Use of TUNEL staining to detect apoptotic cells in the lungs of cattle experimentally infected with Pasteurella haemolytica

Mannheimia haemolytica leukotoxin binds cyclophilin D on bovine neutrophil mitochondria

Mannheimia haemolytica is an important member of the bovine respiratory disease (BRD) complex that causes fibrinous and necrotizing pleuropneumonia in cattle. BRD is characterized by abundant neutrophil infiltration into the alveoli and fibrin deposition. The most important virulence factor of M. haemolytica is its leukotoxin. Previous research in our laboratory has shown that the leukotoxin is able to enter into and traffic to the mitochondria of a bovine lymphoblastoid cell line (BL-3). In this study, we evaluated the ability of LKT to be internalized and travel to mitochondria in bovine neutrophils. We demonstrate that LKT binds bovine neutrophil mitochondria and co-immunoprecipitates with TOM22 and TOM40, which are members of the translocase of the outer mitochondrial (TOM) membrane family. Upon entry into mitochondria, LKT co-immunoprecipitates with cyclophilin D, a member of the mitochondria permeability transition pore. Unlike BL-3 cells, bovine neutrophil mitochondria are not protected against LKT by the membrane-stabilizing agent cyclosporin A, nor were bovine neutrophil mitochondria protected by the permeability transition pore antagonist bongkrekic acid. In addition, we found that bovine neutrophil cyclophilin D is significantly smaller than that found in BL-3 cells. Bovine neutrophils were protected against LKT by protein transfection of an anti-cyclophilin D antibody directed at the C-terminal amino acids, but not an antibody against the first 50 N-terminal amino acids. In contrast, BL-3 cells were protected by antibodies against either the C-terminus or N-terminus of cyclophilin. These data confirm that LKT binds to bovine neutrophil mitochondria, but indicate there are distinctions between neutrophil and BL-3 mitochondria that might reflect differences in cyclophilin D.

Mannheimia haemolytica and its leukotoxin cause neutrophil extracellular trap formation by bovine neutrophils

Mannheimia haemolytica is an important member of the bovine respiratory disease complex, which is characterized by abundant neutrophil infiltration into the alveoli and fibrin deposition. Recently several authors have reported that human neutrophils release neutrophil extracellular traps (NETs), which are protein-studded DNA matrices capable of trapping and killing pathogens. Here, we demonstrate that the leukotoxin (LKT) of M. haemolytica causes NET formation by bovine neutrophils in a CD18-dependent manner. Using an unacylated, noncytotoxic pro-LKT produced by an ΔlktC mutant of M. haemolytica, we show that binding of unacylated pro-LKT stimulates NET formation despite a lack of cytotoxicity. Inhibition of LKT binding to the CD18 chain of lymphocyte function-associated antigen 1 (LFA-1) on bovine neutrophils reduced NET formation in response to LKT or M. haemolytica cells. Further investigation revealed that NETs formed in response to M. haemolytica are capable of trapping and killing a portion of the bacterial cells. NET formation was confirmed by confocal microscopy and by scanning and transmission electron microscopy. Prior exposure of bovine neutrophils to LKT enhanced subsequent trapping and killing of M. haemolytica cells in bovine NETs. Understanding NET formation in response to M. haemolytica and its LKT provides a new perspective on how neutrophils contribute to the pathogenesis of bovine respiratory disease.

Streptococcal pyrogenic exotoxin B-induced apoptosis in a549 cells is mediated by a receptor- and mitochondrion-dependent pathway

It has been shown that streptococcal pyrogenic exotoxin B (SPE B) can induce cells to undergo apoptosis. The present study is to dissect the role of SPE B protease and SPE B protein in the apoptotic process of A549 cells and to elucidate the SPE B-induced apoptotic pathway. Recombinant SPE B (rSPE B) and C192S, a mutant of SPE B without protease activity, were expressed in Escherichia coli and purified by using an affinity column. The apoptosis of A549 cells was assayed by propidium iodide staining, followed by flow cytometry analysis. Our results showed that SPE B induced apoptosis in a dose-dependent manner, whereas C192S did not. When cells were pretreated with rSPE B (2 mug/ml) for as briefly as 5 min and then incubated with C192S of 28 kDa, an apoptosis that is proportional to the period of pretreatment was observed but not with C192S of 42 kDa. These results suggest that the extracellular protease activity of rSPE B is required for the initiation of apoptosis and that the size of SPE B is important for an effective induction of apoptosis. The time course analysis revealed that molecules activated in apoptosis were in the following order: caspase-8 (1.5 h), t-Bid (2.5 h), Bax (3 h), cytochrome c release (6 h), caspase-9 (7 h), and caspase-3 (8 h). The overexpression of Bcl-2 inhibited depolarization of mitochondrial membrane, cytochrome c release, and apoptosis. The results of the present study suggest that SPE B-induced apoptosis is mediated through a receptor-like mechanism and a mitochondrion-dependent pathway.

Inflammatory cytokines enhance the interaction of Mannheimia haemolytica leukotoxin with bovine peripheral blood neutrophils in vitro

Mannheimia (Pasteurella) haemolytica A1 produces several virulence factors that play an important role in the pathogenesis of bovine pneumonic pasteurellosis. Foremost among these is a leukotoxin (LKT) that specifically kills ruminant leukocytes. Recent evidence suggests that M. haemolytica LKT binding to bovine leukocytes is mediated by the beta(2)-integrin CD11a/CD18 (lymphocyte function-associated antigen 1 [LFA-1]), which subsequently induces activation and cytolysis of these cells. Inflammatory cytokines, which are released during viral and bacterial infection, are reported to increase LFA-1 expression and conformational activation. We investigated the effects of the inflammatory cytokines interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and gamma interferon (IFN-gamma) on the interaction of M. haemolytica LKT with bovine peripheral blood neutrophils (PMNs). In this study we demonstrated, by flow cytometry, that bovine PMNs increased their binding to an anti-bovine LFA-1 monoclonal antibody (BAT75A) following in vitro incubation with IL-1beta, TNF-alpha, or IFN-gamma. Incubation with cytokines also increased CD18 expression, as assessed by real-time PCR and by Western blotting. Increased LFA-1 expression by PMNs exposed to cytokines was associated with increased LKT binding and cytotoxicity. The latter represented, at least in part, enhanced PMN apoptosis, as assessed by propidium iodine staining and caspase-3 activation. The results of this study suggest that inflammatory cytokines may play an important role in enhancing the biological response of bovine PMNs to M. haemolytica LKT.

Effect of experimental infection of cattle with bovine herpesvirus-1 (BHV-1) on the ex vivo interaction of bovine leukocytes with Mannheimia (Pasteurella) haemolytica leukotoxin

Mannheimia (Pasteurella) haemolytica A1 produces an extracellular leukotoxin (LKT) that is reported to bind the beta(2)-integrin CD11a/CD18 (LEA-1) on ruminant leukocytes. LKT binding induces activation, and subsequent cytolysis, of these cells. It is well known that active viral infection greatly increases the susceptibility of cattle to pasteurellosis. To better understand the mechanism by which this occurs, we investigated the effects of experimental in vivo infection of cattle with bovine herpes virus-1 (BHV-1) on the ex vivo interaction of bovine leukocytes with the M. haemolytica LKT. In this study, we demonstrated that active BHV-1 infection increased the expression of the beta(2)-integrin CD11a/CD18 (as defined by the mAb BAT75) on bovine peripheral blood neutrophils, enhanced the binding of LKT to bronchoalveolar lavage (BAL) leukocytes and peripheral blood neutrophils, and increased the killing of BAL leukocytes and peripheral blood leukocytes by LKT. In addition, BHV-1 greatly increased the number of BAL, resulting in many more LKT-responsive cells being present in the lungs. These findings might explain in part the increased susceptibility of BHV-1 infected cattle to pneumonic pasteurellosis.

Haemophilus somnus induces apoptosis in bovine endothelial cells in vitro

Haemophilus somnus causes pneumonia, reproductive failure, infectious myocarditis, thrombotic meningoencephalitis, and other diseases in cattle. Although vasculitis is commonly seen as a result of systemic H. somnus infections, the pathogenesis of vascular damage is poorly characterized. In this study, we demonstrated that H. somnus (pathogenic isolates 649, 2336, and 8025 and asymptomatic carrier isolates 127P and 129Pt) induce apoptosis of bovine endothelial cells in a time- and dose-dependent manner, as determined by Hoechst 33342 staining, terminal deoxynucleotidyl transferase-mediated dUTP-FITC nick end labeling, DNA fragmentation, and transmission electron microscopy. H. somnus induced endothelial cell apoptosis in as little as 1 h of incubation and did not require extracellular growth of the bacteria. Viable H. somnus organisms induced greater endothelial cell apoptosis than heat-killed organisms. Since viable H. somnus cells release membrane fibrils and blebs, which contain lipooligosaccharide (LOS) and immunoglobulin binding proteins, we examined culture filtrates for their ability to induce endothelial cell apoptosis. Culture filtrates induced similar levels of endothelial cell apoptosis, as did viable H. somnus organisms. Heat inactivation of H. somnus culture filtrates partially reduced the apoptotic effect on endothelial cells, which suggested the presence of both heat-labile and heat-stable factors. We found that H. somnus LOS, which is heat stable, induced endothelial cell apoptosis in a time- and dose-dependent manner and was inhibited by the addition of polymyxin B. These data demonstrate that H. somnus and its LOS induce endothelial cell apoptosis, which may play a role in producing vasculitis in vivo.

Recombinant bovine interleukin-1beta amplifies the effects of partially purified Pasteurella haemolytica leukotoxin on bovine neutrophils in a beta(2)-integrin-dependent manner

The influx and death of polymorphonuclear leukocytes within the infected lung are hallmarks of bovine pasteurellosis. Recent reports have shown that the Pasteurella haemolytica leukotoxin (LKT) and other RTX toxins bind beta(2)-integrins on target cells. In this study we demonstrate that exposure of bovine neutrophils to recombinant bovine interleukin-1beta upregulates beta(2)-integrins (CD11a/CD18), which in turn enhance the binding and amplify the biological effects of partially purified LKT on these cells. LKT binding and cytotoxicity were inhibited by addition of an anti-integrin antibody (CD11a/CD18). These findings help to clarify the early events that occur in bovine pasteurellosis and support the hypothesis that inflammatory mediators might increase the severity of pasteurellosis by causing upregulation of beta(2)-integrins that serve as an LKT receptor on bovine neutrophils.