Particular aspects show that the signal peptide remains intact on the mature CD18 molecule on ruminant leukocytes rendering these cells susceptible to cytolysis by Lkt. Comparative amino acid sequence analysis of the signal peptide of CD18 of eight ruminants and five non-ruminants revealed that the ruminant CD18 signal peptides contain ‘cleavage-inhibiting’ glutamine (Q), compared to ‘cleavage-conducive’ glycine in non-ruminants, at position −5 relative to the cleavage site. Mutagenesis of Q at position −5 of the bovine CD18 signal peptide to G resulted in the abrogation of Lkt-mediated cytolysis of transfectants expressing bovine CD18 carrying the Q(−5)G mutation. Provided is novel technology to clone cattle and other ruminants expressing CD18 without the signal peptide on their leukocytes, providing ruminants that are less susceptible to M. haemolytica. Methods for treating conditions and/or diseases associated with M. haemolytica (e.g., pneumonic pasteurellosis), comprising administration of polypeptides comprising CD18 signal peptide sequences are also provided.