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Key Event Title
Activation, Inflammatory cytokines, chemokines, cytoprotective gene pathways
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Key Event Components
Key Event Overview
AOPs Including This Key Event
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Key Event Description
The innate immune system plays a crucial role in the initiation of adaptive immune responses. (Poynter, 2012, Salazar and Ghaemmaghami, 2013) It is a first-line of defense against invading microbial pathogens and is activated via a range of pattern recognition receptors (PRRs) that recognize conserved patterns present on pathogens, that is, the toll-like receptors (TLRs) and the nucleotide binding domain leucine-rich repeat containing receptor (NLR) family. These PRRs can be activated by endogenous danger-associated molecular patterns (DAMPs), released under oxidative stress and cell damage and include components of the extracellular matrix generated after tissue injury, for example, hyaluronic acid fragments, intracellular proteins such as heat shock proteins and nonprotein DAMPs such as uric acid crystals. (Kawai and Akira, 2010, Seong and Matzinger, 2004, Wheeler et al., 2009)
NLR protein-3 (NLRP3) is a PRR that belongs to the NLR family, a group of intracellular receptors activated by mitochondrial oxidative stress, for example, by adenosine triphosphate and uric acid. (Kawai and Akira, 2009) On activation, TLR and NLRP3 activate innate immunity signaling pathways leading to the release of proinflammatory cytokines and chemokines. In recent years, increasing attention has been paid to the role of the innate immune system in asthma. The sentinel role of the innate immune systems includes the activation of pathways by pathogen-associated molecular patterns and DAMPs. By this, KEs during sensitization such as activation and migration of DCs are set into motion. (Holgate, 2012) Proinflammatory molecules are also known to induce the expression of surface molecules on immune cells such as antigen-presenting cells (APCs), which are greatly involved in the induction of adaptive immune responses. Thus, whether an immune response or tolerance response is induced in APCs depends not only on the presence of antigenic properties of a substance but also on danger signals.
How It Is Measured or Detected
There are no predictive markers for cellular danger or proinflammatory responses described for respiratory sensitizers yet. The studies performed up until now did not result in any proteins, genes, or molecular pathways that are consistently regulated by a broad range of respiratory sensitizers or genes; (Remy et al., 2014) however, only a few chemicals have been tested. Cytokine production can be measured by ELISA or Bio-Plex systems either in the supernatants or intracellular matrix. Cell systems that can be used include also complex models such as the 3D epithelial cell models, that is, MucilAir™ and PCLS. (Huang et al., 2013, Lauenstein et al., 2014)
Activation of innate immune response can also be assessed using commercial immunoassays for signal transduction pathways, that is, p38 MAPK, JNK 1/2, and ERK 1/2. Other possible detection methods, focusing on ROS production or the induction of cytoprotective pathways, might be used as well to assess the ability of chemicals to generate endogenous danger signals (DAMPs). For ROS production, commercial assays are available that can be applied. The induction of Nrf2-KEAP1 can be assessed using the Keratinosens® (Natsch et al., 2013, Emter et al., 2010) or LuSens (Ramirez et al., 2014) assays (OECD TG 442D) and by measuring gene expression of Nrf2-dependent genes by quantitative polymerase chain reaction (qPCR), that is, HMOX, (Migdal et al., 2013) although the utility of this pathway for respiratory sensitizers is unclear. The BEAS-2B cell line, coupled with microarray analysis, reveals the PTEN pathway as potentially useful. (Verstraelen et al., 2009) The predictivity of these assays has not been studied with a large number of respiratory sensitizers.
Domain of Applicability
It is not fully understood which cell types are the most important sources for the endogenous danger signals involved in sensitization of the respiratory tract. Relevant cell types representing cellular sources for danger signals are probably alveolar and bronchial epithelial cells, keratinocytes, macrophages, DCs, natural killer cells, endothelial cells, and nerve fiber endings. (Verstraelen et al., 2008) In particular, macrophages are able to respond with high levels of, for example, cytokines and ROS after stimulation of PRRs. Human cell lines representative of the cells mentioned above might be used for the measurements of danger signal induction. A limitation of the use of submerged cell lines is that certain respiratory sensitizers hydrolyze in an aqueous environment, which may lead to negative results. (Wanner et al., 2010) Air/liquid exposure in 3D skin or airway models might provide a more robust model although this has not been explored in great detail.
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HOLGATE, S. T. 2012. Innate and adaptive immune responses in asthma. Nat Med, 18, 673-83.
HUANG, S., WISZNIEWSKI, L., CONSTANT, S. & ROGGEN, E. 2013. Potential of in vitro reconstituted 3D human airway epithelia (MucilAir™) to assess respiratory sensitizers. Toxicol In Vitro, 27, 1151-6.
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KAWAI, T. & AKIRA, S. 2010. The role of pattern-recognition receptors in innate immunity: update on Toll-like receptors. Nat Immunol, 11, 373-84.
LAUENSTEIN, L., SWITALLA, S., PRENZLER, F., SEEHASE, S., PFENNIG, O., FÖRSTER, C., FIEGUTH, H., BRAUN, A. & SEWALD, K. 2014. Assessment of immunotoxicity induced by chemicals in human precision-cut lung slices (PCLS). Toxicol In Vitro, 28, 588-99.
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NATSCH, A., RYAN, C. A., FOERTSCH, L., EMTER, R., JAWORSKA, J., GERBERICK, F. & KERN, P. 2013. A dataset on 145 chemicals tested in alternative assays for skin sensitization undergoing prevalidation. J Appl Toxicol, 33, 1337-52.
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REMY, S., VERSTRAELEN, S., VAN DEN HEUVEL, R., NELISSEN, I., LAMBRECHTS, N., HOOYBERGHS, J. & SCHOETERS, G. 2014. Gene expressions changes in bronchial epithelial cells: markers for respiratory sensitizers and exploration of the NRF2 pathway. Toxicol In Vitro, 28, 209-17.
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SEONG, S. Y. & MATZINGER, P. 2004. Hydrophobicity: an ancient damage-associated molecular pattern that initiates innate immune responses. Nat Rev Immunol, 4, 469-78.
VERSTRAELEN, S., BLOEMEN, K., NELISSEN, I., WITTERS, H., SCHOETERS, G. & VAN DEN HEUVEL, R. 2008. Cell types involved in allergic asthma and their use in in vitro models to assess respiratory sensitization. Toxicol In Vitro, 22, 1419-31.
VERSTRAELEN, S., NELISSEN, I., HOOYBERGHS, J., WITTERS, H., SCHOETERS, G., VAN CAUWENBERGE, P. & VAN DEN HEUVEL, R. 2009. Gene profiles of a human alveolar epithelial cell line after in vitro exposure to respiratory (non-)sensitizing chemicals: identification of discriminating genetic markers and pathway analysis. Toxicol Lett, 185, 16-22.
WANNER, R., SONNENBURG, A., QUATCHADZE, M., SCHREINER, M., PEISER, M., ZUBERBIER, T. & STAHLMANN, R. 2010. Classification of sensitizing and irritative potential in a combined in-vitro assay. Toxicol Appl Pharmacol, 245, 211-8.
WHEELER, D. S., CHASE, M. A., SENFT, A. P., POYNTER, S. E., WONG, H. R. & PAGE, K. 2009. Extracellular Hsp72, an endogenous DAMP, is released by virally infected airway epithelial cells and activates neutrophils via Toll-like receptor (TLR)-4. Respir Res, 10, 31.