The Marmoset Model of GB Virus B Infections: Adaptation to Host Phenotypic Variation

Approximately 3% of the world population is chronically infected with hepatitis C virus (HCV). HCV infection is the leading cause of liver transplantation in the United States (1, 40), and liver cancer due to HCV infection is one of the most rapidly increasing types of cancer in the United States (19). The only animal model for HCV infection is the chimpanzee, which has numerous drawbacks. Therefore, GB virus B (GBV-B) infection of tamarins represents an important small primate surrogate model for HCV infections (3, 4). Due to the limited availability of tamarins, our laboratory and others (5, 16, 20, 21) initiated GBV-B studies with common marmosets (Callithrix jacchus), a small New World primate closely related to the tamarin (Saguinus sp.). The tamarin and marmoset represent smaller, less expensive, and more-readily available animal models than the chimpanzee. The history of the GB agent originates with the inoculation of tamarins with serum obtained from a surgeon with hepatitis in an attempt to pass a human hepatitis agent in an animal model (for a review, see reference 4). Subsequently, two viruses were cloned from the tamarin, GBV-A and GBV-B (28, 34). It is now assumed that both viruses were natural tamarin viruses and were not passed to the tamarin from the surgeon; however, GBV-B has not been isolated a second time from tamarins and, the surgeon's serum is no longer available for testing. GBV-B has a very narrow host range for tamarins, marmosets, and other closely related New World monkeys (this study; 6, 21). The organization of the GBV-B genome is very similar to that for HCV. The GBV-B polyprotein encodes 10 proteins, including the structural proteins at the amino terminus (core, E1, E2, and p13) and the nonstructural proteins at the carboxy terminus (NS2, NS3, NS4A, NS4B, NS5A, and NS5B) (15). The RNA genome contains secondary structures in the 5′ and 3′ noncoding regions (NCR), and cis RNA elements are located in the 5′ and 3′ NCR and, if similar to those in HCV, in the NS5B coding region (for a review, see reference 37). In this study, we examined the virus-host interactions of GBV-B during serial passage in marmosets. On the basis of infection profiles, we found that marmosets displayed two phenotypes, the susceptible and partially resistant phenotypes, and that GBV-B was able to rapidly adapt when passaged through a partially resistant marmoset. Once GBV-B adapted, the partially resistant phenotype was not observed again. Furthermore, a modified GBV-B clone was able to replicate in marmosets, and the susceptible and partially resistant phenotypes were reproduced. Sequence mutations observed in GBV-B after passage in partially resistant marmosets may reflect changes that improved interaction with host proteins critical for efficient replication or that allowed innate immune evasion. Furthermore, genetic variation among marmosets may predispose animals to a partially resistant or susceptible phenotype.