Interleukin 8 (IL-8 or chemokine (C-X-C motif) ligand 8, CXCL8) is a chemokine produced by macrophages and other cell types such as epithelial cells, airway smooth muscle cells [3] and endothelial cells. Endothelial cells store IL-8 in their storage vesicles, the Weibel-Palade bodies. [4] [5] In humans, the interleukin-8 protein is encoded by the CXCL8 gene. [6] IL-8 is initially produced as a precursor peptide of 99 amino acids which then undergoes cleavage to create several active IL-8 isoforms. [7] In culture, a 72 amino acid peptide is the major form secreted by macrophages. [7]
There are many receptors on the surface membrane capable of binding IL-8; the most frequently studied types are the G protein-coupled serpentine receptors CXCR1 and CXCR2. Expression and affinity for IL-8 differs between the two receptors (CXCR1 > CXCR2). Through a chain of biochemical reactions, IL-8 is secreted and is an important mediator of the immune reaction in the innate immune system response.
IL-8, also known as neutrophil chemotactic factor, has two primary functions. It induces chemotaxis in target cells, primarily neutrophils but also other granulocytes, causing them to migrate toward the site of infection. IL-8 also stimulates phagocytosis once they have arrived. IL-8 is also known to be a potent promoter of angiogenesis. In target cells, IL-8 induces a series of physiological responses required for migration and phagocytosis, such as increases in intracellular Ca2+, exocytosis (e.g. histamine release), and the respiratory burst.
IL-8 can be secreted by any cells with toll-like receptors that are involved in the innate immune response and has been demonstrated to be a signatory chemokine of CR2+ naive T cells, also known as recent thymic emigrants. [8] Usually, it is the macrophages that see an antigen first, and thus are the first cells to release IL-8 to recruit other cells. Both monomer and homodimer forms of IL-8 have been reported to be potent inducers of the chemokine receptors CXCR1 and CXCR2. The homodimer is more potent, but methylation of Leu25 can block the activity of homodimers.
IL-8 is believed to play a role in the pathogenesis of bronchiolitis, a common respiratory tract disease caused by viral infection.[ citation needed ]
IL-8 is a member of the CXC chemokine family. The genes encoding this and the other ten members of the CXC chemokine family form a cluster in a region mapped to chromosome 4q. [6] [9]
CXCL8 is the primary cytokine involved in the recruitment of neutrophils to the site of damage or infection; in a process called chemotaxis. A number of variables are essential for the successful chemotaxis of neutrophils, including the increased expression of high affinity adhesion molecules to secure the neutrophil to the endothelium near the affected site (and is therefore not washed away into the circulatory system), and that the neutrophil can digest its way through the basement membrane and the extracellular matrix (ECM) to reach affected site. CXCL8 plays a key role in inducing the cell signalling necessary to bring about these changes. [10]
Firstly, at the site of infection histamine release causes vasodilation of the capillaries near the injured area which slows down the blood flow in the region and encourages leukocytes, such as neutrophils, to come closer to the endothelium, and away from the centre of the lumen where the rate of blood flow is highest. Once this occurs weak interactions are made between the selectins expressed on the neutrophil and endothelial cells (expression of which is also increased through the action of CXCL8 and other cytokines). On the neutrophil these are: L selectins, and on the endothelial cell: P and E selectins. This causes the "rolling" phase of chemotaxis.
Once the neutrophil is rolling along the endothelium, it will come into contact with a CXCL8 molecule expressed on the surface which stimulates the cell signalling pathway, mediated through a G-coupled-protein-receptor. The binding of CXCL8 to CXCR1/2 on the neutrophil stimulates the neutrophils to upregulate their expression of the integrin, LFA-1, which takes part in high affinity bonding with ICAM-1 receptors expressed on the endothelium. The expression and affinity of LFA-1 is significantly increased to maximise binding. This causes the neutrophil to slow down more until it is stationary. Another key function of the cell signalling stimulated by CXCL8, is the initiation of the oxidative burst. This process allows the build up of proteolytic enzymes and reactive oxygen species (ROS) which are necessary to break down the ECM and basement membrane. These are released in secretory granules, along with more integrins. The release of ROS and damaging enzymes is regulated to minimise host damage, but continues to reach site of infection at which it will carry out its effector functions. [10]
While neutrophil granulocytes are the primary target cells of IL-8, there are a relatively wide range of cells (endothelial cells, macrophages, mast cells, and keratinocytes) that respond to this chemokine. The chemoattractant activity of IL-8 in similar concentrations to vertebrates was proven in Tetrahymena pyriformis, which suggests a phylogenetically well-conserved structure and function for this chemokine. [11]
Interleukin-8 is a key mediator associated with inflammation where it plays a key role in neutrophil recruitment and neutrophil degranulation. [12] As an example, it has been cited as a proinflammatory mediator in gingivitis [13] and psoriasis.
Interleukin-8 secretion is increased by oxidant stress, which thereby cause the recruitment of inflammatory cells and induces a further increase in oxidant stress mediators, making it a key parameter in localized inflammation. [14] IL-8 was shown to be associated with obesity. [15]
IL-8 has also been implied to have a role in colorectal cancer by acting as an autocrine growth factor for colon carcinoma cell lines [16] or the promotion of division and possible migration by cleaving metalloproteinase molecules. [17] It has also been shown that IL-8 plays an important role in chemoresistance of malignant pleural mesothelioma by inducing expression of transmembrane transporters. [18]
If a pregnant mother has high levels of interleukin-8, there is an increased risk of schizophrenia in her offspring. [19] High levels of Interleukin 8 have been shown to reduce the likelihood of positive responses to antipsychotic medication in schizophrenia. [20]
IL-8 has also been implicated in the pathology of cystic fibrosis. Through its action as a signalling molecule IL-8 is capable of recruiting and guiding neutrophils to the lung epithelium. Overstimulation and dysfunction of these recruited neutrophils within the airways results in release of a number of pro-inflammatory molecules and proteases resulting in further damage of lung tissue. [21]
Some benzodiazepines have inhibitory effects on the adenosine A2B receptor mediated secretion of interleukin-8 in human mast cells. In a 2013 study, diazepam, 4′-chlorodiazepam and flunitrazepam markedly reduced NECA-induced interleukin-8 production in that order of potency, whereas clonazepam showed only a modest inhibition. [22]
The expression of IL-8 is negatively regulated by a number of mechanisms. MiRNA-146a/b-5p indirectly represses IL-8 expression by silencing the expression of IRAK1. [23] Additionally, the 3'UTR of IL-8 contains a A/U-rich element that makes it extremely unstable under certain conditions. IL-8 and other inflammatory cytokines form a vicious cycle with the transcription factor NF-κB in cystic fibrosis. [24] NF-κB regulation represents a novel anti-IL-8 therapy for use in inflammatory diseases such as cystic fibrosis. Pathways leading to the induction of ribosomal protein S6 (rpS6) phosphorylation have also been found to enhance IL-8 protein synthesis. This translational control of IL-8 expression is dependent on A/U-rich proximal sequences (APS), which are found in the 3'UTR of IL-8 immediately after the stop codon. [25]
IL-8 was renamed CXCL8 by the Chemokine Nomenclature Subcommittee of the International Union of Immunological Societies,. [26] Its approved HUGO gene symbol is CXCL8. Its receptors were similarly renamed:
Inflammation is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function.
Cytokines are a broad and loose category of small proteins important in cell signaling. Due to their size, cytokines cannot cross the lipid bilayer of cells to enter the cytoplasm and therefore typically exert their functions by interacting with specific cytokine receptors on the target cell surface. Cytokines have been shown to be involved in autocrine, paracrine and endocrine signaling as immunomodulating agents.
Interleukins (ILs) are a group of cytokines that are expressed and secreted by white blood cells (leukocytes) as well as some other body cells. The human genome encodes more than 50 interleukins and related proteins.
Chemokines, or chemotactic cytokines, are a family of small cytokines or signaling proteins secreted by cells that induce directional movement of leukocytes, as well as other cell types, including endothelial and epithelial cells. In addition to playing a major role in the activation of host immune responses, chemokines are important for biological processes, including morphogenesis and wound healing, as well as in the pathogenesis of diseases like cancers.
P-selectin is a type-1 transmembrane protein that in humans is encoded by the SELP gene.
E-selectin, also known as CD62 antigen-like family member E (CD62E), endothelial-leukocyte adhesion molecule 1 (ELAM-1), or leukocyte-endothelial cell adhesion molecule 2 (LECAM2), is a selectin cell adhesion molecule expressed only on endothelial cells activated by cytokines. Like other selectins, it plays an important part in inflammation. In humans, E-selectin is encoded by the SELE gene.
Macrophage Inflammatory Proteins (MIP) belong to the family of chemotactic cytokines known as chemokines. In humans, there are two major forms, MIP-1α and MIP-1β, renamed CCL3 and CCL4 respectively, since 2000. However, other names are sometimes encountered in older literature, such as LD78α, AT 464.1 and GOS19-1 for human CCL3 and AT 744, Act-2, LAG-1, HC21 and G-26 for human CCL4. Other macrophage inflammatory proteins include MIP-2, MIP-3 and MIP-5.
Interleukin 17 family is a family of pro-inflammatory cystine knot cytokines. They are produced by a group of T helper cell known as T helper 17 cell in response to their stimulation with IL-23. Originally, Th17 was identified in 1993 by Rouvier et al. who isolated IL17A transcript from a rodent T-cell hybridoma. The protein encoded by IL17A is a founding member of IL-17 family. IL17A protein exhibits a high homology with a viral IL-17-like protein encoded in the genome of T-lymphotropic rhadinovirus Herpesvirus saimiri. In rodents, IL-17A is often referred to as CTLA8.
Chemokine ligands 4 previously known as macrophage inflammatory protein (MIP-1β), is a protein which in humans is encoded by the CCL4 gene. CCL4 belongs to a cluster of genes located on 17q11-q21 of the chromosomal region. Identification and localization of the gene on the chromosome 17 was in 1990 although the discovery of MIP-1 was initiated in 1988 with the purification of a protein doublet corresponding to inflammatory activity from supernatant of endotoxin-stimulated murine macrophages. At that time, it was also named as "macrophage inflammatory protein-1" (MIP-1) due to its inflammatory properties.
Chemokine ligand 1 (CCL1) is also known as small inducible cytokine A1 and I-309 in humans. CCL1 is a small glycoprotein that belongs to the CC chemokine family.
The chemokine ligand 1 (CXCL1) is a small peptide belonging to the CXC chemokine family that acts as a chemoattractant for several immune cells, especially neutrophils or other non-hematopoietic cells to the site of injury or infection and plays an important role in regulation of immune and inflammatory responses. It was previously called GRO1 oncogene, GROα, neutrophil-activating protein 3 (NAP-3) and melanoma growth stimulating activity, alpha (MGSA-α). CXCL1 was first cloned from a cDNA library of genes induced by platelet-derived growth factor (PDGF) stimulation of BALB/c-3T3 murine embryonic fibroblasts and named "KC" for its location in the nitrocellulose colony hybridization assay. This designation is sometimes erroneously believed to be an acronym and defined as "keratinocytes-derived chemokine". Rat CXCL1 was first reported when NRK-52E cells were stimulated with interleukin-1β (IL-1β) and lipopolysaccharide (LPS) to generate a cytokine that was chemotactic for rat neutrophils, cytokine-induced neutrophil chemoattractant (CINC). In humans, this protein is encoded by the gene Cxcl1 and is located on human chromosome 4 among genes for other CXC chemokines.
C-X-C motif chemokine 5 is a protein that in humans is encoded by the CXCL5 gene.
CXC chemokine receptors are integral membrane proteins that specifically bind and respond to cytokines of the CXC chemokine family. They represent one subfamily of chemokine receptors, a large family of G protein-linked receptors that are known as seven transmembrane (7-TM) proteins, since they span the cell membrane seven times. There are currently six known CXC chemokine receptors in mammals, named CXCR1 through CXCR6.
Interleukin 8 receptor, beta is a chemokine receptor. IL8RB is also known as CXCR2, and CXCR2 is now the IUPHAR Committee on Receptor Nomenclature and Drug classification-recommended name.
Interleukin 8 receptor, alpha is a chemokine receptor. This name and the corresponding gene symbol IL8RA have been replaced by the HGNC approved name C-X-C motif chemokine receptor 1 and the approved symbol CXCR1. It has also been designated as CD181. The IUPHAR Committee on Receptor Nomenclature and Drug Classification use the HGNC recommended name, CXCR1.
The Interleukin-1 family is a group of 11 cytokines that plays a central role in the regulation of immune and inflammatory responses to infections or sterile insults.
Chemorepulsion is the directional movement of a cell away from a substance. Of the two directional varieties of chemotaxis, chemoattraction has been studied to a much greater extent. Only recently have the key components of the chemorepulsive pathway been elucidated. The exact mechanism is still being investigated, and its constituents are currently being explored as likely candidates for immunotherapies.
C-C motif chemokine ligand 27 is a protein that in humans is encoded by the CCL27 gene.
Interleukin 17F (IL-17F) is signaling protein that is in human is encoded by the IL17F gene and is considered a pro-inflammatory cytokine. This protein belongs to the interleukin 17 family and is mainly produced by the T helper 17 cells after their stimulation with interleukin 23. However, IL-17F can be also produced by a wide range of cell types, including innate immune cells and epithelial cells.