Pulmonary Expression of Tumor Necrosis Factor Alpha, Interleukin-1 Beta, and Interleukin-8 in the Acute Phase of Bovine Pneumonic Pasteurellosis

Inflammatory cytokines are suspected to contribute to the pathogenesis of bovine pneumonic pasteurellosis (BPP) through neutrophil recruitment, leukocyte activation, and the induction of a broad array of soluble inflammatory mediators. An in vivo experimental model of BPP was used to characterize the pulmonary expression kinetics of tumor necrosis factor alpha (TNFα), interleukin-1 beta (IL-1β), and interleukin-8 (IL-8) genes and proteins during the acute phase of disease development. Cytokine expression in bronchoalveolar lavage (BAL) fluid, BAL cells, and pneumonic lung parenchyma was quantitated by northern blot analysis, enzyme-linked immunosorbent assay (ELISA), and in situ hybridization at 2, 4, 8, 16, and 24 hours after endobronchial inoculation of Pasteurella (Mannheimia) haemolytica. Expression of TNFα, IL-1β, and IL-8 was significantly increased in the airways and lung lesions of infected calves as compared with mock-infected controls. Although kinetic patterns varied, peak levels of cytokine mRNA occurred within 8 hours postinfection (PI), and peak cytokine concentrations occurred within 16 hours PI. In all samples, IL-8 was expressed to the greatest extent and TNFα was least expressed. Expression of TNFα was restricted to alveolar macrophages. Alveolar and interstitial macrophages produced IL-1β and IL-8 in the first 4 hours; bronchial and bronchiolar epithelial cells were also significant sources of IL-8 during this period. By 8 hours PI, neutrophils were the dominant source of both IL-1β and IL-8. These findings demonstrate a spatial and temporal association between pulmonary expression of inflammatory cytokines and acute lung pathology, supporting the hypothesis that cytokines contribute to inflammatory lung injury in BPP.

Bivalent vaccination against pneumonic pasteurellosis in domestic sheep and goats with modified-live in-frame lktA deletion mutants of Mannheimia haemolytica

A temperature-sensitive shuttle vector, pBB80C, was utilized to generate in-frame deletion mutants of the leukotoxin structural gene (lktA) of Mannheimia haemolytica serotypes 1, 2, 5, 6, 7, 8, 9, and 12. Culture supernatants from the mutants contained a truncated protein with an approximate molecular weight of 66 kDa which was reactive to anti-leukotoxin monoclonal antibody. No protein reactive to anti-LktA monoclonal antibody was detected at the molecular weight 100-105 kDa of native LktA. Sheep and goats vaccinated intramuscularly with a mixture of serotypes 5 and 6 mutants were resistant to virulent challenge with a mixture of the wild-type parent strains. These vaccinates responded serologically to both vaccine serotypes and exhibited markedly-reduced lung lesion volume and pulmonary infectious load compared to control animals. Control animals yielded a mixture of serotypes from lung lobes, but the proportion even within an individual animal varied widely from 95% serotype 5-95% serotype 6. Cultures recovered from liver were homogeneous, but two animals yielded serotype 5 and the other two yielded serotype 6 in pure culture.

Comparative genome analysis of an avirulent and two virulent strains of avian Pasteurella multocida reveals candidate genes involved in fitness and pathogenicity

BACKGROUND

Pasteurella multocida is the etiologic agent of fowl cholera, a highly contagious and severe disease of poultry causing significant mortality and morbidity throughout the world. All types of poultry are susceptible to fowl cholera. Turkeys are most susceptible to the peracute/acute forms of the disease while chickens are most susceptible to the acute and chronic forms of the disease. The whole genome of the Pm70 strain of P. multocida was sequenced and annotated in 2001. The Pm70 strain is not virulent to chickens and turkeys. In contrast, strains X73 and P1059 are highly virulent to turkeys, chickens, and other poultry species. In this study, we sequenced the genomes of P. multocida strains X73 and P1059 and undertook a detailed comparative genome analysis with the avirulent Pm70 strain. The goal of this study was to identify candidate genes in the virulent strains that may be involved in pathogenicity of fowl cholera disease.

RESULTS

Comparison of virulent versus avirulent avian P. multocida genomes revealed 336 unique genes among the P1059 and/or X73 genomes compared to strain Pm70. Genes of interest within this subset included those encoding an L-fucose transport and utilization system, several novel sugar transport systems, and several novel hemagglutinins including one designated PfhB4. Additionally, substantial amino acid variation was observed in many core outer membrane proteins and single nucleotide polymorphism analysis confirmed a higher dN/dS ratio within proteins localized to the outer membrane.

CONCLUSIONS

Comparative analyses of highly virulent versus avirulent avian P. multocida identified a number of genomic differences that may shed light on the ability of highly virulent strains to cause disease in the avian host, including those that could be associated with enhanced virulence or fitness.

Draft Genome Sequences of Two Virulent Serotypes of Avian Pasteurella multocida

Here we report the draft genome sequences of two virulent avian strains of Pasteurella multocida. Comparative analyses of these genomes were done with the published genome sequence of avirulent P. multocida strain Pm70.

Monomeric expression of bovine beta2-integrin subunits reveals their role in Mannheimia haemolytica leukotoxin-induced biological effects

The ruminant-specific leukotoxin (Lkt) of Mannheimia haemolytica is the key virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Previous studies by us and others indicate that M. haemolytica Lkt binds to CD18, the beta subunit of bovine beta(2)-integrins on leukocytes, and that the species specificity of Lkt-induced effects is resident in the beta subunit CD18 and not in the alpha subunit CD11. However, Lkt also binds to the CD11a subunit of LFA-1. Furthermore, antibodies specific for CD18 or CD11a inhibit signaling events leading to elevation of intracellular [Ca(2+)], tyrosine phosphorylation of the cytosolic domain of CD18, and cytolysis of bovine leukocytes. These observations underscore the need for further investigation to identify the precise subunit of bovine LFA-1 utilized by M. haemolytica Lkt as the functional receptor. For this purpose, monomeric bovine CD18 and CD11a and heterodimeric LFA-1 were expressed in the HEK-293 cell line by transfection, and the resulting transfectants were tested for susceptibility to Lkt-induced effects. All three transfectants effectively bound Lkt. However, Lkt-induced cytolysis was observed only with transfectants expressing monomeric bovine CD18 or LFA-1. Furthermore, intracellular [Ca(2+)] elevation following exposure to Lkt, which is a marker for postbinding signaling leading to cellular activation, was seen only with transfectants expressing monomeric bovine CD18 or LFA-1. These results clearly indicate that the bovine CD18 subunit of beta(2)-integrins is the functional receptor for M. haemolytica Lkt.

Human CD18 is the functional receptor for Aggregatibacter actinomycetemcomitans leukotoxin

Aggregatibacter (Actinobacillus) actinomycetemcomitans is the causative organism of localized aggressive periodontitis, a rapidly progressing degenerative disease of the gingival and periodontal ligaments, and is also implicated in causing subacute infective endocarditis in humans. The bacterium produces a variety of virulence factors, including an exotoxic leukotoxin (LtxA) that is a member of the repeats-in-toxin (RTX) family of bacterial cytolysins. LtxA exhibits a unique specificity to macrophages and polymorphonuclear cells of humans and other primates. Human lymphocyte function-associated antigen 1 (LFA-1) has been implicated as the putative receptor for LtxA. Human LFA-1 comprises the CD11a and CD18 subunits. It is not clear, however, which of its subunits serves as the functional receptor that confers species-specific susceptibility to LtxA. Here we demonstrate that the human CD18 is the receptor for LtxA based on experiments performed with chimeric beta2-integrins recombinantly expressed in a cell line that is resistant to LtxA effects. In addition, we show that the cysteine-rich tandem repeats encompassing integrin-epidermal growth factor-like domains 2, 3, and 4 of the extracellular region of human CD18 are critical for conferring susceptibility to LtxA-induced biological effects.

Integrin-EGF-3 domain of bovine CD18 is critical for Mannheimia haemolytica leukotoxin species-specific susceptibility

Mannheimia haemolytica leukotoxin (LktA) is the primary virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Results from the authors' previous studies demonstrated that the site required for LktA binding leading to susceptibility to its biological effects resides within amino acid residues 500-600 of the extracellular region of bovine CD18. Experiments were designed to identify a much smaller functional domain within this 100 amino acid region of bovine CD18 that is critically required for species-specific susceptibility to LktA effects. Chimeric bovine X human CD18 with different integrin epidermal growth factor(I-EGF) like domains switched between bovine and human CD18 were generated and coexpressed with bovine CD11a in the human K562 cell line. The resulting chimeric transductants were tested for susceptibility to LktA-induced effects. The results demonstrate unequivocally that the I-EGF-3 domain of bovine CD18 (amino acid residues 541-581) is critical for conferring species-specific susceptibility to M. haemolytica leukotoxin.

Characterization of Mannheimia (Pasteurella) haemolytica leukotoxin interaction with bovine alveolar macrophage beta2 integrins

Mannheimia (Pasteurella) haemolytica, the etiologic agent of bovine pneumonic mannheimiosis, produces an exotoxic leukotoxin. The leukotoxin (LktA) is a member of the RTX (repeats in toxin) family of bacterial cytolysins and is distinguished from other toxins by its unique target cell specificity to ruminant leukocytes occurring through binding to a specific receptor. We have demonstrated previously that the beta2 integrin LFA-1 is a receptor for LktA in bovine leukocytes and is involved in leukotoxin-induced biological effects. However the subunits within LFA-1 involved in binding to LktA, and post-binding signaling leading to cellular activation have not been well characterized. The purpose of our study was to pinpoint these precise subunits on bovine alveolar macrophages and to characterize their interaction with LktA. The results in this study indicate that although LktA can efficiently bind to the CD18 subunit of both LFA-1 and Mac-1, post-binding signaling events including elevation of intracellular calcium and CD18 tail phosphorylation are only observed through LFA-1. Furthermore, LktA also binds to the CD11a subunit of LFA-1. LktA binding to CD11a could be inhibited by a small molecule inhibitor of the I(inserted)-domain, the major ligand binding interface on CD11a. I-domain inhibition significantly blunts LktA-induced intracellular calcium elevation and tyrosine phosphorylation of the CD18 tail. Based on our results we suggest that LFA-1 serves as the functional leukotoxin receptor on bovine alveolar macrophages.

Mapping of the binding site for Mannheimia haemolytica leukotoxin within bovine CD18

To map the site involved in Mannheimia haemolytica leukotoxin (LktA) binding and biological activity within bovine CD18, bovine x human CD18 chimeric constructs were generated and coexpressed with bovine CD11a in K562 cells. Studies with the chimeric leukocyte function-associated antigen 1 transductants demonstrate that the site required for LktA binding and biological effects resides within amino acid residues 500 and 600 of the extracellular region of bovine CD18.

Mechanisms underlying Mannheimia haemolytica leukotoxin-induced oncosis and apoptosis of bovine alveolar macrophages

Mannheimia (Pasteurella) haemolytica leukotoxin (LktA) binds to the bovine beta2 integrins (such as LFA-1-CD11a/CD18) and leads to subsequent cellular effects in a dose dependent manner. The objectives of this study were to delineate the mechanisms that underlie LktA-induced oncosis and apoptosis and to examine the role of LktA/LFA-1 interaction in these events. The results demonstrate that LktA-induced oncosis proceeds through a LFA-1 and caspase-1 dependent pathway referred to as 'pyrotosis', as well as through a LFA-1- and caspase-1-independent pathway. LktA-induced apoptosis in alveolar macrophages involves activation of caspase-3 and engages the extrinsic and intrinsic pathways of apoptosis, with the extrinsic pathway being dependent on LFA-1 signaling and TNFalpha.

Recombinant expression of bovine LFA-1 and characterization of its role as a receptor for Mannheimia haemolytica leukotoxin

Mannheimia (Pasteurella) haemolytica leukotoxin (LktA) is the primary virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic mannheimiosis (BPM), a disease which causes major economic loss to the US cattle industry annually. Recent work from our laboratory using an antibody-based approach has shown that LktA binds to bovine LFA-1 in target cells. While this study suggests that LFA-1 might be a specific receptor, it remains to be conclusively shown that LFA-1 is sufficient to induce susceptibility to LktA. It was of interest to determine if functionally active bovine LFA-1 could be reconstituted on a LFA-1 negative cell line and reconstitute susceptibility to LktA. Here, we report the successful recombinant expression of bovine LFA-1 on the cell surface of the human erythroleukemic K562 cell line. The BoLFA-1 transductant expresses bovine CD18 and CD11a as a heterodimer. We found that LktA binds to both the CD18 and CD11a subunits of BoLFA-1 cells. Exposure of BoLFA-1 cells to LktA, induced tyrosine phosphorylation of the CD18 tail, elevation of intracellular calcium, and cytolysis. This is the first report on recombinant expression of functionally active bovine LFA-1 by transduction into an LktA-non-susceptible human cell line.

Pharmacological inhibition of Mannheimia haemolytica lipopolysaccharide and leukotoxin-induced cytokine expression in bovine alveolar macrophages

The inflammatory cytokines tumor necrosis factor-alpha (TNFalpha), interleukin-1 beta (IL-1beta), and interleukin-8 (IL-8) are believed to contribute to the pathogenesis of lung injury in bovine pneumonic mannheimiosis (BPM) caused by Mannheimia (Pasteurella) haemolytica. Inflammatory cytokines may, therefore, represent therapeutic targets to be modulated for the purpose of treating or preventing this important disease of cattle. The purpose of this study was to evaluate the ability of six pharmacological agents to suppress the expression of TNFalpha, IL-1beta, and IL-8 genes and proteins in bovine alveolar macrophages (AM) exposed to M. haemolytica lipopolysaccharide (LPS) and leukotoxin (LktA) in vitro. The compounds tested included dexamethasone (DEX), tetrahydropapaveroline (THP), pentoxifylline (PTX), rolipram (ROL), SB203580 (SB), and thalidomide (THL). Cytokine expression was induced by the addition of purified M. haemolytica LPS and LktA to AM cell cultures following pretreatment with inhibitor compounds. Secretion of TNFalpha, IL-1beta, and IL-8 proteins into the cell culture supernatant was measured using enzyme-linked immunosorbent assays, and steady-state accumulation of cytokine-specific mRNA was measured by northern blot analysis. Dose-dependent inhibition of cytokine secretion occurred in response to pretreatment of AM with DEX (TNFalpha, IL-1beta, IL-8), THP (TNFalpha, IL-1beta, IL-8), PTX (TNFalpha, IL-1beta, IL-8), ROL (TNFalpha, IL-1beta), and SB (TNFalpha, IL-8). Significant dose-dependent inhibition of cytokine mRNA expression occurred in response to pretreatment with DEX (TNFalpha, IL-1beta, IL-8), THP (TNFalpha, IL-1beta, IL-8), and PTX (TNFalpha). DEX was the most effective inhibitor by far; pretreatment with this compound yielded greater than 95% inhibition of cytokine gene and protein expression over a broad range of concentrations. These findings demonstrate that DEX, THP, PTX, ROL, and SB are capable of suppressing inflammatory cytokine secretion by bovine AM in vitro. If pulmonary cytokine secretion may be similarly inhibited in vivo, anti-cytokine therapy may represent a novel strategy for the management of BPM.

Protective effect of dexamethasone in experimental bovine pneumonic mannheimiosis

Clinical and experimental studies provide unequivocal evidence that neutrophils participate in the pathogenesis of lung injury in bovine pneumonic mannheimiosis (BPM). Since the inflammatory cytokines tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-8 play a central role in the recruitment and activation of neutrophils, we hypothesize that pharmacological inhibition of their expression may prevent or reduce the inflammatory lung injury that is characteristic of the disease. The purpose of this study was to determine whether systemic therapy with dexamethasone sodium phosphate (DEX), a potent inhibitor of inflammatory cytokine synthesis, ameliorates disease development in an in vivo experimental model of BPM. Four experimental calves were treated intravenously with DEX (2 mg/kg 6 h prior to infection, 2 mg/kg immediately prior to infection, and 1 mg/kg every 12 h thereafter), while two placebo-treated control calves received dose-matched volumes of sterile saline. Disease was induced in the left lungs of the six calves by endobronchial administration of Mannheimia haemolytica. Clinical disease was characterized using a non-parametric scoring system, and the extent of gross pulmonary pathology affecting the left lung 48 h post-infection (PI) was determined using morphometric methods. Disease scores for DEX-treated calves were significantly lower than those for placebo-treated controls at all time points beyond 2 h PI (P<0.05) and the percent volume of the left lung exhibiting gross pneumonic lesions was significantly lower in DEX-treated calves (6.0+/-1.1%) as compared to controls (68.9+/-13.3%), P<0.05. In addition, histopathological lesions were less severe and extensive in DEX-treated calves. These findings indicate that pharmacological modulation of pulmonary inflammation may represent an alternative approach to control this disease. Successful implementation of this strategy will require additional research to identify drug agents that target the expression of cytokines and other inflammatory mediators without compromising host immune responses.

Prior exposure to Mannheimia haemolytica leukotoxin or LPS enhances beta(2)-integrin expression by bovine neutrophils and augments LKT cytotoxicity

Mannheimia (Pasteurella) haemolytica serotype1 produces a variety of virulence factors that play an important role during the pathogenesis of bovine pneumonic pasteurellosis. Among these, a leukotoxin (LKT) and lipopolysaccharide (LPS) are thought to be the primary virulence factors that contribute to the characteristic pathology of pasteurellosis. Recent evidence suggests that M. haemolytica LKT binding to bovine leukocytes is mediated by the beta(2)-integrin CD11a/CD18 (LFA-1), which subsequently induces activation and death of these cells. Exposure of bovine peripheral blood neutrophils (PMNs) to LKT or LPS induces expression of inflammatory cytokines, which in turn can increase LFA-1 expression and conformational activation. In this study we demonstrated, by flow cytometry and Western blot, that bovine PMNs increased their LFA-1 expression following in vitro exposure to M. haemolytica LKT and LPS. Increased LFA-1 expression by PMNs exposed to LKT and LPS was associated with increased LKT binding and cell death. The results of this study suggest that M. haemolytica LKT and LPS might cooperatively increase LFA-1 expression, and by so doing amplify the lung inflammation that characterizes bovine pasteurellosis.

Biological effects of two genetically defined leukotoxin mutants of Mannheimia haemolytica

Mannheimia(Pasteurella)haemolytica serotype 1 is the primary causative agent responsible for bovine pneumonic mannheimiosis, also known as shipping fever in cattle. The bacterium produces a variety of virulence factors, foremost of which is the exotoxic leukotoxin. The leukotoxin is a calcium-dependent cytolysin that is a member of the RTX (repeats in toxin) family and exhibits a narrow cell-type and species specificity and has biological effects only on ruminant leukocytes and platelets. The genetic organization of the leukotoxin is comprised of four genes: lktC, lktA, lktB and lktD. The lktA structural gene encodes the protoxin (pro-LktA) and lktC encodes a transacylase that post-translationally modifies the inactive pro-LktA to a biologically active wild-type leukotoxin (LktA). The LktA has been implicated as the key factor that contributes to the pathogenesis of lung injury associated with the disease and considerable efforts have been employed in abrogating toxin function while retaining immunogenicity, with an eye towards design of attenuated vaccines. We hypothesized that the pro-LktA retains the ability to cause biological effects on target cells as has been reported in the case of the closely related RTX toxin alpha-hemolysin (HlyA). We also examined the biological effects of an amino-terminal truncation mutant leukotoxin DeltaLktA on target cells. Thus the objectives of our study were to investigate whether two different mutant leukotoxins, one a nonacylated pro-LktA, and the other lacking 344 amino acids at the N-terminal end of the LktA protein; DeltaLktA, are capable of (i). binding to the beta2-integrin leukotoxin receptor, (ii). inducing the elevation of second messenger intracellular calcium ([Ca(2+)](i)), and (iii). inducing inflammatory gene expression, reactive oxygen metabolites (ROMs) and cytolysis in target cells. Our results demonstrate that neither acylation nor the amino terminal 344 amino acids are required for LktA binding but are essential for LktA-induced [Ca(2+)](i) elevation, generation of ROM, generation of the inflammatory cytokine IL-8 and cytolysis in target cells.

Role of Mannheimia haemolytica leukotoxin in the pathogenesis of bovine pneumonic pasteurellosis

Bovine pneumonic pasteurellosis continues to be a major respiratory disease in feedlot cattle despite the recent advances in our understanding of the underlying complexities of causation. The etiological agent, Mannheimia haemolytica, possesses several virulence factors, including capsule, outer membrane proteins, adhesins, neuraminidase, endotoxin and exotoxic leukotoxin. Accumulating scientific evidence implicates leukotoxin as the primary factor contributing to clinical presentation and lung injury associated with this disease. Unlike other virulence factors, leukotoxin shows cell-type- and species-specific effects on bovine leukocytes. Recent investigations have delineated the mechanisms underlying the target-cell-specificity of leukotoxin and how this contributes to the pathogenesis of lung damage. This review summarizes current understanding of the secretion, regulation, mechanisms of action and evolutionary diversity of leukotoxin of M. haemolytica. Understanding the precise molecular mechanisms of leukotoxin is critical for the development of more effective prophylactic and therapeutic strategies to control this complex disease.

Evaluation of cytokine gene expression in porcine spleen cells, peripheral blood mononuclear cells, and alveolar macrophages by competitive RT-PCR

Cytokines act as an important regulator of immune responses. Since cytokine expression levels are generally very low, more accurate and reliable methods of measuring their expression are needed. In this study, a modified competitive reverse transcription-polymerase chain reaction assay was developed to determine the expression levels and patterns of porcine IFN-gamma, IL-2, IL-4, IL-10, IL-12 p 35, and IL-12 p40 in spleen cells, peripheral blood mononuclear cells (PBMC), and alveolar macrophages that were stimulated for 4 h by lipopolysaccharide or phytohemagglutinin. Of these cytokines, the expression level of IFN-gamma was the highest in all examined cells. Constitutive expression of IL-2 and IL-4 was demonstrated in spleen cells and PBMC stimulated with phytohemagglutinin. However, their expression extent was not determinable or extremely low in the lipopolysaccharide-stimulated spleen cells and alveolar macrophages. Moderately high IL-10 expression was observed in all examined cells. IL-12 p 35 expression in alveolar macrophages was always higher than in spleen cells and PBMC. IL-12 p40 expression in alveolar macrophages was higher than in PBMC, but was lower than in spleen cells. In spleen cells, the expression of IL-12 p40 was higher than that of IL-12 p 35. In alveolar macrophages and PBMC, however, IL-12 p 35 showed a higher expression than IL-12 p40. These results indicate that each cytokine has its own characteristic expression profile in different immune cells.

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.

Lipopolysaccharide enhances cytolysis and inflammatory cytokine induction in bovine alveolar macrophages exposed to Pasteurella (Mannheimia) haemolytica leukotoxin

Pasteurella (Mannheimia) haemolytica leukotoxin (Lkt) and lipopolysaccharide (LPS) are the primary virulence factors contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Previous studies have characterized in vitro responses of bovine alveolar macrophages (AMs) to Lkt and LPS. Activation of AMs with Lkt or LPS causes induction of proinflammatory cytokines, and Lkt causes cytolysis of AMs at higher concentrations. Since AMs are exposed to both of these bacterial virulence factors during disease, previous studies may have underestimated the possibility of functional interactions between Lkt and LPS. The purpose of this study was to characterize the effect of simultaneous exposure to both Lkt and LPS on AM cytolysis and proinflammatory cytokine expression. Using cellular leakage of lactate dehydrogenase as an indirect measure of cytolysis, we studied AM responses to Lkt alone, LPS alone and Lkt+LPS. We found that 80-200 pg/ml LPS, which does not itself cause cytolysis, synergistically enhanced the cytolysis induced by 2-5 Lkt units (LU)/ml Lkt. Northern blot analysis demonstrated that synergism between Lkt and LPS resulted in increased levels of IL-8 mRNA, and that the kinetic patterns of TNF-alpha and IL-8 mRNA expression induced by Lkt+LPS differed from those induced by each agent separately. Finally, the WEHI 164 (clone 13) bioassay was used to show that Lkt/LPS synergism resulted in enhanced secretion of biologically active TNF-alpha. These results provide direct evidence of synergism between Lkt and LPS in AM cytolysis and inflammatory cytokine expression. Additional studies to characterize the molecular basis of this phenomenon are indicated.

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.

Serotyping of Mannheimia (Pasteurella) haemolytica isolates from the upper Midwest United States

Mannheimia (Pasteurella) haemolytica biotype A serotype1 (A1) is the primary bacterial agent responsible for the clinical signs and pathophysiologic events in bovine pneumonic pasteurellosis. The goal of this study was to determine the prevalence of other serotypes of M. haemolytica biotype A organisms obtained from the upper Midwest diagnostic laboratories. A total of 147 M. haemolytica isolates were collected from Minnesota, South Dakota, and Michigan. Isolates were tested against M. haemolytica antisera obtained from the National Animal Disease Center, Ames, Iowa. Results indicated that M. haemolytica serotype 1 represented approximately 60%, serotype 6 represented 26%, and serotype 2 represented 7% of the total examined isolates. In addition, 7% of the isolates were serotype 9, 11, or untypable. This finding suggests that M. haemolytica serotypes other than serotype 1 can be isolated from the lung lesions of diseased cattle and seem to be capable of causing the pathologic changes observed in the lung with pneumonic pasteurellosis.

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.

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

Lung sections taken from calves with experimental Pasteurella haemolytica respiratory infection exhibited increased numbers of TUNEL positive cells with time after challenge. This finding suggests that P. haemolytica, or toxins and other components released by the organism, induces apoptosis in bovine leukocytes in vivo. By so doing this might impair host defense and contribute in part to the severe pneumonia that characterizes bovine pasteurellosis.

Pasteurella haemolytica leukotoxin and endotoxin induced cytokine gene expression in bovine alveolar macrophages requires NF-kappaB activation and calcium elevation

In bovine alveolar macrophages (BAMs), exposure to leukotoxin (Lkt) and endotoxin (LPS) from Pasteurella haemolytica results in expression of inflammatory cytokine genes and intracellular calcium ([Ca2+]i) elevation. Leukotoxin from P. haemolytica interacts only with leukocytes and platelets from ruminant species. Upregulation of cytokine genes in different cells by LPS involves activation of the transcription factor NF-kappaB (NF-kappaB), resulting in its translocation from the cytoplasm to the nucleus. Using immunocytochemical staining and confocal imaging, we studied whether NF-kappaB activation represents a common mechanism for the expression of multiple cytokine genes in BAMs (Lkt-susceptible cells) stimulated with Lkt and LPS. Bovine pulmonary artery endothelial cells and porcine alveolar macrophages were used as nonsusceptible cells. The role of Ca2+ and tyrosine kinases in NF-kappaB activation and inflammatory cytokine gene expression was studied, since an inhibitor of tyrosine kinases attenuates LPS-induced [Ca2+]i elevation in BAMs. The results are summarized as follows: (a) Lkt induced NF-kappaB activation and [Ca2+]i elevation only in BAMs, while LPS effects were demonstrable in all cell types; (b) chelation of [Ca2+]i blocked NF-kappaB activation and IL-1beta, TNFalpha, and IL-8 mRNA expression; and (c) tyrosine kinase inhibitor herbimycin A blocked expression of all three cytokine genes in BAMs stimulated with Lkt, while only the expression of IL-1beta was blocked in BAMs stimulated with LPS. We conclude that cytokine gene expression in BAMs requires NF-kappaB activation and [Ca2+]i elevation, and Lkt effects exhibit cell type- and species specificity.

The biphasic mRNA expression pattern of bovine interleukin-8 in Pasteurella haemolytica lipopolysaccharide-stimulated alveolar macrophages is primarily due to tumor necrosis factor alpha

Pasteurella haemolytica serotype 1 is the bacterial agent responsible for the pathophysiological events associated with bovine pneumonic pasteurellosis. Our previous studies support a role for the lipopolysaccharide (LPS) from P. haemolytica in the induction of proinflammatory cytokines. One of the pathological hallmarks of bovine pneumonic pasteurellosis is an influx of neutrophils into the alveolar spaces. This pronounced influx suggests the local production of a chemotactic factor(s) such as interleukin-8 (IL-8). In the context of the lung, the alveolar macrophage appears to be the major producer of IL-8, a proinflammatory cytokine with potent neutrophil chemotactic activity. By using Northern blot analysis, we have examined the kinetics of IL-8 mRNA expression in P. haemolytica LPS-stimulated bovine alveolar macrophages and found that 1 ng of LPS per ml induces maximal expression of IL-8 mRNA. The results also indicate a biphasic time course expression pattern in which IL-8 mRNA levels peak between 1 and 2 h in the first phase and between 16 and 24 h in the second phase (P < 0.01). In addition, monospecific polyclonal antibodies were used to demonstrate the role of tumor necrosis factor alpha (TNF-alpha) in the second phase of IL-8 mRNA expression. Our findings support a role for P. haemolytica LPS and TNF-alpha in the induction of IL-8 from bovine alveolar macrophages.

Pasteurella haemolytica A1-derived leukotoxin and endotoxin induce intracellular calcium elevation in bovine alveolar macrophages by different signaling pathways

Leukotoxin and endotoxin derived from Pasteurella haemolytica serotype 1 are the primary virulence factors contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Activation of bovine alveolar macrophages with endotoxin or leukotoxin results in the induction of cytokine gene expression, with different kinetics (H. S. Yoo, S. K. Maheswaran, G. Lin, E. L. Townsend, and T. R. Ames, Infect. Immun. 63:381-388, 1995; H. S. Yoo, B. S. Rajagopal, S. K. Maheswaran, and T. R. Ames, Microb. Pathog. 18:237-252, 1995). Furthermore, extracellular Ca2+ is required for leukotoxin-induced cytokine gene expression. However, the involvement of Ca2+ in endotoxin effects and the precise signaling mechanisms in the regulation of intracellular Ca2+ by leukotoxin and endotoxin are not known. In fura-2-acetoxymethyl ester-loaded alveolar macrophages, intracellular Ca2+ regulation by leukotoxin and endotoxin was studied by video fluorescence microscopy. Leukotoxin induced a sustained elevation of intracellular Ca2+ in a concentration-dependent fashion by influx of extracellular Ca2+ through voltage-gated channels. In the presence of fetal bovine serum, endotoxin elevated intracellular Ca2+ even in the absence of extracellular Ca2+. Leukotoxin-induced intracellular Ca2+ elevation was inhibited by pertussis toxin, inhibitors of phospholipases A2 and C, and the arachidonic acid analog 5,8,11,14-eicosatetraynoic acid. Intracellular Ca2+ elevation by endotoxin was inhibited by inhibitors of phospholipase C and protein tyrosine kinase, but not by pertussis toxin, or the arachidonic acid analog. To the best of our knowledge, this is the first report of Ca2+ signaling by leukotoxin through a G-protein-coupled mechanism involving activation of phospholipases A2 and C and release of arachidonic acid in bovine alveolar macrophages. Ca2+ signaling by endotoxin, on the other hand, involves activation of phospholipase C and requires tyrosine phosphorylation. The differences in the Ca2+ signaling mechanisms may underlie the reported temporal differences in gene expression during leukotoxin and endotoxin activation.

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

Allelic replacement was used to generate two isogenic lktA deletion mutants of Pasteurella haemolytica serotype 1 that were incapable of synthesizing leukotoxin (Lkt). Southern blot data confirmed that lktA sequences were absent in the two P. haemolytica deletion mutants. Culture supernatants and whole cell lysates from the wild type P. haemolytica, D153 parent strain, but not the lktA deletion mutants, contained immunoreactive and bioactive leukotoxic protein. In addition, only the parent strain was haemolytic when grown on bovine and sheep blood agar plates. Virulence of the lktA deletion mutant, lktA 77, was compared with the parent in an experimentally infected calf model of pneumonic pasteurellosis. Results revealed significant reduction in virulence in the lktA mutant as measured by clinical and lung lesion scores. Notable differences in histological changes such as markedly reduced necrosis and lack of leukocyte degeneration occurred in calves infected with the lktA mutant in comparison with those infected with the parent wild-type strain. Thus, it appears that leukotoxin plays a important role in the pathogenesis of lung injury in bovine pneumonic pasteurellosis.

Effects of Pasteurella haemolytica leukotoxin and lipopolysaccharide on histamine, prostanoid, and leukotriene release by bovine lung parenchyma in vitro

OBJECTIVE

To identify the effect of Pasteurella haemolytica lipopolysaccharide (LPS) and leukotoxin (LKT) on spontaneous and calcium ionophore-induced histamine and inflammatory mediator release from isolated bovine lung parenchyma.

SAMPLE POPULATION

Lungs from 8 healthy cattle.

PROCEDURE

Isolated bovine lung parenchyma was incubated in vitro for 2 hours with LKT or LPS, and spontaneous and induced release of inflammatory mediators was determined.

RESULTS

LKT and LPS increased spontaneous release of histamine and leukotriene B4. In addition, incubation with LPS increased spontaneous release of prostaglandin E2. Moreover, a differential effect of the 2 toxins on calcium ionophore-induced inflammatory mediator release was observed. LKT specifically primed isolated lung parenchyma to release leukotriene B4 and thromboxane B2 in response to calcium ionophore, whereas LPS did not alter the profile of prostanoids released by bovine lung tissue exposed to calcium ionophore.

CONCLUSIONS

Pasteurella haemolytica toxins have a direct effect on bovine lung parenchyma, causing release of inflammatory mediators, which contribute to response to infection. Furthermore, bacterial toxins (LKT in this study) may sensitize tissues to the effects of other irritant stimuli, amplifying the inflammatory response.

Evaluation of efficacy of three commercial vaccines against experimental bovine pneumonic pasteurellosis

The objective of this study was to evaluate the efficacy of three commercial vaccines against experimental pneumonic pasteurellosis in cattle. The three vaccines were: (a) One Shot (SmithKline Beecham, West Chester, PA.), (b) Presponse (Langford Laboratories, Guelph, Ontario) and (c) Once PMH (BioCor, Omaha, NE.). Protective immunity was evaluated in terms of lower clinical and pneumonic lesion scores after endobronchial challenge with virulent P. haemolytica. The results indicate that One Shot elicited antibodies against leukotoxin (Lkt), capsular poly-saccharide (CP) and surface antigens (SA), while Presponse and Once PMH elicited antibodies against CP and SA. There was significant correlation between lung and serum antibody levels against Lkt (P < 0.0001), CP (P < or = 0.0001) and IROMPs (P < or = 0.035). Animals that received the One Shot had significantly (P < or = 0.05) lower mean pneumonic lesion score (36.6 +/- 10.97) as compared to the control group (48.6 +/- 25.92). A significant negative correlation (-0.41; P < or = 0.008) existed between serum antibody levels against Lkt and pneumonic lesion score. High serum antibodies against SA did not correlate with reduction in pneumonic lesion score. In addition, high antibody levels against CP did not correlate consistently with reduced pneumonic lesion scores. The results from this study demonstrates that commercial vaccines evaluated in this trial did not confer optimal protection in vaccinated calves, against experimental pneumonic pasteurellosis. However, One Shot vaccinates showed a better protective immunity compared to the other two vaccine groups.

Induction of nitric oxide production by bovine alveolar macrophages in response to Pasteurella haemolytica A1

We assessed the kinetics of inducible nitric oxide synthase (iNOS) mRNA expression and production of nitric oxide (NO) in bovine alveolar macrophages (AMs) stimulated with purified lipopolysaccharide (LPS) from Pasteurella haemolytica strain 12296. The effect of LPS on iNOS gene expression was dose-dependent and was expressed maximally at 24 h after stimulation with 10 micrograms/ml of LPS. Production of NO measured as secreted nitrite in supernatants took place in a time and dose-dependent manner with peak production at 24 h after LPS stimulation. Recombinant bovine gamma interferon (rb gamma IFN) augmented the LPS-induced iNOS gene expression and production of NO. The ability of LPS to induce iNOS gene expression and NO production either alone or in combination with rb gamma IFN was significantly abrogated by polymyxin B. In addition, the iNOS inhibitor NG-monomethyl-Larginine (L-NMMA) significantly inhibited LPS and rb gamma IFN + LPS induced NO production. Our results also demonstrated that NO produced from an exogenous NO donor sodium nitroprusside (SNP), and NO generated from LPS-stimulated AMs (endogenous) caused cytotoxic injury to bovine pulmonary artery endothelial cells in a dose-dependent manner. The cytotoxic injury caused by NO generated from LPS stimulated AMs was inhibited by polymyxin B or L-NMMA. There was a markedly increased concentration of nitrite in the lung lavage fluids of calves following P. haemolytica infection. These findings support a role for NO in the pathogenesis of lung injury in bovine pneumonic pasteurellosis.

Comparative evaluation of antibodies induced by commercial Pasteurella haemolytica vaccines using solid phase immunoassays

The objective of this study was to evaluate the ability of four commercial vaccines to elicit antibodies against the leukotoxin (Lkt), capsular polysaccharide (CP), iron regulated outer membrane proteins (IROMPs), and whole cell (WC) antigens of Pasteurella haemolytica A1. Modified double antibody sandwich enzyme linked immunosorbent assays (ELISAs) were developed to measure antibody levels against Lkt, CP and IROMPs. An indirect ELISA was developed to measure the levels of antibody against WC antigens. The ideal cut off points for ELISAs were determined on receiver operating characteristic curves, using sera from 30 calves injected subcutaneously with a live P. haemolytica 12296 strain as positive control and sera from 30 colostrum-deprived calves as negative control. The vaccines evaluated were: 'One Shot' (SmithKline Beecham, West Chester, PA) a bacterin-toxoid, 'Presponse' (Langford Laboratories, Guelph, Ontario) a Lkt-rich culture supermatant, 'Once PMH' (BioCor Inc., Omaha, NE) a modified live vaccine, and 'Septimune' (Fort Dodge laboratories, Fort Dodge, IA) an outer membrane extract. Thirty, 4-6 week old Holstein calves were randomized into 5 groups to receive one of the four vaccines or a placebo (sterile phosphate buffered saline). The calves were vaccinated intramuscularly on day 0 and on day 14, and bled on days, 0, 14, and 28 to measure antibody levels against Lkt, CP, IROMPs, and WC antigens of P. haemolytica Al. 'One Shot', and 'Once PMH' vaccinates showed a significant (P < 0.05) increase in antibody levels against Lkt at 28 days. 'Once PMH' vaccinates also showed significant (P < 0.05) increase in antibody levels against IROMPs at 28 days compared to the other four groups but this increase was not significant over time within the 'Once PMH' group. 'Presponse', 'Once PMH' and 'One Shot' vaccinates showed a significant (P < 0.05) increase in antibody levels against CP over time. These groups also had significantly higher antibody levels against CP, compared to controls and 'Septimune' vaccinates at 14 and 28 days (P < 0.05).

Evaluation of three experimental subunit vaccines against pneumonic pasteurellosis in cattle

The objective of this study was to evaluate the efficacy of three Pasteurella haemolytica A1 derived experimental subunit vaccines against pneumonic pasteurellosis in cattle. The three vaccines were: (a) culture supernatant (CS) containing leukotoxin (Lkt), lipopolysaccharide (LPS) and capsular polysaccharide (CP); (b) sodium salicylate extract (SSE) containing iron regulated outer membrane proteins (IROMPs), LPS and CP; (c) and a combination of the above two. Vaccine efficacy was defined in terms of reduction in clinical and pneumonic lesion scores after intrapulmonic challenge with live P. haemolytica. The results indicate that the CS vaccine elicited antibodies against both Lkt and CP, while the SSE vaccine elicited antibodies against IROMPs and CP. Animals inoculated with the combination vaccine showed increased levels of antibodies against IROMPs, Lkt and CP. There was significant correlation between lung and serum antibodies against Lkt, CP and IROMPs. Animals that received the combination vaccine had significantly lower mean pneumonic lung score as compared to SSE and control groups. The animals which received CS vaccine had mean pneumonic lung score significantly lower than that of control group. A strong negative correlation existed between serum antibody levels against Lkt, IROMPs, CP and pneumonic lung scores. The results from this study demonstrate the usefulness of CS vaccine alone or in combination with SSE vaccine in bringing about optimal protection in vaccinated calves, against experimental pneumonic pasteurellosis.

In vitro expression and inhibition of procoagulant activity produced by bovine alveolar macrophages and peripheral blood cells

Local and systemic activation of coagulation is frequently associated with bacterial sepsis. The coagulopathy is due, at least in part, to expression of tissue factor (TF) by monocytes and macrophages. The purpose of this study was to evaluate the expression of procoagulant activity by bovine alveolar macrophages, leukocytes and platelets, and to determine the relative potency of three chemical inhibitors of TF expression (pentoxifylline, retinoic acid, and cyclosporin A). Bovine alveolar macrophages were stimulated with lipopolysaccharide (LPS) derived from Pasteurella haemolytica or recombinant bovine tumour nervous factor (TNF) and dose- and time-dependent effects on TF expression were studied. LPS and TNF induced TF expression in alveolar macrophages and LPS treatment of whole blood induced TF expression in mononuclear cells. Neutrophils and platelets also expressed procoagulant activity, but this activity was not inhibited by anti-bovine TF monoclonal antibody. Pentoxifylline (40 mumol/L), retinoic acid (0.01 mmol/L) and cyclosporin A (0.08 mumol/L) inhibited TF expression when added concurrently with LPS or TNF, but not when added 4 h after stimulation. TF mRNA was not detected in unstimulated alveolar macrophages by Northern blot analysis. In contrast, exposure to LPS or TNF for 6 h induced marked expression of TF mRNA, which was inhibited by treatment with pentoxifylline, retinoic acid and cyclosporin A. Expression of TNF by alveolar macrophages stimulated with LPS was also inhibited by these compounds. Our results indicate that procoagulant activity expressed by alveolar macrophages and monocytes is associated with expression of TF, whereas procoagulant activity expressed by neutrophils and platelets is not. The concentrations of pentoxifylline and retinoic acid necessary for inhibition of TF expression in vitro may not be achievable in vivo owing to their toxic effects. However, the in vitro concentration of cyclosporin A that inhibited TF expression did not exceed the plasma concentration observed in humans, and therefore may be useful for inhibition of TF expression in vivo.

Increased tumor necrosis factor-alpha and interleukin-1 beta expression in the lungs of calves with experimental pneumonic pasteurellosis

We used a well characterized pneumonic pasteurellosis model in calves to determine whether increased proinflammatory cytokines, tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) expression and secretion were associated with pneumonic lesions. Bronchoalveolar lavage fluids, lavage cells consisting of alveolar macrophages and neutrophils with degenerative changes, and lung tissues were analyzed for the presence of TNF-alpha and IL-1 beta approximately 48 h following endobronchial inoculation of logarithmic phase Pasteurella haemolytica 12296 organisms. Levels of TNF-alpha and IL-1 beta mRNA were significantly increased in lavage cells of P. haemolytica-infected animals but not in cells from phosphate buffered saline (PBS) inoculated controls based on in situ hybridization analysis. Significantly increased levels of TNF-alpha, and IL-1 beta mRNA were also expressed within the pneumonic lesions from P. haemolytica-infected calves. In contrast, lung tissues from PBS-inoculated control calves had cytokine mRNAs expressed at extremely low levels. Increased levels of bioactive IL-1 and immunoreactive (not bioactive) TNF-alpha were found in lavage fluids from P. haemolytica-infected calves compared with lavage fluids from PBS-inoculated calves. These findings indicate that the proinflammatory cytokines TNF-alpha and IL-1, may be associated with pathogenesis of lung injury in bovine pneumonic pasteurellosis.

Inflammatory cytokine expression in swine experimentally infected with Actinobacillus pleuropneumoniae

An Actinobacillus pleuropneumoniae infection model in swine was established to study the expression of inflammatory cytokines during acute respiratory disease. Lavage fluid, lavage cells consisting primarily of alveolar macrophages, and lung tissue were analyzed for the presence of various cytokines at 2, 4, 8, and 24 h following endotracheal inoculation of A. pleuropneumoniae. Interleukin-1 beta (IL-1) and IL-8 mRNA levels were elevated within 2 h in lavage cells of animals inoculated with A. pleuropneumonia but not in cells from controls treated with saline-bovine serum albumin, based on Northern (RNA blot) analysis. Tumor necrosis factor (TNF) mRNA was present at low levels in all animals, and the level was not increased at any time point. In situ hybridization was more sensitive than Northern blotting and revealed elevations of all three cytokines in lavage cells within 2 to 4 h of A. pleuropneumoniae inoculation. IL-6 was detected in lavage cells by in situ hybridization but not by Northern blotting. In lung tissue obtained 18 to 24 h after A. pleuropneumoniae instillation, all cytokine mRNAs, including that of IL-6, were detected by Northern blot analysis. The levels of bioactive IL-1 and IL-6 in lavage fluids increased approximately 1,000-fold following A. pleuropneumoniae inoculation, but TNF bioactivity was not detected. Morphological localization of cytokine mRNAs by in situ hybridization indicated markedly increased levels of TNF, IL-1, and IL-8 mRNAs at the periphery of focal lung lesions. These findings indicate that inflammatory cytokines, particularly IL-1 and IL-8, are associated with the development of pleuropneumonia and may contribute to disease severity.

Colorimetric assay using XTT for assessing virulence of avian Pasteurella multocida strains

A colorimetric assay using sodium 3,3'-[1[(phenylamino)carbonyl]3,4- tetrazolium]-bis(4-methoxy-6-nitro) benzene sulfonic acid hydrate (XTT) was adapted to quantitate bactericidal activity of chicken macrophage HD 11 cell line against five Pasteurella multocida strains and an avirulent transposon insertion mutant. The strains used were virulent P1059, and D92, and four avirulent strains including a streptomycin resistant mutant of P1059 (P1059 SmR), two live vaccine strains namely, the Clemson University (CU) and M9, and a transposon insertion mutant PmTn-294. Percentage of bacteria killed by chicken macrophage (HD 11) cells was determined by extrapolation from a standard formazan curve derived by incubating XTT with known bacterial cell numbers of each strain. The amount of formazan as measured by absorption at 450 nm was directly related to the number of viable bacterial cells. The percentages of P1059 SmR, CU, M9 and PmTn-294 killed by HD 11 cells were approximately 50%, 61%, 25% and 34%, respectively. By contrast, the virulent P1059 and D92 strains were resistant to killing, and were able to replicate inside the HD 11 cells. Association of virulence with resistance to phagocytic killing by HD 11 cells as assessed by the colorimetric bactericidal assay, was validated with resistance to complement (C')-mediated killing and a turkey mortality test. Strains P1059 and D92 were resistant to C'-mediated killing, whereas strains P1059 SmR, CU, M9 and PmTn-294 strains were susceptible. All turkeys challenged with P1059 or D92 were dead within 18 hrs. Mortality did not occur in turkeys challenged with strains of P1059 SmR, M9 and PmTn-294. The mortality among CU challenged turkeys ranged from 0 to 40%. The results suggest that the colorimetric bactericidal assay using XTT can be used to quantitate chicken macrophage phagocytic killing of P. multocida strains, and may be a valuable assay to differentiate virulent from avirulent strains of avian P. multocida.

Pasteurella haemolytica serotype 2 contains the gene for a noncapsular serotype 1-specific antigen

An ssa1-homologous genomic fragment cloned from Pasteurella haemolytica serotype 2 (ST2) enabled transformation of Escherichia coli DH5 alpha to a serotype 1 (ST1) phenotype through expression of the ST1-specific antigen (Ssa1). The Ssa1 protein expressed by ssa1-transformed E. coli was susceptible to heat and protease treatment and was distinct from P. haemolytica ST1-specific capsular polysaccharide. Electrophoretic analysis of in vitro-translated proteins, as well as the predicted amino acid sequence, demonstrated that Ssa1 proteins encoded from either ST1- or ST2-derived ssa1 genes were essentially identical. A comparison of the nucleotide sequences of ssa1 genes derived from P. haemolytica ST1 and ST2 revealed greater than 99% homology. Amino acid sequence homology of the predicted products of ST1- and ST2-derived ssa1 genes was greater than 98%. Northern (RNA) blot studies revealed that the presence of an increased level of ssa1 transcript in P. haemolytica ST1 grown as surface-adherent cultures on solid medium was correlated with a serologically detectable Ssa1 protein. Expression of the ssa1 transcript in ST1 was similarly upregulated by a high iron concentration in the growth medium.

Purified Pasteurella haemolytica leukotoxin induces expression of inflammatory cytokines from bovine alveolar macrophages

We obtained biologically active purified leukotoxin (Lkt) from Pasteurella haemolytica serotypel, strain 12296 using preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Three species of Lkt of molecular masses 95, 100, and 104 kDa were obtained. Purity of all three species of Lkt was confirmed by analytical SDS-PAGE and Western blot (immunoblot) analysis. Results from the chromogenic Limulus amebocyte lysate assay and silver staining of SDS-PAGE patterns indicated that the preparations were free of contaminating lipopolysaccharide. We then studied the kinetics of TNF alpha and IL-1 beta mRNA expression in bovine alveolar macrophages stimulated with the purified 104 kDa Lkt. Subcytolytic concentrations of Lkt induced TNF alpha and IL-1 beta gene expression and peak induction was observed at a concentration of 1 leukotoxin unit/ml. Both TNF alpha and IL-1 beta mRNA expression were detectable at 1 h after stimulation with 1 leukotoxin unit/ml. The expression peaked at 2 h, steadily declining up to 6 h, and was undetectable by 10 h. Secreted TNF alpha measured by bioassay peaked at 4-6 h and accumulated at a lesser concentration after 6 h. By contrast, secreted IL-1 peaked at 6 h and decreased significantly by 10 h. The ability of purified Lkt to induce TNF alpha and IL-1 beta gene expression and secretion of bioactive proteins was suppressed by Ca2+ chelating agents, 5 mM EDTA and 5 mM EGTA, but not polymyxin B. Heat-inactivation of the purified Lkt that had lost its cytocidal property completely abrogated induction of TNF alpha and IL-1 beta gene expression and secretion in bovine AMs.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of inflammatory cytokines in bovine alveolar macrophages following stimulation with Pasteurella haemolytica lipopolysaccharide

Bovine tumor necrosis factor alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) cDNAs were generated by reverse transcription and then by PCR amplification from lipopolysaccharide (LPS)-stimulated alveolar macrophage RNA. The amplified cDNAs were cloned into pPow and expressed in Escherichia coli DH5 alpha. The expressed proteins were confirmed as TNF-alpha and IL-1 beta by Western blot (immunoblot) analysis and bioassays. We then used the cloned genes as probes in Northern (RNA) blots and investigated the kinetics of TNF-alpha and IL-1 beta mRNA expression in bovine alveolar macrophages stimulated with purified LPS from Pasteurella haemolytica 12296. The effect of LPS on TNF-alpha and IL-1 beta gene expression was dose dependent, and induction was observed at a concentration of 0.01 microgram/ml. Both TNF-alpha and IL-1 beta mRNA expression were detectable within 0.5 h after stimulation with 1 microgram of LPS per ml, peaked at 1 to 2 h, steadily declined up to 16 h, and were undetectable by 24 h. Secreted TNF-alpha measured by bioassay peaked at 4 h and accumulated at a lesser concentration in conditioned medium throughout the 24 h. By contrast, secreted IL-1 beta was induced at 8 h and reached a maximal concentration at 24 h after stimulation. The ability of LPS to induce TNF-alpha and IL-1 beta gene expression and secretion of bioactive proteins were suppressed by polymyxin B. Our findings support a role for LPS from P. haemolytica in the induction of inflammatory cytokines in bovine pneumonic pasteurellosis.

Expression of iron-regulated outer membrane proteins by porcine strains of Pasteurella multocida

The outer membrane protein (OMP) profiles of two strains of capsular type A Pasteurella multocida isolated from the lungs of pigs with enzootic pneumonia were studied. Sarkosyl extracted OMPs from P. multocida grown under iron-restricted and iron-replete conditions were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis. Results showed that the iron-regulated outer membrane proteins (IROMPs) with molecular masses of 74 kDa, 94 kDa, 99 kDa and 109 kDa were expressed by strain A52, while 74 kDa, 82 kDa, 94 kDa and 99 kDa IROMPs were expressed by strain B80. Swine immune sera, obtained from pigs which were first immunized with a polyvalent P. multocida type A and type D bacterin and subsequently challenged with type A strain of P. multocida, contained antibodies against the IROMPs. These antibodies cross-reacted with the IROMPs expressed by avian strain P1059 of P. multocida. Convalescent-phase serum obtained from turkeys which survived fowl cholera, also cross-reacted with the IROMPs from porcine strains of P. multocida. These results suggested that IROMPs from porcine and avian strains of P. multocida may share common epitopes that were recognized by swine immune serum as well as turkey convalescent-phase serum.

Enhancement of neutrophil-mediated injury to bovine pulmonary endothelial cells by Pasteurella haemolytica leukotoxin

In this study, we used an in vitro coculture system to determine which virulence factor from Pasteurella haemolytica A1 was responsible for augmenting bovine polymorphonuclear neutrophil (PMN)-mediated killing of bovine pulmonary artery endothelial cells (BPAEC). A 51Cr release cytotoxicity assay was used as a measure of BPAEC killing. The mechanisms associated with this BPAEC killing were also studied. Our results demonstrated that the leukotoxin and not the lipopolysaccharide from P. haemolytica was responsible for augmenting the PMN-mediated killing of BPAEC. Furthermore, this augmented killing was related to the stimulation of PMNs by the leukotoxin. Killing of BPAEC by leukotoxin-stimulated PMNs was diminished in the presence of the H2O2 inactivator, catalase. The membrane-permeant H2O2, hydroxyl radical (HO.) scavenger 1,3-dimethyl-2 thiourea, and the HO. scavenger dimethyl sulfoxide but not the myeloperoxidase inhibitor sodium azide attenuated this BPAEC killing. Pretreatment of BPAEC with a 21-aminosteroid (U74500A), a potent iron chelator-antioxidant, provided the most effective protection against BPAEC killing induced by leukotoxin-stimulated PMNs. These data were compatible with the concept that the H2O2 generated by leukotoxin-stimulated PMNs interacts with intracellular iron in the endothelial cell to form highly reactive HO.. We suggest that HO. may be a key factor in BPAEC killing. Furthermore, since the elastase-specific inhibitor N-methoxy-succinyl-Ala-Ala-Pro-Val-chloromethyl ketone (CMK) also attenuated BPAEC killing and both CMK and 1,3-dimethyl-2 thiourea functioned additively in protecting against BPAEC killing, we conclude that both HO. and elastase may jointly contribute to BPAEC killing induced by leukotoxin-stimulated PMNs. This study broadens our understanding of how leukotoxin-stimulated PMNs injure lung endothelial cells and provides new insight into the pathogenesis of bovine pneumonic pasteurellosis.

The role of induced virulence factors produced by Pasteurella haemolytica in the pathogenesis of bovine pneumonic pasteurellosis: review and hypotheses

In the pathogenesis of pneumonic pasteurellosis, there is an abrupt commensal to pathogen shift from a predominance of P. haemolytica serotype 2 (ST2) to serotype 1 (ST1) in the bovine upper respiratory tract (URT) microfloral population. This occurs following periods of stress associated with development of this disease. Data are reviewed from recent publications supporting the contention that surface-expressed ST1-specific factor(s) could be critical in mediating URT adhesion and colonization. Such factors may promote an increase in the number of ST1 organisms deposited through infective droplets into the lungs, beyond that efficiently cleared by normal lung defences. The seeding of these organisms into the lungs may provide numerous foci of infection that eventually progress into characteristic pneumonic lesions seen in the disease.

Effects of Pasteurella haemolytica A1 leukotoxin on bovine neutrophils: degranulation and generation of oxygen-derived free radicals

To further define the role of Pasteurella haemolytica A1 leukotoxin in the pathogenesis of bovine pneumonic pasteurellosis, its in vitro effects on bovine neutrophils were investigated. Leukotoxin-containing culture supernatant, from P. haemolytica, stimulated a neutrophil respiratory burst as measured by the generation of oxygen-derived free radicals O2- and H2O2. This effect was immediate because preincubation of neutrophils with the culture supernatant for 5 min or longer substantially suppressed this respiratory burst. This suppression was due to cytolysis of the neutrophils. Prolonged incubation of neutrophils with the same culture supernatant caused further cytolysis and degranulation. Heat-inactivated P. haemolytica culture supernatant that had lost its cytotoxic properties failed to stimulate respiratory burst by neutrophils. Furthermore, the respiratory burst, cytolysis and degranulation were abrogated only by leukotoxin-neutralizing monoclonal and polyclonal antibodies, but not by antibodies against the lipopolysaccharide. These studies show that the leukotoxin component in the culture supernatant was responsible for the generation of oxygen-derived free radicals and proteolytic enzymes from neutrophils which may participate in direct lung injury.

Pasteurella haemolytica A1 and bovine respiratory disease: pathogenesis

The severe fibrinonecrotic pneumonia associated with pneumonic pasteurellosis usually results from colonization of the lower respiratory tract by Pasteurella haemolytica biotype A, serotype 1(A1). Despite recent research efforts, the authors lack a detailed understanding of the interactions and host response to P. haemolytica in the respiratory tract. The authors hypothesize that management and environmental stress factors or viral infection alters the upper respiratory tract (URT) epithelium allowing P. haemolytica to colonize the epithelium. Once the URT is colonized, large numbers of organisms enter the lung where they interact with alveolar macrophages. Endotoxin, released from the bacteria, crosses the alveolar wall where it activates pulmonary intravascular macrophages, endothelium, neutrophils, lymphocytes, platelets, complement, and Hageman factor leading to complex interactions of cells and mediators. It is the progression of this inflammatory response with neutrophil influx that is ultimately responsible for the pulmonary injury. Leukotoxin is a major virulence factor of P. haemolytica that allows it to survive by destroying phagocytic cells. At subcytolytic concentrations it may also enhance the inflammatory response by activating cells to produce mediators and release reactive oxygen metabolites and proteases.

Genomic distribution of a serotype 1-specific antigen-coding DNA fragment of Pasteurella haemolytica

A genomic fragment of Pasteurella haemolytica biotype A coding for a serotype 1-specific agglutinating antigen was used as a probe in a series of hybridization experiments to determine distribution of the fragment in various P. haemolytica serotypes as well as other bacteria. Results showed presence of the fragment in seven out of the 12 serotypes tested, all of which belonged to biotype A. Two other serotypes belonging to biotype A, all three serotypes belonging to biotype T, two Pasteurella multocida isolates and Escherichia coli did not have the fragment in their genome. Thus the expression of the P. haemolytica biotype A serotype 1-specific agglutinating antigen (PHA1SAA) seems to be due to serotype-specific regulation of protein expression rather than to genetic deletion. Differences in methylation of the PHA1SAA-coding fragment was also noted in DpnI and Sau3AI genomic DNA digests from the various serotypes analyzed by Southern blot. However, no apparent correlation was observed between methylation and PHA1SAA expression. E. coli with a recombinant plasmid containing a homologous genomic fragment derived from P. haemolytica serotype 2 also expressed PHA1SAA.

Alterations in pulmonary morphology and peripheral coagulation profiles caused by intratracheal inoculation of live and ultraviolet light-killed Pasteurella haemolytica A1 in calves

Eighteen male Holstein calves were divided into groups of three and inoculated intratracheally with 5 x 10(9) logarithmic phase or ultraviolet light-killed Pasteurella haemolytica biotype A serotype 1. Serial coagulation profiles were done on one calf from each group during the first 24 hours after inoculation. One calf from each group was necropsied at 4, 12, and 24 hours after inoculation and lesions were characterized with light and transmission electron microscopy. We found that 1) the pulmonary intravascular macrophage may have an important role in the early intravascular inflammatory events; 2) there was morphologic evidence for local initiation of the coagulation cascade in the lung early in the disease process but it was not a consumptive process; and 3) killed-bacteria were capable of causing fibrin exudation, platelet aggregation and alveolar epithelial damage similar to live bacteria, but the degenerative changes in neutrophils, endothelial cells and intravascular fibrin formation that occur with live bacteria were not seen.

Relationship between the iron regulated outer membrane proteins and the outer membrane proteins of in vivo grown Pasteurella multocida

The SDS-PAGE patterns of the outer membrane protein (OMP) extracts of Pasteurella multocida strain P1059, grown under iron-restricted, iron-replete and in vivo conditions, were examined. The results showed that the iron-regulated outer membrane proteins (IROMPs) with molecular masses of 76 kDa, 84 kDa, and 94 kDa were expressed by bacteria grown in iron-restricted media. They were also expressed by in vivo grown P. multocida. Convalescent-phase sera, obtained from turkeys which had survived pasteurellosis, contained antibodies that reacted intensly with th three IROMPs. This indicated that these proteins were expressed in vivo. Bacteria expressing the IROMPs showed greater binding to Congo Red when compared to cells not expressing IROMPs. Cells expressing the IROMPs or its OMP extracts grown in iron-restricted media also showed greater binding to 59Fe-pasteurella siderophore (multocidin) when compared to bacteria or its extracts not expressing IROMPs. Convalescent-phase sera, which contained antibodies against the IROMPs, blocked this specific 59Fe-multocidin binding to IROMPs. Autoradiography was used to determine which of these IROMPs functioned as a receptor for the iron-multocidin complex. The results suggested that these three IROMPs have specific epitopes for binding to the iron multocidin complex.

Changes in blood and bronchoalveolar lavage fluid components in calves with experimentally induced pneumonic pasteurellosis

Pneumonic pasteurellosis was experimentally induced in calves by inoculation of 5 x 10(8) Pasteurella haemolytica organisms into the right diaphragmatic lung lobe. Blood and bronchoalveolar lavage fluid samples were obtained prior to inoculation and at postinoculation hour (PIH) 2, 4, and 6. Calves developed acute lung injury, characteristic of pneumonic pasteurellosis. Lesions were found only in the right diaphragmatic lobe. By PIH 4, significant (P less than 0.01) increases were detected in lavage fluid total cell count, neutrophil count, total protein and albumin concentrations, and alkaline phosphatase (ALP) and lactic dehydrogenase (LD) activities. Myeloperoxidase and elastase activities did not increase. Neutrophil depletion ameliorated the lung lesions and prevented the increase in lavage fluid cell count, total protein, and albumin concentrations and ALP and LD activities. Treatment with the iron chelator, deferoxamine mesylatehydroxyethyl starch, attenuated the increase in total protein and albumin concentrations and ALP and LD activities at PID 4, but not PIH 6. Treatment with a neutrophil function inhibitor, pentoxifylline, prevented the increase in lavage fluid neutrophil numbers, but accentuated the increase in total protein and albumin concentrations, and ALP, LD, myeloperoxidase, and elastase activities.

Morphological and morphometrical analysis of the acute response of the bovine alveolar wall to Pasteurella haemolytica A1-derived endotoxin and leucotoxin

Endotoxin or leucotoxin derived from Pasteurella haemolytica biotype A serotype 1 or saline was deposited by fibreoptic bronchoscopy into the caudal segment of the right anterior lung lobe of calves, and the lesions were characterized by light and transmission electron microscopy. Morphometric techniques were used to determine if changes in the arithmetic mean thickness of the alveolar wall occurred. Group 1 calves (n = 2) were inoculated with 6 ml saline, groups 2 calves (n = 3) received 6 ml of a partially purified leucotoxin preparation, group 3 calves (n = 3) received 96 micrograms of endotoxin in 6 ml of saline and group 4 calves (n = 3) received 2.5 mg of endotoxin in 6 ml of saline. Calves were killed 4 h after inoculation. Lesions in groups 2, 3 and 4 were similar and we found that (a) endotoxin alone is capable of initiating an inflammatory response in the bovine lung, (b) leucotoxin causes cytotoxic changes in alveolar macrophages but not in parenchymal cells of the lung, (c) neutrophil sequestration and platelet aggregation occur in alveolar capillaries in association with pulmonary intravascular macrophages, (d) neutrophils and fibrin were found in the alveolus in close association with alveolar macrophages, (e) disruption of the alveolar epithelial layer occurred in association with neutrophils and (f) there were no significant increases in the arithmetic mean thickness of the alveolar wall.