Interleukin 8 (IL-8) or CXCL8 is a chemokine produced by macrophages and other cell types such as epithelial cells, airway smooth muscle cells[1] and endothelial cells. Endothelial cells store IL-8 in their storage vesicles, the Weibel-Palade bodies.[2][3] In humans, the interleukin-8 protein is encoded by the IL8gene.[4] IL-8 is initially produced as a precursor peptide of 99 amino acids long which then undergoes cleavage to create several active IL-8 isoforms.[5] In culture, a 72 amino acid peptide is the major form secreted by macrophages.[5]

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.

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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 induces 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. 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.{fact}

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.[4][6]

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.[7]

Interleukin-8 is often associated with inflammation. As an example, it has been cited as a proinflammatory mediator in gingivitis[8] and psoriasis.[3]. 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.[9] IL-8 was shown to be associated with obesity.[10]

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[11] or the promotion of division and possible migration by cleaving metalloproteinase molecules.[12]

If a pregnant mother has high levels of interleukin-8, there is an increased risk of schizophrenia in her offspring.[13] High levels of Interleukin 8 have been shown to reduce the likelihood of positive responses to antipsychotic medication in schizophrenia.[14]

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.[16] Additionally, the 3'UTR of IL-8 contains a A/U-rich element that makes it extremely unstable under certain conditions. IL-8 expression is also regulated by the transcription factor NF-κB.[17]NF-κB regulation represents a novel anti-IL-8 therapy for use in inflammatory diseases such as cystic fibrosis.