Lack of adaptation of chimeric GB virus B/hepatitis C virus in the marmoset model: possible effects of bottleneck.

Approximately 3% of the world population is chronically infected with hepatitis C virus (HCV). GB virus B (GBV-B), a surrogate model for HCV, causes hepatitis in tamarins and is the virus phylogenetically most closely related to HCV. Previously we described a chimeric GBV-B containing an HCV insert from the 5' noncoding region (NCR) that was adapted for efficient replication in tamarins (Saguinus species). We have also demonstrated that wild-type (WT) GBV-B rapidly adapts for efficient replication in a closely related species, the common marmoset (Callithrix jacchus). Here, we demonstrate that the chimeric virus failed to adapt during serial passage in marmosets. The chimeric virus was passaged four times through 24 marmosets. During passage, two marmoset phenotypes were observed: susceptible and partially resistant. Although appearing to adapt in a resistant animal during a prolonged and gradual increase in viremia, the chimeric GBV-B failed to replicate efficiently upon passage to a naïve marmoset. The resistance was specific to the chimeric virus, as the chimeric virus-resistant animals were susceptible to marmoset-adapted WT virus during rechallenge studies. Three isolates of the chimeric virus were sequenced, and 20 nucleotide changes were observed, including eight amino acid changes. Three unique changes were observed in the 5' NCR chimeric insert, an area that is highly conserved in HCV. We speculate that the failure of the chimeric virus to adapt in marmosets might be due to a bottleneck that occurs at the time of infection of resistant animals, which may lead to a loss of fitness upon serial passage.

The marmoset model of GB virus B infections: adaptation to host phenotypic variation.

Worldwide, approximately 170 million people are chronically infected with hepatitis C virus (HCV), and chronic infection frequently progresses to serious liver disease, including cirrhosis and hepatocellular carcinoma. GB virus B (GBV-B), the virus phylogenetically most closely related to HCV, causes hepatitis in tamarins. We have demonstrated the suitability of the tamarin as a host for GBV-B and as a surrogate nonhuman primate model for HCV infection, and we have initiated studies of GBV-B infection in a closely related species, the common marmoset (Callithrix jacchus). Here, we demonstrate that marmosets exhibit two phenotypes upon infection with GBV-B: the susceptible phenotype and the partially resistant phenotype. In addition, we identify changes that may correlate with adaptation of the virus to the partially resistant host. GBV-B was serially passaged five times through 14 marmosets as one lineage and two times through 6 marmosets as a second lineage. Virus adapted to the marmosets and eventually exhibited robust infections in two separate lineages, lineages 1 and 2. A third lineage was initiated with a molecular clone, and again, susceptible and partially resistant phenotypes were observed. Three isolates were fully sequenced (from lineage 1), and 21 nucleotide changes were observed, with six amino acid changes. We speculate that the marmoset partially resistant phenotype may be due to a polymorphism in the marmoset population that affects critical virus-host interactions and that wild-type GBV-B is capable of rapidly adapting to this altered host.