Influence of RNA titre and amino acid changes in the NS5A region of GB virus c/hepatitis G virus on the effectiveness of interferon therapy

Human Pegivirus Type 1: A Common Human Virus That Is Beneficial in Immune-Mediated Disease?

Two groups identified a novel human flavivirus in the mid-1990s. One group named the virus hepatitis G virus (HGV) and the other named it GB Virus type C (GBV-C). Sequence analyses found these two isolates to be the same virus, and subsequent studies found that the virus does not cause hepatitis despite sharing genome organization with hepatitis C virus. Although HGV/GBV-C infection is common and may cause persistent infection in humans, the virus does not appear to directly cause any other known disease state. Thus, the virus was renamed “human pegivirus 1” (HPgV-1) for “persistent G” virus. HPgV-1 is found primarily in lymphocytes and not hepatocytes, and several studies found HPgV-1 infection associated with prolonged survival in people living with HIV. Co-infection of human lymphocytes with HPgV-1 and HIV inhibits HIV replication. Although three viral proteins directly inhibit HIV replication in vitro, the major effects of HPgV-1 leading to reduced HIV-related mortality appear to result from a global reduction in immune activation. HPgV-1 specifically interferes with T cell receptor signaling (TCR) by reducing proximal activation of the lymphocyte specific Src kinase LCK. Although TCR signaling is reduced, T cell activation is not abolished and with sufficient stimulus, T cell functions are enabled. Consequently, HPgV-1 is not associated with immune suppression. The HPgV-1 immunomodulatory effects are associated with beneficial outcomes in other diseases including Ebola virus infection and possibly graft-versus-host-disease following stem cell transplantation. Better understanding of HPgV-1 immune escape and mechanisms of inflammation may identify novel therapies for immune-based diseases.

Human pegivirus 1 infection in lung transplant recipients: Prevalence, clinical relevance and kinetics of viral replication under immunosuppressive therapy

BACKGROUND

Human pegivirus 1 (HPgV1) may cause persistent infections in immunocompetent and immunosuppressed individuals. Its clinical relevance, however, has not been determined. Previous studies have described a higher prevalence of HPgV1 infection in organ transplant recipients compared to healthy controls, but its occurrence in lung transplant recipients (LTRs) and its association with immunosuppressive therapy has not been assessed.

OBJECTIVES

The aim of this study was to evaluate the prevalence and clinical significance of HPgV1 infection in LTRs, and to compare HPgV1 loads and kinetics to Torque Teno Virus (TTV) kinetics, which reflects the level of immunosuppression.

STUDY DESIGN

From each of 110 LTRs, five consecutive plasma samples were collected within the first year after transplantation and tested for HPgV1 RNA and TTV DNA loads by quantitative PCR. Data were related to demographic data and clinical parameters followed up for 3 years post transplantation.

RESULTS

HPgV1 prevalence in LTRs was 18,2%. HPgV1 detection was significantly associated with younger age, but not with graft rejections or other microbial infections. The viral replication level remained unaffected by immunosuppressive therapy. This was in contrast to TTV loads which increased after initiation of immunosuppressive therapy, independent of the patients' HPgV1 infection status.

CONCLUSIONS

In contrast to TTV, HPgV1 kinetics do not reflect the level of immunosuppression after lung transplantation, and there is no correlation between the replication of both persistent viruses in the post transplantation follow up. Thus the individual virus host interactions seem to differ substantially and require further investigation.

Hepatitis G virus

A number of new hepatitis viruses (G, TT, SEN) were discovered late in the past century. We review the data available in the literature and our own findings suggesting that the new hepatitis G virus (HGV), disclosed in the late 1990s, has been rather well studied. Analysis of many studies dealing with HGV mainly suggests the lymphotropicity of this virus. HGV or GBV-C has been ascertained to influence course and prognosis in the HIV-infected patient. Until now, the frequent presence of GBV-C in coinfections, hematological diseases, and biliary pathology gives no grounds to determine it as an “accidental tourist” that is of no significance. The similarity in properties of GBV-C and hepatitis C virus (HCV) offers the possibility of using HGV, and its induced experimental infection, as a model to study hepatitis C and to develop a hepatitis C vaccine.

Enhanced and resumed GB virus C replication in HIV-1-infected individuals receiving HAART

In patients co-infected with HIV-1 and GB virus C (GBV-C), the disappearance of GBV-C RNA has been associated with accelerated disease progression. We studied longitudinal GBV-C viral titres in 28 HIV-1/GVB-C co-infected individuals receiving HAART. During HAART, median GBV-C RNA titres increased from 95 to 6000 copies/ml (P < 0.001). In patients interrupting HAART, GBV-C-RNA titres decreased as HIV replication resumed. These findings further support the theory that GBV-C replication could depend on the dynamics of HIV progression.

GB virus type C NS5A sequence polymorphisms: association with interferon susceptibility and inhibition of PKR-mediated eIF2alpha phosphorylation

GB virus type C (GBV-C) causes persistent infection in humans, although the mechanism by which the virus avoids clearance by the host is unknown. To determine if amino acid polymorphisms in the GB virus type C (GBV-C) NS5A and E2 proteins alter response to interferon (IFN) therapy, we studied the sequence of GBVC NS5A and E2 obtained from people receiving IFN therapy. In addition, we expressed recombinant GBVC NS5A protein to determine if it interferes with RNA-activated protein kinase (PKR) function in vitro. GBVC NS5A amplified from a person whose virus was cleared by IFN therapy (IFN sensitive) demonstrated unique amino acid changes occurring in the region that aligns with the hepatitis C virus (HCV) IFN sensitivity-determining region (ISDR) compared with NS5A sequences from individuals who did not clear GBV-C (IFN resistant). There were no differences in the E2 sequences obtained from IFN-sensitive and IFN-resistant isolates. Using a yeast genetic system, IFN-resistant NS5A inhibited PKR-mediated phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha) in yeast, whereas IFN-sensitive NS5A did not inhibit PKR function. GBV-C NS5A amino acid polymorphisms appear to be involved in response to IFN therapy, and IFN-resistant GBV-C NS5A inhibited PKR-mediated eIF2alpha phosphorylation in a yeast genetic system, suggesting a mechanism by which GBV-C may evade clearance by naturally occurring host antiviral responses.

Clinical isolates of GB virus type C vary in their ability to persist and replicate in peripheral blood mononuclear cell cultures

GB virus C/hepatitis G virus (GBV-C) replication in vitro is inefficient and inconsistent. In this study, clinical isolates of GBV-C were evaluated using peripheral blood mononuclear cell (PBMC) based culture methods. Isolates varied consistently in their ability to persistently replicate, and yield increased in cells grown without PHA/IL-2 stimulation. The deduced polyprotein sequence of an isolate that replicated well was determined (GenBank AY196904) and compared to 20 full-length GBV-C sequences. Fourteen of the 16 unique amino acid polymorphisms identified were in the coding regions for nonstructural proteins associated with interferon resistance and RNA replication. These data indicate that clinical GBV-C isolates vary in their ability to persist in culture, do not require PHA/IL-2 stimulation, and that sequence variability in key regulatory regions may affect growth in PBMC cultures. Since GBV-C appears to inhibit HIV replication in a coinfection model, these studies should facilitate determination of the mechanism of this interaction.

Real-time PCR in virology

The use of the polymerase chain reaction (PCR) in molecular diagnostics has increased to the point where it is now accepted as the gold standard for detecting nucleic acids from a number of origins and it has become an essential tool in the research laboratory. Real-time PCR has engendered wider acceptance of the PCR due to its improved rapidity, sensitivity, reproducibility and the reduced risk of carry-over contamination. There are currently five main chemistries used for the detection of PCR product during real-time PCR. These are the DNA binding fluorophores, the 5' endonuclease, adjacent linear and hairpin oligoprobes and the self-fluorescing amplicons, which are described in detail. We also discuss factors that have restricted the development of multiplex real-time PCR as well as the role of real-time PCR in quantitating nucleic acids. Both amplification hardware and the fluorogenic detection chemistries have evolved rapidly as the understanding of real-time PCR has developed and this review aims to update the scientist on the current state of the art. We describe the background, advantages and limitations of real-time PCR and we review the literature as it applies to virus detection in the routine and research laboratory in order to focus on one of the many areas in which the application of real-time PCR has provided significant methodological benefits and improved patient outcomes. However, the technology discussed has been applied to other areas of microbiology as well as studies of gene expression and genetic disease.