Development of an intra- and intergenotypic HCV cell culture method to phenotype and assess antiviral susceptibilities and resistance development of HCV NS3 protease genes from HCV genotypes 1-6
The development of specific antiviral drugs directly targeting the hepatitis C virus (HCV) is clinically important, as the current standard interferon/ribavirin combination treatment is only partially effective, expensive and often associated with severe side effects. Inhibitors of the NS3 protease (PI) therefore represent a promising alternative or additional therapy. To date, the development and in vitro evaluation of PIs is restricted to the genotype 1/2 based replicon and the genotype 2a full length viral cell culture system. However, proteases of the different HCV genotypes vary substantially in their amino acid sequence and secondary structure and require separate evaluation of their efficacy before they go into clinical trials. To address this issue, a panel of intra- and intergenotypic recombinants based on the recombinant infectious clone Jc1 (pFK JFH1/J6/C-846) was developed in this work. The viability of these recombinants was assessed in the Huh7.5 cell culture system, where replicating viruses were detected by HCV-NS5A immunostaining. Intergenotypic recombinants containing genotype 1a, 1b, 3a, 4a and 6a derived proteases were replication defective, whereas the recombinant with genotype 5a derived protease replicated efficiently after acquiring cell culture adaptive mutations. The replacement of not only the NS3 protease gene region, but also its cofactor NS4A, allowed the generation of replication competent intra- and intergenotypic recombinants for all 6 major genotypes. Replacing the NS3 protease of the recombinants with that of patientderived proteases also generated replicating recombinants, greatly expanding the panel of intergenotypic recombinants available for phenotyping and PI evaluation. However, intra- and intergenotypic recombinants showed substantial differences in their replication kinetics, which may be influenced by naturally occurring polymorphism between genotypes and the differential requirement of adaptive/attenuating cell culture mutations. Genotype 1a recombinants replicated very poorly, which may be due to incompatibilities between the type 1a NS3/4A protease and the type 2a backbone. 50% inhibitory concentrations (IC50) of different PIs were measured using Foci Forming Units/ml (FFU/ml) reductions and replication inhibition assays. The different recombinants showed consistent, genotype-associated differences in their susceptibility to the PI BILN 2061, with genotypes 2a, 3a and 5a derived recombinants showing approximately 100-fold lower susceptibility than genotype 1b, 4a and 6a derived recombinants. These observations are consistent with major differences in response rates found in recent treatment trials of genotype 1, 2 and 3 infected patients. Differences in susceptibility were also observed for VX-950, with genotype 1b, 2a and 6a derived recombinants being twice as susceptible than genotype 3a, 4a and 5a derived recombinants. Passaging the intra- and intergenotypic recombinants under increasing concentrations of PI allowed the identification of PI resistance mutations. Resistance mutations to BILN 2061 mapped to the previously identified positions 156 and 168 within the NS3 protease, with a great diversity of amino acid substitutions observed within each genotype. Reintroduction of the identified resistance mutations into the original recombinant viruses conferred increased resistance towards BILN 2061 and some mutations also affected replication kinetics of the recombinants. The developed system will be of major value for the phenotypic characterisation of naturally occurring and treatment induced resistance mutations within all 6 major HCV genotypes towards different PIs. This will allow treatment response predictions for newly developed PIs before they enter clinical trials and the development of individually tailored antiviral treatment regimes.