Genetic and immunologic analyses of PlpE, a lipoprotein important in complement-mediated killing of Pasteurella haemolytica serotype 1

Pasteurella haemolytica serotype 1 is the bacterium most commonly associate d with bovine shipping fever. The presence of antibodies against P, haemoly tica outer membrane proteins (OMPs) correlates statistically,vith resistanc e to experimental P, haemolytica challenge in cattle. Until now, specific P . haemolytica OMPs which elicit antibodies that function in host defense me chanisms have not been identified. In this study, we have cloned and sequen ced the gene encoding one such protein, PlpE. Analysis of the deduced amino acid sequence revealed that PlpE is a lipoprotein and that it is similar t o an Actinobacillus pleuropneumoniae lipoprotein, OmlA. Affinity-purified, anti-PlpE antibodies recognize a protein in all serotypes of P. haemolytica except serotype 11. We found that intact P. haemolytica and recombinant E. coli expressing PlpE are capable of absorbing anti-PlpE antibodies from bo vine immune serum, indicating that PlpE is surface exposed in P. haemolytic a and assumes a similar surface-exposed conformation in E. coli. In complem ent-mediated killing assays, we observed a significant reduction in killing of P. haemolytica when bovine immune serum that was depleted of anti-PlpE antibodies was used as the source of antibody. Our data suggest that PlpE i s surface exposed and immunogenic in cattle and that antibodies against Plp E contribute to host defense against P. haemolytica.