We thank Dr. Dahari and colleagues for their interest in our study and in the new approach we used to determine the half-life of circulating hepatitis B virions. Only recently have intracellular estimates of hepatitis B virus (HBV) been available in patients.1, 2 Before then, and frequently now, HBV DNA levels in serum have been the sole reflection of viral production from infected hepatocytes. It is therefore understandable that the mathematical models used to estimate HBV clearance dynamics have been based on observed HBV DNA levels and unobserved numbers of infected hepatocytes,3, 4 and have been equivalent in form to models used in very different infections such as human immunodeficiency virus (HIV).5
Mathematical models should be constructed to balance knowledge of the replication cycle of the infection with the availability of data to underpin them. Now that more detailed data is available on HBV infection, it is sensible not to ignore it. Our model is a simple extension of those previous models but where levels of intracellular HBV DNA, which was available from liver biopsies of chronically HBV-infected individuals,6 was also included.
HBV has an interesting replication cycle.7 Reverse transcriptase inhibitors such as lamivudine act at very different points of the infection cycle for HBV in comparison to HIV. In HIV, lamivudine inhibits infection of a cell, whereas in HBV it has no effect on infection of a hepatocyte, but rather hinders the generation of mature double-stranded HBV DNA–containing capsids within the cytoplasm of previously infected cells. Moreover, the application of these drugs does not impact on the export of the pool of mature encapsidated double-stranded HBV DNA that was present within infected cells before the application of these drugs.7 Previous models of HBV dynamics have estimated the half-life of HBV virions from the first phase of decaying levels of HBV DNA in serum after the application of inhibitory drugs, at which time HBV particles are still being exported from the diminishing pool of intracellular HBV DNA.8 That none of the studies referenced by Dahari et al.9 produce estimates of HBV virion half-lives consistent with our own is not surprising given that using this approach they have been mainly estimating the export rate of preformed intracellular HBV DNA instead.
Our analysis of chronically infected individuals shows a surprising range of virion half-lives where these were strongly correlated with HBV DNA levels in serum.6 This correlation was not an artefact of our procedure, because the same calculations performed using immunodeficient urokinase plasminogen activator chimera mice failed to exhibit this same correlation. That these mice lacked a specific immune response to HBV, and did not exhibit fast virion clearance rates, strongly suggests that in chronically HBV-infected individuals displaying low viremia and lower virion productivity,10 the immune system is more capable of efficiently removing circulating virions.
However, Dahari et al. raise several valid points. The very fast half-life estimates we obtain in treatment-naïve low viremic patients would be expected to lead to infection clearance.6 The fact that this usually does not occur is mainly due to the presence of the stable viral template, covalently closed circular DNA molecules present in hepatocyte nuclei, which guarantee viral persistence in infected hepatocytes. Furthermore, clearance of virions in serum does not necessarily reflect inhibition of replication in the liver, and this is supported by observations that HBV-specific CD8+ T cells are dysfunctional in chronically infected individuals.11 It is plausible that clearance of circulating virions in individuals undergoing antiviral therapy does not change dramatically and that clearance kinetics may still be slower compared to treatment-naïve patients with similarly low viral titers. Determination of circulating HBV half-life in patients treated with polymerase inhibitors using both intrahepatic and serological measurements may be an important next step.
We do not know why a similarly fast virion half-life should not exist in other viral infections such as HIV and hepatitis C virus. Acute infection is never cleared for HIV, sometimes for hepatitis C virus, and often for HBV in immunocompetent individuals and the magnitude of effect of neutralizing antibodies may reflect this. Given the complex processes involved, we entirely agree that HBV dynamics should be further investigated comparing different approaches, using both clinical studies as well as experimental infection models.