Merging models of hepatitis C virus pathogenesis

Chronic hepatitis C virus (HCV) infection is one of the major causes for development of liver cirrhosis and end-stage liver disease. This article reviews two contrasting models of HCV pathogenesis, discusses the merits of each, and presents a rationale for combining the two models into one. Any successful model of HCV pathogenesis must explain how the characteristic features of cirrhosis and end-stage liver disease arise. These features include the loss of hepatocyte function (low serum albumin and reduced clotting ability); the presence of regenerative nodules; and the deposition of excessive extracellular matrix material, especially collagen (fibrosis), which is associated with the transformation of the liver sinusoids to capillary-like structures leading to portal hypertension. A successful model should explain several observations about the rate of disease progression. HCV is characterized by slow progression of fibrogenesis and, importantly, cirrhosis seems to develop only after a long latency (and only in a subset of patients). Among the prognostic factors of disease progression, the age at infection with the HCV virus and the presence of fibrosis appear to be highly relevant in predicting the development of progressive fibrosis. Traditional models of HCV pathogenesis propose that fibrogenesis is the predominant process. Fibrogenesis is induced by activation of fibrogenic cells, such as stellate cells, which results in excessive collagen deposition. By altering the normal architecture and vasculature, the collagen bands finally lead to cirrhosis and loss of organ function. Activation of stellate cells is induced by inflammation, cytokine signaling, and possibly by hepatocyte apoptosis. The telomere model of HCV pathogenesis suggests that hepatocyte damage plays an essential role in the development of cirrhosis. According to this model, hepatocyte damage leads to increased cell turnover, and to the accelerated shortening of hepatocyte telomeres. Critical telomere shortening leads to hepatocyte senescence, loss of hepatocyte function, exhaustion of hepatocellular regeneration, and to a greatly enhanced fibrotic response to injury. This review summarizes both models and presents evidence that these models are not mutually exclusive but rather can be merged into a comprehensive pathogenesis model that outlines the pathway of HCV-induced cirrhosis.