The precise mechanisms responsible for the failure of intrahepatic hepatitis C virus (HCV)-specific CD8 ؉ T cells to control the virus during persistent infection have not been fully defined. We therefore studied the CD8 ؉ T-cell response in 27 HLA-A2-positive patients using four previously well-defined HLA-A2-restricted HCV epitopes. The corresponding HCV sequences were determined in several patients and compared with the intrahepatic HCV-specific CD8 T here is emerging consensus that cellular immune responses play an important role in the immunopathogenesis of hepatitis C virus (HCV) infection. 1,2 Indeed, the appearance of functional HCVspecific T-cell responses is kinetically associated with control of viremia in the acute phase of infection. [3][4][5][6][7] Studies in chimpanzees revealed that virus-specific T cells accumulate in the liver and that this coincides with liver disease and viral clearance. 8,9 The central role of virusspecific CD4 ϩ and CD8 ϩ T cells in HCV clearance has further been recently demonstrated by cell depletion studies in chimpanzees. 10,11 HCV-specific T cells are also detectable in chronic hepatitis, although at a lower frequency compared with acute resolving infection. HCV-specific CD8 ϩ T-cell lines have also been generated from the human liver. [12][13][14][15] In two of these studies, cytotoxic CD8 ϩ T-cell lines could be established from approximately half of the chronically infected livers. 12,15 The long-term survival of HCV-specific CD8 ϩ T cells in the liver has also been demonstrated by major histocompatibility complex (MHC) class I tetramer analysis. While no tetramer-positive cells were found in the liver in two studies, 16,17 other studies have detected tetramer-positive CD8 ϩ T cells in livers at frequencies often exceeding 1% to 2% of intraheatic CD8 ϩ T cells, [18][19][20] indicating that these cells accumulate at the primary site of infection.The mechanisms by which HCV is able to evade the virus-specific T-cell response that is present in the infected liver are still only poorly understood. Several dif-

The differentiation and functional status of virus-specific CD8؉ T cells is significantly influenced by specific and ongoing antigen recognition. Importantly, the expression profiles of the interleukin-7 receptor alpha chain (CD127) and the killer cell lectin-like receptor G1 (KLRG1) have been shown to be differentially influenced by repetitive T-cell receptor interactions. Indeed, antigen-specific CD8

Hepatitis C virus (HCV) causes chronic infection accompanied by a high risk of liver failure and hepatocellular carcinoma. CD8؉ T cell responses are important in the control of viremia. However, the T cell response in chronic infection is weak both in absolute numbers and in the range of epitopes targeted. In order to explore the biology of this response further, we analyzed expression of a panel of natural killer cell markers in HCV compared with other virus-specific T cell populations as defined by major histocompatibility complex class I tetramers. We found that CD161 was significantly expressed on HCV-specific cells (median 16.8%) but not on CD8؉ T cells specific for human immunodeficiency virus (3.3%), cytomegalovirus (3.4%), or influenza (3.4%). Expression was seen in acute, chronic, and resolved disease and was greatest on intrahepatic HCV-specific T cells (median 57.6%; P < 0.05). Expression of CD161 was also found on hepatitis B virus-specific CD8؉ T cells. In general, CD161؉CD8؉ T cells were found to be CCR7؊ "effector memory" T cells that could produce proinflammatory cytokines (interferon-␥ and tumor necrosis factor-␣) but contained scanty amounts of cytolytic molecules (granzyme B and perforin) and proliferated poorly in vitro. Expression of CD161 on CD8؉ T cells was tightly linked to that of CXCR6, a chemokine with a major role in liver homing. Conclusion: We propose that expression of CD161 indicates a unique pattern of T cell differentiation that might help elucidate the mechanisms of HCV immunity and pathogenesis. (HEPATOLOGY 2008;47:396-406.)

Virus-specific CD8؉ T-cells play an important role in the outcome of acute hepatitis C virus (HCV) infection. In the chronic phase, however, HCV can persist despite the presence of virusspecific T-cell responses. Therefore, we set out to perform a full-breadth analysis of the intrahepatic virus-specific CD8؉ T-cell response, its relation to the peripheral T-cell response, and the overall influence of viral escape and the genetic restriction on intrahepatic CD8؉ T-cell failure. Intrahepatic and peripheral CD8؉ T-cells from 20 chronically HCV infected patients (genotype 1) were comprehensively analyzed using overlapping peptides spanning the entire HCV polyprotein in concert with autologous viral sequences that were obtained for all targeted regions. HCV-specific CD8؉ T-cell responses were detectable in most (90%) chronically HCV-infected patients, and two thirds of these responses targeted novel previously undescribed epitopes. Most of the responses were detectable only in the liver but not in the peripheral blood, indicating accumulation and enrichment at the site of disease. Of note, only approximately half of the responses were associated with viral sequence variations supported by functional analysis as viral escape mutations. Escape mutations were more often associated with HLA-B alleles. Conclusion: Our results show an unexpected high frequency of intrahepatic virus-specific CD8؉ T-cells, a large part of which continue to target the present viral antigens. Thus, our results suggest that factors other than mutational escape contribute to the failure of intrahepatic virus-specific CD8؉ T-cells.

Only a very few HCV-specific CD8+ T cells are required to inhibit HCV replication; inhibition occurs primarily by noncytolytic effector functions.