Accumulation mode particles and LPS exposure induce TLR-4 dependent and independent inflammatory responses in the lung.

BACKGROUND: Accumulation mode particles (AMP) are formed from engine combustion and make up the inhalable vapour cloud of ambient particulate matter pollution. Their small size facilitates dispersal and subsequent exposure far from their original source, as well as the ability to penetrate alveolar spaces and capillary walls of the lung when inhaled. A significant immuno-stimulatory component of AMP is lipopolysaccharide (LPS), a product of Gram negative bacteria breakdown. As LPS is implicated in the onset and exacerbation of asthma, the presence or absence of LPS in ambient particulate matter (PM) may explain the onset of asthmatic exacerbations to PM exposure. This study aimed to delineate the effects of LPS and AMP on airway inflammation, and potential contribution to airways disease by measuring airway inflammatory responses induced via activation of the LPS cellular receptor, Toll-like receptor 4 (TLR-4). METHODS: The effects of nebulized AMP, LPS and AMP administered with LPS on lung function, cellular inflammatory infiltrate and cytokine responses were compared between wildtype mice and mice not expressing TLR-4. RESULTS: The presence of LPS administered with AMP appeared to drive elevated airway resistance and sensitivity via TLR-4. Augmented TLR4 driven eosinophilia and greater TNF-α responses observed in AMP-LPS treated mice independent of TLR-4 expression, suggests activation of allergic responses by TLR4 and non-TLR4 pathways larger than those induced by LPS administered alone. Treatment with AMP induced macrophage recruitment independent of TLR-4 expression. CONCLUSIONS: These findings suggest AMP-LPS as a stronger stimulus for allergic inflammation in the airways then LPS alone.

The bimodal quasi-static and dynamic elastance of the murine lung.

The double sigmoidal nature of the mouse pressure-volume (PV) curve is well recognized but largely ignored. This study systematically examined the effect of inflating the mouse lung to 40 cm H2O transrespiratory pressure (Prs) in vivo. Adult BALB/c mice were anesthetized, tracheostomized, and mechanically ventilated. Thoracic gas volume was calculated using plethysmography and electrical stimulation of the intercostal muscles. Lung mechanics were tracked during inflation-deflation maneuvers using a modification of the forced oscillation technique. Inflation beyond 20 cm H2O caused a shift in subsequent PV curves with an increase in slope of the inflation limb and an increase in lung volume at 20 cm H2O. There was an overall decrease in tissue elastance and a fundamental change in its volume dependence. This apparent "softening" of the lung could be recovered by partial degassing of the lung or applying a negative transrespiratory pressure such that lung volume decreased below functional residual capacity. Allowing the lung to spontaneously recover revealed that the lung required approximately 1 h of mechanical ventilation to return to the original state. We propose a number of possible mechanisms for these observations and suggest that they are most likely explained by the unfolding of alveolar septa and the subsequent redistribution of the fluid lining the alveoli at high transrespiratory pressure.

Absence of cholinergic airway tone in normal BALB/c mice.

Basal airway smooth muscle (ASM) tone has not been demonstrated in mice in vivo. To determine whether basal ASM tone is present in mouse airways we measured respiratory system impedance (Zrs) before and after either atropine or bilateral vagotomy. Zrs was measured using forced oscillations delivered via a wave-tube during slow ( approximately 35s) inflation-deflation maneuvers between transrespiratory pressures (Prs) of 0 and 20 cm H2O. A constant-phase tissue model was applied to the Zrs to calculate airway resistance (R aw), tissue damping (G) and elastance (H). Thoracic gas volume (TGV) was determined plethysmographically at Prs=0 cm H2O and by integration of the inspiratory flow. The relationship between conductance (G aw=1/R aw) and TGV during inflation was also examined. Neither atropine nor vagotomy produced any change in R aw, H, eta (=G/H), TGV or the slope of G aw vs. TGV that was different to that observed in the relevant control groups. These data show that BALB/c mice do not have cholinergic ASM tone in vivo.

Methacholine responsiveness in mice from 2 to 8 wk of age.

Many chronic human lung diseases have their origin in early childhood, yet most murine models used to study them utilize adult mice. An important component of the asthma phenotype is exaggerated airway responses, frequently modelled by methacholine (MCh) challenge. The present study was undertaken to characterize MCh responses in mice from 2 to 8 wk of age measuring absolute lung volume and volume-corrected respiratory mechanics as outcome variables. Female BALB/c mice aged 2, 3, 4, 6, and 8 wk were studied during cumulative intravenous MCh challenge. Following each MCh dose, absolute lung volume was measured plethysmographically at functional residual volume and during a slow inflation to 20-hPa transrespiratory pressure. Respiratory system impedance was measured continuously during the inflation maneuver and partitioned into airway and constant-phase parenchymal components by model fitting. Volume-corrected (specific) estimates of respiratory mechanics were calculated. Intravenous MCh challenge induced a predominantly airway response with no evidence of airway closure in any age group. No changes in functional residual volume were seen in mice of any age during the MCh challenge. The specific airway resistance MCh dose response curves did not show significant differences between the age groups. The results from the present study do not show systematic differences in MCh responsiveness in mice from 2 to 8 wk of age.

Developmental changes in airway and tissue mechanics in mice.

Most studies using mice to model human lung diseases are carried out in adults, although there is emerging interest in the effects of allergen, bacterial, and viral exposure early in life. This study aims to characterize lung function in BALB/c mice from infancy (2 wk) through to adulthood (8 wk). The low-frequency forced oscillation technique was used to obtain impedance data, partitioned into components representing airway resistance, tissue damping, tissue elastance, and hysteresivity (tissue damping/tissue elastance). Measurements were made at end-expiratory pause (transrespiratory system pressure = 2 cmH2O) and during relaxed slow expiration from 20 to 0 cmH2O. Airway resistance decreased with age from 0.63 cmH2O x ml(-1) x s at 2 wk to 0.24 cmH2O x ml(-1) x s at 8 wk (P < 0.001). Both tissue damping and tissue elastance decreased with age (P < 0.001) from 2 to 5 wk, then plateaued through to 8 wk (P < 0.001). This pattern was seen both in measurements taken at end-expiratory pause and during expiration. There were no age-related changes seen in hysteresivity when measured at end-expiratory pause, but the pattern of volume dependence did differ with the age of the mice. These changes in respiratory mechanics parallel the reported structural changes of the murine lung from the postnatal period into adulthood.

Factors influencing the assessment of lung function in mice with influenza-induced lung disease.

BACKGROUND: The constant-phase model (CPM) is commonly fit to respiratory system input impedance (Zrs) to estimate lung mechanics. Driving signal frequencies and the method of model fitting may influence the results, especially in cases of severe lung disease or under severe bronchoconstriction. OBJECTIVE: To illustrate the effects of different CPM fits to Zrs data using a mouse model of influenza-induced lung disease. METHODS: BALB/c mice infected with influenza (or control) were challenged with methacholine. The CPM was fitted to Zrs, measured between 0·25 and 19·625 Hz, using both unweighted and weighted fits. The effect of different lowest frequencies was assessed. RESULTS AND CONCLUSIONS: For influenza-infected mice, the unweighted fit was poor, and airway resistance (Raw) was often biologically impossible. The weighted fit provided more realistic estimates of Raw. Different model fits and minimal frequencies had little effect on tissue mechanics.

Sexual dimorphism in lung function responses to acute influenza A infection.

BACKGROUND: Males are generally more susceptible to respiratory infections; however, there are few data on the physiological responses to such infections in males and females. OBJECTIVES: To determine whether sexual dimorphism exists in the physiological/inflammatory responses of weanling and adult BALB/c mice to influenza. METHODS: Weanling and adult mice of both sexes were inoculated with influenza A or appropriate control solution. Respiratory mechanics, responsiveness to methacholine (MCh), viral titre and bronchoalveolar lavage (BAL) cellular inflammation/cytokines were measured 4 (acute) and 21 (resolution) days post-inoculation. RESULTS: Acute infection impaired lung function and induced hyperresponsiveness and cellular inflammation in both sexes at both ages. Males and females responded differently with female mice developing greater abnormalities in tissue damping and elastance and greater MCh responsiveness at both ages. BAL inflammation, cytokines and lung viral titres were similar between the sexes. At resolution, all parameters had returned to baseline levels in adults and weanling males; however, female weanlings had persisting hyperresponsiveness. CONCLUSIONS: We identified significant differences in the physiological responses of male and female mice to infection with influenza A, which occurred in the absence of variation in viral titre and cellular inflammation.