Protective ability of antibodies against 78- and 40-kilodalton outer membrane antigens of Haemophilus somnus.

The ability of concentrated antibody against the 78- or 40-kilodalton (kDa) outer membrane protein (OMP) of Haemophilus somnus to passively protect calves against H. somnus-induced pneumonia was determined. The 78- and 40-kDa OMPs were evaluated in passive protection experiments, because results of previous studies demonstrated their (i) immunogenicity for cattle, (ii) intense reactivity with convalescent-phase sera which passively protected calves against experimental H. somnus pneumonia, (iii) surface location and accessibility to antibody, and (iv) conservation among a wide range of H. somnus isolates obtained from animals with different diseases and from different geographic locations. The specificity of the two antisera evaluated in this study was verified by (i) immunoblots in which reactivity against the 78- or 40-kDa OMP was present in postimmunization but not preimmunization serum and (ii) immunoblots in which affinity-purified, surface-reactive antibodies in each antisera were used, which demonstrated that essentially only antibody to the 78- or 40-kDa OMP was reactive with the surface of H. somnus. In enzyme-linked immunosorbent assays, the antiserum against the 40-kDa OMP contained immunoglobulin G1 (IgG1), IgG2, and IgM against H. somnus, while the antiserum against the 78-kDa OMP contained IgG1 and IgM but no IgG2 against H. somnus. The antiserum against the 40-kDa OMP contained IgG1 and IgG2 specific for the 40-kDa OMP, as determined by Western blot analysis. Slight reactivity against H. somnus lipopolysaccharide was detected by enzyme-linked immunosorbent assay but not by Western blot analysis. In passive protection experiments, preincubation of bacteria with antibody against the 40-kDa OMP protected calves (P less than 0.025) against H. somnus pneumonia, while antibody against the 78-kDa OMP failed to protect calves against H. somnus pneumonia. Determination of the potential protective capacity of the 78-kDa OMP awaits resolution of the role of anti-78-kDa IgG2 in protection against H. somnus pneumonia. The 40-kDa OMP is, however, a good candidate antigen for evaluation of protective ability against H. somnus pneumonia following active immunization.