Gut 2010;59:867-868 doi:10.1136/gut.2010.210054
Commentary
Tomasz I Michalak
Author Affiliations
Molecular Virology and Hepatology Research Group, Faculty of Medicine, Health Sciences Center, Memorial University, St. John's, Newfoundland, Canada
Correspondence to
Professor Tomasz I Michalak, Molecular Virology and Hepatology Research Group, Faculty of Medicine, Health Sciences Center, 300 Prince Philip Drive, Memorial University, St. John's, NL A1B 3V6, Canada; timich@mun.ca
Infection with hepatitis C virus (HCV) is a global health problem with more than 80% of those infected developing chronic hepatitis C (CHC) that tends to advance to liver cirrhosis and hepatocellular carcinoma. It is estimated that up to three-quarters of HCV-infected individuals are neither diagnosed nor aware that they are infected, and symptomatic liver damage may not appear for years or decades in many patients following virus exposure. The pathogenic and epidemiological significance of HCV could be further magnified by the existence of occult HCV infection (OCI), evidenced by small quantities of HCV RNA in plasma (usually below 100 virus genome copies/ml), liver and/or peripheral blood mononuclear cells (PBMCs), continuing in the context of essentially normal liver function tests after self-limited or therapeutically induced resolution of hepatitis C and OCI in which carriage of HCV RNA traces coincide with moderately elevated liver enzymes of unclear aetiology.1 Although HCV is conventionally known to target hepatocytes, a large and increasing body of experimental and clinical data provides cumulative, hence not yet commonly acknowledged evidence that HCV also propagates at extrahepatic locations, particularly in cells of the immune system.2 3
HCV is a positive single-stranded RNA virus of the Flaviviridae family that demonstrates remarkable genetic variability and typically exists in an infected host as a heterologous population of closely related quasispecies resulting from virus rapid replication driven by an error-prone polymerase. The virus propagates by making a replicative intermediate, designated as the negative or the anti-genomic strand, which detection is a marker of actually progressing virus replication. The 5′-untranslated region (5′-UTR) of virus genome contains an internal ribosome entry site (IRES) essential for viral RNA translation. This sequence is highly conserved among different HCV genomes and arise of variants within this region is an indicator of usually sustained virus change. In this context, HCV derived from extrahepatic locations tend to display variations in the IRES sequence when compared to the genomes from plasma and liver. Some of these substitutions are located at particular nucleotide positions and coincide with modified IRES transcriptional efficiency, suggesting that they may reflect virus adaptation to propagate in a nonhepatic environment.4 In general, compartmentalisation of HCV variants in immune cells is considered as an indicator of hepatocyte-independent virus replication.5
The evidence of HCV replication were found in circulating immune cells, including total PMBC and their B cell, T cell and monocyte/macrophage subsets, dendritic cells, and in lymph nodes in patients with CHC or persistent asymptomatic infection. HCV replication in the immune cells was ascertained by molecular and immunocytochemical analyses, including detection of HCV RNA negative (replicative) strand, viral proteins, and HCV variants distinct from those occurring in patients' plasma and livers. Also, conditions upregulating HCV replication in immune cells and consequently facilitating more readily virus detection in cases with low virus load were established by ex vivo cell stimulation with mitogens differentially activating immune cell subtypes.
Support for the lymphotropic nature of HCV also stemmed from studies applying primary as well as transformed or immortalised lymphoid cell cultures exposed to wild-type virus or cells derived from HCV-infected patients adapted to survive in culture. Along this line, HCV propagation has been demonstrated in Epstein–Barr virus-transformed B cells isolated from PBMC of patients with CHC,6 in vitro infected human T cell lines,7 and in macrophages.8 In regard to infection of primary cells, the ability of human T lymphocytes to support replication of wild-type HCV leading to a release of infectious virus particles with physical properties distinct from those of plasma virions was demonstrated. This T cell infection was associated with emergence of HCV IRES variants, of which some carried mutations identical to those found in immune cells of HCV-infected patients. The infection was prevented following virus neutralisation with antibodies against HCV envelope E2 protein, blocking with antibodies to CD81 (a postulated HCV receptor molecule), and treatment of T cells with interferon α.9 The susceptibility of primary B cells and monocytes to de novo HCV infection was also examined and their ability to support replication, although at low efficiency, has been shown by different groups. Overall, evidence of active propagation of HCV has been uncovered in the main immune cell subtypes independent of whether infection is symptomatic or clinically silent, as well as in in vitro infection experiments with wild-type virus. Due to generally low virus load per cell and relatively small numbers of circulating or in vitro immune cells infected, investigations on HCV lymphotropism remain challenging, require sensitive detection techniques and meticulous approaches.
The propensity of HCV to infect cells of the immune system is consistent with observations of a significantly greater prevalence of HCV infection in lymphoproliferative disorders, such as mixed cryoglobulinaemia and non-Hodgkin's B cell lymphoma. However, a direct pathogenic role of HCV in the development of these diseases is not yet established.
In this issue of Gut, Durand and colleagues10 provide valuable insights into the sequence characteristics and the cell-specific replication potency of HCV variants residing in B cells in patients with a history of CHC and high plasma loads of HCV RNA (see page 934). An interesting and consistent feature of all six patients selected for the study was occurrence in affinity-purified B cells of HCV IRES variants which were not encountered in the genomes in the patients' plasma. Considering this attribute, the authors referred to this infection as occult B cell infection. In the B cell isolates from two cases, HCV RNA negative strand was detected independently confirming progressing replication. By testing translational activity of the plasmid constructs carrying B cell-specific IRES sequences and those from genomes occurring in plasma, the authors demonstrated a poor translational efficacy of the B cell specific IRES-es in liver-derived hepatoma Huh7 cells and primary human hepatocytes but not in B cell lines, such as Raji and Daudi. They also observed a comparable translational efficiency of IRES variants found in different patients in the same compartment (B cells vs. plasma) and a greater translational independence of the B cell specific IRES-es from a potent trans-acting factor (lupus antigen) than those from plasma genomes. Taken together, the data suggest that HCV variants residing in B cells differ in their translational capacity from those occurring in plasma, which should predominantly originate from infected livers, and that they are better adapted to propagate in B cells than hepatocytes. Despite limitations posed by the translation approach used, since only the IRES sequences alone out of the context of the complete virus genome were investigated, the study provides further support for the existence of extrahepatic HCV replication. Propagation of HCV in the immune system may have implications both for spreading virus variants potentially escaping immune and antiviral agent elimination and by modifying immune cell functions in ways promoting virus persistence.
Footnotes
Linked articles 192088.
Competing interests None.
Provenance and peer review Commissioned; not externally peer reviewed.
References
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http://gut.bmj.com/content/59/7/867.full
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