Neutrophil Extracellular Traps Form a Barrier between Necrotic and Viable Areas in Acute Abdominal Inflammation

Abstract

Neutrophils form neutrophil extracellular traps (NETs) of decondensed DNA and histones that trap and immobilize particulate matter and microbial pathogens like bacteria. NET aggregates reportedly surround and isolate large objects like monosodium urate crystals, which cannot be sufficiently cleared from tissues. In the setting of acute necrotizing pancreatitis, massive tissue necrosis occurs, which is organized as pancreatic pseudocysts (1). In contrast to regular cysts, these pseudocysts are not surrounded by epithelial layers. We hypothesize that, instead, the necrotic areas observed in necrotizing pancreatitis are isolated from the surrounding healthy tissues by aggregated NETs. These may form an alternative, putatively transient barrier, separating necrotic areas from viable tissue. To test this hypothesis, we investigated histological samples from the necropsy material of internal organs of two patients with necrotizing pancreatitis and peritonitis accompanied by multiple organ failure. Tissues including the inflammatory zone were stained with hematoxylin and eosin and evaluated for signs of inflammation. Infiltrating neutrophils and NETs were detected by immunohistochemistry for DNA, neutrophil elastase (NE), and citrullinated histone H3. Interestingly, in severely affected areas of pancreatic necrosis or peritonitis, chromatin stained positive for NE and citrullinated histone H3, and may, therefore, be considered NET-derived. These NET structures formed a layer, which separated the necrotic core from the areas of viable tissue remains. A condensed layer of aggregated NETs, thus, spatially shields and isolates the site of necrosis, thereby limiting the spread of necrosis-associated proinflammatory mediators. We propose that necrotic debris may initiate and/or facilitate the formation of the NET-based surrogate barrier.

Ménage-à-Trois: The Ratio of Bicarbonate to CO2 and the pH Regulate the Capacity of Neutrophils to Form NETs

“…We hypothesize that neutrophils detect the border of the inflamed area in part by this pH gradient. NET formation at elevated pH values might then primarily function as a barrier to wall-off infected or inflamed areas and to prevent invasion of the organism with pathogens or to prevent spreading of danger-associated molecular patterns from necroinflammatory areas (28). The pH might also serve as an indicator for neutrophils to sense the progress of inflammation.…”

Frontline Science: Aggregated neutrophil extracellular traps prevent inflammation on the neutrophil-rich ocular surface

“…Later, the accumulation and aggregation of NETs actively prevent the dissemination of the inflammatory stimuli because they degrade inflammatory mediators. Chronic inflammatory responses to necrotic tissue are prevented by NETs that form barriers between the necrotic areas and the healthy surrounding tissues . Neutrophil‐driven inflammations are often terminated by proteolysis of cytokines and chemokines by NET‐born serine proteases limiting the inflammatory focus .…”

“…Chronic inflammatory responses to necrotic tissue are prevented by NETs that form barriers between the necrotic areas and the healthy surrounding tissues. 8,9 Neutrophildriven inflammations are often terminated by proteolysis of cytokines and chemokines by NET-born serine proteases limiting the inflammatory focus. [10][11][12] However, chronic and pathologic accumulation of noncleared NETs reportedly contributes to the perpetuation of autoimmune responses and autoinflammatory tissue damage.…”

Missing in action—The meaning of cell death in tissue damage and inflammation

“…Making use of samples derived from autopsies of two cases of acute necrotizing pancreatitis we characterized the border of the necrotic tissue by immunohistochemistry. 143 A layer of NE-and citrullinated histone H3-positive material surrounded the necrotic core and separated necrotic areas from viable tissue. A further example of the walling off areas of sterile necrotic by NETs was characterized by employing nanodiamonds below the critical size of 50 nm.…”

“…We speculated that neutrophils form a primary provisionary barrier in response to necrotic tissue by forming NETs. Making use of samples derived from autopsies of two cases of acute necrotizing pancreatitis we characterized the border of the necrotic tissue by immunohistochemistry . A layer of NE‐ and citrullinated histone H3‐positive material surrounded the necrotic core and separated necrotic areas from viable tissue.…”