Aop: 40


Each AOP should be given a descriptive title that takes the form “MIE leading to AO”. For example, “Aromatase inhibition [MIE] leading to reproductive dysfunction [AO]” or “Thyroperoxidase inhibition [MIE] leading to decreased cognitive function [AO]”. In cases where the MIE is unknown or undefined, the earliest known KE in the chain (i.e., furthest upstream) should be used in lieu of the MIE and it should be made clear that the stated event is a KE and not the MIE. More help

Covalent Protein binding leading to Skin Sensitisation

Short name
A short name should also be provided that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
Skin Sensitisation AOP

Graphical Representation

A graphical summary of the AOP listing all the KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs should be provided. This is easily achieved using the standard box and arrow AOP diagram (see this page for example). The graphical summary is prepared and uploaded by the user (templates are available) and is often included as part of the proposal when AOP development projects are submitted to the OECD AOP Development Workplan. The graphical representation or AOP diagram provides a useful and concise overview of the KEs that are included in the AOP, and the sequence in which they are linked together. This can aid both the process of development, as well as review and use of the AOP (for more information please see page 19 of the Users' Handbook).If you already have a graphical representation of your AOP in electronic format, simple save it in a standard image format (e.g. jpeg, png) then click ‘Choose File’ under the “Graphical Representation” heading, which is part of the Summary of the AOP section, to select the file that you have just edited. Files must be in jpeg, jpg, gif, png, or bmp format. Click ‘Upload’ to upload the file. You should see the AOP page with the image displayed under the “Graphical Representation” heading. To remove a graphical representation file, click 'Remove' and then click 'OK.'  Your graphic should no longer be displayed on the AOP page. If you do not have a graphical representation of your AOP in electronic format, a template is available to assist you.  Under “Summary of the AOP”, under the “Graphical Representation” heading click on the link “Click to download template for graphical representation.” A Powerpoint template file should download via the default download mechanism for your browser. Click to open this file; it contains a Powerpoint template for an AOP diagram and instructions for editing and saving the diagram. Be sure to save the diagram as jpeg, jpg, gif, png, or bmp format. Once the diagram is edited to its final state, upload the image file as described above. More help


List the name and affiliation information of the individual(s)/organisation(s) that created/developed the AOP. In the context of the OECD AOP Development Workplan, this would typically be the individuals and organisation that submitted an AOP development proposal to the EAGMST. Significant contributors to the AOP should also be listed. A corresponding author with contact information may be provided here. This author does not need an account on the AOP-KB and can be distinct from the point of contact below. The list of authors will be included in any snapshot made from an AOP. More help

Wiki entry based on OECD Series on Testing and Assessment no 168 (4th May 2012)

Corresponding Authors:



Point of Contact

Indicate the point of contact for the AOP-KB entry itself. This person is responsible for managing the AOP entry in the AOP-KB and controls write access to the page by defining the contributors as described below. Clicking on the name will allow any wiki user to correspond with the point of contact via the email address associated with their user profile in the AOP-KB. This person can be the same as the corresponding author listed in the authors section but isn’t required to be. In cases where the individuals are different, the corresponding author would be the appropriate person to contact for scientific issues whereas the point of contact would be the appropriate person to contact about technical issues with the AOP-KB entry itself. Corresponding authors and the point of contact are encouraged to monitor comments on their AOPs and develop or coordinate responses as appropriate.  More help
Agnes Aggy   (email point of contact)


List user names of all  authors contributing to or revising pages in the AOP-KB that are linked to the AOP description. This information is mainly used to control write access to the AOP page and is controlled by the Point of Contact.  More help
  • Brigitte Landesmann
  • Sharon Munn
  • Agnes Aggy


The status section is used to provide AOP-KB users with information concerning how actively the AOP page is being developed, what type of use or input the authors feel comfortable with given the current level of development, and whether it is part of the OECD AOP Development Workplan and has been reviewed and/or endorsed. “Author Status” is an author defined field that is designated by selecting one of several options from a drop-down menu (Table 3). The “Author Status” field should be changed by the point of contact, as appropriate, as AOP development proceeds. See page 22 of the User Handbook for definitions of selection options. More help
Author status OECD status OECD project SAAOP status
Open for citation & comment WPHA/WNT Endorsed 1.1 Included in OECD Work Plan
This AOP was last modified on May 08, 2022 11:33
The date the AOP was last modified is automatically tracked by the AOP-KB. The date modified field can be used to evaluate how actively the page is under development and how recently the version within the AOP-Wiki has been updated compared to any snapshots that were generated. More help

Revision dates for related pages

Page Revision Date/Time
Covalent Binding, Protein November 05, 2020 18:40
sensitisation, skin November 29, 2016 19:28
Activation, Keratinocytes September 16, 2017 10:15
Activation, Dendritic Cells December 03, 2020 10:15
Activation/Proliferation, T-cells November 05, 2020 19:14
Covalent Binding, Protein leads to Activation, Keratinocytes December 03, 2016 16:38
Covalent Binding, Protein leads to Activation, Dendritic Cells November 05, 2020 18:09
Activation, Keratinocytes leads to Activation, Dendritic Cells December 03, 2016 16:38
Activation, Dendritic Cells leads to Activation/Proliferation, T-cells February 11, 2021 18:23
Activation/Proliferation, T-cells leads to sensitisation, skin December 03, 2016 16:38


In the abstract section, authors should provide a concise and informative summation of the AOP under development that can stand-alone from the AOP page. Abstracts should typically be 200-400 words in length (similar to an abstract for a journal article). Suggested content for the abstract includes the following: The background/purpose for initiation of the AOP’s development (if there was a specific intent) A brief description of the MIE, AO, and/or major KEs that define the pathway A short summation of the overall WoE supporting the AOP and identification of major knowledge gaps (if any) If a brief statement about how the AOP may be applied (optional). The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance More help

Skin sensitisation is a term used to denote the regulatory hazards known as human allergic contact dermatitis or rodent contact hypersensitivity, an important health endpoint taken into consideration in hazard and risk assessment of chemicals. Skin sensitisation is an immunological process that is described in two phases: the induction of sensitisation and the subsequent elicitation of the immune reaction. The first phase includes a sequential set of events which are described in this Adverse Outcome Pathway (AOP). The molecular initiating event (MIE) is covalent binding to skin proteins (specifically, to cysteine and/or lysine residues) which leads to keratinocytes' activation, a key event (KE) at cellular level. Another key event at cellular level is activation of dendritic cells, which is caused by hapten-protein complexes as well as by signalling from activated keratinocytes. Dendritic cells subsequently mature and migrate out of the epidermis to the local lymph node where they display major histocompatibility complex molecules, which include part of the hapten-protein complex to naive T-lymphocytes (T-cells). This induces differentiation and proliferation of allergen chemical-specific memory T-cells. This signifies the consecutive KE resulting in the acquisition of sensitisation, the adverse outcome on organ level. A sensitised subject has the capacity then to mount a more accelerated secondary response to the same chemical. Thus, if exposure occurs again, at the same or a different skin site, an aggressive immune response will be elicited resulting in allergic contact dermatitis.

Background (optional)

This optional subsection should be used to provide background information for AOP reviewers and users that is considered helpful in understanding the biology underlying the AOP and the motivation for its development. The background should NOT provide an overview of the AOP, its KEs or KERs, which are captured in more detail below. Examples of potential uses of the optional background section are listed on pages 24-25 of the User Handbook. More help

Summary of the AOP

This section is for information that describes the overall AOP. The information described in section 1 is entered on the upper portion of an AOP page within the AOP-Wiki. This is where some background information may be provided, the structure of the AOP is described, and the KEs and KERs are listed. More help


Molecular Initiating Events (MIE)
An MIE is a specialised KE that represents the beginning (point of interaction between a stressor and the biological system) of an AOP. More help
Key Events (KE)
This table summarises all of the KEs of the AOP. This table is populated in the AOP-Wiki as KEs are added to the AOP. Each table entry acts as a link to the individual KE description page.  More help
Adverse Outcomes (AO)
An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP.  More help
Sequence Type Event ID Title Short name
1 MIE 396 Covalent Binding, Protein Covalent Binding, Protein
2 KE 826 Activation, Keratinocytes Activation, Keratinocytes
3 KE 398 Activation, Dendritic Cells Activation, Dendritic Cells
4 KE 272 Activation/Proliferation, T-cells Activation/Proliferation, T-cells
5 AO 827 sensitisation, skin sensitisation, skin

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarises all of the KERs of the AOP and is populated in the AOP-Wiki as KERs are added to the AOP. Each table entry acts as a link to the individual KER description page.To add a key event relationship click on either Add relationship: events adjacent in sequence or Add relationship: events non-adjacent in sequence.For example, if the intended sequence of KEs for the AOP is [KE1 > KE2 > KE3 > KE4]; relationships between KE1 and KE2; KE2 and KE3; and KE3 and KE4 would be defined using the add relationship: events adjacent in sequence button.  Relationships between KE1 and KE3; KE2 and KE4; or KE1 and KE4, for example, should be created using the add relationship: events non-adjacent button. This helps to both organize the table with regard to which KERs define the main sequence of KEs and those that provide additional supporting evidence and aids computational analysis of AOP networks, where non-adjacent KERs can result in artifacts (see Villeneuve et al. 2018; DOI: 10.1002/etc.4124).After clicking either option, the user will be brought to a new page entitled ‘Add Relationship to AOP.’ To create a new relationship, select an upstream event and a downstream event from the drop down menus. The KER will automatically be designated as either adjacent or non-adjacent depending on the button selected. The fields “Evidence” and “Quantitative understanding” can be selected from the drop-down options at the time of creation of the relationship, or can be added later. See the Users Handbook, page 52 (Assess Evidence Supporting All KERs for guiding questions, etc.).  Click ‘Create [adjacent/non-adjacent] relationship.’  The new relationship should be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. To edit a key event relationship, click ‘Edit’ next to the name of the relationship you wish to edit. The user will be directed to an Editing Relationship page where they can edit the Evidence, and Quantitative Understanding fields using the drop down menus. Once finished editing, click ‘Update [adjacent/non-adjacent] relationship’ to update these fields and return to the AOP page.To remove a key event relationship to an AOP page, under Summary of the AOP, next to “Relationships Between Two Key Events (Including MIEs and AOs)” click ‘Remove’ The relationship should no longer be listed on the AOP page under the heading “Relationships Between Two Key Events (Including MIEs and AOs)”. More help

Network View

The AOP-Wiki automatically generates a network view of the AOP. This network graphic is based on the information provided in the MIE, KEs, AO, KERs and WoE summary tables. The width of the edges representing the KERs is determined by its WoE confidence level, with thicker lines representing higher degrees of confidence. This network view also shows which KEs are shared with other AOPs. More help


The stressor field is a structured data field that can be used to annotate an AOP with standardised terms identifying stressors known to trigger the MIE/AOP. Most often these are chemical names selected from established chemical ontologies. However, depending on the information available, this could also refer to chemical categories (i.e., groups of chemicals with defined structural features known to trigger the MIE). It can also include non-chemical stressors such as genetic or environmental factors. Although AOPs themselves are not chemical or stressor-specific, linking to stressor terms known to be relevant to different AOPs can aid users in searching for AOPs that may be relevant to a given stressor. More help

Life Stage Applicability

Identify the life stage for which the KE is known to be applicable. More help
Life stage Evidence
All life stages

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) can be selected. 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
Term Scientific Term Evidence Link
mouse Mus musculus High NCBI
human Homo sapiens High NCBI

Sex Applicability

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

Overall Assessment of the AOP

This section addresses the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and WoE for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). The goal of the overall assessment is to provide a high level synthesis and overview of the relative confidence in the AOP and where the significant gaps or weaknesses are (if they exist). Users or readers can drill down into the finer details captured in the KE and KER descriptions, and/or associated summary tables, as appropriate to their needs.Assessment of the AOP is organised into a number of steps. Guidance on pages 59-62 of the User Handbook is available to facilitate assignment of categories of high, moderate, or low confidence for each consideration. While it is not necessary to repeat lengthy text that appears elsewhere in the AOP description (or related KE and KER descriptions), a brief explanation or rationale for the selection of high, moderate, or low confidence should be made. More help

1. Concordance of dose-response relationships

While no specific citations were found, an examination of the experimental data for selected compounds (e.g. 1-chloro-2,4-dinitrobenzene) reveals general agreement among the dose-response relationships both within and between intermediate endpoints (see Annex 1[1]). With exceptions, there is agreement between sensitisers initiated by covalent binding to proteins and non-sensitisers tested in mice, guinea-pigs, and humans; this is especially the case for extreme and strong sensitisers but lesser so for weak and non-sensitisers. One problem is that earlier results, especially with the guinea-pig, were not dose response experiments. Chemical reactivity data show very good concordance of dose-response relationships regardless of the method. In general, available data from in vitro assays are fragmentary and often qualitative (i.e., yes/no).

2. Temporal concordance among the key events and adverse effect;

There is good agreement between the sequences of biochemical and physiological events leading to skin sensitisation (see[2];[3];[4];[5];[6];[7]).

3. Strength, consistency, and specificity of association of adverse effect and initiating event

There is excellent strength, as well as good consistency and high specificity, of the association between in vivo skin sensitisation and in chemico protein binding. This is especially true for reactions that have thiol as the preferred molecular target. Based on linear regression analyses, there is excellent interlaboratory/protocol correlations within and between nucleophile depletion and adduct formation methods[8].

4. Biological plausibility, coherence, and consistency of the experimental evidence

The in chemico, in vitro, and in vivo experimental evidence is logical and consistent with the mechanistic plausibility proposed by covalent reactions based on the protein binding theory ([2];[3];[7]). In selected cases, (e.g. 1-chloro-2,4-dinitrobenzene) where the same compound has been examined in a variety of assays (see Annex 1[1]), the coherence and consistency of the experimental data is excellent.

5. Uncertainties, inconsistencies and data gaps.

Uncertainties include the structural and physicochemical cut-offs between theoretical and measured reactivity[8], the significance of the preferred amino acid target (e.g., cysteine versus lysine)[9], the significance of Th1 or type 1 (IFN-γ) versus Th2 or type 2 (IL-2, IL-4, IL-13) cytokine secretion profiles[10], and sensitisation measurements in different in vivo models.

Inconsistencies within the reported data are seen. There are differences between in vitro responses for highly similar chemicals (see[11];[12]). There are differences within and between in vivo test results for highly similar chemicals (see Annex C[13]). Highly hydrophobic chemicals, which are in vivo sensitisers, are not active in aquatic-based in chemico or in vitro assays. The specific nature of the relationship between irritation and sensitisation has yet to be elucidated.

Data gaps: Based on the more than 50 chemical reactions associated with covalent binding to thiol or primary amine moieties[9] in vitro data for keratinocytes, dendritic cells, and T-cell assays, as well as in vivo sensitisation data, is incomplete in that it does not cover the chemical spaces associated with many of these chemical reactions; in chemico data is also incomplete, especially for reactions that favour amino acid targets other than cysteine.

Domain of Applicability

The relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context are defined in this section. Biological domain of applicability is informed by the “Description” and “Biological Domain of Applicability” sections of each KE and KER description (see sections 2G and 3E for details). In essence the taxa/life-stage/sex applicability is defined based on the groups of organisms for which the measurements represented by the KEs can feasibly be measured and the functional and regulatory relationships represented by the KERs are operative.The relevant biological domain of applicability of the AOP as a whole will nearly always be defined based on the most narrowly restricted of its KEs and KERs. For example, if most of the KEs apply to either sex, but one is relevant to females only, the biological domain of applicability of the AOP as a whole would be limited to females. While much of the detail defining the domain of applicability may be found in the individual KE and KER descriptions, the rationale for defining the relevant biological domain of applicability of the overall AOP should be briefly summarised on the AOP page. More help

The molecular initating event of the present AOP is the hapten-protein binding. While covalent reactions with thiol groups and to lesser extent amino groups, are clearly supported by the proposed AOP, reactions targeting other nucleophiles may or may not be supported by the proposed AOP. Limited data on chemical reactivity shows that two competing reactions are possible, the faster reaction dominates. However, this has yet to be proven in vitro or in vivo.

Essentiality of the Key Events

An important aspect of assessing an AOP is evaluating the essentiality of its KEs. The essentiality of KEs can only be assessed relative to the impact of manipulation of a given KE (e.g., experimentally blocking or exacerbating the event) on the downstream sequence of KEs defined for the AOP. Consequently evidence supporting essentiality is assembled on the AOP page, rather than on the independent KE pages that are meant to stand-alone as modular units without reference to other KEs in the sequence.The nature of experimental evidence that is relevant to assessing essentiality relates to the impact on downstream KEs and the AO if upstream KEs are prevented or modified. This includes: Direct evidence: directly measured experimental support that blocking or preventing a KE prevents or impacts downstream KEs in the pathway in the expected fashion. Indirect evidence: evidence that modulation or attenuation in the magnitude of impact on a specific KE (increased effect or decreased effect) is associated with corresponding changes (increases or decreases) in the magnitude or frequency of one or more downstream KEs.When assembling the support for essentiality of the KEs, authors should organise relevant data in a tabular format. The objective is to summarise briefly the nature and numbers of investigations in which the essentiality of KEs has been experimentally explored either directly or indirectly. See pages 50-51 in the User Handbook for further definitions and clarifications.  More help

Since the 1930’s, there has been growing evidence that the main potency-determining step in skin sensitisation of industrial organic compounds is the formation of a stable hapten-protein conjugate (see[2];[3];[37]). Consequently, the molecular initiating event leading to skin sensitisation is postulated in this AOP to be covalent binding of electrophilic chemical species with selected nucleophilic molecular sites of action in skin proteins ([2];[3]). Protein binding reactions are a means of identifying different chemical structures associated with skin sensitisation, which may or may not lead to different expressions in other key events along the AOP.

Support for Essentiality of KEs Defining Question High (Strong) Moderate Low (Weak)
Are downstream KEs and/or the AO prevented if an upstream KE is blocked? Direct evidence from experimental studies illustrating essentiality for at least one of the important KEs. Indirect evidence that sufficient modification of an expected modulating factor attenuates or augments a KE. No or contradictory experimental evidence of the essentiality of any of the KEs.
KE1: Keratinocytes activation Strong When production of IL-1β and IL-18 from keratinocytes was inhibited, it resulted in impaired DC migration[29];[30];[19].
KE2: Dendritic cells activation Strong A study performed in mice showed than when both Langerhans cells and Langerin+ dermal dendritic cells are depleted using DTR KI- mice (in which diphtheria toxin receptor is inserted into the Langerin locus) and subsequently administration of diphtheria toxin (allowing Langerin+ cells to be ablated), the contact hypersensitivity response is abrogated. In contrast, in the bacterial artificial chromosome (BAC)-transgenic mice (in which the diphtheria toxin subunit A (DTA) is cloned into the human Langerin locus, resulting in mice devoid of Langerhans cells) that lack only epidermal Langerhans cells but have normal number of dendritic cells, the contact hypersensitivity is unaffected[38].

Kim et al (2013) showed that exposition of murine dendritic cells to bisabolangelone (inhibitor of dendritic cell functions) attenuated the production of pro-inflammatory cytokines including IL-12, IL-1β, and TNF-alpha, migration to macrophage inflammatory protein-3 beta, and all-T cell activating ability of dendritic cells[39].

KE3: T-cells, activation and proliferation: Strong The use of ACY-1215, an histone deacetylase, prevented the development of contact hypersensitivity in mice in vivo by modulating CD8 T-cell activation and functions[40].

Another study showed that trichomide A exerts immunosuppressive activity against activated T lymphocytes and in an in vivo experiment they demonstrated that trichlomide A significantly ameliorate picryl chloride (PCI)-induced contact hypersensitivity in mice[41].

Evidence Assessment

The biological plausibility, empirical support, and quantitative understanding from each KER in an AOP are assessed together.  Biological plausibility of each of the KERs in the AOP is the most influential consideration in assessing WoE or degree of confidence in an overall hypothesised AOP for potential regulatory application (Meek et al., 2014; 2014a). Empirical support entails consideration of experimental data in terms of the associations between KEs – namely dose-response concordance and temporal relationships between and across multiple KEs. It is examined most often in studies of dose-response/incidence and temporal relationships for stressors that impact the pathway. While less influential than biological plausibility of the KERs and essentiality of the KEs, empirical support can increase confidence in the relationships included in an AOP. For clarification on how to rate the given empirical support for a KER, as well as examples, see pages 53- 55 of the User Handbook.  More help
Support for Biological Plausibility of KERs Defining Question High (Strong) Moderate Low (Weak)
Is there a mechanistic relationship between KEup and KEdown consistent with established biological knowledge? Extensive understanding of the KER based on previous documentation and broad acceptance. KER is plausible based on analogy to,accepted biological relationships, but scientific understanding is incomplete. Empirical support for association between KEs, but the structural or functional relationship between them is not understood.
MIE => KE1: Strong It is well accepted and experimentally proved that upon hapten application, keratinocytes are activated and produce various chemical mediators (e.g. TNFa, IL-1β, and prostaglandin E2) [14];[15].
MIE => KE2: Strong It is accepted and experimentally proved that during skin sensitisation process,immature epidermal and dermal dendritic cells recognize and internalize the hapten-protein complex formed during covalent binding and subsequently mature and migrate to the local lymph nodes. [16];[17];[18].
KE1 => KE2: Moderate Keratinocyte response activates multiple events, including the release of pro-inflammatory cytokines (e.g. IL-18) and the induction of cyto-protective cellular pathways. Under the influence of fibroblast- blood endothelial- and lymph endothelial-chemokines (e.g. CCL19, CCL21) and epidermal cytokines (e.g. IL-1α, IL-1β, IL-18, tumour necrosis factor alpha (TNFα)) maturing dendritic cells migrate from the epidermis to the dermis of the skin and then to the proximal lymph nodes. [19];[20].
KE2 => KE3: Strong It is well accepted and experimentally proved that in the local lymph node, maturedendritic cells present the hapten-protein complex to T-cells via a majorhistocompatibility complex molecule (MHC)[20];[19].

T-cells are typically affected by protein-hapten complexes presented by dendritic cells on MHC molecules. The T-cell will be then activated to form a memory T-cell, which subsequently proliferates[4].

KE3 => AO: Strong It is well known, recognised and experimentally proved that skin sensitisation is a T-cell mediated immune response. [4]
MIE => AO: Strong Haptenation is widely accepted as molecular initiating event for skin sensitisation. In the form of a modified protein [21], the haptenation provides a source of antigen recognised by the immune system as non-self[22];[23];[24].
Empirical Support for KERs Defining Question High (Strong) Moderate Low (Weak)
Does empirical evidence support that a change in KEup leads to an appropriate change in KEdown? Does KEup occur at lower doses, earlier time points, and higher in incidence than KEdown ? Inconsistencies? Multiple studies showing dependent change in both events following exposure to a wide range of specific stressors. No or few critical data gaps or conflicting data. Demonstrated dependent change in both events following exposure to a small number of stressors. Some inconsistencies with expected pattern that can be explained by various factors. Limited or no studies reporting dependent change in both events following exposure to a specific stressor; and/or significant inconsistencies in empirical support across taxa and species
MIE => KE1: Strong Using a series of thiol-reactive cages fluorescent haptens (i.e. bromobimanes) deployed in combination with two photon fluorescence microscopy, immunohistochemistry, and proteomics, Simonson et al. (2011) identified the possible hapten targets in proteins in human skin. Key target found were the basal keratinocytes and the keratins K5 and K14[25].

In a review about murine contact sensitivity, Honda et al.[14] reported that haptens can activate keratinocytes in an NLR-dependent manner. Among the NLR family, NLRP3 controls the production of proinflammatory cytokines through activation of caspase-1. Without NLRP3 or its adaptor protein ASC[26];[27];[28], the production of IL-1β and IL-18 from keratinocytes was inhibited[29];[30];[19].

MIE => KE2: Strong Using an flow-cytometric assay, the influence of contact sensitisers on endocytic mechanisms in murine Langerhans cells was measured. Epidermal cell suspensions were labelled with a monoclonal antibody directed to MHC class II molecules and pH-sensitive fluorochrome-coupled second step reagents. Study reported that stimulation with well-known sensitising compounds resulted in a partial conservation of the fluorescence intensity due to the internalisation of the labelled complexes into less acidic compartments. For untreated Langerhans cells or in the presence of irritants a significant quenching of fluorescence intensity due to the internalization of the MHC-antibody complexes into acidic compartments was noticed[31].

In the h-CLAT assay measuring the expression of CD86 and CD54 protein markers on the surface of the human monocytic leukemia cell line THP-1, the cell exposure to known non sensitisers does not increase cell biomarker expression. On the contrary, exposure to well-known sensitisers leads to an increase of the CD86 and CD54 expression[32];[33].

KE1 => KE2: Moderate Matjeka et al. (2012) exposed HaCaT cell line used as a model of human keratinocytes to skin sensitisers for one hour and then, after washed off, cocultured them with dendritic cells. Data showed that exposure of dendritic cells to chemically treated HaCaT cells led to the activation of dendritic cells measured by CD83 and CD86 upregulation[34].
KE2 => KE3: Strong A recent study showed in mice model that dendritic cells coordinate the interactions that are necessary to initiate polyclonal regulatory T cells proliferation[35].
KE3 => AO: Strong Using dinitrofluorobenzene and mice models, it was shown that cutaneous contact with reactive antigen induces KC/CXC chemokine ligand 1 production and neutrophil infiltration in an antigen, dose-dependent manner. The intensity of neutrophil infiltration into cutaneous antigen challenge sites, in turn, controls the number of antigen-primed T cells recruited into the site and the magnitude of immune response elicited[36].

Quantitative Understanding

Some proof of concept examples to address the WoE considerations for AOPs quantitatively have recently been developed, based on the rank ordering of the relevant Bradford Hill considerations (i.e., biological plausibility, essentiality and empirical support) (Becker et al., 2017; Becker et al, 2015; Collier et al., 2016). Suggested quantitation of the various elements is expert derived, without collective consideration currently of appropriate reporting templates or formal expert engagement. Though not essential, developers may wish to assign comparative quantitative values to the extent of the supporting data based on the three critical Bradford Hill considerations for AOPs, as a basis to contribute to collective experience.Specific attention is also given to how precisely and accurately one can potentially predict an impact on KEdownstream based on some measurement of KEupstream. This is captured in the form of quantitative understanding calls for each KER. See pages 55-56 of the User Handbook for a review of quantitative understanding for KER's. More help

The final aspect of the OECD approach to using the AOP concept is an assessment of the quantitative understanding of an AOP. This includes the evaluation of the experimental data and models used to quantify the molecular initiating event and other key events. It also includes transparent determination of thresholds and response-to-response relationships used to scale in chemico and in vitro effects to in vivo outcomes. For skin sensitisation, a major hurdle is moving from a qualitative AOP to a quantitative AOP. While the assessment of the experimental evidence, empirical data and confidence in the AOP expressed by the Weight-of-Evidence clearly supports the qualitative AOP as a means to identify and characterize the potential for a chemical to be a sensitiser, these same assessments clearly reveal the current lack of ability to consistently predict relative potency. One aspect to be resolved is that of the in vivo data with which to scale the response-to-response ratios. Because the Local Lymph Node Assay (LLNA) can directly quantify the adverse outcome[42], public databases have recently been made available ([43];[44]). LLNA results are often compared with results from alternative methods (e.g.[33]). Such one-to-one comparisons may not be the best approach. As noted by Basketter et al.[42], the LLNA is not without limitations, including variability between EC3 values or any other value (i.e. ECx) within mechanistic classes with equal or near equal chemical reactivity. The specific nature of the in vivo relationship between irritation and sensitisation has yet to be elucidated.

Considerations for Potential Applications of the AOP (optional)

At their discretion, the developer may include in this section discussion of the potential applications of an AOP to support regulatory decision-making. This may include, for example, possible utility for test guideline development or refinement, development of integrated testing and assessment approaches, development of (Q)SARs / or chemical profilers to facilitate the grouping of chemicals for subsequent read-across, screening level hazard assessments or even risk assessment. While it is challenging to foresee all potential regulatory application of AOPs and any application will ultimately lie within the purview of regulatory agencies, potential applications may be apparent as the AOP is being developed, particularly if it was initiated with a particular application in mind. This optional section is intended to provide the developer with an opportunity to suggest potential regulatory applications and describe his or her rationale.To edit the “Considerations for Potential Applications of the AOP” section, on an AOP page, in the upper right hand menu, click ‘Edit.’ This brings you to a page entitled, “Editing AOP.” Scroll down to the “Considerations for Potential Applications of the AOP” section, where a text entry box allows you to submit text. In the upper right hand menu, click ‘Update AOP’ to save your changes and return to the AOP page or 'Update and continue' to continue editing AOP text sections.  The new text should appear under the “Considerations for Potential Applications of the AOP” section on the AOP page. More help

This AOP study[45] describing mechanistic knowledge has supported the development of a number of methods for assessing chemical sensitisation hazard potential or potency without the need for animal testing by measuring the impact of chemical sensitisers on the identified key events[46];[47]. This AOP also forms the mechanistic basis for the development of Integrated Approaches to Testing and Assessment (IATA)[48];[49]. Additionally, data-driven approaches for predicting sensitizer potency also have been developed[50];[51];[52].


List the bibliographic references to original papers, books or other documents used to support the AOP. More help
  1. 1.0 1.1 OECD 2012. The Adverse Outcome Pathway for skin sensitisation initiated by covalent binding to proteins. Part 2: use of the AOP to develop chemical categories and integrated assessment and testing approaches. OECD Environment Directorate Joint Meeting of the Chemicals Committee and the Working Party on chemicals, pesticides and biotechnology. ENV/JM/MONO(2012)10/PART2.
  2. 2.0 2.1 2.2 2.3 Gerberick F, Aleksic M, Basketter D, Casati S, Karlberg AT, Kern P, Kimber I, Lepoittevin JP, Natsch A, Ovigne JM, Rovida C, Sakaguchi H and Schultz T. 2008. Chemical reactivity measurement and the predictive identification of skin sensitisers. Altern. Lab. Anim.36: 215-242.
  3. 3.0 3.1 3.2 3.3 Karlberg A-T, Bergström MA, Börje A, Luthman K and Nilsson JL. 2008. Allergic contact dermatitis- formation, structural requirements, and reactivity of skin sensitizers. Chem. Res. Toxicol. 21: 53-69.
  4. 4.0 4.1 4.2 Vocanson M, Hennino A, Rozieres A, Poyet G, Nicolas JF 2009. Effector and regulatory mechanisms in allergic contact dermatitis. Allergy 64: 1699-1714.
  5. Aeby P, Ashikaga T, Bessou-Touya S, Schapky A, Geberick F, Kern P, Marrec-Fairley M, Maxwell G, Ovigne JM, Sakaguchi H, Reisinger K, Tailhardat M, Martinozzi-Teisser S and Winkler P. 2010. Identifying and characterizing chemical skin sensitizers without animal testing; Colipa’s research and methods development program. Toxicol. In Vitro 24: 1465-1473.
  6. Basketter DA and Kimber I. 2010. Contact hypersensitivity. In: McQueen, C.A. (ed) Comparative Toxicology Vol. 5, 2nd Ed. Elsevier, Kidlington, UK, pp. 397-411.
  7. 7.0 7.1 Adler S, Basketter D, Creton S, Pelkonen O, van Benthem J, Zuang V, Andersen KE, Angers-Loustau A, Aptula A, Bal-Price A, Benfenati E, Bernauer U, Bessems J, Bois FY, Boobis A, Brandon E, Bremer S, Broschard T, Casati S, Coecke S, Corvi R, Cronin M, Daston G, Dekant W, Felter S, Grignard E, Gundert-Remy U, Heinonen T, Kimber I, Kleinjans J, Komulainen H, Kreiling R, Kreysa J, Leite SB, Loizou G, Maxwell G, Mazzatorta P, Munn S, Pfuhler S, Phrakonkham P, Piersma A, Poth A, Prieto P, Repetto G, Rogiers V, Schoeters G, Schwarz M, Serafimova R, Tähti H, Testai E, van Delft J, van Loveren H, Vinken M, Worth A, Zaldivar JM. Alternative (non-animal) methods for cosmetics testing: current status and future prospects-2010.2011. Arch Toxicol.85(5):367-485.
  8. 8.0 8.1 Schwöbel JAH, Koleva YK, Bajot F, Enoch SJ, Hewitt M, Madden JC, Roberts DW, Schultz TW and Cronin MTD. 2011. Measurement and estimation of electrophilic reactivity for predictive toxicology. Chem. Rev. 111: 2562-2596.
  9. 9.0 9.1 OECD 2011. Report of the Expert Consultation on Scientific and Regulatory Evaluation of Organic Chemistry-based Structural Alerts for the Identification of Protein-binding Chemicals. OECD Environment, Health and Safety Publications Series on Testing and Assessment No. 139. ENV/JM/MONO(2011)9.
  10. Hopkins JE, Naisbitt DJ, Kitteringham NR, Dearman RJ, Kimber I, Park BK. 2005. Selective haptenation of cellular or extracellular proteins by chemical allergens: Association with cytokine polarization. Chem. Res. Toxicol. 18: 375-381.
  11. 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.
  12. 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.
  13. European Centre for Ecotoxicological and Toxicological Chemicals. 2010. High information content technologies in support of read-across in chemical risk assessment. Technical report No109. p87.
  14. 14.0 14.1 Honda T, Egawa G, Grabbe S, Kabashima K. 2013. Update of immune events in the murine contact hypersensitivity model: toward the understanding of allergic contact dermatitis. J. Invest. Dermatol. 133: 303-315.
  15. Erkes DA, Selvan RS. 2014. Hapten-induced contact hypersensitivity, autoimmune reactions, and tumour regression: plausibility of mediating antitumor immunity. J. Immunol. Res. Article ID 175265
  16. Ryan CA, Gerberick GF, Gildea LA, Hulette BC, Bettis CJ, Cumberbatch M, Dearman RJ, Kimber I. 2005. Interactions of contact allergens with dendritic cells: opportunities and challenges for the development of novel approaches to hazard assessment. Toxicol. Sci. 88: 4-11.
  17. Ryan CA, Kimber I, Basketter DA, Pallardy M, Gildea LA, Gerberick GF. 2007. Dendritic cells and skin sensitisation. Biological roles and uses in hazard identification. Toxicol. Appl. Pharmacol. 221: 384-394.
  18. Kimber I, Basketter DA, Gerberick GF, Ryan CA, Dearman RJ. 2011. Chemical allergy: Translating biology into hazard characterization. Toxicol. Sci. 120(S1): S238-S268
  19. 19.0 19.1 19.2 19.3 Antonopoulos C, Cumberbatch M, Mee JB, Dearman RJ, Wei XQ, Liew FY, Kimber I, Groves RW. 2008. IL-18 is a key proximal mediator of contact hypersensitivity and allergen induced Langerhans cell migration in murine epidermis. J. Leukoc. Biol. 83: 361-367.
  20. 20.0 20.1 Ouwehand K, Santegoets SJAM, Bruynzeel DP, Scheper RJ, de Gruijl TD, Gibbs S. 2008. CXCL12 is essential for migration of activated Langerhans cells for epidermis to dermis. Eur. J. Immunol. 38: 3050-3059
  21. Lepoittevin JP, Basketter DA, Goossens A, et al. 2011. Allergic contact dermatitis: the molecular basis. Berlin, Germany: Springer.
  22. Martin S, Weltzien HU. 1994. T cell recognition of haptens, a molecular view. Int. Arch. Allergy Immunol. 104: 10-16.
  23. Weltzien HU, Moulon C, Martin S, et al. 1996. T cell immune responses to haptens. Structural models for allergic and autoimmune reactions. Toxicology 107: 141-151.
  24. MacKay C, Davies M, Summerfield V, Maxwell G. 2013. From pathways to people: applying the adverse outcome pathway (AOP) for skin sensitization to risk assessment. ALTEX 30 (4/13):473-486
  25. Simonsson C, Andersson SI, Stenfeldt AL, Bergstrom J, Bauer B, Jonsson CA, Ericson MB, Broo KS. 2011. Caged fluorescent haptens reveal the generation of cryptic epitopes in allergic contact dermatitis. J.Invest. Immunol. 131: 1486-1493.
  26. Sutterwala FS, Ogura Y, Szczepanik M, et al. 2006. Critical role for NALP3/CIAS1/Cryopyrin in innate and adaptive immunity through its regulation of caspase-1. Immunity 24:317-327.
  27. Watanabe H, Gaide O, Petrilli V, et al. 2007. Activation of the IL-1beta-processing inflammasone is involved in contact hypersensitivity. J.Invest. Dermatol. 127:1956-1963.
  28. Watanabe H, Gehrke S, Contassot E, et al. 2008. Danger signalling through the inflammasone acts as a master switch between tolerance and sensitization. J. Immunol. 180:5826-5832.
  29. 29.0 29.1 Antonopoulos C, Cumberbatch M, Dearman RJ, Daniel RJ, Kimber I, Groves RW. 2001. Functional caspase-1 is required for Langerhans cell migration and optimal contact sensitization in mice. J. Immunol. 166: 3672-3677.
  30. 30.0 30.1 Nakae S, Komiyama Y, Narumi S, Sudo K, Horai R, Tagawa Y, Matsushima K, Asano M, Iwakura Y. 2003. IL-1-induced tumor necrosis factor- elicits inflammatory cell infiltration in the skin by inducing IFN-γ-inducible protein 10 in the elicitation phase of the contact hypersensitivity response. Int. Immunol. 15(2): 251-260.
  31. Lempertz U, Kühn U, Knop J and Becker D. 1996. An approach to predictive testing of contact sensitizers in vitro by monitoring their influence on endocytic mechanisms. Internat. Arch. Allergy Immunol. 111: 64-70.
  32. Sakaguchi H, Ashikaga T, Miyazawa M, Kosaka N, Ito Y, Yoneyama K, Sono S, Itagaki H, Toyoda H, Suzuki H. 2009. The relationship between CD86/CD54 expression and THP-1 cell viability in an in vitro skin sensitisation test-human cell line activation test (h-CLAT). Cell Biol. Toxicol. 25: 109-126.
  33. 33.0 33.1 Ashikaga T, Sakaguchi H, Sono S, Kosaka N, Ishikawa M, Nukada Y, Miyazawa M, Ito Y, Nishiyama N, Itagaki H. 2010. A comparative evaluation of in vitro skin sensitisation tests: the human cell-line activation test (h-CLAT) versus the local lymph node assay (LLNA). Altern. Lab. Anim. 38:275-84.
  34. Matjeka T, Summerfield V, Noursadeghi M, Chain BM. 2012. Chemical toxicity to keratinocytes triggers dendritic cell activation via an IL-1 path. J. Allergy Clin. Immunol. Letters to the editor:247-205.
  35. Zou T, Caton AJ, Koretzky GA, Kambayashi T. 2010. Dendritic cells induce regulatory T cell proliferation through antigen-dependent and –independent interactions. J. Immunol. 185:2790-2799.
  36. Engeman T, Gorbachev AV, Kish DD, Fairchild RL. 2004. The intensity of neutrophil infiltration controls the number of antigen-primed CD8 T cells recruited into cutaneous antigen challenge sites. J. Leukocyte Biol. 76:941-949.
  37. Roberts DW, Aptula AO, Patlewicz G, Pease C. 2008. Chemical reactivity indices and mechanism-based read-across for non-animal based assessment of skin sensitisation potential. J. Appl. Toxicol. 28: 443-454.
  38. Christensen AD, Haase C. 2011. Immunological mechanisms of contact hypersensitivity in mice. APMIS 120: 1-27.
  39. Kim HS, Lee YJ, Lee HK, Kim JS, Park Y, Kang JS, Hwang BY, Hong JT, Kim Y, Han SB. 2013. Bisabolangelone inhibits dendritic cells functions by blocking MAPK and NF-ƙB signaling. Food Chem. Tox 59: 26-33.
  40. Tsuji G, Okiyama N, Villaroel VA, Katz S. 2015. Histone deacetylase 6 inhibition impairs effector CD8 T-cell functions during skin inflammation. J. Allergy Clin. Immunol. 135(5): 1228-1239.
  41. Wang X, Zhang A, Gao J, Chen W, Wang S, Wu X, Shen Y, Ke Y, Hua Z, Tan R, Sun Y, Xu Q. 2014. Trichomide A, a natural cyclodepsipeptide, exerts immunosuppressive activity against activated T lymphocytes by upregulating SHP2 activation to overcome contact dermatitis. J. Invest. Dermatol. 134: 2737-2746.
  42. 42.0 42.1 Basketter DA, McFadden JF, Gerberick F, Cochshott A, Kimber I. 2009. Nothing is perfect, not even the local lymph node assay: a commentary and the implications for REACH. Contact Dermatitis 60: 65-69.
  43. Gerberick GF, Ryan, CA, Kern, PS, Schlatter H, Dearman RJ, Kimber I, Patlewicz GY, Basketter DA. 2005. Compilation of historical lymph node data for evaluation of skin sensitisation alternative methods. Dermatitis 16: 157-202.
  44. Kern PS, Gerberick GF, Ryan CA, Kimber I, Aptula A, Basketter BA. 2010. Local lymph node data for the evaluation of skin sensitisation alternatives: A second compilation. Dermatitis 21: 8-32.
  45. MacKay C, Davies M, Summerfield V, Maxwell G. (2013). From pathways to people: applying the adverse outcome pathway (AOP) for skin sensitization to risk assessment. ALTEX.30(4):473-86.
  46. Adler S, Basketter D, Creton S, et al. 2011. Alternative (non-animal) methods for cosmetics testing: current status and future prospects – 2010. Arch. Toxicol. 85, 367-485.
  47. Maxwell G, Aeby P, Ashikaga T, et al. 2011. Skin sensitisation: the Colipa strategy for developing and evaluating non-animal test methods for risk assessment. Altex 28, 50–55.
  48. Tollefsen KE, Scholz S, Cronin MT, Edwards SW, de Knecht J, Crofton K, Garcia-Reyero N, Hartung T, Worth A, Patlewicz G. 2014. Applying Adverse Outcome Pathways (AOPs) to support Integrated Approaches to Testing and Assessment (IATA). Regul. Toxicol. Pharmacol. 70(3):629-40.
  49. Bauch C, Kolle S N, Ramirez T, et al. 2012. Putting the parts together: combining in vitro methods to test for skin sensitizing potentials. Regul. Toxicol. Pharmacol. 63, 489-504.
  50. Jaworska J, Harol A, Kern PS, et al. 2011. Integrating non-animal test information into an adaptive testing strategy – skin sensitization proof of concept case. ALTEX 28, 211-225.
  51. Jaworska J, Dancik Y, Kern P, Gerberick F, Natsch A. 2013. Bayesian integrated testing strategy to assess skin sensitization potency: from theory to practice. J. Appl. Toxicol. 33(11):1353-64.
  52. Maxwell G, MacKay C, Cubberley R, Davies M, Gellatly N, Glavin S, Gouin T, Jacquoilleot S, Moore C, Pendlington R, Saib O, Sheffield D, Stark R, Summerfield V. 2014. Applying the skin sensitisation adverse outcome pathway (AOP) to quantitative risk assessment. Toxicol. In Vitro. 28(1):8-12.