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Event: 901

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

Infiltration, Inflammatory cells

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
Infiltration, Inflammatory cells

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
Level of Biological Organization

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
Organ term

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
Process Object Action
inflammatory response increased

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

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

TNF-induced cytokines and chemokines, such as IL-6, IL-8, GMCSF, CXCL1, and RANTES, can instigate and amplify immune responses through triggering the production of acute phase proteins and the recruitment of neutrophils, macrophages, and basophils to the site of inflammation, and by triggering increased production of monocytes/macrophages from bone marrow[1]. Monocytes are the precursors of macrophages and dendritic cells and circulate in the blood for 1-3 days. Upon secretion of chemokines such as CCL2 which is also referred to as monocyte chemoattractant protein 1 (MCP1), they can migrate towards affected tissue. This was nicely demonstrated when depletion of MCP-1 in supernatants of Fas-stimulated cells was sufficient to block almost all THP-1 monocyte chemotaxis. Using an in vivo mouse model, the authors found that Fas stimulation could trigger phagocyte migration by administration of anti-Fas (Jo2) antibody into C57BL/6 mice within 10 h of anti-Fas administration. This correlated with extensive cell death in the thymus and a dramatic increase of CD11b-positive macrophages in the same tissue[1].

Neutrophils, on the other hand, account for about 50 70 % of all blood leukocytes in the human body [2][3]. Upon an inflammatory event, neutrophil production is upregulated, and its lifetime increases as a response to platelet activating factor (PAF), granulocyte-colony stimulating factor (G-CSF) or various pro-inflammatory cytokines, such as interleukin 1ß (IL-1ß) [3]. The crucial role of PMN in the human immune system is long known. In 1968, Baehner and Karnovsky described a link between a reduced PMN activity and the development of chronic granulomatous disease (CGD) [4]. The important peroxidase-mediated bactericidal role of PMN and the formation of superoxide radicals as one of the main bactericial mechanisms was already described more than 30 years ago [5][6]. A strong negative correlation between the chemotactic ability of PMN and patients with increased bacterial sepsis was demonstrated [7], and clinical morbidity from infections is clearly increased with a reduced number of circulating PMN in the blood [8]. The neutrophilic cytosol contains granules that are filled with a variety of proteins, such as defensins, bactericidal-permeability-increasing protein, proteases (e.g. elastase, cathepsins), and myeloperoxidase (MPO) that consumes hydrogen peroxide (H2O2) and generates hypochlorous acid (HOCl), the most bactericidal oxidant that is produced by PMN [8][2]. Activated neutrophils are capable of producing a variety of pro-inflammatory cytokines, e.g. IL-1ß, IL-6, IL-12 and IL-23, and transport internalised pathogens to lymph nodes to support macrophages and dendritic cells in antigen presentation[9]. Also, contact with pathogens results not only in phagocytosis, but also in the so-called oxidative burst, marked by an increased consumption of molecular oxygen and resulting production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) [10].

Deregulation of this response by constant stimulation of PMNs, as could be shown for nanoparticles for example, ultimately leads to the establishment of a (chronic) inflammation. Here, also macrophages play a vital role. Resident alveolar macrophages, such as Kupffer cells in the liver, that usually phagocyte microorgansims or particles will be activated when overwhelmed by the amount of invading pathogens and in turn release inflammatory cytokines and chemokines. Consequently, neutrophils are recruited and activated as described above [11][12].

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?

Chemotaxis assays can be performed in vitro/ex vivo by using Chemotaxis Chambers (for example Neuro Probe Chambers). Supernatants can be added to the bottom well of the chamber and 3–8 mm nitrocellulose filters are placed on top, while the top chamber contains the inflammatory cells (for example neutrophils). After a certain time period, the number of migrated cells towards the lower chamber can be determined by staining of the cells[1].

Influx of inflammatory cells (mainly neutrophils) can be analysed by tissue staining by using Haematoxylin and eosin [13].

In mice, neutrophil influx can be analysed using a mouse MPO ELISA kit for lysed tissue [14].

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

[1]: human (cells); [13]: human (tissue; representative for general application in patients, as liver inflammation is commonly found in patients with DILI)

<[15][14][16]: mouse (nanomaterial-induced)


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. 1.0 1.1 1.2 1.3 Cullen SP, Henry CM, Kearney CJ, Logue SE, Feoktistova M, Tynan GA, Lavelle EC, Leverkus M, Martin SJ. Fas/CD95-induced chemokines can serve as "find-me" signals for apoptotic cells. Mol Cell. 2013 Mar 28;49(6):1034-48
  2. 2.0 2.1 Freitas M, Lima JL, Fernandes E. Optical probes for detection and quantification of neutrophils' oxidative burst. A review. Anal Chim Acta 2009;649(1):8-23
  3. 3.0 3.1 Wessels I, Jansen J, Rink L, Uciechowski P. Immunosenescence of polymorphonuclear neutrophils. ScientificWorldJournal 2010;10:145-60
  4. Baehner RL, Karnovsky ML. Deficiency of reduced nicotinamide-adenine dinucleotide oxidase in chronic granulomatous disease. Science 1968;162(859):1277-9
  5. Klebanoff SJ. Iodination of bacteria: a bactericidal mechanism. J Exp Med 1967;126(6):1063-78
  6. Klebanoff SJ, Rosen H. The role of myeloperoxidase in the microbicidal activity of polymorphonuclear leukocytes. Ciba Found Symp 1978;(65):263-84
  7. Christou NV, Meakins JL. Neutrophil function in surgical patients: Two inhibitors of granulocyte chemotaxis associated with sepsis. J Surg Res 1979;26(4):355-364
  8. 8.0 8.1 Nauseef WM. How human neutrophils kill and degrade microbes: an integrated view. Immunol Rev 2007;219:88-102
  9. Silva MT. Neutrophils and macrophages work in concert as inducers and effectors of adaptive immunity against extracellular and intracellular microbial pathogens. J Leukoc Biol 2010;87(5):805-13
  10. Babior BM. Phagocytes and oxidative stress. Am J Med 2000;109(1):33-44
  11. Driscoll KE, Deyo LC, Carter JM, Howard BW, Hassenbein DG, Bertram TA. Effects of particle exposure and particle-elicited inflammatory cells on mutation in rat alveolar epithelial cells. Carcinogenesis 1997;18(2):423-30
  12. Knaapen AM, Seiler F, Schilderman PA, Nehls P, Bruch J, Schins RP, Borm PJ. Neutrophils cause oxidative DNA damage in alveolar epithelial cells. Free Radic Biol Med 1999;27(1-2):234-40
  13. 13.0 13.1 Huebscher SG. Histological assessment of non-alcoholic fatty liver disease. Histopathol. 2006;49:450–465
  14. 14.0 14.1 Kermanizadeh A, Brown DM, Hutchison GR, Stone V. Engineered Nanomaterial Impact in the Liver following Exposure via an Intravenous Route–The Role of Polymorphonuclear Leukocytes and Gene Expression in the Organ. Journal of Nanomed & Nanotechnol 2012;04(01):1–7
  15. Cui Y, Liu H, Zhou M, Duan Y, Li N, Gong X, Hu R, Hong M, Hong F. Signaling pathway of inflammatory responses in the mouse liver caused by TiO2 nanoparticles. 2011; J. Biomed. Mater. Res. - Part A 96 A:221–229
  16. Ma L, Zhao J, Wang J, Liu J, Duan Y, Liu H, Li N, Yan J, Ruan J, Wang H, Hong F. The Acute Liver Injury in Mice Caused by Nano-Anatase TiO2. Nanoscale Res Lett. 2009 Aug 1;4(11):1275-85