Clearance of hepatitis C in chimpanzees is associated with intrahepatic T-cell perforin expression during the late acute phase

The liver is the primary site of hepatitis C virus (HCV) replication. Therefore, we undertook detailed intrahepatic studies of T-cell dynamics, apoptosis, and gene expression during the acute phase of infection using liver biopsies from chimpanzees that developed persistent infection or spontaneously cleared the virus. We examined more than 40 liver biopsies histologically and quantitatively for T-cell infiltration, hepatocyte apoptosis and perforin expression. These data were correlated with outcome and viral kinetics. We observed intrahepatic T-cell infiltration in both groups of animals with CD8(+) T cells representing the major population. The appearance of T cells was always associated with apoptosis and mild alanine aminotransferase (ALT) elevations. Apoptosis (5-20% of hepatocytes) always occurred prior to serum ALT peak. Quantification of intrahepatic ALT mRNA revealed no upregulation of gene expression confirming that serum ALT increases were due to release of this enzyme from cells. During the late acute phase, cleared animals showed an increased frequency of hepatocyte apoptosis relative to persistently infected animals (P < 0.05). This correlated with a higher intrahepatic CD8(+) T-cell frequency in the cleared group (P < 0.01) with a greater proportion of lymphocytes expressing perforin compared with the persistent group (P < 0.001). All infected animals mounted intrahepatic immune responses during the acute phase, but these were not maintained in frequency or efficacy in persistent infections. There is a reduction in the numbers of intrahepatic T cells during the late acute phase in infections that become persistent with significantly fewer of these cells functional in clearing the virus by killing infected hepatocytes.