Abstract
Hepatitis B virus (HBV) frequently causes transient infections in adults but chronic infections in infants. The basis of these age-related outcomes is not known. Infection of ducks with duck hepatitis B virus (DHBV) displays a similar dependence of outcome on the age of the host at the time of infection. In this study we compared the infection of ducks at 3 days and 3 weeks of age. We found that the efficiency of infection of hepatocytes by virus in the inoculum was similar between the two age groups but that spread of the infection throughout the liver was severely inhibited in the 3-week-old-old ducks, while a rapid spread of the infection was observed in 3-day-old ducklings. Inhibition of virus spread was accompanied by the appearance in the serum of virus neutralizing activity, as assayed by blocking of infection of primary hepatocyte cultures. Neutralizing activity appeared as early as 1 or 2 days postinfection and increased during the next 2 weeks. Depletion of immunoglobulins from serum eliminated the neutralizing activity. The specific depletion of IgM indicated that IgM appeared as the dominant fraction of neutralizing antibody in the first 2 days postinfection, but declined from day 3 on while IgG antibody rose. We conclude that excess neutralizing antibody arising rapidly in birds inoculated at 3 weeks of age but not in newly hatched ducks prevented secondary cycles of infection, resulting in a limited infection in the liver and contributing to the eventual transient outcome of the infection.