Abstract
Various criteria that could he used for the measurement of local immunity to Newcastle disease virus (NDV) and avian infectious bronchitis virus (IBV) were examined. Virus isolation and the measurement of antibody concentration in the serum and secretions were selected for use in the subsequent studies. Both resistance of the respiratory tract to infection and specific antibody in the upper respiratory tract secretions; could be induced by the administration of live or inactivated lentogenic NDV directly into the respiratory tract. Virus-neutralising antibody appeared in tracheal secretions as early as 6 days after inflection and, after an early peak, declined in titre, whereas antibody did not appear in the nasal secretions until 8 days after infection, but then proved to be more persistent. Antibody was also detected in lung secretions following the intramuscular inoculation of live virus. The direct or indirect exposure of the respiratory tract tissues to antigen was necessary for the development of resistance and the production of local antibody because inactivated virus administered parenterally failed to stimulate either, despite the high concentrations of serum antibody that were usually induced. In passively immunised chicks the circulating; antibody response to NDV was markedly suppressed, but no concomitant suppression. of local immunity could be demonstrated, indicating that the latter was independent of the former. Further evidence in support of a role for local immunity in NDV and IBV infections came from studies on tracheal explants derived from variously immunised or untreated chickens. Such explants were found to differ in their ability to support virus growth. Respiratory tract secretions contained IgA and IgG and variable amounts of IgM, as well as an additional antigenic component that was present in all secretions examined but was absent from the serum. Antibody activity in tracheal secretions appeared to be associated with the IgA fraction.