What is interleukin

Interleukins are a type of cytokine first thought to be expressed by white blood cells (leukocytes) alone but have later been found to be produced by many other body cells ¹. Interleukins play essential roles in the activation and differentiation of immune cells, as well as proliferation, maturation, migration, and adhesion. Interleukins also have pro-inflammatory and anti-inflammatory properties. The primary function of interleukins is, therefore, to modulate growth, differentiation, and activation during inflammatory and immune responses.

Interleukins consist of a large group of proteins that can elicit many reactions in cells and tissues by binding to high-affinity receptors in cell surfaces. Interleukins have both paracrine and autocrine function. Interleukins are also used in animal studies to investigate aspect related to clinical medicine ².

Therapy of human diseases with interleukins

In the immunotherapy of melanoma and renal carcinoma have successfully been used interleukin-2 and interferon-gamma. Their mechanisms of action involve the activation of natural killer (NK) cells and T lymphocytes. The U.S. Food and Drug Administration (FDA) has approved these 2 cytokines for the treatment of these 2 malignancies.

Aldesleukin

Aldesleukin is a recombinant form of human interleukin-2 (IL-2), a cytokine produced by macrophages and lymphocytes that stimulates the proliferation and maturation of T cells, which is used in immune therapy of renal cell cancer and malignant melanoma. Aldesleukin is also known as T cell growth factor, interleukin-2 is a critical cytokine in the adaptive immune process and interacts with specific T cell receptors to activate intracytoplasmic pathways that lead to proliferation and differentiation of immature T lymphoblasts into mature and reactive T cells that play an important role in immune responses to foreign proteins, viruses and bacteria and tumor cells. A recombinant form of interleukin-2 has been developed and shown to be an immune modulator and to have antitumor activity against several malignancies, but most convincingly renal cell cancer and malignant melanoma. Aldesleukin was approved for use in the United States for malignant melanoma in 1992 and indications were subsequently broadened and now include metastatic renal cell carcinoma and metastatic malignant melanoma.

Aldesleukin is available as lyophilized powder in vials of 22 million IU under the brand name Proleukin. The typical dose is 600,000 IU/kg intravenously every 8 hours for a maximum of 14 doses, which can be repeated after a 9 day rest depending upon tolerance. Side effects are common, particularly with high dose interleukin-2 which should be administered in a hospital setting under the supervision of physicians experienced in the use of anticancer agents.

Common side effects include fatigue, fever, chills, nausea, diarrhea, and capillary leak syndrome which can cause peripheral edema, hypotension, renal insufficiency and pulmonary edema. Less common, but potentially severe adverse reactions include shock, anaphylaxis, severe infections, autoimmune conditions and neurologic complications including somnolence, stupor and coma.

Interleukins general properties

• Interleukins (cytokines) are proteins made in response to pathogens and other antigens that regulate and mediate inflammatory and immune responses.
• Interleukin production is a self-limited process. The messenger RNA encoding most interleukins is unstable and causes a transient synthesis. These molecules are rapidly secreted once synthesized.
• Cellular responses to interleukins include up- and down-regulatory mechanisms with the induction and participation of genes that encode inhibitors of the cytokine receptors.
• Interleukins have redundant functions. For instance, interleukin-4, interleukin-5, and interleukin-13 are B-cell growth factors and stimulate B-cell differentiation.
• Interleukins (cytokines) stimulate switching of antibody isotypes in B cells, differentiation of helper T cells into Th-1 and Th-2 subsets, and activation of microbicidal mechanisms in phagocytes.
• Interleukins often influence other interleukin synthesis and actions. For instance, interleukin-1 promotes lymphocyte activation that leads to the release of interleukin-2.
• Cellular responses to interleukins (cytokines) are stimulated and regulated by external signals or high-affinity receptors. For example, stimulation of B-cells by pathogens leads to increased expression of cytokine receptors.
• Most interleukins (cytokines) act either on the same cell that secretes the interleukin (cytokine), for instance, interleukin-2 produced by T cells operates on the same T cells that made it or on a nearby cell. Besides, interleukins (cytokines) may enter the circulation and act far from the site of production, for example, interleukin-1 is an endogenous pyrogen that works on the central nervous system (CNS) and causes fever.
• Small quantities of a cytokine are needed to occupy receptors and elicit biologic effects.

Interleukin function

Interleukin-1

Macrophages, large granular lymphocytes, B cells, endothelium, fibroblasts, and astrocytes secrete interleukin-1. T cells, B cells, macrophages, endothelium and tissue cells are the principal targets. interleukin-1 causes lymphocyte activation, macrophage stimulation, increased leukocyte/endothelial adhesion, fever due to hypothalamus stimulation, and release of acute phase proteins by the liver. It may also cause apoptosis in many cell types and cachexia ³.

Interleukin-1 acts on the hypothalamus to induce fever and is therefore called an endogenous pyrogen. It operates on hepatocytes to increase synthesis of specific serum proteins, such as amyloid A protein and fibrinogen. It causes fall in blood pressure or shock in large amounts. Corticosteroids inhibit the interleukin-1 effect.

Interleukin-2

T cells produce interleukin-2. The principal targets are T cells. Its primary effects are T-cell proliferation and differentiation, increased cytokine synthesis, potentiating Fas-mediated apoptosis, and promoting regulatory T cell development. It causes proliferation and activation of NK cells and B-cell proliferation and antibody synthesis. Also, it stimulates the activation of cytotoxic lymphocytes and macrophages ⁴.

Gene knockout mouse studies have provided evidence that the primary interleukin-2 function in vivo is the suppression of T responses. Mice lacking interleukin-2 or its receptor (CD25) develop lymphadenopathy and T cell-mediated autoimmunity.

Interleukin-3

T cells and stem cells make interleukin-3. It functions as a multilineage colony-stimulating factor ⁵.

Interleukin-4

CD4+T cells (Th2) synthesize interleukin-4, and it acts on both B and T cells. It is a B-cell growth factor and causes IgE and IgG1 isotype selection. It causes Th2 differentiation and proliferation, and it inhibits IFN gamma-mediated activation on macrophages. It promotes mast cell proliferation in vivo ⁴.

Interleukin-5

CD4+T cells (Th2) produce interleukin-5, and its principal targets are B cells. It causes B-cell growth factor and differentiation and IgA selection. Besides, causes eosinophil activation and increased production of these innate immune cells ⁶.

Interleukin-6

T and B lymphocytes, fibroblasts and macrophages make interleukin-6. B lymphocytes and hepatocytes are its principal targets. interleukin-6 primary effects include B-cell differentiation and stimulation of acute phase proteins ⁷.

Interleukin-7

Bone marrow stromal cells produce interleukin-7 that acts on pre-B cells and T cells. It causes B-cell and T-cell proliferation ⁸.

Interleukin-8

Monocytes and fibroblasts make interleukin-8. Its principal targets are neutrophils, basophils, mast cells, macrophages, and keratinocytes. It causes neutrophil chemotaxis, angiogenesis, superoxide release, and granule release ⁹.

Interleukin-9

Th9, Th2, Th17, mast cells, NKT cells, and regulatory T cells produce this cytokine. It enhances T-cell survival, mast cell activation and synergy with erythropoietin ¹⁰.

Interleukin-10

Th2 cells produce interleukin-10. Its principal targets are Th1 cells. It causes inhibition of interleukin-2 and interferon gamma. It decreases the antigen presentation, and MHC class II expression of dendritic cells, co-stimulatory molecules on macrophages and it also downregulates pathogenic Th17 cell responses. It inhibits interleukin-12 production by macrophages ¹¹.

Knockout mice lacking interleukin-10 develop inflammatory bowel disease, probably because of uncontrolled activation of macrophages reacting to enteric microbes.

Interleukin-11

Bone marrow stromal cells and fibroblasts produce interleukin-11. The interleukin-11 principal targets are hemopoietic progenitors and osteoclasts. The interleukin-11 primary effects include osteoclast formation, colony stimulating factor, raised platelet count in vivo, and inhibition of pro-inflammatory cytokine production ¹².

Interleukin-12

Monocytes produce interleukin-12. Its principal targets are T cells. It causes induction of Th1 cells. Besides, it is a potent inducer of interferon gamma production by T lymphocytes and NK cells ¹².

Interleukin-12 overproduction causes allergic disorders. Corticosteroids inhibit the effects of interleukin-12.

Interleukin-13

CD4+T cells (Th2), NKT cells and mast cells synthesize interleukin-13. It acts on monocytes, fibroblasts, epithelial cells and B cells. The interleukin-13 significant effects are B-cell growth and differentiation, stimulates isotype switching to IgE. It causes increased mucus production by epithelial cells, increased collagen synthesis by fibroblasts and inhibits pro-inflammatory cytokine production. Also, interleukin-13 works together with interleukin-4 in producing biologic effects associated with allergic inflammation and in defense against parasites ¹³.

Interleukin-14

T cells produce interleukin-14, and its principal effects are stimulation of activated B cell proliferation and inhibition of immunoglobulin secretion.

Interleukin-15

Monocytes, epithelium, and muscles make interleukin-15. It acts on T cells and activated B cells. It causes the proliferation of both B and T cells. It causes NK cell memory and CD8+ T cell proliferation.

Interleukin-16

Eosinophils and CD8+T cells synthesize interleukin-16. Its principal target is CD4+ T cells. It causes CD4+ T cell chemoattraction.

Interleukin-17

This cytokine is produced by Th-17. It acts on epithelial and endothelial cells. interleukin-17 main effects are the release of interleukin-6 and other pro-inflammatory cytokines. It enhances the activities of antigen-presenting cells. It stimulates chemokine synthesis by endothelial cells ¹⁴.

Interleukin-18

Macrophages mostly make interleukin-18, which can be produced by hepatocytes and keratinocytes. Its principal target is a co-factor in Th1 cell induction. It causes interferon gamma production and enhances NK cell activity.

Interleukin-19

Th2 lymphocytes synthesize interleukin-19 and acts on resident vascular cells in addition to immune cells. It is an anti-inflammatory molecule. It promotes immune responses mediated by regulatory lymphocytes and has substantial activity on microvascular ¹⁵.

Interleukin-19 may be used to induce angiogenesis in ischemic tissue.

Interleukin-20

Immune cells and activated epithelial cells secrete interleukin-20. It acts on epithelial cells. It plays a vital role in the cellular communication between epithelial cells and the immune system under inflammatory conditions.

Interleukin-21

NK cells and CD4+ T lymphocytes make interleukin-21. It acts on various immune cells of innate and the adaptive immune systems. interleukin-21 promotes B and T lymphocyte proliferation and differentiation. It enhances NK cell activity ¹⁶.

The administration of interleukin-21 may be considered for use as a preventive and therapeutic approach when dealing with Th2-mediated allergic diseases.

Interleukin-22

Different cells in both innate and acquired immunities produce interleukin-22, but the primary sources are T cells. Th22 cell is a new line of CD4+ T cells, which differentiated from naive T cells in the presence of various pro-inflammatory cytokines including interleukin-6. interleukin-22 inhibits interleukin-4 production. It also has essential functions in mucosal surface protection and tissue repair ¹⁷.

Interleukin-23

Macrophages and dendritic cells mainly synthesize interleukin-23. It acts on T cells causing maintenance of interleukin-17 producing T cells ¹⁸.

Interleukin-24

Monocytes, T and B cells mostly make interleukin-24. It causes cancer-specific cell death, causes wound healing and protects against bacterial infections and cardiovascular diseases ¹⁹.

Interleukin-25

Dendritic cells produced predominantly interleukin-25. It acts on various types of cells, including Th2 cells. It stimulates the synthesis of Th2 cytokine profile including interleukin-4 and interleukin-13 ²⁰.

Interleukin-26

It is strongly associated inflammatory activity with interleukin-26. Th17 cells produce this interleukin. It acts on epithelial cells and intestinal epithelial cells. It induces interleukin-10 expression, stimulates the production of interleukin-1-beta, interleukin-6, and interleukin-8 and causes Th17 cell generation.

Interleukin-26 shows high expression in psoriatic skin lesions, colonic lesions from individuals with inflammatory bowel disease and synovia of individuals with rheumatoid arthritis. It may constitute a promising target to treat chronic inflammatory disorders.

Interleukin-27

T cells make interleukin-27 that activates STAT-1 and STAT-3, which regulates immune responses. interleukin-27 stimulates interleukin-10 production. It is a pro-inflammatory molecule and upregulates type-2 interferon synthesis by natural killer cells ²¹.

Interleukin-27 was found to exerts anti-inflammatory effects in several experimental autoimmune models. interleukin-27 treatment suppressed autoimmune diabetes.

Interleukin-28 may be sufficient treatment of hepatitis C virus patients.

Interleukin-28

Regulatory T-cells synthesize interleukin-28, which acts on keratinocytes and melanocytes. It stimulates cell presentation of viral antigens to CD8+T lymphocytes. interleukin-28 also upregulates TLR-2 and TLR-3 expression. interleukin-28 enhances the keratinocyte capacity to recognize pathogens in the healthy skin.

Interleukin-29

interleukin-29 is a type-3 interferon and produced by virus-infected cells, dendritic cells, and regulatory T-cells. It upregulates viral protective responses. Virus-infected cells may regulate interleukin-29 genome.

Interleukin-29 is a marker of osteoarthritis as joint inflammation implicates it.

Interleukin-30

Monocytes mainly produce interleukin-30 in response to TLR agonists including bacterial LPS. It acts on monocytes, macrophages, dendritic cells, T and B lymphocytes, natural killer cells, mast cells, and endothelial cells ²¹.

Interleukin-31

interleukin-31 is produced mainly by Th2 cells and dendritic cells. It is a proinflammatory cytokine and a chemotactic factor that direct polymorphonuclear cells, monocytes, and T cells to inflammatory lesions. interleukin-31 induces chemokines production and synthesis of interleukin-6, interleukin-16, and interleukin-32.

Interleukin-32

interleukin-32 is a pro-inflammatory molecule. Natural killer cells and monocytes mainly produce it. interleukin-32 induces the synthesis of various cytokines including interleukin-6, and interleukin-1beta. It inhibits interleukin-15 production ²².

Interleukin-33

Mast cells and Th2 lymphocytes express interleukin-33 that acts on various innate and immune cells including dendritic cells and T and B lymphocytes. It mediates Th2 responses and therefore participates in the protection against parasites and type-I hypersensitivity reaction.

Interleukin-34

Various phagocytes and epithelial cells synthesize Interleukin-34 (interleukin-34). It enhances interleukin-6 production and participates in the differentiation and development of antigen-presenting cells including microglia ²³.

Interleukin-35

Regulatory B cells mainly secrete it. One of the primary functions of this interleukin is its involvement in lymphocyte differentiation. It exhibits an immune-suppressive effect.

Interleukin-36

Phagocytes mainly make interleukin-36. It acts on T lymphocytes and NK cells regulating the IFN-γ synthesis. It stimulates the hematopoiesis and expression of both MHC class I and II molecules as well as intracellular adhesion molecules (ICAM)-1.

Interleukin-36 also seems to play a significant role in human psoriasis. In psoriatic lesion tissues, interleukin-36 levels were found to be elevated, and generalized pustular psoriasis was also discovered, which is rare and life-threatening.

Interleukin-37

interleukin-37 plays an essential role in the regulation of the innate immunity causing immunosuppression. Phagocytes and organs including the uterus, testis, and thymus express it. interleukin-37 upregulates immune responses and inflammation in autoimmune disorders.

In lupus patients were elevated interleukin-37 levels in comparison with healthy controls, and mucocutaneous and renal involvement was correlated with high disease activity.

Interleukin-38

Il-38 acts on T cells and inhibits the synthesis of interleukin-17 and interleukin-22. The placenta, tonsil’s B lymphocytes, spleen, skin, and thymus widely express interleukin-38 ¹³.

Recent studies point to an association between interleukin-38 and autoimmune diseases. Its role in carcinogenesis or cancer growth is unclear.

Interleukin-39

B lymphocytes mainly produce interleukin-39. It acts on neutrophils inducing their differentiation or expansion ²⁴.

Interleukin-39 secreted by activated B cells may be a critical pro-inflammatory cytokine and a potential therapeutic target for the treatment of autoimmune diseases such as systemic lupus erythematosus.

Interleukin-40

interleukin-40 is produced in the bone marrow, fetal liver, and by activated B cells. interleukin-40 plays a vital role in the development of humoral immune responses ¹⁸.

Interleukin-40 expression in several human B-cell lymphomas suggests that it may play a role in the pathogenesis of these diseases.

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