Studies on the immune response of the ovine respiratory tract toparainfluenza 3 virus
Ovine IgG, IgM and IgA and antisera specific for these immunoglobulins were prepared. These reagents were used to estimate immunoglobulin levels in sheep sera and certain other body fluids including the respiratory secretions. The results showed that a secretory IgA system existed in the ovine respiratory tract. Furthermore IgA antibodies to Mycoplasma ovipneumonia were identified in the lung fluid of a sheep clinically affected with pulmonary adenomatosis. IgA antibodies specific for Parainfluenza 3 were demonstrated in the respiratory secretions of lambs which had been experimentally infected with the virus. A large molecular weight non-immunoglobulin substance, which inhibited Parainfluenza 3 and three other paramyxoviruses, was also identified in the respiratory secretions of both conventionally reared and specific pathogen free lambs. It is suggested that in certain reports non-specific inhibitors present in the nasal secretions of calves may have been confused with Parainfluenza 3 specific IgA antibody. Attempts were made to define the protective role of nasal secretion antibody with vaccination-challenge experiments in specific pathogen free lambs. It was shown that live Parainfluenza 3 administered intranasally stimulated comparable serum antibody titres but higher nasal secretion titres than the same dose of live virus given intramuscularly. Inactivated virus inoculated without adjuvant by either route stimulated low or undetectable serum titres and no nasal antibody. Immunity to aerosol challenge, as assessed by viral shedding from the nose and changes in post challenge antibody titres, was best conferred by intranasal inoculation with live virus. Hence there was some evidence that the presence of antibody in the nasal secretions reduced the susceptibility to infection. In subsequent experiments it was found that intramuscular inoculation of inactivated Parainfluenza 3 in complete Freund's adjuvant stimulated high serum and nasal secretion titres, which protected against challenge. However the nasal secretion antibody was IgG^, which was possibly selectively transferred from serum. This contrasted with the earlier finding of IgA antibodies in the respiratory secretions after intranasal inoculation of live virus and showed that a second mucous antibody system existed in the ovine respiratory tract. Results from an experiment with young colostrum fed lambs indicated that maternal antibody alone could prevent infection with Parainfluenza 3 virus. This showed that the presence of cell-mediated immunity may not be essential to prevent sheep becoming infected with this virus. In this and a subsequent experiment it was demonstrated that colostral IgG passed into the nasal and lachrymal secretions of young lambs. This finding supported the earlier suggestion that IgG1 is selectively transferred from serum into the nasal secretions of young lambs. It is suggested that the presence of maternal IgG1 antibodies in the respiratory secretions of newborn suckled lambs could constitute an important defence mechanism against respiratory infections before local antibody synthesis begins at about two weeks of age.