Reply

Occult hepatitis C virus (HCV) infection (OCI) is defined as the presence of the HCV genome in either liver tissue or peripheral blood monocytes, despite constantly undetectable HCV-RNA in serum. The prevalence of OCI has been thought extremely rare in the immunocompetent host.¹ In contrast, this statement may not hold true in cases where antiviral immunity is weakened. In fact, our study results revealed an unexpectedly high prevalence of OCI among recurrent HCV-infected patients after undergoing orthotopic liver transplantation (OLT) who achieved a sustained virologic response (SVR) with direct-acting antiviral agents (DAAs). However, the results are based on the investigation of a subgroup of patients who continue to have abnormal aminotransferases after attaining SVR12.² Consequently, Enomoto et al and De Marco et al both provided valuable and constructive comments and insights. Enomoto et al studied 25 nontransplant patients who normalized serum aspartate and alanine aminotransferases upon SVR with DAAs. This particular group of patients did not harbor the HCV genome in the liver tissue. Of great interest, the majority of these patients had a favorable IL28B genotype, which is requisite for the better suppression of HCV because it augments hepatic antiviral innate immunity. Thus, their study subjects are diametrically opposite to ours. These observations collectively highlight that impaired immunity, including unfavorable IL28B genotype, is the necessary condition for the establishment of OCI.

De Marco et al precisely pointed out the technical limitation with regard to the diagnosis of OCI. Mathematical models suggest that virion production in an infected human liver is estimated to be an average of 1.3 × 10¹² virion per day.³ The half-life of the viral genome in the infected liver is estimated to be much longer than that in the systemic circulation, at 3-11 hours and 40 minutes, respectively.^4,5 Based on these data, the sum of viral genome abundance in the systemic circulation is substantially lower than that in the liver and, along with the dilution factor of the serum volume, there is a large concentration gradient of HCV genome between the liver and the peripheral blood.⁶ Thus, the standard serum HCV-RNA detection methods widely used in clinical practice may fail to detect HCV-RNA if the infected liver sheds only a trace number of viral particles. In fact, it is well-known that undetectable HCV-RNA in the serum does not confer the absence or eradication of HCV from the liver.⁷ This notion is well-exemplified in patients who develop viral breakthrough during antiviral therapy or relapse after antiviral therapy. These observations support the fact that HCV is able to maintain viral replication presumably at a low efficiency and yet still be undetectable in the serum for a long duration. This notion is supported by the recent study investigating the existence of intrahepatic residual HCV-RNA in patients who completed DAA administration while awaiting OLT.⁸ They reported a surprisingly high rate of viral genome positivity in the liver explants. Furthermore, the abundance of intrahepatic HCV genome well correlated to the rate of post-OLT recurrent HCV infection. Taken together, it is plausible that the low-efficiency viral replication that falls under the detection limit is capable of subsisting within the liver tissue and peripheral blood mononuclear cells.

We speculate that OCI in post-OLT patients is the consequence of the multiple events such as DAA-mediated evolution of an antiviral drug-resistant variant that compromises viral replication. Such inefficient viral replication likely results in the inadequate activation of the anti-HCV immune response, especially under the deregulated host immunity against the allograft and compromising effects of immunosuppressants. This suboptimal intrahepatic immunity along with the unfavorable IL28B genotype likely offer a platform for viral latency while exhibiting the liver histology of “viral hepatitis.”

In summary, we agree with the comments and suggestions by Enomoto et al and De Marco et al and leads to our concluding remark that this an exciting time to study the prevalence and significance of OCI, because the post-SVR population is now rapidly growing owing to the recent introduction of DAAs. Toward this goal, a multicenter investigation is necessary to collect the number of study subjects required for statistical power. In addition, it is most important that all investigators use a standardized, sensitive, and specific detection method to define the true prevalence and clinical consequences of OCI, including the potential progression to cirrhosis, graft loss, and the remote development of hepatocellular carcinoma after achieving SVR.