Interleukin 33 (IL-33) is a protein that in humans is encoded by the IL33 gene. [5]
Interleukin 33 is a member of the IL-1 family that potently drives production of T helper-2 (Th2)-associated cytokines (e.g., IL-4). IL33 is a ligand for ST2 (IL1RL1), an IL-1 family receptor that is highly expressed on Th2 cells, mast cells and group 2 innate lymphocytes. [6]
IL-33 is expressed by a wide variety of cell types, including fibroblasts, mast cells, dendritic cells, macrophages, osteoblasts, endothelial cells, and epithelial cells. [7]
IL-33 is a member of the IL-1 superfamily of cytokines, a determination based in part on the molecules β-trefoil structure, a conserved structure type described in other IL-1 cytokines, including IL-1α, IL-1β, IL-1Ra and IL-18. In this structure, the 12 β-strands of the β-trefoil are arranged in three pseudorepeats of four β-strand units, of which the first and last β-strands are antiparallel staves in a six-stranded β-barrel, while the second and third β-strands of each repeat form a β-hairpin sitting atop the β-barrel. IL-33 is a ligand that binds to a high-affinity receptor family member ST2. The complex of these two molecules with IL-1RAcP indicates a ternary complex formation. The binding area appears to be a mix of polar and non-polar regions that create a specific binding between ligand and receptor. The interface between the molecules has been shown to be extensive. Structural data on the IL-33 molecule was determined by solution NMR and small angle X-ray scattering. [8]
Interleukin 33 (IL-33) is a cytokine belonging to the IL-1 superfamily. IL-33 induces helper T cells, mast cells, eosinophils and basophils to produce type 2 cytokines. This cytokine was previously named NF-HEV 'nuclear factor (NF) in high endothelial venules' (HEVs) since it was originally identified in these specialized cells. [9] IL-33 acts intracellularly as a nuclear factor and extracellularly as a cytokine.
IL-33 is constitutively located in the nucleus of structural cells of humans and mice [10] and has a helix-turn-helix domain [9] presumably allowing it to bind to DNA. There is a paucity of research into the nuclear role of IL-33 but amino acids 40-58 in human IL-33 are sufficient for nuclear localisation and histone binding. [11] IL-33 also interacts with the histone methyltransferase SUV39H1 [12] and murine appears to IL-33 interact to NF-κB. [13]
As a cytokine, IL-33 interacts with the receptors ST2 (also known as IL1RL1) and IL-1 Receptor Accessory Protein (IL1RAP), activating intracellular molecules in the NF-κB and MAP kinase signaling pathways that drive production of type 2 cytokines (e.g. IL-5 and IL-13) from polarized Th2 cells. The induction of type 2 cytokines by IL-33 in vivo is believed to induce the severe pathological changes observed in mucosal organs following administration of IL-33. [14] [15] IL-33 is also effective in reversing Alzheimer-like symptoms in APP/PS1 mice, by reversing the buildup and preventing the new formation of amyloid plaques. [16]
Extracellularly, IL-33 is rapidly oxidised. The oxidation process results in the formation of two disulphide bridges and a change in the conformation of the molecule, which prevents it from binding to its receptor, ST2. This is believed to limit the range and duration of the action of IL-33. [17]
IL-33 has been associated with several disease states through Genome Wide Association Studies: asthma, [18] allergy, [19] endometriosis, [20] and hay fever. [21] In particular, a single-nucleotide polymorphism rs928413 (A/G), is located in the 5′ upstream region of IL33 gene, and its minor “G” allele was identified as a susceptible variant for early childhood asthma [22] and atopic asthma [23] development. The rs928413(G) allele creates a binding site for the cAMP responsive element-binding protein 1 transcription factor that may explain the negative effect of the rs928413 minor “G” allele on asthma development. [24] “T” allele of the polymorphism rs4742170 located in the second intron of IL33 gene was linked to specific wheezing phenotype (intermediate-onset wheeze). [25] Risk “T” rs4742170 allele disrupts binding of GR transcription factor to IL33 putative enhancer that may explain the negative effect of the rs4742170 (T) risk allele on the development of wheezing phenotype that strongly correlates with allergic sensitization in childhood. [26]
This protein is one of many that acts as a cytokine and signals inflammation in the body by acting upon macrophages, neutrophils, B cells, Th2 cells, eosinophils, basophils and mast cells. [27] This protein is also thought to cause the itching that is associated with dermatitis. The IL-33 protein resides in keratinocytes of the skin and when subjected to irritation or allergic conditions will communicate with nearby sensory neurons and initiate an itchy feeling. [28] In IL-33 knockout mice, it was discovered that nuclear IL-33 is associated with wound healing as mice without the protein healed significantly slower than mice with the IL-33 protein. [29] Elevated levels of IL-33 are associated with asthma. [30]
In mice, IL-33 was found to effect the production of methionine-enkephalin peptides in group 2 innate lymphocytes, in turn promoting the emergence of beige adipocytes, which leads to increased energy expenditure and decreased adiposity. [31]
Elevated levels of IL-33 have been reported in some patients with nonsmall cell lung carcinomas. The source of elevated serum levels of IL-33 during the early stages could be bronchial and vascular epithelium. [32] IL-33 knockdown showed lower growth of nonsmall cell lung carcinomas, while overexpression of IL-33 resulted in increased growth. Blocking of IL-33 reduced the growth of human nonsmall cell lung carcinomas. I mice model blocking of IL-33 inhibited tumor growth in immunodeficient mice. [33] [34]
In the mouse colon carcinoma model, IL-33 was expressed by tumor stromal cells, while the colon carcinoma cells did not express ST2 with or without IL-33 stimulation. The IL-33 knockout model had higher tumor growth than wild type. Similarly, IFN- γ expression was increased in the IL-33 knockout model as well as the number of T regulatory cells and CD8+ T cells. [35]
Age-related macular degeneration is a retinal disease leading to neovascularization and thus impaired vision. Current treatment includes administration of anti-VEGF but is not sufficient. Retinal pigment epithelial cells can express IL-33 at both mRNA and protein levels. IL-33 expression is upregulated during inflammatory stimuli. IL-33 can inhibit fibroblasts and endothelial cells that express ST2, which can lead to reduced angiogenesis. [36]
In a mouse model of chronic asthma, anti-IL-33 administration decreased antigen-induced immune response. Similar results were found in ST2 deficient mice. IL-33 activated innate lymphoid cells 2 remained in the lymph nodes for several weeks. CD4 + Th2 cells were formed after repeated exposure to IL-33. This type of cells highly produced IL-5. [37]
Chronic inflammation is characteristic for IBD ( inflammatory bowel disease). Under normal conditions, IL-33 is present in healthy intestinal tissue, but during inflammatory conditions its expression is increased. However, IL-33 has also a protective role under inflammatory conditions and is involved in wound healing. [38]
In brain, IL-33 is expressed in oligodendrocytes and astrocytes and is implicated in the pathophysiology of intracerebral hemorrhage. [39]
The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considered essential in B cell antibody class switching, breaking cross-tolerance in dendritic cells, in the activation and growth of cytotoxic T cells, and in maximizing bactericidal activity of phagocytes such as macrophages and neutrophils. CD4+ cells are mature Th cells that express the surface protein CD4. Genetic variation in regulatory elements expressed by CD4+ cells determines susceptibility to a broad class of autoimmune diseases.
Interleukin 12 (IL-12) is an interleukin that is naturally produced by dendritic cells, macrophages, neutrophils, helper T cells and human B-lymphoblastoid cells (NC-37) in response to antigenic stimulation. IL-12 belongs to the family of interleukin-12. IL-12 family is unique in comprising the only heterodimeric cytokines, which includes IL-12, IL-23, IL-27 and IL-35. Despite sharing many structural features and molecular partners, they mediate surprisingly diverse functional effects.
The interleukin 4 is a cytokine that induces differentiation of naive helper T cells (Th0 cells) to Th2 cells. Upon activation by IL-4, Th2 cells subsequently produce additional IL-4 in a positive feedback loop. IL-4 is produced primarily by mast cells, Th2 cells, eosinophils and basophils. It is closely related and has functions similar to IL-13.
Amphiregulin, also known as AREG, is a protein synthesized as a transmembrane glycoprotein with 252 aminoacids and it is encoded by the AREG gene. in humans.
Interleukin 9, also known as IL-9, is a pleiotropic cytokine belonging to the group of interleukins. IL-9 is produced by variety of cells like mast cells, NKT cells, Th2, Th17, Treg, ILC2, and Th9 cells in different amounts. Among them, Th9 cells are regarded as the major CD4+ T cells that produce IL-9.
Interleukin 5 (IL-5) is an interleukin produced by type-2 T helper cells and mast cells.
Interleukin 13 (IL-13) is a protein that in humans is encoded by the IL13 gene. IL-13 was first cloned in 1993 and is located on chromosome 5q31.1 with a length of 1.4kb. It has a mass of 13 kDa and folds into 4 alpha helical bundles. The secondary structural features of IL-13 are similar to that of Interleukin 4 (IL-4); however it only has 25% sequence identity to IL-4 and is capable of IL-4 independent signaling. IL-13 is a cytokine secreted by T helper type 2 (Th2) cells, CD4 cells, natural killer T cell, mast cells, basophils, eosinophils and nuocytes. Interleukin-13 is a central regulator in IgE synthesis, goblet cell hyperplasia, mucus hypersecretion, airway hyperresponsiveness, fibrosis and chitinase up-regulation. It is a mediator of allergic inflammation and different diseases including asthma., and atopic dermatitis.
Interleukin-31 (IL-31) is a protein that in humans is encoded by the IL31 gene that resides on chromosome 12. IL-31 is an inflammatory cytokine that helps trigger cell-mediated immunity against pathogens. It has also been identified as a major player in a number of chronic inflammatory diseases, including atopic dermatitis.
Interleukin-25 (IL-25) – also known as interleukin-17E (IL-17E) – is a protein that in humans is encoded by the IL25 gene on chromosome 14. IL-25 was discovered in 2001 and is made up of 177 amino acids.
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.
Interleukin 19 (IL-19) is an immunosuppressive protein that belongs to the IL-10 cytokine subfamily.
Signal transducer and activator of transcription 6 (STAT6) is a transcription factor that belongs to the Signal Transducer and Activator of Transcription (STAT) family of proteins. The proteins of STAT family transmit signals from a receptor complex to the nucleus and activate gene expression. Similarly as other STAT family proteins, STAT6 is also activated by growth factors and cytokines. STAT6 is mainly activated by cytokines interleukin-4 and interleukin-13.
GATA3 is a transcription factor that in humans is encoded by the GATA3 gene. Studies in animal models and humans indicate that it controls the expression of a wide range of biologically and clinically important genes.
Thymic stromal lymphopoietin (TSLP) is an interleukin (IL)-2-like cytokine, alarmin, and growth factor involved in numerous physiological and pathological processes, primarily those of the immune system. It shares a common ancestor with IL-7.
The interleukin-13 receptor is a type I cytokine receptor, binding Interleukin-13. It consists of two subunits, encoded by IL13RA1 and IL4R, respectively. These two genes encode the proteins IL-13Rα1 and IL-4Rα. These form a dimer with IL-13 binding to the IL-13Rα1 chain and IL-4Rα stabilises this interaction. This IL-13 receptor can also instigate IL-4 signalling. In both cases this occurs via activation of the Janus kinase (JAK)/Signal Transducer and Activator of Transcription (STAT) pathway, resulting in phosphorylation of STAT6. Phosphorylated STAT6 dimerises and acts as a transcription factor activating many genes, such as eotaxin.
Interleukin-17A is a protein that in humans is encoded by the IL17A gene. In rodents, IL-17A used to be referred to as CTLA8, after the similarity with a viral gene.
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.
Interleukin 1 receptor-like 1, also known as IL1RL1 and ST2, is a protein that in humans is encoded by the IL1RL1 gene.
ILC2 cells, or type 2 innate lymphoid cells are a type of innate lymphoid cell. Not to be confused with the ILC. They are derived from common lymphoid progenitor and belong to the lymphoid lineage. These cells lack antigen specific B or T cell receptor because of the lack of recombination activating gene. ILC2s produce type 2 cytokines and are involved in responses to helminths, allergens, some viruses, such as influenza virus and cancer.
In cell biology, TH9 cells are a sub-population of CD4+T cells that produce interleukin-9 (IL-9). They play a role in defense against helminth infections, in allergic responses, in autoimmunity, and tumor suppression.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.