Toll-like receptor 4 deficiency causes pulmonary emphysema

Abstract

TLRs have been studied extensively in the context of pathogen challenges, yet their role in the unchallenged lung is unknown. Given their direct interface with the external environment, TLRs in the lungs are prime candidates to respond to air constituents, namely particulates and oxygen. The mechanism whereby the lung maintains structural integrity in the face of constant ambient exposures is essential to our understanding of lung disease. Emphysema is characterized by gradual loss of lung elasticity and irreversible airspace enlargement, usually in the later decades of life and after years of insult, most commonly cigarette smoke. Here we show Tlr4 -/-mice exhibited emphysema as they aged. Adoptive transfer experiments revealed that TLR4 expression in lung structural cells was required for maintaining normal lung architecture. TLR4 deficiency led to the upregulation of what we believe to be a novel NADPH oxidase (Nox), Nox3, in lungs and endothelial cells, resulting in increased oxidant generation and elastolytic activity. Treatment of Tlr4 -/-mice or endothelial cells with chemical NADPH inhibitors or Nox3 siRNA reversed the observed phenotype. Our data identify a role for TLR4 in maintaining constitutive lung integrity by modulating oxidant generation and provide insights into the development of emphysema. IntroductionTLRs have been intensely studied in the context of microbial challenges, inflammation, and immune cells, but their critical role in non-infectious challenges has newly emerged. We have recently shown that mammalian TLR4 is required for survival during lethal oxidant stress resulting from hyperoxia (1, 2) or bleomycin-induced injury (3). However, even in the absence of injury, the lungs are required to process and adapt to the constant exposure to the inhaled environment. The lungs are exposed continuously to oxidants generated either endogenously from phagocytes and other cell types or exogenously from inhaled oxygen as well as pollutants. In addition, intracellular oxidants, such as those derived from the NADPH oxidase (Nox) system, are involved in many cellular signaling pathways. Under normal circumstances lungs can withstand the oxidant challenges imposed by the ambient environment via the presence of well-developed enzymatic and nonenzymatic antioxidant systems (4). However, when the balance shifts in favor of oxidants, from either an excess of oxidants and/or depletion of its antioxidant responses, oxidative stress occurs. We postulated that TLR4 mediates important antioxidant responses in the lung and that TLR4 deficiency would therefore lead to altered responses to oxidants in the ambient environment. We performed histopathologic and morphometric analyses of lungs isolated from WT and Tlr4 -/-mice from 1 month to 12 months of age. To our surprise, the lungs of Tlr4 -/-mice exhibited age-related changes that resembled pulmonary emphysema in humans both histologically and functionally.

Imbalance of Clara cell-mediated homeostatic inflammation is involved in lung metastasis

“…Although gene knockout of S100A8 resulted in embryonic lethality (Passey et al, 1999) and information on that of SAA3 is currently not available, TLR4-deficient mice displayed an organ-specific disorder, pulmonary emphysema (Zhang et al, 2006). Inhibition of S100A8 abrogated myeloid cell infiltration into pneumonic lungs that requires transendothelial and transepithelial cell migration to reach the pathogens (Kerkhoff et al, 1999;Ryckman et al, 2003;Vogl et al, 2004;Raquil et al, 2008).…”

“…TLR4 activates nuclear factor-kB on which concomitant activation of activating transcription factor 3 (ATF3) has been proposed to give a negative feedback (Gilchrist et al, 2006). TLR4 knockout mice display pulmonary emphysema at high age (Zhang et al, 2006). ATF3-deficient mice suffer from airway inflammation (Gilchrist et al, 2008).…”

Nox enzymes, ROS, and chronic disease: An example of antagonistic pleiotropy

“…TLR4 deficiency, which causes emphysema in mice, up-regulated Nox3 in lung and endothelial cells resulting in increased oxidant generation and elastolytic activity. Treatment of Tlr4 (−/− ) mice or endothelial cells with chemical Nox inhibitors or Nox3 siRNA prevented the disease development [32].…”

NADPH oxidases in lung biology and pathology: Host defense enzymes, and more

“…Exposure of mice to cigarette smoke and/or LPS was found to result in increased lung expression of NOXO1, an activator of NOX1 (214), but the potential significance for cigarette smoke-induced emphysema or lung cancer is unclear. Intriguingly, development of pulmonary emphysema in mice due to genetic deficiency of Toll-like receptor (TLR)-4, was recently associated with upregulation of NOX3 in the pulmonary endothelium, which appears to contribute to elastolytic activity (215).…”