Note: references for individual interleukins are provided on main pages, not so much this page.
Interleukins (ILs) are cytokines that serve as growth factors for blood cells, are involved in haematopoietic differentiation, promote DNA synthesis and regulate the secretion of other inflammatory/immune mediators such as prostaglandins.
Interleukin-1 (IL-1) is a pro-inflammatory interleukin that is secreted by macrophages, amongst other cells and that comes in two forms: IL-1α and IL-1β. Both forms produce virtually the same effects, namely that they trigger fever, promote the production of prostaglandins. The major distinction between the two, besides their differing structures, is that IL-1α is produced active, whereas IL-1β needs to be activated by an enzyme called caspase-1. IL-1 antagonists are used to treat rheumatoid arthritis.
Interleukin-2 (IL-2) is secreted by helper T cells and it stimulates B/T-lymphocyte proliferation, activates lymphokine-activated killer (LAK) cells and NK-cells and stimulates regulatory T-cell development. It also stimulates interferon-gamma (IFN-γ) production. Also used to treat otherwise terminal metastatic tumours such as renal cell carcinoma and malignant melanoma. After the major treatments for acute myeloblastic leukaemia have been performed sometimes a combination of histamine and IL-2 are given so as to better eliminate the cancerous cells and prevent future relapses.
Interleukin-3 is produced by lymphocytes, endothelial cells and astrocytes and promotes the proliferation of haematopoietic cells. While it is not medically used it is under investigation for the treatment of myelosuppression associated with chemotherapy.
Interleukin-5 is produced by helper T cells, mast cells and other granulocytes and it promotes the differentiation and activation of eosinophils, also triggers antigen-induced differentiation of B-cells into plasma cells.
Interleukin-6 is secreted by T-cells, monocytes and fibroblasts where it serves as a pro-inflammatory cytokine that promotes the differentiation of B-cells and serves as a growth factor for plasmacytomas. While it is not medically-utilized in itself, except in aiding the production of monoclonal antibodies for drug development, antagonists are widely used to treat rheumatoid arthritis, inflammatory bowel disease and other inflammatory conditions. Interleukin-6 is also implicated in the pathophysiology of depression, and depressed individuals often have abnormally high levels of this protein in their blood. It may also have a role in blood cancers.
Interleukin-7 is produced by the stromal cells of the bone marrow and promotes the proliferation of T-cells, along with IL-2. It also promotes the growth of B-cell precursors. Currently not medically used but results so far using recombinant IL-7 have been promising in the treatment of chronic viral infections, immunosuppression and cancer immunotherapy.
Interleukin-8, while also having roles as a chemokine, is produced by several cell types and inducing chemotaxis in neutrophils, basophils and T-cells. IL-8 is abnormally highly expressed in oestrogen receptor negative breast, colorectal and stomach cancers; its expression in breast cancer seems to be associated with an increased aggressiveness of the malignancy. It is also implicated in acute lung injury.
Interleukin-9 is produced by TH2 cells, regulates allergic inflammatory responses, promotes the differentiation and growth of mast cells and supports IL-2/IL-4-independent growth of helper T-cells, although its effects on these cells is limited compared to IL-2 and IL-4. It also stimulates IgG and IgE production from IL-4 stimulated B cells. May play a role in oncogenesis and autoimmune diseases.
Interleukin-10 is produced by T-cells, monocytes (including macrophages), dendritic cells and epithelial cells, and inhibits macrophages (including their phagocytic activity) and dendritic cells. Increased production of IL-10 is believed to be involved in the ability of the human papillomavirus' to cause cervical cancer. There's been some interest in developing IL-10 targeting therapeutics for the treatment of a few different cancers. It may also play a role in irritable bowel syndrome and may be used by the Dengue virus to increase its host's susceptibility to the virus.
Interleukin-11 is an interleukin that promotes the proliferation of haematopoietic stem cells and megakaryocyte progenitor cells, also stimulates the maturation of megakaryocytes (hence increasing platelet production) and inhibits the production of adipocytes (fat cells). It is also required embryo implantation, hence also required for conception.
Recombinant human IL-11 (oprelvekin) is used to treat and prevent thrombocytopenia.
Interleukin-12 (IL-12) much like IL-1 comes in two major flavours: IL-12α and IL-12β. They act as growth factors for activated T and NK cells, enhance the lytic activity of NK/LAK cells, and stimulate the production of interferon-gamma by resting peripheral blood mononucleated cells (lymphocytes, macrophages and monocytes). IL-12β is also required for the production of IL-23 (which it forms after combining with IL-23A). IL-23 activates the JAK-STAT pathway and stimulates memory T-cells and the production of pro-inflammatory cytokines. IL-12 also suppresses tumour growth, spread and induces apoptosis in these cells, recombinant IL-12 (edodekin alfa) has been under development for the treatment of cancers and infections (including mycobacterial ones like tuberculosis and viral ones) but results have been disappointing. A monoclonal antibody against IL-12/23 has been under development for the treatment of autoimmune disorders.