Understanding the Structural and Functional Correlates of Acute Lung Inflammation in Two Murine Models

The outcome of lung inflammation is important to host survival as lungs are necessary for oxygen exchange and fighting pathogens or any injurious stimuli. Thus, diagnosing and understanding the kinetics of lung inflammation is an emerging technological area in the field of imaging research and development. Dr. Aulakh’s lab has two separate established models of neutrophilic murine acute lung injury namely, acute low-dose (0.05 ppm) ozone-induced and intranasal bacterial lipopolysaccharide (LPS)-induced lung inflammation. In order to characterize the dynamics of these models, there are two research hypotheses of my project, which are a) acute low-dose ozone exposure causes lung [18F]F-FDG retention because of increased leukocyte glucose uptake due to inflammation as assessed by sequential micro-Positron Emission Tomography-Computed tomography (microPET-CT) in murine lungs, similar to the effects of intranasal bacterial lipopolysaccharide, LPS and b) acute low-dose ozone exposure induces an increase in ultra-small-angle scatter (USAXS) (due to alveolar recruitment), absorption (due to alveolar edema) and a decrease in refraction (due to peri-bronchiolar edema) comparable to intranasal LPS induced changes in these X-ray optical properties as assessed by Lung Multiple Image X-Radiography (MIR). Thus, the premise of my thesis is to test the utility of longitudinal non-invasive imaging modalities, namely sequential [18F]-fluoro-deoxy glucose ([18F]F-FDG) positron emission tomography-computed tomography (PET-CT) and synchrotron multiple image X-radiography (MIR), to assess the progression of acute murine low-dose ozone or intranasal bacterial lipopolysaccharide (LPS) induced lung inflammation over 24 and 70 h time periods, respectively. Both ozone and LPS induced an increase in murine lung [18F]F-FDG standard uptake ratio (SUR) and a heterogenous lung distribution which was unlike the craniocaudal [18F]F-FDG gradient observed in lungs before any exposure (called as baseline or control [18F]F-FDG). The whole-body distribution profiles revealed that lung [18F]F-FDG activity was higher and prolonged up to 28 h in LPS compared to ozone exposed mice. While [18F]F-FDG is a useful marker to highlight areas with high metabolic uptake of glucose in cells such as neutrophils and macrophages recruited during inflammation, the resolution of PET-CT (hundreds of μm) precludes the evaluation of microscopic histopathologic changes especially in the alveoli. Using lung hematoxylin and eosin stained cryosections, the ratios of total lung tissue to air spaces and specifically alveolar parenchyma to air spaces were assessed in mice lungs exposed to 0.05 ppm ozone for 2 h. Results from the X-ray CT lung tissue volume quantifications as well as the histologically derived percent-stained lung or alveolar area quantifications suggest significant damage that is observed as reduced percentage area as well as variability or standard deviation (S.D.) of binary lung images in mice immediately i.e., at 0 h and 6 h after exposure to 2 h of 0.05 ppm ozone. Alveolar damage was also significant at 0 h as shown by reduction in percentage area and S.D. in the binary image region restricted to alveoli. The synchrotron study aimed at following mice lungs before, immediately i.e., at 0 h, and thereafter at 24, 48 or 70 h after saline, bacterial lipopolysaccharide (LPS, 50 μg), or low dose (0.05 ppm for 2 h) ozone exposure. Our results indicate that the lung ultra-small-angle scatter (USAXS), which is a metric of air-tissue boundaries and refraction (which is due to bending of X-rays across air-tissue conducting airways) reduces, especially in the cranial part of left lung, with a corresponding increase in absorption upon exposure to LPS or ozone and is detectable up to 70 h. The changes in lung X-ray optical properties are indicative of the gross inflammatory changes, in response to LPS or ozone exposure, as indicated by increases in lung absorption but reduction in refraction and USAXS. Overall, the results from my project indicate that for a comprehensive analysis of lung inflammation, a combination of lung histological analysis along with objective lung image analysis as described in the longitudinal microPET-CT and lung MIR experiments form powerful techniques for sensitive delineation of inflammatory changes in gross lung structure and function