TLR3 absence confers increased survival with improved macrophage activity against pneumonia.

Toll-like receptor 3 (TLR3) is a pathogen recognition molecule associated with viral infection with double-stranded RNA (dsRNA) as its ligand. We evaluated the role of TLR3 in bacterial pneumonia using Klebsiella pneumoniae (KP). WT and TLR3-/- mice were subjected to a lethal model of KP. Alveolar macrophage polarization, bactericidal activity, and phagocytic capacity were compared. RNA-sequencing was performed on alveolar macrophages from the WT and TLR3-/- mice. Adoptive transfers of alveolar macrophages from TLR3-/- mice to WT mice with KP were evaluated for survival. Expression of TLR3 in postmortem human lung samples from patients who died from gram-negative pneumonia and pathological grading of pneumonitis was determined. Mortality was significantly lower in TLR3-/-, and survival improved in WT mice following antibody neutralization of TLR3 and with TLR3/dsRNA complex inhibitor. Alveolar macrophages from TLR3-/- mice demonstrated increased bactericidal and phagocytic capacity. RNA-sequencing showed an increased production of chemokines in TLR3-/- mice. Adoptive transfer of alveolar macrophages from the TLR3-/- mice restored the survival in WT mice. Human lung samples demonstrated a good correlation between the grade of pneumonitis and TLR3 expression. These data represent a paradigm shift in understanding the mechanistic role of TLR3 in bacterial pneumonia.

Hypoxia-Inducible Factor (HIF)-1α Promotes Inflammation and Injury Following Aspiration-Induced Lung Injury in Mice.

Acid aspiration-induced lung injury is a common disease in the intensive care unit (ICU) and acute respiratory distress syndrome (ARDS). Hypoxia-inducible factor (HIF)-1α is a major transcription factor responsible for regulating the cellular response to changes in oxygen tension. A clear understanding of the function of HIF-1α in lung inflammatory response is currently lacking. Here, we sought to determine the role of HIF-1α in type 2 alveolar epithelial cells (AEC) in the generation of the acute inflammatory response following gastric aspiration (GA). GA led to profound hypoxia at very early time points following GA. This correlated to a robust increase in HIF-1α, tissue albumin and pro-inflammatory mediators following GA in AECs. The extent of lung injury and the release of pro/anti-inflammatory cytokines were significantly reduced in HIF-1α (-/-) mice. Finally, we report that HIF-1α upregulation of the acute inflammatory response is dependent on NF-κB following GA.

Double-Stranded RNA Interacts With Toll-Like Receptor 3 in Driving the Acute Inflammatory Response Following Lung Contusion.

OBJECTIVES: Lung contusion is a major risk factor for the development of acute respiratory distress syndrome. We set to determine the role of toll-like receptor 3 and the binding of double-stranded RNA in the pathogenesis of sterile injury following lung contusion. DESIGN: Toll-like receptor 3 expression was analyzed in postmortem lung samples from patients with lung contusion. Unilateral lung contusion was induced in toll-like receptor 3 (-/-), TIR-domain-containing adapter-inducing interferon-ß (-/-), and wild-type mice. Subsequently, lung injury and inflammation were evaluated. Apoptotic indices, phagocytic activity, and phenotypic characterization of the macrophages were determined. Double-stranded RNA in bronchoalveolar lavage and serum samples following lung contusion was measured. A toll-like receptor 3/double-stranded RNA ligand inhibitor was injected into wild-type mice prior to lung contusion. MEASUREMENTS AND MAIN RESULTS: Toll-like receptor 3 expression was higher in patients and wild-type mice with lung contusion. The degree of lung injury, inflammation, and macrophage apoptosis was reduced in toll-like receptor 3 (-/-), TIR-domain-containing adapter-inducing interferon-ß (-/-), and wild-type mice with toll-like receptor 3 antibody neutralization. Alveolar macrophages from toll-like receptor 3 (-/-) mice had a lower early apoptotic index, a predominant M2 phenotype and increased surface translocation of toll-like receptor 3 from the endosome to the surface. When compared with viral activation pathways, lung injury in lung contusion demonstrated increased p38 mitogen-activated protein kinases, extracellular signal-regulated kinase 1/2 phosphorylation with inflammasome activation without a corresponding increase in nuclear factor-κB or type-1 interferon production. Additionally, pretreatment with toll-like receptor 3/double-stranded RNA ligand inhibitor led to a reduction in injury, inflammation, and macrophage apoptosis. CONCLUSIONS: We conclude that the interaction of double-stranded RNA from injured cells with toll-like receptor 3 drives the acute inflammatory response following lung contusion.

Toll-Like Receptor-9 (TLR9) is Requisite for Acute Inflammatory Response and Injury Following Lung Contusion.

Lung contusion (LC) is a significant risk factor for the development of acute respiratory distress syndrome. Toll-like receptor 9 (TLR9) recognizes specific unmethylated CpG motifs, which are prevalent in microbial but not vertebrate genomic DNA, leading to innate and acquired immune responses. TLR9 signaling has recently been implicated as a critical component of the inflammatory response following lung injury. The aim of the present study was to evaluate the contribution of TLR9 signaling to the acute physiologic changes following LC. Nonlethal unilateral closed-chest LC was induced in TLR9 (-/-) and wild-type (WT) mice. The mice were sacrificed at 5, 24, 48, and 72-h time points. The extent of injury was assessed by measuring bronchoalveolar lavage, cells (cytospin), albumin (permeability injury), and cytokines (inflammation). Following LC, only the TLR9 (-/-) mice showed significant reductions in the levels of albumin; release of pro-inflammatory cytokines IL-1ß, IL-6, and Keratinocyte chemoattractant; production of macrophage chemoattractant protein 5; and recruitment of alveolar macrophages and neutrophil infiltration. Histological evaluation demonstrated significantly worse injury at all-time points for WT mice. Macrophages, isolated from TLR9 (-/-) mice, exhibited increased phagocytic activity at 24 h after LC compared with those isolated from WT mice. TLR9, therefore, appears to be functionally important in the development of progressive lung injury and inflammation following LC. Our findings provide a new framework for understanding the pathogenesis of lung injury and suggest blockade of TLR9 as a new therapeutic strategy for the treatment of LC-induced lung injury.

Activation of hypoxia-inducible factor-1α in type 2 alveolar epithelial cell is a major driver of acute inflammation following lung contusion.

OBJECTIVE: Lung contusion is a major risk factor for the development of acute respiratory distress syndrome. Hypoxia-inducible factor-1α is the primary transcription factor that is responsible for regulating the cellular response to changes in oxygen tension. We set to determine if hypoxia-inducible factor-1α plays a role in the pathogenesis of acute inflammatory response and injury in lung contusion. DESIGN: Nonlethal closed-chest unilateral lung contusion was induced in a hypoxia reporter mouse model and type 2 cell-specific hypoxia-inducible factor-1α conditional knockout mice. The mice were killed at 5-, 24-, 48-, and 72-hour time points, and the extent of systemic and tissue hypoxia was assessed. In addition, injury and inflammation were assessed by measuring bronchoalveolar lavage cells (flow cytometry and cytospin), albumin (permeability injury), and cytokines (inflammation). Isolated type 2 cells from the hypoxia-inducible factor-1α conditional knockout mice were isolated and evaluated for proinflammatory cytokines following lung contusion. Finally, the role of nuclear factor-κB and interleukin-1ß as intermediates in this interaction was studied. RESULTS: Lung contusion induced profound global hypoxia rapidly. Increased expression of hypoxia-inducible factor-1α from lung samples was observed as early as 60 minutes, following the insult. The extent of lung injury following lung contusion was significantly reduced in conditional knockout mice at all the time points, when compared with the wild-type littermate mice. Release of proinflammatory cytokines, such as interleukin-1ß, interleukin-6, macrophage inflammatory protein-2, and keratinocyte chemoattractant, was significantly lower in conditional knockout mice. These actions are in part mediated through nuclear factor-κB. Hypoxia-inducible factor-1α in lung epithelial cells was shown to regulate interleukin-1ß promoter activity. CONCLUSION: Activation of hypoxia-inducible factor-1α in type 2 cell is a major driver of acute inflammation following lung contusion.