This AOP is licensed under the BY-SA license. This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.

AOP: 39


A descriptive phrase which references both the Molecular Initiating Event and Adverse Outcome.It should take the form “MIE leading to AO”. For example, “Aromatase inhibition leading to reproductive dysfunction” where Aromatase inhibition is the MIE and reproductive dysfunction the 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 Binding, Protein, leading to Increase, Allergic Respiratory Hypersensitivity Response

Short name
A name that succinctly summarises the information from the title. This name should not exceed 90 characters. More help
Covalent binding to proteins leads to Respiratory Sensitisation/Sensitization/Allergy
The current version of the Developer's Handbook will be automatically populated into the Handbook Version field when a new AOP page is created.Authors have the option to switch to a newer (but not older) Handbook version any time thereafter. More help
Handbook Version v1.0

Graphical Representation

A graphical representation of the AOP.This graphic should list all KEs in sequence, including the MIE (if known) and AO, and the pair-wise relationships (links or KERs) between those KEs. More help
Click to download graphical representation template Explore AOP in a Third Party Tool


The names and affiliations of the individual(s)/organisation(s) that created/developed the AOP. More help

Jessica Ponder, Physicians Committee for Responsible Medicine Kristie Sullivan, Institute for In Vitro Sciences

Stella Cochrane, Unilever

Steven Enoch, Liverpool John Moores University

Janine Ezendam, RIVM

Joanna Matheson & Kent Carlson, US CPSC

Grace Patlewicz, US EPA

Erwin Roggen, 3RsMC ApS

Katherina Sewald, Fraunhofer ITEM

Point of Contact

The user responsible for managing the AOP entry in the AOP-KB and controlling write access to the page by defining the contributors as described in the next section.   More help
Arthur Author   (email point of contact)


Users with write access to the AOP page.  Entries in this field are controlled by the Point of Contact. More help
  • Kristie Sullivan
  • Jessica Ponder
  • Arthur Author


This field is used to identify coaches who supported the development of the AOP.Each coach selected must be a registered author. More help
  • Jason M. O'Brien

OECD Information Table

Provides users with information concerning how actively the AOP page is being developed and whether it is part of the OECD Workplan and has been reviewed and/or endorsed. OECD Project: Assigned upon acceptance onto OECD workplan. This project ID is managed and updated (if needed) by the OECD. OECD Status: For AOPs included on the OECD workplan, ‘OECD status’ tracks the level of review/endorsement of the AOP . This designation is managed and updated by the OECD. Journal-format Article: The OECD is developing co-operation with Scientific Journals for the review and publication of AOPs, via the signature of a Memorandum of Understanding. When the scientific review of an AOP is conducted by these Journals, the journal review panel will review the content of the Wiki. In addition, the Journal may ask the AOP authors to develop a separate manuscript (i.e. Journal Format Article) using a format determined by the Journal for Journal publication. In that case, the journal review panel will be required to review both the Wiki content and the Journal Format Article. The Journal will publish the AOP reviewed through the Journal Format Article. OECD iLibrary published version: OECD iLibrary is the online library of the OECD. The version of the AOP that is published there has been endorsed by the OECD. The purpose of publication on iLibrary is to provide a stable version over time, i.e. the version which has been reviewed and revised based on the outcome of the review. AOPs are viewed as living documents and may continue to evolve on the AOP-Wiki after their OECD endorsement and publication.   More help
OECD Project # OECD Status Reviewer's Reports Journal-format Article OECD iLibrary Published Version
1.20 Under Development
This AOP was last modified on May 26, 2024 20:39

Revision dates for related pages

Page Revision Date/Time
Increase, Allergic Respiratory Hypersensitivity Response February 07, 2023 13:07
Activation, Dendritic Cells December 03, 2020 10:15
Activation/Proliferation, T-cells November 05, 2020 19:14
Covalent Binding, Protein November 05, 2020 18:40
Increased, secretion of proinflammatory mediators May 17, 2023 15:18
Covalent Binding, Protein leads to Increased proinflammatory mediators September 08, 2022 05:24
Covalent Binding, Protein leads to Increase, Allergic Respiratory Hypersensitivity Response August 01, 2022 19:20
Covalent Binding, Protein leads to Activation, Dendritic Cells November 05, 2020 18:09
Increased proinflammatory mediators leads to Activation, Dendritic Cells August 01, 2022 14:06
Activation, Dendritic Cells leads to Activation/Proliferation, T-cells August 22, 2022 13:46
Activation/Proliferation, T-cells leads to Increase, Allergic Respiratory Hypersensitivity Response August 01, 2022 19:20
Toluene diisocyanate August 01, 2022 14:18


A concise and informative summation of the AOP under development that can stand-alone from the AOP page. The aim is to capture the highlights of the AOP and its potential scientific and regulatory relevance. More help

The assessment of xenobiotics for potential to induce an allergenic response in the respiratory tract is of great regulatory and industrial interest. Ongoing work in this area has hypothesized some differences between the dermal and respiratory sensitisation pathways; however in some cases a lack of strong empirical evidence on a variety of chemistries to test these hypothesis. This AOP represents the currently available data with the aim of identifying knowledge gaps which may be filled with directed research. (Sullivan, et al., 2017)

Sensitization of the respiratory tract is an important occupational health challenge. Here we build on a previously published skin sensitization AOP (AOP 40), relying on literature evidence linked to low-molecular-weight organic chemicals and excluding other known respiratory sensitizers acting via different molecular initiating events. The established key events (KEs) are as follows: (1) covalent binding of chemicals to proteins, (2) activation of cellular danger signals (inflammatory cytokines and chemokines and cytoprotective gene pathways), (3) dendritic cell activation and migration, (4) activation, proliferation, and polarization of T cells, and (5) sensitization of the respiratory tract. There is some evidence that respiratory sensitizers bind preferentially to lysine moieties, whereas skin sensitizers bind to both cysteine and lysine, however this observation may be biased by the limited number of chemicals investigated. Furthermore, exposure to respiratory sensitizers seems to result in cell behavior for KEs 2 and 3, as well as the effector T cell response, in general skewing toward cytokine secretions predominantly associated with T helper 2 (Th2) response.

AOP Development Strategy


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. More help

Sensitization of the respiratory tract by chemicals is the first stage in the development of chemical respiratory allergy, an immune-mediated hypersensitivity reaction to an exogenous low-molecular-weight chemical, which can result in asthma and rhinitis on repeated exposure and is an important occupational health problem. (Mapp et al., 2005) Due to the severity and irreversibility of the adverse effect, identifying chemical respiratory allergens is of considerable regulatory, industrial, and socioeconomic importance. (Boverhof et al., 2008) Efforts to outline a framework for assessment of potential respiratory-sensitizing chemicals are underway. (North et al., 2016) Currently, however, there are no standardized, validated, and regulatory-accepted models for detecting these chemicals or discriminating them from skin sensitizers, potentially due to remaining gaps within the literature as to the exact mechanistic steps leading to respiratory allergy. (Kimber et al., 2011)

Another important issue in the development of predictive test methods is the route of exposure in the sensitization phase. Inhalation exposure is perhaps the most common exposure route of concern for many substances, but there is evidence that sensitization of the respiratory tract can be induced via skin exposure as well. (Tarlo and Malo, 2006, Heederik et al., 2012, Redlich and Herrick, 2008, Kimber and Dearman, 2002) This knowledge has implications for both the mechanistic understanding of the pathway and the potential test methods that may be used to detect respiratory sensitizers. Therefore, this AOP will include information from models using skin and lung exposure (in vivo) and with dermal and respiratory cells and tissues (in vitro/ex vivo).

The outlines of this pathway follow the already-published skin sensitization AOP 40. However, the divergent AOs of the two pathways reflect differences in the effector response (T helper 1 [Th1] vs. T helper 2 [Th2]) and other mechanistic details of at least some KEs;(Kimber et al., 2014) these differences are the focus of this effort. Therefore, the primary evidence relied on to build this AOP must relate directly to known low-molecular-weight organic chemicals to the exclusion of chemicals that act via other mechanisms and therefore require a separate AOP, for example, chloroplatinates.

In brief, the AOP can be summarized as beginning with covalent protein binding, potentially preferentially to lysine nucleophiles in the lung or skin after respiratory or dermal exposure to a low-molecular-weight organic chemical. This protein binding causes the activation of stress response pathways and cellular danger signals, including oxidative stress, cytokines, and chemokines released by epithelial and other cells, leading to dendritic cell (DC) maturation and migration to the draining lymph nodes (DLN). Haptens can also contribute to DC activation directly. Th2-skewed DCs in the DLN signal activation and maturation of T cells, which characterize the sensitization phase, resulting in chemical respiratory allergy. Consistent with regulatory practice, sensitization is considered the AO. (Vandebriel et al., 2011)


Provides a description of the approaches to the identification, screening and quality assessment of the data relevant to identification of the key events and key event relationships included in the AOP or AOP network.This information is important as a basis to support the objective/envisaged application of the AOP by the regulatory community and to facilitate the reuse of its components.  Suggested content includes a rationale for and description of the scope and focus of the data search and identification strategy/ies including the nature of preliminary scoping and/or expert input, the overall literature screening strategy and more focused literature surveys to identify additional information (including e.g., key search terms, databases and time period searched, any tools used). 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 prototypical stressor and the biological system) of an AOP. More help
Key Events (KE)
A measurable event within a specific biological level of organisation. More help
Adverse Outcomes (AO)
An AO is a specialized KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help
Type Event ID Title Short name
MIE 396 Covalent Binding, Protein Covalent Binding, Protein
KE 1496 Increased, secretion of proinflammatory mediators Increased proinflammatory mediators
KE 398 Activation, Dendritic Cells Activation, Dendritic Cells
KE 272 Activation/Proliferation, T-cells Activation/Proliferation, T-cells
AO 313 Increase, Allergic Respiratory Hypersensitivity Response Increase, Allergic Respiratory Hypersensitivity Response

Relationships Between Two Key Events (Including MIEs and AOs)

This table summarizes 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. More help

Network View

This network graphic is automatically generated based on the information provided in the MIE(s), KEs, AO(s), KERs and Weight of Evidence (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

Prototypical Stressors

A structured data field that can be used to identify one or more “prototypical” stressors that act through this AOP. Prototypical stressors are stressors for which responses at multiple key events have been well documented. More help

Life Stage Applicability

The life stage for which the AOP is known to be applicable. More help
Life stage Evidence
All life stages Not Specified

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. More help
Term Scientific Term Evidence Link
human Homo sapiens High NCBI
mouse Mus musculus High NCBI

Sex Applicability

The sex for which the AOP is known to be applicable. More help
Sex Evidence
Unspecific Not Specified

Overall Assessment of the AOP

Addressess the relevant biological domain of applicability (i.e., in terms of taxa, sex, life stage, etc.) and Weight of Evidence (WoE) for the overall AOP as a basis to consider appropriate regulatory application (e.g., priority setting, testing strategies or risk assessment). More help

1. Concordance of dose-response relationships

There is fairly sparse evidence wth more than a few respiratory sensitizers that can offer confidence in dose-response concordance. In BALB/c mice, toluene diisocyanate (TDI) applied to the skin led to TDI-haptenated protein (TDI-hp) (skin keratins and albumin) localization in the stratum corneum, hair follicles, and sebaceous glands within 3 hours, with intensity of staining following a dose–response relationship. (Nayak et al., 2014) provides a detailed dose–response analysis of TDI-induced protein binding, colocalization of immune messenger cells, and migration to Delphian lymph nodes.

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

(Nayak et al., 2014) also provides a detailed temporal analysis of TDI-induced protein binding, colocalization of immune messenger cells, and migration to Delphian lymph nodes.

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

There are clear connections from chemicals with certain reactivity and binding profiles to cellular- and individual-level downstream Th2-related effects leading to respiratory sensitization. Though the number of chemicals studied is quite low, consistent patterns are identified. A better understanding of how differences in haptenation by these chemicals contribute to distinct cellular and immune-system-level responses, and how early DC gene changes contribute (or not) to the expression of maturation markers, will help to increase the specificity of the available test methods.

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

Each of the hypothesized KERs is supported by evidence from studies with at least one, and sometimes a few, known respiratory sensitizers. The events fit with what is known in general for sensitization, and the basic KEs outlined here are consistent with established biological knowledge. However, further research is needed to understand, for a larger number of chemicals, the steps leading to a skewing of the effector response toward Th2 and sensitization of the respiratory tract; therefore, the WoE is considered to be ‘‘moderate.’’

5. Alternative mechanisms

Currently, there are about 80 chemicals identified as respiratory allergens. Exposure occurs primarily in occupational settings. AOs are asthma and rhinitis. The biological mechanisms are often Th2 mediated leading to the production of IgE and eosinophilic inflammation. However, this may not always be the case. For example, human studies reveal PPD to be a respiratory sensitizer, (Helaskoski et al., 2014) but it does not cause a Th2 cytokine response in mice. (Rothe et al., 2011) Specific IgE is induced in some subjects, but not in others, particularly for diisocyanate sensitization. Thus, it is unclear whether IgE is mandatory or not.

Notably, it has to be mentioned that for protein-induced respiratory allergy, the clinical understanding of the disease has been changing dramatically during the last years. For many years, asthma has been considered as a single disease with a defined phenotype. It was assumed that the biology of sensitization is based on Th2-mediated IgE production, migration of mast cells, and subsequent eosinophilic infiltration. Nevertheless, clinical studies of cohort revealed that only about 50% of all patients show a Th2-driven eosinophilic inflammation of the airways. It also covers Th17-driven neutrophilic airway inflammation—an asthmatic phenotype that also can be observed with chemical allergens.

Nowadays, asthma is considered as an umbrella disease with multiple heterogeneous phenotypes, depending on the underlying immunology, pathology, symptoms, and the time of elicitation during lifetime. Furthermore, the concept takes other environmental and genetic influences into consideration. The development of animal models reflecting the heterogeneity of asthma phenotypes is still ongoing and shows in particular the (i) irritant properties of the allergen, (ii) the route of exposure during sensitization and elicitation, and (iii) the dose levels of allergen define whether a Th2 or Th17 phenotype develops.

For chemical allergens, less is known about the influence of atopy, viral infections, and indoor and outdoor environmental pollutants such as cigarette smoke. Of interest is the influence of an additional coexposure to irritant if the chemical allergen is present at low dose. Genetic susceptibility is also a variable of interest. (Yucesoy et al., 2012) and (Wisnewski et al., 2008) among others, have determined factors that may affect the potential for a person’s sensitization potential to diisocyanates, including genetic variants in antioxidant defense genes and PRRs.

A number of studies have looked into the sensitization of transition metal complexes, including one which outlines the evidence for these complexes initiating sensitization not through covalent bond formation, but rather through coordination complexes. (Chipinda et al., 2011) The authors provide evidence that these coordination complexes are not stable enough to survive the antigen processing that a covalent hapten undergoes. Instead an alternative MIE is outlined in which these complexes bind to cell surface proteins like MHC, bypassing the intracellular antigen process. This initiating event fits in with the observed cross-reactivity that appears to transcend the trends one would expect based on the periodic table (for example, complexes of Cr, a group 6 metal, cross sensitizing with complexes of Co, a group 9 metal). (Templeton, 2004) It is thought that the surface protein chelates the metal complex and presents it to T-cells directly, requiring a separate AOP from chemicals acting via covalent binding to proteins.

6. Uncertainties, inconsistencies and data gaps.

A better understanding of how differences in haptenation by these chemicals contribute to distinct cellular responses, and how early DC gene changes contribute (or not) to the expression of maturation markers, will help to increase the specificity of the available test methods. A better understanding of human response and population variability is also needed, along with a better quantitative understanding of the linkages between KEs. Additional studies using human cells and tissues are recommended.

Furthermore, as noted in the evaluation section, efforts to fully understand this pathway and develop toxicological test methods and strategies are hampered by a spare data portfolio, as well as a lack of a robust set of harmonized reference chemicals clearly identified as respiratory sensitizers. Previous authors have gathered preliminary chemical sets with supporting rationale, and collating this information and building a set of harmonized reference chemicals, which can be used to optimize and characterize potential test methods or strategies, are the clear next steps. (Enoch et al., 2010, Cochrane et al., 2015, Enoch et al., 2009)

Domain of Applicability

Addressess the relevant biological domain(s) of applicability in terms of sex, life-stage, taxa, and other aspects of biological context. More help

Sensitizers which do not fit into this AOP:

There have been a number of studies into the sensitisation (and toxicity) of transition metal complexes; key amongst these is a recent study outlining the evidence for these complexes initiating sensitisation via the formation of co-ordination complexes rather than covalent bond formation. (Chipinda et al., 2011) The authors of this study present the evidence that these co-ordination complexes are not stable enough to survive the antigen processing that a covalent hapten undergoes, thus cannot sensitise via this MIE. Instead an alternative MIE is outlined in which these complexes bind to cell surface proteins like MHC, bypassing the intracellular antigen process. This MIE fits in with the observed cross-reactivity that appears to transcend the trends one would expect based on the periodic table (for example, complexes of Cr, a group 6 metal, cross sensitising with complexes of Co, a group 9 metal). (Templeton, 2004) It is thought that the surface protein chelates the metal complex and presents it to T-cells directly. Therefore, transition metals would require a separate AOP from chemicals acting via covalent binding to proteins.

Essentiality of the Key Events

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. More help

Support for Essentiality of KEs

Defining Question

High (Strong)


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.

MIE (KE1): 

Covalent Binding, Protein


There is extensive evidence in the literature for haptenation being the MIE for respiratory sensitization. In general, haptenation can be divided into five types of chemistry, so-called mechanistic domains. These being acylation, aliphatic nucleophilic substitution (SN1/SN2), aromatic nucleophilic substitution (SNAr), Michael addition, and Schiff base formation. (Enoch et al., 2011, Aptula et al., 2005, Aptula and Roberts, 2006, Lalko et al., 2011, Landsteiner and Jacobs, 1935, Landsteiner and Jacobs, 1936, Hopkins et al., 2005)


Activation of Inflammatory Signaling


Strong evidence exists for the essential nature of cellular danger signals in respiratory sensitization. (Silva et al., 2014) found that HDI increased ROS by inhibiting superoxide dismutase (SOD1) in THP-1 cells. This inhibition may further encourage a redox environment via matrix metalloproteinase (MMP reduction). Increased ROS also led to extracellular signal-related kinase (ERK) signaling pathway phosphorylation and the transcription of cytoprotective and maturation pathways (HMOX1 and CD83). Coincubation with the antioxidant n-acetyl cysteine and SOD decreased ERK phosphorylation.


Dendritic  cells activation


Some evidence indicates that IL-10, upregulated by TMA, may block the migration of LC for a short period of time to allow a Th2 phenotype to develop.  Increased IL-4 and IL-10 were detected in the draining lymph nodes of mice after TMA exposure, and DC migration to the DLN was confirmed. Anti-IL-10 antibody ameliorated this response to TMA. (Holden et al., 2008, Cumberbatch et al. 2005)


T-cells, activation and proliferation


In humans, support for the Th2-skewing being associated with sensitization of the respiratory tract rather than the skin comes from studying the responses of individuals who already have an immune response skewed in one direction or the other. (Holden et al., 2008, Newell et al., 2013, Ouyang et al., 2013)


Evidence Assessment

Addressess the biological plausibility, empirical support, and quantitative understanding from each KER in an AOP. More help

Support for Biological Plausibility of KERs

Defining Question

High (Strong)


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 => KE2: 

Covalent Binding, Protein leads to Activation of Inflammatory Signaling


It has been demonstrated with lung cell lines that exposure to haptenated human serum albumin increases reactive oxygen species. (Hur et al., 2009) It is well accepted and experimentally proved in lung cell lines, 3D human airway epithelial cell cultures, and human lung slices that exposure to haptens induces proinflammatory cytokine and chemokine (e.g. IL-1α, TNF-α, IL-6, IL-8, CCL2, CXCL1, CCL5, etc.) release. (Huang et al., 2013, Lauenstein et al., 2014, Verstraelen et al., 2009)

MIE => KE3: 

Covalent Binding, Protein leads to Dendritic Cells Activation


It is well-accepted and experimentally proven that dendritic cells represent the most important antigen-presenting cells in the lung. Immature DCs are distributed above and beneath the basal membrane of the lung epithelium and sample antigens by extending dendrites into the airway lumen. Immature cells mature after encountering antigen, an essential event in the activation of immune response. (Lambrecht and Hammad, 2010, Lambrecht and Hammad, 2003, Lambrecht and Hammad, 2009, Holt et al., 1994)

KE2 => KE3: 

Activation of Inflammatory Signaling leads to Dendritic Cells Activation


DCs express receptors for, and respond to, constitutive and inflammatory chemokines and other chemoattractants, such as platelet-activating factor and formyl peptides. Much investigation has gone into assessing the specific mechanistic events involved in skin sensitizer-caused DC migration. Ex vivo studies with intact human skin, epidermal sheets, and MUTZ-3-derived Langerhans cells (LC) show that fibroblasts mediate migration of cytokine-matured LC via chemokines, including CXCL12, CXCR4, and dermis-derived CCL2 and CCL5. (Ouwehand, et al., 2011) The relevance of these studies for respiratory sensitization is not known.

KE3 => KE4:

Dendritic Cells Activation leads to T-cells, activation and proliferation


It is well-accepted and experimentally proven that a Th2-type T cell polarization is associated with respiratory sensitization. (Hopkins et al., 2005, Huang et al., 2013)

KE4 => AO: 

T-cells, activation and proliferation leads to Sensitisation of the Respiratory Tract 


It is well-demonstrated that clonal expansion of Th2 cells leads to the production of Th2 cytokines that induce Ig class-switching, with clonal expansion of B cells producing antigen-specific IgE. (Dearman et al., 2003)

Empirical Support for KERs

Defining Question

High (Strong)


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 => KE2: 

Covalent Binding, Protein leads to Activation of Inflammatory Signaling


Haptenated peptides generated in vitro can be quantified after 15 minutes. (Hettick, et al., 2009) Most in vitro cellular assay protocols quantify inflammatory readouts after 24 – 48 hours of exposure. TMA induced increased production of IL-10 when incubated with precision cut lung slices (PCLS) for 24 hours. (Lauenstein et al., 2014)

MIE => KE3: 

Covalent Binding, Protein leads to Dendritic Cells Activation


In BALB/c mice, TDI applied to the skin led to TDI-haptenated protein (TDI-hp) (skin keratins and albumin) localization in the stratum corneum, hair follicles, and sebaceous glands within 3 hours, with intensity of staining following a dose–response relationship (Nayak et al. 2014). Subsequently, CD11b+, Langerin (CD207)-expressing DCs, and CD103+ cells migrated to regions of TDI-hp staining. These cells are involved in antigen uptake and stimulation of effector T cells. 

KE2 => KE3: 

Activation of Inflammatory Signaling leads to Dendritic Cells Activation


(Silva et al., 2014) found that HDI increased ROS by inhibiting superoxide dismutase (SOD1) in THP-1 cells. Increased ROS also led to extracellular signal-related kinase (ERK) signaling pathway phosphorylation and the transcription of cytoprotective and maturation pathways (HMOX1 and CD83).

KE3 => KE4:

Dendritic Cells Activation leads to T-cells, activation and proliferation


There is little known about many aspects of antigen processing, such as uptake pathway, peptide generation, and MHC peptide complex stability and density, in chemical sensitization of the respiratory tract. Differences may exist in how skin and respiratory sensitizers are processed that may provide key insight into how to distinguish such chemicals. (Hopkins et al, 2005) found increased expression of type 2 cytokines n mouse lymph node cells after topical exposure to TMA and FITC.

KE4 => AO: 

T-cells, activation and proliferation leads to Sensitisation of the Respiratory Tract 


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 (Vocanson et al., 2009)

Known Modulating Factors

Modulating factors (MFs) may alter the shape of the response-response function that describes the quantitative relationship between two KES, thus having an impact on the progression of the pathway or the severity of the AO.The evidence supporting the influence of various modulating factors is assembled within the individual KERs. More help
Modulating Factor (MF) Influence or Outcome KER(s) involved

Quantitative Understanding

Optional field to provide quantitative weight of evidence descriptors.  More help

Frequency of exposures to toluene diisocyanate exceeding 3 ppb in the time-weighted average (8 hrs, TWA-8) without respiratory protection were found to be associated with incidence. In this study, TWA-8 values above 3 ppb were indicative of peak exposure events, i.e. spills. (Plehiers et al., 2020a and 2020b) This is consistent with a prior report by (Collins et al., 2017) which found a significant link between peak exposure and asthma incidence.

Considerations for Potential Applications of the AOP (optional)

Addressess 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. More help

The construction of an AOP for this endpoint would allow the following: (1) organization of available information to identify remaining uncertainties and prioritize further research, (2) highlighting of differences and similarities between skin and respiratory sensitization pathways, and (3) improvement of existing or identification of novel predictive models that, alone or in an integrated approach, could be used to identify respiratory sensitizers.

Given the available (WoE) outlined above, we propose that the AOP for sensitization of the respiratory tract outlined here allows the identification of gaps in knowledge, research needs, and potential test methods that may be developed further using a larger set of respiratory sensitizers.


List of the literature that was cited for this AOP. More help

APTULA, A. O., PATLEWICZ, G. & ROBERTS, D. W. 2005. Skin sensitization: reaction mechanistic applicability domains for structure-activity relationships. Chem Res Toxicol, 18, 1420-6.

APTULA, A. O. & ROBERTS, D. W. 2006. Mechanistic applicability domains for nonanimal-based prediction of toxicological end points: general principles and application to reactive toxicity. Chem Res Toxicol, 19, 1097-105.

BOVERHOF, D. R., BILLINGTON, R., GOLLAPUDI, B. B., HOTCHKISS, J. A., KRIEGER, S. M., POOLE, A., WIESCINSKI, C. M. & WOOLHISER, M. R. 2008. Respiratory sensitization and allergy: current research approaches and needs. Toxicol Appl Pharmacol, 226, 1-13.

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COCHRANE, S. A., ARTS, J. H. E., EHNES, C., HINDLE, S., HOLLNAGEL, H. M., POOLE, A., SUTO, H. & KIMBER, I. 2015. Thresholds in chemical respiratory sensitisation. Toxicology, 333, 179-194.

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