Peptides derived from the unique region of B19 parvovirus minor capsid protein elicit neutralizing antibodies in rabbits

Structural Shifts of the Parvovirus B19 Capsid Receptor-binding Domain: A Peptide Study

BACKGROUND

Binding appropriate cellular receptors is a crucial step of a lifecycle for any virus. Structure of receptor-binding domain for a viral surface protein has to be determined before the start of future drug design projects.

OBJECTIVES

Investigation of pH-induced changes in the secondary structure for a capsid peptide with loss of function mutation can shed some light on the mechanism of entrance.

METHODS

Spectroscopic methods were accompanied by electrophoresis, ultrafiltration, and computational biochemistry.

RESULTS

In this study, we showed that a peptide from the receptor-binding domain of Parvovirus B19 VP1 capsid (residues 13-31) is beta-structural at pH=7.4 in 0.01 M phosphate buffer, but alpha- helical at pH=5.0, according to the circular dichroism (CD) spectroscopy results. Results of infra- red (IR) spectroscopy showed that the same peptide exists in both alpha-helical and beta-structural conformations in partial dehydration conditions both at pH=7.4 and pH=5.0. In contrast, the peptide with Y20W mutation, which is known to block the internalization of the virus, forms mostly alpha-helical conformation in partial dehydration conditions at pH=7.4. According to our hypothesis, an intermolecular antiparallel beta structure formed by the wild-type peptide in its tetramers at pH=7.4 is the prototype of the similar intermolecular antiparallel beta structure formed by the corresponding part of Parvovirus B19 receptor-binding domain with its cellular receptor (AXL).

CONCLUSION

Loss of function Y20W substitution in VP1 capsid protein prevents the shift into the beta-structural state by the way of alpha helix stabilization and the decrease of its ability to turn into the disordered state.

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.

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.

Identification of AXL as a co-receptor for human parvovirus B19 infection of human erythroid progenitors

Parvovirus B19 (B19V) infects human erythroid progenitor cells (EPCs) and causes several hematological disorders and fetal hydrops. Amino acid (aa) 5-68 of minor capsid protein VP1 (VP1u5-68aa) is the minimal receptor binding domain for B19V to enter EPCs. Here, we carried out a genome-wide CRISPR-Cas9 guide RNA screen and identified tyrosine protein kinase receptor UFO (AXL) as a proteinaceous receptor for B19V infection of EPCs. AXL gene silencing in ex vivo expanded EPCs remarkably decreased B19V internalization and replication. Additions of the recombinant AXL extracellular domain or a polyclonal antibody against it upon infection efficiently inhibited B19V infection of ex vivo expanded EPCs. Moreover, B19V VP1u interacted with the recombinant AXL extracellular domain in vitro at a relatively high affinity (KD = 103 nM). Collectively, we provide evidence that AXL is a co-receptor for B19V infection of EPCs.

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.

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.

Effect of N‑terminal region of human parvovirus B19‑VP1 unique region on cardiac injury in naïve mice

A unique region of human parvovirus B19 virus-VP1 (B19V–VP1u) has been linked to a variety of cardiac disorders. However, the precise role of B19V–VP1u in inducing cardiac injury remains unknown. The present study investigated the effects of B19V–VP1u and different regions of B19V–VP1u, including B19V–VP1uA (residues 1–60), B19V–VP1uB (residues 61–129), B19V–VP1uC (residues 130–195) and B19V–VP1uD (residues 196–227), on inducing cardiac injury in naïve mice by zymography, immunoblotting, H&E staining and cytokine immunoassay. A significantly higher MMP-9/MMP-2 ratio and increased levels of inflammatory cytokines, including IL-6 and IL-1β, were detected in the left ventricles of the mice injected with B19V-non-structural protein 1 (B19V-NS1) and B19V–VP1u, accompanied by increased expression levels of phosphorylated (p-)ERK and p-P38. Significantly upregulated expression levels of atrial natriuretic peptide (ANP), heart-type fatty acid-binding protein (H-FABP) and creatine kinase isoenzyme-MB (CK-MB), which are well-known cardiac injury markers, as well as increased infiltration of lymphocytes, were detected in the left ventricles of the mice injected with B19V–VP1, B19V-NS1 and B19V–VP1u. Moreover, a significantly higher MMP-9/MMP-2 ratio and increased levels of IL-6 and IL-1β were observed in the left ventricles of the mice injected with B19V–VP1u, B19V–VP1u-A, B19V–VP1u-B and B19V–VP1u-C, accompanied by upregulated p-ERK and p-P38 expression. Notably, significantly lower levels of IL-6 and IL-1β were observed in the left ventricles of the mice injected with B19V–VP1uD. Furthermore, significantly increased ANP, H-FABP and CK-MB expression levels were detected in the left ventricles of the mice injected with B19V–VP1u, B19V–VP1u-A and B19V–VP1u-B, along with enhanced infiltration of lymphocytes. Significantly higher serum IL-1β, IL-6, TNF-α and IFN-γ levels were also detected in the mice injected with B19V–VP1u, B19V–VP1u-A and B19V–VP1u-B. To the best of our knowledge, the findings of the present study were the first to demonstrate that the N-terminal region (residues 1–129) of B19V–VP1u induces an increase in the levels of cardiac injury markers, thus providing evidence for understanding the possible functional regions within B19V–VP1u.

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.

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

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

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 role of Th17 cells in viral infections

The present review provides an overview of recent advances regarding the function of Th17 cells and their produced cytokines in the progression of viral diseases. Viral infections alone do not lead to virus-induced malignancies, as both genetic and host safety factors are also involved in the occurrence of malignancies. Acquired immune responses, through the differentiation of Th17 cells, form the novel components of the Th17 cell pathway when reacting with viral infections all the way from the beginning to its final stages. As a result, instead of inducing the right immune responses, these events lead to the suppression of the immune system. In fact, the responses from Th17 cells during persistent viral infections causes chronic inflammation through the production of IL-17 and other cytokines which provide a favorable environment for tumor growth and its development. Additionally, during the past decade, these cells have been understood to be involved in tumor progression and metastasis. However, further research is required to understand Th17 cells' immune mechanisms in the vast variety of viral diseases. This review aims to determine the roles and effects of the immune system, especially Th17 cells, in the progression of viral diseases; which can be highly beneficial for the diagnosis and treatment of these infections.

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.

Vaccine Design Informed by Virus-Induced Immunity

When an individual is exposed to a viral pathogen for the first time, the adaptive immune system is naive and cannot prevent virus replication. The consequence may be severe disease. At the same time, the host may rapidly generate a pathogen-specific immune response that will prevent disease if the virus is encountered again. Parvovirus B19 provides one such example. Children with sickle cell disease can experience life-threatening transient aplastic crisis when first exposed to parvovirus B19, but an effective immune response confers lifelong protection. We briefly examine the induction and benefits of virus-induced immunity. We focus on three human viruses for which there are no licensed vaccines (respiratory syncytial virus, human immunodeficiency virus type 1, and parvovirus B19) and consider how virus-induced immunity may inform successful vaccine design.

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.

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.

Antigenicity analysis of human parvovirus B19-VP1u protein in the induction of anti-phospholipid syndrome

Mounting evidence suggests a connection between human parvovirus B19 (B19) and autoimmune diseases, and especially an association between the B19-VP1 unique region (VP1u) and anti-phospholipid syndrome (APS). However, little is known about the antigenicity of B19-VP1u in the induction of APS-like syndrome. To elucidate the antigenicity of B19-VP1u in the induction of APS, N-terminal truncated B19-VP1u (tVP1u) proteins were prepared to immunize Balb/c mice to generate antibodies against B19-tVP1u proteins. The secreted phospholipase A2 (sPLA2) activities and binding specificity of mice anti-B19-tVP1u antibodies with cardiolipin (CL) and beta-2-glycoprotein I (β2GPI) were evaluated by performing immunoblot, ELISA and absorption experiments. A mice model of passively induced APS was adopted. Although sPLA2 activities were identified in all B19-tVP1u proteins, only amino acid residues 61–227 B19-tVP1u exhibited a higher sPLA2 activity. Autoantibodies against CL and β2GPI exhibited binding activities with all B19-tVP1u proteins. IgG that was purified from mice that had been immunized with amino acid residues 21–227 to 121–227 B19-tVP1u proteins exhibited significantly higher binding activity with CL. IgG that was purified from mice that had been immunized with amino acid residues 21–227, 31–227, 82–227 and 91–227 B19-tVP1u proteins exhibited significantly higher binding activity with β2GPI. Accordingly, significantly higher binding inhibition of CL was detected in the presence of amino acid residues 61–227 and 101–227 B19-tVP1u. Significantly higher binding inhibition of β2GPI was detected in the presence of amino acid residues 21–227, 31–227, 82–227 and 91–227 B19-tVP1u. The mice that received amino acid residues 31–227 or 61–227 anti-tB19-VP1u IgG revealed significant thrombocytopenia and those that received amino acid residues 21–227, 31–227, 61–227, 71–227, 82–227, 91–227, 101–227 or 114–227 anti-tB19-VP1u IgG exhibited significantly prolonged aPTT. These findings provide further information concerning the role of B19-VP1u antigenicity in APS-like autoimmunity.

Genetic analysis of three porcine bocaparvoviruses and identification of a natural recombinant breakpoint in NS1

In this study, we obtained the whole genomes of three porcine bocaparvovirus (PBoV) strains (GD6, GD10, and GD23) by polymerase chain reaction. Sequence analysis showed that all three field strains belonged to PBoV group 3 (G3). The phylogenetic trees based on NS1, NP1, and VP1 differed to the extent that these PBoVs were potentially more closely related to bocaparvoviruses known to infect other animals than to other PBoVs. GD6, GD10, and GD23 all included the conserved sequences YLGPF and HDXXY, with known phospholipase A2 activity. Using recombination-detection software we identified a natural recombinant breakpoint in the NS1 region of PBoV G3. The results of this study will further the epidemiological characterization of PBoVs.

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.

The VP1u Receptor Restricts Parvovirus B19 Uptake to Permissive Erythroid Cells

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

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

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

The Receptor-Binding Domain in the VP1u Region of Parvovirus B19

Parvovirus B19 (B19V) is known as the human pathogen causing the mild childhood disease erythema infectiosum. B19V shows an extraordinary narrow tissue tropism for erythroid progenitor cells in the bone marrow, which is determined by a highly restricted uptake. We have previously shown that the specific internalization is mediated by the interaction of the viral protein 1 unique region (VP1u) with a yet unknown cellular receptor. To locate the receptor-binding domain (RBD) within the VP1u, we analyzed the effect of truncations and mutations on the internalization capacity of the recombinant protein into UT7/Epo cells. Here we report that the N-terminal amino acids 5-80 of the VP1u are necessary and sufficient for cellular binding and internalization; thus, this N-terminal region represents the RBD required for B19V uptake. Using site-directed mutagenesis, we further identified a cluster of important amino acids playing a critical role in VP1u internalization. In silico predictions and experimental results suggest that the RBD is structured as a rigid fold of three α-helices. Finally, we found that dimerization of the VP1u leads to a considerably enhanced cellular binding and internalization. Taken together, we identified the RBD that mediates B19V uptake and mapped functional and structural motifs within this sequence. The findings reveal insights into the uptake process of B19V, which contribute to understand the pathogenesis of the infection and the neutralization of the virus by the immune system.

Human parvovirus B19 VP1u Protein as inflammatory mediators induces liver injury in naïve mice

Human parvovirus B19 (B19V) is a human pathogen known to be associated with many non-erythroid diseases, including hepatitis. Although B19V VP1-unique region (B19-VP1u) has crucial roles in the pathogenesis of B19V infection, the influence of B19-VP1u proteins on hepatic injury is still obscure. This study investigated the effect and possible inflammatory signaling of B19-VP1u in livers from BALB/c mice that were subcutaneously inoculated with VP1u-expressing COS-7 cells. The in vivo effects of B19-VP1u were analyzed by using live animal imaging system (IVIS), Haematoxylin-Eosin staining, gel zymography, and immunoblotting after inoculation. Markedly hepatocyte disarray and lymphocyte infiltration, enhanced matrix metalloproteinase (MMP)-9 activity and increased phosphorylation of p38, ERK, IKK-α, IκB and NF-κB (p-p65) proteins were observed in livers from BALB/c mice receiving COS-7 cells expressing B19-VP1u as well as the significantly increased CRP, IL-1β and IL-6. Notably, IFN-γ and phosphorylated STAT1, but not STAT3, were also significantly increased in the livers of BALB/c mice that were subcutaneously inoculated with VP1u-expressing COS-7 cells. These findings revealed the effects of B19-VP1u on liver injury and suggested that B19-VP1u may have a role as mediators of inflammation in B19V infection.

Blood Groups in Infection and Host Susceptibility

Blood group antigens represent polymorphic traits inherited among individuals and populations. At present, there are 34 recognized human blood groups and hundreds of individual blood group antigens and alleles. Differences in blood group antigen expression can increase or decrease host susceptibility to many infections. Blood groups can play a direct role in infection by serving as receptors and/or coreceptors for microorganisms, parasites, and viruses. In addition, many blood group antigens facilitate intracellular uptake, signal transduction, or adhesion through the organization of membrane microdomains. Several blood groups can modify the innate immune response to infection. Several distinct phenotypes associated with increased host resistance to malaria are overrepresented in populations living in areas where malaria is endemic, as a result of evolutionary pressures. Microorganisms can also stimulate antibodies against blood group antigens, including ABO, T, and Kell. Finally, there is a symbiotic relationship between blood group expression and maturation of the gastrointestinal microbiome.

Th17-related cytokines in systemic lupus erythematosus patients with dilated cardiomyopathies: a possible linkage to parvovirus B19 infection

Dilated cardiomyopathies (DCM) are a major cause of mortality in patients with systemic lupus erythematosus (SLE). Immune responses induced by human parvovirus B19 (B19) are considered an important pathogenic mechanism in myocarditis or DCM. However, little is known about Th17-related cytokines in SLE patients with DCM about the linkage with B19 infection. IgM and IgG against B19 viral protein, and serum levels of Th17-related cytokines were determined using ELISA in eight SLE patients with DCM and six patients with valvular heart disease (VHD). Humoral responses of anti-B19-VP1u and anti-B19-NS1 antibody were assessed using Western blot and B19 DNA was detected by nested Polymerase Chain Reaction (PCR). Levels of interleukin (IL)-17, IL-6, IL-1β, and tumor necrosis factor (TNF)-α were significantly higher in SLE patients with DCM (mean ± SEM, 390.99±125.48 pg/ml, 370.24±114.09 pg/ml, 36.01±16.90 pg/ml, and 183.84±82.94 pg/ml, respectively) compared to healthy controls (51.32±3.04 pg/ml, p<0.001; 36.88±6.64 pg/ml, p<0.001; 5.39±0.62 pg/ml, p<0.005; and 82.13±2.42 pg/ml, p<0.005, respectively). Levels of IL-17 and IL-6 were higher in SLE patients with DCM versus those with VHD (both p<0.01). Five (62.5%) of DCM patients had detectable anti-B19-NS1 IgG and four (50.0%) of them had anti-B19-VP1u IgG, whereas only one (16.7%) of VHD patients had detectable anti-B19-NS1 IgG and anti-B19-VP1u IgG. Serum levels of IL-17, IL-6 and IL-1β were markedly higher in SLE patients with anti-B19-VP1u IgG and anti-B19-NS1 IgG compared to those without anti-B19-VP1u IgG or anti-B19-NS1 IgG, respectively. These suggest a potential association of B19 with DCM and Th17-related cytokines implicated in the pathogenesis of DCM in SLE patients.

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.

Diagnosis of in utero Parvovirus B19 infection and maternal immune response - the relevance of linear epitopes and advanced serologic testing

IMPORTANCE OF THE FIELD

Parvovirus B19 (B19V) infection in utero causing fetal anemia and non-immune hydrops fetalis (NIHF) is a potentially life-threatening event for the fetus. Postexpositional non-invasive diagnosis is based on maternal IgG/IgM response and detection of viral genome in maternal blood. Serologic testing directs prenatal follow-up. Fetal infection is confirmed by polymerase chain reaction or in situ hybridization in fetal blood and/or amniotic fluid cells. The performance of serologic tests is significant in order to direct pre- and perinatal care at rational use of resources. Timing of diagnostic procedures and knowledge of the time course of infection in pregnant, asymptomatic women are critical. IgM negative testing in the presence of prolonged viremia may complicate individual risk analysis in pregnancy. Recently, advanced IgG avidity assays and epitope-type specific assays (IgG ETS EIA) have been re-evaluated.

AREAS COVERED IN THIS REVIEW

Epidemiology, clinical relevance and management of B19V infection in pregnancy. A review of the current literature (November 1984 - May 2009) and evaluation of current information on performance and predictive value of molecular and VP1/VP2 antigen-based IgG tests directed at the diagnosis of materno-fetal B19V infection and detection of past immunity. New aspects of B19V-associated fetal disease other than anemia/NIHF are also covered.

WHAT THE READER WILL GAIN

An overview of immunology and clinical relevance of B19V infection in pregnancy, of the potential value of advanced serologic testing and fields of future research.

TAKE HOME MESSAGE

In the absence of a commercially available vaccine, serologic tests remain important tools in individual risk analysis of pregnant women exposed to B19V. Sequential application of IgG avidity and IgG ETS EIAs may improve risk stratification and timing of invasive testing in B19V-exposed pregnancies, in particular with IgM-negativity and/or persistent DNAemia. Prospective evaluation of these test systems correlated to fetal outcome in order to reduce fetal morbidity and mortality as well as the overall burden of disease of B19V with regard to fetal malformation may be subject to future research.

Molecular characterization of the newly identified human parvovirus 4 in the family Parvoviridae

Human parvovirus 4 (PARV4) is an emerging human virus, and little is known about the molecular aspects of PARV4 apart from its incomplete genome sequence, which lacks information of the termini. We analyzed the gene expression profile of PARV4 using a nearly full-length HPV4 genome in a replication competent system in 293 cells. We found that PARV4 utilizes two promoters to transcribe non-structural protein- and structural protein-encoding mRNAs, respectively, which were polyadenylated at the right end of the genome. Three major proteins, including the large non-structural protein NS1a, whose mRNA is spliced, and capsid proteins VP1 and VP2, were detected. Additional functional analysis of the NS1a revealed its capability to induce cell cycle arrest at G2/M phase in ex vivo-generated human hematopoietic stem cells. Taken together, our characterization of the molecular features of PARV4 suggests that PARV4 represents a new genus in the family Parvoviridae.

The globoside receptor triggers structural changes in the B19 virus capsid that facilitate virus internalization

Globoside (Gb4Cer), Ku80 autoantigen, and α5β1 integrin have been identified as cell receptors/coreceptors for human parvovirus B19 (B19V), but their role and mechanism of interaction with the virus are largely unknown. In UT7/Epo cells, expression of Gb4Cer and CD49e (integrin alpha-5) was high, but expression of Ku80 was insignificant. B19V colocalized with Gb4Cer and, to a lesser extent, with CD49e. However, only anti-Gb4Cer antibodies could disturb virus attachment. Only a small proportion of cell-bound viruses were internalized, while the majority became detached from the receptor. When added to uninfected cells, the receptor-detached virus showed superior cell binding capacity and infectivity. Attachment of B19V to cells triggered conformational changes in the capsid leading to the accessibility of the N terminus of VP1 (VP1u) to antibodies, which was maintained in the receptor-detached virus. VP1u became similarly accessible to antibodies following incubation of B19V particles with increasing concentrations of purified Gb4Cer. The receptor-mediated exposure of VP1u is critical for virus internalization, since capsids lacking VP1 could bind to cells but were not internalized. Moreover, an antibody against the N terminus of VP1u disturbed virus internalization, but only when present during and not after virus attachment, indicating the involvement of this region in binding events required for internalization. These results suggest that Gb4Cer is not only the primary receptor for B19V attachment but also the mediator of capsid rearrangements required for subsequent interactions leading to virus internalization. The capacity of the virus to detach and reattach again would enhance the probability of productive infections.

Human bocavirus capsid structure: insights into the structural repertoire of the parvoviridae

Human bocavirus (HBoV) was recently discovered and classified in the Bocavirus genus (family Parvoviridae, subfamily Parvovirinae) on the basis of genomic similarity to bovine parvovirus and canine minute virus. HBoV has been implicated in respiratory tract infections and gastroenteric disease in children worldwide, yet despite numerous epidemiological reports, there has been limited biochemical and molecular characterization of the virus. Reported here is the three-dimensional structure of recombinant HBoV capsids, assembled from viral protein 2 (VP2), at 7.9-A resolution as determined by cryo-electron microscopy and image reconstruction. A pseudo-atomic model of HBoV VP2 was derived from sequence alignment analysis and knowledge of the crystal structure of human parvovirus B19 (genus Erythrovirus). Comparison of the HBoV capsid structure to that of parvoviruses from five separate genera demonstrates strong conservation of a beta-barrel core domain and an alpha-helix, from which emanate several loops of various lengths and conformations, yielding a unique surface topology that differs from the three already described for this family. The highly conserved core is consistent with observations for other single-stranded DNA viruses, and variable surface loops have been shown to confer the host-specific tropism and the diverse antigenic properties of this family.

Antibody-mediated opsonization of red blood cells in parvovirus B19 infection

Red blood cells (RBCs) express abundantly parvovirus B19 receptor, and their role in the dissemination or clearance of B19 infection is unknown. In this study, we report that in early, acute or persistent infection, B19 viremia is mostly associated with RBCs. The capacity of different patients' plasma or IgG to opsonize RBCs collected from patients with early B19 infection, was investigated. The highest opsonization activity was observed with plasma or IgG fractions from patients with past B19 infection. In contrast, IgG samples from patients with acute or persistent infection showed no or little opsonization activity. The depletion of antibodies specific to B19 VP1, but not VP2, from IgG samples, resulted in a significant suppression of opsonization. Furthermore, IgG samples preincubated with heated B19 particles exposing VP1-unique (VP1u) region were unable to opsonize RBCs. These observations clearly suggest a role for anti-VP1u IgG in the opsonization of RBC-bound B19 particles.

Effects of human parvovirus B19 VP1 unique region protein on macrophage responses

Background

Activity of secreted phospholipase A (sPLA2) has been implicated in a wide range of cellular responses. However, little is known about the function of human parvovirus B19-VP1 unique region (VP1u) with sPLA2 activity on macrophage.

Methods

To investigate the roles of B19-VP1u in response to macrophage, phospholipase A2 activity, cell migration assay, phagocytosis activity, metalloproteinase assay, RT-PCR and immunoblotting were performed.

Results

In the present study, we report that migration, phagocytosis, IL-6, IL-1β mRNA, and MMP9 activity are significantly increased in RAW264.7 cells by B19-VP1u protein with sPLA2 activity, but not by B19-VP1uD175A protein that is mutated and lacks sPLA2 activity. Additionally, significant increases of phosphorylated ERK1/2 and JNK proteins were detected in macrophages that were treated with B19-VP1u protein, but not when they were treated with B19-VP1uD175A protein.

Conclusion

Taken together, our experimental results suggest that B19-VP1u with sPLA2 activity affects production of IL-6, IL-1β mRNA, and MMP9 activity, possibly through the involvement of ERK1/2 and JNK signaling pathways. These findings could provide clues in understanding the role of B19-VP1u and its sPLA2 enzymatic activity in B19 infection and B19-related diseases.

Interaction of parvovirus B19 with human erythrocytes alters virus structure and cell membrane integrity

The unique region of the capsid protein VP1 (VP1u) of B19 virus (B19V) elicits a dominant immune response and has a phospholipase A(2) (PLA(2)) activity required for the infection. Despite these properties, we have observed that the VP1u-PLA(2) motif occupies an internal position in the capsid. However, brief exposure to increasing temperatures induced a progressive accessibility of the PLA(2) motif as well as a proportional increase of the PLA(2) activity. Similarly, upon binding on human red blood cells (RBCs), a proportion of the capsids externalized the VP1u-PLA(2) motif. Incubation of B19V with RBCs from 17 healthy donors resulted in extensive virus attachment ranging between 3,000 and 30,000 virions per cell. B19V empty capsids represent an important fraction of the viral particles circulating in the blood (30 to 40%) and bind to RBCs in the same way as full capsids. The extensive B19V binding to RBCs did not cause direct hemolysis but an increased osmotic fragility of the cells by a mechanism involving the PLA(2) activity of the exposed VP1u. Analysis of a blood sample from an individual with a recent B19V infection revealed that, at this particular moment of the infection, the virions circulating in the blood were mostly associated to the RBC fraction. However, the RBC-bound B19V was not able to infect susceptible cells. These observations indicate that RBCs play a significant role during B19V infection by triggering the exposure of the immunodominant VP1u including its PLA(2) constituent. On the other hand, the early exposure of VP1u might facilitate viral internalization and/or uncoating in target cells.

Visualization of the externalized VP2 N termini of infectious human parvovirus B19

The structures of infectious human parvovirus B19 and empty wild-type particles were determined by cryoelectron microscopy (cryoEM) to 7.5-A and 11.3-A resolution, respectively, assuming icosahedral symmetry. Both of these, DNA filled and empty, wild-type particles contain a few copies of the minor capsid protein VP1. Comparison of wild-type B19 with the crystal structure and cryoEM reconstruction of recombinant B19 particles consisting of only the major capsid protein VP2 showed structural differences in the vicinity of the icosahedral fivefold axes. Although the unique N-terminal region of VP1 could not be visualized in the icosahedrally averaged maps, the N terminus of VP2 was shown to be exposed on the viral surface adjacent to the fivefold beta-cylinder. The conserved glycine-rich region is positioned between two neighboring, fivefold-symmetrically related VP subunits and not in the fivefold channel as observed for other parvoviruses.

Serodiagnosis of human bocavirus infection

Background. A new human-pathogenic parvovirus, human bocavirus (HBoV), has recently been discovered and associated with respiratory disease in small children. However, many patients have presented with low viral DNA loads, suggesting HBoV persistence and rendering polymerase chain reaction-based diagnosis problematic. Moreover, nothing is known of HBoV immunity. We examined HBoV-specific systemic B cell responses and assessed their diagnostic use in young children with respiratory disease.

Patients and methods. Paired serum samples from 117 children with acute wheezing, previously studied for 16 respiratory viruses, were tested by immunoblot assays using 2 recombinant HBoV capsid antigens: the unique part of virus protein 1 and virus protein 2.

Results. Virus protein 2 was superior to the unique part of virus protein 1 with respect to immunoreactivity. According to the virus protein 2 assay, 24 (49%) of 49 children who were positive for HBoV according to polymerase chain reaction had immunoglobulin (Ig) M antibodies, 36 (73%) had IgG antibodies, and 29 (59%) exhibited IgM antibodies and/or an increase in IgG antibody level. Of 22 patients with an increase in antibody levels, 20 (91%) had a high load of HBoV DNA in the nasopharynx, supporting the hypothesis that a high HBoV DNA load indicates acute primary infection, whereas a low load seems to be of less clinical significance. In a subgroup of patients who were previously determined to have acute HBoV infection (defined as a high virus load in the nasopharynx, viremia, and absence of other viral infections), 9 (100%) of 9 patients had serological evidence of primary infection. In the control group of 68 children with wheezing who had polymerase chain reaction results negative for HBoV in the nasopharynx, 9 (13%) had IgM antibodies, including 5 who displayed an increase in IgG antibody levels and were viremic. No cross-reactivity with human parvovirus B19 was detected.

Conclusions. Respiratory infections due to HBoV are systemic, elicit B cell immune responses, and can be diagnosed serologically. Serological diagnoses correlate with high virus loads in the nasopharynx and with viremia. Serological testing is an accurate tool for disclosing the association of HBoV infection with disease.

Induction of antiphospholipid antibodies and antiphospholipid syndrome-like autoimmunity in naive mice with antibody against human parvovirus B19 VP1 unique region protein

BACKGROUND

Previous studies have postulated a connection between human parvovirus B19 (B19) infection and anti-phospholipid antibodies (APhL). B19 infection and anti-phospholipid syndrome (APS) exhibit congruent symptoms. Recently, phospholipase A2 (PLA2)-like activity has been linked to the VP1 unique region (VP1u) of B19. However, the precise role of B19-VP1u in pathogenesis of autoimmunity is still obscure.

METHODS

To elucidate the roles of VP1u in B19 infection and autoimmunity, the reactivity of B19-VP1u proteins with various autoantibodies were evaluated by ELISA and immunoblotting. Rabbits were immunized with purified recombinant B19-VP1u protein to generate anti-sera. Absorption experiments were conducted to determine the binding specificity of rabbit anti-sera against B19-VP1u, cardiolipin (CL) and beta-2-glycoprotein I (beta2GPI). Moreover, the effects of passive transfer of polyclonal rabbit anti-B19-VP1u IgG antibodies on platelets, activated partial thromboplastin time (aPTT), and autoantibodies were assessed.

RESULTS

Autoantibodies against CL, beta2GPI, and phospholipid (PhL) in sera from patients with B19 infection, were cross-reactive with B19-VP1u. Consistently, sera from rabbits immunized with recombinant B19-VP1u protein displayed raised detectable immunoglobulins against B19-VP1u, CL, beta2GPI and PhL. Additionally, the mice immunized with anti-B19-VP1u IgG developed thrombocytopenia, prolongation of aPTT, and autoantibody against beta2GPI and PhL.

CONCLUSIONS

These experimental results suggested the association between B19-VP1u and production of anti-beta2GPI antibodies, APhL, and APS-like autoimmunity. Altogether, it may provide a clue in understanding the role of B19-VP1u in inducing autoantibodies and B19-associated APS manifestations.

The association of VP1 unique region protein in acute parvovirus B19 infection and anti-phospholipid antibody production

BACKGROUND

Previous studies have postulated a connection between human parvovirus B19 (B19) infection and anti-phospholipid antibodies (aPL). Recently, the phospholipase domain of B19 has been linked to B19-VP1 unique region (VP1u). To elucidate the roles of VP1u in B19 infection and aPL production, the major reactivity of anti-B19-VP1u, anti-cardiolipin antibody (aCL), and anti-beta2-glycoprotein I (beta2GPI) antibody was evaluated.

METHODS

Sera from 102 clinically suspected cases of B19 infection were analyzed by nested PCR and ELISA. Humoral responses of anti-B19-VP1u and anti-B19-VP1uD175A IgM/IgG antibodies, aCL and the anti-beta2GPI antibody were assessed by Western blot and ELISA. Absorption experiments were also performed to determine the binding specificity of immunoglobulins to B19-VP1u, CL and beta2GPI.

RESULTS

Sera from patients with the diagnostic pattern DNA+/IgM+/IgG+ had a high frequency (57%) for recognition of CL and beta2GPI. Furthermore, adsorption experiments were performed by adding purified B19-VP1u, which partially suppressed the reactivity of anti-B19VP1u to CL and beta2GPI.

CONCLUSIONS

Serum from patients with acute B19 infection has a high frequency in recognition of CL and beta2GPI, and the phospholipase domain observed in the B19-VP1u may have contributed to the production of aPL. These findings may provide a clue for understanding the roles of B19-VP1u in B19 infection and aPL production.

Conformational changes in the VP1-unique region of native human parvovirus B19 lead to exposure of internal sequences that play a role in virus neutralization and infectivity

The unique region of the capsid protein VP1 (VP1u) of human parvovirus B19 (B19) elicits a dominant immune response and has a phospholipase A(2) (PLA(2)) activity, which is necessary for the infection. In contrast to the rest of the parvoviruses, the VP1u of B19 is thought to occupy an external position in the virion, making this region a promising candidate for vaccine development. By using a monoclonal antibody against the most-N-terminal portion of VP1u, we revealed that this region rich in neutralizing epitopes is not accessible in native capsids. However, exposure of capsids to increasing temperatures or low pH led to its progressive accessibility without particle disassembly. Although unable to bind free virus or to block virus attachment to the cell, the anti-VP1u antibody was neutralizing, suggesting that the exposure of the epitope and the subsequent virus neutralization occur only after receptor attachment. The measurement of the VP1u-associated PLA(2) activity of B19 capsids revealed that this region is also internal but becomes exposed in heat- and in low-pH-treated particles. In sharp contrast to native virions, the VP1u of baculovirus-derived B19 capsids was readily accessible in the absence of any treatment. These results indicate that stretches of VP1u of native B19 capsids harboring neutralizing epitopes and essential functional motifs are not external to the capsid. However, a conformational change renders these regions accessible and triggers the PLA(2) potential of the virus. The results also emphasize major differences in the VP1u conformation between natural and recombinant particles.

Human parvovirus B19 infection in pregnancy: should screening be offered to the low-risk population?

Human parvovirus B19 infection during pregnancy can result in fetal hydrops and death. Intra-uterine transfusion in hydropic fetuses affected by the virus can reduce perinatal deaths. Up to 33% of women with this infection are asymptomatic. In view of the significant adverse outcomes, the role of routine screening among low-risk pregnant women is discussed.

Identification and characterization of a second novel human erythrovirus variant, A6

Parvovirus B19 (B19), currently the only accepted member of the Erythrovirus genus, is the only parvovirus known to be pathogenic in humans. Recently a viral sequence, tentatively termed V9 which showed 11% variability from the published B19 sequences, was described from a patient with aplastic crisis. To search for additional parvovirus variants, we used the new NS1/7.5EC PCR assay whose primers were designed from a conserved region of the B19/V9 sequence and encompasses an MfeI restriction enzyme site that would allow differentiation between B19- and V9-like sequences. Screening of 225 serum and bone marrow samples and 62 plasma pools identified one new atypical parvovirus sequence, A6, from an anemic HIV-positive patient. A6 exhibited 88% similarity to B19 and 92% to V9, compared to >98% correspondence between reported B19 isolates. Based on the genome similarity to B19, an RT-PCR for A6 capsid transcripts was developed and used to test for A6 infectivity of UT7/Epo/S1 cells. Despite high viral titers, A6 viral transcripts were not detected. Thus, although the prevalence of B19 variants probably is low, the true clinical significance remains unknown. Current PCR analyses are unlikely to detect novel variants without the design of specific primers to the A6/V9/B19 common sequences.

Peptide dendrimers: applications and synthesis

Peptide dendrimers are radial or wedge-like branched macromolecules consisting of a peptidyl branching core and/or covalently attached surface functional units. The multimeric nature of these constructs, the unambiguous composition and ease of production make this type of dendrimer well suited to various biotechnological and biochemical applications. Applications include use as biomedical diagnostic reagents, protein mimetics, anticancer and antiviral agents, vaccines and drug and gene delivery vehicles. This review focuses on the different types of peptide dendrimers currently in use and the synthetic methods commonly employed to generate peptide dendrimers ranging from stepwise solid-phase synthesis to chemoselective and orthogonal ligation.

The VP1 N-terminal sequence of canine parvovirus affects nuclear transport of capsids and efficient cell infection

The unique N-terminal region of the parvovirus VP1 capsid protein is required for infectivity by the capsids but is not required for capsid assembly. The VP1 N terminus contains a number of groups of basic amino acids which resemble classical nuclear localization sequences, including a conserved sequence near the N terminus comprised of four basic amino acids, which in a peptide can act to transport other proteins into the cell nucleus. Testing with a monoclonal antibody recognizing residues 2 to 13 of VP1 (anti-VP1-2-13) and with a rabbit polyclonal serum against the entire VP1 unique region showed that the VP1 unique region was not exposed on purified capsids but that it became exposed after treatment of the capsids with heat (55 to 75 degrees C), or urea (3 to 5 M). A high concentration of anti-VP1-2-13 neutralized canine parvovirus (CPV) when it was incubated with the virus prior to inoculation of cells. Both antibodies blocked infection when injected into cells prior to virus inoculation, but neither prevented infection by coinjected infectious plasmid DNA. The VP1 unique region could be detected 4 and 8 h after the virus capsids were injected into cells, and that sequence exposure appeared to be correlated with nuclear transport of the capsids. To examine the role of the VP1 N terminus in infection, we altered that sequence in CPV, and some of those changes made the capsids inefficient at cell infection.

Identification of an immunodominant peptide in the parvovirus B19 VP1 unique region able to elicit a long-lasting immune response in humans

The immune response against parvovirus B19 is mainly directed against the two structural proteins, VP1 and VP2. The amino terminal half of the VP1 unique region has been shown to elicit a dominant immune response in humans, more effective than other linear epitopes and also it has been seen to contain significant neutralizing linear epitopes. Three overlapping recombinant peptides corresponding to amino acids 2-40 (VP1-A), amino acids 32-71 (VP1-B), and amino acids 60-100 (VP1-C) of the VP1 unique region were produced by a procaryotic expression system. These peptides were used as antigens in a Western blot assay to detect specific immunoglobulin G (IgG) in serum samples from blood donors of different age groups with documented signs of a past B19 infection. Fragment VP1-C appeared significantly immunodominant over the other peptides, reacting with specific IgG in 86% of serum samples. The fragment VP1-C corresponds to a sequence with a known neutralizing activity and seems able to elicit a long-lasting immune response because specific IgG were present in blood donors of all age groups. VP1-C would therefore appear to be an attractive candidate as a component of a subunit vaccine.

Differential IgM response to conformational and linear epitopes of parvovirus B19 VP1 and VP2 structural proteins

The IgM immune response against conformational and linear epitopes of B19 structural proteins VP1 and VP2 was examined in serum samples with a suspect B19 infection to determine the most suitable antigen for use in IgM detection and also to evaluate a possible relationship between the course of B19 infection and the presence of epitope type-specific IgM. The detection of IgM against conformational epitopes was performed by ELISA using undenatured VP1 and VP2 antigens whereas the detection of IgM against linear epitopes was performed by Western blot assays using denatured VP1 and VP2. IgM immune response against VP1 conformational epitopes appeared dominant, being detected in all serum samples positive for specific IgM, whereas IgM against VP2 linear antigen were found less frequently, being identified in less than half of the B19 IgM positive sera. In the examination of the course of infection, IgM against VP1 conformational epitopes appeared in the active phase of B19 infection at the same time and with the same frequency as IgM anti VP2 conformational epitopes and anti linear VP1 epitopes. IgM against VP1 conformational epitopes were seen to be long-lasting because in the recent phase of infection they were still present when other specific IgM were absent. During the active phase of B19 infection, IgM against VP2 linear epitopes were less frequently found than other specific IgM and in the recent phase they underwent a rapid temporal diminution. The data demonstrate that a sensitive B19 IgM test needs to be performed in diagnostic laboratories by ELISA using conformational B19 antigens; Western blot assays can be used only as confirmatory tests using VP1 linear antigens.

Localization of a new neutralizing epitope on the mumps virus hemagglutinin-neuraminidase protein

Four protein fragments which span the entire hemagglutinin-neuraminidase protein (HN) of mumps virus were expressed in HeLa cells and cell extracts were tested for their capability to induce neutralizing antibodies in mice. Fragment HN3 (aa 213-372) was able to induce the production of hemagglutination-inhibiting and neutralizing antibodies. When a subfragment of HN3, the synthetic peptide NSTLGVKSAREF (aa 329-340 of HN) was used for immunization, hemagglutination-inhibiting and neutralizing antibodies against mumps wild type virus but not against the Urabe Am9 vaccine virus were raised. The peptide could, therefore, contain a new epitope, which may be critical for protective host humoral immune response.