Self-assembled B19 parvovirus capsids, produced in a baculovirus system, are antigenically and immunogenically similar to native virions

Infectious parvovirus B19 circulates in the blood coated with active host protease inhibitors

The lack of a permissive cell culture system has limited high-resolution structures of parvovirus B19 (B19V) to virus-like particles (VLPs). In this study, we present the atomic resolution structure (2.2 Å) of authentic B19V purified from a patient blood sample. There are significant differences compared to non-infectious VLPs. Most strikingly, two host protease inhibitors (PIs), inter-alpha-trypsin inhibitor heavy chain 4 (ITIH4) and serpinA3, were identified in complex with the capsids in all patient samples tested. The ITIH4 binds specifically to the icosahedral fivefold axis and serpinA3 occupies the twofold axis. The protein-coated virions remain infectious, and the capsid-associated PIs retain activity; however, upon virion interaction with target cells, the PIs dissociate from the capsid prior to viral entry. Our finding of an infectious virion shielded by bound host serum proteins suggests an evolutionarily favored phenomenon to evade immune surveillance and escape host protease activity.

Viral protein-based nanoparticles (part 2): Pharmaceutical applications

Viral protein nanoparticles (ViP NPs) such as virus-like particles and virosomes are structures halfway between viruses and synthetic nanoparticles. The biological nature of ViP NPs endows them with the biocompatibility, biodegradability, and functional properties that many synthetic nanoparticles lack. At the same time, the absence of a viral genome avoids the safety concerns of viruses. Such characteristics of ViP NPs offer a myriad of opportunities for theirapplication at several points across disease development: from prophylaxis to diagnosis and treatment. ViP NPs present remarkable immunostimulant properties, and thus the vaccination field has benefited the most from these platforms capable of overcoming the limitations of both traditional and subunit vaccines. This was reflected in the marketing authorization of several VLP- and virosome-based vaccines. Besides, ViP NPs inherit the ability of viruses to deliver their cargo to target cells. Because of that, ViP NPs are promising candidates as vectors for drug and gene delivery, and for diagnostic applications. In this review, we analyze the pharmaceutical applications of ViP NPs, describing the products that are commercially available or under clinical evaluation, but also the advances that scientists are making toward the implementation of ViP NPs in other areas of major pharmaceutical interest.

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.

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.

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.

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.

Novel Surrogate Neutralizing Assay Supports Parvovirus B19 Vaccine Development for Children with Sickle Cell Disease

Children with sickle cell disease (SCD) suffer life-threatening transient aplastic crisis (TAC) when infected with parvovirus B19. In utero, infection of healthy fetuses may result in anemia, hydrops, and death. Unfortunately, although promising vaccine candidates exist, no product has yet been licensed. One barrier to vaccine development has been the lack of a cost-effective, standardized parvovirus B19 neutralization assay. To fill this void, we evaluated the unique region of VP1 (VP1u), which contains prominent targets of neutralizing antibodies. We discovered an antigenic cross-reactivity between VP1 and VP2 that, at first, thwarted the development of a surrogate neutralization assay. We overcame the cross-reactivity by designing a mutated VP1u (VP1uAT) fragment. A new VP1uAT ELISA yielded results well correlated with neutralization (Spearman’s correlation coefficient = 0.581; p = 0.001), superior to results from a standard clinical diagnostic ELISA or an ELISA with virus-like particles. Virus-specific antibodies from children with TAC, measured by the VP1uAT and neutralization assays, but not other assays, gradually increased from days 0 to 120 post-hospitalization. We propose that this novel and technically simple VP1uAT ELISA might now serve as a surrogate for the neutralization assay to support rapid development of a parvovirus B19 vaccine.

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.

Comparison of 4 commercial enzyme immunoassays for serology testing of human parvovirus B19 infection

BACKGROUND

Parvovirus B19 is a pathogenic virus often diagnosed by serology, yet little is known about analytical performance of commercial enzyme immunoassays (EIAs).

OBJECTIVE

To investigate performance of 4 EIAs for parvovirus B19 IgM and IgG: Liaison, Euroimmun, Mikrogen and Virion/Serion.

STUDY DESIGN

To compare 4 EIAs to Biotrin's ELISA on 168 samples and determine consensus score for discordant samples using Mikrogen's confirmatory line assay.

RESULTS

Two thirds of results for IgM/IgG were identical for all 4 EIAs and Biotrin. Liaison shows the highest IgM sensitivity, but has low specificity. Euroimmun lacks IgM sensitivity. Mikrogen had a good overall performance, but had the lowest IgG specificity. Virion/Serion had variable performance with a low IgM specificity and the most borderline and cross-reactive results.

CONCLUSIONS

Liaison and Mikrogen have similar performance to Biotrin's ELISA. Euroimmun lacks sensitivity and Virion/Serion produced many borderline and cross-reactive results.

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.

Bombyx mori Pupae Efficiently Produce Recombinant AAV2/HBoV1 Vectors with a Bombyx mori Nuclear Polyhedrosis Virus Expression System

Recombinant adeno-associated virus (AAV) vectors have broad application prospects in the field of gene therapy. The establishment of low-cost and large-scale manufacturing is now the general agenda for industry. The baculovirus-insect cell/larva expression system has great potential for these applications due to its scalability and predictable biosafety. To establish a more efficient production system, Bombyx mori pupae were used as a new platform and infected with recombinant Bombyx mori nuclear polyhedrosis virus (BmNPV). The production of a chimeric recombinant adeno-associated virus (rAAV) serotype 2/human bocavirus type-1 (HBoV1) vector was used to evaluate the efficiency of this new baculovirus expression vector (BEV)–insect expression system. For this purpose, we constructed two recombinant BmNPVs, which were named rBmNPV/AAV2Rep-HBoV1Cap and rBmNPV/AAV2ITR-eGFP. The yields of rAAV2/HBoV1 derived from the rBmNPV/AAV2Rep-HBoV1Cap and rBmNPV/AAV2ITR-eGFP co-infected BmN cells exceeded 2 × 10⁴ vector genomes (VG) per cell. The rBmNPV/AAV2Rep-HBoV1Cap and rBmNPV/AAV2ITR-eGFP can express stably for at least five passages. Significantly, rAAV2/HBoV1 could be efficiently generated from BmNPV-infected silkworm larvae and pupae at average yields of 2.52 × 10¹² VG/larva and 4.6 × 10¹² VG/pupa, respectively. However, the vectors produced from the larvae and pupae had a high percentage of empty particles, which suggests that further optimization is required for this platform in the future. Our work shows that silkworm pupae, as an efficient bioreactor, have great potential for application in the production of gene therapy vectors.

Virus-Like Particle: Evolving Meanings in Different Disciplines

Virus-like particle (VLP) is a term that has been in use for about 80 years. Usually, VLP has meant a particle that is like a virus, generally by appearance, but without either proven or actual virus functionality. Initially VLP referred to particles seen in electron microscope images of tissues. More recently, VLP has come to mean other things to other researchers. A key divergence has been use of VLP in association with vaccine and biotechnology applications versus use of VLP in enumeration of viruses in environmental samples. To these viral ecologists, a VLP is a particle that is virus sized, has nucleic acid, and could be a functional virus. But to vaccine developers and biotechnology researchers a VLP instead is a viral structure that intentionally lacks a viral genome. In this study, we look at the history of use of VLP, following changes in meaning as the technology to study VLPs changed.

A CRISPR-Cas9 repressor for epigenetic silencing of KRAS

KRAS is one of the most frequently mutated oncogenes in cancers. Currently no direct and effective anti-KRAS therapies are available. Using the powerful CRISPR-Cas9 technology to target the mutant KRAS promoter, we designed an epigenetic repressor to silence KRAS through epigenome editing. Catalytically dead Cas9 (dCas9) functioned as a DNA binding device, which was fused with a transcriptional repressor histone deacetylase 1 (HDAC1). We designed a panel of three CRISPR RNAs (crRNAs) covering 1500-bp range of the KRAS promoter and identified that crRNA1 and crRNA2 efficiently silenced KRAS. The suppression of K-Ras resulted in a significant inhibition of cell growth, suppression of colony formation in soft agar and induction of cell death in cancer cells with KRAS mutations. In addition, the chromatin immunoprecipitation (ChIP) assay demonstrated dCas9-HDAC1 modified histone acetylation on the KRAS promoter. Furthermore, transfection of dCas9-HDAC1 protein and gRNA ribonucleoprotein complex also inhibited K-Ras and suppressed cell proliferation. In summary, we have developed a new strategy that combines CRISPR-Cas9 technology with HDAC1 epigenetic silencing to target cancers driven by KRAS mutations.

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.

Parvovirus Capsid-Antibody Complex Structures Reveal Conservation of Antigenic Epitopes Across the Family

The parvoviruses are small nonenveloped single stranded DNA viruses that constitute members that range from apathogenic to pathogenic in humans and animals. The infection with a parvovirus results in the generation of antibodies against the viral capsid by the host immune system to eliminate the virus and to prevent re-infection. For members currently either being developed as delivery vectors for gene therapy applications or as oncolytic biologics for tumor therapy, efforts are aimed at combating the detrimental effects of pre-existing or post-treatment antibodies that can eliminate therapeutic benefits. Therefore, understanding antigenic epitopes of parvoviruses can provide crucial information for the development of vaccination applications and engineering novel capsids able to escape antibody recognition. This review aims to capture the information for the binding regions of ∼30 capsid-antibody complex structures of different parvovirus capsids determined to date by cryo-electron microscopy and three-dimensional image reconstruction. The comparison of all complex structures revealed the conservation of antigenic regions among parvoviruses from different genera despite low sequence identity and indicates that the available data can be used across the family for vaccine development and capsid engineering.

Filling Adeno-Associated Virus Capsids: Estimating Success by Cryo-Electron Microscopy

Adeno-associated viruses (AAVs) have been employed successfully as gene therapy vectors in treating various genetic diseases for almost two decades. However, transgene packaging is usually imperfect, and developing a rapid and accurate method for measuring the proportion of DNA encapsidation is an important step for improving the downstream process of large scale vector production. In this study, we used two-dimensional class averages and three-dimensional classes, intermediate outputs in the single particle cryo-electron microscopy (cryo-EM) image reconstruction pipeline, to determine the proportion of DNA-packaged and empty capsid populations. Two different preparations of AAV3 were analyzed to estimate the minimum number of particles required to be sampled by cryo-EM in order for robust calculation of the proportion of the full versus empty capsids in any given sample. Cost analysis applied to the minimum amount of data required for a valid ratio suggests that cryo-EM is an effective approach to analyze vector preparations.

Emerging Human Parvoviruses: The Rocky Road to Fame

Parvoviruses are structurally simple viruses with linear single-stranded DNA genomes and nonenveloped icosahedral capsids. They infect a wide range of animals from insects to humans. Parvovirus B19 is a long-known human pathogen, whereas adeno-associated viruses are nonpathogenic. Since 2005, many parvoviruses have been discovered in human-derived samples: bocaviruses 1-4, parvovirus 4, bufavirus, tusavirus, and cutavirus. Some human parvoviruses have already been shown to cause disease during acute infection, some are associated with chronic diseases, and others still remain to be proven clinically relevant-or harmless commensals, a distinction not as apparent as it might seem. One initially human-labeled parvovirus might not even be a human virus, whereas another was originally overlooked due to inadequate diagnostics. The intention of this review is to follow the rocky road of emerging human parvoviruses from discovery of a DNA sequence to current and future clinical status, highlighting the perils along the way.

Evolutionary aspects of Parvovirus B-19V associated diseases and their pathogenesis patterns with an emphasis on vaccine development

Parvovirus B-19, a single human pathogenic member of the Parvoviridae family with it's small ssDNA and non-enveloped structure, is known to cause diseases in erythroid progenitor cells. But a wide range of clinical association of this virus with cells of non-erythroid origins has recently been discovered and many of those are being investigated for such association. Higher substitution rates in due course of evolution has been suggested for this cellular tropism and persistence. In this report, we have summarized the different disease manifestations of B-19 virus and have tried to find out a pattern of pathogenesis. Finally, we have focused on the vaccination strategies available against B-19 virus to correlate these with the mechanisms involved in various diseases caused by this virus.

Validation of monoplex assays detecting antibodies against Corynebacterium diphtheriae and Clostridium tetani toxins, rubella virus and parvovirus B19 for incorporation into Multiplex Serology

Serological assays detecting antibodies in serum or plasma samples are useful and versatile instruments to investigate an individual's infection and vaccination history, e.g. for clinical diagnosis, personal risk evaluation, and seroepidemiological studies. Multiplex Serology is a suspension bead array-based high-throughput methodology for simultaneous measurement of antibodies against multiple pathogens in a single reaction vessel, thus economizing sample volume, measurement time, and costs. We developed and validated bead-based pathogen-specific Monoplex Serology assays, i.e. assays including only antigens for the respective pathogen, to detect antibodies against Corynebacterium diphtheriae and Clostridium tetani toxins, rubella virus and parvovirus B19. The developed assays expand the portfolio of existing pathogen-specific bead-based serology assays and can be efficiently incorporated into larger Multiplex Serology panels. The newly developed Monoplex Serology assays consist of only one antigen per infectious agent, expressed as Glutathione S-transferase-fusion proteins in E. coli. Specificity, sensitivity and Cohen's kappa statistics in comparison with routine clinical diagnostic assays were calculated for serum dilutions 1:100 and 1:1000. All pathogen-specific assays were successfully validated at both serum dilutions with the exception of rubella Monoplex Serology which showed impaired sensitivity (57.6%) at dilution 1:1000. Specificities of successfully validated Monoplex Serology assays ranged from 85.6% to 100.0% (median: 91.7%), and sensitivities from 81.3% to 95.8% (median: 90.9%); agreement with the reference assays ranged from substantial to almost perfect (kappa: 0.66-0.86, median: 0.78). Statistical performance and slim assay design enable efficient incorporation of the developed assays into Multiplex Serology.

Construction of protein-functionalized virus-like particles of parvovirus B19

Decoration of virus-like particles (VLPs) expands the repertory of functions these particles can display. In the last years, VLPs have successfully been used as scaffolds to present different molecules, frequently through the specific reaction of chemical groups on the surface of the particles, or by protein engineering when the presentation of peptides or proteins is the primary goal. VLPs of parvovirus B19 (B19V), have been previously produced in vitro and its stability and ability to assemble into hybrid particles composed of wild-type and chimeric proteins evidenced their potential as research tools. Herein, we report the presentation of functional proteins on the surface of B19V VLPs, through the fusion of the gene coding for the heterologous protein within the gene coding for the structural protein VP2. Two model proteins were used for the construction of chimeras, a lipase from Bacillus pumilus (BplA) and the enhanced green fluorescent protein (EGFP). Both chimeras were folded and successfully assembled in vitro into VLPs. While the BplA chimera exhibited esterase activity, the chimera of EGFP showed no fluorescence. We replaced the EGFP by its fast-folding derivative "super folder GFP" (sfGFP) flanked by larger linkers to increase its movement freedom, which resulted in fluorescent protein able to assemble fluorescent VLPs. These results expand the toolbox for VLP decoration as well as for the construction of new nanobiomaterials.

Expression and characterization of codon-optimized Crimean-Congo hemorrhagic fever virus Gn glycoprotein in insect cells

Crimean-Congo hemorrhagic fever virus (CCHFV) is a major cause of tick-borne viral hemorrhagic disease in the world. Despite of its importance as a deadly pathogen, there is currently no licensed vaccine against CCHF disease. The attachment glycoprotein of CCHFV (Gn) is a potentially important target for protective antiviral immune responses. To characterize the expression of recombinant CCHFV Gn in an insect-cell-based system, we developed a gene expression system expressing the full-length coding sequence under a polyhedron promoter in Sf9 cells using recombinant baculovirus. Recombinant Gn was purified by affinity chromatography, and the immunoreactivity of the protein was evaluated using sera from patients with confirmed CCHF infection. Codon-optimized Gn was successfully expressed, and the product had the expected molecular weight for CCHFV Gn glycoprotein of 37 kDa. In time course studies, the optimum expression of Gn occurred between 36 and 48 hours postinfection. The immunoreactivity of the recombinant protein in Western blot assay against human sera was positive and was similar to the results obtained with the anti-V5 tag antibody. Additionally, mice were subjected to subcutaneous injection with recombinant Gn, and the cellular and humoral immune response was monitored. The results showed that recombinant Gn protein was highly immunogenic and could elicit high titers of antigen-specific antibodies. Induction of the inflammatory cytokine interferon-gamma and the regulatory cytokine IL-10 was also detected. In conclusion, a recombinant baculovirus harboring CCHFV Gn was constructed and expressed in Sf9 host cells for the first time, and it was demonstrated that this approach is a suitable expression system for producing immunogenic CCHFV Gn protein without any biosafety concerns.

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.

Epidemiology of two human protoparvoviruses, bufavirus and tusavirus

Two human parvoviruses were recently discovered by metagenomics in Africa, bufavirus (BuV) in 2012 and tusavirus (TuV) in 2014. These viruses have been studied exclusively by PCR in stool and detected only in patients with diarrhoea, although at low prevalence. Three genotypes of BuV have been identified. We detected, by in-house EIA, BuV1-3 IgG antibodies in 7/228 children (3.1%) and 10/180 adults (5.6%), whereas TuV IgG was found in one child (0.4%). All children and 91% of the adults were Finnish, yet interestingly 3/6 adults of Indian origin were BuV-IgG positive. By competition EIA, no cross-reactivity between the BuVs was detected, indicating that the BuV genotypes represent distinct serotypes. Furthermore, we analysed by BuV qPCR stool and nasal swab samples from 955 children with gastroenteritis, respiratory illness, or both, and found BuV DNA in three stools (0.3%) and for the first time in a nasal swab (0.1%). This is the first study documenting the presence of BuV and TuV antibodies in humans. Although the seroprevalences of both viruses were low in Finland, our results indicate that BuV infections might be widespread in Asia. The BuV-specific humoral immune responses appeared to be strong and long-lasting, pointing to systemic infection in humans.

Peptide presentation on primate erythroparvovirus 1 virus-like particles: In vitro assembly, stability and immunological properties

Virus-like particles (VLPs) have demonstrated to be valuable scaffolds for the display of heterologous peptides for vaccine development and other specific interactions. VLPs of primate erythroparvovirus 1, generally referred as parvovirus B19 (B19V), have already been produced in-vivo and in-vitro from the recombinant VP2 protein of this virus. In this study, chimeric forms of B19V VP2 were constructed, and their ability to assemble into VLPs was evaluated. Chimeras were composed of the VP2 protein fused, at its N-terminus, with two peptides derived from the fusion glycoprotein (F) of the respiratory syncytial virus (RSV). The chimeric proteins self-assembled into VLPs morphologically similar to B19V virions. Stability of these VLPs was analyzed under denaturation conditions with guanidinium chloride (GdnHCl). Our results indicate that the presence of the heterologous fragments increased the stability of VLPs assembled by any of the VP2 chimeras. Specific proteolysis assays shown that a fraction of the N-termini of the chimeric proteins is located on the outer surface of the VLPs. Immunogenicity of VLPs against RSV was evaluated and the results indicate that the particles can elicit a humoral immune response, although these antibodies did not cross-react with RSV in ELISA tests. These results provide novel insights into the localization of the N-termini of B19V VP2 protein after in vitro assembly into VLPs, and point them to be attractive sites to display peptides or proteins without compromise the assembly or stability of VLPs.

Enhanced assembly and colloidal stabilization of primate erythroparvovirus 1 virus-like particles for improved surface engineering

Virus-like particles (VLPs) are the product of the self-assembly, either in vivo or in vitro, of structural components of viral capsids. These particles are excellent scaffolds for surface display of biomolecules that can be used in vaccine development and tissue-specific drug delivery. Surface engineering of VLPs requires structural stability and chemical reactivity. Herein, we report the enhanced assembly, colloidal stabilization and fluorescent labeling of primate erythroparvovirus 1 (PE1V), generally referred to as parvovirus B19. In vitro assembly of the VP2 protein of PE1V produces VLPs, which are prone to flocculate and hence undergo limited chemical modification by thiol-specific reagents like the fluorogenic monobromobimane (mBBr). We determined that the addition of 0.2M l-arginine during the assembly process produced an increased yield of soluble VLPs with good dispersion stability. Fluorescent labeling of VLPs suspended in phosphate buffered saline (PBS) added with 0.2M l-Arg was achieved in significantly shorter times than the flocculated VLPs assembled in only PBS buffer. Finally, to demonstrate the potential application of this approach, mBBr-labeled VLPs were successfully used to tag human hepatoma HepG2 cells. This new method for assembly and labeling PE1V VLPs eases its applications and provides insights on the manipulation of this biomaterial for further developments.

STATEMENT OF SIGNIFICANCE

Application of virus-derived biomaterials sometimes requires surface modification for diverse purposes, including enhanced cell-specific interaction, the inclusion of luminescent probes for bioimaging, or the incorporation of catalytic properties for the production of enzyme nanocarriers. In this research, we reported for the first time the colloidal stabilization of the primate erythroparvovirus 1 (PE1V) virus-like particles (VLPs). Also, we report the chemical modification of the natural Cys residues located on the surface of these VLPs with a fluorescent probe, as well as its application for tagging hepatoma cells in vitro. Keeping in mind that PE1V is a human pathogen, virus-host interactions already exist in human cells, and they can be exploited for therapeutic and research aims. This study will impact on the speed in which the scientific community will be able to manipulate PE1V VLPs for diverse purposes. Additionally, this study may provide insights on the colloidal properties of these VLPs as well as in the effect of different protein additives used for protein stabilization.

Self-assembling protein nanoparticles in the design of vaccines

For over 100 years, vaccines have been one of the most effective medical interventions for reducing infectious disease, and are estimated to save millions of lives globally each year. Nevertheless, many diseases are not yet preventable by vaccination. This large unmet medical need demands further research and the development of novel vaccines with high efficacy and safety. Compared to the 19th and early 20th century vaccines that were made of killed, inactivated, or live-attenuated pathogens, modern vaccines containing isolated, highly purified antigenic protein subunits are safer but tend to induce lower levels of protective immunity. One strategy to overcome the latter is to design antigen nanoparticles: assemblies of polypeptides that present multiple copies of subunit antigens in well-ordered arrays with defined orientations that can potentially mimic the repetitiveness, geometry, size, and shape of the natural host-pathogen surface interactions. Such nanoparticles offer a collective strength of multiple binding sites (avidity) and can provide improved antigen stability and immunogenicity. Several exciting advances have emerged lately, including preclinical evidence that this strategy may be applicable for the development of innovative new vaccines, for example, protecting against influenza, human immunodeficiency virus, and respiratory syncytial virus. Here, we provide a concise review of a critical selection of data that demonstrate the potential of this field. In addition, we highlight how the use of self-assembling protein nanoparticles can be effectively combined with the emerging discipline of structural vaccinology for maximum impact in the rational design of vaccine antigens.

Expression and characterization of hepatitis E virus-like particles and non-virus-like particles from insect cells

The hepatitis E virus (HEV) capsid antigen expressed in insect cell has been proposed as a candidate subunit vaccine for the prevention of hepatitis E. However, the expression and purification of HEV virus-like particles (VLPs) from insect cells have not been explored. We aimed to optimize the procedure to obtain HEV VLPs. In this study, two conformations of the HEV capsid proteins were expressed in insect cells, VLPs and non-VLPs, and they were purified separately. The physicochemical properties and the humoral immune responses induced by the two forms were analyzed and compared. We found that HEV VLPs were more immunogenic in mice than HEV non-VLPs. Therefore, we optimized the conditions that yielded high VLPs expression in insect cell cultures and developed an efficient purification method. The results suggest that the distinction and isolation of VLPs from non-VLPs are essential to generate a more immunogenic vaccine.

The Mammalian Cell Cycle Regulates Parvovirus Nuclear Capsid Assembly

It is unknown whether the mammalian cell cycle could impact the assembly of viruses maturing in the nucleus. We addressed this question using MVM, a reference member of the icosahedral ssDNA nuclear parvoviruses, which requires cell proliferation to infect by mechanisms partly understood. Constitutively expressed MVM capsid subunits (VPs) accumulated in the cytoplasm of mouse and human fibroblasts synchronized at G0, G1, and G1/S transition. Upon arrest release, VPs translocated to the nucleus as cells entered S phase, at efficiencies relying on cell origin and arrest method, and immediately assembled into capsids. In synchronously infected cells, the consecutive virus life cycle steps (gene expression, proteins nuclear translocation, capsid assembly, genome replication and encapsidation) proceeded tightly coupled to cell cycle progression from G0/G1 through S into G2 phase. However, a DNA synthesis stress caused by thymidine irreversibly disrupted virus life cycle, as VPs became increasingly retained in the cytoplasm hours post-stress, forming empty capsids in mouse fibroblasts, thereby impairing encapsidation of the nuclear viral DNA replicative intermediates. Synchronously infected cells subjected to density-arrest signals while traversing early S phase also blocked VPs transport, resulting in a similar misplaced cytoplasmic capsid assembly in mouse fibroblasts. In contrast, thymidine and density arrest signals deregulating virus assembly neither perturbed nuclear translocation of the NS1 protein nor viral genome replication occurring under S/G2 cycle arrest. An underlying mechanism of cell cycle control was identified in the nuclear translocation of phosphorylated VPs trimeric assembly intermediates, which accessed a non-conserved route distinct from the importin α2/β1 and transportin pathways. The exquisite cell cycle-dependence of parvovirus nuclear capsid assembly conforms a novel paradigm of time and functional coupling between cellular and virus life cycles. This junction may determine the characteristic parvovirus tropism for proliferative and cancer cells, and its disturbance could critically contribute to persistence in host tissues.

Cellular and viral life cycles are connected through multiple, though poorly understood, mechanisms. Parvoviruses infect humans and a broad spectrum of animals, causing a variety of diseases, but they are also used in experimental cancer therapy and serve as vectors for gene therapy. Parvoviruses can only multiply in proliferating cells providing essential replicative and transcriptional functions. However, it is unknown whether the cell cycle regulatory machinery may also control parvovirus assembly. We found that the nuclear translocation of parvovirus MVM capsid subunits (VPs) was highly dependent on physiological cell cycle regulations in mammalian fibroblasts, including: quiescence, progression through G1/S boundary, DNA synthesis, and cell to cell contacts. VPs nuclear translocation was significantly more sensitive to cell cycle controls than viral genome replication and gene expression. The results support nuclear capsid assembly as the major driving process of parvoviruses biological hallmarks, such as pathogenesis in proliferative tissues and tropism for cancer cells. In addition, disturbing the tight coupling of parvovirus assembly with the cell cycle may determine viral persistence in quiescent and post-mitotic host tissues. These findings may contribute to understand cellular regulations on the assembly of other nuclear eukaryotic viruses, and to develop cell cycle-based avenues for antiviral therapy.

Gene expression analysis of potential genes and pathways involved in the pathogenic mechanisms of parvovirus B19 in human colorectal cancer

In order to investigate the pathogenic mechanisms of parvovirus B19 in human colorectal cancer, plasmids containing the VP1 or VP2 viral capsid proteins or the NS1 non-structural proteins of parvovirus B19 were constructed and transfected into primary human colorectal epithelial cells and LoVo cells. Differential gene expression was detected using a human genome expression array. Functional gene annotation analyses were performed using Database for Annotation, Visualization and Integrated Discovery v6.7 software. Gene ontology (GO) analyses revealed that VP1-related functions included the immune response, immune system process, defense response and the response to stimulus, while NS1-associated functions were found to include organelle fission, nuclear division, mitosis, the M-phase of the mitotic cell cycle, the mitotic cell cycle, M-phase, cell cycle phase, cell cycle process and cell division. Pathway expression analysis revealed that VP1-associated pathways included cell adhesion molecules, antigen processing and presentation, cytokines and the inflammatory response. Moreover, NS1-associated pathways included the cell cycle, pathways in cancer, colorectal cancer, the wnt signaling pathway and focal adhesion. Among the differential genes detected in the present study, 12 genes were found to participate in general cancer pathways and six genes were observed to participate in colorectal cancer pathways. NS1 is a key molecule in the pathogenic mechanism of parvovirus B19 in colorectal cancer. Several GO categories, pathways and genes were selected and may be the key targets through which parvovirus B19 participates in colorectal cancer pathogenesis.

Insect cells as a production platform of complex virus-like particles

Virus-like particles (VLPs) are multiprotein structures that resemble the conformation of native viruses but lack a viral genome, potentiating their application as safer and cheaper vaccines. The production of VLPs has been strongly linked with the use of insect cells and the baculovirus expression vector system, especially those particles composed of two or more structural viral proteins. In fact, this expression platform has been extensively improved over the years to address the challenges of coexpression of multiple proteins and their proper assembly into complexes in the same cell. In this article, the role of insect cell technology in the development and production of complex VLPs is overviewed; recent achievements, current bottlenecks and future trends are also highlighted.

Exacerbating effects of human parvovirus B19 NS1 on liver fibrosis in NZB/W F1 mice

Systemic lupus erythematosus (SLE) is an autoimmune disorder with unknown etiology that impacts various organs including liver. Recently, human parvovirus B19 (B19) is recognized to exacerbate SLE. However, the effects of B19 on liver in SLE are still unclear. Herein we aimed to investigate the effects of B19 on liver in NZB/W F1 mice by injecting subcutaneously with PBS, recombinant B19 NS1, VP1u or VP2, respectively. Our experimental results revealed that B19 NS1 protein significantly enhanced the TGF-β/Smad fibrotic signaling by increasing the expressions of TGF-β, Smad2/3, phosphorylated Smad2/3, Smad4 and Sp1. The consequent fibrosis-related proteins, PAI-1 and α-SMA, were also significantly induced in livers of NZB/W F1 mice receiving B19 NS1 protein. Accordingly, markedly increased collagen deposition was also observed in livers of NZB/W F1 mice receiving B19 NS1 protein. However, no significant difference was observed in livers of NZB/W F1 mice receiving B19 VP1u or VP2 as compared to the controls. These findings indicate that B19 NS1 plays a crucial role in exacerbating liver fibrosis in NZB/W F1 mice through enhancing the TGF-â/Smad fibrotic signaling.

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.

Human parvovirus B19 NS1 protein aggravates liver injury in NZB/W F1 mice

Human parvovirus B19 (B19) has been associated with a variety of diseases. However, the influence of B19 viral proteins on hepatic injury in SLE is still obscure. To elucidate the effects of B19 viral proteins on livers in SLE, recombinant B19 NS1, VP1u or VP2 proteins were injected subcutaneously into NZB/W F1 mice, respectively. Significant expressions of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were detected in NZB/W F1 mice receiving B19 NS1 as compared to those mice receiving PBS. Markedly hepatocyte disarray and lymphocyte infiltration were observed in livers from NZB/WF 1 mice receiving B19 NS1 as compared to those mice receiving PBS. Additionally, significant increases of Tumor Necrosis Factor -α (TNF-α), TNF-α receptor, IκB kinase -α (IKK-α), nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor (IκB) and nuclear factor-kappa B (NF-κB) were detected in livers from NZB/W F1 mice receiving B19 NS1 as compared to those mice receiving PBS. Accordingly, significant increases of matrix metalloproteinase-9 (MMP9) and U-plasminogen activator (uPA) were also detected in livers from NZB/W F1 mice receiving B19 NS1 as compared to those mice receiving PBS. Contrarily, no significant variation on livers from NZB/W F1 mice receiving B19 VP1u or VP2 was observed as compared to those mice receiving PBS. These findings firstly demonstrated the aggravated effects of B19 NS1 but not VP1u or VP2 protein on hepatic injury and provide a clue in understanding the role of B19 NS1 on hepatic injury in SLE.

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.

[Observations on human parvovirus B19 infection diagnosed in 2011]

INTRODUCTION

The incidence of human parvovirus B19 infection is unknown.

AIM

A retrospective analysis of clinical and laboratory findings was carried out in patients diagnosed with human parvovirus B19 infection in 2011 in a virologic laboratory of a single centre in Hungary.

METHODS

Clinical and laboratory data of patients with proven human parvovirus B19 infection were analysed using in- and out-patient files.

RESULTS

In 2011, 72 patients proved to have human parvovirus B19 infection with the use of enzyme immunoassay. The clinical diagnoses of these patients were as follows: human parvovirus B19 infection (30.6%), transient aplastic crisis (16.7%), arthritis (8.3%) and acute hepatitis (4.1%). Symptoms of each of the four phases of the infection occurred in various combinations with the exception of the monophase of cheek exanthema. This occurred without the presence of other symptoms in some cases. Leading symptoms and signs were exanthema (in 74.6% of cases), haematological disorders (in 69% of cases), fever (in 54.9% of cases) and arthritis (in 33.8% of cases). Several atypical dermatological symptoms were also observed. Acute arthritis without exanthema was noted in 8 patients. Of the 72 patients with proven human parvovirus B19 infection there were 7 pregnant women, and one of them had hydrops foetalis resulting spontaneous abortion. In 16 patients (22.5%) human parvovirus B19 IgG was undetectable despite an optimal time for testing.

CONCLUSION

The observations of this study may contribute to a better recognition of clinical symptoms of human parvovirus B19 infection.

Virus-like particles as a highly efficient vaccine platform: diversity of targets and production systems and advances in clinical development

Virus-like particles (VLPs) are a class of subunit vaccines that differentiate themselves from soluble recombinant antigens by stronger protective immunogenicity associated with the VLP structure. Like parental viruses, VLPs can be either non-enveloped or enveloped, and they can form following expression of one or several viral structural proteins in a recombinant heterologous system. Depending on the complexity of the VLP, it can be produced in either a prokaryotic or eukaryotic expression system using target-encoding recombinant vectors, or in some cases can be assembled in cell-free conditions. To date, a wide variety of VLP-based candidate vaccines targeting various viral, bacterial, parasitic and fungal pathogens, as well as non-infectious diseases, have been produced in different expression systems. Some VLPs have entered clinical development and a few have been licensed and commercialized. This article reviews VLP-based vaccines produced in different systems, their immunogenicity in animal models and their status in clinical development.

Production of parvovirus B19 vaccine in insect cells co-infected with double baculoviruses

Recombinant human parvovirus B19 virus-like particles (VLPs), a candidate vaccine, were produced using the insect cell (Sf-9)-baculovirus (AcNPV) expression system. The synthesis and assembly of the particles in Sf-9 cells are directed by double infections with one recombinant virus (bacVP1) expressing the parvovirus minor viral protein VP1 and a second virus (bacVP2) expressing the major viral protein VP2. Previous animal studies demonstrated that the polypeptide composition of the VLPs strongly affects the elicitation of virus neutralizing antibodies. The key factor controlling the production of an immunologically potent product in bioreactors was identified to be the multiplicity of infection (MOI) of bacVP1 and bacVP2 used for infection. A probabilistic model, which correlates well with the experimental results, was employed to facilitate the selection of MOIs and to provide a better understanding of the baculovirus co-infection process. A novel production process based on secondary infections was developed to ensure product consistency and to simplify large-scale logistics. The effects of other critical process parameters, such as temperature, dissolved oxygen concentration, lactate concentration, cell concentration at infection, and harvest time, were also investigated. (c) 1996 John Wiley & Sons, Inc.

Seroprevalence of immunoglobulin G antibody to parvovirus B19 in Ontario

The prevalence of antibody to parvovirus B19 was assessed in two populations. In a group of 494 residents from Ontario and the Maritimes, virus-specific immunoglobulin (Ig) M antibody, a marker of acute infection, was found throughout the year but was most prevalent during the late winter and early spring months. The overall prevalence of IgG antibody in this group was 30.3%. In an effort to examine age-specific prevalence in this population, a second group of sera from 210 pediatric patients at The Hospital for Sick Children, Toronto, Ontario and from Red Cross blood donors was tested for the presence of B19-specific IgG, and of these, 31.4% of the samples were positive. This prevalence varied from 3.3% in the under five-year-old age group to 66.7% in the 35- to 45-year-old age group. Eighty per cent of sera from females of this group were seropositive. This study provides insight into the prevalence of parvovirus B19 IgG antibody in the population.

Co-expression vs. co-infection using baculovirus expression vectors in insect cell culture: Benefits and drawbacks

The baculovirus expression vector system (BEVS) is a versatile and powerful platform for protein expression in insect cells. With the ability to approach similar post-translational modifications as in mammalian cells, the BEVS offers a number of advantages including high levels of expression as well as an inherent safety during manufacture and of the final product. Many BEVS products include proteins and protein complexes that require expression from more than one gene. This review examines the expression strategies that have been used to this end and focuses on the distinguishing features between those that make use of single polycistronic baculovirus (co-expression) and those that use multiple monocistronic baculoviruses (co-infection). Three major areas in which researchers have been able to take advantage of co-expression/co-infection are addressed, including compound structure-function studies, insect cell functionality augmentation, and VLP production. The core of the review discusses the parameters of interest for co-infection and co-expression with time of infection (TOI) and multiplicity of infection (MOI) highlighted for the former and the choice of promoter for the latter. In addition, an overview of modeling approaches is presented, with a suggested trajectory for future exploration. The review concludes with an examination of the gaps that still remain in co-expression/co-infection knowledge and practice.