Cross-reactivity and antigenic heterogeneity among Actinobacillus pleuropneumoniae strains of serotypes 4 and 7.

Actinobacillus pleuropneumoniae strains of serotypes 4 and 7 were studied for their antigenic properties by means of agglutination, coagglutination, indirect hemagglutination, immunodiffusion, and counterimmunoelectrophoresis tests. Strains of serotype 4 showed cross-reactivity with those of serotype 7 in various serological tests. Serotype 7 strains were antigenically heterogeneous and shared common antigens with several other serotypes. By using boiled whole-cell saline extract as the antigen in the immunodiffusion test, serotype 7 strains could be divided into four subgroups. Subgroup I strains did not have antigens in common with other serotypes, whereas subgroup II strains had antigens in common with serotype 4; subgroup III strains had antigens in common with serotype 10, and subgroup IV had antigens in common with serotypes 1, 9, and 11. The indirect hemagglutination test using unheated whole-cell saline extract as the antigen detected serotype-specific activity. Quantification of serotype-specific and group-specific antigens by coagglutination and immunodiffusion tests was found useful for identifying strains that belonged to serotype 4 or 7.