This section will provide information on the general properties of cytokines and help to understand how cytokines are able to regulate the immune system.
PROPERTIES OF CYTOKINES
Cytokines are essentially small proteins that have been secreted in order to regulate and mediate the processes of inflammation, hematopoiesis and immunity. On the whole they act in a short time span, over a short distance and mostly at very low concentration levels (this is for majority of cases but not always). There are also always produced de novo during a response to an immune stimulus. Some of the responses to cytokines can include reducing or elevating membrane receptor protein expression, stimulating proliferation, differentiation and secretion of specific effector molecules. In order for them to function to generate an appropriate response, they need to bind to specific membrane receptors, which leads to the transmission of a signal to the cells through the action of second messengers (usually tyrosine kinases) in order to alter the behaviour of the cell (known as gene expression).
Different cell types are able to secrete the same cytokine and it is also possible for a single cytokine to act on several different cell types (this is referred to as pleiotropy). They are often produced in a cascade, which means that one cytokine is responsible for stimulating its target cells to produce additional cytokines. However, some cytokines can be redundant in their activity because their functions are possible to be stimulated by many other different cytokines. The actions of many cytokines can be described as either being antagonistic (cytokines having opposite actions) or synergistic (two or more cytokines acting together).
Cytokine is most often used as a general name, however, there are more specific terms which are used to identify where the cytokines has been made from for example monokine (cytokines produced by monocytes), lymphokine (cytokines produced by lymphocytes), interleukin (cytokines produced by interleukins) and chemokine (cytokines that possess a chemotactic activity). There action can be described as being autocrine (acting on cells that secreted them), endocrine (acting on distance cells) and paracrine (acting on nearby cells).
Due to their pleiotropy, short half-life, redundancy and low plasma concentrations, these have all made the process of isolating and characterising cytokines more complicated. This has led to identifying new cytokines at the DNA level by using known cytokine genes in order to identify similar newer ones.
Activities of cytokines can be often characterised using a purified cell population in vitro with recombinant cytokines or using knockout mice in order to characterise cytokine function in vivo for individual cytokine genes. They are made up of a very large number of cell populations, but two of the pre-dominant producers of cytokines are macrophages and helper T cells (Th).
The most popular groups of cytokines are interleukins, chemokines and interferons.
- Interleukins are the largest group of cytokines and these play an important role in stimulating immune cell proliferation and differentiation. Some of the most common members of this group include: Interleukin 1 (IL-1): responsible for activating T cells. Interleukin 2 (IL-2): functions to stimulate the proliferation of antigen-activated T and B cells. Interleukin 4, 5 and 6 (IL-4, IL-5 and IL-6): these are vital in stimulating proliferation and differentiation of B cells. Interferon gamma (IFNg) and Tumor Gowth Factor beta (TGF-b): role in activating macrophages. Interleukin 3 and 7 and Granulocyte Monocyte Colony-Stimulating Factor (IL-3 and IL-7): crucial in stimulation of hematopoiesis.
- Chemokines are very important in attracting leukocytes to sites of infection. There are characterised by having conserved cysteine residues and this is used to assign them into four groups. The four groups which are representative chemokines are C-C chemokines (e.g. MIP-1a, MIP-1b, MCP-1 and RANTES), C-X-C chemokines (e.g. IL-8), C chemokines (e.g. Lymphotactin) and CXXXC chemokines (e.g. Fractalkine). There are however, some cytokines which are predominantly inhibitory, such as IL-10 and IL-13 are known inhibit inflammatory cytokine production by macrophages.
- Interferons which include IFN-alpha and IFN-beta (function to inhibit virus replication in infected cells) and IFN-gamma (functions in stimulating antigen-presenting cell MHC expression).
T cells are initially activated as Th0 cells and these can produce IL-2, IL-4 and IFN-gamma. The cytokine environment nearby is then responsible for influencing differentiation into Th1 or Th2 cells. IL-12 can promote Th1 activities whereas IL-4 is able suppress Th1 activity and stimulate Th2 activity. The activities of Th1 and Th2 are described as being antagonistic in nature. Th1 cytokines have the ability to cause IFN-gamma to inhibit the proliferation of Th2 cells, whereas IL-2 and IFN-gamma can stimulate B cells in order to secrete IgG2a and also to inhibit the secretion of IgG1 and IgE. Th2 cytokine can cause IL-10 to inhibit Th1 secretion of IFNg and IL-2 and it can also suppress Class II MHC expression, inflammatory cytokines by macrophages and the production of bacterial killing molecules. The balance between Th1 and Th2 activity can used to steer the immune response in the direction of humoral or cell-mediated immunity.
Th1 cells are able to produce IL-2, IFN-gamma, and TNF-beta, which can then activate macrophages and Tc to stimulate inflammation and cellular immunity. Th1 cells are found to secrete GM-CSF and IL-3 which can lead to stimulating the bone marrow in order to produce more leukocytes. Whereas Th2 cells have the ability to secrete IL-4, IL-5, IL-6, and IL-10 and this is responsible for stimulating the production of antibody by B cells.
Helper T cells are vital for two major functions which are to stimulate B cells to produce antibody and to stimulate cellular inflammation and immunity. There are two functionally distinct subsets of T cells that are able to secrete cytokines that are responsible for promoting these different activities of helper T cells.
Cytokines need to bind to specific membrane receptors in order to act on their target cells. The specific receptors and their corresponding cytokines can be classified into a number of different families that is based on their structures and activities.
- Chemokine family receptors: These are usually containing seven trans-membrane helices and have the ability to interact with G protein. Members of this family include RANTES, MIP-1 and IL-8 and also chemokine receptors CXCR4 and CCR5 have been found to be used by human immunodeficiency virus (HIV) to preferentially enter T cells and macrophages.
- Hematopoietin family receptors: Are often either dimers or trimers which possess a conserved cysteines within their extracellular domains and they also contain a conserved Trp-Ser-X-Trp- Ser sequence. Examples of members for this family are GM-CSF and IL-2 through IL-7.
- Interferon family receptors: These contain a conserved cysteine residues but do not possess a Trp-Ser-X-Trp-Ser sequence. Examples for this family include IFN-alpha, IFN-beta and IFN-gamma.
- Tumor Necrosis Factor family receptors: These consists of four extracellular domains. Examples of receptors for this family include membrane-bound CD40, Fas, soluble TNF-alpha and TNF-beta.
One of the best characterised is the hematopoietin cytokine receptors and these are usually made up of two subunits (one which is cytokine-specific and the other is responsible for signal transduction). Classical example of this is the GM-CSF subfamily, which consists of a unique alpha subunit which has the ability to specifically bind either GM-CSF, IL-3, or IL-5 with low affinity and a shared beta subunit which is a signal transducer and can also elevate the cytokine-binding affinity. The binding of cytokine can promote the dimerization process of the alpha and beta subunits and this then leads to the association with cytoplasmic tyrosine kinases in order to phosphorylate proteins which are essential for activating mRNA transcription. GM-CSF and IL-3 both are able to act on hematopoietic stem cells, progenitor cells and even activate monocytes. Along with IL-5, they are able to also stimulate basophil degranulation and eosinophil proliferation. However, all three of these receptors can only phosphorylate the same cytoplasmic protein. Antagonistic activities of GM-CSF and IL-3 have been discovered and this is sometimes used to explain them competing for limited amounts of beta subunit.
The TNF receptor family molecules such as Fas and CD40 can bind to cell surface ligands on effector T cells (FasL and CD40L). CD40 is mostly expressed on either B cell or macrophage plasma membranes. The binding of T cell CD40L to B cell CD40 can result in stimulateing B cell proliferation and also isotype switching. Whereas, the binding of T cell CD40L to macrophage CD40 can lead to stimulating macrophages to secrete TNF-alpha and thereby become more sensitive to IFN-gamma. The binding of T cell FasL to Fas can initial activation of caspase proteases that can lead to apoptosis of the cell expressing membrane Fas. It is well known that activated lymphocytes can express Fas, so that FasL-positive Tc cells are able to regulate the immune response by eliminating the cells that are activated. A mutant Fas which is found to over-proliferated lymphocytes is often associated to an immune deficiency disease.
The IL-2R subfamily of receptors for IL-2, IL-4, IL-7, IL-9 and IL-15 all are known to have a common signal-transducing gamma chain. Each one of these has a unique cytokine-specific alpha chain. IL-2 and IL-15 are trimers and also can share IL-2R beta chain. Monomeric IL-2R alpha is found to have low affinity for IL-2, where the dimeric IL-2R beta/gamma has intermediate affinity, and trimeric IL-2R alpha/beta/gamma bind to IL-2 with high affinity. IL-2R alpha chain (Tac) is expressed only by activated but not resting T cells, whereas resting T cells and NK cells are able to constitutively express low numbers of IL-2R beta/gamma. The activation of antigen causes stimulation of T cell expression with high affinity IL-2R trimers, in addition to the secretion of IL-2. This process can allow the autocrine stimulation of T cell proliferation in an antigen-specific manner. The antigen specificity of the immune response is essentially maintained by the close proximity of antigen-presenting B cells and macrophages with their respective helper T cells, in order that cytokines are able to secrete in the direction of and close to the membrane of the target cell. There is evidence for X-linked severe combined immunodeficiency (X-scid) which is the result of a defect in IL-2R family gamma chain and this often leads to loss of activity from this family of cytokines.
During an immune responses the fragments of membrane receptors can be shed and these then are able to compete for cytokine binding. The activity of cytokines can therefore be blocked by these antagonists (i.e. molecules which can bind cytokines or their receptors). An example of this is when IL-1 has a specific antagonist that can block the binding of IL-1 alpha and IL-1 beta to their receptor. Finally, microbes can also directly influence cytokine activities. For example, the Vaccinia virus (Cowpox and Smallpox) is encoding soluble molecules that are able to bind IFN-gamma, whereas the Epstein-Barr virus (Infectious Mononucleosis) is able to encode a molecule homologous to IL-10 that has the ability to suppress the immune function in the host.