Upstream eventRelease, Cytokine
Infiltration, Inflammatory cells
Key Event Relationship Overview
AOPs Referencing Relationship
Life Stage Applicability
Key Event Relationship Description
Binding of damage- or pathogen-associated molecular patterns (DAMPs or PAMPs) to pattern recognition receptors (PPRs) such as toll-like receptors (TLRs) can lead to the activation of, amongst others, nuclear factor-κB (NF-κB) or the transcription factor AP-1. This leads to an upregulation of chemokines and inflammatory cytokines, such as tumor necrosis factor-α (TNF-α), interleukins or proteases . TLRs are found expressed in most cells, including liver cells such as hepatocytes, Kupffer cells (KCs) or hepatic stellate cells (HPCs) . Upon cytokine secretion, polymorphonuclear neutrophils (PMNs) that, amongst others, circulate in the blood, can become attracted. PMNs are potent phagocytes, but they also lead to pathogen destruction upon oxidative bursting and are for their part capable of pro-inflammatory cytokine production as well. Various endothelial adhesion molecules, such as the intercellular adhesion molecule 1 (ICAM-1), mediate neutrophil adhesion to endothelial cells. ICAM-1 expression on the luminal surface of the capillary is increased during inflammation, and interacts with ß2 integrin, which is expressed on the surface of PMN. Subsequent to adhesion, neutrophils begin to migrate across the endothelium and towards the center of inflammation . Interleukin-8 (IL-8) is known to be one of the most potent chemoattractants for the recruitment and activation of neutrophils into various organs (e.g. lung, intestine), and binds to the human CXC chemokine receptor 1 (CXCR1) and CXC chemokine receptor 2 (CXCR2) on the surface of the PMN . But not only IL-8, also macrophage inflammatory protein-2 (MIP-2), growth-regulated oncogenes-α, -β, and -γ, as well as the rodent peptides cytokine-induced neutrophil chemoattractant and KC are all members of the CXC subfamily of chemokines and chemoattractants for inflammatory cells
Cullen and co-workers could further confirm the importance of specific chemokines for chemotaxis of different inflammatory cells. They depleted certain chemokines by using respective antibodies in supernatants of Fas-stimulated HeLa cells (see Relationship:924 for explanation on Fas and its role on cytokine induction) and subsequently assessed the chemotactic activity of immune cells. Only depletion of MCP-1 was sufficient to block almost all THP-1 monocyte chemotaxis. On the other hand, chemotaxis of primary human peripheral blood neutrophils was depending mainly on secreted IL-8. 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.
Evidence Supporting this KER
Secreted chemokines are signalling proteins that attract immune cells to migrate to the infected or damaged tissue, in order to trigger tissue repair, removal of cell bodies or bacteria.
Include consideration of temporal concordance here
Infiltration of the hepatic parenchyma by neutrophils was found coinciding with chemokine induction. When chemokines have been neutralized by the addition of neutralizing monoclonal antibodies, a study found KC mainly responsible for abrogating an inflammatory response to Fas-induced hepatic inflammation. In this study, chemokine induction in the livers of anti-Fas-treated mice was not associated with activation of NF-kB, but it coincided with nuclear translocation of activator protein-1 (AP-1). AP-1 activation in the liver was detected shortly (1–2 h) after anti-Fas treatment, suggesting a connection to the onset of apoptosis. When apoptosis was prevented by pre-treatment of the mice with a caspase-3 inhibitor, AP-1 activation and hepatic chemokine production were both significantly reduced. Moreover the authors report a reduction of the hepatic inflammation by 70%.
Addition of 200ng/ml anti-Fas antibody to HeLa cells resulted in the secretion of about 0.7ng/ml IL-8 into the supernatant. In the same study, they could show that supernatants of cells treated with 250ng/ml anti-Fas induced the strongest infiltration of neutrophils (which was almost abolished when the supernatants were treated with an anti-IL-8 antibody). This infiltration was strongly decreased upon dilution of the supernatants. Thus, this allows for a rough quantification of the IL-8 concentrations that are needed for potent chemoattraction of neutrophils.
A further study on quantification of neutrophil migration in dependence of IL-8 concentration found a biphasic exhibition of migration, with an optimum random neutrophil motility at 3 nM of IL-8 .
Uncertainties and Inconsistencies
Studies exist that report an optimal IL-8 concentration for strongest neutrophil motility, so this KER can actually be quantified. However, this is usually performed ex vivo. Isolated neutrophils are very sensitive towards manual handling and need to be treated with care and within a very short time frame. Therefore, results give an indication on necessary concentrations, but need to be carefully considered with regards to direct transferability to the in vivo situation. Moreover, not only IL-8 is responsible for the recruitment of neutrophils, but also other chemokines can contribute to attraction of inflammatory cells. As they play a minor role, they are usually not considered and included in ex vivo studies.
Quantitative Understanding of the Linkage
Is it known how much change in the first event is needed to impact the second? Are there known modulators of the response-response relationships? Are there models or extrapolation approaches that help describe those relationships?
This KER can be described quantitatively, as useful chemotaxis-assays are available that make use of isolated immune cells ex vivo. Those results give an indication on concentrations necessary for cell migration, but need to be carefully considered with regards to direct transferability to the in vivo situation. Moreover, not only IL-8 is responsible for the recruitment of neutrophils, but also other chemokines can contribute to attraction of inflammatory cells. However, additional proof for this KER is provided by the neutralization of chemokines, which prevented a further onset of inflammation.
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
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- Arrese M, Cabrera D, Kalergis AM, Feldstein AE. Innate Immunity and Inflammation in NAFLD/NASH. Dig Dis Sci. 2016 May;61(5):1294-303
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- Kunkel SL, Standiford T, Kasahara K, Strieter RM. Interleukin-8 (IL-8): the major neutrophil chemotactic factor in the lung. Exp Lung Res 1991;17(1):17-23
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- Buanne P, Di Carlo E, Caputi L, Brandolini L, Mosca M, Cattani F, Pellegrini L, Biordi L, Coletti G, Sorrentino C, Fedele G, Colotta F, Melillo G, Bertini R. Crucial pathophysiological role of CXCR2 in experimental ulcerative colitis in mice. J Leukoc Biol 2007;82(5):1239-46
- Faouzi S, Burckhardt BE, Hanson JC, Campe CB, Schrum LW, Rippe RA, Maher JJ. Anti-Fas induces hepatic chemokines and promotes inflammation by an NF-kappa B-independent, caspase-3-dependent pathway. J Biol Chem. 2001 Dec 28;276(52):49077-82
- 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
- Lin F, Nguyen CM, Wang SJ, Saadi W, Gross SP, Jeon NL. Effective neutrophil chemotaxis is strongly influenced by mean IL-8 concentration. Biochem Biophys Res Commun. 2004 Jun 25;319(2):576-81