Histophilus somni causes extracellular trap formation by bovine neutrophils and macrophages

Abstract

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.

Macrophage Extracellular Traps: A Scoping Review

“…These dyes have also been used to construct spectofluorometric assays as an effort to quantify MET release by measuring fluorescence staining of extracellular DNA [6,24,25]. Typical definitions of METs, like NETs, include extracellular fibers composed of DNA that extend outside the bounds of the cell and are degraded by treatment with DNase I or micrococcal nuclease (Fig.…”

“…Several studies have examined the fundamental question of whether METs have cidal activity for different microorganisms. In order to assess microbicidal activity, most MET killing assays compare macrophages infected with or without the presence of DNase, and compare the recovered microorganisms [24,25]. In order to separate killing that results from MET-dependent and MET-independent mechanisms (mainly phagocytosis), some studies have used the actin cytoskeletal inhibitor, cytochalasin D, to inhibit macrophage phagocytosis [24,28].…”

“…In order to assess microbicidal activity, most MET killing assays compare macrophages infected with or without the presence of DNase, and compare the recovered microorganisms [24,25]. In order to separate killing that results from MET-dependent and MET-independent mechanisms (mainly phagocytosis), some studies have used the actin cytoskeletal inhibitor, cytochalasin D, to inhibit macrophage phagocytosis [24,28]. There have been mixed results using this technique as some groups have noted decreased MET release in cells pretreated with cytochalasin D [25,26].…”

Activation of the stress response in macrophages alters the M1/M2 balance by enhancing bacterial killing and IL-10 expression

“…Thus, we evaluated the effect of the SR on other Mϕ killing mechanisms such as antibacterial DNA-based extracellular trap (ET) formation [32][33][34][35][36] and the expression of antimicrobial peptides (AMPs). Mϕs were infected with live GBS bacteria and ET formation visualized by fluorescent microscopy.…”

The DNA fibers of shrimp hemocyte extracellular traps are essential for the clearance of Escherichia coli

“…Similar results have also reported for vertebrate ETs. For example,~40% more bacteria were killed by ETs from bovine neutrophils than by phagocytosis, while suppression of phagocytosis alone had no significant effect on the number of bacteria killed by the bovine neutrophils (Hellenbrand et al, 2013). In septic vertebrate blood too, more bacteria were captured and cleared by ETs than were phagocytosed by neutrophils or macrophages (Clark et al, 2007).…”