Macrophages have important functional roles in regulating the timely promotion and resolution of inflammation. While many of the intracellular signaling pathways involved in the pro-inflammatory responses of macrophages are well characterized, the components that regulate macrophage reparative properties are less well understood. We identified the MEK1/2 pathway as a key regulator of macrophage reparative properties. Pharmacological inhibition of the MEK1/2 pathway (MEKi) significantly increased expression of IL-4/IL-13 (M2) responsive genes in murine bone marrow-derived and alveolar macrophages. Deletion of the MEK1 gene using LysMCre+/+MEK1fl/fl macrophages as an alternate approach yielded similar results. MEKi enhanced STAT6 phosphorylation, and MEKi induced changes in M2 polarization were dependent on STAT6. In addition, MEKi-treatment significantly increased both murine and human macrophage efferocytosis of apoptotic cells (AC) independent of macrophage polarization and STAT6. These phenotypes were associated with increased gene and protein expression of Mertk, Tyro3, and Abca1, three proteins that promote macrophage efferocytosis. We also studied the effects of MEKi on in vivo macrophage efferocytosis and polarization. MEKi treated mice had increased efferocytosis of apoptotic PMNs instilled into the peritoneum. Furthermore, administration of MEKi after LPS-induced lung injury led to improved recovery of weight, fewer neutrophils in the alveolar compartment, and greater macrophage M2 polarization. Collectively, these results show that MEK1/2 inhibition is capable of promoting reparative properties of both murine and human macrophages. These studies suggest that the MEK1/2 pathway may be a therapeutic target to promote the resolution of inflammation via modulation of macrophage functions.

BackgroundThere are pulmonary consequences to obesity, including increased prevalence of asthma, greater susceptibility to influenza, and possibly reduced susceptibility to lung injury. Although it is well established that obesity is associated with alterations to the immune system, little is known about obesity-associated changes to pulmonary immune cells.ObjectivesWe hypothesized that obesity would alter the inflammatory milieu in the unchallenged lung and circulation; thereby contributing to altered susceptibility to lung injury.MethodsWe used a murine model of diet-induced obesity and evaluated bone marrow and blood leukocytes at 3 months, and pulmonary leukocytes at 3 and 6 months for changes in their adhesion and chemokine receptors, markers of activation states, and cell numbers. We also evaluated the inflammatory response to LPS in obese mice.ResultsIn the lung, diet-induced obesity was associated with increased leukocyte numbers over-time. Adhesion receptors were increased in a cell- and site-specific fashion, and there was an evolution of macrophage and neutrophil polarization toward M1 and N1, respectively. After LPS-challenge, obesity was associated with increased neutrophil recruitment to the lung with impaired migration into the alveolar space. Associated with these changes, obesity increased LFA-1 and ICAM-1 neutrophil expression and altered CXCL1 gradients.ConclusionOur results highlight the effects of diet-induced obesity on the murine blood and lung leukocyte populations, including increases in adhesion receptor expression that may contribute to altered recruitment or retention within the lung. Translation of these findings to people with obesity will be critical for determining the basic inflammatory underpinnings of pulmonary disease susceptibility.Electronic supplementary materialThe online version of this article (doi:10.1186/s12931-016-0341-8) contains supplementary material, which is available to authorized users.

Chronic obstructive pulmonary disease (COPD) comprises chronic bronchitis and emphysema, and is a leading cause of morbidity and mortality. Because tissue destruction is the prominent characteristic of emphysema, extracellular proteinases, particularly those with elastolytic ability, are often considered to be key drivers in this disease. Several human and mouse studies have implicated roles for matrix metalloproteinases (MMPs), particularly macrophage-derived proteinases, in COPD pathogenesis. MMP-28 is expressed by the pulmonary epithelium and macrophage, and we have found that it regulates macrophage recruitment and polarization. We hypothesized that MMP-28 has contributory roles in emphysema via alteration of macrophage numbers and activation. Because of the established association of emphysema pathogenesis to macrophage influx, we evaluated the inflammatory changes and lung histology of Mmp28 mice exposed to 3 and 6 months of cigarette smoke. At earlier time points, we found altered macrophage polarization in the smoke-exposed Mmp28 lung consistent with other published findings that MMP-28 regulates macrophage activation. At both 3 and 6 months, Mmp28 mice had blunted inflammatory responses more closely resembling nonsmoked mice, with a reduction in neutrophil recruitment and CXCL1 chemokine expression. By 6 months, Mmp28 mice were protected from emphysema. These results highlight a previously unrecognized role for MMP-28 in promoting chronic lung inflammation and tissue remodeling induced by cigarette smoke and highlight another potential target to modulate COPD.

We tested the role of Stat5 in dendritic cell (DC) and alveolar macrophage homeostasis in the lung using CD11c-cre mediated deletion (Cre+5f/f). We show that Stat5 is required for CD103+ dendritic cell (DC) and alveolar macrophage development. We found that fetal monocyte maturation into alveolar macrophages was impaired in Cre+5f/f mice, and we also confirmed impaired alveolar macrophage development of progenitor cells using mixed chimera experiments. In the absence of Stat5 signaling in alveolar macrophages, mice developed alveolar proteinosis with altered lipid homeostasis. In addition, loss of Stat5 in CD11c+ cells was associated with exaggerated LPS-induced inflammatory responses and vascular leak. In Cre+5f/f mice, there was loss of immune-dampening effects on epithelial cells, a key source of CCL2 that serves to recruit monocytes/macrophages. These findings demonstrate the critical importance of Stat5 signaling in maintaining lung homeostasis and underscore the importance of resident macrophages in moderating tissue damage and excess inflammation.

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This study was designed to test the therapeutic potential of a MEK1/2 inhibitor (MEKi) in an experimental model of Pseudomonas aeruginosa pneumonia. The study found that treatment with MEKi reduced alveolar neutrophilic inflammation and led to faster recovery of weight compared to carrier-treated mice, without impairing bacterial clearance. Alveolar macrophages isolated from MEKi-treated mice also had increased M2 gene and protein expression, supporting the concept that MEKi modulates in vivo macrophage inflammatory responses. In summary, this report demonstrates the potential of MEKi to promote the resolution of inflammation in vivo during a primary lung infection without impairing bacterial clearance.

Matrix metalloproteinases (MMPs) are transcriptionally regulated proteases that have multiple roles in modifying the extracellular matrix (ECM) and inflammatory response. Our previous work identified Mmp28 as a key regulator of inflammation and macrophage polarization during experimental models of pulmonary infection, fibrosis, and chronic smoke exposure. However, the signaling pathways responsible for regulation of macrophage Mmp28 expression remain undefined. This study utilized murine macrophages obtained from wild type, Tlr2 À/À , Tlr4(Trif mutant ), and Ifnar1 À/À mice to test the hypothesis that macrophage Mmp28 expression was dependent on TRIF and type I IFN. Our results support the hypothesis, demonstrating that increased macrophage Mmp28 expression was dependent on type I IFN after LPS and poly(I:C) stimulation. To gain further insight into the function of MMP28, we explored the inflammatory response of macrophages derived from wild type or Mmp28 À/À mice to stimulation with poly(I:C). Our data support a role for MMP28 in regulating the macrophage inflammatory response to poly(I:C) because expression of Ccl2, Ccl4, Cxcl10, and Il6 were increased in Mmp28 À/À macrophages. Together, these data support a model in which macrophages integrate TRIF-and type I IFN-dependent signaling to coordinate regulation of proteins with the capacity to modify the ECM.

A multiscale model was proposed to calculate the circumferential stress (CS) and wall shear stress (WSS) and analyze the effects of global and local factors on the CS, WSS and their synergy on the arterial endothelium in large straight arteries. A parameter pair [Zs, SP A] (defined as the ratio of CS amplitude to WSS amplitude and the phase angle between CS and WSS for different harmonic components, respectively) was proposed to characterize the synergy of CS and WSS. The results demonstrated that the CS or WSS in the large straight arteries is determined by the global factors, i.e. the preloads and the afterloads, and the local factors, i.e. the local mechanical properties and the zero-stress states of arterial walls, whereas the Zs and SP A are primarily determined by the local factors and the afterloads. Because the arterial input impedance has been shown to reflect the physiological and pathological states of whole downstream arterial beds, the stress amplitude ratio Zs and the stress phase difference SP A might be appropriate indices to reflect the influences of the states of whole downstream arterial beds on the local blood flow-dependent phenomena such as angiogenesis, vascular remodeling and atherosgenesis.