Parvovirus B19

1.2.1 Epidemiology and natural history

Parvovirus B19 (B19) was occasionally discovered by Cossart et al. during testing for hepatitis B virus surface antigen in 1974 [20]. The infection with this virus is worldwide especially in childhood. The prevalence of anti-B19 IgG antibody is 2-15% in young children (1-5 years), 15-60% in school age children (6-19 years), 30- 60% in adults, and more than 85% in the senior population [21-24]. Pathways of transmission are respiratory route, blood-derived products and vertically from mother to fetus. B19 is a pathogen with a variety of clinical manifestations, ranging from asymptomatic courses in immunocompetent individuals to lethal cytopenias in immunocompromised patients [25]. The most common disease associated with B19 infection is erythema infectiosum, also termed fifth disease or “slapped-cheek” disease.

The duration of viremia has been reported from weeks to months in immunocompetent hosts following the development of virus specific antibodies.

However, Lindblom etal [26] reported that viremia was detectable during the whole follow-up period (128 weeks) in 4 of 5 immunocompetent individuals after acute B19 infection in spite of the resolution of clinical symptoms. Thus, the clearance of B19 viremia may be slower than previously thought. Bone marrow is the primary site of replication [25].

Role of Parvovirus B19 infection in hepatitis C -- Introduction

1.2.2 Virology

B19 is a single-stranded nonenveloped DNA virus, 22-24 nm diameters, containing 5,596 nucleotides (nt), and is member of the family Parvoviridae known to be pathogenic in humans.

The B19 genome has two large open reading frames, encoding for the non-structural protein (NS1) and for non-structural capsid proteins (VP1 and VP2). NS1 manifests site-specific DNA-binding, subserves multiple replicative functions and is cytotoxic to host cells [27, 28]. VP2 is the major structural protein to make up 96% of the total capsid protein [29]. VP1 accounts for the remaining 4% and is identical to VP2 with the addition of 227 amino acids (termed the VP1 unique region, VP1u) at the amino terminus [29, 30]. Importantly, the VP1 protein comprises a viral phospholipase A2 activity [31] which was suggested to be critical for efficient transfer of the viral genome to initiate replication. The Erythrocyte P antigen has been identified as the B19 receptor for entering into cell to initiate infection [32]. In addition, a5b1-integrin and the Ku80 autoantigen have been described as co-receptors [33-35].

Unlike HCV, the nucleotide sequence of B19 is rather conserved, containing only 6% divergence. So far three genotypes: B19-, LaLi-, and V9-related viruses have been identified [36]. However, the variation of B19 sequence has no correlation with specific disease symptoms and persistent infection [25, 37, 38].

1.2.3 Immunology of B19 infection

Humoral immune response against B19 is correlated with the eradication of the virus and has a long lasting protection against re-infection [39]. B19-specific cellular immune responses have been studied only in the recent decade. NS1, VP1 and VP2 can be targets of the host’s cellular immune responses. B19 specific CD8⁺ and CD4⁺ T cell responses can be found in acute infection, persistent infection and remote infection [40-47]. The maintenance of B19-specific T cell responses may indicate persistence of low loads of residual virus as it has been shown in the case of recovery after HBV infection [48]. Importantly, specific T cell epitopes have been characterized in recent years. One group [47] has described two particularly frequently detected epitopes in healthy B19 IgG-positive but IgM/DNA-negative individuals. Although one epitope (LASEESAFYVLEHSSFQLLG) was DRB1*1501 restricted, positive T cell responses were detectable in DRB1*1501 negative

Role of Parvovirus B19 infection in hepatitis C -- Introduction

individuals too. In 6 serologically recovered B19 infected individuals with positive responses against this epitope only 2 were DRB1*15 positive. The lack of an efficient T cell response against B19 may lead to persistent infection [49].

B19-specific T cell responses have not been studied yet in the context of hepatitis virus infections. There is no information on frequency, specificity and strength of B19 specific T cell immunity in hepatitis C versus responses in healthy controls. Thus, one aim of this thesis project was to address this question accordingly.

1.2.4 Therapy

Normally, B19 infection requires no specific antiviral treatment. In immunocompetent individuals, some patients may need symptomatic treatment (NSAIDs)because of B19-induced arthralgia. Erythrocyte transfusion is required in cases of B19-induced transient aplastic crisis [50]. In immunocompromised patients, infusion of immunoglobulin is an effective therapy to against chronic anemia or cytopenia due to persistent B19 infection. Fetal blood transfusions is a curative treatment in severe related hydrops fetalis [25, 51, 52]. In patients with B19-related myocarditis, interferon beta is a kind of successful treatment contributes to virus clearance and protects left ventricular function [53]. Currently, larger trials are exploring the efficacy of interferon beta treatment of B19-related mycoarditits.

1.2.5 B19 infection and liver disease

In the past decade, a growing number of studies have been published to reveal an association between B19 and liver disease especially in the etiology of fulminant liver failure and the persistence of B19 in liver [54-63]. The summary of these studies is shown in Table 1. Recently, an investigation aimed to evaluate the effect of B19-infection on the course of HBV-associated liver disease by Toan and colleague demonstrated that B19-infection was not only frequent in HBV-infected Vietnamese patients but also obviously associated with severe hepatitis B-associated liver disease.

In this study, total of 463 Vietnamese individuals was recruited. Of these, 399 were HBV-infected patients (311 symptomatic HBV-infected patients with well-characterized clinical profiles, 88 asymptomatic chronic HBV carriers with no liver disease) and 64 were healthy individuals. Sera of these individuals were tested for the presence of B19-DNA by nPCR and quantity real-time PCR and DNA-sequencing.

Paralleled liver biochemical and serological testing were also assayed. The prevalence

Role of Parvovirus B19 infection in hepatitis C -- Introduction

of B19 DNA was significantly higher in HBV-infected patients compared to the healthy control group (24.1% vs 4.7%, p<0.001). Moreover, it showed a significantly higher prevalence of B19 DNA in the HCC subgroup compared with no-HCC subgroup (38.6% vs 18.5%, p<0.001). The prevalence of B19 DNA in “severe (LC and HCC)” patients group and “mild (AHB, CHB, and ASYM)” patients group were 29.9% and 18.5%, respectively (p=0.008). By using multivariate analysis they also demonstrated the serum B19 viral load is correlated with the HBV viral load and serum ALT levels. Based on these findings the authors concluded that B19 may be persistent in HBV patients and B19 may play an important role in the pathogenesis of HBV in Vietnamese.