Pasteurella haemolytica leukotoxin-induced increase in phospholipase A2 activity in bovine neutrophils

Complete Closed Genome Sequences of a Mannheimia haemolytica Serotype A1 Leukotoxin Deletion Mutant and Its Wild-Type Parent Strain

Mannheimia haemolytica is a bacterial pathogen that secretes leukotoxin (LktA) which binds to leukocyte membranes via CD18, causing bacterial pneumonia in ruminants. We report the complete closed genome sequences of a leukotoxin mutant and its parent strain that are frequently used in respiratory disease studies.

Mannheimia haemolytica and bovine respiratory disease

Mannheimia haemolytica is the principal bacterium isolated from respiratory disease in feedlot cattle and is a significant component of enzootic pneumonia in all neonatal calves. A commensal of the nasopharynx, M. haemolytica is an opportunist, gaining access to the lungs when host defenses are compromised by stress or infection with respiratory viruses or mycoplasma. Although several serotypes act as commensals, A1 and A6 are the most frequent isolates from pneumonic lungs. Potential virulence factors include adhesin, capsular polysaccharide, fimbriae, iron-regulated outer membrane proteins, leukotoxin (Lkt), lipopolysaccharide (LPS), lipoproteins, neuraminidase, sialoglycoprotease and transferrin-binding proteins. Of these, Lkt is pivotal in induction of pneumonia. Lkt-mediated infiltration and destruction of neutrophils and other leukocytes impairs bacterial clearance and contributes to development of fibrinous pneumonia. LPS may act synergistically with Lkt, enhancing its effects and contributing endotoxic activity. Antibiotics are employed extensively in the feedlot industry, both prophylactically and therapeutically, but their efficacy varies because of inconsistencies in diagnosis and treatment regimes and development of antibiotic resistance. Vaccines have been used for many decades, even though traditional bacterins failed to demonstrate protection and their use often enhanced disease in vaccinated animals. Modern vaccines use culture supernatants containing Lkt and other soluble antigens, or bacterial extracts, alone or combined with bacterins. These vaccines have 50-70% efficacy in prevention of M. haemolytica pneumonia. Effective control of M. haemolytica pneumonia is likely to require a combination of more definitive diagnosis, efficacious vaccines, therapeutic intervention and improved management practices.

Phospholipase A2 functions in Pseudomonas aeruginosa-induced apoptosis

Pseudomonas aeruginosa, a gram-negative, facultative pathogen, causes severe and often even lethal infections in immunocompromised patients, as well as cystic fibrosis patients. We show here that a variety of P. aeruginosa strains activate phospholipase A2 (PLA2), cultured epithelial cells, and fibroblasts, resulting in increased intracellular and extracellular arachidonic acid release. The use of different PLA2 inhibitors revealed that P. aeruginosa-induced arachidonic acid release is mediated by activation of cytosolic PLA2 (cPLA2), whereas iPLA2 or sPLA2 do not seem to be involved in the response to P. aeruginosa. Likewise, the cPLA2-specific inhibitors MAFP and AACOCF3 prevented apoptosis of cultured epithelial cells upon P. aeruginosa infection, whereas inhibitors specific for iPLA2 or sPLA2 were without effect. The physiological significance of these findings is indicated by an inhibition of apoptosis in tracheal epithelial cells upon in vivo infection with P. aeruginosa. The data indicate that arachidonic acid generation by activation of cPLA2 during P. aeruginosa infection plays an important role in the induction of host cell death.

Determination of specific receptor sites for platelet activating factor in bovine neutrophils

OBJECTIVE

To identify and characterize a platelet activating factor (PAF) receptor in bovine neutrophils by use of radioligand binding, reverse transcription-polymerase chain reaction (RT-PCR) assay, and western blot analysis.

ANIMALS

4 healthy adult cows.

PROCEDURE

Bovine neutrophil membranes were isolated for association, dissociation, and saturation binding experiments with PAF labeled with hydrogen 3 (3H-PAF). The RT-PCR assay was performed with appropriate human primers, and western blot analysis was developed with a polyclonal antibody obtained from a peptide of bovine PAF receptor.

RESULTS

Analysis of kinetic binding data supported a single class of PAF receptor. Binding of 3H-PAF to membrane preparations was selectively displaced by PAF and a nonhydrolyzable analogue of guanine triphosphate (Gpp[NH]p) and by lyso-PAF (a biologically inactive analogue of PAF) to a lesser extent. Among other PAF receptor antagonists, 14-deoxyandrographolide and WEB 2086 were the most effective in inhibiting 3H-PAF binding sites in neutrophil membranes; 2 lignans, schisandrin-A and gamma-schisandrin were also effective, but 2 gingkolides (BN52020 and BN52021) only mildly inhibited 3H-PAF binding. Results of RT-PCR assay and western blot analysis of neutrophil crude membranes confirmed the presence of a PAF receptor.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that bovine neutrophils express only 1 type of PAF receptor, and it is likely that this receptor is involved in inflammatory responses. The most effective PAF antagonists were 14-deoxyandrographolide and WEB 2086; these PAF antagonists may be potentially useful in the treatment of inflammatory processes in cattle.

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.

Construction and characterization of an acapsular mutant of Mannheimia haemolytica A1

The nmaA and nmaB genes, which code for UDP-GlcNAc-2-epimerase and UDP-ManNAc-dehydrogenase, respectively, are involved in capsular polysaccharide biosynthesis in Mannheimia haemolytica A1. A chloramphenicol resistance (Cm(r)) cassette cloned behind an M. haemolytica A1 promoter, plpcat, was created and used to interrupt nmaA and nmaB. A 1.3-kbp DNA fragment that encompasses part of nmaA and nmaB was replaced by the 1.0-kbp plpcat, resulting in a knockout mutant which is Cm(r) and unable to synthesize N-acetylmannosamine (ManNAc) and N-acetylmannosaminuronic acid (ManNAcA). The DNA replacement was confirmed by Southern hybridization and PCR analyses of the nmaA and nmaB loci. Electron microscopy examination of the mutant showed the absence of capsular materials compared to the parent strain. The loss of NmaA and NmaB activity was confirmed by analysis of carbohydrate moieties using capillary electrophoresis. Serum sensitivity assays indicated that the acapsular mutant is as resistant as the encapsulated parent to complement-mediated killing by colostrum-deprived calf serum but is more sensitive to killing by immune bovine serum. Analysis of lipopolysaccharide prepared from the acapsular mutant and encapsulated parent confirmed that these strains have long O-polysaccharide chains, possibly conferring resistance to serum-mediated killing.

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.

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.

Response of the ruminant respiratory tract to Mannheimia (Pasteurella) haemolytica

Pneumonia is a leading cause of loss to the sheep and cattle industry throughout the world. Mannheimia (Pasteurella) haemolytica is one of the most important respiratory pathogens of domestic ruminants and causes serious outbreaks of acute pneumonia in neonatal, weaned and growing lambs, calves, and goats. M. haemolytica is also an important cause of pneumonia in adult animals. Transportation, viral infections with agents such as infectious bovine rhinotracheitis virus, parainfluenza-3 virus or bovine respiratory syncytial virus, overcrowding, housing of neonates and weaned animals together and other stressful conditions predispose animals to M. haemolytica infection [1, 2]. This review assimilates some of the findings key to cellular and molecular responses of the lung from a pathologist's perspective. It includes some of what is known and underscores areas that are not fully understood.

Role of phospholipase D in Pasteurella haemolytica leukotoxin-induced increase in phospholipase A(2) activity in bovine neutrophils

The effects of Pasteurella haemolytica leukotoxin (LKT) on the activity of phospholipase D (PLD) and the regulatory interaction between PLD and phospholipase A(2) (PLA(2)) were investigated in assays using isolated bovine neutrophils labeled with tritiated phospholipid substrates of the two enzymes. Exposure of [(3)H]lysophosphatidylcholine-labeled neutrophils to LKT caused concentration- and time-dependent production of phosphatidic acid (PA), the product of PLD. LKT-induced generation of PA was dependent on extracellular calcium. Both production of PA and metabolism of [(3)H]-arachidonate ([(3)H]AA)-labeled phospholipids by PLA(2) were inhibited when ethanol was used to promote the alternative PLD-mediated transphosphatidylation reaction, resulting in the production of phosphatidylethanol rather than PA. The role of PA in regulation of PLA(2) activity was then confirmed by means of an add-back experiment, whereby addition of PA in the presence of ethanol restored PLA(2)-mediated release of radioactivity from neutrophil membranes. Considering the involvement of chemotactic phospholipase products in the pathogenesis of pneumonic pasteurellosis, development and use of anti-inflammatory agents that inhibit LKT-induced activation of PLD and PLA(2) may improve therapeutic management of the disease.

Role of intracellular calcium in Pasteurella haemolytica leukotoxin-induced bovine neutrophil leukotriene B4 production and plasma membrane damage

Isolated neutrophils were used to study the intracellular calcium ([Ca2+]i) dependency of Pasteurella haemolytica leukotoxin-induced production of leukotriene B4 and plasma membrane damage. Exposure of neutrophils to leukotoxin caused a rapid and concentration-dependent increase in [Ca2+]i, followed by simultaneous plasma membrane damage and production of leukotriene B4. Removal of extracellular Ca2+, replacement of Ca2+ with other divalent cations, or exposure to high concentration of verapamil, an inhibitor of voltage-dependent calcium channels, inhibited leukotoxin-induced increases in [Ca2+]i, leukotriene B4 production, and membrane damage, thus indicating that influx of extracellular Ca2+ is necessary to produce these leukotoxin-induced neutrophil responses.