Reply:

We appreciate the comments made by Carreno et al. because they reflect the complexity and controversy of data available on “occult” hepatitis C virus (HCV) infection, which were discussed in detail within our review article.1 If clinically significant replication of HCV takes place in patients with “occult” HCV infection and sustained virologic response (SVR) after interferon-alfa (IFN)–based antiviral therapy for chronic HCV infection, high rates of late virologic relapse may be expected. However, late virologic relapse rates are overall very low in patients with SVR, irrespective of whether their immune system is impaired or not.1 Carreno et al. argue that “cumulative evidence” strongly supports HCV replication in patients with “occult” HCV infection. Because this conclusion would have substantial consequences for patients considered cured of their HCV infection as well as for the healthcare system, data must be interpreted most carefully and differentiated. Indeed, MacParland et al. reported de novo infection of T cells exposed to plasma of patients with SVR after IFN-based antiviral treatment as demonstrated by detection of genomic and antigenomic HCV RNA as well as detection of HCV-like particles by immunoelectron microscopy.2 In contrast, other data indicate reasonable doubt whether HCV can replicate in peripheral blood mononuclear cells (PBMCs) at all.3 HCV cell entry receptors are CD81, scavenger receptor class B type I (SR-BI), claudin-1, and occludin, of which claudin-1, SR-BI, and occludin are typically not expressed on PBMCs.3-5 Possible infection of PBMCs was investigated using the currently most robust and suitable in vitro system to fully recapitulate HCV entry, replication, and virus production.3,6 Cell culture–produced HCV did not replicate in PBMCs, infection of PBMC subsets with HCV pseudoparticles (HCV-pp) failed, and even artificial expression of claudin-1 on a special B cell line also expressing CD81 and SR-BI did not enhance HCV-pp infectivity of PBMCs.3 Directly transfected HCV RNA was also not translated and did not replicate in PBMC subsets.3 Furthermore, Carreno et al. mention unpublished data reporting a higher incidence of serologic markers of HCV infection in family members of patients with “occult” HCV infection in comparison to patients with chronic hepatitis C.7 These data may be interpreted in detail after full publication, but prima facie it seems difficult to understand why immune responses to HCV in family members of subjects with undetectable HCV RNA in serum was more frequent than in family members of patients with detectable HCV RNA. Moreover, HCV-specific CD4 T cell response may occur because of ongoing viral replication but also may reflect persistence of specific T cells after resolved HCV infection as discussed within our article.1 Similar considerations may be taken into account for other indirect markers of prior infection, such as anti-HCV core antibodies.8 We fully agree with Carreno et al. that the data of studies investigating the topic of “occult” HCV infection are heterogeneous.1,10 However, it must be noted that several studies have failed to confirm “occult” HCV infection under various circumstances using particularly sensitive tests.1,9,11,12 Reports on detection of HCV proteins in patients with “occult” HCV infection have been discussed within our review article but must be interpreted cautiously with respect to methods used and results of other studies.1,13 In summary, data about the presence or absence of HCV RNA in liver and/or PBMCs despite negative HCV RNA in serum of patients with previous HCV infection are conflicting. Further well-designed, multicenter, prospective trials are necessary not only for current IFNbased therapies but also in the future for antiviral therapies based on specifically targeted antiviral therapy for HCV (STAT-C), with and without IFN, to finally resolve these conflicting data.