X antigen/antibody markers in hepadnavirus infections. Antibodies to the X gene product(s).

Antibodies to the X antigen of hepatitis B virus and woodchuck hepatitis virus were assayed in serial sera from infected individuals and compared with other markers of infection. Antibody to the X antigen was found in 11 of 17 (65%) patients and 17 of 40 (42%) woodchucks that were surface-antigen positive. In comparison, this antibody was found in 5 of 14 (36%) patients and in none of 4 woodchucks that were surface-antigen negative. In 5 of 6 patients showing seroconversion from hepatitis B e antigen to antibody, antibody to X appeared at or near the time of seroconversion. In patients persistently positive for e antigen, X antibody often appeared when viral DNA became undetectable in the serum. In 14 of 17 (82%) woodchucks positive for antibody to X antigen, it also appeared near or after the time that viral DNA in serum disappeared. X antibodies were detected with great frequency only in populations with high frequencies of other hepatitis B virus markers. The results are consistent with the conclusion that antibody to X antigen is a marker of hepadnavirus infections that seems to be associated with a decrease in viral replication. Antibodies to the X antigen, then, may be a host response to the replication complex of the virus.