Neutralizing injurious stimuli, proinflammatory mediator catabolism, and polymorphonuclear leukocyte (PMN) clearance are determinants of inflammatory resolution. To this, we recently added innate-type lymphocyte repopulation as being central for restoring postinflammation tissue homeostasis with a role in controlling innate immune–mediated responses to secondary infection. However, although macrophages dominate resolution, their phenotype and role in restoring tissue physiology once inflammation abates are unknown. Therefore, we isolated macrophages from the resolving phase of acute inflammation and found that compared with classically activated proinflammatory M1 cells, resolution-phase macrophages (rMs) possess weaker bactericidal properties and express an alternatively activated phenotype but with elevated markers of M1 cells including inducible cyclooxygenase (COX 2) and nitric oxide synthase (iNOS). This phenotype is controlled by cAMP, which, when inhibited, transforms rM to M1 cells. Conversely, elevating cAMP in M1 cells transforms them to rMs, with implications for cAMP in the resolution of systemic inflammation. It transpires that although rMs are dispensable for clearing PMNs during self-limiting inflammation, they are essential for signaling postresolution lymphocyte repopulation via COX 2 lipids. Thus, rM macrophages are neither classically nor alternatively activated but a hybrid of both, with a role in mediating postresolution innate-lymphocyte repopulation and restoring tissue homeostasis.

Patients with advanced cirrhosis experience frequent infections leading to sepsis, which carries high mortality. While innate immune dysfunction underlies this vulnerability, the precise cause remains elusive. We found prostaglandin (PGE 2 ) elevated in acutely decompensated (AD) patients at immunosuppressive levels. Plasma from AD and end-stage liver disease (ESLD) patients suppressed macrophage cytokine secretion and bacteria killing in a PGE 2 receptor-dependent manner, effects not seen in stable cirrhosis. Mouse models (bile duct ligation and CCL4-liver injury) also demonstrated elevated PGE 2 , which when inhibited completely restored immune competence and survival following infection. Importantly, albumin binds/inactivates PGE 2 resulting in greater PGE 2 bioavailability. This results in enhanced immunosuppressive effects of AD plasma in patients with low albumin levels. Administering albumin to AD patients reversed immunosuppressive properties of their plasma; protective effects recapitulated in rodent survival studies. Thus, elevated PGE 2 combined with hypoalbuminemia mediates immunosuppression in AD and ESLD patients, which can be reversed with albumin. Cyclooxygenase (COX)-derived lipid mediators have broad immunosuppressive effects12-15 that could explain the aetiology of infection susceptibility in cirrhosis patients. Thus, we used a number of in vitro and in vivo assays using plasma from patients with AD and ESLD derived from clinical trials to investigate the role of bioactive lipid mediators in immunosuppression as well as animal models of liver injury for survival analyses. ResultsProstaglandin E 2 (PGE 2 ) is elevated in patients with acute decompensation at levels that are immunosuppressive via its effect on the EP2/3 receptor ESI/LC-MS/MS analysis of acutely decompensated patient plasma (day 1-2 of hospital admission) demonstrated significantly elevated PGE 2 , PGF 2 α, 5-and 15-HETE compared to HV ( Figure 1A and supplementary Figure 1E-G). However, only PGE 2 dampened TNFα release from LPS-stimulated human monocyte-derived macrophages when pre-treated with the mean concentrations observed in AD patients (0.1ng/ml) ( Figure 1B).Human monocyte-derived macrophages were incubated with culture media supplemented with 25% (vol./vol.) plasma from AD patients (see Table 1 for clinical characteristics).Compared to macrophages treated with media supplemented with HV plasma, AD plasma caused a significant decrease in LPS-stimulated TNFα that was reversed by pre-incubating Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts macrophages with the E-prostanoid (EP) 1-3/D-prostanoid (DP) 1 receptor antagonist, AH6809 ( Figure 1C). Additionally, macrophages were incubated with E. coli in the presence/absence of HV or AD plasma. Compared to macrophages treated with HV plasma, those with AD plasma exhibited reduced bacterial killing, an effect reversed by pretreatment with AH6809 (50μM) ( Figure 1D). AH6809 had no direct bactericidal effect while cell viability was unaffected b...

Aspirin is a unique nonsteroidal anti-inflammatory drug; at high doses (aspirinhigh, 1g), it is anti-inflammatory stemming from the inhibition of cyclooxygenase and proinflammatory signaling pathways including NF-κB, but is cardioprotective at lower doses (aspirinlow, 75 mg). The latter arises from the inhibition of thromboxane (Tx) B2, a prothrombotic eicosanoid also implicated in polymorphonuclear leukocyte trafficking. As a result, aspirinlow is widely used as a primary and secondary preventative against vascular disease. Despite this and its ability to synthesize proresolution 15-epi-lipoxin A4 it is not known whether aspirinlow is anti-inflammatory in humans. To address this, we generated skin blisters by topically applying cantharidin on the forearm of healthy male volunteers, causing an acute inflammatory response including dermal edema formation and leukocyte trafficking. Although not affecting blister fluid volume, aspirinlow (75 mg, oral, once daily/10 days) reduced polymorphonuclear leukocyte and macrophage accumulation independent of NF-κB-regulated gene expression and inhibition of conventional prostanoids. However, aspirinlow triggered 15-epi-lipoxin A4 synthesis and up-regulated its receptor (FPRL1, ALX). From complimentary in vitro experiments, we propose that 15-epi-lipoxin A4 exerts its protective effects by triggering antiadhesive NO, thereby dampening leukocyte/endothelial cell interaction and subsequent extravascular leukocyte migration. Since similar findings were obtained from murine zymosan-induced peritonitis, we suggest that aspirinlow possesses the ability to inhibit mammalian innate immune-mediated responses. This highlights 15-epi-lipoxin A4 as a novel anti-inflammatory working through a defined receptor and suggests that mimicking its mode of action represents a new approach to treating inflammation-driven diseases.

Macrophages are either classically (M1) or alternatively-activated (M2). Whereas this nomenclature was generated from monocyte-derived macrophages treated in vitro with defined cytokine stimuli, the phenotype of in vivo-derived macrophages is less understood. We com-

Key Points Resolving, but not hyperinflammatory stimuli create a microenvironment conducive for the optimal development of adaptive immunity. After onset and resolution, we introduce a third phase to acute inflammatory responses dominated by macrophages and lymphocytes.