The role of hepatitis B virus-specific memory CD8 T cells in the control of viral replication

Abstract

Background/Aims: The aim of this study is to clarify the differences of host immune responses between acute self-limited and chronic persistent hepatitis B virus (HBV) infections by quantitative and qualitative analysis of HLA-A*2402-restricted HBV-specific CD8⁺ T cells.

Methods: HBV-specific CD8⁺ T cells in peripheral blood mononuclear cells (PBMCs) from patients infected with HBV were analyzed by flow cytometry using two HLA-A*2402–HBV peptide tetrameric complexes.

Results: High numbers of HBV-specific CD8⁺ T cells were detected in acute phase PBMCs from most individuals with acute HBV infection while the number of these cells was greatly reduced in recovery phase PBMCs. HBV-specific CD8⁺ T cells were not detected in PBMCs from individuals with chronic HBV infection except for one patient during acute exacerbation. HBV-specific CD8⁺ T cells were induced by in vitro peptide stimulation in PBMCs from chronic HBV carriers with a low level of serum HBV-DNA but not from those with a high level of serum HBV-DNA. CD28CD45RA phenotype analysis showed that HBV-specific CD8⁺ T cells in acute phase PBMCs predominantly express a memory T cell phenotype.

Conclusions: HBV-specific memory CD8⁺ T cells may play a crucial role in complete clearance of HBV from patients with acute HBV hepatitis.

Introduction

HBV-specific cytotoxic T lymphocytes (CTLs) are believed to play a major role both in virus clearance and in the pathogenesis of liver cell injury [1], [2]. A vigorous CTL response specific for HBV-encoded proteins was detected in peripheral blood from individuals with acute HBV infection who ultimately cleared the virus [3], [4], [5], [6], [7], [8]. In contrast, only a weak CTL response specific for HBV core antigen was detected in peripheral blood from patients with chronic HBV infection [3], [6], [7]. In support of these observations, a recent study using HLA class I tetrameric complexes detected a high number of HLA-A2-restricted HBV-specific CD8⁺ T cells in peripheral blood from patients with acute HBV infection [9], [10]. In contrast, a very low number of HBV-specific CD8⁺ T cells was detected in peripheral blood from patients with chronic HBV infection [11]. Similar findings have been observed in patients with acute and chronic HCV infection by two other studies using HLA class I tetrameric complexes [12], [13]. Together these observations suggest that high numbers of CTLs specific for hepatitis virus are essential for viral clearance.

Sixteen of the 21 identified HBV-specific CTL epitopes are HLA-A2-restricted [1], [3], [4], [5], [6], [7], [8], [14], [15]. Therefore, most previous studies of HBV-specific CTLs analyzed HLA-A2-restricted CTLs from HLA-A2⁺ HBV-infected patients. We recently identified two HBV-specific CTL epitopes presented by HLA-A*2402, the most common HLA class I allele in East Asia [16]. CTL responses specific for these epitopes (HBV core 117–125: EYLVSFGVW; and pol 756–764: KYTSFPWLL) were frequently found in HLA-A*2402⁺ patients with acute HBV infection [16]. As the amino acid sequences of these epitopes and their flanking regions are highly conserved in HBV isolates, these CTL epitopes are useful for characterization of HBV-specific CTLs in HBV-infected Asian individuals.

Our recent study using an HLA-B*3501-tetrameric complex showed that expanding HCV-specific CTLs in acute HCV infection have a memory T cell phenotype, while those from patients with chronic HCV infection do not predominantly express this phenotype [13]. Unlike acute HCV infection, most adult patients with acute HBV infection are able to clear the virus. It is likely that chronic HBV infection is caused by vertical transmission during labor or horizontal transmission during babyhood or early childhood. The immune responses in HBV infections are clearly different from those in HCV infection.

In the present study, we used HLA-A*2402–HBV peptide tetrameric complexes to quantitatively analyze HBV-specific CD8⁺ T cells in peripheral blood from patients with acute and chronic HBV infection. Furthermore, the characteristics of HBV-specific CD8⁺ T cells were investigated by four-color flow cytometric analysis. This is the first study that analyzed HBV-specific CD8⁺ T cells by using HLA class I tetramer except for HLA-A*0201 and in non-Caucasian.