Interleukin 33
Interleukin 33 (IL-33) is a protein that in humans is encoded by the IL33 gene.
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.
IL-33 is expressed by a wide variety of cell types, including fibroblasts, mast cells, dendritic cells, macrophages, osteoblasts, endothelial cells, and epithelial cells.

# Structure
IL-33 is a member of the IL-1 superfamily of cytokines, a determination based in part on the molecules -trefoil structure, a conserved structure type described in other IL-1 cytokines, including IL-1, IL-1, IL-1Ra and IL-18. In this structure, the 12 -strands of the -trefoil are arranged in three pseudorepeats of four -strand units, of which the first and last -strands are antiparallel staves in a six-stranded -barrel, while the second and third -strands of each repeat form a -hairpin sitting atop the -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.

# Function
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. IL-33 acts intracellularly as a nuclear factor and extracellularly as a cytokine.

## Nuclear role
IL-33 is constitutively located in the nucleus of structural cells of humans and mice and has a helix-turn-helix domain 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. IL-33 also interacts with the histone methyltransferase SUV39H1 and murine appears to IL-33 interact to NF-B.

## Cytokine role
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-B 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. 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.

## Regulation
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.

# Clinical significance
IL-33 has been associated with several disease states through Genome Wide Association Studies: asthma, allergy, endometriosis, and hay fever.
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.  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. 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. Elevated levels of IL-33 are associated with asthma.
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.