Inflammatory Cytokines Function & Release
Cytokines are essential communicators in the immune system, orchestrating cell interactions and responses to maintain balance and combat diseases.
- Cytokines are small proteins crucial for cell communication, particularly in the immune system.
- They can be pro-inflammatory or anti-inflammatory, affecting cell interactions and responses.
- Cytokines are produced by various cell types, including immune cells like macrophages, B cells, and T cells.
- They differ from chemokines in structure and function, with chemokines primarily acting as chemoattractants.
- Cytokines play vital roles in inflammation, cancer development, autoimmune diseases, and immune response regulation.
What are Cytokines?
Cytokines are small secreted proteins released by cells that are a major communication mechanism used by the immune system. They also have specific effects on the interactions between cells. Cytokines can either have pro or anti-inflammatory effects on the host depending on the family of the cytokine
Cytokines interact with cell surface receptors in order to be activated and carry out their specific functions. The are particularly important in the immune system as they regulate the balance between cell-based and humoural responses. They also modulate the maturation, growth, and activity of certain cell populations. Cytokines also have the ability to enhance or inhibit the action of other cytokines. They can do this by increasing or dampening down specific signals that stimulate the production, activation and migration of other cytokines to a site of infection.
Cytokines can be produced by a variety of different cell types including immune cells such as macrophages, B cells and T cells. T cells that produce cytokines include T helper cells (CD4+) which include Th1, Th2, Th17 and regulatory T cells (Treg). Additionally, cytotoxic T cells also produce a variety of cytokines. Other cells involved in the production of pro inflammatory cytokines and anti inflammatory cytokines include mast cells, endothelial cells, fibroblasts and various stromal cells (connective tissue cells of any organ). Certain cytokines can be produced by more than one cell type.
Cytokines vs Chemokines
Cytokines and chemokines are both cytokines, signaling molecules that play important roles in the immune system. However, there are some key differences between the two.
For one, cytokines are produced by a variety of cells, including T cells, B cells, macrophages, and more. Chemokines, on the other hand, are mostly produced bywhite blood cells known as neutrophils.
Additionally, cytokines typically exist as dimers or oligomers, while chemokines are monomers. This difference in structure leads to different functions for each type of cytokine. Cytokines generally act as growth factors or regulators of cell proliferation and differentiation, while chemokines typically function as chemoattractants, helping to direct the movement of cells.
Finally, cytokines are often named for their function, while chemokines are typically named for their structure. For instance, IL-1 is a cytokine that regulates inflammation, while CXCL8 is a chemokine that attracts neutrophils to an infection site.
Cytokines are classified according to their cell of origin or their mechanism of action and are produced throughout the body. Cytokine is an umbrella term and these secreted molecules are given other names based on their effector function, cell of secretion or target of action. For example, cytokines that are produced by lymphocytes and are involved in the interactions between those cells are called lymphokines or interleukins (ILs). Additionally, cytokines produced by macrophages and monocytes are called monokines.
Proinflammatory cytokines are a class of molecules that play an important role in inflammation. These molecules are produced by both B cells and T cells, and they help to regulate the inflammatory response. proinflammatory cytokines can be divided into two groups: those that are secreted by immune cells, and those that are produced by other cell types in response to inflammation. The best-known proinflammatory cytokines include interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha). These cytokines play a key role in the development of inflammation, and they are involved in a wide range of inflammatory diseases.
Proinflammatory cytokines & cancer development
Proinflammatory cytokines are also involved in the development of cancer. Some proinflammatory cytokines, such as IL-1 and TNF-alpha, can promote the growth of cancer cells. Other proinflammatory cytokines, such as IL-6 and IL-10, can suppress the immune system's ability to fight cancer. Cancer cells often take advantage of the proinflammatory cytokines to help them grow and spread.
Proinflammatory cytokines & autoimmune disease
Proinflammatory cytokines are also important in the development of autoimmune diseases. Autoimmune diseases occur when the body's immune system attacks healthy tissues. Proinflammatory cytokines play a key role in the development of these diseases by promoting inflammation. Many autoimmune diseases, such as rheumatoid arthritis and Crohn's disease, are characterized by chronic inflammation. Proinflammatory cytokines are also involved in the development of allergies and asthma. Allergies and asthma are both characterized by inflammation of the airways. Proinflammatory cytokines play a key role in the development of these conditions by promoting inflammation.
What are chemokines?
What are chemokines?
Chemokines are a type of cytokine that functions by attracting cells to a site of infection. They are very small in size ranging between 8 and 10 kDa. Their name signifies their function, chemo relates to the fact they use chemotaxis, or movement in response to a chemical stimulus to stimulate the migration of cells from one area to another. Whereas -kines relates to the fact that they are a type of cytokine.
chemokines are important mediators of the immune response and are involved in a variety of inflammatory diseases. chemokines are produced by various cells, including leukocytes, endothelial cells, fibroblasts, and epithelial cells. They bind to chemokine receptors on the surface of leukocytes and stimulate them to migrate towards the site of infection. chemokines are classified according to their structure into four families: CXC, CC, CX3C, and XC. Each family has several subtypes that differ in their amino acid sequence and binding affinity for chemokine receptors. The most well-studied chemokine is interleukin-8 (IL-8), which belongs to the CXC family.
How are chemokine produced?
Chemokines are produced by various cells in response to inflammation and serve as chemoattractants for leukocytes, playing an important role in the immune response. They are small proteins that bind to chemokine receptors on the surface of leukocytes and stimulate them to migrate towards the site of infection. chemokines are classified according to their structure into four families: CXC, CC, CX3C, and XC. Each family has several subtypes that differ in their amino acid sequence and binding affinity for chemokine receptors. The most well-studied chemokine is interleukin-8 (IL-8), which belongs to the CXC family.
Chemokines play major roles in immunological reactions, in which they are the secondary pro-inflammatory signalling proteins that are produced as a result of the production of primary cytokines. They also function to maintain homeostasis, in which they regulate cell movement in order to ensure that the immune system functions at its optimal state. Chemokines must bind to their receptor in order to become activated. Chemokine receptors are mainly present on the surface of white blood cells (lymphocytes).
Chemokines differ in their structure however they all share similar structural homology. Most chemokines share the presence of four cysteine amino acids, two of which are used to classify all chemokines into four types. Specifically, the two amino acids close to the N-terminal region of the chemokine are used for the classification into CC, CXC, C, and CX3C types. The X symbolizes an amino acid and differs depending on the type of chemokine.CC chemokines have two cysteines that are next to each other, for example, CCL2 contains two cysteines next to each other and two leucine’s at the carboxyl terminus. CXC chemokines have one amino acid between the two cysteine residues, for example CXCL1. C chemokines have two cysteines instead of four, one of which is at the N-terminus and CX3C chemokines have three amino acids between the two cysteines.
Cytokine release syndrome/cytokine storm
Cytokine release syndrome, also called cytokine storm, is a systemic inflammatory response characterised by a large and dysregulated release of pro-inflammatory cytokines.
Under homeostatic conditions, cytokines help to coordinate the immune system’s defense against pathogens. However, large quantities of cytokines can spur the recruitment of mass amounts of immune cells, causing widespread inflammation along with a decrease in the functionality of local organs affected.
Certain types of immunotherapy can cause CRS, with monoclonal antibodies and CAR-T cells the most common treatments to induce the syndrome. The immune cells affected by immunotherapy, as well as the therapeutics cells themselves, contribute to the rapid outpouring of cytokines. Markers and symptoms for CRS are discussed in detail below. Although most patients experience only a mild reaction, NCI and othr institutions state that the condition can be life-threatening.
IL-6 in Cytokine Storm
Elevated levels of IL-6 have been reported in patients with CRS and in murine studies. IL-6 is unique in that it signals through two different modalities: ‘trans’ signalling via soluble receptors and ‘classic’ membrane-bound receptor interactions. Both ultimately utilize the membrane-bound gp130 receptor subunit for signal transduction. After IL-6 binds to its soluble receptor, it can bind the gp130 present on cell types which do not express the membrane-bound IL-6 receptor. Thus, it can induce and initialise signalling in a wide range of cells. This feature may be part of what makes IL-6 such a pervasive mediator of CRS.
IL-6 is thought to contribute to some of the most common symptoms of cytokine release syndrome. The soluble receptor ‘trans’ signalling is key in this feature. Hallmarks of severe CRS like complement activation, vascular leakage, and disseminated intravascular coagulation are thought to be mediated by soluble receptor/IL-6 signalling paths. Trans signalling can also intensify the effects of IL-6 on target cells by enhancing TNF and IL-1 activity when concentrations of TNF and IL-1 soluble receptors are high.
IFNγ & IL-10 in Cytokine Release Syndrome
IFNγ can cause many flu-like symptoms, including fever, headache, dizziness and fatigue. It can also induce the activation of different immune cells, including macrophages. Once active, these cells can produce massive quantities of many different cytokines, including IL-6, TNFα and IL-10. In turn, TNFα can generate further flu-like symptoms (fever, fatigue, etc.) and mediate vascular leakage, acute phase protein production, and lung injury.
Endothelial Cells in Cytokine Storm
Emerging markers of severe CRS include Ang-2 and von Willebrand factor, both signs of endothelial cell activation. These are often found to be elevated in CRS patients. Endothelial activation implies that these cells play a crucial role in the disease course of CRS, both as targets and as inducers of further inflammatory responses. These cells mediate the vascular dysfunction which contributes to the symptoms seen in severe cases, such as hypotension and vascular leakage. Endothelial cells are also thought to be another source of IL-6 in severe cases of cytokine storm.
Symptoms of Cytokine Release Syndrome
CRS most commonly begins with a fever and flu-like symptoms, but can quickly worsen. Patients who develop the syndrome generally recover within two weeks; most do not suffer repercussions or long-term effects after CRS has been treated.
Common symptoms of CRS are outlined below. However, it should be noted that CRS is a heterogeneous condition which can present with different symptoms in varying levels of severity.
Cytokines released by th1 and th2 cells
Th1 cells are one type of CD4+ T cell subset that helps fight off intracellular pathogens. There are two main subsets of th1 cells, cytotoxic t lymphocytes (CTLs) and natural killer (NK) cells. These th1 cells produce cytokines IL-2, IFN gamma, TNF alpha, and play a key role in the development of cell-mediated immunity against th1 specific antigens. In contrast to th2 cells which help fight off extracellular pathogens, th1 cells only help fight off intracellular pathogens.
Macrophage & Dendritic Cell activation of th1 cells
The cytokine IL-12 is secreted by macrophages and DCs to activate th1 cells. IL-12 also has the ability to activate NK cells, which secrete cytokines that can stimulate th1 cell differentiation and proliferation. TNFα is released during infections by th1 specific pathogens and activates th 1 cells as well as NK cells to become cytotoxic or proliferate. IFNγ not only activates macrophages and th1 cells, it also activates T helper type 2 (Th2) cells to become mature. Another cytokine that is released by th1 cells when they are activated is IL-18. This cytokine can activate th 1 cells in the absence of antigen presentation by DCs, but it will elicit stronger th1 cytokine responses in the presence of th2 cytokines IL-4 or IL-10.
Cytokines released by th2 cells
IL4: binds to the IL-4 receptor, which is composed of an α and β subunit that are bound together by a disulfide bond. This receptor triggers multiple pathways. One route leads to the activation of RNA polymerase II, which induces transcription of several genes involved in immune response such as those encoding immunoglobulin (Ig) or T–cell receptors (TCR). Other activated pathways lead to the activation of mitogen-activated protein (MAP) kinase, which activates several transcription factors involved in immune response.
IL5: induces B cells to produce IgG2a or IgG1 antibodies and is important for eosinophil survival and differentiation. IL6 is secreted by macrophages in response to interferon-γ (IFN-gamma), TNF-alpha, thrombin, lipopolysaccharide (LPS), or transforming growth factor (TGF)-β. It acts on epithelial cells to upregulate adhesion molecules that mediate neutrophil adherence during inflammation.IL10: inhibits the activity of Th1 cytokines such as IFN-γ, TNF-alpha, and IL2. It also has the effect of promoting class switching to IgG1 and inhibits production of proinflammatory cytokines such as TNF-α, IL1B, and IL6.
IL13: enhances activation, proliferation, differentiation, migration, survival, and memory of B cells so that they produce large amounts of IgE antibodies in response to antigens containing polysaccharides. Eosinophils respond to IL4 by producing eotaxin which recruits these cells into inflammatory tissues. Th2 cytokines induce expression of other peptide or protein cytokines including granulocyte macrophage colony stimulating factor (GM-CSF) from macrophages and Trp65 from keratinocytes, which leads to the production of IL-1 and TNF-α.
Proinflammatory cytokine IL-6
Interleukin-6 (IL-6) is a 21 kDa glycoprotein that was discovered in the 1980s. IL-6 was originally termed B cell stimulatory factor 2 and thought to be a member of the interferon (IFN) family; however, cloning of IL-6 led to the discovery that IL-6 does not have any IFN-inducing ability, and instead has since been identified to have many diverse functions including cell growth and robust pro-inflammatory responses (Hirano et al, 1988). IL-6 is primarily expressed by antigen presenting cells (APCs) such as macrophages and dendritic cells and B cells, although secretion is also described CD4+ T cells (Dienz et al, 2009; Hirano, 1998).
IL-6 and inflammation
As excess levels of IL-6 or the IL6R are associated with pathogeneses, blocking the IL-6R has revealed promising results in certain inflammatory diseases and cancer (Allocca et al, 2013; Coward et al, 2011). Targeting IL-6 signalling molecules is being explored as a therapeutic strategy (Aparicio-Siegmund et al, 2014). STAT3 inhibitors are currently being tested in clinical trials to treat various cancers, where the anti-apoptotic role of STAT3 is targeted (Page et al, 2011).
IL-6 and JAK/STAT
Moreover, several efforts have been made to inhibit the activities of JAK1 and JAK2 in an effort to suppress the robust inflammatory response induced by the JAK/STAT pathway (Leonard and O’Shea, 1998), however an antibody specifically targeting the IL-6R, called tocilizumab, may prove to be more beneficial for treating inflammatory disease (Garbers et al, 2015). Additionally, another monoclonal antibody, targeting IL-6, called Siltuximab, has shown positive results in clinical trials treating prostate cancer and multiple myeloma (Dorff et al, 2010; Rossi et al, 2010).
Proinflammatory cytokine IL-1
The cytokine, IL-1, has been the subject of copious research since it was first identified. This pro-inflammatory cytokine is known to play a dichotomous role in disease by inducing pathogenesis of auto-inflammatory disorders, while simultaneously defending against invading pathogens . IL-1 is implicated in the development of numerous inflammatory diseases including ulcerative colitis. This has resulted in the mechanism behind IL-1 signalling becoming the subject of much interest. Likewise, novel members of both the IL-1 family of cytokines and the IL-1 receptor (IL-1R) family are also the focus of intense research in order to determine their involvement in the host response to disease. I will now discuss these cytokines and receptors in more detail, focusing specifically on the IL-33/ST2 and IL-36/ IL-36R pathways as these are the subject of this thesis.
IL-1 and IL-1R Family Members
The IL-1R/ toll-like receptor (TLR) superfamily consists of two subgroups: the IL-1R family whose members contain three extracellular immunoglobulin domains (Ig) and the Toll-like receptor subgroup whose members contain extracellular leucine-rich repeats. All family members share a similar intracellular Toll-IL1R (TIR) domain, which is required for binding of adaptor proteins. The IL-1 family is divided into three subfamilies based on the length of the N-terminal domain (Table 1). The IL-1 subfamily consists of IL-1α, IL-1β, IL-1Ra and IL-33. The IL-18 subfamily is composed of IL-18 and IL-37, and finally the IL-36 subfamily contains IL-36α, IL-36β, IL-36γ, IL-36RN and IL-38.
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Type 1 cytokines
Type-1 cytokines are cytokines produced by Th1 CD4+ T cells and include IL-2, IFN-gamma, IL-12 and TNF-beta. Type-2 cytokines are those produced by Th2 CD4+ T cells and include IL-4, IL-5, IL-6, IL-10 and IL-13.
Cytokines can also be split into family groups depending on their secondary and tertiary structures. For example, the cytokines IL-6, IL-11, CNTF, LIF, OSM (Oncostatin-M), Epo (Erythropoietin), G-CSF, Growth Hormone (GH), Prolactin (PRL), IL-10, IFN-alpha and IFN-beta all form long chain 4 helix bundles.
On the other hand, the cytokines, IL-2, IL-3, IL-4, IL-5, IL-7, IL-9, IL-13, GM-CSF, M-CSF, SCF, IFN-gamma all form short chain 4 helix bundles giving a different structure to the cytokines mentioned above.
Type I Cytokine Receptors
Type I cytokine receptors have conserved motifs in their extracellular n-terminal domain however they do not possess intrinsic protein tyrosine kinase activity. This family includes receptors for IL-2 (beta subunit,) IL-3, IL-4, IL-5, IL-6, IL-7, IL-9, IL-11, IL-12, GM-CSF, G-CSF, Epo, LIF and CNTF. Type I cytokine receptor family is subdivided into three groups based on the ability of the cytokine receptor family members to form complexes with one of three different types of receptor signalling components. These include gp130, common beta, and common gamma which is also known as the gamma chain of the IL-2 receptor.
Type II cytokine receptors
Type II cytokine receptors are multimeric receptors and are made up of heterologous subunits. The type II cytokine receptors are mainly used by the interferons. This family includes receptors for the cytokines, IFN-alpha, IFN-beta, IFN-gamma, IL-10 and IL-22. The extracellular domains of type II cytokine receptors are similar in structure to their ligand-binding domain. Within the various receptor subgroups, there are several conserved intracellular sequence motifs which are proposed to function function as binding sites for the intracellular proteins JAK and STAT proteins in the signal transduction cascade.
Cytokines are essential controllers of cells and they control cell growth, migration, development and differentiation. Their effects may be suppressive or enhancing based on the family which they are divided in to, and their biological structure and function.
Additionally, cytokines can exert cytotoxic effects on infectious agents such as tumour cells, either directly or by activating cells with cytotoxic potential. Any given cytokine may have many different biologic effects, however, two different cytokines from opposing families may have similar or identical activities. Cytokines play crucial roles in tissue damage repair, in tumour cell development and progression, in the control of cell replication and apoptosis (programmed cell-death), and in the regulation of a variety of immune responses.
Some examples of cytokines involved in cellular and systemic activities include those that are involved in the promotion of pro-inflammatory responses which include, IL-1 beta, IL-18, IL-33 and IL-17. Those that are involved in the mediation of innate immunity include Type I IFNs, TNF-α and IL-17. Cytokines involved in the regulation of lymphocyte growth activation and differentiation include IL-2, IL-4, IL-5, IL-12 and IL-5. Those involved in the activation of inflammatory cells include Type II IFNs and IFN-γ. Cytokines involved in the stimulation of haematopoiesis, which is the production of the cellular components of blood and blood plasma include IL-3, IL-5, IL-7 and GM-CSF.
Although there are is a multitude of cytokines, there are certain groups which carry out the majority of physiological actions in the host. Some examples of these are interleukins, interferons, tumour necrosis factor and chemokines.
Interleukins are glycoproteins that play a role in the activation and differentiation of immune cells. They are produced by lymphocytes, monocytes and various other cells in the body. They are involved in the proliferation, maturation and migration of cells as well as pro- and anti-inflammatory activities. Interleukins stimulate various up-regulatory and down-regulatory mechanisms which impacts the activation and repression of genes. Additionally, interleukins can also influence the synthesis and functioning of other interleukins within the family.
Interleukins belong to a superfamily that is made up of proteins with varying sizes. Currently over 43 members of this superfamily have been identified and these range from IL-1 to IL-43. Within the superfamily of interleukins, the functions of the cytokines differ depending on their structure, biological activity and receptor that they bind to.
The interferon family consists of various cytokines that are widely expressed throughout the body. Like other cytokines, interferons are also released by cells of the host's immune system in response to invading organisms such as bacteria and viruses. Interferons are expressed by a variety of cells including viral infected cells (Type I IFNs) and T cells, natural killer cells (NK cells) and macrophages. Interferons aid in the recruitment of effector molecules that protect the cells against infection. For example, as interferons stimulate the activation and production of NK cells and macrophages, they contribute to the destruction of both the viruses and infected cells. Three types of interferons have been identified thus far and these include Type I interferons, Type II interferons, and Type III interferons.
Type I Interferons
Type I interferons are divided into two major groups, IFN-α and IFN-β. There are additional members of the type I IFN family that include IFN-κ, IFN-ω, and IFN-δ, however, these members are less characterised. Only one type of IFN-β exists whereas IFN-α is subdivided into different subtypes including IFN-α1, IFN-α2, IFN-α3, IFN-α4, IFN-α5 and IFN-α6.
The major functions of Type I IFNs (IFN-α and IFN-β) are to protect against viral infection which is achieved through the destruction of viral mRNA in virally infected cells which is required for viral replication. Type I IFNs also inhibit the translation of viral proteins thereby preventing the production of a viral progeny. Additionally, Type I IFNs promote an increase in ligands to the receptors of NK cells which thereby stimulates the cells to attack and lyse infected cells. Type I IFNs also stimulate the activation of pathways that leads to the destruction of infected cells by NK cells and macrophages.
Type II Interferon
Type II interferon is made up of a single cytokine known as IFN-gamma (IFN-γ). This cytokine is mainly produced by Th1 T cells, Natural Killer cells, CD8+ T cells (effector T cells) and in some cases antigen-presenting cells such as dendritic cells and macrophages and B cells.
Type II interferon is different from the other interferons due to the fact that it does not possess a potent antiviral effect, however, IFN-γ can contribute to the long-term control of virally infected cells by activating MHC complexes. Its main function is to activate effector cells giving the cytokine a predominantly pro-inflammatory effect. In innate immunity, IFN-γ is produced mainly by NK cells, whereas in adaptive immunity, it is produced largely by T cells and increased production of IFN-γ is stimulated by the cytokines, IL-12 and IL-18. The cytokines IL-4 and IL-10 are involved in the negative regulation of IFN-γ production. Some other functions of IFN-γ include macrophage activation which promotes the phagocytic and pinocytic activities of these cells. This therefore contributes to pathogen destruction. Additionally, IFN-γ inhibits cell growth and promotes apoptosis of non-self cells or infected cells.
Type III interferons
Type III interferons are divided into three different cytokines that include IFN-λ1 (IL-28a) IFN-λ2 (IL28b), and IFN-λ3 (IL-29). Type III interferons share structural homology to the signalling proteins in the IL-10 family. However, the signalling pathways of Type III interferons is similar to that of Type I interferons due to the fact that activation of Type III interferons are dependent on the actions of interferon regulatory factors (IRFs) and the transcription factor NF-kB. Whilst Type III interferons share a number of similar functions with Type I interferons, they primarily function in mucosal epithelial cells and liver cells where they serve to protect host cells from viral infections.
TNF cytokine family
Tumour Necrosis Factor (TNF) consists of a group of proteins involved in a variety of physiological and pathological processes. There are currently 40 members within the TNF superfamily, however TNF-α and TNF-β are the most notable and well characterised.
TNF-α is a multifunctional cytokine that plays a role in immunity and programmed cell death. Its major role in immunity is to attract certain immune cells to the site of infection site by stimulating the expression and production of adhesion molecules by vascular endothelial cells. This allows immune cells to adhere to blood vessel walls which promotes their migration to the infected site and thereby allows for the destruction of invading pathogens. Additionally, TNF-α also stimulates the production of chemokines that are involved in inflammatory responses. This promotes the migration of immune cells to the site of infection.
TNF-α also promotes the programmed cell death of tumour cells via apoptosis or necrosis. This is carried out by promoting the recruitment of proteins involved in cell death signalling. When produced in large amounts, TNF-α has the ability to reduce blood pressure or shock during events as severe infections. However, when this cytokine is produced in large concentrations, it can result in low blood sugar concentration and intravascular thrombosis (condition caused by the development of small blood clots throughout the bloodstream which can block small blood vessels causing excessive bleeding.)
TNF-β, also known as Lymphotoxin-alpha is a transmembrane protein. It is produced by activated lymphocytes and is involved in the regulation of cell survival, proliferation, differentiation and apoptosis. TNF-β plays a major role in innate immune regulation and its production has been conveyed to prevent tumour growth and destroy cancerous cell lines. However, uncontrolled expression of TNF-β can result in uncontrolled cellular growth and the production of tumours. TNF-β has similar functions to TNF-α in that it is a potent regulator of various immune and inflammatory responses. Additionally, it also plays a role in the coagulation of blood.
Cytokine signalling is an essential component of the regulation of the body. In order for the immune system to function effectively, cytokine signalling is essential. For example, macrophages and dendritic cells phagocytose foreign particles and send a cytokine signal to inactivated white blood cells nearby. The cytokines released bind to receptors on the lymphocytes (white blood cells) which in turn allow the lymphocytes to become activated and carry out their designated functions. The combination of the macrophages or dendritic cells releasing cytokines and the activation of lymphocytes through cytokine signalling aid in the regulation of homeostasis in the body.
Cytokine receptors contain one to three chains. The ligand-binding subunit of a receptor is called the alpha chain. Other signal transducing subunits are named beta chains or gamma chains. Cytokine receptors are all associated with one or more members of JAKs, which stimulate tyrosine phosphorylation after the ligand (cytokine) binds to its receptor. This in turn recruits various signalling proteins (STATs) to the receptor complex initiating a response depending on the cytokine.
There are two main signal transduction pathways that are involved in cytokine activation. Both lead to the activation of protein kinases that stimulate phosphorylation which subsequently stimulate secondary signal transduction and amplification of the signal.
The first pathway, used by mitogenic cytokines such as EGF which induces cell growth and proliferation, contains tyrosine kinases as the main signal transducers which can sometimes be intrinsically built into the receptors for the designated cytokine.
The second pathway involves activation of phospholipases, which stimulate the production small proteins that activate serine-threonine kinases and raise calcium levels inside the cell. This pathway is used by TGF-beta.
Both of the major pathways also involve effector molecules downstream of the signalling pathways such as GDP-binding proteins, calcium-binding proteins, phosphatases and the products of proto-oncogenes (genes that promote tumour production). Although receptors may differ depending on the cytokine family, the respective ligand or cytokine can share similar signal transduction pathways despite the fact that receptor families may be different.