Upstream eventIncreased pro-inflammatory mediators
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Endocytic lysosomal uptake leading to liver fibrosis||adjacent||High|
|Increased DNA damage leading to increased risk of breast cancer||adjacent||Moderate||Not Specified|
|Increased reactive oxygen and nitrogen species (RONS) leading to increased risk of breast cancer||adjacent||Moderate||Not Specified|
Life Stage Applicability
|All life stages|
Key Event Relationship Description
Circulating blood leukocytes are required to migrate to sites of injury and infection with the aim to eliminate the primary inflammatory trigger and contribute to tissue repair. In this process are involved selectins (expressed both on leukocytes and endothelium) and integrins (expressed on leukocytes) (von Andrian et al., 1991), with the essential role of the vascular endothelium.
Fast activation of the endothelium with inflammatory stimuli such as histamine and PAF (type I) or slow activation with tumor necrosis factor (TNF) or cytokine interleukin-1 β (IL-1β) (type II), makes the surface of endothelium adhesive (Bevilacqua and Gimbrone, 1987; Pober and Sessa, 2007). This transformation is mediated by a transcriptionally regulated program involving the nuclear factor NF-kB dependent pathway triggered by pro-inflammatory cytokines or bacterial endotoxins (reviewed by Collins et al., 1995).
Integrins mediate attachment between cells or to basement membrane. The β2 integrin family is exclusively expressed on leukocytes and is essential for leukocyte arrest on the endothelium and for migration across the endothelium (Ley et al., 2007). In unstimulated leukocytes integrins are usually in a conformation with low binding affinity, until they receive signals from other receptors, such as chemokine receptors (G-protein-coupled receptors), when they change their conformation and display high affinity for ligands (Luo et al., 2007). Chemokines activate β1 or β2 integrins on monocytes, neutrophils, and lymphocytes and as such serve as chemoattractant for these cells during inflammation (Huber et al., 1991; Tanaka et al., 1993; Gunn et al., 1998).
The chemokines are a family of structurally related cytokines that can act as pro-inflammatory agents (Baggiolini et al., 1994; Vaddi et al., 1997). They have the ability to attract leukocyte subsets to specific sites. They recruit neutrophils, monocytes, natural killer cells (NK) and natural killer T (NKT) cells, all of which express inflammatory chemokine receptors and immature dendritic cells (DCs) that provide the link between innate and adaptive immunity (Oo et al., 2010). After antigen-specific activation of lymphocytes by activated DCs, inflammatory chemokines then attract antigen-specific effector T cells to the inflammatory site (Heydtmann and Adams, 2002).
During diapedesis, leukocytes migrate across the endothelium and basement membrane to enter tissue (Ley et al., 2007; Yadav et al., 2003). Once in tissue, the leukocyte follows chemokine gradients to sites of inflammation, using chemokine-mediated changes in the actin cytoskeleton to propel migration. For example, it was demonstrated that chemokines CXCL9, CXCL10 and CXCL11 are important not only in adhesion, but also in transmigration of effectors T lymphocytes through hepatic endothelium (Curbishley et al., 2005; Eksteen et al., 2004). Intracellular actin reorganization is a prerequisite for cell movement, and it has been shown that chemokines such as SDF-1 induce and increase intracellular filamentous actin in lymphocytes (Bleul et al., 1996).
There is essential role of interleukins, but also other factors such as tumor necrosis factor (TNF), interferon (IFN) in leukocyte recruitment and production of chemokines.
Normally, IL-1β binds to IL-1R1 receptor on the surface of target cells. Following ligand binding the adaptor molecule, myeloid differentiation factor-88 (MyD88), interacts with IL-1R1 via its toll interleukin receptor (TIR) domain (O'Neill, 2008). Signal transduction leads to activation of both mitogen-activated protein kinases (MAPKs) and the transcription factor NF-kB, and resulting in pro-inflammatory cytokine expression. For example, chemokine RANTES production requires the transcription factor NF-kB and the activation of mitogen-activated protein kinases (MAPKs) (Genin et al., 2000; Miyamoto et al., 2000; Kujime et al., 2000, Maruoka et al., 2000; Yang et al., 2000).
TNF-α cleavage produces an intracellular domain that translocates to the nucleus and induces pro-inflammatory cytokine signalling, particularly the expression of IL-12 (Friedman et al., 2006). IL-18 induces natural killer and natural killer T cells to produce IFN-γ (Okamura et al., 1998), but it requires IL-12 to induce IFN-γ production by Th1 cells (Nakanishi et al., 2001). There is an essential role of IFN-I in promoting the chronic recruitment of Ly6Chi monocytes. IFN-I production is elicited via a toll like receptor-7 (TLR-7) and MyD88-dependent pathway (Lee et al., 2008).
While CXC-chemokines, e.g. IL-8, act mostly on neutrophils (Springer, 1995), members of the CC-chemokines, e.g. RANTES and macrophage inflammatory protein have been shown to exert function on monocytes, eosinophils and lymphocytes (Baggiolini et al., 1994; Carr et al., 1994). This depends on the receptors that are expressed on leukocytes. Th1 express preferentially CCR5 and CXCR3, while Th2 cells have CCR3, CCR4 and CCR8 on their surface (Syrbe et al., 1999). Monocytes and macrophages express CCR5 and other receptors for RANTES (Weber et al., 2000).
RANTES chemokine is produced by many cells in the extravascular compartment, including fibroblasts, epithelial cells, and tissue-infiltrating lymphocytes and monocytes (MacEwan, 2002; Hehlgans and Männel, 2002; Black et al., 1997). It acts as a potent chemoattractant for monocytes, memory T cells, eosinophils, and basophils (Schall et al., 1988, 1990; Baggiolini and Dahinden, 1994). Elevated levels of RANTES transcripts are detected within hours of exposure to pro-inflammatory stimuli, including IL-1β, TNF-α, IFN-γ, viruses and LPS (Barnes et al., 1996).
Evidence Supporting this KER
There is much evidence that application of chemokines attract leukocytes to specific site in different species (Beck et al., 1997; Lee et al., 2000; Fahy et al., 2001; Nikiforou et al., 2016).
It was shown that a number of chemokines destabilize the rolling of lymphocytes on L-selectin ligands, suggesting that chemokines are capable of regulating the rolling process (Grabovsky et al., 2002).
Jorgensen and colleagues showed that exposure of mice to FliCind strain S. Typhimurium triggered a significant neutrophil influx in the spleen of wild-type mice, but not Il1b−/−Il18−/− mice (Jorgensen et al., 2016).
The expression of chemokines CCL2, CCL7, and CCL12 was reduced dramatically in MyD88-/- mice (Lee et al., 2009).
Miyamoto and colleagues showed that exposure of cells to IL-1β, TNF-α, and IFN-γ resulted in the induction of RANTES mRNA and protein (Ortiz et al., 1996; Miyamoto et al., 2000). The levels of RANTES production by the fibroblasts in the presence of IL-1β or TNF-α were significantly elevated compared with those in the absence of these factors (Yamada et al., 2001).
In the mouse, IFN-γ administration induces high levels of IP-10 expression in liver and kidneys, with lower levels in spleen (Narumi et al., 1992).
CAPE inhibitor of NF-kB blocked partially IL-1β induced expression of chemokines MIP-1a and MIP-1b (Guo et al., 2003).
CCR2 and CCR5 receptors on the CD8 T cells are enriched in the inflamed human liver, and CCR1 is important in the regulation of hepatic inflammation in murine models (Shields et al., 1999; Boisvert et al., 2003).
Intradermal injection of RANTES induces a potent T-lymphocyte and eosinophils recruitment (Fahy et al., 2001; Beck et al., 1997). Intradermal administration of MIP-1a resulted in accumulation of monocytes, lymphocytes, eosinophils and recruitment of neutrophils (Lee et al., 2000).
Direct IL-1α exposure to the gut resulted in increased numbers of CD3+ cells in the fetal sheep ileum when compared with control animals on the first day after the exposure. The number of white blood cells, monocytes, and neutrophils was increased in cord blood after 6 days of IL-1α exposure to the lung and chorioamnion/skin. The number of lymphocytes on the day 6 was increased for the lung. Compared with controls, gut mRNA levels of TNF-α and IL-1 was significantly increased at 6 days after IL-1α exposure to the GI tract (Nikiforou et al., 2016).
IL-1 β induced up-regulation of CXCR4 in certain cancer cells, but in order to do so necessary is that these cells have IL-1R1. Presence of IL-1R antagonist significantly inhibited the up-regulation of CXCR4 induced by IL-1β at both mRNA and protein level (Sun et al., 2015).
SDF-1 is an efficacious chemoattractant and showed a similar dose response for murine lymphocytes and human monocytes, but was not active on human or murine neutrophils. SDF-1 is a highly effective transendothelial chemoattractant (Bleul et al., 1996).
Uncertainties and Inconsistencies
Lloyd and colleagues found that several chemokines can stimulate the adherence of peripheral blood lymphocytes to ICAM-1 coated slides (Loyd et al., 1996). However, by using a parallel plate flow chamber, other study failed to observe such an effect (Carr et al., 1996).
Quantitative Understanding of the Linkage
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
Human (Bleul et al., 1996; Miyamoto et al., 2000; Yamada et al., 2001; Sun et al., 2015)
Sheep (Nikiforou et al., 2016)
Mouse (Narumi et al., 1992; Fahy et al., 2001; Lee et al., 2009)
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