Transcriptional transactivation of parvovirus B19 promoters in nonpermissive human cells by adenovirus type 2

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.

Enhanced Transduction of Human Hematopoietic Stem Cells by AAV6 Vectors: Implications in Gene Therapy and Genome Editing

We have reported that of the 10 most commonly used adeno-associated virus (AAV) serotype vectors, AAV6 is the most efficient in transducing primary human hematopoietic stem cells (HSCs) in vitro, as well as in vivo. More recently, polyvinyl alcohol (PVA), was reported to be a superior replacement for human serum albumin (HSA) for ex vivo expansion of HSCs. Since HSA has been shown to increase the transduction efficiency of AAV serotype vectors, we evaluated whether PVA could also enhance the transduction efficiency of AAV6 vectors in primary human HSCs. We report here that up to 12-fold enhancement in the transduction efficiency of AAV6 vectors can be achieved in primary human HSCs with PVA. We also demonstrate that the improvement in the transduction efficiency is due to PVA-mediated improved entry and intracellular trafficking of AAV6 vectors in human hematopoietic cells in vitro, as well as in murine hepatocytes in vivo. Taken together, our studies suggest that the use of PVA is an attractive strategy to further improve the efficacy of AAV6 vectors. This has important implications in the optimal use of these vectors in the potential gene therapy and genome editing for human hemoglobinopathies such as β-thalassemia and sickle cell disease.

Human parvovirus B19: a mechanistic overview of infection and DNA replication

Human parvovirus B19 (B19V) is a human pathogen that belongs to genus Erythroparvovirus of the Parvoviridae family, which is composed of a group of small DNA viruses with a linear single-stranded DNA genome. B19V mainly infects human erythroid progenitor cells and causes mild to severe hematological disorders in patients. However, recent clinical studies indicate that B19V also infects nonerythroid lineage cells, such as myocardial endothelial cells, and may be associated with other disease outcomes. Several cell culture systems, including permissive and semipermissive erythroid lineage cells, nonpermissive human embryonic kidney 293 cells and recently reported myocardial endothelial cells, have been used to study the mechanisms underlying B19V infection and B19V DNA replication. This review aims to summarize recent advances in B19V studies with a focus on the mechanisms of B19V tropism specific to different cell types and the cellular pathways involved in B19V DNA replication including cellular signaling transduction and cell cycle arrest.

High-efficiency transduction of primary human hematopoietic stem cells and erythroid lineage-restricted expression by optimized AAV6 serotype vectors in vitro and in a murine xenograft model in vivo

We have observed that of the 10 AAV serotypes, AAV6 is the most efficient in transducing primary human hematopoietic stem cells (HSCs), and that the transduction efficiency can be further increased by specifically mutating single surface-exposed tyrosine (Y) residues on AAV6 capsids. In the present studies, we combined the two mutations to generate a tyrosine double-mutant (Y705+731F) AAV6 vector, with which >70% of CD34⁺ cells could be transduced. With the long-term objective of developing recombinant AAV vectors for the potential gene therapy of human hemoglobinopathies, we generated the wild-type (WT) and tyrosine-mutant AAV6 vectors containing the following erythroid cell-specific promoters: β-globin promoter (βp) with the upstream hyper-sensitive site 2 (HS2) enhancer from the β-globin locus control region (HS2-βbp), and the human parvovirus B19 promoter at map unit 6 (B19p6). Transgene expression from the B19p6 was significantly higher than that from the HS2-βp, and increased up to 30-fold and up to 20-fold, respectively, following erythropoietin (Epo)-induced differentiation of CD34⁺ cells in vitro. Transgene expression from the B19p6 or the HS2-βp was also evaluated in an immuno-deficient xenograft mouse model in vivo. Whereas low levels of expression were detected from the B19p6 in the WT AAV6 capsid, and that from the HS2-βp in the Y705+731F AAV6 capsid, transgene expression from the B19p6 promoter in the Y705+731F AAV6 capsid was significantly higher than that from the HS2-βp, and was detectable up to 12 weeks post-transplantation in primary recipients, and up to 6 additional weeks in secondary transplanted animals. These data demonstrate the feasibility of the use of the novel Y705+731F AAV6-B19p6 vectors for high-efficiency transduction of HSCs as well as expression of the b-globin gene in erythroid progenitor cells for the potential gene therapy of human hemoglobinopathies such as β-thalassemia and sickle cell disease.

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

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

The prevalence of human parvovirus B19 DNA and antibodies in blood donors from four Chinese blood centers

BACKGROUND

Human parvovirus B19 is a common human pathogen that causes a variety of diseases with outcomes ranging from asymptomatic to severe, especially in immunocompromised patients. The B19 virus can be transmitted via blood and/or blood products and its resistance to common viral inactivation and/or removal methods raises the importance of B19-related blood safety. However, the existence, variation, and loading of B19 in Chinese blood donors have not been determined.

STUDY DESIGN AND METHODS

Quantitative polymerase chain reaction (PCR) was developed to detect all three genotypes of the human erythrovirus DNA in plasma samples. In total, 3957 donations from four Chinese blood centers were screened for B19 by real-time minipool nucleic acid amplification technology (NAT). The positive samples were then confirmed by nested PCR and subjected to sequence analysis and alignment for phylogenetic studies. An enzyme-linked immunosorbent assay-based experiment was also performed to identify the prevalence of immunoglobulin (Ig)G and/or IgM antibodies specific to the B19 structural proteins in acquired samples.

RESULTS

Of 3957 blood donors, 23 (0.58%) specimens were found positive for B19 DNA. The quantitative DNA levels ranged from 2.48 × 10(2) to 6.38 × 10(4) copies/mL. The phylogenic analyses showed that the prevalent genotypes in Chinese blood donors belong to B19 Genotype 1. A total of 448 samples from Chinese blood donors were investigated for the seroprevalence of B19 antibodies, among which 24.6 and 6.9% of specimens were seropositive for B19 IgG and IgM antibodies, respectively. A total of 2.5% of these samples were positive for both antibody isotypes.

CONCLUSIONS

Whether B19 NAT screening of blood and blood products should be launched in China, larger studies are needed to facilitate an informed decision.

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

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

The genome of human parvovirus b19 can replicate in nonpermissive cells with the help of adenovirus genes and produces infectious virus

Human parvovirus B19 (B19V) is a member of the genus Erythrovirus in the family Parvoviridae. In vitro, autonomous B19V replication is limited to human erythroid progenitor cells and in a small number of erythropoietin-dependent human megakaryoblastoid and erythroid leukemic cell lines. Here we report that the failure of B19V DNA replication in nonpermissive 293 cells can be overcome by adenovirus infection. More specifically, the replication of B19V DNA in the 293 cells and the production of infectious progeny virus were made possible by the presence of the adenovirus E2a, E4orf6, and VA RNA genes that emerged during the transfection of the pHelper plasmid. Using this replication system, we identified the terminal resolution site and the nonstructural protein 1 (NS1) binding site on the right terminal palindrome of the viral genome, which is composed of a minimal origin of replication spanning 67 nucleotides. Plasmids or DNA fragments containing an NS1 expression cassette and this minimal origin were able to replicate in both pHelper-transfected 293 cells and B19V-semipermissive UT7/Epo-S1 cells. Our results have important implications for our understanding of native B19V infection.

Co-infection of human parvovirus B19 in Vietnamese patients with hepatitis B virus infection

BACKGROUND/AIMS

Human parvovirus B19 (B19) has been identified in the serum of hepatitis B virus (HBV) infected patients. However, the effect of B19-infection on the course of HBV-associated liver disease has not previously been investigated. We examined the prevalence of B19-DNA in HBV-infected Vietnamese patients and analysed the association between co-infection and the clinical outcome of HBV-infection.

METHODS

Serum samples from 399 HBV-infected patients and 64 healthy individuals were analysed for the presence of B19-DNA by PCR and DNA-sequencing.

RESULTS

B19-DNA was detected in 99/463 (21.4%) individuals. The proportion of HBV-infected patients who were also co-infected with B19 was higher than the healthy controls (P<0.001). B19-DNA was detected more frequently in patients with HBV-associated hepatocellular carcinoma compared to patients with acute and chronic HBV, HBV-associated liver cirrhosis and healthy subjects (P<0.006). A positive correlation was also found between B19-DNA loads and both serum HBV-DNA loads and alanine aminotransferase (rho>0.250 and P<0.05).

CONCLUSIONS

Our findings demonstrate that B19-infection is frequent in HBV-infected Vietnamese patients. Also, a significant correlation exists between HBV/B19 co-infection and a greater likelihood of progression to more severe hepatitis B-associated liver disease. Further studies are required to determine the role of B19-infection on HBV-associated pathogenesis.

Parvovirus B19-induced apoptosis of hepatocytes

Parvovirus B19 (B19 virus) can persist in multiple tissues and has been implicated in a variety of diseases, including acute fulminant liver failure. The mechanism by which B19 virus induces liver failure remains unknown. Hepatocytes are nonpermissive for B19 virus replication. We previously reported that acute fulminant liver failure associated with B19 virus infection was characterized by hepatocellular dropout. We inoculated both primary hepatocytes and the hepatocellular carcinoma cell line Hep G2 with B19 virus and assayed for apoptosis by using annexin V staining. Reverse transcriptase PCR analysis and immunofluorescence demonstrated that B19 virus was able to infect the cells and produce its nonstructural protein but little or no structural capsid protein. Infection with B19 virus induced means of 28% of Hep G2 cells and 10% of primary hepatocytes to undergo apoptosis, which were four- and threefold increases, respectively, over background levels. Analysis of caspase involvement showed that B19 virus-inoculated cultures had a significant increase in the number of cells with active caspase 3. Inhibition studies demonstrated that caspases 3 and 9, but not caspase 8, are required for B19 virus-induced apoptosis.

Parvovirus B19 and erythroid cells

Parvovirus B19 is a human erythrovirus, i.e. which induces the death of erythroid progenitors. In such cells, until now only ubiquitous transcription factors have been described to regulate promoter driven gene expression. Their possible interactions with erythroid specific transcription factors merit further investigations. Effectively, the high level of replication of B19 in erythroid cells is not well understood. In addition to apoptosis, necrosis or inhibition of cell growth, the death of B19 infected erythroid progenitors has been never clearly reported as the result of immunological attack: this mecanism will merit further investigations. The interactions with other cell types in vitro remain at present not well defined but many obstacles have been mentioned which counteract B19 expression.

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

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

Adeno-associated virus 2-mediated transduction and erythroid lineage-restricted expression from parvovirus B19p6 promoter in primary human hematopoietic progenitor cells

Human parvovirus B19 gene expression from the viral p6 promoter (B19p6) is restricted to primary human hematopoietic cells undergoing erythroid differentiation. We have demonstrated that expression from this promoter does not occur in established human erythroid cell lines in the context of a recombinant parvovirus genome (Ponnazhagan et al. J Virol 69:8096-8101, 1995). However, abundant expression from this promoter can be readily detected in primary human bone marrow cells (Wang et al. Proc Natl Acad Sci USA 92:12416-12420, 1995; Ponnazhagan et al. J Gen Virol 77:1111-1122, 1996). In the present studies, we investigated the pattern of expression from the B19p6 promoter in primary human bone marrow-derived CD34+ HPC undergoing differentiation into myeloid and erythroid lineages. CD34+ cells were transduced with recombinant adeno-associated virus 2 (AAV) vectors containing the beta-galactosidase (lacZ) gene under the control of the following promoters/enhancers: the cytomegalovirus promoter (vCMVp-lacZ), B19p6 promoter (vB19p6-lacZ), B19p6 promoter with an upstream erythroid cell-specific enhancer element (HS-2) from the locus control region (LCR) from the human beta-globin gene cluster (vHS2-B19p6-lacZ), and the human beta-globin gene promoter with the HS-2 enhancer (vHS2-beta p-lacZ). Transgene expression was evaluated either 48 h after infection or following erythroid differentiation in vitro for 3 weeks. Whereas high-level expression from the CMV promoter 48 h after infection diminished with time, low-level expression from the B19p6 and the beta-globin promoters increased significantly following erythroid differentiation. Furthermore, in HPC assays, there was no significant difference in the level of expression from the CMV promoter in myeloid or erythroid cell-derived colonies. Expression from the B19p6 and the beta-globin promoters, on the other hand, was restricted to erythroid cell colonies. These data further corroborate that the B19p6 promoter is erythroid cell-specific and suggest that the recombinant AAV-B19 hybrid vectors may prove useful in gene therapy of human hemoglobinopathies in general and sickle cell anemia and beta-thalassemia in particular.

Recombinant human parvovirus B19 vectors: erythroid cell-specific delivery and expression of transduced genes

A novel packaging strategy combining the salient features of two human parvoviruses, namely the pathogenic parvovirus B19 and the nonpathogenic adeno-associated virus type 2 (AAV), was developed to achieve erythroid cell-specific delivery as well as expression of the transduced gene. The development of such a chimeric vector system was accomplished by packaging heterologous DNA sequences cloned within the inverted terminal repeats of AAV and subsequently packaging the DNA inside the capsid structure of B19 virus. Recombinant B19 virus particles were assembled, as evidenced by electron microscopy as well as DNA slot blot analyses. The hybrid vector failed to transduce nonerythroid human cells, such as 293 cells, as expected. However, MB-02 cells, a human megakaryocytic leukemia cell line which can be infected by B19 virus following erythroid differentiation with erythropoietin (N. C. Munshi, S. Z. Zhou, M. J. Woody, D. A. Morgan, and A. Srivastava, J. Virol. 67:562-566, 1993) but lacks the putative receptor for AAV (S. Ponnazhagan, X.-S. Wang, M. J. Woody, F. Luo, L. Y. Kang, M. L. Nallari, N. C. Munshi, S. Z. Zhou, and A. Srivastava, J. Gen. Virol. 77:1111-1122, 1996), were readily transduced by this vector. The hybrid vector was also found to specifically target the erythroid population in primary human bone marrow cells as well as more immature hematopoietic progenitor cells following erythroid differentiation, as evidenced by selective expression of the transduced gene in these target cells. Preincubation with anticapsid antibodies against B19 virus, but not anticapsid antibodies against AAV, inhibited transduction of primary human erythroid cells. The efficiency of transduction of primary human erythroid cells by the recombinant B19 virus vector was significantly higher than that by the recombinant AAV vector. Further development of the AAV-B19 virus hybrid vector system should prove beneficial in gene therapy protocols aimed at the correction of inherited and acquired human diseases affecting cells of erythroid lineage.

Rescue and autonomous replication of adeno-associated virus type 2 genomes containing Rep-binding site mutations in the viral p5 promoter

The Rep proteins encoded by the adeno-associated virus type 2 (AAV) play a crucial role in the rescue, replication, and integration of the viral genome. In the absence of a helper virus, little expression of the AAV Rep proteins occurs, and the AAV genome fails to undergo DNA replication. Since previous studies have established that expression of the Rep78 and Rep68 proteins from the viral p5 promoter is controlled by the Rep-binding site (RBS) and the YY1 factor-binding site (YBS), we constructed a number of recombinant AAV plasmids containing mutations and/or deletions of the RBS and the YBS in the p5 promoter. These plasmids were transfected in HeLa or 293 cells and analyzed for the potential to undergo AAV DNA rescue and replication. Our studies revealed that (i) a low-level rescue and autonomous replication of the wild-type AAV genome occurred in 293 but not in HeLa cells; (ii) mutations in the RBS resulted in augmented expression from the p5 promoter, leading to more efficient rescue and/or replication of the AAV genome in 293 but not in HeLa cells; (iii) little rescue and/or replication occurred from plasmids containing mutations in the YBS alone in the absence of coinfection with adenovirus; (iv) expression of the adenovirus E1A gene products was insufficient to mediate rescue and/or replication of the AAV genome in HeLa cells; (v) autonomously replicated AAV genomes in 293 cells were successfully encapsidated in mature progeny virions that were biologically active in secondary infection of HeLa cells in the presence of adenovirus; and (vi) stable transfection of recombinant AAV plasmids containing a gene for resistance to neomycin significantly affected stable integration only in 293 cells, presumably because rescue and autonomous replication of the AAV genome from these plasmids occurred in 293 cells but not in HeLa or KB cells. These data suggest that in the absence of adenovirus, the AAV Rep protein-RBS interaction plays a dominant role in down-regulating viral gene expression from the p5 promoter and that perturbation in this interaction is sufficient to confer autonomous replication competence to AAV in 293 cells.