Bone marrow-derived stromal cells (BMSCs) protect against acute lung injury (ALI). To determine the role of BMSC mitochondria in the protection, we airway-instilled mice first with lipopolysaccharide (LPS), then with mouse BMSCs (mBMSCs). Live optical studies revealed that mBMSCs formed connexin 43 (Cx43)-containing gap junctional channels (GJCs) with the alveolar epithelium, releasing mitochondria-containing microvesicles that the epithelium engulfed. The presence of BMSC mitochondria in the epithelium was evident optically, as also by the presence of human mitochondrial DNA in mouse lungs in which we instilled human BMSCs (hBMSCs). The mitochondrial transfer increased alveolar ATP. LPS-induced ALI, indicated by alveolar leukocytosis and protein leak, inhibition of surfactant secretion and high mortality, was markedly abrogated by wild type mBMSCs, but not by mutant, GJC-incompetent mBMSCs, or by mBMSCs with dysfunctional mitochondria. This is the first evidence that BMSCs protect against ALI by restituting alveolar bioenergetics through Cx43-dependent alveolar attachment and mitochondrial transfer.

Tissue-resident macrophages of barrier organs constitute the first line of defense against pathogens at the systemic interface with the ambient environment. In lung, resident alveolar macrophages (AMs) provide sentinel function against inhaled pathogens1. Bacterial constituents ligate toll-like receptors (TLRs) on AMs2, causing AMs to secrete proinflammatory cytokines3 that activate alveolar epithelial receptors4, leading to recruitment of neutrophils that engulf pathogens5,6. However, since the AM-induced immune response could itself cause tissue injury, it is unclear how AMs modulate the response to prevent injury. Here, through real-time alveolar imaging in situ, we show that a subset of AMs attached to the alveolar wall, formed connexin 43 (Cx43)-containing gap junctional channels (GJCs) with the epithelium. During lipopolysaccharide (LPS)-induced inflammation, the AMs remained alveolus-attached and sessile, and they established intercommunication through synchronized Ca2+ waves, using the epithelium as the conducting pathway. The intercommunication was immunosuppressive, involving Ca2+ dependent activation of Akt, since AM-specific knockout of Cx43 enhanced alveolar neutrophil recruitment and secretion of proinflammatory cytokines in the bronchoalveolar lavage (BAL). The picture emerges of a novel immunomodulatory process in which a subset of alveolus-attached AMs intercommunicates immunosuppressive signals to reduce endotoxin-induced lung inflammation.

Alveolar macrophages have been investigated for years by approaches involving macrophage extraction from the lung by bronchoalveolar lavage, or by cell removal from lung tissue. Since extracted macrophages are studied outside their natural milieu, there is little understanding of the extent to which alveolar macrophages interact with the epithelium, or with one another to generate the lung’s innate immune response to pathogen challenge. Here, we review new evidence of macrophage-epithelial interactions in the lung and we address the emerging understanding that the alveolar epithelium plays an important role in orchestrating the macrophage driven immune response.

Westphalen K, Monma E, Islam MN, Bhattacharya J. Acid contact in the rodent pulmonary alveolus causes proinflammatory signaling by membrane pore formation. Am J Physiol Lung Cell Mol Physiol 303: L107-L116, 2012. First published May 4, 2012 doi:10.1152/ajplung.00206.2011Although gastric acid aspiration causes rapid lung inflammation and acute lung injury, the initiating mechanisms are not known. To determine alveolar epithelial responses to acid, we viewed live alveoli of the isolated lung by fluorescence microscopy, then we microinjected the alveoli with HCl at pH of 1.5. The microinjection caused an immediate but transient formation of molecule-scale pores in the apical alveolar membrane, resulting in loss of cytosolic dye. However, the membrane rapidly resealed. There was no cell damage and no further dye loss despite continuous HCl injection. Concomitantly, reactive oxygen species (ROS) increased in the adjacent perialveolar microvascular endothelium in a Ca 2ϩ -dependent manner. By contrast, ROS did not increase in wild-type mice in which we gave intraalveolar injections of polyethylene glycol (PEG)-catalase, in mice overexpressing alveolar catalase, or in mice lacking functional NADPH oxidase (Nox2). Together, our findings indicate the presence of an unusual proinflammatory mechanism in which alveolar contact with acid caused membrane pore formation. The effect, although transient, was nevertheless sufficient to induce Ca 2ϩ entry and Nox2-dependent H2O2 release from the alveolar epithelium. These responses identify alveolar H2O2 release as the signaling mechanism responsible for lung inflammation induced by acid and suggest that intra-alveolar PEG-catalase might be therapeutic in acid-induced lung injury.calcium; hydrogen peroxide; nitric oxide synthase 2; nitric oxide; reactive oxygen species; catalase; flash photolysis; resealing; clodronate THE GASTRIC AND THE PULMONARY epithelial surfaces are potentially exposed to highly concentrated HCl. The gastric epithelium secretes the acid at pH 1-2; the pulmonary epithelium encounters the acid following aspiration of gastric contents. Although a 200-m-thick mucus layer protects the gastric epithelium from acid injury (4), the much thinner layer of surfactant (6) is likely to provide relatively little protection to alveoli. Large aspirations cause severe lung inflammation, leading to acute lung injury (ALI) that associates with high mortality and morbidity (21). Mild aspirations prime the lung for secondary pneumonitis (30,34). No specific treatment is available for acid-induced ALI.An unsolved problem in the understanding of acid-induced ALI relates to the question of how the injury initiates following the first contact of acid with the alveolar membrane. Particularly lacking is the absence of studies defining the immediate alveolar response to the acid contact. Although animal studies confirm that airway instillation of highly concentrated acid (pH Ͻ2) causes ALI, these studies have largely focused on lung responses occurring well after the injury has alread...