Characterization of immunodominant surface antigens of Haemophilus somnus

An immunodominant Haemophilus somnus outer membrane protein with an apparent molecular mass of 40 kDa on Western blots (immunoblots) of gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels was characterized because a monospecific antibody against this antigen was protective. This monospecific antibody was used for immunoaffinity purification of the antigen. The immunoaffinity-purified antigen reacted with a polyclonal antibody to the 40-kDa antigen but not with a monoclonal antibody (3G9) which reacted with the 40-kDa antigen in gradient gels. On 8 or 10% gels, the approximately 40-kDa antigen was resolved as two bands, a 40-kDa band which reacted with the protective monospecific polyclonal antibody (p40) and a band of lower molecular mass which reacted with monoclonal antibody 3G9. The latter antigen was designated p39. Both antigens were conserved in all H. somnus isolates tested. The specific antibodies were also used to detect cross-reacting antigens in other gram-negative bacteria. Antibody to p40 reacted with proteins of 55 to 28 kDa, with the greatest intensity shown among proteins from other members of the family Pasteurellaceae. Antibody to p40 was reduced by absorption with live H. somnus or other members of the family Pasteurellaceae, so the antigen appears to be surface exposed. Antibody to p39 only cross-reacted with a broad band (38 to 40 kDa) in Haemophilus agni. Since H. agni is not a bovine pathogen and since convalescent-phase serum from H. somnus-infected animals did recognize p39, the latter may be a good immunodiagnostic antigen, if the lack of cross-reactivity with antigens in other gram-negative bacteria is confirmed with a polyclonal antibody to p39. The cross-reactivity of antiserum to p40 with antigens of members of the family Pasteurellaceae and the ability of this antiserum to protect against H. somnus pneumonia indicate that p40 may be a useful vaccine antigen for H. somnus disease and perhaps even diseases caused by other members of the family Pasteurellaceae.