Histophilus somni stimulates bovine monocyte-derived macrophages to release microparticles that increase fibrin clot formation in vitro.

Histophilus somni is a Gram-negative coccobacillus that causes diffuse vasculitis and intravascular thrombosis that can lead to multiple organ failure in cattle. Macrophages are important cellular mediators of fibrin deposition and removal at sites of inflammation. It has become evident that macrophages and other cells release microparticles (MPs) that have an array of biological activities, including pro-coagulant activity. We sought to determine whether monocyte-derived macrophages exposed to H. somni in vitro release MPs that activate the clotting cascade in a manner that could lead to thrombus formation. Bovine monocyte-derived macrophages were incubated with H. somni (at a 10:1 ratio) in RPMI with 10% heat inactivated fetal bovine serum for 6 h at 37 °C with 5 % CO2. Membrane-shed MPs were isolated from the conditioned media, washed twice with Ca2+ and Mg2+ free HBSS, and pro-coagulant activity assessed by a one-step plasma clotting assay. We observed greater pro-coagulant activity for MPs from H. somni stimulated macrophages than from unstimulated controls. Microparticle pro-coagulant activity was inhibited by addition of an anti-tissue factor antibody. We also observed co-localization of fluorescein-labeled H. somni cells and annexin V staining as evaluated by confocal microscopy. These results demonstrate that exposure to H. somni cells causes bovine monocyte-derived macrophages to release MPs that contain tissue factor, the first such report for bovine macrophages. We infer that if similar events occur in vivo they could amplify thrombus formation in bovine histophilosis.

Procoagulant activity of bovine neutrophils incubated with conditioned media or extracellular vesicles from Histophilus somni stimulated bovine brain endothelial cells.

Histophilus somni is a Gram negative coccobacillus that causes respiratory, reproductive and central nervous system disease in cattle. The hallmark of H. somni infection is diffuse vasculitis and intravascular thrombosis that can lead to an acute central nervous system disease known as thrombotic meningoencephalitis (TME). Because neutrophils are major players in the pathophysiology of septic meningitis, we sought to determine their role in H. somni-induced fibrin clot formation in vitro. Bovine brain endothelial cells (TBBE cells) were exposed to H. somni cells at a 1:25 ratio, respectively. Conditioned media (CM) were collected after a 6 h incubation at 37 °C with 5% CO2, and then incubated with bovine peripheral blood polymorphonuclear neutrophils (PMNs). Following incubation, fibrin clot formation and tissue factor activity were assessed by a re-calcified plasma clotting assay. We found greater tissue factor activity in cell lysates and CM from H. somni-stimulated TBBE cells than unstimulated control TBBE cells. In addition, PMNs exposed to CM or extracellular vesicles from H. somni-stimulated TBBE cells expressed von Willenbrand factor, exhibited increased fibrin clot formation, and displayed greater tissue factor activity than PMNs exposed to CM or extracellular vesicles from unstimulated control TBBE cells. These results suggest that bovine PMNs might acquire extracellular vesicles from endothelial cells that leads to thrombus formation in bovine brain microvasculature and contribute to the process that characterizes TME.

Histophilus somni causes extracellular trap formation by bovine neutrophils and macrophages.

Histophilus somni (formerly Haemophilus somnus) is a Gram-negative pleomorphic coccobacillus that causes respiratory, reproductive, cardiac and neuronal diseases in cattle. H. somni is a member of the bovine respiratory disease complex that causes severe bronchopneumonia in cattle. Previously, it has been reported that bovine neutrophils and macrophages have limited ability to phagocytose and kill H. somni. Recently, it was discovered that bovine neutrophils and macrophages produce extracellular traps in response to Mannheimia haemolytica, another member of the bovine respiratory disease complex. In this study, we demonstrate that H. somni also causes extracellular trap production by bovine neutrophils in a dose- and time-dependent manner, which did not coincide with the release of lactate dehydrogenase, a marker for necrosis. Neutrophil extracellular traps were produced in response to outer membrane vesicles, but not lipooligosacchride alone. Using scanning electron microscopy and confocal microscopy, we observed H. somni cells trapped within a web-like structure. Further analyses demonstrated that bovine neutrophils trapped and killed H. somni in a DNA-dependent manner. Treatment of DNA extracellular traps with DNase I freed H. somni cells and diminished bacterial death. Treatment of bovine monocyte-derived macrophages with H. somni cells also caused macrophage extracellular trap formation. These findings suggest that extracellular traps may play a role in the host response to H. somni infection in cattle.

Bovine herpesvirus type 1 infection of bovine bronchial epithelial cells increases neutrophil adhesion and activation.

Respiratory infection of cattle with bovine herpesvirus type 1 (BHV-1) predisposes cattle to secondary pneumonia with Mannheimia haemolytica as part of the bovine respiratory disease complex (BRD). One cell type that has received limited investigation for its role in the inflammation that accompanies BRD is the respiratory epithelial cell. In the present study we investigated mechanisms by which BHV-1 infection of respiratory epithelial cells contributes to the recruitment and activation of bovine polymorphonuclear neutrophils (PMNs) in vitro. Primary cultures of bovine bronchial epithelial (BBE) cells were infected with BHV-1 and assessed for cytokine expression by real-time PCR. We found that BHV-1 infection elicits a rapid IL-1, IL-8 and TNF-alpha mRNA response by BBE cells. Bovine PMNs exhibited greater adherence to BHV-1 infected BBE cells than uninfected cells. The increased adherence was significantly reduced by the addition of an anti-IL-1beta antibody or human soluble TNF-alpha receptor (sTNF-alphaR). Pre-incubation of bovine PMNs with conditioned media from BHV-1 infected BBE cells increased PMN migration, which was inhibited by addition of an anti-IL-1beta antibody, sTNF-alphaR, or an IL-8 peptide inhibitor. Conditioned media from BHV-1 infected BBE cells activated bovine PMNs in vitro as demonstrated by PMN shape change, production of reactive oxygen species and degranulation. PMNs also exhibited increased LFA-1 expression and susceptibility to M. haemolytica LKT following incubation with BHV-1 infected BBE cell conditioned media. Our results suggest that BHV-1 infection of BBE cells triggers cytokine expression that contributes to the recruitment and activation of neutrophils, and amplifies the detrimental effects of M. haemolytica LKT.