EVIDENCE THAT HEPATOCYTE TURNOVER IS REQUIRED FOR RAPID CLEARANCE OF DUCK HEPATITIS-B VIRUS DURING ANTIVIRAL THERAPY OF CHRONICALLY INFECTED DUCKS

Duck hepatitis B virus (DHBV) DNA synthesis in congenitally infected d ucks is inhibited by 2'-deoxycarbocyclic guanosine (2'-CDG). Three mon ths of therapy reduces the number of infected hepatocytes at least 10 fold (W. S. Mason, J. Cullen, J. Saputelli, T.-T. Wu, C. Liu, W. T. Lo ndon, E. Lustbader, P. Schaffer, A. P. O'Connell, I. Fourel, C. E. Ald rich, and A. R. Jilbert, Hepatology 19:393-411 1994). The present stud y was performed to determine the kinetics of disappearance of infected hepatocytes and to evaluate the role of hepatocyte turnover in this p rocess. Essentially all hepatocytes were infected before drug therapy. Oral treatment with 2'-CDG resulted in a prompt reduction in the numb er of infected hepatocytes. After 2 weeks, only 30 to 50% appeared to still be infected, and less than 10% were detectably infected after 5 weeks of therapy. To assess the possible role of hepatocyte turnover i n these changes, 5-bromo-2'-deoxyuridine (BUdR) was administered 8 h b efore liver biopsy to label host DNA in hepatocytes passing through S phase, and stained nuclei were detected in tissue sections by using an antibody reactive to BUdR. The extent of nuclear labeling after 5 wee ks was the same as that before therapy (ca. 1%). However, biopsies tak en after 2 weeks of therapy showed a ca. 10-fold elevation in the numb er of nuclei labeled with BUdR. This result suggested that a rapid cle arance of infected hepatocytes by 2'-CDG was caused not just by the in hibition of viral replication but also by an acceleration of the rate of hepatocyte turnover. To test this possibility further, antiviral th erapy was carried out with another strong inhibitor of DHBV DNA synthe sis, 5-fluoro-2',3'-dideoxy-3'-thiacytidine (524W), which did not acce lerate hepatocyte turnover in ducks. 524W administration led to a stro ng inhibition of virus production but to a slower rate oi decline in t he number of infected hepatocytes, so that ca. 50% (and perhaps more) were still infected after 3 months of therapy. In addition, histopatho logic evaluation of 2'-CDG-treated ducks revealed liver injury, especi ally at the start of therapy. No liver damage was observed during 524W therapy. These results imply that clearance of infected hepatocytes f rom the liver is correlated with hepatocyte turnover. Thus, in the abs ence of immune clearance or other sources for the accelerated eliminat ion df infected hepatocytes, inhibitors of virus replication would hav e to be administered for a long period to substantially reduce the bur den of infected hepatocytes in the liver.