Exudate macrophages attenuate lung injury by the release of IL-1 receptor antagonist in gram-negative pneumonia.

RATIONALE: Exudate macrophages are key players in host defense toward invading pathogens. Their antiinflammatory and epithelial-protective potential in gram-negative pneumonia, however, remains elusive. OBJECTIVES: We investigated whether exudate macrophages contributed to preservation of alveolar epithelial barrier integrity and analyzed the molecular pathways involved. METHODS: We evaluated the antiinflammatory and epithelial-protective effects of exudate macrophages in a model of LPS- and Klebsiella pneumoniae-induced lung injury comparing wild-type and CC-chemokine receptor 2 (CCR2)-deficient mice with defective lung macrophage recruitment and in in vitro studies using primary alveolar epithelial cells. MEASUREMENTS AND MAIN RESULTS: CCR2(-/-) mice exhibited enhanced alveolar epithelial cell apoptosis and lung leakage on intratracheal LPS treatment, which could be attributed to lack of exudate macrophage recruitment from the circulating pool as demonstrated in a model of wild-type/CCR2(-/-) bone-marrow chimeric mice. Among various antiinflammatory and proliferative mediators analyzed, the endogenous counterpart of resident macrophage-expressed IL-1ß, IL-1 receptor antagonist (IL-1ra), was highly up-regulated in flow-sorted exudate macrophages in LPS-treated wild-type mice. LPS/IL-1ß-induced impairment of alveolar epithelial cell integrity was antagonized by IL-1ra in vitro. Finally, intratracheal substitution of IL-1ra or intravenous adoptive transfer of IL-1ra(+/+) but not IL-1ra(-/-) blood mononuclear cells attenuated alveolar inflammation, epithelial apoptosis, and loss of barrier function in LPS-challenged or K. pneumoniae-infected CCR2(-/-) mice and enhanced survival after K. pneumoniae infection. CONCLUSIONS: We conclude that recruited lung macrophages attenuate IL-1ß-mediated acute lung injury in gram-negative pneumonia by release of IL-1ra.

Lung epithelial apoptosis in influenza virus pneumonia: the role of macrophage-expressed TNF-related apoptosis-inducing ligand.

Mononuclear phagocytes have been attributed a crucial role in the host defense toward influenza virus (IV), but their contribution to influenza-induced lung failure is incompletely understood. We demonstrate for the first time that lung-recruited "exudate" macrophages significantly contribute to alveolar epithelial cell (AEC) apoptosis by the release of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in a murine model of influenza-induced pneumonia. Using CC-chemokine receptor 2-deficient (CCR2(-/-)) mice characterized by defective inflammatory macrophage recruitment, and blocking anti-CCR2 antibodies, we show that exudate macrophage accumulation in the lungs of influenza-infected mice is associated with pronounced AEC apoptosis and increased lung leakage and mortality. Among several proapoptotic mediators analyzed, TRAIL messenger RNA was found to be markedly up-regulated in alveolar exudate macrophages as compared with peripheral blood monocytes. Moreover, among the different alveolar-recruited leukocyte subsets, TRAIL protein was predominantly expressed on macrophages. Finally, abrogation of TRAIL signaling in exudate macrophages resulted in significantly reduced AEC apoptosis, attenuated lung leakage, and increased survival upon IV infection. Collectively, these findings demonstrate a key role for exudate macrophages in the induction of alveolar leakage and mortality in IV pneumonia. Epithelial cell apoptosis induced by TRAIL-expressing macrophages is identified as a major underlying mechanism.

Lung dendritic cells elicited by Fms-like tyrosine 3-kinase ligand amplify the lung inflammatory response to lipopolysaccharide.

RATIONALE: Strategically located beneath the alveolar epithelial barrier, dendritic cells (DCs) of the lung are centrally involved in the sampling and processing of inhaled antigens. However, the contribution of DCs to acute lung inflammation induced by inhaled bacterial toxins is largely unknown. OBJECTIVES: To determine the effect of increased lung DC numbers elicited by Fms-like tyrosine kinase-3 ligand (Flt3L) on the acute lung inflammatory response to Escherichia coli lipopolysaccharide (LPS) and Klebsiella pneumoniae infection. METHODS: Mice were pretreated with Flt3L either in the absence or presence of anti-CD11a antibodies to block the Flt3L-elicited lung DC accumulation or were made transiently neutropenic and then challenged with E. coli LPS or K. pneumoniae. MEASUREMENTS AND MAIN RESULTS: Flt3L-pretreated mice challenged with LPS responded with drastically increased numbers of both lung parenchymal and alveolar DCs together with an aggravated neutrophilic alveolitis, elevated tumor necrosis factor-alpha and IL-12 levels, and a strongly increased lung permeability compared with LPS- or Flt3L-only-treated mice. Anti-CD11a-mediated blockade of lung DC accumulation significantly attenuated the lung permeability developing in response to LPS, whereas transient neutropenia did not affect lung permeability changes. Finally, Flt3L-pretreated mice responded with increased lung permeability and decreased survival upon infection with K. pneumoniae. CONCLUSIONS: Lung DCs actively participate in the early inflammatory response to both inhaled bacterial toxins and live bacteria and play a yet unrecognized role in regulating lung barrier integrity.

Alveolar epithelial cells direct monocyte transepithelial migration upon influenza virus infection: impact of chemokines and adhesion molecules.

Influenza A virus pneumonia is characterized by severe lung injury and high mortality. Early infection elicits a strong recruitment of monocytes from the peripheral blood across the endo-/epithelial barrier into the alveolar air space. However, it is currently unclear which of the infected resident lung cell populations, alveolar epithelial cells or alveolar macrophages, elicit monocyte recruitment during influenza A virus infection. In the current study, we investigated whether influenza A virus infection of primary alveolar epithelial cells and resident alveolar macrophages would elicit a basal-to-apical monocyte transepithelial migration in vitro. We found that infection of alveolar epithelial cells with the mouse-adapted influenza A virus strain PR/8 strongly induced the release of monocyte chemoattractants CCL2 and CCL5 followed by a strong monocyte transepithelial migration, and this monocytic response was strictly dependent on monocyte CCR2 but not CCR5 chemokine receptor expression. Analysis of the adhesion molecule pathways demonstrated a role of ICAM-1, VCAM-1, integrin-associated protein (CD47), and junctional adhesion molecule-c on the epithelial cell surface interacting with monocyte beta(1) and beta(2) integrins and integrin-associated protein in the monocyte transmigration process. Importantly, addition of influenza A virus-infected alveolar macrophages further enhanced monocyte transmigration across virus-infected epithelium in a TNF-alpha-dependent manner. Collectively, the data show an active role for virus-infected alveolar epithelium in the regulation of CCL2/CCR2-dependent monocyte transepithelial migration during influenza infection that is essentially dependent on both classical beta(1) and beta(2) integrins but also junctional adhesion molecule pathways.