User:Kinkreet/Immunology/Cytokines

Cytokines describe any intercellular signalling molecule which is secreted by cells; within this group are the chemokines and interleukines. Chemokines are cytokines which attracts or defer cell movement along its concentration gradient; interleukines are a general name for cytokines which communicates between immune cells.

Most cytokines are between 15-25 kDa. Most act locally in an autocrine and paracrine manner, though some (e.g. IL-1) can work in an endocrine manner. They bind to receptors with high affinity and avidity, and thus can produce effects even in very low concentrations. Cytokines tend not to be expressed constitutively, and most are induced only for a short period of time; mRNA of cytokines have repeated regions of UAUU, and thus are degraded easily, typically having a half-life of no more than 30-120 minutes. Furthermore, after a cell has transcribed the cytokine gene (for ~12 hours), most cell tend to be unresponsive for a few days; this is thought to prevent over-inflammation.

IL-1β is produced as part of the resposne of many Toll-like receptors (TLRs). IL-1R is a receptor for IL-1 which is often co-expressed on the surface of the same TLR-bearing cell, and it shares the same downstream signalling pathway as TLRs using the MyD88 pathway; therefore, the signal for IL-1 would get amplified with time during an immune response, which could be beneficial for acquiring a quick response, but if dysregulated, can lead to chronic inflammation.

IL-2
In the 1970s, supernatant from activated T cells was shown to promote T cell growth, the factor identified was IL-2. IL-2 also promote proliferation, differentiation and survival of mature T and B cells, as well as the cytolytic activity of Natural Killer (NK) cells

The interleukin 2 receptor is made up of 3 subunits - IL-2Rα (CD25), IL-2Rβ (CD122) and γc(CD132) - which associates non-covalently. The IL-2Rα and IL-2Rβ subunit are involved in IL-2 binding, whereas the IL-2Rβ and γc subunit are involved in signal transduction. JAK3 associate with γc, Janus kinase 1 (JAK1) associates with IL-2Rβ and are activated when IL-2 binds; together, they activate the MAP kinase (MAPK) pathway, the Phosphoinositide 3-kinase (PI3K) pathway, and the JAK-STAT pathway, all signalling for growth, survival, transcriptional regulation and effector differentiation. In the JAK-STAT pathway, activated JAK phosphorylates tyrosines on STAT, STAT then dimerize by using its SH2 domain to bind to phosphotyrosine on another STAT, and vice versa. The STAT dimer then enter the nucleaus and act as a transcription factor. This pathway is used by most class I and II cytokine receptors.

IL-2 first binds with low affinity (binding affinity Kd≈10 nM) to IL-2Rα, and upon binding, this stabilizes a secondary binding site on the IL-2 for IL-2Rβ to bind, albeit with very low affinity (Kd≈100 nM). However, the bound IL-2Rβ then recruits γc to form a quaternary complex; the complex of IL-2Rβ and γc binds with intermediate affinity (Kd≈1nM), and altogether, the whole quaternary complex binds with high affinity (Kd≈10μM). In NK cells, macrophages, and resting T cells, the receptor expressed is not the trimeric receptor, but just the IL-2Rβ/γc heterodimer.

The γc subunit is shared amongst the receptors for IL-4, IL-7, IL-9, IL-15 and IL-21. The gene for the γc subunit is found on the X-chromosome, so people with X-linked severe combined immunodeficiency disease (X-SCID) is not responsive to IL-2, 4, 7, 9, 15 and 21. The IL-2Rα is specific to IL-2, whereas the IL-2Rβ subunit can also be found in the IL-15 receptor.

The function of IL-2 was elucidated in 1995 where IL-2 knockout mice are observed to mount an autoimmune response, indicated by a high level of T helper cells, antibodies and lesions in several organs. Treatment of the knockout mice with anti-CD40L prevented this reaction. As CD40-CD40L are involved in the positive costimulatory pathway, IL-2 is thought to provide a signal for self tolerance. This is confirmed in a separate study where IL-2 KO mouse mount an attack, using T cells of both kinds, against islet cell allografts.

TNF-α

IFN-α, -β and -γ induce the transcription of most HLA genes, to produce MHC molecules to be expressed on the surfaces of cells, even those that do not normally express MHC. IFN-γ also prime macrophages, B cell class switching to IgG3 (the best at fixing complement), enhances antigen processing and MHC loading.

Modes of action
One cytokine can produce many effects, this is called pleiotropy; IL-4 can promote the proliferation of thymocytes, B cells and mast cells, but also the class switching to IgE. Likewise, many cytokines can produce the same effect, this is called redundancy; IL-2, IL-4 and IL-5 all promote B cell activation. Sometimes two cytokines are required together to signal for an effect, this is called synergy; IL-4 and IL-5 can each separately signal for the class switching of B cells to secrete IgE, but together they produce a stronger signal than the sum of both individually. Then there are competing cytokines with opposite effects acting against each other, this is called antagonism; IFN-γ is a TH1 response cytokine which blocks IL-4, a TH2 response cytokine.

Types of cytokines
There are four structurally distinctive families of cytokines: haematopoietins, interferons, chemokines and tumour necrosis factors (TNFs).

Interferons are produced by many cell types in the innate immune system in response to viral infections; it is released by infected cells to make non-infected neighbouring cells more resistent to the virus. Interferons prevent protein synthesis and viral replication, by producing ribonuclease L (RNase L, for latent) which degrades all RNA within the cell, both cellular and viral. There are two types of interferons, of the α and β families; these are different from IFNγ, which are predominantly produced by TH1 cells.

Tumour necrosis factors (TNFs) were originally found by regression in certain tumours, and thus they are known to be able to combat tumour cells by inducing apoptosis and inflammation. There are two types of TNFs - TNFα produced by macrophages, and TNFβ produced by T-cells - the two are 30% identical, and binds to the same receptor.

TNF production is induced by the binding of LPS by the Toll-like receptor TLR4.

Just as there are four types of cytokines, there are 5 types of cytokine receptors: immunoglobulin superfamily receptors, Class I cytokine receptors

The immunoglobulin superfamily receptors can bind to IL-1, M-CSF and C-Kit.

Class I cytokine receptors (haematopoietin receptor) are dimers with each subunit having multiple extracellular domains, the domain closest to the membrane contains a conserved WSXWS sequence, while the second closest domain contains 4 conserved cysteines. The class I receptors are split into three subfamilies, depending on the common signalling subunit it employs; the common subunit complexes with another subunit, and it is that subunit which binds to a specific cytokine and gives the receptor its specificity. Members of the GM-CSF receptor subfamily have a common β subunit for signal transduction, and can bind to GM-CSF, IL-3 and IL-5; members of the IL-6 receptor subfamily share a common gp130 subunit, and can bind to IL-6, IL-11, CNTF and LIF/OSM; members of the IL-2 receptor subfamily share a common γ subunit, and is activated by binding to IL-2, IL-15, IL-7, IL-9 and IL-4. This sharing of signal transduction subunits can explain redundancy when different cytokine receptors uses the same subunit for signal transduction; it also explains antagonism, where a cytokine with a higher affinity will outcompete with another cytokine for the common subunit, but then again, the antagonism may not be obvious as the cytokines are redundant.

Class II cytokine receptors (interferon receptor) binds interferons of all classes and IL-10. IL-10 is an anti-inflammatory cytokine expressed by Treg alongside TGFβ. Treg regulate by generating extracellular adenosine, which have an immunosuppressive effect when bound by adenosine receptors.

TNF receptors, or death receptor, is a homotrimeric complex which binds to TNF, CD40 (costimulatory), nerve growth factor (NGF) and FAS. Each subunit has four repeated domains, each containing 3 conserved regions.

Chemokine receptors are all G-protein coupled receptors (GPCRs), and binds to 8-10kDa chemokines such as IL-8, RANTES, MIP-1, PF4, MCAF, NAP-2. Once bound, the G protein is though to dissociate, with the α-subunit dissociating to activate adenylyl cyclase, producing cAMP, a mark of stress. The β- and γ-subunits then activates multiple enzymes involved in actin polymerization, adhesion, cytoskeletal rearrangement, differentiation and porliferation, all of which leads to the chemotaxis of the chemokine receptor-bearing cell to move along the concentration gradient.