Early intrahepatic CD8 responses in HBV pathogenesis: A new piece of the puzzle
Article Outline
It is now generally accepted that HBV is not directly cytopathic and liver damage is immune mediated. Several steps in research have been crucial in leading to the present understanding of the immune mechanisms involved in HBV clearance. Almost twenty-five years ago the first experiments addressing the role of CTL in the immunopathogenesis of HBV-induced liver injury showed that core-specific T cells could be cytotoxic for autologous liver cells derived from patients with chronic HBV infection [1]. Ten years later, experiments addressing the role of intrahepatic T cells in the immunopathogenesis of acute viral hepatitis demonstrated that in patients with acute hepatitis A, liver derived CD8+ T cell clones could kill HAV-infected target cells [2]. At that time this study raised several ethical concerns, but demonstrated that HAV liver cell damage was an immune mediated event. Five years later by in vitro T cell stimulation with synthetic peptides mimicking the natural endogenous processing of antigens, HBV-specific cytotoxic T cells (CTL) could be readily detected in the peripheral blood of patients with acute HBV infection. Even if at that time a precise kinetics of HBV-specific CD8 cells frequency in association with liver cell damage was not available, a strict association between cytotoxic T cells, liver cells damage and HBV clearance was defined [3], [4]. This was further confirmed by the advent of the HLA/peptide tetramers, which allowed us to more precisely enumerate peripheral blood CD8 cells and to more deeply analyze their function. However several aspects of HBV immunopathogenesis still needed to be uncovered. Even if in vitro cytotoxicity experiments could demonstrate CTL killing of target cells pulsed with synthetic peptides or target cells expressing endogenously synthesized HBV antigens a definite causative effect in HBV pathogenesis was not demonstrated; moreover, if CTL were responsible for viral clearance by elimination of infected hepatocytes this was difficult to reconcile with the evidence that virtually all liver cells are infected during the early stages of infection. Massive liver necrosis and failure would have been the rule rather than the exception of an acute HBV infection.
A great contribution to the further development of the research in the field came from the transgenic mouse and the chimpanzee animal models. In the transgenic mouse a causative effect of CTL in hepatocellular injury was demonstrated by adoptive transfer of HBV-specific CD8 cells that could induce different degrees of hepatocellular damage up to a model of fulminant hepatitis [5], [6]. However the most important achievement was the understanding that CTL could not only kill their targets but also even with greater efficiency clear HBV by non-cytolytic mechanisms. This led to reinterpretation of the nature of the cell-mediated immune events occurring during acute HBV infection and introduced a new paradigm, the concept that HBV-specific CTL can eliminate this virus from infected cells without killing them. Not only the adaptive CTL response can clear HBV by non-cytopathic mechanisms but also the innate immune response by activation of type I interferons and by cytokines secreted by resident intrahepatic NK and NKT cells can substantially reduce viral load in the early phases of infection when liver enzymes are still in the normal range. This was demonstrated in the chimpanzee model of infection; in fact the peak of liver HBV–DNA and the initial reduction of viral load are detectable earlier than liver T cell infiltration [7]. The extent to which cytopathic and non-cytopathic mechanisms play a role in HBV clearance is in favor of the non-cytopathic way hat the innate and adaptive immune response employ against HBV. The non-cytopathic mechanisms in fact seem to be more efficient in limiting both virus spread and replication and in reducing the number of target cells and therefore tissue damage of the host [8]. In order to terminate the infection, as in self-limiting acute hepatitis, CTL have also to do the dirty job killing target hepatocytes still harboring HBV antigens. Finally, similar to a real war when control networks have to be organized after the victory for future maintenance of power, as HBV may persist in its “occult” state, antigen-specific T cells will be active years after infection [9], [10].
In this issue of the Journal, Sprengers et al. analyze for the first time HBV-specific cytotoxic T-cells during and after acute HBV infection in the liver compartment in humans [11]. This had not been previously attempted because of ethical and safety constraints that do not allow to perform a needle biopsy in this stage of HBV infection. The authors, however, demonstrated that a reliable cytometric analysis of the intrahepatic T cell infiltrate can be safely achieved by a minimal invasive technique, as a fine needle aspiration biopsy [12]. By this approach intrahepatic T cells were derived and studied in five patients with acute HBV infection at the clinical onset of disease, at the time of HBsAg seroconversion and 90 days post-HBsAg seroconversion.
By advanced immunological tools that allow ex vivo analysis of antigen-specific CD8 cells, previous studies in patients with acute hepatitis were able to describe the kinetics of circulating HBV-specific CD8 cells in terms of frequency and function [13]; however virus-specific T cell response at the site of virus replication and antigen expression had never been studied before. Sprengers et al. show intrahepatic compartmentalization of HBV-specific CD8 cells at all time points of the study with a progressive decline of the HLA-DR activation marker on intrahepatic HBV-specific CD8 cells, suggesting either that CD8 cells infiltrate the liver parenchyma and subsequently modify their functional state from effector to memory T cells or that liver CD8 cells recruitment continues also after clinical resolution of acute hepatitis when the contraction phase of the expanded CTL clones occurs and memory CD8 cells emerge. In several experimental models CD8 cells accumulate and go on to apoptosis into the liver [14], [15]; this, however, does not exclude that less activated CD8 cells persist where minimal amounts of viral antigen are produced and contribute to keep the virus under check. Even if, based on the results of this study, it is not possible to understand the timing of cytolytic and non-cytolytic mechanisms carried on by the intrahepatic CD8 cells, we can speculate that HLA-DR positive CD8 cells exerted their anti-HBV activity both by cytotoxic and non-cytotoxic mechanisms, while CD8 cells persisting late after seroconversion would operate by prevalent non-cytotoxic mechanisms even though local expansion of activated CD8 clones with minimal cytotoxic effect cannot be excluded. This could resemble what has been shown in asymptomatic HBV carriers that present a rich intrahepatic HBV-specific CTL infiltrate [16], with no enzyme elevation and low stable levels of HBV–DNA.
In summary, the study of Sprengers et al. provides an additional novel piece of information on HBV immunopathogenesis. New further questions arise from our desire of knowledge: which are the phenotypic and functional characteristics of the intrahepatic HBV-specific T cell infiltrate years after self-limited hepatitis and do these cells maintain a cytotoxic potential? Answers to these questions will contribute to understand the immunological mechanisms of viral persistence and how to manipulate the cellular immune response for the development of a successful therapeutic vaccine.
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PII: S0168-8278(06)00277-7
doi:10.1016/j.jhep.2006.05.004
© 2006 European Association for the Study of the Liver. Published by Elsevier Inc. All rights reserved.
