Interleukin 2
Interleukin-2 (IL-2) is an interleukin, a type of cytokine signaling molecule in the immune system. It is a 15,5 - 16 kDa protein that regulates the activities of white blood cells (leukocytes, often lymphocytes) that are responsible for immunity. IL-2 is part of the body's natural response to microbial infection, and in discriminating between foreign ("non-self") and "self". IL-2 mediates its effects by binding to IL-2 receptors, which are expressed by lymphocytes.

# Signaling pathway
IL-2 is a member of a cytokine family, each member of which has a four alpha helix bundle; the family also includes IL-4, IL-7, IL-9, IL-15 and IL-21. IL-2 signals through the IL-2 receptor, a complex consisting of three chains, termed alpha, beta and gamma. The gamma chain is shared by all family members.
The IL-2 Receptor (IL-2R)  subunit has low affinity for its ligand but has the ability (when bound to the  and  subunit) to increase the IL-2R affinity 100-fold. Heterodimerization of the  and  subunits of IL-2R is essential for signalling in T cells.
Gene expression regulation for IL-2 can be on multiple levels or by different ways. One of the checkpoints is signaling through TCR receptor, antigen receptor of T-lymphocytes after recognizing MHC-peptide complex. Signaling pathway from TCR then goes through phospholipase-C (PLC) dependent pathway. PLC activates 3 major transcription factors and their pathways: NFAT, NFkB and AP-1. After costimulation from CD28 the optimal activation of expression of IL-2 and these pathways is induced.
At the same time Oct-1 is expressed. It helps the activation. Oct1 is expressed in T-lymphocytes and Oct2 is induced after cell activation.
NFAT has multiple family members, all of them are located in cytoplasm and signaling goes through calcineurin, NFAT is dephosphorylated and therefore translocated to the nucleus.
AP-1 is a dimer and is composed of c-Jun and c-Fos proteins. It cooperates with other transcription factors including NFkB and Oct.
NFkB is translocated to the nucleus after costimulation through CD28. NFkB is a heterodimer and there are two binding sites on the IL-2 promoter.

# Function
IL-2 has essential roles in key functions of the immune system, tolerance and immunity, primarily via its direct effects on T cells. In the thymus, where T cells mature, it prevents autoimmune diseases by promoting the differentiation of certain immature T cells into regulatory T cells, which suppress other T cells that are otherwise primed to attack normal healthy cells in the body. IL-2 also promotes the differentiation of T cells into effector T cells and into memory T cells when the initial T cell is also stimulated by an antigen, thus helping the body fight off infections.   Its expression and secretion is tightly regulated and functions as part of both transient positive and negative feedback loops in mounting and dampening immune responses. Through its role in the development of T cell immunologic memory, which depends upon the expansion of the number and function of antigen-selected T cell clones, it plays a key role in enduring cell-mediated immunity.

# Role in disease
While the causes of itchiness are poorly understood, some evidence indicates that IL-2 is involved in itchy psoriasis.

# Medical use

## Pharmaceutical analogues
Aldesleukin is a form of recombinant interleukin-2. It is manufactured using recombinant DNA technology and is marketed as a protein therapeutic and branded as Proleukin. It has been approved by the Food and Drug Administration (FDA) and in several European countries for the treatment of cancers (malignant melanoma, renal cell cancer) in large intermittent doses and has been extensively used in continuous doses.
Interking is a recombinant IL-2 with a serine at residue 125, sold by Shenzhen Neptunus.
Neoleukin 2/15 is a computationally designed mimic of IL-2 that was designed to avoid common side effects. It it currently being commercialized into a therapeutic.

### Dosage
Various dosages of IL-2 across the United States and across the world are used. The efficiency and side effects of different dosages is often a point of disagreement.
Usually, in the U.S., the higher dosage option is used, affected by cancer type, response to treatment and general patient health. Patients are typically treated for five consecutive days, three times a day, for fifteen minutes. The following approximately 10 days help the patient to recover between treatments. IL-2 is delivered intravenously on an inpatient basis to enable proper monitoring of side effects.
A lower dose regimen involves injection of IL-2 under the skin typically on an outpatient basis. It may alternatively be given on an inpatient basis over 13 days, similar to and often including the delivery of chemotherapy.
Intralesional IL-2 is commonly used to treat in-transit melanoma metastases and has a high complete response rate.

### Toxicity
IL-2 has a narrow therapeutic window, and the level of dosing usually determines the severity of the side effects.
Some common side effects:
- flu-like symptoms (fever, headache, muscle and joint pain, fatigue)
- nausea/vomiting
- dry, itchy skin or rash
- weakness or shortness of breath
- diarrhea
- low blood pressure
- drowsiness or confusion
- loss of appetite
More serious and dangerous side effects sometimes are seen, such as capillary leak syndrome, breathing problems, serious infections, seizures, allergic reactions, heart problems or a variety of other possible complications.
Intralesional IL-2 used to treat in-transit melanoma metastases is generally well-tolerated.

## Pharmaceutical derivative
Eisai markets a drug called denileukin diftitox (trade name Ontak), which is a recombinant fusion protein of the human IL-2 ligand and the diphtheria toxin. This drug binds to IL-2 receptors and introduces the diphtheria toxin into cells that express those receptors, killing the cells. In some leukemias and lymphomas, malignant cells express the IL-2 receptor, so denileukin diftitox can kill them. In 1999 Ontak was approved by the U.S. Food and Drug Administration (FDA) for treatment of cutaneous T-cell Lymphoma (CTCL).

# Clinical research
IL-2 has been used in clinical trials for the treatment of chronic viral infections and as a booster (adjuvant) for vaccines. The use of large doses of IL-2 given every 68 weeks in HIV therapy, similar to its use in cancer therapy, was found to be ineffective in preventing progression to an AIDS diagnosis in two large clinical trials published in 2009. More recently low dose IL-2 has shown early success in modulating the immune system in disease like type 1 diabetes and vasculitis. There are also promising studies looking to use low dose IL-2 in ischaemic heart disease.

# History
According to an immunology textbook: "IL-2 is particularly important historically, as it is the first type I cytokine that was cloned, the first type I cytokine for which a receptor component was cloned, and was the first short-chain type I cytokine whose receptor structure was solved. Many general principles have been derived from studies of this cytokine including its being the first cytokine demonstrated to act in a growth factorlike fashion through specific high-affinity receptors, analogous to the growth factors being studied by endocrinologists and biochemists".:712
In the mid-1960s, studies reported "activities" in leukocyte-conditioned media that promoted lymphocyte proliferation.:16 In the mid-1970s, it was discovered that T-cells could be selectively proliferated when normal human bone marrow cells were cultured in conditioned medium obtained from phytohemagglutinin-stimulated normal human lymphocytes.:712 The key factor was isolated from cultured mouse cells in 1979 and from cultured human cells in 1980. The gene for human IL-2 was cloned in 1982 after an intense competition.:76
Commercial activity to bring an IL-2 drug to market was intense in the 1980s and '90s. By 1983, Cetus Corporation had created a proprietary recombinant version of IL-2 (Aldesleukin, later branded as Proleukin), with the alanine removed from its N-terminal and residue 125 replaced with serine.:7677:201 Amgen later entered the field with its own proprietary, mutated, recombinant protein and Cetus and Amgen were soon competing scientifically and in the courts; Cetus won the legal battles and forced Amgen out of the field.:151  By 1990 Cetus had gotten aldesleukin approved in nine European countries but in that year, the U.S. Food and Drug Administration (FDA) refused to approve Cetus' application to market IL-2. The failure led to the collapse of Cetus, and in 1991 the company was sold to Chiron Corporation. Chiron continued the development of IL-2, which was finally approved by the FDA as Proleukin for metastatic renal carcinoma in 1992.
By 1993 aldesleukin was the only approved version of IL-2, but Roche was also developing a proprietary, modified, recombinant IL-2 called teceleukin, with a methionine added at is N-terminal, and Glaxo was developing a version called bioleukin, with a methionine added at is N-terminal and residue 125 replaced with alanine. Dozens of clinical trials had been conducted of recombinant or purified IL-2, alone, in combination with other drugs, or using cell therapies, in which cells were taken from patients, activated with IL-2, then reinfused. Novartis acquired Chiron in 2006 and sold the aldesleukin business to Prometheus Laboratories in 2010.