The interleukin-2 receptor (IL-2R) is a

heterotrimeric thumbnail, 400px, Trimeric form of a TNF-α mutant In biochemistry, a protein trimer is a macromolecular complex formed by three, usually non-covalently bound, macromolecules like proteins or nucleic acids. A protein trimer often occurs from the ...

protein expressed on the surface of certain

immune cells White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...

, such as

lymphocyte A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include T cells (for cell-mediated and cytotoxic adaptive immunity), B cells (for humoral, antibody-driven adaptive immunity), an ...

s, that binds and responds to a

cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...

called

IL-2 The Ilyushin Il-2 (Russian language, Russian: Илью́шин Ил-2) is a Ground attack aircraft, ground-attack plane that was produced by the Soviet Union in large numbers during the World War II, Second World War. The word ''shturmovík'' (C ...

Composition

IL-2 binds to the IL-2 receptor, which has three forms, generated by different combinations of three different proteins, often referred to as "chains": α (alpha) (also called IL-2Rα, CD25, or Tac antigen), β (beta) (also called IL-2Rβ, or CD122), and γ (gamma) (also called IL-2Rγ, γ_c,

common gamma chain The common gamma chain (γc) (or CD132), also known as interleukin-2 receptor subunit gamma or IL-2RG, is a cytokine receptor sub-unit that is common to the receptor complexes for at least six different interleukin receptors: IL-2, IL-4, IL-7, ...

, or CD132); these subunits are also parts of receptors for other cytokines. The β and γ chains of the IL-2R are members of the

type I cytokine receptor Type I cytokine receptors are transmembrane receptors expressed on the surface of cells that recognize and respond to cytokines with four α-helical strands. These receptors are also known under the name '' hemopoietin receptors'', and share a ...

family.

Structure-activity relationships of the IL-2/IL-2R interaction

The three receptor chains are expressed separately and differently on various cell types and can assemble in different combinations and orders to generate low, intermediate, and high affinity IL-2 receptors. The α chain binds IL-2 with low affinity, the combination of β and γ together form a complex that binds IL-2 with intermediate affinity, primarily on

memory T cells Memory T cells are a subset of T lymphocytes that might have some of the same functions as memory B cells. Their lineage is unclear. Function Antigen-specific memory T cells specific to viruses or other microbial molecules can be found in both ...

and

NK cells Natural killer cells, also known as NK cells, are a type of cytotoxic lymphocyte critical to the innate immune system. They are a kind of large granular lymphocytes (LGL), and belong to the rapidly expanding family of known innate lymphoid cell ...

; and all three receptor chains form a complex that binds IL-2 with high affinity (Kd ~ 10⁻¹¹ M) on activated

T cells T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their ce ...

and

regulatory T cells The regulatory T cells (Tregs or Treg cells), formerly known as suppressor T cells, are a subpopulation of T cells that modulate the immune system, maintain immune tolerance, tolerance to self-antigens, and prevent autoimmune disease. Treg ...

. The intermediate and high affinity receptor forms are functional and cause changes in the cell when IL-2 binds to them. The structure of the stable complex formed when IL-2 binds to the high affinity receptor has been determined using

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The structure supports a model wherein IL-2 initially binds to the α chain, then the β is recruited, and finally γ.

Signaling

The three IL-2 receptor chains span the

cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

and extend into the cell, thereby delivering biochemical signals to the cell interior. The alpha chain does not participate in signaling, but the beta chain is complexed with an

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

called

Janus kinase 1 Janus kinase 1 (JAK1) is a human tyrosine kinase protein essential for signaling for certain type I and type II cytokines. It interacts with the common gamma chain (γc) of type I cytokine receptors, to elicit signals from the IL-2 receptor f ...

(JAK1), that is capable of adding phosphate groups to molecules. Similarly the gamma chain complexes with another

tyrosine kinase A tyrosine kinase is an enzyme that can transfer a phosphate group from ATP to the tyrosine residues of specific proteins inside a cell. It functions as an "on" or "off" switch in many cellular functions. Tyrosine kinases belong to a larger cla ...

called JAK3. These enzymes are activated by IL-2 binding to the external domains of the IL-2R. As a consequence, three intracellular signaling pathways are initiated, the MAP kinase pathway, the

Phosphoinositide 3-kinase Phosphoinositide 3-kinases (PI3Ks), also called phosphatidylinositol 3-kinases, are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, motility, survival and intracellular trafficking, which i ...

(PI3K) pathway, and the

JAK-STAT pathway The JAK-STAT signaling pathway is a chain of interactions between proteins in a cell, and is involved in processes such as immunity, cell division, cell death, and tumor formation. The pathway communicates information from chemical signals outs ...

. Once IL-2 binds to the high affinity receptor, the complex is rapidly internalized and has only a short time to signal. IL-2, IL-2Rβ, and γ_c are rapidly degraded, but IL-2Rα is recycled to the cell surface. Thus, the concentration of IL-2 and its receptor available determines the tempo, magnitude and extent of T cell immune responses. IL-2 and its receptor have key roles in key functions of the immune system, tolerance and

immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity ...

, primarily via their direct effects on

. In the

thymus The thymus (: thymuses or thymi) is a specialized primary lymphoid organ of the immune system. Within the thymus, T cells mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. The thymus ...

, where T cells mature, they prevent

autoimmune diseases An autoimmune disease is a condition that results from an anomalous response of the adaptive immune system, wherein it mistakenly targets and attacks healthy, functioning parts of the body as if they were foreign organisms. It is estimated that ...

by promoting the differentiation of certain immature T cells into

, which kill off other T cells that are primed to attack normal healthy cells in the body. IL-2/IL2R also promotes the differentiation of T cells into

effector T cells 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 considere ...

and into

when the initial T cells is also stimulated by an

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

, thus helping the body fight off infections. Through their 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, they also have a key role in enduring cell-mediated immunity.

Clinical implications

Drugs that inhibit IL-2 receptors, such as

basiliximab Basiliximab, sold under the brand name Simulect, is a monoclonal antibody used to prevent rejection in kidney transplants. It is a chimeric mouse-human monoclonal antibody to the α chain (CD25) of the IL-2 receptor of T cells. It is used in c ...

and

daclizumab Daclizumab (trade name Zinbryta) is a therapeutic humanized monoclonal antibody which was used for the treatment of adults with relapsing forms of multiple sclerosis (MS). Daclizumab works by binding to CD25, the alpha subunit of the IL-2 receptor ...

are used in conjunction with other drugs to prevent immune rejection of transplants.

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 factor–like fashion through specific high-affinity receptors, analogous to the growth factors being studied by endocrinologists and biochemists".

References

External links

* {{Interleukin receptor modulators Type I cytokine receptors Integral membrane proteins