Protection of hepatocytes from cytotoxic T cell mediated killing by interferon-alpha.

BACKGROUND: Cellular immunity plays a key role in determining the outcome of hepatitis C virus (HCV) infection, although the majority of infections become persistent. The mechanisms behind persistence are still not clear; however, the primary site of infection, the liver, may be critical. We investigated the ability of CD8+ T-cells (CTL) to recognise and kill hepatocytes under cytokine stimulation. METHODS/PRINCIPLE FINDINGS: Resting hepatocytes cell lines expressed low levels of MHC Class I, but remained susceptible to CTL cytotoxicity. IFN-alpha treatment, in vitro, markedly increased hepatocyte MHC Class I expression, however, reduced sensitivity to CTL cytotoxicity. IFN-alpha stimulated hepatocyte lines were still able to present antigen and induce IFN-gamma expression in interacting CTL. Resistance to killing was not due to the inhibition of the FASL/FAS- pathway, as stimulated hepatocytes were still susceptible to FAS-mediated apoptosis. In vitro stimulation with IFN-alpha, or the introduction of a subgenomic HCV replicon into the HepG2 line, upregulated the expression of the granzyme-B inhibitor-proteinase inhibitor 9 (PI-9). PI-9 expression was also observed in liver tissue biopsies from patients with chronic HCV infection. CONCLUSION/SIGNIFICANCE: IFN-alpha induces resistance in hepatocytes to perforin/granzyme mediate CTL killing pathways. One possible mechanism could be through the expression of the PI-9. Hindrance of CTL cytotoxicity could contribute to the chronicity of hepatic viral infections.

Quantification and localisation of FOXP3+ T lymphocytes and relation to hepatic inflammation during chronic HCV infection.

BACKGROUND/AIMS: T lymphocyte-mediated immune reactions are closely involved in the pathogenesis of HCV-induced chronic liver disease. Regulatory T cells are able to suppress HCV-specific T lymphocyte proliferation and cytokine secretion during chronic HCV infection. We wished to address to what extent regulatory T cells exist in the livers of HCV+ individuals, and what the role of such cells might be in disease progression. METHODS: We analysed the frequency and distribution of FOXP3+ cells, along with CD4, CD8 and CD20+ cells, in liver biopsies of 28 patients with chronic HCV and 14 patients with PBC, and correlated these data with histological parameters. RESULTS: A striking number of FOXP3+ cells were present in the portal tract infiltrates of HCV-infected livers. FOXP3+ cells were largely CD4+ and there was a remarkably consistent ratio of total CD4+:FOXP3+ cells in liver across a wide range of disease states of around 2:1. This differed substantially from the ratio observed in PBC (10:1, P=0.001). CONCLUSIONS: An unexpectedly high proportion of the cellular infiltrate in persistent HCV infection comprises FOXP3+ cells. The relative proportion of FOXP3+ cells remains similar in both mild and severe fibrosis. These cells are likely to play a critical role in intrahepatic immune regulation, although their overall role in disease progression remains to be determined.

Virus-specific CD8+ T lymphocytes within the normal human liver.

The frequency and phenotype of human antiviral memory CD8(+) T cells in blood are well studied, yet little is known about their distribution within tissues. Analysis of antiviral CD8(+) T cell populations derived from a unique set of normal liver and blood samples identified a consistent population of virus-specific cells within the liver. In comparison to the circulating T cells, the liver-derived T cells were present at frequencies which were variably enriched compared to that in the blood, and showed significant differences with regard to the expression of CD45RA, CD45RO, CD95, CCR7, CD27 and CD28. The differences in these cell surface markers are consistent with a mature 'effector memory' phenotype of antigen-specific CD8(+) T cells within the liver. An enrichment of an activated subset of NKT cells (V alpha 24/V beta 11) was also observed, a finding which may be relevant to the regulation of the antiviral populations.

Ultra-sensitive class I tetramer analysis reveals previously undetectable populations of antiviral CD8+ T cells.

A major breakthrough in cellular immunology has been the development of HLA class I tetramers to analyze CD8(+) T cell responses. However, in many situations, including persistent virus infection, specific T cell responses are rarely detected using this technology. This raises the question of whether such responses are 'deleted' (or 'exhausted') or present below the conventional detection limit for class I tetramer staining. In particular, persistent hepatitis C virus (HCV) infection is characterized by very weak or apparently absent specific CD8(+) T cell responses, even though they are readily detectable in acute disease. Therefore, we assessed the use of anti-PE-labeled magnetic beads to enrich tetramer-positive HCV-specific T cells and identify previously undetectable populations. Using the enrichment technique, HCV-specific T cells could be detected in the majority of infected individuals, whereas these responses were not detected using conventional tetramer staining (8/15 vs. 1/15; p=0.01). Magnetic enrichment could reliably detect very rare HCV-specific responses at frequencies of >0.0011% of CD8(+) T cells (approximately 1/million PBMC), and phenotypic analysis of these rare populations was possible. Therefore, this direct ex vivo technique revealed the persistence of very low frequencies of virus-specific CD8(+) T cells during chronic virus infection and is readily transferable to the study of other viral, self- or tumor-specific T cells.