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Event: 397

Key Event Title

The KE title should describe a discrete biological change that can be measured. It should generally define the biological object or process being measured and whether it is increased, decreased, or otherwise definably altered relative to a control state. For example “enzyme activity, decreased”, “hormone concentration, increased”, or “growth rate, decreased”, where the specific enzyme or hormone being measured is defined. More help

Response, Keratinocytes

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. The short name should be less than 80 characters in length. More help
Response, Keratinocytes

Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. Note, KEs should be defined within a particular level of biological organization. Only KERs should be used to transition from one level of organization to another. Selection of the level of biological organization defines which structured terms will be available to select when defining the Event Components (below). More help

Cell term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Organ term

Further information on Event Components and Biological Context may be viewed on the attached pdf.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable. More help

Key Event Components

Further information on Event Components and Biological Context may be viewed on the attached pdf.Because one of the aims of the AOP-KB is to facilitate de facto construction of AOP networks through the use of shared KE and KER elements, authors are also asked to define their KEs using a set of structured ontology terms (Event Components). In the absence of structured terms, the same KE can readily be defined using a number of synonymous titles (read by a computer as character strings). In order to make these synonymous KEs more machine-readable, KEs should also be defined by one or more “event components” consisting of a biological process, object, and action with each term originating from one of 22 biological ontologies (Ives, et al., 2017; See List). Biological process describes dynamics of the underlying biological system (e.g., receptor signalling). The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signalling by that receptor).Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description. To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons. If a desired term does not exist, a new term request may be made via Term Requests. Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add. More help

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help


This is a structured field used to identify specific agents (generally chemicals) that can trigger the KE. Stressors identified in this field will be linked to the KE in a machine-readable manner, such that, for example, a stressor search would identify this as an event the stressor can trigger. NOTE: intermediate or downstream KEs in one AOP may function as MIEs in other AOPs, meaning that stressor information may be added to the KE description, even if it is a downstream KE in the pathway currently under development.Information concerning the stressors that may trigger an MIE can be defined using a combination of structured and unstructured (free-text) fields. For example, structured fields may be used to indicate specific chemicals for which there is evidence of an interaction relevant to this MIE. By linking the KE description to a structured chemical name, it will be increasingly possible to link the MIE to other sources of chemical data and information, enhancing searchability and inter-operability among different data-sources and knowledgebases. The free-text section “Evidence for perturbation of this MIE by stressor” can be used both to identify the supporting evidence for specific stressors triggering the MIE as well as to define broad chemical categories or other properties that classify the stressors able to trigger the MIE for which specific structured terms may not exist. More help

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected from an ontology. In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help

Life Stages

The structured ontology terms for life-stage are more comprehensive than those for taxa, but may still require further description/development and explanation in the free text section. More help

Sex Applicability

The authors must select from one of the following: Male, female, mixed, asexual, third gender, hermaphrodite, or unspecific. More help

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. For example, the biological state being measured could be the activity of an enzyme, the expression of a gene or abundance of an mRNA transcript, the concentration of a hormone or protein, neuronal activity, heart rate, etc. The biological compartment may be a particular cell type, tissue, organ, fluid (e.g., plasma, cerebrospinal fluid), etc. The role in the biology could describe the reaction that an enzyme catalyses and the role of that reaction within a given metabolic pathway; the protein that a gene or mRNA transcript codes for and the function of that protein; the function of a hormone in a given target tissue, physiological function of an organ, etc. Careful attention should be taken to avoid reference to other KEs, KERs or AOPs. Only describe this KE as a single isolated measurable event/state. This will ensure that the KE is modular and can be used by other AOPs, thereby facilitating construction of AOP networks. More help

Haptens can also react with cell surface proteins and activate response pathways in keratinocytes (see [1]). Uptake of the hapten by keratinocytes activates multiple events, including the release of pro-inflammatory cytokines and the induction of cyto-protective cellular pathways. Activation of the pro-inflammatory cytokine IL-18 results from cleavage of inactive IL-18 precursor protein by inflammasome-associated caspase-1[2]. Sensitizers can activate the inflammasome ([3];[4]) and in so doing induce IL-18 production. Intracellular Nodlike receptors (NLR) contain sensors for a number of cellular insults. Upon activation (by a currently unknown mechanism), NLRs oligomerise form molecular complexes (i.e. inflammasomes) that are involved in the activation of inflammatory-associated caspases, including caspase-1. Inductions of intracellular levels of IL-18 exhibit responses upon exposure to sensitizers which can be used to establish potency[5].

Keratinocyte exposure to sensitizers also results in induction of antioxidant/electrophile response element ARE/EpRE-dependent pathways[6]. Briefly, reactive chemicals bind to Keap1 (Kelch-like ECH-associates protein 1) that normally inhibit the nuclear erythroid 2-related factor 2 (Nrf2). Released Nrf2 interacts with other nuclear proteins and binds to and activates ARE/EpREdependent pathways, including the cytoprotective genes NADPH-quinone oxidoreductase 1 (NQ01) and glutathione S-transferase (GSHST), among others ([6];[7]). An in vitro reporter assay based on activation via the ARE/EpRE response element has been shown to be responsive to known sensitizers in HaCaT keratinocytes[8]. Expression of ARE/EpRE-dependent genes and other cytoprotective genes (including CYP1A1, MT1 and MT2) in HaCaT cells are part of a proprietary in vitro battery approach to determining sensitisation potency[9]. Both the Natsch and McKim groups have shown that this signalling pathway responds in a quantitative fashion, which is related to LLNA potency (e.g. strong, moderate, and weak).

How It Is Measured or Detected

One of the primary considerations in evaluating AOPs is the relevance and reliability of the methods with which the KEs can be measured. The aim of this section of the KE description is not to provide detailed protocols, but rather to capture, in a sentence or two, per method, the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements. Methods that can be used to detect or measure the biological state represented in the KE should be briefly described and/or cited. These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA).Key considerations regarding scientific confidence in the measurement approach include whether the assay is fit for purpose, whether it provides a direct or indirect measure of the biological state in question, whether it is repeatable and reproducible, and the extent to which it is accepted in the scientific and/or regulatory community. Information can be obtained from the OECD Test Guidelines website and the EURL ECVAM Database Service on Alternative Methods to Animal Experimentation (DB-ALM). ?

Methods that have been previously reviewed and approved by a recognized authority should be included in the Overview section above. All other methods, including those well established in the published literature, should be described here. Consider the following criteria when describing each method: 1. Is the assay fit for purpose? 2. Is the assay directly or indirectly (i.e. a surrogate) related to a key event relevant to the final adverse effect in question? 3. Is the assay repeatable? 4. Is the assay reproducible?

Biochemical Pathways Related to Skin Sensitisation

Biochemical or intracellular pathways affected by the action of reactive chemicals are in general thought to be related to skin sensitisation; however, current knowledge is incomplete. The c-Jun Nterminal kinases and the p38 kinases have been shown to be activated upon exposure to protein-binding chemicals [10]. However, these studies are based on only a few compounds, in particular, 1-chloro-2,4-dinitrobenzene. A few investigations have examined this possibility by measuring changes in kinase expression in different signal transduction pathways (e.g. p38 MAPK, ERK, PGK, and NFκB) (see [11]). Miyazawa and co-workers ([12] and [13]) examined P38 MAPK (mitogen-activated protein kinase) and ERK, (extracellular signal-regulated kinase) in THP-1 cells and showed that 1-chloro-2,4-dinitrobenzene, an extreme sensitizer, activates both cellular pathways and stimulates TNF-α release and subsequent phenotypic changes in the cells. As noted, the Keap1/Nrf2/ARE/EpRE cell signalling assay is a potential cellular marker for sensitisation because Keap1 is a thiol-rich sensor protein which has been shown to be covalently modified by electrophiles that leads to activation of ARE-dependent genes[14].

Events in Keratinocyte

In recent years, investigations have focused on the DNA antioxidant-response element (ARE), also known as electrophile response element (see [15]). The KeratinoSens system of Natsch’s group uses a luciferase reporter gene under control of a single copy of the ARE element of the human AKR1C2 gene stably inserted into immortalized human keratinocytes (HaCaT cells)[8]. The experimental design is robust with chemicals routinely tested at twelve concentrations in triplicate before evaluating for significant induction of gene activity [16]. One advantage of this assay is that it appears to address the issue of metabolism[8]. While several variants of the luciferase-based ARE assay have been developed based on data derived from the assay described by Natsch and Emter[6], Natsch et al.[17] evaluated the predictive performances of the ARE assay evaluated against LLNA data for more than 100 chemicals. They report a 79%, concordance, 79% sensitivity and 81% specificity[17]. An in vitro assay based on IL-18 induction in human keratinocytes (cell line NCTC 2544) can also distinguish between sensitizers and irritants ([18];[19]). Other studies have described chemokines (e.g. CCL2, CCL4) and receptor (e.g. CCR7) (see [11]).

The Keap1/Nrf2/ARE/EpRE cell signalling assay is also the mechanistic basis for the work on skin sensitisation chemicals at CeeTox Inc.[9]. Briefly, this work includes quantitative realtime polymerase chain reaction measurements of the relative abundance of mRNA for eleven selected genes. While most of the data is proprietary, the reported results are highly promising. Interestingly, both Emter et al.[8] and McKim et al.[9] combine their cell signalling results with chemical reactivity data in algorithms, which can be viewed as a first step in using the AOP in quantitative assessment.


Keratinocytes are the major cell type of the epidermis of the skin. They are known to be the primary site of skin metabolism and play an important role in epithelial DC activation. Using human keratinocytes (HaCaT cells), McKim et al.[9] evaluated selected genes associated with three cell signalling pathways (Keap1/Nrf 2/ARE/EpRE, ARNT/AhR/XRE, and Nrf1/MTF/MRE) which are known to be activated by sensitizing agents. Briefly, the relative abundance of eleven genes whose expression is controlled by one of these pathways, was measured. The Nrf2/ARE/EpRE-controlled genes are: 1) NQO1, 2) AKR1C2, 3) thioredoxin (TXN), 4) interleukin 8 (IL8), 5) aldehyde dehydrogenase 3A (ALDH3A), 6) heme-oxygenase 1 (HMOX1), 7) musculoaponeurotic fibrosarcoma (MafF), and 8) GCLC. The XRE-controlled gene is 9) CYP1A1. The MRE-controlled genes are 10) metallothionein 1 (MT1), and 11) metallothionein 2 (MT2). Gene expression was reported on a scale of 4+, 3+, 2+, + and NC). For DNCB McKim et al, report: NQO1 expression = 4+; AKR1C2 = 2+; TXN = NC; IL8 = +; CYP1A1 = +; ALDH3A = NC; HMOX = NC; MafF = +; GCLC = NC; MT1A = NC, and MT2A = 2+. Microarray analysis of DCNB-treated HaCaT cells by Vanderbriel et al. (2010) show similar results.

The KeratinoSens assay[8] examines dose-responses (routinely twelve concentrations in triplicate) for significant induction of gene activity in an in vitro assay with a luciferase reporter gene under control of a single copy of the ARE-element of the human AKR1C2 gene stably inserted into HaCaT keratinocyte cell line. Using a standard operating procedure[8], experimental data was generated and the average maximal induction of gene activity (Imax) and the average concentration inducing gene activity >50% above control values (EC1.5) were determined. The latter calculations were performed with linear extrapolation from the values above and below the induction threshold (as for the EC3 value determination in the LLNA. Intra- and inter-laboratory testing of DNCB with the KeratioSens assay[16] report repeatable and reproducible results (Table 2). Table 2. Intra- and inter-laboratory testing of DNCB with the KeratinoSens assay.


Imax (fold induction)

EC 1.5 (μM)

A (historical)


















Cultures of human keratinocyte cell line NCTC 2544 were exposed to DNCB and cell-associated IL-18 evaluated 24 hours later[18]. Intracellular IL-18 content was assessed by specific sandwich ELISA, with results expressed in pg/mg of total intracellular protein. DNCB induces a significant dose-response increase in IL-18 production; however, production is modest as compared to other sensitizers that were tested.

Overview table: How it is measured or detected

Method(s) Reference URL Regulatory


Validated Non


ARE-Nrf2 Luciferase Test Method (Keratinosens) TG442D [1] X X
DB-ALM [2]
RhE-IL-18 Gibbs et al., 2013 [3] X

Domain of Applicability

This free text section should be used to elaborate on the scientific basis for the indicated domains of applicability and the WoE calls (if provided). While structured terms may be selected to define the taxonomic, life stage and sex applicability (see structured applicability terms, above) of the KE, the structured terms may not adequately reflect or capture the overall biological applicability domain (particularly with regard to taxa). Likewise, the structured terms do not provide an explanation or rationale for the selection. The free-text section on evidence for taxonomic, life stage, and sex applicability can be used to elaborate on why the specific structured terms were selected, and provide supporting references and background information.  More help


List of the literature that was cited for this KE description. Ideally, the list of references, should conform, to the extent possible, with the OECD Style Guide ( (OECD, 2015). More help
  1. Weltzien H, Corsini E, Gibbs S, Lindstedt M, Borrebaeck C, Budde P, Schulz-Knappe P, Thierse H-J, Martin S, Roggen E. 2009. Safe cosmetics without animal testing? Contributions of the EU Project Sens-it-iv. J. für Verbraucherschutz und Lebensmittelsicherheit. 4: 41-48.
  2. Martinon, F., Mayor, A. and Tschopp, J. 2009. The inflammasomes: guardians of the body. Ann. Rev. Immunol. 27: 229-265.
  3. Sutterwala FS, Ogura Y, Szczepanik M, Lara-Tejero M, Lichtenberger GS, Grant EP, Bertin J, Coyle AJ, Galán JE, Askenase PW, Flavell RA. 2006. Critical role for NALP3/CIAS1/Cryopyrin in innate and adaptive immunity through its regulation of caspase-1. Immunity 24: 317-327.
  4. Watanabe H, Gaide O, Pétrilli V, Martinon F, Contassot E, Roques S, Kummer JA, Tschopp J, French LE. 2007. Activation of the IL-1beta-processing inflammasome is involved in contact hypersensitivity. J. Invest. Dermatol. 127: 1956-1963.
  5. Van Och FMM, Van Loveren H, Van Wolfswinkel JC, Machielsen AJC, Vandebriel RJ. 2005. Assessment of potency of allergenic activity of low molecular weight compounds based on IL-1α and IL-18 production by a murine and human keratinocyte cell line. Toxicology 210: 95-109.
  6. 6.0 6.1 6.2 Natsch A and Emter R. 2008. Skin sensitizers induce antioxidant response element dependent genes: Application to the in vitro testing of the sensitisation potential of chemicals. Toxicol. Sci. 102:110-119
  7. Ade N, Leon F, Pallardy M, Pfeiffer JL, Kerdine-Romer S, Tissier MH, Bonnet PA, Fabre I Ourlin JC. 2009. HMOX1 and NQO1 genes are upregulated in response to contact sensitizers in dendritic cells and THP-1 cell line: role of the Keap1/Nrf2 pathway. Toxicol. Sci. 107: 451-460.
  8. 8.0 8.1 8.2 8.3 8.4 8.5 Emter R, Ellis G, Natsch A. 2010. Performance of a novel keratinocyte-based reporter cell line in screen skin sensitizers in vitro. Toxicol. Appl. Pharmacol. 245: 281-290.
  9. 9.0 9.1 9.2 9.3 McKim JM Jr, Keller DJ III, Gorski JR. 2010. A new in vitro method for identifying chemical sensitizers combining peptide binding with ARE/EpRE-mediated gene expression in human skin cells. Cutan. Ocul. Toxicol. 29: 171-192.
  10. Trompezinski S, Migdal C, Tailhardat M, Le Varlet B, Courtellemont P, Haftek M, Serres M. 2008. Charaterization of early events involved in human dendritic cell maturation induced by sensitizers: cross talk between MAPK signalling pathways. Toxicol. Appl. Pharmacol. 230: 397- 406.
  11. 11.0 11.1 dos Santos GG, Reinders J, Ouwhand K, Rustemeyer T, Scheper RJ, Gibbs S. 2009. Progress on the development of human in vitro dendritic cell based assays for assessment of skin sensitizing potential of compounds. Toxicol. Appl. Pharmacol. 236: 372-382.
  12. Miyazawa, M. Ito, Y., Kosaka, N., Nukada, Y., Sakaguchi, H., Suzuki, H., and Nishiyama, N. 2008a. Role of MAPK signalling pathway in the activation of dendritic type cell line, THP-1, induced by DNBC and NiSO4. Toxicol. Sci. 33: 51-59.
  13. Miyazawa, M., Ito, Y., Kosaka, N., Nukada, Y., Sakaguchi, H., Suzuki, H., and Nishiyama, N. 2008b. Role of TNF-alpha and extracellular ATP in THP-1, cell activation following allergen exposure. Toxicol. Sci. 33: 51-59.
  14. Dinkova-Kostova AT, Holtzclaw WD, Kensler TW. 2005. The role of Keap1 in cellular protective responses. Chem. Res. Toxicol. 18: 1779-1791.
  15. Natsch A. 2010. The Nrf2-Keap1-ARE toxicity pathway as a cellular sensor for skin sensitizers-functional relevance and a hypothesis on innate reactions to skin sensitizers. Toxicol. Sci. 113: 284-292.
  16. 16.0 16.1 Natsch A, Bauch C, Foertsch L, Gerberick F, Norman K, Hilberer A, Inglis H, Landsiedel R, Onken S, Reuter H, Schepky A, Emter R. 2011. The intra- and inter-laboratory reproducibility and predictivity of the KeratinoSens assay to predict skin sensitizers in vitro: results of a ring-study in five laboratories. Toxicol. In Vitro 25: 733-44.
  17. 17.0 17.1 Natsch A, Emter R, Ellis G. 2009. Filling the concept with data: Integrating data from different in vitro and in silico assays on skin sensitizers to explore the battery approach for animal-free skin sensitisation testing. Toxicol. Sci. 107: 106-121.
  18. 18.0 18.1 Corsini E, Mitjans M, Galbiati V, Lucchi L, Galli CL, Marinovich M. 2009. Use of IL-18 production in a human keratinocyte cell line to discriminate contact sensitizers from irritants and low molecular weight respiratory allergens. Toxicol. In Vitro 23: 769-796.
  19. Mitjans M, Galbiati V, Lucchi L, Viviani B, Marinovich M, Galli CL, Corsini E. 2010. Use of IL-8 release and p38 MAPK activation in THP-1 cells to identify allergens and to assess their potency in vitro. Toxicol. In Vitro 24: 1803-1809.