The VP1u Receptor Restricts Parvovirus B19 Uptake to Permissive Erythroid Cells

Human parvovirus B19 virus-like particle formation in Nicotiana benthamiana

There has been a surge in the interest to utilize plants as hosts for producing vaccine antigens. In this study, we demonstrated the successful expression of the human parvovirus B19 (B19V) capsid protein (VP2) in Nicotiana benthamiana cells. The B19V VP1 and VP2 genes were cloned under the control of estrogen-inducible promoters and transiently expressed in N. benthamiana leaves using the agroinfiltration method. The addition of estrogen significantly boosted the expression of VP2. Furthermore, codon optimization of the VP2 sequence resulted in over a 30-fold increase in its expression compared with that of the wild-type. Analysis of negatively stained samples by sucrose density gradient ultracentrifugation and electron microscopy revealed that the expressed VP2 proteins formed spherical particles with diameters of approximately 20 nm. Immunostaining analysis of protoplasts derived from VP2-expressing N. benthamiana leaves indicated that VP2 signals were predominantly localized in the cytoplasm. These findings strongly suggested that B19V VP2 assembles and formed virus-like particles (VLPs) within the cytoplasm of N. benthamiana cells, presenting a promising method for producing B19V VLPs in plant systems.

Backbone NMR resonance assignments for the VP1u N-terminal receptor-binding domain of the human parvovirus pathogen B19

Parvovirus B19 (B19V) is a human pathogen that is the causative agent of several diseases in infants and adults. Due to a lack of antivirals against this virus, treatment options are limited. The minor capsid protein of B19V has a unique N terminus, named VP1u, which is essential for infection. The VP1u encodes a receptor binding domain (RBD), necessary for host cell entry, and a phospholipase A2 (PLA2) domain, crucial for endosomal escape during cellular trafficking. Both domains are indispensable for infection, making the RBD a plausible drug target for inhibitors against B19V, as it is located on the exterior surface of the virus. To date, no experimental structural information has been available for the VP1u component for any Parvovirus. Here we report the backbone NMR resonance assignments for the RBD of B19V and demonstrate it forms a stable structure. The backbone chemical shifts are in good agreement with a structure predicted by AlphaFold, validating that the RBD contains three helices connected by tight turns. This RBD construct can now be used for further NMR studies, including assignment of full-length VP1u, determination of protein-protein interaction interfaces, and development of B19 antivirals specific to the RBD domain.

Parvovirus B19 in Rheumatic Diseases

Parvovirus B19 (B19V) is a human pathogen belonging to the Parvoviridae family. It is widely diffused in the population and responsible for a wide range of diseases, diverse in pathogenetic mechanisms, clinical course, and severity. B19V infects and replicates in erythroid progenitor cells (EPCs) in the bone marrow leading to their apoptosis. Moreover, it can also infect, in an abortive manner, a wide set of different cell types, normally non-permissive, and modify their normal physiology. Differences in the characteristics of virus-cell interaction may translate into different pathogenetic mechanisms and clinical outcomes. Joint involvement is a typical manifestation of B19V infection in adults. Moreover, several reports suggest, that B19V could be involved in the pathogenesis of some autoimmune rheumatologic diseases such as rheumatoid arthritis (RA), juvenile idiopathic arthritis (JIA), systemic sclerosis (SSc), systemic lupus erythematosus (SLE), or vasculitis. This review provides basic information on the B19 virus, highlights characteristics of viral infection in permissive and non-permissive systems, and focuses on recent findings concerning the pathogenic role of B19V in rheumatologic diseases.

Globoside Is an Essential Intracellular Factor Required for Parvovirus B19 Endosomal Escape

Human parvovirus B19 (B19V), like most parvoviruses, possesses phospholipase A2 (PLA2) activity, which is thought to mediate endosomal escape by membrane disruption. Here, we challenge this model and find evidence for a mechanism of B19V entry mediated by the glycosphingolipid globoside without endosome disruption and retrograde transport to the Golgi. We show that B19V PLA2 activity requires specific calcium levels and pH conditions that are not optimal in endosomes. Accordingly, endosomal membrane integrity was maintained during B19V entry. Furthermore, endosomes remained intact when loaded with MS2 bacteriophage particles pseudotyped with multiple B19V PLA2 subunits, providing superior enzymatic potential compared to native B19V. In globoside knockout cells, incoming viruses are arrested in the endosomal compartment and the infection is blocked. Infection can be rescued by promoting endosomal leakage with polyethyleneimine (PEI), demonstrating the essential role of globoside in facilitating endosomal escape. Incoming virus colocalizes with Golgi markers and interfering with Golgi function blocks infection, suggesting that globoside-mediated entry involves the Golgi compartment, which provides conditions favorable for the lipolytic PLA2. Our study challenges the current model of B19V entry and identifies globoside as an essential intracellular receptor required for endosomal escape.

Feasibility of Domain Segmentation of B19V VP1u Using Intein Technology for Structural Studies

INTRODUCTION

Parvovirus B19 (B19V) is a human pathogen, and the minor capsid protein of B19V possesses a unique N terminus called VP1u that plays a crucial role in the life cycle of the virus.

OBJECTIVES

The objective of this study was to develop a method for domain segmentation of B19 VP1u using intein technology, particularly its receptor binding domain (RBD) and phospholipase A2 (PLA2) domain.

METHODS

RBD and PLA2 domains of VP1u were each fused to the DnaE split inteins derived from the Nostoc punctiforme. Each of these precursor proteins was expressed in E. coli. Combining the purified precursors in equal molar ratios resulted in the formation of full-length VP1u. Furthermore, Circular Dichroism (CD) spectroscopy and PLA2 assays were used to probe the structure and activity of the newly formed protein.

RESULTS

The CD spectrum of the full length VP1u confirmed the secondary structure of protein, while the PLA2 assay indicated minimal disruption in enzymatic activity.

CONCLUSION

This method would allow for the selective incorporation of NMR-active isotopes into either of the VP1u domains, which can reduce signal overlap in NMR structural determination studies.

Therapeutic Potential of Engineered Virus-like Particles of Parvovirus B19

Virus-like particles (VLPs) comprise one or many structural components of virions, except their genetic material. Thus, VLPs keep their structural properties of cellular recognition while being non-infectious. VLPs of Parvovirus B19 (B19V) can be produced by the heterologous expression of their structural proteins VP1 and VP2 in bacteria. These proteins are purified under denaturing conditions, refolded, and assembled into VLPs. Moreover, chimeric forms of VP2 have been constructed to harbor peptides or functional proteins on the surface of the particles without dropping their competence to form VLPs, serving as presenting nanoparticles. The in-vitro assembly approach offers exciting possibilities for the composition of VLPs, as more than one chimeric form of VP2 can be included in the assembly stage, producing multifunctional VLPs. Here, the heterologous expression and in-vitro assembly of B19V structural proteins and their chimeras are reviewed. Considerations for the engineering of the structural proteins of B19V are also discussed. Finally, the construction of multifunctional VLPs and their future potential as innovative medical tools are examined.

Globoside and the mucosal pH mediate parvovirus B19 entry through the epithelial barrier

Parvovirus B19 (B19V) is transmitted primarily via the respiratory route, however, the mechanism involved remains unknown. B19V targets a restricted receptor expressed in erythroid progenitor cells in the bone marrow. However, B19V shifts the receptor under acidic conditions and targets the widely expressed globoside. The pH-dependent interaction with globoside may allow virus entry through the naturally acidic nasal mucosa. To test this hypothesis, MDCK II cells and well-differentiated human airway epithelial cell (hAEC) cultures were grown on porous membranes and used as models to study the interaction of B19V with the epithelial barrier. Globoside expression was detected in polarized MDCK II cells and the ciliated cell population of well-differentiated hAEC cultures. Under the acidic conditions of the nasal mucosa, virus attachment and transcytosis occurred without productive infection. Neither virus attachment nor transcytosis was observed under neutral pH conditions or in globoside knockout cells, demonstrating the concerted role of globoside and acidic pH in the transcellular transport of B19V. Globoside-dependent virus uptake involved VP2 and occurred by a clathrin-independent pathway that is cholesterol and dynamin-dependent. This study provides mechanistic insight into the transmission of B19V through the respiratory route and reveals novel vulnerability factors of the epithelial barrier to viruses.

Non-Permissive Parvovirus B19 Infection: A Reservoir and Questionable Safety Concern in Mesenchymal Stem Cells

Mesenchymal stromal/stem cells (MSCs) are multipotent cells with differentiation, immunoregulatory and regenerative properties. Because of these features, they represent an attractive tool for regenerative medicine and cell-based therapy. However, MSCs may act as a reservoir of persistent viruses increasing the risk of failure of MSCs-based therapies and of viral transmission, especially in immunocompromised patients. Parvovirus B19V (B19V) is a common human pathogen that infects bone marrow erythroid progenitor cells, leading to transient or persistent anemia. Characteristics of B19V include the ability to cross the placenta, infecting the fetus, and to persist in several tissues. We thus isolated MSCs from bone marrow (BM-MSCs) and fetal membrane (FM-MSCs) to investigate their permissiveness to B19V infection. The results suggest that both BM- and FM- MSCs can be infected by B19V and, while not able to support viral replication, allow persistence over time in the infected cultures. Future studies are needed to understand the potential role of MSCs in B19V transmission and the conditions that can favor a potential reactivation of the virus.

Recombinant Virus-like Particles of Human Parvovirus B19 with the Internal Location of VP1 Unique Region Produced by Hansenula polymorpha

Human parvovirus B19 (HPV B19) is pathogenic to human, which can cause fifth disease, transient aplastic crisis, arthritis, myocarditis, autoimmune disorders, hydrops fetalis, and so on. Currently, no approved vaccines or antiviral drugs are available against HPV B19, and thus the development of effective vaccines is needed. The capsid of HPV B19 is composed of two types of proteins, i.e., the major capsid protein VP2 and the minor protein VP1. Previous experimental studies have shown that the dominant immune responses against HPV B19 are elicited by VP1, especially the unique region on the N-terminus of VP1. It has been found that VP2 alone or VP2 and VP1 together can assemble into virus-like particle (VLP). The VLP structure formed by VP2 has been resolved, however, the location of VP1 in the capsid, especially the location of VP1 unique region with strong immunogenicity, is still not clear. In the present work, using the Hansenula polymorpha expression system developed by our laboratory, two kinds of recombinant HPV B19 VLPs were expressed, i.e., the VLP co-assembled by VP1 and VP2 (VP1/VP2 VLP) and the VLP whose VP1 content was improved (VP1h/VP2 VLP). The expression, purity, and morphology of these two VLPs were characterized, and then their immunogenic properties were investigated and compared with those of the VLP containing VP2 alone (VP2 VLP) previously developed by our group. Furthermore, the location of the VP1 unique region in the VLPs was determined by using the immunogold electron microscopy (IGEM). Our experimental results show that the VP1h/VP2 VLP elicits a stronger neutralization against the HPV B19 than VP2 and VP1/VP2 VLPs, which implies that the increase of VP1 content significantly improves the level of neutralizing antibodies. In addition, the IGEM observations suggest that the unique region of VP1 may be located inside the recombinant VLP. The VLPs recombinantly expressed by our Hansenula polymorpha system may serve as a promising candidate immunogen for HPV B19 vaccine development.

Safety and immunogenicity of parvovirus B19 virus-like particle vaccine lacking phospholipase A2 activity

Parvovirus B19 (B19) belongs to the Erythroparvovirus genus and is known to cause the fifth disease in children. Primary infection of pregnant women is associated with a high risk of hydrops fetalis and stillbirth due to severe fetal anemia. Virus-like particle (VLP) vaccine candidates for B19 have been developed, although none have been approved so far. The B19 phospholipase A2 domain (B19 PLA2), located in the VP1 unique region, is believed to be associated with adverse inflammatory reactions, and previous effective attempts to improve this vaccine modality inserted a mutation to impair the PLA2 activity of VLPs. In this study, we designed VLPs with a deletion mutant of PLA2 (⊿PLA2 B19 VLP), devoid of PLA2 activity, and confirmed their immunogenicity and safe use in vivo. These results were supported by the lack of histological inflammatory reactions at the site of immunization or the production of IL-6 in ⊿PLA2 B19 VLP-immunized mice, that were observed in mice immunized with B19 VLPs. CD4⁺ T cells from mice vaccinated with VLPs and B19-seropositive human samples were not activated by B19 PLA2 stimulation, suggesting that the B19 PLA2 domain does not constitute a major CD4⁺ T cell epitope. Most importantly, the ⊿PLA2 B19 VLPs induced neutralizing antibodies against B19, in levels similar to those found in B19-seropositive human samples, indicating that they could be used as a safe and effective vaccine candidate against B19.

Structural Dynamics and Activity of B19V VP1u during the pHs of Cell Entry and Endosomal Trafficking

Parvovirus B19 (B19V) is a human pathogen that is the causative agent of fifth disease in children. It is also known to cause hydrops in fetuses, anemia in AIDS patients, and transient aplastic crisis in patients with sickle cell disease. The unique N-terminus of Viral Protein 1 (VP1u) of parvoviruses, including B19V, exhibits phospholipase A2 (PLA2) activity, which is required for endosomal escape. Presented is the structural dynamics of B19V VP1u under conditions that mimic the pHs of cell entry and endosomal trafficking to the nucleus. Using circular dichroism spectroscopy, the receptor-binding domain of B19V VP1u is shown to exhibit an α-helical fold, whereas the PLA2 domain exhibits a probable molten globule state, both of which are pH invariant. Differential scanning calorimetry performed at endosomal pHs shows that the melting temperature (Tm) of VP1u PLA2 domain is tuned to body temperature (37 °C) at pH 7.4. In addition, PLA2 assays performed at temperatures ranging from 25-45 °C show both a temperature and pH-dependent change in activity. We hypothesize that VP1u PLA2 domain differences in Tm at differing pHs have enabled the virus to "switch on/off" the phospholipase activity during capsid trafficking. Furthermore, we propose the environment of the early endosome as the optimal condition for endosomal escape leading to B19V infection.

Towards the Antiviral Agents and Nanotechnology-Enabled Approaches Against Parvovirus B19

Parvovirus B19 (B19V) as a human pathogenic virus, would cause a wide range of clinical manifestations. Besides the supportive and symptomatic treatments, the only FDA-approved antiviral drug for the treatment of B19V is intravenous immunoglobulins, which however, have limited efficacy and high cost. By far, there are still no virus-specific therapeutics clinically available to treat B19V infection. Therefore, exploiting the potential targets with a deep understanding of the life cycle of B19V, are pivotal to the development of B19V-tailored effective antiviral approaches. This review will introduce antiviral agents via blocking viral invasion, inhibiting the enzymes or regulatory proteins involved in DNA synthesis, and so on. Moreover, nanotechnology-enabled approaches against B19V will also be outlined and discussed through a multidisciplinary perspective involving virology, nanotechnology, medicine, pharmaceutics, chemistry, materials science, and other fields. Lastly, the prospects of the antiviral agents and nanosystems in terms of fabrication, clinical translation and potential breakthroughs will be briefly discussed.

A Conserved Receptor-Binding Domain in the VP1u of Primate Erythroparvoviruses Determines the Marked Tropism for Erythroid Cells

Parvovirus B19 (B19V) is a human pathogen with a marked tropism for erythroid progenitor cells (EPCs). The N-terminal of the VP1 unique region (VP1u) contains a receptor-binding domain (RBD), which mediates virus uptake through interaction with an as-yet-unknown receptor (VP1uR). Considering the central role of VP1uR in the virus tropism, we sought to investigate its expression profile in multiple cell types. To this end, we established a PP7 bacteriophage-VP1u bioconjugate, sharing the size and VP1u composition of native B19V capsids. The suitability of the PP7-VP1u construct as a specific and sensitive VP1uR expression marker was validated in competition assays with B19V and recombinant VP1u. VP1uR expression was exclusively detected in erythroid cells and cells reprogrammed towards the erythroid lineage. Sequence alignment and in silico protein structure prediction of the N-terminal of VP1u (N-VP1u) from B19V and other primate erythroparvoviruses (simian, rhesus, and pig-tailed) revealed a similar structure characterized by a fold of three or four α-helices. Functional studies with simian parvovirus confirmed the presence of a conserved RBD in the N-VP1u, mediating virus internalization into human erythroid cells. In summary, this study confirms the exclusive association of VP1uR expression with cells of the erythroid lineage. The presence of an analogous RBD in the VP1u from non-human primate erythroparvoviruses emphasizes their parallel evolutionary trait and zoonotic potential.

Fusion of parvovirus B19 receptor-binding domain and pneumococcal surface protein A induces protective immunity against parvovirus B19 and Streptococcus pneumoniae

BACKGROUND

Parvovirus B19 (B19) is a well-known cause of fifth disease in children, but infection during pregnancy may cause hydrops fetalis and stillbirth. The receptor-binding domain (RBD) of the VP1 unique capsid plays a pivotal role in infection. Here, we aimed to improve the immunogenicity of an RBD-based vaccine by genetically fusing it with Streptococcus pneumoniae surface protein A (PspA).

METHODS

Mice were intramuscularly injected with RBD-based vaccines. Antigen-specific antibodies and neutralizing activity against B19 were measured. Protective immunity against S. pneumoniae was evaluated by monitoring the survival of mice nasally challenged with bacteria and determining antigen-specific T cell activation in splenic cells.

RESULTS

RBD alone failed to generate neutralizing antibodies against B19, but fusion with PspA induced higher levels of neutralizing IgG compared to B19 virus-like particles. Furthermore, a comparable level of PspA-specific IgG was induced by RBD-PspA and PspA alone, which was sufficient to protect mice against pneumococcal infection. Stimulation with PspA, but not RBD, induced cytokine production in splenic cells from mice immunized with RBD-PspA, suggesting that PspA-specific T cells supported immunoglobulin class switching of both RBD- and PspA-specific B cells.

CONCLUSIONS

RBD-PspA should be an effective bivalent vaccine against B19 and S. pneumoniae infections.

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.

The VP1u of Human Parvovirus B19: A Multifunctional Capsid Protein with Biotechnological Applications

The viral protein 1 unique region (VP1u) of human parvovirus B19 (B19V) is a multifunctional capsid protein with essential roles in virus tropism, uptake, and subcellular trafficking. These functions reside on hidden protein domains, which become accessible upon interaction with cell membrane receptors. A receptor-binding domain (RBD) in VP1u is responsible for the specific targeting and uptake of the virus exclusively into cells of the erythroid lineage in the bone marrow. A phospholipase A₂ domain promotes the endosomal escape of the incoming virus. The VP1u is also the immunodominant region of the capsid as it is the target of neutralizing antibodies. For all these reasons, the VP1u has raised great interest in antiviral research and vaccinology. Besides the essential functions in B19V infection, the remarkable erythroid specificity of the VP1u makes it a unique erythroid cell surface biomarker. Moreover, the demonstrated capacity of the VP1u to deliver diverse cargo specifically to cells around the proerythroblast differentiation stage, including erythroleukemic cells, offers novel therapeutic opportunities for erythroid-specific drug delivery. In this review, we focus on the multifunctional role of the VP1u in B19V infection and explore its potential in diagnostics and erythroid-specific therapeutics.

Globoside Is Dispensable for Parvovirus B19 Entry but Essential at a Postentry Step for Productive Infection

Globoside (Gb4) is considered the primary receptor of parvovirus B19 (B19V); however, its expression does not correlate well with the attachment and restricted tropism of the virus. The N terminus of VP1 (VP1u) of B19V interacts with an as-yet-unknown receptor required for virus internalization. In contrast to Gb4, the VP1u cognate receptor is expressed exclusively in cells that B19V can internalize. With the aim of clarifying the role of Gb4 as a B19V receptor, we knocked out the gene B3GalNT1 coding for the enzyme globoside synthase in UT7/Epo cells. Consequently, B3GalNT1 transcripts and Gb4 became undetectable in the knockout (KO) cells without affecting cell viability and proliferation. Unexpectedly, virus attachment, internalization, and nuclear targeting were not disturbed in the KO cells. However, NS1 transcription failed, and consequently, genome replication and capsid protein expression were abrogated. The block could be circumvented by transfection with a B19V infectious clone, indicating that Gb4 is not required after the generation of viral double-stranded DNA with resolved inverted terminal repeats. While in wild-type (WT) cells, occupation of the VP1u cognate receptor with recombinant VP1u disturbed virus binding and blocked the infection, antibodies against Gb4 had no significant effect. In a mixed population of WT and KO cells, B19V selectively infected WT cells. This study demonstrates that Gb4 does not have the expected receptor function, as it is dispensable for virus entry; however, it is essential for productive infection, explaining the resistance of the rare individuals lacking Gb4 to B19V infection.IMPORTANCE Globoside has long been considered the primary receptor of B19V. However, its expression does not correlate well with B19V binding and uptake and cannot explain the pathogenesis or the remarkable narrow tissue tropism of the virus. By using a knockout cell line, we demonstrate that globoside does not have the expected function as a cell surface receptor required for B19V entry, but it has an essential role at a postentry step for productive infection. This finding explains the natural resistance to infection associated with individuals lacking globoside, contributes to a better understanding of B19V restricted tropism, and offers novel strategies for the development of antiviral therapies.

Advances in the Development of Antiviral Strategies against Parvovirus B19

Parvovirus B19 (B19V) is a human pathogenic virus, responsible for an ample range of clinical manifestations. Infections are usually mild, self-limiting, and controlled by the development of a specific immune response, but in many cases clinical situations can be more complex and require therapy. Presently available treatments are only supportive, symptomatic, or unspecific, such as administration of intravenous immunoglobulins, and often of limited efficacy. The development of antiviral strategies against B19V should be considered of highest relevance for increasing the available options for more specific and effective therapeutic treatments. This field of research has been explored in recent years, registering some achievements as well as interesting future perspectives. In addition to immunoglobulins, some compounds have been shown to possess inhibitory activity against B19V. Hydroxyurea is an antiproliferative drug used in the treatment of sickle-cell disease that also possesses inhibitory activity against B19V. The nucleotide analogues Cidofovir and its lipid conjugate Brincidofovir are broad-range antivirals mostly active against dsDNA viruses, which showed an antiviral activity also against B19V. Newly synthesized coumarin derivatives offer possibilities for the development of molecules with antiviral activity. Identification of some flavonoid molecules, with direct inhibitory activity against the viral non-structural (NS) protein, indicates a possible line of development for direct antiviral agents. Continuing research in the field, leading to better knowledge of the viral lifecycle and a precise understanding of virus–cell interactions, will offer novel opportunities for developing more efficient, targeted antiviral agents, which can be translated into available therapeutic options.

Parvovirus B19 Uncoating Occurs in the Cytoplasm without Capsid Disassembly and It Is Facilitated by Depletion of Capsid-Associated Divalent Cations

Human parvovirus B19 (B19V) traffics to the cell nucleus where it delivers the genome for replication. The intracellular compartment where uncoating takes place, the required capsid structural rearrangements and the cellular factors involved remain unknown. We explored conditions that trigger uncoating in vitro and found that prolonged exposure of capsids to chelating agents or to buffers with chelating properties induced a structural rearrangement at 4 °C resulting in capsids with lower density. These lighter particles remained intact but were unstable and short exposure to 37 °C or to a freeze-thaw cycle was sufficient to trigger DNA externalization without capsid disassembly. The rearrangement was not observed in the absence of chelating activity or in the presence of MgCl₂ or CaCl₂, suggesting that depletion of capsid-associated divalent cations facilitates uncoating. The presence of assembled capsids with externalized DNA was also detected during B19V entry in UT7/Epo cells. Following endosomal escape and prior to nuclear entry, a significant proportion of the incoming capsids rearranged and externalized the viral genome without capsid disassembly. The incoming capsids with accessible genomes accumulated in the nuclear fraction, a process that was prevented when endosomal escape or dynein function was disrupted. In their uncoated conformation, capsids immunoprecipitated from cytoplasmic or from nuclear fractions supported in vitro complementary-strand synthesis at 37 °C. This study reveals an uncoating strategy of B19V based on a limited capsid rearrangement prior to nuclear entry, a process that can be mimicked in vitro by depletion of divalent cations.

Evidence of Human Parvovirus B19 Infection in the Post-Mortem Brain Tissue of the Elderly

After primary exposure, the human parvovirus B19 (B19V) genome may remain in the central nervous system (CNS), establishing a lifelong latency. The structural characteristics and functions of the infected cells are essential for the virus to complete its life cycle. Although B19V has been detected in the brain tissue by sequencing PCR products, little is known about its in vivo cell tropism and pathogenic potential in the CNS. To detect B19V and investigate the distribution of its target cells in the CNS, we studied brain autopsies of elderly subjects using molecular virology, and optical and electron microscopy methods. Our study detected B19V in brain tissue samples from both encephalopathy and control groups, suggesting virus persistence within the CNS throughout the host’s lifetime. It appears that within the CNS, the main target of B19V is oligodendrocytes. The greatest number of B19V-positive oligodendrocytes was found in the white matter of the frontal lobe. The number was significantly lower in the gray matter of the frontal lobe (p = 0.008) and the gray and white matter of the temporal lobes (p < 0.0001). The morphological changes observed in the encephalopathy group, propose a possible B19V involvement in the demyelination process.

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.