Antienvelope antibodies are protective against GBV-C reinfection: Evidence from the liver transplant model

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.

Clinical and molecular aspects of human pegiviruses in the interaction host and infectious agent

Background

Human pegivirus 1 (HPgV-1) is a Positive-sense single-stranded RNA (+ ssRNA) virus, discovered in 1995 as a Flaviviridae member, and the closest human virus linked to HCV. In comparison to HCV, HPgV-1 seems to be lymphotropic and connected to the viral group that infects T and B lymphocytes. HPgV-1 infection is not persuasively correlated to any known human disease; nevertheless, multiple studies have reported a connection between chronic HPgV-1 infection and improved survival in HPgV-1/HIV co-infected patients with a delayed and favorable impact on HIV infection development. While the process has not been thoroughly clarified, different mechanisms for these observations have been proposed. HPgV-1 is categorized into seven genotypes and various subtypes. Infection with HPgV-1 is relatively common globally. It can be transferred parenterally, sexually, and through vertical ways, and thereby its co-infection with HIV and HCV is common. In most cases, the clearance of HPgV-1 from the body can be achieved by developing E2 antibodies after infection.

Main body

In this review, we thoroughly discuss the current knowledge and recent advances in understanding distinct epidemiological, molecular, and clinical aspects of HPgV-1.

Conclusion

Due to the unique characteristics of the HPgV-1, so advanced research on HPgV-1, particularly in light of HIV co-infection and other diseases, should be conducted to explore the essential mechanisms of HIV clearance and other viruses and thereby suggest novel strategies for viral therapy in the future.

Characterization of a peptide domain within the GB virus C envelope glycoprotein (E2) that inhibits HIV replication

GB virus C (GBV-C) infection is associated with prolonged survival in HIV-infected cohorts, and GBV-C E2 protein inhibits HIV entry when added to CD4+ T cells. To further characterize E2 effects on HIV replication, stably transfected Jurkat cell lines expressing GBV-C E2 or control sequences were infected with HIV and replication was measured. HIV replication (all 6 isolates studied) was inhibited in all cell lines expressing a region of 17 amino acids of GBV-C E2, but not in cell lines expressing E2 without this region. In contrast, mumps and yellow fever virus replication was not inhibited by E2 protein expression. Synthetic GBV-C E2 17mer peptides did not inhibit HIV replication unless they were fused to a tat-protein-transduction-domain (TAT) for cellular uptake. These data identify the region of GBV-C E2 protein involved in HIV inhibition, and suggest that this GBV-C E2 peptide must gain entry into the cell to inhibit HIV.

The natural history of non-human GB virus C in captive chimpanzees

GB virus C (GBV-C) is a common, non-pathogenic human virus that infects lymphocytes. Persistent GBV-C infection of humans with coexistent human immunodeficiency virus (HIV) infection is associated with prolonged survival, and GBV-C replication inhibits HIV replication in vitro. A GBV-C virus variant was identified in chimpanzees in 1998 and was named GBV-C_trog or GBV-C_cpz. The prevalence and natural history of GBV-C in chimpanzees remains uncharacterized. We examined the sera from 235 captive chimpanzees for the presence of GBV-C viraemia, viral persistence and clearance, E2 antibody kinetics and RNA sequence diversity. Sequences from six isolates shared more sequence identity with GBV-C_cpz than with human GBV-C. The prevalence of GBV-C_cpz viraemia and E2 antibody in chimpanzees (2.5 and 11 %, respectively) was similar to human GBV-C prevalence in healthy human blood donors (1.8 and 9 %, respectively). Persistent GBV-C_cpz infection occurred in two of the six viraemic animals and was documented for 19 years in one animal. Host subspecies troglodyte GBV-C isolates and published verus GBV-C isolates shared a high degree of sequence identity, suggesting that GBV-C in chimpanzees should be identified with a chimpanzee designation (GBV-C_cpz). The prevalence and natural history of chimpanzee GBV-C variant (GBV-C_cpz) appears to be similar to human GBV-C infection. The chimpanzee could serve as an animal model to study HIV–GBV-C co-infection.

Characterization of an immunodominant antigenic site on GB virus C glycoprotein E2 that is involved in cell binding

GB virus type C (GBV-C) is a human flavivirus that may cause persistent infection, although most infected individuals clear viremia and develop antibodies to the envelope glycoprotein E2. To study GBV-C E2 antigenicity and cell binding, murine anti-E2 monoclonal antibodies (MAbs) were evaluated to topologically map immunogenic sites on GBV-C E2 and for the ability to detect or block recombinant E2 binding to various cell lines. Five competition groups of MAbs were identified. Groups I and II did not compete with each other. Group III competed with both groups I and II. Group IV did not compete with group I, II, or III. One MAb competed with all of the other MAbs, suggesting that the epitopes bound by these MAbs are intimately related. Individually, none of the MAbs competed extensively with polyclonal human convalescent antibody (PcAb); however, combinations of all five MAb groups completely blocked PcAb binding to E2, suggesting that the epitopes bound by these MAbs form a single, immunodominant antigenic site. Only group I and III MAbs detected purified recombinant E2 bound to cells in binding assays. In contrast, group II MAbs neutralized the binding of E2 to cells. Both PcAb and MAbs were conformation dependent, with the exception of one group II MAb (M6). M6 bound to a five-amino-acid sequence on E2 if the peptide included four C-terminal or eight N-terminal residues, suggesting that the GBV-C E2 protein contains a single immunodominant antigenic site which includes a complex epitope that is involved in specific cellular binding.

Drug interactions between psychoactive substances and antiretroviral therapy in individuals infected with human immunodeficiency and hepatitis viruses

The liver disease characteristic of alcohol dependence encompasses three main related entities: steatosis, alcoholic hepatitis, and cirrhosis. Alcoholic cirrhosis is a leading cause of global morbidity and mortality. Alcohol intake among injecting drug users is a major contributor to transmission of viral infections, such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), and hepatitis C viruses (HCV). HIV and HCV coinfected patients develop liver diseases earlier and more severely than the monoinfected individuals, including hepatocellular carcinoma. Interactions exist between the therapeutic drugs used to minimize and control the drug and alcohol dependence. Furthermore, drug-drug interactions occur between the highly active antiretroviral therapy (HAART) and alcohol, different HAART components and methadone, or each one of the therapies with the other, thus contributing to a higher toxicity level. With the evolution of effective antiretroviral therapy, survival of persons with HIV, and the syndrome it causes, acquired immunodeficiency syndrome (AIDS) has increased dramatically. Drug-drug interactions may appear between alcohol and anti-HBV or anti-HCV, therapy in the presence or absence of anti-HIV therapy. Several other medical-, social-, and drug-related factors of this population have to be considered when providing HAART. Because many coinfected patients also have problems with substance use, dealing with their drug dependence is an important first step in an attempt to improve adherence to and tolerance of antiviral therapy. It is necessary to minimize the risk of liver disease acceleration and/or reinfection with hepatitis viruses. Knowledge of potential drug interactions between methadone, antiretroviral therapy, psychoactive drugs, and antipsychotics and the role of coinfection with HBV or HCV and the drugs used in eradicating viral hepatitis permits suitable antiretroviral combinations.

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.

GB virus C genotype determination in GB virus-C/HIV co-infected individuals

Several recent studies have indicated that patients infected with human immunodeficiency virus (HIV) exhibit a beneficial effect of co-infection with GB virus C (GBV-C). The benefit is demonstrated by slower progression to acquired immunodeficiency syndrome (AIDS) and prolonged survival time after the development of AIDS. In some but not all studies, a significant association between GBV-C/HIV co-infection and increased CD4(+) cell counts has been reported. To understand further the possible role that GBV-C might play in the reduced morbidity and mortality among HIV-infected patients, we sought to examine the presence of different GBV-C genotypes in a cohort of co-infected patients. PCR products derived from the 5'-untranslated region (5'-UTR) and the second envelope gene (E2) were sequenced directly and genotyped by phylogenetic analysis. While 5'-UTR analysis delineated the major type, analysis of the complete E2 gene was required for identification of group 2 subtypes, designated 2a and 2b. Among 35 patients tested, GBV-C genotype was determined for 33: two patients were infected with genotype 1, 12 with type 2a, and 19 with type 2b. Clinical data were available for 25 genotyped patients: one infected with genotype 1, nine with genotype 2a, and 15 with type 2b. CD4 cell counts tended to be lower in patients infected with genotype 2a compared with those with genotype 2b (310 +/- 136 vs 430 +/- 199, P = 0.054). Additional studies with larger cohorts from separate geographical regions are needed to determine whether a particular GBV-C genotype is associated with reduced morbidity or mortality among HIV co-infected patients.

Non-A to E hepatitis

PURPOSE OF THE REVIEW

The list of possible hepatotropic viruses continues to grow with the discovery of the GB virus-C, the TT virus and the SEN virus. There is emerging data on the biology of these newly discovered :In spite of continuing research into the pathogenicity of the GB virus-C and the TT virus, definite evidence linking them to acute or chronic liver disease is lacking. The SEN virus was reported in 2000, and although there seems to be an association between virus and transfusion-related hepatitis, more data are awaited before definite conclusions can be drawn. The effect of GB virus-C, the TT virus and the SEN virus co-infection on other viral and non-viral hepatitides has also been studied in some detail. Again, there is no definite evidence so far that these viruses modify other liver diseases.

SUMMARY

At the present time, diagnostic testing for these viruses does not seem to be warranted outside of clinical studies. The discovery of these viruses, however, paves the way for further research into novel viral agents that infect humans, other among hosts.

GB virus C/hepatitis G virus (GBV-C/HGV): still looking for a disease

GB Virus C and Hepatitis G Virus (GBV-C/HGV) are positive, single-stranded flaviviruses. GBV-C and HGV are independent isolates of the same virus. Transmission via the blood-borne route is the commonest mode, although vertical and sexual transmission is well documented. GBV-C/HGV is distributed globally; its prevalence in the general population is 10 fold higher in African countries than in non-African countries. High prevalences of GBV-C/HGV have been found in subjects with frequent parenteral exposure and in groups at high risk of exposure to blood and blood products. The clinical significance of human infection with GBV-C/HGV is currently unclear. The virus can establish both acute and chronic infection and appears to be sensitive to interferon. Only some 12-15% of chronic Non-A, B, C hepatitis cases are infected with GBV-C/HGV. A direct association with liver pathology is still lacking and it is not yet clear as to whether GBV-C/HGV is indeed a hepatotropic virus. Current evidence suggests that the spectrum of association of GBV-C/HGV infection with extrahepatic diseases ranges from haematalogical diseases, aplastic anaemia, human immunodeficiency virus (HIV)-positive idiopathic thrombocytopenia and thalassemia, through to common variable immune deficiency and cryoglobunemia.

What do we need to know about non-A-to-E viral hepatitis?

The list of potential hepatotrophic viruses continues to grow, with the recent discovery of the GB virus-C, the TT virus, and the SEN virus. Prevalence rates of the GB virus-C have ranged from 1.2% to 13% among healthy blood donors from all over the world. Higher prevalence rates have been reported among intravenous drug users. Similarly, the TT virus has a global distribution. However, in spite of numerous reports of the presence of both of these viruses in various kinds of liver diseases, definite evidence linking it to a specific disease or illness is lacking. The SEN virus is thought to be a novel viral agent that may be linked to cryptogenic chronic hepatitis, but data are awaited.