Cytokines (Greek cyto-, cell; and -kinos, movement) are small cell-signaling protein molecules that are secreted by numerous cells and are a category of signaling molecules used extensively in intercellular communication. Cytokines can be classified as proteins, peptides, or glycoproteins; the term "cytokine" encompasses a large and diverse family of regulators produced throughout the body by cells of diverse embryological origin.<ref>Template:Cite book</ref>
The term "cytokine" has been used to refer to the immunomodulating agents, such as interleukins and interferons. Biochemists disagree as to which molecules should be termed cytokines and which hormones. As we learn more about each, anatomic and structural distinctions between the two are fading. Classic protein hormones circulate in nanomolar (10Template:Sup) concentrations that usually vary by less than one order of magnitude. In contrast, some cytokines (such as IL-6) circulate in picomolar (10Template:Sup) concentrations that can increase up to 1,000-fold during trauma or infection. The widespread distribution of cellular sources for cytokines may be a feature that differentiates them from hormones. Virtually all nucleated cells, but especially endo/epithelial cells and resident macrophages (many near the interface with the external environment) are potent producers of IL-1, IL-6, and TNF-α.<ref>Boyle, J. J. (2005). Macrophage activation in atherosclerosis: pathogenesis and pharmacology of plaque rupture. Curr Vasc Pharmacol, 3(1), 63-68. PMID=15638783</ref> In contrast, classic hormones, such as insulin, are secreted from discrete glands (e.g., the pancreas).<ref name="Cannon2000">Template:Cite journal</ref> As of 2008, the current terminology refers to cytokines as immunomodulating agents. However, more research is needed in this area of defining cytokines and hormones.
Part of the difficulty with distinguishing cytokines from hormones is that some of the immunomodulating effects of cytokines are systemic rather than local. For instance, to use hormone terminology, the action of cytokines may be autocrine or paracrine in chemotaxis and endocrine as a pyrogen. Further, as molecules, cytokines are not limited to their immunomodulatory role. For instance, cytokines are also involved in several developmental processes during embryogenesis<ref>Template:Cite journal</ref><ref group="nb">Saito explains "much evidence has suggested that cytokines and chemokines play a very important role in the reproduction, i.e. embryo implantation, endometrial development, and trophoblast growth and differentiation by modulating the immune and endocrine systems."(15)</ref><ref>Template:Cite journal</ref><ref group="nb">Chen explains the regulatory activity of LIF in human and murine embryos: "In conclusion, human preimplantation embryos express LIF and LIF-R mRNA. The expression of these transcripts indicates that preimplantation embryos may be responsive to LIF originating either from the surrounding environment or from the embryos themselves and exerting its function in a paracrine or autocrine manner."(719)</ref>
Each cytokine has a matching cell-surface receptor. Subsequent cascades of intracellular signalling then alter cell functions. This may include the upregulation and/or downregulation of several genes and their transcription factors, resulting in the production of other cytokines, an increase in the number of surface receptors for other molecules, or the suppression of their own effect by feedback inhibition.
The effect of a particular cytokine on a given cell depends on the cytokine, its extracellular abundance, the presence and abundance of the complementary receptor on the cell surface, and downstream signals activated by receptor binding; these last two factors can vary by cell type. Cytokines are characterized by considerable "redundancy", in that many cytokines appear to share similar functions.
It seems to be a paradox that cytokines binding to antibodies have a stronger immune effect than the cytokine alone. This may lead to lower therapeutic doses.
Said et al. showed that inflammatory cytokines cause an IL-10-dependent inhibition of <ref>Said EA, et al. Programmed death-1-induced interleukin-10 production by monocytes impairs CD4+ T cell activation during HIV infection. Nat Med. 2010 Apr;16(4):452-9.</ref> T-cell expansion and function by up-regulating PD-1 levels on monocytes which leads to IL-10 production by monocytes after binding of PD-1 by PD-L.<ref>Elias A. Said et al. 2009, PD-1 Induced IL10 Production by Monocytes Impairs T-cell Activation in a Reversible Fashion" Nature Medicine 2010; 452-9.</ref>
Adverse reactions to cytokines are characterized by local inflammation and/or ulceration at the injection sites. Occasionally such reactions are seen with more widespread papular eruptions.<ref name="Andrews">James, William; Berger, Timothy; Elston, Dirk (2005). Andrews' Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0.</ref>
Cytokines have been classed as lymphokines, interleukins, and chemokines, based on their presumed function, cell of secretion, or target of action. Because cytokines are characterised by considerable redundancy and pleiotropism, such distinctions, allowing for exceptions, are obsolete.
- The term interleukin was initially used by researchers for those cytokines whose presumed targets are principally leukocytes. It is now used largely for designation of newer cytokine molecules discovered every day and bears little relation to their presumed function. The vast majority of these are produced by T-helper cells.
- The term chemokine refers to a specific class of cytokines that mediates chemoattraction (chemotaxis) between cells.
Structural homology has been able to partially distinguish between cytokines that do not demonstrate a considerable degree of redundancy so that they can be classified into four types:
- The four-α-helix bundle family - Member cytokines have three-dimensional structures with four bundles of α-helices. This family, in turn, is divided into three sub-families:
- the IL-1 family, which primarily includes IL-1 and IL-18
- the IL-17 family, which has yet to be completely characterized, though member cytokines have a specific effect in promoting proliferation of T-cells that cause cytotoxic effects.
A classification that proves more useful in clinical and experimental practice divides immunological cytokines into those that enhance cellular immune responses, type 1 (IFN-γ, TGF-β, etc.), and type 2 (IL-4, IL-10, IL-13, etc.), which favor antibody responses.
A key focus of interest has been that cytokines in one of these two sub-sets tend to inhibit the effects of those in the other. Dysregulation of this tendency is under intensive study for its possible role in the pathogenesis of autoimmune disorders.
Several inflammatory cytokines are induced by oxidant stress.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> The fact that cytokines themselves trigger the release of other cytokines<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> and also lead to increased oxidant stress makes them important in chronic inflammation.
In recent years, the cytokine receptors have come to demand the attention of more investigators than cytokines themselves, partly because of their remarkable characteristics, and partly because a deficiency of cytokine receptors has now been directly linked to certain debilitating immunodeficiency states. In this regard, and also because the redundancy and pleiomorphism of cytokines are, in fact, a consequence of their homologous receptors, many authorities think that a classification of cytokine receptors would be more clinically and experimentally useful.
A classification of cytokine receptors based on their three-dimensional structure has, therefore, been attempted. Such a classification, though seemingly cumbersome, provides several unique perspectives for attractive pharmacotherapeutic targets.
- Immunoglobulin (Ig) superfamily, which are ubiquitously present throughout several cells and tissues of the vertebrate body, and share structural homology with immunoglobulins (antibodies), cell adhesion molecules, and even some cytokines. Examples: IL-1 receptor types.
- Haemopoietic Growth Factor (type 1) family, whose members have certain conserved motifs in their extracellular amino-acid domain. The IL-2 receptor belongs to this chain, whose γ-chain (common to several other cytokines) deficiency is directly responsible for the x-linked form of Severe Combined Immunodeficiency (X-SCID).
- Interferon (type 2) family, whose members are receptors for IFN β and γ.
- Tumor necrosis factors (TNF) (type 3) family, whose members share a cysteine-rich common extracellular binding domain, and includes several other non-cytokine ligands like CD40, CD27 and CD30, besides the ligands on which the family is named (TNF).
- Seven transmembrane helix family, the ubiquitous receptor type of the animal kingdom. All G protein-coupled receptors (for hormones and neurotransmitters) belong to this family. Chemokine receptors, two of which act as binding proteins for HIV (CD4 and CCR5), also belong to this family.Template:Citation needed
- Interleukin-17 receptor (IL-17R) family, which shows little homology with any other cytokine receptor family. Structural motifs conserved between members of this family include: an extracellular fibronectin III-like domain, a transmembrane domain and a cytoplasmic SERIF domain. The known members of this family are as follows: IL-17RA, IL-17RB, IL-17RC, IL17RD and IL-17RE.<ref>Template:Cite journal</ref>
Adverse effects of cytokines have been linked to many disease states and conditions ranging from major depression<ref name="pmid20015486">Template:Cite journal</ref> and Alzheimer's disease<ref name="pmid20692646">Template:Cite journal</ref> to cancer<ref name="pmid11239407">Template:Cite journal</ref> with levels either being elevated or changed. Over-secretion of cytokines can trigger a dangerous syndrome known as a cytokine storm; this may have been the cause of severe adverse events during a clinical trial of TGN1412. Cytokine storms also were the main cause of death in the 1918 "Spanish Flu" pandemic. Deaths were weighted more heavily towards people with healthy immune systems, due to its ability to produce stronger immune responses, like increasing cytokine levels. Another important<ref>Template:Cite journal</ref> example of cytokine storm is seen in acute pancreatitis. Cytokines are integral and implicated in all angles of the cascade resulting in the systemic inflammatory response syndrome and multi organ failure associated with this intra-abdominal catastrophe.
Plasma levels of various cytokines may give information on the presence, or even predictive value of inflammatory processes involved in autoimmune diseases such as rheumatoid arthritis,<ref>Kokkonen, H. Arthritis & Rheumatism, Feb. 2, 2010; vol 62: pp 383–391</ref> as well as immunomodulatory effects of foods or drugs.<ref>Template:Cite journal</ref> In addition, elevated levels of IL-7, an important cytokine involved in T cell homeostasis, have been detected in the plasma of HIV-infected patients.<ref>Template:Cite journal</ref>
- Active Hexose Correlated Compound
- Cytokine secretion assay
- Cytokine storm
- ELISA assays
- ELISPOT assays
- FluoroSpot assays
- Granulocyte-colony stimulating factor
- Signal transduction
Template:Commons category <references group="nb" />
- Cytokine Gene Summary, Ontology, Pathways and More - Immunology Database and Analysis Portal (ImmPort)
- Cytokine Tutorial. dead link.
- Cell Interactions: Cytokines. inactive link
- Reperfusion Injury in Stroke.
- CopeWithCytoKines Cytokines Online Pathfinder Encyclopaedia
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