensitivity of ELISA compared with WB (78% vs. 33%, P = 0.001), contradicting perpetuated claims that B19 NS1 IgG is detected primarily in patients with arthralgia or chronic infection. Previous reports of the absence of NS1 IgG during the initial phase of infection (< 6 weeks) were proved incorrect by the detection of NS1 IgG in 60% of samples from patients recently infected by B19. Including conformational epitopes in the ELISA increases the diagnostic sensitivity, although immunologically, a temporal (years) attenuation of NS1 antibodies appears to take place. This novel diagnostic tool may be useful as a supplement in case of borderline results by VP2 ELISA and for monitoring the efficacy of future capsid-based B19 vaccines.

Recombinant human parvovirus B19 vectors

In an attempt to exploit the remarkable tissue-tropism of the human parvovirus B19 to target human hematopoietic cells of the erythroid lineage, recombinant human adeno-associated virus 2 genomes were encapsidated in parvovirus B19 capsids. Although efficient transduction of primary human hematopoietic cells in the erythroid lineage occurred, a low-level of transgene expression in non-erythroid cells was also detected. These studies suggest that cell surface expression of P antigen, the primary receptor for parvovirus B19, is necessary but not sufficient for parvovirus B19 vector-mediated transduction of human hematopoietic cells. These studies also suggest the existence of a putative cell surface co-receptor, which is required for successful infection of human hematopoietic cells by parvovirus B19.

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.

Recombinant human parvovirus B19 vectors: erythrocyte P antigen is necessary but not sufficient for successful transduction of human hematopoietic cells

The blood group P antigen, known to be abundantly expressed on erythroid cells, has been reported to be the cellular receptor for parvovirus B19. We have described the development of recombinant parvovirus B19 vectors with which high-efficiency, erythroid lineage-restricted transduction can be achieved (S. Ponnazhagan, K. A. Weigel, S. P. Raikwar, P. Mukherjee, M. C. Yoder, and A. Srivastava, J. Virol. 72:5224-5230, 1998). However, since a low-level transduction of nonerythroid cells could also be detected and since P antigen is expressed in nonerythroid cells, we reevaluated the role of P antigen in the viral binding and entry into cells. Cell surface expression analyses revealed that approximately 75% of primary human bone marrow mononuclear erythroid cells and approximately 31% of cells in the nonerythroid population were positive for P antigen. Two human erythroleukemia cell lines, HEL and K562, and a human promyelocytic leukemia cell line, HL-60, were also examined for P antigen expression and binding and entry of the vector. HEL and K562 cells showed intermediate levels, whereas HL-60 cells demonstrated high levels of expression of P antigen. However, the efficiency of vector binding to these cells did not correlate with P antigen expression. Moreover, despite P antigen positivity and efficient viral binding, HEL, K562, and HL-60 cells could not be transduced with the vector. Low levels of P antigen expression could also be detected in two primary cell types, human umbilical vein endothelial cells (HUVEC) and normal human lung fibroblasts (NHLF). In addition, vector binding occurred in both cell types and was inhibited by globoside, indicating the involvement of P antigen in virus binding to these cells. These primary cells could be efficiently transduced with the recombinant vector. These data suggest that (i) P antigen is expressed on a variety of cell types and is involved in binding of parvovirus B19 to human cells, (ii) the level of P antigen expression does not correlate with the efficiency of viral binding, (iii) P antigen is necessary but not sufficient for parvovirus B19 entry into cells, and (iv) parvovirus B19 vectors can be used to transduce HUVEC and NHLF. These studies further suggest the existence of a putative cellular coreceptor for efficient entry of parvovirus B19 into human cells.

Different patterns of restriction to B19 parvovirus replication in human blast cell lines

B19 parvovirus can replicate in erythroid progenitor cells and in a small number of human blast cell lines. To better understand and analyze the B19 virus replicative cycle, we performed and compared the infection of bone marrow cells and of different blast cell lines with erythroblastoid and megakaryoblastoid phenotypic characteristics (UT-7, TF-1, M-07, and B1647). Following in vitro infection, B19-specific nucleic acids were characterized with regard to the genome-replicative intermediates, the transcription pattern, and the localization of virus-specific nucleic acids inside infected cells. While all cell lines tested proved to be susceptible to B19 virus infection, two different patterns of restriction to replication of B19 virus were observed. In the first restriction pattern, observed in UT-7 cells, the single-stranded viral DNA was converted to double-stranded replicative intermediates, identical to those found in bone marrow cells, and a full set of viral transcripts were observed. However, replication and transcription were restricted to a small subset of cells, and production of capsid proteins was not detected. In the second restriction pattern, observed in TF-1, M-07, and B1647 cells, the single-stranded viral DNA was not converted to double-stranded replicative intermediates.

Identification of a novel RNA splicing pattern as a basis of restricted cell tropism of erythrovirus B19

Prior studies on the transcription of erythrovirus B19 have identified a short leader sequence associated with all spliced viral transcripts. While some variability has been observed in the acceptor for this first intron, studies to date in both permissive and nonpermissive cell types have reported a unique splice donor site. In the semipermissive MB-02 cell line, we have found that splicing of this first intron proceeds almost exclusively via a cryptic CT donor downstream of the previously reported GT donor at nucleotide 406. The resulting messages for the viral structural proteins and 11-kDa protein are thereby made bicistronic, with the first expressible polypeptide being a 34 amino acid fusion of the NS-1 and 7.5-kDa proteins. The presence of an upstream open-reading frame on these messages is likely to block effective translation of the downstream structural protein products. We propose this as a significant mechanism in determining B19's tropism on the basis of host cell splicing machinery, and present evidence in support of this model. Additionally, this is the first report of usage of a noncanonical splice donor in B19, and to our knowledge the first report of a CT-AG splice in any system.

Possible interactions between the NS-1 protein and tumor necrosis factor alpha pathways in erythroid cell apoptosis induced by human parvovirus B19

Human erythroid progenitor cells are the main target cells of the human parvovirus B19 (B19), and B19 infection induces a transient erythroid aplastic crisis. Several authors have reported that the nonstructural protein 1 (NS-1) encoded by this virus has a cytotoxic effect, but the underlying mechanism of NS-1-induced primary erythroid cell death is still not clear. In human erythroid progenitor cells, we investigated the molecular mechanisms leading to apoptosis after natural infection of these cells by the B19 virus. The cytotoxicity of NS-1 was concomitantly evaluated in transfected erythroid cells. B19 infection and NS-1 expression induced DNA fragmentation characteristic of apoptosis, and the commitment of erythroid cells to undergo apoptosis was combined with their accumulation in the G(2) phase of the cell cycle. Since B19- and NS-1-induced apoptosis was inhibited by caspase 3, 6, and 8 inhibitors, and substantial caspase 3, 6, and 8 activities were induced by NS-1 expression, there may have been interactions between NS-1 and the apoptotic pathways of the death receptors tumor necrosis factor receptor 1 and Fas. Our results suggest that Fas-FasL interaction was not involved in NS-1- or B19-induced apoptosis in erythroid cells. In contrast, these cells were sensitized to tumor necrosis factor alpha (TNF-alpha)-induced apoptosis. Moreover, the ceramide level was enhanced by B19 infection and NS-1 expression. Therefore, our results suggest that there may be a connection between the respective apoptotic pathways activated by TNF-alpha and NS-1 in human erythroid cells.

Genetic heterogeneity of the immunogenic viral capsid protein region of human parvovirus B19 isolates obtained from an outbreak in a pediatric ward

Whereas human parvovirus B19 commonly infects children and causes erythema infectiosum, it causes more severe diseases when it infects adults. In order to examine whether different clinical outcomes of B19 infection can be ascribed to the viral genetic heterogeneity, we have determined the nucleotide sequence of highly immunogenic portions of the B19 genome obtained from six patients with various clinical manifestations in a single outbreak. Our observations demonstrated that although the B19 sequences showed a significant heterogeneity, it was not correlated with the clinical manifestation. It was thus suggested that the host immune response to B19 infection may be a major determinant of clinical presentations associated with acute B19 infection.

Regulation of human B19 parvovirus promoter expression by hGABP (E4TF1) transcription factor

The genetic expression of human B19 parvovirus is only dependent on one promoter in vivo and in vitro. This is the P6 promoter, which is located on the left side of the genome and is a single-stranded DNA molecule. This led us to investigate the regulation of the P6 promoter and the possible resulting variability of the nucleotide sequence. After analysis of the promoter region of 17 B19 strains, only 1.5% variability was found. More exciting was the finding of mutations that were clustered around the TATA box and defined a highly conserved region (nucleotides 113-210) in the proximal part of the P6 promoter. HeLa and UT7/Epo cell extracts were found to protect this region, which contained a core motif for Ets family proteins, with YY1 and Sp1 binding sites on either side. Gel mobility shift assays performed with nuclear proteins from HeLa and UT7/Epo cells identified DNA-binding proteins specific for these sites. By supershift analysis, we demonstrated the binding of the hGABP (also named E4TF1) protein to the Ets binding site and the fixation of Sp1 and YY1 proteins on their respective motifs. In Drosophila SL2 cells, hGABPalpha and -beta stimulated P6 promoter activity, and hGABPalpha/hGABPbeta and Sp1 exerted synergistic stimulation of this activity, an effect diminished by YY1.