Interleukins (IL) are the most potent cytokines maintaining balances within the immune system. To date, 36 interleukins are known.
They play essential roles in the activation and differentiation of immune cells, as well as proliferation, maturation, migration, and adhesion and play essential roles in both innate and adaptive immunity. The major function involves modulating growth, differentiation, and activation during inflammatory and immune responses.
Interleukins consist of a large group of proteins that induce many reactions in cells and tissues by binding to high-affinity receptors in cell surfaces.
- IL-1 (alpha and beta) is produced by macrophages, dendritic cells and some other cell types; activates differentiation of T cells presumably into Th1 helper type stimulating cell-mediated immunity (a pro-inflammatory cytokine); it activates hypothalamic receptors possessing strong pyrogenic activity with fever development. Also, it stimulates other types of immune cells.
- IL-2 is produced by T helper cells (predominantly of Th1 type); it stimulates the large sets of immune cells (T- and B cells, monocytes, NK cells) maintaining cell proliferation and differentiation.
- IL-3 is a potent hematopoietic factor, produced by bone marrow cells and T cells. It stimulates bone marrow precursors of immunocytes, initiating their differentiation.
- IL-4 is expressed by T helper-2 (Th2) cell subset; it stimulates early B cell proliferation, conversion of “naive” Th0-cells into Th2 type, supporting humoral immune response and allergic reactions with switching to IgE production.
- IL-5 is secreted presumably by T cells (Th2) and stimulates eosinophil and basophil maturation, immunoglobulin synthesis; as a result, it takes part in allergy development.
- IL-6 is a pro-inflammatory cytokine with wide pleiotropic effects produced by different cell lines (T cells, macrophages, keratinocytes, endothelial cells, etc.); it stimulates B cell differentiation and promotes cellular inflammation.
- IL-7 is another important hematopoietic factor, produced by bone marrow stromal cells and some other cell types. It induces the proliferation of early bone marrow precursors of immune cells, especially T-lymphocytes.
- IL-8 is a strong chemotaxis agent (chemokine) secreted by macrophages, endothelial and epithelial cells and other cell types. It stimulates directed migration of neutrophils and basophils, their adhesion and metabolic activation.
- IL-10 is the cytokine with suppressive activity for cell immune reactions. It is considered to be produced by macrophages and by Treg or regulatory T cells. It blocks IL-1, IL-2, IL-6 and alpha-tumour necrosis factor synthesis (an anti-inflammatory cytokine).
- IL-12 is produced by macrophages and dendritic cells, activates differentiation of T cells into Th1 helper type, thus redirecting the immune response towards the cell-mediated immunity. It is potent pro-inflammatory cytokine enforcing IL-2 and γ-interferon synthesis by T cells.
- IL-13 is synthesized by T helper-2 (Th2) cells and mast cells; it stimulates B cells, activates allergic reactions with IgE production, stimulates mucus secretion by epithelial cells, suppresses inflammation (an anti-inflammatory cytokine).
- IL-17 is produced by a special subpopulation of T helper cells (Th17); it demonstrates highly pleiotropic effects and activates the broad set of immune cells (T cells, neutrophils, macrophages and some others), thereby promoting inflammation (a pro-inflammatory cytokine); it also activates hematopoiesis of myeloid cell lines, endothelial and epithelial cells. On the other hand, IL-17 is capable of stimulating antibody synthesis.
- IL-18 is secreted by macrophages, dendritic cells, keratinocytes; it is also potent pro-inflammatory cytokine that stimulates transition Th0 into Th1 helper type, accelerates the synthesis of IL-2 and γ-interferon by Th1 cells, and activates NK cells.
- IL-21 is produced by Th17 and Th2; it stimulates Th17 development, differentiation of B cells, maturation of NK cells.
- IL-23 is expressed by macrophages and dendritic cells; its main function is the maintenance of Th17 helper cells and T memory cells.
- IL-28 and IL-29 are referred to as lambda-interferons