Hepatocyte turnover in transient and chronic hepadnavirus infections

Hepatocyte turnover appears to be an important feature in the resolution of transient and progression of chronic hepadnavirus infections. Hepatocyte death, initiated through attack by antiviral cytotoxic T-lymphocytes (CTL), and compensatory hepatocyte proliferation, are both believed to be major contributing factors in the loss of virus DNA during immune resolution of transient infections. Noncytopathic curing of hepatocytes is also suggested to occur, though this mechanism does not prevent the death of large numbers of hepatocytes. Hepatocyte death, proliferation and curing are also important features of chronic infections, though the outcomes are different. In particular, immune selection due to persistent attack by antiviral CTL is thought to play a role in the emergence of hepatocytes infected with mutant strains of hepatitis B virus (HBV) (e.g. HBV e antigen-negative strains) and in the emergence of hepatocytes that appear refractory to HBV infection. In both instances, clonal expansion of subpopulations of hepatocytes may be inferred to have taken place. Interestingly, foci of altered hepatocytes and hepatocellular carcinomas (HCC) typically do no support virus replication. Thus, immune selection of hepatocytes by antiviral CTL, by inducing clonal expansion, may also play an important role in the progression to HCC. In this review, we discuss the evidence in support of roles for hepatocyte turnover in the resolution of transient and progression of chronic HBV infections.