X-deficient woodchuck hepatitis virus mutants behave like attenuated viruses and induce protective immunity in vivo

The X protein (HBX) of the hepatitis B virus (HBV) has been shown to be imp ortant for the establishment of HBV infection in vivo. Our previous studies suggested that interaction of HBX with the proteasome complex may underlie the pleiotropic functions of HBX. In this study, we generated a series of woodchuck hepatitis virus (WHV) X mutants, including mutants of the domain interacting with the proteasome, and studied their infectivity in woodchuck s. Many of the mutants were defective in transactivation but none of them w ere completely replication defective in vitro. In vivo, all the wild-type a nd some X mutant-transfected animals demonstrated evidence of infection wit h and-WHc and/or anti-WHs seroconversion. Most of the wild-type- and X muta nt-transfected animals had transient viremia. Some animals were later chall enged with infectious WHV. Animals inoculated with X mutants, including tho se with no serologic evidence of infection, were protected from the challen ge, suggesting previous infection with resulting protective immunity. Our s tudy demonstrates that the previously described functional domains of HBX a re biologically important and the X-defective mutants, possibly as attenuat ed viruses, are not completely replication defective in vivo.