Antigenic specificity of convalescent serum from cattle with Haemophilus somnus-induced experimental abortion

Abstract

The antigens of Haemophilus somnus recognized by convalescent bovine serum were studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting with a protein A-peroxidase conjugate. The same two 76K and 40K antigens were predominant in whole-bacterium preparations and in outer-membrane-enriched, Triton X-100-insoluble fractions. The surface location of these antigens was confirmed by absorbing antiserum with whole, live bacteria. Absorption with Haemophilus somnus removed antibody reactivity for the 76K antigen and reduced reactivity for the 40K antigen. Absorption with Pasteurella multocida, Actinobacillus equuli, or Escherichia coli did not reduce reactivity, and results with Pasteurella haemolytica were equivocal. The two immunodominant antigens detected in this study were conserved in isolates of Haemophilus somnus from thromboembolic meningoencephalitis, pneumonia, reproductive failure, or asymptomatic carriers. Convalescent sera from nearly all 17 cattle studied recognized these two antigens. Other antigens were recognized less consistently. Although other antigens may also be involved, the 76K and 40K surface antigens of Haemophilus somnus appear to be important candidates for a subunit vaccine or an immunodiagnostic assay.