A monoclonal antibody against a Pasteurella multocida outer membrane protein protects rabbits and mice against pasteurellosis

Monoclonal antibodies (MAbs) directed against the 37.5-kDa outer membrane protein were produced by fusing myeloma cells with spleen cells obtained from mice immunized with a pathogenic strain of Pasteurella multocida isolated from a rabbit. Desirable MAbs were selected by enzyme-linked immunosorbent assay, whole-cell radioimmunoprecipitation (WC-RIP), and Western blot (immunoblot) analysis. WC-RIP and Western blot analyses, using MAb 1608 adsorbed with intact P. multocida cells and the eluted MAb, demonstrated that the antigen recognized by this MAb is exposed on the cell surface, is antibody accessible, and has an estimated molecular mass of 37.5 kDa. Treatment of outer membrane vesicles of P. multocida with proteinase K totally abrogated the MAb 1608 activity, indicating that this MAb binds to a protein antigenic determinant. Furthermore, MAb 1608 was nonreactive to purified lipopolysaccharide in Western blot analysis. Passive transfer studies showed that nine rabbits inoculated intranasally with MAb 1608 and homologously challenged intranasally had significantly reduced mortality, severity of pneumonia, prevalence of P. multocida colonization in nonrespiratory organs, and numbers of p. multocida in nasal cavities compared with the controls. Furthermore, the number of P. multocida in lungs was reduced 84,750-fold. Similarly, passive transfer experiments indicated that MAb 1608 protected mice against homologous and heterologous challenges with P. multocida strains bearing the antigenic determinant recognized by MAb 1608. However, no protection was afforded by MAb 1608 when mice were challenged with a P. multocida strain lacking the antigenic determinant recognized by MAb 1608. This study establishes that the 37.5-kDa outer membrane protein is the target for a protective MAb.