TLR9 and the NLRP3 Inflammasome Link Acinar Cell Death With Inflammation in Acute Pancreatitis

Abstract

BACKGROUND & AIMS Acute pancreatitis is characterized by early activation of intracellular proteases followed by acinar cell death and inflammation. Activation of damage-associated molecular pattern (DAMP) receptors and a cytosolic complex termed the inflammasome initiates forms of inflammation. In this study, we examined whether DAMP-receptors and the inflammasome provide the link between cell death and the initiation of inflammation in pancreatitis. METHODS Acute pancreatitis was induced by caerulein stimulation in wild-type mice and mice deficient in components of the inflammasome (ASC, NLRP3, caspase-1), Toll-like receptor 9 (TLR9), or the purinergic receptor P2X7. Resident and infiltrating immune cell populations and pro-IL-1β expression were characterized in control and caerulein-treated adult murine pancreas. TLR9 expression was quantified in pancreatic cell populations. Additionally, wild-type mice were pretreated with a TLR9 antagonist prior to induction of acute pancreatitis by caerulein or retrograde bile duct infusion of taurolithocholic acid 3-sulfate (TLCS). RESULTS Caspase-1, ASC, and NLPR3 were required for inflammation in acute pancreatitis. Genetic deletion of Tlr9 reduced pancreatic edema, inflammation, and pro-IL-1β expression in pancreatitis. TLR9 was expressed in resident immune cells of the pancreas, which are predominantly macrophages. Pretreatment with the TLR9 antagonist IRS954 reduced pancreatic edema, inflammatory infiltrate, and apoptosis. Pretreatment with IRS954 reduced pancreatic necrosis and lung inflammation in TLCS-induced acute pancreatitis. CONCLUSIONS Components of the inflammasome, specifically ASC, caspase-1, and NLRP3, are required for the development of inflammation in acute pancreatitis. TLR9 and P2X7 are important DAMP receptors upstream of inflammasome activation, and their antagonism could provide a new therapeutic strategy for treating acute pancreatitis.

Cell Death and DAMPs in Acute Pancreatitis

“…(nDAMPs) and mitochondrial DAMPs (mitDAMPs) (10,(37)(38)(39). In addition to necrosis and apoptosis, additional pathways (for example, necroptosis, pyroptosis and autophagy) regulate DAMP release and affect AP progression (Figure 2).…”

“…Perhaps paradoxically, intracellular heat shock proteins (HSPs) protect pancreatic acinar cells partly by inhibition of apoptosis (56,57), suggesting a pathogenic role of apoptosis in AP (58). Subsequently, researchers also observed that pancreatic acinar cells undergoing apoptosis can release histones, DNA and HMGB1, which facilitate pancreatic injury and the inflammatory response (10,11). Whether induction or inhibition of apoptosis would be beneficial in a clinical setting remains unproven.…”

“…Both activation of TLR9 and the NLRP3 inflammasome contribute to pancreatic acinar cell death and sterile inflammation in AP (10). Thus, genetic deletion of NLRP3, caspase-1 and TLR9 in mice protects against cerulein-induced pancreatitis (10).…”

“…Both activation of TLR9 and the NLRP3 inflammasome contribute to pancreatic acinar cell death and sterile inflammation in AP (10). Thus, genetic deletion of NLRP3, caspase-1 and TLR9 in mice protects against cerulein-induced pancreatitis (10). The potential mechanism responsible for these phenomena is involved in increased mitochondrial DNA (mitDNA) and nuclear DNA (nDNA) release during apoptosis, which activates TLR9 as well as NLRP3 inflammasome pathways (10,38,39).…”

A computer-based automated algorithm for assessing acinar cell loss after experimental pancreatitis

“…They include features of the acinar cell such as the premature intra-acinar activation of proteases [19,20,21], as well as extra-acinar sequalae. Damage signals from the pancreas, starting with the acinar cell [22], initiate a robust inflammatory infiltration into the pancreatic parenchyma with accompanying stromal edema and individual acinar cell swelling [23,24]. In this new inflammatory milieu and with larger-sized acini, the computer-learning algorithm must develop a prediction rule that can still accurately quantify acini containing these variations.…”