Detection of an erythrovirus sequence distinct from B19 in a child with acute anaemia

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.

Genotypes of erythrovirus B19, their geographical distribution & circulation in cases with various clinical manifestations

Erythrovirus B19 (B19V) is one of the erythroviruses known to be pathogenic in humans. B19V is classified into three distinct genotypes; 1, 2 and 3, differing from each other by 2-13 per cent. Genotype 1 consists of the prototype B19V isolates, genotype 2 comprises the A6, LaLi and their related isolates while genotype 3 includes the V9- and V9-related isolates. The classification of genotype 1 into two subtypes (1A and 1B) and genotype 3 into two subtypes (3a and 3b) with an estimated nucleotide difference of about 5 per cent has been done. Predominance of genotype 1 across all the continents is seen followed by genotypes 2 and 3. There are no disease-specific genotypes. All the three genotypes have been found in symptomatic as well as asymptomatic individuals and have been reported from several countries across the world. The prevalence of genotype 2 in older populations and its absence from current circulation in Northern Europe has also been reported. The present review focuses on geographic distribution and association of genotypes of B19V with different clinical manifestations.

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.

Parvovirus B19

Primary parvovirus B19 infection is an infrequent, but serious and treatable, cause of chronic anemia in immunocompromised hosts. Many compromised hosts have preexisting antibody to B19 and are not at risk. However, upon primary infection, some patients may be able to mount a sufficient immune response to terminate active parvovirus B19 infection of erythroid precursors. The most common consequence of B19 infection in the compromised host is pure red-cell aplasia, resulting in chronic or recurrent anemia with reticulocytopenia. Anemia persists until neutralizing antibody is either produced by the host or passively administered. Parvovirus B19 should be suspected in compromised hosts with unexplained or severe anemia and reticulocytopenia, or when bone-marrow examination shows either giant pronormoblasts or absence of red-cell precursors. Diagnosis is established by detection of B19 DNA in serum in the absence of IgG antibody to B19. In some cases, IgG antibody is detected but is not neutralizing. Anti-B19 IgM may or may not be present. Therapy includes any or all of the following: red-cell transfusion, adjustment in medications to restore or improve the patient’s immune system, and administration of intravenous immunoglobulin (IVIG). Following treatment, patients should be closely monitored, especially if immunosuppression is unchanged or increased. Should hematocrit trend downward and parvovirus DNA trend upward, the therapeutic options above should be revisited. In a few instances, monthly maintenance IVIG may be indicated. Caregivers should be aware that B19 variants, though rarely encountered, can be missed or under-quantitated by some real-time polymerase-chain reaction methods.

Update of the human parvovirus B19 biology

Since its discovery, the human parvovirus B19 (B19V) has been associated with many clinical situations in addition to the prototype clinical manifestations, i.e. erythema infectiosum and erythroblastopenia crisis. The clinical significance of the viral B19V DNA persistence in sera after acute infection remains largely unknown. Such data may constitute a new clinical entity and is discussed in this manuscript. In 2002, despite the genetic diversity among B19V viruses has been reported to be very low, the description of markedly distinct sequences showed a new organization into three genotypes. The most recent common ancestor for B19V genotypes was estimated at early 1800s. B19V replication is enhanced by hypoxia and this might to explain the high viral load detected by quantitative PCR in the sera of infected patients. The minimum infectious dose necessary to transmit B19V infection by the transfusion of labile blood products remains unclear. At the opposite, the US Food and Drug Administration proposed a limit of 10(4)IU/mL of viral DNA in plasma pools used for the production of plasma derivatives. Recently, a new human parvovirus (PARV4) has been discovered. The consequences on blood transfusion of this blood-borne agent and its pathogenicity are still unknown.

Human immunodeficiency virus/human parvovirus B19 co-infection in blood donors and AIDS patients in Sichuan, China

BACKGROUND

Human parvovirus B19 (B19) is a common pathogen which causes a variety of diseases. Persistent B19 infection is related to the degree of host immunodeficiency in patients with human immunodeficiency virus (HIV) infection. However, the existence, loading, virus evolution and distribution of B19 in Chinese HIV-positive patients have not been determined.

MATERIALS AND METHODS

We investigated 573 HIV-positive blood donors and AIDS patients in Sichuan, China in the last two decades. Bl9-specific serology and quantitative polymerase chain reaction were used to determine the prevalence of B19/HIV co-infection. Viral genome fragments were subjected to phylogeny and haplotype analysis.

RESULTS

B19 genomic DNA was found in 26 of 573 (4.5%) HIV-positive individuals, a higher prevalence than in blood donors. DNA levels ranged from 5.3×10(2)-1.1×10(5) copies/mL. The seroprevalence of IgG was significantly lower in HIV-positive samples than in HIV-negative blood donors, indicating deficient production of B19-specific IgG in the former. The B19 isolates were genotype-1 subtype B19-1A which formed a monophyletic group; seven distinct haplotypes were discovered with 60% of the B19/HIV co-infected variants sharing one central haplotype.

DISCUSSION

This study on the prevalence, phylogeny and distribution of human parvovirus B19 in Sichuan, China, demonstrates the persistence of B19 in the circulation of both immunocompetent and immunocompromised subjects, with implications for blood safety.

Advances in human B19 erythrovirus biology

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

Simultaneous persistence of multiple genome variants of human parvovirus B19

The species human parvovirus B19 (B19V) can be divided into three genotypes. In this study, we addressed the question as to whether infection of an individual is restricted to one genotype. As viral DNA is detectable in tissue for long times after acute infection, we examined 87 liver specimens from adults for the presence of B19V DNA. Fifty-nine samples were found to be positive, 32 of them for genotype 1, 27 for genotype 2 and four for genotype 3. In four samples, DNA of two genotypes was detected; samples from three individuals were positive for genotypes 1 and 2 and a sample from one individual was positive for genotypes 1 and 3. Surprisingly, significant sequence heterogeneity was observed at approximately 1 % of the nucleotides of the genotype 1 genomes from individuals with double genotype 1 and 2 infection. Controls using different enzymes for genome amplification and dilutions of the template verified that nucleotide heterogeneity was due to the presence of three or more genome variants of genotype 1. In summary, the evidence shows that individuals can be infected with two different genotypes, and B19V DNA can persist as a population of different genomes. The results may have implications for the understanding of the antiviral immune response and the development of vaccines against B19V.

Tissue persistence and prevalence of B19 virus types 1–3

Human parvovirus B19 is a minute ssDNA virus that causes a wide variety of diseases, including erythema infectiosum, arthropathy, anemias and fetal death. In addition to the B19 prototype, two new variants (B19 types 2 and 3) have been identified. After primary infection, B19 genomic DNA has been shown to persist in solid tissues of not only symptomatic but also of constitutionally healthy, immunocompetent individuals. The viral DNA persists as an intact molecule without persistence-specific mutations, and via a storage mechanism with life-long capacity. Thus, the mere presence of B19 DNA in tissue cannot be used as a diagnostic criterion, although a possible role in the pathology of diseases, for example through mRNA or protein production, cannot be excluded. The molecular mechanism, host-cell type and possible clinical significance of tissue persistence are yet to be elucidated.

Biological and immunological relations among human parvovirus B19 genotypes 1 to 3

The human parvovirus B19 is now divided into three genotypes: type 1 (prototype), type 2 (A6- and LaLi-like), and type 3 (V9-like). In overall DNA sequence, the three virus types differ by approximately 10%. The most striking DNA dissimilarity, of >20%, is observed within the p6 promoter region. Because of the scarcity of data on the biological activities and pathogenetic potentials of virus types 2 and 3, we examined the functional characteristics of these virus types. We found the activities of the three p6 promoters to be of equal strength and to be most active in B19-permissive cells. Virus type 2 capsid protein VP2, alone or together with VP1, was expressed with the baculovirus system and was shown to assemble into icosahedral parvovirus-like particles, which were reactive in the hemagglutination assay. Furthermore, sera containing DNA of any of the three B19 types were shown to hemagglutinate. The infectivities of these sera were examined in two B19-permissive cell lines. Reverse transcription-PCR revealed synthesis of spliced B19 mRNAs, and immunofluorescence verified the production of NS and VP proteins in the infected cells. All three genotypes showed similar functional characteristics in all experiments performed, showing that the three virus types indeed belong to the same species, i.e., human parvovirus B19. Additionally, the antibody activity in sera from B19 type 1- or type 2-infected subjects (long-term immunity) was examined with homo- and heterologous virus-like particles. Cross-reactivity of 100% was observed, indicating that the two B19 genotypes comprise a single serotype.

Reactivity of genotype-specific recombinant proteins of human erythrovirus B19 with plasmas from areas where genotype 1 or 3 is endemic

Human erythrovirus (parvovirus) B19 (B19) is a common human pathogen. The recent discovery of three genotypes, 1 to 3, raised issues related to the ability of genotype-specific antigens to cross-react with antibodies elicited by other genotypes. This study assessed antibody capture and immunoglobulin G (IgG) cross-reactivity between genotype 1 and genotype 3 recombinant antigens and analyzed the potential gain of adding VP1 protein to VP2 capsid antigen. Plasma samples from genotype 1- or genotype 3-infected populations were blindly tested with blindly prepared reagents. The IgG reactivity was assessed with baculovirus-expressed VP2 or VP1 and VP2 recombinant genotype 1 or genotype 3 proteins in a standardized enzyme immunoassay (EIA). A high degree of agreement (>95%) between EIA results was observed, with Spearman correlation coefficient and kappa reliability coefficient results of >/=0.95 for samples from the United Kingdom and >/=0.77 for samples from Ghanaian children, respectively. Most discrepant results were related to equivocal reactivity. The addition of VP1 to VP2 capsids did not significantly impact antibody detection. These data suggest that the currently available genotype-1-based IgG EIA is suitable to detect antibody to B19 genotype 3 in Ghanaian children. However, samples from the Ghanaian adult population exhibited atypical results in the assay, possibly due to the high levels of nonspecific IgG antibodies found in adults living in this region. Within these limitations, the study demonstrates that genotype 1 and genotype 3 antigens are equally effective in detecting either antibody species.

Effects of transfusion on human erythrovirus B19-susceptible or -infected pediatric recipients in a genotype 3-endemic area

BACKGROUND

Human erythrovirus (parvovirus) B19 is transmitted by transfusion of blood, blood components, and plasma derivatives and is resistant to most viral inactivation methods. B19 genotype 3 is prevalent in Ghana, and no related clinical information is available.

STUDY DESIGN AND METHODS

This study assessed the transmission of B19 genotype 3 by transfusion and the potential effect of transfused B19 antibodies in viremic recipients. Immunological aspects of B19 genotype 3 infection in children mainly transfused for acute malarial anemia were examined. Molecular and serologic methods adapted to genotype 3 were developed and used.

RESULTS

Among 114 donor-recipient pairs from Ghana, two donations contained B19 DNA and specific antibodies, and no evidence of transmission was found. B19 immunoglobulin G (IgG)-containing whole blood was transfused to 14 B19 DNA-positive recipients. Three recipients with detectable levels of IgG to B19 failed to clear viremia 1 to 2.3 months after transfusion. Ten recipients without IgG to VP2 before transfusion cleared the virus but failed to develop an immune response to B19 within 1 to 2 months after transfusion. Only 1 patient who received little specific IgG by transfusion produced detectable antibodies.

CONCLUSION

Low levels of B19 genotype 3 DNA associated with specific IgG are not infectious by transfusion. Viral clearance and apparent down regulation of immune response to B19 may be related to removal of the viral antigens by transfused antibodies and/or immunomodulatory effect of transfusion.

Clinical aspects of parvovirus B19 infection

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

Serologic and molecular detection of human Parvovirus B19 infection

Following its identification by Yvonne Cossart in 1975, human Parvovirus B19 has been recognized as the causative agent of a wide range of diseases. In childhood, the most common disease is a typical exanthema called "fifth disease". In adults, viral infection may be responsible for fetal loss and for aplastic anaemia in immuno-compromised patients. Because persistent viral infection may induce an autoimmune response, Parvovirus B19 is emerging as an environmental factor linked to the pathogenesis of autoimmunity. As a result of its expanding disease spectrum, Parvovirus B19 is the subject of intense efforts to clarify the pathogenesis of virus-related disorders as well as improve diagnostic laboratory testing including standardization of serological and nucleic acid-based detection assays. Enzymatic immunoassays based on conformational antigens have proven to be the most important tools for accurate diagnosis in the majority of cases. In other selected clinical cases, the detection of Parvovirus B19 infection can be complemented by PCR and, more recently, by the real-time PCR.

Characterization of Parvovirus B19 genotype 2 in KU812Ep6 cells

An infectious parvovirus B19 (B19V) genotype 2 variant was identified as a high-titer contaminant in a human plasma donation. Genome analysis revealed a 138-bp insertion within the p6 promoter. The inserted sequence was represented by an additional 30 bp from the end of the inverted terminal repeat adjacent to a 108-bp element found also, in inverted orientation, at the extreme right end of the unique sequence of the genome. However, despite the profound variations in the promoter region, the pattern of gene expression and DNA replication did not differ between genotype 1 and genotype 2 in permissive erythroid KU812Ep6 cells. Capsid proteins of both genotypes differ in their amino acid sequences. However, equivalent kinetics of virus inactivation at 56 degrees C or pH 4 indicated a comparable physicochemical stability of virus capsids. Sera from six individuals infected by B19V genotype 1 were investigated on cross-neutralization of B19V genotype 2 in vitro. Similar neutralization of both B19V genotypes was observed in sera from three individuals, while the sera from three other individuals showed weaker cross-neutralization for genotype 2. In conclusion, the in vitro replication characteristics and physical stability of B19V capsids are very similar between human parvovirus B19 genotypes 1 and 2, and cross-neutralization indicates a close antigenic relation of genotypes 1 and 2.

Evaluation of different assays for the detection of parvovirus B19 DNA in human plasma

Human parvovirus B19 is a frequent contaminant of blood and plasma-derived medicinal products and transmission of this virus has been shown to occur through the administration of contaminated products. Inactivation of the virus has proved difficult and as a consequence, manufacturers of blood products have implemented screening measures to reduce the load of parvovirus B19 in manufacturing plasma pools by the use of nucleic acid amplification techniques (NAT). We present data comparing the performance of two commercially available kits for the detection and quantitation of parvovirus B19 DNA using the LightCycler, and determine their applicability for the detection of recently discovered variants of the virus. Parvovirus B19 DNA was readily quantified using both commercial assays. However, one kit failed to detect any of the variant viruses. The second kit detected the variant viruses although there was a marked difference in the sensitivity of detection of the different virus genotypes. To improve the detection of these variant viruses a novel assay has been developed and data are presented to show its use for screening pooled plasma for the presence of parvovirus B19 DNA and the variants identified recently.

Detection and differentiation of human parvovirus variants by commercial quantitative real-time PCR tests

Parvovirus B19 causes a variety of diseases in humans, with outcomes ranging from asymptomatic to severe, such as chronic anemia in immunocompromised patients or fetal hydrops and death after maternal infection during pregnancy. The virus may be transmitted via plasma-derived products. According to the results of solvent-detergent safety studies, an upper limit of B19 DNA in plasma pools was recently defined. To restrict the input of B19 virus into production pools, a quantitative nucleic acid test is a prerequisite. We examined the suitability of the two commercial quantitative B19 PCR tests, LightCycler-Parvovirus B19 quantification kit (Roche Diagnostics) and RealArt Parvo B19 LC PCR (Artus) for detection, quantification, and differentiation of the three known B19 genotypes, including the newly described erythrovirus variants (genotypes 2 and 3). The former kit was highly sensitive for genotype 1 but was not suitable for detection of genotype 2 or one of two genotype 3 strains. The latter kit detected and differentiated all three genotypes, albeit with lower sensitivity for one of the genotype-3 strains. We furthermore assessed the prevalence of the three B19 virus genotypes in blood donors, by screening pooled plasma samples derived from 140,160 Finnish blood-donor units. None of the pools contained detectable levels of B19 virus genotypes 2 or 3. The origin, mode of transmission, and clinical significance of these genotypes are unknown and deserve further study. The RealArt Parvo B19 LC PCR is suitable for detection, quantification, and differentiation of all three B19 virus genotypes in molecular and clinical research.

Advances in the biology, diagnosis and host-pathogen interactions of parvovirus B19

Increased recognition of parvovirus B19 (B19), an erythrovirus, as a significant human pathogen that causes fetal loss and severe disease in immunocompromised patients has resulted in intensive efforts to understand the pathogenesis of B19-related disease, to improve diagnostic strategy that is deployed to detect B19 infection and blood-product contamination and, finally, to elucidate the nature of the cellular immune response that is elicited by the virus in diverse patient cohorts. It is becoming clear that at least three related erythrovirus strains (B19, A6/K71 and V9) are circulating in the general population and that viral entry into target cells is mediated by an expanding range of cellular receptors, including P antigen and beta-integrins. Persistent infection by B19 is emerging as a contributory factor in autoimmune disease, a hypothesis that is constrained by the detection of B19 in the skin of apparently healthy individuals. B19 infection during pregnancy may account for thousands of incidences of fetal loss per annum in Europe, North America and beyond, yet there is currently only minimal screening of pregnant women to assess serological status, and thereby risk of infection, upon becoming pregnant. Whilst major advances in diagnosis of B19 infection have taken place, including standardization of serological and DNA-based detection methodologies, blood donations that are targeted at high-risk groups are only beginning to be screened for B19 IgG and DNA as a means of minimizing exposure of at-risk patients to the virus. It is now firmly established that a Th1-mediated cellular immune response is mounted in immunocompetent individuals, a finding that should contribute to the development of an effective vaccine to prevent B19 infection in selected high-risk groups, including sickle-cell anaemics.

LightCycler consensus PCR for rapid and differential detection of human erythrovirus B19 and V9 isolates

Until recently, B19 virus was considered to be the only human pathogen of the genus erythrovirus. However, other non-B19 virus strains, such as V9, have now been isolated and are thought to cause infections clinically and serologically indistinguishable from those caused by B19 virus. Whereas B19 virus related isolates have a low genetic diversity of only 1-2%, nucleotide disparity of up to 12% was found for the new isolates, suggesting that non-B19 virus isolates may not be detectable using B19 virus specific PCR methods. To overcome this problem, we designed consensus primers and probes to enable the simultaneous detection of both B19 and non-B19 virus and subsequent discrimination of the two lineages by melting temperature (T(m)) analysis. A total of 196 clinical specimens, from 185 patients with a history of or an anamnesis resembling B19 virus infection, were analyzed using the consensus PCR test. Erythrovirus DNA was detected in 37 of these samples and was found to be B19 virus specific in each case, confirming previous results using B19 virus specific PCR. Although no non-B19 virus DNA was detected in any of the clinical samples tested in this study, more extensive studies are warranted. The routine use of erythrovirus consensus PCR in the diagnosis of B19 virus infection should provide valuable information on the epidemiology and clinical role of non-B19 virus isolates; its use in screening would increase the safety of blood products.

B19 virus genome diversity: epidemiological and clinical correlations

Genetic analysis of parvovirus B19 has been carried out mainly to establish a framework to track molecular epidemiology of the virus and to correlate sequence variability with different pathological and clinical manifestations of the virus. A good amount of information regarding B19 virus sequence variability is available, and presently there are about 400 sequence records deposited in the nucleotide database of NCBI. A few are almost complete genomic sequences, and these allow the construction of a global alignment framework. Many others are partial genomic sequences, limited to selected regions, and these allow comparison of a higher number of isolates from well-defined epidemiological settings and/or pathological conditions. Most studies showed that the genetic variability of B19 virus is low, that molecular epidemiology is possible only on a limited geographical and temporal setting, and that no clear correlations are present between genome sequence and distinctive pathological and clinical manifestations. More recently, several viral isolates have been identified that show remarkable sequence diversity with respect to reference sequences. The identification of variant isolates added to the knowledge of genetic diversity in this virus group and allowed the identification of three divergent genetic clusters, about 10% divergent from each other and still quite distinct from other parvoviruses, that can be thought of as different genotypes within the human erythrovirus group and that show clearly resolved phylogenetic relationship. These variant isolates pose interesting questions regarding the real extent of genetic variability in the human erythroviruses, the relevance of these viruses in terms of epidemiology and their possible implication in the pathogenesis of erythrovirus-related diseases.

A new parvovirus genotype persistent in human skin

Parvovirus B19 is the exclusive human pathogen of the Erythrovirus genus. In classical view, the B19 DNA sequence shows little variability, with no disease-specific or tissue type specific associations. We examined skin biopsies from patients with B19-unrelated skin disease or from constitutionally healthy adults by polymerase chain reaction assays for four different genomic regions of the B19 virus. Sequencing showed that the skin-derived viral DNA differed within the protein-coding region from the B19 reference sequences by 10.8% and from the V9 variant by 8.6% and within the noncoding region (covering nucleotides 189-435 of the promoter region) by 26.5 and 17.2%, respectively. Despite this sequence difference, the promoter region was shown by a luciferase gene expression assay to be biologically active. We have detected a new B19 virus genotype, K71, which differs extensively from the known B19-virus genotypes and is persistently carried in human skin.

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.

Genetic diversity within human erythroviruses: identification of three genotypes

B19 virus is a human virus belonging to the genus Erythrovirus: The genetic diversity among B19 virus isolates has been reported to be very low, with less than 2% nucleotide divergence in the whole genome sequence. We have previously reported the isolation of a human erythrovirus isolate, termed V9, whose sequence was markedly distinct (>11% nucleotide divergence) from that of B19 virus. To date, the V9 isolate remains the unique representative of a new variant in the genus Erythrovirus, and its taxonomic position is unclear. We report here the isolation of 11 V9-related viruses. A prospective study conducted in France between 1999 and 2001 indicates that V9-related viruses actually circulate at a significant frequency (11.4%) along with B19 viruses. Analysis of the nearly full-length genome sequence of one V9-related isolate (D91.1) indicates that the D91.1 sequence clusters together with but is notably distant from the V9 sequence (5.3% divergence) and is distantly related to B19 virus sequences (13.8 to 14.2% divergence). Additional phylogenetic analysis of partial sequences from the V9-related isolates combined with erythrovirus sequences available in GenBank indicates that the erythrovirus group is more diverse than thought previously and can be divided into three well-individualized genotypes, with B19 viruses corresponding to genotype 1 and V9-related viruses being distributed into genotypes 2 and 3.

Nucleic acid testing for emerging viral infections

The development of new technologies leads to the discovery of new viruses. For each of these new infectious agents, relevance to transfusion, including transmissibility by transfusion, pathogenicity, prevalence in blood donors, persistence and the availability of screening assays needs to be assessed. Since 1995, one virus and a new family of viruses have been identified. GB virus-C/hepatitis G virus (GBV-C/HGV), a flavi virus with some homology with and epidemiological features of HCV, is not related to post-transfusion hepatitis but seems to positively interfere with human immunodeficiency virus replication. Human circoviruses include TT virus (TTV) and SEN-V. Both are highly variable, constituting a large family of distantly related viruses. They appear ubiquitous, infecting humans very early in life and are largely persistent. No clinical symptoms or pathogenicity is associated with TTV, but SEN-V might be associated with some non-A-E post-transfusion hepatitis. Parvovirus B19 has been known for many years, but its transmission to recipients of plasma derivatives despite viral inactivation raised the issue of screening plasma pools by nucleic acid testing. Most fractionators quantify B19 DNA in plasma pools to ensure a viral load of <10(4) IU mL-1.

Baculovirus expression of erythrovirus V9 capsids and screening by ELISA: serologic cross-reactivity with erythrovirus B19

Diagnosis of erythrovirus B19 (B19) relies on serology and the detection of viral DNA. Recently, a distinct erythrovirus isolate termed V9, markedly different from erythrovirus B19 (> 11% nucleotide disparity), was isolated. Standard B19 PCR assays were inconclusive and serologic tests failed to categorize V9 as an acute B19-like infection. Sequencing, combined with PCR studies, have since demonstrated the need for specific and differentiated techniques when examining samples for possible B19 or V9 viremia. The antigenic properties of the V9 capsid proteins have not been characterized previously. To address this question, V9 VP1 and VP2 open reading frames were cloned and expressed in insect cells using a baculovirus vector. Large quantities of purified recombinant V9 capsid protein were produced and electron micrographs revealed self-assembly of V9 VP1/VP2 and VP2 capsids into empty icosahedral erythrovirus-like particles with a diameter of approximately 23 nm. Screening of a panel of 270 clinical samples for the presence of V9 IgM and IgG antibodies in ELISA showed 100% serologic cross-reactivity between B19 and V9 when comparing V9 VP2 capsids to a commercial B19 VP2 assay. This suggests that both a V9 and a B19 antibody response may be diagnosed equally well by ELISA using either V9 or B19 recombinant capsids as antigen source. Retrospectively, translation of the V9 sequence indicates that despite a significant genetic variation on the DNA level, the majority of the discrepant DNA sequence represents silent mutations leading to an amino acid sequence very similar to the known B19 strains (96-97% homology).

Parvovirus B19 and parvovirus V9 are not associated with Henoch-Schönlein purpura in children

BACKGROUND

Based on single case reports, parvovirus B19 (B19) has repeatedly been proposed as an etiologic agent in patients with Henoch-Schönlein purpura (HSP), perhaps causing vasculitis by direct invasion of vascular endothelial cells because of the tissue distribution of the cellular B19 receptor. A cohort of children with HSP and other vasculitic diseases was investigated and compared with healthy control children to assess the role of B19 as well as parvovirus V9 (a putative emerging B19-like virus).

PATIENTS AND METHODS

Serum samples from 36 children with HSP (n = 29) or other vasculitic diseases (n = 7) were examined, and 38 healthy bone marrow donors were used as controls. The presence of specific B19 and V9 IgM and IgG antibodies was determined with a recently developed enzyme-linked immunosorbent assay, and viral DNA was detected by a novel nested PCR.

RESULTS

Specific IgM was not present in any of the patient or control serum samples. B19 DNA was detected in one patient, a previously healthy 8-year-old boy diagnosed with HSP, whereas none of the controls was B19-positive. V9 was not detected in any of the clinical or control samples. It seems likely that B19 infection might have triggered the development of HSP in the B19-positive patient, because B19 viremia is otherwise uncommon.

CONCLUSIONS

Although causality is difficult to construe in single cases, the data indicate that B19 is not a common contributing factor in the pathogenesis of vasculitis and that this pathogen is only rarely associated temporally with HSP or vasculitic diseases in children.

Novel PCR assay for differential detection and screening of erythrovirus B19 and erythrovirus V9

Diagnosis of erythrovirus B19 relies on serology and the detection of viral DNA. These techniques were believed to detect all field isolates because erythrovirus B19 has been known to undergo little genetic variation (1-2%). Recently, a distinct erythrovirus isolate termed V9, markedly different from erythrovirus B19 (>11% nucleotide disparity), was isolated from a child in France suffering from transient aplastic anemia. Standard PCR assays and serological tests failed to demonstrate an acute erythrovirus B19 infection. Subsequent sequencing of the erythrovirus V9 genome shows that the nucleotide discrepancies encompass the entire genome, indicating that standard erythrovirus B19 PCR assays will not reliably detect erythrovirus V9 DNA. As a tool for studying the epidemiological role and medical importance of this erythrovirus variant, a PCR assay is described that allows simultaneous detection of, and distinction between, erythrovirus B19 and the V9 isolate. Examination of 100 erythrovirus B19 IgM positive samples as well as plasma pools representing 100,000 Danish blood donor units for the presence of B19 and V9 DNA was performed. Despite the apparent absence of erythrovirus V9 in the clinical samples at present, the DNA sequence variability demonstrates that the erythrovirus group may be more divergent than thought previously and the child harboring this isolate may herald erythrovirus V9 as a possible emerging virus.

Parvovirus B19-related anemia in HIV-infected patients

Persistent infection with parvovirus B19 (B19) is an important treatable cause of anemia in HIV-infected patients. B19 has a tropism for erythroid progenitors and causes pure red cell aplasia (PRCA). The failure to produce neutralizing antibodies to the virus following B19 infection in immunodeficient persons may result in persistent viremia and chronic PRCA (B19-PRCA). The seroprevalence rates for B19 in unselected persons with HIV infection are high, similar to those seen in the general population. Reports of B19-related anemia in HIV infected patients, however, are infrequent. A partial explanation may be that B19-PRCA is predominantly a complication associated with advanced immunodeficiency. The condition is probably underdiagnosed as well. The finding of an unexplained normocytic anemia with absent reticulocytes, in an afebrile HIV-infected patient without renal dysfunction suggests a diagnosis of B19-PRCA. The diagnosis is established when the following criteria are met: (1) bone marrow biopsy showing PRCA, (2) serum or bone marrow positivity for B19 DNA by PCR or dot-blot hybridization, and (C) no alternate explanation for the PRCA. Serological methods are unreliable for the diagnosis because these patients often lack IgM and IgG antibodies to B19. Nearly all patients with B19-PRCA respond to treatment with intravenous immunoglobulin (IVIg) with a rise in the hemoglobin to levels appropriate for the clinical condition of the patient. An alternative explanation for the anemia must be sought in patients not responding to IVIg. Most patients with CD4+ T-lymphocyte counts of < or = 100 cells/mm3 relapse to anemia, usually within 6 months of IVIg therapy. Such patients must be retreated with IVIg 2 g/kg given over 2 to 5 days. The routine use of maintenance IVIg 0.4 g/kg q 4wk may be considered in these patients to prevent relapse.

Novel human erythrovirus associated with transient aplastic anemia

Erythrovirus (formerly parvovirus) B19 causes a wide range of diseases in humans, including anemia due to aplastic crisis. Diagnosis of B19 infection relies on serology and the detection of viral DNA by PCR. These techniques are usually thought to detect all erythrovirus field isolates, since the B19 genome is known to undergo few genetic variations. We have detected an erythrovirus (V9) markedly different from B19 in the serum and bone marrow of a child with transient aplastic anemia. The B19 PCR assay yielded a product that hybridized only very weakly to the B19-specific probe and whose sequence diverged more from those of 24 B19 viruses (11 to 14%) than the divergence found within the B19 group (</=6.65%). Restriction enzyme analysis of the V9 genome revealed that this genetic divergence extended beyond the amplified region. Interestingly, serological tests failed to demonstrate a response characteristic of acute B19 infection. V9 could be a new erythrovirus, and new diagnostic tests are needed for its detection.