Significance Neutrophil accumulation is fundamental to acute inflammation. In response to tissue injury, circulating neutrophil–platelet aggregates (N-PAs) form for secondary capture. Counterregulation of acute inflammatory processes by specialized proresolving mediators is essential to mitigate collateral injury to healthy bystander tissue. Here, we identified a biosynthetic route in human platelets for the proresolving mediator maresin 1 (MaR1) that is amplified during platelet–neutrophil interactions. In a self-resolving murine model of acute lung injury, N-PAs rapidly formed and a MaR1 counterregulatory circuit was engaged to restrain N-PAs and acute inflammation and restore homeostasis of the injured lung.

Bacterial pneumonia is a leading cause of morbidity and mortality worldwide. Host responses to contain infection and mitigate pathogen-mediated lung inflammation are critical for pneumonia resolution. Aspirin-triggered resolvin D1 (AT-RvD1; 7S,8R,17R trihydroxy-4Z,9E,11E,13Z,15E,19Z docosahexaenoic acid) is a lipid mediator that displays organ protective actions in sterile lung inflammation, and regulates pathogen-initiated cellular responses. Here, in a self-resolving murine model of Escherichia coli pneumonia, lipid mediator metabololipidomics performed on lungs obtained at baseline, 24 hours and 72 hours after infection uncovered temporal regulation of endogenous AT-RvD1 production. Early treatment with exogenous AT-RvD1 (1 hr post-infection) enhanced clearance of E.coli and Pseudomonas aeruginosa in vivo, and lung macrophage phagocytosis of fluorescent bacterial particles ex vivo. Characterization of macrophage subsets in the alveolar compartment during pneumonia identified efferocytosis by infiltrating macrophages (CD11bHi CD11cLow) and exudative macrophages (CD11bHi CD11cHi). AT-RvD1 increased efferocytosis by these cells ex vivo, and accelerated neutrophil clearance during pneumonia in vivo. These anti-bacterial and pro-resolving actions of AT-RvD1 were additive to antibiotic therapy. Taken together, these findings suggest that the pro-resolving actions of AT-RvD1 during pneumonia represent a novel host-directed therapeutic strategy to complement the current antibiotic centered approach to combatting infections.

Rationale: Inducible nitric oxide synthase (iNOS) has been implicated in the development of acute lung injury. Recent studies indicate a role for mechanical stress in iNOS and endothelial NOS (eNOS) regulation. Objectives: This study investigated changes in lung NOS expression and activity in a mouse model of ventilatorinduced lung injury. Methods: C57BL/6J (wild-type [WT]) and iNOSdeficient (iNOS Ϫ/Ϫ ) mice received spontaneous ventilation (control) or mechanical ventilation (MV; VT of 7 and 20 ml/kg) for 2 hours, after which NOS gene expression and activity were determined and pulmonary capillary leakage assessed by the Evans blue albumin assay. Results: iNOS mRNA and protein expression was absent in iNOS Ϫ/Ϫ mice, minimal in WT control mice, but significantly upregulated in response to 2 hours of MV. In contrast, eNOS protein was decreased in WT mice, and nonsignificantly increased in iNOS Ϫ/Ϫ mice, as compared with control animals. iNOS and eNOS activities followed similar patterns in WT and iNOS Ϫ/Ϫ mice. MV caused acute lung injury as suggested by cell infiltration and nitrotyrosine accumulation in the lung, and a significant increase in bronchoalveolar lavage cell count in WT mice, findings that were reduced in iNOS Ϫ/Ϫ mice. Finally, Evans blue albumin accumulation in lungs of WT mice was significant (50 vs. 15% increase in iNOS Ϫ/Ϫ mice compared with control animals) in response to MV and was prevented by treatment of the animals with the iNOS inhibitor aminoguanidine. Conclusion: Taken together, our results indicate that iNOS gene expression and activity are significantly upregulated and contribute to lung edema in ventilator-induced lung injury.Keywords: inducible nitric oxide synthase; lung permeability; mechanical ventilation Nitric oxide (NO) is involved in many physiologic and pathologic conditions, such as blood vessel relaxation, neurotransmission, and host defense. NO also plays a critical role in tissue injury in the context of various inflammatory conditions (1). NO is produced by three isoforms (neuronal, endothelial, and inducible) of NO synthase (NOS). Endothelial NOS (eNOS), which is calcium/calmodulin (Ca 2ϩ /CaM)-dependent and activated by agonists (e.g., acetylcholine), produces a low level of NO output, whereas inducible NOS (iNOS) is independent of Ca 2ϩ /CaM, is transcriptionally regulated by proinflammatory products (e.g., LPS) and cytokines (interleukin 1␤ [IL-1␤], tumor necrosis factor ␣ [TNF-␣], IFN-␥), and results in sustained and elevated (Received in original form November 19, 2004; accepted in final form May 20, 2005) Supported by awards from the National Heart, Lung, and Blood Institute (NIH R01 HL049441 and P50 HL 73994).* These authors contributed equally to the work presented in this article. release of NO (2). Therefore, overproduction of NO, in particular in the setting of superoxide production (3), leads to oxidative stress and tissue injury in conditions such as endotoxin-induced acute lung injury (ALI) (4).ALI is characterized by a severe inflammatory process, pr...

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Xanthine oxidoreductase (XOR) plays a prominent role in acute lung injury because of its ability to generate reactive oxygen species. We investigated the role of XOR in ventilator-induced lung injury (VILI). Male C57BL/6J mice were assigned to spontaneous ventilation (sham) or mechanical ventilation (MV) with low (7 ml/kg) and high tidal volume (20 ml/kg) for 2 h after which lung XOR activity and expression were measured and the effect of the specific XOR inhibitor allopurinol on pulmonary vascular leakage was examined. In separate experiments, rat pulmonary microvascular endothelial cells (RPMECs) were exposed to cyclic stretch (5% and 18% elongation, 20 cycles/min) for 2 h before intracellular XOR activity measurement. Lung XOR activity was significantly increased at 2 h of MV without changes in XOR expression. There was evidence of p38 MAP kinase, ERK1/2, and ERK5 phosphorylation, but no change in JNK phosphorylation. Evans blue dye extravasation and bronchoalveolar lavage protein concentration were significantly increased in response to MV, changes that were significantly attenuated by pretreatment with allopurinol. Cyclic stretch of RPMECs also caused MAP kinase phosphorylation and a 1.7-fold increase in XOR activity, which was completely abrogated by pretreatment of the cells with specific MAP kinase inhibitors. We conclude that XOR enzymatic activity is significantly increased by mechanical stress via activation of p38 MAP kinase and ERK and plays a critical role in the pathogenesis of pulmonary edema associated with VILI.

Acute lung injury is a life-threatening condition caused by disruption of the alveolar-capillary barrier leading to edema, influx of inflammatory leukocytes, and impaired gas exchange. Specialized proresolving mediators biosynthesized from essential fatty acids, such as docosahexaenoic acid, have tissue protective effects in acute inflammation. Herein, we found that the docosahexaenoic acidederived mediator resolvin D3 (RvD3): 4S,11R,17S-trihydroxydocosa-5Z,7E,9E,13Z,15E,19Z-hexaenoic acid was present in uninjured lungs, and increased significantly 24 to 72 hours after hydrochloric acideinitiated injury. Because of its delayed enzymatic degradation, we used aspirin-triggered (AT)-RvD3: 4S,11R,17R-trihydroxydocosa-5Z,7E,9E,13Z,15E,19Z-hexaenoic acid, a 17R-epimer of RvD3, for in vivo experiments. Histopathological correlates of acid injury (alveolar wall thickening, edema, and leukocyte infiltration) were reduced in mice receiving AT-RvD3 1 hour after injury. AT-RvD3etreated mice had significantly reduced edema, as demonstrated by lower wet/dry weight ratios, increased epithelial sodium channel g expression, and more lymphatic vessel endothelial hyaluronan receptor 1-positive vascular endothelial growth factor receptor 3-positive lymphatic vessels. Evidence for counterregulation of NF-kB by RvD3 and AT-RvD3 was seen in vitro and by AT-RvD3 in vivo. Increases in lung epithelial cell proliferation and bronchoalveolar lavage fluid levels of keratinocyte growth factor were observed with AT-RvD3, which also promoted cutaneous re-epithelialization. Together, these data demonstrate protective actions of RvD3 and AT-RvD3 for injured mucosa that accelerated restoration of epithelial barrier and function. (Am J Pathol 2016 http://dx.doi.org/10.1016/j.ajpath.2016 Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are serious conditions characterized by loss of normal epithelial and endothelial barrier function, leading to pulmonary edema and infiltration of leukocytes culminating in significant impairments in gas exchange. 1 ARDS and its milder form, ALI, are serious conditions characterized by loss of normal epithelial and endothelial barrier function, leading to pulmonary edema and infiltration of leukocytes culminating in significant impairments in gas exchange. 1 ALI is a common condition, affecting approximately 200,000 patients a year in the United States alone. 2 Although our understanding of the pathophysiological changes associated with ALI/ARDS has improved since its original description, effective management of this condition still proves difficult, and mortality remains approximately 40%. 2 Aspiration pneumonia/pneumonitis is a common cause of ALI/ARDS characterized by direct damage to lung epithelia (because of the low pH of gastric contents) followed by extensive sloughing of the injured epithelial cells. 3 Despite significant damage to the epithelial barrier,

Specialized proresolving mediators (SPMs) decrease NF-κB activity to prevent excessive tissue damage and promote the resolution of acute inflammation. Mechanisms for NF-κB regulation by SPMs remain to be determined. In this study, after LPS challenge, the SPMs 15-epi-lipoxin A (15-epi-LXA), resolvin D1, resolvin D2, resolvin D3, and 17-epi-resolvin D1 were produced in vivo in murine lungs. In LPS-activated human bronchial epithelial cells, select SPMs increased expression of the NF-κB regulators A20 and single Ig IL-1R-related molecule (SIGIRR). Of interest, 15-epi-LXA induced and in an lipoxin A receptor/formyl peptide receptor 2 (ALX/FPR2) receptor-dependent manner in epithelial cells and in murine pneumonia. This SPM regulated NF-κB-induced cytokines to decrease pathogen-mediated inflammation. In addition to dampening lung inflammation, surprisingly, 15-epi-LXA also enhanced pathogen clearance with increased antimicrobial peptide expression. Taken together, to our knowledge these results are the first to identify endogenous agonists for A20 and SIGIRR expression to regulate NF-κB activity and to establish mechanisms for NF-κB regulation by SPMs for pneumonia resolution.

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