Antibodies to outer membrane proteins but not to lipopolysaccharide inhibit pulmonary proliferation of Pasteurella multocida in mice

The role of rabbit antibodies against Pasteurella multocida outer membrane proteins and lipopolysaccharides (LPS) in resistance remains unknown. Pooled immune sera against P. multocida outer membranes were prepared from specific-pathogen-free rabbits immunized with sucrose gradient-purified P. multocida outer membranes. Western immunoblotting showed that purified outer membrane protein antibodies reacted strongly against the outer membrane proteins but not the purified LPS. Affinity-purified LPS antibodies exhibited strong reactivity against purified LPS and very little reactivity against outer membrane vesicles. Mice were inoculated intranasally with immune serum or normal rabbit serum, challenged intranasally with 10⁶ CFU of P. multocida, and euthanatized 48 h later to determine the number of P. multocida organisms in the lungs. Mice inoculated with pooled immune serum had a 3,300-fold reduction (P < 0.001) in the numbers of P. multocida in the lungs as compared with the controls. Similarly, mice inoculated with purified outer membrane protein antibodies had a 201-fold reduction (P < 0.001) in the numbers of P. multocida. Conversely, mice inoculated with affinity-purified LPS antibodies had a 1.1-fold reduction (P > 0.50) in the numbers of P. multocida. These results show that antibodies against the outer membrane proteins but not the LPS are the components of rabbit immune sera which inhibit P. multocida proliferation in mouse lungs.