Mannheimia haemolytica serotype 1 (S1) is the most common bacterial isolate found in shipping fever pneumonia in beef cattle. Currently used vaccines against M. haemolytica do not provide complete protection against the disease. Research with M. haemolytica outer membrane proteins (OMPs) has shown that antibodies to one particular OMP from S1, PlpE, may be important in immunity. In a recently published work, members of our laboratory showed that recombinant PlpE (rPlpE) is highly immunogenic when injected subcutaneously into cattle and that the acquired immunity markedly enhanced resistance to experimental challenge (A. W. Confer, S. Ayalew, R. J. Panciera, M. Montelongo, L. C. Whitworth, and J. D. Hammer, Vaccine 21:2821-2829, 2003). The objective of this work was to identify epitopes of PlpE that are responsible for inducing the immune response. Western blot analysis of a series of rPlpE with nested deletions on both termini with bovine anti-PlpE hyperimmune sera showed that the immunodominant region is located close to the N terminus of PlpE. Fine epitope mapping, in which an array of overlapping 13-mer synthetic peptides attached to a derivatized cellulose membrane was probed with various affinity-purified anti-PlpE antibodies, identified eight highly reactive regions, of which region 2 (R2) was identified as the specific epitope. The R2 region is comprised of eight imperfect repeats of a hexapeptide (QAQNAP) and is located between residues 26 and 76. Complementmediated bactericidal activity of affinity-purified anti-PlpE bovine antibodies confirmed that antibodies directed against the R2 region are effective in killing M. haemolytica.Bovine respiratory disease arises from the interaction of numerous contributing factors, including physical stresses associated with weaning, shipment, inclement weather, and overcrowding coupled with viral and bacterial infections (8,31,32,59,63). The result in severe cases is colonization of the lungs with pathogenic bacteria resulting in severe pneumonia. Pasteurella multocida, Haemophilus somnus, and Mannheimia (formerly Pasteurella) haemolytica are associated with bovine pneumonia. However, M. haemolytica serotype 1 (S1) is by far the most important bacterial pathogen in the development of the often-fatal fibrinous pleuropneumonia in beef cattle known as pneumonic pasteurellosis or shipping fever (31, 32).Immunity against M. haemolytica is thought to be primarily through production of serum antibodies that neutralize the secreted leukotoxin (LKT) and antibodies against surface antigens (45). The mechanism of activity of antisurface antibodies and the specific surface antigens involved in anti-M. haemolytica immunity are not known; however, complementmediated bacterial lysis and bacterial phagocytosis and killing are thought to be important in defense against M. haemolytica infection (45). Complement-mediated bactericidal activity against M. haemolytica and phagocytosis of M. haemolytica by bovine neutrophils has been demonstrated with bovine immune serum (12,17,40,46).Litt...