Construction of an isogenic leukotoxin deletion mutant of Pasteurella haemolytica serotype 1: characterization and virulence

Leukotoxins of gram-negative bacteria

Leukotoxins are a group of exotoxins that produce their primary toxic effects against leukocytes, especially polymorphonuclear cells (PMNs). Leukotoxins include a variety of chemicals ranging from 9,10-epoxy 12-octadecenoate, a fatty acid derivative secreted by leukocytes themselves, to proteins such as RTX (repeats in toxin). This review focuses on leukotoxins of three species of gram-negative bacteria, Mannheimia (Pasteurella) haemolytica, Actinobacillus actinomycetemcomitans, and Fusobacterium necrophorum.

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.

Mannheimia haemolytica leukotoxin activates a nonreceptor tyrosine kinase signaling cascade in bovine leukocytes, which induces biological effects

The leukotoxin (LktA) produced by Mannheimia haemolytica binds to bovine lymphocyte function-associated antigen 1 (LFA-1) and induces biological effects in bovine leukocytes 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 effects must entail both binding to LFA-1 and activation of signaling pathways which are present in bovine leukocytes. However, the signaling pathways leading to biological effects upon LktA binding to LFA-1 have not been characterized. In this context, several reports have indicated that ligand binding to LFA-1 results in activation of a nonreceptor tyrosine kinase (NRTK) signaling cascade. We designed experiments with the following objectives: (i) to determine whether LktA binding to LFA-1 leads to activation of NRTKs, (ii) to examine whether LktA-induced NRTK activation is target cell specific, and (iii) to determine whether LktA-induced NRTK activation is required for biological effects. We used a biologically inactive mutant leukotoxin (DeltaLktA) for comparison with LktA. Our results indicate that LktA induces tyrosine phosphorylation (TP) of the CD18 tail of LFA-1 in bovine leukocytes. The DeltaLktA mutant does not induce TP of the CD18 tail, albeit binding to bovine LFA-1. LktA-induced TP of the CD18 tail was attenuated by an NRTK inhibitor, herbimycin A; a phosphatidylinositol 3'-kinase (PI 3-kinase) inhibitor, wortmannin; and a Src kinase inhibitor, PP2, in a concentration-dependent manner. Furthermore, LktA induces TP of the CD18 tail in bovine, but not porcine, leukocytes. Moreover, LktA-induced intracellular calcium ([Ca2+]i) elevation was also inhibited by herbimycin A, wortmannin, and PP2. Thus, our data represent the first evidence that binding of LktA to bovine LFA-1 induces a species-specific NRTK signaling cascade involving PI 3-kinase and Src kinases and that this signaling cascade is required for LktA-induced biological effects.

Bordetella bronchiseptica fimbrial protein-enhanced immunogenicity of a Mannheimia haemolytica leukotoxin fragment

Leukotoxin produced by Mannheimia (Pasteurella) haemolytica is an important virulence factor in shipping fever pneumonia in feedlot cattle and is a critical protective antigen. In this study, the immune response to a chimeric protein generated by combining a gene fragment encoding neutralizing epitopes of M. haemolytica leukotoxin and a fimbrial protein gene (fim N) from Bordetella bronchiseptica was evaluated. The recombinant gene was cloned in a bacterial expression vector under the control of the tac promoter and expressed as a fusion protein with glutathione-S-transferase (GST) in Escherichia coli. Immunization of mice with the recombinant protein, GST-LTXFIM elicited a significantly stronger anti-leukotoxin antibody response than comparable immunizations with GST-LTX fusion proteins lacking FIM N. The GST-LTXFIM was also more stable than GST-LTX during storage at -80 degrees C, thus alleviating a stability problem inherent to leukotoxin. This chimeric protein may be a candidate for inclusion in new generation vaccines against shipping fever pneumonia.

Pasteurella (Mannheimia) haemolytica leukotoxin-induced cytolysis of bovine leukocytes: role of arachidonic acid and its regulation

Pasteurella (Mannheimia) haemolytica leukotoxin (Lkt) is the major factor that contributes to lung injury in bovine pneumonic pasteurellosis. Lkt is a pore-forming exotoxin that has the unique property of inducing cytolysis only in ruminant leukocytes and platelets. Cytolysis of many cell types is mediated by arachidonic acid (AA) and its generation by phospholipases is regulated by G-protein-coupled receptors. However, the contribution of Lkt-induced AA generation to cytolysis and the signalling cascade underlying AA generation in bovine leukocytes have not been determined. We have determined whether AA mediates Lkt-induced cytolysis and delineated the signalling mechanisms underlying AA generation in bovine leukocytes. Bovine lymphoma cells were used as an experimental system to investigate the Lkt-induced [(3)H] AA release, an index of AA generation and lactate dehydrogenase release, an index of cytolysis. The results indicate that Lkt induces AA release and cytolysis in a concentration- and time-dependent fashion. The AA analog, 5,8,11,14-eicosatetraynoic acid inhibited Lkt-induced cytolysis, but not AA release. Lkt-induced AA release and cytolysis were inhibited by pertussis toxin, inhibitors of cytosolic phospholipase A(2)(cPLA(2)), phospholipase C and protein kinase C (PKC), and by chelation of intracellular calcium. Furthermore, Western blot analysis revealed the presence of G(i), G(s)and G(q)type G-proteins. These results demonstrate that AA metabolites from cPLA(2)activation contribute to Lkt-induced cytolysis and G(i)type G-proteins, Ca(2+)and PKC, regulate the cPLA(2)activity.

Inactivation of Pasteurella (Mannheimia) haemolytica leukotoxin causes partial attenuation of virulence in a calf challenge model

The leukotoxin of Pasteurella (Mannheimia) haemolytica is believed to play a significant role in pathogenesis, causing cell lysis and apoptosis that lead to the lung pathology characteristic of bovine shipping fever. Using a system for Cre-lox recombination, a nonpolar mutation within the lktC transacylase gene of the leukotoxin operon was created. The lktC locus was insertionally inactivated using a loxP-aph3-loxP cassette, and then the aph3 marker was excised from the chromosome by Cre recombinase expressed from a P. haemolytica plasmid. The resulting lktC strain (SH2099) secretes inactive leukotoxin and carries no known antibiotic resistance genes. Strain SH2099 was tested for virulence in a calf challenge model. We inoculated 3 x 10(8) or 3 x 10(9) CFU of wild-type or mutant bacteria into the lungs of healthy, colostrum-deprived calves via transthoracic injection. Animals were observed for clinical signs and for nasal colonization for 4 days, after which they were euthanized and necropsied. The lower inoculum (3 x 10(8) CFU) caused significantly fewer deaths and allowed lung pathology to be scored and compared, while the 3 x 10(9) CFU dose of either the wild-type or mutant was lethal to >/=50% of the calves. The estimated 50% lethal dose of SH2099 was four times higher than that of the wild-type strain. Lung lesion scores were reduced twofold in animals inoculated with the mutant, while clinical scores were nearly equivalent for both strains. The wild-type and mutant strains were equally capable of colonizing the upper respiratory tracts of the calves. In this study, the P. haemolytica lktC mutant was shown to be less virulent than the parent strain.

FnrP interactions with the Pasteurella haemolytica leukotoxin promoter

Pasteurella haemolytica FnrP is homologous to Fnr, the global transcriptional regulator of anaerobic respiration in Escherichia coli. To investigate the role of O(2) in the expression of P. haemolytica leukotoxin, we tested a lktC::lacZ fusion constructed in E. coli for a FnrP-mediated regulatory effect under aerobic and anaerobic growth conditions. Both E. coli Fnr and FnrP suppressed leukotoxin transcription under aerobic conditions. Under anaerobic conditions, Fnr suppressed transcription, while FnrP increased transcription. These results were confirmed using FnrP*, a mutant form of FnrP that activates anaerobically inducible genes under aerobic conditions. In mobility shift assays, partially purified FnrP bound to a potential regulatory site in a P. haemolytica lktC promoter fragment.

Lymphocyte function-associated antigen 1 is a receptor for Pasteurella haemolytica leukotoxin in bovine leukocytes

Pasteurella (Mannheimia) haemolytica leukotoxin (Lkt) causes cell type- and species-specific effects in ruminant leukocytes. Recent studies indicate that P. haemolytica Lkt binds to bovine CD18, the common subunit of all beta2 integrins. We designed experiments with the following objectives: to identify which member of the beta2 integrins is a receptor for Lkt; to determine whether Lkt binding to the receptor is target cell (bovine leukocytes) specific; to define the relationships between Lkt binding to the receptor, calcium elevation, and cytolysis; and to determine whether a correlation exists between Lkt receptor expression and the magnitude of target cell cytolysis. We compared Lkt-induced cytolysis in neutrophils from control calves and from calves with bovine leukocyte adhesion deficiency (BLAD), because neutrophils from BLAD-homozygous calves exhibit reduced beta2 integrin expression. The results demonstrate for the first time that Lkt binds to bovine CD11a and CD18 (lymphocyte function-associated antigen 1 [LFA-1]). The binding was abolished by anti-CD11a or anti-CD18 monoclonal antibody (MAb). Lkt-induced calcium elevation in bovine alveolar macrophages (BAMs) was inhibited by anti-CD11a or anti-CD18 MAb (65 to 94% and 37 to 98%, respectively, at 5 and 50 Lkt units per ml; P < 0.05). Lkt-induced cytolysis in neutrophils and BAMs was also inhibited by anti-CD11a or anti-CD18 MAb in a concentration-dependent manner. Lkt bound to porcine LFA-1 but did not induce calcium elevation or cytolysis. In neutrophils from BLAD calves, Lkt-induced cytolysis was decreased by 44% compared to that of neutrophils from control calves (P < 0.05). These results indicate that LFA-1 is a Lkt receptor, Lkt binding to LFA-1 is not target cell specific, Lkt binding to bovine LFA-1 correlates with calcium elevation and cytolysis, and bovine LFA-1 expression correlates with the magnitude of Lkt-induced target cell cytolysis.

Ultrastructural characterization of apoptosis in bovine lymphocytes exposed to Pasteurella haemolytica leukotoxin

OBJECTIVE

To characterize ultrastructural changes of bovine lymphocytes exposed to Pasteurella haemolytica leukotoxin (LKT).

SAMPLE POPULATION

Partially purified LKT from a wild type P. haemolytica A1 strain and inactive pro-LKT from an isogeneic mutant Phaemolytica strain. Isolated bovine lymphocytes were obtained from 2 healthy calves.

PROCEDURE

Isolated bovine lymphocytes were incubated with various concentrations of LKT and pro-LKT for 3 hours at 37 C and examined by use of transmission electron microscopy. A cytochemical Klenow DNA fragmentation assay was used to examine lymphocytes for DNA fragmentation.

RESULTS

Lymphocytes incubated with LKT at a high concentration (1.0 toxic U/ml) had ultrastructural evidence of cytoplasmic and nuclear membrane rupture and swelling or lysis of mitochondria. Low concentrations of leukotoxin (0.1 toxic U/ml) induced DNA fragmentation in 80% of lymphocytes. Ultrastructurally, these cells had nuclear membrane blebbing, cytoplasmic vaculation, chromatin condensation, nuclear fragmentation, and membrane-bound apoptotic bodies. Incubation of lymphocytes with LKT at extremely low concentrations (0.001 toxic U/ml) or with pro-LKT did not alter their ultrastructure. Inclusion of 0.5 mM ZnCl2 in the medium blocked leukotoxin-induced ultrastructural changes in bovine lymphocytes.

CONCLUSIONS AND CLINICAL RELEVANCE

Low concentrations of LKT induce apoptosis and high concentrations induce oncotic cell lysis in bovine lymphocytes. The ability of low LKT concentrations to induce apoptosis in host leukocytes may allow bacteria to escape host immune surveillance and colonize the host.

Cloning and characterization of the gene encoding Pasteurella haemolytica FnrP, a regulator of the Escherichia coli silent hemolysin sheA

A Pasteurella haemolytica A1 gene was identified from a recombinant library clone that expressed hemolysis in host Escherichia coli cells. The gene, designated fnrP, had sequence identity to E. coli fnr, a global transcriptional regulator of genes required for conversion to anaerobic growth. FnrP complemented anaerobic deficiencies of a fnr-null mutant strain of E. coli and increased expression of the Fnr-dependent, anaerobic terminal reductase gene, frdA. FnrP was purified, identified by immunoblotting, and shown to be nonhemolytic. When FnrP was expressed in E. coli DeltasheA, a null mutant of the cryptic hemolysin SheA, the transformants were nonhemolytic, indicating that FnrP activates this silent hemolysin.

Pasteurella haemolytica leukotoxin induced apoptosis of bovine lymphocytes involves DNA fragmentation

It has been reported that Pasteurella haemolytica leukotoxin (LKT) induces morphologic changes in bovine leukocytes consistent with apoptosis in vitro, but DNA fragmentation was not observed. We investigated whether bovine lymphocytes undergo DNA fragmentation during LKT-induced apoptosis. Bovine peripheral blood lymphocytes were isolated by density gradient centrifugation and exposed to LKT or inactive pro-LKT protein from a lktC- mutant strain. After exposure, DNA fragmentation in lymphocytes was quantified colorimetrically by diphenylamine assay and visualized by agarose gel electrophoresis. At high LKT concentrations, bovine lymphocytes were lysed, but at low concentrations, LKT caused DNA fragmentation characteristic of apoptosis. Maximal DNA fragmentation in bovine lymphocytes was induced by 0.1 TU ml(-1) LKT following 3 h exposure, but only background level of DNA fragmentation was observed with the inactive pro-LKT. Equine lymphocytes that are resistant to LKT intoxication did not show DNA fragmentation following exposure to LKT. Preincubation of LKT with a neutralizing anti-LKT monoclonal antibody inhibited LKT-induced DNA fragmentation. Electrophoresis of DNA from bovine lymphocytes treated with 0.1 TU ml(-1) LKT demonstrated the typical 'ladder' pattern of internucleosomal DNA cleavage, the hallmark of apoptosis associated with activation of endonucleases. LKT-induced DNA fragmentation was inhibited by 0.5 mM ZnCl2, an endonuclease inhibitor. The results indicated that LKT at low concentrations induced apoptotic cell death of bovine lymphocytes, which may play a role in initiation and persistence of P. haemolytica infection.