Immunodiffusion and neutralization studies on porcine adenoviruses

Precipitating antigens were prepared from porcine kidney cell cultures infected with the four approved serotypes of porcine adenoviruses (PAV) and from human amnion cell cultures infected with serotype 5 human adenovirus. For extracellular precipitating antigens (EPA) concentration by ammonium sulphate precipitation from cell culture fluids was used. Cell-associated precipitating antigens (CAPA) were extracted from cell sediments by repeated freezing and thawing. All antigens reacted alike and formed a single coalescent precipitin line of identity when tested against sera collected from a sow infected in the field or from weaners intranasally infected with serotypes 3 or 4 of PAV. In order to determine the optimal time of harvesting cell culture materials for the preparation of precipitating antigens, the kinetics of production and release of infectious virus and precipitating activity of PAV serotype 3 in porcine kidney cell cultures were studied. Precipitating activity first appeared with CAPA 24 h p.i. and 12 h later with EPA. Infectivity titers did not correlate with precipitating activities of EPA or CAPA beyond that stage. At the time the infectivity of EPA was decreasing, its precipitating titer continued to increase. The peaks of precipitating activities of CAPA and EPA were demonstrated at 96 and 144 h p.i., respectively. Three 7-week-old weaners with serum neutralizing antibodies against the four serotypes of PAV, but without detectable precipitating antibodies, were inoculated intranasally with serotype 3 of PAV. Serum samples collected 1 week p.i. showed precipitating activities and steep increases in neutralizing antibody titers against the homologous serotype 3 and the heterologous serotypes 1, 2 and 4 of PAV. The serum neutralizing antibody titers remained nearly constant over a period of 18 weeks p.i. while the intensity of the precipitating reaction decreased. Intranasal infection of the pigs with serotype 4 PAV induced heterotypic anamnestic neutralizing antibody response as well as an increase of the precipitating antibodies.