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Key Event Title
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP||Point of Contact||Author Status||OECD Status|
|Latent TGFbeta1 activation leads to pulmonary fibrosis||AdverseOutcome||Cataia Ives (send email)||Under development: Not open for comment. Do not cite|
|Substance interaction with the lung cell membrane leading to lung fibrosis||AdverseOutcome||Cataia Ives (send email)||Under development: Not open for comment. Do not cite||EAGMST Under Review|
|TLR4 activation, PPAR gamma activation and Pulmonary fibrosis||AdverseOutcome||Arthur Author (send email)||Under development: Not open for comment. Do not cite|
Key Event Description
Pulmonary fibrosis is broadly defined as the thickening or scarring of lung tissue, due to excessive deposition of extracellular matrix. In the normal human lung, the nasopharynx and the conducting airways are mainly covered by epithelium composed of ciliated, mucous secreting cells in direct contact with the basement membrane with submucosal glands containing goblet, duct, and serous cells also contributing to the fluid balance and mucous production (Koval & Sidhaye, 2017). Within this epithelium, basal cells are found which are stimulated to proliferate and differentiate in response to injury (Koval & Sidhaye, 2017). Further down the lung, in the terminal bronchiole region, the epithelium does not contain submucosal glands, but instead contains club cells which produce pulmonary surfactant and can differentiate into bronchiolar or alveolar epithelial cells. Finally, in the terminal airspaces, the epithelium is made up entirely of type I and type II alveolar epithelial cells. In between the two adjacent alveoli are two layers of alveolar epithelium resting on basement membrane, which consists of interstitial space, pulmonary capillaries, elastin and collagen fibres. Thus, the alveolar capillary membrane, where gas exchange takes place, is made up of the alveolar epithelium and alveolar endothelium (Gracey et al, 1968). In pulmonary fibrosis, damage to the pulmonary epithelium results in excessive deposition of collagen by constitutively activated myofibroblasts during the wound healing response. This causes a pronounced decrease in the number of capillaries within the alveolar septa with asymmetric deposition of collagen and cells between part of the surface of a capillary and the nearby alveolar lining. In areas where capillaries are not present, the alveolar capillary membrane is occupied with collagen and cells.
How It Is Measured or Detected
In vivo, histopathological analysis is used for assessing fibrotic lung disease. Morphometric analysis of the diseased area versus total lung area is used to quantitatively stage the fibrotic disease. Although, some inconsistencies can be introduced during the analysis due to the experience of the individual scoring the disease, the histological stain, etc., a numerical scale with grades from 0 to 8, originally developed by Ashcroft et al., 1988 is assigned to indicate the amount of fibrotic tissue in histological samples. This scale is applied to diagnose lung fibrosis in both human and animal samples. Modifications to this scoring system were proposed (Hubner et al., 2008), which enables morphological distinctions thus enabling a better grading of the disease. Using the modified scoring system, bleomycin induced lung fibrosis in rats was scored as follows: Grade 0 – normal lung, Grade 1 – isolated alveolar septa with gentle fibrotic changes, Grade 2 – knot like formation in fibrotic areas in alveolar septa, Grade 3 – contiguous fibrotic walls of alveolar septa, Grade 4 – single fibrotic masses, Grade 5 – confluent fibrotic masses, Grade 6 – large contiguous fibrotic masses, Grade 7 – air bubbles and Grade 8 – fibrotic obliteration. Further morphometric analysis can be conducted to quantify the total disease area (Nikota et al., 2017).
Lungs are formalin fixed and paraffin embedded such that an entire cross section of lung can be presented on a slide. The entire cross section is captured in a series of images using wide field light microscope. Areas of alveolar epithelium thickening and consolidated air space are identified. ImageJ software (freely available) is used to trace the total area (green line) and the diseased area (red line) imaged and quantified. The diseased area is equal to disease area/total area (Nikota et al., 2017).
In vitro, there is no single assay that can measure the alveolar thickness. However, a combination of assays spanning various KEs described above provide a measure of the extent of fibrogenesis potential of tested substances. qRT-PCR and ELISA assays measuring increased collagen, TGFβ1 and various pro-inflammatory mediators are used as sensitive markers of potential of substances to induce the adverse outcome of lung fibrosis.
Domain of Applicability
Regulatory Significance of the Adverse Outcome
1. Ashcroft, T., J.M. Simpson, and V. Timbrell. 1988. Simple method of estimating severity of pulmonary fibrosis on a numerical scale. J. Clin. Pathol. 41:467-470.
2. Gracey DR, Divertie MB and Brown Jr. AL. Alveolar-Capillary Membrane in Idiopathic Interstitial Pulmonary Fibrosis. 1968. American Review of Respiratory Disease, 98(1), pp. 16–21.
3. Hübner, R. H., Gitter, W., El Mokhtari, N. E., Mathiak, M., Both, M., Bolte, H., Freitag-Wolf, S., & Bewig, B. (2008). Standardized quantification of pulmonary fibrosis in histological samples. BioTechniques, 44(4), 507–517.
4. Koval, M., & Sidhaye, V. (2017). Introduction: The Lung Epithelium. In Lung Epithelial Biology in the Pathogenesis of Pulmonary Disease (pp. xiii-xviii). Elsevier.
5. Nikota J, Banville A, Goodwin LR, Wu D, Williams A, Yauk CL, Wallin H, Vogel U, Halappanavar S. Stat-6 signaling pathway and not Interleukin-1 mediates multi-walled carbon nanotube-induced lung fibrosis in mice: insights from an adverse outcome pathway framework. Part Fibre Toxicol. 2017 Sep 13;14(1):37.