H epatitis C virus (HCV) infects an estimated 180 million people worldwide, and about 75% of newly infected patients progress toward a chronic infection, which constitutes a risk for severe liver diseases such as cirrhosis and hepatocarcinoma (1-4). HCV is a hepacivirus in the Flaviviridae family characterized by the error-prone replication and quasispecies dynamics typical of RNA viruses (2,(5)(6)(7). No vaccine is available to prevent HCV infections or disease, and the current standard of care treatment consists of the combination of pegylated alpha interferon (IFN-␣) and the purine nucleoside analogue ribavirin (1-␤-D-ribofuranosyl-1-H-1,2,4-triazole-3-carboxamide) (8-10). Type I interferons (IFNs) such as IFN-␣ are members of a family of cytokines that constitute key components of the innate immune response to viruses, and their induction results in an antiviral state of the cell and suppression of viral replication (11)(12)(13)(14). Ribavirin is currently used to treat a number of human viral infections, and it can display multiple mechanisms of action, including enhancement of Th1 antiviral immune responses, upregulation of IFN-stimulated genes (ISGs), depletion of GTP pools, or viral RNA mutagenesis (see reviews in references 15, 16 and 17). However, on average only about 60% of the chronically infected patients show a sustained virological response to treatment with IFN-␣ plus ribavirin that results in virus clearance (18-21). The molecular and physiological bases of the therapeutic activity of the current combination treatment with IFN-␣ plus ribavirin are poorly understood. It might be possible to establish new treatment protocols based on recently developed directly acting antiviral agents (DAAs), with or without .Highly variable viruses exploit a number of strategies to counteract selective constraints intended to prevent their replication (reviewed in reference 7). Mutations that confer resistance to DAAs generally map in the target protein or in a protein that interacts with the target (28-30). However, the complexity of the cellular IFN response pathway is expected to require greater diversity of viral resistance mutations. Current evidence suggests that the response of HCV to IFN-␣-based therapy is influenced by several host (i.e., interleukin-28B [IL-28B] gene polymorphisms) and viral genetic (i.e., viral genotype and population complexity) factors (31-37).The advent of cell culture systems for sustained replication of HCV offers new prospects to approach the evolutionary dynamics of HCV and the host cell-virus relationship, including the anti-HCV activity of IFN-␣. Using these systems, Garaigorta and Chisari documented that HCV-induced protein kinase R (PKR) phosphorylation inhibited translation of IFN-stimulated genes (ISGs) in infected hepatocytes (38). Here we describe the adaptation of the HCV replication system of genotype 2a (39-41) to perform serial passages of HCV in the Huh-7.5 hepatocyte cell line with sustained, efficient viral replication for at least 100 serial passages. This cell ...

Passage of hepatitis C virus (HCV) in human hepatoma cells resulted in populations that displayed partial resistance to alpha interferon (IFN-␣), telaprevir, daclatasvir, cyclosporine, and ribavirin, despite no prior exposure to these drugs. Mutant spectrum analyses and kinetics of virus production in the absence and presence of drugs indicate that resistance is not due to the presence of drug resistance mutations in the mutant spectrum of the initial or passaged populations but to increased replicative fitness acquired during passage. Fitness increases did not alter host factors that lead to shutoff of general host cell protein synthesis and preferential translation of HCV RNA. The results imply that viral replicative fitness is a mechanism of multidrug resistance in HCV. IMPORTANCEViral drug resistance is usually attributed to the presence of amino acid substitutions in the protein targeted by the drug. In the present study with HCV, we show that high viral replicative fitness can confer a general drug resistance phenotype to the virus. The results exclude the possibility that genomes with drug resistance mutations are responsible for the observed phenotype. The fact that replicative fitness can be a determinant of multidrug resistance may explain why the virus is less sensitive to drug treatments in prolonged chronic HCV infections that favor increases in replicative fitness. Selection of viral mutants resistant to antiviral agents is a major problem for the successful treatment of viral diseases. In the case of RNA viruses, high mutation rates during genome replication provide viral populations with an ample reservoir of phenotypic variants, including mutants that can escape selective constraints. Resistance to a single drug that targets a viral protein develops at a rate that depends on the genetic barrier (number and types of mutations needed to acquire resistance) and the phenotypic barrier (fitness cost) imposed by the resistance mutations (1-16). When drug resistance mutations do not entail a significant fitness cost-either because the mutations per se do not critically affect viral functions or because compensatory mutations are acquired-they may reach detectable levels despite no prior exposure of the viral population to the drug (1, 16-27).Control of hepatitis C virus (HCV) infections is hampered by the complexity of HCV quasispecies replicating in the liver (16,28,29). Directly acting antiviral agents (DAAs)-some currently in use and others under development-offer great promise for control of HCV either as a substitute for or complement of the standard-of-care (SOC) therapy based on treatment using a combination of pegylated alpha interferon (IFN-␣) and ribavirin (30)(31)(32)(33)(34)(35)(36). Combinations that include the polymerase inhibitor sofosbuvir have produced sustained viral responses that in some cases have been higher than 90% in clinical trials (37-40), but the possible impact of resistance mutations is not known; sofosbuvir resistance substitution S282T in NS5B is present in the ...