|Briggs, Robert - Bob|
Submitted to: Infection and Immunity
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/6/2001
Publication Date: N/A
Citation: Interpretive Summary: Bacterial pneumonia caused by Mannheimia haemolytica is a major economic problem to the beef and dairy cattle industries in the United States and in Western Europe. Current strategies to control the disease include vaccination and antibiotic medication. The former method is largely ineffective due to marketing limitations in regard to the timing of administration. Antibiotic medication is costly and leads to adverse ecological consequences with increasing microbial antibiotic resistance. Alternative therapeutic/ prevention strategies are therefore desirable. Leukotoxin, a toxin elaborated by this bacterium, is the primary virulence factor that contributes to the progression and damage caused by the disease. Several lines of evidence, however, indicate that it is the animal's response to the toxin rather than the toxin itself that causes the damage. The toxin specifically binds to receptors on certain cell types and elicits an inflammatory cascade of events. It was found in this experiment that leukotoxin induces non-receptor tyrosine kinases, that the activation is target-cell specific, that tyrosine kinase activation is required for subsequent inflammatory cell responses, and that mere binding of toxin is insufficient to trigger tyrosine kinase activation. A better understanding of the mechanisms by which Mannheimia haemolytica leukotoxin interacts with cell receptors and the subsequent release of inflammatory mediators will provide new avenues for effective control of bacterial pneumonia in cattle.
Technical Abstract: The leuktoxin (LktA) produced by Mannheimia haemolytica binds to bovine lymphocyte function-associated antigen 1 (LFA-1) and induces biological effects in bovine leukotcytes in a cellular and species-specific fashion. We have previously shown that LktA also binds to porcine LFA-1 without eliciting any effects. These findings suggest that the specificity of LktA Aeffects must entail both binding to LFA-1 and, separately, activation of signaling pathways which are present in bovine leukocytes. In this context, several reports have indicated that ligand binding to LFA-1 results in activation of a non-receptor tyrosine kinase (NRTK) signaling cascade. We designed experiments: 1) to determine whether LktA binding to LFA-1 leads to activation of NRTKs; 2) to examine whether LktA-induced NRTK activation is target cell specific; and 3) to determine whether LktA- induced NRTK activation is required for biological effects. We used a biologically inactive mutant leukotoxin (deltaleukotoxin) for comparison with LktA. Our results indicate that LktA induces tyrosine phosphorylation (TP) of the CD18 tail of LFA-1 in bovine leukotcytes. The deltaleukotoxin does not induce TP of the CD18 tail but still binds to bovine LFA-1. LktA-induced TP of the CD18 tail and LktA-induced intracellular calcium elevation was attenuated by a NRTK inhibitor, herbimycin, wortmannin, and a Src-kinase inhibitor (PP2) in a concentration dependent manner. Furthermore, LktA induces TP of the CD18 tail in bovine, but not in porcine leukocytes. These data represent the first evidence that binding of LktA to bovine LFA-1 induces a species-specific NRTK signaling cascade involving PI-3- and Src-kinases, and this signaling cascade is required for LktA- induced biological effects.