Alveolar macrophage and neutrophil interactions in Pasteurella haemolytica-induced endothelial cell injury

Effect of TRIF on permeability and apoptosis in bovine microvascular endothelial cells exposed to lipopolysaccharide

Bovine respiratory disease complex (BRDC) can be caused by several Gram negative bacteria. Lung endothelial cells may be damaged by the release of lipopolysaccharide (LPS) from these organisms. Toll-like receptor (TLR-4) signaling pathways include the myeloid differentiation primary response gene 88 (MyD88) and the Toll/interleukin (IL)-1 receptor (TIR) domain-containing adapter-inducing interferon-β (TRIF) pathways. The aim of this study was to determine which of these pathways is responsible for permeability changes, apoptosis and cytokine production in bovine lung microvascular cells exposed to LPS. Bovine lung endothelial cells were treated with a peptide to inhibit MyD88 signaling or small interfering RNA (siRNA) to inhibit TRIF signaling. Effects were measured using trans-well endothelial electrical resistance to determine cell monolayer permeability, annexin staining to estimate apoptosis and real-time PCR to measure levels of expression of IL-1β and tumor necrosis factor (TNF)-α mRNA. Inhibition of TRIF signaling reduced permeability changes and apoptosis in endothelial cells exposed to LPS. In contrast, MyD88 inhibition reduced expression of IL-1β and TNF-α mRNA in LPS treated cells, but had no effect on permeability. It was concluded that TRIF signaling in LPS-stimulated lung endothelial cells results in permeability changes and apoptosis.

Neutrophil adhesion and transmigration through bovine endothelial cells in vitro by protein H and LPS of Pasteurella multocida

This study describes an in vitro investigation on the role of Pasteurella multocida cells and its isolated protein H and LPS on neutrophil adhesion and migration through bovine endothelial cell monolayers. P. multicoda cells, protein H and LPS increased the adhesion and transmigration of neutrophils through BAEC. The bacteria/cell ratio of 100 for P. multocida, protein H concentration 0.05-0.2 microM and LPS concentration 0.5-1.0 microM respectively, induced the maximum adhesion and transmigration of neutrophils through BAEC. The optimal time of incubation with bacteria or bacterial products was 4-6 h. Our results confirm the role of Gram-negative bacteria and of components of the outer membrane such as protein H or LPS in activating the neutrophils and in promoting the adhesion and cells transmigration from the vessels to the site of inflammation.

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 platelet-activating factor in alveolar septal injury associated with experimentally induced pneumonic pasteurellosis in calves

OBJECTIVE

To determine whether platelet-activating factor (PAF) is involved in acute lung microvascular injury associated with pneumonic pasteurellosis in calves.

ANIMALS

15 healthy 2- to 4-week-old male Holstein calves.

PROCEDURE

Calves were anesthetized and inoculated intrabronchially with saline (0.9% NaCl) solution (n = 5) or 1x10(9) Pasteurella haemolytica organisms (n = 10). Of the 10 calves inoculated with P haemolytica, 5 also were treated with WEB 2086, a potent inhibitor of PAF, and 5 were treated with vehicle. Blood and bronchoalveolar lavage samples were collected before and 1, 2, 4, and 6 hours after inoculation of P. haemolytica. Blood samples were analyzed to evaluate total number and differential counts of leukocytes, dilute whole-blood leukocyte deformability, size of neutrophils, and neutrophil CD11b expression. Bronchoalveolar lavage samples were analyzed for total number and differential counts of nucleated cells, total protein concentration, and hemoglobin concentration. Size and gross and histologic appearance of lung lesions also was determined.

RESULTS

Treatment of calves with WEB 2086 reduced size of lung lesions, attenuated the increase in microvascular permeability, and reduced neutrophil infiltration in the first 4 hours after inoculation. Treatment with WEB 2086 also attenuated a decrease in leukocyte deformability, increase in size of neutrophils, and CD11b expression by circulating neutrophils.

CONCLUSIONS AND CLINICAL RELEVANCE

It appears that PAF is a major mediator for altered lung microvascular permeability and activation of circulating neutrophils in the first 4 hours after onset of pneumonic pasteurellosis in calves.

Coculture system to assess biocompatibility of candidate orthopaedic materials

When examining the biocompatibility of materials for orthopaedic use, it is important to examine the in vitro response of osteoblasts and macrophages to the material. The purpose of the current study was to develop an in vitro system that more accurately models the dynamic macrophage and osteoblast interactive response to biomaterials. A filter coculture system was designed that enables two cell types to be separated, while permitting cellular mediators to diffuse through its porous membrane. Using the filter coculture system, macrophage and osteoblast interaction in response to polymethylmethacrylate particle exposure was examined. As a marker of bone resorption potential, the level of prostaglandin E2 released from the macrophage and osteoblast coculture was compared with traditionally used in vitro culture systems. Cells interacting in the filter coculture system were found to produce prostaglandin E2 levels significantly greater than those of traditional conditioned media transfer systems. This filter coculture system offers an in vitro model that accounts for the continual cell to cell mediator interaction of two cell types simultaneously exposed to an implant material. This system may be useful in examining the biocompatibility of candidate materials at the bone interface, and thus elucidating the mechanisms of material induced bone resorption.

Identification of immunogenic, surface-exposed outer membrane proteins of Pasteurella haemolytica serotype 1

Pasteurella haemolytica serotype 1 (S1) is the bacterium most frequently recovered from the lungs of cattle that have succumbed to shipping fever pneumonia. P. haemolytica outer membrane proteins (OMPs) are important immunogens in the development of resistance to pneumonic pasteurellosis. The purpose of this study was to identify the repertoire of immunogenic, surface-exposed P. haemolytica (S1) OMPs, that could be important in the development of protective immunity. We determined surface exposure of OMPs by (1) their susceptibility to protease treatment and (2) their ability to adsorb out antibodies from bovine immune sera. For a comprehensive identification of immunogenic, surface-exposed OMPs, we used bovine antisera from calves that were resistant to experimental P. haemolytica challenge after (1) natural exposure to P. haemolytica, (2) vaccination with live P. haemolytica, or (3) vaccination with P. haemolytica OMPs. We identified 21 immunogenic, surface-exposed P. haemolytica OMPs. Most were recognized by all three immune sera. However, some were recognized by one or two of the three antisera. Our analyses identified surface-exposed, immunogenic proteins that were not identified in previous studies.

Binding of Pasteurella haemolytica leukotoxin to bovine leukocytes

Pasteurella haemolytica is the principal bacterial pathogen in the bovine respiratory disease complex. This organism produces an exotoxin (referred to as leukotoxin) during logarithmic-phase growth that is a potent leukocyte-modulating agent. At low concentrations, it activates neutrophils and mononuclear phagocytes to release inflammatory mediators, while at the same time making these cells destined to undergo apoptotic cell death. At higher concentrations, the toxin causes rapid swelling and loss of cell viability. In this study, we demonstrated that toxin binding can be directly evaluated by flow cytometry with biologically active biotinylated leukotoxin. Leukotoxin binding was blocked by the addition of a neutralizing anti-leukotoxin monoclonal antibody and was not detected when bovine leukocytes were incubated with culture filtrates from a mutant strain of P. haemolytica that does not produce biologically active leukotoxin. In addition, treatment of bovine leukocytes with protease K eliminated subsequent binding of leukotoxin, suggesting that there is a protein on the leukocyte surface that is either a leukotoxin binding site or is required for stabilization of leukotoxin binding. We did not detect binding of biotinylated leukotoxin to porcine or human leukocytes, which have been reported previously to be resistant to the lytic effects of the leukotoxin. These findings suggest that there may be a specific binding site for P. haemolytica leukotoxin on bovine but not on porcine or human leukocytes and that it might be involved in the activation and lytic activities of the leukotoxin.

Pasteurella haemolytica leukotoxin induces bovine leukocytes to undergo morphologic changes consistent with apoptosis in vitro

Infection of the bovine lung with Pasteurella haemolytica results in an acute respiratory disorder known as pneumonic pasteurellosis. One of the key virulence determinants used by this bacterium is secretion of an exotoxin that is specific for ruminant leukocytes (leukotoxin). At low concentrations, the leukotoxin can activate ruminant leukocytes, whereas at higher concentrations, it inhibits leukocyte functions and is cytolytic, presumably as a result of pore formation and subsequent membrane permeabilization. We have investigated the possibility that the activation-inhibition paradox is explained in part by leukotoxin-mediated apoptosis (i.e., activation-induced cell death) of bovine leukocytes. Incubation of bovine leukocytes with P. haemolytica leukotoxin caused marked cytoplasmic membrane blebbing (zeiosis) and chromatin condensation and margination, both of which are hallmarks of apoptosis. The observed morphologic changes in bovine leukocytes were leukotoxin dependent, because they were significantly diminished in the presence of an anti-leukotoxin monoclonal antibody. In addition, bovine leukocytes incubated with culture supernatant from a mutant strain of P. haemolytica that does not produce any detectable leukotoxin failed to exhibit the morphologic changes characteristic of cells undergoing apoptosis. These observations may represent an important mechanism by which P. haemolytica overwhelms host defenses, contributing to the fibrinous pleuropneumonia characteristic of bovine pasteurellosis.

Pasteurella haemolytica lipopolysaccharide-induced cytotoxicity in bovine pulmonary artery endothelial monolayers: inhibition by indomethacin

Exposure of bovine pulmonary artery endothelial cells to Pasteurella haemolytica lipopolysaccharide caused severe morphologic changes. Initially, there was dilatation of the rough endoplasmic reticulum and mitochondrial swelling followed by cell retraction, membrane bleb formation, and cell detachment. The affected endothelial cells had severe membrane damage resulting in the leakage of lactate dehydrogenase. Indomethacin in concentrations of 0.5 mM or greater caused marked decreases in the lipopolysaccharide-induced leakage of lactate dehydrogenase. Indomethacin at 5 mM also caused a marked reduction of the lipopolysaccharide-induced morphologic changes resulting in apparent maintenance of the monolayer integrity for 8 hours versus 1 hour in the lipopolysaccharide-treated control. A marked decrease in the cell and nuclear membrane effects resulted, but the rough endoplasmic reticulum dilatation and mitochondrial changes proceeded. These results indicate that indomethacin does not prevent lipopolysaccharide binding but interferes with later events in lipopolysaccharide-induced cytotoxicity in the bovine pulmonary endothelial cell. The concentration of indomethacin required to produce this inhibition suggests that the primary mechanism is not cyclooxygenase inhibition.

Knockout mutants of Actinobacillus pleuropneumoniae serotype 1 that are devoid of RTX toxins do not activate or kill porcine neutrophils

The Actinobacillus pleuropneumoniae RTX-toxins ApxI, ApxII, and ApxIII are important virulence factors of this swine pathogen. It is hypothesized that the Apx toxins are deleterious to defense cells of the host, enabling the bacterium to infect the host. To confirm this, we studied the effect on porcine polymorphonuclear neutrophils of mutant strains of A. pleuropneumoniae that were devoid of Apx toxins. For this purpose, we developed a system for targeted mutagenesis of A. pleuropneumoniae based on the conditionally replicating plasmid pVE6063 and insertional mutagenesis by homologous recombination. Employing this system on the reference strain of serotype 1, a strain that secretes ApxI and ApxII, we generated mutant strains that were devoid of ApxI and/or ApxII. We compared the ability of the parent strain and the mutant strains to provoke an oxidative burst in porcine neutrophils and to kill these cells. The parent strain and mutants that secreted either ApxI or ApxII provoked an oxidative burst and killed the neutrophils, whereas mutant strains that were devoid of ApxI and ApxII did not. These experiments indicate the importance of ApxI and ApxII to these profound effects on neutrophils and emphasize the importance of ApxI and ApxII in pathogenesis.

Regulation of major histocompatibility complex class II expression by Pasteurella haemolytica leukotoxin

Many properties have been associated with Pasteurella haemolytica leukotoxin and other repeat-in-toxin toxins, including their cytotoxic activity on various cells of the lymphoid and nonlymphoid systems as well as their ability to modulate the immunological activity of lymphocytes and monocytes. In this study, we assessed the ability of P. haemolytica leukotoxin to affect the expression major histocompatibility complex (MHC) class II molecules on bovine peripheral monocytes. Peripheral blood mononuclear cells were isolated from P. haemolytica leukotoxin-seronegative calves and incubated with various concentrations of authentic leukotoxin as well as the recombinant lktA gene product (LktA). Expression of MHC class II antigen on cells was evaluated by flow cytometric methods. The results indicated that both a crude, authentic leukotoxin preparation and LktA were able to affect MHC class II expression by inducing a marked downregulation of MHC class II expression on bovine monocytes. However, when cells were activated with gamma interferon (IFN-gamma), LktA and Lkt had little or no detectable effect. By using a cell line which expresses MHC class II only after activation by INF-gamma, we were able to confirm the observation that LktA had no effect on the expression of MHC class II after IFN-gamma treatment. Leukotoxin affected the functional capacity of monocytes to present antigen, as demonstrated by the ability of LktA or authentic leukotoxin to totally inhibit a mixed lymphocyte culture from MHC-mismatched calves. Thus, leukotoxin was able to downregulate constitutive expression of MHC class II expression, and we propose that this is a novel way in which this molecule can affect the immune function of monocytes, playing an important role in bacterial pathogenesis and survival of organisms at the infection site.

Tissue factor expression in bovine endothelial cells induced by Pasteurella haemolytica lipopolysaccharide and interleukin-1

Pasteurella haemolytica in cattle produces fibrino-hemorrhagic pleuropneumonia characterized by extensive pulmonary microvascular thrombosis and parenchymal necrosis. The purpose of this in vitro study was to determine if P. haemolytica lipopolysaccharide (LPS) promotes vascular thrombosis by inducing a procoagulant state in vascular endothelial cells. After treatment of confluent monolayers of bovine pulmonary artery endothelial cells with various concentrations of either P. haemolytica LPS or Escherichia coli LPS, the procoagulant activity of the endothelial cells was determined using a chromogenic assay dependent on cellular tissue factor expression. The LPS treatment induced significant increases in cellular tissue factor expression in a LPS concentration- and time-dependent manner. Highest levels of tissue factor were present at 22 hours after treatment, although high LPS concentrations induced moderate tissue factor levels at 5 hours after treatment. Interleukin-1 also induced tissue factor expression in endothelial cells and enhanced the LPS-induced effects. This interleukin-1 effect could be diminished by concurrent use of an interleukin-1 receptor antagonist. These results demonstrate that LPS and cytokine promotion of a procoagulant state in endothelial cells occurs in vitro. Similar mechanisms may play a role in P. haemolytica-mediated pulmonary vascular thrombosis.